Effects of haloperidol and SCH 23390 on acoustic startle and prepulse inhibition under basal and stimulated conditions.

1. Adult Sprague-Dawley rats underwent startle testing for assessment of baseline startle amplitude and prepulse inhibition (PPI) of the startle reflex. 2. Animals were tested after administration of either: saline, a selective D1 dopamine (DA) receptor antagonist, a relatively selective D2 DA antagonist, or combined low dose D1 and D2 antagonists. 3. Changes due to antagonists were assessed with and without administration of the D1/D2 agonist apomorphine. 4. Testing without apomorphine stimulation showed that both D1 and D2 antagonists reduce baseline startle and enhance PPI. Further, the two antagonists exhibited a synergistic interaction. 5. Testing with apomorphine showed that D1 and D2 antagonists reduce apomorphine-induced startle enhancement. Again, the two exhibited a synergistic interaction. 6. For PPI, the D2 but not D1 antagonist reduced the apomorphine effect. However, the D1 antagonist potentiated the effect of the D2 antagonist.