Abstract:
The inventive subject matter relates to novel glutamate receptors and utilization thereof; more specifically to a glutamate receptor, DNA which encodes the receptor, a transformed cell expressing the receptor, a method for producing the receptor, a method for identifying an agonist, antagonist, or allosteric modulator for glutamic acid, a method for identifying an agonist for glutamic acid, an antibody to the receptor, and processes for making glutamate receptor modulators and pharmaceutical compositions comprising said modulator.

Description:
BACKGROUND OF THE INVENTIVE SUBJECT MATTER  
       [0001]     1. Technical Field of the Inventive Subject Matter  
         [0002]     The inventive subject matter relates to novel glutamate receptors and utilization thereof; more specifically to a glutamate receptor, DNA which encodes the receptor, a transformed cell expressing the receptor, a method for producing the receptor, a method for identifying an agonist, antagonist, or allosteric modulator for glutamic acid, a method for identifying an agonist for glutamic acid, an antibody to the receptor, and processes for making glutamate receptor modulators and pharmaceutical compositions comprising said modulator.  
         [0003]     2. Background  
         [0004]     Glutamic acid is a major excitatory neurotransmitter in the central nervous system, and it is widely accepted that its abnormal control is involved in progressive encephalopathies such as memory disorders, ischemic encephalopathy, amyotropic lateral sclerosis (ALS), Parkinson&#39;s disease, and Huntingon&#39;s chorea (Meldrum, B. S., Neurology, 1994 November; 44 (11 Supple 8):S14-23; Nishizawa, Y., Life Sci. 2001, Jun. 15; 69(4):369-81). Therefore, many studies concerning glutamate receptors have been carried out up to now in cranial nerve system. Many receptors (three kinds of ionotropic receptors and eight kinds of metabotropic receptors) have been found in the central nervous system with their splicing variants as well. Particularly, since 1992 when metabotropic glutamate receptor type I (mGluRla) was cloned by Nakanishi, et al., at least three splicing variants (mGluRlb, mGluRlc and mGluRld) have been confirmed as mGluR1 variants (As to details, refer to Hermans, E. and Challiss, R. A., Biochemical J., 359:465-484, 2001). In all of those variants, the C-terminal region of mGluRla becomes short, and their existence in nerve cells and glia cells has been confirmed. On the basis of such abundant receptor information, development for working drugs which are specific to each receptor has been extensively carried out. Even today new therapeutic drugs in the treatment of the above-described diseases are being developed (As to details, refer to Barnard, E. A., Trends Pharmacol. Sci., 1997, May; 18(5):141-8; Schoepp, D. D., Conn. P. J., Trends Pharmacol. Sci., 1993, January; 14(1):13-10).  
         [0005]     Nowadays, we have several pieces of knowledge that suggest physiological functions of the peripheral glutamate receptor (Berk, M., Plein, H., Ferreira, D., Clin. Neuropharmacol., 2001, May-June; 24(129-32; Karim, F., J. Neurosci. 2001, Jun. 1; 21(11):3771-9; Berk, M., Plein, H., Belsham, B., Life Sci. 2000; 66(25):2427-32; Carlton, S. M., Goggeshall, R. E., Brain Res. 1999, Feb. 27; 820(1-2):63-70; Haxhij. M. A., Erokwu, B., Dreshaj, I. A., J. Auton. Nerv. Syst. 1997, Dec. 11; 67(3):192-9; Inagaki, N., FASEB J. 1995, May; 9(8):686-91; Erdo, S. L., Trends Pharamcol. Sci., 1991, November; 12(11):426-9; Aas, P., Tanso, R., Formum, F., Eur. J. Pharamacol. 1989, May 2; 164(1):93-102; Said, S. I., Dey, R. D., Dickman, K., Trends Pharmacol. Sci. 2001, July; 22(7):344-5; Skerry, T. M., Genever, P. G., Trends Pharamacol., Sci. 2001, April; 22(4):174-81). However, those peripheral glutamate receptors are expressed in peripheral nerves, smooth muscle and immune tissues. There has been no report for their expression in epithelium of tongue and digestive tract. In mammals including humans to maintain normal growth and health, it is necessary to orally take up required amounts of nutrients at a specific timing and excrete disposable matter. This is actually done by the digestive tract, which is a single tube consisting of oral cavity, stomach, small intestine and large intestine. The process of digestion and absorption is controlled by intrinsic intestinal neuroplexus and extrinsic cranial nerves.  
         [0006]     The judgment as to whether or not to take a necessary nutrient is the result of brain integration of a signaling pathway that the individual is aware of taste with an autonomous signaling pathway that the individual is unaware of visceral sense. It is considered that salty taste (sodium, potassium, etc.) serves as a marker of minerals and is required for maintaining the osmotic pressure of the body fluid; sweetness (glucose) serves as a marker of carbohydrates and is required for supplementing energy; umami (sodium glutamate) serves as protein marker and is useful for supplementing energy and essential amino acids; and bitterness serves as a marker for toxic substances. That is, necessary nutrients are taken up relying on the tastes thereof. Then, if necessary amounts are ingested, satiation is determined by a series of intracerebral processes coming from the signal input to the solitary tract nucleus. Those signals are derived from activated vagus afferent fibers through nutrient sensors existing in the stomach, small intestine, and hepatoportal vein (Bray, G. A., Proc. Nutr. Soc., 2000; 59:373-84; Bray G. A., Med. Clin. North. Am. 1989:73:29).  
         [0007]     On the other hand, physiological studies on the mechanism for chemical sensation in the digestive tract have been performed for a long time. It is supposed that there are sensors that detect the content of the digestive tract (for the details, reference is made to Mei, N., J. Auton. Nerv. Syst., 1983; 9:199-206; Mei, N., Lucchini, S., J. Auton, Nerv. Syst., 1992; 41:15-8). The digestive chemosensory system includes a glucose sensor (Mei, N., J. Physiol. (Lond.) 1978, 282, 485-5-6), a temperature sensor (El Ouazzani, T., Mei, N., Exp. Brain Res. 1979; 15; 34:419-34), an osmotic pressure sensor (Mei, N., Garnier, L., J. Auton. Nerv. Syst., 1986; 16:159-70), a pH sensor, an amino acid sensor (Mei, N., Physiol. Rev., 1985; 65:211-37), and a stretch sensor (Barber, W. D., Burks, T. F., Gastroenterol Clin. North. Am. 1987; 16:521-4).  
         [0008]     In particular, a sensor that recognizes glutamic acid was suggested by Niijima et al. from neural excitation that occurred when glutamic acid was administered in the digestive tract. In this experiment, the technique of recording neural discharge activity was used for the stomach branch and abdominal cavity branch of the vagus nerve. Those vagal branches control mainly the stomach and small intestine and responded to glutamic acid; therefore was assumed that there is a mechanism that recognizes this amino acidat the vagus nerve ending (Niijima, A., Physiol. Behav., 1991; 49:1025-8). However, no cloning has been made for such a supposed sensor that recognizes glutamic acid until Applicants&#39; present work.  
       DISCLOSURE OF THE INVENTION  
       [0009]     Although many studies have been made on glutamate receptors and digestive tract sensors as described above, to date, glutamate perception is unclear and no progress has been made in recent works. Failure of receptor isolation from tissues containing glutamate sensors (receptor, transporter, etc.) necessary for nutrient recognition in the mucous membrane of the digestive tract prevented the progress in this research field. Applicants expect that elucidation of the umami-like substances that bind to glutamate sensors in the digestive tract would enable development of drugs and the like directed to control of the nutrient recognition mechanism described below.  
         [0010]     That is, the nutrient recognition mechanism also plays an important role on satiety or surfeit and improves poor physical condition in edacity and imbalance when indulging nutrients in eating disorders. It is considered that abnormal recognition of nutrients in the digestive tract naturally results in disturbance in the overall process of digestion and absorption, thus causing edacity, eating disorders, inappetence, indigestion, diarrhea, constipation, etc. Medically, there are many factors involved in the development of digestive diseases such as ulcers (stomach ulcer, duodenum ulcer) due to psychogenetic hyperphagia, cibophobia, obesity, anomaly of acid secretion, anomaly of blood flow in digestive tract, anomaly of secretion of digestive enzymes, etc., stress ulcers, drug-caused (NSAIDs, etc.) acute ulcers, ischemic ulcer (ischemic colitis), diabetes due to anomaly of secretion of insulin or anomaly of secretion of digestive tract hormone, heavy stomach, nausea, constipation, diarrhea, hypersensitivity bowel syndrome, etc. due to anomaly of gastrointestinal motility and so forth.  
         [0011]     Further, in recent years, the abrupt increase in obesity incidence is a social phenomenon. Many of those who are obese are said to have decreased basal metabolism and tend to eat too much. How to control the appetite of obese individuals is of great social concern. Many try to be on an excessive diet. However, in most cases, they are unsuccessful. Thus, improving the mechanism of nutrient recognition in the digestive tract and achieving satiety with a normal meal is very important to those who are obese.  
         [0012]     The second object of the inventive subject matter is derived from the above-described viewpoint, and the matter to be solved is identification of an actual glutamate-like substance which binds to glutamate sensors in the epithelium of the digestive tract andmethods forutilizing such sensors are provided.  
         [0013]     Applicants have investigated a receptor distribution in the epithelium of the tongue and in the digestive tract by way of an immunohistological methods using antibodies that recognize the intracellular domain of the metabotropic glutamate receptor type 1 (mGluR1). As a result, it has been found that cells in the epithelium of the tongue and the mucous membrane layer of the stomach are positive for mGluR1 where the receptor is present. In the tongue epithelium, the apical site of taste cells from taste buds are positive for mGluR1. Whereas in the stomach, mucus-secreting cells (neck mucus cells) and pepsinogen-secreting cells (chief cells) at the body of the stomach and mucous cells at the antrum of the stomach are positive for mGluR1. cDNA cloning from tongue epithelium was first performed, which has produced novel glutamate receptors, including that having the nucleic acid sequence of SEQ ID NO: 19 and is gustatory bud type mGluR1β, type A (hereinafter referred to “taste mGluR1” or “taste mGluR1 variant”). The taste mGluR1 is found in the taste buds and in the mucosal cells in the stomach. It is expected that this glutamate receptor is a novel umami taste receptor. Furthermore, Applicants are assiduously investigating whether the stomach contains another mGluR1 variant in the mucosal cells. It is expected that this would be a digestive tract glutamate sensor, which was previously unknown, and that the receptor cDNA, a purified receptor, and the receptor-expressing cells are useful for screening for modulators of digestive tract glutamate sensor.  
         [0014]     The inventive subject matter has been achieved on the basis of the above findings and its summary is as follows.  
         [0015]     (1) An isolated protein of glutamate receptor of following (A) or (B): 
        (A) a protein which comprises the amino acid sequence of SEQ ID NO: 2;     (B) a protein which comprises the amino acid sequence of SEQ ID NO: 2 with at least one substitution from: 
            (a) His 26 to Tyr,     (b) Arg 39 to Ser, and     (c) Val 51 to Ile.    
               
