Peripheral control of the receptor discharge by the efferent system in frog labyrinth: an intracellular study

Frog labyrinthine receptor activity is controlled by an efferent inhibitory system whose fibres directly synapse with the hair cells. EPSP and spike discharge, recorded at rest and during rotatory stimulation from fibres of the posterior nerve in the isolated labyrinth, were in fact reduced and eventually abolished by electrical stimulation (10-200/sec) either of the proximal end of the same nerve or of the central stumps of the anterior-horizontal nerve. Such inhibitory effect was selectively cancelled by: 1) d-tubocurarine (10-6 M); 2) ACh and carbachol (10-4); 3) increasing K+ (10 mM) or decreasing Cl- (<25 mM) concentration in the bath; 4) cutting the VIII nerve at its entrance in the brain stem and allowing degeneration of the efferent fibres. Denervation rules out the possibility that reoeptor-receptor fibres sustain the observed inhibitory effect, which is most likely mediated by efferent fibres that extensively branch in the VIII nerve and depress the hair cell activity by releasing ACh. The transmitter most likely acts by opening the K+ and Cl- channels. In some unita the same stimulation pattern evoked, instead of an inhibition, a consistent increase in both EPSP and spike discharge. Such "facilitation" was unaffected by drugs and ionic modifications whioh impair the activation of the efferent system. This effect is tentatively ascribed to an accumulation of K+ in the extracellular space following sustained activity