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

The Phytoestrogenic Effect of Hibiscus sabdariffa Involves Estrogen Receptor α in Ovariectomized Wistar Rats

The calyxes of Hibiscus sabdariffa present multiple pharmacological effects primarily attributed to their high anthocyanin content; however, little is known about their phytoestrogenic effect. Ovarian hypofunction (OH) is a process characterized by the rapid detention of the production of ovarian hormones, which compromises reproductive and cognitive functions. Hormone replacement therapy (HRT) efficiently compensates for OH; nevertheless, questions have been raised on its secondary effects and safety. One of the alternatives to tackling OH involves using phytoestrogens such as anthocyanins for their structural similarity to natural estrogens. In a Wistar rat model of ovariectomy (OVX), we recently reported the beneficial properties of an anthocyanin-rich extract prepared from the calyces of H. sabdariffa (HSE) in hindering the adverse effects of OH on memory performance and highlighted a possible phytoestrogenic impact through the modulation of estrogen receptor (ER) expression. We now report that HSE and estradiol differentially affected the expression of ERα and ERβ. ERα was more sensitive to HSE; meanwhile, estradiol preferentially modulated ERβ. Thus, our study leads to further research on using H. sabdariffa as a nutrition-based alternative to HRT.

The interaction of membrane estradiol receptors and metabotropic glutamate receptors in adaptive and maladaptive estradiol-mediated motivated behaviors in females

Estrogen receptors were initially identified as intracellular, ligand-regulated transcription factors that result in genomic change upon ligand binding. However, rapid estrogen receptor signaling initiated outside of the nucleus was also known to occur via mechanisms that were less clear. Recent studies indicate that these traditional receptors, estrogen receptor α and estrogen receptor β, can also be trafficked to act at the surface membrane. Signaling cascades from these membrane-bound estrogen receptors (mERs) can rapidly alter cellular excitability and gene expression, particularly through the phosphorylation of CREB. A principal mechanism of neuronal mER action has been shown to occur through glutamate-independent transactivation of metabotropic glutamate receptors (mGlu), which elicits multiple signaling outcomes. The interaction of mERs with mGlu has been shown to be important in many diverse functions in females, including driving motivated behaviors. Experimental evidence suggests that a large part of estradiol-induced neuroplasticity and motivated behaviors, both adaptive and maladaptive, occurs through estradiol-dependent mER activation of mGlu. Herein we will review signaling through estrogen receptors, both "classical" nuclear receptors and membrane-bound receptors, as well as estradiol signaling through mGlu. We will focus on how the interactions of these receptors and their downstream signaling cascades are involved in driving motivated behaviors in females, discussing a representative adaptive motivated behavior (reproduction) and maladaptive motivated behavior (addiction).

Membrane estrogen signaling in female reproduction and motivation

Estrogen receptors were initially identified in the uterus, and later throughout the brain and body as intracellular, ligand-regulated transcription factors that affect genomic change upon ligand binding. However, rapid estrogen receptor signaling initiated outside of the nucleus was also known to occur via mechanisms that were less clear. Recent studies indicate that these traditional receptors, estrogen receptor-α and estrogen receptor-β, can also be trafficked to act at the surface membrane. Signaling cascades from these membrane-bound estrogen receptors (mERs) not only rapidly effect cellular excitability, but can and do ultimately affect gene expression, as seen through the phosphorylation of CREB. A principal mechanism of neuronal mER action is through glutamate-independent transactivation of metabotropic glutamate receptors (mGluRs), which elicits multiple signaling outcomes. The interaction of mERs with mGluRs has been shown to be important in many diverse functions in females, including, but not limited to, reproduction and motivation. Here we review membrane-initiated estrogen receptor signaling in females, with a focus on the interactions between these mERs and mGluRs.

Optogenetic Activation of β-Endorphin Terminals in the Medial Preoptic Nucleus Regulates Female Sexual Receptivity

Estrogen and progesterone (P4) act in neural circuits to elicit lordosis, the stereotypical female sexual receptivity behavior. Estradiol acts through membrane receptors to rapidly activate a limbic-hypothalamic circuit consisting of the arcuate (ARH), medial preoptic (MPN), and ventromedial (VMH) nuclei of the hypothalamus. This initial activation results in a transient but necessary inhibition of lordosis, which appears to be a result of the release of β-endorphin (β-End) from proopiomelanocortin (POMC) terminals onto cells containing the µ-opioid receptor (MOR) in the MPN. To functionally examine the role of the MOR in the hypothalamic lordosis circuit, we transfected a channelrhodopsin (ChR2) adeno-associated virus into POMC cell bodies in the ARH and photostimulated POMC/β-End axon terminals in the MPN in sexually receptive female Pomc-cre mice. Following estrogen and P4 priming, sexual receptivity was assessed by measuring the lordosis quotient (LQ). Following an initial trial for sexual receptivity, mice were photostimulated during behavioral testing, and brains were processed for MOR immunohistochemistry (IHC). Photostimulation decreased the LQ only in ChR2-expressing Pomc-cre mice. Furthermore, photostimulation of ChR2 in POMC/β-End axon terminals in the MPN resulted in the internalization of MOR, indicating activation of the receptor. Our results suggest that the activation of the MOR in the MPN is sufficient to attenuate lordosis behavior in a hormone-primed, sexually receptive female mouse. These data support a central role of MOR in female sexual behavior, and provide further insight into the hypothalamus control of sexual receptivity.

Estradiol Membrane-Initiated Signaling in the Brain Mediates Reproduction

Over the past few years our understanding of estrogen signaling in the brain has expanded rapidly. Estrogens are synthesized in the periphery and in the brain, acting on multiple receptors to regulate gene transcription, neural function, and behavior. Various estrogen-sensitive signaling pathways often operate in concert within the same cell, increasing the complexity of the system. In females, estrogen concentrations fluctuate over the estrous/menstrual cycle, dynamically modulating estrogen receptor (ER) expression, activity, and trafficking. These dynamic changes influence multiple behaviors but are particularly important for reproduction. Using the female rodent model, we review our current understanding of estradiol signaling in the regulation of sexual receptivity.

Integrating Neural Circuits Controlling Female Sexual Behavior

The hypothalamus is most often associated with innate behaviors such as is hunger, thirst and sex. While the expression of these behaviors important for survival of the individual or the species is nested within the hypothalamus, the desire (i.e., motivation) for them is centered within the mesolimbic reward circuitry. In this review, we will use female sexual behavior as a model to examine the interaction of these circuits. We will examine the evidence for a hypothalamic circuit that regulates consummatory aspects of reproductive behavior, i.e., lordosis behavior, a measure of sexual receptivity that involves estradiol membrane-initiated signaling in the arcuate nucleus (ARH), activating β-endorphin projections to the medial preoptic nucleus (MPN), which in turn modulate ventromedial hypothalamic nucleus (VMH) activity-the common output from the hypothalamus. Estradiol modulates not only a series of neuropeptides, transmitters and receptors but induces dendritic spines that are for estrogenic induction of lordosis behavior. Simultaneously, in the nucleus accumbens of the mesolimbic system, the mating experience produces long term changes in dopamine signaling and structure. Sexual experience sensitizes the response of nucleus accumbens neurons to dopamine signaling through the induction of a long lasting early immediate gene. While estrogen alone increases spines in the ARH, sexual experience increases dendritic spine density in the nucleus accumbens. These two circuits appear to converge onto the medial preoptic area where there is a reciprocal influence of motivational circuits on consummatory behavior and vice versa. While it has not been formally demonstrated in the human, such circuitry is generally highly conserved and thus, understanding the anatomy, neurochemistry and physiology can provide useful insight into the motivation for sexual behavior and other innate behaviors in humans.

