Ventral pallidal microinjections of receptor-selective opioid agonists produce differential effects on circling and locomotor activity in rats.

Locomotor activity was investigated following microinjections of receptor-selective opioid agonists into the ventral pallidum (VP) of rats. In Expt. 1, male Long-Evans rats were treated with unilateral microinjections of the mu agonist [D-Ala2-MePhe4, Gly-ol5]-enkephalin (DAGO), the delta agonist [D-Pen2, D-Pen5]-enkephalin (DPDPE) or the kappa agonist U50,488H, and the rate and duration of circling behaviour were measured. DAGO (0.01, 0.1, 1.0 nmol) produced a dose-dependent increase in contralateral circling; pretreatment with 1.0 mg/kg naltrexone blocked the circling induced by the highest dose. The behavioral effect was largest when injections were targeted at the VP rather than structures dorsal to the VP. In contast to DAGO, intrapallidal DPDPE (0.01, 0.1, 1.0, 10.0 nmol) produced a slight increase in contralateral circling only at the highest dose and U50, 488H (0.01, 0.1, 1.0, 10.0 nmol) produced no effect. In Expt. 2, the effects of bilateral injections of DAGO, DPDPE and U50,488H were tested in photocell activity boxes. DAGO produced a dose-dependent increase in locomotor activity and this increase was decreased by 1.0 mg/kg naltrexone. A slight increase in activity was observed with the highest dose of DPDPE, and a slight decrease was observed with the highest dose of U50,488H. These findings confirm that opiate actions in the VP contribute to opiate-induced locomotion and suggest that mu and to some extent delta receptors are involved in this behavior.