We studied taste transduction in sensory receptor cells. Specifically, we examined the actions of glutamate, a significant taste stimulus, on the membrane properties of taste cells by applying whole cell patch-clamp techniques to cells in rat taste buds isolated from foliate and vallate papillae. In 55 of 91 taste cells, bath-applied glutamate, at concentrations that elicit taste responses in the intact animal (10-20 mM), produced one of two different responses when the cell membrane was held near its presumed resting potential, -85 mV. 'Sustained' glutamate responses were observed in the majority of taste cells (51 of 55) and consisted of an outward current (reduction of the maintained inward current). Sustained glutamate responses were voltage dependent, were decreased by membrane depolarization, and were accompanied by a reduction in membrane conductance. An analysis of the reversal potential of sustained responses in different ionic conditions and the effect of ion substitutions suggested that the currents were carried by cations. The data suggest that sustained responses are mediated by the closure of nonselective cation channels. Other taste cells (4 of 55) responded to glutamate with a transient inward current-so called 'transient' responses. Transient glutamate responses were voltage dependent and Na+ dependent, and appeared to be generated by nonspecific cation channels activated by glutamate. L(+)-2-amino-4-phosphonobutyric acid (L-AP4), a specific agonist of a metabotropic glutamate receptor (mGluR4) recently identified in rat taste cells and believed to be involved in taste transduction, mimicked the sustained glutamate responses. These findings indicate that glutamate, at concentrations at or slightly above threshold for taste in rats, produces two different membrane currents. The properties of these two responses suggest that there may be two different sets of nonspecific cation channels in taste cells, one closed by glutamate (sustained response) and the other opened (transient response). Our findings on the effect of L-AP4 suggest that the sustained response is the membrane mechanism mediating, at least in part, taste transduction for glutamate.
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