Abstract:
It is intended to disclose a glutamate sensor in taste sensation and digestion and provide techniques using the same. A glutamate-receptor protein having the following characteristics is reacted with a substance binding thereto in the presence of a test substance and thus the inhibition or acceleration of the reaction is detected to thereby search for a glutamate agonist or antagonist or an allosteric modulator: (1) having a transmembrane domain and an intracellular domain common to a type 1 metabolic glutamate receptor protein; and (2) having an extracellular domain shorter by about 409 or 481 amino acid residues than the type 1 metabolic glutamate receptor protein.

Description:
TECHNICAL FIELD  
       [0001]     The present invention 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.  
       BACKGROUND ART  
       [0002]     Human sense of taste is believed to be constituted of five basic tastes—salty, sweet, acidic, bitter and umami taste (relishable taste). Each taste is generated by the binding of a taste substances to each receptor specifically expressed in taste cells existing in taste buds of the tongue. Until now, ENaC/Deg. (salty taste receptor), EnaC, ASIC, HCN (acidic taste), T2R family (bitter taste receptor), T1R2/T1R3 (sweet taste receptor), Taste mGluR4 (umami taste receptor), etc. have been cloned as candidates of the taste receptors (As to details, refer to Lindemann, B., Nature, 413; 13, 219-225, 2001; the cited document is incorporated by reference in the present specification; that is the same hereinafter as well).  
         [0003]     A low-affinity glutamate receptor expressed in taste bud cells in the rat found by Chaudhari, N., Landin, A. M., Roper, S. D., et al. as an umami taste receptor has been a convincing evidence for proving the umami taste-receiving mechanism at molecular level ( Nat. Neurosci.,  February 2000;3(2):113-9). The umami taste receptor has the same host gene as that in type 4 (mGluR4) which is a subtype of glutamate receptor of a rat brain type/a metabotropic type (Tanabe, Y., et al.,  Neuron,  January 1992;8(1):169-79; Flor, P. J., et al.,  Neuropharmacology,  February 1995;34(2):149-55); and since taste type mGluR4 holds a partially deficient extracellular domain by a splicing variation, the finding of specific working substances other than glutamic acid that can utilize the present new type variant as a peripheral glutamate receptor has been receiving public attention.  
         [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, November 1994;44 (11 Supple8):S14-23; Nishizawa, Y., Life Sci. Jun. 15, 2001;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 (mGluR1a) was cloned by Nakanishi, et al., at least three splicing variants (mGluR1b, mGluR1c and mGluR1d) 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 mGluR1a 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-mentioned diseases are being developed (As to details, refer to Barnard, E. A.,  Trends Pharmacol. Sci.,  May 1997;18(5):141-8; Schoepp, D. D., Conn. P. J.,  Trends Pharmacol. Sci.,  January 1993;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., May-June 2001;24(129-32; Karim, F., J. Neurosci. Jun. 1, 2001;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. Feb. 27, 1999;820(1-2):63-70; Haxhij. M. A., Erokwu, B., Dreshaj, I. A., J. Auton. Nerv. Syst. Dec. 11, 1997;67(3):192-9; Inagaki, N., FASEB J. May 1995;9(8):686-91; Erdo, S. L., Trends Pharamcol. Sci., November 1991;12(11):426-9; Aas, P., Tanso, R., Fonnum, F., Eur. J. Pharamacol. May 2, 1989;164(1):93-102; Said, S. I., Dey, R. D., Dickman, K., Trends Pharmacol. Sci. July 2001;22(7):344-5; Skerry, T. M., Genever, P. G., Trends Pharamacol., Sci. April 2001;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 an 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 acid at the vagus nerve ending (Niijima, A., Physiol. Behav., 1991;49:1025-8).  
       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. The inventors of the present invention 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 present invention is derived from the above-mentioned 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 tongue and the digestive tract and methods for utilizing such sensors are provided.  
         [0013]     The present inventors 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 successfully performed, which has produced a novel glutamate receptor. It is expected that this glutamate receptor is a novel umami taste receptor or 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 umami taste receptor and digestive tract glutamate sensor.  
         [0014]     The present invention has been achieved on the basis of the above findings and its summary is as follows.  
         [0015]     (1) A glutamate receptor protein comprising:  
         [0016]     (A) a transmembrane domain and an intracellular domain common to the type 1 metabotropic glutamate receptor; and  
         [0017]     (B) an extracellular domain which is shorter than type 1 metabotropic glutamate receptor by 481 or 409 amino acid residues.  
         [0018]     (2) The glutamate receptor protein according to (1), wherein it is expressed on epithelium of tongue of rat.  
         [0019]     (3) The glutamate receptor protein according to (1), wherein it has an amino acid sequence represented by SEQ ID NO: 6 and NO: 8 or an amino acid sequence represented by amino acid nos. 73 to 790 and 73 to 497 in the above amino acid sequence.  
         [0020]     (4) The glutamate receptor protein according to (3), wherein it contains substitution, deletion, insertion or addition of one or plural amino acid residue(s) and is able to generate a second messenger by binding with glutamic acid.  
