OPIOID ANTAGONISTS AND METHODS OF THEIR USE

Abstract: The present invention relates to a novel mammalian anti-opioid receptor protein (OFQR), peptide ligands (such as OFQ) that bind to OFQR, and methods of using the OFQ peptide and analogues to reverse the physiologic effects of opiates such as morphine. The isolation, characterization and pharmacological use of the endogenous peptide ligand is described. A particular embodiment of the OFQ peptide is a heptadecapeptide having an FGGF aminoterminal motif. The peptide specifically binds to an OFQ receptor protein heterologously expressed in mammalian cells. The peptide does not bind with high affinity to mu , delta or kappa receptors, but it antagonizes opioid mediated effects (such as analgesia and hypothermia) without increasing nociceptive sensitivity. Tyrosine substitution variants of the peptide ligand specifically bind to the opioid receptor and can be radioiodinated. Also provided are methods of making such peptide ligands and OFQR antagonists, and methods of using the ligands for diagnostic and therapeutic uses and for the identification of other naturally-occurring or synthetic opioid receptor ligands.

Description: OPIOID ANTAGONISTS AND METHODS OF THEHl USE This invention was made at least in part with government support under National Institutes of Health grant ROI MH48991. The government may have certain rights to this invention. BACKGROUND OF THE INVENTION Field of the Invention This invention relates to receptors from mammalian species and ligands specific for such receptors, which are active in the antagonism of opioid action. Specifically, the invention relates to the isolation of an endogenous peptide ligand specific for a novel mammalian receptor, the recognition of its anti-opioid properties, and use of the ligand for reversing physiologic effects of opiates such as morphine. The invention also relates to the construction of analogues, derivatives and peptide mimetics of this endogenous mammalian receptor ligand, and their use as opiate antagonists. Specifically provided is a mammalian hypothalamus-derived endogenous opioid receptor ligand, synthetic embodiments and analogues thereof, and methods of making and using such ligands. Background of the Invention The use (and abuse) of opiates, such as opium and morphine, have been known since antiquity (reviewed in Brownstein, 1993, Proc. Natl. Acad. Sci. USA 90: 5391-5393). Since the nineteenth century, chemical characterization and synthesis of many morphine analogues have been achieved in an effort to discover a compound having the analgesic effects of morphine but lacking its addictive potential. These efforts have heretofore been unsuccessful. The biology behind the reasons for the analgesic and addictive properties of morphine and morphine-like compounds was first elucidated by the discovery of endogenous morphine-like compounds termed enkephalins (see DiChara & North, 1992, Trends in Pharmacol. Sci. 13: 185- 193 for review). Accompanying this finding of an endogenous opioid was the biochemical evidence for a family of related but distinct opiate receptors, each of which displays a unique pharmacological profile of response to opiate agonists and antagonists (see McKnight & Rees, 1991, Neurotransmissions 7: 1-6 for review). To date, four distinct opiate receptors have been described by their pharmacological profiles and anatomical distribution: these comprise the μ, δ, K, and a receptors (the σ receptor has been determined to be a non-opioid receptor displaying cross-reactivity with some opioid agonists). Thus, mammalian opioid receptors are known in the art, and some of these proteins have been isolated biochemically and their corresponding genes have been recently cloned using genetic engineering means. Kieffer et al., 1992, Proc. Natl. Acad. Sci. USA 89: 12048-12052 disclosed the isolation of a cDNA copy of the mouse δ-opioid receptor by expression cloning. Evans et al., 1992, Science 258: 1952-1955 disclosed the isolation of a cDNA copy of the mouse δ-opioid receptor by expression cloning. Chen et al., 1993, Molec. Pharmacol. 44: 8-12 disclosed the isolation of a cDNA copy of the rat μ-opioid receptor. Yasuda et al, 1993, Proc. Natl. Acad. Sci. USA 90: 6736-6740 disclosed the isolation of a cDNA copy of each of the mouse K- and δ-opioid receptor. Bzdega et al, 1993, Proc. Natl. Acad. Sci. USA 90: 9305-9309 disclose the isolation and chromosomal location of the δ-opioid receptor in the mouse. The present inventors have cloned, expressed and functionally characterized a novel mammalian receptor gene, disclosed in co-owned and co-pending U.S. Patent Application No. 08/149,093, filed November 8, 1993, which is hereby incorporated by reference in its entirety. Specifically disclosed therein are nucleic acids encoding the novel mammalian receptor gene, recombinant expression constructs comprising this receptor gene, cells containing such constructs and expressing the novel receptor gene, and methods for making and using such nucleic acids, as well as constructs and cells for opioid detection and novel drug screening. The nucleic acid sequence of the gene and die deduced amino acid sequence of the cognate receptor protein were also disclosed in this prior application. The receptor gene was previously referred to in the co-pending case as an MSOR (methadone specific opioid receptor) gene because the receptor bound methadone with a high affinity. Such high affinity constitutes a specificity for methadone, although this term is not intended to imply that only methadone binds to the receptor. Indeed, as shown herein, an endogenous ligand of the present invention binds to the same receptor with startlingly high affinity (producing a physiologic response when administered in a picomole or nanomole dose i.c.v.). Now that the endogenous ligand has been discovered, and disclosed in co-owned and co-pending U.S. Patent Application No. 08/514,451 filed August 11, 1995, the MSOR terminology is no longer needed. The ligand has been named orphanin FQ (OFQ) from the initial and terminal amino acids of the disclosed sequence (F for phenylalanine and Q for glutamine). Hence the receptor will now be referred to as the o hanin FQ receptor (OFQR), which is an anti-opioid receptor (AOR). A great advantage in efforts for developing novel psychotropic drugs which exert their activity (analgesic and otherwise) via binding to mammalian opioid or anti-opioid receptors would be to identify the endogenous ligand(s) which bind to such receptors. Certain such opioid ligands have been isolated in the prior art, including the peptides comprising the endorphins and enkephalins (see Jaffe and Martin, 1990, "Opioid Analgesics and Antagonists", in Goodman and Gilman, eds., The Pharmacological Basis of Therapeutics. 7th ed. (Pergammon Press, Inc.: New York), Chapter 21, p. 491-531). Although opiates are powerful therapeutic agents, their therapeutic index is sufficiently low to cause many problems in their clinical use. Such problems include stupor, respiratory compromise, or profound analgesia that may be undesired (and mask symptoms of an acute surgical emergency, such as appendicitis). Physical dependence on morphine or other opiates can also develop, and may be difficult for a clinician to diagnose. It has been recognized that administration of an antagonist to reverse opiate effects is clinically useful in these circumstances, either for reducing physiologic effects of the opiate (undesired analgesia or apnea), or precipitating a withdrawal syndrome to demonstrate addiction. In spite of the importance of opioid antagonists, progress has only slowly been made in developing additional members of this class of drugs. The best known such antagonist is naloxone, an organically synthesized molecule that resembles oxymorphone and is a competitive antagonist at μ, δ, K and σ opioid receptors. Naltrexone appears to be another relatively pure antagonist, but with higher oral efficacy and a longer duration of action. These drugs are believed to interact with and competitively bind to the opioid receptors to exert their antagonistic action. They reverse signs of opiate intoxication such as apnea and stupor, and they can also stimulate a withdrawal syndrome in drug dependent subjects characterized by anxiety, chills, abdominal cramping, tachycardia lacrimation and sweating. The search for additional opioid antagonists has continued, because of a clinical need for such drugs. Endogenous antagonists have been particularly pursued because of their predicted potency, and the invaluable information they would provide about the body's physiologic pathways for developing tolerance and addiction. In spite of these advantages, such endogenous anti-opioids have eluded investigators for years. SUMMARY OF THE INVENTION The present invention takes advantage of the discovery that an anti-opioid biological system physiologically counteracts or antagonizes the actions mediated by the binding of opioids to opioid receptors. The OFQ anti-opioid receptor is the same receptor (MSOR) disclosed in the commonly assigned U.S. Patent Application Serial No. 08/149,093. The ligand of the present invention is the ligand disclosed in commonly assigned U.S. Patent Application Serial No. 08/514,451. The method of treatment of the present application was made possible by the present inventors' recognition that the receptor is an anti-opioid receptor that, when bound to a ligand of the present invention, antagonizes the physiologic effects of an opioid. This invention provides small, readily-produced peptides that are ligands for a novel mammalian anti-opioid receptor protein having the amino acid sequence identified as SEQ ID Nos.: 5 and 6. Peptides of the invention are characterized as having the amino acid sequence identified as SEQ ID Nos.: 5 and 6 or a subsequence thereof, amino acid sequence variants of the sequence or subsequence, as well as analogues and derivatives thereof, that are ligands for the • mammalian opioid receptor protein having the amino acid sequence identified as SEQ ID Nos.: 5 and 6, as well as analogues and derivatives thereof. The method of the present invention antagonizes a physiological effect (such as analgesia or hypothermia) of an opioid (such as morphine or heroin) in an animal (such as a rodent or a human). The method includes administering to the animal a pharmaceutically effective amount of the peptide, which specifically binds to a receptor encoded by the DNA molecule set forth in SEQ. ID. No. 3. The peptide is preferably an heptadecapeptide having an aminoteπninal FGGF motif, and the peptide does not bind with high specificity to mu, delta, or kappa opioid receptors. It antagonizes physiologic opioid effects (such as analgesia or hypothermia) without increasing nociceptive sensitivity beyond the baseline sensitivity of the animal to painful stimuli in the absence of the exogenous opioid or stress induced analgesia. Stress induced analgesia is a normal physiologic mechanism, believed to be mediated by endogenous opioids, for reducing pain perception in response to stressful environmental stimuli. A pharmaceutically effective amount of the peptide is an amount sufficient to subjectively or objectively dimmish the effects of the opiate. An example would be an amount sufficient to reverse the degree of analgesia to painful stimuli induced by an opioid, or an amount sufficient to raise body temperature of an animal experiencing opiate induced hypothermia. In some of the described embodiments, this amount would be a picomolar or nanomolar icv dose. For example, a pharmaceutically effective dose in a mouse would be a 50 picomole to 10 nanomole intracranial ventricular (icv) dose, in 2μl vehicle. The present invention may be administered by many routes, including intravenously (iv), subcutaneously (sc), or intrathecally (it). Given the marked anti-opioid potency of the peptide, relatively low doses of the peptide could even be given iv. Human doses given icv and intrathecally would be expected to be 1-10 nanomoles in a 2 microliter volume. In yet other embodiments, the peptide may be an analogue, derivative or mimetic of the peptides. An analogue is a peptide having the same anti-opioid activity as the disclosed peptides, but differing in primary amino acid sequence. The variations in the primary amino acid sequence could be made using the techniques described in this specification, and known in the art. A mimetic of the peptide is a molecule, either a peptide or other biopharmacεutical (such as an organically synthesized compound) that specifically binds to a receptor encoded by SEQ. ID. No. 3 and antagonizes a physiological effect of an opioid in an animal. In other embodiments of the invention, the method includes administering to the animal a second anti-opioid antagonist, wherein the second anti-opioid antagonist and anti-opioid peptide are administered in a sufficient amount in combination to antagonize physiological effects of the opioid. The first and second antagonists may be given either in series or administered together in a mixture. For example, the anti-opioid peptide can be administered in a vehicle (such as isotonic saline solution) in admixture with naloxone (at a pharmaceutically effective dose of 1 mg/kg body weight of the animal). Alternatively, the anti-opioid peptide can be administered in combination with other anti-opioid peptides (such as analogues of the peptide of the present invention, or other endogenous or synthetic peptides that have an anti-opioid activity). The peptides of the invention include linear and cyclized peptides, and synthetic analogues and variants thereof. Certain embodiments of such variants include substitution variants, wherein an amino acid residue at one or more positions in the peptide is replaced with a different amino acid residue (including atypical amino acid residues) from that found in the corresponding position of the amino acid sequence of the parent peptide of the invention. Certain other embodiments of peptide variants of the invention include addition variants, wherein such variant peptides may include up to about a total of 10 additional amino acids, covalently linked to either the amino- terminal or carboxyl-terminal extent, or both, of the parent peptide of the present invention. Such additional amino acids may also include atypical amino acids. Linear and cyclized embodiments of the amino acid substitution and addition variant peptides are also provided as peptides of the invention. In addition, peptides of the invention may be provided as fusion proteins with other functional targeting agents, such as immunoglobulin fragments. Derivatives of the peptides of the invention also include modifications of the amino- and carboxyl-termini and amino acid side chain chemical groups such as amines, carboxylic acids, alkyl and phenyl groups. In a first aspect, the invention provides peptides of the formula: (Xaa) n -Phe-Gly-Gly-Phe-(A 1 )-(A 2 )-(A 3 )-(A 4 )-(A 5 )-(A 6 )-(A 7 )-(A 8 )-(A 9 )-(A 10 )-(A 11 )-(A 12 )-Gln-(Xaa) m wherein A 1 is Thr, Leu or Met; A 2 is Gly, Arg or Thr; A 3 is Ala, Arg or Ser; A 4 is Arg, Ile, Glu or Gin; A 5 is Lys, Arg or Phe; A 6 is Ser, Pro or Lys; A 7 is Ala, Lys, Gin or Val; A 8 is Arg, Leu, Thr or Val; A 9 is Lys, Pro or Thr; A 10 is Tyr, Leu or Trp; A" is Ala, Asp or Val; A 12 is Asn, or Thr; (Xaa) is any amino acid; n and m are integers wherein n + m is no more than 82, preferably no more than 20, or even no more than 10; and the amino acids are each individually in either the D or L stereochemical configuration and the peptide specifically binds to a mammalian opioid receptor having an amino acid sequence identified by SEQ ID No.: 3. In a preferred embodiment, the peptide has the formula: Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln (SEQ ID No.: 5). In an another preferred embodiment, the peptide has the