 
         [0021]     (2) The glutamate receptor protein according to (1), wherein said protein is expressed in mucosal cells of rat stomach.  
         [0022]     (3) An isolated DNA of following (a), (b), or (c): 
        (a) DNA encoding glutamate receptor protein having amino acid sequence of SEQ ID NO: 2,     (b) DNA which comprises nucleic acid sequence of SEQ ID NO: 1 or 442-2169 of SEQ ID NO: 1,     (c) DNA which hybridizes with a DNA molecule having the nucleotide sequence of SEQ ID NO: 1 under stringent conditions and followed with two washes at 60° C. in a solution comprising a salt concentration of 0.1×SSC and 0.1% SDS.        
 
         [0026]     (4) A cell which holds DNA coding for the glutamate receptor protein described in (3) in an expressible form.  
         [0027]     (5) A method for the search of agonist, antagonist or allosteric modulator for glutamic acid, characterized in that, the glutamate receptor protein described in any of (1) to (2) is made to react with a substance which bonds to that protein in the presence of a substance to be tested whereupon inhibition or promotion of the reaction is detected.  
         [0028]     (6) A method for the search of agonist for glutamic acid, characterized in that, the glutamate receptor protein described in any of (1) to (2) is made to react with a substance to be tested whereupon the reaction is detected.  
         [0029]     (7) The method according to (6), wherein the glutamate receptor protein from the cell of (4) or a membrane fraction prepared from the cell is used.  
         [0030]     (8) The method according to (5), wherein inhibition or promotion of the above bond is detected by a second messenger generated by the glutamate receptor protein.  
         [0031]     (9) The method according to (7), wherein the glutamate receptor protein from the cell of (4) or a membrane fraction prepared from the cell is used.  
         [0032]     (10) An antibody which specifically bonds to the glutamate receptor protein described in any of (1) to (2).  
         [0033]     (11) A method for the manufacture of a drug for the adjustment of a second messenger which is generated by bonding of glutamic acid to a glutamate receptor comprising: 
        a step where the glutamate receptor protein described in any of (1) to (2) is made to react with a substance which bonds to said protein in the presence of a substance to be tested to detect inhibition or promotion of the reaction whereby agonist, antagonist or allosteric modulator for glutamic acid is searched; and     a step where a pharmaceutical composition is prepared using the agonist, antagonist or allosteric modulator for glutamic acid prepared in the above step as an effective ingredient.        
 
         [0036]     (12) A method for the manufacture of a drug for the adjustment of a second messenger which is generated by bonding of glutamic acid to a glutamate receptor comprising: 
        a step where the glutamate receptor protein described in any of (1) to (2) is made to react with a substance to be tested to detect inhibition or promotion of the reaction whereby agonist for glutamic acid is searched; and     a step where a pharmaceutical composition is prepared using the agonist for glutamic acid prepared in the above step as an effective ingredient.        
 
