delta-Opioid receptor-mediated regulation of central dopaminergic neurons in the rat

Effects of intraventricular injections of the delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) and antagonist 17-cyclopropylmethyl-6,7-dehydro-4,5-epoxy-3,14-dihydroxy-6,7,2',3'-indo l morphinan (naltrindole) hydrochloride were determined on the activities of mesolimbic, nigrostriatal, tuberoinfundibular and periventricular-hypophysial dopaminergic neurons in brains of male rats. Dopaminergic neuronal activity was estimated by measuring concentrations of 3,4-dihydroxyphenylacetic acid (DOPAC) and accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of the decarboxylase inhibitor 3-hydroxybenzylhydrazine in regions of the brain (nucleus accumbens, striatum, median eminence) and the intermediate lobe of the pituitary which contain terminals of these neurons. DPDPE produced a dose- and time-related increase in concentrations of DOPAC and accumulation of DOPA in nucleus accumbens and median eminence, but had no effect in striatum or intermediate lobe of the pituitary. Naltrindole hydrochloride had no effect per se, but blocked the ability of DPDPE to increase DOPAC concentrations in nucleus accumbens and median eminence. These results reveal that activation of delta-opioid receptors selectively increases the activities of mesolimbic and tuberoinfundibular but not of nigrostriatal or periventricular-hypophysial dopaminergic neurons.