Effects of hydroxyflutamide in the medial preoptic area or lateral septum on reproductive behaviors in male rats

Neurobiology of the lateral septum: regulation of social behavior

Social interactions are essential for mammalian life and are regulated by evolutionary conserved neuronal mechanisms. An individual's internal state, experiences, and the nature of the social stimulus are critical for determining apt responses to social situations. The lateral septum (LS) - a structure of the basal forebrain - integrates abundant cortical and subcortical inputs, and projects to multiple downstream regions to generate appropriate behavioral responses. Although incoming cognitive information is indispensable for contextualizing a social stimulus, neuromodulatory information related to the internal state of the organism significantly influences the behavioral outcome as well. This review article provides an overview of the neuroanatomical properties of the LS, and examines its neurochemical (neuropeptidergic and hormonal) signaling, which provide the neuromodulatory information essential for fine-tuning social behavior across the lifespan.

Effects of gonadectomy and dihydrotestosterone on neuronal plasticity in motivation and reward related brain regions in the male rat

Gonadal hormones affect neuronal morphology to ultimately regulate behaviour. In female rats, oestradiol mediates spine plasticity in hypothalamic and limbic brain structures, contributing to long-lasting effects on motivated behaviour. Parallel effects of androgens in male rats have not been extensively studied. Here, we investigated the effect of both castration and androgen replacement on spine plasticity in the nucleus accumbens shell and core (NAcSh and NAcC), caudate putamen (CPu), medial amygdala (MeA) and medial preoptic nucleus (MPN). Intact and castrated (gonadectomy [GDX]) male rats were treated with dihydrotestosterone (DHT, 1.5 mg) or vehicle (oil) in three experimental groups: intact-oil, GDX-oil and GDX-DHT. Spine density and morphology, measured 24 hours after injection, were determined through three-dimensional reconstruction of confocal z-stacks of DiI-labelled dendritic segments. We found that GDX decreased spine density in the MPN, which was rescued by DHT treatment. DHT also increased spine density in the MeA in GDX animals compared to intact oil-treated animals. By contrast, DHT decreased spine density in the NAcSh compared to GDX males. No effect on spine density was observed in the NAcC or CPu. Spine length and spine head diameter were unaffected by GDX and DHT in the investigated brain regions. In addition, immunohistochemistry revealed that DHT treatment of GDX animals rapidly increased the number of cell bodies in the NAcSh positive for phosphorylated cAMP response-element binding protein, a downstream messenger of the androgen receptor. These findings indicate that androgen signalling plays a role in the regulation of spine plasticity within neurocircuits involved in motivated behaviours.

Effects of progesterone treatment during adulthood on consummatory and motivational aspects of sexual behavior in male rats

In males of a variety of species, administration of progesterone during adulthood has been shown to decrease the expression of consummatory sexual behaviors and androgen receptors. However, it remains to be determined if the progesterone-induced decrease in androgen-receptor signaling and consummatory sexual behaviors correspond with less of a preference for a sexually receptive female relative to another male, a behavioral phenotype indicative of sexual motivation. Consistent with the effects of progesterone reported in males of other species, progesterone-treated rats, relative to vehicle-treated rats, exhibited fewer intromissions and ejaculations. Correspondingly, the weights of the androgen sensitive bulbourethral glands were lighter in progesterone-treated rats. In addition, unlike vehicle-treated rats, progesterone-treated rats did not exhibit a preference for a female rat during the early stages of testing. However, across the entire test, both treatment groups exhibited a preference for a female rat, and consequently, there were no differences between the conditions in overall sexual motivation. Progesterone treatment did not alter activity or anxiety-like behaviors. The results of the current study suggest that the lower levels of androgen-receptor signaling and consummatory sexual behaviors in males following progesterone treatment are associated with a transient deficit in the preference for a female sexual incentive.