         [0039]     The inventive subject matter will now be illustrated in detail as hereunder.  
         [0040]     Typically, an inventive glutamate receptor protein is a protein having an amino acid sequence represented by amino acid nos. 1 to 576 in SEQ ID NO: 2 in the Sequence Listing. An open reading domain of a base sequence of rat cDNA coding for an inventive protein is shown in SEQ ID NO: 1.  
         [0041]     Since a variant of the glutamate receptor protein as such is a metabotropic glutamate type 1 receptor (mGluR1) of a stomach type found from mucosal cells, Applicants named it as stomach mGluR1. In mGluR1, there have been two known types, i.e. type A (mGluR1a) and type B (mGluR1b), depending upon the splicing variation of the C-terminal. An inventive protein encoded by SEQ ID NO: 1 is a variation of type A (mGluR1a). Hereinafter, the glutamate receptor proteins of the inventive subject matter may be generally referred to as mGluR1 variant in the present specification. When an appropriate promoter is linked to the upstream region of the base sequence represented by SEQ ID NO: 1 and is expressed within an appropriate cell line, Applicants have produced active glutamate receptors.  
         [0042]     Comparison of the amino acid sequence of the inventive subject matter with that of brain-type metabotropic glutamate type 1 receptor (hereinafter referred to as mGluR1, accession number: M61099, SEQ ID NO: 14), an inventive receptor has a truncated N-terminus. The first methionine for the stomach mGluR1 corresponds to the residue M410 in the brain-type metabotropic glutamate receptor. The mGluR1 variant isolated from taste tissue also contains this truncation at the amino end of the receptor. The rest of the amino acid sequence for all currently known variants, including brain-type, taste-type, and stomach-type mGluR1, is identical until the sequence encoding the intracellular domain. At the C-terminus, the stomach-type mGluR1 is spliced at the K952 residue (numbering corresponds to the receptor sequence for the brain-type receptor). After the corresponding lysine 952, an inventive protein contains a novel peptide sequence of 33 amino acids shown in SEQ ID NO: 18. This amino acid sequence is not present in the brain-type mGluR1. The sequence detail is shown in  FIGS. 2 and 3 .  
         [0043]     Thus, the mGluR1 variants of the inventive subject matter have the same transmembrane domain as type 1 metabotropic glutamate receptor protein, but demonstrate differences in the intracellular domain and the extracellular domain when compared with the type 1 receptor. The extracellular domain of the inventive mGluR1 is the active site for glutamic acid but binding affinity is different from brain mGluR1. Other brain mGluR1 agonists such as quisqualic acid, ibotenic acid, ACPD (1-aminocyclopentane-trans-1,3-dicarboxylic acid) and so on may function as ligands with inventive mGluR1.  
         [0044]     Despite the fact that the intracellular domain of the mGluR1 variant of the inventive subject matter is different from that of mGluR1, the binding site for G proteins at the C-terminus is conserved. A shorter C-terminus seems to affect the electrophysiological response induced by receptor activation (Mary et al., J Biol. Chem. 1998 Jan. 2; 273 (1):425-32); nevertheless, the mGluR1 variant is still considered to be a functional receptor which is able to generate a second messenger.  
         [0045]     The stomach mGluR1 of the inventive subject matter may be derived from a rat. Alternatively, so long as it can generate a second messenger when glutamic acid is bound thereto, the mGluR1 variant may be derived from any animal, including mammals such as human, monkey, mouse, dog, cow, rabbit, birds, and fish.  
         [0046]     In the case where the mGluR1 variant is used as a component of pharmaceutical composition, it is preferably derived from a mammal. The truncation site at the N-terminus has an amino acid sequence highly conserved among the rat, mouse, and human. The nucleotide sequence at the intron site where the N-terminus splicing site occurs in the rat is very similar to the mouse, as shown in  FIG. 1 . The intron structure that yields the N-terminal truncation in the rat stomach and gustatory (taste) mGluR1 variants seems to be also present in the mouse. Therefore, those conserved sequences are an indication that a corresponding variant is expected to exist in mouse and human.  
         [0047]     The mGluR1 variant of the inventive subject matter may be a protein having the amino acid sequence of SEQ ID NO: 2, including substitution, deletion, insertion or addition of one or a plurality of amino acids at one or a plurality of sites, so long as the mGluR1 variant has the property of generating a second messenger when glutamic acid is bound thereto. In particular, such substitutions, deletions, insertions or additions may occur in the same manner that species-differences occur among rat, mouse, human, monkey, dog, cow, and rabbit. Since an exemplary sequence of the inventive subject matter derives from rat, the candidate amino acid for such substitution is easily found by sequence comparison using commercially available homology comparison software. An exemplary partial comparison is shown in  FIG. 1 . Particularly, preferable substitutions are His in 26th position for Tyr, Arg in 39th position for Ser, and Val in 51th position for Ile (position numbers corresponds to those in SEQ ID NO: 2). The inventive subject matter includes all such variations as long as the variant-specific, truncated sites are conserved.  
         [0048]     The term “plurality” as used herein varies depending on the positions of amino acid residues in the three-dimensional structure of the protein and the types of the amino acids. However, the number may be such that the homology with the amino acid sequence shown by SEQ ID NO: 2 is 80% or more, preferably 90% or more. More particularly, a plurality is 2 to 115 amino acids, preferably 2 to 58 amino acids, more preferably 2 to 30 amino acids.  
         [0049]     An inventive glutamate receptor may be in a purified or isolated form; however, when the activity is required, it is preferably in a form that is expressed in a suitable cell and localized in the membrane of the cell or in a form contained in a membrane fraction prepared from a cell in which the mGluR1 variant was expressed. Thus, the inventive subject matter also includes cells that express an mGluR1 variant, or a membrane fraction prepared from such cells.  
         [0050]     An inventive mGluR1 variant can be obtained, for example, by introducing DNA that encodes the mGluR1 variant into a suitable host cell to express the mGluR1 variant. The above-described DNA includes DNA that encodes the mGluR1 variant, isolated from the chromosome of a cell of a mammal such as mouse. When chromosomal DNA is used, it is preferable that cDNA is used since it is considered necessary to control a post-transcriptional process such as splicing so that mGluR1 variant can be generated.  
         [0051]     The cDNA of an mGluR1 variant can be cloned by amplifying the cDNA of mGluR1 variant using RNA prepared from the epithelium of the tongue of a mammal such as a rat as a template, and oligonucleotides shown in the embodiments as primers. In addition, since the structure of an mGluR1 variant, particularly the unique structure in the N-terminal region, has been determined as described herein, the cloning and identification of the cDNA of mGluR1 variant can be performedeasily based on the disclosed structures. Theopen reading frame nucleotide sequence of the cDNA of mGluR1 variant thus obtained is shown in SEQ ID NO: 1.  
         [0052]     Thus, another feature of the inventive subject matter is a polynucleotide coding for an inventive mGluR1 variant. With regard to the polynucleotide coding for an inventive mGluR1 variant, any polynucleotide which contains a nucleic acid base sequence, whether DNA or RNA, preferably DNA, coding for an above-described mGluR1 variant of the inventive subject matter may be used, provided that the polynucleotide does not code for brain-type mGluR1. Such a polynucleotide is DNA or RNA, such as mRNA, coding for the mGluR1 variant of the inventive subject matter and may be double-stranded or single-stranded. In the case of a double-stranded polynucleotide, it may be double-stranded DNA, double-stranded RNA or a DNA:RNA hybrid. In the case of a single-stranded polynucleotide, it may be a sense or coding strand, or an anti-sense or non-coding strand. Typically, the polynucleotide is a polynucleotide having a base sequence represented by SEQ ID NO: 1.  
         [0053]     The DNA which encodes the mGluR1 variant includes, in addition to the nucleotide sequence shown in SEQ ID NO: 1, DNA which hybridizes with DNA having this nucleotide sequence of SEQ ID NO: 1, or a probe that can be prepared from the same nucleotide sequence under stringent conditions and that encodes the mGluR1 variant. The term “stringent conditions” means conditions whereby a specific hybrid is formed, but nonspecific hybrids are not formed. It is difficult to clearly express the conditions by numeric values; examples thereof include those conditions whereby DNAs having high homology, for example, DNAs having 50% or more, preferably 75% or more homology hybridize with each other but DNAs having a lower homology than that will not hybridize with each other, or those conditions whereby DNAs hybridize with each other under ordinary washing conditions in southern hybridization, i.e., at 60° C. in a salt concentration corresponding to 1×SSC, 0.1% SDS, preferably 0.1×SSC, 0.1% SDS. Alternatively, when the probe having nucleic acid sequence of SEQ ID NO: 16 is used for the hybridization, stomach-specific hybrid is expected to be formed.  
         [0054]     Cells into which DNA encoding the mGluR1 variant is introduced preferably include animal cells, insect cells or yeast when the activity of mGluR1 variant is required to be maintained, with animal cells being particularly preferable. Examples of cells that are expected to enable transient expression of mGluR1 activity by introducing a recombinant vector containing DNA encoding the mGluR1 variant include  Xenopus laevis  oocyte, Chinese hamster ovary (CHO) cell, baby hamster kidney (BHK) cell, human embryonic kidney (HEK) cell, Sf-9 insect cell, PC12 cell, and CACO-2 cell. In addition, when DNA encoding the mGluR1 variant is incorporated into chromosomal DNA to express the mGluR1 variant permanently, the cells described, other than the  Xenopus laevis  oocyte, are suitable.  
         [0055]     With regard to a method for introduction of DNA coding for mGluR1 variant, publicly known methods may be used. Technique which is necessary for the operations such as an operation of introduction of DNA into cells is described in Sambrook, J., Fritsch, E. F. and Maniatis, T. “Molecular Cloning, A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press (1989), etc.  
         [0056]     On the other hand, when no physiological activity is necessary such as the case where the mGluR1 variant is used as an immunogen for preparing antibody that specifically binds to the mGluR1 variant, cells to which DNA encoding the mGluR1 variant is introduced may be those cells that do not express the mGluR1 variant in an active form. As such cells, microbial cells that are usually used for the production of heterologous protein, including  Escherichia coli  may be used.  
         [0057]     To produce the mGluR1 variant in the host cell, DNA, which encodes the mGluR1 variant, is ligated to an expression regulation sequence such as promoter or enhancer suitable for the host cell. The DNA which encodes the mGluR1 variant may include a processing information site, for example, a ribosome binding site, an RNA splicing site, a polyadenylation site, and a transcription terminator sequence as necessary. Preferable expression control sequences include promoters derived from immunoglobulin gene, SV40, adenovirus, bovine papilloma virus, and cytomegalovirus.  
         [0058]     The techniques necessary for the manipulation of cells such as introduction of DNA therein are described in, for example, Sambrook, J., Fritsch, E. F., and Maniatis, T., “Molecular Cloning A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press, (1989).  
         [0059]     The mGluR1 variant and a cell that retains the mGluR1 variant can be produced by cultivating a cell that harbors the DNA encoding the mGluR1 variant obtained as described above in an expressible form in a medium to produce the mGluR1 variant.  
