GLUTAMATE RECEPTOR COMPOSITIONS AND METHODS

Abstract: The present invention discloses novel DNAs that encode proteins having electrophysiological and pharmacological properties characteristic of glutamate receptors. The glutamate receptors are exemplified by proteins encoded by representative cDNA clones GluR1, GluR2, GluR3, GluR4, GluR5, GluR6 and GluR7, fragments thereof, and functional combinations of these glutamate receptor proteins and/or fragments. DNA sequences from the cDNA clones for GluR1, GluR2, GluR3, GluR4 and GluR5 are especially useful as probes, thus enabling those skilled in the art to identify, without undue experimentation, other members of the L-glutamate receptor family.

Description: GLUTAMATE RECEPTOR COMPOSITIONS AND METHODS Field of the Invention The present invention relates to a family of novel DNA sequences and encoded receptors proteins that comprise the glutamate neurotransmitter system. The invention also relates to methods for making the glutamate receptors and for using the receptor proteins in assays designed to identify and characterize glutamate agonists and antagonists. Background of the Invention The amino acid L-glutamate is a major excitatory neurotransmitter in the mammalian central nervous system. Anatomical, biochemical and electrophysiological analyses suggest that glutamatergic systems are involved in a broad array of neuronal processes, including fast excitatory synaptic transmission, regulation of neurotransmitter release, long-term potentiation, learning and memory, developmental synaptic plasticity, hypoxic-ischemic damage and neuronal cell death, epileptiform seizures, as well as the pathogenesis of several neurodegenerative disorders. See generally, Monaghan et al . , Ann. Rev. Pharmacol . Toxicol . 29, 365-402 (1989). This extensive repertoire of functions, especially those related to learning, neurotoxicity and neuropathology, has stimulated recent attempts to describe and define the mechanisms through which glutamate exerts its effects. Currently, glutamate receptor classification schemes are based on pharmacological criteria which serve to define five receptor subtypes or classes: those activated by N-methyl-D-aspartic acid (NMDA) , kainic acid (KA) , c_-amino-3-hydroxy-5-methyl- isoxazole-4-propionic acid (AMPA, formerly called the quisqualic acid or QUIS receptor) , 2-amino-4- phosphonobutyric acid (AP4 or APB) , and 1-amino- cyclopentyl-l,3-dicarboxylic acid (ACPD) . The effects of glutamate are mediated primarily through interactions with cation-selective, ionotropic receptors (Foster and Fagg, Brain Res. Rev. 7, 103-164 (1984); Strange, Biochem. J. 249, 309-318 (1988)). An exception is the ACPD receptor subtype which has the properties of a metabotropic receptor. This class of glutamate receptors alters synaptic physiology via GTP-binding proteins and the second messengers diacylglycerol and inositol 1,4,5-triphosphate (Gundersen, et al . , Proc. R. Soc. London Ser. B 221, 127 (1984); Sladeczek, et al . , Nature 317, 717 (1985); Nicoletti, et al . , J. Neurosci. 6, 1905 (1986); Sugiyama, et al . , Nature 325, 531 (1987)). The electrophysiological and pharmacological properties of the glutamate receptors have been studied extensively and are now well established. See for example, Foster and Fagg, Brain Res. Rev. 7, 103 (1984); Cotman et al. , Trends Neurosci. 10, 263 (1987); Mayer and Westbrook, Prog. Neurobiol . 28, 197 (1987) ; atkins and Olvermann, Trends Neurosci. 10, 265 (1987); and Blair et al . , Science 242, 577 (1988). This is in contrast to their biochemical characteristics and struc- ture at the molecular level, which, until the teaching of the present invention, remained largely unknown. Definitions In the present specification and claims, reference will be made to phrases and terms of art which are expressly defined for use herein as follows: As used herein, glutamate receptors refer to neurotransmitter receptor proteins that are activated by L-glutamate and related compounds. These receptor proteins are classified on the basis of their "pharmacology". Currently there are five classes of receptors: those activated by N-methyl-D-aspartate (NMDA) , kainic acid (KA) , α-amino-3-hydroxy-5-methyl- isoxazole-4-propionic acid (AMPA, formerly called the quisqualic acid or QUIS receptor) , 2-amino-4- phosphonobutyric acid (AP4 or APB) , and 1-amino- cyclopentyl-l,3-dicarboxylic acid (ACPD). As used herein, NMDA means N-methyl- D-aspartate, which is a ligand (agonist) for the NMAD glutamate receptor subtype. As used herein, AMPA means α-amino-3- hydroxy-5-methyl-isoxazole-4-propioniate, which is a ligand (agonist) for the AMPA glutamate receptor subtype. The AMPA receptor was formerly called the QUIS (quisqualate) receptor. As used herein, QUIS means quisqualic acid or quisqualate, which is a ligand (agonist) for the pharmacologically defined receptor subtype previously referred to as the QUIS (quisqualate) receptor. The receptor previously referred to as the QUIS receptor is now referred to as the AMPA receptor. As used herein, KA means kainic acid or kainate, which is a ligand (agonist) for the kainate glutamate receptor subtype. As used herein, APB means 2-amino-4- phosphonobutyrate, which is a ligand (agonist) for the APB glutamate receptor subtype. The acronym AP4 is also sometimes used to refer to the 2-amino-4- phosphonobutyrate glutamate receptor subtype. As used herein, ACPD means 1-amino- cyclopentyl-l,3-dicarboxylic acid, which is a ligand (agonist) for the ACPD glutamate receptor subtype. As used herein, when used as a modifier of glutamate receptor protein(s) of the present invention, the phrase "having electrophysiological and pharmaco¬ logical properties characteristic of a glutamate receptor" means that the neuronal signal(s) generated by receptor protein in response to glutamate or glutamate- like ligands will be comparable to those of known glutamate receptors. See also, the definition for "functional", which is given below. As used herein, homomeric means receptors comprised of only one type of subunit. As used herein, heteromeric means receptors comprised of more than one type of subunit. As used herein, functional, when used as a modifier of glutamate receptor protein(s) of the present invention, means that binding of glutamate (or glutamate-like) ligand to receptor protein(s) causes membrane "ion channels" to open. This allows ions to move across the membrane, which in turn depolarizes the cell and generates a neuronal signal. Stated another way, functional means that a neuronal signal is generated as a consequence of ligand binding to receptor protein(s) . As used herein, GABA means 7 -aminobutyric acid. As used herein, γ-DGG means 7 -D-glutamyl- glycine. As used herein, GAMS means 7 -D-glutamylamino- methyl-sulphonate. As used herein, PDA means 2,3-cis-piperidine dicarboxylic acid. As used herein, GDEE means glutamate diethyl- ester. As used herein, TMDs mean transmembrane domains. As used herein, APV means 2-amino-5-phospho- novaleric acid. As used herein, CPP means 3-(2-carboxypiper- azin-4- 1)propy1-1-phosphate. As used herein, nAChRs mean neuronal nicotinic acetylcholine receptors. As used herein, CNS means central nervous system. As used herein, PNS means peripheral nervous system. As used herein, an agonist binding subunit is a receptor subunit that contains a binding site for a pharmacological compound which, upon binding, activates the receptor. As used herein, a non-agonist binding subunit is a receptor subunit that does not bind agonists. As used herein, the term antagonist refers to a substance that interferes with receptor function. Antagonists are of two types: competitive and non- competitive. A competitive antagonist (also known as a competitive blocker) competes with an agonist for overlapping binding sites. A non-competitive antagonist or blocker inactivates the functioning of the receptor by binding to a site on the receptor other than the agonist binding site. As used herein, GluRl refers to the cDNA clone which encodes a single glutamate receptor subunit protein of the same name having a M_ = ca. 99,769 daltons. GluRl was the first glutamate receptor subunit encoding cDNA to be isolated; it was previously referred to as GluR-Kl. GluR-Kl has been renamed glutamate receptor subunit gene 1 or, more simply, GluRl. Additional glutamate receptor subunits or subunit related genes are called GluR2, GluR3, GluR4, GluR5, GluR6, GluR7 and so forth. GluRl cDNA has been deposited with the American Type Culture Collection; it has been accorded ATCC No. 68134. As used herein, GluR2 refers to a cDNA clone which encodes a single glutamate receptor subunit protein of the same name having a M p = ca. 96,400 daltons. GluR2 cDNA has been deposited with the American Type Culture Collection; it has been accorded ATCC No. 68132. As used herein, GluR3 refers to a cDNA clone which encodes a single glutamate receptor subunit protein of the same name having a M p = ca. 98,000 daltons. GluR3 cDNA has been deposited with the American Type Culture Collection; it has been accorded ATCC No. 68133. As used herein, GluR4 refers to a cDNA clone which encodes a single protein of the same name having a M_ = ca. 98,500. GluR4 cDNA has been deposited with the American Type Culture Collection; it has been accorded ATCC No. 68375. As used herein, GluR5 refers to a GluR5 cDNA clone which encodes a single protein of the same name having a M p = ca. 100,000. G1UR5 cDNA (as GluR5-l) has been deposited with the American Type Culture Collection; it has been accorded ATCC No. 68374. (There are two length variants of GluR5 cDNA, referred to herein as GluR5-l and GluR5-2. Translation of the GluR5 cDNAs predicts a single long open reading frame of 920 amino acids. The difference between GluR5-l and GluR5-2 DNA derives from an insertion of 45 nucleotides (15 amino acids) in the GluR5-l DNA which does not interrupt this reading frame. The 15 amino acid insertion in the GluR5-l receptor protein is unique among the receptor proteins disclosed herein; thus the shorter GluR5-2 variant is the counterpart of the GluRl, GluR2, GluR3, GluR4, GluR6 and GluR7 subunits.) As used herein, GluR6 refers to a cDNA clone which encodes a single protein of the same name having a M_ = ca. 100,000. As used herein, GluR7 refers to a cDNA clone which encodes a single protein of the same name. GluR7 encoding fragments 35-T3 and U35 are shown in Figure 12. As used herein, GluRl, GluR2, GluR3, GluR4, GluR5,GluR6 and GluR7 are each used interchangeably to refer to genes, cDNA clones and the glutamate receptor proteins they encode. Use of the phrase "substantial sequence homology" in the present specification and claims means that the DNA, RNA or amino acid sequences which have slight and non-consequential sequence variations from the actual sequences disclosed and claimed herein are considered to be equivalent to the sequences of the present invention, and as such are within the scope of the appended claims. In this regard, "slight and non- consequential sequence variations" mean that "homologous" sequences (i.e., the sequences that have substantial sequence homology with the DNA, RNA, or proteins disclosed and claimed herein) will be functionally equivalent to the sequences disclosed and claimed in the present invention. Functionally equivalent sequences will function in substantially the same manner to