Abstract
The present study was undertaken to determine the role of dopamine (DOP) in the excitability of the peripheral vestibular system in the frog. For this purpose, using multi-unit recording of action potentials from the semicircular ampulla posterior whole nerve in the isolated preparation with the aid of external perfusion, we investigated the effects of DOP agonists that are involved in modulatory actions on synaptic transmission in the frog semicircular canals. External application of DOP (0.1-1 mM), D1 agonist chloro-APB hydrobromide (CAPB; 50-100 ?M) and D2 agonist quinerolane (QUI; 50-100 ?M) induced a dosedependent and reversible decline in the resting discharge frequency. 100 ?M CAPB and 100 ?M QUI induced a decrease in firing frequency (mean 3.5?1.2 and 61.5?6.2% of the control level, respectively, n=6). Firing evoked by 1 ?? AMPA, 50 ?? NMDA and 300 ?? ACPD could be depressed by administration of 50 ?M CAPB by 67.1?9.4% (n=5, *P<0.05), 39.1?16.8% (n=5, *P<0.05) and 49.5?10.2% (n=5, *P<0.05), respectively. In similar conditions, firing evoked by AMPA, NMDA and ACPD could be depressed by administration of 50 ?M QUI by 25.7?8.4% (n=4, *P<0.05), 38.8?14.2% (n=5, *P<0.05) and 34.2?9.8% (n=4, *P<0.05), respectively. The inhibitory action of DOP agonists on l-glutamate responses persisted in high Mg2+ solution in conditions of selective activation of postsynaptic membrane. The results obtained suggest that DOP may interact with both D1 and D2 receptor subtypes. The inhibition of NMDA, AMPA, ACPD responses by DOP agonists suggests that DOP exerts inhibitory control over both ionotropic and metabotropic types of l-glutamate receptors, and that one possible site for DOP action is on the postsynaptic membrane of the synapse. This mechanism may result in the reduction of the activated firing rate, thus, preventing over-excitation and excitotoxic injury of the afferent dendrites after external application of l-glutamate and excessive receptor stimulation.
Author(s): Andrianov GN, Ryzhova IV, Tobias TV