Low Daytime Light Intensity Disrupts Male Copulatory Behavior, and Upregulates Medial Preoptic Area Steroid Hormone and Dopamine Receptor Expression, in a Diurnal Rodent Model of Seasonal Affective Disorder

Seasonal affective disorder (SAD) involves a number of psychological and behavioral impairments that emerge during the low daytime light intensity associated with winter, but which remit during the high daytime light intensity associated with summer. One symptom frequently reported by SAD patients is reduced sexual interest and activity, but the endocrine and neural bases of this particular impairment during low daylight intensity is unknown. Using a diurnal laboratory rodent, the Nile grass rat (Arvicanthis niloticus), we determined how chronic housing under a 12:12 h day/night cycle involving dim low-intensity daylight (50 lux) or bright high-intensity daylight (1,000 lux) affects males’ copulatory behavior, reproductive organ weight, and circulating testosterone. We also examined the expression of mRNAs for the aromatase enzyme, estrogen receptor 1 (ESR1), and androgen receptor (AR) in the medial preoptic area (mPOA; brain site involved in the sensory and hormonal control of copulation), and mRNAs for the dopamine (DA) D1 and D2 receptors in both the mPOA and nucleus accumbens (NAC; brain site involved in stimulus salience and motivation to respond to reward). Compared to male grass rats housed in high-intensity daylight, males in low-intensity daylight displayed fewer mounts and intromissions when interacting with females, but the groups did not differ in their testes or seminal vesicle weights, or in their circulating levels of testosterone. Males in low-intensity daylight unexpectedly had higher ESR1, AR and D1 receptor mRNA in the mPOA, but did not differ from high-intensity daylight males in D1 or D2 mRNA expression in the NAC. Reminiscent of humans with SAD, dim winter-like daylight intensity impairs aspects of sexual behavior in a male diurnal rodent. This effect is not due to reduced circulating testosterone and is associated with upregulation of mPOA steroid and DA receptors that may help maintain some sexual motivation and behavior under winter-like lighting conditions.

Hormone-dependent medial preoptic/lumbar spinal cord/autonomic coordination supporting male sexual behaviors

Testosterone (T) can act directly through neural androgen receptors (AR) to facilitate male sexual behavior; however, T's metabolites also can play complicated and interesting roles in the control of mating. One metabolite, dihydrotestosterone (DHT) binds to AR with significantly greater affinity than that of T. Is that important behaviorally? Another metabolite, estradiol (E), offers a potential alternative route of facilitating male mating behavior by acting through estradiol receptors (ER). In this review we explore the roles and relative importance of T as well as E and DHT at various levels of the neuroaxis for the activation of male sex behavior in common laboratory animals and, when relevant research findings are available, in man.

Contributions of testosterone and territory ownership to sexually-motivated behaviors and mRNA expression in the medial preoptic area of male European starlings

Animals integrate social information with their internal endocrine state to control the timing of behavior, but how these signals are integrated in the brain is not understood. The medial preoptic area (mPOA) may play an integrative role in the control of courtship behavior, as it receives projections from multiple sensory systems, and is central to the hormonal control of courtship behavior across vertebrates. Additionally, data from many species implicate opioid and dopaminergic systems in the mPOA in the control of male courtship behavior. We used European starlings to test the hypothesis that testosterone (T) and social status (in the form of territory possession) interact to control the timing of courtship behavior by modulating steroid hormone-, opioid- and dopaminergic-related gene expression in the mPOA. We found that only males given both T and a nesting territory produced high rates of courtship behavior in response to a female. T treatment altered patterns of gene expression in the mPOA by increasing androgen receptor, aromatase, mu-opioid receptor and preproenkephalin mRNA and decreasing tyrosine hydroxylase mRNA expression. Territory possession did not alter mRNA expression in the mPOA, despite the finding that only birds with both T and a nesting territory produced courtship behavior. We propose that T prepares the mPOA to respond to the presence of a female with high rates of courtship song by altering gene expression, but that activity in the mPOA is under a continuous (i.e. tonic) inhibition until a male starling obtains a nesting territory.

Lack of anti-androgenic effects of equol on reproductive neuroendocrine function in the adult male rat

