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

Conditioned preferences: Gated by experience, context, and endocrine systems

Central to the navigation of an ever-changing environment is the ability to form positive associations with places and conspecifics. The functions of location and social conditioned preferences are often studied independently, limiting our understanding of their interplay. Furthermore, a de-emphasis on natural functions of conditioned preferences has led to neurobiological interpretations separated from ecological context. By adopting a naturalistic and ethological perspective, we uncover complexities underlying the expression of conditioned preferences. Development of conditioned preferences is a combination of motivation, reward, associative learning, and context, including for social and spatial environments. Both social- and location-dependent reward-responsive behaviors and their conditioning rely on internal state-gating mechanisms that include neuroendocrine and hormone systems such as opioids, dopamine, testosterone, estradiol, and oxytocin. Such reinforced behavior emerges from mechanisms integrating past experience and current social and environmental conditions. Moreover, social context, environmental stimuli, and internal state gate and modulate motivation and learning via associative reward, shaping the conditioning process. We highlight research incorporating these concepts, focusing on the integration of social neuroendocrine mechanisms and behavioral conditioning. We explore three paradigms: 1) conditioned place preference, 2) conditioned social preference, and 3) social conditioned place preference. We highlight nonclassical species to emphasize the naturalistic applications of these conditioned preferences. To fully appreciate the complex integration of spatial and social information, future research must identify neural networks where endocrine systems exert influence on such behaviors. Such research promises to provide valuable insights into conditioned preferences within a broader naturalistic context.

Opioid Receptor Mu 1 Gene (OPRM1) A118G Polymorphism and Emotional Modulation of Pain

Purpose

The A118G polymorphism in the opioid receptor mu 1 gene (OPRM1) is associated with decreased opioid receptor availability, altered emotion, and increased pain. Given that emotions modulate pain (positive emotions inhibit pain, negative emotions enhance pain), we predicted that G allele carriers would experience impaired emotional modulation of pain compared to non-G allele carriers.

Patients and Methods

Emotional pictures (ie, erotica, neutral, attack) from the International Affective Picture System were used by permission from the authors to experimentally manipulate emotions in 64 adult participants while painful electrocutaneous stimulations were delivered in a cross-sectional study. Ratings of arousal and valence/pleasure were made in response to pictures, and pain ratings and a physiological measure of spinal nociception (ie, nociceptive flexion reflex, NFR) were collected in response to painful stimulations. Secondary analyses were conducted to examine the relationship between the A118G polymorphism and emotional modulation of pain/NFR.

Results

Exposure to emotional pictures elicited similar changes in valence, but G-carriers rated erotic pictures as more arousing. In non-carriers, pain was facilitated by attack pictures and pain and NFR were inhibited by erotic pictures relative to neutral pictures. Among G-carriers, pain was facilitated by negative emotional pictures but there was no pain or NFR inhibition by positive emotional pictures.

Conclusion

The altered response to pleasant stimuli further supports the role of opioids in appetitive behavior and describes how the A118G polymorphism may prevent carriers from inhibiting pain during pleasure.

Endogenous Opioid Release After Orgasm in Man: A Combined PET/Functional MRI Study

The endogenous μ-opioid receptor (MOR) system plays a key role in the mammalian reward circuit. Human and animal experiments suggest the involvement of MORs in human sexual pleasure, yet this hypothesis currently lacks in vivo support. Methods: We used PET with the radioligand [¹¹C]carfentanil, which has high affinity for MORs, to quantify endogenous opioid release after orgasm in man. Participants were scanned once immediately after orgasm and once in a baseline state. Hemodynamic activity was measured with functional MRI during penile stimulation. Results: The PET data revealed significant opioid release in the hippocampus. Hemodynamic activity in the somatosensory and motor cortices and in the hippocampus and thalamus increased during penile stimulation, and thalamic activation was linearly dependent on self-reported sexual arousal. Conclusion: Our data show that endogenous opioidergic activation in the medial temporal lobe is centrally involved in sexual arousal, and this circuit may be implicated in orgasmic disorders.

The effect of intraperitoneal and intra-RMTg infusions of CTAP on rats' social interaction

Social interaction is necessary for the development of individuals and society. Social interaction behaviors are rewarding. Similar to exogenous opioids, social interaction behaviors are able to induce rewarding effects that are regulated by the endogenous opioid system as well. As one type of opioid receptor, μ-opioid receptors (MORs), are densely expressed in the rostromedial tegmental nucleus (RMTg), which results in the RMTg being extremely sensitive to rewarding effects induced by exogenous and endogenous opioids. Here, we investigated how RMTg MORs played a role in rewarding effects induced by social interaction behaviors of male Wistar rats, using a conditioned place preference (CPP) model. Results showed that the CPP induced by social interaction behaviors was inhibited when the function of MORs was blocked via injecting CTAP (a selective MOR antagonist) intraperitoneally, and intra-RMTg injections of lower doses of CTAP affected the CPP in the same way. In addition, injecting CTAP intraperitoneally significantly inhibited the expression of pouncing behavior, while intra-RMTg injections of CTAP significantly inhibited the expression of all three types of social behaviors. These results suggest that RMTg MORs may be a crucial target and remain to be further explored in order to better understand the mechanism of the rewarding effects of social interaction behaviors.

