Short-term increase and long-term reversion of striatal cell activity after degeneration of the nigrostriatal dopamine system.

The spontaneous activity of neurons in the head of the striatum was studied in rats 3 days and more than 1 year after a 6-hydroxydopamine-induced lesion of the nigrostriatal dopamine system in comparison to unlesioned animals. Cells were detected and tracked by stimulating the excitatory corticostriatal pathway. In unlesioned animals striatal cells discharged at very low frequencies, with a median of 0.04 impulse/second. The activity was increased to 0.28 impulses/second 3 days after the lesion. This increase was related to the degree of dopamine depletion. More than 1 year after the lesion, the frequency had decreased to a level indistinguishable from that measured in unlesioned animals, with a median of 0.03 impulses/second. Cells in 3-day lesioned animals discharged a higher number of bursts at shorter intervals as compared to unlesioned animals, while in long-term denervated animals the bursting pattern was similar to that in unlesioned animals. This demonstrates that removal of the dopaminergic input results in increased activity only during an initial phase and that adaptive processes subsequently occur. The data from this Parkinsonian model suggest that symptoms of this disease cannot simply be related to an increased striatal cellular activity. The fact that the initially increased spontaneous activity adapted indicates that functional effects of a lesion can only be evaluated when studying the resulting changes throughout a time course.