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

In equine species, luteinising hormone (LH) and prolactin (PRL) release are reduced throughout pregnancy but increase at foaling. The present experime nts were designed to study a possible opioidergic regulation of LH and PRL release ill 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 +/- 48, 282.7 +/- 3 .4 and 319.8 +/- 2.1). mares were injected with the opioid antagonist nalox one (0.5 mg/kg body weight) and saline. The two treatments were always sepa rated by 2 days, and mares served as their own controls. Two hours after be ing given naloxone and saline, mares were given the gonadotrophin-releasing hormone (GnRH) analogue buserelin (5 mug per animal). The naloxone experim ent 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 Blood for the determination of LH and PRL was withdrawn at 15 min intervals for 240 min, and naloxone or sal ine was injected after 60 min. Naloxone induced significant (P <0.05) LH re lease on days 172, 236 and 283 of pregnancy but not on days 26, 76 and 330 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. Th e experiments indicate that endogenous opioids are involved in the inhibiti on of LH release during the secund half of pregnancy in equine species. The deactivation of opioid effects on LH release might be a prerequisite for t he onset of ovarian activity postpartum. Plasma PRL concentrations increase d significantly (P < 005) after naloxone administration on days 236, 282 an d 320 of pregnancy. The naloxone-induced PRL release was most pronounced to rwards term, indicating an increase in the naloxone-releasable pool and/or the absence of other PRL-release inhibitory mechanisms.