Sexual experience increases oxytocin, but not vasopressin, receptor densities in the medial preoptic area, ventromedial hypothalamus, and central amygdala of male rats

Oxytocin (OT) and vasopressin (VP) are considered to be principal neurochemical substrates of bonding in monogamous species. We have reported previously that conditioning of a sexual partner preference in male rats resulted in conditioned activation of OT and VP neurons in hypothalamic paraventricular and supraoptc nuclei. Here we asked whether such conditioning would also alter OT or VP receptor densities. Sexually naïve male rats were assigned to one of three groups (n = 15/group). The Paired group received 9 copulatory training trials with sexually receptive females scented with a neutral almond odor. The Unpaired group received 9 copulatory training trials with unscented sexually receptive females. The Naïve group were not given sexual experience. Paired and Unpaired males were given a final test in an open field with two receptive females, one scented and the other unscented, to assess the development of conditioned ejaculatory preference (CEP), which was expressed significantly in the Paired group. Brains from rats in the three groups were then assessed for OT receptor (OTR) or VP1a receptor (VPR) densities within cortical, limbic and hypothalamic structures using autoradiography with selective 125I-labeled receptor ligands. Sexual experience alone increased OTR significantly in the medial preoptic area (mPOA), ventromedial hypothalamus (VMH), and central nucleus of the amygdala (CeA) in both Paired- and Unpaired-trained males compared to sexually Naïve males. No differences were found for experience on VPR densities in any region. These data add to a growing body of evidence that sexual experience alters brain function and processing of sex-related cues, and suggest that enhanced activation of OTRs in the mPOA, VMH, and CeA by conditioned OT release in those regions may underlie CEP in the male rat.

Behavioral, Neural, and Molecular Mechanisms of Conditioned Mate Preference: The Role of Opioids and First Experiences of Sexual Reward

Although mechanisms of mate preference are thought to be relatively hard-wired, experience with appetitive and consummatory sexual reward has been shown to condition preferences for partner related cues and even objects that predict sexual reward. Here, we reviewed evidence from laboratory species and humans on sexually conditioned place, partner, and ejaculatory preferences in males and females, as well as the neurochemical, molecular, and epigenetic mechanisms putatively responsible. From a comprehensive review of the available data, we concluded that opioid transmission at μ opioid receptors forms the basis of sexual pleasure and reward, which then sensitizes dopamine, oxytocin, and vasopressin systems responsible for attention, arousal, and bonding, leading to cortical activation that creates awareness of attraction and desire. First experiences with sexual reward states follow a pattern of sexual imprinting, during which partner- and/or object-related cues become crystallized by conditioning into idiosyncratic “types” that are found sexually attractive and arousing. These mechanisms tie reward and reproduction together, blending proximate and ultimate causality in the maintenance of variability within a species.

μ-opioid receptor availability is associated with sex drive in human males

The endogenous mu-opioid receptor (MOR) system modulates a multitude of social and reward-related functions, and exogenous opiates also influence sex drive in humans and animals. Sex drive shows substantial variation across humans, and it is possible that individual differences in MOR availability underlie interindividual of variation in human sex drive. We measured healthy male subjects’ (n = 52) brain’s MOR availability with positron emission tomography (PET) using an agonist radioligand, [¹¹C]carfentanil, that has high affinity for MORs. Sex drive was measured using self-reports of engaging in sexual behaviour (sex with partner and masturbating). Bayesian hierarchical regression analysis revealed that sex drive was positively associated with MOR availability in cortical and subcortical areas, notably in caudate nucleus, hippocampus, and cingulate cortices. These results were replicated in full-volume GLM analysis. These widespread effects are in line with high spatial autocorrelation in MOR expression in human brain. Complementary voxel-based morphometry analysis (n = 108) of anatomical MR images provided limited evidence for positive association between sex drive and cortical density in the midcingulate cortex. We conclude that endogenous MOR tone is associated with individual differences in sex drive in human males.

