Alterations in pallidal neuronal responses to peripheral sensory and striatal stimulation in symptomatic and recovered Parkinsonian cats

Abstract

The spontaneous activity, responses to peripheral sensory and ipsilateral caudate nucleus stimulation of globus pallidus (GP) and entopeduncular nucleus (ENTO) neurons were studied in cats while normal, symptomatic for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induced parkinsonism, and when spontaneously recovered from gross parkinsonian motor deficits. Administration of MPTP resulted in parkinsonian motor symptoms that spontaneously recovered approximately 4–6 weeks after the MPTP administration. Post-mortem dopamine levels in recovered animals was approximately 95% below levels previously measured in normal animals. In symptomatic animals, the mean spontaneous firing rate for GP units was decreased by 50% and increased by 55% for ENTO units recorded. Spontaneous firing rates for GP and ENTO units in recovered cats were not significantly different from those observed in normal cats. In normal cats, 31.4% of GP and 29% of ENTO units tested responded to tactile stimulation of the face. Only 12.2% of GP and 13% of ENTO units responded to such stimulation in parkinsonian animals while the responses were generally less specific (larger receptive fields, more bilateral receptive fields, and more responses to multiple stimulation types) than normal. In recovered cats GP and ENTO responses resembled those observed in normal cats. There was no difference in the overall percentage of pallidal units responding to striatal stimulation across the 3 experimental conditions. There was, however, an increase in the percentage of units responding with complex response sequences (i.e. decrease in activity followed by an increase in activity) in symptomatic animals as compared to normal and recovered animals. The results suggest that loss of striatal dopamine in parkinsonian animals has profound effects on the sensory responsiveness of GP and ENTO neurons and that these effects coincide with the appearance of and recovery from parkinsonian motor deficits. These data further support the notion that sensory information processing by the basal ganglia may play an important role in influencing motor output.