Interactions between dopamine D1 and D2 receptors in the model of thyrotropin-releasing hormone (TRH)-induced behaviour in rabbits.

The interactions between dopamine D1 and D2 receptors in the model of thyrotropin-releasing hormone (TRH)-induced behavioural excitation were assessed in adult male rabbits. As we had previously observed with the dopamine D1 receptor antagonist SCH 23390 (0.01 mg/kg i.v.), sulpiride (12 mg/kg i.v.), a dopamine D2 receptor antagonist, also significantly antagonized the scratching behaviour elicited by TRH (100 micrograms/10 microliters i.c.v.). Sulpiride (6 and 12 mg/kg i.v.) also induced marked grooming. SKF 38393 (10 mg/kg i.v.), a dopamine D1 receptor agonist, did not modify the TRH-induced scratching. However, LY 17155 (0.5 mg/kg i.v.), a dopamine D2 receptor agonist, significantly increased it. The potentiating effects of LY 171555 were completely antagonized by SCH 23390. These results demonstrate that a concomitant activation of both dopamine D1 and D2 receptors is required for the expression of TRH-induced scratching. They also suggest that scratching behaviour may be a D2-dependent, D1-enabled response. The involvement of dopamine D1 and D2 receptors on grooming behaviour is also discussed.