Antagonism by 8-OH-DPAT, but not ritanserin, of catalepsy induced by SCH 23390 in the rat

Abstract

In the present experiments, it was shown that the catalepsy induced by the dopamine D₁ antagonist SCH 23390 (0.2 mgkg⁻¹ sc), was completely antagonised by the administration of 8-OH-DPAT (0.1 mg kg⁻¹ sc) for the duration of the effect of SCH 23390 (approx. 120 min). Neither the catalepsy induced by raclopride (16 mg kg⁻¹ sc) nor that induced by SCH 23390 (0.2 mg kg⁻¹ sc) could be antagonised by treatment with the 5-HT₂ receptor antagonist ritanserin (0.13–2.0 mg kg⁻¹ sc). Administration of SCH 23390 (0.0125–0.2 mg kg⁻¹ sc) produced a significant suppression of avoidance behavior at all doses, and also produced a significant decrease in the number of intertriai crosses. At the higher doses, 0.05 and 0.2 mg kg⁻¹ sc, there were also escape failures. In contrast to the finding in our previous report that raclopride and 8-OH-DPAT in a synergistic manner produce a suppression of conditioned avoidance behavior, no such interaction was found between 8-OH-DPAT (0.1 mg kg⁻¹ sc) and SCH 23390 (6 μg kg⁻¹ sc) in the present study.