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

Progesterone: A Steroid with Wide Range of Effects in Physiology as Well as Human Medicine

Progesterone is a steroid hormone traditionally linked with female fertility and pregnancy. In current reproductive medicine, progesterone and its analogues play crucial roles. While the discovery of its effects has a long history, over recent decades, various novel actions of this interesting steroid have been documented, of which its neuro- and immunoprotective activities are the most widely discussed. Discoveries of the novel biological activities of progesterone have also driven research and development in the field of progesterone analogues used in human medicine. Progestogen treatment has traditionally and predominately been used in maintaining pregnancy, the prevention of preterm labor, various gynecological pathologies, and in lowering the negative effects of menopause. However, there are also various other medical fields where progesterone and its analogues could find application in the future. The aim of this work is to show the mechanisms of action of progesterone and its metabolites, the physiological and pharmacological actions of progesterone and its synthetic analogues in human medicine, as well as the impacts of its production and use on the environment.

The effect of progesteron for expression delta (δ) opioid receptor spinal cord through peripheral nerve injury

Background

Neuropathic pain is a major problem to date because of its high prevalence and lack of effective treatment. Neuropathic pain processes can be influenced by many factors and at various levels of the nervous system, including progesterone and the opioid system. The various mechanisms of the effect of progesterone on pain are still controversial, while the effect of progesterone on the activation of the opioid system also needs to be proven. This study aimed to determine the effect of progesterone on pain through the modulation mechanism of the opioid system.

Methods

This research is a completely randomized experimental study using male wistar rats aged around three months at the Experimental Animal Laboratory, Department of Medical Biochemistry, Faculty of Medicine, Airlangga University.

Results

The result was analyzed by using statistical analysis of two independent samples (t-test). The t value was obtained at 6.880, p = 0.000 (p < 0.05).

Conclusion

It was shown that there was a significant difference in the delta (δ) opioid receptor expression between the control group and the progesterone group, which indicated that progesterone causes an increase in the delta (δ) opioid receptor expression in the spinal cord.

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Experimental study on mice subjects to determine the difference delta (δ) opioid receptors in the spinal cord.

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Determine the effect of progesterone on pain through the modulation mechanism of the opioid system.

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The administration of progesterone has a positive effect on the expression of delta (δ) opioid receptors.

Mapping of Morphine-Induced OPRM1 Gene Expression Pattern in the Adult Zebrafish Brain

Morphine is a potent analgesic opiate commonly used in treating pain, and it is also a substance of abuse and highly addictive. Hence, it is vital to discover the action sites of morphine in the brain to increase its efficacy of treatment. In the present study, we aimed at identifying comprehensive neuroanatomical locations that are sensitive to morphine in the adult zebrafish (Danio rerio). We performed in situ hybridization to localize the mu opioid receptor (oprm1) gene and to map the morphine sensitive brain areas using neuronal PAS domain-containing protein 4a (npas4a), an early gene marker. Real-time PCR was used to detect changes in mRNA levels of oprm1 and npas4a in control and acute morphine treated fish (2 mg/L; 20 min). Intense positive oprm1 signals were seen in the telencephalon, preoptic area, habenula, hypothalamic area and periventricular gray zone of the optic tectum. Acute morphine exposure significantly increased oprm1 and npas4a mRNA levels in the medial zone of dorsal telencephalon (Dm), ventral region of the ventral telencephalon (Vv), preoptic area, and in the hypothalamus but a decrease in oprm1 and npas4a signals in the dorsal habenula. This study provides a detailed map of oprm1 localization in the brain, which includes previously unreported oprm1 in the habenula of teleost. Presence of oprm1 in multiple brain sites implies multiple action targets of morphine and potential brain functions which could include reward, cognitive and negative emotions.

