Dopaminergic modulation of afferent synaptic transmission in the semicircular canals of frogs

Using multiunit recording of action potentials from the whole nerve with the aid of external perfusion, we investigated the effects of dopamine (DOP) agonists that are involved in modulatory actions on synaptic transmission in the isolated labyrinth preparations of frogs. The external application of DOP (0.1-1 mM), the D(1) agonist chloro-APB hydrobromide (CAPB, 50-100 microM) and the D2 agonist quinerolane (QUI, 50-100 microM) induced a dose-dependent and reversible decline in the resting discharge frequency. In this concentration range, the potency of applied CAPB considerably exceeded that of QUI. AMPA, NMDA and ACPD responses were inhibited by the D1 and D2 agonists, implicating both subtypes of DOP receptors in the modulation of both ionotropic and metabotropic glutamate receptors. The inhibitory action of the DOP agonists on L-glutamate responses persisted in a high Mg2+ solution in conditions of selective activation of the postsynaptic membrane. The results obtained suggest that DOP may interact with both D1 and D2 receptor subtypes, most likely located postsynaptically on the afferent nerve fibers. This dopaminergic control mechanism may result in the reduction of the activated firing rate, thus preventing over-excitation and excitotoxic injury of the afferent dendrites after the external application of L-glutamate and excessive receptor stimulation.