         [0021]     (5) DNA which codes for the glutamate receptor protein mentioned in any of (1) to (4) and does not express the type 1 metabotropic glutamate receptor protein.  
         [0022]     (6) A cell which holds DNA coding for the glutamate receptor protein mentioned in any of (1) to (4) in an expressible form.  
         [0023]     (7) A method for the manufacture of a glutamate receptor protein, characterized in that, cells which hold DNA coding for the glutamate receptor protein mentioned in any of (1) to (4) in an expressible form are incubated in a medium whereupon the glutamate receptor protein is produced and then the glutamate receptor protein is collected from the above-mentioned cells.  
         [0024]     (8) A method for the search of agonist, antagonist or allosteric modulator for glutamic acid, characterized in that, the glutamate receptor protein mentioned in any of (1) to (4) is made to react with a substance which binds to that protein in the presence of a substance to be tested whereupon inhibition or promotion of the reaction is detected.  
         [0025]     (9) A method for the search of agonist for glutamic acid, characterized in that, the glutamate receptor protein mentioned in any of (1) to (4) is made to react with a substance to be tested whereupon the reaction is detected.  
         [0026]     (10) The method according to (8), wherein the glutamate receptor protein from the cell of (6) or a membrane fraction prepared from the cell is used.  
         [0027]     (11) The method according to (10), wherein inhibition or promotion of the above binding is detected by a second messenger generated by the glutamate receptor protein.  
         [0028]     (12) The method according to (9), wherein the glutamate receptor protein from the cell of (6) or a membrane fraction prepared from the cell is used.  
         [0029]     (13) The method according to (12), wherein inhibition or promotion of the above binding is detected by a second messenger generated by the glutamate receptor protein.  
         [0030]     (14) An antibody which specifically binds to the glutamate receptor protein mentioned in any of (1) to (4).  
         [0031]     (15) A method for the manufacture of a drug for the adjustment of a second messenger which is generated by binding of glutamic acid to a glutamate receptor comprising  
         [0032]     a step where the glutamate receptor protein mentioned in any of (1) to (4) is made to react with a substance which binds 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  
         [0033]     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.  
         [0034]     (16) A method for the manufacture of a drug for the adjustment of a second messenger which is generated by binding of glutamic acid to a glutamate receptor comprising  
         [0035]     a step where the glutamate receptor protein mentioned in any of (1) to (4) 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  
         [0036]     a step where a pharmaceutical composition is prepared using the agonist for glutamic acid prepared in the above step as an effective ingredient.  
         [0037]     The present invention will now be illustrated in detail as hereunder.  
         [0038]     Typically, the glutamate receptor protein of the present invention is any of a protein having an amino acid sequence represented by amino acid nos. 1 to 718 in the SEQ ID NO: 2 in the Sequence Listing, a protein having an amino acid sequence represented by amino acid nos. 1 to 425 in the SEQ ID NO: 4 in the Sequence Listing, a protein having amino acids 1 to 790 in the SEQ ID NO: 6 in the Sequence Listing and a protein having amino acids 1 to 497 in the SEQ ID NO: 8 in the Sequence Listing. An open reading domain of a base sequence of rat cDNA coding for the present protein is shown in SEQ ID NOS: 1, 3, 5, 7, 9 and 10.  
         [0039]     Since a variant of the glutamate receptor protein as such is a metabotropic glutamate type 1 receptor (mGluR1) of a taste type found from epithelial cells of tongue, the present inventors named it as mGluRT; further, in view of homology of the sequences, the protein coded by SEQ ID NOS: 1 and 3 was named mGluRTα, the protein coded by SEQ ID NOS: 5 and 7 was named mGluRTβ and the protein coded by SEQ ID NOS: 9 and 10 was named mGluRTγ. In mGluR1, there have been known two types, i.e. type A (mGluR1a) and type B (mGluR1b), depending upon the splicing variation of C-terminal, and in the proteins of the present invention, proteins coded by SEQ ID NOS: 1 and 3, 5 and 7 and 9 and 10 are also variants corresponding to type A and type B, respectively. So the protein coded by SEQ ID NO: 1 was named mGluRTαa; the protein coded by SEQ ID NO: 3 was named mGluRTαb; the protein coded by SEQ ID NO: 5 was named mGluRTβa; the protein coded by SEQ ID NO: 7 was named mGluRTβb; the protein coded by SEQ ID NO: 9 was named mGluRTγa; and the protein coded by SEQ ID NO: 10 was named mGluRTγb. Hereinafter, the glutamate receptor proteins of the present invention may be generally referred to as mGluR1 variants in the present specification. When an appropriate promoter is linked to upstream region of the base sequences represented by SEQ ID NOS: 1, 3, 5, 7, 9 and 10 and is expressed within an appropriate cells, it is possible to produce active glutamate receptors.  