Diverse actions of estradiol on anorexigenic and orexigenic hypothalamic arcuate neurons

Contribution to Special Issue on Fast effects of steroids. There is now compelling evidence for membrane-associated estrogen receptors in hypothalamic neurons that are critical for the hypothalamic control of homeostatic functions. It has been known for some time that estradiol (E2) can rapidly alter hypothalamic neuronal activity within seconds, indicating that some cellular effects can occur via membrane initiated events. However, our understanding of how E2 signals via membrane-associated receptors and how these signals impact physiological functions is only just emerging. Thus, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell excitability and even gene transcription in hypothalamic neurons. One population of hypothalamic neurons, the anorexigenic proopiomelanocortin (POMC) neurons, has long been considered to be a target of E2's actions based on gene (Pomc) expression studies. However, we now know that E2 can rapidly alter POMC neuronal activity within seconds and activate several intracellular signaling cascades that ultimately affect gene expression, actions which are critical for maintaining sensitivity to insulin in metabolically stressed states. E2 also affects the orexigenic Neuropeptide Y/Agouti-related Peptide (NPY/AgRP) neurons in similarly rapid but antagonistic manner. Therefore, this review will summarize our current state of knowledge of how E2 signals via rapid membrane-initiated and intracellular signaling cascades in POMC and NPY/AgRP neurons to regulate energy homeostasis.

Estradiol Membrane-Initiated Signaling and Female Reproduction

The discoveries of rapid, membrane-initiated steroid actions and central nervous system steroidogenesis have changed our understanding of the neuroendocrinology of reproduction. Classical nuclear actions of estradiol and progesterone steroids affecting transcription are essential. However, with the discoveries of membrane-associated steroid receptors, it is becoming clear that estradiol and progesterone have neurotransmitter-like actions activating intracellular events. Ultimately, membrane-initiated actions can influence transcription. Estradiol membrane-initiated signaling (EMS) modulates female sexual receptivity and estrogen feedback regulating the luteinizing hormone (LH) surge. In the arcuate nucleus, EMS activates a lordosis-regulating circuit that extends to the medial preoptic nucleus and subsequently to the ventromedial nucleus (VMH)--the output from the limbic and hypothalamic regions. Here, we discuss how EMS leads to an active inhibition of lordosis behavior. To stimulate ovulation, EMS facilitates astrocyte synthesis of progesterone (neuroP) in the hypothalamus. Regulation of GnRH release driving the LH surge is dependent on estradiol-sensitive kisspeptin (Kiss1) expression in the rostral periventricular nucleus of the third ventricle (RP3V). NeuroP activation of the LH surge depends on Kiss1, but the specifics of signaling have not been well elucidated. RP3V Kiss1 neurons appear to integrate estradiol and progesterone information which feeds back onto GnRH neurons to stimulate the LH surge. In a second population of Kiss1 neurons, estradiol suppresses the surge but maintains tonic LH release, another critical component of the estrous cycle. Together, evidence suggests that regulation of reproduction involves membrane action of steroids, some of which are synthesized in the brain.

Sex-dependent consequences of pre-pubertal gonadectomy: Social behavior, stress and ethanol responsivity

Alcohol consumption can be enhanced or moderated by sensitivity to its aversive and appetitive properties, including positive social outcomes. These differences emerge post-pubertally, suggesting a potential role of gonadal hormones. To determine the role of gonadal hormones in sensitivity to the social impairing and social context-related attenuations in the aversive effects of ethanol, prepubertal male and female rats were gonadectomized (GX) or sham (SH) operated on postnatal day (P) 25, or left non-manipulated (NM). In adulthood (P70), rats were restrained for 90 min prior to challenge with 0.0 or 1.0 g/kg ethanol and social interaction (SI) testing. At P77, groups of 4 same-sex littermates from the same surgical condition were given access to a supersaccharin (SS) solution (3% sucrose, 0.125% saccharin), followed by an intraperitoneal injection of ethanol (0.0, 0.50, 1.0, 1.5 g/kg). Intakes of SS were examined 24h later for expression of conditioned taste aversions. Acute stress prior to SI testing increased frequency of play fighting in both sexes, whereas there were no GX effects on this measure, social investigation nor contact. GX, however, decreased baseline social preference (a social anxiety-like effect) in males, while inducing anxiolytic-like increases in baseline social preference in females. The social drinking test revealed that females developed ethanol conditioned taste aversions at a lower dose relative to males, regardless of surgical condition. These findings suggest a potential role for gonadal hormones in moderating social-anxiety like behaviors but not sensitivity to the social impairing effects of ethanol or ethanol's aversive consequences in a social context.

