Activation of the efferent system in the isolated frog labyrinth: effects on the afferent EPSPs and spike discharge recorded from single fibres of the posterior nerve.

Intra-axonal recordings were obtained from single afferent fibres of the posterior nerve in the isolated labyrinth of the frog (Rana esculenta). EPSPs and spike discharge were recorded both at rest and during rotatory stimulation of the canal. Electrical stimulation of either the distal end of the cut posterior nerve or of the central stumps of the anterior-horizontal nerves elicited a frequency-dependent inhibitory effect on the afferent discharge arising from the posterior canal. Denervation experiments revealed that inhibition is mediated by efferent fibres exhibiting a high degree of branching in the proximal part of the eighth nerve. The inhibitory effect was selectively cancelled by (1)D-tubocurarine 10(-6) M; (2) atropine 5 x 10(-5) M; (3) acetylcholine or carbachol 10(-4) M; (4) eserine 10(-5) M. Inhibition is thus most likely to be sustained by the release of acetylcholine from the efferent nerve terminals. Experiments in which the ionic composition of the external medium was modified suggest that the transmitter acts mainly by opening the chloride ion channels of the hair cell membrane. In some units the same stimulation pattern evoked a consistent increase in both EPSP and spike discharge, instead of inhibition. Such facilitation was unaffected by drugs for ionic modifications which block the efferent synapse, but disappeared after denervation. Inhibition and facilitation, therefore, act as two control mechanisms which are able to modify substantially, at the first stage of processing, the sensory information which is sent to the vestibular second order neurons.