Behavioral studies of umami: tales told by mice and rats.

Psychophysical research with rats and mice has been instrumental in understanding umami taste transduction and perception. Although early studies suggested that an NMDA-like receptor detected substances that elicit an umami taste, studies using behavioral methods with both rats and mice indicate that the picture is much more complex. When the G protein-coupled receptor T1R1+T1R3 was discovered, it was believed to be the umami receptor and a more broadly tuned L-amino acid receptor. However, since then a number of behavioral studies, like molecular and physiological studies, report evidence that other receptors may contribute to umami taste. For example, T1R3 knockout mice (KO) have only slightly elevated detection thresholds for monosodium glutamate (MSG) and L-alanine. In conditioned taste aversion studies, T1R3 KO mice show bidirectional generalization of the aversion between MSG and L-alanine, suggesting that these substances have similar tastes. However, these KO mice can discriminate between the tastes of the two substances, indicating other receptors also respond to these amino acids. (RS)-alpha-cycloprophy-4-phosphonophenylglycine (CPPG), a potent mGluR4 antagonist, decreases an aversion to MSG in rats while increasing the strength of generalization of the aversion to L-arginine or L-serine. These behavioral studies suggest that glutamate can activate several putative receptors, most notably T1R1+T1R3 and taste-mGluR4, and possibly NMDA-like receptors or taste-mGluR1. These receptors generate similar but not identical sensations which, when combined, form a complex perception identified as umami. Further, these studies suggest that afferent signaling from T1R1+T1R3 and taste-mGluR4 likely combine to generate the taste sensations associated with other L-amino acids.