L-glutamate (Glu) has at least two sites of action in the frog semicircular canal: the hair cell (presynaptic) and the primary afferent nerve fibres (postsynaptic). Glu's action on the hair cell results in an increased release of the natural transmitter which is responsible for a substantial increase in the frequency of firing in primary afferents. The presynaptic action of Glu is antagonized by D-alpha-aminoadipate (D alpha AA). Glu produces a long-lasting depolarization on the afferent nerve fibres which does not by itself elicit any afferent discharge of impulses when the release of the natural transmitter is prevented. Glu-induced nerve depolarization is only partially antagonized by D alpha AA. The difficulty of reconciling some of the observations made of the effects of Glu in semicircular canals with its presumed role as an afferent transmitter in this organ is discussed, but this role is not definitely rejected.
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