Alterations in peripheral and central dopamine receptor sensitivity after subchronic treatment with fluphenazine and sulpiride.

The effects of subchronic treatment with the antipsychotic drugs fluphenazine and sulpiride on the sensitivity of peripheral neuronal dopamine receptors and central dopamine autoreceptors were evaluated. The ability of apomorphine, a dopamine agonist, to inhibit electrically induced sympathetic vasoconstriction in pithed rats, and apomorphine-induced inhibition of spontaneous locomotor activity in awake rats were used as indices of peripheral and central dopamine receptor sensitivity, respectively. A single injection of fluphenazine decanoate, a long-acting preparation of fluphenazine, enhanced the central locomotor inhibitory effect of low doses of apomorphine 4 and 6 weeks after drug administration, whereas the antidopaminergic effect on peripheral dopamine receptors was prolonged and persisted at least up to 6 weeks. In another set of experiments rats were treated with fluphenazine hydrochloride and sulpiride for 10 days and subsequently challenged with apomorphine after various withdrawal times. Both antipsychotic drugs augmented the inhibitory effect of apomorphine in the periphery, although the time courses of the potentiation were different. Both treatments also enhanced the locomotor inhibitory effect of apomorphine. These results are in line with our previous finding that long-term treatment with dopamine antagonists can induce neuronal dopamine receptor up-regulation also outside the central nervous system. Peripheral neuronal dopamine receptors thus show similar adaptive responses to long-term blockade as central dopamine autoreceptors, and may serve as a useful experimental model in studies concerned with mechanisms of dopaminergic autoregulation in the central nervous system.