Modulation by sex steroids of brain opioid receptors: implications for the control of gonadotropins and prolactin secretion

Long-Term Estrogen Receptor Beta Agonist Treatment Modifies the Hippocampal Transcriptome in Middle-Aged Ovariectomized Rats

Estradiol (E2) robustly activates transcription of a broad array of genes in the hippocampal formation of middle-aged ovariectomized rats via estrogen receptors (ERα, ERβ, and G protein-coupled ER). Selective ERβ agonists also influence hippocampal functions, although their downstream molecular targets and mechanisms are not known. In this study, we explored the effects of long-term treatment with ERβ agonist diarylpropionitrile (DPN, 0.05 mg/kg/day, sc.) on the hippocampal transcriptome in ovariectomized, middle-aged (13 month) rats. Isolated hippocampal formations were analyzed by Affymetrix oligonucleotide microarray and quantitative real-time PCR. Four hundred ninety-seven genes fulfilled the absolute fold change higher than 2 (FC > 2) selection criterion. Among them 370 genes were activated. Pathway analysis identified terms including glutamatergic and cholinergic synapse, RNA transport, endocytosis, thyroid hormone signaling, RNA degradation, retrograde endocannabinoid signaling, and mRNA surveillance. PCR studies showed transcriptional regulation of 58 genes encoding growth factors (Igf2, Igfb2, Igf1r, Fgf1, Mdk, Ntf3, Bdnf), transcription factors (Otx2, Msx1), potassium channels (Kcne2), neuropeptides (Cck, Pdyn), peptide receptors (Crhr2, Oprm1, Gnrhr, Galr2, Sstr1, Sstr3), neurotransmitter receptors (Htr1a, Htr2c, Htr2a, Gria2, Gria3, Grm5, Gabra1, Chrm5, Adrb1), and vesicular neurotransmitter transporters (Slc32a1, Slc17a7). Protein-protein interaction analysis revealed networking of clusters associated with the regulation of growth/troph factor signaling, transcription, translation, neurotransmitter and neurohormone signaling mechanisms and potassium channels. Collectively, the results reveal the contribution of ERβ-mediated processes to the regulation of transcription, translation, neurogenesis, neuromodulation, and neuroprotection in the hippocampal formation of ovariectomized, middle-aged rats and elucidate regulatory channels responsible for DPN-altered functional patterns. These findings support the notion that selective activation of ERβ may be a viable approach for treating the neural symptoms of E2 deficiency in menopause.

Modulation of brain β-endorphin concentration by the specific part of the Y chromosome in mice

BACKGROUND

Several studies in animal models suggest a possible effect of the specific part of the Y-chromosome (Y(NPAR)) on brain opioid, and more specifically on brain β-endorphin (BE). In humans, male prevalence is found in autistic disorder in which observation of abnormal peripheral or central BE levels are also reported. This suggests gender differences in BE associated with genetic factors and more precisely with Y(NPAR).

METHODOLOGY/PRINCIPAL FINDINGS

Brain BE levels and plasma testosterone concentrations were measured in two highly inbred strains of mice, NZB/BlNJ (N) and CBA/HGnc (H), and their consomic strains for the Y(NPAR). An indirect effect of the Y(NPAR) on brain BE level via plasma testosterone was also tested by studying the correlation between brain BE concentration and plasma testosterone concentration in eleven highly inbred strains. There was a significant and major effect (P<0.0001) of the Y(NPAR) in interaction with the genetic background on brain BE levels. Effect size calculated using Cohen's procedure was large (56% of the total variance). The variations of BE levels were not correlated with plasma testosterone which was also dependent of the Y(NPAR).

CONCLUSIONS/SIGNIFICANCE

The contribution of Y(NPAR) on brain BE concentration in interaction with the genetic background is the first demonstration of Y-chromosome mediated control of brain opioid. Given that none of the genes encompassed by the Y(NPAR) encodes for BE or its precursor, our results suggest a contribution of the sex-determining region (Sry, carried by Y(NPAR)) to brain BE concentration. Indeed, the transcription of the Melanocortin 2 receptor gene (Mc2R gene, identified as the proopiomelanocortin receptor gene) depends on the presence of Sry and BE is derived directly from proopiomelanocortin. The results shed light on the sex dependent differences in brain functioning and the role of Sry in the BE system might be related to the higher frequency of autistic disorder in males.