         [0040]     Comparison of the amino acid sequence of the present invention with that of brain-type metabotropic glutamic type 1 receptor (hereinafter referred to as mGluR1) which has been confirmed to be present in the brain is shown in  FIG. 1 . The C-terminal side of mGluRTαa (amino acid numbers 1 to 718 in SEQ ID NO: 2) is identical with each C-terminal side of mGluR1a, and the C-terminal side of mGluRTαb (amino acid numbers 1 to 425 in SEQ ID NO: 4) is identical with C-terminal side of mGluR1b, respectively; but as compared with mGluR1, both N-terminal side were shorter to an extent of 481 amino acid residues. On the other hand, mGluRTβ and mGluRTγ had the same coding regions, and in both of them, C-terminal sides (amino acid numbers 1 to 790 in SEQ ID NO: 6) of type A (mGluRTβa and mGluRTγa) are identical with that of mGluR1a; while, C-terminal sides (amino acid numbers 1 to 497 in SEQ ID NO: 8) of type B (mGluRTβb and mGluRTγb) are identical with that of mGluR1b; but, as compared with mGluR1, N-terminal side was shorter to an extent of 409 amino acid residues. As will be mentioned later, it is believed that the glutamate receptor of the present invention is a splicing variant derived from gene which is common to mGluR1. Hereinafter, the glutamate receptor protein may be wholly called as mGluR1 variants in the present specification.  
         [0041]     When the cDNA sequence coding for mGluR1 variants are compared with mGluR1 mRNA sequence, it was suggested that they were derived from common gene. Thus, mGluR1 variants are presumed to be the result where exon in mGluR1 gene was deleted by an alternative splicing. The detail is shown in  FIG. 1 . Brain-type mGluR1 comprises exons 1 to 9; and for subtypes, type A (mGluR1a) comprises exons 1 to 7 and 9, and type B (mGluR1b) comprises exons 1 to 8 (refer to  FIG. 1A ). On the other hand, among mGluR1 variants of the present invention, mGluRTαa comprises exons 5 to 7 and 9, mGluRTαb comprises exons 5 to 8 (refer to  FIG. 1B ), mGluRTγa comprises exons 3 to 7 and 9, mGluRTβb comprises exons 3 to 8, mGluRTγa comprises exons 4 to 7 and 9, and mGluRTγb comprises exons 4 to 8 (refer to  FIG. 1C ). Since initiation methionine codon is not present in exon 3 constituting mGluRTβ, coding region of mGluRTβ starts from methionine in exon 4; as a result, coding regions of mGluRTβ and mGluRTγ are common. Here, exons 1 to 6, exon 7 and exons 8 to 9 correspond to extracellular domain, seven transmembrane domain and intracellular domain, respectively.  
         [0042]      FIG. 2  shows the structures of mGluR1 and mGluR1 variants. mGluR1 comprises intracellular domain (exons 1 to 6), seven transmembrane domain (exon 7) and extracellular domain (type A: exon 9; type B: exon 8). mGluR1 variants also have the same intracellular domain and seven transmembrane domain as mGluR1 has, and it has the same sequence as those of mGluR1.  
         [0043]     Incidentally, mGluRTα starts from exon 5; therefore, the amino acid sequence of SEQ ID NO: 2 (mGluRTαa) corresponds to 73 to 790 of the amino acid sequence in SEQ ID NO: 6 (mGluRTβa and mGluRTγa), and the amino acid sequence of SEQ ID NO: 4 (mGluRTαb) corresponds to 73 to 497 of the amino acid sequence in SEQ ID NO: 8 (mGluRTβb and mGluRTγb).  
         [0044]     Thus, the mGluR1 variants of the present invention have the same transmembrane domain and intracellular domain as those of type 1 metabotropic glutamate receptor protein and has extracellular domain which is shorter to an extent of 409 or 481 amino acid residues than type 1 metabotropic glutamate receptor protein. Thus, the mGluR1 variant of the present invention is different from mGluR1 in terms of extracellular domain which is an acting site to ligand; therefore, it is presumed to be different from mGluR1 in terms of affinity to ligand. Meanwhile, it is common to mGluR1 in intracellular domain which is an effector domain of seven transmembrane G protein conjugate receptor (GPCR); therefore, it is a functional receptor which is able to generate a second messenger.  
         [0045]     The mGluR1 variant of the present invention 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 mammalian such as human, monkey, mouse, dog, cow, and rabbit, birds, and fin. In the case where the mGluR1 variant is used as a component of pharmaceutical composition, it is preferably derived from a mammalian.  
         [0046]     The mGluR1 variant of the present invention may be a protein having the amino acid sequence of SEQ ID NO: 6 or the amino acid sequence consisting of amino acid numbers 73 to 790 in SEQ ID NO: 6 (SEQ ID NO: 2), a protein having the amino acid sequence of SEQ ID NO: 8 or the amino acid sequence consisting of amino acid numbers 73 to 7497 in SEQ ID NO: 8 (SEQ ID NO: 4), or a protein having the amino acid sequence any of SEQ ID NOS: 2, 4, 6 and 7 including substitution, deletion, insertion or addition of one or a plurality of amino acids at one or a plurality of sites so long as it has properties of generating a second messenger when glutamic acid is bound thereto.  
