Oxytocin in the medial preoptic area facilitates male sexual behavior in the rat.

Oxytocin (OT) is a versatile neuropeptide that is involved in a variety of mammalian behaviors, and its role in reproductive function and behavior has been well established. The majority of pharmacological studies of the effects of OT on male sexual behavior have focused on the paraventricular nucleus (PVN), ventral tegmental area (VTA), hippocampus, and amygdala. Less attention has been given to the medial preoptic area (MPOA), a major integrative site for male sexual behavior. The present study investigated the effects of intra-MPOA administration of OT and (d(CH2)51, Tyr(Me)2, Thr4, Orn8, Tyr-NH29)-vasotocin, an OT antagonist (OTA), on copulation in the male rat. The relationship between OT receptor (OTR) binding levels in the MPOA and sexual efficiency was also explored. Microinjection of OT into the MPOA facilitated copulation in sexually experienced male rats, whereas similar injections of an OTA inhibited certain aspects of copulation but had no significant effect on locomotor activity in an open field. Contrary to expectation, sexually efficient males had lower levels of OTR binding in the rostral MPOA compared to inefficient animals. The present data suggest that OT activity in the MPOA is not necessary for the expression of male sexual behavior but is sufficient to facilitate copulatory behaviors and improve sexual efficiency in sexually experienced male rats. These data also suggest that OTR activity in the MPOA stimulates anogenital investigation, facilitates the initiation of copulation, and plays a role in the sensitization effect of the first ejaculation on subsequent ejaculations.

Neuroendocrine Regulation of Male Sexual Behavior.

The major brain areas that control males' sexual motivation and performance include the amygdala, the bed nucleus of the stria terminalis (BNST), the medial preoptic area (MPOA), and paraventricular nucleus (PVN) of the hypothalamus, as well as the mesolimbic dopamine (DA) system. The MPOA, PVN, and brain stem and spinal nuclei control the genital reflexes. Sensory and motor aspects are integrated and elicited by the amygdala, BNST, MPOA, PVN, and the mesolimbic and nigrostriatal DA tracts, which are integral for other social behaviors, as well. Developmental hormonal effects organize the network to elicit specific behaviors, which are activated by those hormones in adolescence and adulthood. Steroid hormones primarily work through slow genomic mechanisms that increase enzymes, receptors, or structural proteins, although they may also activate membrane receptors for rapid effects. © 2019 American Physiological Society. Compr Physiol 9:1383-1410, 2019.

A role for hypocretin (orexin) in male sexual behavior.

The role of hypocretin (orexin; hcrt/orx) neurons in regulation of arousal is well established. Recently, hcrt/orx has been implicated in food reward and drug-seeking behavior. We report here that in male rats, Fos immunoreactivity (ir) in hcrt/orx neurons increases markedly during copulation, whereas castration produces decreases in hcrt/orx neuron cell counts and protein levels in a time course consistent with postcastration impairments in copulatory behavior. This effect was reversed by estradiol replacement. Immunolabeling for androgen (AR) and estrogen (ER alpha) receptors revealed no colocalization of hcrt/orx with AR and few hcrt/orx neurons expressing ER alpha, suggesting that hormonal regulation of hcrt/orx expression is via afferents from neurons containing those receptors. We also demonstrate that systemic administration of the orexin-1 receptor antagonist SB 334867 [N-(2-methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl urea] impairs copulatory behavior. One locus for the prosexual effects of hcrt/orx may be the ventral tegmental area (VTA). We show here that hcrt-1/orx-A produces dose-dependent increases in firing rate and population activity of VTA dopamine (DA) neurons in vivo. Activation of hcrt/orx during copulation, and in turn, excitation of VTA DA neurons by hcrt/orx, may contribute to the robust increases in nucleus accumbens DA previously observed during male sexual behavior. Subsequent triple immunolabeling in anterior VTA showed that Fos-ir in tyrosine hydroxylase-positive neurons apposed to hcrt/orx fibers increases during copulation. Together, these data support the view that hcrt/orx peptides may act in a steroid-sensitive manner to facilitate the energized pursuit of natural rewards like sex via activation of the mesolimbic DA system.

