A possible dual regulation of prolactin release by the serotoninergic system in rats at pro-oestrus and during late pregnancy: role of ovarian hormones

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Abstract
The effect of para-chlorophenylalanine (pCPA), an inhibitor of serotonin synthesis, on prolactin release was studied in rats on the day of pro-oestrus and at the end of pregnancy (day 19). The surges of prolactin normally seen in the afternoon of pro-oestrus in intact rats and in rats ovariectomized on dioestrous day 2 and primed with oestrogen were significantly inhibited by pCPA treatment. Administration of 5-hydroxy-tryptophan reversed the inhibitory action of pCPA on prolactin release. Treatment with progesterone also completely reversed the inhibitory effect of pCPA on prolactin release in pro-oestrous rats and partially reversed it in ovariectomized oestrogen-treated rats. Ovariectomy on day 19 of pregnancy induced a significant release of prolactin 12 and 24 h later. Administration of pCPA on day 18 of pregnancy produced a marked increase in serum concentrations of prolactin on days 19 and 20 in rats left intact or ovariectomized on day 19. Administration of 5-hydroxy-tryptophan significantly reversed this stimulatory effect of pCPA on prolactin release but did not modify the release of prolactin induced by ovariectomy. Methiothepin (1-[10,11-dihydro-8-(methylthio) thiepin-10,41]-4-methylpiperazine maleate), a serotonin receptor blocker, also induced a significant increase in serum concentrations of prolactin on day 20 of pregnancy in rats left intact or ovariectomized on day 19. These results suggest the existence of different serotoninergic actions in the regulation of prolactin release at pro-oestrus and in late pregnancy. Serotonin facilitates the surges of serum prolactin released at pro-oestrus and in ovariectomized rats treated with oestrogen; progesterone enhances this effect. On the other hand, during late pregnancy, when progesterone tonically inhibits prolactin release, serotonin inhibits the release of prolactin. J. Endocr. (1987) 112, 367–374