In situ Ca2+ imaging reveals neurotransmitter receptors for glutamate in taste receptor cells

The neurotransmitters at synapses in taste buds are not yet known with conf idence. Here we report a new calcium-imaging technique for taste buds that allowed us to test for the presence of glutamate receptors (GluRs) in livin g isolated tissue preparations. Taste cells of rat foliate papillae were lo aded with calcium green dextran (CaGD). Lingual slices containing CaGD-labe led taste cells were imaged with a scanning confocal microscope and superfu sed with glutamate (30 muM to 1 mM), kainate (30 and 100 muM), AMPA (30 and 100 muM), or NMDA (100 muM). Responses were observed in 26% of the cells t hat were tested with 300 muM glutamate. Responses to glutamate were localiz ed to the basal processes and cell bodies, which are synaptic regions of ta ste cells. Glutamate responses were dose-dependent and were induced by conc entrations as low as 30 muM. The non-NMDA receptor antagonists CNQX and GYK I 52466 reversibly blocked responses to glutamate. Kainate, but not AMPA, a lso elicited Ca2+ responses. NMDA stimulated increases in [Ca2+](i) when th e bath medium was modified to optimize for NMDA receptor activation. The su bset of cells that responded to glutamate was either NMDA-unresponsive (54% ) or NMDA-responsive (46%), suggesting that there are presumably two popula tions of glutamate-sensitive taste cells-one with NMDA receptors and the ot her without NMDA receptors. The function of GluRs in taste buds is not yet known, but the data suggest that glutamate is a neurotransmitter there. Glu Rs in taste cells might be presynaptic autoreceptors or postsynaptic recept ors at afferent or efferent synapses.