Estrogenic regulation of histamine receptor subtype H1 expression in the ventromedial nucleus of the hypothalamus in female rats

Female sexual behavior is controlled by central estrogenic action in the ventromedial nucleus of the hypothalamus (VMN). This region plays a pivotal role in facilitating sex-related behavior in response to estrogen stimulation via neural activation by several neurotransmitters, including histamine, which participates in this mechanism through its strong neural potentiating action. However, the mechanism through which estrogen signaling is linked to the histamine system in the VMN is unclear. This study was undertaken to investigate the relationship between estrogen and histamine receptor subtype H1 (H1R), which is a potent subtype among histamine receptors in the brain. We show localization of H1R exclusively in the ventrolateral subregion of the female VMN (vl VMN), and not in the dorsomedial subregion. In the vl VMN, abundantly expressed H1R were mostly colocalized with estrogen receptor α. Intriguingly, H1R mRNA levels in the vl VMN were significantly elevated in ovariectomized female rats treated with estrogen benzoate. These data suggest that estrogen can amplify histamine signaling by enhancing H1R expression in the vl VMN. This enhancement of histamine signaling might be functionally important for allowing neural excitation in response to estrogen stimulation of the neural circuit and may serve as an accelerator of female sexual arousal.

Anabolic-androgenic steroid effects on nociception and morphine antinociception in male rats

The purpose of this study was to investigate the effects of acute and chronic administration of anabolic-androgenic steroids (AAS) on nociception and morphine antinociception in acute pain models, as well as on chronic inflammatory nociception. In Experiment 1, adult, gonadally intact male rats were injected s.c. for 28 days with either 5 mg/kg testosterone (T), dihydrotestosterone (DHT), stanozolol (STAN), or safflower oil vehicle (N=12-25/group). On day 28, rats in each group were tested on acute thermal and mechanical nociceptive assays, before and after morphine treatment. In Experiment 2, rats in each group (N=8-10/group) were injected with mineral oil or complete Freund's adjuvant (CFA) into one hindpaw after 28 days of AAS treatment, and then tested for thermal hyperalgesia, mechanical allodynia, inflammation and locomotor suppression intermittently for 28 days. Experiment 3 replicated nociceptive measurements in Experiments 1 and 2, but with a single AAS or vehicle injection occurring 3h prior to testing (N=10-12/group). While chronic AAS administration tended to decrease body weight gain and alter reproductive organ weights in the expected manner, it did not significantly alter acute nociception nor attenuate the development of various chronic pain indices after CFA administration. Morphine antinociceptive potency was significantly decreased by chronic DHT on the hot plate test only. Acute AAS administration also did not significantly alter acute or chronic nociception, or morphine antinociceptive potency. Comparisons between acute and chronic AAS administration suggest that steroid tolerance did not occur in rats treated with AAS chronically. Taken together, these data do not support the hypothesis that AAS exposure alters nociception or morphine antinociception in gonadally intact males.

Distribution of mRNAs encoding classical progestin receptor, progesterone membrane components 1 and 2, serpine mRNA binding protein 1, and progestin and ADIPOQ receptor family members 7 and 8 in rat forebrain

Several lines of evidence suggest the existence of multiple progestin receptors that may account for rapid and delayed effects of progesterone in the CNS. The delayed effects have been long attributed to activation of the classical progestin receptor (Pgr). Recent studies have discovered novel progestin signaling molecules that may be responsible for rapid effects. These include progesterone receptor membrane component 1 (Pgrmc1), Pgrmc2, progestin and adipoQ receptor 7 (Paqr7) and Paqr8. The functions of these molecules have been investigated extensively in non-neural, but not in neural tissues, partly because it is unclear which are expressed in the brain and where they are expressed. To address these issues, we compared the distributions of mRNAs encoding Pgr, Pgrmc1, Pgrmc2, Paqr7 and Paqr8 using in situ hybridization with radiolabeled oligodeoxynucleotidyl probes in forebrain tissues of estradiol-treated female rats. We also examined the distribution of serpine mRNA binding protein 1 (Serbp1), a putative binding partner of Pgrmc1. Analyses of adjacent brain sections showed that the highest expression of mRNAs encoding Pgr, Pgrmc1, Pgrmc2 and Serbp1 was detected in several hypothalamic nuclei important for female reproduction. In contrast, expression patterns of Paqr7 and Paqr8 were low and homogeneous in the hypothalamus, and more abundant in thalamic nuclei. The neuroanatomical distributions of these putative progestin signaling molecules suggest that Pgrmc1 and Pgrmc2 may play roles in neuroendocrine functions while Paqr7 and Paqr8 are more likely to regulate sensory and cognitive functions.