Importance of neuroanatomical data from domestic animals to the development and testing of the KNDy hypothesis for GnRH pulse generation

Work during the last decade has led to a novel hypothesis for a question that is half a century old: how is the secretory activity of GnRH neurons synchronized to produce episodic GnRH secretion. This hypothesis posits that a group of neurons in the arcuate nucleus (ARC) that contain kisspeptin, neurokinin B (NKB), and dynorphin (known as KNDy neurons) fire simultaneously to drive each GnRH pulse. Kisspeptin is proposed to be the output signal to GnRH neurons with NKB and dynorphin acting within the KNDy network to initiate and terminate each pulse, respectively. This review will focus on the importance of neuroanatomical studies in general and, more specifically, on the work of Dr Marcel Amstalden during his postdoctoral fellowship with the authors, to the development and testing of this hypothesis. Critical studies in sheep that laid the foundation for much of the KNDy hypothesis included the report that a group of neurons in the ARC contain both NKB and dynorphin and appear to form an interconnected network capable of firing synchronously, and Marcel's observations that the NKB receptor is found in most KNDy neurons, but not in any GnRH neurons. Moreover, reports that almost all dynorphin-NKB neurons and kisspeptin neurons in the ARC contained steroid receptors led directly to their common identification as "KNDy" neurons. Subsequent anatomical work demonstrating that KNDy neurons project to GnRH somas and terminals, and that kisspeptin receptors are found in GnRH, but not KNDy neurons, provided important tests of this hypothesis. Recent work has explored the time course of dynorphin release onto KNDy neurons and has begun to apply new approaches to the issue, such as RNAscope in situ hybridization and the use of whole tissue optical clearing with light-sheet microscopy. Together with other approaches, these anatomical techniques will allow continued exploration of the functions of the KNDy population and the possible role of other ARC neurons in generation of GnRH pulses.

Anatomical Markers of Activity in Hypothalamic Neurons

The scientific community has searched for years for ways of examining neuronal tissue to track neural activity with reliable anatomical markers for stimulated neuronal activity. Existing studies that focused on hypothalamic systems offer a few options but do not always compare approaches or validate them for dependence on cell firing, leaving the reader uncertain of the benefits and limitations of each method. Thus, in this article, potential markers will be presented and, where possible, placed into perspective in terms of when and how these methods pertain to hypothalamic function. An example of each approach is included. In reviewing the approaches, one is guided through how neurons work, the consequences of their stimulation, and then the potential markers that could be applied to hypothalamic systems are discussed. Approaches will use features of neuronal glucose utilization, water/oxygen movement, changes in neuron-glial interactions, receptor translocation, cytoskeletal changes, stimulus-synthesis coupling that includes expression of the heteronuclear or mature mRNA for transmitters or the enzymes that make them, and changes in transcription factors (immediate early gene products, precursor buildup, use of promoter-driven surrogate proteins, and induced expression of added transmitters. This article includes discussion of methodological limitations and the power of combining approaches to understand neuronal function. © 2020 American Physiological Society. Compr Physiol 10:549-575, 2020.

Differential disruption of conditioned ejaculatory preference in the male rat based on different sensory modalities by micro-infusions of naloxone to the medial preoptic area or ventral tegmental area

RATIONALE

Male rats trained to associate a neutral odor or rodent jacket on a female with their post-ejaculatory reward state display a preference to ejaculate with females bearing the odor or jacket. This conditioned ejaculatory preference (CEP) can be shifted by systemic administration of the opioid antagonist naloxone (NAL) during training, such that NAL-trained males distribute their ejaculations to females without the cue, relative to saline (SAL)-trained males.

OBJECTIVE

The present study examined two brain sites, the medial preoptic area (mPOA) or ventral tegmental area (VTA), where the opioid reward state might be induced.

METHODS

Sexually naïve Long-Evans males were implanted with bilateral guide cannula aimed at either site before they underwent multi-ejaculatory conditioning trials at 4-day intervals with sexually receptive females that bore either an almond odor or rodent tethering jacket. Infusions of NAL (1 μl/side) or SAL (1 μl/side) were made prior to each conditioning trial. All males were infused with SAL prior to a final open-field choice test with two sexually receptive females, one scented and the other unscented, or one jacketed and the other unjacketed.

RESULTS

Males previously conditioned with SAL in either region showed significant CEP. In contrast, prior infusions of NAL to the mPOA shifted the preference towards the unfamiliar female, whereas prior infusions to the VTA abolished CEP for the odor. Subsequent detection of Fos protein induced by the cue showed that, relative to SAL-treated males, prior experience with NAL in the mPOA suppressed Fos in both the mPOA and VTA, whereas prior experience with NAL in to the VTA suppressed Fos in the VTA alone.

CONCLUSIONS

Opioid antagonism in the mPOA produces a state of non-reward whereas in the VTA, it produces a state in which the odor does not acquire incentive properties.

