Early withdrawal from repeated cocaine administration upregulates muscarinic and dopaminergic D2-like receptors in rat neostriatum.

The present results show an increase in locomotor activity 24 h following repeated cocaine administration only with the higher dose (10 mg/kg, i.p., daily for 1 week) compared to controls (administered with saline). Binding assays were done and the ligands used were [3H]N-methylscopolamine ([3H]-NMS), [3H]-SCH 23390, and [3H]-spiroperidol to determine muscarinic (M1- and M2-like), D1 and D2 receptors, respectively. Scatchard analyses revealed alterations in Bmax not only for muscarinic, but also for D2-like receptors that were significantly increased. On the other hand, no alterations were detected on D1-like receptors densities and dissociation constant values. However, the Kd value was significantly increased for D2 receptors. The changes in muscarinic receptors were observed predominantly on M2-like, which presented an increase of 84% with the 10 mg/kg, i.p., dose only. On D2-like receptors, increases of 63 and 54% were demonstrated with the doses of 5 and 10 mg/kg, i.p.. The preferential effects of cocaine on muscarinic and D2-like receptors were also demonstrated in vitro where decreases in [3H]-NMS and [3H]-spiroperidol binding were observed. The results indicate that the effects of cocaine on muscarinic and dopaminergic postsynaptic receptors are functions of dose, duration of treatment, and time of drug withdrawal.