Abstract

Decerebrate or encéphale isolé cats were tested with controlled acceleratory motion in order to identify vestibular neurons activated by motion. The effects of d-amphetamine, an agent recently shown effective in preventing motion sickness, were studied on both neuronal unit activity and field potentials evoked by stimulation of a branch of the VIIIth nerve. d-Amphetamine (100 µg/kg) administered intravenously markedly increased the spontaneous and motion-evoked activity in midcollicular decerebrate cats. In encéphale isolé or precollicular animals, d-amphetamine decreased unit firing. A decrease in the polysynaptic field potential was noted although the monosynaptic field potential was unaffected by the drug. These results indicate that d-amphetamine effects vestibular neurons through pathways other than those associated with primary afferent activity of the VIIIth cranial nerve. Central nervous system structures rostral to the inferior colliculus appear to be implicated in the d-amphetamine depression of vestibular firing.