Establishment of partner preference in male rats: Effect of prenatal letrozole and sexual experience

Repeated testing for masculine sexual behavior influences female sex preference in males. Males perinatally treated with aromatase inhibitors show male preference, but also copulate with the receptive female. Such copulation modifies sex preference most likely because of its rewarding properties. In this study, we intended to equal the incentive value of both stimuli -in the sex preference test- by using receptive females with vaginal occlusion. Vehicle and letrozole-treated (0.56 μg/kg, gestation days 10-21) males were repeatedly tested for sex preference at 40, 55, 70, 85 and 100 days of age. These ages were selected because males of 40 days are unable to copulate, while by 100 days of age almost all males show the complete repertoire of masculine sexual behavior. At 40 days of age, males of all groups fail to show sex preference and none of them was able to copulate. In controls of 100 days of age all males showed female-sex preference and all intromitted the female. A large proportion (44%) of vehicle-treated males that could not copulate the female showed male preference. Twenty to 30% of the prenatally letrozole treated males also had same-sex preference even if they could copulate; and most of them (67%) had a male preference when copulation was precluded. These data support the idea that copulation is crucial for developing a female preference in control animals. The results suggest that brain changes produced by estrogens along early development and stimuli coming from the partner are essential for shaping sex preference.

Randomized, Double-Blind, Parallel-Group, Placebo-Controlled Trial of Bupropion as Treatment for Methadone-Emergent Sexual Dysfunction in Men

Methadone is largely recognized as an effective treatment for opiate-dependent patients; however, it causes reduced brain dopaminergic action resulting in significant sexual dysfunction. Bupropion is a dopamine reuptake inhibitor which can potentially improve erectile function among male patients on methadone (MMT). This is a phase II, randomized, double-blind, parallel-group, placebo-controlled trial, involving 80 MMT male patients (73.4%) with mean age of 42.83 years ±9.68. These MMT male patients were randomly assigned into two groups to receive bupropion and placebo, respectively. The primary efficacy outcome measure was the difference between the two groups in end-point mean improvement scores using the measurement of Clinical Global Impression Scale adapted for Sexual Function (CGI-SF) at baseline (week 0) and at weeks 2, 4, and 6. Malay version of the sexual desire inventory-2 (SDI-2-BM) and Malay version of International Index of Erectile Function 15 (Mal-IIEF-15) domain scores were evaluated as secondary parameters. Improvement of the end-point mean from baseline were seen across the scores of SDI-2-BM (mean difference = 11.77 ± 2.90, 95% confidence interval (CI) [3.89, 19.54], p < .001) and Mal-IIEF-15 (mean difference = 8.37 ± 2.71, 95% CI [15.75, 0.99], p = .02), and the total plasma testosterone level (mean difference = 4.03, 95% CI [0.90, 7.15], p = .01). A categorical improvement of "much/very much improved" (CGI-SF score = 2) was reported by 58.3% ( n = 21/36) of bupropion SR-assigned versus 27.7% ( n = 10/36) placebo-assigned patient. Bupropion was well tolerated with no serious adverse events reported other than insomnia (17.7%). Six weeks of bupropion SR treatment reported significant improvement in key aspects of sexual function among male opiate-dependent patients on methadone maintenance treatment with emergent sexual dysfunction.

Evidence That Dynorphin Acts Upon KNDy and GnRH Neurons During GnRH Pulse Termination in the Ewe

A subpopulation of neurons located within the arcuate nucleus, colocalizing kisspeptin, neurokinin B, and dynorphin (Dyn; termed KNDy neurons), represents key mediators of pulsatile GnRH secretion. The KNDy model of GnRH pulse generation proposes that Dyn terminates each pulse. However, it is unknown where and when during a pulse that Dyn is released to inhibit GnRH secretion. Dyn acts via the κ opioid receptor (KOR), and KOR is present in KNDy and GnRH neurons in sheep. KOR, similar to other G protein-coupled receptors, are internalized after exposure to ligand, and thus internalization can be used as a marker of endogenous Dyn release. Thus, we hypothesized that KOR will be internalized at pulse termination in both KNDy and GnRH neurons. To test this hypothesis, GnRH pulses were induced in gonad-intact anestrous ewes by injection of neurokinin B (NKB) into the third ventricle and animals were euthanized at times of either pulse onset or termination. NKB injections produced increased internalization of KOR within KNDy neurons during both pulse onset and termination. In contrast, KOR internalization into GnRH neurons was seen only during pulse termination, and only in GnRH neurons within the mediobasal hypothalamus (MBH). Overall, our results indicate that Dyn is released onto KNDy cells at the time of pulse onset, and continues to be released during the duration of the pulse. In contrast, Dyn is released onto MBH GnRH neurons only at pulse termination and thus actions of Dyn upon KNDy and GnRH cell bodies may be critical for pulse termination.

