Activity of pallidal and striatal tonically active neurons is correlated in MPTP-treated monkeys but not in normal monkeys

The goal of this study is to assess the function of tonically active neuron s (TANs) of the striatum and their malfunction in the parkinsonian state. W e recorded multiple spike trains of striatal TANs and pallidal neurons, whi ch are the main target of striatal projections. Recordings were performed i n two vervet monkeys before and after the induction of tremulous parkinsoni sm by systemic injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hy drochloride (MPTP). We then calculated cross-correlograms between TANs and pallidal neurons to evaluate the interactions between them. In the normal monkeys, only 1.3% (2/152) of the cross-correlograms displaye d significant peaks, and 8.6% (13/152) displayed significant oscillations. After MPTP treatment, 42.8% (83/194) of the cross-correlograms displayed si gnificant peaks or troughs, or both, and 58.8% (114/194) displayed signific ant 3-19 Hz periodic oscillations. The frequency content of the coherent os cillations matched the frequency content of the activity of individual TANs , but was only weakly related to that of individual pallidal cells. These results confirm the notion that in the normal state neurons in the ba sal ganglia tend to fire independently, whereas in the parkinsonian state t hey exhibit synchronized oscillatory activity. The low level of correlated activity in the normal state demonstrates that TANs have only a slight effe ct on pallidal activity during execution of familiar behavior. The high lev el of oscillatory correlated activity in the parkinsonian state further sug gests that coherent oscillations of the whole basal ganglia circuitry under lie the clinical features of Parkinson's disease.