Abstract
Glutamate is the neurotransmitter of the synapse between vestibular type I hair cells and the afferent nerve calyx. This calyx may also be involved in local feedback, which may modify sensory cell activity via N-methyl- D-aspartate (NMDA) receptors. Glycine is the co-agonist of glutamate in NMDA receptor activation. Both agents have been detected by immunocytochemistry in the nerve calyx. Glutamate and NMDA stimulations cause changes in the intracellular calcium concentration ([Ca 2+] i) of isolated type I sensory cells. We investigated the effect of glycine stimulation on [Ca 2+] i in guinea pig type I sensory cells by spectrofluorimetry with fura-2. Glycine application to isolated type I sensory cells induced a rapid and transient increase in [Ca 2+] i. The fluorescence ratio increased by 55% above the resting level. The peak was reached in 9 s and the return to basal level took about 20 s. A specific antagonist of the glycine site on NMDA receptors, 7-chlorokynurenate (10 μM), decreased the calcium response to glycine by 60%. Glycine may activate NMDA receptors. Glycine may also activate the strychnine-sensitive glycine receptor-gated channel. Strychnine (50 μM) decreased the calcium response to glycine by 60%. Thus, glycine probably induces calcium concentration changes in type I vestibular sensory cells via NMDA receptors and/or glycine receptors.
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