Activation of MU opioid receptors in the pre-optic-anterior hypothalamus releases prolactin in the conscious male rat

Microinjection of opioid agonists into the pre-optic-anterior hypothalamus ( PO AHA) was used to determine the identity of the opioid receptor subtype(s) involved in the stimulation of prolactin release. The mu agonist DAGO [(D-Ala2, NMe-Phe4, Gly-o 15)-enkephalin] was the only opioid agonist to show dose-dependent release of prolactin, the lowest significant dose being 0.001 nmoles. Neither the specific delta agonist DPDPE [(D-Pen2, D-Pen5)-enkephalin] nor the specific kappa agonist U50,488H [(trans-3,4-dichloro-N-methyl-N-(2-(1-pyrrodinyl)-cyclohexyl)-benzene acetamide] showed dose-dependent increase of prolactin secretion, or indeed any significant increase in prolactin secretion in the dose range 0.01-1 nmoles and 0.01-10 nmoles respectively. We suggest that mu (and not kappa or delta) opioid receptors in the PO AHA are involved in the opioid stimulated release of prolactin in the conscious male rat.