Evidence for NMDA receptor in the afferent synaptic transmission of the vestibular system.

This study aimed to define the pharmacology and physiological role of the N-methyl-D-aspartate (NMDA) receptor in the synapse between the hair cells and primary afferent neurons in the vestibular system. The spontaneous and mechanically evoked spike discharges of vestibular nerve fibers were extracellularly recorded in isolated inner ear from the axolotl (Ambystoma tigrinum). Pressure ejection of NMDA (10(-6) to 10(-3) M) elicited a dose-dependent increase of the basal spike discharge from the vestibular nerve fibers. Extracellular magnesium antagonized the NMDA effect in a dose-dependent manner. D(-)-2-amino-5-phosphonovaleric acid (AP5, 10(-5) to 10(-3) M) and 7-chloro-kynurenic acid (7ClKyn, 10(-6) to 10(-3) M) inhibited the basal activity of the vestibular nerve fibers. 7ClKyn also diminished the responses elicited by the mechanical stimulation of the preparation. Glycine (10(-9) to 10(-6) M) applied by bath substitution enhanced the NMDA response, and the glycine agonist D-serine partially reversed the 7ClKyn inhibitory action. These results suggest that NMDA receptors participate in the generation of the basal spike discharge of vestibular system primary afferent neurons, but its activation is not critical for the response to brief mechanical stimuli.