Journal of Neuroscience, Vol 4, 2106-2116, Copyright © 1984 by Society for Neuroscience
Pharmacology of the vestibular hair cell-afferent fiber synapse in the frog
JM Annoni, SL Cochran and W Precht
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.
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