Sexually-motivated song is predicted by androgen-and opioid-related gene expression in the medial preoptic nucleus of male European starlings (Sturnus vulgaris)

Across vertebrates, communication conveys information about an individual's motivational state, yet little is known about the neuroendocrine regulation of motivational aspects of communication. For seasonally breeding songbirds, increases in testosterone in spring stimulate high rates of sexually-motivated courtship song, though not all birds sing at high rates. It is generally assumed that testosterone or its metabolites act within the medial preoptic nucleus (POM) to stimulate the motivation to sing. In addition to androgen receptors (ARs) and testosterone, opioid neuropeptides in the POM influence sexually-motivated song production, and it has been proposed that testosterone may in part regulate song by modifying opioid systems. To gain insight into a possible role for androgen-opioid interactions in the regulation of communication we examined associations between sexually-motivated song and relative expression of ARs, mu opioid receptors (muORs), and preproenkephalin (PENK) in the POM (and other regions) of male European starlings using qPCR. Both AR and PENK expression in POM correlated positively with singing behavior, whereas muOR in POM correlated negatively with song. Furthermore, the ratio of PENK/muOR expression correlated negatively with AR expression in POM. Finally, in the ventral tegmental area (VTA), PENK expression correlated negatively with singing behavior. Results support the hypothesis that ARs may alter opioid gene expression in POM to fine-tune singing to reflect a male's motivational state. Data also suggest that bidirectional relationships may exist between opioids and ARs in POM and song, and additionally support a role for opioids in the VTA, independent of AR activity in this region.

Orphanin FQ-ORL-1 regulation of reproduction and reproductive behavior in the female

Orphanin FQ (OFQ/N) and its receptor, opioid receptor-like receptor-1 (ORL-1), are expressed throughout steroid-responsive limbic and hypothalamic circuits that regulate female ovarian hormone feedback and reproductive behavior circuits. The arcuate nucleus of the hypothalamus (ARH) is a brain region that expresses OFQ/N and ORL-1 important for both sexual behavior and modulating estradiol feedback loops. Within the ARH, the activation of the OFQ/N-ORL-1 system facilitates sexual receptivity (lordosis) through the inhibition of β-endorphin neuronal activity. Estradiol initially activates ARH β-endorphin neurons to inhibit lordosis. Simultaneously, estradiol upregulates coexpression of OFQ/N and progesterone receptors and ORL-1 in ARH β-endorphin neurons. Ovarian hormones regulate pre- and postsynaptic coupling of ORL-1 to its G protein-coupled signaling pathways. When the steroid-primed rat is nonreceptive, estradiol acts pre- and postsynaptically to decrease the ability of the OFQ/N-ORL-1 system to inhibit ARH β-endorphin neurotransmission. Conversely, when sexually receptive, ORL-1 signaling is restored to inhibit β-endorphin neurotransmission. Although steroid signaling that facilitates lordosis converges to deactivate ARH β-endorphin neurons, estradiol-only facilitation of lordosis requires the activation of ORL-1, but estradiol+progesterone does not, indicating that multiple circuits mediate ovarian hormone signaling to deactivate ARH β-endorphin neurons. Research on the role of OFQ/N-ORL-1 in ovarian hormone feedback loops is just beginning. In the rat, OFQ/N may act to terminate gonadotropin-releasing hormone and luteinizing hormone release under positive and negative feedbacks. In the ewe, it appears to directly inhibit gonadotropin-releasing hormone release to mediate progesterone-negative feedback. As a whole, the localization and actions of OFQ/N-ORL-1 system indicate that it may mediate the actions of estradiol and progesterone to synchronize reproductive behavior and ovarian hormone feedback loops.

Membrane-initiated non-genomic signaling by estrogens in the hypothalamus: cross-talk with glucocorticoids with implications for behavior

The estrogen receptor and glucocorticoid receptor are members of the nuclear receptor superfamily that can signal using both non-genomic and genomic transcriptional modes. Though genomic modes of signaling have been well characterized and several behaviors attributed to this signaling mechanism, the physiological significance of non-genomic modes of signaling has not been well understood. This has partly been due to the controversy regarding the identity of the membrane ER (mER) or membrane GR (mGR) that may mediate rapid, non-genomic signaling and the downstream signaling cascades that may result as a consequence of steroid ligands binding the mER or the mGR. Both estrogens and glucocorticoids exert a number of actions on the hypothalamus, including feedback. This review focuses on the various candidates for the mER or mGR in the hypothalamus and the contribution of non-genomic signaling to classical hypothalamically driven behaviors and changes in neuronal morphology. It also attempts to categorize some of the possible functions of non-genomic signaling at both the cellular level and at the organismal level that are relevant for behavior, including some behaviors that are regulated by both estrogens and glucocorticoids in a potentially synergistic manner. Lastly, it attempts to show that steroid signaling via non-genomic modes may provide the organism with rapid behavioral responses to stimuli.

Estradiol upregulates progesterone receptor and orphanin FQ colocalization in arcuate nucleus neurons and opioid receptor-like receptor-1 expression in proopiomelanocortin neurons that project to the medial preoptic nucleus in the female rat

BACKGROUND

Ovarian steroids regulate sexual receptivity in the female rat by acting on neurons that converge on proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH) that project to the medial preoptic nucleus (MPN). Estradiol rapidly activates these neurons to release β-endorphin that activates MPN μ-opioid receptors (MOP) to inhibit lordosis. Lordosis is facilitated by the subsequent action of progesterone that deactivates the estradiol-induced MPN MOP activation. Orphanin FQ (OFQ/N; also known as nociceptin) infusions into the ARH, like progesterone, deactivate MPN MOP and facilitate lordosis in estradiol-primed rats. OFQ/N reduces the activity of ARH β-endorphin neurons through post- and presynaptic mechanisms via its cognate receptor, ORL-1.

METHODS

We tested the hypotheses that progesterone receptors (PR) are expressed in ARH OFQ/N neurons by immunohistochemistry and ORL-1 is expressed in POMC neurons that project to the MPN by combining Fluoro-Gold injection into the MPN and double-label fluorescent in situ hybridization (FISH). We also hypothesized that estradiol increases coexpression of PR-OFQ/N and ORL-1-POMC in ARH neurons of ovariectomized rats.

RESULTS

The number of PR- and OFQ/N-immunopositive ARH neurons was increased as was their colocalization by estradiol treatment. FISH for ORL-1 and POMC mRNA revealed a subpopulation of ARH neurons that was triple labeled, indicating these neurons project to the MPN and coexpress ORL-1 and POMC mRNA. Estradiol was shown to upregulate ORL-1 and POMC expression in MPN-projecting ARH neurons.

CONCLUSION

Estradiol upregulates the ARH OFQ/N-ORL-1 system projecting to the MPN that regulates lordosis.

Temporal and concentration-dependent effects of oestradiol on neural pathways mediating sexual receptivity

The acceptance of oestradiol signalling through receptors found in the cell membrane, as well as, the nucleus, has provided for a re-examination of the timing and location of the actions of oestradiol on neural circuits mediating sexual receptivity (lordosis). Oestradiol membrane signalling involves the transactivation of metabotrophic glutamate receptors (mGluRs) that transduce steroid information through protein kinase C signalling cascades producing rapid activation of lordosis-regulating circuits. It has been known for some time that oestradiol initially produces an inhibition of the medial preoptic nucleus. We have demonstrated that underlying this inhibition is oestradiol acting in the arcuate nucleus to induce β-endorphin release, which inhibits the medial preoptic nucleus through a μ-opioid receptor mechanism. This transient inhibition is relieved by either subsequent progesterone treatment or longer exposure to higher doses of oestradiol to facilitate lordosis behaviour. We review recent findings about oestradiol membrane signalling inducing dendritic spine formation in the arcuate nucleus that is critical for oestradiol induction of sexual receptivity. Moreover, we discuss the evidence that, in addition to oestrogen receptor α, several other putative membrane oestrogen receptors facilitate lordosis behaviour through regulation of the arcuate nucleus. These include the GRP30 and the STX activated Gq-mER. Finally, we report on the importance of GABA acting at GABAB receptors for oestradiol membrane signalling that regulates lordosis circuit activation and sexual receptivity.

