Responses to monosodium glutamate and guanosine 5'-monophosphate in rat fungiform taste cells.

Monosodium glutamate (MSG) elicits a unique taste sensation called umami. The umami sensation is potentiated by the presence of 5'-ribonucleotides such as guanosine 5'-monophosphate (5'-GMP). We have used giga-seal whole cell recording to examine glutamate transduction in individual cells of isolated rat fungiform taste buds. Approximately 56% of fungiform taste cells responded to bath application of glutamate. Three types of responses occurred: decrease in inward holding current; increase in inward holding current; and a biphasic response, with an increase followed by a decrease in holding current. Similar responses were observed in response to 5'-GMP. Further, responses to 5'-GMP may occur in cells that are glutamate-insensitive, suggesting that different receptors mediate the transduction of glutamate and 5'-GMP. Simultaneous bath application of glutamate and 5'-GMP resulted in a synergistic response in some taste cells.