Differential effects of morphine and naltrexone on the in vitro LH secretion from male and female carp pituitary gland

The in vitro effects of morphine (10(-10), 10(-8), 10(-6) or 10(-5) M) or/and naltrexone (10(-6) or 10(-8) M) on LH release from male and female carp (Cyprinus carpio L.) dispersed pituitary cells (obtained from fish at the time of late gonad recrudescence) were investigated. Morphine alone at the lowest tested concentration (10(-10) M) increased LH secretion from the cells of males. On the contrary, in female cell incubations the highest concentrations of morphine (10(-6) or 10(-5) M) significantly lowered LH levels. Naltrexone alone (at both tested concentrations) had no influence on LH secretion, neither in males nor in females. However in the incubations of female cells it antagonised the influence of morphine at 10(-10) or 10(-8) M. In male cell incubations naltrexone abolished the stimulatory action of morphine at 10(-10) M. The results suggest that in the in vitro culture of carp pituitary cells LH secretion is modulated by the opioids which affect the release of this gonadotropin through the typical opioid receptors and that the mu type of these receptors is involved in this process. The effects of opioid agonist and antagonist depend on the stage of gonadal maturity and the sex of fish i.e. the actual level of sex steroids.

Sex differences in opioid analgesia: clinical and experimental findings

Sex differences in analgesic responses to opioids have received increasing attention in recent years. This article examines the literature on sex differences in opioid analgesia, including the results of studies from the authors' own laboratories. In general, nonhuman animal studies suggest more robust opioid analgesic responses in males relative to females; however, the human studies completed to date seem to indicate greater opioid analgesia among females. The most consistent evidence of sex differences in analgesia comes from studies of kappa-agonist-antagonists administered to patients following oral surgery. These data indicate more robust analgesia in females, and dose-response characteristics suggest that these agents possess both analgesic and antianalgesic properties, and the agonists may produce these effects in different proportions for women versus men. In contrast, the data from laboratory pain models in humans suggest greater analgesic effects in women in response mu-opioid agonists but not kappa-agonist-antagonists. Multiple mechanisms may explain sex differences in opioid analgesia, including gonadal hormonal effects, pharmacokinetics and pharmacodynamics, genetic influences, balance of analgesic/antianalgesic processes, and psychological factors. However, the disparity of results obtained from different pain models--animals versus humans and clinical pain versus experimental pain in humans--suggests that the models themselves are mechanistically different. Additional investigation is warranted in order to further explicate the nature of sex differences in opioid analgesia and to elucidate the underlying mechanisms.

Cortical and striatal μ-opioid receptors are altered by gonadal hormone treatment but not by prenatal morphine exposure in adult male and female rats

The cerebral cortex (CX), cingulate CX (cgCX), and striatum (STR) play an important role in locomotion, cognition, emotion, and reward-motivated behaviors, and are altered by prenatal morphine exposure. We have demonstrated that delta-opioid receptors in the CX and STR of adult male and female rats are altered by prenatal morphine exposure and gonadal hormonal treatment. Because morphine binds with greater affinity to mu- than delta-opioid receptors, the present study examined the effect of prenatal morphine exposure on mu-opioid receptor density in the CX, cgCX, and STR of adult male and female rats using receptor autoradiography. In Experiment 1, three groups of adult male rats were analyzed: intact, gonadally intact; GNX, gonadectomized; and TP, GNX and testosterone propionate (TP)-treated. In Experiment 2, four groups of adult females were analyzed: OVX, ovariectomized; EB, OVX and estradiol benzoate (EB)-treated; P, OVX and progesterone (P)-treated; and EB+P, OVX and EB- and P-treated. In male rats, GNX and TP males had lower mu-opioid receptor densities in all three brain regions than gonadally intact males regardless of prenatal drug exposure. In female rats, OVX, EB+P-treated females had lower mu-opioid receptor density in the STR than OVX only females regardless of prenatal drug exposure. There were no drug or gonadal hormone effects in the CX or in the cgCX of female rats. Thus, the present study demonstrates that gonadal hormones, and not prenatal morphine exposure, alter the density of mu-opioid receptors in the CX, cgCX, and STR of adult male and female rats.