Curvilinear relationships between mu-opioid receptor labeling and undirected song in male European starlings (Sturnus vulgaris)

Female-directed communication in male songbirds has been reasonably well studied; yet, relatively little is known about communication in other social contexts. Songbirds also produce song that is not clearly directed towards another individual (undirected song) when alone or in flocks. Although the precise functions of undirected song may differ across species, this type of song is considered important for flock maintenance, song learning or practice. Past studies show that undirected song is tightly coupled to analgesia and positive affective state, which are both mediated by opioid activity. Furthermore, labeling for the opioid met-enkephalin in the medial preoptic nucleus (POM) correlates positively with undirected song production. We propose that undirected song is facilitated and maintained by opioid receptor activity in the POM and other brain regions involved in affective state, analgesia, and social behavior. To provide insight into this hypothesis, we used immunohistochemistry to examine relationships between undirected song and mu-opioid receptors in male starlings. Polynomial regression analyses revealed significant inverted-U shaped relationships between measures of undirected song and mu-opioid receptor labeling in the POM, medial bed nucleus of the stria terminalis (BSTm), and periaqueductal gray (PAG). These results suggest that low rates of undirected song may stimulate and/or be maintained by mu-opioid receptor activity; however, it may be that sustained levels of mu-opioid receptor activity associated with high rates of undirected song cause mu-opioid receptor down-regulation. The results indicate that mu-opioid receptor activity in POM, BSTm, and PAG may underlie previous links identified between undirected song, analgesia, and affective state.

Modulation of the arcuate nucleus-medial preoptic nucleus lordosis regulating circuit: a role for GABAB receptors

Estradiol rapidly activates a microcircuit in the arcuate nucleus of the hypothalamus (ARH) that is needed for maximal female sexual receptivity. Membrane estrogen receptor-α complexes with and signals through the metabotropic glutamate receptor-1a stimulating NPY release within the ARH activating proopiomelanocortin (POMC) neurons. These POMC neurons project to the medial preoptic nucleus (MPN) and release β-endorphin. Estradiol treatment induces activation/internalization of MPN μ-opioid receptors (MOR) to inhibit lordosis. Estradiol membrane action modulates ARH gamma-aminobutyric acid receptor-B (GABAB) activity. We tested the hypothesis that ARH GABAB receptors mediate estradiol-induced MOR activation and facilitation of sexual receptivity. Double-label immunohistochemistry revealed expression of GABAB receptors in NPY, ERα and POMC expressing ARH neurons. Approximately 70% of POMC neurons expressed GABAB receptors. Because estradiol initially activates an inhibitory circuit and maintains activation of this circuit, the effects of blocking GABAB receptors were evaluated before estradiol benzoate (EB) treatment and after at the time of lordosis testing. Bilateral infusions of the GABAB receptor antagonist, CGP52432, into the ARH prior to EB treatment of ovariectomized rats prevented estradiol-induced activation/internalization of MPN MOR, and the rats remained unreceptive. However, in EB-treated rats, bilateral CGP52432 infusions 30 min before behavior testing attenuated MOR internalization and facilitated lordosis. These results indicated that GABAB receptors were located within the lordosis-regulating ARH microcircuit and are necessary for activation and maintenance of the estradiol inhibition of lordosis behavior. Although GABAB receptors positively influence estradiol signaling, they negatively regulate lordosis behavior since GABAB activity maintains the estradiol-induced inhibition.

Fluorescently-Labeled Estradiol Internalization and Membrane Trafficking in Live N-38 Neuronal Cells Visualized with Total Internal Reflection Fluorescence Microscopy

Estradiol is a steroid hormone that binds and activates estradiol receptors. Activation of these receptors is known to modulate neuronal physiology and provide neuroprotection, but it is not completely understood how estradiol mediates these actions on the nervous system. Activation of a sub-population of estradiol receptor-α (ERα), originally identified as a nuclear protein, localizes to the plasma membrane and appears to be a critical step in neuroprotection against brain injury and disease. Previously we showed that estradiol stimulates the rapid and transient trafficking of plasma membrane ERα in primary hypothalamic neurons, and internalization of membrane-impermeant estradiol (E6BSA-FITC) into cortical neuron endosomes in vitro. These findings support the concept that estradiol activates and down-regulates plasma membrane ERα by triggering endocytosis. Here, we use TIRFM (total internal reflection fluorescence microscopy) to image the trafficking of E6BSA-FITC, and GFP-labeled ERα, in live cells in real time. We show that activation of plasma membrane ERs by E6BSA-FITC result in internalization of the fluorescent ligand in live N-38 neurons, an immortalized hypothalamic cell line. Pretreatment with ER antagonist ICI 182,780 decreased the number of E6BSA-FITC labeled puncta observed. We also observed in live N-38 neurons that E6BSA-FITC co-localized with FM4-64 and LysoTracker fluorescent dyes that label endosomes and lysosomes. Our results provide further evidence that plasma membrane ERα activation results in endocytosis of the receptor.

Estradiol signaling in the regulation of reproduction and energy balance

Our knowledge of membrane estrogenic signaling mechanisms and their interactions that regulate physiology and behavior has grown rapidly over the past three decades. The discovery of novel membrane estrogen receptors and their signaling mechanisms has started to reveal the complex timing and interactions of these various signaling mechanisms with classical genomic steroid actions within the nervous system to regulate physiology and behavior. The activation of the various estrogenic signaling mechanisms is site specific and differs across the estrous cycle acting through both classical genomic mechanisms and rapid membrane-initiated signaling to coordinate reproductive behavior and physiology. This review focuses on our current understanding of estrogenic signaling mechanisms to promote: (1) sexual receptivity within the arcuate nucleus of the hypothalamus, (2) estrogen positive feedback that stimulates de novo neuroprogesterone synthesis to trigger the luteinizing hormone surge important for ovulation and estrous cyclicity, and (3) alterations in energy balance.

The Neurosteroid Progesterone Underlies Estrogen Positive Feedback of the LH Surge

Our understanding the steroid regulation of neural function has rapidly evolved in the past decades. Not long ago the prevailing thoughts were that peripheral steroid hormones carried information to the brain which passively responded to these steroids. These steroid actions were slow, taking hours to days to be realized because they regulated gene expression. Over the past three decades, discoveries of new steroid receptors, rapid membrane-initiated signaling mechanisms, and de novo neurosteroidogenesis have shed new light on the complexity of steroids actions within the nervous system. Sexual differentiation of the brain during development occurs predominately through timed steroid-mediated expression of proteins and long term epigenetic modifications. In contrast across the estrous cycle, estradiol release from developing ovarian follicles initially increases slowly and then at proestrus increases rapidly. This pattern of estradiol release acts through both classical genomic mechanisms and rapid membrane-initiated signaling in the brain to coordinate reproductive behavior and physiology. This review focuses on recently discovered estrogen receptor-α membrane signaling mechanisms that estradiol utilizes during estrogen positive feedback to stimulate de novo progesterone synthesis within the hypothalamus to trigger the luteinizing hormone (LH) surge important for ovulation and estrous cyclicity. The activation of these signaling pathways appears to be coordinated by the rising and waning of estradiol throughout the estrous cycle and integral to the negative and positive feedback mechanisms of estradiol. This differential responsiveness is part of the timing mechanism triggering the LH surge.