         [0060]     Active mGluR1 variant, that is, mGluR1 variant that can generate a second messenger when glutamic acid is bound thereto can be utilized for screening agonist, antagonist or allosteric modulator of glutamic acid. For example, the mGluR1 variant and a substance that binds to the mGluR1 variant are reacted in the presence of a test substance, and inhibition or promotion of the reaction is detected, thereby screening agonist, antagonist or allosteric modulator of glutamic acid (hereinafter, these may be referred to collectively as “ligand”). The allosteric modulator binds to a site other than the binding site between the mGluR1 variant and glutamic acid to exhibit similar function to that of the agonist or antagonist.  
         [0061]     Further, the agonist of glutamic acid may be screened by reacting the mGluR1 variant with a test substance and detecting the reaction.  
         [0062]     The active mGluR1 variant may include cells that express the mGluR1 variant or membrane fractions prepared from such cells. Such membrane fractions may be prepared by allowing cells to express active mGluR1 variant, ultrasonically disrupting the cells, and subjecting the sonicate to density gradient centrifugation to collect a membrane fraction.  
         [0063]     Further, examples of the substance that binds to the above-described mGluR1 variant include glutamic acid, glutamic acid agonist, or known ligands that bind to mGluR1 (L-AP4, CPPG, MAP-4, or the like). The substances that modulate the activity of the mGluR1 variant include drugs that influence the intracellular concentration of calcium (calcium channel and sodium channel opener, Na/K pump inhibitor, Na/Ca exchange agonist, Ca-ATPase inhibitor, protein kinase C agonist), drugs that influence intracellular cAMP concentration (phosphodiesterase agonist, adenylate cyclase agonist), and drugs that influence intracellular cGMP concentration (cGMP-dependent phosphodiesterase agonist, guanylate cyclase agonist) and so forth.  
         [0064]     Inhibition or promotion of the reaction between mGluR1 variant and a substance that binds thereto can be detected by measuring a second messenger that is generated by binding of a ligand such as glutamic acid to the mGluR1 variant. Alternatively, the above-described inhibition or promotion of reaction can also be detected by measuring the binding of a labeled known ligand to the mGluR1 variant instead of detecting the second messenger.  
         [0065]     Further, the reaction between the mGluR1 variant and the agonist of glutamic acid can be detected by measuring a second messenger that is generated by binding of the mGluR1 variant to the agonist of glutamic acid.  
         [0066]     The intracellular domain of stomach mGluR1 variant lacks 267 amino acid (about 800 bp) from that of the brain type mGluR1, type A. Despite such difference, brain, gustatory bud and stomach type mGluR1 have the same basic intracellular signal transmitting mechanism. The above-described second messenger is a rise in intracellular calcium concentration accompanied by the production of inositol triphosphate (IP3) as a result of activation of Gq (GTP binding protein) followed by activation of phospholipase C. In the downstream area of calcium variation in signal transduction, there are functional adjustments of the critical stage by phosphorylation of cytoplasmic and membrane proteins, and by gene expression adjustment via intracellular calcium-dependent protein kinase. Therefore, it is possible to detect second messengers other than IP3 and calcium by measurement of intracellular cAMP, cGMP changes and channel function change as a result of activation of calcium-dependent phosphodiesterase, protein phosphorylation of cell membrane fraction, etc.  
         [0067]     Hereinafter, specific methods for searching a ligand using mGluR1 variant will be exemplified.  
         [0068]     (1) mGluR1 variant cRNA is expressed in oocytes of  Xenopus  and a ligand acting on mGluR1 variant is searched by a two-electrode voltage cramp method using increase or decrease in intracellular calcium-depending chloride current (Pin, J. P., et al., Proc. Natl. Acad. Sci. USA, 1992 Nov. 1; 89(21):10331-5; Kasahara, J., Sugiyama, H., FEBS Lett., 1994 Nov. 21; 355(1):41-4; Takahashi, K., et al., J. Biol. Chem., 1993 Sep. 15; 268)26):19341-5).  
         [0069]     (2) A candidate compound for ligand and known ligand acting on mGluR1 (such as glutamic acid, quisqualic acid, ibotenic acid, ACPD (1-aminocyclopentane-trans-1,3-dicarboxylic acid), CHPG ((RS)-2-chloro-5-hydroxy-phenylglycine), MPEP (2-methyl-6-(phenylethynyl)-pyridine), LY367385, etc.) are acted on a mGluR1 variant-expressing cell or a membrane fraction prepared from that cell for a certain period and amount of the known ligand bound to cell membrane of the mGluR1 variant-expressing cell or the membrane fraction is measured to conduct a ligand search (Naples, M. A., Neuropharmacology, 2001; 40(2):170-7; Thomsen, C., Neuropharmacology, 1997 January; 36(1):21-30; H. I. Yamamura, S. J. Enna and M. J. Kuhar, eds. 1958, Neurotransmitter Receptor Binding, 2nd ed., Raven Press, New York). Amount of the known ligand is able to be measured by the amount of radioactivity bound to the cell membrane or the membrane fraction after a radioactive labeling of a part of such substances.  
         [0070]     (3) A calcium-sensitive dye (for example, Fura-2, Indo-1, Fluo-3 or the like) is introduced into an mGluR1 variant expressing cell in advance, and a ligand candidate compound and the mGluR1 variant expressing cells are allowed to contact for a certain period of time, and then ligands are screened by using as an index a change in a ratio of intensities of fluorescence (intracellular calcium concentration). Alternatively, screening of ligand is performed by a change in a ratio of intensities of fluorescence (intercellular calcium concentration) obtained when an mGluR1 variant agonist, a candidate compound for ligand, and an mGluR1 variant expressing cells into which a calcium-sensitive dye is introduced are allowed to contact for a certain period of time.  
         [0071]     (4) Screening of ligands is performed by using as an index a change in a ratio of intensities of fluorescence (intracellular cAMP concentration) obtained when a cAMP-sensitive fluoroprotein (for example, FICRhR or the like) is introduced into an mGluR1 variant expressing cell in advance and then a ligand candidate compound and the mGluR1 variant expressing cells are allowed to contact for a certain period of time (Adams S R, Nature 1991 Feb. 21; 349(6311):694-7).  
         [0072]     (5) Screening of ligands is performed by using as an index the production amount of proton obtained when a candidate compound for ligand and an mGluR1 variant expressing cells are allowed to contact for a certain period of time, or when an mGluR1 variant agonist, a candidate compound for ligand and an mGluR1 variant expressing cells are allowed to contact for a certain period of time and measured by a cytosensor (McConnell H M, Science 1992 Sep. 25; 257(5078):1906-12).  
         [0073]     A food additive containing agonist, antagonist or allosteric modulator of glutamic acid searched as described above as an effective ingredient is able to be used as a novel umami taste-adjusting substance. Further, a pharmaceutical composition containing agonist, antagonist or allosteric modulator of glutamic acid searched as described above as an effective ingredient is able to be used as a drug for the adjustment of second messenger generated by binding of glutamic acid to a glutamate receptor. When the second messenger is adjusted, it is now possible that diseases and symptoms caused by abnormality of the glutamate receptor are improved and prevented.  
         [0074]     The anomalies of control of vagus nerve include anomaly of afferent pathway (disorder of nutrient recognition) and anomaly of efferent pathway. The diseases or pathology due to the anomaly of afferent pathway include hyperphagia, cibophobia, obesity and so on. On the other hand, those due to the anomaly of efferent pathway include digestive ulcers (stomach ulcer, duodenal ulcer) due to psychogenetic hyperphagia, cibophobia, obesity, anomaly of acid secretion, anomaly of blood flow in digestive tract, anomaly of secretion of digestive enzymes, etc., stress ulcers, drug-caused (NSAIDs, etc.) acute ulcers, ischemic ulcer (ischemic colitis), diabetes due to anomaly of secretion of insulin or anomaly of secretion of digestive tract hormone, heavy stomach, nausea, constipation, diarrhea, hypersensitivity vowel syndrome, etc. due to anomaly of motility and so forth.  
         [0075]     Use of mGluR1 variant as an immunogen enables preparation of an antibody that specifically binds to the mGluR1 variant. In particular, since the mGluR1 variant has a novel amino acid sequence in the C-terminus, antibody, particularly monoclonal antibody, that contains this portion as an epitope is expected to bind to the mGluR1 variant and not to bind to other glutamate receptors. The antibody specific to the mGluR1 variant can be used in immunostaining specific to the mGluR1 variant. Further, when the amino acid residue of the novel C-terminal intracellular domain is estimated from the three-dimensional structure forecast, it is possible to prepare an mGluR1 variant-specific antibody. An antibody which is specific to mGluR1 variant is able to be used for an immunostaining which is specific to mGluR1 variant, etc. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0076]      FIG. 1   a  is a sequence alignment which shows mGluR1 protein homology across species: rat mGluR1, mouse mGluR1, human mGluR1, mGluR4 and T1R1.  
         [0077]      FIG. 1   b  is a sequence alignment which shows 5′ transcript sequence homology between rat and mouse.  
         [0078]      FIG. 2   a  is a sequence alignment which shows a sequence comparison of mGluR1 in C-terminus; the 3′ mGluR1 sequence cloned from stomach is aligned with the corresponding mGluRla splicing variant (Brain type mGluR1, upper line, accession number M61099).  
         [0079]      FIG. 2   b  is a sequence alignment which shows how a truncated C-terminus comes from the conjugation of proximal and distal ends between truncation as indicated in the diagram. There is around 800 bp missing.  
         [0080]      FIG. 3  is a drawing which shows an illustration comparing the brain mGluR1 with the stomach mGluR1 variant.  
         [0081]      FIG. 4  is a series of photographs which shows stomach mucosa containing cells expressing mGluR1 variant. Neck mucous, chief and parietal cells hybridized with mGluR1 antisense probe at the stomach body as shown at pictures in the left panel.  
         [0082]      FIG. 5  is a series of photographs which shows in situ hybridization of mGluR1 variant in cerebellum control tissue. Purkinje cells from cerebellum stained in blue expresses the mGluR1 variant transcript. In the left panel, tissue sections are hybridized with an antisense probe. The right panel shows sense hybridization.  
         [0083]      FIG. 6   a  is a drawing which shows crossover PCR and PCR primers used in the Examples herein. Primers were designed to specifically target the truncated region NPR-2 and CFP-2.  
         [0084]      FIG. 6   b  is a photograph of an agarose gel which depicts the full sequence cDNA produced by linking the PCR products of the reaction shown in  FIG. 6   a  in a final reaction generating a combined fragment.  
         [0085]      FIG. 7   a  is a graph which depicts changes in membrane currency when brain variant mGluR1 is expressed in oocytes of  Xenopus  and sodium glutamate is acted thereon.  
         [0086]      FIG. 7   b  is a graph which depicts changes in membrane currency when stomach mGluR1 variant is expressed in oocytes of  Xenopus  and sodium glutamate is acted thereon.  
         [0087]      FIG. 8  is a graph which shows current responses to serial concentrations of glutamate on stomach or brain mGluR1 variant. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0088]     The inventive subject matter relates to an isolated glutamate receptor protein, comprising:  
         [0089]     (A) the amino acid sequence of SEQ ID NO: 2; or  
         [0090]     (B) the amino acid sequence of SEQ ID NO: 2 with at least one amino acid substitution selected from the group consisting of: 
        (a) His 26 to Tyr,     (b) Arg 39 to Ser,     (c) Val 51 to Ile, and     (d) combinations thereof.        
 
