Opioidergic inhibition of luteinising hormone and prolactin release changes during pregnancy in pony mares

Abstract

In equine species, luteinising hormone (LH) and prolactin (PRL) release are reduced throughout pregnancy but increase at foaling. The present experiments were designed to study a possible opioidergic regulation of LH and PRL release in pregnant Shetland mares (n=6). At various stages of pregnancy (days 26·4 0·6, 75·4 5·4, 171·8 2·4, 226·2 4·8, 282·7 3·4 and 319·8 2·1), mares were injected with the opioid antagonist naloxone (0·5 mg/kg body weight) and saline. The two treatments were always separated by 2 days, and mares served as their own controls. Two hours after being given naloxone and saline, mares were given the gonadotrophin-releasing hormone (GnRH) analogue buserelin (5 µg per animal). The naloxone experiment was repeated at 2 days after foaling. Blood for the determination of LH and PRL was withdrawn at 15 min intervals for 240 min, and naloxone or saline was injected after 60 min. Naloxone induced significant (P<0·05) LH release on days 172, 226 and 283 of pregnancy but not on days 26, 76 and 320 and 2 days after foaling. Buserelin caused a significant (P<0·05) increase in plasma LH concentrations on days 172, 226, 282 and 320 of pregnancy. The experiments indicate that endogenous opioids are involved in the inhibition of LH release during the second half of pregnancy in equine species. The deactivation of opioid effects on LH release might be a prerequisite for the onset of ovarian activity postpartum. Plasma PRL concentrations increased significantly (P<0·05) after naloxone administration on days 226, 282 and 320 of pregnancy. The naloxone-induced PRL release was most pronounced towards term, indicating an increase in the naloxone-releasable pool and/or the absence of other PRL-release inhibitory mechanisms.

Dominant follicle and gonadotropin dynamics before ovulation in postpartum lactating mares

“…Furthermore, although not evaluated in this study, a potential negative energy balance due to the expected high milk yield in the postpartum crossbred draft mares may have contributed, at least in part, to the inhibition of GnRH and gonadotropins. However, an inhibitory effect of endogenous opioids on GnRH release has not been observed after parturition in mares (Aurich et al, 2001). Another novel finding of the present study is the confirmation in the PP Lactating group (also observed in N-PP Cycling group) of the temporal relationship between FSH and LH, with the…”

“…In mares, the mean levels of gonadotropins are basal during almost the whole gestation period (Aurich et al, 2001;Ginther, 1992). Periodic FSH surges continue to occur mainly until the first 60 days of gestation; consequently, FSH reaches basal levels after 150 days, has a posterior resurgence 5 days before parturition, and continues to increase until 1 day before or at the parturition day, probably due to several hormonal changes that initiate the foaling process (Ginther, 1992).…”

Induced Lactation with a Dopamine Antagonist in Mares: Different Responses between Ovariectomized and Intact Mares

“…In this study, the ovulation took place at the end of autumn, when PRL decreases to undetectable levels, in non-pregnant mares (Guillaume 2001). Aurich et al (2001) report a nadir of prolactin concentrations on days 172 and 226 of gestation, and a significant increase at the end of gestation. In equine embryo transfer, the major difficulty is the synchrony of the time of ovulation between the donor and recipient mares.…”

Endocrine changes during the peripartal period related to colostrogenesis in mammalian species

“…In horses, gestagen concentration at parturition is still at about 50% of the peak level (Nagel et al, 2012;Legacki et al, 2016a), even though the placenta seems to selectively repress gestagen synthesis near term (Legacki et al, 2018). As an ungulate species the newborn foal requires immediate colostrum uptake after birth, and instead of the absolute concentration of both gestagens and E2, the changing ratio of their concentrations mainly based on a steep increase of E2 is assumed to be the most important trigger of lactogenesis (Aurich et al, 2001). In contrast to sheep and assumedly horses, primates including human cannot change the pathway of placental steroid synthesis from P4 to E2 (Figure 1).…”

“…In sows, PRL additionally stimulates the proliferation and development of mammary parenchyma, which requires elevated PRL already during the last trimester of pregnancy (Farmer et al, 2000;Farmer and Quesnel, 2020). Based on Aurich et al (2001) PRL in horses remains elevated until two weeks postpartum. In contrast to other species, PRL levels in cats and dogs remain elevated throughout the entire lactation (Concannon et al, 1978;Banks et al, 1983).…”