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

Abstract

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.

The Glutamate Receptor Subunit δ1 Is Highly Expressed in Hair Cells of the Auditory and Vestibular Systems

“…The functional site could be either postsynaptic to the hair cell or postsynaptic to the lateral efferent input to spiral ganglion cell dendrites beneath IHCs. The presence of ␦1 postsynaptic to hair cells is consistent with the hypothesis that an excitatory amino acid is the neurotransmitter of hair cells (Annoni et al, 1984;Soto and Vega, 1988;Bledsoe et al, 1989; for review, see Eybalin, 1993;Puel, 1995). Spiral and vestibular ganglion neurons express many glutamate receptors including AMPA, kainate, and NMDA ionotropic receptors and several metabotropic receptors (Ryan et al, 1991;Eybalin, 1992b, 1995;Demêmes et al, 1995;Niedzielski and Wenthold, 1995;Usami and Ottersen, 1995).…”

“…Similar studies have suggested that the vestibular afferent neurotransmitter is an excitatory amino acid (Annoni et al, 1984;Soto and Vega, 1988;Bledsoe et al, 1989).…”

“…First, they may f unction as autoreceptors. Several lines of evidence indicate that the neurotransmitter of the inner ear hair cells is an excitatory amino acid (Bledsoe et al, 1981(Bledsoe et al, , 1989Annoni et al, 1984;Soto and Vega, 1988;Puel et al, 1991aPuel et al, ,b, 1994Kataoka and Ohmori, 1994), and ␦1 may regulate its release from the hair cell. Restriction of ␦1 expression to IHCs in the organ of Corti is consistent with the fact that most (90 -95%) afferent transmission is from IHCs (Spoendlin, 1972;Berglund and Ryugo, 1987); OHCs may not need to regulate their excitatory amino acid release, which may occur only at modest levels, because OHCs form synapses on the remaining 5% of type II SGNs.…”

Effects of excitatory amino acid antagonists on evoked and spontaneous excitatory potentials in guinea‐pig hippocampus.

“…regions (Perkins & Stone, 1982;Cochran, 1983;Ganong et al 1983Ganong et al , 1986Ganong & Cotman, 1986), and interact with paired pulse potentiation of the perforant path in a manner consistent with a post-synaptic receptor blocker (Harris & Cotman, 1985). In addition, kynurenate blocks spontaneous synaptic events at the vestibular hair cell afferent, synapse of the frog (Annoni et al 1984). These observations are consistent with an action of these antagonists at a post-synaptic receptor type common to many excitatory amino acid synapses.…”

“…In addition, kynurenate and and pBB-PzDA are potent synaptic antagonists in several other C.N.s. excitatory pathways (Cochran, 1983;Ganong et al 1983;Annoni et al 1984;Davies et al 1984;Herrling, 1985).…”