produce substantially the same compositions as the nucleic acid and amino acid compositions disclosed and claimed herein. Use of the phrase "substantially pure" in the present specification and claims as a modifier of DNA, RNA, polypeptides or proteins means that the DNA, RNA, polypeptides or proteins so designated have been separated from their in vivo cellular environment by means of human effort; as a result of this separation and purification, the substantially pure DNAs, RNAs, polypeptides and proteins are useful in ways that the non-separated, impure DNAs, RNAs, polypeptides or proteins are not. The amino acids which comprise the various amino acid sequences appearing herein may be identified according to the following three-letter or one-letter abbreviations: The nucleotides which comprise the various nucleotide sequences appearing herein have their usual single-letter designations (A, G, T, C or U) used routinely in the art. As used herein, the words "protein", "peptide" and polypeptide" are considered to be equivalent terms and are used interchangeably. As used herein, bp means base pairs. Kbp means kilo base pairs, or base pairs times 1000. As used herein, all temperatures are given in degrees Centigrade unless indicated otherwise. Deposits cDNA clones encoding-representative glutamate receptor protein subunits of the present invention have been deposited with the American Type Culture Collection, Rockville, Maryland, U.S.A. (ATCC) to provide those skilled in the art with the best mode of making and using the probes necessary to identify additional members of the L-glutamate family of genes. The deposits have been made under the terms of the Budapest Treaty on the International Recognition of Deposits of Microorganisms for Purposes of Patent Procedure and the Regulations promulgated under this Treaty. Samples of the cloned DNA sequences are and will be available to industrial property offices and other persons legally entitled to receive them under the terms of the Treaty and Regulations and otherwise in compliance with the patent laws and regulations of the United States of America and all other nations or inter¬ national organizations in which this application, or an application claiming priority of this application, is filed or in which any patent granted on any such application is granted. The ATCC Deposit Numbers and Deposit Dates for the deposited representative cDNA clones are as follows: GluRl Clone ATCC No. 68134, October 19, 1989 GluR2 Clone ATCC No. 68132, October 19, 1989 GluR3 Clone ATCC No. 68133, October 19, 1989 GluR4 Clone ATCC No. 68375, August 2, 1990 GluR5 Clone ATCC No. 68374, August 2, 1990 Summary of the Invention The present invention discloses a family of novel glutamate receptor proteins and DNA sequences that encode them. The glutamate receptors of the invention have electrophysiological and pharmacological properties characteristic of glutamate receptors of the central nervous system. The glutamate receptors of the present invention are exemplified by cation-selective ion channel-type proteins encoded by cDNA clones GluRl, GluR2, GluR3, GluR4, GluR5, GluR6 and GluR7. In addition to being useful for the production of glutamate receptor proteins, these cDNAs are also useful as probes, thus enabling those skilled in the art, without undue experimentation, to identify and isolate additional proteins in the glutamate receptor family. The novel functional glutamate receptors of the present invention can be assembled from either individual GluR subunits (homomeric) or from combinations of subunit polypeptides (heteromeric) . GluRl, G1UR2, GluR3, GluR4 and GluR5 are examples of presently preferred subunits for forming homomeric receptors, while the combinations of GluRl, GluR2, GluR3, GluR4 and GluR5 are examples of presently preferred subunits for forming heteromeric receptors. In addition to disclosing novel DNAs and proteins the present invention also comprises methods for making glutamate receptors and using them to identify and characterize agonists and antagonists of glutamate receptor function. The invention also comprises methods for determining whether unknown protein(s) are functional as glutamate receptors. Description of the Invention The present invention discloses a family of glutamate receptors and novel DNA sequences that encode these receptors. More specifically, in one aspect, the present invention comprises substantially pure functional proteins, or functional fragments thereof, or functional combinations of these proteins and/or fragments wherein the proteins or fragments or functional combinations have electrophysiological and pharmacological properties characteristic of a glutamate receptor. In another aspect, the invention comprises substantially pure functional proteins, or functional fragments thereof, or functional combinations of these proteins and/or fragments, wherein the proteins or functional fragments or combinations have electrophysio¬ logical and pharmacological properties characteristic of a glutamate receptor subtype selected from the group consisting of N-methyl-D-aspartate (NMDA) , α-amino-3- hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) , kainate (KA) and 2-amino-4-phosphonobutyrate (APB) . In another aspect, the invention comprises substantially pure functional proteins, or functional fragments thereof, or functional combinations of these proteins and/or fragments, wherein the proteins or functional fragments have electrophysiological and pharmacological properties characteristic of KA and/or AMPA glutamate receptor subtypes. In still another aspect, the invention comprises substantially pure proteins, or fragments thereof, or functional combinations of the proteins and/or the fragments, wherein the proteins or fragments or functional combinations have electrophysiological and pharmacological properties of a glutamate receptor and are encoded by DNA having at least about 40% homology with at least one member of the group consisting of GluRl DNA, G1UR2 DNA, GluR3 DNA, GluR4 DNA, GluR5 DNA, G1UR6 DNA and G1UR7 DNA. In still another aspect, the invention comprises substantially pure proteins, or fragments thereof, or functional combinations of the proteins and/or the fragments, wherein the proteins or fragments or functional combinations have electrophysiological and pharmacological properties of a glutamate receptor and have at least about 40% amino acid homology with at least one member of the group consisting of GluRl (Fig. 1), G1UR2 (Fig. 3), G1UR3 (Fig. 4), G1UR4 (Fig. 9), GluR5 (Fig. 10), GluR6 (Fig.11) and GluR7 (Fig. 12). In another aspect, the invention comprises substantially pure protein selected from the group of proteins consisting of GluRl, GluR2, GluR3, GluR4, GluR5, GluR6 and GluR7, and combinations thereof wherein said combinations are functional as glutamate receptor(s) . In another aspect, the invention comprises a substantially pure proteins having M_s of about 99,769 (GluRl), 94,400 (GluR2) , 98,000 (GluR3) , 98,500 (GluR4) , 100,000 (GluR5) and 100,000 (GluR6) that form ion channels which possess the electrophysiological and pharmacological properties characteristic of glutamate receptors of the KA and/or AMPA subtypes. In yet another aspect the invention comprises substantially pure functional proteins having substantial sequence homology with the substantially pure functional proteins of the invention. The present invention also includes substantially pure DNA encoding functional proteins, or functional fragments thereof, having electrophysio¬ logical and pharmacological properties characteristic of glutamate receptors. In another aspect, the invention comprises substantially pure DNA encoding functional proteins or functional fragments having electrophysiological and pharmacological properties characteristic of a glutamate receptor subtype selected from the group consisting of N-methyl-D-aspartate (NMDA) , <_-amino-3-hydroxy-5-methyl- isoxazole-4-propionic acid (AMPA) , kainate (KA) and 2-amino-4-phosphonobutyrate (APB) . Still further the invention comprises DNA encoding KA and/or AMPA glutamate receptors. In another aspect, the invention comprises substantially pure DNA selected from the group consisting Of GluRl DNA, GluR2 DNA, G1UR3 DNA, GluR4 DNA, GluR5 DNA, GluR6 DNA and GluR7 DNA. Still further the invention comprises substantially pure DNA having at least about 40% homology with at least one member of the group consisting of GluRl DNA, GluR2 DNA, GluR3 DNA, GluR4 DNA, GluR5 DNA, GluR6 DNA, and GluR7 DNA. In another related aspect, the invention comprises DNA encoding proteins having at least about 40% homology with at least one member of the group consisting of GluRl (Fig. 1) , GluR2 (Fig. 3) , GluR3 (Fig. 4), GluR4 (Fig. 9), GluR5 (Fig. 10), GluR6 (Fig. 11), and G1UR7 (Fig. 12). The invention also comprises substantially pure DNA encoding substantially pure proteins having M r s of about 99,769 (GluRl), 96,400 (GluR2) , 98,000 (GluR3) , 98,500 (G1UR4) , 100,000 (GluR5) and 100,000 (GluR6) that form ion channels which possess the electrophysiological and pharmacological properties characteristic of a glutamate receptor of the KA and/or AMPA subtypes. The invention further comprises substantially pure DNA that is functionally equivalent to any of the substantially pure DNA of the invention wherein functionally equivalent means that the substantially pure DNA will encode proteins, or functional fragments thereof, which will form ion channel(s) in response to ligands for glutamate receptors. In another aspect, the invention comprises substantially pure sense or antisense mRNA transcribed from the DNAs of the invention wherein the DNAs encode substantially pure functional proteins that have electrophysiological and pharmacological properties characteristic of a glutamate receptor. In another aspect, the. invention comprises cDNA clones that encode the amino acid sequences of functional peptides having electrophysiological and pharmacological properties characteristic of a glutamate receptor. Representative clones have been deposited with the American Type Culture Collection to provide to those skilled in the art necessary starting materials (i.e., probes) to identify and isolate additional members of the glutamate receptor family. The representative cDNA clones include: GluRl (ATCC No. 68134), GluR2 (ATCC No. 68132), GluR3 (ATCC No. 68133), GluR4 (ATCC No. 68375), an GluRδ (ATCC No. 65374) . Either the full length cDNA clones or fragments thereof can be used as probes. When fragments are used as probes, preferably the DNA sequences will be from the carboxyl encoding portion of the DNA, and most preferably will include ion channel encoding portions of the DNA sequences. In another aspect, the invention comprises functional peptide fragments, and functional combinations thereof, encoded by the DNAs of the invention. Such functional peptide fragments can be produced by those skilled in the art, without undue experimentation, by eliminating some or all of the amino acids in the sequence not essential for the peptide to function as a glutamate receptor. A determination of the amino acids that are essential for glutamate receptor function is made, for example, by systematic digestion of the DNAs encoding the peptides and/or by the introduction of deletions into the DNAs. The modified (e. g. , deleted or digested) DNAs are expressed, for