         [0047]     The “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 any of SEQ ID NOS: 2, 4, 6 and 8 is 80% or more, preferably 90% or more. More particularly, the plurality is 2 to 115, preferably 2 to 58, more preferably 2 to 30.  
         [0048]     The glutamate receptor of the present invention 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 cells 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 glutamate receptor of the present invention also includes cells that express mGluR1 variant or a membrane fraction prepared from such cells.  
         [0049]     The 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-mentioned DNA includes gene that encodes the mGluR1 variant, isolated from the chromosome of a cell of a mammalian such as mouse. When chromosomal gene 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.  
         [0050]     The cDNA of mGluR1 variant can be cloned by amplifying the cDNA of mGluR1 variant using RNA prepared from the epithelium of 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 mGluR1 variant, particularly unique structure on the N-terminal region has been made clear by the present invention, cloning and identification of the cDNA of mGluR1 variant can be performed easily based on the structures. The open reading frame nucleotide sequence of the thus obtained cDNA of mGluR1 variant is shown in each SEQ ID NOS: 1, 3, 5, 7, 9 and 10.  
         [0051]     Thus, another feature of the present invention is a polynucleotide coding for any of mGluR1 variants of the present invention. With regard to the polynucleotide coding for any of mGluR1 variants of the present invention, anything which contains a base sequence (DNA or RNA, preferably DNA) coding for the above-mentioned mGluR1 variant of the present invention may be used provided that it does not code for brain-type mGluR1. Such a polynucleotide is DNA and RNA such as mRNA coding for the mGluR1 variant of the present invention and may be double- or single-stranded. In the case of a double-stranded one, it may be a double-stranded DNA, a double-stranded RNA or a hybrid of DNA:RNA. In the case of a single-stranded one, it may be sense chain (code chain) or anti-sense chain (non-code chain). Typically, the polynucleotide is a polynucleotide having base sequences represented by SEQ ID NOS: 1, 3, 5, 7, 9 and 10.  
         [0052]     The DNA which encodes the mGluR1 variants includes in addition to the nucleotide sequence shown in SEQ ID NOS: 1, 3, 5, 7, 9 and 10, DNA which hybridizes with DNA having this nucleotide sequence or a probe that can be prepared from the same nucleotide sequence under stringent conditions and that encodes the mGluR1 variant. The “stringent conditions” means conditions whereby 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. and a salt concentration corresponding to 1×SSC, 0.1% SDS, preferably 0.1×SSC, 0.1% SDS.  
         [0053]     Cells to which DNA encoding the mGluR1 variant is introduced include preferably animal cells, insect cells or yeast when the activity of mGluR1 variant is necessary, with animal cells being particularly preferable. Examples of cells that are considered to enable transient expression of the function 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 COCA-2 cell. In addition, when DNA encoding the mGluR1 variant is incorporated in chromosomal DNA to express the mGluR1 variant permanently, those cells other than the  Xenopus laevis  oocyte are suitable.  
         [0054]     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 mentioned in Sambrook, J., Fritsch, E. F. and Maniatis, T. “Molecular Cloning, A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press (1989), etc.  
         [0055]     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.  
         [0056]     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.  
         [0057]     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).  
         [0058]     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.  
         [0059]     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.  
         [0060]     Further, the agonist of glutamic acid may be screened by reacting the mGluR1 variant with a test substance and detecting the reaction.  
         [0061]     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.  
         [0062]     Further, examples of the substance that binds to the above-mentioned 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.  
         [0063]     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-mentioned 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.  
         [0064]     Further, the reaction between the mGluR1 variant and the agonist of glutamic acid can be detected by detecting a second messenger that is generated by binding of the mGluR1 variant to the agonist of glutamic acid.  
         [0065]     The intracellular domain of mGluR1 variant is the same as the brain type and gustatory bud type mGluR1 and the brain type and gustatory bud type mGluR1 have the same intracellular signal transmitting mechanism. Accordingly, the above-mentioned 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 transmittance, there are function adjustment of acute stage by phosphorylation of cytoplasm and membrane protein and that by gene expression adjustment via intracellular calcium-dependent protein kinase. Therefore, it is possible to detect a second messenger 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.  
         [0066]     Hereinafter, specific methods for searching a ligand using mGluR1 variant will be exemplified. (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,  Nov. 1, 1992;89(21):10331-5; Kasahara, J., Sugiyama, H.,  FEBS Lett.,  Nov. 21, 1994;355(1):41-4; Takahashi, K., et al.,  J. Biol. Chem.,  Sep. 15, 1993;268)26):19341-5).  
         [0067]     (2) A candidate compound for ligand and known ligand acting on mGluR1 (such as glutamic acid, quisqualic acid, CHPG, MPEP, 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,  January 1997;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.  
         [0068]     (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.  
         [0069]     (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 Feb. 21, 1991; 349(6311): 694-7).  
         [0070]     (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 Sep. 25, 1992; 257(5078): 1906-12).  