Membrane-initiated estradiol signaling regulating sexual receptivity

Estradiol has profound actions on the structure and function of the nervous system. In addition to nuclear actions that directly modulate gene expression, the idea that estradiol can rapidly activate cell signaling by binding to membrane estrogen receptors (mERs) has emerged. Even the regulation of sexual receptivity, an action previously thought to be completely regulated by nuclear ERs, has been shown to have a membrane-initiated estradiol signaling (MIES) component. This highlighted the question of the nature of mERs. Several candidates have been proposed, ERα, ERβ, ER-X, GPR30 (G protein coupled estrogen receptor), and a receptor activated by a diphenylacrylamide compound, STX. Although each of these receptors has been shown to be active in specific assays, we present evidence for and against their participation in sexual receptivity by acting in the lordosis-regulating circuit. The initial MIES that activates the circuit is in the arcuate nucleus of the hypothalamus (ARH). Using both activation of μ-opioid receptors (MOR) in the medial preoptic nucleus and lordosis behavior, we document that both ERα and the STX-receptor participate in the required MIES. ERα and the STX-receptor activation of cell signaling are dependent on the transactivation of type 1 metabotropic glutamate receptors (mGluR1a) that augment progesterone synthesis in astrocytes and protein kinase C (PKC) in ARH neurons. While estradiol-induced sexual receptivity does not depend on neuroprogesterone, proceptive behaviors do. Moreover, the ERα and the STX-receptor activation of medial preoptic MORs and augmentation of lordosis were sensitive to mGluR1a blockade. These observations suggest a common mechanism through which mERs are coupled to intracellular signaling cascades, not just in regulating reproduction, but in actions throughout the neuraxis including the cortex, hippocampus, striatum, and dorsal root ganglias.

Estradiol rapidly regulates membrane estrogen receptor alpha levels in hypothalamic neurons

Estrogen receptors (ERs) and estrogen-binding proteins have been localized intracellularly and on the cell surface. The membrane-associated proteins initiate signaling that activates a myriad of cellular responses including the modulation of ion channels and ultimately transcription. Although many of the downstream actions of membrane ERs, including ERα and ERβ, have been characterized, the mechanisms regulating membrane ER levels have remained elusive in the nervous system. In the present study, we used surface biotinylation to identify and study the estradiol regulation of membrane ERα in mixed-sex, cultured hypothalamic neurons from rat. Following surface biotinylation, Western blot analysis revealed full-length 66 kDa ERα and several ERα splice variants, most notably a biotinylated 52 kDa ERα-immunoreactive protein. Treatment of the neurons with estradiol caused a rapid and transient increase of the biotinylated 52 kDa and 66 kDa ERα proteins in the plasma membrane. Exposure of the neurons to estradiol also significantly increased internalization of 52 kDa and 66 kDa ERα membrane proteins, a measure of receptor activation. In the hypothalamus, membrane ERα signaling depends on transactivation of metabotropic glutamate receptor-1a (mGluR1a). Estradiol treatment increased the internalization of mGluR1a in parallel with ERα, a finding consistent with the hypothesis of an ERα-mGluR1a signaling unit. These results demonstrate that estradiol regulates the amount of ERα in the membrane, suggesting estradiol can regulate its own membrane signaling.

Rapid delivery of internalized signaling receptors to the somatodendritic surface by sequence-specific local insertion

The recycling pathway is a major route for delivering signaling receptors to the somatodendritic plasma membrane. We investigated the cell biological basis for the remarkable selectivity and speed of this process. We focused on the mu-opioid neuropeptide receptor and the beta(2)-adrenergic catecholamine receptor, two seven-transmembrane signaling receptors that traverse the recycling pathway efficiently after ligand-induced endocytosis and localize at steady state throughout the postsynaptic surface. Rapid recycling of each receptor in dissociated neuronal cultures was mediated by a receptor-specific cytoplasmic sorting sequence. Total internal reflection fluorescence microscopy imaging revealed that both sequences drive recycling via discrete vesicular fusion events in the cell body and dendritic shaft. Both sequences promoted recycling via "transient"-type events characterized by nearly immediate lateral spread of receptors after vesicular insertion resembling receptor insertion events observed previously in non-neural cells. The sequences differed in their abilities to produce distinct "persistent"-type events at which inserted receptors lingered for a variable time period before lateral spread. Both types of insertion event generated a uniform distribution of receptors in the somatodendritic plasma membrane when imaged over a 1 min interval, but persistent events uniquely generated a punctate surface distribution over a 10 s interval. These results establish sequence-directed recycling of signaling receptors in CNS neurons and show that this mechanism has the ability to generate receptor-specific patterns of local surface distribution on a timescale overlapping that of rapid physiological signaling.

Mixing pleasures: review of the effects of drugs on sex behavior in humans and animal models

Drugs of abuse act on the brain circuits mediating motivation and reward associated with natural behaviors. There is ample evidence that drugs of abuse impact male and female sexual behavior. First, the current review discusses the effect of drugs of abuse on sexual motivation and performance in male and female humans. In particular, we discuss the effects of commonly abused drugs including psychostimulants, opiates, marijuana/THC, and alcohol. In general, drug use affects sexual motivation, arousal, and performance and is commonly associated with increased sexual risk behaviors. Second, studies on effects of systemic administration of drugs of abuse on sexual behavior in animals are reviewed. These studies analyze the effects on sexual performance and motivation but do not investigate the effects of drugs on risk-taking behavior, creating a disconnect between human and animal studies. For this reason, we discuss two studies that focus on the effects of alcohol and methamphetamine on inhibition of maladaptive sex-seeking behaviors in rodents. Third, this review discusses potential brain areas where drugs of abuse may be exerting their effect on sexual behavior with a focus on the mesolimbic system as the site of action. Finally, we discuss recent studies that have brought to light that sexual experience in turn can affect drug responsiveness, including a sensitized locomotor response to amphetamine in female and male rodents as well as enhanced drug reward in male rats.

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

INTRODUCTION

The ventromedial hypothalamus (VMH) controls female rodent copulatory behavior, which can be modulated by injection of various compounds into the VMH. Aim. The aim was to determine whether evoked excitatory postsynaptic potentials (EPSPs) or single-unit activity within the VMH ex vivo is a better parameter to predict lordosis.

METHODS

VMH slices were placed onto a 64 microelectrode chip and spontaneous single-unit activity was recorded or slices stimulated to evoke EPSPs.

MAIN OUTCOME MEASURES

The sodium channel blocker, tetrodotoxin and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) inhibited EPSPs, confirming EPSPs were glutamatergic in origin. The GABA(A) antagonist bicuculline potentiated EPSPs implying endogenous GABA tone. Single-unit activity was abolished by tetrodotoxin but unaffected by DNQX or bicuculline.

RESULTS

Glutamatergic neurotransmission was greatest during metestrous and following ovariectomization. The number of regions within the VMH eliciting single-unit activity was reduced following ovariectomy without changing spike frequency. Adrenergic agents increasing lordosis via the VMH in vivo, decreased glutamate neurotransmission but increased single-unit activity. Conversely, agents decreasing lordosis via the VMH increased glutamatergic neurotransmission and inhibited single-unit activity (8-OH-DPAT, [D-Ala(2), N-Me-Phe(4), Gly-ol(5)]-enkephalin, corticotropin releasing factor, bicuculline). Melanocortin and pituitary adenylate cyclase-activating polypeptide agonists had no effect.