example by transcribing the DNA and then introducing the resulting mRNA into Xenopus oocytes, where translation of the mRNAs will occur. Functional analysis of the proteins thus expressed in the oocytes is accomplished by exposing the oocytes to ligands known to bind to and functionally activate glutamate receptors, and then monitoring the oocytes to see if the expressed fragments form ion channel(s). If ion channel(s) are detected, the fragments are functional as glutamate receptors. In still another aspect, the invention comprises cells transformed with DNAs of the invention. In still another aspect, the invention comprises Xenopus oocytes to which mRNA of the invention has been introduced, e. g. , by injection. Still further, the invention comprises novel glutamate receptors made by expression of DNA sequences of the invention, or translation of the corresponding mRNAs. Such novel receptors include the individual GluRl, GluR2, G1UR3, GluR4, GluR5, GluR6 and GluR7 receptors, fragments thereof plus functional combinations of the receptors or fragments. Still further, the invention comprises DNA, RNA and proteins that are functionally equivalent to the DNAs, RNAs and proteins of the present invention. Such functionally equivalent DNAs, RNAs and proteins will function in substantially the same manner as the DNAs, RNAs and proteins of the invention. In addition to DNA, RNA and protein compositions of matter, the invention comprises methods. A first method of the invention is a method for identifying DNA that is homologous to DNA known to encode glutamate receptor protein(s) . This method comprises contacting an "unknown" or test sample of DNA with a glutamate receptor DNA probe (e. g. , GluRl, GluR2, GluR3, G1UR4, GluR5, GluR6, GluR7, etc. ) under low and/or high stringency hybridization conditions, and then identifying as glutamate receptor homologous DNA that "unknown" or test DNA which hybridizes with the glutamate probe DNA. A second method of the invention is directed to identifying functional glutamate receptors (i.e. , glutamate receptors that form ion channels) . This method comprises contacting proteins, preferably in an oocyte expression system, with at least one ligand known to activate glutamate receptors, measuring ion channel response to the ligand(s) , and identifying as functional glutamate receptor(s) those proteins which exhibit an ion channel response as a consequence of the contact. A third method of the invention is a method for determining whether a substance is a functional ligand for glutamate receptor protein. According to this method, proteins known to function as glutamate receptors are contacted with an "unknown" or test substance, the ion channel activity of the known glutamate receptor is monitored subsequent to the contact with the "unknown" or test substance, and those substances which effect an ion channel response in the known glutamate receptor(s) are identified as functional ligands for receptor proteins. Turning now to some of the specific DNAs of the invention, cDNA clone GluRl was isolated from a rat forebrain cDNA library by screening for expression of kainate-gated ion channels in Xenopus oocytes. An insert from clone GluRl was used as a probe to screen cDNA brain libraries (first under low-stringency hybridization conditions and then under higher stringency conditions) in order to find cDNA clones that encode other members of the glutamate receptor family. Use of the GluRl probe cDNA led to the identification and isolation of the GluR2 and GluR3 clones. A probe from GluR2 was used to identify and isolate clones GluR4 and GluR5, and GluR5 was used to isolate clones for GluR6 and GluR7. cDNA clone GluRl encodes a functional glutamate receptor subunit which consists of a single protein of about M r = 99,769, before any post-translational modifications. This protein forms an ion channel which possesses the electrophysiological and pharmacological properties of KA and AMPA receptors. The proteins encoded by the GluRl, GluR2, GluR3, GluR4, GluR5, GluR6 and GluR7 genes exhibit considerable inter-subunit amino acid sequence identity. See Table 2. These proteins have been found to form distinct homomeric and heteromeric KA/AMPA-sensitive ion channels in Xenopus oocytes. For example, single protein subunits of glutamate receptors, GluRl, GluR2, GluR3, GluR4 and GluR5 are sufficient to form homomeric functional receptor ion-channel complexes activated by KA, AMPA, QUIS but not by NMDA and APB. While GluR2 subunits can form functional homomeric complexes, this subunit more efficiently assembles receptor-ion channel complexes in heteromeric combination with GluRl or GluR3 subunits. The GluR5 protein forms in Xenopus oocytes a homomeric ion channel which is weakly responsive to glutamate but not to N-methyl-D-aspartate, kainate, quisqualate and 2-amino-4-phosphonobutyrate. The fact that oocytes expressing GluR5 are responsive to L- glutamate but not to KA, quisqualate or AMPA may indicate that this protein can participate in the formation of receptors with a different pharmacological profile than the KA/AMPA subunits. This suggestion is supported by in situ hybridization data. During embryonic and postnatal development the GluR5 gene is expressed in subsets of neuronal cells in the CNS and PNS and the spatial and temporal expression pattern of the GluR5 gene is largely overlapping with the KA/AMPA subunit GluR4. However, in adult brains, the GluR5 gene is expressed in a pattern distinct from those of the KA/AMPA subunit genes, consistent with the suggestion that GluR5 represents a subtype of glutamate receptors different from the KA/AMPA receptors. Without further elaboration, it is believed that one of ordinary skill in the art can, using the preceding description, and the following Examples and Detailed Descriptions of the Drawings, utilize the present invention to its fullest extent. The material disclosed in the examples, unless otherwise indicated, is disclosed for illustrative purposes and therefore should not be construed as being limiting in any way of the appended claims. Examples Example 1 Production of Brain cDNA Libraries Poly(A) + RNA is purified by the guanidine thiocyanate-CsCl method (Chirgwin et al . , Biochem. 18, 5294 (1979) from various regions of the brain, eg. , mammalian brain, or a suitable cell line, eg. , the NCB- 20 cell line. The purified RNA is used as a template to prepare double strand cDNA. A poly-dT primer linked to a Xho I restriction site is used as a primer to prime the Moloney reverse transcriptase for the synthesis of the first strand using 5-methyl dCTP instead of cCTP as a precursor. RNase H and DNA polymerase I are added to complete the second strand. The cDNA is blunt-ended with T4 DNA polymerase which increases the chance of making a full-length cDNA. Eco RI adapters are ligated to the blunt-end and the ends are kinased. The cDNA is then digested with the Xho I restriction enzyme. This enzyme cannot cleave hemimethylated DNA so it only cleaves the un-methylated Xho I restriction site that had been attached to the dT-primer at the 3' end of the mRNA. The resulting double stranded cDNA has a Xho I restriction site at the 3' end of the mRNA and an Eco RI site at the 5' end. This cDNA is then placed into an appropriate vector, such as λZAP vector, which is part of the λZAP-cDNA cloning system from Statagene. When the λZAP vector is used, the cDNA is placed into the vector so that the 5' end of the mRNA is near the lacZ promoter. (The λZAP vector has a T3 RNA polymerase promoter at one end of the cDNA insert and a T7 RNA polymerase promoter at the other end which makes it possible to synthesize either sense or antisense RNA for further experiments, including expression in oocytes. Example 2 Low and High Stringency Hybridization cDNA libraries are preferably made with the λZAP-cDNA system described in Example 1. A hybridiza- tion probe is preferably made from DNA obtained from Glul, GluR2, G1UR3, GluR4, GluR5, GluR6 or GluR7 clones or another suitable clone or source. The DNA is labeled by the random prime method, preferably using the Amersham Multiprime DNA labeling kit. Preferably, low stringency hybridization conditions are used, at least at first. A suitable hybridization solution is as: (1 M Nacl, 50 mM Tris [pH 8.0], 0.5% SDS, 0.1% sodium pyrophosphate, 100 mg/ml denatured herring sperm DNA, 0.1% [w/v] each of Ficoll, polyvinylpyrrolidone and bovine serum albumin) . For low stringency screening, a temperature of 50° C. is preferable, and library filters are washed in 2 X SSPE (1 X SSPE is 180 mM NaCl, 9 mM Na 2 HP0 4 , 0.9 mM NaH 2 P0 4 and 1 mM EDTA, pH 7.4) at room temperature and exposed to Kodak XAR-5 film at -70° C. For high stringency screening, the temperature is adjusted to 65° C. and the filters are washed at this temperature in 0.2X SSPE containing 0.5% sodium dodecyl sulfate. Filters are exposed to Kodak XAR-5 film with Cronex Quanta II/III intensifying screens at -70° C. for 18-48 hr. Under these low-stringency hybridization conditions, about one in two thousand brain cDNA clones show some hybridization to the probe made from the GluRl cDNA insert. Example 3 Analysis of Clones Identified by Hybridization At least two different approaches can be used to analyze clones that are identified by low-stringency hybridization screening. One approach is to pick positive λZAP clones and pool them into mixtures of about 100 clones. mRNA is made in vitro from these pools of λZAP clones and the mRNA is injected into oocytes in order to test the ability of the mRNA to direct synthesis of functional glutamate receptors. If a positive clone is found, the individual λZAP cDNA clone is isolated by subdividing the pool until the functional clone is isolated. (See Example 5, below, for a discussion of how this approach was used to isolate the GluRl clone.) A second approach that can be used to evaluate positive clones is to analyze each insert individually. Although this is tedious, the "individual clone" approach has the advantage that initially it does not require functional expression. When the "individual clone" approach is used, each clone is plaque purified and the cDNA insert is analyzed individually. This is facilitated by the fact that, at least in the λZAP cDNA system, the cDNA is cloned into a cassette flanked by the bacteriophage fl origin of replication. The cDNA is contained within a pBluescript plasmid which can be rescued from the λZAP bacteriophage by helper infection. (Once this is done the cDNA is in the small pBluescript plasmid which is much easier to work with than the much larger λbacteriophage.) Sense or antisense RNA is made from the cDNA insert in the pBluescript plasmid using either the T3 (sense) or T7 (anti-sense) promoter. The cDNA insert is also preferably mapped with restriction enzymes. For example, the cDNA inserts are cut with frequent cutting restriction enzymes. The resulting fragments are size fractionated on a gel and the fragments transferred to a filter for Southern blot analysis. The filters are hybridized with a probe made from known glutamate receptors, eg. , GluRl, GluR2, GluR3, GluR4, GluR5, GluR6 or GluR7. The hybridizing fragments from each clone are subcloned into the single- stranded vector M13, (mplδ or mpl9) , and the fragment sequenced. DNA sequencing is performed using the dideoxynucleotide chain termination method of Sanger et al . , Proc. Natl . Acad. Sci . USA 74, 5463 (1977), or an automatic sequencer such as one manufactured by Applied Biosystems. The sequence is preferably analyzed by computer using software such as the programs developed by Intelligenetics, Staden and the University of Wisconsin. mRNA made from full-length or nearly full- length clones are expressed in the oocyte system and the functional properties of the new receptors are characterized. If a clone is not functional when expressed by itself, it is tested in the presence of mRNA made from other candidate clones. Example 4 Expression Cloning and Assay in Xenopus Oocytes This assay is an adaptation of the assay of Masu et al . , Nature 329, 836 (1987). It depends upon the fact that when foreign mRNA is injected into Xenopus oocytes, the mRNA is translated into functional protein. DNA templates, either a λZAP cDNA preparation or a plasmid containing the cDNA to be tested, are cut downstream from the cDNA insert with a restriction enzyme. The post-restriction digest is digested with Proteinase K and then extracted with two phenol: chloroform (1:1) extractions. The DNA template is then ethanol precipitated. The template is mixed with either T3 RNA polymerase, to make sense strand, or T7 RNA polymerase, to make anti-sense strand, plus rATP, rCTP, rGTP, rUTP, and RNase inhibitor. Simultaneously, the RNA transcripts are capped with a 7meGpppG cap. Throughout the procedure gloves are worn and the water is treated with diethylpyrocarbonate to inactivate RNases. The in vitro synthesized mRNA transcripts are injected into Xenopus oocytes. 