Curare and the Efferent Vestibular System: An Electrophysiological Study in the Frog

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Abstract
The action of d-tubocurarine on the spontaneous discharge recorded from the whole horizontal semicircular canal (HC) nerve and from single HC afferent fibers was studied in frogs. 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 .mu.l, 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 .apprx. 60% of the preparations and a decrease in 6-8% of the cases, whether the contralateral 8th nerve was cut or not, while Ringer had no effect. After severing of the ipsilateral 8th 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 .apprx. 200 afferent fibers was 21.4 .+-. 1.7 spikes/s after curare treatment, while it was only 15.6 .+-. 1.3 spikes/s after Ringer injection; these 2 values are highly significantly different. Apparently, the ampullary receptors are tonically subject to an inhibitory influence probably due to the tonic activity of the cholinergic efferent vestibular system. The activity of the hair cells is probably also controlled by another, as yet unknown cholinergic mechanism.