THE P2X3 RECEPTOR, METHODS OF ALTERING P2X3 RECEPTOR ACTIVITY AND USES THEREOF

Abstract: The subject invention relates to the P2X3 receptor, methods of modulating the activity of the P2X3 receptor and to uses of these methods. In particular, such methods may be used, for example, to accelerate the rate of resensitization of a desensitized receptor.

Description: THE P2X, RECEPTOR, METHODS OF ALTERING P2X 3 RECEPTOR ACTIVITY AND USES THEREOF The present application is a Continuation- In-Part of pending U.S. patent application Ser. No. 09/191,136, filed on November 13, 1998, which is Continuation- In-Part of U.S. patent application Ser. No. 09/008,526, filed on January 16, 1998, now abandoned, and U.S. patent application Ser. No. 09/008,185, filed on January 16, 1998, now abandoned. The present application also claims priority from U.S. provisional application Ser. No. 60/130,339, filed on April 21, 1999. BACKGROUND OF THE INVENTION Technical Field The subject invention relates to the P2X 3 receptor, methods of modulating the activity of the P2X 3 receptor and to uses of these methods. In particular, such methods may be used, for example, to accelerate the rate of receptor resensitization, when the receptor has been desensitized. Furthermore, the present invention also encompasses the use of receptor antagonists, in particular, a P2X 3 receptor antagonist, to minimize the sensation of pain in a mammal. Background Information P2X receptors function as homomultimeric cation-permeable ion channels and, in some cases, as heteromeric channels consisting of two different P2X receptor subtypes (Lewis et al., Nature 377:432-435 (1995); Le et al . , J. Neurosci. 18:7152-7159 (1998); Torres et al . , Mol . Pharmacol . 54:989-993 (1998)). At least one pair of P2X receptor subtypes, P2X 2 and P2X 3 , functions as a heteromeric channel in rat nodose ganglion neurons where it exhibits distinct pharmacological and electrophysiological properties (Lewis et al . , supra (1995)). With respect to individual receptors, the rat P2X 2 receptor is expressed in the spinal cord, and in the nodose and dorsal root ganglia (Brake et al . , Nature 371:519-523 (1994)), while rat P2X 3 receptor expression is found primarily in a subset of neurons of the sensory ganglia (Chen et al . , Nature 377:428-430 (1995); Vulchanova et al . , Neuropharmacol . 36:1229-1242 (1997)). The distribution of both receptors is consistent with a role in pain transmission. The P2X 2 and P2X 3 receptor subunits form functional channels when expressed alone, and can also form a functional heteromultimeric channel that has properties similar to currents seen in native sensory channels when co-expressed (Lewis et al . , Nature 377:432-435 (1995) ) . Evidence from studies in rat nodose ganglia indicate that both P2X 2 /P2X 3 heteromeric channels and P2X 2 homomeric channels contribute to ATP-induced currents (Virginio et al . , J. Phvsiol. (Lond) 510:27-35 (1998); Thomas et al . , J. Phvsiol. (Lond) 509 (Pt 2) .411-417 (1998)); Vulchanova et al . , Proc. Natl. Acad. Sci. USA 93:8063-8067 (1996); Simon et al . , Mol. Pharmacol. 52:237-248 (1997)). ATP, which activates P2X 2 , P2X 3 , and P2X 2 /P2X 3 receptors, functions as an excitatory neurotransmitter in the spinal cord dorsal horn and in primary afferents from sensory ganglia (Holton et al . , J. Phvsiol. (Lond.) 126:124-140 (1954)). ATP- induced activation of P2X receptors on dorsal root ganglion nerve terminals in the spinal cord stimulates the release of glutamate, a key neurotransmitter involved in nociceptive signaling (Gu et al . , Nature 389:749-753 (1997)). Thus, ATP released from damaged cells may evoke pain by activating P2X 2 , P2X 3 , or P2X 2 /P2X 3 receptors on nociceptive nerve endings of sensory nerves. This is consistent with the induction of pain by intradermally applied ATP in the human blister-base model (Bleehen, Br. J. Pharmacol. 62:573-577 (1978)), the identification of P2X 3 receptors on nociceptive neurons in the tooth pulp (Cook et al . , Nature 387:505-508 (1997)), and with reports that P2X receptor antagonists are analgesic in animal models (Driessen et al . , Naunvn Schmiedebergs Arch. Pharmacol . 350:618-625 (1994)). This evidence suggests that P2X 2 and P2X 3 function in nociception, and that modulators of these human P2X receptors may be useful as analgesics. The utility of cibacron blue [i.e., Reactive Blue-2; 2- Anthracenesulfonic acid, l-amino-4- [ [4- [ [4-chloro-6- [ (2- sulfophenyl) amino] -1,3, 5-triazin-2-yl] amino] -3- sulfophenyl] amino] -9, 10-dihydro-9, 10-dioxo-] , an anthraquinone sulfonic acid derivative, as an inhibitor of ATP-mediated signaling and P2X and P2Y receptor activation, has been well documented (Ralevic et al . , Pharmacological Reviews 50:413-492 (1998) ) . Cibacron blue functions as an antagonist of several diverse ATP-mediated physiological responses, including rat urinary bladder smooth muscle contraction (Hashimoto et al . , Br. J. Pharmacol. 115:636-640 (1995)), rat cecum inhibitory junction potentials (Manzini et al . , Eur . J. Pharmacol . 127:197-204 (1986)), phospholipid secretion from rat isolated alveolar type II cells (Rice et al . , Br. J. Pharmacol. 97:1258-162 (1989)), and calcium influx in rat parotid acinar cells (Soltoff et al . , Biochem. Biophys. Res. Commun. 165:1279-1285 (1989)). Cibacron blue also functions both as an antagonist of P2 receptor-operated inward currents and calcium influx in PC12 cells (Nakazawa et al . , Pflugers Arch 418:214-219 (1991); Michel et al . , Schmiedebergs Arch. Pharmacol . 354:562-571 (1996); Surprenant , A., Ciba Found. Symp. 198:208-219 (1996)), and as an inhibitor of ecto- nucleotidase activity in Xenopus oocytes (Ziganshin et al . , Biochem. Pharmacol. 51:897-901 (1996)). Recombinant rat P2X X and P2X 2 receptors are also sensitive to inhibition by cibacron blue (Surprenant, A., Ciba Found. Svmp . 198:208-219 (1996)). Although the effects of cibacron blue on P2X receptor function appear to be primarily inhibitory, one report has described its potentiating activity at the P2X 4 receptor (Miller et al . , Neuropharmacology 37:1579-1586 (1998)). In HEK293 cells expressing the rat P2X 4 receptor, pre-treatment with cibacron blue mediated a 4 -fold increase in the potency of ATP without affecting the maximum response (Miller et al . , supra (1998) ) . The nociceptive effects of exogenously administered ATP and P2X receptor agonists have also been demonstrated in laboratory animals (Bland-Ward et al . , Br. J. Pharmacol. 122:366-371 (1997); Hamilton et al . , Br. J. Phamacol . 126:326- 332 (1999) ) . A selective P2 receptor-mediated increase in ectopic neuronal excitability that is localized to damaged sensory afferents has also been recently reported in rats following chronic constriction nerve injury (Chen et al., NeuroReport 10:2779-2782 (1999)). In addition to the peripheral nociceptive actions of P2X receptor activation, stimulation of spinal P2X receptors may also contribute to nociception as indicated by the ability of intrathecally (i.t.) administered P2 receptor agonists to increase sensitivity to acute and persistent noxious stimuli in rodents (Driessen et al . , Brain Res. 666:182-188 (1994); Tsuda et al . , Br. J. Pharmacol. 127:449-456 (1999); Tsuda et al . , Br. J. Pharmacol. 128:1497- 1504 (1999) ) . The utility of available purinergic ligands to evaluate the role of individual P2 receptor subtypes in mammalian physiology has been complicated by the susceptibility of P2 receptor agonists to undergo enzymatic degradation, and by the lack of P2 receptor subtype-selective agonists and antagonists (King et al . , Trends in Pharmacol . Sci . 19:506-514 (1998); Ralevic et al . , Pharmacol. Rev. 50:413-492 (1998)). However, the recent availability of recombinant mammalian P2 receptor subtypes has allowed for the systematic characterization of the pharmacology of specific P2 receptor subtypes (King et al . , supra (1998); Bianchi et al . , Europ. J. Pharmacol. 376:127-138 (1999) ) and led to further clarification of the pharmacological selectivity of ligands acting at P2X receptors. For example, 2' ,3' - O- (2,4, 6-trinitrophenyl) -ATP (TNP-ATP), a fluorescent ATP analog with antinociceptive actions following i.t. administration in mice (Tsuda et al . , Br. J. Pharmacol. 127:449-456 (1999); Tsuda et al . , Br. J. Pharmacol. 128:1497- 1504 (1999) ) , has been found to be a potent nanomolar antagonist at the recombinant rat P2X 1# P2X 3 , and P2X 2/3 receptors (Lewis et al . , Br. J. Pharmacol. 124:1463-1466 (1998); Thomas et al . , J. Phvsiol. 5092:411-417 (1998)). Since subtype-selective ligands for the individual P2 receptors have yet to be identified, efforts to elucidate the specific P2X receptor subtypes involved in the transmission of nociceptive signals has been largely based on receptor localization and functional studies using immunohistochemical techniques. These studies have shown that both the homomeric P2X 3 and heteromeric P2X 2/3 receptor subtypes are selectively localized to the central and peripheral terminals of small diameter sensory neurons (Chen et al . , Nature 377:428-431 (1995); Lewis et al . , Nature 377:432-435 (1995); Vulchanova et al., Neurooharmacol . 36:1229-1242 (1997); Vulchanova et al . , Euro. J. Neurosci. 10:3470-3478 (1998)). Further, recent data has shown that P2X 3 specific immunoreactivity is significantly increased in both the injured dorsal root ganglion and in the ipsalateral spinal dorsal horn following chronic constriction injury of the rat sciatic nerve (Novakovic et al . , Pain 80:273- 282 (1999) ) . Taken together, the functional and immunohistochemical localization of P2X 3 containing receptors (P2X 3 and/or P2X 2/3 ) on sensory nerves indicates that these P2X receptors may have a primary role in mediating the nociceptive effects of exogenous ATP. Thus, compounds which block or inhibit activation of P2X 3 receptors serve to block the pain stimulus. Receptor antagonists to compounds which normally activate the P2X 3 receptor and/or P2X 2 /P2X 3 heteromeric channels, such as ATP, could successfully block the transmission of pain. In view of the above, methods are certainly needed which provide the ability to regulate or control the P2X receptors, for example, P2X 3 . Control of such receptors provides the ability to minimize pain in patients in need of such treatment. All U.S. patents, patent applications and publications cited herein, whether supra or infra, are hereby incorporated by reference in their entirety. SUMMARY OF THE INVENTION The present invention includes an isolated polynucleotide encoding a human P2X 3 receptor polypeptide or a receptor having a nucleotide sequence at least 90% identical to said polynucleotide sequence encoding the human P2X3 receptor. The polynucleotide may be a polydeoxyribonucleotide (DNA) or a polyribonucleotide (RNA) . More specifically, the DNA may comprise the sequence represented by SEQ ID NO: 15. The invention also includes a host cell comprising the polynucleotide. This host cell may be, for example, a bacterial cell, a mammalian cell, a yeast cell or an amphibian cell. Additionally, the present invention encompasses an expression vector comprising a polynucleotide, as described above, operably linked to at least one control sequence that directs transcription of the polynucleotide. The polypeptide encoded by the polynucleotide may be the human P2X3 which may comprise the amino acid sequence of SEQ ID NO: 16. The present invention also includes a host cell comprising this expression vector. Furthermore, the present invention includes a method for producing a human P2X 3 receptor polypeptide, the method comprising the steps of: (a) culturing a host cell, described above, for a time and under conditions sufficient for expression of said polypeptide; and (b) recovering said polypeptide. The invention also includes a purified human P2X 3 receptor polypeptide, wherein the polypeptide comprises the amino acid sequence of SEQ ID NO: 16. Additionally, the present invention encompasses a method for identifying compounds that modulate P2X receptor activity, the method comprising the steps of: (a) providing a host cell that expresses a P2X receptor comprising a human P2X 3 polypeptide; (b) mixing a test compound with the P2X receptor; and (c) measuring either: (i) the effect of the test compound on the activation of the P2X receptor or the cell expressing the P2X receptor, or (ii) the binding of the test compound to the cell or the P2X receptor. The cell may be as described above. The measurement of step (c) (ii) may be performed by measuring a signal generated by a detectable moiety. The detectable moiety may be, for example, selected from the group consisting of a fluorescent label, a radiolabel, a chemiluminescent label and an enzyme. The measurement of step (c) (i) may be performed by measuring a signal generated by a radiolabeled ion, a chromogenic reagent, a fluorescent probe or an electrical current. In the method, the human P2X 3 receptor polypeptide may comprise the amino acid sequence of SEQ ID NO: 16. Moreover, the present invention also includes a method for detecting a target polynucleotide of a P2X 3 receptor in a test sample, the method comprising the steps of: (a) contacting the target polynucleotide with at least one human P2X 3 receptor-specific polynucleotide probe or a complement thereof to form a target-probe complex; and (b) detecting the presence of the target-probe complex in the test sample. Additionally, the present invention encompasses a method for detecting cDNA of human P2X 3 receptor mRNA in a test sample, the method comprising the steps of: (a) performing reverse transcription in order to produce cDNA; (b) amplifying the cDNA obtained from step (a) ; and (c) detecting the presence of the human P2X 3 receptor in the test sample. In this method, the detection step (c) comprises utilizing a detectable moiety capable of generating a measurable signal . The invention also includes an isolated polynucleotide encoding a human P2X 3 receptor or a portion thereof and capable of selectively hybridizing to a nucleic acid encoding a human P2X 3 receptor polypeptide, wherein said polynucleotide comprises the sequence of SEQ ID NO: 15 or a portion thereof. The polynucleotide may be produced by recombinant or synthetic techniques. The present invention also includes a purified polypeptide encoded a by human P2X 3 receptor polynucleotide wherein said polypeptide comprises the amino acid sequence of SEQ ID NO: 16 or a portion thereof. The polypeptide may be produced by recombinant or synthetic techniques . Also, the present invention includes a monoclonal antibody which specifically binds to a human P2X 3 receptor comprising the amino acid sequence of SEQ ID NO: 16 or an immunoreactive fragment thereof . It should also be noted that the invention encompasses a method for detecting human P2X 3 receptor in a test sample, the method comprising the steps of: (a) contacting the test sample with an antibody or a fragment thereof which specifically binds to the human P2X 3 receptor, for a time and under conditions sufficient for the formation of a resultant complex; and (b) detecting the resultant complex containing the antibody, wherein the antibody specifically binds to human P2X 3 receptor amino acid comprising the amino acid sequence of SEQ ID NO: 16 or a fragment thereof. Additionally, the present invention includes an isolated polynucleotide encoding a human P2X receptor polypeptide or a variant thereof, wherein the receptor is P2X 3 . The invention also includes a therapeutic method for relieving pain comprising: (a) presenting an individual afflicted with pain; and (b) administering to the individual an effective amount of a P2X 3 antagonistic compound. The antagonistic compound may be effective against P2X 3 heteromultimeric channels. Furthermore, the present invention also includes a method of potentiating the effects of an agonist which activates a P2X 3 receptor comprising the steps of: (a) incubating cells comprising said P2X 3 receptor with a triazene dye; and (b) exposing the incubated cells to the agonist for a time and under conditions sufficient for the agonist to bind to the P2X 3 receptor, wherein the triazene dye of step (a) potentiates the effect of the agonist of step (b) . The receptor may be derived from a mammal such as a human or a rodent . The triazene dye may be selected from the group consisting of, for example, cibacron blue, basilen blue, reactive blue 5 and reactive red 2. The agonist may be, for example, adenosine 5 ' -triphosphate disodium (ATP). The present invention also includes a method of blocking the inhibitory activity of a non-selective P2 receptor antagonist on a P2X 3 receptor comprising the steps of: (a) incubating P2X 3 -expressing cells with a triazene dye; and (b) exposing the incubated cells to a non-selective P2 receptor antagonist, wherein the triazene dye of step (a) blocks the inhibitory activity of the antagonist. The P2X receptor may be derived from a mammal such as a rodent or a human. The antagonist may be pyridoxal-5-posphate-6-azophenyl-2' , 4' -disulfonic acid (PPADS) . The triazene dye may be cibacron blue or one of the other triazene dyes described above . Additionally, the present invention encompasses a method of accelerating the rate of P2X 3 receptor resensitization of desensitized P2X 3 receptor-expressing cells comprising the step of exposing the densensitized P2X 3 receptor- expressing cells to a triazene dye, wherein the triazene dye accelerates said rate of