50 nl of an RNA solution, 0.2-0.6 mg/ml, is injected into the stage V oocytes. The injected oocytes are incubated at 16° C. in Barth's medium for 2-7 days before they are analyzed for the presence of functional receptors. Voltage recordings are made by penetrating the oocyte with a micro-electrode filled with 3M KCL and connected to the bridge circuit of an appropriate voltage clamp unit, eg. , the Dagan 8500 voltage clamp unit. Voltage clamp studies are preferably carried out with two electrodes, a voltage electrode filled with 3M KCl and a current electrode filled with 0.25 M CsCl, 0.25 M CsF and 50 mM EGTA. (See Example 6, below, for a discussion of results of recordings from oocytes injected with RNA from GluRl cDNA coding for a KA/AMPA glutamate receptor.) Before injection the oocytes are prepared by taking ovaries from anesthetized adult female Xenopus . The ovary tissue is treated with collagenase, 2 mg/ml, for two hours and then the ovarian epithelium and follicular cells are dissected away. Example 5 Expression Cloning of the GluRl Receptor Xenopus oocytes were injected with poly(A) + RNA isolated from rat forebrain. 2-10 days later, the oocytes were tested electrophysiologically for their ability to respond to selective agonists for glutamate receptor subtypes. Both glutamate and quisqualate remove induced depolarizations displaying a biphasic pattern composed of a fast acting, smooth, presumably ligand-gated ion channel response, and a longer lasting, fluctuating, probably second-messenger mediated response. NMDA and KA elicited smooth responses with fast onsets. APB gave no response. A directional cDNA library (λZAPII RTB1; complexity: 8 x 10 5 elements) , consisting of 18 independent sublibraries of 44,000 clones each, was constructed from this poly(A) + RNA using the bacteriophage expression vector λZAPII. A pool of in vitro transcripts, comprised of transcripts made separately from all 18 amplified sublibraries, was injected into oocytes. Small depolarizations (1-3 mV) were seen in voltage recordings from oocytes challenged with lOOμM kainate 10 days after injection. No responses to NMDA or quisqualate were detected. Neither uninjected oocytes nor water-injected oocytes showed any responses to glutamate receptor agonists. Subsequently, pools of 44,000 clones (= the single sublibraries), 4000, 400 and 40 clones were tested. In each of these tests at least one pool responded to KA. The following criteria were used throughout the screening procedure to assure that the very small responses observed initially were not recording artifacts: (a) responses in a given oocyte were reproducible, (b) responses were fast (within one second of agonist application) , (c) responses were readily reversible upon superfusion of the oocyte with control Ringer solution, (d) lOμM domoate gave a response similar to the one elicited by lOOμM kainate. The pools yielding the largest responses at each stage were selected for further subdivision. The clones in the final positive pool of 40 clones were analyzed for their insert size, and the 12 clones with the largest inserts (all > 2 kb) were tested individually for their ability to direct the synthesis of a functional kainate receptor. Only one clone, carrying a 3.0 kb insert, was found to elicit kainate responses and was named λZAPII-GluRl. Example 6 Electrophysiological and Pharmacological Characterization of the GluRl Clone The plasmid pGluRl was subsequently rescued from bacteriophage λZAPII-GluRl, and upon transcription in vitro sense RNA but not antisense RNA induced kainate responses when injected into oocytes. As little as lOpg of GluRl sense transcript under voltage-clamp conditions (-70mV holding potential) gave detectable responses to lOOμM KA. In order to rule out the possibility that GluRl codes for a transcription factor, oocytes injected with pGluRl in vitro transcripts were kept in medium supplemented with 50μg/ml actinomycin D to inhibit endogenous transcription. These oocytes exhibited the same responses to kainate as those kept in control medium. Therefore, injection-induced transcription from the oocyte genome seems unlikely to contribute to the observed responses. L-glutamate evoked much smaller responses than did KA, and even at lmM elicited only 50% of the depolarization seen with 30μM KA. This is consistent with previous reports that glutamate is only a weak agonist for the KA receptor subtype (Monaghan et al . , Nature 306, 176 (1983)). Other glutamate receptor agonists such as NMDA, quisqualate and L-aspartate evoked no responses when applied at 150μM, lOμM, and lOOμM, respectively. Unrelated neurotransmitter receptor agonists such as glycine, GABA A , serotonin and nicotine also failed to evoke responses, even when tested at concentrations as high as lmM. Glycine did not potentiate the KA response. Dose-response curves for KA and domoate were recorded and EC 50 values of 39μM and 1.8μM, respectively, were derived. The average reversal potential extrapolated from current responses to lOμM kainate obtained at a series of holding potentials between 0 and -130mV was lOmV. Responses to KA did not desensitize, not even after prolonged (up to 10 min) superfusion with high concentrations (lOOμM) of the agonist. These findings are also consistent with earlier reports from experiments with KA receptors expressed from total pol (A) + RNA (Verdoorn & Dingledine, Molecular Pharmacology 34, 298 (1988)). Similarly, the pharmacological profile of GluRl (see Table 1) , as revealed by the inhibiting properties of various known glutamate receptor antagonists, is consistent with previous reports for KA receptors in systems where total poly(A) + RNA was used as a source of kainate receptor message (Hirono et al . , Neurosci . Res. 6, 106 (1988); Lerma et al . , Proc. Natl . Acad. Sci . USA 86, 2083 (1989)). Of all the compounds tested (each at lmM), the broad specificity glutamate receptor antagonist kynurenic acid clearly was the most potent inhibitor of the kainate-evoked depolarizations in oocytes injected with GluRl RNA synthesized in vitro. To a lesser extent, 7 -D-glutamylglycine ( 7 -DGG) , reported to preferentially block KA and NMDA receptors but not quisqualate receptors (Davies and Watkins, Brain Res. 206, 172 (1981)), inhibited kainate responses, as did 7 -D-glutamylaminomethyl-sulphonate (GAMS) , which reportedly prefers KA and AMPA receptors (Fagg, Trends Neurosci . 8, 207 (1985)). Similarly, 2,3-cis-piperidine dicarboxylic acid (PDA) , which is known to block all subtypes of glutamate receptors (Foster and Fagg, Brain Res. Rev. 7, 103 (1984)), blocked the GluRl response. Glutamate diethylester (GDEE) , which is thought to preferentially inhibit AMPA-type glutamate receptors (Foster and Fagg, Brain Res. Rev. 7, 103 (1984)), did not block the response significantly, but instead displayed weak agonist properties. The NMDA receptor antagonists 2-amino-5-phosphonovaleric acid (APV) and 3-(2-carboxypiperazin-4-yl)propyl-l-phosphate (CPP) , as well as NMDA itself, slightly inhibited the KA responses, thus acting as weak antagonists. They did not show any agonist properties. Taken together, the electrophysiological properties observed in oocytes injected with GluRl transcript, as well as the observed pharmacological properties, indicate that GluRl represents a functional KA receptor indistinguishable from the one observed in oocytes injected with total poly(A) + RNA. Thus, the single protein subunit encoded by GluRl is sufficient to form an active receptor-ion channel complex. Example 7 Seguencing and Primary Structure of GluRl The cDNA insert of the plasmid pGluRl was subcloned into M13mpl9 and sequenced (Fig. 1) . An open reading frame of 2721 bp was found within the total length of 2992 bp. Thus, the predicted protein consists of 889 amino acids, with a calculated M r of about 99,769. The deduced protein sequence contains a putative signal peptide of 18 amino acids at its N-terminus, with a predicted cleavage site that conforms to empirical rules (von Heijne, Nucl . Acids Res. 14, 4683 (1986)). The N-terminus of the protein therefore is expected to be located extracellularly. There are 6 possible N-glycosylation sites present in the presumed extracellular domain. Sequence comparisons with other sequenced ligand-gated ion channels, the nicotinic acetylcholine receptors, GABA A receptors and glycine receptors, reveals little over-all homology. All ligand-gated ion channel subunits sequenced previous to this disclosure have a conserved extracellular region characterized by 2 cysteine residues spaced 14 amino acids apart from each other, with conserved proline and aspartate residues located 8 and 10 amino acids, respectively, downstream from the first of these two cysteines (Barnard et al . , Trends Neurosci . 