Rolls E T, Critchley H D, Wakeman E A, Mason R
Department of Experimental Psychology, University of Oxford, England.
Physiol Behav. 1996 Apr-May;59(4-5):991-1000. doi: 10.1016/0031-9384(95)02178-7.
To investigate the neural encoding of glutamate taste in the primate, recordings were made from taste responsive neurons in the cortical taste areas in macaques. Most of the neurons were in the orbitofrontal cortex taste area, with a small number in adjacent taste areas. First, it was shown that single neurons that had their best responses to sodium glutamate also had good responses to glutamic acid. The correlation between the responses to these two tastants was higher than between any other pair of tastants, which included glucose (sweet), sodium chloride (salty), HCl (sour), and quinine HCl (bitter). Accordingly, the responsiveness to glutamic acid clustered with the response to monosodium glutamate in a cluster analysis with this set of stimuli, and glutamic acid was close to sodium glutamate in a space created by multidimensional scaling. Second, it was shown that the responses of these neurons to the nucleotide umami tastant inosine 5'-monophosphate were more correlated with their responses to monosodium glutamate than to any prototypical tastant. Third, concentration response curves showed that concentrations of monosodium glutamate as low as 0.001 M were just above threshold for some of these neurons. Fourth, neurons have not yet been found in this cortical region that showed synergism of monosodium glutamate and the nucleotide inosine 5'-monophosphate: it was shown that mixtures of 0.0001 M inosine 5'-monophosphate with different concentrations (0.001, 0.01, and 0.1 M) of monosodium glutamate did not have a greater effect than the monosodium glutamate alone. Fifth, some neurons in the orbitofrontal region, which responded to monosodium glutamate and other food tastes, decreased their responses after feeding with monosodium glutamate to behavioural satiety. In some cases this reduction was sensory-specific. These findings show that the taste neurons activated by monosodium glutamate can also be activated by other umami tastants, including glutamic acid and the nucleotide inosine 5'-monophosphate. The responses to these umami tastants were more similar to each other than to any of the other prototypical tastants, providing evidence that in this system umami is encoded differently from the other tastants. Moreover, the findings with these tastants provide additional evidence that the responses to monosodium glutamate are not due just to activation of a sodium taste channel.
为了研究灵长类动物中谷氨酸味觉的神经编码，研究人员对猕猴大脑皮质味觉区域中对味觉有反应的神经元进行了记录。大多数神经元位于眶额皮质味觉区，少数位于相邻的味觉区。首先，研究表明，对谷氨酸钠反应最佳的单个神经元对谷氨酸也有良好反应。这两种味觉物质反应之间的相关性高于其他任何一对味觉物质，其他味觉物质包括葡萄糖（甜味）、氯化钠（咸味）、盐酸（酸味）和盐酸奎宁（苦味）。因此，在对这组刺激进行聚类分析时，对谷氨酸的反应性与对谷氨酸钠的反应聚集在一起，并且在多维标度创建的空间中，谷氨酸与谷氨酸钠接近。其次，研究表明，这些神经元对核苷酸鲜味剂5'-肌苷单磷酸的反应与其对谷氨酸钠的反应的相关性高于对任何典型味觉物质的反应。第三，浓度反应曲线表明，低至0.001 M的谷氨酸钠浓度对其中一些神经元来说刚好高于阈值。第四，在这个皮质区域尚未发现显示谷氨酸钠与核苷酸5'-肌苷单磷酸协同作用的神经元：研究表明，0.0001 M 5'-肌苷单磷酸与不同浓度（0.001、0.01和0.1 M）的谷氨酸钠的混合物并不比单独的谷氨酸钠有更大的作用。第五，眶额区域中一些对谷氨酸钠和其他食物味道有反应的神经元，在喂食谷氨酸钠至行为饱腹感后，其反应会降低。在某些情况下，这种降低是感觉特异性的。这些发现表明，被谷氨酸钠激活的味觉神经元也可以被其他鲜味剂激活，包括谷氨酸和核苷酸5'-肌苷单磷酸。对这些鲜味剂的反应彼此之间比与任何其他典型味觉物质的反应更相似，这证明在这个系统中鲜味的编码方式与其他味觉物质不同。此外，这些味觉物质的研究结果提供了额外的证据，表明对谷氨酸钠的反应不仅仅是由于钠味觉通道的激活。
