Pharmacology of the vestibular hair cell-afferent fiber synapse in the frog.

The isolated, intact, membranous labyrinth of the frog (Rana temporaria) has been investigated electrophysiologically in vitro to determine the nature of the transmitter substance at the synapse between the vestibular hair cells and afferent fibers. Spontaneous synaptic activity can be monitored with intra-axonal recordings from the afferents. Increased K+ in the bath results in an increase in frequency of presynaptic release, as indicated by an increased frequency of spontaneous synaptic potentials. Adding Mg2+ and lowering Ca2+ results in a decrease in synaptic potential frequency (often to zero) with no change in their mean amplitude, indicating pre-synaptic blockade. Extracellular recordings from individual vestibular afferents indicate that bath-applied glutamate and related acidic amino acids consistently increase the firing rates of these afferents in a dose-dependent manner with no evidence of desensitization. In the presence of presynaptic blockade (high Mg2+/low Ca2+), bath application of glutamate and its agonists results in a reversible depolarization of vestibular afferents, suggesting a postsynaptic action of these substances. 2-Amino-5-phosphonovaleric acid, kynurenic acid, and other acidic amino acid antagonists reversibly decrease the amplitudes of spontaneously occurring synaptic potentials without affecting their frequency, indicating subsynaptic blockade. These antagonists also block the postsynaptic depolarizations due to glutamate and its agonists. GABA and its agonists and antagonists have no consistent effect upon afferent activity. These findings suggest that glutamate, aspartate, or a related compound is the transmitter at this synapse. However, the antagonists used, or the receptors themselves, are not selective enough to discriminate adequately between the agonists. Therefore, which of these glutamate agonists are actually involved in synaptic transmission remains to be determined.