Differental Properties of Behavioral Responses to Bromocriptine and Apomorphine in Rats

The study was carried out in an attempt to understand the contribution of dopamine D-1 and D-2 receptor activities to yawing responses induced by bromocriptine (BRC) in rats. Intraperitoneal (I.p.)injection of BRC ranging from 1 to 20 mg/kg produced yawing behavior. The yawing behavior. The yawing response were increased by reserpine (2mg/kg) and α-methyl-p-tyrosine (α-MPT;200mg/kg) but decreased by α-MPT (400mg/kg).Reserpine (2mg/kg) plus α-MPT (200mg/kg) completely inhibited the yawing induced by BRC. The yawing response were stimulated by a low dose of apomorphine (0.1mg/kg). A dopamine receptor agonist, but were inhibited by a high dose (1mg/kg). Furthermore, BRC alone did not produce stereotypy but potentiated, in a dose-dependent manner, apomorphine (1mg/kg)^induced stereotypy, which was inhibited by sulpride (20mg/kg), a dopamine D-2 recepter antagonist. Theresult suggest that the important factors in the occurrence of yawing and stereotyped behaviors, include not only dopamine D-2 recepter stimulation, but also endogenous dopamine or D-1 recepter activation, and seem to depent upon the ratio and potency of D-1 recepter versus D-2 recepter activity.