CONCLUSIONS

Here we present a novel, robust VMH in vitro technique that (i) is consistent with the hypothesis that glutamate via non-NMDA receptors inhibits lordosis; (ii) glutamate is under the endogenous tone of GABA and steroid hormones; (iii) inhibition of lordosis during metestrous and following ovariectomy potentiates glutamatergic neurotransmission; (iv) activation of G(q)- and G(i)-coupled receptors decreases and increases glutamate neurotransmission, respectively, with an inverse correlation on single-unit activity; (v) activation of G(s)-coupled receptors has no direct effect on glutamate or single-unit activity; and (vi) potency, receptor subtypes and localization can be determined prior to in vivo studies.

Estrogen actions on neuroendocrine glia

Astrocytes are the most abundant cells in the central nervous system (CNS). It appears that astrocytes are as diverse as neurons, having different phenotypes in various regions throughout the brain and participating in intercellular communication that involves signaling to neurons. It is not surprising then that astrocytes in the hypothalamus have an active role in the CNS regulation of reproduction. In addition to the traditional mechanism involving ensheathment of neurons and processes, astrocytes may have a critical role in regulating estrogen-positive feedback. Work in our laboratory has focused on the relationship between circulating estradiol and progesterone synthesized de novo in the brain. We have demonstrated that circulating estradiol stimulates the synthesis of progesterone in adult hypothalamic astrocytes, and this neuroprogesterone is critical for initiating the LH surge. Estradiol cell signaling is initiated at the cell membrane and involves the transactivation of metabotropic glutamate receptor type 1a (mGluR1a) leading to the release of intracellular stores of calcium. We used surface biotinylation to demonstrate that estrogen receptor-alpha (ERalpha) is present in the cell membrane and has an extracellular portion. Like other membrane receptors, ERalpha is inserted into the membrane and removed via internalization after agonist stimulation. This trafficking is directly regulated by estradiol, which rapidly and transiently increases the levels of membrane ERalpha, and upon activation, increases internalization that finally leads to ERalpha degradation. This autoregulation temporally limits membrane-initiated estradiol cell signaling. Thus, neuroprogesterone, the necessary signal for the LH surge, is released when circulating levels of estradiol peak on proestrus and activate progesterone receptors whose expression has been induced by the gradual rise of estradiol during follicular development.

Membrane estradiol signaling in the brain

While the physiology of membrane-initiated estradiol signaling in the nervous system has remained elusive, a great deal of progress has been made toward understanding the activation of cell signaling. Membrane-initiated estradiol signaling activates G proteins and their downstream cascades, but the identity of membrane receptors and the proximal signaling mechanism(s) have been more difficult to elucidate. Mounting evidence suggests that classical intracellular estrogen receptor-alpha (ERalpha) and ERbeta are trafficked to the membrane to mediate estradiol cell signaling. Moreover, an interaction of membrane ERalpha and ERbeta with metabotropic glutamate receptors has been identified that explains the pleomorphic actions of membrane-initiated estradiol signaling. This review focuses on the mechanism of actions initiated by membrane estradiol receptors and discusses the role of scaffold proteins and signaling cascades involved in the regulation of nociception, sexual receptivity and the synthesis of neuroprogesterone, an important component in the central nervous system signaling.

Membrane estrogen receptors activate metabotropic glutamate receptors to influence nervous system physiology

Until recently, the idea that estradiol could affect cellular processes independent of nuclear estrogen receptors was often dismissed as artifact. This in spite of a large number of carefully controlled studies performed both within and outside the nervous system demonstrating estrogens regulate various intracellular signaling pathways by acting at the membrane surface of cells and/or at biological rates incompatible with the time course of genomic-initiated events. The concept that estradiol can act on surface membrane receptors to regulate nervous system function is now far less controversial. However, there is evidence that there may be multiple types of estrogen receptors on the membrane surface of cells. Determining the physiological relevance of each of these receptors is currently underway. Two important membrane estrogen receptors are in fact the classical estrogen receptor-alpha (ERalpha) and estrogen receptor-beta (ERbeta) proteins, which is somewhat surprising based upon their well-established role in nuclear gene transcription. This review will focus on the mechanism by which surface-localized ERalpha and ERbeta stimulate intracellular signaling events in cells of the nervous system through activation of metabotropic glutamate receptors (mGluRs). This mechanism of estrogen receptor function also requires caveolin proteins, which provide the subcellular compartmentalization of the particular signaling components required for appropriate cell stimulation. The review will conclude with several examples of physiological processes under the apparent regulation of ER/mGluR signaling.

17beta-estradiol-mediated neuroprotection and ERK activation require a pertussis toxin-sensitive mechanism involving GRK2 and beta-arrestin-1

17-beta-Estradiol (E2) is a steroid hormone involved in numerous bodily functions, including several brain functions. In particular, E2 is neuroprotective against excitotoxicity and other forms of brain injuries, a property that requires the extracellular signal-regulated kinase (ERK) pathway and possibly that of other signaling molecules. The mechanism and identity of the receptor(s) involved remain unclear, although it has been suggested that E2 receptor alpha (ERalpha) and G proteins are involved. We, therefore, investigated whether E2-mediated neuroprotection and ERK activation were linked to pertussis toxin (PTX)-sensitive G-protein-coupled effector systems. Biochemical and image analysis of organotypic hippocampal slices and cortical neuronal cultures showed that E2-mediated neuroprotection as well as E2-induced ERK activation were sensitive to PTX. The sensitivity to PTX suggested a possible role of G-protein- and beta-arrestin-mediated mechanisms. Western immunoblots from E2-treated cortical neuronal cultures revealed an increase in phosphorylation of both G-protein-coupled receptor-kinase 2 and beta-arrestin-1, a G-protein-coupled receptor adaptor protein. Transfection of neurons with beta-arrestin-1 small interfering RNA prevented E2-induced ERK activation. Coimmunoprecipitation experiments indicated that E2 increased the recruitment of beta-arrestin-1 and c-Src to ERalpha. These findings suggested that ERalpha is regulated by a mechanism associated with receptor desensitization and downregulation. In support of this idea, we found that E2 treatment of cortical synaptoneurosomes resulted in internalization of ERalpha, whereas treatment of cortical neurons with the ER agonists E-6-BSA-FITC [beta-estradiol-6-(O-carboxymethyl)oxime-bovine serum albumin conjugated with fluorescein isothiocyanate] and E-6-biotin [1,3,5(10)-estratrien-3,17beta-diol-6-one-6-carboxymethloxime-NH-propyl-biotin] resulted in agonist internalization. These results demonstrate that E2-mediated neuroprotection and ERK activation involve ERalpha activation of G-protein- and beta-arrestin-mediated mechanisms.

Nervous system physiology regulated by membrane estrogen receptors

Our understanding of estrogen signaling in the nervous system has undergone a significant shift in recent years. For over three decades, the idea that all estradiol actions were explained by direct regulation of transcription held sway. Within the past decade, the idea that in addition to classical effects, membrane-initiated actions of estradiol are important has gained traction. While several novel putative membrane estrogen receptors (ERs) have been described, a large fraction of measured responses appear to be due to membrane-localized estrogen receptor-alpha (ER alpha) and estrogen receptor-beta (ER beta), the same proteins that regulate gene expression. These membrane-localized ERs participate in the regulation of the synthesis of neuroprogesterone, dorsal root ganglion (DRG) neuron excitation, and female sexual receptivity. This is achieved by the modulation of intracellular cell signaling pathways usually associated with the activation of G protein-coupled receptors (GPCRs). ER alpha and ER beta are themselves not GPCRs that directly activate G proteins to regulate physiological responses, but rather interact with traditional GPCRs to initiate cell signaling. This review presents results that support a direct protein-protein interaction between ER alpha and ER beta with metabotropic glutamate receptors (mGluRs), allowing estradiol to signal through mGluRs. This ER/mGluR hypothesis explains how estradiol can activate a wide-range of intracellular pathways and provides an underlying mechanism for the hitherto seemingly unrelated rapid membrane actions in the nervous system.

