Mechanisms of compensation for vestibular deficits in the frog. II. Modification of the inhibitory Pathways.

In hemilabyrinthectomized frogs inhibitory responses of central vestibular neurons to electrical stimulation of the remaining vestibular nerve were recorded extra- and intracellularly at different stages (0, 3, and 60 days) after the operation. In acute animals inhibition of vestibular neurons following stimulation of the VIIIth nerve is rarely observed. In chronic animals about 30% of the vestibular neurons on the partially deafferented side and about 15% of the vestibular neurons on the intact side are inhibited. The distribution of the latencies of these inhibitory responses is bimodal with ranges from 4 to 14 ms and 18 to 24 ms. Removal of the cerebellum reduced the number of inhibited vistibular neurons and picrotoxin abolished all inhibitory responses. The vestibular input to the cerebellar dorsal rim is bilateral. In chornic animals excitation of Purkinje cells was similar as in acute preparations but many more cells were disfacilitated. Inhibition of partially deafferented vestibular neurons by cerebellar and brain stem neurons increases in parallel with their excitatory commissural input. The balance between these plastic changes may be crucial for the functional recovery of appropriate compensatory reflexes.