Effect of oestrus cyclicity on the number of brain opioid mu receptors in the rat

Estradiol and Mu opioid-mediated reward: The role of estrogen receptors in opioid use

Opioid use and opioid use disorder are characterized by sex and gender differences, and some of these differences may be mediated by differences in the hormonal milieu within and across individuals. This review focuses on the role of ovarian hormones, and particularly estradiol, on the endogenous mu opioid receptor system. There is an abundance of data indicating that estradiol influences the activity of endogenous mu opioid peptides, the activation of mu opioid receptors, and the internalization and desensitization of mu opioid receptors. These effects have functional consequences on behaviors mediated by endogenous mu opioid receptor activity and on sensitivity to mu opioid agonists and antagonists. Recent behavioral data suggest these consequences extend to mu opioid reward, and preclinical studies report that estradiol decreases self-administration of mu opioid receptor agonists across a range of experimental conditions. Data collected in human laboratory studies suggest that estradiol may have functionally similar effects in clinical populations, and thus estrogen receptors may be a potential target in the development of novel therapeutics. This review summarizes data from cellular assays to clinical trials to explore how estradiol influences mu opioid receptor activity, as well as potential ways in which estrogen receptors may be targeted to address the problems of opioid use.

Role of steroid hormones and morphine treatment in the modulation of opioid receptor gene expression in brain structures in the female rat

This study determined the effects of acute treatment with morphine on the expression of the Oprm1, Oprk1, and Oprd1 genes (which encode μ, κ, and δ receptors, respectively) in the striatum, hypothalamus, and periaqueductal gray (PAG) in ovariectomized female rats treated with estrogen. Ovariectomized female rats were divided into five equal groups. Two groups received estrogen (50 µg/kg, 54 h before testing) and saline (ES group) or 3.5 mg/kg morphine (EM group) 2 h before euthanasia. The SS group received saline solution 54 and 2 h before the experiments. The SM group received saline 54 h and 3.5 mg/kg morphine 2 h before the experiments. The W group remained undisturbed. The genes expression were evaluated. Oprm1 and Oprk1 expression were activated, respectively, in the hypothalamus and PAG and in the striatum and PAG by morphine only in estrogen-treated animals. Oprd1 expression in the hypothalamus and PAG was activated by morphine in both estrogen-treated and -nontreated animals. The Oprm1 and Oprk1 gene response to morphine might depend on estrogen, whereas the Oprd1 gene response to morphine might not depend on estrogen, supporting the hypothesis of a functional role for ovarian hormones in opioid receptor-mediated functional adaptations in the female brain.

[Gender aspects in anesthesia : modified approach in research and treatment?]

Gender differences can have a relevant influence on the perioperative outcome as male and female patients are affected differently by adverse events, e.g. side effects of drugs. Furthermore, differences relating to specific drug effects, comorbidities and outcome after anesthesia or intensive care have been demonstrated. There seems to be a gender bias in diagnosis and therapy. While the knowledge regarding this field is still growing certain aspects have already been integrated into clinical practice: prevention of postoperative nausea and vomiting (PONV), target controlled infusion (TCI) model and male only policy with production of blood products. There is a need to study the influence of gender, age and race in order to optimize treatment towards a more individualized therapy. This article highlights already identified differences and discusses potential underlying mechanisms.

Role of endogenous opiates in glucoprivic inhibition of the luteinizing hormone surge and fos expression by preoptic gonadotropin-releasing hormone neurones in ovariectomized steroid-primed female rats