         [0095]     In another aspect of the inventive subject matter, said glutamate receptor protein is expressed by the mucosal cells in the stomach of rat.  
         [0096]     The inventive subject matter further relates to an isolated DNA molecule, comprising: 
        (a) a nucleic acid sequence encoding a glutamate receptor protein selected from the group consisting of:     (A) the amino acid sequence of SEQ ID NO: 2, or     (B) the amino acid sequence of SEQ ID NO: 2 with at least one amino acid substitution selected from the group consisting of: 
            (i) His 26 to Tyr,     (ii) Arg 39 to Ser,     (iii) Val 51 to Ile, and     (iv) combinations thereof;    
            (b) a nucleic acid sequence of SEQ ID NO: 1;     (c) a nucleic acid sequence of residues 442-2169 of SEQ ID NO: 1; or     (d) a nucleic acid sequence which hybridizes with a DNA molecule having the nucleotide sequence of SEQ ID NO: 1 under stringent conditions and followed with two washes at 60° C. in a solution comprising a salt concentration of 0.1×SSC and 0.1% SDS.        
 
         [0107]     The inventive subject matter additionally relates to a host cell transformed with an isolated DNA molecule coding for the glutamate receptor protein, as described above, in an expressible form.  
         [0108]     In a preferred embodiment, said isolated DNA molecule in an expressible form comprises a vector. One of ordinary skill in the art will understand that there a many expression vectors known in the art and commercially available today.  
         [0109]     In addition, the inventive subject matter relates to a method for identifying an agonist, antagonist, or allosteric modulator for glutamic acid, comprising the steps of: 
        (a) in the presence of a substance to be tested, reacting a glutamate receptor protein according to claim  1  with a substance which binds to said glutamate receptor protein; and     (b) detecting inhibition or promotion of said reaction.        
 
