Tonic influence from one labyrinth onto the contralateral one. An electrophysiological study in the frog

In the frog's isolated head preparations, spontaneous activity was recorded from the whole nerve of the left horizontal semicircular canal (HC) for 6 min before and 16 min after destruction of the right labyrinth by heating or administration of d-tubocurarine (0.5 microliter, 5.10(-6)M) into the perilymph of the right labyrinth. Just after destruction on the right labyrinth, spontaneous activity of the left HC nerve abruptly increased by 20-400% in 24 preparations out of the 40 studied; activity then increased slowly and regularly and in most cases reached a steady level. In the 16 other preparations such destruction had no effect (15 preparations) or elicited a slight decrease of the HC nerve activity (one preparation). After curare administration, the spontaneous activity of the left HC nerve decreased by 20-100% in 27 preparations out of the 40 studied; in most cases such a decrease was reversed 25-80 min after administration of the drug. The spontaneous discharges were unaffected in 11 preparations and slightly increased in the two others. Destruction of the right labyrinth or administration of curare never modified spontaneous activity recorded from the left HC nerve when the connections between the two labyrinths had been interrupted either by sagittal section of the medulla oblongata or section of the right vestibular nerve close to the brain stem. These results demonstrate that one labyrinth has a tonic inhibitory influence on the contralateral one.