formula: Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Tyr-Ala-Asn-Gln (SEQ ID No.: 6). Naturally-occurring embodiments of the peptides, purified using well-established techniques from the cells or tissues producing the peptide, and synthetic embodiments, are within the scope of the invention. In other embodiments the peptides are of the general formula: (Xaa)--Phe-Gly-Gly-Phe-(A 1 )-(A 2 )-(A 3 )-(A 4 )-(A 5 )-(A 6 )-(A 7 )-(A 8 )-(A )-(A 10 )-(A")-(A 12 )-Gln-(Xaa) m and having amino acid substituents as described above wherein such peptides are radiolabeled by conjugation with or binding to a radioactive isotope. In a preferred embodiment, the peptide has the formula: Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Tyr-Ala-Asn-Gln (SEQ ID No.: 6) and the radiolabel is a radioisotope of iodine. In other preferred embodiments, the peptide is covalently linked to a radiolabel binding moiety and die radiolabel is a radioisotope of indium, gallium, technetium, rhenium or other useful radioisotope. The invention also provides pharmaceutical compositions of the peptides of the invention. In a first aspect, the peptides of the invention are provided in a pharmaceutical composition comprising an acceptable carrier or diluent, or a pharmaceutical dosage form such as a vial of predetermined dose of the peptide, suspended in an injectable or inhalable vehicle such as sterile water or isotonic saline. Other dosage forms include tablets or capsules, particularly for synthetic mimetics more suitable for oral administration. In a second aspect, such pharmaceutical compositions are provided in detectably-labeled embodiments, useful for diagnostic identification of sites of both normal and pathological peptide ligand receptor binding in vivo and in vitro. In such embodiments, the peptides of die invention are detectably labeled, for example, with a radioisotope such as 1-123, 1-125 or 1-131, conjugated to the peptide via, inter alia, a tyrosine residue that is non-essential for receptor binding. Additional radioisotopes, such as In-Ill, Ga-67, Re-186, Re-188 and Tc-99m, can be conjugated to such peptides using methods well-understood in the art. In such embodiments, die pharmaceutical composition is radiolabeled to an appropriate specific activity, and administered to an animal, preferably a human, at a diagnostically-effective and non-toxic dose. Methods and routes of administration may be selected by those with skill in the art based on well-established techniques and in a clinically-appropriate fashion. Alternatively, the radiolabeled ligands can be used to identify specific brain regions where the ligand receptor is present. DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates the nucleotide (SEQ ID No.: 3) and amino acid (SEQ ID No.: 4) sequences of the mammalian anti-opioid receptor of the invention. Fig. 2 presents an amino acid sequence comparison between the novel mammalian OFQR protein of the invention (designated LCI 32) and the rat μ-opioid receptor, and die mouse δ- and .-opioid receptor proteins. Fig. 3 illustrates in situ hybridization of rat brain sections with a nucleic acid hybridization probe specific for the mammalian OFQR of the invention. Fig. 4 presents affinity binding experiment results of 3 H -methadone binding to COS-7 cells (Panel A) and to COS-7 cells expressing the novel mammalian OFQR of the invention. Fig. 5 shows the extent of inhibition of forskolin-stimulated cAMP accumulation in a CHO cell heterologously expressing OFQR of the invention by fractions of acetic acid-extracted peptides from porcine hypothalamus, wherein A 2g0 represents absorbance at 280nm, shaded bars represent results obtained with untransfected CHO cells and open bars represent results obtained with OFQR transfected CHO cells. Fig. 6 shows the extent of inhibition of forskolin-stimulated cAMP accumulation in CHO cell heterologously expressing the OFQR of the invention by fractions of reverse-phase HPLC fractionated acetic acid-extracted peptides from porcine hypothalamus, wherein A 2U represents absorbance at 214nm, and shaded bars represents results obtained with OFQR transfected CHO cells. Fig. 7 is a comparison of the amino acid sequence of oφhanin FQ (OFQ) and opioid receptor binding peptides; identical amino acid residues shared between the peptides are shown in boldface. Fig. 8 is a graph illustrating the extent of inhibition of forskolin-stimulated cAMP accumulation in OFQR transfected CHO cells by the peptides Oφhanin FQ (filled circles); Y14- oφhanin FQ (filled squares); monoiodinated Y14-oφhanin FQ (open squares) and Leu- Enkephalin (open circles). Oφhanin FQ-treated untransfected CHO cells are shown as filled triangles. Fig. 9 is a graph of ( 125 I)-labeled Y14-θφhanin FQ binding to isolated membranes from OFQR transfected CHO cells. The inset is a Scatchard analysis of these results. Fig. 10 shows the effects on horizontal activity (Panel A) and vertical activity (Panel B) in mice treated with varying concentrations of oφhanin FQ peptide. Fig. 11 illustrates the effects of oφhanin FQ peptide on nociception in mice as determined using a hotplate assay. Fig. 12 shows the effects of varying icv doses of oφhanin FQ peptide on nociception in mice as determined using a tail-flick assay. Fig. 13A is a bar graph, illustrating the anti-opioid effect of OFQ on stress induced analgesia, showing average tail withdrawal latency change in animals that have not received moφhine analgesia, after administration of saline, naloxone, and OFQ. Fig. 13B is a bar graph showing the results of a writhing test, demonstrating reversal of moφhine analgesia by OFQ. Fig. 14 is a graph that demonstrates reversal of moφhine analgesia in animals by comparing tail withdrawal latency time in animals that have received varying doses of OFQ. Fig. 15 is a graph of core body temperature vs. time, demonstrating attenuation of moφhine hypothermia by OFQ. Fig. 16 is a graph of withdrawal jumping precipitated by OFQ in moφhine dependent mice, demonstrating precipitation of withdrawal signs in the mice. Figs. 17A, 17B, and 17C is a series of graphs showing that OFQ does not ind with high affinity to mu, delta and kappa opioid receptors. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a method of antagonizing a physiologic effect of an opioid in an animal. As used in this specification, the terms "opiate" and "opioid" will be used as defmed in Goodman and Gilman, The Pharmacological Basis of Therapeutics, 7th edition (hereinafter "Goodman and Gilman"), page 492. "Opiate" will be used to designate drugs derived from opium, such as moφhine, codeine, and the many semi-synthetic congeners of moφhine. "Opioid" will refer in a generic sense to all drugs or biological substances, natural and synthetic, widi moφhine-like actions. Hence the term "opioid" will include such synthetic moφhine-like compounds as propoxyphene, as well as the endogenous opioid peptides that include the enkephalins, endoφhins, and dynoφhins. The term "anti-opioid" refers to a peptide or other chemical compound mat antagonizes the physiological effects of an opioid. "Antagonizes" refers to a biochemical or pharmaceutical reversal or reduction of a physiologic effect. The peptides of the present invention specifically bind to an anti-opioid receptor, which is believed to exist separate from opioid receptors, and mediates physiologic antagonism of effects mediated by opioid receptors. "High specificity" (selectivity) refers to binding with high affinity to a receptor, which in this specification shall be defined to bind wim a Kj or IC 50 of about 100 nM or less. In more particular embodiments wherein specific binding is particularly high, the IC. or IC 50 is 10-100 nM, particularly 0.1 to 10 nM, and especially 0.1 nM or less. Methods of determining binding selectivity are well known, and for example are described in Goldstein, TIPS Reviews, pages 456-459, December 1987 (Vol. 8), which is incoφorated by reference. The expression Kj is used as known in the art, and defined in J. Neurochem. 64:18-23 (1995), which is incoφorated by reference. See particularly Table 1 of that incoφorated reference. As described in the incoφorated reference, Kj values are calculated based on the IC 50 value for the high affinity site using the equation of Cheng and Prusoff, Biochem. Pharmacol. 22:3099- 3108 (1973): where IC.*, is the concentration of unlabeled ligand that inhibits 50% of the labeled ligand (L) binding, [L] is me total concentration of ligand provided in the assay, and the K D is defined as the dissociation constant. ICjo may be determined by me method described in Example 12. The dissociation constant K D is defined at Goodman and Gilman, page 41, which is incoφorated by reference. Determination of the dissociation constant for an agonist receptor complex is well known in the art, and is, for example, determined by the methods described in Munson, Principles of Pharmacology (Chapman & Hall), 1995, Chapter 1, which is incoφorated by reference. Determination of die affinity constant is particularly discussed at pages 17-18 thereof, and is a technique well known in die an. It may be determined from a conventional Scatchard analysis, wherein the slope of the Scatchard line is -1/K D . Alternatively, K D is determined from kinetic experiments in which K D = k,/k 2 where k, is the dissociation rate constant and k 2 is the association rate constant. A physiologic effect of an opioid includes (witiiout limitation) one or more of those biochemical changes and/or clinical signs and symptoms noted after administration of an opioid to an animal, and includes those effects enumerated at pages 497-504 of Goodman and Gilman. Such effects include one or more of inhibition of cellular cAMP accumulation, analgesia, drowsiness, changes in mood (including euphoria), pupillary responses, respiratory depression, nausea, peripheral vasodilation, hypothermia, and reduction of gastrointestinal motility. The mvention also provides methods for designing OFQ receptor ligands and analogues, derivatives and mimetics thereof, using the amino acid sequence of die peptide ligands of the invention. Also encompassed wi in the scope of this invention are such analogues, derivatives and mimetics produced by die methods of me invention. In a second aspect, the invention provides nucleic acids encoding a novel mammalian opioid-specific receptor protein having an amino acid sequence identified as SEQ ID No.: 4, recombinant eukaryotic expression constructs capable of expressing the novel mammalian opioid- specific receptor of the invention in cultures of transformed cells, as well as cultures of transformed eukaryotic cells that synthesize the receptor of the invention. The invention also provides homogeneous compositions of the receptor protein having an amino acid sequence identified as SEQ ID No.: 4, and antibodies against and epitopes of the receptor protein of the invention. Methods for characterizing these receptor proteins and metiiods for using these proteins in the development of agents having pharmacological uses related to these receptors are also provided by the invention. In this aspect of the invention is provided a nucleic acid having a nucleotide sequence encoding a mammalian OFQ receptor. In a preferred embodiment, the nucleic acid encodes 1452 nucleotides of die cDNA comprising 1101 nucleotides of coding sequence, 181 nucleotides of 5' untranslated sequence and 170 nucleotides of 3' untranslated sequence, depicted in Fig. 1 and identified as SEQ ID No.: 3. Encompassed in this aspect of the invention is the disclosed sequence and allelic variants of this sequence, either naturally occurring or the product of in vitro chemical or genetic modification. The corresponding receptor protein, having the deduced amino acid sequence shown in Fig. 1 and identified as SEQ ID No.: 4, is also an aspect of the invention. A particular embodiment of this aspect of the invention is a homogeneous composition of the 47 kD mammalian OFQ receptor protein and derivatives thereof, said size being understood to be the size of the protein before any post-translational modifications. The amino acid sequence of die 47 kD receptor protein is depicted in Fig. 1 and identified as SEQ ID No.: 4. This invention provides botii nucleotide and amino acid sequence probes derived from the sequences herein provided, isolated from either cDNA or genomic DNA, as well as probes made synthetically with the sequence information derived therefrom. The invention specifically includes but is not limited to oligonucleotide, nick-translated, random primed, or in vitro amplified probes made using cDNA or genomic clones embodying this aspect of the invention, and oligonucleotide and otiier synthetic probes synthesized chemically using die nucleotide sequence information of cDNA or genomic clone embodiments of the invention. The present invention also includes syntiietic peptides made using the nucleotide sequence information comprising the cDNA embodiments of the receptor protein embodiments of the invention. The invention includes eitiier naturally occurring or synthetic peptides which may be used as antigens for the production of receptor-specific antibodies, or useful as competitors of receptor molecules for agonist, antagonist or drug binding, or to be used for the production of inhibitors of the binding of agonists or antagonists or analogues tiiereof to such OFQ receptor molecules. Particularly preferred embodiments of this aspect of the invention are such peptides that interact with the peptide ligand embodiment of the invention and enhance or inhibit peptide ligand binding to the receptor protein. The present invention also provides antibodies against and epitopes of the mammalian opioid receptor molecules of the invention, including antisera and both polyclonal and monoclonal embodiments of such antibodies and hybridoma cell lines producing such monoclonal antibodies. The present invention provides recombinant expression constructs comprising a nucleic acid encoding a mammalian OFQ receptor of the invention wherein the construct is capable of expressing the encoded OFQ in cultures of cells transformed with the construct. Preferred embodiments of such constructs comprise the OFQ receptor cDNA depicted in Fig. 1 (SEQ ID No.: 3), such constructs being capable of expressing the OFQ receptor encoded therein in cells transformed with the construct. The invention also provides culmres of cells transformed with the recombinant expression constructs of the invention, each such culmre being capable of and in fact expressing the mammalian OFQ receptor encoded in the transforming construct; and protein preparations of prokaryotic and eukaryotic cell membranes containing the OFQR protein of the invention, derived from culmres of prokaryotic or eukaryotic cells, respectively, transformed with the recombinant expression constructs of the invention. The invention also provides methods for screening compounds for their ability to inhibit, facilitate or modulate the biochemical activity of the mammalian OFQ receptor molecules of die invention, for use in the in vitro screening of novel agonist and antagonist compounds. In preferred embodiments, cells transformed wim a recombinant expression construct of the invention are contacted widi such a compound, and die binding capacity of the compounds, as well as the effect of the compound on binding of otiier, known opioid agonists and antagonists, is assayed. Additional preferred embodiments comprise quantitative analyses of such effects. The invention specifically provides a method for screening a compound for a capacity to bind to a mammalian OFQ receptor in cells expressing the receptor, and using a competitive assay between the compound to be tested and the peptide ligands of die invention. The method comprises the steps of: (a) transforming a culture of eukaryotic or prokaryotic cells with a recombinant