Preoptic glutamate facilitates male sexual behavior.

The medial preoptic area (MPOA) is a critical regulatory site for the control of male sexual behavior. We first measured glutamate in 2 min microdialysate samples from the MPOA before, during, and after copulation by male rats. There was a slight [approximately 140% of baseline (BL)] rise in extracellular glutamate when the female was presented, a significant increase (approximately 170% of BL) during periods of mounting and intromitting, and a very large increase in samples collected during ejaculation (approximately 300% of BL). A precipitous fall in levels occurred in the first postejaculatory sample; the magnitude of this fall was highly correlated with the length of the postejaculatory interval of quiescence. In experiment 2, we reverse-dialyzed a mixture of glutamate uptake inhibitors into the MPOA before and during mating; control animals received artificial CSF. The mixture increased extracellular glutamate (approximately 280% of BL), increased the number of ejaculations in the 40 min test, decreased ejaculation latency, and decreased the postejaculatory latency to resume copulation. These data, together with other findings that glutamate in the MPOA can elicit genital reflexes in anesthetized rats and that glutamate receptor antagonists in the MPOA impair copulation, strongly suggest that MPOA glutamate is a major facilitator of copulation and that the postejaculatory fall in glutamate regulates the postejaculatory interval.

Melanin concentrating hormone and estrogen receptor-alpha are coexstensive but not coexpressed in cells of male rat hypothalamus.

In male rats, estradiol (E(2)) exerts marked anorectic effects. One mechanism proposed for this effect is an E(2)-mediated down-regulation of the orexigenic neuropeptide melanin concentrating hormone (MCH). Previous anatomical work has shown that both MCH and estrogen receptor alpha (ERalpha) are found in quantity in the lateral hypothalamic area (LHA), a structure long associated with appetite and ingestive behavior. It has been hypothesized that the most direct manner by which E(2) could affect MCH expression and feeding would be via classical nuclear ERalpha located in MCH neurons. To evaluate this notion, we performed double-label immunohistochemistry for MCH and ERalpha in male rat hypothalamus. We report here that MCH neurons do not contain ERalpha, suggesting that the primary locus for estrogenic control of feeding is not the MCH neurons themselves. Rather, we show substantial overlap in the anatomical distribution of both cell types, raising the possibility that E(2) influences MCH signaling indirectly via adjacent ERalpha-containing cells.

Increased expression of carbon monoxide-producing enzymes in the MPOA after sexual experience in male rats.

The hypothalamus contains numerous nuclei involved in the regulation of reproductive, stress, circadian, and homeostatic behaviors, with many of these nuclei concentrated within the preoptic and anterior regions. The gaseous neurotransmitter, nitric oxide (NO), has already been shown to have an important regulatory role within the medial preoptic area (MPOA) of the anterior hypothalamus, where it facilitates sexual behaviors. However, little is known about the role of other gaseous neurotransmitters in this area. Here, we report that the carbon monoxide (CO) producing enzymes HO-1 and HO-2 are present in the MPOA and are differentially influenced by sexual experience in a manner similar to that previously reported for NO enzymes. Immunohistochemical staining of brains collected after 0, 1, or 7 sexual experiences reveals that HO-1 is expressed transiently after the first sexual experience, while HO-2 increases only with repeated experience. This increase appears to be specific to the MPOA, as nearby brain areas do not exhibit this degree or pattern of expression. We observed a transient increase in HO-2 colocalization with neuronal nitric oxide synthase (nNOS) after a single sexual experience, but these cells appear to be otherwise disparate, despite the fact that both express within the central nucleus of the MPOA. Together, these findings suggest that endogenous CO may be behaviorally relevant within the MPOA and that CO and NO may be differentially regulated there.

Sexual experience increases nitric oxide synthase in the medial preoptic area of male rats.

