Opioidergic regulation of prolactin secretion during pregnancy: role of ovarian hormones

SOAJE M; DEIS RP

JOURNAL OF ENDOCRINOLOGY

BIOSCIENTIFICA LTD

Lugar: Bristol; Año: 1997 vol. 155 p. 99 - 106

0022-0795

We have recently demonstrated the existence of a neuromodulatoryregulation of prolactin secretion by the opioidsystem showing a paradoxical opioid-induced prolactinsuppression at the end of pregnancy. The aim of this studywas to determine a possible interaction between the opioidsystem and ovarian hormones on the release of prolactinduring pregnancy.Serum prolactin levels measured at 1800 h were significantlyhigher on days 3 and 6 of pregnancy whencompared with the other days of gestation. These increasesin serum prolactin were reduced significantly by naloxone(2 mg/kg) administered at 1730 h and by RU-486(10 mg/kg) administered at 0800 h. The response inducedby RU-486 was potentiated by naloxone only on day 3.On days 7, 13 and 16, prolactin secretion was not modifiedby RU-486 and/or naloxone treatment. In RU-486pretreated rats, naloxone administration increased serumprolactin levels only on day 16 of pregnancy. Interestingly,progesterone treatment (0·5 mg/rat) on days 13, 14 and15 of pregnancy prevented the high increase in serumprolactin induced by RU-486 and naloxone on day 16 ofpregnancy. The progressive increase and decrease of serumprogesterone concentration during pregnancy was notmodified by naloxone treatment.The participation of oestrogen in the regulation ofprolactin secretion by the opioid system on days 3, 16 and19 was examined by treating these groups of rats withoestradiol benzoate or tamoxifen citrate. Oestradiol(2 μg/rat) significantly increased serum prolactin levels onday 3 and naloxone administration did not modify thisincrease. No effect was observed after oestradiol (5 μg/rat)and naloxone treatment on days 16 and 19 of pregnancy.Oral administration of tamoxifen (500 μg/kg) the previousday did not modify the serum prolactin concentrationmeasured at 1800 h in oil-treated rats on days 3, 16 and 19of pregnancy. The antioestrogen completely abolished thenaloxone-induced prolactin secretion on day 16 in ratspretreated with RU-486 but no effect was observed onday 19. When tamoxifen was administered on days 14 and15 of pregnancy, the high serum prolactin levels on day 19induced by treatment with RU-486 and naloxone weresignificantly reduced.In conclusion, these results provide an important newinsight into the existence of a dual neuromodulatoryregulation of prolactin secretion by the opioid systemduring pregnancy. After a stimulatory action during thefirst days, there is a change to an inhibitory control atthe end of pregnancy, starting around day 16. Moreover,the activation of the inhibitory modulation of the opioidsystem on prolactin secretion on days 16 and 19 ofpregnancy seems to be mediated by changes in theoestrogen and progesterone action.