The actions of dopamine receptors in the guinea pig cochlea

Dopamine, a major lateral olivocochlear efferent neurotransmitter, exerts both excitatory and inhibitory effects on the central nervous system depending on the receptor involved. We investigated the effects of different dopamine receptors on the cochlea by perilymphatic perfusion with D(1/5), D(2) and D(3) receptor agonists and antagonists and recording neural and hair cell responses (compound action potential - CAP; summating potential - SP) before, during and after perfusions. The D(1/5) agonist resulted in marked suppression of CAP amplitudes whilst leaving SP amplitudes unchanged, suggesting an inhibitory action of these receptors on afferent dendrites. The D(1/5) antagonist had little or no effect, suggesting that there is no influence of tonic dopamine release on these receptors. In contrast, perfusing a D(2) receptor antagonist resulted in marked suppression of CAP suggesting an excitatory action of the receptors and a strong influence of tonic dopamine release on the D(2) receptors. The D(2) agonist had little effect, implying that tonic dopamine release is maximally activating this class of dopamine receptors. D(2) antagonists resulted in reduction of SP, cochlear microphonic and distortion product otoacoustic emission amplitudes, suggesting that D(2) receptor action is not confined to afferent dendrites. Perfusion with D(3) agonists and antagonists had no effect.