resensitization of P2X 3 receptors of said desensitized P2X 3 -receptor expressing cells. Again, the P2X 3 receptor may be derived from a mammal such as a human or a rodent . The triazene dye may be cibacron blue or one of the other triazene dyes described above. A method of inducing antinociceptive effects in a mammal comprising the step of administering a P2X receptor antagonist to a patient in need of such antinociceptive effects in an amount sufficient to effect the antinociceptive effects. Again, the mammal may be a human or a rat. The P2X 3 receptor antagonist induces antinociceptive effects on a P2X 3 -containing receptor. The receptor may be, for example, P2X3. The antagonist may be, for example, 2' ,3' -O- (2, 4, 6-trinitrophenyl) -ATP (TNP-ATP). BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts the sequence of the P2X 3 5 ' RACE product of Example 2 (SEQ ID NO: 13), in which the sequences of primers are underlined and the predicted initiation codon (ATG) is shown in boldface. Figure 2 depicts the sequence of the P2X 3 3 ' RACE product of Example 3 (SEQ ID NO: 14), in which the sequences of primers are underlined and the predicted termination codon (TAG) is shown in boldface. Figure 3 depicts the sequence of the complete open reading frame of cDNA encoding human P2X 3 receptor polypeptide (SEQ ID NO: 15) . The initiation (ATG) and termination (TAG) codons are shown in boldface; 5' and 3' flanking sequences introduced during plasmid construction, including the EcoRI (GAATTC) and Not I (GCGGCCGC) restriction sites, are underlined. Figure 4 depicts the aligned predicted amino acid sequences of human (hP2X 3 ) (SEQ ID NO: 16) and rat (r P2X 3 ) (SEQ ID NO: 17) receptor polypeptides. Identical residues are identified by boxing. Figure 5 illustrates the potentiation effect of cibacron blue on calcium influx mediated by ATP-stimulated human P2X 3 receptors. A, 1321-P2X 3 cells loaded with the Ca 2+ indicator Fluo-4 were treated with ATP in the presence ( solid line) or absence (dashed line) of cibacron blue. Relative fluorescence is shown as the percent of the maximum response obtained in the absence of cibacron blue. B, Xenopus oocyte expressing hP2X 3 receptors were challenged with ATP in the absence (small current) and presence (large current) of cibacron blue. ATP application is denoted by the horizontal bar. Figure 6 illustrates that cibacron blue (CB) significantly increases the potency of ATP-induced hP2X 3 receptor activation in a concentration dependent manner. ATP concentration-effect curves were determined in the absence or presence of cibacron blue by measuring Ca 2+ influx, as determined by Fluo-4 fluorescence, in 1321Nl-hP2X 3 cells: B, without cibacron blue (ATP EC 50 = 356 ± 47 nM, E max = 102 ± 3%) ; A , 1 μM cibacron blue (ATP EC 50 = 64 ± 7 nM* , E max = 267 ± 6%*) ; τ , 3 μM cibacron blue (ATP EC 50 = 46 ± 8 nM* , E max = 330 ± 5%*) ; ♦ , 10 μM cibacron blue (ATP EC 50 = 60 ± 12 nM* , E max = 345 ± 6%*) . Data are shown as a percent of the maximal response to 10 μM ATP and are the means (± sem) of three experiments (Statistical analysis based on pEC 50 values; *P < 0.05 as compared to control) . Figure 7 illustrates that the potency of cibacron blue to potentiate hP2X 3 receptor activation is similar for prototypic P2X 3 agonists. Cibacron blue concentration-effect curves were determined for each of four prototypic P2X 3 receptor agonists by measuring Ca 2+ influx, as determined by Fluo-4 fluorescence, in 1321Nl-hP2X 3 cells. The half-maximal concentrations of cibacron blue required to mediate full potentiation were as follows: ■ *" , 10 μM ATP (cibacron blue EC 50 = 1.4 ± 0.5 μM, E max = 504 + 15%); ^ , 10 μM 2-meSATP (cibacron blue EC 50 = 1.4 ± 0.2 μM, E raax = 555 ± 18%); ■, 10 μM BzATP (cibacron blue EC 50 = 0.9 ± 0.1 μM, E max = 562 ± 12%); ♦, 10 μM αβ-meATP (cibacron blue EC 50 = 1.4 ± 0.2 μM, E max = 537 ± 14%) . Data are shown as a percent of the maximal response to 10 μM ATP and are the means (± sem) of three experiments. Concentration-effect curves were fitted using a four-parameter logistic equation in GraphPad Prism. Figure 8 illustrates the potentiation of hP2X 3 receptor activity with various triazene dyes. Concentration-effect curves for four structurally related triazene dyes were determined by measuring ATP-activated Ca 2+ influx in 1321N1- hP2X 3 cells: ■, reactive red 2 (EC 50 = 55 ± 10 μM, E max = 600%, fixed parameter); ♦, basilen blue (EC 50 = 1.2 ± 0.6 μM, E max = 373 ± 17%*); A , reactive blue 5 (EC 50 = 1.4 ± 0.5 μM, E max = 534 ± 14%); , cibacron blue (EC 50 = 1.2 ± 0.2 μM, E max = 566 ± 17%) . Data are shown as a percent of the maximal response to 10 μM ATP and are the means (± sem) of three experiments (Statistical analysis based on pEC 50 values; *P < 0.05 as compared to control) . Figure 9 illustrates that cibacron blue blocks the inhibitory activity of PPADS. A, Concentration-effect curves for the inhibition of ATP-activated hP2X 3 receptor activity by PPADS were determined in the presence and absence of cibacron blue. PPADS and cibacron blue were co-applied 3 minutes before the addition of 3 μM ATP: β # without cibacron blue (PPADS IC 50 = 8.6 ± 3 μM, E raax = 101 ± 4%); •, 1 μM cibacron blue (PPADS IC 50 = 14 + 3 μM, E max = 280 ± 6%*); ♦, 10 μM cibacron blue (PPADS IC 50 = 51 ± 4 μM* , E max = 437 ± 6%*); A , 100 μM cibacron blue (PPADS IC S0 = 220 ± 186 μM* , E max = 488 ± 9%*) . Inset, Data are normalized to the maximal signal observed at each concentration of cibacron blue. B, Concentration-effect curves for cibacron blue potentiation of hP2X 3 responses, activated by 3 μM ATP, were determined in the presence and absence of PPADS: B, without PPADS (cibacron blue EC S0 = 3.8 ± 0.4 μM, E max = 738 ± 22%); A , 5 μM PPADS (cibacron blue EC 50 = 4.5 ± 0.3 μM, E max = 682 ± 15%);^, 10 μM PPADS (cibacron blue EC 50 = 7.5 ± 0.2 μM* , E max = 730 ± 7%); , 50 μM PPADS (cibacron blue EC 50 = 15 ± 1.4 μM* , E max = 653 ± 10%) . Data are shown as a percent of the maximal response to 3 μM ATP and are the means (± sem) of three experiments (Statistical analysis based on pEC 50 values; *P < 0.05 as compared to control) . Figure 10 illustrates that cibacron blue significantly increases the rate of hP2X 3 receptor recovery from desensitization. Cibacron blue concentration-effect curves were determined in non-desensitized and acutely desensitized 1321-hP2X 3 cells: B, non-desensitized (cibacron blue EC S0 = 1.1 ± 0.1 μM, E max = 288 ± 5%); •, desensitized (cibacron blue EC 50 = 6.4 + 0.4 μM*, E max = 302 ± 5%) . Data are shown as a percent of the maximal response to 3 μM ATP and are the means (± sem) of three experiments (Statistical analysis based on pEC 50 values; *P < 0.05 as compared to control) . Figure 11 illustrates that cibacron blue accelerates the recovery of hP2X 3 receptors from acute desensitization. A, 1321-hP2X 3 cells were pre-treated with ATP or D-PBS (control curve) , washed twice to remove excess extracellular ATP, and incubated for the time periods shown prior to re-challenge with various concentrations of ATP. The control curve (dashed line) shows the concentration-effect of ATP on mock- desensitized (D-PBS-treated) cells. B, 1321-hP2X 3 cells were pre-treated with ATP, washed twice to remove excess extracellular ATP, and incubated for the time periods shown in the presence cibacron blue prior to re-challenge with various concentrations of ATP. The control curve (dashed line) shows the concentration-effect of ATP on mock-desensitized (D-PBS- treated) cells pre-treated with 50 μM cibacron blue. C, The rates of receptor recovery are shown as a function of % of the non-desensitized response over time. Curves are solutions of %control = max (1-exp (-K t *time) ) , where ^control is the percentage of receptor activity as compared to non- desensitized receptors, max is the ^control activity observed at 61.5 min, time is the time in minutes and K t is the time constant. t (half-time of receptor resensitization) was calculated as ln(0.5)/-K. Figure 12 illustrates the time course for the acutenociceptive effects of intradermal BzATP in the rat. Figure 13 illustrates dose-response determinations for acute nociceptive effects of P2X receptor agonists following intradermal administration into the rat hindpaw (n=6 per dose group). Values represent +/- S.E.M. for the cumulative nociceptive paw flinching responses occurring 15 minustes post injection. *P < 0.05 as compared to vehicle-treated rats. Figure 14 represents concentration-effect determinations for TNP-ATP (diamonds, IC 50 = 40 nM) , TNP-ADP (squares, IC 50 = 120 nM) and TNP-AMP (triangles, IC 50 > 3,000 nM) to inhibit 10 μM BzATP-stimulated calcium flux in 1321N1 cells expressing rP2X 3 receptors. RFU = relative fluorescence units. Values represent mean + S.E.M from three separate experiments . Figure 15 illustrates the effects of intradermal co- administration of TNP-ATP (F(3,20) = 8.20, P < 0.05), but not TNP-AMP (F(3,20) = 0.30, P > 0.05), with BzATP (1000 nmol/paw) dose-dependently attenuates acute nociceptive paw flinching in the rat (n = 6 per dose group) . Values represent mean + S.E.M. for the cumulative nociceptive paw flinching responses occurring 15 minutes post-injection. * P < 0.05 as compared to vehicle-treated rats. Figure 16 represents the effects of intradermal co-administration of TNP-ATP (black bars) , but not TNP-AMP (gray bars) , with 5% formalin attenuated acute nociceptive paw flinching in the rat (n = 6 per dose group) . Phase I represents cumulative acute nociceptive responses occurring 15 minutes immediately following intradermal administration (F(2,27) = 5.15, P < 0.05). Phase II represents cumulative nociceptive responses recorded for a 20 minute period beginning 30 minutes post formalin injection (F(2,27) = 6.97, P < 0.05). Values represent mean + S.E.M., * P < 0.05 as compared to vehicle- treated rats. Figure 17 represents concentration-effect determinations for the effects of cibacron blue on agonist activation of rat P2X 3 and P2X 2/3 receptors. (Left panel) Representative concentration-effect curves for cibacron blue (EC 50 = 2 μM) to enhance BzATP (1 μM) and α, ?-meATP (10 μM) activation of rat P2X 3 receptors. (Right panel) Representative concentration-effect curves for cibacron blue to enhance BzATP (1 μM) and α,?-meATP (10 μM) activation of rat P2X 2/3 receptors. RFU = relative fluorescence units. Figure 18 illustrates the nociceptive effects of intradermal co-administration of BzATP and cibacron blue into the hindpaw of the rat (F(16,352) = 7.30, P < 0.05). Vehicle responses indicate the effects of saline (open square) or co- administration of saline and BzATP (filled circles) on acute (cumulative responses for the first 15 minutes post-injection) paw flinching behavior. The nociceptive effects of cibacron blue alone are indicated by the open squares and dotted lines. The effects of co-administration of cibacron blue and BzATP are indicated by the filled circles and solid lines. Values represent mean + S.E.M. from three separate experiments (n = 6 per dose group), * P < 0.05 as compared to the nociceptive effects of BzATP alone, + P < 0.05 as compared to the nociceptive effects of cibacron blue alone. Figure 19 illustrates that cibacron blue increases nociceptive paw flinching in both Phase I and Phase II of the rat formalin test. Cibacron blue (30 and 100 nmol/paw) was co- administered with intradermal formalin (1-5%) into the rat hind paw. Phase I nociceptive responses were recorded for the first 15 min following administration. Phase II nociceptive responses were recorded for a 20 minutes period begining 30 minutes after administration. Intradermal administration of increasing doses of formalin alone produced significant increases in nociceptive responses ( + P < 0.05) as compared to vehicle injections. The black bars indicate nociceptive responses of formalin or vehicle alone. The gray bars indicate nociceptive responses of cibacron blue (30 nmol/paw) in the presence of formalin or vehicle. The hatched bars indicate nociceptive responses of cibacron blue (100 nmol/paw) in the presence of formalin or vehicle. Values represent mean + S.E.M. (n = 6 per dose group), * P < 0.05 as compared formalin alone (vehicle responses at each formalin dose), + P < 0.05 as compared to vehicle alone . DETAILED DESCRIPTION OF THE INVENTION The subject invention relates to the P2X 3 receptor, the nucleic acid sequence of this receptor, the amino acid sequence of the receptor, methods of producing this receptor, and methods of altering the activity of the P2X 3 receptor by use of various chemicals (e.g., cibacron blue and/or TNP-ATP). The ability to externally regulate the receptor may allow one, for example, to control sensations such as, pain, following a traumatic accident, during the course of a terminal illness, during surgery, after surgery or during any situation during which a patient's pain must be managed by a medical provider. In particular, the present invention provides a method for screening a plurality of compounds for specific binding to a purinoreceptor to identify a compound that modulates the activity of the receptor. The method comprises (a) providing a cell that expresses the human (or other mammalian) purinoreceptor polypeptide coding sequence, (b) mixing a test compound with the cell, and (c) measuring the effect of the test compound on the activation of the purinoreceptor or the cell expressing the purinoreceptor receptor. In addition, the invention provides a method for determining the amount of a receptor agonist or antagonist in a test sample. The method comprises (a) providing a cell that expresses the human (or other mammalian) purinoreceptor polypeptide coding sequence, (b) mixing the cell with a test sample, and (c) measuring the effect of the test compound on the activation of the purinoreceptor or the cell expressing the purinoreceptor receptor. The invention also encompasses a host cell that encodes the purinoreceptor of interest. The host cell is genetically engineered with a vector, also encompassed by the present invention, which may be a cloning vector or an expression vector. The vector comprises a polynucleotide sequence encoding a purinoreceptor operably linked to control sequences that control its expression. Preferably, the host cell is stably transfected to express the purinoreceptor. More preferably, the host cell is a purinoreceptor null cell which, if not already lacking endogenous purinoreceptor expression, has been so engineered. The practice of the present invention will employ, unless otherwise indicated, conventional techniques of molecular biology, microbiology, recombinant DNA technology, electrophysiology, and pharmacology that are within the skill of the art. Such techniques are explained fully in the literature. See, for example, Sambrook, Fritsch & Maniatis, Molecular Cloning: A Laboratory Manual, Second Edition (1989); DNA Cloning, Vols. I and II (D.N. Glover Ed. 1985); Perbal, B., A Practical Guide to Molecular Cloning (1984); the series, Methods In Enzymology (S. Colowick and N. Kaplan eds., Academic Press, Inc.); Transcription and Translation (Hames et al . eds. 1984); Gene Transfer Vectors For Mammalian Cells (J. H. Miller et al . eds. (1987) Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.); Scopes, Protein Purification: Principles and Practice (2nd ed., Springer- Verlag) ; and PCR: A Practical Approach (McPherson et al . eds. (1991) IRL Press) . As used in this specification and the appended claims, the singular forms "a, " "an" and "the" include plural references unless the content clearly dictates otherwise. Thus, for example, reference to "a primer" includes two or more such primers, reference to "an amino acid" includes more than one such amino acid, and the like. Definitions : In describing the present invention, the following terms will be employed and are defined as indicated below: The term "P2 receptor" intends a purinergic receptor for the ligand ATP and/or other purine or pyrimidine nucleotides, whether natural or synthetic. P2 receptors are broadly subclassified as "P2X" or "P2Y" receptors. These types differ in their pharmacology, structure, and signal transduction mechanisms. The P2X receptors are generally ligand-gated ion channels, while the P2Y receptors operate generally through a G protein-coupled system. Moreover, and without intending to be limited by theory, it is believed that P2X receptors comprise multimers of receptor polypeptides, which multimers may be of either the same or different subtypes. Consequently, the term "P2X receptor" refers, as appropriate, to the individual receptor subunit or subunits, as well as to the homomeric and heteromeric receptors comprised thereby. The term "subunit" when used in reference to purinoreceptors intends a polypeptide which, either alone or in combination with one or more other polypeptides, forms a functional purinoreceptor. Where a purinoreceptor comprises more than one polypeptide subunit, the subunits may be either identical (forming a homomeric multimer) or different (forming a heteromeric multimer.) The term "P2X n " intends a P2X receptor subtype wherein n is an integer of at least 1. At the time of the invention, at least 7 P2X n receptor subtypes have been isolated and/or characterized . A "P2X 3 receptor agonist" is a compound that binds to and activates a P2X 3 receptor. By "activates" is intended the elicitation of one or more pharmacological, physiological, or electrophysiological responses. Such responses may include, but are not limited to, an increase