10, 502 (1987)). This hypothetical signature for neurotransmitter receptor-channel complexes is poorly conserved in the protein encoded by GluRl. The proline and aspartate residues are present, but the first cysteine residue is located only 7 residues upstream from the proline residue, and the second cysteine residue is absent. A hydropathy plot analysis of GluRl revealed several regions which are candidates for transmembrane domains (TMDs) . The region between amino acids 455 and 810 was notable because its hydropathy profile resembled that seen in the other ligand-gated ion channels: three closely spaced putative TMDs which are separated by ca. 175 amino acid residues from a fourth putative TMD which is located close to the C-terminus of the protein. Within this region, the following four transmembrane regions are assigned in GluRl: TMD I, located between amino acid residues 463 and 480, TMD II between residues 521 and 539, TMD III between residues 596 and 614, and TMD IV between residues 788 and 808. Example 8 Isolation and Characterization of GluR2 and GluR3 cDNA clones encoding the GluR2 and GluR3 genes were isolated from an adult rat forebrain library using a low-stringency hybridization screening protocol (see Example 2) and a radiolabeled fragment of the GluRl cDNA as probe. Figures 3 and 4 show the nucleotide and derived amino acid sequence of clones ARB14 (GluR2) and ARB312 (GluR3) . The calculated molecular weights for the mature, non-glycosylated forms of GluR2 and GluR3 are 96,400 daltons (862 amino acids) and 98,000 daltons (866 amino acids) , respectively. Potential N-linked glycosylation sites occur in the GluR2 protein at Asn- 235, Asn-349, Asn-385, Asn-392, and Asn-835 and in the GluR3 protein at Asn-35, Asn-238, Asn-352, Asn-387, and Asn-394. Like GluRl, the hydrophobicity profile for both GluR2 and GluR3 reveals five strongly hydrophobic regions: one such domain is located at the amino terminus of each protein and has characteristics of a signal peptide, while four additional hydrophobic regions probably form membrane-spanning helices (MSR I- IV) and are located in the carboxy-terminal half of each polypeptide. Figure 5 is an alignment of deduced amino acid sequences for the proteins encoded by the GluRl, GluR2 and GluR3 (and GluR4 and GluR5) genes. As the aligned sequences demonstrate, there is significant sequence identity between GluRl and both GluR2 (70%) and GluR3 (69%) as well as between GluR2 and GluR3 (74%) . (Also see Table 2) . The sequence identity is most pronounced in the carboxy-terminal half of each protein. Example 9 Electrophysiological Comparison: GluRl, GluR2 and GluR3 The strong sequence similarity between the proteins encoded by GluRl, GluR2 and GluR3 suggested that, as with GluRl, the GluR2 and GluR3 proteins might function as homomeric, kainate-sensitive ion channels in Xenopus oocytes. To test this hypothesis, oocytes were injected with in vitro synthesized RNA transcripts derived from individual cDNA clones. Figure 6A shows that both GluR2 and GluR3 injected oocytes depolarize in response to bath application of lOOμM KA. The amplitudes of the KA responses were not equivalent for the three glutamate receptor subunits: with equal amounts of injected RNA (2 ng) , responses in GluR3 RNA-injected oocytes were invariably larger than GluRl responses. KA-invoked depolarizations in GluR2- injected oocytes were the weakest and could only be detected in oocytes injected with much larger amounts of RNA (10 to 25 ng) . The data in Figure 6B show that, in addition to KA, oocytes injected with GluRl or GluR3 RNA also respond to QUIS (lOμM) , AMPA (50μM) , glutamate (GLU, 100 μM) . No detectable responses were obtained with NMDA (30μM plus lOμM glycine) or APB (50μM) . Responses obtained from oocytes injected with GluR2 RNA were too small for reproducible quantitation and were, therefore, excluded from the analysis. For GluRl-injected oocytes the responses to AMPA and QUIS were typically 35-40% of the maximal KA response, while for GluR3 they were about 10% of the KA response. Relative to KA, the response of GluRl to domoic acid (DOM, lOμM) is about 6-fold greater than that seen for GluR3. Taken together these data demonstrate that receptors assembled from GluRl or GluR3 subunits are pharmacologically distinct. Furthermore, the observation that homomeric GluRl and GluR3 receptors respond to both QUIS and AMPA, albeit with reduced efficiencies, provides direct evidence that KA, QUIS and AMPA can bind to the same glutamate receptor poly¬ peptide. Figure 6B also shows that the pharmacological profile of oocytes injected with individual GluRl or GluR3 subunit RNA is significantly different than that seen in oocytes injected with rat brain hippocampus poly(A) + RNA. This finding suggests that the responses seen in oocytes injected with hippocampus RNA are mediated by heteromeric glutamate receptors assembled from various combinations of GluRl, GluR2 and GluR3 subunit polypeptides. This suggestion is supported by the fact that all three GluR subunit genes are actively transcribed in the hippocampus. Example 10 Pharmacological Comparison of GluRl. GluR2 and GluR3 This example addresses the question of whether glutamate receptors assembled from mixtures of the proteins encoded by the GluRl, GluR2 and ,GluR3 subunit genes have pharmacological properties significantly different from each other or from those observed for single subunit receptors. A comparison of the data in Figure 6B and 7B suggests that, for the agonists tested, there are few substantial differences in the pharmacology. However, the responses to QUIS, AMPA and GLU are, relative to GluRl, significantly reduced in the oocytes expressing the subunit combinations. Except for the NMDA response, the overall agonist profiles for oocytes injected with GluR subunit combinations are more similar to oocytes containing hippocampus pol (A) + RNA than to those injected with either GluRl or GluR3 subunit RNA alone. Example 11 Comparison of KA-Activated Currents Recorded from GluRl, GluR2 and GluR3 Figure 7A compares KA-activated currents recorded from oocytes injected with mixtures of GluRl, GluR2 and GluR3 subunit (stippled columns) with the summed currents measured for the individual subunits (blank columns) . The principle finding is a significant potentiation of KA-evoked currents in oocytes coexpressing GluRl plus GluR2 subunits (approximately 4- fold increase over the summed responses for singly- injected oocytes) or GluR2 plus GluR3 subunits (approximately 2-fold increase) . Injection of all three subunit RNAs results in an average 2.5 fold-increase in KA-evoked currents. These results support the conclusion that in oocytes individual glutamate receptor subunit polypeptides do not behave in a simple independent fashion but rather interact with each other. It is probable that such interaction results in the generation of heteromeric glutamate receptors with properties which are distinct from the receptors comprised of solitary glutamate.receptor subunits. Example 12 Current-Voltage Relationships for KA-Evoked Responses This example examines the current-voltage (I/V) relationships for KA-evoked responses measured in oocytes injected with individual GluR subunits, combinations thereof or hippocampus pol (A) + RNA. The I/V data are illustrated in Figure 8 where oocytes injected with individual glutamate receptor subunit RNAs are shown in panel A, oocytes injected with combinations of subunits are shown in panels B and C, and for purposes of comparison, oocytes expressing hippocampus poly(A) + RNA are also shown in panel A. The KA responses measured in oocytes injected with brain poly(A) + RNA show an approximately linear I/V relationship with a reversal potential of about -lOmV. This result is in marked contrast to the I/V curves obtained for oocytes injected with single GluRl or GluR3 subunit RNA. The GluRl and GluR3 I/V curves show strong inward rectification and reversal potentials near -40mV. From these data it is clear that the KA-sensitive receptors present in oocytes injected with hippocampus RNA are different from those assembled by oocytes injected with GluRl or GluR3 subunit RNAs. In Figure 8B the I/V curve for the GluRl plus GluR2 combination is noticeably different from that observed for the GluRl subunit alone. Oocytes injected with this pair of RNAs show a nearly linear I/V plot and have a reversal potential of approximately -lOmV. This plot is strikingly similar to that seen with hippocampus RNA-injected oocytes (panel A) . In contrast, the I/V curve for the GluRl plus GluR3 combination is only marginally different from those measured in oocytes expressing the individual subunits. Figure 8C shows some inward rectification in the I/V curve for the GluR2 plus GluR3 subunit combination, as well as a reversal potential somewhat more negative (-20mV) than those determined for GluRl plus GluR2 or hippocampus RNA I/V plots (-lOmV) . When all three glutamate receptor subunit RNAs are combined in a single oocyte, the resulting I/V curve approximates that seen for the GluRl plus GluR2 combination in both reversal potential and slope; however, the responses with the three subunits show a pronounced inward rectification not observed with GluRl plus GluR2. Example 13 Distribution of GluRl, GluR2 and GluR3 mRNA In The Mammalian Central Nervous System The hypothesis that proteins encoded by the GluRl, GluR2 and GluR3 genes assemble to form heteromeric glutamate receptors in vivo can be disproved by a showing that the individual subunit genes are transcribed in different neuroanatomical loci. Therefore, the distribution of GluRl, GluR2 and GluR3 RNAs in the adult rat brain was examined using radiolabeled anti-sense RNA probes and in situ hybridization histochemistry essentially as described by Deneris, et al . , J. Biol . Chem. 264, 6268 (1989). The results indicated that the hybridization patterns obtained with the GluRl, GluR2 and GluR3 probes were nearly identical, with the strongest hybridization seen in the CA1-CA3 regions of the hippocampus and the dentate gyrus. High-resolution analysis of these areas suggests that the hybridization signal originates in the pyramidal cell layer of regions CA1-CA3 and the granule cell layer of the dentate gyrus. Somewhat weaker hybridization of all three probes was seen in the piriform cortex, caudate-putamen, amygdala, and hypothalamus. Low levels of hybridization were detected in the thalamus, with little or no signal observed in fiber tracts. While differential hybridization was seen in the medial habenula and neocortex, the overall patterns of expression for the GluRl, GluR2 and GluR3 subunit genes showed substantial concordance. Example 14 Isolation of cDNAs for GluR4 and GluR5 Using a fragment of the GluR2 cDNA (nucleotides 1793-2240) as probe and a low stringency hybridization protocol, several GluR4 and R5 clones were isolated from a rat forebrain library. Sequence analysis demonstrated that none of the cDNA clones contained an entire open reading frame. Northern blots with mRNA from different adult rat brain tissues indicated that the GluR4 and GluR5 transcripts were most abundant in the cerebellum. Consequently, the partial GluR4 and GluR5 cDNA clones were used as probes under high stringency screening conditions to isolate cDNAs encoding large open reading frames from an adult rat cerebellum cDNA library. Of the cDNA clones thus isolated, two GluR4- related clones (ACER112 and ACER121B) encoded only portions of the GluR4 gene but possessed sufficient overlap to engineer a full-length, expressible construct in the pBS SK (+) vector (Stratagene Cloning Systems) . The nucleotide sequence of this GluR4 construct, designated pK45, was determined and the deduced amino acid sequence is presented in Figure 9. Among the 29 GluR5-related cDNAs isolated from the cerebellum library, three clones, specified ARB12, ARB15 and ΛRB20, were identified which encode an identical large open reading frame. The sequence of cDNA clone ARB20 (GluR5-l) is shown in Figure 10. ARB15 and ARB12 are shorter than ARB20 at the 5' end. The ARB20 cDNA consists of a 187 bp 5' untranslated region, a continuous open reading frame of 2760 bp, and a 3' untranslated region of 303 bp. The 5' untranslated region ends with the sequence AAGATGG which is characteristic of a translational start site. Three additional cDNA clones originally isolated from the forebrain library were also examined. The sequences of these cDNAs are identical to ARB20 in the predicted translation initiation site region. Sequence analysis revealed that two variants of the GluR5 cDNA are represented in the forebrain and the cerebellum libraries. This heterogeneity derives from the insertion of 45 nucleotides found in ARB20 and 17 of the 29 GluR5-related cDNA clones isolated (Figure 10) . This insertion does not interrupt the open reading frame. Furthermore, consensus splice donor and acceptor sites are absent (Breathnach and Chambon, Ann. Rev. Biochem. 