GLUTAMATE EXCITATORY EFFECTS ON AMPULLAR RECEPTORS OF THE FROG

The action of glutamate on frog ampullar receptors was investigated to assess the potential role of this excitatory amino acid as an afferen t transmitter in the hair cell system. Intracellular recordings from s ingle afferent units in the isolated labyrinth revealed that glutamate and the glutamate receptor agonists, N-methyl-D-aspartic acid, quisqu alic acid and kainic acid increase dose-dependently the frequency of t he resting afferent discharge of EPSPs and spikes and produce long las ting depolarizations. After blocking synaptic transmission by using 5 mM Co2+, the same compounds elicited only depolarizations of amplitude comparable to those observed in normal saline. Quisqualic acid and ka inic acid were much more potent than N-methyl-D-aspartic acid in incre asing the frequency of afferent discharge and in causing axonal depola rizations. The depolarization caused by glutamate was reduced dose-dep endently by the competitive non-NMDA receptor antagonist 6-cyano-7-nit roquinaxoline-2,3 dione and disappeared almost completely in Na+-free Ringer solution. These results are consistent with the hypothesis that glutamate is the afferent transmitter in vestibular organs and indica te that receptors mainly of the non-NMDA type are present not only at postsynaptic level but also in hair cells. Presynaptic glutamate recep tors may function as autoreceptors controlling by a positive feed-back mechanism the release of the afferent transmitter.