Glutamate-induced cobalt uptake reveals non-NMDA receptors in rat taste cells

Taste receptor cells are chemical detectors in the oral cavity. Taste cells form synapses with primary afferent neurons that convey the gustatory info rmation to the central nervous system. Taste cells may also synapse with ot her taste cells within the taste buds. Furthermore, taste cells may receive efferent connections. However, the neurotransmitters at these synapses hav e not been identified. Glutamate, a major excitatory neurotransmitter in ot her sensory organs, might act at synapses in taste buds. We used a cobalt s taining technique to detect Ca2+-permeable glutamate receptors in taste bud s and thus establish whether there might be glutamatergic synapses in gusta tory end organs. When 500 mu m slices of foliate and vallate papillae were briefly exposed t o 1 mM glutamate in the presence of CoCl2, a subset of spindle-shaped taste cells accumulated Co2+ Cobalt uptake showed concentration-dependency in th e range from 10 mu m to 1 mM glutamate. Interestingly, higher glutamate con centrations depressed cobalt uptake. This concentration-response relation f or cobalt uptake suggests that synaptic glutamate receptors, not receptors for glutamate taste, were activated. Sensory axons and adjacent non-sensory epithelium were not affected by these procedures. Glutamate-stimulated cob alt uptake in taste cells was antagonized by the non-NMDA receptor antagoni st CNQX. Depolarization with 50 mM K+ and application of NMDA (300 mu M) di d not increase the number of stained taste cells. This pharmacological char acterization of the cobalt uptake suggests that non-NMDA receptors are pres ent in taste cells. These receptors might be autoreceptors at afferent syna pses, postsynaptic receptors of a putative efferent system, or postsynaptic receptors at synapses with other taste cells. (C) 2000 Wiley-Liss, Inc.