Curare and the efferent vestibular system. An electrophysiological study in the frog

In the frog we have studied the action of d-tubocurarine on the spontaneous discharge recorded from the whole horizontal semicircular canal (HC) nerve and from single HC afferent fibres. Gross activity was recorded in isolated head preparations. In each case, the spontaneous frequency was measured 6 min before and 16 min after a drop of D-tubocurarine (0.1 microliter, 5.10(-6)M) dissolved in Ringer or after a similar drop of Ringer was injected into the perilymph near the HC ampulla. Curare elicited a significant increase of the discharge frequency in about 60% of the preparations and a decrease in 6-8% of the cases, whether the contralateral eighth nerve was cut or not, while Ringer had no effect. After severing of the ipsilateral eighth nerve anterior branch, curare had no significant effect on the mean resting rate (calculated from 25 preparations), while in a few cases it evoked an increase or a decrease of the discharge frequency. Single afferent discharges were recorded in intact frogs. The mean frequency calculated from about 200 afferent fibres was 21.4 +/- 1.7 spikes/s after curare treatment, while it was only 15.6 +/- 1.3 spikes/s after Ringer injection; these two values are highly significantly different. From these results we conclude that the ampullary receptors are tonically subject to an inhibitory influence probably due to the tonic activity of the cholinergic efferent vestibular system. Moreover, the activity of the hair cells is probably also controlled by another, as yet unknown cholinergic mechanism.