in receptor-specific cellular depolarization. A "P2X 3 receptor antagonist" is a substance that binds to a P2X 3 receptor and prevents agonists from activating the receptor. Pure antagonists do not activate the receptor, but some substances may have mixed agonist and antagonist properties . The term "polynucleotide" as used herein means a polymeric form of nucleotides of any length, either ribonucleotides or deoxyribonucleotides . This term refers only to the primary structure of the molecule. Thus, the term includes double- and single-stranded DNA, as well as double- and single-stranded RNA. It also includes modifications, such as by methylation and/or by capping, and unmodified forms of the polynucleotide. The term "variant" is used to refer to an oligonucleotide sequence which differs from the related wild- type sequence in the insertion, deletion or substitution of one or more nucleotides. When not caused by a structurally conservative mutation (see below) , such a variant oligonucleotide is expressed as a "protein variant" which, as used herein, indicates a polypeptide sequence that differs from the wild-type polypeptide in the insertion, deletion or substitution of one or more amino acids. The protein variant differs in primary structure (amino acid sequence) , but may or may not differ significantly in secondary or tertiary structure or in function relative to the wild-type. The term "mutant" generally refers to an organism or a cell displaying a new genetic character or phenotype as the result of change in its gene or chromosome. In some instances, however, "mutant" may be used in reference to a variant protein or oligonucleotide and "mutation" may refer to the change underlying the variant . "Identity" is defined as an exact nucleotide to nucleotide or amino acid to amino acid correspondence of two polynucleotide sequences or polypeptide sequences, respectively. Two or more nucleotide sequences can be compared by determining their "percent identity" . Two or more amino acid sequences can also be compared by determining their "percent identity" . The programs available in the Wisconsin Sequence Analysis Package, Version 8 (available from Genetics Computer Group, Madison, Wl), for example, the GAP program, are capable of calculating both the identity between two polynucleotides and the identity between two polypeptide sequences, respectively. Other programs for calculating percent identity are known in the art . "Similarity" means the exact amino acid to amino acid comparison of two or more polypeptides at the appropriate place, where amino acids are identical or possess similar chemical and/or physical properties such as charge or hydro hobicity. Thus, a "percent similarity" then can be determined between the compared polypeptide sequences. Techniques for determining amino acid sequence identity, as well as nucleic acid sequence identity, are well known in the art and include determining the nucleotide sequence of the mRNA for the gene (usually via a cDNA intermediate) and determining the amino acid sequence encoded thereby, and comparing this to a second amino acid sequence (see discussion of percent identity above) . "Polypeptide" and "protein" are used interchangeably herein and indicate a molecular chain of amino acids linked through peptide bonds. The terms do not refer to a specific length of the product. Thus, peptides, oligopeptides, and proteins are included within the definition of polypeptide. The terms include post-translational modifications of the polypeptide, for example, glycosylations, acetylations, phosphorylations and the like. In addition, protein fragments, analogs, mutated or variant proteins, fusion proteins and the like are included within the meaning of polypeptide, provided that such fragments, etc. retain the binding or other characteristics necessary for their intended use . A "functionally conservative mutation" as used herein intends a change in a polynucleotide encoding a derivative polypeptide in which the activity is not substantially altered compared to that of the polypeptide from which the derivative is made. Such derivatives may have, for example, amino acid insertions, deletions, or substitutions in the relevant molecule that do not substantially affect its properties. For example, the derivative can include conservative amino acid substitutions, such as substitutions which preserve the general charge, hydrophobicity/hydrophilicity, side chain moiety, and/or steric bulk of the amino acid substituted, for example, Gly/Ala, Val/lle/Leu, Asp/Glu, Lys/Arg, Asn/Gln, Thr/Ser, and Phe/Trp/Tyr. By the term "structurally conservative mutant" is intended a polynucleotide containing changes in the nucleic acid sequence but encoding a polypeptide having the same amino acid sequence as the polypeptide encoded by the polynucleotide from which the degenerate variant is derived. This can occur because a specific amino acid may be encoded by more than one "codon, " or sequence of three nucleotides, i . e . , because of the degeneracy of the genetic code. "Recombinant host cells," "host cells," "cells," "cell lines," "cell cultures," and other such terms denoting microorganisms or higher eukaryotic cell lines cultured as unicellular entities refer to cells which can be, or have been, used as recipients for recombinant vectors or other transfer DNA, immaterial of the method by which the DNA is introduced into the cell or the subsequent disposition of the cell. The terms include the progeny of the original cell which has been transfected. Cells in primary culture as well as cells such as oocytes also can be used as recipients. A "vector" is a replicon in which another polynucleotide segment is attached, such as to bring about the replication and/or expression of the attached segment. The term includes expression vectors, cloning vectors, and the like. A "coding sequence" is a polynucleotide sequence that is transcribed into mRNA and/or translated into a polypeptide. The boundaries of the coding sequence are determined by a translation start codon at the 5' -terminus and a translation stop codon at the 3 '-terminus. A coding sequence can include, but is not limited to, mRNA, cDNA, and recombinant polynucleotide sequences. Variants or analogs may be prepared by the deletion of a portion of the coding sequence, by insertion of a sequence, and/or by substitution of one or more nucleotides within the sequence. Techniques for modifying nucleotide sequences, such as site-directed mutagenesis, are well known to those skilled in the art. See, for example, Sambrook et al . , supra; DNA Cloning, Vols . I and II, supra; Nucleic Acid Hybridization, supra. "Operably linked" refers to a situation wherein the components described are in a relationship permitting them to function in their intended manner. Thus, for example, a control sequence "operably linked" to a coding sequence is ligated in such a manner that expression of the coding sequence is achieved under conditions compatible with the control sequences. A coding sequence may be operably linked to control sequences that direct the transcription of the polynucleotide whereby said polynucleotide is expressed in a host cell. The term "transfection" refers to the insertion of an exogenous polynucleotide into a host cell, irrespective of the method used for the insertion, or the molecular form of the polynucleotide that is inserted. The insertion of a polynucleotide per se and the insertion of a plasmid or vector comprised of the exogenous polynucleotide are included. The exogenous polynucleotide may be directly transcribed and translated by the cell, maintained as a nonintegrated vector, for example, a plasmid, or alternatively, may be stably integrated into the host genome. "Transfection" generally is used in reference to a eukaryotic cell while the term "transformation" is used to refer to the insertion of a polynucleotide into a prokaryotic cell. "Transformation" of a eukaryotic cell also may refer to the formation of a cancerous or tumorigenic state. The term "isolated, " when referring to a polynucleotide or a polypeptide, intends that the indicated molecule is present in the substantial absence of other similar biological macromolecules. The term "isolated" as used herein means that at least 75 wt.%, more preferably at least 85 wt.%, more preferably still at least 95 wt.%, and most preferably at least 98 wt.% of a composition is the isolated polynucleotide or polypeptide. An "isolated polynucleotide" that encodes a particular polypeptide refers to a polynucleotide that is substantially free of other nucleic acid molecules that do not encode the subject polypeptide; however, the molecule may include functionally and/or structurally conservative mutations as defined herein. A "test sample" as used herein intends a component of an individual's body that is a source of one of the P2X receptors, including P2X 3 . These test samples include biological samples which can be evaluated by the methods of the present invention described herein and include body fluids such as whole blood, tissues and cell preparations. The following single-letter amino acid abbreviations are used throughout the text : Alanine A Arginine R Asparagine N Aspartic acid D Cysteine C Glutamine Q Glutamic aci .d E Glycine G Histidine H Isoleucine I Leucine L Lysine K Methionine M Phenyla1anine F Proline P Serine S Threonine T Tryptophan W Tyrosine Y Valine V As noted above, mammalian P2X 3 receptors, polynucleotides encoding variant receptors or polypeptide subunits thereof, and methods of making these receptors are provided herein. The invention includes not only the above P2X receptor but also methods for screening compounds using the receptor and cells expressing the receptor. Further, polynucleotides and antibodies which can be used in methods for detection of the receptor, as well as the reagents useful in these methods, are provided. Compounds and polynucleotides useful in regulating the receptor and its expression also are provided as disclosed hereinbelow. In one preferred embodiment, the polynucleotide encodes the aforementioned human P2X receptor polypeptide or protein variants thereof containing conservative amino acid substitutions. DNA encoding the above mentioned human P2X receptor, and variants thereof, can be derived from genomic or cDNA, prepared by synthesis, or by a combination of techniques. The DNA can then be used to express the human P2X receptor or as a template for the preparation of RNA using methods well known in the art (see, Sambrook et al . , supra), or as a molecular probe capable of selectively hybridizing to, and therefore detecting the presence of, other P2X-encoding nucleotide sequences. cDNA encoding the P2X 3 receptor may be obtained from an appropriate DNA library. cDNA libraries may be probed using the procedure described by Grunstein et al . (1975) Proc. Natl. Acad. Sci. USA 73:3961. The cDNA thus obtained can then be modified and amplified using the polymerase chain reaction ("PCR") and primer sequences to obtain the specific DNA encoding the human P2X receptor. More particularly, PCR employs short oligonucleotide primers (generally 10-20 nucleotides in length) that match opposite ends of a desired sequence within the DNA molecule. The sequence between the primers need not be known. The initial template can be either RNA or DNA. If RNA is used, it is first reverse transcribed to cDNA. The cDNA is then denatured, using well-known techniques such as heat, and appropriate oligonucleotide primers are added in molar excess. Primer extension is effected using DNA polymerase in the presence of deoxynucleotide triphosphates or nucleotide analogs. The resulting product includes the respective primers at their 5' -termini, covalently linked to the newly synthesized complements of the original strands. The replicated molecule is again denatured, hybridized with primers, and so on, until the product is sufficiently amplified. Such PCR methods are described in for example, U.S. Patent Nos. 4,965,188; 4,800,159; 4,683,202; 4,683,195; incorporated herein by reference in their entireties. The product of the PCR is cloned and the clones containing the P2X receptor DNA, derived by segregation of the primer extended strand, selected. Selection can be accomplished using a primer as a hybridization probe. Alternatively still, the respective P2X receptor DNA could be generated using a RT-PCR (reverse transcriptase - polymerase chain reaction) approach starting with human RNA. Human RNA may be obtained from cells or tissue in which the specific P2X receptor is expressed, for example, brain, spinal cord, uterus or lung, using conventional methods. For example, single-stranded cDNA is synthesized from human RNA as the template using standard reverse transcriptase procedures and the cDNA is amplified using PCR. This is but one example of the generation of P2X receptor variants from a human tissue RNA template. Reverse transcription of human RNAs can also be accomplished utilizing reagents from the Superscript Preamplification System (GibcoBRL, Gaithersburg, MD) and the following method: Poly A+ RNA (1 microgram) derived from pituitary gland tissue (Clontech, Inc. Palo Alto, CA) and 1 μl (50 nanograms) random hexamer primers are combined in a final volume of 12 μl dH 2 0. This mixture is heated to 70°C for 10 minutes and chilled on ice for 1 minute. The following components are added: 2 μl 10X PCR buffer (200 mM Tris-HCl pH 8.4, 500mM KCl), 2 μl 25 mM MgCl 2 . 1 μl lOmM dNTP mix, and 2 μl 0.1 M dithiothreitol . The reaction is equilibrated for 5 minutes at 25°C after which 1 μl (200 units) Superscript II reverse transcriptase is added and incubation continued at 25°C for 10 minutes, followed by 50 minutes at 42°C. Alternatively, 10 picomoles Oligo dT primer can be substituted for the random hexamer primers in the above reaction mixture. In this case, equilibration is carried out at 42°C for 2 minutes after which the reverse transcriptase is added and incubation continued at 42°C for 50 minutes. The reverse transcription reaction is terminated by incubation at 70°C for 15 minutes and chilled on ice. Rnase H (1 μl; 2 units) is added and the mixture incubated for 20 minutes at 37°C, then stored on ice. Synthetic oligonucleotides may be prepared using an automated oligonucleotide synthesizer such as that described by Warner (1984) DNA 3:401. If desired, the synthetic strands may be labeled with 32 P by treatment with polynucleotide kinase in the presence of 32 P-ATP, using standard conditions for the reaction. DNA sequences, including those isolated from genomic or cDNA libraries, may be modified by known methods which include site-directed mutagenesis as described by Zoller (1982) Nucleic Acids Res. 10:6487. Briefly, the DNA to be modified is packaged into phage as a single stranded sequence. It is then converted to a double stranded DNA with DNA polymerase using, as a primer, a synthetic oligonucleotide complementary to the portion of the DNA to be modified, and having the desired modification included in its own sequence. Culture of the transformed bacteria, which contain replications of each strand of the phage, are plated in agar to obtain plaques. Theoretically, 50% of the new plaques contain phage having the mutated sequence, and the remaining 50% have the original sequence. Replicates of the plaques are hybridized to labeled synthetic probe at temperatures and conditions suitable for hybridization with the correct strand, but not with the unmodified sequence. The sequences that have been identified by hybridization are recovered and cloned. Alternatively, it may be necessary to identify clones by sequence analysis if there is difficulty in distinguishing the variant from wild type by hybridization. In any case, the DNA would be sequence-confirmed. Once produced, DNA encoding the specific P2X receptor, or DNA about 60-80% identical to the nucleotide sequence encoding the specific P2X receptor, and more preferably, DNA about 90% identical to the nucleotide sequence encoding the specific P2X receptor, may then be incorporated into a cloning vector or an expression vector for replication in a suitable host cell. Vector construction employs methods known in the art. Generally, site-specific DNA cleavage is performed by treating with suitable restriction enzymes under conditions that generally are specified by the manufacturer of these commercially available enzymes. After incubation with the restriction enzyme, protein is removed by extraction and the DNA recovered by precipitation. The cleaved fragments may be separated using, for example, polyacrylamide or agarose gel electrophoresis methods, according to methods known by those of skill in the art. Sticky end cleavage fragments may be blunt ended using E. coli DNA polymerase 1 (Klenow) in the presence of the appropriate deoxynucleotide triphosphates (dNTPs) present in the mixture. Treatment with SI nuclease also may be used, resulting in the hydrolysis of any single stranded DNA portions . Ligations are performed using standard buffer and temperature conditions using T4 DNA ligase and ATP. Alternatively, restriction enzyme digestion of unwanted fragments can be used to prevent ligation. Standard vector constructions generally include specific antibiotic resistance elements. Ligation mixtures are transformed into a suitable host, and successful transformants selected by antibiotic resistance or other markers. Plasmids from the transformants can then be prepared according to methods known to those in the art usually following a chloramphenicol amplification as reported by Clewell et al . , J. Bacteriol . 