17β-estradiol and progesterone regulate multiple progestin signaling molecules in the anteroventral periventricular nucleus, ventromedial nucleus and sexually dimorphic nucleus of the preoptic area in female rats

Recent work identified novel progestin signaling molecules, including progesterone receptor membrane component 1 (Pgrmc1), Pgrmc2, serpine mRNA binding protein 1 (Serbp1), progestin and adiponectin receptors 7 (Paqr7) and Paqr8. These molecules mediate rapid progesterone (P(4)) effects in non-neural tissue and we recently mapped their expression in the brain. Many rapid effects of P(4) require 17β-estradiol (E(2)) and P(4) priming; therefore, we examined the effects of ovarian hormones on the expression of these non-classical progestin signaling molecules. We focused specifically on the anteroventral periventricular nucleus (AVPV), the sexually dimorphic nucleus of the preoptic area (SDN-POA) and the ventrolateral portion of the ventromedial nucleus (VMNvl). These brain nuclei are important for female reproduction. Ovariectomized adult female rats were implanted with capsules containing sesame oil or E(2), and injected 48 h later with sesame oil or P(4). Brains were collected 8 h later and RNA was isolated from the AVPV, SDN-POA and VMNvl. We assessed the effects of ovarian hormones on mRNA levels using quantitative polymerase chain reaction (QPCR). In the AVPV, Serbp1 mRNA levels were increased by P(4) in the presence of E(2), and Paqr8 was downregulated by P(4) alone. In the SDN-POA, combined E(2) and P(4) increased Pgrmc1 and Serbp1 mRNA levels, and E(2) alone increased Paqr8 mRNA levels. Finally, in the VMNvl, P(4) increased mRNA levels encoding Pgrmc1, Pgrmc2 and Serbp1, and the combination of E(2) and P(4) increased Pgrmc1 and Serbp1 mRNA levels. Paqr7 was not regulated by E(2) or P(4) in any brain region examined. In summary, we showed that ovarian hormones regulate novel progestin signaling molecules in brain regions important for the neuroendocrine control of reproduction.

Mu and kappa opioid receptor expression in the mediobasal hypothalamus and effectiveness of selective antagonists on prolactin release during lactation

Endogenous opioid peptides are involved in prolactin release during lactation, in part by decreasing tuberoinfundibular dopaminergic (TIDA) neuronal activity. Both mu (mu) and kappa (kappa) opioid receptors have a role in the suckling-induced prolactin rise after 4-5 h up deprivation. The aim of this study was to investigate effects of mu opioid receptor antagonist, beta-funaltrexamine (beta-FNA), and kappa opioid receptor antagonist, nor-binaltorphimine (nor-BNI), on prolactin secretion and TIDA neuronal activity in lactating rats after 18 h pup deprivation. After 4 h separation from pups, the suckling-induced prolactin rise was abolished by 16 microg nor-BNI and 5 microg beta-FNA, coincident with increased dihydroxyphenylacetic acid (DOPAC):dopamine ratio in the stalk-median eminence (SME). However, after 18 h pups separation, these same doses of nor-BNI and beta-FNA did not alter the prolactin surge or DOPAC:dopamine ratios in the SME. Higher doses of nor-BNI (32 microg) and beta-FNA (10 microg) were required to inhibit suckling-induced prolactin secretion. beta-FNA (10 microg) increased the DOPAC:dopamine ratio in the SME, whereas nor-BNI (32 microg) treatment had no effect. The mu and kappa opioid receptor mRNA levels in the mediobasal hypothalamus were similar to suckled control rats after 4 h pup deprivation, but increased 1.4-fold after 18 h pup deprivation. These data support involvement of endogenous opioidergic systems in the suckling-induced prolactin rise after a prolonged (18 h) period of pup deprivation, as well as the shorter (4 h) pup deprivation period previously reported. Suppression of TIDA neuronal activity likely played a part in mu opioid receptor input to the suckling-induced prolactin rise after both 4 h and 18 h separation, whereas non-dopaminergic input was implicated with kappa opioid receptors after 18 h pup deprivation. Increased mu and kappa opioid receptors gene expression in the mediobasal hypothalamus may contribute to reduced effectiveness of opioid receptor antagonists to block suckling-induced prolactin release after 18 h pup deprivation.