expression construct capable of expressing a mammalian OFQ receptor having an amino acid sequence identified as SEQ ID No.: 4, wherein the cells of the transformed cell culture express the receptor; (b) assaying the transformed cell culmre for binding of an amount of a detectably- labeled peptide according to Claim 1 in competition with varying amounts of tiie compound; and (c) determining whether the compound competitively binds to the receptor by calculating the extent of inhibition of binding of the detectably-labeled peptide in the presence of the compound. In additional embodiments of this method is included the additional step of: (d) comparing the binding capacity of the compound with the binding capacities of additional compounds that are known to bind to mammalian opioid receptors, wherein said additional compounds comprise naturally-occurring and synthetic opioid receptor agonists and antagonists. The present invention is also useful for the detection of analogues, agonists or antagonists, known or unknown, of the mammalian OFQ receptor of the invention, either naturally occurring or embodied as a drug. In preferred embodiments, such analogues, agonists or antagonists may be detected in blood, saliva, semen, cerebrospinal fluid, plasma, lymph, or any other bodily fluid. In particularly preferred embodiments the peptide ligands of the invention are used in competitive binding assays to quantitatively evaluate novel ligand binding to die OFQ receptor. Specific preferred embodiments of the present invention will become evident from the following more detailed description of certain preferred embodiments and the claims. The term "novel mammalian OFQ receptor" and "OFQR" as used herein refers to proteins comprising or consisting essentially of, and having substantially the same biological activity as, the protein encoded by the nucleic acid depicted in Fig. 1 (SEQ ID No.:3). This definition is intended to encompass natural allelic variations in the disclosed OFQR sequence. Cloned nucleic acid provided by the present invention may encode OFQR protein of any species of origin, including, for example, mouse, rat, rabbit, cat, and human, but preferably the nucleic acid provided by die invention encodes OFQR receptors of mammalian, most preferably rat and human, origin. The nucleic acids of the invention comprise nucleic acid hybridization probes comprising DNA or RNA consisting essentially of the nucleotide sequence of the OFQ receptor, depicted in Fig. 1 (SEQ ID No.:3), or any portion thereof effective in nucleic acid hybridization. Mixtures of such nucleic acid hybridization probes are also within the scope of this embodiment of the invention. Nucleic acid probes as provided herein are useful for detecting OFQ receptor gene expression in cells and tissues using techniques well-known in the art, included but not limited to Northern blot hybridization, in situ hybridization and Southern hybridization to reverse transcriptase-polymerase chain reaction product DNAs. The probes provided by the present invention, including oligonucleotide probes derived therefrom, are also useful for Southern hybridization of mammalian, preferably human, genomic DNA for screening for restriction fragment length polymoφhisms (RFLPs) associated with certain genetic disorders. The invention provides an isolated and purified, naturally-occurring, endogenous mammalian peptide ligand that specifically binds to the OFQ receptor of the invention. For the puφoses of this invention it will be understood tiiat the peptide is any biologically active molecule that specifically binds to an OFQ receptor of the invention. The peptides of die invention include synthetic embodiments of the naturally-occurring peptide ligand isolated as described herein, as well as analogues, derivatives and variants of this peptide that specifically bind to the OFQR. Such analogues include substitution variants, wherein an amino acid is substituted conservatively with another amino acid that does not ablate the specific binding properties of the peptide ligand. Specifically provided are substitution variants wherein the substimted amino acid is Leu 14 , wherein this residue is substimted with tyrosine. Each peptide ligand of the invention is comprised of a sequence of amino acids. The term amino acid as used in this invention is intended to include all L- and D- amino acids, naturally occurring and otherwise. Generally, those skilled in the art will recognize that peptides as described herein may be modified by a variety of chemical techniques to produce compounds having essentially me same activity as the unmodified peptide, and optionally having other desirable properties. For example, carboxylic acid groups of the peptide, whether carboxyl-terminal or sidechain, may be provided in the form of a salt of a pharmaceutically-acceptable cation or esterified to form a C,-C 16 ester, or converted to an amide of formula NR,R 2 wherein R, and R 2 are each independently H or C r C 16 alkyl, or combined to form a heterocyclic ring, such as a 5- or 6- membered ring. Amino groups of the peptide, whether amino-teπninal or sidechain, may be in the form of a pharmaceutically-acceptable acid addition salt, such as the HCl, HBr, acetic, benzoic, toluene sulfonic, maleic, taitaric and otiier organic salts, or may be modified to C,-C 16 alkyl or dialkyl amino or further converted to an amide. Hydroxyl groups of the peptide sidechain may be converted to C,-C, 6 alkoxy or to a C.-C b ester using well-recognized techniques. Phenyl and phenolic rings of the peptide sidechain may be substimted with one or more halogen atoms, such as fluorine, chlorine, bromine or iodine, or wi C,-C 16 alkyl, C,-C I6 alkoxy, carboxylic acids and esters thereof, or amides of such carboxylic acids. Methylene groups of the peptide sidechains can be extended to homologous C : - C 4 alkylenes. Thiols can be protected with any one of a number of well-recognized protecting groups, such as acetamide groups. Those skilled in the art will also recognize metiiods for introducing cyclic structures into the peptides of this invention to select and provide conformational constraints to the strucmre that result in enhanced binding and/or stability. For example, a carboxyl-terminal or amino-terminal cysteine residue can be added to die peptide, so that when oxidized die peptide will contain a disulfide bond, thereby generating a cyclic peptide. Other peptide cyclizing methods include die formation of thioethers and carboxyl- and amino- teπninal amides and esters. It will be apparent to one skilled in the art that the anti-opioid activity of the peptides disclosed herein lies not in the precise amino acid sequence, but rather in the epitopes inherent in the amino acid sequences encoded by the DNA sequences. It will therefore also be apparent that it is possible to recreate the anti-opioid activity of one of these peptides by recreating the epitope, without necessarily recreating the exact amino acid sequence. This could be achieved either by directly synthesizing the peptide (thereby circumventing the need to use the DNA sequences) or, alternatively, by designing a nucleic acid sequence that encodes for the epitope, but which differs, by reason of the redundancy of the genetic code, from the sequences disclosed herein. Similarly, the OFQR DNA sequence may also be varied, while still producing a functional OFQR protein. Accordingly, the degeneracy of the genetic code further widens the scope of the present invention as it enables major variations in the nucleotide sequence of a DNA molecule while maintaining the amino acid sequence of tiie encoded protein. The genetic code and variations in nucleotide codons for particular amino acids is presented in Tables 1 and 2. Based upon die degeneracy of the genetic code, variant DNA molecules may be derived from the DNA sequences disclosed herein using standard DNA mutagenesis techniques, or by synthesis of DNA sequences. Table 1 The Genetic Code First Third Position Position (5' end) Second Position (3' end) | T A Phe Ser Tyr Cys Phe Ser Tyr Cys C T 'Leu Ser ' Stop (och) Stop A Leu Ser Stop (amb) Tφ G 1 1 Leu Pro H}s Arg Leu Pro His Arg C C 'Leu Pro ' Gin Arg A Leu Pro Gin Arg G 1 1 Ile Thr Asn Ser Ile Thr Asn Ser C A 'lie Thr ' Lys Arg A Met Thr Lys Arg G 1 1 Val Ala Asp Gly Val Ala Asp Gly C G ■ Val Ala ' Glu Gly A Val (Met) Ala Glu Gly G 1 1 "Stop (och)" stands for the ocre termination triplet, and "Stop (amb)" for the amber. ATG is the most common initiator codon; GTG usually codes for valine, but it can also code for methionine to initiate an mRNA chain. Table 2 The Degeneracy of the Genetic Code Number of Total Synonymous Number of Codons Amino Acid Codons 6 Leu, Ser, Arg 18 4 Gly, Pro, Ala, Val, Thr 20 3 Ile 3 2 Phe, Tyr, Cys, His, Gin, 18 Glu, Asn, Asp, Lys 1 Met, Tφ _2 Total number of codons for amino acids 61 Number of codons for termination _3 Total number of codons in genetic code 64 Additionally, standard mutagenesis techniques may be used to produce peptides which vary in amino acid sequence from the peptides encoded by the DNA molecules disclosed herein. However, such peptides will retain the essential characteristic of the peptides encoded by the DNA molecules disclosed herein, i.e. the ability to antagonize opioid action by binding to the OFQR. This characteristic can readily be determined by the assay technique described herein for cAMP accumulation. Screening for retention of anti-opioid effects is similarly achieved in a forthright manner using the methods of Examples 8-11 below. Variant peptides include those with variations in amino acid sequence, including minor deletions, additions and substimtions. While the site for introducing an amino acid sequence variation is predetermined, the mutation per se need not be predetermined. For example, in order to optimize the performance of a mutation at a given site, random mutagenesis may be conducted at the target codon or region and the expressed protein variants screened for the optimal combination of desired activity. Techniques for making substitution mutations at predetermined sites in DNA having a known sequence as described above are well known. In order to maintain a functional peptide, preferred peptide variants will differ by only a small number of amino acids from the peptides encoded by the native DNA sequences. Preferably, such variants will be amino acid substimtions of single residues. Substitutional variants are those in which at least one residue in the amino acid sequence has been removed and a different residue inserted in its place. Such substimtions generally are made in accordance with the following Table 3 when it is desired to finely modulate the characteristics of the protein. Table 3 shows amino acids which may be substimted for an original amino acid in a protein and which are regarded as conservative substimtions. As noted, all such peptide variants are tested to confirm that they retain the ability to antagonize opiate action. TABLE 3 Original Residue Conservative Substimtions Ala ser Arg lys Asn gin, his Asp glu Cys ser Gin asn Glu asp Gly pro His asn; gin Ile leu, val Leu ile; val Lys arg; gin; glu Met leu; ile Phe met; leu; tyr Ser thr Thr ser Tφ tyr Tyr tφ; phe Val ile; leu Substantial changes in biological activity may be made by selecting substimtions that are less conservative than those in Table 2, i.e., selecting residues that differ more significantly in their effect on maintaining (a) the strucmre of the polypeptide backbone in the area of the substitution, for example, as a sheet or helical conformation, (b) the charge or hydrophobicity of the molecule at the target site, or (c) the bulk of the side chain. Peptidomimetic and organomimetic embodiments are also hereby explicitly declared to be within the scope of the present invention, whereby the three-dimensional arrangement of the chemical constituents of such peptido- and organomimetics mimic the three-dimensional arrangement of the peptide backbone and component amino acid sidechains in the peptide, resulting in such peptido- and organomimetics of the peptides of mis invention having substantial biological activity. For computer modelling applications, a pharmacophore is an idealized, three- dimensional definition of the structural requirements for biological activity. Peptido- and organomimetics can be designed to fit each pharmacophore with current computer modelling software (computer aided drug design). The degree of overlap between the specific activities of pharmacophores remains to be determined. It will be understood tiiat mimetics prepared using such techniques that specifically bind to the OFQ receptor and developed using the peptide ligands of the invention will fall within the scope of the appended claims. The peptides provided by the present invention can be chemically synthesized by any of a number of manual or automated methods of synthesis known in the art. Automated synthetic routines such as those available for use with automated peptide synthesizers are also intended to come within the scope of the present invention. Chemical derivatization, using the methods disclosed in this specification or other methods well known in the art, of naturally- occurring peptides or peptides purified from mixtures of protein degradation products, degraded by enzymatic or chemical means, are also within the scope of this invention, as are peptides made by molecular or genetic engineering means. Preferably, solid phase peptide synthesis (SPPS) is carried out on a 0.25 millimole (mmole) scale using an Applied Biosystems Model 431A Peptide Synthesizer and using 9-fluorenylmethyloxycarbonyl (Fmoc) amino-terminus protection, coupling with dicyclohexylcarbodiimide/hydroxybenzotriazole or 2-(lH-benzo-triazol-l-yl)-l, 1,3,3- tetramethyluronium hexafluorophosphate/ hydroxybenzotriazole (HBTU/HOBT), and using p- hydroxymethylphenoxymethylpolystyrene (HMP) or Sasrin™ resin for carboxyl-teπninus acids or Rink amide resin for carboxyl-terminus amides. Fmoc-derivatized amino acids are prepared from the appropriate precursor amino acids by tritylation and triphenylmethanol in trifluoroacetic acid, followed by Fmoc derivitization as described by Atherton et al. (1989, Solid Phase Peptide Synthesis. IRL Press: Oxford). Sasrin™ resin-bound peptides are cleaved using a solution of 1 % TFA in dichloromethane to yield the protected peptide. Where appropriate, protected peptide precursors are cyclized between the amino- and carboxyl-termini by reaction of the amino-terminal free amine and carboxyl-terminal free acid using diphenylphosphorylazide in nascent peptides wherein the amino acid sidechains are protected. HMP or Rink amide resin-bound products are routinely cleaved and protected sidechain-containing cyclized peptides deprotected using a solution comprised of trifluoroacetic acid (TFA), optionally also comprising water, thioanisole, and ethanedithiol, in ratios of 100 : 5 : 5 : 2.5, for 0.5 - 3 hours at room temperature. Crude peptides are purified by preparative high pressure liquid chromatography (HPLC) using a Waters Delta-Pak C18 column and gradient elution with 0.1% TFA in water modified with acetonitrile. After column elution, acetonitrile is evaporated from the eluted fractions, which are then lyophilized. The identity of each product so produced and purified is confirmed by fast atom bombardment mass spectroseopy (FABMS) or electrospray mass spectroseopy (ESMS). The production of proteins such as the OFQ receptor proteins of the invention from cloned genes by genetic engineering means is well known in tiiis art. The discussion which follows is accordingly intended as an overview of this field, and is not intended to reflect the full state of the art. DNA encoding an opioid receptor may be obtained, in view of the instant disclosure, by chemical synthesis, by screening reverse transcripts of mRNA from appropriate