Maruyama Yutaka, Pereira Elizabeth, Margolskee Robert F, Chaudhari Nirupa, Roper Stephen D
Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, Florida 33136, USA.
J Neurosci. 2006 Feb 22;26(8):2227-34. doi: 10.1523/JNEUROSCI.4329-05.2006.
A number of gustatory receptors have been proposed to underlie umami, the taste of L-glutamate, and certain other amino acids and nucleotides. However, the response profiles of these cloned receptors have not been validated against responses recorded from taste receptor cells that are the native detectors of umami taste. We investigated umami taste responses in mouse circumvallate taste buds in an intact slice preparation, using confocal calcium imaging. Approximately 5% of taste cells selectively responded to L-glutamate when it was focally applied to the apical chemosensitive tips of receptor cells. The concentration-response range for L-glutamate fell approximately within the physiologically relevant range for taste behavior in mice, namely 10 mm and above. Inosine monophosphate enhanced taste cell responses to L-glutamate, a characteristic feature of umami taste. Using pharmacological agents, ion substitution, and immunostaining, we showed that intracellular pathways downstream of receptor activation involve phospholipase C beta2. Each of the above features matches those predicted by studies of cloned and expressed receptors. However, the ligand specificity of each of the proposed umami receptors [taste metabotropic glutamate receptor 4, truncated metabotropic glutamate receptor 1, or taste receptor 1 (T1R1) and T1R3 dimers], taken alone, did not appear to explain the taste responses observed in mouse taste cells. Furthermore, umami responses were still observed in mutant mice lacking T1R3. A full explanation of umami taste transduction may involve novel combinations of the proposed receptors and/or as-yet-undiscovered taste receptors.
许多味觉受体被认为是鲜味（L-谷氨酸、某些其他氨基酸和核苷酸的味道）的基础。然而，这些克隆受体的反应谱尚未与从鲜味味觉的天然探测器——味觉受体细胞记录的反应进行验证。我们使用共聚焦钙成像技术，在完整切片制备中研究了小鼠轮廓乳头味蕾中的鲜味味觉反应。当L-谷氨酸局部施加到受体细胞的顶端化学敏感尖端时，约5%的味觉细胞对其有选择性反应。L-谷氨酸的浓度-反应范围大致落在小鼠味觉行为的生理相关范围内，即10 mM及以上。肌苷一磷酸增强了味觉细胞对L-谷氨酸的反应，这是鲜味味觉的一个特征。通过使用药物制剂、离子替代和免疫染色，我们表明受体激活下游的细胞内信号通路涉及磷脂酶Cβ2。上述每个特征都与对克隆和表达受体的研究所预测的特征相匹配。然而，单独考虑每个提议的鲜味受体（味觉代谢型谷氨酸受体4、截短的代谢型谷氨酸受体1或味觉受体1（T1R1）和T1R3二聚体）的配体特异性，似乎无法解释在小鼠味觉细胞中观察到的味觉反应。此外，在缺乏T1R3的突变小鼠中仍观察到鲜味反应。鲜味味觉转导的完整解释可能涉及提议受体的新组合和/或尚未发现的味觉受体。