Neurogenesis in the olfactory bulb induced by paced mating in the female rat is opioid dependent

The possibility to control the rate of sexual stimulation that the female rat receives during a mating encounter (pacing) increases the number of newborn neurons that reach the granular layer of the accessory olfactory bulb (AOB). If females mate repeatedly, the increase in the number of neurons is observed in other regions of the AOB and in the main olfactory bulb (MOB). It has also been shown that paced mating induces a reward state mediated by opioids. There is also evidence that opioids modulate neurogenesis. In the present study, we evaluated whether the opioid receptor antagonist naloxone (NX) could reduce the increase in neurogenesis in the AOB induced by paced mating. Ovariectomized female rats were randomly divided in 5 different groups: 1) Control (not mated) treated with saline, 2) control (not mated) treated with naloxone, 3) females that mated without controlling the sexual interaction (no-pacing), 4) females injected with saline before pacing the sexual interaction and 5) females injected with NX before a paced mating session. We found, as previously described, that paced mating induced a higher number of new cells in the granular layer of the AOB. The administration of NX before paced mating, blocked the increase in the number of newborn cells and prevented these cells from differentiating into neurons. These data suggest that opioid peptides play a fundamental role in the neurogenesis induced by paced mating in female rats.

Anatomy and physiology of genital organs - men

Male sexual functions involve a number of organs and structures in genitalia whose role is to produce fertilizing gametes and to allow female-partner insemination. The testes belong to the reproductive and endocrine systems as they synthesize spermatozoa and androgens, and are under finely regulated hormonal control by the hypothalamopituitary axis. Sexual responses are controlled by a complex and coordinated interplay of both the somatic and the autonomic nervous system in multiple components of the brain, spinal cord, and relevant peripheral organs. Erectile bodies are an essential element of the penis and engorgement of the penis with blood leads to penile tumescence. Blood engorgement is due to relaxation of smooth-muscle cells of erectile tissue and endothelium of the penile arteries. The penis gains additional rigidity when the ischiocavernosus muscles contract. Stimuli from peripheral and/or central origins activate particular spinal nuclei, causing penile erection. Ejaculation consists of two phases, emission and expulsion, which correspond, respectively, to secretion of the different components of the semen by sex glands and forceful expulsion of semen due to rhythmic contractions of the bulbospongiosus muscle. A spinal generator of ejaculation integrates genital stimuli and sexual cues and, when the excitatory threshold is reached, triggers ejaculation by orchestrating the activation of autonomic and somatic pathways commanding the peripheral events of ejaculation.

μ-Opioid receptors within subregions of the striatum mediate pair bond formation through parallel yet distinct reward mechanisms

The prairie vole is a socially monogamous rodent that is an excellent animal model for studies of the neurobiology of social attachment. Such studies have demonstrated that activation of reward circuitry during social interactions facilitates pair bond formation. Within this circuitry, μ-opioid receptors (MORs) modulate naturally rewarding behavior in an anatomically segregated manner; MORs located throughout the striatum (dorsal striatum, NAc core, and the entire NAc shell) are implicated in general motivational processes, whereas those located specifically within the dorsomedial NAc shell mediate positive hedonics (and are referred to as a "hedonic hotspot"). The purpose of the present study was to determine whether MORs within these distinct subregions differentially mediate pair bond formation. We first used receptor autoradiography to compare MOR binding densities between these regions. MOR binding was significantly higher in the NAc core and dorsomedial NAc shell compared with the ventral NAc shell. We next used partner preference testing to determine whether MORs within these subregions differentially mediate pair bonding. Blockade of MORs using 1 or 3 μg of H-d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 within the dorsal striatum decreased mating during the cohabitation period and inhibited partner preference formation. In contrast, blockade of MORs within dorsomedial NAc shell inhibited partner preference formation without effecting mating behavior, whereas other regions were not involved. Thus, MORs within the dorsal striatum mediate partner preference formation via impairment of mating, whereas those in the dorsomedial NAc shell appear to mediate pair bond formation through the positive hedonics associated with mating.

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.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Nucleus accumbens μ-opioid receptors mediate social reward