Equol (EQ), a metabolite of the soy isoflavone daidzein, has well known estrogenic properties. Data from animal studies suggested that EQ may act also as an anti-androgen. However, data regarding how EQ may affect brain functions like the regulation of neuroendocrine activity and reproductive outcomes in adult male rats are still lacking. We therefore investigated the effects of EQ on sex-steroid regulated gene expression in the brain [medial preoptic area/anterior hypothalamus (MPOA/AH) and medial basal hypothalamus/median eminence (MBH/ME)], pituitary, and prostate as a reference androgen-dependent organ. Furthermore reproductive outcomes were evaluated. The anti-androgen flutamide (FLUT) served as reference compound. Male rats (n=12 per group) were treated by gavage for 5 days with either EQ (100 or 250 mg/kgBW/day), or FLUT 100 mg/kgBW/day. All vehicle- and EQ-treated males showed successful reproductive outcomes, whereas FLUT-exposed males had severe reproductive impairments resulted in infertility. FLUT decreased relative weights of prostate, seminal vesicles and epididymides, and increased serum levels of luteinizing hormone, follicle-stimulating hormone, testosterone and 5α-dihydrotestosterone without altering prolactin levels, whereas EQ exerted opposite effects. Both EQ and FLUT decreased gonadotropin releasing hormone (GnRH) expression in the MPOA/AH. Only FLUT upregulated levels of GnRH receptor expression both in the MBH/ME and pituitary. While EQ downregulated the hypothalamic ERα and ERβ expressions, but FLUT did not. In the prostate, only FLUT upregulated both ERα and AR mRNA expression levels. Taken together, our findings are the first data that EQ did not induce anti-androgenic effects on brain, prostate and male reproductive parameters, however, estrogenic neuroendocrine and reproductive effects of EQ were observed.

Chemosignals and hormones in the neural control of mammalian sexual behavior

Males and females of most mammalian species depend on chemosignals to find, attract and evaluate mates and, in most cases, these appetitive sexual behaviors are strongly modulated by activational and organizational effects of sex steroids. The neural circuit underlying chemosensory-mediated pre- and peri-copulatory behavior involves the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), medial preoptic area (MPOA) and ventromedial hypothalamus (VMH), each area being subdivided into interconnected chemoreceptive and hormone-sensitive zones. For males, MA-BNST connections mediate chemoinvestigation whereas the MA-MPOA pathway regulates copulatory initiation. For females, MA-MPOA/BNST connections also control aspects of precopulatory behavior whereas MA-VMH projections control both precopulatory and copulatory behavior. Significant gaps in understanding remain, including the role of VMH in male behavior and MPOA in female appetitive behavior, the function of cortical amygdala, the underlying chemical architecture of this circuit and sex differences in hormonal and neurochemical regulation of precopulatory behavior.

Species differences in the immunoreactive expression of oxytocin, vasopressin, tyrosine hydroxylase and estrogen receptor alpha in the brain of Mongolian gerbils (Meriones unguiculatus) and Chinese striped hamsters (Cricetulus barabensis)

Species differences in neurochemical expression and activity in the brain may play an important role in species-specific patterns of social behavior. In the present study, we used immunoreactive (ir) labeling to compare the regional density of cells containing oxytocin (OT), vasopressin (AVP), tyrosine hydroxylase (TH), or estrogen receptor alpha (ERα) staining in the brains of social Mongolian gerbils (Meriones unguiculatus) and solitary Chinese striped hamsters (Cricetulus barabensis). Multiple region- and neurochemical-specific species differences were found. In the anterior hypothalamus (AH), Mongolian gerbils had higher densities of AVP-ir and ERα-ir cells than Chinese striped hamsters. In the lateral hypothalamus (LH), Mongolian gerbils also had higher densities of AVP-ir and TH-ir cells, but a lower density of OT-ir cells, than Chinese striped hamsters. Furthermore, in the anterior nucleus of the medial preoptic area (MPOAa), Mongolian gerbils had higher densities of OT-ir and AVP-ir cells than Chinese striped hamsters, and an opposite pattern was found in the posterior nucleus of the MPOA (MPOAp). Some sex differences were also observed. Females of both species had higher densities of TH-ir cells in the MPOAa and of OT-ir cells in the intermediate nucleus of the MPOA (MPOAi) than males. Given the role of these neurochemicals in social behaviors, our data provide additional evidence to support the notion that species-specific patterns of neurochemical expression in the brain may be involved in species differences in social behaviors associated with different life strategies.

Relationship between sexual satiety and motivation, brain androgen receptors and testosterone in male mandarin voles

Androgen receptors participate in the neuroendocrine regulation of male sexual behavior, primarily in brain areas located in the limbic system. Males of many species present a long-term inhibition of sexual behavior after several ejaculations, known as sexual satiety. It has been shown in rats that androgen receptor expression is reduced 24h after a single ejaculation, or mating to satiety, in the medial preoptic area, nucleus accumbens and ventromedial hypothalamus. The aim of this study was to analyze these processes in another animal, the mandarin vole (Microtus mandarinus). We compared differences in androgen receptor (AR) and testosterone (T) expression in various brain areas between male mandarin voles sexually satiated and those exposed to receptive females but not allowed to mate. Sexual satiety was associated with decreased AR and T expression in the lateral septal nucleus (LS), medial amygdala (MeA), medial preoptic area (mPOA) and ventromedial hypothalamic nucleus (VMH). Males exposed to receptive females showed an increase in AR and T expression in the bed nucleus of the stria terminalis (BNST), LS, MeA and VMH. Serum testosterone levels remained unchanged after 24h in males exposed to receptive females or males mated to satiety. These data suggest a relationship between sexual activity and a decrease in AR and T expression in specific brain areas, and a relationship between sexual motivation and increased AR and T expression in other brain areas, independently of testosterone levels.

