Short-term inhibition of prolactin secretion by naloxone treatment in the pregnant gilt

Hormone profiles and nest-building behavior during the periparturient period in rabbit does

This study was done to elucidate the relationship between the hormones beta-estradiol, progesterone, prolactin and beta-endorphin and nest-building behavior in rabbit does during the periparturient period. Beta-estradiol increased as parturition approached with a significant increase starting 2 days before parturition (day -2). Progesterone decreased with the progress of gestation, but a significant decrease was observed starting on day -2. Prolactin concentration started to increase on day -2 prepartum but a significant increase in prolactin concentration was not noted until the last day of pregnancy. The concentration of beta-endorphin was highly variable during the sampling period and could not be correlated with the other hormones or nest-building behavior. To assess the role of prolactin in nest-building behavior, groups of rabbits were treated with bromocryptine, bromocryptine plus prolactin or saline (controls). Treatment with 4 mg bromocryptine or 4 mg bromocryptine + 1.5 mg bovine prolactin on days 25 and 27 of pregnancy did not affect the number of live kits born or the gestation length. The mean nest scores, a measure of the nest quality, were not affected by bromocryptine treatment, but treatment with bromocryptine plus prolactin resulted in lower nest scores (P < 0.05). Injection of 8 mg bromocryptine from day 28 of gestation to kindling resulted in an extended gestation (P < 0.05). Injection of 4 or 8 mg bromocryptine resulted in fewer live kits born (P < 0.05), reduced nest scores (P < 0.01) and blocked milk production, as determined from the palpable mammary tissue. These results indicate that prolactin has less influence on nest-building behavior than on milk production. The hormones most likely to influence nest building are beta-estradiol and progesterone because the levels of these hormones started to change at the time when the rabbits started to prepare nests. Further study is required to determine the influence of these hormones on nest-building behavior.

Treatment of lactating sows with the dopamine agonist Cabergoline: effects on LH and prolactin secretion and responses to challenges with naloxone and morphine

This study was conducted to examine the effects of chronic administration of a long-acting dopamine agonist, Cabergoline, on LH and prolactin secretion during lactation in the sow. The effect of the administration of the opioid antagonist naloxone and the agonist morphine in Cabergoline treated animals was also evaluated. In Part I of the experiment, 16 sows were treated as either CONT sows (n = 4; control, no treatment); CAB sows (n = 4; treated with Cabergoline from days 10 to 26 of lactation); CAB + NAL sows (n = 4; received Cabergoline treatment and naloxone challenges); CAB + MORP sows (n = 4; treated with Cabergoline and morphine challenges). Plasma LH and prolactin concentrations were measured in blood samples taken from all sows during 6-h periods at days 12, 19 and 26 of lactation. To extend the results at the most critical response period at day 26, another 11 sows were allocated in Part II to either Control (n = 3), Cabergoline (n = 4) or Cabergoline and morphine (n = 4) treatments as for Part I, but the effect of treatments were only confirmed in a single period of sampling at day 26 of lactation. Cabergoline treatment alone increased (P < 0.001) mean plasma LH concentrations at day 26 but not at days 12 and 16 of lactation. In contrast, naloxone challenges given in the presence of Cabergoline treatment increased (P < 0.05) mean LH at days 12 and 19 of lactation but not at day 26. Morphine challenges in the presence of Cabergoline treatment decreased (P < 0.05) mean LH concentrations only at day 26 of lactation, but did not completely reverse the effect of Cabergoline. No treatment differences in plasma oestradiol-17 beta were detected at any time. Plasma prolactin decreased (P < 0.001) in response to treatment with Cabergoline but there were no additional effects of naloxone or morphine. These data provide evidence for the existence of dopaminergic and opioidergic regulation of LH secretion in lactation in the sow and the relative influence of these systems changes as lactation progresses. Furthermore, the data suggest that the stimulatory effect of Cabergoline treatment on LH secretion in late lactation may be mediated by its effects on an inhibitory opioidergic mechanism. Finally, the data provide conclusive proof that prolactin does not directly influence LH secretion or estrogenic activity of the ovary during lactation in the sow.