MORPHINE, D-PEN(2), D-PEN(5) ENKEPHALIN AND U50,488H DIFFERENTIALLY AFFECT THE LOCOMOTOR-ACTIVITY AND BEHAVIORS INDUCED BY QUINPIROLE IN GUINEA-PIGS

The effects of morphine D-Pen2, D-Pen5 enkephalin (DPDPE) and U50,488H on the behavioural syndrome elicited by the dopamine (DA) D-2 agonist quinpirole, were investigated. Morphine (1, 5 and 15 mg/kg SC) and morphine administered intracerebroventricularly (ICV) (2 x 5-mu-l, 10(-3) M; total dose = 10 nmol) produced piloerection and sedation. DPDPE ICV (2 x 5-mu-l and 2 x 10-mu-l, 10(-3) M; total doses = 10 and 20 nmol) produced piloerection and sedation similar to morphine. U50,488H (I mg/kg SC) induced locomotor activity and some stereotyped behaviour, whereas U50,488H (5 and 10 mg/kg SC) induced muscle rigidity and dystonic-like movements. The locomotor and behavioural response elicited by quinpirole (3 mg/kg IP) was attenuated in guinea-pigs pretreated witb morphine (1, 5 and 15 mg/kg SC), morphine-ICV (2 x 5-mu-l, 10(-3) M), and DPDPE-ICV (2 x 5-mu-l and 2 x 10-mu-l, 10(-3) M). These effects were reversed by naloxone (15 mg/kg SC). U50,488H (I mg/kg SC) increased the quinpirole-induced locomotor activity, whereas U50,488H (5 and 10 mg/kg SC) decreased the locomotor activity and stereotyped behaviours produced by quinpirole. These results indicate that the gross behavioural effects of mu, delta and kappa opioids differ in guinea-pigs compared to other rodent species, and suggest differential involvement of these opioid receptor subtypes with DA D-2 receptor-mediated activity.