         [0071]     A food additive containing agonist, antagonist or allosteric modulator of glutamic acid searched as mentioned 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 mentioned 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.  
         [0072]     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, 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 vowel syndrome, etc. due to anomaly of motility and so forth.  
         [0073]     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 N-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 N-terminal intracellular domain (cell surface exposure part) 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  
       [0074]      FIG. 1  is a graph which shows an outline of splicing of mGluR1 and mGluR1 variants (mGluRTα, mGluRTβ and mGluRTγ).  
         [0075]      FIG. 2  is a graph which shows an outline of structures of mGluR1 and mGluR1 variants (mGluRTα, mGluRTβ and mGluRTγ).  
         [0076]      FIG. 3A  is a photograph which shows the result of immunostaining of taste bud (circumvallate papilla) of rat by an anti-mGluR1 antibody.  
         [0077]      FIG. 3B  is a photograph which shows the result of immunostaining of stomach of rat by an anti-mGluR1 antibody.  
         [0078]      FIG. 4  is a drawing which shows the action of L-glutamic acid on vagal gastric branch-afferent nerve activity. The abscissa stands for time while the ordinate stands for nerve activity.  
         [0079]      FIG. 5  is a drawing in which expression of nucleotide coding for mGluR1 variant by an RT-PCR method is confirmed ( FIG. 5A : mGluRTα;  FIG. 5B : mGluRTβ). It has been confirmed that mGluR1 variant is expressed in taste bud.  
         [0080]      FIG. 6  is a drawing which shows changes in membrane currency when mGluR1 variant is expressed in oocytes of  Xenopus  and sodium glutamate is acted thereon. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0081]     The present invention will now be more specifically illustrated by way of the following Examples although the present invention is not limited thereto.  
       EXAMPLE 1  
     Cloning of Novel Metabotropic Glutamate Receptor cDNA from Circumvallate Papillae of Rat  
       [0082]     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 PCR product was subjected to electrophoresis using 2% agarose gel and the sequences were analyzed by an ABI Sequencer Model 3100 (ABI Co., Ltd.).  
         [0083]     In six kinds of mGluR1 variant cDNA (mGluRTαa, mGluRTαb, mGluRTβa, mGluRTβb, mGluRTγa and mGluRTγb) found from circumvallate papillae, there are unique sequences at 5′-side, and in that areas, there are stop codons. The upstream side thereof is the same as that in the known substance, and that is quite similar to the sequence of mGluR1 of type A or type B. Such a unique part is not translated; therefore, all of the six kinds of mGluR1 variant cDNA are the same as a part of amino acid sequence of mGluR1; however, the chain length is short.  
         [0084]     Forward primers specific to the six kinds of mGluR1 variant cDNA were prepared by Hokkaido System Science (the primers used are shown in Table 1) while, with regard to reverse primers, the followings were prepared from brain type mRNA sequence (Masu, et al.,  Nature,  349:760, 1991) (mGluR1-4253R 5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′ (SEQ ID NO: 17) and mGluR1-4198R 5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′ (SEQ ID NO: 18) for type A and mGluR1-3266R 5′-GGG TAT TGT CCT CTT CTT CCA CA-3′ (SEQ ID NO: 19) for type B).  
         [0085]     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. Condition for the PCR was that GeneAmp PCR System 9700 was used where a cycle of 94° C.→20 seconds, 56° C.→1 minute and 68° C.→3 minutes was carried out for 30 cycles; finally, 10 minute extension o 68° C. was done. Further, the second PCR was conducted and the resulting template was subjected to a cloning using pCRII-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.  
         [0086]     As a result, novel cDNAs mentioned in SEQ ID NOS: 1, 3, 5, 7, 9 and 10 were found. The resulting clones were found to be splicing variants of mGluR1 having novel extracellular domain.  