110:667 (1972). The DNA is isolated and analyzed usually by restriction enzyme analysis and/or sequencing. Sequencing may be by the well-known dideoxy method of Sanger et al . , Proc. Natl. Acad. Sci. USA 74:5463 (1977) as further described by Messing et al . , Nucleic Acid Res . 9:309 (1981), or by the method reported by Maxam et al., Meth. Enzymol. 65:499 (1980). Problems with band compression, which are sometimes observed in GC rich regions, are overcome by use of, for example, T-deazoguanosine or inosine, according to the method reported by Barr et al . , Biotechniques 4:428 (1986). Host cells are genetically engineered with the vectors of this invention, which may be a cloning vector or an expression vector. The vector may be in the form of a plasmid, a viral particle, a phage, etc. The engineered host cells can be cultured in conventional nutrient media modified as appropriate for activating promoters, selecting transformants/transfectants or amplifying the subunit- encoding polynucleotide. The culture conditions, such as temperature, pH and the like, generally are similar to those previously used with the host cell selected for expression, and will be apparent to those of skill in the art. Both prokaryotic and eukaryotic host cells may be used for expression of desired coding sequences when appropriate control sequences that are compatible with the designated host are used. For example, among prokaryotic hosts, Escherichia coli is frequently used. Also, for example, expression control sequences for prokaryotes include but are not limited to promoters, optionally containing operator portions, and ribosome binding sites. Transfer vectors compatible with prokaryotic hosts can be derived from, for example, the plasmid pBR322 that contains operons conferring ampicillin and tetracycline resistance, and the various pUC vectors, that also contain sequences conferring antibiotic resistance markers. These markers may be used to obtain successful transformants by selection. Commonly used prokaryotic control sequences include but are not limited to the lactose operon system (Chang et al . , Nature 198:1056 (1977) ) , the tryptophan operon system (reported by Goeddel et al., Nucleic Acid Res. 8:4057 (1980)) and the lambda-derived Pi promoter and N gene ribosome binding site (Shimatake et al., Nature 292:128 (1981)), the hybrid Tac promoter (De Boer et al., Proc. Natl. Acad. Sci. USA 292:128 (1983)) derived from sequences of the trp and lac UV5 promoters . The foregoing systems are particularly compatible with E. coli ; however, other prokaryotic hosts such as strains of Bacillus or Pseudomonas may be used if desired. Eukaryotic hosts include yeast and mammalian cells in culture systems. Pichia pastoris, Saccharomyces cerevisiae and S. carl sber gens is are commonly used yeast hosts. Yeast- compatible vectors carry markers that permit selection of successful transformants by conferring protrophy to auxotrophic mutants or resistance to heavy metals on wild- type strains. Yeast-compatible vectors may employ the 2-μ origin of replication (Broach et al . , Meth. Enzymol . 101:307 (1983) ) , the combination of CEN3 and ARS1 or other means for assuring replication, such as sequences that will result in incorporation of an appropriate fragment into the host cell genome . Control sequences for yeast vectors are known in the art and include but are not limited to promoters for the synthesis of glycolytic enzymes, including the promoter for 3 -phosphoglycerate kinase. See, for example, Hess et al . , J. Adv. Enzyme Reg. 7:149 (1968), Holland et al . , Biochemistry 17:4900 (1978) and Hitzeman, J. Biol. Chem. 255:2073 (1980). For example, some useful control systems are those that comprise the glyceraldehyde-3 -phosphate dehydrogenase (GAPDH) promoter or alcohol dehydrogenase (ADH) regulatable promoter, or the hybrid yeast promoter ADH2/GAPDH described in Cousens et al . , Gene 61:265-275 (1987), terminators also derived from GAPDH, and, if secretion is desired, leader sequences from yeast alpha factor. In addition, the transcriptional regulatory region and the transcriptional initiation region which are operably linked may be such that they are not naturally associated in the wild-type organism. Mammalian cell lines available as hosts for expression are known in the art and are available from depositories such as the American Type Culture Collection. These include but are not limited to HeLa cells, human embryonic kidney (HEK) cells, Chinese hamster ovary (CHO) cells, baby hamster kidney (BHK) cells, and others. Suitable promoters for mammalian cells also are known in the art and include viral promoters such as that from Simian Virus 40 (SV40) , Rous sarcoma virus (RSV) , adenovirus (ADV) , bovine papilloma virus (BPV) and cytomegalovirus (CMV) . Mammalian cells also may require terminator sequences and poly A addition sequences; enhancer sequences which increase expression also may be included, and sequences which cause amplification of the gene also may be desirable. These sequences are known in the art. Vectors suitable for replication in mammalian cells may include viral replicons, or sequences which ensure integration of the appropriate sequences encoding the P2X receptors into the host genome. An example of such a mammalian expression system is described in Gopalakrishnan et al . , Eur . J . Pharmacol. Mol. Pharmacol. 290:237-246 (1995). Other eukaryotic systems are also known, as are methods for introducing polynucleotides into such systems, such as amphibian cells, using standard methods such as described in Briggs et al . , Neuropharmacol . 34:583-590 (1995) or Stϋhmer, Meth. Enzymol. 207:319-345 (1992), insect cells using methods described in Summers and Smith, Texas Agricultural Experiment Station Bulletin No. 1555 (1987), and the like. The baculovirus expression system can be used to generate high levels of recombinant proteins in insect host cells. This system allows for high level of protein expression, while post-translationally processing the protein in a manner similar to mammalian cells. These expression systems use viral promoters that are activated following baculovirus infection to drive expression of cloned genes in the insect cells (O'Reilly et al . (1992) Baculovirus Expression Vectors: A Laboratory Manual, IRL/Oxford University Press) . Transfection may be by any known method for introducing polynucleotides into a host cell, including packaging the polynucleotide in a virus and transducing a host cell with the virus, by direct uptake of the polynucleotide by the host cell, and the like, which methods are known to those skilled in the art. The transfection procedures selected depend upon the host to be transfected and are determined by the rountineer. The expression of the receptor may be detected by use of a radioligand selective for the receptor. However, any radioligand binding technique known in the art may be used to detect the receptor (see, for example, Winzor et al . (1995) Quantitative Characterization of Ligand Binding, Wiley-Liss, Inc., NY; Michel et al . , Mol . Pharmacol . 51:524-532 (1997)). Alternatively, expression can be detected by utilizing antibodies or functional measurements, i.e., ATP-stimulated cellular depolarization using methods that are well known to those skilled in the art. For example, agonist-stimulated Ca 2+ influx, or inhibition by antagonists of agonist-stimulated Ca 2+ influx, can be measured in mammalian cells transfected with the recombinant P2X 2 receptor cDNA, such as COS, CHO or HEK cells. Alternatively, Ca 2+ influx can be measured in cells that do not naturally express P2 receptors, for example, the 1321N1 human astrocytoma cell line, have been prepared using recombinant technology to transiently or stably express the P2X 3 and receptor. The P2X polypeptide is recovered and purified from recombinant host cell cultures expressing the same by known methods including ammonium sulfate or ethanol precipitation, acid extraction, anion or cation exchange chromatography, phosphocellulose chromatography, hydrophobic interaction chromatography, hydroxyapatite chromatography or lectin chromatography. Protein refolding steps can be used, as necessary, in completing configuration of the protein. Finally, high performance liquid chromatography (HPLC) can be employed for final purification steps. Those polypeptides encompassed by the present invention are preferably about 40-60% percent similar to the amino acid sequence corresponding to the P2X 3 receptor, more preferably about 70-85% percent similar to the amino acid sequence of the P2X 3 receptor, and even more preferably at least about 90% percent similar to the amino acid sequence of the P2X 3 receptor. The human P2X receptor polypeptide, or fragments thereof, of the present invention, also may be synthesized by conventional techniques known in the art, for example, by chemical synthesis such as solid phase peptide synthesis. In general, these methods employ either solid or solution phase synthesis methods. See, for example, J. M. Stewart and J. D. Young, Solid Phase Peptide Synthesis, 2nd Ed., Pierce Chemical Co., Rockford, IL (1984) and G. Barany and R. B. Merrifield, The Peptides: Analysis, Synthesis, Biology, editors E. Gross and J. Meienhofer, Vol. 2, Academic Press, New York, (1980), pp. 3-254, for solid phase peptide synthesis techniques; and M. Bodansky, Principles of Peptide Synthesis, Springer-Verlag, Berlin (1984) and E. Gross and J. Meienhofer, Eds., The Peptides: Analysis, Synthesis, Biology, supra, Vol. 1, for classical solution synthesis. In one preferred system, either the DNA or the RNA derived therefrom, each of which encode the specific human P2X receptor, may be expressed by direct injection into a cell, such as a Xenopus laevis oocyte . Using this method, the functionality of the human P2X 3 receptor encoded by the DNA or the mRNA can be evaluated as follows. A receptor- encoding polynucleotide is injected into an oocyte for translation into a functional receptor subunit. The function of the expressed variant human P2X 3 receptor can be assessed in the oocyte by a variety of techniques including electrophysiological techniques such as voltage-clamping, and the like. Receptors expressed in a recombinant host cell may be used to identify compounds that modulate P2X 3 . In this regard, the specificity of the binding of a compound showing affinity for the receptor is demonstrated by measuring the affinity of the compound for cells expressing the receptor or membranes from these cells. This may be done by measuring specific binding of labeled (for example, radioactive) compound to the cells, cell membranes or isolated receptor, or by measuring the ability of the compound to displace the specific binding of a standard labeled ligand. See, Michel et al . , supra. Expression of variant receptors and screening for compounds that bind to, or inhibit the binding of labeled ligand to these cells or membranes, provide a method for rapid selection of compounds with high affinity for the receptor. These compounds may be agonists, antagonists or modulators of the receptor. Expressed receptors also may be used to screen for compounds that modulate P2X receptor activity. One method for identifying compounds that modulate P2X activity, comprises providing a cell that expresses a specific human P2X receptor polypeptide, combining a test compound with the cell and measuring the effect of the test compound on that P2X receptor activity. The cell may be a bacterial cell, a mammalian cell, a yeast cell, an amphibian cell, an insect or any other cell expressing the receptor. Preferably, the cell is a mammalian cell or an amphibian cell. Thus, for example, a test compound is evaluated for its ability to elicit an appropriate response, e.g., the stimulation of cellular depolarization or increase in intracellular calcium levels due to calcium ion influx if a P2X purinoreceptor is expressed in the host cell, the stimulation of an increase in intracellular calcium ion levels and/or inositolphospholipid hydrolysis and the formation of inositol phosphate if a P2Y purinoreceptor is expressed, or for the compound's ability to modulate the response to a P2X or P2Y purinoreceptor agonist or antagonist. The level of intracellular calcium may be analyzed using a calcium ion-sensitive fluorescent indicator. Cellular fluorescence may be monitored using a fluorometer. Examples of calcium ion-sensitive fluorescent dyes include, for example, quin-2 ( see, e . g. , Tsien et al . , J. Cell. Biol. 94:325 (1982)), fura-2 (see, e . g. , Grynkiewicz et al . , J. Biol. Chem. 260:3440 (1985)), calcium green-1, indo-1 (see, e . g. , Grynkiewicz et al . , supra) , fluo-3 (see, e . g. , Kao et al., J. Biol. Chem. 264:8179 (1989)) and rhod-2 ( see, e . g. , Tsien et al . , J " . Biol . Chem . abstract 89a (1987)), and the nonspecific esterase-hydrolyzable acetoxymethyl esters thereof, all of which are commercially available (Molecular Probes, Eugene, OR; Sigma Chemical Co., St. Louis, MO). Membrane depolarization of cells genetically engineered to express a P2X n purinoreceptor may be monitored using a fluorescent dye that is sensitive to changes in membrane potential. For example, the potential-sensitive fluorescent dye partitions into a membrane upon depolarization and results in a detectable increase in cellular fluorescence. Examples of such membrane potential-sensitive fluorescent dyes include carbocyanines, such as 3 , 3 ' -dipentyloxacarbocyanine iodide (DiOCs) and 3 , 3 ' -dipropylthiadicarbocyanine iodide (DiSC 3 ), oxonols, such as bis- (1, 3-dibutylbarbituric acid) pentamethine oxonol (DiBAC 4 (5)) or bis- (1, 3-dibutylbarbituric acid) pentamethine oxonol (DiBAC 4 (5) ) , or the like. In order to calibrate the fluorescence emission of these dyes in si tu, an agent that quenches the fluorescence emission may be used. Thus, for example, anti-fluorescein (Molecular Probes) quenches approximately 87% of the fluorescence of a 5 nM solution of fluo-3 at pH 7.0, and may used to calibrate the fluorescence emission of this dye. When acetoxymethyl ester dye derivatives are use, incomplete hydrolysis of the ester may result in a fluorescent indicator that is flourescent but insensitive to calcium ions. Controls for such a situation include transporting saturating amounts of calcium ions into the cell by an ionophore to achieve the maximum fluorescence response and transport of manganese ions into the cell to quench the fluorescence of the indicator if all acetoxymethyl esters have been hydrolyzed. One means by which such ions can be transported into cells is with the use of an ionophore, such as A23187 ( see, e . g. , Pressman et al . (1976) Ann . Rev. Biochem. 45:501) (Sigma Chemical Co.), the brominated derivative thereof ( see, e . g. , Deber et al . (1985) Anal. Biochem . 146:349) (Molecuar Probes), or other ionophores well known in the art. In addition, it may be desirable to quantify the amount of intracellular calcium ion from the fluorescence emission of a cell by comparing the fluorescence data obtained from the test compounds to a calibration curve that was generating by a series of calibrators each having a known calcium ion concentration. Thus, calcium ion standards are made having a range of concentrations bu preparing a stock solution of, e.g., CaCl2, from which dilutions may be made to attain the desired standard concentration (s) . The fluorescence emission of the standards in the presence of the calcium ion-sensitive fluorescent indicator dye is used to construct a standard curve and the intracellular calcium ion concentration of the genetically engineered cell in the assay is determined from the standard curve. Alternatively, cells previously treated with a calcium ionophore may be incubated with the indicator dye and the calcium ion standards used to generate the standard curve . The assay may be conducted manually or using an automated system. For a high capacity functional screening assay identifying human purinoreceptor ligands, an automated system is preferred. An example of such an automated system comprises providing a 96 -well culture plate in each well of which is cultured a cell genetically engineered to encode and express a human purinoreceptor polypeptide. The plate is loaded into a fluorescence imaging plate reader ("FLIPR"), which simultaneously measures the kinetics of intracellular calcium flux in each of the 96 wells. Such an FLIPR is commercially available from Molecular Devices Corp. (Sunnyvale, CA) . The FLIPR is capable of quantitatively transferring fluids into and from each well of the 96 -well plate and thus can be used to add the calcium-ion sensitive fluorescent indicator dye, a candidate compound, a purinoreceptor agonist, e.g., ATP, UTP, 2-methylthioATP, or the like, and/or a purinoreceptor antagonist, e.g., suramin, cibacron blue, PPADS, or the like. The FLIPR collects fluorescence data throughout the course of the assay. In a similar manner, the presence of a purinoreceptor agonist or antagonist in a test sample may be determined using a manual or an automated system. An automated system for practicing the method comprises providing a 96-well culture plate in each well of which a genetically engineered cell that expresses a purinoreceptor is cultured. The fluorescent indicator dye, test sample, and/or purinoreceptor agonist are added to each well and the fluorescence emission from each well is simultaneously monitored by an FLIPR. P2X purinoreceptor drugs