Influence of melatonin and oestradiol on the opioidergic regulation of LH and prolactin release in pony mares

Abstract

The aim of this study was to investigate the influence of oestradiol, melatonin and season on the opioid regulation of LH and prolactin release. Effects of the opioid antagonist naloxone (0.5 mg/kg) on LH and prolactin secretion were determined in ovariectomized pony mares. In experiment 1, mares in January (n = 6) were pretreated with oestradiol benzoate (5 micrograms/kg) for 20 days. In experiment 2, beginning in May, mares (n = 7) received melatonin (15 mg) for 15 days and subsequently a combination of melatonin plus oestradiol for 20 days. In experiment 3, beginning in May, mares (n = 6) were pretreated with oestradiol for 30 days, left untreated for 12 days and then given melatonin for 35 days. In all experiments the animals were injected with the opioid antagonist naloxone and saline on 2 consecutive days prior to treatment. In experiment 1, animals received naloxone and saline on days 10 and 11 and 20 and 21 following oestradiol treatment. In experiment 2, naloxone and saline were administered on days 15 and 16 following melatonin treatment and on days 10 and 11 and 20 and 21 of melatonin plus oestradiol treatment. In experiment 3, the animals received naloxone and saline on days 10 and 11, 20 and 21 and 30 and 31 of oestradiol treatment, prior to melatonin treatment and on days 15 and 16, 25 and 26 and 35 and 36 following melatonin. In January (experiment 1), naloxone evoked a significant (P < 0.05) LH release at all times, however the LH increment in response to naloxone increased during oestradiol pretreatment (P < 0.05). During the breeding season (experiments 2 and 3), naloxone induced a significant (P < 0.05) increase in plasma LH concentrations when mares had not been pretreated with oestradiol or melatonin and after oestradiol pretreatment. Basal LH concentrations and the LH increment in response to naloxone increased significantly (P < 0.05) during the 30-day oestradiol pretreatment. Melatonin decreased the naloxone-induced LH release and the LH release in response to naloxone and saline no longer differed after 25 and 35 days of melatonin pretreatment. When melatonin was given together with oestradiol for 20 days, again a significant (P < 0.05) LH release in response to naloxone occurred. Prolactin release was significantly (P < 0.05) increased by naloxone when mares had been pretreated with only melatonin. The opioid antagonist did not affect prolactin release in mares that had not been pretreated or received oestradiol either alone or in combination with melatonin. In conclusion, in long-term ovariectomized mares, opioids inhibit LH secretion independent from ovarian factors. This opioid inhibition of LH secretion is enhanced by oestradiol and reduced by melatonin. Although short-term melatonin treatment inactivates the opioid regulation of LH release, a prolonged influence of melatonin as occurs in winter does not prevent activation of the opioid system. This indicates that effects of melatonin on the opioid regulation of LH release change with time. An opioid inhibition of prolactin secretion...

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

“…Naloxone-inducible PRL release does not exist in intact, non-pregnant mares but can be activated by oestradiol-progesterone and melatonin treatment in ovariectomised mares (Aurich et al 1994(Aurich et al , 1997. Thus, pregnancy is the only reproductive state in which opioids inhibit PRL secretion in non-pretreated mares.…”

“…In cyclic mares, the opioidergic inhibition of LH secretion is activated by a sequence of oestrogen and progesterone influence characteristic of the oestrous cycle. In seasonal anoestrous mares, the opioidergic inhibition of LH release can be stimulated by a prolonged period of oestrogen exposure or independently of ovarian factors (Aurich et al 1994(Aurich et al , 1995(Aurich et al , 1997. Opioids inhibit PRL secretion in steroid-treated (Aurich et al 1995) and melatonin-treated (Aurich et al 1997) ovariectomised mares but not in non-pregnant, ovaryintact mares.…”

“…In seasonal anoestrous mares, the opioidergic inhibition of LH release can be stimulated by a prolonged period of oestrogen exposure or independently of ovarian factors (Aurich et al 1994(Aurich et al , 1995(Aurich et al , 1997. Opioids inhibit PRL secretion in steroid-treated (Aurich et al 1995) and melatonin-treated (Aurich et al 1997) ovariectomised mares but not in non-pregnant, ovaryintact mares. In the non-pregnant mare, PRL release is primarily inhibited by dopamine (Aurich et al 2000), and the physiological relevance of opioids for the regulation of PRL release remains to be established.…”

Prolactin in the Horse: Historical Perspective, Actions and Reactions, and Its Role in Reproduction

“…In 1997 [79], they reported that naloxone, an opioid antagonist agent, stimulated prolactin secretion in mares that had previously been administered melatonin but not in mares that had received estradiol or melatonin plus estradiol. Naloxone administration to otherwise untreated mares in the breeding and nonbreeding season had no effect on prolactin secretion [80].…”

142 Factors affecting basal and post-exercise prolactin secretion in horses

“…Aurich et al (1995b) reported that prolactin secretion was induced by naloxone treatment in ovariectomized pony mares that had been pretreated with estradiol benzoate. However, Aurich et al (1997) reported that prolactin release was significantly increased by naloxone administration in ovariectomized pony mares after mares had been pretreated with only melatonin, but not when they received a combination of estradiol benzoate and melatonin. Thus, the literature is contradictory on the effects of opioid antagonists on prolactin secretion in horses.…”