Activation of dopamine cell firing by repeated L-DOPA administration to dopamine-depleted rats: its potential role in mediating the therapeutic response to L-DOPA treatment.

The administration of L-dihydroxyphenylalanine (L-DOPA) to patients with Parkinson's disease is known to produce acute effects that include the reduction of rigidity as well as delayed therapeutic actions involving the resumption of complex motor behavior. In order to examine the potential role of dopamine (DA) cell activity in mediating these responses, the effects of acute and repeated L-DOPA administration on the electrophysiological activity of the residual dopamine (DA) neurons were examined in rats that had received partial 6-hydroxydopamine (6-OHDA)-induced DA lesions. DA cell activity was assessed along three dimensions: (1) the relative proportion of DA neurons exhibiting spontaneous spike firing, (2) their basal firing rate, and (3) their firing pattern. Following 6-OHDA-induced DA depletion, rats were treated for 1 month with saline or L-DOPA. In addition, rats from each group received either an acute injection of L-DOPA or saline on the day of recording. In rats receiving repeated saline treatment, the DA neurons recorded following acute L-DOPA administration were firing at significantly slower basal firing rates and exhibited less burst firing when compared to saline-pretreated rats given acute saline. In contrast, DA cells recorded from rats that had received repeated L-DOPA administration for 4 weeks followed by an acute saline injection did not exhibit any significant differences from DA cells of intact control rats with respect to basal firing rate or firing pattern; however, there was a substantial increase in the proportion of DA neurons exhibiting spontaneous spike firing after correcting for 6-OHDA-induced cell loss. In addition, in rats receiving repeated L-DOPA treatment, the DA cells recorded following acute administration of L-DOPA showed significantly less of a reduction in firing rate when compared to the cells recorded following acute L-DOPA in the saline treatment group. These results show that: (1) acute L-DOPA administration appears to exert its actions by DA autoreceptor stimulation, whereas (2) repeated L-DOPA administration increases the proportion of spontaneously active DA neurons in partially lesioned rats. As a result, repeated L-DOPA administration would be expected to cause an increase in spike-dependent DA release as a consequence of the greater proportion of DA cells showing spontaneous activity. This may be the major factor underlying the delayed therapeutic benefits of L-DOPA therapy in the treatment of Parkinson's disease.