50, 349-383 (1981)), which suggests that the insertion does not arise from an unspliced intron and is, most likely, the result of an alternative splice event. Nucleotide sequence analysis indicates that the two GluR5 variants are otherwise identical. No cDNA clone was found for the shorter splice variant encoding the entire open reading frame. Therefore a clone (ARBΔ20) was constructed that is missing the 45 nucleotide insertion found in ARB20 (GluR5-l) but is otherwise identical to that clone. The shorter splice variant clone (ARBΔ20) is referred to as GluR5-2. Both ARB20 and ARBΔ20 were used in Xenopus oocyte expression experiments (see Example 16) and the variant proteins encoded were named GluR5-l and GluR5-2, respectively. The GluR4 mRNA was detected on Northern blots of cerebellum RNA as a 4.5 kb species. The smaller size mRNA may represent splice variants. Northern blot analysis also indicated that the major GluR5 mRNA has a size of 6 kilobases. Example 15 Structural Features of GluR5 cDNA and Protein Translation of the cDNA nucleotide sequence for GluR5-l predicts a single long open reading frame of 920 amino acid residues (Figure 10) . The GluR5 sequence has overall amino acid sequence identity with each of the KA/AMPA subunits (Figure 5, and Table 2). The 15 amino acid insertion in GluR5-l is unique among the proteins listed, thus the shorter GluR5-2 variant is the counterpart to the KA/AMPA subunits characterized. Table 2 shows that GluR5 is thus far the most dissimilar glutamate receptor subunit identified and the comparison of GluR5 with the KA/AMPA subunits highlights the most conserved sequence elements (Figure 5) . Within other ligand-gated ion channel families, the neuronal acetylcholine receptor, the GABA A receptor and the glycine receptor families, the N-terminal extracellular domain is most conserved while the C-terminal sequences diverge between the membrane-spanning regions (MSR) III and IV. In the glutamate receptor subunit gene family, in contrast, the regions N-terminal of the proposed MSR I (Hollmann, et al . , Nature 342, 643 (1989)), have only 17% identity and are less similar than the regions C-terminal of MSR I which have 45% identity. The 'Cys-Cys loop 1 , a signature for ligand-gated neurotransmitter receptor channel complexes (Barnard, et al . , Trends Neurosci . 10, 502 (1987)) is not conserved in the glutamate receptor subunit family (Figure 5) . The C-terminal half of glutamate receptor subunits are suggested to be involved in channel formation and contain the postulated membrane spanning helices (MSR I- IV; Figure 5) (Hollmann, et al . , Nature 342, 643 (1989)). The presumed MSR III is the most conserved continuous sequence, with only one conservative amino acid exchange (Val to Ile) in the GluR5 protein (Figure 5) . As mentioned above, in other ligand-gated channel families the segment between MSR III and IV is divergent in length and sequence. In the glutamate receptor subunit family the similarity in this postulated segment is high (48%) and only GluR5 exhibits a sequence length variation. The KA/AMPA receptors and the GluR5 protein are generally divergent C-terminal of the proposed MSR IV. The hydrophobicity plot for GluR5 is similar to those of the KA/AMPA receptors, suggesting a conserved secondary structure in the proposed ion channel forming portion of the protein. However, the N- terminal half of the GluR5 hydrophobicity plot is unusual. In this region, GluR5, as compared to the KA/AMPA subunits, is more hydrophobic and contains several segments that could span the membrane. Based on algorithms that search for membrane-associated helices, four (Rao and Argos, Biochim. Biophys. Acta 869, 197-214 (1986)) or seven (Eisenberg, et al . , J. Mol . Biol . 179, 125-142 (1984)) putative transmembrane regions can be assigned to GluR5. A comparison of the C-terminal regions of all five glutamate receptor subunits with the frog (Gregor, et al . , Nature 342, 689 (1989)) and chicken (Wada, et al . , Nature 342, 684 (1989)) KA-binding proteins demonstrates a similar extent of sequence conservation (35-40% amino acid identity) . A FASTA search (Pearson and Lipman, Proc. Natl . Acad. Sci . 85, 2444 (1988)) of the GenBank, EMBL and SWISS-PROT databases with the GluR5 sequence uncovered no significant similarities to other proteins. Example 16 Electrophysiological Properties of GluR5 mRNA Injected Oocvtes Exposed to L-Glutamate In vitro synthesized GluR5-l and GluR5-2 cRNAs were individually expressed in Xenopus oocytes. With either cRNA, the glutamate receptor agonists KA (lOOμM) , AMPA (50 μM) , quisqualate (10 μM) , APB (lOOμM) and NMDA (100μM, applied with lOμM glycine) did not elicit membrane depolarizations in cRNA injected oocytes. However, weak membrane depolarizations induced by L- glutamate (100 μM) were recorded in oocytes injected with GluR5-l cRNA (maximal depolarization 3.5 mV) and GluR5-2 cRNA (maximal depolarization 4.5 mV) . Significantly stronger membrane depolarizations were not found in response to L-glutamate in oocytes co-injected with GluR5-l and GluR5-2 cRNA as compared to oocytes injected with GluR5-2 cRNA alone. For any particular oocyte injected with GluR5-l or GluR5-2 cRNA, the depolarizations were reproducible, showed fast onset, and were slowly (within 5 min) reversed when agonist superfusion was switched to buffer superfusion. Neither un-injected oocytes nor water-injected oocytes showed a response to the glutamate receptor agonists tested. Responses to L-glutamate were recorded in 7 (out of 7) oocytes for GluR5-l (membrane depolarization 2.29 + 0.26 mV s.e.m) and 29 (out of 33) oocytes for GluR5-2 (2.27 + 0.19 mV s.e.m) . Example 17 Distribution of GluR4 and GluR5 mRNA In the Developing Central and Peripheral Nervous Systems For the developmental study of GluR4 and R5 gene expression, sections of mice from embryonic day 10 (E10) through postnatal day 21 (P21) were analyzed using in situ hybridization and histochemistry. In the entire CNS, a diffuse expression of the GluR4 and R5 genes was detected at E10. These first hybridization signals originate from postmitotic neurons. This is best demonstrated in the myelencephalon at E12. The ependymal layer is facing the neural canal and contains dividing neuroblasts. No hybridization was detectable in these cells. The postmitotic cells are located in the exterior part of the neural tube and express both genes. Later in development transcripts for GluR5 and, to a lesser extent, GluR4 were particularly pronounced in areas where neurons differentiate and assemble into nuclei. These temporal changes in the hybridization pattern were best observed for GluR5 in the primary sensory nuclei of the medulla oblongata and the nuclei of the pons which hybridized more intensely than surrounding structures At E14, GluR5 gene expression was particularly intense in several discrete brain nuclei, whereas GluR4 gene expression was detectable over the entire rostral and caudal parts of the brain. During postnatal development, the spatial distribution of GluR4 gene transcripts did not change but usually smaller amounts of mRNA were detectable than at late embryonic stages. In contrast, GluR5 gene expression appeared to become more restricted spatially during development and transcript levels were down-regulated. Extreme changes in the temporal GluR5 hybridization pattern were apparent in the cerebellar cortex. Until P12, high GluR5 transcript levels were detected in the granular and Purkinje cell layer. Later, the intensity of hybridization signals in the granular cell layer was reduced relative to the Purkinje cell layer and starting at P14, only a faint hybridization signal was detected in the granular cell layer. In general, those regions of the brain that exhibited a dense labeling during embryonic development also had detectable transcript levels in adults. In P21 animals the highest GluR4 transcript levels were observed in the cell layers of the olfactory bulb, the hippocampus, the cerebellum and the retina. In the retina, strong hybridization was found in the ganglion cell layer and in the amacrine cells of the inner nuclear layer. No expression was detected in Muller cells. For GluR5, the strongest hybridization signals at P21 were found in the olfactory bulb, the amygdala, the colliculi and some hypothalamic nuclei. In the developing peripheral nervous system (PNS) , the hybridization assays showed that the GluR4 and R5 genes are expressed to varying degrees in the cranial ganglia (e. g. , trigeminal ganglion, acoustic ganglia) , dorsal root ganglia and the mural ganglia of the intestinal organs. Comparable to the CNS, transcripts in the PNS are detected by ElO for GluR4 and by Ell for GluR5. During development, hybridization signals for GluR4 continuously increase until early postnatal stages and then persist with similar intensity in adults. Hybridization signals for GluR5 increase up to E16 and remain with comparable intensity in later developmental stages. In postnatal animals, the dorsal root ganglia (GluR5) and the mural ganglia of the intestinal organs (G1UR4 and GluR5) exhibit higher levels of hybridization than the CNS. High resolution autoradiography in the dorsal root ganglia demonstrates hybridization of the GluR5 probe over neuronal cells whereas satellite cells are unlabeled. Example 18 Distribution of GluR4 and GluR5 mRNA in the Adult Mammalian (Rat) Brain The distribution of the GluR4 and GluR5 mRNA transcripts in the adult CNS was studied by in situ hybridization. In the forebrain region, high levels of GluR4 transcripts were detected in the CAl and the dentate gyrus of the hippocampus, in the medial habenula and particularly in the reticular thalamic nucleus. The hippocampus showed only weak expression of the GluR5 gene and no transcripts were detected in the medial habenula. The GluR5 hybridization signal was intense in the cingulate and piriform cortex, several hypothalamic nuclei, the amygdala and the lateral septum. In the cerebellum, the hybridization patterns for GluR4 and R5 probes were overlapping but distinct. Both probes were detected at high levels in the Purkinje cell layer. In the granular cell layer the GluR4 probe produced strong labeling, while GluR5 probe labeling was weak. Example 19 Isolation of GluR6 and GluR7 cDNA clones encoding the GluR6 and GluR7 genes were isolated from a adult rat forebrain library using a low-stringency hybridization screening protocol (see Example 2) and a radiolabeled fragment of about 1.2 kbp (nucleotides 705-2048) of the GluR5 cDNA as probe. The selected clones were identified by restriction digest map and sequencing. Figure 11 shows the nucleotide and derived amino acid sequence of the GluR6 clone; Figure 12 shows the same for fragments 35-T3 (Fig. 12 A) and U35 (Fig. 12 B) from