are considered potential therapeutic agents in several disorders including, without limitation, central nervous system or peripheral nervous system conditions, e.g., epilepsy, pain, depression, neurodegenerative diseases, and the like, and in disorders of the reproductive system, asthma, peripheral vascular disease, hypertension, immune system disorders, irritable bowel disorder or premature ejaculation. In addition, the DNA, or RNA derived therefrom, can be used to design oligonucleotide probes for DNAs that express specific P2X receptors. As used herein, the term "probe" refers to a structure comprised of a polynucleotide, as defined above, which contains a nucleic acid sequence complementary to a nucleic acid sequence present in a target polynucleotide. The polynucleotide regions of probes may be composed of DNA, and/or RNA, and/or synthetic nucleotide analogs. Such probes could be useful in in vi tro hybridization assays to distinguish P2X 2 , and P2X 4 variants from wild-type message, with the proviso that it may be difficult to design a method capable of making such a distinction given the small differences that may exist between sequences coding the wild-type and a variant P2X receptor. Alternatively, a PCR-based assay could be used to amplify the sample RNA or DNA for sequence analysis. Furthermore, each specific P2X polypeptide or fragment (s) thereof can be used to prepare monoclonal antibodies using techniques that are well known in the art. The specific P2X receptor or relevant fragments can be obtained using the recombinant technology outlined below, i . e . , a recombinant cell that expresses the receptor or fragments can be cultured to produce quantities of the receptor or fragment that can be recovered and isolated. Alternatively, the specific P2X polypeptide or fragment (s) thereof can be synthesized using conventional polypeptide synthetic techniques as known in the art . Monoclonal antibodies that display specificity and selectivity for a particular P2X polypeptide can be labeled with a measurable and detectable moiety, for example, a fluorescent moiety, radiolabels, enzymes, chemiluminescent labels and the like, and used in in vi tro assays. It is theorized that such antibodies could be used to identify wild-type or variant P2X receptor polypeptides for immuno-diagnostic purposes. For example, antibodies have been generated to detect amyloid bl- 40 v. 1-42 in brain tissue (Wisniewski et al . (1996) Biochem. J. 313:575-580; also see, Suzuki et al . (1994) Science 264:1336-1340; Gravina et al . (1995) J. Biol. Chem. 270:7013- 7016; and Turnet et al . (1996) J. Biol. Chem. 271:8966-8970). Allosteric Modulation of Receptor Activity: Activation of the P2X receptors by ATP and other P2X receptor agonists regulates ion gradients across the cell membrane, modulates the cytosolic concentrations of cations, including Ca 2+ , Na + and K + , and has a role in the regulation of cell membrane potential. Allosteric modulators of receptor activity generally enhance agonist induced receptor activation by binding to secondary sites on the receptor. With respect to cibacron blue, the present invention relates to the discovery that this P2X receptor antagonist has the ability to allosterically modulate the effects of a P2X 3 receptor present in a mammal such as a human or a rat . More specifically, in cells expressing the human P2X 3 receptor, cibacron blue has the ability to mediate approximately a 3-7 fold increase in the magnitude and potency of ATP-activated Ca 2+ influx and transmembrane currents. The half-maximal concentration of cibacron blue required to mediate maximal potentiation is independent of the agonist used to activate the hP2X 3 receptor. Since cibacron blue enhances both agonist potency and the absolute magnitude of P2X 3 receptor activition, these allosteric actions are significantly distinct from the previously reported effects of cibacron blue on P2X4 receptor activity (Miller et al . , Naunyn Schmiedebergs Arch Pharmacol . 354:562-571 (1998)). Thus, consistent with the properties of other ligand-gated ion channels, P2X 3 receptor activity may be allosterically modulated by a ligand distinct from the endogenous agonist. With respect to the effect of cibacron blue on antagonists, the non-selective P2 receptor antagonist, PPADS, for example, causes a rightward shift of the cibacron blue concentration-effect curve, whereas increasing concentrations of cibacron blue attenuate PPADS antagonism. The rat homolog of the P2X 3 receptor produces similar results to those presented above and in the examples, upon exposure to cibacron blue, suggesting that the modulatory activity of cibacron blue is not species-dependent. It should be noted that the mechanism of cibacron blue- mediated P2X 3 receptor potentiation is not a secondary result of its previously described inhibitory effect on ectonucleotidases (Stout et al., Biochem. Mol. Biol. Int. 36:927-934 (1995)). If cibacron blue-mediated ecto-ATPase activity were a contributing factor, it would be expected that cibacron blue alone would mediate agonist-like activity by increasing the level of endogenous ATP in the medium. However, as evidenced by the data corresponding to the examples below, there are no intrinsic effects of cibacron blue on hP2X 3 receptor activity. Furthermore, the accumulation of endogenous agonist as a result of ecto-ATPase inhibition would be expected to affect all P2 receptor subtypes, rather than the P2X 3 receptor alone. In addition to ATP, in accordance with the present invention, cibacron blue may be utilized to potentiate hP2X 3 receptor activation by other P2X receptor agonists including, for example, 2-meSATP, BzATP and αβ-meATP. In each case, the half-maximal concentration of cibacron blue required to mediate full potentiation is similar, suggesting that the effect of cibacron blue on the receptor is independent of the agonist. Thus, any agonist, as deemed to be appropriate by a medical practitioner, may be utilized in combination with cibacron blue. In addition to mediating an increase in the magnitude of the maximal P2X 3 receptor signal, cibacron blue enhances agonist potency by causing a leftward shift of the ATP concentration-response curve. In particular, in the presence of 3 μM cibacron blue, ATP is 7-fold more potent than in its absence, suggesting that cibacron blue may have an affect on the affinity and/or the efficacy of ATP for the hP2X 3 receptor, or serves to enhance the cooperativity of ATP binding to the multimeric receptor. The modulatory activity of cibacron blue may be corroborated by the observation that the inhibitory potency of a non-competitive P2X 3 antagonist, PPADS, is inversely related to the concentration of cibacron blue. Cibacron blue, while increasing the magnitude of P2X 3 receptor activation, causes a rightward shift of the PPADS concentration-effect curve demonstrating that this allosteric modulator reduces antagonist activity. This effect of cibacron blue is independent of ATP concentration and is thus not a consequence of an apparent increase in receptor occupancy. The cibacron blue-mediated leftward shift of the agonist concentration-effect curves and rightward shift of the antagonist concentration-effect curves, support the conclusion that cibacron blue functions as an allosteric modulator of P2X 3 receptor activity. Furthermore, the mutual exclusivity of PPADS-mediated inhibition and cibacron blue-mediated potentiation suggests a complex interaction between regulatory ligands that modulate P2X 3 receptor function. The modulatory effect of cibacron blue described above may be observed at both rat and human P2X 3 receptors, and is at least 1000-fold more potent than the actions of Ca 2+ , suggesting that endogenously expressed P2X 3 receptors might be subject to functional regulation by a multiplicity of low and high-affinity interactions. Acceleration of Receptor Recovery Subsequent to Desensitization : In addition to potentiating the effects of ATP at the hP2X 3 receptor, cibacron blue also has the ability to produce approximately a 6-fold increase in the rate of hP2X 3 receptor recovery from desensitization, as evidenced by its ability to restore ATP-responsiveness to acutely desensitized receptors. Therefore, in accordance with the present invention, cibacron blue may be administered to a patient in order to increase the rate of P2X 3 receptor resensitization following a phase of desensitization . Furthermore, it should be noted that the potentiation of both the human and rat P2X 3 receptors by cibacron blue occurs concomitantly with accelerated receptor resensitization. Thus, in accordance with the present invention, one may potentiate the receptor while simultaneously resensitizing the receptor. The apparent rate of recovery from desensitization is increased six- fold in the presence of 50 μM cibacron blue. The decrease in the half-life of the refractory period after desensitization suggests that endogenously expressed P2X 3 receptors may be subject to modulatory mechanisms which facilitate their functional recovery. It is believed that the binding of cibacron blue to the P2X 3 receptor leads to a rapid conformational change, resulting in the potentiation of ATP-mediated P2X 3 receptor activation. This conformational change also mediates a slower, long-term effect on the desensitization state of the receptor, such that allosteric modulation and functional resensitization occur sequentially and may share a common mechanism of action. In view of the above, the present invention relates to the novel discovery that cibacron blue selectively modulates human and rat P2X 3 receptors by enhancing agonist potency and efficacy, as well as facilitating receptor resensitization following acute agonist-induced desensitization. Thus, one may administer cibacron blue to a patient in order to both modulate the receptor, if desired, for example, in sensory deficit studies, as well as facilitate resensitization, or to achieve either one of these two effects. Therapeutic Implications: In accordance with the present invention, one may also peripherally administer P2 receptor antagonists to a patient in order to, for example, reduce nociception (i.e., the sensation of pain). For example, TNP-ATP (i.e., a potent P2X receptor agonist) may be utilized to reduce acute or persistent nociception in a patient. Peripherally administered TNP-ATP attenuates both acute and persistent nociception in the formalin (i.e., inflammatory stimulus) test that provides evidence for a contribution of both P2X 3 and/or P2X 2/3 receptors to peripheral nociceptive neurotransmission. This view is further supported by the ability of cibacron blue to specifically enhance P2X 3 a d P2X 2/3 receptor activation in vi tro and to enhance both acute and persistent nociception in vivo . The pronociceptive effects of cibacron blue in vivo appear to be pharmacologically specific since another anthraquinone sulfonic acid derivative, reactive orange, which does not alter P2X 3 receptor function, does not enhance the nociceptive effects of intradermal formalin. In summary, in view of the above and as evidenced by the examples presented below, activation of ATP-gated P2X 3 and P2X 2/3 receptors, which are highly localized on capsaicin- sensitive primary sensory neurons (Vulchanova et al . , Neuropharmacol . 36:1229-1242 (1997)), contribute to nociceptive neurotransmission. The peripheral administration of the P2X receptor agonists initiate acute nociceptive responses in laboratory animals and enhance the nociceptive effects of other noxious stimuli including carrageenan, formalin, and capsacin as illustrated herein (see also Bland-Ward et al . , Br. J. Pharmacol . 122:366-371 (1997); Hamilton et al . , Br. J. Pharmacol . 126:326-332 (1999); Sawynok et al . , Eur. J. Pharmacol . 330:115-121 (1997); Tsuda et al . , Br. J. Pharmacol. 127:449-456 (1999); Tsuda et al . , Br. J. Pharmacol. 128:1497- 1504 (1999) ) . The demonstration that peripherally administered TNP-ATP attenuates both acute and persistent nociception in the rat formalin test provides evidence for a contribution of both P2X 3 and/or P2X 2/3 receptors to peripheral nociceptive neurotransmission. This concept is further supported by the ability of cibacron blue to selectively enhance P2X 3 and P2X 2/3 receptor activation in vi tro and to enhance both acute and persistent pain in vivo . Thus, the pharmacological modulation by TNP-ATP and cibacron blue of nociceptive responses produced by a P2 receptor agonist (BzATP) or by an inflammatory stimulus (formalin) provide evidence for a specific role of P2X 3 and/or P2X 2/3 receptor activation in nociceptive neurotransmission. The examples presented below relate to specific embodiments for carrying out the present invention. The examples are offered for illustrative purposes only, and are not intended to limit the scope of the present invention in any way. Efforts have been made to ensure accuracy with respect to numbers used ( e . g. , amounts, temperatures, etc.), but some experimental error and deviation should, of course, be allowed for. EXAMPLE 1 IDENTIFICATION OF A HUMAN cDNA SEQUENCE LIKELY TO ENCODE P2X, POLYPEPTIDE The predicted amino acid sequence of the rat P2X 3 receptor (NCBI sequence I.D. number 1103623) was used to search for human DNA sequences which would code for similar polypeptides. The TBLASTN database search tool (Altschul (1993) J " . Mol . Evol . 36:390-300) was used, which allows querying nucleotide databases with a protein sequence by dynamically translating the DNA sequences into all 6 possible reading frames. A search of the Genbank sequence-tagged sites (STS) database revealed a human genomic fragment, 229 basepairs in length, containing an open reading fame which would be predicted to encode a polypeptide having a high degree of homology to a region of the rat P2X 3 receptor. The deposited sequence for this fragment (Genbank accession number G03901) was as follows: CCCGAATCGG TGGACTGCTT CTCCACTGTG GTCTGGTCGC TGGGGTACAC TGGGTTGGTC AAAGCCGCGA TTTTCAGTGT AGTCTCATTC ACNTGNAGGC GAAAGAGCTG GTGTTGTCAA GTTCTGACTA TGGGCAATGT CCTCTTTTGT GACCCCATTT GACAGACTCA GCAGTGGGCG CCCATGACCT AGTCATGAGG GGAGCCAGGA CATCTGTGTG ATCCCAAGG (SEQ ID NO:l) Where "N" represents any of the bases A, T, G and C. EXAMPLE 2 IDENTIFICATION OF THE 5' END OF THE PX2, cDNA Based on the sequence of G03901, primers were designed for use in reverse-transcription polymerase chain reaction (RT-PCR) procedures in an effort to isolate the intact open reading frame for this receptor. The primers used in the reactions described below were as follows: Primer Is (SEQ ID. NO: 2) : 5 ' -TTTACCAACCCAGTGTACCC-3 ' Primer 2s (SEQ ID. NO: 3) : 5 ' -ACCACAGTGGAGAAGCAGTC-3 ' Primer 3as (SEQ ID. NO:4) : 5 ' -GAATCGGTGGACTGCTTCTC-3 ' Primer 4as (SEQ ID. NO: 5) : 5 ' CGATTTTCAGTGTAGTCTCATTC-3 ' Primer 5as (SEQ ID. NO:6): 5 ' GGGGTACACTGGGTTGGTAA-3 ' 5 'RACE Anchor Primer (SEQ ID. NO: 7): 5 ' CUACUACUACUAGGCCACGCGTCGACTAGTACGGGIIGGGIIGGGIIG-3 ' Universal Adapter Primer (SEQ ID. NO: 8) : 5 ' -CUACUACUACUAGGCCACGCGTCGACTAGTAC-3 ' Adapter Primer (SEQ ID. NO: 9): 5 ' -GGCCACGCGTCGACTAGTACTTTTTTTTTTTTTTTTT-3 ' Abridged Universal Adapter Primer (SEQ ID. NO: 10) : 5 ' -GGCCACGCGTCGACTAGTAC-3 ' 5'hP2X 3 Primer (SEQ ID. NO: 11) : 5 ' -CACCATGAACTGCATATCCGACTTC-3 ' 3'hP2X 3 Primer (SEQ ID. NO: 12) : 5 ' CTAGTGGCCTATGGAGAAGGC-3 ' To identify the 5' end of the cDNA which is derived from the genomic region which sequence G03901 is part of, the RACE technique (Rapid Amplification of cDNA Ends) (Frohman et al . (1988), Proc . Na tl . Acad . Sci . U. S . A . 