Role of opiates in striatal D-1 dopamine receptor supersensitivity induced by chronic L-dopa treatment

Repeated administration of L-dihydroxyphenylalanine (L-dopa) to rats lesioned with monolateral intranigral injections of 6-hydroxydopamine counteracted the increased density of striatal [3H]spiroperidol binding sites induced by the lesion. On the contrary, the treatment with L-DOPA further enhanced the hypersensitivity of adenylate cyclase to dopamine stimulation that follows striatal denervation. In addition, the apomorphine-induced rotations were strongly potentiated. The latter effect was antagonized by morphine given acutely shortly before the dopamine agonist. On the other hand, the efficacy of [D-Ala2]-methionine enkephalinamide to inhibit striatal adenylate cyclase was decreased in 6-hydroxydopamine-lesioned rats chronically treated with L-dopa. Moreover, in these animals, when naltrexone was given chronically together with L-dopa, the supersensitivity of the enzyme to dopamine stimulation did not develop. Finally, in 6-hydroxydopamine-lesioned rats, chronic morphine, similarly to L-dopa, further enhanced the responses of adenylate cyclase to dopamine stimulation. These data suggest that prolonged indirect activation of striatal opiate receptors and their consequent desensitization could be among the causes of the hyperactivity of D-1 dopamine receptors that follows chronic L-dopa treatment.