Positive social interactions are essential for emotional well-being and proper behavioral development of young individuals. Here, we studied the neural underpinnings of social reward by investigating the involvement of opioid neurotransmission in the nucleus accumbens (NAc) in social play behavior, a highly rewarding social interaction in adolescent rats. Intra-NAc infusion of morphine (0.05-0.1 μg) increased pinning and pouncing, characteristic elements of social play behavior in rats, and blockade of NAc opioid receptors with naloxone (0.5 μg) prevented the play-enhancing effects of systemic morphine (1 mg/kg, s.c.) administration. Thus, stimulation of opioid receptors in the NAc was necessary and sufficient for morphine to increase social play. Intra-NAc treatment with the selective μ-opioid receptor agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]enkephalin (DAMGO) (0.1-10 ng) and the μ-opioid receptor antagonist Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) (0.3-3 μg) increased and decreased social play, respectively. The δ-opioid receptor agonist DPDPE ([D-Pen(2),D-Pen(5)]-enkephalin) (0.3-3 μg) had no effects, whereas the κ-opioid receptor agonist U69593 (N-methyl-2-phenyl-N-[(5R,7S,8S)-7-(pyrrolidin-1-yl)-1-oxaspiro[4.5]dec-8-yl]acetamide) (0.01-1 μg) decreased social play. Intra-NAc treatment with β-endorphin (0.01-1 μg) increased social play, but met-enkephalin (0.1-5 μg) and the enkephalinase inhibitor thiorphan (0.1-1 μg) were ineffective. DAMGO (0.1-10 ng) increased social play after infusion into both the shell and core subregions of the NAc. Last, intra-NAc infusion of CTAP (3 μg) prevented the development of social play-induced conditioned place preference. These findings identify NAc μ-opioid receptor stimulation as an important neural mechanism for the attribution of positive value to social interactions in adolescent rats. Altered NAc μ-opioid receptor function may underlie social impairments in psychiatric disorders such as autism, schizophrenia, or personality disorders.

Future sexual medicine physiological treatment targets

INTRODUCTION

Sexual function in men and women incorporates physiologic processes and regulation of the central and peripheral nervous systems, the vascular system, and the endocrine system. There is need for state-of-the-art information as there is an evolving research understanding of the underlying molecular biological factors and mechanisms governing sexual physiologic functions.

AIM

To develop an evidence-based, state-of-the-art consensus report on the current knowledge of the major cellular and molecular targets of biologic systems responsible for sexual physiologic function.

METHODS

State-of-the-art knowledge representing the opinions of seven experts from four countries was developed in a consensus process over a 2-year period.

MAIN OUTCOME MEASURES

Expert opinion was based on the grading of evidence-based medical literature, widespread internal committee discussion, public presentation, and debate.

RESULTS

Scientific investigation in this field is needed to increase knowledge and foster development of the future line of treatments for all forms of biological-based sexual dysfunction. This article addresses the current knowledge of the major cellular and molecular targets of biological systems responsible for sexual physiologic function. Future treatment targets include growth factor therapy, gene therapy, stem and cell-based therapies, and regenerative medicine.

CONCLUSIONS

Scientific discovery is critically important for developing new and increasingly effective treatments in sexual medicine. Broad physiologic directions should be vigorously explored and considered for future management of sexual disorders.

Lesions of orexin neurons block conditioned place preference for sexual behavior in male rats

The hypothalamic neuropeptide orexin (hypocretin) mediates reward related to drugs of abuse and food intake. However, a role for orexin in sexual reward has yet to be investigated. Orexin neurons are activated by sexual behavior, but endogenous orexin does not appear to be essential for sexual performance and motivation in male rats. Therefore, the goal of the current study was to test the hypothesis that orexin is critically involved in processing of sexual reward in male rats. First, it was demonstrated following exposure to conditioned contextual cues associated with sexual behavior in a conditioned place preference paradigm that cFos expression is induced in orexin neurons, indicating activation of orexin neurons by cues predicting sexual reward. Next, orexin-cell specific lesions were utilized to determine the functional role of orexin in sexual reward processing. Hypothalami of adult male rats were infused with orexin-B-conjugated saporin, resulting in greater than 80% loss of orexin neurons in the perifornical-dorsomedial and lateral hypothalamus. Orexin lesions did not affect expression of sexual behavior, but prevented formation of conditioned place preference for a sexual behavior paired chamber. In contrast, intact sham-treated males or males with partial lesions developed a conditioned place preference for mating. Orexin lesioned males maintained the ability to form a conditioned place aversion to lithium chloride-induced visceral illness, indicating that orexin lesions did not disrupt associative contextual memory. Overall, these findings suggest that orexin is not essential for sexual performance or motivation, but is critical for reward processing and conditioned cue-induced seeking of sexual behavior.

Mu-opioid receptor densities are depleted in regions implicated in agonistic and sexual behavior in male European starlings (Sturnus vulgaris) defending nest sites and courting females

Social status and resource availability can strongly influence individual behavioral responses to conspecifics. In European starlings, males that acquire nest sites sing in response to females and dominate other males. Males without nest sites sing, but not to females, and they do not interact agonistically with other males. Little is known about the neural regulation of status- or resource-appropriate behavioral responses to conspecifics. Opioid neuropeptides are implicated in birdsong and agonistic behavior, suggesting that opioids may underlie differences in the production of these behaviors in males with and without nest sites. Here, we examined densities of immunolabeled mu-opioid receptors in groups of male starlings. Males that defended nest boxes dominated other males and sang at higher rates when presented with a female than males without nest boxes, independent of testosterone concentrations. Multiple regression analyses showed nest box ownership (not agonistic behavior or singing) predicted the optical density of receptor labeling in the medial bed nucleus of stria terminalis, paraventricular nucleus, ventral tegmental area and the medial preoptic nucleus. Compared to males without nest boxes, males with nest boxes had lower densities of immunolabeled mu-opioid receptors in these regions. Singing additionally predicted the area covered by labeling in the ventral tegmental area. The results suggest that elevated opioid activity in these regions suppresses courtship and agonistic behavioral responses to conspecifics in males without nest boxes. The findings are consistent with a dynamic role for opioid receptors in adjusting social behavior so that it is appropriate given the resources available to an individual.