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.

Sex-specific responses to opiates: animal and human studies

It is widely reported that analgesic drugs acting at mu, kappa, and delta opioid-receptors display quantitative and qualitative differences in effect in males and females. These sex-related differences are not restricted to the analgesic/antinociceptive properties of opioids, but are also present in opioid-induced side effects, such as changes in respiration, locomotor activity, learning/memory, addiction, and changes in the cardiovascular system. An increasing number of well-controlled animal and human studies directly examining the issue of sex in the potency of opioids show that, although sex may affect opioid analgesia, the direction and magnitude of sex differences depend on many interacting variables. These include those specific to the drug itself, such as dose, pharmacology, and route and time of administration, and those particular to the subject, such as species, type of pain, genetics, age, and gonadal/hormonal status. In the current review, we systematically present these animal and human studies and discuss the data in relation to the depending variables. Although the observed sex differences in opioid effect may be clinically relevant, lack of knowledge on other factors involved in the large variability in patient opioid analgesic sensitivity should compel practitioners to customize their dosing regimens based on individual requirements.

CNS arousal mechanisms bearing on sex and other biologically regulated behaviors

It now seems possible to move beyond analyzing only the mechanisms for specific sexual behaviors to the analysis of 'generalized arousal' that underlies all motivated behaviors. Our science has advanced sufficiently to attack mechanisms linking specific motivations to these general arousal mechanisms that intrinsically activate all biologically-regulated behaviors including ingestive behaviors. Learning from the well-developed reproductive behavior paradigm, we know that sex hormone effects on hypothalamic neurons have been studied to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been worked out, and several functional genomic regulations have been discovered. Here we focus for the first time on three chemical systems that signal 'generalized arousal' and which impact hormone-dependent hypothalamic neurons of importance to sexual arousal: histamine, norepinephrine and enkephalin. Progress in linking generalized arousal to specific motivational mechanisms is reviewed.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

Interactions of galanin and opioids in nociceptive modulation in the arcuate nucleus of hypothalamus in rats

The fact that galanin, beta-endorphin and their receptors are present in the arcuate nucleus of hypothalamus (ARC), coupled with our previous observation that both beta-endorphin and galanin play antinociceptive roles in pain modulation in the ARC, made it of interest to study their interactions. The hindpaw withdrawal latency (HWL) in response to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. We showed that the antinociceptive effect induced by intra-ARC injection of galanin was dose-dependently attenuated by the following intra-ARC injection of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) attenuated the increased HWL induced by intra-ARC injection of galanin in a dose-dependent manner, while the delta-opioid receptor antagonist naltrindole or the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) did not. Moreover, intra-ARC injection of a galanin receptor antagonist galantide attenuated intraperitoneal morphine-induced increases in HWLs. These results demonstrate that the antinociceptive effect of galanin was related to the opioid system, especially mu-opioid receptor was involved in, and that systemic morphine induced antinociception involves galanin in the ARC.

Response of neuropeptide Y-induced feeding to μ-, δ- and κ-opioid receptor antagonists in the neonatal chick

It is known that opioid antagonists reduce the orexigenic effect of neropeptide Y (NPY) in mammals. We studied the effect of three opioid antagonists on NPY-induced feeding in male broiler chicks. Beta-funaltrexamine (beta-FNA), naloxonazine (NAL), ICI-174,864 (ICI) or nor-binaltorphimine (nor-BNI), antagonists of mu-, mu1-, delta- or kappa-receptors, and NPY were co-injected in chicks. Food intake was measured 30 min after treatment. Co-injection of beta-FNA or NAL was effective in reducing NPY-induced feeding, whereas ICI and nor-BNI had little effect on NPY-induced feeding. These data suggest that the mu-opioid receptor, especially the mu1-opioid has some relation to NPY-induced feeding, and implies that an endogenous ligand, such as beta-endorphin, participates in the orexigenic effect of NPY in neonatal chicks.