                                                                   TABLE 1                           Primers                    SEQ               Name   Primer Name   ID NO   Sequence                    Brain   PCR-1   Forward   mGluR1-50F   20   5′-GAG ACC AAT AGC TGT GTC TAC CC-3′           mGluR1a       Reverse   mGluR1-4253R   17   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   mGluR1-114F   21   5′-TGG ACA CCT GAT CCA CAC ACC TT-3′               Reverse   mGluR1-4198R   18   5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′               Brain   PCR-1   Forward   (same)   20   5′-GAG ACC AAT AGC TGT GTC TAC CC-3′       mGluR1b       Reverse   (same)   17   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   (same)   21   5′-TGG ACA CCT GAT CCA CAC ACC TT-3′               Reverse   (same)   18   5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′               Brain   PCR-1   Forward   mGluR1-718-2F   11   5′-GTG AAT CAG AGG AAG TGT TCA GA-3′       mGluRTαa       Reverse   mGluR1-4253R   17   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   mGluR1-718-3F   12   5′-AAT GTA ACA GTC ACT GGT GCT GGG-3′               Reverse   mGluR1-4198R   18   5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′               Brain   PCR-1   Forward   mGluR1-718-2F   11   5′-GTG AAT CAG AGG AAG TGT TCA GA-3′       mGluRTαb       Reverse   mGluR1-3266R   19   5′-GGG TAT TGT CCT CTT CTT CCA CA-3′           PCR-2   Forward   mGluR1-718-3F   12   5′-AAT GTA ACA GTC ACT GGT GCT GGG-3′               Reverse   mGluR1-3266R   19   5′-GGG TAT TGT CCT CTT CTT CCA CA-3′               Brain   PCR-1   Forward   mGluR1-790-1F   13   5′-GGG ACT CTC TCC TGT CTT GTG AG-3′       mGluRTβa       Reverse   mGluR1-4253R   17   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   mGluR1-790-2F   14   5′-AGC ATA ACA GGG AAT TGC AGT GG-3′               Reverse   mGluR1-4198R   18   5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3               Brain   PCR-1   Forward   (same)   13   5′-GGG ACT CTC TCC TGT CTT GTG AG-3′       mGluRTβb       Reverse   (same)   17   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   (same)   14   5′-AGC ATA ACA GGG AAT TGC AGT GG-3′               Reverse   (same)   18   5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3               Brain   PCR-1   Forward   mGluR1-1599-200F   15   5′-CAG ACA GAA TAT AAT AGT CGG TC-3′       mGluRTγa       Reverse   mGluR1-4253R   17   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   mGluR1-1599-221F   16   5′-ACA AGT ACA AAA CAA GCT CTG C-3′               Reverse   mGLuR1-4198R   18   5′-ATA ATT CAA GAG TCA CAA TCC TGG-C3′               Brain   PCR-1   Forward   (same)   15   5′-CAG ACA GAA TAT AAT AGT CGG TC-3′       mGluRTγb       Reverse   (same)   17   5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′           PCR-2   Forward   (same)   16   5′-ACA AGT ACA AAA CAA GCT CTG C-3′               Reverse   (same)   18   5′-ATA ATT CAA GAG TCA CAA TCC TGG-C3′                  
 
       EXAMPLE 2  
     Identification of Localization of Glutamate Receptor by Immunostaining Means  
       [0087]     &lt;1&gt; Preparation of slice specimen of tongue of rat Under anesthetization with ether, right auricle of heart of rat (Wistar strain; male; 10 to 15 weeks age) was incised and blooded; immediately after that, tongue site was collected.  
         [0088]     The cut-out tongue specimen was shaken for one night and day with 4% paraformaldehyde (4° C.) and immobilized by dipping therein. After that, it was dipped for 3 to 4 days in 20% sucrose-PBS to cryoprotect, embedded in Tissue-Tek R  (OCT compound) and sliced into 5 to 7 μm using a cryostat. The slices were dried at room temperature and stored at 4° C. until subjecting to staining.  
         [0089]     &lt;2&gt; Immunostaining by anti-metabotropic glutamate receptor type 1 antibody  
         [0090]     Immunostaining of the slices was carried out according to a method mentioned in Drengk, A. C., et al.,  J. Auto. Nerv. Sys.,  78:109-112, 2000 and Miampamba, M., et al.,  J. Auto. Nerv. Sys.  77:140-151, 1999. After the slices were washed with PBS firstly, they were treated with 3% hydrogen peroxide methanol in order to inhibit the reaction by intrinsic peroxidase. After that, the slices were washed with PBS and were subjected to a blocking for 1 hour using 1% normal bovine serum albumin-added PBS (1% BSA-PBS) containing 10% normal equine serum. After washing with PBS once again, they were made to react with a primary antibody (anti-mGluR1a, rabbit, polyclonal, Chemicon, cat# AB 1551) diluted with 1% BSA-PBS containing 1% normal equine serum at 4° C. for two nights. Then the slices were washed with PBS and made to react with the secondary antibody (anti-mGluR1a, rabbit, polyclonal, Chemicon, cat# AB 1551) diluted with 1% BSA-PBS at room temperature for one hour. Finally, reaction with ABC (avidin-biotin complex) was carried out using a Vectorstain Elite Kit (Vector) and colorized using 0.025% diaminobenzidine-0.25% nickel chloride-0.01% H 2 O 2 . After completion of the reaction, the slices were washedwith PBS, dehydrated with ethanol-xylene, sealed and observed under a microscope. That which was not treated with a primary antibody was used as a negative control.  
         [0091]     &lt;3&gt;Results  
         [0092]     The result of the immunostaining is shown in  FIG. 3 . In the tongue specimen, taste cells were stained by the anti-mGluR1 antibody ( FIG. 3A ). It has been usually believed that no mGluR1 receptor is expressed in taste cells. Therefore, it is believed that mGluR1 variant is expressed in the taste cells; and functionally, relation to the umami reception was suggested. In the stomach specimens ( FIG. 3B ), each of cell which produces viscous liquid of pylorus and main cell and auxiliary cell of stomach body was stained by the anti-mGluR1. It has been generally believed no mGluR1 receptor is expressed in those cells. Therefore, mGluR1 variant was expressed in viscous liquid producing cell and main cell, and relation to secretion of viscous liquid and secretion of digestive enzyme was suggested, functionally.  