cells or cell line culmres, by screening genomic libraries from appropriate cells, or by combinations of these procedures, as illustrated below. Screening of mRNA or genomic DNA may be carried out with oligonucleotide probes generated from the nucleic acid sequence information from the receptor (OFQR) disclosed herein. Probes may be labeled with any detectable group and used in conventional hybridization assays, as described in greater detail in the Examples below. In the alternative, amino acid transporter-derived nucleic acid sequences may be obtained by use of the polymerase chain reaction (PCR) procedure, using PCR oligonucleotide primers corresponding to nucleic acid sequence information derived from an OFQR as provided herein. See U.S. Patent Nos. 4,683,195 to Mullis et al. and 4,683,202 to Mullis. OFQR protein may be synthesized in host cells transformed with a recombinant expression construct comprising a nucleic acid encoding the OFQ receptor cDNA. For the pmposes of this invention, a recombinant expression construct is a replicable DNA construct in which a nucleic acid encoding an opioid receptor is operably linked to suitable control sequences capable of effecting the expression of the opioid receptor in a suitable host. Generally, control sequences include a transcriptional promoter, an optional operator sequence to control transcription, a sequence encoding suitable mRNA ribosomal binding sites, and sequences which control the termination of transcription and translation. See, Sambrook et al., 1990, Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Press: New York). Vectors useful for practicing the present invention include plasmids, viruses (including phage), retroviruses, and integratable DNA fragments (i.e., fragments integratable into the host genome by homologous recombination). A preferred vector is RcRVS (Invitrogen, San Diego, CA). Cultures of cells derived from multicellular organisms are a desirable host for recombinant opioid receptor protein synthesis. Transformed host cells are cells which have been transformed or transfected with recombinant expression constructs made using recombinant DNA techniques and comprising nucleic acid encoding an amino acid transporter protein. Transformed host cells may express the OFQR protein; when expressed, the OFQ receptor of the invention will typically be located in the host cell membrane. See, Sambrook et al., ibid. In principal, any higher eukaryotic cell culture is useful, whether from vertebrate or invertebrate culture. However, mammalian cells are preferred, as illustrated in the Examples. Propagation of such cells in cell culture has become a routine procedure. See Tissue Culmre. Academic Press, Kruse & Patterson, editors (1973). Examples of useful host cell lines are human 293 cells, VERO and HeLa cells, Chinese hamster ovary (CHO) cell lines, mouse Ltk cell lines and WI138, BHK, COS-7, CF, and MDCK cell lines. CHO cells, COS-7 cells and Ltk cells are preferred. The recombinant expression constructs of the present invention are useful to transform cells which do not ordinarily express an OFQ receptor to thereafter express the receptor. Such cells are useful as intermediates for making cell membrane preparations useful for receptor binding activity assays, which are in turn useful for screening drugs or new peptides made in accordance with the present invention. The recombinant expression constructs of the present invention thus provide a method for obtaining reagents for screening potentially useful peptides and otiier drugs at advantageously lower cost than conventional animal screening protocols. While not completely eliminating the need for ultimate in vivo activity and toxicology assays, the constructs and culmres of the invention provide an important first screening step for the vast number of potentially useful psychoactive drugs synthesized, discovered or extracted from natural sources each year. The recombinant expression constructs of the present invention are useful to detect, isolate, characterize and identify other novel endogenous receptors for anti-opioid agonists (such as OFQ) and antagonists found in plasma, serum, lymph, cerebrospinal fluid, seminal fluid, or other potential sources of such compounds. The utility of the present invention for using the nucleic acids of the invention to produce cell membranes containing the receptor protein encoded thereby, and the demonstrated utility of this aspect of the invention to permit the isolation, characterization and identification of a novel peptide, termed oφhanin FQ (OFQ) herein, as an endogenous receptor-binding ligand, enables rational drug design of novel therapeutically-active drugs using currently-available techniques (see Walters, "Computer-Assisted Modeling of Drugs", in Klegeπnan & Groves, eds., 1993, Pharmaceutical Biotechnology. Inteφhaπn Press: Buffalo Grove, IL, pp. 165-174). The invention provides homogeneous compositions of a novel mammalian receptor protein produced by transformed eukaryotic cells as provided herein. Each such homogeneous composition is intended to be comprised of the OFQR protein that comprises at least 90% of the protein in such homogenous composition. The invention also provides membrane preparations from cells expressing the OFQR protein as the result of transformation with a recombinant expression construct, as described herein. Mammalian receptor proteins made from cloned genes in accordance with the present invention may be used for screening analogues of an anti-opioid system that operates in parallel with and in functional antagonism to the opioid system. The receptor proteins may also be useful to find agonists or antagonists of opioid binding, or for determining the amount of such agonists or antagonists that are present in a solution of interest (e.g., blood plasma, cerebrospinal fluid or serum). For example, host cells may be transformed with a recombinant expression construct of the present invention, a mammalian OFQR expressed in those host cells, and the cells or membranes thereof used to screen compounds for their effect on anti-opioid receptor binding activity. By selection of host cells that do not ordinarily express an OFQR, pure preparations of membranes containing the transporter can be obtained. The invention also provides ligands for such OFQR proteins, including naturally- occurring ligands and synthetic embodiments thereof. Methods for identifying otiier, alternative ligands, and agonists and antagonists of OFQ peptide ligand binding are also provided by the invention. Such methods include competitive binding assays, wherein the peptide ligands of the invention are detectably labeled and incubated under competitive binding conditions with varying amounts of any putative ligand compound to be tested. The extent of inhibition of labeled ligand binding is useful in characterizing the extent and affinity of binding of novel ligands to the OFQR. Specificity of binding can also be deteπnined using such assays. Preferably, the peptide ligand of the invention being used in such competition experiments is detectably labeled with a radioisotope, such as 1-123, 1-125 and 1-131. The invention also provides antibodies that are immunologically reactive to the opioid receptor protein or epitopes thereof provided by the invention. The antibodies provided by the invention may be raised using methods well known in the art. The present invention also provides monoclonal antibodies tiiat are immunologically reactive with an epitope derived from an OFQ receptor of the invention, or fragment thereof. Such antibodies are made using methods and techniques well known to those of skill in the "art. Monoclonal antibodies provided by the present invention are produced by hybridoma cell lines, that are also provided by die invention and that are made by methods well known in the art. The ligands of the invention are useful as diagnostic and tiierapeutic agents when constimted as pharmaceutical compositions with the appropriate carriers or diluents. In diagnostic embodiments, detectably-labeled peptide ligands are used in methods for diagnosing diseases or pathological conditions related to ligand binding of OFQ receptors in vivo. Similarly, therapeutic methods of treatment are encompassed by die invention and provided using pharmaceutical compositions of such peptides administered in vivo in therapeutically-effective amounts. Preparation of pharmaceutically acceptable compositions of the peptides of die present invention can be accomplished using methods well known to those with skill in the art. Any of the common carriers such as sterile saline solution, plasmas, etc., can be utilized with the peptides provided by the invention. Routes of administration include but are not limited to oral, intracranial ventricular (icv), intrathecal (it), intravenous (iv), parenteral, rectal, topical ophthalmic, subconjunctival, nasal, aural and transdermal. Peptides of die invention may be administered intravenously in any conventional medium for intravenous injection such as an aqueous saline medium, or in blood plasma medium. Such medium may also contain conventional pharmaceutical adjunct materials such as, for example, pharmaceutically acceptable salts to adjust the osmotic pressure, buffers, preservatives and the like. Among the preferred media are normal saline and plasma. Embodiments of the invention comprising medicaments can be prepared with conventional pharmaceutically acceptable carriers, adjuvants and counterions as would be known to those of skill in the art. The medicaments are preferably in the form of a unit dose in solid, semi-solid and liquid dosage forms such as tablets, pills, powders, liquid solutions or suspensions, and injectable and infusible solutions, for example a unit dose vial. Effective dosage ranges included in the unit dose container vary from about 100 μg/kg to about 10 mg/kg of body weight are contemplated. The invention also provides methods for detecting the amount of an analyte in a solution wherein the analyte is an endogenous peptide ligand of the invention in a biological sample, such as blood, serum, plasma or cerebrospinal fluid. In such methods is provided a first mixture comprised of cells or membranes heterologously expressing the OFQ opioid receptor of the invention, a second mixmre comprised of a standard amount of a detectably-labeled embodiment of the peptide ligands of die invention, and a third mixmre comprised of a diagnostically-significant tissue sample or bodily fluid. From these mixtures is produced a specific binding reaction mixmre by contacting the first, second and third mixtares and incubating the reaction mixture for a time sufficient to allow binding between the peptide ligand and die OFQ receptor. The extent of the binding reaction is determined by calculating the amount of the detectably-labeled peptide ligand of the invention bound to die OFQ receptor, and comparing the amount of binding of the peptide ligand to die OFQ receptor in die presence of the tissue sample or bodily fluid of the third provided mixture with the amount of binding found in the absence of the tissue sample or bodily fluid of the third provided mixmre. Throughout these examples, injections are sometimes given intrathecally or intraventricularly. The intrathecal injection method allows the application of small quantities of drug directly into the spinal cord, without surgical intervention. The mouse is briefly exposed to halothane, immediately prior to all injections. Intrathecal injection is achieved with disposable 30 g, l Λ inch needles mated to a 10μl luer tip syringe (Hamilton, Reno, NV) are used. Drug is administered in artificial cerebral spinal fluid and injection volumes are 2-5 / χl. The mouse is placed on a flat surface, with its head in a towel cupped gently with the heel of the palm, and die thumb and forefinger lightly pushing down on the iliac crests with enough force to stretch the skin across the lower back. The needle is inserted into the tissue just medial to die L5 spinous process so that it enters the groove between the spinous and transverse processes. This site maximizes vertebral accessibility and minimizes the possibility of spinal damage (into the cauda equina). The needle is then moved carefully forward to the intervertebral space as the angle of the syringe is decreased from about 30° to 15°. The tip of the needle is inserted so tiiat approximately 0.5 cm is within the vertebral column which is reliably indicated by a flick of the mouse's tail. In a human, conventional lumbar puncture with a spinal needle can be used to access the epidural or intrathecal space. For icv injections, a 26g V4 inch needle is fitted with a piece of polyethylene tubing so that 3mm of uncovered needle enter the mouse's skull. (The sleeve ensures that the depth of the needle is appropriate for the depth of die lateral ventrical). Mice are briefly anesthetized with halothane. The needle is held at a 45° angle and inserted at Bregma, 1-2 mm lateral to midline. The needle readily punctures the skull at this suture, so no incision is required for accurate placement. The needle tip is towards the tail, and die 3mm depth allows direct placement of the drug into a lateral ventricle. The Examples which follow illustrate specific embodiments of the invention, and various uses thereof. They are set forth for explanatory puφoses only, and are not to be taken as limiting the invention. EXAMPLE 1 Isolation of a Mammalian Opioid Receptor Probe by Random PCR Amplification of Rat Brain-derived cDNA Using Degenerate Oligonucleotide Primers In order to clone novel mammalian G-protein coupled receptors, cDNA prepared from RNA from different regions of mammalian (rat) brain was used as template for a polymerase chain reaction (PCR)-based random cloning experiment. PCR was performed using a pair of degenerate oligonucleotide primers derived from a mouse δ-opioid receptor (Kieffer et al. , 1992, Proc. Natl. Acad. Sci. USA 89:12048-12052; Evans et al., 1992, Science 258:1952-1955). PCR products obtained in this experiment were characterized by nucleotide sequencing and used to isolate a full-length cDNA from a rat brain cDNA library. The PCR amplification experiments were performed as described in co-owned and co-pending U.S. Serial No. 08/149,093, incoφorated by reference, using the following primers: Primer III (sense): ATGAATTCAC(G/A/C/T)(A/G)T(G/C)ATGAG(C/T)GT(G/C)GAC(C/A)G(C/A)TA (SEQ ID NO:l) and Primer VII (antisense): TTGTCGAC(G/A)TA(G/A)AG(A/G)A(T/C)(G/A/C/T)GG(G/A)TT (SEQ ID NO:2). Amplified products of die PCR reaction were separated on a 1.0% agarose gel (see Sambrook et al., ibid.), and fragments ranging in size from 400 basepairs (bps) to 750 bp were subcloned in the plasmid vector pBluescript (Stratagene, LaJolla, CA). A multiplicity of bacterial colonies comprising each of the subcloned fragments were used to make bacterial colony lifts on nitrocellulose filters using conventional techniques (see Sambrook, et al, ibid.). Such filters were hybridized with a [ 32 P]-dCTP-labeled radioactive nucleic acid probe comprising a full-length mouse δ-opioid receptor cDNA at a concentration of 1 x 10 6 cpm/mL under low stringency hybridization conditions [35% formamide, 5X standard citrate saline (SSC; wherein IX SSC is 0.15M NaCl/0.015M sodium citrate, pH 7.0), 5X Denhardt's solution (wherein IX Denhardt's solution is 0.02g/mL each of bovine serum albumin, Ficoll and polyvinylpyrrolidone)] at 37°C overnight. After hybridization, die filters were washed in a solution of 2X SSC/0.1 % sodium dodecyl sulfate (SDS) at 55°C and then exposed to X-ray film (XAR-5, Eastman-Kodak, Rochester, NY) for 2 days at -70°C using tungsten-impregnated intensifying screens (DuPont- NEN, Wilmington, DE). Plasmid DNA from hybridizing clones was purified and die nucleotide sequence of die insert cDNA determined by the dideoxynucleotide chain termination method (Sanger et al, 1977, Proc. Natl. Acad. Sci. USA 74:5463-5467) using Sequenase ® (U.S. Biochemical Coφ., Cleveland, OH). EXAMPLE 2 Isolation of a Novel Mammalian Opioid Receptor cDNA One of the PCR products (termed LCI 32) was isolated and sequenced in this way and found to have a high degree of homology to the mouse δ-opioid receptor sequence (Evans et al, ibid, and Kieffer et al, ibid.). A full-length cDNA clone corresponding to this PCR fragment was isolated from a cDNA library prepared in the cloning vector λgtl 1 comprising oligo(dT)-primed rat brain cDNA, as described in co-owned and co-pending U.S. Serial No. 07/149,093. Nucleotide sequence analysis performed essentially as described (see, Sambrook, ibid.) revealed the sequence shown in Fig. 1 (SEQ ID No.:3). The putative protein product of the gene is also shown in Fig. 1 (SEQ ID No.:4). The sequence was found to have an open reading frame comprising 1101 nucleotides encoding a protein 367 amino acids in length, and having a predicted molecular weight of 47 kilodaltons prior to post-translational modification. The sequence immediately 5' to tiie proposed initiation codon was found to contain several translation termination codons in-frame with the open reading frame, supporting the assignment of the translation start site. Predicted transmembrane domains [using the algorithm of Eisenberg et al. (1984, J. Molec. Biol. 179:125-142)] are boxed and identified by Roman numerals (I- VII), and three sites of possible N-linked glycosylation are identified in the amino-teπninal portion of the protein with solid triangles. Potential protein phosphorylation sites found in predicted cytoplasmic loops are marked with an asterisk. Further, a pair of cysteine residues conserved among known opioid receptors were found in the first and second predicted extracellular loops. On the basis of this analysis, this cloned nucleic acid was determined to be a novel mammalian receptor related to opioid receptors, such as the μ, δ and K receptors. The predicted amino acid sequences of this novel receptor, the rat μ-opioid receptor (Chen et al., ibid.)