In female mammals, the preovulatory luteinizing hormone (LH) 'surge' elicits ovulation and the subsequent transformation of Graafian follicles into corpora lutea, and is thus a critical component of successful reproduction. In light of evidence that this surge is impaired as a consequence of caloric restriction, the following experiments utilized pharmacological strategies to determine whether glucose substrate homeostasis influences the magnitude and/or duration of this pivotal hormonal event. Groups of oestrogen-and progesterone-primed ovariectomized (OVX) rats were injected intravenously (i.v.) with the glucose antimetabolite, 2-deoxy-D-glucose (2DG: 100 or 400 mg/kg), or the vehicle, saline, prior to onset of the expected LH surge. Other rats were pretreated with 2DG (100 microg/rat) or saline by an intracerebroventricular (i.c.v) route. While glucoprivation did not abolish the afternoon LH surge in these animals, mean plasma LH levels were significantly decreased in groups injected with the higher i.v. dose of 2DG or treated with this drug by an i.c.v route, relative to their vehicle-injected controls. In other studies, i.c.v delivery of the opioid receptor antagonist, naltrexone (NALT), partially reversed the inhibitory effects of 2DG on the gonadal steroid-induced LH surge. Dual-label immunocytochemistry of tissue sections from the preoptic area and anterior hypothalamus of OVX, steroid-primed rats revealed nuclear Fos-immunoreactivity (-ir) in a subpopulation of gonadotropin-releasing hormone-(GnRH-)immunopositive neurones prior to maximal preovulatory LH release. Animals pretreated with 2DG i.c.v showed a significant decrease in mean numbers of GnRH neurones exhibiting Fos-ir, whereas coadministration of 2DG and NALT resulted in numbers of double-labelled neurones that were similar to those detected in the non-drug-treated controls. These studies show that magnitude of the LH surge is decreased by glucose substrate imbalance, and that regulatory effects of this metabolic challenge on the reproductive neuroendocrine axis is correlated with alterations in the transcriptional activation of preoptic GnRH neurones by gonadal steroid positive feedback. The present results also support a role for central opiatergic neurotransmission in glucoprivic regulation of cyclic LH secretion in this animal model.

Antiprogestins inhibit the binding of opioids to mu-opioid receptors in nervous membrane preparations

The present study showed that the glucocorticoid/progesterone antagonists, 17 beta-hydroxy-1 1 beta-(4-dimethylamino-phenyl-1)-17-(prop-1-ynyl)estra-4,9-dien+ ++-3-one (RU486) and 17 beta-hydroxy-11 beta-(4-dimethylamino-phenyl-1)-17-(propan-3-ol)estra-4,9-dien-3-o ne (ZK 98299), inhibit the binding of labeled dihydromorphine to mu-opioid receptors present on membrane preparations derived from rat and mouse brain, as well as from human neuroblastoma cells. The inhibitory effect of RU486 was dose-dependent and linked to a decrease of the affinity of labeled dihydromorphine to the mu-opioid receptors. Kinetic experiments have shown that RU486 induces a decrease of the association rate constant (k + 1) of dihydromorphine. RU486 also proved able to dissociate the dihydromorphine-mu-opioid receptor complex, although at a rate slower than that exhibited by unlabeled dihydromorphine. Finally, the addition of NaCl (100 mM) to the incubation buffer induced a 50% decrease of the inhibitory effect of RU486. A 6-day treatment of neuroblastoma cells with RU486 eliminated the inhibitory effect morphine exerts on the intracellular accumulation of cyclic AMP induced by prostaglandin E1. These results indicate that RU-486 may interact with brain mu-opioid receptors in vitro, by decreasing the affinity of opioid ligands.

Prenatal exposure to morphine alters brain mu opioid receptor characteristics in rats

Prenatal morphine exposure alters neither the binding capacity nor the affinity of ligand binding to mu opioid receptors of adult male brains. However, males have significantly higher Bmax in the hypothalamus than ovariectomized females. In females, prenatal exposure to morphine reduces the Bmax of mu opioid receptors 25% in the hypothalamus and preoptic area. Estrogen treatment increases the Bmax of mu opioid receptors in the striatum of all ovariectomized females but in the hypothalamus only of morphine-exposed females, thereby eliminating the sex difference observed in control animals.