Nitric oxide in the medial preoptic area (MPOA) is important for the expression and sensitization of male sexual behavior. In this article, the authors report that repeated sexual experience (mating for 2 hr on each of 3 days) increased levels of nitric oxide synthase (NOS) in the MPOA of male rats, regardless of whether they mated on the day they were given an overdose of sodium phenobarbital. This effect resulted from the previous experience and not acute mating, as NOS was not increased 2 hr after the first mating in previously naive males. Experience-induced increases in NOS in the MPOA may be one mechanism through which sexual experience facilitates sexual behavior in male rats.

Getting his act together: roles of glutamate, nitric oxide, and dopamine in the medial preoptic area.

Gonadal hormones have primarily slow, genomically mediated effects, but copulation requires rapid interactions with a partner. A major way in which hormones facilitate male sexual behavior is by increasing production of neurotransmitter receptors or of enzymes that regulate neurotransmitter synthesis or release. Dopamine is an important facilitative neurotransmitter, and the medial preoptic area (MPOA) is a critical integrative site for male sexual behavior. MPOA dopamine is released before and during mating and facilitates copulation, genital reflexes, and sexual motivation. Gonadal hormones regulate dopamine release in the MPOA of male rats in part by increasing nitric oxide synthase (NOS) in the MPOA; the resultant increase in production of nitric oxide (NO) increases both basal and female-stimulated dopamine release. Glutamate also increases dopamine release via increased production of NO. At least some of the glutamatergic inputs to the MPOA are from the medial amygdala (MeA) and bed nucleus of the stria terminalis (BNST), which mediate the female-stimulated increase in dopamine, which in turn enhances copulatory ability. Extracellular glutamate in the MPOA increases during copulation, especially during ejaculation, and increased glutamate facilitates copulation and genital reflexes. Previous sexual experience also facilitates copulation and confers resistance to impairment by various lesions, drugs, and stress. Experience enhances processing of sexual stimuli, and its effects require activation of glutamate NMDA receptors and NOS in the MPOA. Neuronal NOS is increased in the MPOA of experienced males. Therefore, glutamate, NO, and dopamine interact in the MPOA to facilitate mating and to enhance future sexual responsiveness.

The role of ΔfosB in the medial preoptic area: Differential effects of mating and cocaine history.

The transcription factor deltaFosB (ΔFosB) is induced in the nucleus accumbens (NAc) by repeated exposure to drugs of abuse and natural rewards. Less is known about its role in other brain areas. Here, we compared the effects of mating versus cocaine history on induction of ΔFosB in the medial preoptic area (MPOA), an integral site for reproductive behavior, and in the NAc. ΔFosB immunoreactivity (ir) was increased in the MPOA of previously naïve and experienced male rats that mated the day before euthanasia, compared to unmated controls and experienced males with recent mating abstinence. Western immunoblots confirmed that the 35-37-kDa isoform of ΔFosB was increased more in recently mated males. Conversely, previous plus recent cocaine did not increase ΔFosB-ir in the MPOA, despite an increase in the NAc. Next, a viral vector expressing ΔFosB, its dominant negative antagonist ΔJunD, or green fluorescent protein (GFP) control, were microinjected bilaterally into the MPOA. ΔFosB overexpression impaired copulation and promoted female-directed aggression, compared to ΔJunD and control males. These data suggest that ΔFosB in the mPOA is expressed in an experience-dependent manner and affects systems that coordinate mating and aggression. (PsycINFO Database Record

Neuronal nitric oxide synthase and gonadal steroid interaction in the MPOA of male rats: co-localization and testosterone-induced restoration of copulation and nNOS-immunoreactivity.

