Synergistic blockade of some dopamine-mediated behaviours by (-)-sulpiride and SCH 23390 in the rat.

Several studies have indicated that the D2 dopamine receptors mediate the antidopaminergic activity of the neuroleptics; nevertheless, the selective blocker (-)-sulpiride weakly inhibits dopamine-mediated behaviour. The present study investigated whether the concomitant injection of doses of the D1 antagonist SCH 23390, which by themselves are without effect, would enable (-)-sulpiride to express fully neuroleptic activity in the rat. The benzamide YM 09151-2 that strongly inhibits dopamine-mediated behaviour was also studied. Rats receiving different doses of (-)-sulpiride, YM 09151-2 and SCH 23390 given alone or in combination were tested for exploratory activity, apomorphine-induced stereotyped behaviour and hyperactivity elicited by the D2 agonist LY 171555. When given alone, (-)-sulpiride (10, 20 and 40 mg/kg IP) had no effect on exploratory activity and stereotypy. When (-)-sulpiride was administered in combination with an ineffective dose of SCH 23390 (5 micrograms/kg) both responses were significantly inhibited. The combined administration of subthreshold doses of (-)-sulpiride (2.5 mg/kg) and SCH 23390 (2.5 micrograms/kg) significantly inhibited hypermotility induced by LY 171555. Moreover, the combined administration of ineffective doses of YM 09151-2 with subthreshold doses of SCH 23390 strongly inhibited all the behavioural responses. The results indicate that SCH 23390 allowed (-)-sulpiride to exhibit a wider spectrum of neuroleptic activity and potentiated the antidopaminergic activity of YM 09151-2.