Androgen receptors, sex behavior, and aggression

Androgens are intricately involved in reproductive and aggressive behaviors, but the role of the androgen receptor in mediating these behaviors is less defined. Further, activity of the hypothalamic-pituitary-gonadal axis and hypothalamic-pituitary-adrenal axis can influence each other at the level of the androgen receptor. Knowledge of the mechanisms for androgens' effects on behaviors through the androgen receptor will guide future studies in elucidating male reproductive and aggressive behavior repertoires.

Brain structures involved in the sexual behaviour of Ile de France rams with different sexual preferences and levels of sexual activity

Using Fos, as a marker, we analysed the brain structures of rams, with different libidos or sexual preferences that had been activated by contact with males or females. Ile de France rams aged from 1.5 to 7 years were used. Fos immunoreactivity (Fos IR) was analysed in rams with high (HL) or low libido (LL) after 90 min of direct contact with females (HL DirF n=7 or LL DirF n=7) or in rams of high libido having indirect contact through a fence, with females (HL IndF n=6) or males (HL IndM n=5) and finally, in males who preferred other males as partners by indirect contact through a fence with males (MO IndM n=4). Direct or indirect contact with a preferred sexual partner (LL DirF, HL Dir F, HL IndF, MO IndM) induced the appearance of Fos-IR cells in several diencephalic and cortical structures. Conversely, indirect contact with males did not induce Fos-IR in males interested in females (HL IndM). In the medial preoptic area (MPOA), the paraventricular nucleus and the medial bed nucleus of the stria terminalis the cell density of Fos IR cells was higher in HL Dir F than in LL DirF suggesting involvement in sexual motivation whereas only the MPOA seemed involved the consummatory component of sexual behaviour (Fos IR density HL DirF>HL IndF). The enthorinal cortex was the only structure specifically activated by males attracted to other males (Fos IR density MO IndM>HL IndM) whereas Fos IR density did not differ between the HL IndF and HL IndM groups.

Cholinergic control of male mating behavior in hamsters: effects of central oxotremorine treatment

The responses of rats to intracranial injections of cholinergic drugs implicate acetylcholine in the control of male mating behavior and suggest specific brain areas as mediators of these effects. In particular, past work has linked the medial preoptic area (MPOA) to the control of intromission frequency but implicated areas near the lateral ventricles in effects on the initiation and spacing of intromissions. Studies of responses to systemic cholinergic treatments suggest that acetylcholine is even more important for the control of mating behavior in male hamsters but provide no information on the relevant brain areas. To fill this gap, we observed the effects of central injections of the cholinergic agonist oxotremorine that approached the MPOA along contrasting paths. Both studies suggest that increased cholinergic activity in or near the MPOA can facilitate behavior by reducing the postejaculatory interval and possibly affecting other parts of the mechanisms controlling the initiation of copulation and the efficiency of performance early in an encounter. In addition, oxotremorine caused other changes in behavior that could not be tied to the MPOA and may reflect actions at more dorsal sites, possibly including the bed nucleus of the stria terminalis and medial septum. These effects were notably heterogeneous, including facilitatory and disruptive effects on male behavior along with a facilitation of lordosis responses to manual stimulation. These results emphasize the number and diversity of elements of sexual behavior in hamsters that are under the partial control of forebrain cholinergic mechanisms.

Copulation modifies AR and ERα mRNA expression in the male rat brain

BACKGROUND/AIMS

One day after male sexual behavior [one ejaculation or copulation to satiety (ad libitum copulation during 4h with the same female)] androgen receptor immunoreactivity (AR-ir) is decreased and estrogen receptor alpha immunoreactivity (ERα-ir) increased in various brain areas related with its control. Seven days after sexual satiety there was a limited recovery of sexual behavior accompanied by a partial recuperation in the AR-ir. In this study we evaluated if these changes in AR-ir and ERα-ir were paralleled by variations in their respective mRNA.