Neuronal nitric oxide synthase (nNOS) in the medial preoptic area (MPOA) has been implicated in various physiological functions, including male rat copulation. Based on their apparent sensitivity to gonadal steroid manipulation, we hypothesized that nNOS cells contain steroid receptors, and the testosterone-induced restoration of nNOS-immunoreactivity in castrates should accompany the restoration of copulation. In Experiment 1, we investigated co-localization of nNOS with the androgen receptor (AR) and the estrogen receptor alpha (ERalpha) using immunocytochemistry. We found regionally specific co-localizations of nNOS-AR and nNOS-ERalpha. In Experiment 2, we investigated the relationship between MPOA nNOS-immunoreactivity (ir) and copulatory measures in the testosterone-induced restoration paradigm in castrates. The restoration of various copulatory measures was accompanied by an increase in optical density of nNOS-ir, but not in the number of nNOS-ir cells. These data provide additional evidence supporting the role of MPOA nitric oxide in male rat copulation.

Dopamine, the medial preoptic area, and male sexual behavior.

The medial preoptic area (MPOA), at the rostral end of the hypothalamus, is important for the regulation of male sexual behavior. Results showing that male sexual behavior is impaired following MPOA lesions and enhanced with MPOA stimulation support this conclusion. The neurotransmitter dopamine (DA) facilitates male sexual behavior in all studied species, including rodents and humans. Here, we review data indicating that the MPOA is one site where DA may act to regulate male sexual behavior. DA agonists microinjected into the MPOA facilitate sexual behavior, whereas DA antagonists impair copulation, genital reflexes, and sexual motivation. Moreover, microdialysis experiments showed increased release of DA in the MPOA as a result of precopulatory exposure to an estrous female and during copulation. DA may remove tonic inhibition in the MPOA, thereby enhancing sensorimotor integration, and also coordinate autonomic influences on genital reflexes. In addition to sensory stimulation, other factors influence the release of DA in the MPOA, including testosterone, nitric oxide, and glutamate. Here we summarize and interpret these data.

A nitric oxide synthesis inhibitor in the medial preoptic area inhibits copulation and stimulus sensitization in male rats.

Dopamine in the medial preoptic area (MPOA) facilitates copulation in male rats, and nitric oxide (NO) regulates basal and female-stimulated MPOA dopamine release. Microinjection of L-nitro-arginine methyl ester (L-NAME, an NO synthesis inhibitor) into the MPOA blocked copulation in naive rats and impaired copulation in sexually experienced males. In other naive rats, L-NAME or saline was microinjected into the MPOA before each of 7 daily exposures to a receptive female placed over their cage. In a drug-free test on Day 8, copulation by L-NAME-treated rats was similar to that of unexposed controls and was impaired relative to saline-treated males. Therefore, NO in the MPOA is important for copulation and stimulus sensitization in male rats.

An NMDA antagonist impairs copulation and the experience-induced enhancement of male sexual behavior in the rat.

Sexual experience facilitates subsequent male sexual behavior; activation of the N-methyl-D-aspartate (NMDA) glutamate receptor may play a role in this experience-induced enhancement. In this article, the authors report that systemic injections of MK-801, an NMDA receptor antagonist, impaired male sexual behavior in sexually naive and sexually experienced male rats. Furthermore, saline-treated rats that received 7 daily exposures to an inaccessible estrous female instead of sexual experience displayed enhancement of copulation on the following day. Injections of MK-801 before each of these exposures inhibited the experience-induced enhancement on the drug-free test on Day 8. These data suggest that stimulation of NMDA receptors enhances sexual performance immediately and mediates the experience-induced enhancement of subsequent copulatory behavior.

Lysergic acid diethylamide and [-]-2,5-dimethoxy-4-methylamphetamine increase extracellular glutamate in rat prefrontal cortex.

The ability of hallucinogens to increase extracellular glutamate in the prefrontal cortex (PFC) was assessed by in vivo microdialysis. The hallucinogen lysergic acid diethylamide (LSD; 0.1 mg/kg, i.p.) caused a time-dependent increase in PFC glutamate that was blocked by the 5-HT(2A) antagonist M100907 (0.05 mg/kg, i.p.). Similarly, the 5-HT(2A/C) agonist [-]-2,5-dimethoxy-4-methylamphetamine (DOM; 0.6 mg/kg, i.p.), which is a phenethylamine hallucinogen, increased glutamate to 206% above saline-treated controls. When LSD (10 microM) was directly applied to the PFC by reverse dialysis, a rapid increase in PFC glutamate levels was observed. Glutamate levels in the PFC remained elevated after the drug infusion was discontinued. These data provide direct evidence in vivo for the hypothesis that an enhanced release of glutamate is a common mechanism in the action of hallucinogens.

