Effects of prolactin-releasing peptide on tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in estrogen-treated female rats

Both systemic and central effects of a newly discovered prolactin (PRL)-releasing factor (PRF), prolactin-releasing peptide (PrRP), were determined in this study. Systemic injection of PrRP (1 and 10 microg/rat, i.v.) stimulated PRL secretion in ovariectomized, estrogen-treated rats similar to the effect of another PRF, thyrotropin-releasing hormone (TRH). Pretreatment with a dopamine D2 receptor antagonist, sulpiride (1 microg/rat, i.v.), potentiated the stimulatory effect of both PrRP and TRH on PRL secretion. Using the double-labeling immunohistochemical method, PrRP-immunoreactive terminals were found in close contact with tyrosine-hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus. Central administration of PrRP (0.1-1,000 ng/rat, i.c.v.) stimulated tuberoinfundibular but not nigrostriatal dopaminergic neuronal activity in 15 min. Levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence and striatum were used as indices for tuberoinfundibular dopaminergic (TIDA) and nigrostriatal dopaminergic neuronal activities, respectively. The serum PRL level, however, was not significantly changed. Similar treatment with TRH (10 ng/rat, i.c.v.) stimulated and inhibited TIDA neuronal activity and serum PRL, respectively, at 30 min. In summary, PrRP may play a role in both the central and peripheral control of PRL secretion.