Physiology of membrane oestrogen receptor signalling in reproduction

The best characterised oestrogen receptors (ERs) that are responsible for membrane-initiated oestradiol signalling are the classic ERs, ERalpha and ERbeta. When in the nucleus, these proteins are oestradiol activated transcription factors but, when trafficked to the cell membrane, ERalpha and ERbeta rapidly activate protein kinase pathways, alter membrane electrical properties, modulate ion flux and can mediate long-term effects through gene expression. To initiate cell signalling, membrane ERs transactivate metabotropic glutamate receptors (mGluRs) to stimulate Gq signalling through pathways using PKC and calcium. In this review, we discuss the interaction of membrane ERalpha with metabotropic glutamate receptor 1a (mGluR1a) to initiate rapid oestradiol cell signalling and its critical roles in female reproduction: sexual behaviour and oestrogen positive feedback of the luteinising hormone (LH) surge. Although long considered to be regulated by the long-term actions of oestradiol on gene transcription, recent results indicate that membrane oestradiol cell signalling is vital for a full display of sexual receptivity. Similarly, the source of pre-ovulatory progesterone necessary for initiating the LH surge is hypothalamic astrocytes. Oestradiol rapidly amplifies progesterone synthesis through the release of intracellular calcium stores. The ERalpha-mGluR1a interaction is necessary for critical calcium flux. These two examples provide support for the hypothesis that membrane ERs are not themselves G-protein receptors; rather, they use mGluRs to signal.

Membrane-localised oestrogen receptor alpha and beta influence neuronal activity through activation of metabotropic glutamate receptors

Until recently, the idea that oestradiol could affect cellular processes independent of nuclear oestrogen receptors (ERs) was controversial. This was despite the large number of carefully controlled studies performed both within and outside the nervous system demonstrating that oestrogens regulate various intracellular signalling pathways by acting at the membrane surface of cells and/or at biological rates incompatible with the time course of genomic-initiated events. At present, it is far less controversial that oestradiol acts at surface membrane receptors to regulate nervous system function. Recent studies have demonstrated that the classical intracellular ERs, ERalpha and ERbeta, are major players in mediating the actions of oestradiol on the membrane surface. This review focuses on one potential mechanism by which surface-localised ERalpha and ERbeta stimulate intracellular signalling events in cells of the nervous system. After oestradiol treatment, both ERalpha and ERbeta are capable of activating different classes of metabotropic glutamate receptors (mGluRs). Oestradiol activation of mGluRs is independent of glutamate, but requires expression of several different caveolin proteins to compartmentalise the different ERs with mGluRs into functional signalling microdomains. ER/mGluR signalling is a potential means by which oestrogens can both rapidly and for extended periods, influence a variety of intracellular signalling processes and behaviours.

Protein kinase C signaling in the hypothalamic arcuate nucleus regulates sexual receptivity in female rats

Rapid membrane-mediated estradiol signaling regulating sexual receptivity requires the interaction of the estrogen receptor (ER)-alpha and the metabotropic glutamate receptor 1a (mGluR1a). A cell signaling antibody microarray revealed that estradiol activated 42 proteins in the arcuate nucleus of the hypothalamus (ARH). To begin an analysis of various signaling pathways, protein kinase A and protein kinase C (PKC)-theta, whose signaling pathways have been implicated in the estradiol regulation of sexual receptivity, were examined. In the ARH sample, the increase in phospho-protein kinase A could not be confirmed by Western blotting, in either cytosolic or membrane fractions. However, the increase in phosphorylated PKCtheta seen with the pathway array was verified by Western blotting. To study whether rapid estradiol activation of PKC regulates the ARH-medial preoptic nucleus pathway regulating lordosis, mu-opioid receptor (MOR) internalization and lordosis reflex were tested. Blocking PKC in ARH with 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]3-(1H-indol-3-yl) maleimide significantly attenuated estradiol-induced MOR internalization. Furthermore, disruption of PKC signaling within the ARH at the time of estradiol treatment significantly diminished the lordosis reflex. Moreover, blocking PKC prevented MOR internalization when the circuit was activated by the mGluR1a agonist, (RS)-3,5-dihydroxyphenylglycine. Activation of PKC with phorbol 12, 13-dibutyrate induced MOR internalization, indicating that PKC was a critical step for membrane ERalpha-initiated mGluR1a-mediated cell signaling and phorbol 12, 13-dibutyrate significantly facilitated the lordosis reflex. Together these findings indicate that rapid membrane ERalpha-mGluR1a interactions activate PKCtheta cell signaling, which regulates female sexual receptivity.

Estrogen and progesterone modulate [35S]GTPgammaS binding to nociceptin receptors

Sex steroids modulate reproduction by altering the response of steroid-activated opioid circuits in the hypothalamus and limbic system, by inducing release of endogenous opioids and activation of their cognate receptors. Many studies have concentrated on steroid regulation of exogenous opioid peptides, but steroids also have important actions on opioid receptors inducing receptor trafficking. Opioid receptors are G protein-coupled receptors and their activation catalyzes the exchange of GTP for GDP initiating intracellular signaling cascades. Kinetics of G protein activation were studied using [(35)S]GTPgammaS binding. Catalytic amplification, the number of G proteins activated per occupied receptor, was used as a measure of receptor/transducer amplification. The present study examined whether estrogen and progesterone treatment altered the kinetics of nociceptin opioid receptor (ORL1) in plasma membranes from the medial preoptic area and mediobasal hypothalamus. These hypothalamic regions are important in the gonadal steroid hormone regulation of sexual receptivity. In the mediobasal hypothalamus, estrogen increased ORL1 (B(max)) receptor number 2-fold and maximal GTPgammaS binding (E(max)) 3.9-fold. Subsequent progesterone treatment further increased ORL1 E(max )6.9-fold above baseline, despite a 2-fold decrease in the catalytic amplification factor. In the medial preoptic area, estrogen alone did not increase E(max), but both estrogen and progesterone were able to increase ORL1 B(max) 2.2-fold and E(max) 3-fold, despite having a 3-fold decrease in the catalytic amplification factor. These effects are interesting because they indicate actions of steroids that increase the number of ORL1 but decrease the catalytic amplification suggesting that the steroid effects on opioid receptors are complex and may involve modulation by other signals.

Membrane estrogen receptor-alpha interactions with metabotropic glutamate receptor 1a modulate female sexual receptivity in rats

In rats, female sexual behavior is regulated by a well defined limbic-hypothalamic circuit that integrates sensory and hormonal information. Estradiol activation of this circuit results in mu-opioid receptor (MOR) internalization in the medial preoptic nucleus, an important step for full expression of sexual receptivity. Estradiol acts through both membrane and intracellular receptors to influence neuronal activity and behavior, yet the mechanism(s) and physiological significance of estradiol-mediated membrane responses in vivo have remained elusive. Recent in vitro evidence found that stimulation of membrane-associated estrogen receptor-alpha (ER alpha) led to activation of metabotropic glutamate receptor 1a (mGluR1a). Furthermore, mGluR1a signaling was responsible for the observed downstream effects of estradiol. Here we present data that show that ER alpha and mGluR1a directly interact to mediate a rapid estradiol-induced activation of MOR in the medial preoptic nucleus, leading to female sexual receptivity. In addition, blockade of mGluR1a in the arcuate nucleus of the hypothalamus resulted in a significant attenuation of estradiol-induced MOR internalization, leading to diminished female sexual behavior. These results link membrane-initiated estradiol actions to neural events modulating behavior, demonstrating the physiological importance of ER alpha-to-mGluR1a signaling.