Effects of perinatal exposure to delta 9-tetrahydrocannabinol on operant morphine-reinforced behavior

The present study examined the effects of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) when administered during the perinatal period on morphine self-administration in adulthood. To this end, pregnant Wistar rats were daily exposed to Delta(9)-THC from the fifth day of gestation up to pup weaning, when they were separated by gender and left to mature to be used for analyses of operant food- and morphine-reinforced behavior in a progressive ratio (PR) schedule. We also analyzed dopaminergic activity (DOPAC/DA) in reward-related structures during specific phases of the behavioral study. In both reinforcement paradigms, food and morphine, females always reached higher patterns of self-administration than males, but this occurred for the two treatment groups, Delta(9)-THC or vehicle. These higher patterns measured in females corresponded with a higher DOPAC/DA in the nucleus accumbens prior to the onset of morphine self-administration in comparison to males. Interestingly, DOPAC/DA was lower in Delta(9)-THC-exposed females compared to oil-exposed females and similar to oil- and Delta(9)-THC-exposed males. In addition, Delta(9)-THC-exposed females also exhibited a reduction in DOPAC/DA in the ventral tegmental area, which did not exist in males. All these changes, however, disappeared after 15 days of morphine self-administration and they did not reappear after 15 additional days of extinction of this response. Our data suggest that females are more vulnerable than males in a PR schedule for operant food and morphine self-administration; perinatal Delta(9)-THC exposure is not a factor influencing this vulnerability. The neurochemical analysis revealed that the activity of limbic dopaminergic neurons prior to morphine self-administration was higher in females than males, as well as that the perinatal Delta(9)-THC treatment reduced the activity of these neurons only in females, although this had no influence on morphine vulnerability in these animals.

Visualizing activation of opioid circuits by internalization of G protein-coupled receptors

Mu-opioid receptor (MOR) and opioid receptor-like receptor (ORL-1) circuits in the limbic hypothalamic system are important for the regulation of sexual receptivity in the female rat. Sexual receptivity is tightly regulated by the sequential release of estrogen and progesterone from the ovary suggesting ovarian steroids regulate the activity of these neuropeptide systems. Both MOR and ORL-1 distributions overlap with the distribution of estrogen and progesterone receptors in the hypothalamus and limbic system providing a morphological substrate for interaction between steroids and the opioid circuits in the brain. Both MOR and ORL-1 are receptors that respond to activation by endogenous ligands with internalization into early endosomes. This internalization is part of the mechanism of receptor desensitization or down regulation. Although receptor activation and internalization are separate events, internalization can be used as a temporal measure of circuit activation by endogenous ligands. This review focuses on the estrogen and progesterone regulation of MOR and ORL-1 circuits in the medial preoptic nucleus and ventromedial nucleus of the hypothalamus that are central to modulating sexual receptivity.

Endogenous opioid regulation of oxytocin and ACTH secretion during pregnancy and parturition

Progress of parturition in the rat is optimal when there is increased oxytocin secretion, thus ensuring quick birth and otherwise risking adverse neonatal health. To ensure that the mechanisms for this are available, oxytocin neurons adapt in pregnancy and this includes development of a tonic inhibition by endogenous opioids. Endogenous opioid inhibition of oxytocin secretion increases in pregnancy, initially acting on the nerve terminals in the posterior pituitary and later on oxytocin cell bodies and their inputs. This inhibition enhances stores of oxytocin and enables restraint of oxytocin neuron responsiveness to selected excitatory inputs. The hypothalamic neurons which mediate stress also adapt in late pregnancy so that hypothalamo-pituitary-adrenal axis and oxytocin secretory responses to stressor exposure are attenuated. This is also partly due to endogenous opioid inhibition. Thus, in pregnancy oxytocin and hypothalamo-pituitary-adrenal axis secretion in response to stimulation is restrained, protecting the unborn fetus(es) from premature delivery and glucocorticoid exposure and preparing the oxytocin neurons for their important secretory role during parturition. In parturition itself, endogenous opioids continue to inhibit these neurons. Stress exposure during parturition delays births, probably due to endogenous opioid inhibition of pulsatile oxytocin secretion. On the other hand, basal ACTH and corticosterone secretion are reduced in parturition through inhibition by endogenous opioids. So, opioids continue to regulate the activity of oxytocin and hypothalamo-pituitary-adrenal mechanisms in labor; inhibition of oxytocin neurons at this time may control the spacing of pup births.

