Sex differences in the motor inhibitory and stimulatory role of dopamine D1 receptors in rats

We investigated sex differences in the motor responses to the fall and selective dopamine D1-like receptor agonist,(+/-)-6-chloro-7,8-dihydroxyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF-81297: 0.3, 3, and 10 mg/kg, s.c.), in non-habituated adult rats. In general, SKF-81297 produced a biphasic effect on motor activity (including locomotion, rearing and exploratory activity) which consisted of an initial short inhibition followed by a long-lasting stimulation. These effects were dose- and sex-dependent. The inhibitory phase was more pronounced in males than females while the opposite was true for the stimulatory phase. Importantly, the motor inhibitory effects of SKF-81297 were not due to an increase in stereotypy (e.g., grooming activity), These biphasic effects on several motor parameters suggest the presence of two distinct dopamine D I receptor populations which have opposite effects on motor activity and which are, in part, sexually dimorphic. (C) 2002 Published by Elsevier Science B.V.