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The Journal of Neurophysiology Vol. 82 No. 5 November 1999, pp. 2061-2069
Copyright ©1999 by the American Physiological Society
Department of Anatomy and Neurobiology, Colorado State University, Fort Collins 80523; and Rocky Mountain Taste and Smell Center, University of Colorado Health Sciences Center, Denver, Colorado 80262
Lin, Weihong and
Sue C. Kinnamon.
Physiological Evidence for Ionotropic and Metabotropic Glutamate
Receptors in Rat Taste Cells. J. Neurophysiol. 82: 2061-2069, 1999. Monosodium glutamate (MSG)
elicits a unique taste in humans called umami. Recent molecular studies
suggest that glutamate receptors similar to those in brain are present
in taste cells, but their precise role in taste transduction remains to
be elucidated. We used giga-seal whole cell recording to examine the
effects of MSG and glutamate receptor agonists on membrane properties of taste cells from rat fungiform papillae. MSG (1 mM) induced three
subsets of responses in cells voltage-clamped at 
80 mV: a decrease in
holding current (subset I), an increase in holding current (subset II),
and a biphasic response consisting of an increase, followed by a
decrease in holding current (subset III). Most subset II glutamate
responses were mimicked by the ionotropic glutamate receptor (iGluR)
agonist N-methyl-D-aspartate (NMDA). The
current was potentiated by glycine and was suppressed by the NMDA
receptor antagonist D()-2-amino-5-phosphonopentanoic acid (AP5). The group III metabotropic glutamate receptor (mGluR) agonist L-2-amino-4-phosphonobutyric acid (L-AP4)
usually mimicked the subset I glutamate response. This hyperpolarizing
response was suppressed by the mGluR antagonist
(RS)-
-cyclopropyl-4-phosphonophenylglycine (CPPG) and by
8-bromo-cAMP, suggesting a role for cAMP in the transduction pathway.
In a small subset of taste cells, L-AP4 elicited an
increase in holding current, resulting in taste cell depolarization under current clamp. Taken together, our results suggest
that NMDA-like receptors and at least two types of group III mGluRs are
present in taste receptor cells, and these may be coactivated by MSG.
Further studies are required to determine which receptors are located
on the apical membrane and how they contribute to the umami taste.
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