Opioid regulation of gonadotropin release: role of signal transduction cascade

The present investigation elucidates the opioidergic modulation of gonadotropin releasing hormone release mechanism by signal transduction cascade in discrete brain regions from estrogen-progesterone primed ovariectomized rats. The effects of mu-opioid agonist morphine and its antagonist naloxone followed by morphine were studied (in two different groups of rats) on protein kinase A, adenosine 3',5' cyclic monophosphate, protein kinase C and calcium/calmodulin protein kinase-II as well as phospholipase C, phospholipase A(2), diacylglycerol and inositol 1,4, 5-triphosphate. Significant decline in phosphoinositide metabolism was observed after morphine treatment as depicted by decrease in phospholipase C and phospholipase A2 activities as well as inositol 1,4,5-triphosphate and diacylglycerol contents from discrete brain regions. Protein kinase A activity showed translocation from membrane bound to cytosolic form along with a decrease in its activator adenosine 3',5'-cyclic monophosphate levels in morphine-treated group. Calcium/calmodulin dependent protein kinase II activity also declined, whereas, protein kinase C activity increased in the cytosolic fraction after 45 min of morphine administration. Naloxone was seen to counteract the changes induced by morphine in most of the brain regions studied. Morphine also suppressed luteinizing hormone levels, whereas, follicle stimulating hormone level did not change. The present investigation provides evidence for opioidergic mediated suppression of gonadotropin release through the downregulation of signal transduction cascade.

The neurobiology of cannabinoid dependence: sex differences and potential interactions between cannabinoid and opioid systems

Cannabis is the most widely used illicit drug in many western countries. Its psychoactive ingredient, delta9-tetrahydrocannabinol (THC), produces a variety of effects in animals and humans that are probably mediated by specific cannabinoid receptors in the brain and interactions with several neurotransmitter and neuromodulator systems. For instance, recent research has revealed an important mutual functional relationship between cannabinoids and endogenous opioid systems in mediating the pharmacological and behavioral actions produced by these agents, including their reinforcing effects. Perinatal exposure to and interactions between cannabinoids and opioids might also have long-term behavioral consequences lasting into adulthood. In this work, we present preliminary evidence examining the potential effects of maternal exposure to THC on the motivational properties of morphine in male and female adult rats, as measured by an intravenous opiate self-administration paradigm.

The development of POMC gene expression in the medial basal hypothalamus of prepubertal rats

Alterations in brain opioid gene expression may underlie the dramatic change in the latency to display parental behavior in juvenile rats. Male and female juvenile rats (18-25 days of age) exhibit parental behavior either immediately or within 1-2 days after coming in contact with foster pups. By 30 days of age, however, their response latencies increase to adult levels of 5-10 days. Given the established involvement of the endogenous opioid system in adult maternal and juvenile parental behaviors, the objective of the present report was to determine possible changes in proopiomelanocortin (POMC) gene expression in the medial basal hypothalamus (MBH) during this early developmental window. We compared POMC gene expression in the MBH of male and female juvenile rats from 21 to 33 days of age by in situ hybridization histochemistry. A significant increase in the number of POMC cells in males and females was detected at 30 days of age in the central portion of the arcuate nucleus. This increase in POMC mRNA may contribute to the shift in parental behavior that occurs in male and female juvenile rats.

Brain opioid receptor measurements by positron emission tomography in normal cycling women: relationship to luteinizing hormone pulsatility and gonadal steroid hormones

The regulation of central mu-opioid receptors in women during the menstrual cycle was explored with positron emission tomography and the selective radiotracer [11C]carfentanil. Ten healthy women were studied twice, during their follicular and luteal phases. Plasma concentrations of estradiol, progesterone, testosterone, and beta-endorphin were determined immediately before scanning. LH pulsatility was measured over the 9 h preceding each of the two positron emission tomography scans. No significant differences in the binding potential of mu-opioid receptors (binding capacity/Kd) were observed between phases of the menstrual cycle. However, significant negative correlations were observed between circulating levels of estradiol during the follicular phase and mu-receptor binding measures in the amygdala and hypothalamus, two regions thought to be involved in the regulation of GnRH pulsatility. LH pulse amplitude was positively correlated with mu binding in the amygdala, whereas LH pulse number was negatively correlated with binding in this same region. No significant associations were noted between LH pulse measures and the hypothalamus for this sample. These results suggest that amygdalar mu-opioid receptors exert a modulatory effect on GnRH pulsatility, and that circulating levels of estradiol also regulate central mu-opioid function.