       Example 3  
     Presumption of Function of mGluR1 Variant  
       [0093]     Rats (Wistar strain, males, 8 to 10 weeks age; Nippon Charles Liver) were fasted for 18 hours, laparotomy was conducted under anesthetization with urethane (1 g/kg; i.p.) and vagal gastric branch was exfoliated to an extent of about 5 mm. After vagal bundle was cut, it was placed on a small operation stand (8×6 mm), fat and bonded tissues around that were carefully detached and the terminal fiber at the side of organ was placed on a dipole electrode made of platinum for recording and insulated from the surrounding tissues by a mixture of liquid paraffin-vaseline (1:1). In the meanwhile, as a route for administration of MSG (sodium L-glutamate; manufactured by Ajinomoto Co., Inc.), a silicon tube for oral administration was implanted in the stomach.  
         [0094]     Nerve action potential was amplified to an extent of 10,000-fold by a micropotential amplifier (DAM-80 manufactured by WPI); and after noise was reduced by a Vessel filter (4-pole, High Cut 10 Hz, Low Cut 1 KHz), it was subjected to an A/D conversion (Powerlab 4sp, manufactured by ADI Instruments, Inc.) and incorporated into a computer (sampling rate 3 KHz, iBook). At the same time, an amplified signal was separated by a window discriminator (DSE-435 manufactured by Daiya Medical Co., Ltd.) into a noise component and a nerve signal component together with monitoring by an oscilloscope, integrated for 5 seconds by a spike counter (DSE-335P manufactured by Daiya Medical Co., Ltd.) and recorded in a chart recorder (WT-465G manufactured by Nippon Koden Corporation). In analysis of the spike wave shape, a SHE software (manufactured by ADI Instruments, Inc.) was used.  
         [0095]     The result is shown in  FIG. 4 . Centripetal activity of vagal gastric branch upon administration of 150 mM of MSG to stomach was accelerated. Vagal centripetal path is believed to be a signal transmittance path which sends visceral sense, particularly nutrition information from stomach and intestine, to bulbar nucleus of solitary tract and conducts digestion adjustment by after-meal sense such as satisfactory and unpleasant senses and adjustment of vagal centrifugal path. Accordingly, the fact that vagal centripetal activity was accelerated by administration of MSG to digestive tracts shows the possibility that MSG is a cause for generation of its signal and that mGluR1 variant expressed in lumen of digestive tract mediates its signal generation.  
       Example 4  
     Expression of mGluR1 Variant in Tissues by RT-PCR Method  
       [0000]     Whole Coding Sequence  
         [0096]     cDNA prepared by reverse transcription of total RNA of brain and circumvallate papilla with transcriptase was used. With regard to the 1st PCR primer, 30 cycles of PCR were carried out using mGluR1-790-1F (forward) (SEQ ID NO: 13), mGluR1-4253R (reverse) (SEQ ID NO: 17) and Z-Taq and the resulting PCR product diluted to an extent of 10-fold was used as a template for the 2nd PCR. For primer in the 2nd PCR, mGluR1-718-3F (forward; 5′-AAT GTA ACA GTC ACT GGT GCT GGG-3′) (SEQ ID NO: 12) and mGluR1-3266R (reverse: 5′-GGG TAT TGT CCT CTT CTT CCA CA-3′) (SEQ ID NO: 19) were used in the case of mGluRTα; while, in the case of mGluRTβ, mGluR1-790-2F (forward) (SEQ ID NO: 14) and mGluR1-4198R (reverse) (5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′) were used. That was conducted in 30 cycles as well using Z-Taq. The resulting PCR products were already confirmed by an ABI Sequencer Model 3100. Incidentally, the type of the device used for the PCR was GeneAmp PCR System 9700. As a result, bands were confirmed near 1760 bp (type A) and 1900 bp (type B) for mGluRTα; while, for mGluRTβ, a band was confirmed near 2000 bp, and the expression was confirmed in circumvallate papilla ( FIG. 5A  and  FIG. 5B ).  
       Example 5  
     Analysis of Function: Oocyte Isolation  
       [0097]     Oocyte-expressing strain of  Xenopus  was used for analysis of function of mGluR1 mRNA derived from circumvallate papilla of rats.  
         [0098]     Female  Xenopus  (purchased from Watanabe Zoshoku) was bred in a fish tank until oocytes were isolated. The  Xenopus  was anesthetized with tricain methanesulfonate (MS 222, Sigma) dissolved in deionized water in a concentration of 1 g/L followed by buffering with NaHCO 3  (500 mg/L). Anatomy was conducted by hand and oocytes in stages V and VI were recovered from ovary and incubated (for 30 minutes to 1 hour) in a 0.2% collagenase S-1 (Nitta Gelatin) until dissociation took place. After treating with collagenase, the oocytes were washed; follicles were removed; selection was conducted under a microscope; and incubation was carried out at 18° C. for one night.  