Neuroprogesterone: key to estrogen positive feedback?

In the cycling female rat, estradiol and progesterone induce reproductive behavior and the surge of luteinizing hormone (LH) needed for ovulation. Circulating estradiol of ovarian origin induces progesterone receptors in the preoptic area and hypothalamus. Sequential activation of estrogen receptors (ER) and progesterone receptors coordinates reproductive physiology and behavior. In ovariectomized and adrenalectomized (ovx/adx) rats, administration of estradiol alone is sufficient to initiate an LH surge, and central infusion of aminoglutethimide (AGT), a blocker of the P450 side chain cleavage enzyme, disrupted the estrous cycle of intact rats without affecting peripheral estradiol levels, suggesting that an endogenous source of progesterone remains in these animals. In ovx/adx rats, progesterone levels in the hypothalamus increase prior to the LH surge, and inhibition of progesterone synthesis prevents the LH surge, suggesting that hypothalamic neuroprogesterone is necessary for estrogen positive feedback. In support of the idea that estradiol induces neuroprogesterone, estradiol increased expression of the progesterone-synthesizing enzyme 3beta-hydroxysteroid dehydrogenase (3beta-HSD) in the hypothalamus before the LH surge. Further, in vitro experiments demonstrate that estradiol stimulates progesterone synthesis in astrocytes, considered to be the most active steroidogenic cells in the CNS. To stimulate neurosteroidogenesis, estradiol acts through membrane ER and type 1a metabotropic glutamate receptors (mGluR1a) to increase free cytoplasmic calcium ([Ca(2+)](i)) via activation of the PLC-IP(3) pathway. Estradiol-induced progesterone synthesis is mimicked by thapsigargin-induced release of IP(3) receptor-sensitive Ca(2+) stores in astrocyte cultures. Thus, estradiol-induced progesterone synthesis is dependent on membrane ERs that act through mGluR1a to activate the PLC-IP(3) pathway. This neuroprogesterone also facilitated proceptive behavior. Blocking either progesterone synthesis or progesterone receptor in estrogen-primed ovx/adx prevented proceptive but not receptive behaviors.

Release of orphanin FQ/nociceptin in the medial preoptic nucleus and ventromedial nucleus of the hypothalamus facilitates lordosis

Opioid regulation of reproduction has been widely studied. However, the role of opioid receptor-like 1 receptor (NOP; also referred to as ORL-1 and OP4) and its endogenous ligand orphanin FQ/nociceptin (OFQ/N) have received less attention despite their extensive distribution throughout nuclei of the limbic-hypothalamic system, a circuit that regulates reproductive behavior in the female rat. Significantly, the expression of both receptor and ligand is regulated in a number of these nuclei by estradiol and progesterone. Activation of NOP in the ventromedial nucleus of the hypothalamus (VMH) of estradiol-primed nonreceptive female rats facilitates lordosis. NOPs are also expressed in the medial preoptic nucleus (MPN), however, their roles in reproductive behavior have not been studied. The present experiments examined the role of NOP in the regulation of lordosis in the MPN and tested whether endogenous OFQ/N in the MPN and VMH mediates reproductive behavior. Activation of NOP by microinfusion of OFQ/N in the MPN facilitated lordosis in estradiol-primed sexually nonreceptive female rats. Passive immunoneutralization of OFQ/N in either the MPN or the VMH reduced lordosis in estradiol-primed females, but had no effect on lordosis in estradiol+progesterone-primed sexually receptive rats. These studies suggest that OFQ/N has a central role in estradiol-only induced sexual receptivity, and that progesterone appears to involve additional circuits that mediate estradiol+progesterone sexual receptivity.

Site-specific estrogen and progestin regulation of orphanin FQ/nociceptin and nociceptin opioid receptor mRNA expression in the female rat limbic hypothalamic system

The distributions of orphanin FQ (OFQ/N; also known as nociceptin) and its cognate receptor, opioid receptor-like receptor-1 (NOP), overlap steroid-responsive regions throughout reproductive circuits of the limbic system and hypothalamus. For example, in the ventromedial nucleus of the hypothalamus (VMH), OFQ/N facilitates lordosis in female rats through estrogen and progesterone regulation of nociceptin activity. We studied estrogen and progesterone regulation of OFQ/N and NOP mRNA expression in limbic-hypothalamic reproductive circuits. Ovariectomized rats were treated with 17beta-estradiol-benzoate (2 microg) and 26 hours later with oil or progesterone (500 microg) and were killed 30 hours after initial treatment. Alternate brain sections were processed for OFQ/N or NOP mRNA in situ hybridization. High levels of hybridization for NOP and OFQ/N and overlapping distributions were observed throughout the limbic hypothalamic reproductive circuits; however, in VMH, only NOP expression was observed. Estrogen treatment increased NOP mRNA expression in anteroventral periventricular nucleus (AVPV), median preoptic nucleus, and VMH. Subsequent progesterone treatment did not alter estrogen-induced expression of NOP mRNA in VMH or median preoptic nucleus but reduced expression in the AVPV. OFQ/N mRNA levels were also regulated by steroids. In the caudal part of the posterodorsal medial amygdala, estrogen increased OFQ/N mRNA levels, and progesterone did not alter this increase, whereas, in the medial part of the medial preoptic nucleus, estrogen and progesterone were needed to increase OFQ/N mRNA levels. Steroid regulation of OFQ/N and NOP in the medial preoptic nucleus and VMH is consistent with emerging data indicating that this opioid system regulates female reproduction.

Estrogen and CCK1 receptor modification of mu-opioid receptor binding in the cortex of female rats

Cholecystokinin (CCK) in the nervous system has effects opposite to those of opioids. However, the mechanism by which CCK opposes the effect of opioids at the receptor or cellular level is still unknown. In the brain, distributions of CCK receptors and opioid receptors have been demonstrated to overlap. The present study was undertaken to determine the mechanism of CCK-opioid interactions in the cortex of ovariectomized rats. Furthermore, because estrogen is a powerful regulator of CCK and opioid activity, we examined whether estrogen state also modulates the interactions of these neuropeptides. mu-Opioid (MOP) receptor binding was examined in cortical membranes that were preincubated with CCK-8S and CCK receptor agonist and antagonist followed with 3H-DAMGO. Pharmacological results revealed that CCK-8S suppressed 3H-DAMGO binding in cortical membranes of ovariectomized rats. The same result was obtained using a CCK1 receptor agonist (JMV-180), whereas a CCK2 receptor agonist (CCK-4) failed to suppress 3H-DAMGO binding. Antagonism of the CCK1 receptor by JMV-179 blocked both CCK-8S and JMV-180 suppression of 3H-DAMGO binding. Furthermore, estrogen treatment to female rats resulted in a suppression of 3H-DAMGO binding in cortical membranes. These results demonstrate an estrogen regulation of the MOP receptor and a protein-protein interaction between CCK1 receptor and MOP receptor.

Sexual receptivity is reduced in the female mu-opioid receptor knockout mouse

Activation of mu-opioid receptors is critical to steroid regulation of female sexual behavior, lordosis, in rodents. Estrogen treatment activates mu-opioid receptors in the medial preoptic area inhibiting lordosis, but ultimately appears important for progesterone facilitation of lordosis. We investigated the role of mu-opioid receptors in the expression of sexual receptivity in mice lacking mu-opioid receptors. Although estrogen and progesterone facilitated lordosis in mu-opioid receptor knockout mice, they exhibited deficits in lordosis quotient and score compared with wild-type females, indicating reduced sexual receptivity. In contrast, wild-type and mu-opioid receptor knockout female mice did not differ in either active or passive avoidance of the male. These data are most consistent with the hypothesis that mu-opioid receptor activation is necessary for estrogen and progesterone to maximally facilitate lordosis.

