Involvement of Brain Dopamine in Prolactin Secretion Induced by a Synthetic Met5-enkephalin Analogue in Rats

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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 100μg/100g BW i. p.) caused a rapid and dose-related increase in plasma PRL in urethane-anesthetized male rats. Naloxone (125μg/100g BW i.v.) abolished PRL responses to FK 33-824. In the rat pretreated with either reserpine (2 mg/100 g BW i. p.), α-methyl-p-tyrosine (30 mg/100g BW i. p.) or pimozide (50μg/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 (50μg/rat, i. c. v.) nor diphenhydramine (100μg/100g BW i. v.) treatment influenced the plasma PRL response to FK 33-824. FK 33-824 (10-8-5×10-5 M) did not stimulate PRL release from dispersed anterior pituitary cells in vitro nor attenuate the inhibitory effect of dopamine (5×10-7 M). These results suggest that central dopaminergic mechanisms are involved in PRL release induced by the opioid peptide.
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