         [0112]     In a preferred embodiment, said method for detecting inhibition or promotion of said binding is by detecting a second messenger generated by the glutamate receptor protein.  
         [0113]     In another aspect, said glutamate receptor protein is prepared from a cell as described above, or a membrane fraction prepared from said cell.  
         [0114]     The inventive subject matter also relates to a method for identifying an agonist for glutamic acid, comprising the steps of: 
        (a) reacting a glutamate receptor protein according to claim  1  with a substance to be tested; and     (b) detecting said reaction.        
 
         [0117]     In a preferred embodiment, said method for detecting inhibition or promotion of said binding is by detecting a second messenger generated by the glutamate receptor protein.  
         [0118]     In an alternate aspect, said glutamate receptor protein is prepared from a cell as described above, or a membrane fraction prepared from said cell.  
         [0119]     The inventive subject matter further relates to an antibody which specifically binds to a glutamate receptor protein as described above.  
         [0120]     Additionally, the inventive subject matter relates to an active agent for modulating a second messenger which is generated by binding of glutamic acid to a glutamate receptor, produced by a process comprising the steps of: 
        (a) in the presence of a substance to be tested, reacting a glutamate receptor protein according to claim  1  with a substance which binds to said protein;     (b) detecting inhibition orpromotion of said reaction; and     (c) analyzing said inhibition or promotion of said reaction by said substance to be tested, and determining whether said substance to be tested is an agonist, antagonist, or allosteric modulator for glutamic acid.        
 
         [0124]     The inventive subject matter additionally relates to a pharmaceutical composition comprising: 
        (a) an active agent for modulating a second messenger which is generated by binding of glutamic acid to a glutamate receptor, produced by a process comprising the steps of: 
            (i) in the presence of a substance to be tested, reacting a glutamate receptor protein according to claim  1  with a substance which binds to said protein;     (ii) detecting inhibition or promotion of said reaction; and     (iii) analyzing said inhibition or promotion of said reaction by said substance to be tested, and determining whether said substance to be tested is an agonist, antagonist, or allosteric modulator for glutamic acid; and    
            (b) a pharmaceutically acceptable carrier.        
 
         [0130]     Further, the inventive subject matter relates to an active agent for modulating a second messenger which is generated by binding of glutamic acid to a glutamate receptor, produced by a process comprising the steps of: 
        (a) in the presence of a substance to be tested, reacting a glutamate receptor protein according to claim  1  with a substance which binds to said protein;     (b) detecting inhibition or promotion of said reaction; and     (c) analyzing said inhibition or promotion of said reaction by said substance to be tested, and determining whether said substance to be tested is an agonist for glutamic acid.        
 
         [0134]     Finally, the inventive subject matter relates to a pharmaceutical composition comprising: 
        (a) an active agent for modulating a second messenger which is generated by binding of glutamic acid to a glutamate receptor, produced by a process comprising the steps of: 
            (i) in the presence of a substance to be tested, reacting a glutamate receptor protein according to claim  1  with a substance which binds to said protein;     (ii) detecting inhibition or promotion of said reaction; and     (iii) analyzing said inhibition or promotion of said reaction by said substance to be tested, and determining whether said substance to be tested is an agonist for glutamic acid; and    
            (b) a pharmaceutically acceptable carrier.        
 