., die mouse δ-opioid receptor (Evans et al, ibid, and Kieffer et al., ibid.) and die mouse κ-opioid receptor (Yasuda et al., ibid.) are aligned in Fig. 2. Overbars indicate predicted transmembrane regions I through VII in the protein product of the genes. Amino acid residues that are found in common between all four mammalian opioid receptors are presented in boldface. Overall, the novel mammalian receptor disclosed herein had 47% overall identity with the other mammalian opioid receptors, which similarity rose to 67% when only the predicted transmembrane domains were considered. Comparisons were made individually at each transmembrane domain (TMI-TMVII), as an average over all transmembrane domains (TMavg) and as the average degree of amino acid sequence homology for each protein as a whole (avg/all). In total, 145 of the 367 residues are shared with the other mammalian opioid receptors, confirming the conclusion that the novel mammalian receptor disclosed herein is related to the opioid receptors. The inventors have now surprisingly discovered, however, that the receptor mediates an anti-opioid effect. EXAMPLE 3 Construction of a Recombinant Expression Construct, DNA Transfection and Functional Expression of the Novel Mammalian Opioid Receptor In order to biochemically characterize the novel mammalian opioid receptor described in Example 2, and to confirm that it encodes a novel opioid receptor, the cDNA was cloned into a mammalian expression construct, the resulting recombinant expression construct transfected into COS-7 cells (for transient expression assays) and mouse Ltk " cells (for stable expression assays), and cell membranes (COS-7) or cell lines (Ltk) were generated that expressed the receptor protein in cellular membranes at the cell surface. Such cells and membranes isolated from such cells were used for biochemical characterization experiments as described in U.S. Serial No. 149,093. The entire coding region of the receptor cDNA insert was subcloned into the RcRSV vector (Invitrogen, San Diego, CA) using conventional techniques (see Sambrook et al., ibid.). Such recombinant expression constructs were introduced into COS-7 cells using die calcium-phosphate precipitation technique (Chen & Okayam, 1987, Molec. Cell Biol. 7:2745- 2752), the transfected cells allowed to express the receptor for between 24-96 hours, and then cell membranes containing the receptor were isolated. The protein concentration was adjusted to a 15-80 μg/sample for each of the binding studies described below. These recombinant expression constructs were also introduced into Ltk" cells using the calcium-phosphate precipitation technique, and stably-transfected clones were selected by growth in e mammalian neomycin analog G418 (Grand Island Biological Co., Long Island, NY), as the vector RcRSV contains a functional copy of a bacterial neomycin resistance gene. Stable cells lines were then selected for membrane binding studies based on mRNA expression levels of individual neomycin-resistant transfected clones determined by Northern analysis (see Sambrook et al., ibid.). Cell membranes were prepared and used as described above for COS-7 cell transfectants. Specific binding assays using a variety of opioid receptor agonists and antagonists were performed on membranes from both transient and stable transfectants. Ligand binding experiments were performed essentially as described in Bunzow et al. (1988, Nature 336:783- 787), and U.S. Serial No. 08/149,093). In binding experiments, increasing amounts of membrane protein (from 15-80μg) were incubated with the radioactively-labeled opioid agonist or antagonist to be tested for 120 min at 22°C in a total volume of 1 ml. However, in these experiments no specific binding was found for the following compounds (their known receptor binding specificities are noted in parentheses): [ 3 H]-Tyr-DAla-Gly-Met-Phe-Gly-ol (DAMGO; μ- opioid receptor agonist), [ 3 H]-c[D-penicillamine 2 , D-penicillamine 5 ]enkephalin (DPDPE; δ agonist), [ 3 H]-U-69,593 (K agonist), [ 3 H]-diprenoφhine (μ agonist), [ 3 H]-bremacozine (#c agonist), [ 3 H]- dihydromoφhine (μ agonist), [ 3 H]-ethylketocyclazocine (K agonist) or [ 125 I]-β-endoφhin. Although low levels of specific binding were seen using [ 3 H]-naloxone (μ antagonist), the significance of these results was compromised by the fact that untransfected COS-7 and Ltk" cells also shown endogenous low levels of specific [ 3 H]-naloxone binding. Suφrisingly, however, specific binding was found using [ 3 H]-methadone. The results of Scatchard analysis of the methadone binding data are shown in Fig. 4. For Scatchard analysis experiments, 0.25 ml aliquots of crude plasma membrane homogenate from transfected cell culmres was incubated in duplicate with increasing concentrations of pHJmethadone (70.3 Ci/mmol; 10-3000 pM final concentration) under conditions described above. The estimated value for B^ was derived from these data using the LIGAND computer program. Panel A of Fig. 4 shows the results of radiolabeled methadone binding with untransfected COS-7 cells; similar results were found with Ltk " cell membranes. These results demonstrate no or negligible amounts of endogenous methadone binding by these cell membranes. Panel B shows the results using COS-7 cells transfected widi the RcRSV-LC132 expression construct. The levels of specific binding shown in this graph correspond to a dissociation constant (K D ) of about 10 10 M for methadone and a B^ of about 400-450 fentomoles/μg protein for the novel mammalian opioid receptor expressed by these cells. Thus, the novel mammalian opioid receptor disclosed herein has the heretofore unknown property of exhibiting specific binding to the opiate analog, methadone, while showing no specific binding to a variety of other known opioid receptor agonists and antagonists. These results support the conclusion that the receptor disclosed herein is a completely novel and heretofore unsuspected member of the opioid receptor family, termed herein therefore OFQ. EXAMPLE 4 Brain Tissue Distribution of OFQR Expression The distribution of mRNA corresponding to expression of the OFQ receptor gene in various regions of the rat brain was determined by in situ hybridization of rat brain slices, as described in U.S. Serial No. 08/149,093. Results of these experiments are shown in Fig. 3. Panel A shows a section through the frontal cortex, preoptic area and caudate putamen; Panel B shows a section through the hypothalamus, thalamus and hippocampus; and Panel C shows a section tiirough the pons and cerebellum. These experiments localized high level OFQR expression in the hypothalamus (arcuate (Arc), posterior (PH), lateral (LH) and ventromedial (VMH) hypothalamic nuclei, Panel B), certain nuclei of the thalamus (paraventricular thalamic nuclei (PVP), Panel B), the medial habenula (MHb, Panel B), the CA regions of the hypothalamus, the dentate gyms (DG, Panel B), the locus coeruleus and certain cortical areas (medial preoptic are (MPA), Panel A and the cortex (CX), Panel B), Virtually no signal was seen in the caudate putamen (Cpu, Panel A) or cerebellum (Cb, Panel C). Strong hybridization was also detected in sections of die brainstem (Panel C) and the spinal cord (not shown). These results demonstrate that the OFQ receptor disclosed herein is expressed in rat brain in a variety of anatomically-distinct sites, suggesting an important role for this receptor in both higher brain function and central nervous system control of motor and sensory nerve signalling. The anti-opioid effects mediated by mis receptor, however, were completely unpredictable. EXAMPLE 5 Identification of an Endogenously-occurring Peptide Ligand for the OFQ Receptor Due to the high level of sequence homology between the OFQ receptor the and μ-, δ- and κ-opioid receptors found as disclosed in Example 2, it was appreciated tiiat the novel receptor disclosed herein might specifically recognize an endogenous peptide ligand, and respond to binding such a ligand using a second messenger signalling system similar to those found associated with the previously-described opioid receptors. Since the OFQR had been found to be expressed in cells in the central nervous system in Example 4, and since OFQR mRNA is highly expressed in hypothalamus, porcine hypothalamic homogenates were screened for receptor binding activity. Inhibition of forskolin-stimulated, cyclic adenosine monophosphate (cAMP) accumulation in cells heterologously expressing OFQR was used as an assay, which assay had been used to characterize the interaction of opioid receptors and their ligands. Acetic acid extracts of porcine hypothalamic tissues were prepared as follows. 4.5kg of freshly frozen porcine hypothalamic tissue were extracted in 9L of a solution of 0.5M acetic acid/lOmM ascorbic acid/lmM EDTA. The extract was centrifuged to remove insoluble debris and the supernatant absorbed batchwise onto a C 18 silica matrix. Unbound material was removed by washing with water, and specifically-bound material was then eluted in a solution of 80% methanol. A total of 2L of methanolic eluate were then concentrated by rotary evaporation of a final volume of 44 mL, and material having a molecular weight less than 10 kilodaltons was obtained by ultrafiltration using an Amicon Centripep 10 column (Amicon, Beverl, MA). This material was applied in ten separate experiments to a cation exchange HPLC column (Protein Pak SP 8HR, lOxlOOmm, Waters) equilibrated with lOmM NH 4 COO. The column was developed with a linear gradient of NH,COOin 10% methanol at a flow rate of lmL/min; a total of 80 ImL fractions were collected. 10% (lOOμL) of each of 5 consecutive fractions were pooled and lyophilized, resulting in 16 pools. 5% of each pool was tested in duplicate for the capacity to inhibit forskolin-stimulated cAMP accumulation in cells heterologously expressing the OFQ opioid receptor. Assays of ligand binding-associated inhibition of forskolin-stimulated cAMP accumulation in cells heterologously expressing the OFQ receptor were performed as follows. CHO cells, deficient in dihydrofolate reductase (dhfr) were transfected with OFQR-encoding cDNA cloned into the expression vector pRcRSV (Invitrogen), as described in Example 3 above, using calcium phosphate co-precipitation (Okayama and Chen, ibid.) Stably-transfected clones were selected using G418, and screened for OFQ expression using a reverse transcriptase-PCR protocol (RT-PCR). One clone that tested positively for OFQR expression (LC-7) was used in ligand binding experiments. For determination of cAMP levels, receptor-transfected CHO cells (i.e., LC-7) or untransfected CHO/dhfr cells, were plated in 24-well culture plates and grown to confluency. After removal of culture media, aliquots of HPLC fractions prepared as described above in a total volume of 0.2mL DMEM containing lOmM HEPES buffer, pH 7.4, ImM forskolin and ImM Ro 20-1724 (Rolipram, RBI) were added per well and cells incubated at 37°C for 10 min. Cellular reactions were halted by the addition of 0.5mL ice-cold ethanol, and plates were stored for 12h or overnight at 80°C. Frozen plates were centrifuged and aliquots of die supernatant were removed and dried onto 96-well plates for cAMP determinations. cAMP determinations were performed using a commercially-available kit (Biotrak SPA, Amersham) essentially according to the manufacturer's instructions. The results of these experiments are shown in Fig. 5. Open bars represent results obtained widi OFQ-expressing transfected CHO cells, and shaded bars represent results obtained with untransfected CHO cells. Pools 13 and 14 (corresponding to fractions 61-70) consistently showed adenyl cyclase inhibitory activity in transfected versus untransfected cells. The large increase in cAMP detected in fraction 2 is due to endogenous cAMP extracted from the tissue. Fractions 60-67 were found to contain most of the detected bioactivity, and were pooled for further purification by reverse-phase HPLC. Reverse-phase HPLC was performed on pooled fractions 60-67 as follows. The complete bioactive material was loaded onto an octyl silica column (Superspher RP Select B, 2x125mm, Merck) and eluted at a flow rate of 0.12mL/min with a linear gradient of acetonitrile in 0.1 % trifluoroacetic acid (TFA). Aliquots of fractions from this gradient were tested as described above, and the results of these assays shown in Fig. 6. Inhibition of forskolin- stimulated cAMP in OFQR-transfected CHO cells (shaded bars) was detected only in the major peak of eluted protein (measured as absorbance at 214nm (A 214 )). The isolated material having adenyl cyclase inhibitory activity was analyzed by mass spectrometry and sequenced by Edman degradation. The material was determined to be a peptide having the sequence: Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln (SEQ ID No.: 5). (Abbreviations for amino acids can be found in Zubay, Biochemistry 2d ed., 1988, MacMillan Publishing: New York, p. 33). Final yields of this peptide were about 200 picomoles from 4.5kg hypothalamic tissue (wet weight). A computer database search revealed that this peptide had not been reported, either as a unique entity or as part of a larger protein. This peptide is designated oφhanin FQ (OFQ) herein. The primary strucmre of this novel peptide was compared with the primary structure of a number of naturally-occurring opioid peptides, as shown in Fig. 7. The amino terminal tetrapeptide sequence motif YGGF of the known opioid receptor ligands is strikingly similar to the amino terminal sequence FGGF found in OFQ. Further, two clusters of basic amino acids found in the oφhanin FQ peptide resemble the arrangement of positively-charged residues in Dynoφhin A and β-Endoφhin. However, none of these peptides could be shown to bind to