Effects of steroids on the brain opioid system

The experiments reported here add further evidence in support of the view that sex steroids may influence the binding characteristics of brain opioid receptors. In particular, it has been shown that: (a) the number of mu-opioid receptors varies in the hypothalamus of regularly cycling female rats according to the different phases of the estrous cycle, which are characterized by fluctuations of circulating levels of sex steroids; (b) the number of mu-opioid receptors decreases in the hypothalamus and in the corpus striatum when ovariectomized rats are submitted to treatments with estradiol and progesterone able to induce a "positive" feedback effect on LH release. A treatment with estrogen alone able to induce a "negative" feedback effect on LH release brings about an increase of the number of mu-opioid receptors in the thalamus and in the hippocampus; (c) in addition to the mu-receptors, receptors of the delta type may also be involved in the control of gonadotropin secretion; recent results here presented indicate that a line of immortalized hypothalamic cells (GT1 cells), which synthesize and secrete LHRH, present delta opioid receptors on their membranes; these are apparently involved in the control of LHRH release from these cells.

Changes of [3H]naloxone binding in oestrogen stimulated rat uterus

Effects of a single dose of oestradiol (Oe) on [3H]naloxone (Nal) binding in ovariectomized rat uterus were studied. Specific [3H]Nal binding was assessed by saturation analysis in 800 g supernatants and pellets of uterine homogenates. Two binding sites with higher (Kd approximately 1 nM) and lower affinity (Kd approximately 15 nM) for Nal were observed, their binding capacities and affinities have changed after Oe treatment in a time-dependent manner. The high affinity binding sites, detected only in the cytoplasmic fraction, disappeared after 1 h and only became detectable again at 24 h after hormone treatment, the lower affinity binding sites, after an initial drop, slowly increased, peaking at the 9th hour of hormone injection. The competition experiments indicate the involvement of different opiate receptor subpopulations in Oe induced changes. In the nuclear fraction, the Bmax values started to increase at 15 h, reaching the highest level at 18 h. The Kd values of lower affinity sites, in both studied compartments, were increased, i.e. the affinity decreased in the second half of the examined period.

Effects of ovarian hormones on brain opioid binding sites in castrated female rats

These experiments were performed to analyze whether treatments of ovariectomized female rats with ovarian steroid regimens able to induce either an increase (positive feedback effect) or a decrease (negative feedback effect) of serum levels of luteinizing hormone (LH) have some impact on the characteristics of mu-opioid binding sites in circumscribed areas of the brain. The increase of serum levels of LH elicited by a treatment with estradiol benzoate (EB) plus progesterone (P; positive feedback effect) was accompanied by a significant decrease in the number of mu-binding sites in the hypothalamus and in the corpus striatum. The decrease in serum levels of LH induced by a treatment with EB alone (negative feedback effect) brought about a significant increase of the number of mu-binding sites in the thalamus and in the hippocampus. These results seem to suggest that the release of LH induced by EB plus P may involve a decrease of hypothalamic mu-binding sites. Apparently, the inhibitory effect on LH release exerted by EB alone does not involve any change of the density of these binding sites in the hypothalamus.

Testosterone and postnatal ontogenesis of hypothalamic mu ([3H]dihydromorphine) opioid receptors in the rat