Dopamine and serotonin: influences on male sexual behavior.

Steroid hormones regulate sexual behavior primarily by slow, genomically mediated effects. These effects are realized, in part, by enhancing the processing of relevant sensory stimuli, altering the synthesis, release, and/or receptors for neurotransmitters in integrative areas, and increasing the responsiveness of appropriate motor outputs. Dopamine has facilitative effects on sexual motivation, copulatory proficiency, and genital reflexes. Dopamine in the nigrostriatal tract influences motor activity; in the mesolimbic tract it activates numerous motivated behaviors, including copulation; in the medial preoptic area (MPOA) it controls genital reflexes, copulatory patterns, and specifically sexual motivation. Testosterone increases nitric oxide synthase in the MPOA; nitric oxide increases basal and female-stimulated dopamine release, which in turn facilitates copulation and genital reflexes. Serotonin (5-HT) is primarily inhibitory, although stimulation of 5-HT(2C) receptors increases erections and inhibits ejaculation, whereas stimulation of 5-HT(1A) receptors has the opposite effects: facilitation of ejaculation and, in some circumstances, inhibition of erection. 5-HT is released in the anterior lateral hypothalamus at the time of ejaculation. Microinjections of selective serotonin reuptake inhibitors there delay the onset of copulation and delay ejaculation after copulation begins. One means for this inhibition is a decrease in dopamine release in the mesolimbic tract.

Women's orgasm.

An orgasm in the human female is a variable, transient peak sensation of intense pleasure, creating an altered state of consciousness, usually with an initiation accompanied by involuntary, rhythmic contractions of the pelvic striated circumvaginal musculature, often with concomitant uterine and anal contractions, and myotonia that resolves the sexually induced vasocongestion and myotonia, generally with an induction of well-being and contentment. Women's orgasms can be induced by erotic stimulation of a variety of genital and nongenital sites. As of yet, no definitive explanations for what triggers orgasm have emerged. Studies of brain imaging indicate increased activation at orgasm, compared to pre-orgasm, in the paraventricular nucleus of the hypothalamus, periaqueductal gray of the midbrain, hippocampus, and the cerebellum. Psychosocial factors commonly discussed in relation to female orgasmic ability include age, education, social class, religion, personality, and relationship issues. Findings from surveys and clinical reports suggest that orgasm problems are the second most frequently reported sexual problems in women. Cognitive-behavioral therapy for anorgasmia focuses on promoting changes in attitudes and sexually relevant thoughts, decreasing anxiety, and increasing orgasmic ability and satisfaction. To date there are no pharmacological agents proven to be beneficial beyond placebo in enhancing orgasmic function in women.

Influences of dopamine and glutamate in the medial preoptic area on male sexual behavior.

Several brain nuclei interact to orchestrate the appetitive and consummatory aspects of male sexual behavior. Of these structures, the medial preoptic area (mPOA) of the hypothalamus is of particular interest, as it receives input from all sensory modalities, and damage to this region disrupts copulation in a wide variety of taxa. Furthermore, the mPOA is both responsive to gonadal hormones and involved in endocrine regulation. Neurochemical studies have demonstrated that both dopamine and glutamate levels rise in the mPOA in response to sexual activity, while antagonism of these neurotransmitters impairs male sexual response. Here we review how dopamine and glutamate act in the mPOA to modulate male sexual behavior.

Sexual experience increases oxytocin receptor gene expression and protein in the medial preoptic area of the male rat.