         [0140]     The following examples are illustrative of the inventive subject matter and are not intended to be limitations thereon. Unless otherwise indicated, all percentages are based upon 100% by weight of the final composition.  
       EXAMPLE 1  
     Cloning of Novel Metabotropic Glutamate Receptor cDNA from Circumvallate Papillae of Rat  
       [0141]     Total RNA derived from circumvallate papillae of ten rats of Wistar strain of 16 weeks age were extracted and subjected to a reverse transcription reaction to give cDNA (kit used: SuperScript, Gibco-BRL). cDNA coding for full length of mGluR1 was used as a template and a PCR was carried out by Z-Taq. This enzyme has a good replication efficiency at 3′-side and is suitable for a TOPO TA cloning reaction after that. The PCRproductwas subjected to electrophoresis using 2% agarose gel and the sequences were analyzed by an ABI Sequencer Model 3100 (ABI Co., Ltd.).  
         [0142]     Taste mGluR1β type A was cloned from circumvallate papillae, with unique sequence at 5′-side Forward primers specific to mGluR1β type A variant cDNA prepared by Hokkaido System Science; the primers used are shown in Table 1. The following reverse primers were prepared from brain type mRNA sequence (mGluR1-4253R 5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′ (SEQ ID NO: 4) and mGluR1-4198R 5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′ (SEQ ID NO: 11) for type A (Masu, et al., Nature, 349:760, 1991).  
         [0143]     cDNA (150 ng) was used as a template, then 10 μM of forward and reverse primers, 10×LA PCR buffer, 2.5 mM of MgCl 2  and 2.5 mM of dNTP were mixed and 0.25 units of Z-Taq enzyme was placed therein to make the total volume 10 μl. Conditions for the PCR reactions: GeneAmp PCR System 9700 was used where a cycle of 94° C. for 20 seconds, 56° C. for 1 minute and 68° C. for 3 minutes was carried out for 30 cycles; finally, 10 minute extension for 68° C. was done. Further, the second PCR was conducted and the resulting template was subjected to a cloning using pCR11-TOPO vector by a TOPO TA Cloning Kit (Invitrogen). Positive clones were subjected to a colony PCR while plasmids were purified by a Hispeed Plasmid Maxi-Kit (Quiagen) followed by subjecting to a functional analysis.  
         [0144]     As a result, mGluR1β Type A cDNA described in SEQ ID NO: 19 was found.  
                                                                   TABLE 1                           Primers                    SEQ                       ID       Name   Primer Name   NO   Sequence                    Brain   PCR-1   Forward   mGluR1-50F   21   5′-GAG ACC AAT AGC TGT GTC TAC CC-3′           mGluR1a       Reverse   mGluR1-4253R   4   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   mGluR1-114F   12   5′-TGG ACA CCT GAT CCA CAC ACC TT-3′               Reverse   mGluR1-4198R   11   5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′               Taste   PCR-1   Forward   mGluR1-790-1F   22   5′-GGG ACT CTC TCC TGT CTT GTG AG-3′       mGluR1βa       Reverse   mGluR1-4253R   4   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   mGluR1-790-2F   23   5′-AGC ATA ACA GGG AAT TGC AGT GG-3′               Reverse   mGluR1-4198R   11   5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′                  
 
       EXAMPLE 2  
     In Situ Hybridization of Stomach mGluR1  
       [0145]     Rat stomach mucosa was prepared as described previously (Hoshino et al., 1999, and Yoshida et al., 2001). Hybridization was performed with probes at concentrations of 200-500 ng/ml in a hybridization solution (50% formamide, 5×SSC, 1% SDS, 50 μg/ml tRNA, and 50 μg/ml heparin) at 55° C. for 16 h. Antisense probes with nucleotide sequence common to all mGluR1 variants (SEQ ID NO: 17) were labeled with digoxigenin and sections incubated with anti digoxigenin alkaline phosphatase conjugate antibodies (Roche Molecular Biochemicals). Signals were developed with BM purple substrate (Roche Molecular Biochemicals).  
         [0146]     As a result, in situ hybridization this analysis revealed that the stomach cells that contain mGluR1 transcripts are: neck mucous, chief and parietal cells as shown in the pictures of the left side  FIG. 4  using an mGluR1 anti-sense probe.  
       EXAMPLE 3  
     Cloning of Novel Metabotropic Glutamate Receptor cDNA from Stomach of Rat  
       [0147]     Tissue and RNA. Stomach was scraped from 20 adult (12 to 16-week old) Sprague-Dawley rats (Charles River, Japan). Rat Cerebellum was sampled to clone mGluRla as control. Total RNA was then extracted with ISOGEN reagent (Wako, Osaka, Japan) and first-strand 5′ RACE (rapid amplification of cDNA ends) synthesized using SuperScript reverse transcriptase, oligo (dT) 12-18 primer (both from Invitrogen, USA) and SMART II oligonucleotide (SMART RACE cDNA amplification kit, Clontech Laboratories, USA).  
         [0148]     3′ end PCR. The C-terminal sequence corresponding to the truncated C-terminal was determined by a series of PCR reactions. Sequence was analyzed with an ABI Sequencer Model 3100. In an intend to produce a full-length stomach variant mGluR1, two sequences were yielded by PCR combining the N-Terminal forward primer-1 [NFP-1] (5′-GGGACTCTCTCCTGTCTTGTGAG-3′; SEQ ID NO: 3), homologous to the truncated N-terminal sequence, with C-Terminal reverse primer designed from the mGluRla splicing variant sequence (mGluR1 4253R 5′-TACCATATGGAATTGTGCTTTGTCA-3′; SEQ ID NO: 4). Sequence analysis revealed that one of the sequences was identical to mGluRla C-terminus and the other showed a unique truncation that is unalike any mGluR1 splicing variants (Soloviev at al., 1999). Both C-terminal regions were confirmed by connecting the NFP-1 forward primer with either a specific C-terminal reverse primer homologous to the truncated region (mGluR1-COOR variant 5′-TTGACACTCCTTGGTGCTGGCAT-3′; SEQ ID NO: 5) or a primer homologous only to mGluR1 type a (mGluR1 3241Ra 5′-GTAAAGGGTCTTGGTGCTGGCAT-3′; SEQ ID NO: 6) ( FIG. 6 ).  
         [0149]     Crossover PCR and cloning. After sequence analysis, the whole coding sequence of the mGluR1 stomach variant was constructed by crossover PCR using the following primers:  
                                                                                                                 (SEQ ID NO: 3)                    N-Terminal forward primer-1 [NFP-1]               5′-GGGACTCTCTCCTGTCTTGTGAG-3′                        (SEQ ID NO: 7)                    N-Terminal reverse primer-1 [NRP-1]               5′-GTATTGTCCTCTTCTTCCACATTGTAAAGGGTCTTGGTGCT           GGCAT-3′                        (SEQ ID NO: 8)                    C-Terminal forward primer [CFP-2]               5′-AATGTGGAAGAAGAGGACAATACCCCTTCT-3′                        (SEQ ID NO: 9)                    C-Terminal reverse primer [CRP-2]               5′-TACCATATGGAATTGTGCTTTGTCA-3′          
 
         [0150]     Fragments yielded by NFP-1&amp;NRP1 and CFP-2&amp;CRP-2 were combined to obtained the final stomach mGluR1 cDNA variant by the next primers:  
                                                                 (SEQ ID NO: 10)                    [NFP-2]               5′-AGCATAACAGGGAATTGCAGTGG-3′                        (SEQ ID NO: 11)                    mGluR1-4198R               5′-ATAATTCAAGAGTCACAATCCTGGC-3′          
 