OFQR, suggesting a structural/functional divergence of the oφhanin FQ peptide from these other opioid receptor ligands. In order to verify that the isolated material had die observed properties of the pooled fraction from which it is isolated, a synthetic peptide having the deduced sequence was made and was found to be identical to die isolated peptide in its elution profile in reverse-phase HPLC assay and in molecular weight as determined by mass spectrometry. Moreover, the synthetic peptide was also found to inhibit adenyl cyclase production in forskolin-stimulated CHO cells transfected with and heterologously expressing OFQR, having an E o of 1.58nM ( O.666nM) and showing maximal inhibition (80%) at a concentration of about lOOnM. This high potency is comparable to that of other neuropeptides identified as the natarally-occurring ligands of other CNS-specific receptors. Thus, the isolated sequence, termed oφhanin FQ herein, represents a novel endogenous peptide ligand for the OFQR. To further characterize ligand binding of this novel ligand to OFQR, a series of peptide analogs were prepared that were tyrosine-substituted at different positions in the peptide sequence. Then cAMP inhibition assays were performed using botii the tyrosine-substituted peptides and monoiodotyrosine substimted peptides. Iodinated peptide was synthesized using die chloramine T method of Hunter and Greenwood (1962, Nature J94: 495). From these studies it was determined at the peptide analog having tyrosine at position 14 (Y-14: SEQ ID No.: 6) and its monoiodo form (I-Y14) were OFQR agonists of almost equivalent potency to the oφhanin FQ peptide, having EC 50 values of InM and 3nM, respectively. Fig. 8 shows a comparison of adenyl cyclase inhibitory activity of oφhanin FQ (filled circles), Y14 oφhanin FQ (filled squares), monoiodo Y14 oφhanin FQ (open squares) and Leu-Enkephalin (open circles) in transfected CHO cells heterologously expressing OFQR; untransfected CHO cells were treated with oφhanin FQ peptide as a control (filled triangles). Data were normalized so that cAMP levels in forskolin-stimulated, untransfected CHO cells was equal to 100%. cAMP levels were determined with all incubations being done at least twice in triplicate. Fig. 8 shows the results of a representative experiment. The Y14 peptide was used to quantitatively assay receptor binding affinity for the novel ligand. Membranes from OFQR transfected CHO cells (LC-7) were used at a concentration of 55μg membrane protein/assay. Membranes were incubated widi increasing concentrations of ( 1 5 I)-Y14-oφhanin FQ in a final volume of 0.2mL binding buffer (50 mM Hepes, pH 7.4, lOmM NaCI, ImM MgC12, 2.5mM CaC12, 0.1 % bovine serum albumin, 0.025% bacitracin) containing lmg wheat germ agglutinin-coated SA beads (Amersham). Iodinated peptide was synthesized as above using the chloramine T method. The monoiodinated species was obtained as a single peak, having a specific activity of 2200 Ci/mmole on the day of synthesis. Assays were performed in 96-well plates (OptiPlate, Caberra Packard), and the mixtares were incubated with shaking for lh. Bound ligand-associated radioactivity were determined by scintillation proximity (see, for example, Nelson, 1987, Anal. Biochem. 165: 287; Bosworth and Towers, 1989, Nature 341: 167) using a TopCount microplate scintillation counter (Canberra packard). Concentrations of free ligand were calculated by subtracting the amount of specifically-bound ligand from the total amount of radioligand added. The results of these experiments are shown in Fig. 9. A Scatchard analysis is shown in the inset. The radiolabeled monoiodo-Y14-oφhanin FQ peptide displayed saturable and displaceable binding to membranes of OFQR-expressing transfected cells, having a K D (slope of Scatchard line = -1/K D ) of O. lnM (80 picomole ± .02 nanomole) and a B^ (Scatchard line abscissa intercept) of 800fmol/mg membrane protein. K D was determined as in Munson, Principles of Pharmacology (Chapman & Hall), 1995, Chapter 1 (which is incoφorated by reference). These experiments demonstrate that the novel endogenous ligand peptide, oφhanin FQ, and Y14-tyrosine substimted analogs thereof, bind saturably to OFQR with high affinity, further confirming the conclusion that this peptide is a naturally-occurring, endogenous ligand for this receptor. These experiments also indicate tiiat the Y14-substitated peptide can be used as a radioligand to detect and quantify OFQ opioid receptor levels. EXAMPLE 6 Functional Characterization of the Orphanin FQ Peptide In order to study the physiological activity of the oφhanin FQ peptide, the in vivo activity of the peptide was investigated on unrestricted animals, and specifically, on nociception in such animals. For these experiments, the peptide was administered intracerebroventricularly (icv) and intrathecally (it) in mice. Immediately after peptide administration, mice were placed in transparent boxes in groups of three, and behavioral signs recorded. Emphasis was placed on signs indicative of depressant, stimulant and autonomic effects of the peptide (as described by Irwin, 1968, Psychopharmacologia 13: 222). In open-field observation experiments, the peptide at larger doses (e.g. above lnmole/2μL/mouse) was noted to have a profound influence on locomotor activity, as well as a decrease in muscle tone, a loss of righting reflex, and ataxia. A quantitative investigation showed a decrease of both horizontal and vertical locomotion following icv administration of the peptide at concentrations of 0.1-10nmole/2μL/mouse. These results are shown in figure 10. The effects of the peptide on horizontal and vertical locomotor activity was measured using Digiscan Animal Activity Monitors (Omnitech, Columbus, OH). The values for horizontal and vertical locomotor activity represent the total number of interruptions of the horizontal and vertical sensors during the first 10 minutes following administration of the peptide. Data are shown as the mean ±_ standard error of the mean. On the other hand, the oφhanin FQ peptide showed little analgesic effect in mese mice, as recorded in a hot plate test, as shown in Fig. 11. Groups of 6-8 male MORO mice (weight =22g) were administered phosphate buffered saline (open circles) or varying doses of die oφhanin peptide. Reaction time represents the time taken for the mice to lick their paws. The hot plate was set to 58°C, and a cutoff time of 60 sec was used. No significant decrease in reaction time was detected, with the exception of mice administered die highest levels of peptide, lOnmole/mouse, icv (about .45 nmole/g) designated by an asterisk. This observation was most likely related to a decrease in locomotor activity and muscle tone. At all doses tested, all animals exhibited normal toe- and tail-pinch reflexes, as shown in Fig. 12. No analgesic effect was observed when oφhanin FQ was administered intrathecally at 2.5-10nmole/4μL/animal; however, at the highest dose, hind limb paralysis and decrease in locomotor activity was observed. No analgesic effect was observed in any mouse with this peptide at doses that did not produce significant decreases in motor activity and muscle tone. These results indicate that, despite strucmral homology with analgesia-producing opioid receptor ligands as described in Example 5, the oφhanin FQ peptide appears to be pharmacologically distinct and to lack appreciable analgesic activity (which is defined as no significant decrease in reaction time as measured in this Example 6). EXAMPLE 7 Stress Induced Analgesia Reversed by Naloxone and OFQ In spite of the structural homology between OFQ and die opioid peptides, icv injected animals do not display unconfounded antinociception or analgesia, which would be observable as a decreased sensitivity to pain. One report has been made that OFQ instead induces hyperalgesia, a supersensitivity to pain, in both the tail flick and hot-plate assays. Meunier et al., Nature 377,532-535 (1995). Contrary to the expectations of others, the present inventors have found that OFQ does not produce hyperalgesia, but instead potently and dose- dependently antagonizes opiate actions such as analgesia. Hence OFQ is observed to act as an endogenous anti-opioid peptide tiiat is capable of reversing effects of either endogenous or exogenous opioids in an animal, such as a mouse or human. The anti-opioid effects of the OFQ peptide are demonstrated in this example, which shows the effects of OFQ in the tail-withdrawal (TW) test, an assay of response to acute thermal pain. The results are shown in Fig. 13A. Swiss-Webster mice of both sexes (20-35 g; Simonsen Inc.; N=7-8 per group) were tested for nociceptive sensitivity to immersion of the distal half of the tail in 46°C water before and after (10, 20 and 30 min) subcutaneous (sc) injection with saline (Sal; lOml/mg) and naloxone (NAL, lmg/kd) and icv injection (under light halothane anesthesia) with Veh (2.5 microliter of artificial cerebrospinal fluid (CSF)) or 10 nmole OFQ. Separate groups of mice received only sc injections of saline or naloxone (labeled None to indicate no icv injection in Fig. 13A). The latency to respond to die heat stimulus by a vigorous flexion of the tail was measured to the nearest 0.1 s by an experienced observer blind to the drug condition of the animal. Mice were lightly restrained in a cloth/cardboard holder during testing. In the absence of a withdrawal reflex or other indication of distress, the tail was removed from the water after a cut-off latency of 15 s. To improve accuracy, three separate TW latency determinations, separated by 20 s, were made and averaged at each test. In this and all subsequent experiments, no main effects nor interactions of sex were found to be significant, so data from both sexes were pooled. Because there were no changes in post-injection TW latencies in any group with repeated testing at 10, 20 and 30 min post-injection, data from these time points were pooled. An ANOVA (analysis of variants) performed on averaged TW change data revealed significant main effects of sc injection and icv injection (F2,44=9.31, p< .05, respectively). Planned contrasts demonstrate that the Sal/Veh group is significantly different from all others. Although there was no evidence of hyperalgesia produced by OFQ, vehicle-treated animals displayed higher TW latencies (ie were less sensitive to pain) than OFQ-treated animals (p< .01)(Fig. 13). The vehicle treated animals were displaying stress-induced-analgesia (SI A). SIA is mediated by endogenous opioids, hence die effect of both the opiate antagonist naloxone and OFQ on the SIA was investigated. Naloxone and OFQ both attenuated the 4 s post-injection increase in TW latency displayed by vehicle-treated animals (p < .05). The combined administration of naloxone and OFQ had no additional effect. In no case did any mouse treated with OFQ or naloxone exhibit hyperalgesia; administration of these drugs instead merely returned pain sensitivity to baseline by reversing SIA. Hyperalgesia would have been detected by a shorter tail withdrawal latency than with vehicle alone, which was not observed with OFQ. EXAMPLE 8 Reversal of morphine analgesia by OFQ in Writhing Test To maximize the observation of hyperalgesia, the acetic acid AC assay was performed to determine the effect of OFQ on the abdominal constriction test. This test is an extremely sensitive measure of tonic, chemically-induced pain stimulus. The results are given in Fig. 13B. The AC assay was a modification of the technique originally described by Koster et al. Mice (N=9-15 per group) were acclimated to individual cylindrical Plexiglass observation chambers for > 30 minutes. They were then weighed and given an sc injection of saline (Sal; 10 ml/mg) or naloxone (NAL; 1 mg/kg and 10 mg/kg), or handled but given no injection (None). Immediately thereafter, mice were lightly anesthetized with halothane and given and icv injection of vehicle (VEH; 2.5μl of artificial CSF) or 2.5 nmole OFQ in 2.5μl vehicle (OFQ-2.5). A separate group of Sal-treated mice received no icv injection (None). Between 5 and 10 minutes later, all mice were given an intraperitoneal (ip) injection of 0.9% acetic acid and placed back in their observation chambers. For the next 30 minutes, the number of abdominal constrictions (lengthwise stretches of the torso with a concomitant concave arching of the back) were counted and recorded. Four mice in separate chambers were observed simultaneously by a single experimenter blind to drug condition. ANOVA revealed a significant group differences. The results are shown in Fig. 13B. The decreased number of constrictions in VEH mice is believed to be due to SIA, which was reversed by NAL and OFQ-2.5. The 2.5 nmole icv dose of OFQ was pharmaceutically sufficient to inhibit the analgesic effect of moφhine within 30 minutes after the OFQ was administered. Example 9 Reversal of Morphine Analgesia by OFQ in Tail Withdrawal Latency OFQ reverses the analgesic actions of moφhine, which are mediated by the opioid SIA substrate. In this example, moφhine is administered to mice immediately following assessment of their baseline nociceptive sensitivity. It produced a profound analgesia that peaked at 30 minutes post-injection (Fig. 14) and lasted about 2 hours. But OFQ reversed this analgesia in a dose-dependent manner (at doses of 2.5-25 nmole). Swiss-Webster mice of both sexes (N=6-ll per group) were tested for baseline nociceptive sensitivity on die 49°C TW test (as described in Example 7), injected s.c. with moφhine sulfate (MOR; 5 mg/kg), and then retested 20 min later to establish the existence of moφhine analgesia. Analgesic mice (those with TW latencies of at least double their baseline; all except 3) were then lightly anesthetized with halothane, injected i.c.v. with vehicle (Veh; 2.5 μl artificial CSF) or OFQ (2.5, 5, 10 or 25 nmole in 2.5μl vehicle), and retested for TW latencies 10, 25, 40 and 100 min later by an experimenter blind to OFQ dose. The 25 nmol OFQ dose produced marked atonia and flaccid paralysis in up to 50% of mice. These side-effects should not, however, be considered a confound of the present data because mice injected with mis high OFQ dose actually reacted faster to the thermal stimulus. Fig. 14 shows time-course data demonstrating moφhine analgesia and its dose- dependent reversal by four doses of OFQ. Symbols represent mean (±SEM) TW latencies from 49°C water at each time point. A linear regression analysis was used to calculate the dose of OFQ causing a 50% inhibition of the effect of 5 mg/kg moφhine analgesia, and determined tiiat this ED 50 was 7.5 nmol in a 2.5μl volume. The ED 50 is the dose required to produce a response of 50% of maximal effect. A significant inhibition of analgesia would be a 50% inhibition of opiate analgesia. The ED^ would be a pharmaceutically sufficient amount of OFQ to achieve significant inhibition. EXAMPLE 10 Attenuation of morphine hypothermia by OFQ This example demonstrates the ability of OFQ to functionally antagonize opioid effects other than analgesia. The non-nociceptive action of moφhine stadied here was the ability to induce hypothermia in rodents at high doses. The DBA/2J mice strain was used to study attenuation of hypothermia because of its known robust hypothermic response. Core body temperature of DBA/2J mice (15-30 g; The Jackson Laboratory; N = 7- 8 per group) was assessed immediately before and 30 min after systemic administration of saline (Sal; lOml/kg) or moφhine (MOR; 20 mg/kg, i.p.). Immediately thereafter, mice were injected i.c.v. with vehicle (Veh; 2.5 μl artificial CSF) or 5 or 10 nmole OFQ in 2.5μl vehicle (OFQ-5 and OFQ-10), and retested 15, 30 and 60 min later. As shown in Fig. 15, there was a 6°C decrease in body temperature produced by MOR, in addition to a transient hypothermia produced by halothane. The body temperamre of saline-treated mice remained constant at about 