         [0000]     Metabotropic Glutamate cRNA Preparation  
         [0099]     Variant mGluR1 cDNA was constructed by an RT-PCR method from total RNA of taste papilla and brain of rat using the following primers (mGluRTβ: 1st PCR: mGluR1-790-1F 5′-GGG ACT CTC TCC TGT CTT GTG AG-3′ (SEQ ID NO: 13), mGluR1-4253R 5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′ (SEQ ID NO: 17), 2nd PCR: mGluR1 790-2F forward 5′-AGC ATA ACA GGG AAT TGC AGT GG-3′ (SEQ ID NO: 14), mGluR1 4198 reverse 5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′ (SEQ ID NO: 18), mGluRTγ: 1st PCR: mGluR1-1599-200F 5′-CAG ACA GAA TAT AAT AGT CGG TC-3′ (SEQ ID NO: 15), mGluR1-4253R 5′-TAC CAT ATG GAA TTG TGC TTT GTC A-3′ (SEQ ID NO: 17), 2nd PCR: mGluR1-1599-221F 5′-ACA AGT ACA AAA CAA GCT CTG C-3′ (SEQ ID NO: 16), mGluR1 4198 reverse 5′-ATA ATT CAA GAG TCA CAA TCC TGG C-3′ (SEQ ID NO: 18)) and constitutive enzyme Z-Taq DNA polymerase. Since the polymerase remains blunt end in PCR fragments, PCR-amplified mGluRTβa and mGluRTγa template DNA were inserted into pCRII-TOPO vector (Invitrogen) by a TOPO-TA cloning reaction. pCRII-TOPO/mGluRTγ vector and pCRII-TOPO/mGluRTβ vector were made into straight lines using EcoRV and XbaI, respectively, extracted with a mixture of phenol and chloroform and precipitated with ethanol together with sodium acetate. After a sequence analysis, cRNA was prepared using a transcription kit for Sp6 promoter made by Ambion (mMESSAGE MMACHINE kit) binding to pCRII-TOPO promoter. Briefly, transcription of straight-chain template DNA of about 1 μg was carried out using 2 μL of enzyme mixture (the final concentration was made 20 μL volume using 10 μL of 2XNTP/Cap and 10× reaction buffer). For the synthesis of cRNA, the reaction solution was incubated at 37° C. for 3 hours and the residual template was decomposed by addition of 1 μL of DNase 1 for 15 minutes. The transcribed product was purified by extracting with phenol-chloroform and precipitating with isopropanol. cRNA was reconstructed in water treated with diethylpyrocarboxylic acid; and before injecting into oocytes, quantitative determination was conducted under irradiation with UV.  
         [0000]     Microinjection of cRNA  
         [0100]     After 24 hours from the recovery, 100 ng of mRNA per 25 nL of glass capillary tube having a standard diameter of 12 μm (Microinjector, WPI) was injected into oocytes of healthy  Xenopus  having transparent animal pole and vegetal pole. Incubation was carried out for 72 hours in an MBS solution [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  and 0.91 mM CaCl 2 ; pH 7.5] to which 2 mM pyruvic acid and 0.5 mM theophylline were added; and after that, the oocytes were used for an electrophysiological assay (Sanna, et al., 1994).  
         [0000]     Method for Measurement of Membrane Current  
         [0101]     Membrane current of oocytes was measured by a two-electrode membrane potential fixation method using Geneclamp Amplifier (Axon Instruments). Glass microelectrode used for the measurement was prepared by a puller (Shutter Co., Ltd.) and that where electrode resistance upon filling with 3M KCl was 1 to 3 MΩ was used. The treated oocytes were transferred to a chamber for the measurement; glass electrode was inserted under a stereoscopic microscope; and under fixation of potential to −70 mV, calcium-dependent chloride current upon stimulation with glutamic acid was measured. The experiment was carried out for both of the cases of cells which express mGluR1 variant of rat and which do not.  
         [0102]     The result of record of measurement of membrane current of oocytes where mGluRTβa was expressed is shown in  FIG. 6 . When the medium in an inner area of the measuring bath was substituted with a medium containing 50 mM of sodium glutamate (MSG), continuous introvert current was induced and the introvert current was confirmed to disappear by performing substitution of the medium again. That is believed to be due to the fact that glutamic acid acted on the receptor whereupon a calcium-dependent chloride channel via an intracellular information transmittance system was activated; and as a result, that was measured as introvert current. The same introvert current was also observed in mGluRTγa-expressing oocytes. Incidentally, although not shown here, such a reaction was not observed in oocytes into which cRNA was not injected. Therefore, it has now been proved that the mGluR1 variant which is a cloning gene product of the present invention has an action of conducting the glutamic acid reception and inducing an intracellular calcium mobilization. According to the same proceeding, it has been also confirmed that mGluRTα has a ligand action and it is also possible to search agonist, antagonist or allosteric modulator.  
       INDUSTRIAL APPLICABILITY  
       [0103]     In accordance with the present invention, 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.  
         [0104]     The invention 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 invention and all such modifications and variations are intended to be included within the scope of the following claims.