Hormonal symphony: steroid orchestration of gene modules for sociosexual behaviors

Genes induced by estrogens in the mammalian forebrain influence a variety of neural functions. Among them, reproductive behavior mechanisms are very well understood. Their functional genomics provide a theoretical paradigm for linking genes to neural circuits to behavior. We propose that estrogen-induced genes are organized in modules: Growth of hypothalamic neurons; Amplification of the estrogen effect by progesterone; Preparative behaviors; Permissive actions on sex behavior circuitry; and Synchronization of mating behavior with ovulation. These modules may represent mechanistic routes for CNS management of successful reproduction. Moreover, new microarray results add estrogen-dependent genes, including some expressed in glia, suggesting possible hormone-dependent neuronal/glial coordination.

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.

Sex-related differences in the distribution of opioid receptor-like 1 receptor mRNA and colocalization with estrogen receptor mRNA in neurons of the spinal trigeminal nucleus caudalis in the rat

We recently reported that exogenously applied orphanin FQ, the endogenous ligand for opioid receptor-like 1 (ORL(1)) receptor, produces sex-specific modulation of trigeminal nociception, and that estrogen contributes to these sex-related differences. Estrogen could produce these sex-related differences by altering the expression of the ORL(1)-receptor gene in the trigeminal nucleus caudalis. Utilizing in situ hybridization, we compared levels of ORL(1) receptor mRNA and investigated its colocalization with estrogen receptor mRNA in trigeminal neurons. Our results showed that in male rats, ORL(1) receptor mRNA is abundantly expressed in the rostral part of the trigeminal nucleus caudalis, and at the junction of caudalis and interpolaris (Vc/Vi). In comparison with males, levels of ORL(1) receptor mRNA were not significantly different in proestrus females, but were significantly higher in the rostral trigeminal nucleus caudalis and at the junction of Vc/Vi of diestrus females. In addition, ovariectomy raised the levels in the rostral trigeminal nucleus caudalis, and at the junction of Vc/Vi. Levels were reduced to proestrus levels in these regions following estradiol replacement. Our results also showed that ORL(1) receptor mRNA is present in majority of estrogen receptor (alpha and/or beta) mRNA-containing neurons. We conclude that there are sex-related differences in the ORL(1)-receptor gene expression in the trigeminal nucleus caudalis, which appear to be determined in part by estrogen levels.

Involvement of endogenous beta-endorphin in antinociception in the arcuate nucleus of hypothalamus in rats with inflammation

Although exogenous administration of beta-endorphin to the arcuate nucleus of hypothalamus (ARC) had been shown to produce antinociception, the role of endogenous beta-endorphin of the ARC in nociceptive processing has not been studied directly. The aim of the present study was to investigate the effect of endogenous beta-endorphin in the ARC on nociception in rats with carrageenan-induced inflammation. The hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation was assessed by the hot-plate test and the Randall Selitto Test. Intra-ARC injection of naloxone had no significant influence on the HWL to thermal and mechanical stimulation in intact rats. The HWL decreased significantly after intra-ARC injection of 1 or 10 microg of naloxone in rats with inflammation, but not with 0.1 microg of naloxone. Furthermore, intra-ARC administration of the selective mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) decreased the nociceptive response latencies to both stimulation in a dose-dependent manner in rats with inflammation, while intra-ARC administration of the selective delta-opioid receptor antagonist naltrindole or the selective kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) showed no influences on the nociceptive response latency. The antiserum against beta-endorphin, administered to the ARC, also dose-dependently reduced the HWL in rats with inflammation. The results indicate that endogenous beta-endorphin in the ARC plays an important role in the endogenous antinociceptive system in rats with inflammation, and that its effect is predominantly mediated by the mu-opioid receptor.

