In vivo and in vitro evidence of dopaminergic system down regulation induced by chronic L-DOPA

The biochemical modifications which occur in the dopaminergic system after chronic administration of L-DOPA are investigated. Levels of DA and of its metabolite 3-methoxytyramine (3-MT), an expression of the amount of DA released, were raised to the same extent in controls given a single dose of 1-DOPA and in chronically treated rats given 100 mg/kg of 1-DOPA plus 25 mg/kg of benserazide twice a day for 24 days. However, the reduction in neuronal function expressed by the decrease in 3-MT which follows treatment with DA agonists such as piribedil and apomorphine was less pronounced in the chronically L-DOPA treated rats. This suggests that such treatment causes a down regulation of DA receptors. These in vivo results were confirmed by in vitro analysis of DA receptor activity after chronic L-DOPA. Under these conditions there was a significant reduction in the number of [3H]-spiperone and [3H]-ADTN binding sites with no changes in their affinity. The in vivo and in vitro findings both suggest the involvement of a subsensitive compensatory mechanism or down regulation of dopaminergic neurons after chronic treatment with L-DOPA.