Estrogen Modulation of Mu-Opioid Receptor-Stimulated [³⁵S]-GTP-Gamma-S Binding in Female Rat Brain Visualized by in vitro Autoradiography

Abstract

The µ-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 µ-ORs are steroid hormone-dependent, few studies have shown how steroid hormones such as estrogen and/or progesterone can affect µ-OR function. Therefore, the present study investigated if in vivo estrogen or estrogen plus progesterone treatment of ovariectomized (OVX) rats affects µ-OR coupling to its G proteins. We used autoradiographic analysis of agonist-stimulated [³⁵S]-GTPγS binding, in which brain sections were incubated in the presence or absence of the µ-OR agonist [D-Ala², N-Me-Phe⁴, Gly²ol]-enkephalin (DAMGO). Film images were quantified using calibrated [¹⁴C] 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 [³⁵S]-GTPγS binding in the mPOA when compared to control OVX animals. In addition, estrogen increased µ-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 µ-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.

An ex vivo Multi-Electrode Approach to Evaluate Endogenous Hormones and Receptor Subtype Pharmacology on Evoked and Spontaneous Neuronal Activity within the Ventromedial Hypothalamus; Translation from Female Receptivity

“…The EC 50 of DAMGO was selective for µ opioid receptors [56] and was blocked with the µ opioid selective antagonist CTOP. Autoradiographic studies confirm µ opioid receptors in the VMH [57]. 8‐OH‐DPAT also potentiated EPSPs and inhibited SUA.…”

A maternal “junk‐food” diet reduces sensitivity to the opioid antagonist naloxone in offspring postweaning

“…The differences in fat intake between males and females may be attributed to differences in the rate of development of the opioid system between the sexes, however this is poorly explored in the literature and a need for further investigation remains. Another possible explanation for the sex‐specific effect observed is differences in the concentrations of gonadal hormones between the sexes, as estrogen has previously been implicated in the regulation of the endogenous opioid system (37, 38).…”

Dose‐ and time‐dependent estradiol modulation of morphine antinociception in adult female rats

“…One mechanism that likely underlies E2 modulation of nociception and opioid antinociception is E2 modulation of CNS opioid neurotransmission. E2 increases preproenkephalin gene expression (Holtzman et al, 1997;Amandusson et al, 1999;Sinchak et al, 2000) and decreases mu opioid receptor density (Hammer, 1990;Weiland and Wise, 1990;Š lamberová et al, 2003), translocation (Eckersell et al, 1998), and intracellular signaling (Kelly et al, 1999;Acosta-Martinez and Etgen, 2002) in various brain areas and/or spinal cord. Some of these effects have been shown to be time-dependent; for example, peak pre-proenkephalin levels in ventromedial hypothalamus and posterodorsal medial amygdala occurred 24-48 h after s.c. estradiol benzoate injection (Sinchak et al, 2000), and opioid peptide levels and receptor density are also known to fluctuate across the rat estrous cycle (Hammer, 1990;Barden et al, 1981;Knuth et al, 1983;Genazzani et al, 1990;Maggi et al, 1991;Chang et al, 2000).…”