38°C, except for a brief hypothermic episode from exposure to halothane. OFQ itself did not affect body temperature. However, both doses of OFQ significantly reversed 20 mg/kg MOR hypothermia at 60-90 minutes post-MOR injection ( 433 =44.94 ,p < .001). A reversal of hypothermia would be at least a 1 °C increase in temperature under these conditions. The dose of OFQ given in this Example is pharmaceutically sufficient to achieve this reversal. The high dose of moφhine (20 mg/kg) used in this experiment produced Straub tail, which is a muscular rigidity of the tail accompanying exposure to opiates in virtually all mice. Exposure to doses of at least 5 nmole OFQ effectively eliminated Straub tail in the vast majority of animals within 1-2 minutes following icv injection. The 5 nmole icv dose in the 15- 30g rat would be an example of a pharmaceutically sufficient amount to eliminate Straub tail in a majority of animals within 1-2 minutes following icv injection. This finding further generalizes OFQ's functional antagonism of opiate mechanisms. EXAMPLE 11 Precipitation of Opiate Withdrawal Syndrome in Morphine Dependent Mice Opiate tolerance, dependence and withdrawal can not be explained by simple alterations in opiate receptor function. Moφhine tolerance and addiction is believed to involve action of (until-now) ill-defined anti-opioid activities. Specifically, moφhine administration is believed to cause the release of anti-opioid peptides, which in turn may counteract the effects of moφhine and contribute to maintenance of physiologic homeostasis. An anti-opioid peptide should precipitate withdrawal symptoms by interfering with this homeostatic state. This Example illustrates that OFQ indeed causes an opiate withdrawal syndrome in moφhine dependent mice, further supporting its role as an anti-opioid. C57BL/6J mice of both sexes (15-25 g; The Jackson Laboratory; N=6-ll per group) were made dependent on moφhine using a sustained-release preparation. The preparation was 35 mg/ml moφhine sulfate in saline solution, vortexed with a 6: 1 light mineral oil; mannide monooleate (Arlacel A, Sigma) mixture in a 1:7 ratio of moφhine:oil solution. The viscous white emulsion was administered s.c. at a dose of 200 mg/kg moφhine (in an 8 ml/kg volume) using a 21-gauge needle. At the time of testing (5-6 h post-injection) moφhine was still present based on Straub tail and 49°C TW latency elevations. Mice were administered s.c. saline (Sal; 10 ml/kg) or NAL (10 mg/kg) followed immediately by i.c.v. vehicle (Veh; 2.5 μl artificial CSF) or OFQ (0.1-10 nmole in 2.5μl vehicle). Since all doses of OFQ used appeared to be equally effective in the precipitation of withdrawal, these data were combined. Each animal was then placed on individual 17 x 27 cm platforms suspended 32 sm above a table top. The number of jumps was recorded for 20 min, as were occurrences of tremor/"wet-dog shakes" and the presence of ptosis. OFQ doses >5 nmole produced atonia in some moφhine-dependent mice that may have hindered jumping behavior. Withdrawal jumping precipitated by OFQ is shown in Fig. 16. Bars represent mean (±SEM) number of withdrawal jumps in the 20-min testing period. Ratios above the error bars describe the number of animals exhibiting jumping behavior relative to the total number tested. The withdrawal syndrome produced by i.c.v. OFQ was unequivocal, albeit modest in comparison to that produced by systemic naloxone (NAL). Other withdrawal symptoms, including wet-dog shakes and ptosis, were observed in mice treated with OFQ, but not in mice merely given injections of vehicle. Precipitation of withdrawal shall be defined as increasing total withdrawal jumps in the protocol of Example 11. Example 12 Determination of Affinity of OFQ Receptor Agonists The peptide of the present invention does not bind with high specificity to μ, n or δ opioid receptors. This example will demonstrate that lack of receptor specificity, and demonstrate how specificity of ligand-receptor binding can be determined. [ 3 H]diprenoφhine was used as a competitive antagonist to assay receptor binding affinity for OFQ. Membranes from OFQR transfected CHO cells ((LC-7) were used at a concentration of lOOμg membrane protein/assay. Stable transfected-CHO cells were rinsed three times with 50 mM Tris-HCI buffer (pH 7.7), harvested and homogenized in the same buffer using a glass-Teflon homogenizer at 4°C. The homogenate was centrifuged at 42,000 g at 4°C for 15 minutes. The pellet was resuspended in 50 mM Tris-HCI buffer and incubated at room temperamre for one hour. Following pre-incubation,the homogenate was centrifuged as described above and the pellet was stored at 70°C until used. The pellet was thawed, washed with 50 mM Tris-HCI buffer and the homogenate was centrifuged as above. The pellet was resuspended in me binding buffer and the protein concentration was adjusted to lOOμg/μl as determined by a standard curve determined by Lowry assay using bovine serum albumin as the standard. In equilibrium competition experiments [ 3 H]diprenoφhine was utilized as an antagonist radioligand to label μ, K and δ receptors, respectively, in CHO cells expressing the μ, K or δ receptors, and OFQ was used as a displacer. In addition, DAMGO, U50488H and DPDPE were also used as a displacer in the μ, K and δ receptor expressed CHO cells in these assays. Binding reactions were conducted in an incubation tube with 50 mM Tris-HCI buffer (pH 7.7) in the presence of 10 micromolar peptidase inhibitor bestatin at 25 °C for 60 minutes in a total volume of 1 ml using a final [ 3 H]diprenoφhine concentration of 0.36 nM. All of the tabes contained the membrane preparation and [ 3 H]diprenoφhine. A ligand of interest was also added to each membrane/diprenoφhine preparation. A subset of the tabes contained 10 micromolar naloxone as the ligand of interest, to permit determination of non-specific binding. Another subset of the tabes contained OFQ in the presence of the [ 3 H]diprenoφhine to determine displacement of OFQ from each class of receptor by the diprenoφhine displacer. Another subset of tubes contained DPDPE, while yet another subset contained U50488H as die ligand of interest. Each of the ligands of interest was present in a concentration from zero in order of magnitude up to 100 micromolar. The concentration of radioactive diprenoφhine displacer was varied, in different sets of tabes, across the range shown in Fig. 17. At the end of die 60 minutes period of incubation, the incubation was terminated by vacuum filtration using a Brandel CellHarvester over polyethylenimine (0.5%)-soaked GF/B filters. The Cell Harvester has a manifold that simultaneously emptied out all of the reaction tabes in a controlled manner and deposits the contents on a filter disk that corresponds to each mbe, washed 2 times with 2 ml each of ice-cold 50 mM Tri-HCl buffer, and the contents again deposited on die corresponding filter disk. Filter disks were placed in minivials, allowed to elute overnight in EcoLume scintillation fluid (ICN, Costa Mesa, CA), and then counted in a Beckman scintillation counter. The resulting data were radioactive counts, which were proportional to percent total bound diprenoφhine radioligand. The percent bound ligand of interest (e.g., OFQ) was inversely proportional to the amount of radioactivity detected. As shown in Fig. 17, the percent of total OFQ at the μ, K and δ receptors was stable over a broad range of displacer concentrations. The DAMGO, DPDPE and U50488H ligands (known to bind to me μ, K and δ receptors) were, however, competitively displaced. For each set of ligand displacement studies, the results obtained with the scintillation counter were analyzed using an IBM computer system equipped with a GraphPad Inplot (from GraphPad, Inc., of San Diego, CA) software program. This program performs a nonlinear regression fit of a logistic equation to the data, and produced the graphs shown in Fig. 17. Using this data, the program further computes IC 50 (the concentration of unlabeled ligand, such as OFQ) that inhibits 50% of the radioactively labeled ligand binding, such as [ 3 H]diprenoφhine). IC 50 and Kj are related by the equation Kj = 10^(1 +[L]/K D ) where [L] is the concentration of labeled ligand diprenoφhine, and K D is defined as the dissocation constant for [ 3 H]diprenoφhine. IC 50 data are shown in the following Table 1: TABLE 1 Affinity of opioid receptor agonists and OFQ at mu, delta and kappa receptors expressed in CHO cells Compound IC J0 *(nM) DAMGO 2.0±0.1 DPDPE 0.59±0.01 U50488H 1.6±0.2 OφhanFQ 2083 ±432 2246±339 754 ±89 *IC JO values were derived from competition experiments using an opioid antagonist radioligand [ 3 H] diprenoφhine (0.36 nM). Data are mean SE values derived from three independent experiments. The ligand of interest has a low IC j o ( ≤ lOOnM) if it binds with high affinity to the receptor. All ligands of interest bound with high affinity to one of the opioid receptors, except for OFQ which had an ICso greater 2000 at μ and δ, and 754 at die K site. These values show that OFQ does not specifically bind to μ, K and δ receptors. As used herein, specifically binding to a receptor shall mean having an affinity expressed by an IC 50 of less than about lOOnm. In preferred embodiments, the affinity is 0.1-lOnm, preferably less than lnm, and most preferably less than O.lnm. Example 13 The affinity of any of the peptides of the present invention can be determined by an affinity study as described in Example 12. Hence, an OFQ analogue with amino acid substitutions can be substituted for the OFQ of Example 12. An OFQR transfected CHO cell membrane would be substituted for the μ, K and δ receptor expressing cell membranes of Example 12 to determine affinity of the peptide analogue to the OFQ receptor. This provides a straightforward screening method for determining whether the peptide binds specifically (K, or IC 50 < lOOnm) to the OFQR. Example 14 Protocol for Obtaining OFQ Anti-Opioid Antagonist and Using the Antagonist Antagonism of the OFQ/OFQR system will have substantial clinical utility. Although the antagonist will not have analgesic actions itself, it is expected to be effective as an adjunct to moφhine pharmacotherapy. By interfering with the body's homeostatic anti-opioid mechamsm, the OFQR antagonist is expected to allow use of lower therapeutic doses of moφhine to achieve greater analgesic effects. Lower doses of moφhine have the advantage of producing fewer side-effects (such as respiratory depression or constipation), and will also help avoid development of tolerance or addiction that can be experienced following prolonged use of high doses of moφhine. The OFQR antagonist is also expected to lessen me signs and symptoms of opiate withdrawal in addicts. Use of the antagonist in this manner has been made possible by die present inventors' recognition that the OFQ/OFQR interaction mediates an anti-opioid effect. Although a method of using an anti-opioid antagonist has not been known to the art, oligonucleotides have previously been reported tiiat antagonize the effect of OFQ. See Meunier et al., Nature 377:532-535 (1995), which is incorporated by reference. The oligonucleotides reported in that paper were antisense oligonucleotides to translated regions of the OFQR. Antisense oligonucleotide mAS[25,9] was the 17-mer 3'-GGAGAAAGGACGGGGTA-5' complementary to bases 9-25 of the translated region of the mouse ORL, mRNA. Control missense oligonucleotide hAS[25,9], was die 17-mer 3'-GGAGAAGGGGCGCGGCA-5' complementary to bases 9-25 of the translated region of the human ORL, mRNA. The oligonucleotides were dissolved in sterile physiological saline at the final concentration of 2 mg ml. Male Swiss CDI mice (20-25g; Charles River) were manually injected with 10μl of solution directly into the lateral brain ventrical every day for 4 consecutive days. Expression of the receptor was inhibited by the antisense oligonucleotides. Other oligonucleotides of similar length could be mapped to bases 9-25 (using the genetic code of Table 1) to provide similar antagonistic activity. Other antagonists are used in the present method to treat an opiate withdrawal syndrome. A heptadecapeptide is synthesized with a sequence that is substantially homologous to me OFQ amino acid sequence, but with one or more amino acid substitutions made. The synthesized heptadecapeptide (or other oligopeptide) is then screened using the cAMP accumulation assay in OFQ transfected CHO cells. Peptides that do not inhibit cAMP accumulation (using the assay described in Example 5) are then selected as a partial agonist that acts as an antagonist. The selected peptides obtained by this straightforward assay are then administered to animals (as in Example 11) to demonstrate attentuation of opate withdrawal, as evidenced by reduction in withdrawal jumping compared to animals not receiving the OFQ antagonist. Synthetic peptide combinatorial libraries may also be screened to find an agonist- antagonist peptide that does not inhibit cAMP accumulation in the disclosed assay. Screening is performed using the techniques described in Houghten et al., Generation and use of synthetic peptide combinatorial libraries for basic research and drug discovery, Nature 354:84-86, incoφorated by reference. The soluble, nonsupport bound peptide libraries appear useful in virtually all in vitro and in vivo assays, as reported by Ostresh, "Libraries from libraries: Chemical transformation of combinatorial libraries to extend the range and repertoire of chemical diversity," PNAS. The successful use of these libraries has been reported for the development of receptor-active opioid peptides. See Dooley, "An All D-Amino Opioid Peptide with Central Analgesic Activity from a Combinatorial Library," Science 266:2019-2022, 1994, which is incoφorated by reference. Antagonists discovered by these techniques are tested for interference with opiate withdrawal signs, as in Example 8. It should be understood that the foregoing disclosure emphasizes certain specific embodiments of the invention and that all modifications or alternatives equivalent thereto are within the spirit and scope of die invention as set forth in the appended claims. SEQUENCE LISTING (1) GENERAL INFORMATION (i) APPLICANTS: Grisel, Judith E. Grandy, David K. Mogil, Jeffrey S. Bunzow, James R. Civelli, Olivier Reinscheid, Rainer Klaus Nothacker, Hans-Peter Monsma, Frederick James (ii) TITLE OF INVENTION: Opioid Antagonists and Methods of Their Use (iii) NUMBER OF SEQUENCES: 10 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Klarquist Sparkman Campbell Leigh & Whinston LLP (B) STREET: 121 S.W. Salmon, Suite 1600 (C) CITY: Portland (D) STATE: Oregon (E) COUNTRY: USA (F) ZIP: 97204 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy Disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: Patentin Release #1.0, Version WP5.1 ASCII text (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (C) CLASSIFICATION: (vii) PRIOR APPLICATION DATA: (A) APPLICATION NUMBER: 08/533,058 (B) FILING DATE: 3 November 1995 (C) APPLICATION NUMBER: 08/514,541 (D) FILING DATE: 11 August 1995 (viii) ATTORNEY/AGENT INFORMATION (A) NAME: Richard J. Polley, Esq. (B) REGISTRATION NUMBER: 28,107 (C) REFERENCE/DOCKET NUMBER: 899-45532/RJP (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: (503) 226-7391 (B) TELEFAX: (503) 228-9446 (2) INFORMATION FOR SEQ ID NO: 1 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 31 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: (A) DESCRIPTION: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 1: ATGAATTCAC NRTSATGAGY GTSGACHGHT A 31 (3) INFORMATION FOR SEQ ID NO: 2 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 24 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: (A) DESCRIPTION: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: TTGTCGACRT ARRAGRAYNG GRTT 24 (4) INFORMATION FOR SEQ ID NO: 3 