Role of opioidergic and monoaminergic neurotransmission in the GnRH release mechanism of EBP-primed OVX rats

We examined the effect of intracerebroventricular (i.c.v.) administration of mu-opioid agonist, morphine, and its antagonist naloxone followed by morphine on the activities of monoamine-metabolizing enzymes, namely tyrosine hydroxylase (TH) and monoamine oxidase (MAO) along with adenosinetriphosphatase (Na+, K+ -ATPase), the enzyme responsible for the maintenance of ionic gradients across the membrane, in seven discrete regions of brain from estrogen- and progesterone-primed ovariectomized rats. TH activity decreased after morphine treatment in some areas such as the median eminence-arcuate region (ME-ARC), the amygdala, and the thalamus, showing statistically significant change. MAO activity increased in all the areas studied, but more appreciable change was observed in medial preoptic area (mPOA), the ME-ARC region, and the cortex. Pronounced increase in Na+, K+ -ATPase enzyme activity was observed after the drug treatment. Naloxone given prior to morphine injection resulted in recovery of the enzyme activities in most of the areas studied. Our study may provide insights into the precise opioidergic modulation of gonadotropin releasing hormone (GnRH) release mechanisms through the involvement of monoaminergic system, elucidating the basis of various neuronal dysfunctions and their management in opioid addicts.

Progesterone increases levels of mu-opioid receptor mRNA in the preoptic area and arcuate nucleus of ovariectomized, estradiol-treated female rats

Estradiol (E2) and progesterone (P) play different roles in generating the preovulatory surge release of luteinizing hormone-releasing hormone (LH-RH) and luteinizing hormone (LH). Results of our previous studies suggest that at least some of these steroid-specific effects may be mediated by beta-endorphinergic neurons. However, it is also possible that E2 and P differentially regulate responsiveness to opioids by altering mu-opioid receptor gene expression. To test this hypothesis, we used quantitative in situ hybridization histochemistry (ISHH) to measure the effects of E2 and P on mu-opioid receptor mRNA levels in cells of the preoptic area (POA) and arcuate nucleus (Arc). We examined several groups of animals in the morning and afternoon on the day of LH surge release: (1) 1-week ovariectomized (OVX) rats with or without E2 treatment sacrificed between 09:00 and 09:30 h (48 h after E2 capsules inserted); (2) OVX with or without E2 treatment sacrificed between 15:30 and 16:00 h; and (3) OVX with both E2 and P treatment sacrificed between 15:30 and 16:00 h (approximately 54 h after E2 and 6 h after P administration). We found that E2 had no effect on morning or afternoon levels of mu-opioid receptor mRNA levels in either the POA or Arc. In contrast, P treatment increased afternoon levels of mu-opioid receptor mRNA in both regions. These findings indicate that differential effects of E2 and P on LH-RH release may be mediated by steroid-specific effects on mu-opioid receptor gene expression in neurons of the POA and/or Arc.

Opioid infidelity: novel opioid peptides with dual high affinity for delta- and mu-receptors

Deltorphins represent the paragon of delta-opioid-receptor ligands of natural origin, since they exceed the affinity and selectivity of the endogenous enkephalins by orders of magnitude. A series of opioid peptides have been developed in which the change in a single amino acid causes an extraordinary increase in mu-receptor binding while maintaining high affinity for the delta-receptor. The peptides appear to have a similar extended conformation in solution with a type-I beta-turn in the N-terminus region, suggesting that tertiary architecture plays a pivotal role in enabling the peptide to bind indiscriminately to mu- and delta-receptors. These dual-affinity peptide ligands can serve to mask delta- and mu-receptors while mapping kappa-receptors in the nervous system, to provide an understanding of the differences and similarities in the structure of the binding domains of delta- and mu-receptors, and might lead to a comprehensive new regime for the clinical management of acute and chronic pain.

