Haloperidol conditioned catalepsy in rats: a possible role for D1-like receptors.

Decreases in brain dopamine (DA) lead to catalepsy, quantified by the time a rat remains with its forepaws resting on a suspended horizontal bar. Low doses of the DA D2 receptor-preferring antagonist haloperidol repeatedly injected in a particular environment lead to gradual day-to-day increases in catalepsy (catalepsy sensitization) and subsequent testing following an injection of saline reveal conditioned catalepsy. We tested the hypothesis that D1-like and D2 receptors play different roles in catalepsy sensitization and in acquisition and expression of conditioned catalepsy. Rats were repeatedly treated with the DA D1-like receptor antagonist SCH 23990 (0.05, 0.1 and 0.25 mg/kg i.p.), the D2 receptor-preferring antagonist haloperidol (0.1, 0.25 and 0.5 mg/kg i.p.) or a combination of the two drugs and tested for catalepsy each day in the same environment. Following 10 drug treatment days, rats were injected with saline and tested for conditioned catalepsy in the previously drug-paired environment. Haloperidol did not elicit cataleptic responses in the initial session; however, rats developed sensitization with repeated testing. Significant catalepsy sensitization was not observed in rats repeatedly tested with SCH 23390. When rats were injected and tested with saline following haloperidol sensitization they exhibited conditioned catalepsy in the test environment; conditioned catalepsy was not seen following SCH 23390. Rats treated with 0.05 mg/kg SCH 23390+0.25 mg/kg haloperidol showed catalepsy sensitization but failed to show conditioned catalepsy. Conversely, SCH 23390 (0.05 mg/kg) given on the test day after sensitization to haloperidol (0.25 mg/kg) failed to block conditioned catalepsy. Repeated antagonism of D2 receptors leads to catalepsy sensitization with repeated testing in a specific environment. Conditioned catalepsy requires intact D1-like receptor function during sensitization sessions but not during test sessions. In conclusion, repeated antagonism of D2, but not D1-like receptors leads to catalepsy sensitization with repeated testing in a specific environment. Conditioned catalepsy requires functional D1-like receptors during sensitization sessions but not during test sessions.