Activation of Opioid Receptors in the Mediobasal Hypothalamus Stimulates Prolactin Secretion in the Conscious Rat

In an attempt to localize the opioid receptor(s) (mu, delta and kappa) involved in opioid‐stimulated prolactin release in the conscious male rat, opioid agonists were microinjected into the mediobasal hypothalamus and prolactin levels measured before and after injection. The specific mu agonist, DAGO ((D‐Ala², NMe‐Phe⁴, Gly‐ol⁵)‐enkephalin) was the most effective in eliciting prolactin release, the smallest effective dose being 0.01 nmoles. The specific delta agonist, DPDPE ((D‐Pen², D‐Pen⁵)‐enkephalin) had no significant effect even at the highest dose of 10 nmoles. The specific kappa agonist, U50,488H ((trans‐3,4‐dichloro‐N‐methyl‐N‐(2‐(1‐pyrrolidinyl)‐cyclohexyl)‐benzene acetamide) was effective at the doses 1.0 and 10 nmoles. We conclude that mu and kappa opioid receptors in the mediobasal hypothalamus are involved in opioid‐stimulated prolactin release and that delta receptors are not.