Site-specific opioid receptor blockade allows prepubertal guinea pigs to display progesterone-facilitated lordosis

The role of the endogenous opioid system in polycystic ovary syndrome

OBJECTIVE

To review the complex role of the opioid system in reproduction and carbohydrate metabolism, abnormalities in the opioid system in women with polycystic ovary syndrome (PCOS), and the role of opioid antagonists in the management of PCOS-related infertility.

DESIGN

Pertinent articles were identified through a computer PubMed search. References of selected articles were hand searched for additional citations.

CONCLUSION(S)

Endogenous opioids are generally considered inhibitory central neurotransmitters. Peripherally, opioids are involved in the regulation of pancreatic islet function, hepatic insulin clearance, and glucose metabolism, potentially contributing to the pathogenesis of hyperinsulinemia and insulin resistance in PCOS. The presence of sex steroids is required for normal function of the opioid system in both GnRH secretion and carbohydrate metabolism. In women with PCOS, growing evidence suggests dysregulation of the opioid system both centrally and peripherally, with complex interactions. The opioid system effects on carbohydrate metabolism appear to be modulated by obesity. Finally, naltrexone has been demonstrated to successfully augment traditional ovulation induction regimens, but has limited support as a single ovulation induction agent for PCOS.

Estrogen modulation of mu-opioid receptor-stimulated [35S]-GTP-gamma-S binding in female rat brain visualized by in vitro autoradiography

The mu-opioid receptor (OR) is involved in several aspects of female reproductive neuroendocrinology, such as the control of gonadotropin release and the display of lordosis behavior. Even though the neuroendocrine events modulated by mu-ORs are steroid hormone-dependent, few studies have shown how steroid hormones such as estrogen and/or progesterone can affect mu-OR function. Therefore, the present study investigated if in vivo estrogen or estrogen plus progesterone treatment of ovariectomized (OVX) rats affects mu-OR coupling to its G proteins. We used autoradiographic analysis of agonist-stimulated [(35)S]-GTPgammaS binding, in which brain sections were incubated in the presence or absence of the mu-OR agonist [D-Ala(2), N-Me-Phe(4), Gly(2)ol]-enkephalin (DAMGO). Film images were quantified using calibrated [(14)C] standards. Analysis was performed in steroid-responsive hypothalamic regions such as the medial preoptic area (mPOA) and the ventromedial nucleus of the hypothalamus, as well as in non-hypothalamic brain regions. Treatment with estrogen, alone or with progesterone, significantly increased DAMGO-stimulated [(35)S]-GTPgammaS binding in the mPOA when compared to control OVX animals. In addition, estrogen increased mu-OR coupling in the caudate putamen. Steroid treatment had no effect on either basal or DAMGO-stimulated binding in the other brain regions examined. These findings suggest that estrogen modulates mu-OR function in a brain region-specific fashion. This could have important implications in terms of how these hormones synchronize reproductive behavior and gonadotropin release.

Presence of mother and unfamiliar female alters levels of testosterone, progesterone, cortisol, adrenocorticotropin, and behavior in maturing Guinea pigs

Although the guinea pig is characterized by precocial physical development and minimal active maternal care, studies suggest the presence of the mother can influence neuroendocrine and behavioral activity of offspring even well beyond weaning. Previous results may have been influenced by the procedure of housing weaned subjects with the mother to within 2 days of testing. The present study examined approximately 40-day-old guinea pigs housed apart from the mother for 0 (not rehoused), 2, or 10 days. Rehousing without the mother led to elevations in plasma testosterone (measured in males), progesterone (measured in females), cortisol, and adrenocorticotropin (ACTH) (both measured in males and females). Offspring housed without the mother for 10 days had the highest progesterone, cortisol, and ACTH levels. Testosterone elevations were observed in 2-day-, but not 10-day-, rehoused animals. Regardless of rehousing condition, 60 min isolation in a novel test cage elevated progesterone, cortisol, and ACTH, and reduced testosterone. These effects were all moderated if the subject was tested with the mother or another female. Sexual behavior toward the mother was observed frequently, but only in males housed apart from her prior to testing. Overall, males and females that had been housed apart from the mother interacted with her as they would an unfamiliar female. Our results corroborate previous findings, suggest the effect of housing apart from the mother on male testosterone is transitory, and indicate that continuous housing with the mother past weaning suppresses circulating progesterone in females and cortisol and ACTH in both sexes.