Mu-opioid receptors in seizure-controlling brain structures are altered by prenatal morphine exposure and by male and female gonadal steroids in adult rats

The present study used autoradiography to examine the effect of prenatal morphine exposure on mu-opioid receptor density in epileptic seizure-controlling brain structures including the substantia nigra pars compacta (SNC), substantia nigra pars reticulata (SNR), superior colliculus (SC), and subthalamic nucleus (STN) of adult male and female rats. The results demonstrate that prenatal morphine exposure increases the mu-opioid receptor density in the SNC and STN, but not in the SNR or in the SC of gonadally intact adult male rats. The density of mu-opioid receptors in the SNC and STN is, however, decreased following gonadectomy in morphine-exposed males, and testosterone treatment fails to restore this decrease to the level of gonadally intact males. Further, in the SC, the density of mu receptors was lower in both saline-exposed, gonadectomized (GNX) and GNX, TP-treated males and in morphine-exposed, GNX, TP-treated males relative to gonadally intact saline- and morphine-exposed males, respectively. In ovariectomized (OVX) female rats, the same prenatal morphine exposure increases the mu-opioid receptor density in the SNC and SNR, but decreases it in the STN. The density of mu-opioid receptors is also decreased in the SNC and SC of OVX estrogen-treated females and in the SNR and SC of OVX, progesterone-treated females. Thus, the present study demonstrates that mu-opioid receptors in seizure-controlling brain structures are sex-specifically altered by prenatal morphine exposure in adult progeny. Further, prenatal morphine exposure alters gonadal hormone effects on the density of mu receptors in adult, OVX females.

Orphanin FQ/nociceptin and mu-opioid receptor mRNA levels in human SH-SY5Y neuroblastoma cells: effects of activating the cAMP-PKA signal transduction pathway

Responses to opioid agonists vary, depending on past opioid exposure and the physiological state. The intracellular signaling pathway mediated by cAMP and protein kinase A (PKA) has been linked to regulation of opioid receptor responsiveness. The role of the cAMP-PKA pathway in regulating opioid receptor gene expression is incompletely defined. Mu-opioid receptor (MuOR) and orphanin FQ/nociceptin receptor (ORL(1)) transcripts were measured after activating this pathway in human neuroblastoma cells. Human SH-SY5Y neuroblastoma cells were maintained in continuous monolayer culture. Cells were incubated with combinations of agents which activate the cAMP-PKA signal transduction pathway, including forskolin and choleratoxin (CTX). MuOR and ORL(1) transcript levels were measured by hybridization to specific probes. Activation of the cAMP-PKA signal transduction pathway with forskolin in the presence of phosphodiesterase inhibitors was associated with a time-dependent decrease in the level of MuOR mRNA; partial recovery was observed with prolonged incubations. Forskolin effects were mimicked by CTX, but not by dideoxyforskolin. The PKA inhibitor H89 blunted the actions of forskolin. However, forskolin responses persisted despite coincubation with protein synthesis inhibitors. ORL(1) transcript levels did not significantly change, but vasoactive intestinal polypeptide (VIP) transcripts exhibited substantial increases, in the presence of forskolin or CTX. These observations support a role for cAMP in regulating MuOR responsiveness through actions at the level of receptor gene expression. ORL(1) transcript levels are not effected, suggesting that the cAMP-PKA pathway has differential effects on the expression of mRNA for different, but biochemically closely related, opioid receptor subtypes.

Prenatal exposure to morphine differentially alters gonadal hormone regulation of delta-opioid receptor binding in male and female rats

