Synergistic effects between D-1 and D-2 dopamine antagonists on catalepsy in rats

The effect of selective D-1 and D-2 dopamine agonists on catalepsy induced by various dopamine antagonists was studied. A potent and selective D-2 antagonist, YM-09151 (YM-09151-2) at a dose of 1.2 mg/kg, SC and a selective D-1 antagonist, SCH 23390 at 1.0 mg/kg, SC induced catalepsy in rats. Mixed D-1/D-2 antagonists, haloperidol (HPD) and cis-flupentixol (FLU) also induced catalepsy at doses of 2.0 and 0.8 mg/kg, SC, respectively. A mixed D-1/D-2 agonist, apomorphine (1.0 mg/kg, SC), a selective D-2 agonist, bromocriptine (10 mg/kg, IP) and a muscarinic antagonist, scopolamine (1.0 mg/kg, SC), prevented or markedly reduced the incidence of catalepsy by the tested antagonists. In contrast, a selective D-1 agonist, SKF 38393 (4.0 mg/kg, SC) did not reduce the cataleptogenic effects of HPD, FLU and SCH 23390, but did reduce the effect of YM-09151. Moreover, co-administration of YM-09151 with SCH 23390 produced a marked increase in the incidence of catalepsy. The incidence seen after the combination of YM-09151 and SCH 23390 at low doses was significantly different from that seen after each drug alone at the doubled dose. Thus, D-1 and D-2 antagonists potentiated each other's effect in producing catalepsy. These results suggest an important role of both D-1 and D-2 receptors in the catalepsy and the existence of synergistic effects of D-1 and D-2 receptor blockade.