Effects of endogenous opioid peptides and their analogs on the activities of hypothalamic arcuate neurons in brain slices from diestrous and ovariectomized rats

Various endogenous opioid peptides and some of their analogs were used in this study to test their effects on the membrane activities of hypothalamic arcuate neurons in brain slices. Both ovariectomized and diestrous rats were used in the study, and freshly prepared brain slices from these animals were used for extracellular single-unit recording studies. All of the opioids exhibited potent inhibitory effects on the firing of arcuate neurons, viz., beta-endorphin inhibited 55% (n = 33), DAGO 62% (n = 21), dynorphin A 55% (n = 11), U50,488 36% (n = 39), Met-enkephalin 35% (n = 54), and DPDPE 50% (n = 8) of tested arcuate neurons from ovariectomized rats. Significantly higher percentage of inhibition was observed in slice preparations from diestrous rats for DAGO 86% (n = 22), and slightly higher for dynorphin A 59% (n = 22) and U50,488 53% (n = 15). Pretreatment with naloxone prevented most of the actions by beta-endorphin and DAGO, and nor-binaltorphimine prevented those by dynorphin A and U50,488. Most of the effects of Met-enkephalin could also be blocked by nor-binaltorphimine (67%, n = 6), but less by naltrindole (25%, n = 8). Naltrindole, however, seemed to be more effective in blocking the action of [D-Pen2,5]-enkephalin (100%, n = 2). In summary, all opioids tested exerted potent inhibitory effects upon the firing of arcuate neurons possibly through multiple opioid receptors, and the presence of ovarian hormones may have an effect on the neuron's responsiveness to opioid acting on mu type receptors.

Brain soluble and membrane-bound Tyr-aminopeptidase activities during the stages of estrous and proestrous in the female rat

Brain Tyr-aminopeptidase activity levels during two stages of the rat estrous cycle (estrous and proestrous), are described in this research. The study includes the activities of the soluble and the membrane-bound forms. Soluble Tyr-aminopeptidase activity shows a significant increase during the proestrous stage in the hypothalamus. The complete membrane-bound activity did not significantly change at any point. However, a significant rise was found in the membrane-bound puromycin insensitive aminopeptidase activity in the hypothalamus and the pituitary gland during the proestrous phase.

Effect of adrenal and sex hormones on opioid analgesia and opioid receptor regulation

The role of endocrine factors on opioid analgesia (antinociception) and opioid receptors was studied in male and female Swiss-Webster mice. Morphine was more potent in male than in female mice, although this difference appears to be due to greater availability of morphine to the brain in males. Saturation binding studies indicated that the density and affinity of brain mu- and delta-opioid binding sites were equivalent in males and females. Males and females were implanted SC with naltrexone (NTX) or placebo pellets for 8 days, and then the pellets were removed. This treatment increased the density of mu and delta binding sites in brain and increased the potency of morphine for both sexes, although the increase in antinociceptive effects for males was greater than for females. Adrenalectomy (ADX) in male mice increased the potency of morphine and methadone but did not alter the brain levels of either drug. ADX did not alter brain opioid binding of either mu or delta ligands. When male ADX and control mice were treated with NTX, the potency of morphine and brain opioid binding sites were increased equivalently in both groups. Gonadectomy (GDX) in male mice tended to decrease morphine potency, although this was not found to be a very reliable effect. When male GDX and control mice were implanted with NTX, brain opioid binding was increased similarly in both groups, although morphine potency was increased less in GDX mice. Overall, these studies show that sex differences and hormones of the adrenals and gonads in male mice do not alter brain opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The female reproductive axis and its modifications during the post-partum period

The female reproductive axis in mammals is a highly complex dynamic system which goes through different transient or absorbent states during the course of a life-time. Little is known about the mechanisms controlling this system during fetal life and at birth, although it has been shown in numerous species, including primates, that the whole machinery is already functioning (Brooks et al., 1990; Plant, 1986). After a delay ranging from a few days to a few weeks, according to the species, the reproductive axis becomes quiescent and activity apparently resumes only at the time of puberty. Here again understanding of the phenomenon is still limited (Ojeda, 1991).