Role of the spinocerebellum in adaptive gain control of cat's vestibulospinal reflex.

In decerebrate cats, a 3-h period of sustained roll tilt of the head (at 0.15 Hz. +/- 10) leading to selective stimulation of labyrinth receptors, associated with a synchronous roll tilt of the body (at 0.15 Hz., +/- 12.5) leading to 2.5 degrees out-of-phase neck rotation produced an adaptive increase in gain of the vestibulospinal reflex (VSR) elicited by roll tilt of the animal at 0.15 Hz, +/- 10 degrees. This increase reached the maximum at the end of the third h of stimulation and persisted unmodified during the first h after stimulation. Microinjection into zone B of the cerebellar anterior vermis of the GABA-A agonist muscimol (0.25 microliter at 8 micrograms/microliters saline), producing only a slight or negligible depression of the VRS gain in non-adaptive conditions, prevented the occurrence of the adapted increase in gain of the VSR following a 3-h period of sustained head-body rotation. Moreover, intravermal injection of the GABA-A agonist muscimol or the GABA-B agonist baclofen (0.25 microliter at 8 or 2 micrograms/microliters saline, respectively) suppressed the already adapted VSR gain. It is postulated that the adaptive increase in gain of the VSR following a sustained neck-vestibular stimulation depends on plastic changes which affect the Purkinje cells of the cerebellar anterior vermis.