         [0151]     The first amplification was performed with pfu DNA polymerase enzyme (Promega, USA) while the crossover PCR was carried out with Easy-A high-fidelity PCR cloning enzyme (Stratagene, USA).  
         [0152]     The final template was cloned into the pcDNA3.1/V5-His vector through a TOPO cloning reaction (TOPO TA Expression Kit, Invitrogen, USA).  
         [0153]     The forward primer used to amplify mGluR1 from rat Cerebellum as a control for functional analysis was the mGluR1-114F (5′TGGACACCTGATCCACACACCTT-3′; SEQ ID NO: 12) and the reverse primer mGluR1-4198R (SEQ ID NO: 11).  
         [0154]     As a result, novel stomach type, mGluR1β cDNA described in SEQ ID NO: 1 was found. The N-terminus for stomach mGluR1 resulted to have exactly the same sequence that the one in taste tissue called mGluR1β type A variant of Example 1, which was described by Applicants in PCT Publication No. WO 03/068818. Details for the C-terminal sequence are indicated in  FIG. 2 . The upper panel indicates the native nucleotide sequence of brain mGluR1 type A aligned with the corresponding sequence cloned from stomach. At the upper line in capital and bold letters are the related amino acids to the brain sequence. After the splicing site the sequence in cloned stomach-mGluR1 continues with the original brain cDNA sequence further down the stop codon. This 3′ end also contains a stop codon in frame with the open reading frame. The resulting receptor from stomach contains a shorter C-terminal amino acid sequence compared to the brain with 33 additional amino acids at the end specific to this variant. The different forms of the brain and stomach-mGluR1 transcripts are represented in the lower panel of  FIG. 2 . The discrepancy at the 3′ region between both RNAs, brain and stomach, is that around 800 bases are missing in the stomach sequence. The putative protein structure for the brain and stomach-mGluR1 are shown in  FIG. 3 .  
         [0155]      FIG. 1  upper panel illustrates the high homology that exists among rat, mouse and human mGluR1 amino acid sequence at the N-terminal region where taste and stomach mGluR1 protein starts being synthesized (M410 in the mouse). The homology is also compared to other glutamate (mGluR4) and taste (T1R1) receptors from the same family at the equivalent peptide sequence site. The lower panel of  FIG. 1  shows the nucleotide homology between the mouse and the rat at the intron site where stomach and taste 5′ cDNA for mGluR1 begins. This highly conserved amino acid sequence suggests that variant beta is likely found in others species as well. In addition, the structure described for the beta variant is maintained in the mouse 5′ end.  
         [0156]     To study what cells in the stomach express mGluR1 in situ hybridization was performed on stomach sections. This analysis revealed that the stomach cells that contain mGluR1 transcripts are: neck mucous, chief and parietal cells, as shown in the pictures of the left side of  FIG. 4  using an mGluR1 anti-sense probe.  FIG. 5  is a positive control indicating the abundant mGluR1 expression at the left site panel colored in blue in Purkinje cells from cerebellum applying the same mGluR1 anti-sense probe than that was used in the stomach.  
         [0157]     To study its function, truncated stomach mGluR1 was synthesized by crossover PCR. The primer combination for PCR reaction as well as the final product is shown in  FIG. 6 . Primers were designed to specifically target the truncated region (NRP-2 and CFP-2). The full sequence cDNA was produced by linking the PCR products in a final reaction to generate the template shown in the agarose gel at the figure. Sequence analysis of the PCR end product was confirmed and used for electrophysiological studies.  
       EXAMPLE 3  
     Functional Analysis  
       [0158]     cRNA synthesis. The resulting pcDNA3.1/V5-His vector was used as a template to synthesize the corresponding stomach and brain mGluR1 cRNA. Target DNA was amplified again with pfu DNA polymerase enzyme (Promega, USA) including the T7 promoter sequence (T7 PCR Forward primer 5′-TATTTAATACGACTCACTATAGGATAAGCATAACAGGGAATTGCAGTGG-3′; SEQ ID NO: 13) with the reverse primer mGluR1-4198R (SEQ ID NO: 11). Capped RNA was synthesized with a T7 transcription kit (mMessage mMachine, Ambion, USA). Reaction mixture was incubated for 2 hours at 37° C. for complete RNA synthesis and remaining template DNA was degraded by adding 1 mL of DNase 1 during 15 minutes. Transcripts were purified by phenol-chloroform extraction and isopropanol precipitation. cRNA was reconstituted in diethyl pyrocarbonate-treated (DEPC) water and quantitated by UV light absorbance before oocyte injection.  
         [0159]     Oocyte injection. Twenty-four hours after collection, healthy  Xenopus  oocytes retaining clear animal and vegetal pole were injected (microinjector, WPI) with about 25 nL containing 100 ng of CRNA using a standard-bore glass capillary tube of 12 mm diameter at the tip. Electrophysiological recording was performed at 24 and 48 hours post injection in MBS buffer [88 mM NaCl, 1 mM KCl, 2.4 mM NaHCO 3 , 10 mM HEPES, 0.82 mM MgSO 4 , 0.33 mM Ca(NO 3 ) 2 , 0.91 mM CaCl 2 , pH 7.5] supplemented with 2 mM pyruvate and 0.5 mM theophylline at 18° C. (28).  
         [0160]     Voltage-clamp. Oocytes were placed in a recording chamber and perfused with MBS at room temperature. Recording and clamping microelectrodes were pulled from 1.5 mm (outside diameter) capillary tubing filled with 3 M KCL. The electrodes were then impaled into the animal pole and voltage-clamped at −70 mV using a Geneclamp amplifier (Axon Instruments, USA). L-glutamate was perfused into the recording chamber and Ca 2+  dependent Cl −  peak current in oocytes expressing rat mGluR1 recorded. Data recording and analysis was done using pClamp software (Axon Instruments, USA).  
         [0161]     Results. Receptor activity was assessed in  Xenopus  oocytes by in vitro cRNA synthesis from full-length mGluR1 clone and posterior oocyte microinjection. The stomach variant was functionally compared with the already established response elicited by brain-mGluR1. Current responses after L-glutamate application during 30 seconds were recorded fromxenopus oocytes injectedwith the in vitro synthesized mGluR1 cRNA for either the brain (left) or stomach (right) variant. Recordings were done under −70 mV voltage clamp Downward deflection an inward current. Both brain and stomach-mGluR1 activated a Ca 2+  dependent Cl −  channel. But the brain variant achieved maximum amplitude using 100 mM L-glutamate as stimuli, while taste-mGluR1 required a much higher glutamate concentration for maximum stimulation (25 mM, in accordance with the amount found in foodstuffs). In addition, glutamate evoked a larger inward current in oocytes expressing the brain-mGluR1 opposed to oocytes bearing the stomach variant.  
         [0162]     Current responses to serial concentrations of glutamate as stimuli were recorded from oocytes injected with either the stomach (blue) or brain (pink) variant mGluR1. Adose-responsecurve ( FIG. 8 ) representingthemean of 2 to 3 sets of data from each group was produced showing that the stomach-mGluR1 has a lower affinity for its ligand than the receptor found in the brain probably due to its short N-terminal.  
       INDUSTRIAL APPLICABILITY  
       [0163]     In accordance with the inventive subject matter, there is provided a novel metabotropic glutamate receptor. This glutamate receptor is able to be used for the search of agonist, antagonist or allosteric modulator for glutamic acid. It is also able to be used as a food additive as a novel umami-tasting substance and also as a drug for improving diseases and symptoms caused by metabolism abnormality in digestive tracts.  
         [0164]     The inventive subject matter being thus described, it will be obvious that the same may be modified or varied in many ways. Such modifications and variations are not to be regarded as a departure from the spirit and scope of the inventive subject matter and all such modifications and variations are intended to be included within the scope of the following claims.