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 1452 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: (A) DESCRIPTION: cDNA (ix) FEATURE: (A) NAME/KEY: 5'UTR (B) LOCATION: 1..181 (ix) FEATURE: (A) NAME/KEY: CDS (B) LOCATION: 182..1282 (ix) FEATURE: (A) NAME/KEY: 3'UTR (B) LOCATION: 1283..1452 (xi) SEQUENCEDESCRIPTION: SEQID NO:3: CCGAGGAGCC ATTCCCAGCC GCAGCAGACC CCAATCTAGA GTGAGAGTCA TTGCTCAGTC 60 CACTGTGCTC CTGCCTGCCC GCCTTTCTGC TAAGCATTGG GGTCTATTTT GCGCCCAGCT120 TCTGAAGAGG CTGTGTGTGC CGTTGGAGGA ACTGTACTGA GTGGCTTTGC AGGGTGACAG180 C ATG GAG TCC CTC TTT CCT GCT CCA TAC TGG GAG GTC TTG CAT GGC 226 Met Glu Ser Leu Phe Pro Ala Pro Tyr Trp Glu Val Leu His Gly 1 5 10 15 AGC CAC TTT CAA GGG AAC CTG TCC CTC CTA AAT GAG ACC GTA CCC CAC 274 Ser His Phe Gin Gly Asn Leu Ser Leu Leu Asn Glu Thr Val Pro His 20 25 30 CAC CTG CTC CTC AAT GCT AGT CAC AGC GCC TTC CTG CCC CTT GGA CTC 322 His Leu Leu Leu Asn Ala Ser His Ser Ala Phe Leu Pro Leu Gly Leu 35 40 45 AAG GTC ACC ATC GTG GGG CTC ATC TTG GCT GTG TGC ATC GGG GGG CTC 370 Lys Val Thr Ile Val Gly Leu Ile Leu Ala Val Cys Ile Gly Gly Leu 50 55 60 CTG GGG AAC TGC CTC GTC ATG TAT GTC ATC CTC AGG ACA CCC AAG ATG 418 Leu Gly Asn Cys Leu Val Met Tyr Val Ile Leu Arg Thr Pro Lys Met 65 70 75 AAG ACA GCT ACC AAC ATT TAC ATA TTT AAT CTG GCA CTG GCT GAT ACC 466 Lys Thr Ala Thr Asn Ile Tyr Ile Phe Asn Leu Ala Leu Ala Asp Thr 80 85 90 95 CTG GTC TTG CTA ACA CTG CCC TTC CAG GGC ACA GAC ATC CTA CTG GGC 514 Leu Val Leu Leu Thr Leu Pro Phe Gin Gly Thr Asp Ile Leu Leu Gly 100 105 110 TTC TGG CCA TTT GGG AAA GCA CTC TGC AAG ACT GTC ATT GCT ATC GAC 562 Phe Trp Pro Phe Gly Lys Ala Leu Cys Lys Thr Val Ile Ala Ile Asp 115 120 125 TAC TAC AAC ATG TTT ACC AGC ACT TTT ACT CTG ACC GCC ATG AGC GTA 610 Tyr Tyr Asn Met Phe Thr Ser Thr Phe Thr Leu Thr Ala Met Ser Val 130 135 140 GAC CGC TAT GTG GCT ATC TGC CAC CCT ATC CGT GCC CTT GAT GTT CGG 658 Asp Arg Tyr Val Ala Ile Cys His Pro Ile Arg Ala Leu Asp Val Arg 145 150 155 ACA TCC AGC AAA GCC CAG GCT GTT AAT GTG GCC ATA TGG GCC CTG GCT 706 Thr Ser Ser Lys Ala Gin Ala Val Asn Val Ala Ile Trp Ala Leu Ala 160 165 170 175 TCA GTG GTT GGT GTT CCT GTT GCC ATC ATG GGT TCA GCA CAA GTG GAA 754 Ser Val Val Gly Val Pro Val Ala Ile Met Gly Ser Ala Gin Val Glu 180 185 190 GAT GAA GAG ATC GAG TGC CTG GTG GAG ATC CCT GCC CCT CAG GAC TAT 802 Asp Glu Glu Ile Glu Cys Leu Val Glu Ile Pro Ala Pro Gin Asp Tyr 195 200 205 TGG GGC CCT GTA TTC GCC ATC TGC ATC TTC CTT TTT TCC TTC ATC ATC 850 Trp Gly Pro Val Phe Ala Ile Cys Ile Phe Leu Phe Ser Phe Ile Ile 210 215 220 CCT GTG CTG ATC ATC TCT GTC TGC TAC AGC CTC ATG ATT CGA CGA CTT 898 Pro Val Leu Ile Ile Ser Val Cys Tyr Ser Leu Met Ile Arg Arg Leu 225 230 235 CGT GGT GTC CGT CTG CTT TCA GGC TCC CGG GAG AAG GAC CGA AAC CTG 946 Arg Gly Val Arg Leu Leu Ser Gly Ser Arg Glu Lys Asp Arg Asn Leu 240 245 250 255 CGG CGT ATC ACT CGA CTG GTG CTG GTA GTG GTG GCT GTG TTT GTG GGC 994 Arg Arg Ile Thr Arg Leu Val Leu Val Val Val Ala Val Phe Val Gly 260 265 270 TGC TGG ACG CCT GTG CAG GTG TTT GTC CTG GTT CAA GGA CTG GGT GTT 1042 Cys Trp Thr Pro Val Gin Val Phe Val Leu Val Gin Gly Leu Gly Val 275 280 285 CAG CCA GGT AGT GAG ACT GCA GTT GCC ATC CTG CGC TTC TGC ACA GCC 1090 Gin Pro Gly Ser Glu Thr Ala Val Ala Ile Leu Arg Phe Cys Thr Ala 290 295 300 CTG GGC TAT GTC AAC AGT TGT CTC AAT CCC ATT CTC TAT GCT TTC CTG 1138 Leu Gly Tyr Val Asn Ser Cys Leu Asn Pro Ile Leu Tyr Ala Phe Leu 305 310 315 GAT GAG AAC TTC AAG GCC TGC TTT AGA AAG TTC TGC TGT GCT TCA TCC 1186 Asp Glu Asn Phe Lys Ala Cys Phe Arg Lys Phe Cys Cys Ala Ser Ser 320 325 330 335 CTG CAC CGG GAG ATG CAG GTT TCT GAT CGT GTG CGG ACG ATT GCC AAG 1234 Leu His Arg Glu Met Gin Val Ser Asp Arg Val Arg Thr Ile Ala Lys 340 345 350 GAT GTT GGC CTT GGT TGC AAG ACT TCT GAG ACA GTA CCA CGG CCA GCA 1282 Asp Val Gly Leu Gly Cys Lys Thr Ser Glu Thr Val Pro Arg Pro Ala 355 360 365 TGACTAGGCG TGGACCTGCC CATGGTGCCT GTCAGCCCAC AGAGCCCATC CTACACCCAA342 CACGGAGCTC ACACAGGTCA CTGCTCTCTA GGTTGACCCT GAACCTTGAG CATCTGGAGI402 CTTGAATGGC TTTTCTTTTG GATCAGGATG CTCAGTCCTA GAGGAAGACC 1452 (5) INFORMATION FOR SEQ ID NO: 4 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 367 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: (A) DESCRIPTION: protein (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4 : Met Glu Ser Leu Phe Pro Ala Pro Tyr Trp Glu Val Leu His Gly Ser 1 5 10 15 His Phe Gin Gly Asn Leu Ser Leu Leu Asn Glu Thr Val Pro His His 20 25 30 Leu Leu Leu Asn Ala Ser His Ser Ala Phe Leu Pro Leu Gly Leu Lys 35 40 45 Val Thr Ile Val Gly Leu Ile Leu Ala Val Cys Ile Gly Gly Leu Leu 50 55 60 Gly Asn Cys Leu Val Met Tyr Val Ile Leu Arg Thr Pro Lys Met Lys 65 70 75 80 Thr Ala Thr Asn Ile Tyr Ile Phe Asn Leu Ala Leu Ala Asp Thr Leu 85 90 95 Val Leu Leu Thr Leu Pro Phe Gin Gly Thr Asp Ile Leu Leu Gly Phe 100 105 110 Trp Pro Phe Gly Lys Ala Leu Cys Lys Thr Val Ile Ala Ile Asp Tyr 115 120 125 Tyr Asn Met Phe Thr Ser Thr Phe Thr Leu Thr Ala Met Ser Val Asp 130 135 140 Arg Tyr Val Ala Ile Cys His Pro Ile Arg Ala Leu Asp Val Arg Thr 145 150 155 160 Ser Ser Lys Ala Gin Ala Val Asn Val Ala Ile Trp Ala Leu Ala Ser 165 170 175 Val Val Gly Val Pro Val Ala Ile Met Gly Ser Ala Gin Val Glu Asp 180 185 190 Glu Glu Ile Glu Cys Leu Val Glu Ile Pro Ala Pro Gin Asp Tyr Trp 195 200 205 Gly Pro Val Phe Ala Ile Cys Ile Phe Leu Phe Ser Phe Ile Ile Pro 210 215 220 Val Leu Ile Ile Ser Val Cys Tyr Ser Leu Met Ile Arg Arg Leu Arg 225 230 235 240 Gly Val Arg Leu Leu Ser Gly Ser Arg Glu Lys Asp Arg Asn Leu Arg 245 250 255 Arg Ile Thr Arg Leu Val Leu Val Val Val Ala Val Phe Val Gly Cys 260 265 270 Trp Thr Pro Val Gin Val Phe Val Leu Val Gin Gly Leu Gly Val Gin 275 280 285 Pro Gly Ser Glu Thr Ala Val Ala Ile Leu Arg Phe Cys Thr Ala Leu 290 295 300 Gly Tyr Val Asn Ser Cys Leu Asn Pro Ile Leu Tyr Ala Phe Leu Asp 305 310 315 320 Glu Asn Phe Lys Ala Cys Phe Arg Lys Phe Cys Cys Ala Ser Ser Leu 325 330 335 His Arg Glu Met Gin Val Ser Asp Arg Val Arg Thr Ile Ala Lys Asp 340 345 350 Val Gly Leu Gly Cys Lys Thr Ser Glu Thr Val Pro Arg Pro Ala 355 360 365 ( 6 ) INFORMATION FOR SEQ ID NO : 5 : ( i ) SEQUENCE CHARACTERISTICS : (A) LENGTH : 17 amino acids (B) TYPE : amino acid (D) TOPOLOGY : linear (ii) MOLECULE TYPE : (A) DESCRIPTION : peptide (xi) SEQUENCE DESCRIPTION : SEQ ID NO : 5 : Phe Gly Gly Phe Thr Gly Ala Arg Lys Ser Ala Arg Lys Leu Ala Asn 1 5 10 15 Gin (7) INFORMATION FOR SEQ ID NO: 6: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 17 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: (A) DESCRIPTION: peptide (xi) SEQUENCE DESCRIPTION: SEQ ID NO: 6: Phe Gly Gly Phe Thr Gly Ala Arg Lys Ser Ala Arg Lys Tyr Ala Asn 1 5 10 15 Gin (8) INFORMATION FOR SEQ ID NO. (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 17 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: peptide (xi) SEQUENCE DESCRIPTION: SEQ ID NO. Tyr Gly Gly Phe Leu Arg Arg Ile Arg Pro Lys Leu Lys Trp Asp Asn Gin 1 5 10 15 (9) INFORMATION FOR SEQ ID NO. 8: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 16 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: peptide (xi) SEQUENCE DESCRIPTION: SEQ ID NO. 8 Tyr Gly Gly Phe Met Thr Ser Glu Lys Ser Gin Thr Pro Leu Val Thr 1 5 10 15 (10) INFORMATION FOR SEQ ID NO : 9 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 13 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: peptide (xi) SEQUENCE DESCRIPTION: SEQ ID NO. 9 Tyr Gly Gly Phe Leu Arg Arg Gin Phe Lys Val Val Thr 1 5 10 (11) INFORMATION FOR SEQ ID NO: 10 (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 5 amino acids (B) TYPE: amino acid (D) TOPOLOGY: linear (ii) MOLECULE TYPE: peptide (xi) SEQUENCE DESCRIPTION: SEQ ID NO. 10 Tyr Gly Gly he Leu 1 5

Claims: hat we claim is: 1. A method of antagonizing a physiological effect of an opioid in an animal, comprising administering to the animal a pharmaceutically effective amount of a peptide that has an activity of binding to a mammalian OFQ receptor with high specificity. 2. The method of claim 1 wherein the peptide has an FGGF motif. 3. The method of claim 1 wherein the peptide does not bind to mu, delta, or kappa opioid receptors with high specificity. 4. The method of claim 1 wherein the peptide antagonizes opioid analgesia without increasing nociceptive sensitivity. 5. The method of claim 1 wherein the mammalian OFQ receptor is encoded by the DNA molecule set forth in SEQ. ID. No. 3. 6. A method of antagonizing physiologic effects of an opioid in an animal, comprising the step of administering to the animal an amount of an anti-opioid peptide sufficient to reduce a physiological effect of the opioid, wherein the peptide: (a) has an FGGF motif; (b) binds with high specificity to a receptor encoded by SEQ. ID No. 3; (c) does not bind to mu, delta or kappa opioid receptors with high specificity; and (d) antagonizes opioid analgesia without increasing nociceptive sensitivity. 7. The method of claim 6, wherein the peptide antagonizes opiate induced hypothermia in animals. 8. The method of claim 6, wherein the peptide is administered to the animal to antagonize morphine induced analgesia. 9. The method of claim 6 wherein the FGGF motif is an amino-terminal motif. 10. The method of claim 6, wherein the peptide is a heptadecapeptide. 11. The method of claim 1, wherein the peptide is of the formula: (Xaa) a -Phe-Gly-Gly-Phe- (A 1 ) - (A 2 ) -A 3 ) - (A 4 ) - (A s ) - (A 6 ) - (A 7 ) - (A 8 ) - (A 9 ) - (A 10 ) - (A 11 ) - (A 12 ) -Gin- (Xaa) m wherein A 1 is Thr, Leu or Met; A 2 is Gly, Arg or Thr; A 3 is Ala , Arg or Ser ; A 4 is Arg, Ile , Glu or Gin; A 5 is Lys , Arg or Phe ; A 6 is Ser , Pro or Lys ; A 7 is Ala, Lys , Gin or Val ; A 8 is Arg, Leu, Thr or Val ; A 9 is Lys , Pro or Thr ; A 10 is Tyr , Leu or Trp ; A 11 is Ala, Asp or Val; A 12 is Asn, or Thr; and (Xaa) is any amino acid; n and m are integers wherein n + m is no more than 82,- and wherein the amino acids are each individually in either the D or L stereochemical configuration. 12. The method of claim 11 wherein the peptide is selected from the group consisting of amino acid sequences selected from the group consisting of Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg- Lys-Leu-Ala-Asn-Gln and Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala- Arg-Lys-Tyr-Ala-Asn-Gln. 13. The method of claim 1, wherein the peptide is an analogue, derivative or mimetic of the peptide of claim 11. 14. The method of claim 6, further comprising administering to the animal a second anti-opioid antagonist, wherein the second anti-opioid antagonist and anti-opioid peptide are administered in a sufficient amount in combination to antagonize physiological effects of the opioid. 15. A method of antagonizing a physiologic effect of morphine on an animal, comprising the step of administering to the animal, in a sufficient amount to diminish the physiologic effect, a peptide having an amino acid sequence consisting of Phe-Gly-Gly-Phe- Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln. 16. A pharmaceutical dosage form of the peptide of claim 1. 17. The pharmaceutical dosage form of claim 16, comprising the peptide and a pharmaceutical carrier. 18. The pharmaceutical dosage form of claim 16, further comprising instructions for administering the pharmaceutical dosage form to an animal to diminish opioid intoxication. 19. A method of antagonizing opioid effects caused by exogenous administration of an opioid in an animal, comprising administering to the animal a pharmaceutically effective amount of an anti-opioid peptide sufficient to reduce analgesia induced by the exogenous opioid, wherein the anti-opioid peptide: (a) is a heptadecapeptide having an aminoterminal FGGF motif; (b) binds to a receptor encoded by SEQ. ID. No. 3 with an IC S0 of lOOnm or less; (c) does not bind to mu, delta or kappa opioid receptors with an IC 50 of lOOnm or less; (d) antagonizes opioid analgesia without increasing nociceptive sensitivity; and (e) antagonizes morphine induced hypothermia in animals when given at a pharmaceutically effective dose to inhibit a morphine induced hypothermic response. 20. The method of claim 19 wherein the exogenous opioid is morphine. 21. A peptide of formula: (Xaa) n -Phe-Gly-Gly-Phe- (A 1 ) - (A 2 ) - (A 3 ) - (A 4 ) - (A 5 ) - (A 6 ) - (A 7 ) - (A 8 ) - (A 9 ) - (A 10 ) - (A 11 ) - (A 12 ) -Gin- (Xaa) m wherein A 1 is Thr, Leu or Met; A 2 is Gly, Arg or Thr; A 3 is Ala, Arg or Ser; A 4 is Arg, Ile, Glu or Gin; A 5 is Lys, Arg or Phe; A 6 is Ser, Pro Lys; A 7 is Ala, Lys, Gin or Val; A 8 is Arg, Leu, Thr or Val; A 9 is Lys, Pro or Thr; A 10 is Tyr, Leu or Trp; A 11 is Ala, Asp or Val; A 12 is Asn, or Thr; and (Xaa) is any amino acid; n and are integers wherein n + m is no more than 82; and wherein the amino acids are each individually in either the D or L stereochemical configuration and wherein the peptide specifically binds to a mammalian opioid receptor having an amino acid sequence identified by Seq ID No.:4. 22. A peptide according to claim 21 having the formula Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln, wherein n + m equals zero. 23. A peptide according to claim 21 having the formula Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Tyr-Ala-Asn-Gln, wherein n + m equals zero. 24. The peptide of claim 23 that is radiolabeled with a radioisotope of iodine. 25. A peptide according to claim 21 that is radiolabeled with a radioactive isotope. 26. A pharmaceutical composition comprising a peptide according to claims 21, 22, 23, 24 or 25 and a pharmaceutically- acceptable carrier or diluent. 27. A pharmaceutical composition of claim 26 that is a diagnostic agent. 28. A pharmaceutical composition of claim 26 that is a therapeutic agent. 29. A method of diagnosing a locomotor disease in an animal, comprising the step of administering a diagnostically effective amount of the pharmaceutical composition of claim 27 to the animal. 30. A method of treating locomotor disease in an animal, comprising the step of administering a therapeutically effective amount of the pharmaceutical composition of claim 28 to the animal. 31. A method of designing an MSOR opioid receptor ligant comprising the step of preparing an analogue, derivative or mimetic of the peptide of claim 21. 32. An analogue, derivative or mimetic of an MSOR opioid receptor prepared by the method of claim 31. 33. A method of screening a compound for a capacity to bind to a mammalian MSOR opioid receptor in cells expressing the receptor, the method comprising the steps of: (a) transforming a culture of eukaryotic or prokaryotic cells with a recombinant expression construct capable of expressing a mammalian MSOR opioid receptor having an amino acid sequence identified as SEQ ID No. :4, wherein the cells of the transformed cell culture express the opioid receptor; (b) assaying the transformed cell culture for binding of an amount of a detectably-labeled peptide according to claim 21 in competition with varying amounts of the compound; and (c) determining whether the compound competitively binds to the opioid receptor by calculating the extend of inhibition of binding of the detectably-labeled peptide in the presence of the compound. 34. A method of claim 33 comprising the additional step of: (d) comparing the binding capacity of the compound with the binding capacities of additional compounds that are known to bind to mammalian opioid receptors, wherein said additional compounds comprise naturally-occurring and synthetic opioid receptor agonists and antagonists.

Inventors: BUNZOW JAMES R, CIVELLI OLIVIER, GRANDY DAVID K, GRISEL JUDITH E, MOGIL JEFFREY S, MONSMA FREDERICK JAMES, NOTHACKER HANS-PETER, REINSCHEID RAINER KLAUS, BUNZOW, JAMES, R., CIVELLI, OLIVIER, GRANDY, DAVID, K., GRISEL, JUDITH, E., MOGIL, JEFFREY, S., MONSMA, FREDERICK, JAMES, NOTHACKER, HANS-PETER, REINSCHEID, RAINER, KLAUS
Applicants: UNIV OREGON HEALTH SCIENCES, OREGON HEALTH SCIENCES UNIVERSITY
Application year: 1996
Main classification: C12N15
IPC codes: A61K38, A61K51, A61P25, A61P39, C07K14, C12N5, C12N15