Oxytocin (OT) promotes social and reproductive behaviors in mammals, and OT deficits may be linked to disordered social behaviors like autism and severe anxiety. Male rat sexual behavior is an excellent model for OT regulation of behavior, as its pattern and neural substrates are well characterized. We previously reported that OT microinjected into the medial preoptic area (MPOA), a major integrative site for male sexual behavior, facilitates copulation in sexually experienced male rats, whereas intra-MPOA injection of an OT antagonist (OTA) inhibits copulation. In the present studies, copulation on the day of sacrifice stimulated OTR mRNA expression in the MPOA, irrespective of previous sexual experience, with the highest levels observed in first-time copulators. In addition, sexually experienced males had higher levels of OTR protein in the MPOA than sexually naïve males and first-time copulators. Finally, intra-MPOA injection of OT facilitated mating in sexually naive males. Others have reported a positive correlation between OT mRNA levels and male sexual behavior. Our studies show that OT in the MPOA facilitates mating in both sexually naive and experienced males, some of the behavioral effects of OT are mediated by the OTR, and sexual experience is associated with increased OTR expression in the MPOA. Taken together, these data suggest a reciprocal interaction between central OT and behavior, in which OT facilitates copulation and copulation stimulates the OT/OTR system in the brain.

Dopamine D1 receptors and phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein-32 in the medial preoptic area are involved in experience-induced enhancement of male sexual behavior in rats.

The medial preoptic area (MPOA) is an integral site for male sexual behavior. Dopamine is released in the MPOA before and during copulation and facilitates male rat sexual behavior. Repeated sexual experience and noncopulatory exposures to an estrous female facilitate subsequent copulation. However, the neurobiological mechanisms that mediate such enhancement remain unclear. Here, we examined the role of dopamine D1 receptors in the MPOA in experience-induced enhancement of male sexual behavior in rats. In experiment 1, microinjections of the D1 antagonist SCH-23390 into the MPOA before each of seven daily 30-min noncopulatory exposures to a receptive female impaired copulation on a drug-free test on Day 8, compared to vehicle-treated female-exposed animals. Copulatory performance in drug-treated animals was similar to that of vehicle-treated males that had not been preexposed to females. This effect was site specific. There were no group differences in locomotor activity in an open field on the copulation test day. In experiment 2, a separate cohort of animals was used to examine phosphorylation of dopamine- and cAMP-regulated phosphoprotein (DARPP-32) in the MPOA of animals with acute and/or chronic sexual experience. DARPP-32 is a downstream marker of D1 receptor signaling and substrate of cAMP-dependent protein kinase (PKA). Western immunoblot analysis revealed that p-DARPP-32 expression was greatest in the MPOA of males that received both acute and chronic sexual experience, compared to all other mated conditions and naïve controls. These data suggest that D1 receptors in the MPOA contribute to experience-induced enhancement of male sexual behavior, perhaps through a PKA regulated mechanism.

An NMDA antagonist in the MPOA impairs copulation and stimulus sensitization in male rats.

Systemic injections of an NMDA antagonist have been shown to impair mating in male rats. One site where glutamate and its NMDA receptors may contribute to mating is the medial preoptic area (MPOA), which is vital for male sexual behavior. Glutamate is released in the MPOA during copulation, and especially at the time of ejaculation. We report here that the NMDA antagonist MK-801, microinjected into the MPOA, impaired copulatory behavior in sexually naïve as well as experienced males. In rats tested both as naïve and after sexual experience, drug treatment produced more profound impairment in naïve males. In addition, MK-801, microinjected into the MPOA before each of 7 noncopulatory exposures to receptive female rats, resulted in copulatory impairments on a drug-free test on Day 8, relative to aCSF-treated rats; their behavior was similar to that of males that had not been preexposed to females. Therefore, NMDA receptors in the MPOA contribute to the control of copulation and stimulus sensitization. Glutamate, acting via NMDA receptors, regulates many neural functions, including neuronal plasticity. This is the first demonstration that a similar mechanism in the MPOA sensitizes male rats to the stimuli from a receptive female, and thereby enhances their behavior.