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.

Neurokinin 3 receptor immunoreactivity in the septal region, preoptic area and hypothalamus of the female sheep: colocalisation in neurokinin B cells of the arcuate nucleus but not in gonadotrophin-releasing hormone neurones

Recent evidence has implicated neurokinin B (NKB) in the complex neuronal network mediating the effects of gonadal steroids on the regulation of gonadotrophin-releasing hormone (GnRH) secretion. Because the neurokinin 3 receptor (NK3R) is considered to mediate the effects of NKB at the cellular level, we determined the distribution of immunoreactive NK3R in the septal region, preoptic area (POA) and hypothalamus of the ewe. NK3R cells and/or fibres were found in areas including the bed nucleus of the stria terminalis, POA, anterior hypothalamic and perifornical areas, dopaminergic A15 region, dorsomedial and lateral hypothalamus, arcuate nucleus (ARC) and the ventral premammillary nucleus. We also used dual-label immunocytochemistry to determine whether a neuroanatomical basis for direct modulation of GnRH neurones by NKB was evident. No GnRH neurones at any rostral-caudal level were observed to contain NK3R immunoreactivity, although GnRH neurones and fibres were in proximity to NK3R-containing fibres. Because NKB fibres formed close contacts with NKB neurones in the ARC, we determined whether these NKB neurones also contained immunoreactive NK3R. In luteal-phase ewes, 64% +/- 11 of NKB neurones colocalised NK3R. In summary, NK3R is distributed in areas of the sheep POA and hypothalamus known to be involved in the control of reproductive neuroendocrine function. Colocalisation of NK3R in NKB neurones of the ARC suggests a potential mechanism for the autoregulation of this subpopulation; however, the lack of NK3R in GnRH neurones suggests that the actions of NKB on GnRH neurosecretory activity in the ewe are mediated indirectly via other neurones and/or neuropeptides.

Evidence for opioid involvement in the motivation to sing

Songbirds produce high rates of song within multiple social contexts, suggesting that they are highly motivated to sing and that song production itself may be rewarding. Progress has been made in understanding the neural basis of song learning and sensorimotor processing, however little is known about neurobiological mechanisms regulating the motivation to sing. Neural systems involved in motivation and reward have been conserved across species and in songbirds are neuroanatomically well-positioned to influence the song control system. Opioid neuropeptides within these systems play a primary role in hedonic reward, at least in mammals. In songbirds, opioid neuropeptides and receptors are found throughout the song control system and within several brain regions implicated in both motivation and reward, including the medial preoptic nucleus (POM) and ventral tegmental area (VTA). Growing research shows these regions to play a role in birdsong that differs depending upon whether song is sexually motivated in response to a female, used for territorial defense or sung as part of a flock but not directed towards an individual (undirected song). Opioid pharmacological manipulations and immunocytochemical data demonstrate a role for opioid activity possibly within VTA and POM in the regulation of song production. Although future research is needed, data suggest that opioids may be most critically involved in reinforcing song that does not result in any obvious form of immediate externally mediated reinforcement, such as undirected song produced in large flocks or during song learning. Data are reviewed supporting the idea that dopamine activity underlies the motivation or drive to sing, but that opioid release is what makes song production rewarding.

Maternal separation leads to persistent reductions in pain sensitivity in female rats

We determined responses to noxious thermal stimuli, before and after morphine, and mu-opioid receptor binding in brain regions involved in nociception in maternally separated (MS), neonatally handled (H) and nonhandled (NH) female rats. Long-Evans dams were randomly assigned to either 180-minute (MS) or 15-minute (H) minute daily separations from their litters or left undisturbed (NH). At 120 days of age, paw lick latency (50 degrees C hot plate) was determined in offspring during diestrous. Rats were then given 1, 2, 5, or 10 mg/kg morphine and paw lick latency was measured. Rats were killed during diestrous and mu-opioid receptor binding was determined in discrete brain regions, using [(3)H]DAMGO autoradiography. MS rats had significantly longer (P < .05) paw lick latencies compared with H rats. The percent maximal possible effect of morphine was significantly (P < .05) lower in MS compared with H rats for the 5 mg/kg dose. Mu-Opioid receptor binding capacity was significantly greater (P < .05) in MS rats compared with H rats in the medial preoptic nucleus. In conclusion, MS and H treatments led to antipodal differences in pain sensitivity in female rats and differential mu-opioid receptor binding in the medial preoptic nucleus.

