Chaudhari N, Yang H, Lamp C, Delay E, Cartford C, Than T, Roper S
Department of Physiology, Colorado State University, Ft. Collins 80523, USA.
J Neurosci. 1996 Jun 15;16(12):3817-26. doi: 10.1523/JNEUROSCI.16-12-03817.1996.
Receptor proteins for photoreception have been studied for several decades. More recently, putative receptors for olfaction have been isolated and characterized. In contrast, no receptors for taste have been identified yet by molecular cloning. This report describes experiments aimed at identifying a receptor responsible for the taste of monosodium glutamate (MSG). Using reverse transcriptase (RT)-PCR, we found that several ionotropic glutamate receptors are present in rat lingual tissues. However, these receptors also could be detected in lingual tissue devoid of taste buds. On the other hand, RT-PCR and RNase protection assays indicated that a G-protein-coupled metabotropic glutamate receptor, mGluR4, also is expressed in lingual tissues and is limited only to taste buds. In situ hybridization demonstrated that mGluR4 is detectable in 40-70% of vallate and foliate taste buds but not in surrounding nonsensory epithelium, confirming the localization of this metabotropic receptor to gustatory cells. Expression of mGluR4 in taste buds is higher in preweaning rats compared with adult rats. This may correspond to the known higher sensitivity to the taste of MSG in juvenile rodents. Finally, behavioral studies have indicated that MSG and L-2-amino-4-phosphonobutyrate (L-AP4), a ligand for mGluR4, elicit similar tastes in rats. We conclude that mGluR4 may be a chemosensory receptor responsible, in part, for the taste of MSG.
光感受器的受体蛋白已被研究了几十年。最近，嗅觉的假定受体已被分离和鉴定。相比之下，味觉受体尚未通过分子克隆被鉴定出来。本报告描述了旨在鉴定负责谷氨酸钠（味精）味道的受体的实验。使用逆转录酶（RT）-PCR，我们发现大鼠舌组织中存在几种离子型谷氨酸受体。然而，在没有味蕾的舌组织中也能检测到这些受体。另一方面，RT-PCR和核糖核酸酶保护试验表明，一种G蛋白偶联的代谢型谷氨酸受体mGluR4也在舌组织中表达，并且仅局限于味蕾。原位杂交表明，在40%-70%的轮廓乳头和叶状乳头味蕾中可检测到mGluR4，但在周围的非感觉上皮中未检测到，这证实了这种代谢型受体在味觉细胞中的定位。与成年大鼠相比，断奶前大鼠味蕾中mGluR4的表达更高。这可能与幼年啮齿动物对味精味道已知的更高敏感性相对应。最后，行为学研究表明，味精和mGluR4的配体L-2-氨基-4-膦酰丁酸（L-AP4）在大鼠中引发相似的味道。我们得出结论，mGluR4可能是一种化学感受器，部分负责味精的味道。
