NMDA and non-NMDA receptors mediate responses in the primary gustatory nucleus in goldfish

Primary gustatory afferents from the oropharynx of the goldfish, Carassius auratus, terminate in the vagal lobe, a laminated structure in the dorsal m edulla comparable to the gustatory portion of the nucleus of the solitary t ract in mamma Is. We utilized an in vitro brain slice preparation to test t he role of different ionotropic glutamate receptor subtypes in synaptic tra nsmission of gustatory information by recording changes in field potentials after application of various glutamate receptor antagonists. Electrical st imulation of the vagus nerve (NX) evokes two short-latency postsynaptic fie ld potentials from sensory layers of the vagal lobe. 6,7-Dinitroquinoxaline -2,3-dione and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione, two non-N -methyl-D-aspartate (NMDA) ionotropic receptor antagonists, blocked these s hort-latency potentials. Slower potentials that were revealed under Mg2+-fr ee conditions, were abolished by the NMDA receptor antagonist, D(-)-2-amino -5-phosphonovaleric acid (APV). Repetitive stimulation produced short-term facilitation, which was attenuated by application of APV. These results ind icate that the synaptic responses in the vagal lobe produced by stimulation of the gustatory roots of the NX involve both NMDA and non-NMDA receptors. An NMDA receptor-mediated facilitation may serve to amplify incoming burst s of primary afferent activity.