the GluR7 clone. Example 20 GluR-Related Assays The GluR cDNAs, mRNAs, proteins and functional fragments thereof, are useful in various assays designed to identify and characterize L-glutamate receptors and ligands. For example, the cDNAs are useful as probes to identify additional members of the glutamate receptor gene family. mRNAs transcribed from the DNAs of the invention are especially useful in assays designed to identify and characterize both functional receptors and ligands. This use is especially important for the identification and design of compounds that can affect L-glutamate receptor function. In an assay for identifying and characterizing functional receptors, mRNA is transcribed from DNAs of the invention (either full length or fragments thereof produced by deletions, substitutions, synthesis, etc. ) and then translated to produce GluR proteins. In preferred form, the mRNAs are translated in oocytes, preferably Xenopus oocytes. The expressed glutamate receptor proteins are then exposed to ligands known to functionally bind to and activate glutamate receptors. The electrophysiological characteristics of the glutamate receptor proteins are measured, and those that form functional ion channels are concluded to be functional glutamate receptor. In a related assay designed to identify functional ligands for glutamate receptors, proteins known to functionally bind to glutamate receptor activating compound(s) are contacted with at least one "unknown" or test compound whose ability to induce glutamate receptors to form ion channels is sought to be determined. The electrophysiological properties of the glutamate receptors are measured following exposure to the test compound(s) , and those that can effect an ion channel response are concluded to be functional ligands for glutamate receptors. DRAWINGS Brief Description of the Drawings The following is a brief description of the drawings. The drawings comprise 10 Figures, of which: Figure 1 is a drawing that shows the nucleotide and deduced amino acid sequence of the clone GluRl. Figure 2 (a and b) comprises two sequence homology analyses. Figure 2a compares the extracellular region located between amino acid residues 89 and 106 of GluRl with the "Cys-Cys-loop" region found in all other ligand-gated ion channels, showing sequence homology. Figure 2b compares the putative TMD II region of GluRl with hypothetical TMD II regions of other ligand-gated ion channels, suggesting protein sequence conservation. Figure 3 is a drawing that shows the nucleotide and deduced amino acid sequence of the clone GluR2. Figure 4 is a drawing that shows the nucleotide and deduced amino acid sequence of the clone GluR3. Figure 5 is a drawing which shows the alignment of deduced amino acid sequences for the GluRl, GluR2, GluR3, GluR4 and GluR5 (GluR5-l) subunits of the glutamate receptor gene family. Figure 6 (A and B) is comprised of two graphs which compare current responses measured in Xenopus oocytes injected with individual GluRl, GluR2 and GluR3 subunit RNAs (Figure 6A) or rat brain hippocampus poly(A) + RNA (Figure 6B) . Figure 7 (A and B) is comprised of two graphs which compare current responses measured in Xenopus oocytes injected with combinations of GluRl, GluR2 and GluR3 RNAs. Figure 8 is comprised of three graphs which illustrate the dependence of the current responses to lOOmM KA upon the membrane potential. Figure 9 is a drawing that shows the nucleotide and deduced amino acid sequence of the clone GluR4. Figure 10 is a drawing that shows the nucleotide and deduced amino acid sequence of the cDNA clone encoding glutamate receptor subunit GluR5-l. Figure 11 is a drawing that shows the nucleotide and deduced amino acid sequence of the clone GluR6. Figure 12 (A and B) is a drawing that shows the nucleotide and deduced amino acid sequence of fragments 35-T3 and U35 from the clone GluR7. Detailed Description of the Drawings Figure 1. The nucleotide and deduced amino acid sequence of the clone pGluRl. Nucleotides are numbered in 5'- to 3'-terminal direction, starting with the first nucleotide of the codon for the putative amino-terminal residue of the mature protein. Cleavage of the assumed signal peptide is predicted (von Heijne, Nucl . Acids Res. 14, 4683 (1986)) to occur at position 1. Nucleotides located upstream of the predicted 5'-terminal base have been assigned negative numbers. Nucleotides -1 to -54 encode a putative signal peptide (von Heijne, supra) , and bases -55 through -251 represent a 5•-untranslated region. At the 3'-terminus of the clone 71 nucleotides of untranslated sequence were found. The deduced amino acid sequence for GluRl is shown above the nucleotide sequence, the numbering starting with the first residue of the mature protein. Possible extracellular N-glycosylation sites are marked with an asterisk above the amino acid residue. The region marked by two arrows (amino acids 89-103) bears some resemblance to the ligand-gated ion channel signature postulated by some workers (Grenningloh et al . , Nature 330, 25 (1987); Barnard et al . , TINS 10, 502 (1987)) . Figure 1 Methods. The insert cDNA of the bacteriophage clone AZAPII-GluR-Kl was cut out with Eco Rl/Xho I, blunt-ended and subcloned into the Sma I-site of the bacteriophage vector M13mpl9 (Messing et al . , Proc. Natl . Acad. Sci . USA, 74, 3642 (1977)), yielding clones with both orientations of the cDNA. Overlapping deleted subclones were constructed for each strand or¬ ientation using the Cyclone™ kit from USB. Single-strand sequencing by the dideoxynucleotide chain-termination method (Sanger et al . , Proc. Natl . Acad. Sci. USA, 74, 5463 (1977)), was carried out with all 45 subclones, and additionally, 10 oligonucleotide primers were synthesized to facilitate sequencing across areas were compressions or gaps were encountered in the sequences derived from the deletion subclones. Complete sequences for both strands were thus obtained. Intelli- Genetics software packages (IntelliGenetics version 5.0, and PC/Gene™) were used for analyzing sequences. Figure 2a. Comparison of the extracellular region located between amino acid residues 89 and 106 of GluRl with the "Cys-Cys-loop" region found in all other ligand-gated ion channels, showing sequence homology. Sequences of nAChR subunits αl, βl , 7 , δ are from mouse muscle (Heinemann et al . , in: Molecular Neurobiology : Recombinant DNA Approaches (ed. Heinemann, S. & Patrick, J.) 45-96 (Plenum Press, New York, 1987)), those of nAChR subunits α2, α3, α4, β2 , β3 , and β are from rat brain (Deneris et al . , J. Biol. Chem. 264, 6268 (1989); Duvoisin et al . , Neuron 3, 487 (1989)). GABA A subunits α and β are from calf brain (Barnard et al . , Trends Neurosci . 10, 502 (1987)). GlyR 48k is the M_ = 48 kDa subunit of the glycine receptor from rat brain (Grenningloh et al . , Nature 328, 215-220 (1987)). Boxed amino acid residues are found at identical positions in GluRl as well as in at least one of the other receptor sequences. One gap has been introduced arbitrarily. Figure 2b. Comparison of the putative TMD II region of GluRl with hypothetical TMD II regions of other ligand-gated ion channels (Barnard et al . . Trends Neurosci . 10, 502 (1987); Deneris et al . , J. Biol . Chem. 264, 6268 (1989); Duvoisin et al . , Neuron 3, 487 (1989); Grenningloh et al . , Nature 328, 215 (1987); Heinemann et al . , in: Molecular Neurobiology: Recombinant DNA Approaches (ed. Heinemann, S. & Patrick, J.) 45-96 (Plenum Press, New York, 1987))1987), suggesting protein sequence conservation. Figure 3. Nucleotide and deduced amino acid sequence of cDNA clone 1RB14 encoding the rat glutamate receptor subunit gene GluR2. The nucleotides are numbered in the 5' to 3' direction, starting with the first nucleotide in the codon corresponding to the putative amino-terminal residue in the mature protein. Nucleotides extending 5' of base 1 are designated with negative numbers and include the region encoding the putative signal peptide and 5•-untranslated region. There are five potential N-linked glycosylation sites located at Asn-235, Asn-349, Asn-385, Asn-392, and Asn-835. The calculated molecular mass of the mature, non-glycosylated form of the GluR2 protein is 96,400 daltons. The asterisk indicates a translation termination codon. Figure 4. Nucleotide and deduced amino acid sequences of cDNA clone 1RB312 encoding the rat glutamate receptor subunit gene GluR3. The nucleotides are numbered in the 5' to 3' direction, starting with the first nucleotide in the codon corresponding to the putative amino-terminal residue in the mature protein. Nucleotides extending 5' of base 1 are designated with negative numbers and include the region encoding the putative signal peptide and 5'-untranslated region. There are five potential N-linked glycosylation sites located at Asn-35, Asn-238, Asn-352, Asn-387, and Asn-394. The calculated molecular mass of the mature, non-glycosylated form of the GluR3 protein is 98,000 daltons. The asterisk indicates a translation termination codon. Figure 5. Alignment of deduced amino acid sequences for the rat glutamate receptor subunits. Identical residues in all compared sequences are boxed, spaces were introduced to maximize homology. Predicted signal peptides and four proposed membrane spanning regions (MSR I-IV) are indicated. Two arrowheads (▼) delimit a region in GluRl proposed (Hollman, et al . , Nature 342, 643 (1989)) to correspond to the 'Cys-Cys' loop found in other ligand-gated ion channel subunits (Barnard, et al . , Trends Neurosci . 10, 502 (1987)). The hatched line denotes the insertion of 15 amino acid residues found in the GluR5-l, but not in the GluR5-2 protein. Figure 6. Comparison of current responses measured in Xenopus oocytes injected with individual GluRl, GluR2 and GluR3 subunit RNAs or rat brain hippocampus poly(A) + RNA. (A) Responses of oocytes to lOOmM KA measured 3 days after injection of individual GluRl (2ng) , GluR2 (lOng) or GluR3 (2ng) RNA. The insert shows examples of voltage recording traces obtained from such oocytes except that the GluR2 response was obtained 5 days after injection of 25ng RNA. (B) Responses of oocytes to the indicated agonists measured 3 days after injection with GluRl (2ng) or GluR3 (2ng) RNA or adult rat brain hippocampus poly(A) + RNA ( ca. 50ng) . All values are normalized to the response obtained with lOOmM KA and are presented as the mean ± S.E.M. with n > 3 for all measurements. All oocytes were voltage-clamped to -70mV and recordings were performed as described by Hollmann, et al . , Nature 342, 643 (1989). Figure 7. Comparison of current responses measured in Xenopus oocytes injected with combinations of GluRl, GluR2 and GluR3 RNAs. (A) Responses of oocytes to lOOmM KA measured 3 days after injection of 2ng RNA for each of the indicated GluR subunits. The open columns represent the sum of the responses measured in oocytes expressing the individual GluR subunit RNAs, while the stippled columns show the measured amplitudes after co-expression of the GluR subunit RNAs in individual oocytes. (B) Responses of oocytes to the indicated agonists measured 3 days after injection of 2ng RNA for each of the indicated GluR subunits or 50ng rat brain hippocampus poly(A) + RNA. Values have been normalized to the response obtained with lOOmM KA and are presented as the mean ± S.E.M. with n > 3 for all measurements. All