Regulation by endogenous opioids of suckling-induced prolactin secretion in pregnant and lactating rats: role of ovarian steroids

SOAJE M; DEIS RP

JOURNAL OF ENDOCRINOLOGY

BIOSCIENTIFICA LTD

Lugar: Bristol; Año: 2002 vol. 172 p. 255 - 261

0022-0795

Evidence suggests that endogenous opioid peptides areimplicated in the suckling-induced prolactin rise. Weexplored the role of the opioid system and the participationof ovarian hormones in the regulation of prolactin inducedby the suckling stimulus at the end of pregnancy in ratswith developed maternal behavior, and during lactation.Suckling for 24 h induced a significant increase in serumprolactin on day 19 of pregnancy, which was increasedmore than three times when naloxone (2 mg/kg s.c.) ormifepristone (2 mg/kg) was administered. The combinationof naloxone and mifepristone did not increase serumprolactin more than either compound alone. Administrationof tamoxifen (500 μg/kg orally) on days 14 and 15of pregnancy completely abolished the effect of naloxone,indicating a role for estrogens in establishing this inhibitoryrole of opioids. To examine the participation of the opioidsystem during lactation, we used groups of rats on days 1,3, 5, 12 and 19 postpartum either (i) isolated from the pupsfor 4 h, or (ii) isolated from the pups for 3·5 h and reunitedwith them and suckled for 30 min. Naloxone, given justbefore replacing the pups, prevented the increase in serumprolactin levels observed in the suckled group of rats buthad no effect on the basal levels of the isolated rats. Toexamine whether the participation of the opioid system inthe release of prolactin is dependent on the variation ofprogesterone levels, rats on day 20 of pregnancy wereimplanted with two cannulae containing progesterone(that blocked postpartum ovulation) or cholesterol, andcesarean surgery was performed on day 21. To maintainlactation, pups (1?3 days old) were replaced every 24 h,and 4 days after the cesarean eight pups were placed in thecage at 1800 h to maintain a strong suckling stimulusduring the following 24 h. Naloxone administrationsignificantly reduced serum prolactin levels in control(cholesterol) rats but progesterone implants prevented theinhibitory effect of naloxone and this effect was notmodified by treatment with estrogen.These results indicate that the opioid system modulatessuckling-induced prolactin secretion, passing from aninhibitory action before delivery to a stimulatory actionduring lactation. This regulatory shift seems to bedependent on the fall in progesterone concentration at theend of pregnancy and the subsequent increase after thepostpartum ovulation and luteal phase