85:8998-9002) was employed. Extension of the cDNA identified through the RT-PCR step was accomplished using the 5 'RACE™ reagent system (Life Technologies, Gaithersburg, MD) . One microgram of poly A+ RNA derived from human pituitary gland tissue (Cat. # 65894-1, Lot # 6080167; Clontech Laboratories, Palo Alto, CA) was used in a reaction using reagents provided in the kit as described; lμl (1 μg) of RNA was combined with 3 μl (3pmol) Primer 3as and 11 μl Rnase-Free water (water treated with diethylpyrocarbonate, or DEPC) and heated to 70°C for 10 minutes followed by 1 minute on ice. 2.5 μl lOx reaction buffer (200 mM Tris-HCl pH 8.4, 500mM KCl), 3 μl 25 mM MgCl 2 , 1 μl 10 mM dNTP mix, and 2.5 μl 0.1 M DTT were added. The mix was incubated at 42°C for 2 minutes after which 1 μl Superscript II™ reverse transcriptase (Life Technologies) was added. The reaction was incubated for an additional 30 minutes at 42°C, 15 minutes at 70 °C, and on ice for 1 minute. One microliter of RNase H (2 units) was added and incubated at 55°C for 20 minutes. The cDNA was purified using the GlassMax™ columns included in the kit. The cDNA was eluted from the column in 50 μl distilled water (dH 2 0) , lyophilized, and resuspended in 21 μl dH 2 0. Tailing of the cDNA was accomplished in the following reaction: 7.5 μl dH 2 0, 2.5 μl reaction buffer (200 mM Tris-HCl pH 8.4, 500mM KCl), 1.5 μl 25 mM MgCl 2 , 2.5 μl 2mM dCTP, and 10 μl of the cDNA were incubated at 94°C for 3 minutes, then 1 minute on ice, followed by 10 minutes at 37°C. Finally, the mixture was incubated at 70°C for 10 minutes and then placed on ice. PCR amplification of the cDNA was performed in the following steps : 5 μl of the cDNA was included in a reaction which also contained 5 μl lOx GeneAmp™ PCR buffer (Perkin Elmer, Foster City, CA) (500mM KCl, lOOmM Tris-HCl pH 8.3, 15mM MgCl 2 , and 0.01% (w/v) gelatin), 1 μl 10 mM dNTP mix, lμl (10 pmol) Anchor Primer, 1 μl (10 pmol) Primer 5as, and 35 μl dH 2 0. The reaction was heated to 95°C for 1 minute, then held at 80°C for 2 minutes, during which 0.5 μl (2.5 units) Amplitaq™ polymerase (Perkin-Elmer) was added. The reaction was cycled 35 times under these conditions: 94°C for 15 seconds, 52°C for 20 seconds, and 72°C for 1 minute. After the amplification, the reaction products were purified utilizing the QiaQuick™ PCR product purification system (Qiagen, Inc., Chatsworth CA) as per the manufacturer's instructions. The products were eluted from the columns with 50 μl TE buffer (lOmM Tris, ImM EDTA pH 8.0), and one microliter of the eluent was utilized as template DNA in a PCR reaction to increase levels of specific product for subsequent isolation. The reamplification also included: 5 μl lOx GeneAmp™ PCR buffer, 1 μl 10 mM dNTP mix, 1 μl (10 pmol) Universal Amplification Primer, 1 μl (10 pmol) Primer 4as, and 40.5 μl dH 2 0. The reaction was heated to 95°C for 1 minute, then held at 80°C during which 0.5 μl (2.5 units) Amplitaq™ polymerase was added. The reaction was cycled 35 times under these conditions: 94°C for 15 seconds, 50°C for 20 seconds, and 72°C for 1 minute. Amplification products were analyzed via 0.8% agarose gel electrophoresis and a predominant product of approximately 1.3 kilobase pairs in length was detected. This product was excised from the gel and purified via the QiaQuick™ purification system. The product was eluted from the column with 50 μl dH 2 0 and lyophilized to 10 μl volume. Three microliters of the resulting DNA was used in a ligation reaction with pCR 2.1 vector (Invitrogen, Carlsbad, CA) incubated at 14°C overnight. The ligation products were used to transform E. coli from the cloning kit using standard manufacturer's protocols. Insert sizes of resulting clones were determined using EcoRI digestions of the plasmids and clones containing inserts of the approximate size of the PCR product were sequenced using fluorescent dye-terminator reagents (Prism™, Perkin Elmer Applied Biosystems Division, Foster City, CA) and an Applied Biosystems Model 373 DNA sequencer. The sequence of the 5 'RACE product including the EcoRI sites from the pCR 2.1 vector is shown in Figure 1 (SEQ ID NO: 13) . The sequences of the amplimers (Universal Amplification Primer and the complement to Primer 4as) are underlined. Example 3 IDENTIFICATION OF THE 3' END OF THE P2X, cDNA To identify the sequence surrounding the termination codon of the open reading frame encoding the human P2X 3 receptor, the Life Technologies 3 ' RACE™ System was employed with primers designed to STS G03901. Poly A+ RNA (500 nanograms) derived from pituitary gland tissue (see Example 2, above) was used in the reaction as follows: The RNA and 10 picomoles Adapter Primer were combined in a final volume of 12 μl dH 2 0. This mixture was heated to 70°C for 10 minutes and chilled on ice for 1 minute. The following components were added: 2 μl lOx PCR buffer (200 mM Tris-HCl pH 8.4, 500 mM KCl), 2 μl 25 mM MgCl 2 , 1 μl lOmM dNTP mix, and 2 μl 0.1M dithiothreitol . The reaction was equilibrated to 42°C for 2 minutes after which 1 μl (200 units) Superscript II™ reverse transcriptase was added and incubation continued at 42°C for 50 minutes. The reaction was terminated by incubation at 70°C for 15 minutes and chilled on ice. Rnase H (1 μl; 2 units) was added and the mixture was incubated for 20 minutes at 37°C, then stored on ice. Amplification of the 3 ' end of the P2X 3 cDNA was accomplished in the following reactions: 2μl of the first strand cDNA synthesized above was used in a PCR mixture also including 5 μl lOx GeneAmp™ PCR buffer, 1 μl lOmM dNTPs, 1 μl (10 picomoles) , Primer Is, lμl (10 picomoles) Abridged Universal Amplification Primer (AUAP) and 39.5 μl dH 2 0. The reaction was heated to 95°C for 1 minute, then held at 80°C for 2 minutes, during which 0.5 μl (2.5 units) Amplitaq™ polymerase was added. The reaction was cycled 35 times under these conditions: 94°C for 15 seconds, 54°C for 20 seconds, and 72°C for 2 minutes. After cycling, the reaction was incubated for 10 minutes at 70°C and stored at 4°C. After the amplification, the reaction products were purified utilizing the QiaQuick™ PCR product purification system as per the manufacturer's instructions. The products were eluted from the columns with 50 μl TE buffer (lOmM Tris, 0. ImM EDTA pH 8.0) and one microliter of the eluent was utilized as template DNA in a PCR reaction to increase levels of specific product for subsequent isolation. The reamplification also included: 5 μl lOx GeneAmp™ PCR buffer, 1 μl 10 mM dNTP mix, 1 μl (10 pmol) AUAP, 1 μl (10 pmol) Primer 2s, and 40.5 μl dH 2 0. The reaction was heated to 95°C for 1 minute, then held at 80 °C during which 0.5 μl (2.5 units) Amplitaq™ polymerase was added. The reaction was cycled 35 times under these conditions: 94°C for 15 seconds, 54°C for 20 seconds, and 72°C for 2 minutes. Amplification products were analyzed via 0.8% agarose gel electrophoresis and a predominant product of approximately 700 base pairs in length was detected. This product was excised from the gel and purified via the Qiaquick™ purification system. The product was eluted from the column with 50 μl dH 2 0 and lyophilized to 10 μl volume. Three microliters of the above DNA was used in a ligation reaction with pCR 2.1 vector (Invitrogen) incubated at 15°C for 3.5 hours. The ligation products were used to transform E. coli from the cloning kit . Insert sizes of resulting clones were determined using EcoRI digestions of the plasmids and clones containing inserts of the approximate size of the PCR product were sequenced using fluorescent dye-terminator reagents (Prism, Applied Biosystems) and an Applied Biosystems 373 DNA sequencer. The sequence of the 3 ' RACE product including the EcoRI sites from the pCR 2.1 vector is shown in Figure 2 (SEQ ID NO: 14), in which the sequences of the amplimers (AUAP and the complement to Primer 2s) are underlined. Example 4 ISOLATION OF cDNA CONTAINING THE INTACT OPEN READING FRAME OF HUMAN P2X-, Using information on the sequence surrounding the initiation and termination codons of the human P2X 3 message, oligonucleotide primers were designed and synthesized to enable RT-PCR of the intact open reading frame of the mRNA. The sequences of these primers, designated 5 ' hP2X 3 and 3'hP2X 3 , are shown above. PCR amplification was performed on a portion (2ul) of the pituitary gland cDNA described in Example 3. A proofreading thermostable polymerase (Cloned Pfu DNA Polymerase, Strategene, La Jolla, CA) was used in the amplification to ensure high-fidelity amplification. The reaction mixture consisted of 2 μl cDNA, 5 μl lOx cloned Pfu polymerase reaction buffer (200 mM Tris-HCl (pH 8.8), lOOmM KCl, lOOmM (NH 4 ) 2 S0 4 , 20mM MgS0 4 , 1% Triton X-100, 1 mg/ml nuclease-free bovine serum albumin) , 1 μl dNTP mix, lμl (10 picomoles) 5'hP2X 3 Primer, lμl (10 picomoles) 3 ' hP2X 3 Primer, and 39.5 μl dH 2 0. The reaction was heated to 95°C for 1 minute, then held at 80°C for 2 minutes, during which time 0.5 μl (1.25units) cloned Pfu polymerase was added. The reaction was cycled 35 times under the following conditions: 94°C for 20 seconds, 52°C for 20 seconds, and 72°C for 3.5 minutes. After cycling, the reaction was incubated for 10 minutes at 70°C. The reaction products were separated on a 0.8 % agarose gel and a product of approximately 1.2 kilobases was excised and purified via the QiaQuick™ gel purification system. The DNA was eluted with 50 μl dH 2 0, lyophilized and resuspended in 10 μl dH 2 0. One microliter of this DNA was use in a reamplification reaction which also inluded 5 μl lOx Pfu reaction buffer, 1 μl dNTP mix, lμl (10 picomoles) 5 ' hP2X 3 Primer, lμl (10 picomoles) 3 ' hP2X 3 Primer, and 40.5 μl dH 2 0. The reaction was heated to 95°C for 1 minute, then held at 80°C for 2 minutes, during which 0.5 μl (1.25units) cloned Pfu polymerase was added. The reaction was cycled 15 times under the following conditions: 94°C for 20 seconds, 52°C for 20 seconds, and 72°C for 3.5 minutes. After cycling, the reaction was incubated for 10 minutes at 70°C. The reaction products were separated on a 0.8 % agarose gel and the 1.2 kilobase product was excised and purified via the QiaQuick™ gel purification system. The DNA was eluted with 50 μl dH 2 0, lyophilized and resuspended in 15 μl dH 2 0. Three microliters of the purified PCR product was used in a ligation reaction using the pCRscript™ cloning system (Stratagene) which also included 0.5μl (5ng) of the pCRScript™ Amp SK(+) vector, lμl of pCRScript™ lOx Reaction Buffer, 0.5 μl of lOmM ATP, lμl (5 units) Srf I restriction enzyme, lμl (4 units) T4 DNA ligase, and 3 μl dH 2 0. The reaction mixture was incubated at room temperature for one hour, then at 65°C for 10 minutes. One microliter of this reaction product was used to transform XL-2 blue ultracompetent cells (Stratagene) as per standard manufacturer's protocols. Resulting clones were screened by restriction analysis and sequenced using fluorescent dye-terminator reagents (Prism, Applied Biosystems) and an Applied Biosystems Model 310 DNA sequencer. The sequence of the intact open reading frame is shown in Figure 3 (SEQ ID NO: 15) . A comparison of the predicted protein sequence of the human P2X 3 of the present invention (SEQ ID NO: 16) with that of the corresponding rat polypeptide (SEQ ID NO: 17) is depicted in Figure 4. Example 5 EXPRESSION AND ELECTROPHYSIOLOGICAL ANALYSIS OF RECOMBINANT P2X, RECEPTORS IN XENOPUS OOCYTES Oocytes of Xenopus laevis were prepared and injected with receptor DNA of the present invention, and receptor responses were measured using two-electrode voltage-clamp, according to procedures previously described (Briggs et al . (1995), supra) . Oocytes were maintained at 17-18 °C in normal Barth's solution (90 mM NaCl, 1 mM KCl, 0.66 mM NaN0 3 , 0.74 mM CaCl 2 , 0.82 mM MgCl 2 , 2.4 mM NaHC0 3 , 2.5 mM sodium pyruvate, and 10 mM Na N- (2-hydroxy-ethyl) -piperazine-N' - (2-ethanesulfonic acid) ("HEPES") buffer, final pH 7.55) containing 100 μg/ml gentamicin. Responses were measured at a holding potential of -60 mV in modified Barth's solution containing 10 mM BaCl 2 and lacking CaCl 2 and MgCl 2 (final pH 7.4) . However, in some experiments, the cell potential was intentionally varied in order to determine the response current-voltage relationship. Agonist was applied briefly using a computer-controlled solenoid valve and a push/pull applicator positioned to within 200-400 μm from the oocyte. Responses were recorded by computer in synchrony with agonist application. Antagonists were included with agonist in the push/pull applicator and were applied to the bath by superfusion for at least 3 minutes before application of agonist. Responses were quantified by measuring the peak amplitude. DNA for injection into oocytes was the P2X 3 insert from pCDNA3.1 prepared as described in Example 2. The clone was grown up and prepared in large scale using the QIAgen maxiprep DNA preparation system according to the manufacturer's instructions. The DNA was ethanol precipitated and resuspended in TE buffer. For functional anaysis of human P2X 3 receptors, 10 ng of human P2X 3 DNA prepared as described above were injected into the nucleus of Xenopus oocytes . Oocytes were incubated in normal Barth's solution containing 100 μg/ml gentamicin for 2- 7 days following injection. The response to 10 μM ATP was then recorded. The results of the above expression and analysis show the receptors of the present invention to be functional. Oocytes injected with human P2X 3 DNA responded to extracellular application of ATP by exhibiting a mixed-conductance cation current (100-6000 nA) . Oocytes injected with an appropriate amount of water did not respond to ATP . An approximate ATP EC 50 of 0.7 μM was obtained from concentration-response relationships (0.01-1000 μM) from these oocytes. ATP-induced current-voltage relationships were also recorded from these oocytes. These revealed a reversal potential of approximately zero mV, with pronounced inward rectification recorded at negative membrane potentials. Another P2X receptor agonist, α, β-methylene-ATP, elicited maximal currents similar to those evoked by ATP, although it was slightly less potent (EC S0 = 2.1 μM) . Application of a third P2X receptor agonist, 2-methylthio-ATP, was slighly more potent (EC S0 = 0.4 μM) than either ATP or α, β-methylene-ATP . Functional antagonism of responses was determined by application of the non-specific P2X receptor antagonists suramin or pyridoxal-phosphate-6 -azophenyl-2 ' , 4 ' -disulfonic acid (PPADS) . Both antagonists produced a complete block of ATP (0.3 μM) -induced currents, with suramin displaying increased potency (IC 50 = 0.3 μM) relative to PPADS (IC 50 = 1 μM) . In summary, injection of human P2X 3 receptor DNA into Xenopus oocytes resulted in expression of functional P2X 3 receptors on the cell surface, and these receptors function as ligand-gated non-specific cation channels. These receptors responded to extracellular P2 receptor agonists with a rank order potency of 2-methylthio-ATP > ATP > α, β-methylene-ATP. They also exhibit inward rectification and are blocked by both P2 receptor antagonists PPADS and suramin. Example 6 Measurement of Intracellular Calcium Levels Subsequent to Cibacron Blue Exposure The following materials, cell lines and culture apply to this example and all examples which follow, where noted. Materials Adenosine 5 ' -triphosphate disodium (ATP), 2-methylthio- ATP tetrasodium (2-meSATP) , and β-methylene ATP dilithium (αβ-meATP) were obtained from Research Biochemicals International (Natick, MA). 2' & 3 ' -0- (4-benzoylbenzoyl) -ATP tetraethylammonium salt (mixed isomers) (BzATP) and cibacron blue were obtained from Sigma Chemical Company (St. Louis, MO) . G418 sulfate was obtained from Calbiochem-Novabiochem Corp. (La Jolla, CA) . Dulbecco's modified Eagle's medium (D- MEM) (with 4.5 mg ml "1 glucose and 4 mM L-glutamine) and fetal bovine serum (FBS) were obtained from Hyclone Laboratories, Inc. (Logan, Utah). Dulbecco's phosphate-buffered saline (D- PBS) (with 1 mg ml "1 glucose and 3.6 mg l "1 Na pyruvate, without phenol red) , hygromycin and Lipofectamine were obtained from Life Technologies (Grand Island, NY) . Fluo-4 AM was purchased from Molecular Probes (Eugene, OR) . Stable cell lines and cell culture The rat P2X 3 receptor cDNA was 100% identical to the previously published sequence (Garcia-Guzman et al . , Brain Res. Mol. Brain. Res. 47:59-66 (1997)). The human P2X 3 receptor was essentially identical to that reported by Garcia- Guzman et al, supra (1997) (Genbank accession #Y07683) . A single exception was at amino acid residue 126, where an arginine was encoded; the published sequence encodes a proline at this position. Multiple replications of cloning the human P2X 3 receptor yielded the same sequence, suggesting that the observed difference is not the result of a cloning artifact or a sequencing error. 