Brain opioids modulate the activity of the hypothalamo-pituitary complex by binding to specific receptors which have been subdivided at least into 3 subclasses (mu, kappa, delta, etc). mu-Receptors and their ligands seem to be particularly involved in the control of gonadotropin and prolactin release. It is known that the neuroendocrine system, as well as the brain opioid systems and their receptors, are not fully mature at birth; it is also known that the postnatal maturation of many brain machineries is under the control of androgens secreted by the developing testes. Consequently, it has been investigated whether the presence or the absence of testosterone at time of birth may induce changes of the binding characteristics of hypothalamic mu-opioid receptors. The experiments have been performed by evaluating the maximal binding capacity (Bmax, an index of the number of receptors), and the affinity constant (Ka) of the specific mu-ligand dihydromorphine in hypothalamic plasma membrane preparations derived from normal male rats, normal female rats, male rats orchidectomized 2 days after birth and female rats treated 2 days after birth with 1.25 mg of testosterone propionate. Animals belonging to the 4 groups were killed at days 16, 26 and 60 of age. The results obtained show that, at 16 days of age, in the 4 groups of rats the number of hypothalamic mu receptors is identical. At 26 days a significant increase in the number of mu-receptors occurs in normal female animals, while their levels remain similar to those found at 16 days in the other 3 groups of animals. At 60 days of age, the number of mu-receptors in normal females remains elevated, while the number of mu-receptors increases to reach normal female levels in the hypothalamus of neonatally castrated males. At 60 days, there were no changes in normal males or in androgenized females. The variations here reported took place without any change of the Ka of dihydromorphine for the mu-receptors. These data show a sexual dimorphism of hypothalamic mu-receptors and suggest that their ontogenetic development may be linked to the presence or the absence of androgens at time of birth.

Modulation of the binding characteristics of hypothalamic mu opioid receptors in rats by gonadal steroids

Recent evidence suggests that the effects of the opioids on gonadotropin release may depend on the endocrine status existing in the experimental animal. In the brain, the effects of the opioids are exerted through the interaction with different classes of opioid receptors (mu, delta, kappa, etc.). Among these, the mu receptors appear to be particularly relevant to the control of gonadotropin secretion. Different groups of experiments have been performed in the rat in order to analyze whether changes of circulating levels of sex steroids may have an impact on the binding characteristics of hypothalamic mu opioid receptors, as evaluated by a receptor binding assay performed on plasma membrane preparations, using [3H]dihydromorphine as a mu ligand. In a first series of experiments, it has been observed that the ontogenesis of hypothalamic mu opioid receptors is different in male and in female rats: the concentration of mu sites, similar in animals of the two sexes at 16 days of age, increases in females, but not in males, between day 16 and day 26 of life. This sexual difference persists in 60-day old animals, when the brain is fully mature. It has also been observed that the pattern of maturation of hypothalamic mu receptors can be reversed by neonatal castration of males and by neonatal testosterone treatment of females. In a second series of experiments, it has been shown that in the hypothalamus of regularly cycling female rats the concentration of mu receptors varies during the different phases of the estrous cycle. In particular, a rather high density of mu sites during diestrus day 2 and the morning of the day of proestrus was found; this is followed by a progressive decline during the afternoon of the day of proestrus and the day of estrus, with a minimum value of the concentration of mu receptors being recorded in the first day of diestrus. These fluctuations seem to be linked to the physiological changes of serum levels of ovarian steroids: in fact, in a third series of experiments, it has been found that the positive feedback effect on LH release, exerted by the treatment of ovariectomized female rats with estrogens plus progesterone, is accompanied by a significant decrease of the concentration of hypothalamic mu opioid receptors; treatments with estrogens alone, able to induce a negative feedback effect on LH secretion, are not associated with modifications of hypothalamic mu receptors. These data seem to indicate that hypothalamic mu receptors may be involved in the positive but not in the negative feedback control of LH secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Possible hyperendorphinergic pathophysiology of the Rett syndrome

The Rett syndrome is a postnatal developmental and neurological disorder seen only in girls. Many of the symptoms of this disorder, such as microcephaly, stereotypy, respiratory disturbances and seizures, are analogous to the effects of the administration of beta-endorphin or other opioids in animals. Preliminary reports of elevated beta-endorphin-like immunoreactivity in the cerebrospinal fluid of girls with the Rett syndrome, as well as improvement in some of their symptoms during the administration of the opioid antagonist naltrexone, are suggestive of endorphinergic hyperactivity. Thus, the pathophysiology of the Rett syndrome might involve excessive stimulation of opioid receptors in the central nervous system by beta-endorphin or other endogenous opioids.