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.