1321N1 human astrocytoma cells stably expressing rP2X 3 , or hP2X 3 receptors (1321rX 3 -3, and 1321hX 3 -l l , respectively) were constructed using standard lipid-mediated transfection methods. All cell lines were maintained in D-MEM containing 10% FBS and antibiotics as follows: 1321rX 3 -3 and 1321hX 3 -ll cells, 300 μg ml "1 G418; and 1321rX 2 -l cells, 1 00 μg ml "1 hygromycin. Cells were grown at 37°C in a humidified atmosphere containing 5% C0 2 . Measurement of intracellular Ca 2+ levels P2X receptor function was determined on the basis of agonist-mediated increases in cytosolic Ca 2+ concentration. A fluorescent Ca + chelating dye (Fluo-4) was used as an indicator of the relative levels of intracellular Ca 2+ in a 96- well format using a Fluorescence Imaging Plate Reader (FLIPR, Molecular Devices, Sunnyvale, CA) . Cells were grown to confluence in 96 -well black-walled tissue culture plates and loaded with the acetoxymethylester (AM) form of Fluo-4 (1 μM) in D-PBS for 1-2 hours at 23°C. Cibacron blue (50 μl of 4x concentration) was added 3 min before the addition of agonists (50 μl of 4x concentration) (final volume = 200 μl) . Fluorescence data was collected at 1 to 5 sec intervals throughout each experimental run. Data shown in Figure 5a are based on the peak increase in relative fluorescence units as compared to basal fluorescence. Concentration-effect curves for all cell types are shown as a percentage of the maximum ATP-mediated signal measured in the absence of cibacron blue. Concentration response data were analyzed using a four-parameter logistic Hill equation in GraphPad Prism (San Diego, CA) . All data are expressed as mean ± sem. Statistical analysis was performed using Student's t-test (P < 0.05) on the basis of pIC 50 values. As evidenced by the data, ATP activation caused a rapid and transient increase in the levels of cytoplasmic Ca 2+ . The shapes of the Ca 2+ influx curves were qualitatively similar to electrophysiological data (see Example 9 below) measured in Xenopus oocytes (Fig 5b) , and were consistent with previously reported observations (Bianchi et al , 1999) . Pre-incubation of the cells for 3 min with cibacron blue (10 μM) led to a 3-7 fold increase in the magnitude of the maximal ATP-activated response (E max ) , as measured both by Ca 2+ influx (Fig 5a) and transmembrane currents (Fig 5b) . Cibacron blue mediated a similar 307 fold potentiation of the maximal ATP response using cells expressing the rat P2X 3 receptor homolog (data not shown) . Pilot experiments showed that the onset of the cibacron blue effect occurred in less than 1 min, thus a 3 min pre- application time was selected to ensure full activity. Cibacron blue alone exhibited no intrinsic effect on Ca 2+ influx at concentrations up to 200 μM and did not measurably affect the pH of the assay buffer (pH 7.2) at concentrations up to 1 mM. The potentiating effect of cibacron blue was specific for the P2X 3 receptor, since concentrations of cibacron blue up to 1 mM did not alter agonist activation of hP2X x , hP2X 2 and hP2X 7 receptors expressed in 1321N1 cells (data not shown) . Cibacron blue (10 μM) did enhance the potency of ATP activation of hP2X 4 receptor-mediated Ca 2+ influx in the presence of sub-maximal concentrations of agonist, as has previously been described (Miller et al . , 1998) . However, no increase in the maximal ATP activated hP2X 4 response was observed. Example 7 Characterization of Receptor Electrophysiology Following Exposure to Cibacron Blue Electrophysiology : The hP2X 3 receptor subtype expressed in Xenopus oocytes was characterized using standard two-electrode voltage-clamp techniques. Briefly, oocytes were denuded of overlying follicle cells and intranuclear injections of 12 nl cDNA (1 μg/μl) were performed on each oocyte. Oocytes were used for recordings 1-5 days post-injection and were perfused (3.5 ml/min) with a standard recording solution containing (mM) : 96 NaCl, 2.0 KCl, 1.8 CaCl 2 , 1.0 MgCl 2 , 5.0 Na-pyruvate and 5.0 Na-Hepes (pH 7.4). Electrodes (1.5-2.0 MΩ) were filled with 120 mM KCl. ATP was applied using a solenoid-driven drug application pipette positioned close to the oocyte in the perfusion chamber. ATP was applied every 3.5 min, and application duration typically lasted 5 sec. Cibacron blue was bath applied for at least 3 min before being co-applied with ATP through the drug pipette. Cells were voltage-clamped at -60 mV. Data were acquired and analyzed using pClamp software (Axon Instruments, Foster City, CA) . As evidenced by the data of Figure 5b, cibacron blue (1 μM) produced a potentiation of the peak amplitude of 1 μM ATP- activated currents to 213 ± 49% of control (Fig 5b) . The effect of cibacron blue on the E max of the hP2X 3 receptor- mediated transmembrane current was long-lasting, such that full potentiation was observed up to 9 min after a brief (1 min) exposure to cibacron blue. The onset of the cibacron blue potentiation effect was rapid (<1 min; data not shown) . Co-application of cibacron blue and ATP resulted in potentiation of the Ca 2+ influx signal, albeit with lower apparent potency and E max than observed after a 3 min pre- incubation period. Cibacron blue (10 μM) had no apparent effect on the kinetics of the ATP-activated Ca 2+ flux response (Fig 5a) or on the acute desensitization kinetics of the hP2X 3 receptor (Fig 5b) . The potentiation of ATP-activated human P2X 3 receptors by cibacron blue was concentration-dependent (Fig 6) , with an observed half-maximal response (EC 50 ) of 1.4 ± 0.5 μM (Fig 6). In addition to increasing the E max of the ATP-activated hP2X 3 receptor response, cibacron blue also caused a concentration- dependent leftward shift of the ATP concentration-effect curve (Fig 6) . In the presence of 3 μM cibacron blue, the magnitude of ATP-activated hP2X 3 receptor signaling was increased more than 3-fold (E max = 330 ± 5%) whereas the EC 50 of ATP was decreased from 356 ± 100 nM to 46 ± 8 nM (Fig 6) . The EC 50 of cibacron blue required to mediate potentiation was similar irrespective of the agonist used to activate hP2X 3 receptors. The E max of hP2X 3 receptor activation by maximal (10 μM) concentrations of either ATP, BzATP, 2-meSATP or αβ-meATP, all of which are known agonists for the P2X 3 receptor, was similar at all cibacron blue concentrations (Fig 7) . Cibacron blue did not confer agonist activity to nucleotides previously shown to be inactive at the P2X 3 receptor (Garcia-Guzman et al . , supra (1997); Bianchi et al . , Eur . J . Pharmacol . 376:127- 138 (1999) , including ADP, UTP, and UDP (100 μM, data not shown) . The potentiating effect of cibacron blue was unaffected by depletion of intracellular Ca 2+ stores using thapsigargin but was completely abrogated in the presence of excess extracellular EGTA, suggesting that the increased magnitude of the ATP-activated response was due to increased Ca 2+ flow across the plasma membrane (data not shown) . Example 8 Ability of Triazene Dyes to Potentiate Receptor Activation by ATP Triazene dyes structurally related to cibacron blue, including basilen blue, reactive blue 5, reactive red 2, reactive orange 14 and reactive yellow 2, were tested for their ability to potentiate hP2X 3 receptor activation by ATP (Fig 8) . Whereas reactive orange 14 and reactive yellow 2 exhibited little or no potentiating activity, basilen blue, reactive blue 5 and reactive red 2 mediated significant hP2X 3 receptor potentiation. The anthraquinone sulfonic acid derivatives, basilen blue and reactive blue 5, exhibited half- maximal concentrations of hP2X 3 receptor potentiation similar to cibacron blue (EC 50 values of 1.2 + 0.6 μM and 1.4 ± 0.5 μM, respectively) . Reactive red 2 was significantly less potent as a potentiator of hP2X 3 receptor activation by ATP (EC 50 = 55 + 10 μM) (Fig 8) . None of the triazene dyes tested were intrinsically fluorescent, nor did they affect the pH of the assay buffer at concentrations up to 1 mM. Example 9 Effect of Cibacron Blue on Inhibitory Activity of PPADS and Effect of PPADS on Potentiation Activity of Cibacron Blue The inhibition of hP2X 3 receptors by PPADS, a non- selective P2 receptor antagonist, has been demonstrated previously (Garcia-Guzman et al . , Brain Res. Mol. Brain Res. 47:59-66 (1997)). In the absence of cibacron blue, PPADS inhibited ATP-mediated hP2X 3 activation with a half-maximal concentration (IC 50 ) of 8.6 ± 3 μM (Fig 9a). Pre-treatment of the hP2X 3 -expressing cells with 10 μM cibacron blue increased both the maximal ATP-activated signal (E max = 437 + 6%) and the apparent IC 50 (51 + 3 μM) of PPADS. To determine whether cibacron blue mediates this effect by increasing the effective potency of ATP, the experiment was performed using 1, 3, 10 or 30 μM ATP. For all concentrations of ATP, cibacron blue produced a similar concentration-dependent rightward shift of the PPADS concentration-effect curves. For example, in the absence of cibacron blue, the apparent IC 50 values for PPADS at each ATP concentration were 3.64 ± 1.1 μM (1 μM ATP), 3.11 ± 1.0 μM (3 μM ATP) , 4.81 ± 1.1 μM (10 μM ATP) , 2.67 ± 0.7 μM (30 μM ATP) , confirming that PPADS is a non-competitive antagonist at the P2X 3 receptor. Similarly, PPADS was found to be non-competitive with ATP at concentrations of cibacron blue up to 100 μM (data not shown) . The effect of cibacron blue on the inhibitory potency of PPADS was thus found to be independent of ATP concentration, suggesting that cibacron blue and PPADS exhibit mutually exclusive effects at the hP2X 3 receptor. In the converse of this experiment, the effect of PPADS on cibacron blue potentiation of the hP2X 3 receptor was determined (Fig 9b) . PPADS caused a concentration-dependent rightward shift of the concentration-effect curve of cibacron blue, while simultaneously reducing the initial magnitude of ATP activation (Fig 9b) . Although 50 μM PPADS was sufficient to fully inhibit ATP-activated hP2X 3 receptors, cibacron blue overcame the inhibitory activity of PPADS in a concentration- dependent manner. Example 10 Effect of Cibacron Blue As Modulator of Receptor Activity in Non-Desensitized and Acutely Desensitized Receptors The potency of cibacron blue as a modulator of hP2X 3 receptor activity was determined in non-desensitized and acutely desensitized receptors (Fig 10) . 1321Nl-hP2X 3 cells were exposed to 10 μM ATP for 1 min to acutely desensitize the hP2X 3 receptors. As described in Fig 10, the EC 50 of cibacron blue required to fully potentiate non-desensitized hP2X 3 receptors was 1.1 ± 0.2 μM (Fig 10). However, acutely desensitized hP2X 3 receptors appeared to be less sensitive to cibacron blue-mediated potentiation (EC 50 = 6.4 ± 0.5 μM) , such that 100 μM cibacron blue was required to achieve a maximal signal. Regardless of the initial state of the hP2X 3 receptors (non-desensitized or acutely desensitized) , cibacron blue pre- treatment ultimately led to a similar agonist-activated maximal activity, suggesting that the size of the receptor pool was comparable under both conditions (Fig 10) . Example 11 Ability of Cibacron Blue to Restore Functional Activation to Acutely Desensitized Receptors Following Exposure to ATP hP2X 3 receptor-expressing cells were desensitized by pretreatment with ATP (10 μM) for 1 min, washed to remove extracellular ATP and, after various time periods of incubation with or without cibacron blue, desensitized receptors were rechallenged with ATP. Figure 11a demonstrates the lack of hP2X 3 response to a second challenge with ATP immediately after desensitization (time 1.5 min). Extension of the incubation time between desensitization and subsequent challenge with ATP revealed the progressive recovery of hP2X 3 receptor activity, approaching the control (non-desensitized) signal by 61.5 min. The addition of 50 μM cibacron blue during the incubation period following ATP-induced desensitization appeared to increase both the apparent potency of ATP and the rate of recovery from desensitization (Fig lib) . After a 15 min incubation with cibacron blue, the desensitized cells showed almost full activity as compared to control (non-desensitized) cells, indicating a considerably shorter refractory period after desensitization. Note that the inclusion of cibacron blue in the incubation buffer leads to i) enhanced rate of recovery of hP2X 3 receptors from desensitization, ii) increased final E max , and iii) increased potency of the agonist (Fig lib) . Figure lie shows the maximal receptor signal at various time points following acute desensitization as a percentage of the control (non-desensitized) signal in the presence and absence of 50 μM cibacron blue (see dashed lines in Figures 11a and b) . The calculated half-times (t M ) of the refractory period (defined as the time required to restore 50% of the activity observed at 60 min) were 15.9 min (K t = 0.0436 min "1 ) in the absence, and 2.6 min (K t = 0.2626 min "1 ) in the presence, of cibacron blue. Thus, cibacron blue increases the rate of hP2X 3 receptor recovery from desensitization by 6-fold. Example 12 Nociceptive Effects of BzATP Subjects : Adult male Sprague-Dawley rats, 230-350 g, (Charles River, Wilmington, MA) were housed in groups of five per cage and given free access to food and water. Animals were on a 12 hr light-dark cycle of 6:00 - 18:00 hr. Animals were only used once in each experiment. All experimental protocols and animal handling procedures were approved by an institutional animal care and use committee (IACUC) . Drugs : Morphine sulfate was obtained from Mallinckrodt, Inc. (St. Louis, MO) and was dissolved in a 0.9% saline solution. Adenosine 5' -triphosphate disodium (ATP), 2- methylthio-ATP tetrasodium (2-meSATP) , and αβ-methylene ATP dilithium (αβ-meATP) were obtained from Research Biochemicals International (Natick, MA). 2' & 3 ' -0- (4-benzoylbenzoyl) -ATP tetraethylammonium salt (mixed isomers) (BzATP) and cibacron blue (Reactive blue-2) were obtained from Sigma Chemical Company (St. Louis, MO). TNP-ATP and Fluo-4AM was purchased from Molecular Probes (Eugene, OR) . All compounds were freshly dissolved and diluted in 0.9% saline. G418 sulfate was obtained from Calbiochem-Novabiochem Corp. (La Jolla, CA) . Dulbecco's modified Eagle's medium (D-MEM) (with 4.5 mg ml "1 glucose and 4 mM L-glutamine) and fetal bovine serum (FBS) were obtained from Hyclone Laboratories, Inc. (Logan, Utah). Dulbecco's phosphate-buffered saline (D-PBS) (with 1 mg ml "1 glucose and 3.6 mg l "1 Na pyruvate, without phenol red), hygromycin and Lipofectamine were obtained from Life Technologies (Grand Island, NY) . Nociceptive testing: Nociceptive responses were assessed using procedures previously described for the formalin test of chemically-induced persistent pain (Abbott et al., Pain 60:91- 102 (1995); Tjosen et al . , Pain 51:5-17 (1992)). Experimentally naive animals were placed in individual plexiglass cages and allowed 30 minutes to acclimate to the testing environment. Following this period, animals received subcutaneous injections of either a formalin solution (1, 2.5, 5%) , different doses of BzATP alone, or in combination with TNP-ATP or cibacron blue, into the dorsal surface of the right hind paw using an insulin gauge (29G1/2) needle. The volume of injection was 50 μl for all treatments. To assess acute nociception, animals were observed immediately following drug injections and the number of flinch behaviors (paw withdrawals) was recorded over a 1-minute period. Additional observations were conducted at sequential 5 -minute intervals during the first 15-20 minutes following drug injections (Phase I, acute phase of the formalin test) . For some experiments, observations began 30 minutes post formalin injection and continued for 20 minutes thereafter (Phase II, persistent phase of the formalin test) . For each individual experiment, 6 rats were used in separate experimental and control groups . Mean cumulative flinch responses were analyzed by analysis of variance and post-hoc comparisons were conducted using Fisher's least significant difference test (GB-STAT, Dynamics Microsystems, Inc., Silver Spring, MD) . Statistical significance was determined at P < 0.05. The intradermal administration of BzATP (100-1000 nmol/paw) into the dorsal surface of the rat hindpaw produced a dose-dependent paw flinching response (Figure 12) . The magnitude of nociceptive paw flinching following 1000 nmol/paw BzATP was equivalent to that observed following the acute intradermal administration of 5% formalin (Phase I of the formalin test) . The duration of this effect was short lasting with the majority of paw flinch responses occurring in the first 5-minute interval following drug injection. By 20 minutes post drug administration the number of BzATP-induced paw flinch responses was not significantly different (P > 0.05) from vehicle injected animals. BzATP did not produce a second phase of prolonged nocifensive paw flinching behavior (data not shown) like that which is characteristically observed following intradermal formalin administration (Phase II of the formalin test) (Tjosen et al . , Pain 51:5-17 (1992); Sawynok et al . , Eur. J. Pharmacol. 330:115-121 (1997). The ability of intradermal BzATP to produce nocifensive behavior in the rat was supported by the ability of systemically administered morphine to dose- dependently (ED 50 = 4 mg/kg, s.c.) reduce BzATP (1000 nmol/paw) induced hindpaw flinching (data not shown) . The nociceptive effects of BzATP were similar to those of other P2 receptor agonists including α,β-meATP (Figure 13) which is less susceptible to metabolic degradation than other P2 receptor agonists (Ralevic et al . , Pharmacol. Rev. 50:413- 492 (1999) ) . Other nucleotide agonists including ATP and 2meSATP also produced acute nociceptive paw flinching (Figure 13) but the maximal responses at the doses tested were significantly less than that observed for BzATP. ADP has been previously shown not to activate P2X 3 receptors (Bianchi et al., Eur . J . Pharmacol . 376:127-138 (1999)) and intradermal administration of ADP did not produce any nociceptive responding (P > 0.05) . This pattern of in vivo activity is consistent with previous pharmacological evaluations of prototypic P2 receptor agonists to activate the recombinant rat P2X 3 receptor in vi tro (Bianchi et al . , supra (1999)). The rank order of potency for these P2 receptor agonists in vi tro was BzATP (EC 50 = 32 nM) > 2meSATP (EC 50 = 220 nM) > ATP (EC 50 = 340 nM) > α,β-meATP (EC 50 = 510 nM) >> ADP (EC 50 >100,000 nM) (Bianchi et al . , supra (1999)). A similar pharmacological profile was also observed for the human P2X 3 receptor (Bianchi et al. , supra (1999) ) . Example 13 Antinociceptive Effects of TNP-ATP Example 12 above presents the protocol utilized to administer the TNP-ATP to the rats. In terms of results, the novel P2X receptor antagonist, TNP-ATP potently inhibited BzATP-stimulated calcium flux (see Example I) in 1321N1 cells expressing the rat P2X 3 receptor (Figure 14) . As has been shown for the human P2X 3 receptor (Lewis et al . , Br. J. Pharmacol. 