PERSPECTIVE

This article describes the persistent impact of early life adversity on pain sensitivity and the analgesic potency of morphine. Clinically, early life history may play an important role in pain symptoms and responses to opioid analgesics.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Opioid receptor function in social attachment in young domestic fowl

Opioid systems are implicated in social attachment processes. This research sought to determine the functional contribution of each opioid receptor in modulating social attachment/separation distress. Following ICV administration of opiate probes, 7-day-old cockerels were isolated from conspecifics for a 3 min test period under either a mirror or no-mirror condition. Vocalizations served as the measure of separation-stress. Opioid receptor probes included: the mu agonist DAMGO (0.02, 0.19, 1.95 nmol), the mu antagonist CTOP (0.009, 0.09, 0.9 nmol), the delta agonist SNC80 (0.3, 1.0, 3.0 micromol), the delta antagonist naltrindole (0.2, 2.2, 22.2 nmol), the kappa agonist U50, 488 (1, 30, 100 nmol), the kappa antagonist norBNI (1.3, 13.6, 136.1 nmol), the NOP agonist N/OFQ (0.01, 0.1, 1.0 nmol), and the NOP antagonist UFP-101 (0.1, 1.0, 10.0 nmol). DAMGO attenuated separation distress vocalizations. No other drug probe enhanced or attenuated distress vocalizations. Further, the non-selective opiate antagonist naloxone (0.3, 8.3, 27.5 nmol) did not exacerbate distress vocalizations. These results suggest that only the mu receptor modulates social attachment in young domestic fowl.

Ejaculation induces long-lasting behavioural changes in male rats in the forced swimming test: evidence for an increased sensitivity to the antidepressant desipramine

Copulation to exhaustion induces a sexual inhibitory state featured by a decreased motivation. Since diminished motivation is a key symptom of depression, we analysed if sexually exhausted animals showed increased levels of depressive-like behaviour in the forced swimming test (FST). Besides, sexual activity has been reported to have reinforcing properties. Thus, we analysed whether different levels of sexual activity modified the development of the behavioural despair in the FST. Finally, the effect of a sub-threshold dose of desipramine (DMI, 2.5mg/kg) was evaluated in animals with different sexual conditions. Male adult rats were divided into: (a) naive rats and (b) animals executing one intromission (1-INTR) or (c) one ejaculation (1-EJ) and (d) sexually satiated rats, classified as sexually responsive (R) and non-responsive (NR). No differences were found in immobility behaviour between sexually exhausted and naive rats. In the pre-test sessions of the FST males attaining ejaculation (1-EJ, R and NR) had lower levels of immobility or showed a tendency towards such a diminution, while animals with sexual activity not involving ejaculation did not. Data suggest a "protective" effect of ejaculation against the development of depressive-like behaviour. Finally, the sub-effective dose of DMI produced an antidepressant-like action in all animals that ejaculated. In conclusion, sexual experience involving ejaculation modifies the behavioural expression of rats in the FST and their sensitivity to antidepressant drugs like DMI. Both responses reflect brain plastic changes induced by ejaculation.

Central regulation of ejaculation

Ejaculation is a reflex mediated by a spinal control center, referred to as a spinal ejaculation generator. This spinal ejaculation generator coordinates sympathetic, parasympathetic and motor outflow to induce the two phases of ejaculation, i.e., emission and expulsion. In addition, the spinal ejaculation generator integrates this outflow with inputs that are related to the summation of sexual activity prior to ejaculation that are required to trigger ejaculation. Recently, a group of spinothalamic neurons in the lumbar spinal cord (LSt cells) were demonstrated to comprise an integral part of the spinal ejaculation generator. Specifically, lesions of LSt cells completely ablate ejaculatory function. Moreover, LSt cells are activated following ejaculation, but not following other components of sexual behavior. Furthermore, based on their relationship with autonomic nuclei, motoneurons and genital sensory inputs, LSt cells are also in the ideal anatomical position to integrate sensory inputs and autonomic and motor outflow. Additionally, the spinal ejaculation generator is under inhibitory and excitatory influence of supraspinal sites, including the nucleus paragigantocellularis (nPGi), the paraventricular nucleus of the hypothalamus (PVN) and the medial preoptic area (MPOA). Finally, sensory information related to ejaculation is processed in the spinal cord and brain, possibly contributing to the rewarding properties of ejaculation. One candidate pathway for relay of ejaculation-related cues consists of LSt cells and their projections to the parvocellular subparafascicular thalamic nucleus. Moreover, neural activation specifically related to ejaculation is observed in the brain and may reflect of processing of ejaculation-related sensory cues.