Endogenous opioid peptide modulation of LH secretion in the ewe lamb: possible involvement of 5-hydroxytryptamine

Abstract

Evidence from several species suggest that the endogenous opioid peptides participate in the regulation of gonadotrophin and prolactin secretion. The aim of the present study involving intact and ovariectomized prepubertal ewe lambs was to compare the effects in vivo of an opioid peptide agonist [D-Ala2,N-Phe4,Met(0)ol5]-enkephalin (FK 33-824) and antagonist, naloxone, on concentrations of LH and prolactin in plasma, and levels of neurotransmitter metabolites in cerebrospinal fluid (CSF), with their effects in vitro on the release of gonadotrophin-releasing hormone (GnRH) and neurotransmitters from isolated median eminences. Infusion of FK 33-824 (0.5 mg/30 min) in vivo depressed plasma LH levels in both intact and ovariectomized lambs; this effect could be reversed by naloxone. In ovariectomized lambs, the inhibitory action of FK 33-824 on plasma LH levels was associated with a 13% rise in the concentration of the metabolite of 5-hydroxytryptamine, 5-hydroxyindolacetic acid (5-HIAA). Concurrent administration of naloxone resulted in an abrupt 33% fall in CSF levels of 5-HIAA. No significant changes in plasma concentrations of prolactin or CSF concentrations of the metabolites of dopamine were observed in response to the administration of FK 33-824 or FK 33-824 plus naloxone. That FK 33-824 inhibited LH release through a central mechanism was confirmed using superfused median eminences in vitro. Thus FK 33-824 (1 mumol/l) greatly diminished the release of GnRH induced by the introduction of a depolarizing stimulus (36 mmol K+/l) in tissue obtained from both intact and ovariectomized ewe lambs.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Beta‐Endorphin on Pulsatile Luteinizing Hormone and Prolactin Secretion During the Follicular Phase in the Ewe

“…This result agrees with studies in the rat where /?-endorphin increases prolactin levels (2) probably via the m u -or kappa-receptor (3,7). If similar receptors are involved in tllc control of prolactin secretion in the ewe, it then becomes dlfkxlt to explain why the mixed mu-and kappa-receptor agonist F K 33-824 does not increase prolaction secretion but does inhibit 1.H secretion in the ewe lamb (5). Presumably this may reflect differences in the ontogeny of the opioid receptors associated with LH and prolactin secretion during sexual maturation of the CWC.…”

“…/?-Endorphin in the ewe 125 to inhibit G n R H release. Certainly it has been shown that the opioid agonist FK 33-824 inhibits GnRH release from the isolated median eminence (5). Anatomical evidence can be used to support either possibility; the /?-endorphin terminals interact with the GnRH neurons in a t least two loci, the preoptic anterior hypothalamus and the median eminence (9).…”

“…control of prolactin secretion. Opioid agonists clearly stimu-1~1~ prolactin secretion in the rat (3); however in sheep, although nlorphine stimulates prolactin secretion (4), the mixed mu-and kappa-receptor agonist FD 33-824 has no effect suggesting that Prolactin secretion may be under different opioid control (5).…”

A Role for Catecholaminergic Neurons in the Suppression of Pulsatile Luteinizing Hormone Secretion in the Prepubertal Ewe Lamb

“…In particular, since catecholamines do not appear to be inhibitory prior to 22 weeks of age, some other neurotransmitter is probably holding LH in check during this period. This inhibitory neurotransmit ter may be an endogenous opioid, since opioid antagonists in crease LH release in prepubertal lambs [26][27][28], while opioid agonists inhibit LH [33,35]. Arguing against this possibility is the report that there was no change in response to the opioid antagonist, naloxone, between 9 and 27 weeks of age [28].…”

Seasonal endocrine cycles in the European hedgehog, Erinaceus europaeus

“…There is also evidence that the inhibition of gonadotrophin secretion by opioids acts via the stimulation of the release of a precursor of melatonin, and other reproductively active pineal índoles (Reiter, 1984), 5-hydroxytryptamine (Stansfield et al, 1988). Similarly, the inverse relation¬ ship between prolactin and testosterone indicates short-day stimulation of testicular prolactin receptors in the hedgehog, the opposite of events in another hibernator, the hamster (Klemcke et al, 1983).…”