Participation of ionotropic and metabotropic glutamate receptors in taste cell responses to MSG

Abstract

We have previously reported that monosodium glutamate (MSG) stimulation elicited three types responses (transient inward current, sustained inward current and outward current) while L-AP4 (a potent mGluR4 agonist) evoked only outward currents in C57BL/6J mouse taste cells. The outward current responses to MSG and L-AP4 appeared to be mediated by metabotropic glutamate type 4 receptors (mGluR4). In this study, we examined whether agonists of ionotropic glutamate receptor agonists (NMDA, AMPA and ibotenic acid) also elicit responses in mouse taste cells. NMDA (1 mM) elicited transient inward currents, similar to those often observed with MSG, indicating the presence of NMDA receptor/channels that are permeable to Ca2+ ions and are activated by MSG in some taste cells. The sustained inward current response to MSG appeared to result from activation of a nonselective cation conductance, but it is not known if this response is coupled to ionotropic or metabotropic receptors. AMPA (1 mM) elicited small outward currents in all responding cells. Ibotenic acid, which produces a considerably stronger umami taste than MSG in humans, elicited two types of responses in isolated cells; transient inward currents and sustained inward current, suggesting that inward, as well as outward, currents are related to umami transduction. Also, we confirmed that MSG, AP4 (2 mM) and NMDA all can increase [Ca2+]i in taste cells. These results indicate that some cells have both metabotropic and ionotropic receptors, while other taste cells have only one or the other type of receptor.