RESPONSES OF STRIATAL NEURONS TO PERIPHERAL SENSORY STIMULATION IN SYMPTOMATIC MPTP-EXPOSED CATS

Extracellular single unit activity was recorded in the dorsal lateral caudate nucleus of awake cats and the responses of these neurons to somatosensory, visual and auditory stimuli were assessed. Recordings were obtained when animals were normal and when they were symptomatic for a Parkinson-like syndrome as a result of exposure to the dopaminergic toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In normal animals, 22% of recorded neurons responded to tactile stimulation of the face, 7% had auditory responses, 6% had visual responses, and 6% were multimodal. The post-MPTP period was divided into an 'early' period, in which cats had received several MPTP injections but still remained asymptomatic, and a 'late' period, in which cats had severe motor and sensorimotor impairments. Unit responsiveness was essentially normal in the 'early' period but grossly abnormal in the 'late' period. When animals were symptomatic, only 6% of sampled neurons had responses to somatosensory stimulation, 0.8% had auditory responses and no cells were found with visual responses. Those cells that did respond to somatosensory stimulation did so in a non-specific fashion. Symptomatic animals had 93-96% depletion of dorsal striatal dopamine and extensive loss of substantia nigra pars compacta neurons. The results suggest that sensorimotor and motor abnormalities that accompany severe loss of striatal dopamine are at least in part due to a dopamine-dependent loss of sensory processing abilities of striatal neurons.