oocytes were clamped to -70mV and recordings performed as described by Hollmann, et al . , Nature 341, 643 (1989). Figure 8. The dependence of the current responses to lOOmM KA upon the membrane potential. (A) Data obtained from oocytes injected with rat brain hippocampus poly(A) + RNA (50 ng, Solid Square I) , GluRl (Open Square D) or GluR3 RNA (Open circle, O) ; (B) GluRl plus GluR2 RNAs (Solid Square I) or GluRl plus GluR3 RNAs (Open Square D) ; and (C) GluR2 plus G1UR3 RNAs (Solid Square I) or all "three GluR subunit RNAs (Open Square D) . Recordings were made from oocytes 3 days after injection of 2ng RNA for each GluR subunit. Voltages were stepped by lOmV between -150mV and +50mV and all values are normalized to the response measured at -70mV. Figure 10. Nucleotide and amino acid sequence of a single cDNA clone (ARB20) encoding the rat glutamate receptor subunit GluR5-l. The deduced amino acid sequence is shown above the nucleotide sequence. Cleavage of the assumed signal peptide is predicted at amino acid position 1 (von Heijne, Nucl . Acids Res. 14, 4683 (.1986)). This cleavage site is after a proline residue, which is atypical. Amino acids encoding the signal peptide were assigned negative numbers. Nucleotides are numbered in the 5' to 3' direction, beginning with the first residue of the codon for the putative amino-terminal residue of the mature protein. The nucleotides on the 5' side of amino acid residue l are indicated by negative numbers. Stop codons flanking the open reading frame (ORF) are denoted with 'END', the polyadenylation signals are underlined. Sites of potential N-linked glycosylation are indicated by asterisks (*) . Two arrowheads (▼) delimit the insertion of 45 nucleotides between nucleotides 1113 and 1159 found in ARB20, but not in 12 of the 29 cDNA clones isolated. Tables Table 1 Pharmacology of the glutamate receptor encoded by GluRl: properties of various glutamate receptor antagonists as measured in oocytes injected with GluRl in vitro RNA a = % of the response evoked by 30μM kainate immediately before drug application b = % of the response seen with 30μM kainate alone immediately before application of drug/kainate mixture Oocytes had been injected with 1.25ng of GluRl in vitro sense RNA 3 days prior to the recording. The oocytes were voltage-clamped at -70mV, and the test compounds (all at lmM, except kainate, which was 30μM) applied by rapid superfusion, with 5 min intervals between drugs. Peak currents were recorded, and each number represents the average of 3 recordings from 3 different oocytes, ± SEM. The 100% current response corresponds to 40 - 200nA, depending on the oocyte. Table 2. The percent amino acid sequence identity among pairwise combinations of members of the glutamate receptor subunit gene family. GluRl G1UR2 GluR3 GluR4 GluR5 GluRl 100 GluR2 GluR3 GluR4 G1UR5 The sequences were compared using sequence analysis software from the University of Wisconsin Genetics Computer Group (Devereux, et al . , Nucl . Acids Res. 12, 387 (1984)). The percent sequence identity between paired sequences was calculated by dividing the number of aligned positions with identical amino acids by the total number of aligned positions in the shortest of the sequences examined and multiplying the quotient by 100. Specification Summary From the foregoing description, one of ordinary skill in the art can understand that the present invention discloses substantially pure DNA segments encoding a family of glutamate receptor subunit proteins. The glutamate receptors of the invention are proteins, or functional fragments thereof, that have electrophysiological and pharmacological properties characteristic of glutamate receptors, including glutamate receptor subtypes selected from the group consisting of N-methyl-D-aspartate (NMDA) , AMPA or quis¬ qualate, kainate (KA) and 2-amino-4-phosphonobutyrate (APB) . The new glutamate receptors of the present invention are exemplified by proteins encoded by cDNA clones GluRl, GluR2, GluR3, GluR4 and GluR5, GluR6 and GluR7 and functional combinations thereof, as well as functional peptide fragments encoded thereby. Both the glutamate receptor genes and proteins of the present invention are useful for drug screening. In addition, the cDNAs from GluRl-GluR7 can be used as probes, by those skilled in the art, without undue experimentation, to identify and isolate other new and novel glutamate receptors. Such other new and novel glutamate receptors are expressly within the scope of the present invention. Without departing from the spirit and scope of this invention, one of ordinary skill can make various changes and modifications to the invention to adapt it to various usages and conditions. As such, these changes and modifications are properly, equitably, and intended to be, within the full range of equivalence of the following claims.

Claims: WHAT IS CLAIMED IS: 1. Substantially pure functional proteins, or functional fragments thereof, or functional combinations of said proteins and/or said fragments, wherein said proteins or fragments or combinations have electrophysiological and pharmacological properties characteristic of glutamate receptor(s) . 2. Proteins, or fragments thereof, or combinations of said proteins and/or said fragments of Claim 1 wherein said proteins or fragments or combinations of said proteins and/or said fragments have electrophysiological and pharmacological properties characteristic of a glutamate receptor subtype selected from the group consisting of N-methyl-D-aspartate (NMDA) , α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) , kainate (KA) and 2-amino-4-phosphono- butyrate (APB) subtypes. 3. Proteins, or fragments thereof, or combinations of said proteins and/or said "" fragments of Claim 2 wherein said proteins or fragments or combinations have electrophysiological and pharmaco¬ logical properties characteristic of a glutamate receptor of the KA and/or AMPA subtypes. 4. Proteins, or fragments thereof, or combinations of said proteins and/or said fragments according to Claim 1, wherein said proteins or fragments or combinations have are encoded by DNA having at least about 40% nucleotide sequence homology with at least one member of the group consisting of GluRl DNA, GluR2 DNA, G1UR3 DNA, G1UR4 DNA, GluR5 DNA, GluR6 DNA, and GluR7 DNA. 5. Proteins, or fragments thereof, or combinations of said proteins and/or said fragments according to Claim 1, wherein the proteins or fragments or combinations have at least about 40% amino acid homology with at least one member of the group consisting of GluRl (Fig. 1), GluR2 (Fig. 3), GluR3 (Fig. 4) , GluR4 (Fig. 9) , GluR5 (Fig. 10) , GluR6 (Fig. 11) and GluR7 (Fig. 12) . 6. Substantially pure protein selected from the group of proteins consisting of GluRl, GluR2, GluR3, GluR4, GluR5, GluR6 and GluR7, and combinations thereof wherein said combinations are functional as glutamate receptor(s) . 7. Proteins of Claim 1 wherein said proteins are selected from the group consisting of GluRl (M r ca. 99,769), G1UR2 (M_ ca. 96,400), GluR3 (M_ ca. 98,000), G1UR4 (M r 98,500), GluR5 (M_ 100,000), GluR6 (M_ 100,000) and GluR7. 8. Substantially pure DNA encoding functional proteins, or functional fragments thereof, having electrophysiological and pharmacological properties characteristic of glutamate receptor(s) . 9. DNA of Claim 8 wherein said encoded proteins or fragments have electrophysiological and pharmacological properties characteristic of a glutamate receptor subtype selected from the group consisting of N-methyl-D-aspartate (NMDA) , α-amino-3-hydroxy-5-methyl- isoxazole-4-propionic acid (AMPA) , kainate (KA) and 2-amino-4-phosphonobutyrate (APB) subtypes. 10. DNA of Claim 9 wherein said encoded proteins or fragments have electrophysiological and pharmacological properties of glutamate receptors of the KA and/or AMPA subtypes. 11. DNA according to Claim 8 wherein said DNA is selected from the group consisting of GluRl DNA, G1UR2 DNA, GluR3 DNA, GluR4 DNA, GluR5 DNA, GluR6 DNA and GluR7 DNA. 12. DNA of Claim 8 wherein said DNA has at least about 40% nucleotide sequence homology with at least one member of the group consisting of GluRl DNA, G1UR2 DNA, GluR3 DNA, GluR4 DNA, GluR5 DNA, GluR6 DNA, and GluR7 DNA. 13. DNA of Claim 8 wherein said DNA encodes proteins having at least about 40% amino acid homology with at least one member of the group consisting of GluRl, G1UR2, G1UR3, GluR4, GluR5, GluR6 and GluR7. 14. DNA according to Claim 8 wherein said DNA encodes glutamate receptor protein selected from the group consisting of GluRl (M p ca. 99,769), GluR2 (M p ca. 96,400), G1UR3 (M p ca.98,000), GluR4 (M_ ca. 98,500), GluR5 (M. ca. 100,000), GluR6 (M r ca. 100,000) and GluR7. 15. DNA of Claim 8 wherein said DNA is functionally equivalent to DNA selected from the group consisting of GluRl, GluR2, GluR3, GluR4, GluR5, GluR6 and GluR7. 16. DNA probes comprising GluRl, GluR2, GluR3, GluR4, GluR5, GluR6 and GluR7 cDNA, or fragments thereof. 17. DNA probes of Claim 16 wherein said probes are at least about 20 base pairs in length. 18. DNA probes of Claim 16 wherein said probes are from the carboxyl encoding ends of said cDNA sequences. 19. DNA probes of Claim 16 wherein said probes are from ion channel encoding portions of said cDNA sequences. 20. Sense or antisense mRNA sequences transcribed from DNA according to Claim 8. 21. Oocytes expressing the sense or antisense mRNA of Claim 20. 22. A method for identifying, in a test sample, DNA homologous to DNA encoding glutamate receptor protein(s) , said method comprising contacting test DNA with probe DNA of Claim 16 under high or low stringency hybridization conditions and identifying that test DNA which hybridizes with said probe as being homologous to DNA encoding glutamate receptor protein(s) . 23. A method according to Claim 22 wherein said DNA sequences used as probes are cDNA sequences encoding the carboxyl end of the GluRl, GluR2, GluR3,GluR4, GluR5, GluR6 and GluR7 receptors. 24. A method according to Claim 22 wherein said DNA sequences used a probes include the ion channel encoding portion(s) of GluRl, GluR2, GluR3, GluR4, G1UR5, GluR6 and GluR7. 25. A method for identifying glutamate receptor functioning as an ion channel, said method comprising contacting proteins with at least one ligand known to activate glutamate receptors, measuring ion channel response to said ligand(s) and identifying as functional glutamate receptor those proteins which exhibit an ion channel response as a consequence of said contact. 26. A method according to Claim 25 wherein said proteins are contacted with said ligands in an oocyte expression system. 27. A method for determining whether a substance is a functional ligand for glutamate receptor protein, said method comprising contacting functional glutamate receptor with said substance, monitoring ion channel activity and identifying as functional ligands those substances which effect an ion channel response.

Inventors: BETTLER BERNHARD, BOULTER JAMES RICHARD, HEINEMANN STEPHEN FOX, HOLLMANN MICHAEL, JENSEN JAN EGEBIERG, BETTLER, BERNHARD, BOULTER, JAMES, RICHARD, HEINEMANN, STEPHEN, FOX, HOLLMANN, MICHAEL, JENSEN, JAN, EGEBIERG
Applicants: SALK INST FOR BIOLOGICAL STUDI, THE SALK INSTITUTE FOR BIOLOGICAL STUDIES
Application year: 1990
Main classification: C12N15
IPC codes: C07K14, C07K19, C12N5, C12N15, C12P21, C12Q1, C12R1, G01N33