Psychophysical and electrophysiological studies indicated that
the umami substances have no enhancing activity on other primary
tastes. Experiments using amiloride clearly show that the umami
component of canine chorda tympani nerve response to umami substances
is independent of the salt component. Single fiber analysis
of the responses of the mouse glossopharyngeal nerve and the
monkey primary taste cortex neuron show that the responses to
umami substances are independent of other primary tastes. A
large synergism between monosodium glutamate (MSG) and disodium
5'-inosinate (IMP) or disodium 5'-guanylate (GMP) is observed
in dogs and is explained in terms of allosteric effect. The
order of intensity of umami taste induced by a mixture of 0.5
mM GMP and 1.5 mM of various agonists for the glutamate receptors
was glutamate>ibotenate>dl(+)-2-amino-4-phosphonobutyric
acid (DL-AP4)-(+)-1-aminocyclopentane-
trans-1,3-dicarboxylic
acid (
trans-ACPD). Kainate,
N-methyl-d-aspartate (NMDA) and
(RS)-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA),
which are agonists for ionotropic receptors, have no umami taste.
It was concluded that the umami receptor is not identical to
any of known glutamate receptors, and there seems to be a unique
receptor for umami.