Activation of mu-opioid receptors inhibits lordosis behavior in estrogen and progesterone-primed female rats

The present study investigated the effect of highly selective mu-opioid receptor (OR) agonists on lordosis behavior in ovariectomized rats treated with 3 microg of estradiol benzoate followed 48 h later by 200 microg of progesterone. Ventricular infusion of the endogenous mu-OR agonists endomorphin-1 and -2 suppressed receptive behavior in a time- and dose-dependent fashion. At 6 microg, both endomorphin-1 and -2 inhibited lordosis behavior within 30 min. However, while the effect of endomorphin-1 lasted 60 min, endomorphin-2 inhibition lasted up to 120 min after infusion. Pretreatment with naloxone (5 mg/kg sc) was able to block both endomorphin-1 and endomorphin-2 effects on lordosis. Site-specific infusions of endomorphin-1 or endomorphin-2 into the medial preoptic area (mPOA), the ventromedial nucleus of the hypothalamus (VMH), or into the mesencephalic central gray did not affect receptivity. In contrast, infusion of 1 mug of either compound into the medial septum/horizontal diagonal band of Broca inhibited lordosis in a pattern very similar to that seen after intraventricular infusions. Infusion of the potent synthetic mu-OR agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (0.08 microg) into the VMH and mPOA inhibited lordosis behavior for at least 60 min after infusion. The nonspecific opioid receptor antagonist naloxone was able to facilitate lordosis in partially receptive female rats when infused into the mPOA but not when infused into the VMH. The behavioral effects of the agonists and antagonist used in this study suggest that the endogenous mu-opioid system modulates estrogen and progesterone-induced lordosis behavior.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Lordosis-enhancing medial preoptic area lesions do not alter hypothalamic estrogen receptor- or progestin receptor-immunoreactivity in prepubertal female guinea pigs

Female guinea pigs rarely display adult-typical lordosis responses to ovarian steroid hormones until 40-50 days of age. Behavioral hyporesponsiveness in prepubertal females may be due, in part, to deficiencies in hypothalamic estrogen receptors and/or estradiol-induced progestin receptors. This study was designed to test the hypothesis that bilateral medial preoptic area (MPOA) lesions, which enhance the display of progesterone-facilitated lordosis in juvenile females, increase levels of hypothalamic estrogen receptors and/or estradiol-induced progestin receptors. Hartley guinea pigs were ovariectomized at 11-12 days of age and at 14-15 days of age received bilateral electrolytic or sham lesions aimed at the MPOA. At approximately 3 weeks of age, lesioned and sham-lesioned animals were either tested for the display of progesterone-facilitated lordosis or perfused, and their hypothalamic tissue processed for estrogen receptor- or estradiol-induced progestin receptor-immunostaining. Although a significantly higher percentage of MPOA-lesioned than sham-lesioned guinea pigs displayed progesterone-facilitated lordosis (85.7% vs. 5. 8%, respectively, p<0.05), there were no significant lesion-related differences in the number or staining intensity of cells containing estrogen receptor- or estradiol-induced progestin receptor-immunoreactivity in the ventrolateral hypothalamus or arcuate nucleus. These data do not support the hypothesis that the enhanced display of progesterone-facilitated lordosis in prepubertal guinea pigs following MPOA lesions is due to increased hypothalamic concentrations of estrogen receptors or estradiol-induced progestin receptors.