The present study tested the hypothesis that exposure to morphine on gestation days 11-18 differentially alters delta-opioid receptors in the brain of adult male and female rats. In Experiment 1, the binding characteristics of delta-opioid receptors were examined in membrane homogenates from six brain regions, including the hypothalamus (HYP), preoptic area, frontal cortex (CX), ventral tegmental area, striatum (STR) and cerebellum of adult male and female rats. In Experiment 2, the density of delta-opioid receptors was assessed in the CX and STR using receptor autoradiography. Prenatal morphine exposure has no effects on delta-opioid receptors in the brain of gonadally intact, adult male rats regardless of methodology. However, when male rats were gonadectomized in Experiment 2, morphine-exposed males have fewer delta-opioid receptors than controls in the CX but not in the STR. These reductions in cortical delta-opioid receptors are restored by testosterone replacement, demonstrating that prenatal morphine exposure alters testosterone regulation in the CX of male rats. In ovariectomized (OVX) female rats, prenatal morphine exposure increases the density of delta-opioid receptors in the frontal CX. Interestingly, this up-regulation of delta-opioid receptors is not present when the CX is investigated by autoradiography. Moreover, progesterone given alone or in combination with estrogen reduces the density of delta-opioid receptors in the CX and STR of both saline- and morphine-exposed, OVX females. Thus, mid to late gestational morphine exposure differentially alters the influence of adult gonadal hormones on delta-opioid receptors in the CX, decreasing the sensitivity in females and increasing it in males. This is also the first report to demonstrate that gonadal hormones regulate delta receptor densities in brain regions other than the HYP of OVX females.

Localization of promoter elements in the human mu-opioid receptor gene and regulation by DNA methylation

The regulation of mu-opioid receptor gene expression was investigated using several molecular techniques. Genomic clones containing portions of the human mu-opioid receptor gene were sequenced. 5'-RACE analysis of human brain cDNA confirmed the presence of mRNAs up to -313 from the start codon. As was found for the mouse and rat genes, transcription apparently initiates in the absence of a discernable TATA box. To characterize promoter function, portions of the 5'-flanking region were linked to a reporter gene in transient transfection experiments. Two approximately 50 bp adjacent segments had potent, orientation specific promoter activity. More down-stream segments also had promoter activity. None of the 5'-flanking region constructs showed tissue specificity. The potential role of DNA methylation in preventing ectopic expression was investigated by surveying the methylation state of a CpG rich region straddling the start codon. A neural derived cell line (SH-SY5Y) that expresses the mu-opioid receptor lacked virtually any CpG methylation. In contrast, two neural derived cell lines that do not express the mu-opioid receptor were nearly totally methylated while non-neural cell lines had intermediate levels of CpG methylation. Additional transient transfection experiments revealed that CpG methylation of the 5'-flanking region suppressed reporter gene expression. These results indicate that CpG methylation plays an important role in regulating mu-opioid receptor expression in neural cells; however, no association was found with regulation of expression in non-neural cells.

Ovarian steroids differentially regulate the expression of PRL-R in neuroendocrine dopaminergic neuron populations: a double label confocal microscopic study

The aims of this study were (1) to identify the possible hypothalamic targets for a short prolactin (PRL) feedback in the adult female rat by identifying DAergic neuron populations expressing PRL receptor (PRL-R); (2) to describe the effect of ovarian steroids on the expression of PRL-R and (3) to compare the distribution of both the extracellular (EC) and ligand binding (LB) domains of the PRL-R on the hypothalamic dopaminergic neurons by applying double label immunocytochemistry for the different domains of PRL-R and for tyrosine hydroxylase (TH). Five- to six-month-old female rats were ovariectomized (OVX) and implanted with either 17 beta-estradiol (E2), progesterone (P4) or received an E2 and a P4 implant (E2 + P4) at the same time. In the periventricular nucleus and in the dorsomedial portion of the middle arcuate nucleus, a dramatic increase in PRL-REC immunoreactivity was observed in E2 implanted rats. This increase was attenuated in E2 + P4 rats, but P4 treatment alone had no effect. Changes in PRL-REC expression were paralleled by changes in serum PRL levels. Interestingly, PRL-REC expression in the rostral arcuate nucleus decreased in P4 implanted rats, however, P4 did not attenuate the E2-induced increase in PRL-REC density. PRL-REC immunostaining was observed on the membrane, in the cytoplasm and in the nucleus. PRL-RLB immunoreactivity was also detectable in the TH positive neurons, but no nuclear staining was observed with this antibody. However, we found a strong PRL-RLB immunostaining in the ependymal lining of the 3rd ventricle and in the processes of tanycytes projecting to the median eminence. These data indicate that (1) all neuroendocrine DAergic cells can be targets for PRL, (2) expression of PRL-R is differentially affected by ovarian steroids in the different TH cell populations, (3) PRL-RLB domain may be involved in trafficking PRL in the median eminence.