Involvement of brain dopamine in prolactin secretion induced by a synthetic Met5-enkephalin analogue in rats.

The effect of a potent opioid peptide FK 33-824, [D-Ala2, MePhe4, Met(O)5-ol] enkephalin, on prolactin (PRL) release from rat anterior pituitary was investigated in vivo and in vitro. Systemic administration of FK 33-824 (1, 10 and 100 micrograms/100 g BW i.p.) caused a rapid and dose-related increase in plasma PRL in urethane-anesthetized male rats. Naloxone (125 micrograms/100 g BW i.v.) abolished PRL responses to FK 33-824. In the rat pretreated with either reserpine (2 mg/100 g BW i.p.), alpha-methyl-p-tyrosine (30 mg/100 g BW i.p.) or pimozide (50 micrograms/100 g BW i.v.), basal plasma PRL levels were elevated and FK 33-824 injection did not further increase plasma PRL. In contrast, neither 5,6-dihydroxy-tryptamine (50 micrograms/rat, i.c.v.) nor diphenhydramine (100 micrograms/100 g BW i.v.) treatment influenced the plasma PRL response to FK 33-824. FK 33-824 (10(-8)-5 X 10(-5) M) did not stimulate PRL release from dispersed anterior pituitary cells in vitro nor attenuate the inhibitory effect of dopamine (5 X 10(-7) M). These results suggest that central dopaminergic mechanisms are involved in PRL release induced by the opioid peptide.