EFFECTS OF A DOPAMINERGIC AGONIST IN THE GUINEA-PIG COCHLEA

This study investigates the role of dopamine, a putative lateral effer ent neurotransmitter/modulator, in cochlear physiology and physiopatho logy. Cochlear potentials were recorded in guinea pigs after intracoch lear perfusion of increasing doses (0.1-1 mM) of piribedil, an agonist of the D2/D3 receptors. A dose-dependent reduction in the amplitude o f auditory nerve compound action potential (CAP) was observed, predomi nantly at high-intensity tone-burst stimulations, and without signific ant effect on CAP threshold. There was no variation of cochlear microp honic and summating potential. When 1 mM piribedil was perfused into t he cochlea during continuous 130 dB SPL pure tone exposure (6 kHz, 15 min), CAP threshold shifts were significantly less than in control ani mals with artificial perilymph-perfused cochleas. No dendritic damage was observed, although there was evident hair cell damage. Similarly, radial dendrites were clearly protected against ischemia-induced damag e when 1 mM piribedil was applied prior to a 10-min ischemia. These re sults suggest that dopamine modulates the activity of radial afferent fibers via D2/D3 receptors. The protective effect of piribedil during acoustic trauma or ischemia suggests that this modulation corresponds to a prevention of excitotoxicity due to dysfunction of inner hair cel l neurotransmission.