124:1463-1466 (1998)), sequential removal of the terminal phosphate groups significantly reduces antagonist potency at the rat P2X 3 receptor with TNP-AMP showing little inhibitory activity at concentrations up to 30 μM. A similar rank order of potency for these P2X receptor antagonists was observed at the rat P2X 2/3 receptor (data not shown) . Co-administration of intradermal TNP-ATP (30-300 nmol/paw) with BzATP (1000 nmol/paw) into the dorsal surface of the rat hind paw, produced a significant (P < 0.05) and dose-dependent reduction in nociceptive paw flinching behavior (Figure 15) . The antinociceptive effects of TNP-ATP appear to be pharmacologically specific since co-administration of TNP-AMP with BzATP did not reduce BzATP-induced paw flinching behavior. Similarly, co-administration of TNP-ATP with 5% formalin into the dorsal surface of the rat hindpaw also dose- dependently reduced nocifensive behaviors in the acute (Phase I) portion of the formalin test (Figure 16) . In addition, the antinociceptive effects of TNP-ATP were also evident in the persistent phase (Phase II) of the formalin test where a significant 30% reduction in formalin-induced paw flinching was observed at both doses (30 and 100 nmol/paw) of TNP-ATP. Consistent with its antinociceptive activity against BzATP, TNP-ATP, but not TNP-AMP, attenuated nociceptive responses in both the acute (Phase I) and persistent (Phase II) components of the formalin test. Example 14 Rat P2X 3 and P2X 2/3 Receptor Activation Enhancement by Cibacron Blue Example 6 illustrates the protocol utilized to evaluate the effects of cibacron blue on rat P2X 3 and P2X 2/3 receptors. With respect to the results observed, consistent with its allosteric actions at the human P2X 3 receptor (see Examples I and II, above), cibacron blue produced a concentration- dependent increase in both BzATP (1 μM) and α,β-meATP (10 μM) stimulated calcium flux (EC S0 values = 580 and 720 nM, respectively) in 1321N1 cells expressing the rat P2X 3 receptor (Figure 17) . While the maximal enhancing effects of cibacron blue were observed at concentrations up to 100 μM, higher concentrations of cibacron blue produced variable results and were generally less effective in enhancing agonist activation of the rat P2X 3 receptor (data not shown) . This latter phenomenon may be attributable to the intrinsic antagonist activity of cibacron blue (Ralevic et al . , Pharmacol. Rev. 50:413-492 (1998) ) . Cibacron blue over the concentration range of 0.3 - 10 μM also enhanced the activation of the rat P2X 2/3 receptor by BzATP (1 μM) and α,β-meATP (10 μM) (Figure 17). However, these effects were biphasic with concentrations of cibacron blue greater than 10 μM producing less enhancement of agonist- mediated activation of the P2X 2/3 receptor. Additionally, concentrations of cibacron blue greater than 30 μM antagonized the activation of the rat P2X 2/3 receptor by α,?-meATP . A contribution of homomeric P2X 2 receptors to these biphasic effects appears unlikely since cibacron blue produced similar effects on both BzATP- and α, β-meATP-mediated activation of rat P2X 2/3 receptors and α,β-meATP does not activate P2X 2 receptors (Lewis et al . , Nature 377:432-435 (1995); Binachi et al . , Eur. J. Pharmacol. 376:127-138 (1999)). Consistent with these observations, cibacron blue was found to only inhibit ATP- induced activation of the rat P2X 2 receptor (IC 50 = 8 μM) (data not shown) . Additionally, cibacron blue has been previously shown not to enhance agonist activation of the human P2X X , P2X 2 , and P2X 7 receptors (Alexander et al . , J. Pharmacol. Exp. Ther. 291:1135- 1142 (1999) ) . While cibacron blue produced a 10-fold enhancement of P2X 3 receptor activition, only a maximal 2.5-4.5-fold increase in P2X 2/3 receptor activation was observed. While the exact reasons for theses differential effects remain unclear, the intrinsic antagonist activity of cibacron blue (Ralevic et al . , Pharmacol. Rev. 50:413-492 (1998)) may contribute to the observed differences in the magnitude of the apparent allosteric enhancement of the P2X 3 and P2X 2/3 receptors. Example 15 Pronociceptive Effects of Cibacron Blue Since specific concentrations of cibacron blue selectively can enhance the activation of the rat P2X 3 and P2X 2/3 receptors in vi tro, as evidenced by the results presented above, additional studies were conducted to investigate the potential of cibacron blue to enhance the nociceptive effects of BzATP in the rat. Consistent with the data shown in Figure 12, the intradermal administration of BzATP alone (10-300 nmol/paw) into the rat hindpaw produced a dose-dependent increase in nociceptive behavior (Figure 17a-d) . The intradermal administration of cibacron blue alone (10-300 nmol/paw) produced only a mild, but statistically significant nociceptive response at a dose of 100 nmol/paw (Figure 18a-d) . The intradermal co-administration of cibacron blue with BzATP into the rat hindpaw produced significant and biphasic effects in nociceptive paw flinching behavior relative to the nociceptive effects of BzATP alone (Figure 18a-d) . At a low dose of BzATP (10 nmol/paw) , cibacron blue produced a small, but statistically significant (P < 0.05) enhancement in nociceptive responding as compared to the effects of BzATP alone (Figure 18a) . The pronociceptive effects of cibacron blue were significantly greater when combined with a minimally nociceptive dose of BzATP (30 nmol/paw) (Figure 17b) . At a higher dose of BzATP (100 nmol/paw) , the ability of cibacron blue to enhance nociception was observed only at the dose of 30 nmol/paw (Figure 17c) . In contrast, the intradermal co-administration of cibacron blue with a high dose of BzATP (300 nmol/paw) produced a dose-dependent inhibition of paw flinching responses as compared to the nociceptive effects of BzATP alone (Figure 18d) . Example 16 Enhancement of Formalin Nociception by Cibacron Blue Since intermediate doses of cibacron blue (30 and 100 nmol/paw) were found to be most effective in enhancing the nociceptive effects of intradermal BzATP, these doses of cibacron blue were also examined for their ability to enhance nociception in the formalin test. In the acute phase (Phase I) of the formalin test, intradermal cibacron blue alone produced a significant and dose-dependent nociceptive response (Figure 16a) . Co-administration of intradermal cibacron blue (30 and 100 nmol/paw) with various concentrations of formalin (1, 2.5 and 5%) also produced greater nociception in the acute phase (Phase I) of the formalin test as compared to the effects of formalin alone (Figure 19a) . However, these effects appeared to be additive with formalin since a significant interaction between the nociceptive effects of formalin and cibacron blue was not observed (P > 0.05) (Figure 19a) . During the persistent nociceptive component (Phase II) of the formalin test, intradermal cibacron blue alone did not produce a significant (P < 0.05) nociceptive response (Figure 19b) . However, co-administration of cibacron blue (30 and 100 nmol/paw) with formalin (1 and 2.5%) produced significantly greater paw flinching behavior relative to the nociceptive effects of either formalin or cibacron blue administered alone (Figure 19b) . In this persistent portion of the formalin test, cibacron blue significantly potentiated the nociceptive effects of 1% and 2.5 % formalin as indicated by a significantly (P < 0.05) greater than additive interaction between the nociceptive effects of formalin and cibacron blue. At a minimally nociceptive dose of formalin (1%) , the pronociceptive effects of cibacron blue were biphasic with 30 nmol/paw of cibacron blue producing a significantly larger enhancement of persistent nociception as compared to the higher dose of cibacron blue (100 nmol/paw) (Figure 19b) . The intradermal administration of 5% formalin produced significantly greater nociception relative to lower doses of formalin, however, co-administration of cibacron blue with this dose of formalin did not produce a further enhancement of paw flinching behavior. The pro-nociceptive enhancing effects of cibacron blue appear to be pharmacologically specific since a structurally similar cibacron blue analog, reactive orange, which does not allosterically modulate P2X 3 receptor activation (Alexander et al., J. Pharmacol. Exp. Ther. 291:1135-1142 (1999)), did not produce nociception alone following intradermal administration (30 and 100 nmol/paw) and had no effect on acute or persistent nociceptive paw flinching when co-administered with formalin (data not shown) .

Claims: CLAIMS : 1. An isolated polynucleotide encoding a human P2X 3 receptor polypeptide or a receptor having a nucleotide sequence at least 90% identical to said polynucleotide sequence encoding said human P2X3 receptor. 2. A polynucleotide according to Claim 1, wherein the polynucleotide is a polydeoxyribonucleotide (DNA) . 3. A polynucleotide according to Claim 1, wherein the polynucleotide is a polyribonucleotide (RNA) . 4. A polynucleotide according to Claim 2, wherein the DNA comprises the sequence of SEQ ID NO: 15. 5. A host cell comprising a polynucleotide according to Claim 1 or Claim 4. 6. A host cell according to Claim 5, wherein said cell is selected from the group consisting of a bacterial cell, a mammalian cell, a yeast cell and an amphibian cell. 7. A host cell according to Claim 6, wherein the cell is an amphibian cell. 8. A host cell according to Claim 6, wherein the cell is a mammalian cell. 9. An expression vector comprising a polynucleotide according to Claim 1 operably linked to at least one control sequence that directs transcription of the polynucleotide. 10. The expression vector according to Claim 9, wherein said polynucleotide encodes a human P2X3 receptor polypeptide, wherein said polypeptide comprises the amino acid sequence of SEQ ID NO: 16. 11. A host cell comprising an expression vector according to Claim 9. 12. A host cell according to Claim 11, wherein the cell is selected from the group consisting of a bacterial cell, a mammalian cell, a yeast cell and an amphibian cell. 13. A host cell according to Claim 12, wherein the cell is an amphibian cell. 14. A host cell according to Claim 12, wherein the cell is a mammalian cell. 15. A host cell comprising the expression vector of Claim 10. 16. A host cell according to Claim 15, wherein the cell is selected from the group consisting of a bacterial cell, a mammalian cell, a yeast cell and an amphibian cell. 17. A host cell according to Claim 16, wherein the cell is an amphibian cell. 18. A host cell according to Claim 16, wherein the cell is a mammalian cell. 19. A method for producing a human P2X 3 receptor polypeptide, the method comprising the steps of: (a) culturing a host cell according to Claim 11 for a time and under conditions sufficient for expression of said polypeptide; and (b) recovering said polypeptide. 20. A method for producing a human P2X 3 receptor polypeptide, the method comprising the steps of: (a) culturing a host cell according to Claim 15 for a time and under conditions suitable for expression of said polypeptide; and (b) recovering said polypeptide. 21. A purified human P2X 3 receptor polypeptide, wherein said polypeptide comprises the amino acid sequence of SEQ ID NO: 16. 22. A method for identifying compounds that modulate P2X receptor activity, the method comprising the steps of: (a) providing a cell that expresses a P2X receptor comprising a human P2X3 polypeptide; (b) mixing a test compound with the P2X receptor; and (c) measuring either (i) the effect of the test compound on the activation of the P2X receptor or the cell expressing the P2X receptor, or (ii) the binding of the test compound to the cell or the P2X receptor. 23. A method according to Claim 22, wherein the host cell is selected from the group consisting of a bacterial cell, a mammalian cell, a yeast cell and an amphibian cell. 24. A method according to Claim 22, wherein said measurement of step (c) (ii) is performed by measuring a signal generated by a detectable moiety. 25. A method according to Claim 24, wherein said detectable moiety is selected from the group consisting of a fluorescent label, a radiolabel, a chemiluminescent label and an enzyme . 26. A method according to Claim 22, wherein said measurement of step (c) (i) is performed by measuring a signal generated by a radiolabeled ion, a chromogenic reagent, a fluorescent probe or an electrical current. 27. A method according to Claim 23, wherein the host cell is a mammalian cell. 28. A method according to Claim 23, wherein the host cell is an amphibian cell. 29. A method according to Claim 22, wherein the human P2X 3 receptor polypeptide comprises the amino acid sequence of SEQ ID NO: 16. 30. A method for detecting a target polynucleotide of a P2X 3 receptor in a test sample, the method comprising the steps of: (a) contacting the target polynucleotide with at least one human P2X 3 receptor-specific polynucleotide probe or a complement thereof to form a target-probe complex; and (b) detecting the presence of the target-probe complex in the test sample . 31. A method for detecting cDNA of human P2X 3 receptor mRNA in a test sample, the method comprising the steps of: (a) performing reverse transcription in order to produce cDNA; (b) amplifying the cDNA obtained from step (a) ; and (c) detecting the presence of the human P2X 3 receptor in the test sample. 32. A method according to Claim 31, wherein said detection step (c) comprises utilizing a detectable moiety capable of generating a measurable signal. 33. An isolated polynucleotide encoding a human P2X 3 receptor or a portion thereof and capable of selectively hybridizing to a nucleic acid encoding a human P2X 3 receptor polypeptide, wherein said polynucleotide comprises the sequence of SEQ ID NO: 15 or a portion thereof. 34. An isolated polynucleotide according to Claim 33, wherein the polynucleotide is produced by recombinant techniques . 35. A purified polypeptide encoded by a human P2X 3 receptor polynucleotide, wherein said polypeptide comprises the amino acid sequence of SEQ ID NO: 16 or a portion thereof. 36. A purified polypeptide according to Claim 35 produced by recombinant techniques . 37. A purified polypeptide according to Claim 35 produced by synthetic techniques. 38. A monoclonal antibody which specifically binds to human P2X 3 receptor comprising the amino acid sequence of SEQ ID NO: 16 or an immunoreactive fragment thereof. 39. A method for detecting human P2X 3 receptor in a test sample, the method comprising the steps of: (a) contacting the test sample with an antibody or a fragment thereof which specifically binds to the human P2X 3 receptor, for a time and under conditions sufficient for the formation of a resultant complex; and (b) detecting the resultant complex containing the antibody, wherein said antibody specifically binds to human P P2X 3 receptor amino acid comprising the amino acid sequence of SEQ ID NO: 16 or a fragment thereof. 40. An isolated polynucleotide encoding a human P2X receptor polypeptide or a variant thereof wherein said receptor is P2X 3 . 41. A therapeutic method for relieving pain comprising: (a) presenting an individual afflicted with pain; and (b) administering to said individual an effective amount of a P2X 3 antagonistic compound. 42. The method of claim 41 wherein said antagonistic compound is effective against P2X 3 heteromultimeric channels. 43. A method of potentiating the effects of an agonist which activates a P2X 3 receptor comprising the steps of: a) incubating cells comprising said P2X 3 receptor with a triazene dye; b) exposing said incubated cells to said agonist for a time and under conditions sufficient for said agonist to bind to said P2X 3 receptor, wherein said triazene dye of step (a) potentiates said effect of said agonist of step (b) . 44. The method of claim 43 wherein said P2X 3 receptor is derived from a mammal . 45. The method of claim 44 wherein said mammal is a rodent or a human. 46. The method of claim 43 wherein said triazene dye is selected from the group consisting of cibacron blue, basilen blue, reactive blue 5 and reactive red 2. 47. The method of claim 46 wherein said triazene dye is cibacron blue . 48. The method of claim 43 wherein said agonist is adenosine 5 ' -triphosphate disodium (ATP). 49. A method of blocking the inhibitory activity of a non-selective P2 receptor antagonist on a P2X 3 receptor comprising the steps of : a) incubating P2X 3 -expressing cells with a triazene dye; b) exposing said incubated cells to a non-selective P2 receptor antagonist, wherein said triazene dye of step (a) blocks said inhibitory activity of said antagonist . 50. The method of claim 49 wherein said P2X receptor is derived from a mammal . 51. The method of claim 50 wherein said mammal is a rodent or a human. 52. The method of claim 49 wherein said antagonist is pyridoxal-5-posphate-6-azophenyl-2' , 4 ' -disulfonic acid (PPADS) . 53. The method of claim 49 wherein said triazene dye is cibacron blue. 54. A method of accelerating the rate of P2X 3 receptor resensitization of desensitized P2X 3 receptor-expressing cells comprising the step of exposing said densensitized P2X 3 receptor-expressing cells to a triazene dye, wherein said triazene dye accelerates said rate of resensitization of P2X 3 receptors of said desensitized P2X 3 -receptor expressing cells. 55. The method of claim 54 wherein said P2X 3 receptor is derived from a mammal. 56. The method of claim 55 wherein said mammal is a rodent or a human. 57. The method of claim 54 wherein said triazene dye is cibacron blue . 58. A method of inducing antinociceptive effects in a mammal comprising the step of administering a P2X receptor antagonist to a patient in need of such antinociceptive effects in an amount sufficient to effect said antinociceptive effects. 59. The method of claim 58 wherein said mammal is a human or a rat . 60. The method of claim 58 wherein said P2X receptor antagonist induces antinociceptive effects on a P2X 3 -containing receptor. 61. The method of claim 60 wherein said P2X 3 -containing receptor is P2X 3 . 62. The method of claim 58 wherein said antagonist is 2' ,3' -0- (2,4, 6-trinitrophenyl) -ATP (TNP-ATP) .

Inventors: BURGARD EDWARD C, JARVIS MICHAEL F, KOWALUK ELIZABETH A, LYNCH KEVIN J, VANBIESEN TIMOTHY, BURGARD, EDWARD, C., JARVIS, MICHAEL, F., KOWALUK, ELIZABETH, A., LYNCH, KEVIN, J., VANBIESEN, TIMOTHY
Applicants: ABBOTT LAB, ABBOTT LABORATORIES
Application year: 2000
Main classification: C12N15
IPC codes: A61K31, A61K38, A61K45, A61P25, A61P29, A61P43, C07K14, C07K16, C12N5, C12N15, C12P21, C12Q1, C12R1, G01N33