Detection of beta-endorphin in the cerebrospinal fluid after intrastriatal microinjection into the rat brain

We have investigated to what extent microinjected beta-endorphin could migrate from the rat brain parenchyma into the CSF compartment. Exogenous rat beta-endorphin (0.1 nmol) was microinjected into the left striatum 1 mm from the lateral ventricle in anesthetized male rats. CSF samples were collected at different time points up to 2 h post-injection from a catheter affixed to the atlanto-occipital membrane of the cisterna magna. Radioimmunoassay and mass spectrometry were performed on the CSF samples, and brain sections were immunostained for beta-endorphin and mu-opioid receptors. The beta-endorphin injected rats showed a marked increase in beta-endorphin immunoreactive (IR) material in the CSF, with a peak at 30-45 min post-injection, and this beta-endorphin-IR material existed mainly as the intact beta-endorphin peptide. The immunohistochemistry results revealed the appearance of distinct beta-endorphin-IR cell bodies in the globus pallidus and the bed nucleus of stria terminalis supracapsular part, regions distant from the injection site, at 2 h post-injection of exogenous beta-endorphin. The beta-endorphin-IR in several of the globus pallidus cell bodies colocalized with the mu-opioid receptor-IR at the cell surface. These findings show that upon delivery of synthetic beta-endorphin, there is a significant intracerebral spread of the injected peptide, reaching regions far from the site of injection via diffusion in the extracellular space and flow in the cerebrospinal fluid. This may be of relevance when interpreting studies based on intracerebral injections of peptides, and advances our knowledge regarding the migration of compounds within the brain.

Different stages in the temporal course of estrogen treatment produce opposite effects on voluntary alcohol consumption in male rats

The purpose of this study was to examine alcohol consumption in different stages of the estrogen treatment. Three groups of castrated male Wistar rats were used. One group was treated with 5 microg of estradiol benzoate (E) per day per rat for 6 days and oil from days 7 to 12 (EO group). The second group was treated with oil for 6 days and E from days 7 to 12 (OE) and the third with E for 12 consecutive days (EE). The three groups were exposed to a choice of both water and ethanol (10%) before treatment (PreT), from days 7 to 12 of the oil or the E treatment (T2), and during 6 additional days in the post-treatment period (PosT). Alcohol was not available from days 1 to 6 of the oil or the E treatment (T1). Alcohol consumption in the EO group during T2 was higher than in PreT and PosT periods and all periods in the other two groups. In contrast, alcohol consumption during T2 was significantly lower than during the PreT of the OE group and T2 of the EE group. At the same time in the EE group, alcohol intake in the T2 was higher than in the PreT and the PosT periods. These results reveal the opposite effects of estrogen treatment on alcohol consumption, which apparently depended on the physiological conditions produced by the temporal course of hormone treatment.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Medial preoptic area δ-opioid receptors inhibit lordosis

Endogenous opioid peptides that activate the delta-opioid receptor (DOR) are thought to facilitate female receptive behavior. This facilitation of lordosis has been demonstrated by intracerebroventricular infusions and injection of DOR-active ligands into the ventromedial hypothalamic nucleus, an area with robust DOR binding. However, DOR binding is distributed throughout the hypothalamus, and the role of DOR in other areas of the hypothalamus has not been examined. In the current study, we demonstrated DOR immunoreactivity in the medial preoptic area (MPO), in particular medial preoptic nucleus (MPN) of the preoptic area. DOR immunoreactive processes were sparsely distributed in the medial and lateral parts of the MPN. Larger DOR immunoreactive fibers were localized in the ventrolateral aspect of the lateral MPN. The MPN is involved in the modulation of female sexual receptivity and the distribution of DOR in this area suggested to us that DOR may regulate lordosis. Ovariectomized rats with unilateral cannulae aimed at the MPN were given 5microg 17beta-estradiol benzoate (EB), once every 4 days and tested for lordosis. [D-Pen(2), D-Pen(5)]-enkephalin (DPDPE), a DOR agonist, microinfused into the MPO, 52-54h after EB-priming, inhibited lordosis when compared with the aCSF (vehicle) control (P <== 0.05). The inhibitory effects of DPDPE were reversed by microinjection of naltrindole, a DOR antagonist (P <== 0.05). Interestingly, the DOR inhibition of lordosis is similar to the micro-opioid receptor inhibition of lordosis in the MPN. These results indicate that DOR in the MPO, particularly in the MPNm, plays an important role in the regulation of lordosis.

Estrogen-induced mu-opioid receptor internalization in the medial preoptic nucleus is mediated via neuropeptide Y-Y1 receptor activation in the arcuate nucleus of female rats

The endogenous peptides beta-endorphin (beta-END) and neuropeptide Y (NPY) have been implicated in regulating sexual receptivity. Both beta-END and NPY systems are activated by estrogen and inhibit female sexual receptivity. The initial estrogen-induced sexual nonreceptivity is correlated with the activation and internalization of mu-opioid receptors (MORs), in the medial preoptic nucleus (MPN). Progesterone reverses the estrogen-induced activation/internalization of MOR and induces the sexual receptive behavior lordosis. To determine whether NPY and endogenous opioids interact, we tested the hypothesis that estrogen-induced MOR activation is mediated through NPY-Y1 receptor (Y1R) activation. Retrograde tract tracing demonstrated Y1Ron beta-END neurons that projected to the MPN. Sex steroid modulation of MOR in the MPN acts through NPY and the Y1R. Estradiol administration or intracerebroventricular injection of NPY activated/internalized Y1R in the arcuate nucleus and MOR in the MPN of ovariectomized (OVX) rats. Moreover, the selective Y1R agonist [Leu31, Pro34]-Neuropeptide Y (LPNY) internalized MOR in the MPN of OVX rats. The Y1R antagonist (Cys31, Nva34)-Neuropeptide Y (27-36)2 prevented estrogen-induced Y1R and MOR activation/internalization. NPY reversed the progesterone blockade of estradiol-induced Y1R and MOR internalization in the arcuate nucleus and MPN, respectively. Behaviorally, LPNY inhibited estrogen plus progesterone-induced lordosis, and the MOR-selective antagonist D-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr amide reversed LPNY-induced inhibition of lordosis. These results suggest that a sequential sex steroid activation of NPY and MOR circuits regulates sexual receptivity.

Activation of μ opioid receptors in the medial preoptic area following copulation in male rats

The current study tested the hypothesis that sexual behavior is a biological stimulus for release of endogenous opioid peptides. In particular, activation of mu opioid receptors (MOR) in the medial preoptic area (MPOA), a key area for regulation of male sexual behavior, was studied in male rats. MOR endocytosis or internalization was used as a marker for ligand-induced receptor activation, utilizing confocal, electron, and bright microscopic analysis. Indeed, mating including one ejaculation induced receptor activation in the MPOA, demonstrated by increased immunoreactivity for MOR, increased numbers of endosome-like particles immunoreactive for MOR inside the cytoplasm of neurons, and increased percentage of neurons with three or more endosome-like particles inside the cytosol. Moreover, it was demonstrated that MOR activation occurred within 30 min following mating and was still evident after 6 h. Mating-induced internalization was prevented by treatment with the opioid receptor antagonist naloxone before mating, suggesting that mating-induced receptor activation is a result of action of endogenous MOR ligands. i.c.v. injections of MOR ligand [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin resulted in internalization of the MOR in a similar manner observed following mating. Finally, mating induced Fos expression in MOR containing neurons in the MPOA. However, naloxone pretreatment did not prevent Fos activation of MOR neurons, suggesting that Fos induction was not the result of MOR activation. In summary, these results provide further evidence that endogenous opioid peptides are released in the MPOA during male sexual behavior.