Ovine Brain Areas Sensitive to Naloxone-Induced Stimulation of Luteinizing Hormone Release

Abstract

Previous work established that intravenous administration of the opioid receptor antagonist naloxone abruptly increased release of luteinizing hormone (LH) and decreased release of prolactin (PRL) in suckled anestrous ewes and also increased LH release in cyclic luteal ewes. The goal of the present research was to identify brain sites at which local unilateral infusions of naloxone would consistently duplicate the previously noted effects of intravenous naloxone. Intracerebral guide tubes were surgically implanted into the brain of 13 nonpregnant and 16 pregnant ewes at least 4 weeks prior to experimentation. Intracerebral infusion (20–40 µl each through an inner cannula) was performed once daily during postpartum anestrus in suckled fall-lambing ewes and during recurring luteal states of the estrous cycle. Naloxone infusion (n = 142) usually consisted of 50 or 100 µg naloxone, although 5 ewes received 200 and 400 µg per infusion. Control infusions (n = 103) consisted of the vehicle for naloxone (i.e., 0.9% NaCl). Serum concentrations of LH and PRL were quantified at 10-min intervals from 90 min before to 100 min after infusion. Hormone data from individual ewes were grouped for least-squares analysis of variance based upon postmortem neuroanatomical identification of each infusion site. Unilateral intracerebral administration of naloxone consistently induced an increase in LH release within 20 min in the following two neuroanatomical groups: basal forebrain (n = 9 ewes) and chiasmatic area (n = 4 ewes). These naloxone-sensitive brain areas constituted an apparent continuum of tissue sites located as far rostral as ventrolateral septum, diagonal band of Broca and nucleus accumbens and continuing caudally into the preoptic area in and around the organum vasculosum of the lamina terminalis. Brain sites at which the present unilateral infusions of naloxone did not consistently stimulate release of LH included those hypothalamic areas caudal to the preoptic area such as anterior, ventromedial and lateral hypothalamic area including the arcuate nucleus and third ventricle. Therefore, neuroanatomical sites at which local unilateral infusion of naloxone stimulated (i.e., disinhibited) release of LH were very similar to the location of LH-releasing hormone (LHRH) perikarya reported by others for the ovine brain and quite distant from the pituitary gland. Although intravenous naloxone was shown previously to decrease PRL release in suckled anestrous ewes, the present infusion of naloxone did not consistently affect PRL release in such ewes. In summary, local unilateral antagonism of opioid receptors in the vicinity of LHRH perikarya was sufficient to disinhibit release of LHRH/LH, but opioid mechanisms stimulatory to PRL release were not antagonized by intracerebral infusions of naloxone.

Blockade of the Oestrogen-Induced Luteinizing Hormone Surge in Ovariectomized Ewes by a Highly Selective Opioid µ-Receptor Agonist: Evidence for Site of Action

“…The data are, however, consistent with such a hypothesis, and consistent with the evidence for dual sites of opioid regulation of GnRH secretion in the luteal and follicular phases of the oestrous cycle [14, 15, 16, 17]. …”

“…Most studies have used LH secretion as an indirect measure of GnRH secretion, as GnRH and LH pulsatile release are synchronous [33], and there is no evidence in sheep for regulation of LH secretion by opiates at the level of the pituitary [8, 9]. Unilateral infusion of the opiate antagonist naloxone into the basal forebrain, MPOA, MBH or the anterior hypothalamic area increased plasma LH concentrations in luteal phase ewes [16], although in another study from the same laboratory, naloxone infusion into MBH had no effect on LH secretion [17]. Infusion of β-endorphin antiserum into MPOA, but not MBH, increased plasma LH levels, whereas met-enkephalin antiserum had the same effect when infused into MBH or anterior hypothalamic area, but not MPOA [16].…”

“…Previous studies have shown that infusion of opiate antagonists into either the MPOA or the mediobasal hypothalamus (MBH), close to the ME, can increase GnRH and LH secretion in sheep during the luteal and follicular phases of the oestrus cycle [14, 15, 16, 17]. This suggests that there may be dual sites of opioidergic regulation of GnRH secretion.…”

Intracerebral Immunoneutralization of Beta-Endorphin and Met-Enkephalin Disinhibits Release of Pituitary Luteinizing Hormone in Sheep

“…Intravenous administration of naloxone to sheep during postpartum anestrus or during the luteal phase of recurring estrous cycles abruptly disinhibited serum LH [9][10][11], Previous work in this laboratory attempted to determine by intracerebral infusion the naloxone-sensitive neural sites which suppressed LHRH/LH in sheep [12], Because regional differences in the disinhibition of LH by intracerebral naloxone were observed, it seemed possible that intracerebral infusion of EO antisera might also produce region-specific disinhibition of LHRH/LH release. Such an approach has the potential to neurochemically identify brain sites at which each specific EO peptide suppres ses LHRH.…”

Effect of Growth Retardation on Pituitary Luteinizing Hormone and Hypothalamic Neuropeptide Y in Ovariectomized Sheep

“…Serum concentrations of LH were quantified by a heterologous double-antibody radioimmunoassay as described previously [16] for the NIADDK anti-oLH-l serum. The final dilution of this antise rum was 1:800,000, and radiolabeled bovine LH (USDA bLH-l-l) was used as the labeled antigen.…”