METHODS

Sexually experienced male rats were sacrificed at different intervals: immediately, 24h or seven days after sexual satiety or 24h after one ejaculation. The changes in AR and ERα mRNA were analyzed by in situ hybridization using digoxigenine-labeled oligonucleotide probes in the MPOA, LSV and the bed nucleus of the stria terminalis, medial division, anterior (BSTMA).

RESULTS

AR mRNA density was decreased in the MPOA and the LSV immediately and 24h after one ejaculation or sexual satiety. Seven days after copulating to satiety, there was a recovery of AR mRNA. In the BSTMA the different behavioral conditions did not modify the AR mRNA expression. In the MPOA, LSV and BSTMA the ERα mRNA increased after a single ejaculation and at all intervals after sexual satiety.

CONCLUSION

In some brain areas and after some intervals of sexual activity, the changes in steroid protein receptors expression seem to be consequence of parallel changes in the expression of the respective mRNA.

Different subregions of the medial preoptic area are separately involved in the regulation of copulation and sexual incentive motivation in male rats: a behavioral and morphological study

The purpose of this study was to investigate whether sexual incentive motivation and copulatory performance are regulated by different subregions of the medial preoptic area (MPOA). Sexual incentive motivation was measured by means of a partner preference test. Both copulatory behavior and sexual incentive motivation were tested in male rats treated with 50mg/kg of either EGb 761 or a vehicle (distilled water) by gavage for 14 days. Administration of EGb 761 increased the number of intromissions, but had no effect on the number of mounts, mount latency, intromission latency, ejaculation latency, or post-ejaculatory interval. In the partner preference test, the total duration of visits to estrous female rats in both of the groups was significantly different from the total duration of visits to sexually active males. EGb 761 treatment increased the number of ejaculations compared both to vehicle-treated controls on day 14 and the same group on day 0. In comparison with the controls, the EGb 761-treated group showed a significant increase in the number of tyrosine hydroxylase-expressing cells in the dorsal, but not the ventral, subregion of the MPOA, and significantly high dopamine levels in the MPOA. These results indicate that EGb 761 does not affect sexual incentive motivation, but facilitates copulatory performance in male rats, suggesting that the mechanisms responsible for sexual incentive motivation and copulatory performance may be associated with differential functions of MPOA subregions.

The effects of nitric oxide-cGMP pathway stimulation on dopamine in the medial preoptic area and copulation in DHT-treated castrated male rats

Dopamine (DA) in the medial preoptic area (MPOA) provides important facilitative influence on male rat copulation. We have shown that the nitric oxide-cGMP (NO-cGMP) pathway modulates MPOA DA levels and copulation. We have also shown that systemic estradiol (E(2)) maintains neuronal NO synthase (nNOS) immunoreactivity in the MPOA of castrates, as well as relatively normal DA levels. This effect of E(2) on nNOS probably accounts for at least some of the previously demonstrated behavioral facilitation by intra-MPOA E(2) administration in castrates. Therefore, we hypothesized that stimulation of the MPOA NO-cGMP pathway in dihydrotestosterone (DHT)-treated castrates should restore DA levels and copulatory behaviors. Reverse-dialysis of a NO donor, sodium nitroprusside (SNP), increased extracellular DA in the MPOA of DHT-treated castrates and restored the ability to copulate to ejaculation in half of the animals. A cGMP analog, 8-Br-cGMP, also increased extracellular DA, though not as robustly, but did not restore copulatory ability. The effectiveness of the NO donor in restoring copulation and MPOA DA levels is consistent with our hypothesis. However, the lack of behavioral effects of 8-Br-cGMP, despite its increase in MPOA DA, suggests that NO may have additional mediators in the MPOA in the regulation of copulation. Furthermore, the suboptimal copulation seen in the NO donor-treated animals suggests the importance of extra-MPOA systems in the regulation of copulation.

Gonadal steroid receptors colocalize with central nervous system neurons projecting to the rat prostate gland

Mating-induced Fos-immunoreactive (-ir) cells are colocalized with androgen receptors (AR), estrogen receptors (ER), or both in limbic and hypothalamic areas known to mediate male rat mating behavior. A steroid-responsive neural network might govern copulatory behavior in male laboratory rats that is analogous to the network described in female rats that governs the lordosis response. This hypothesized network in males may synchronize and coordinate sexual behavioral responses with physiological responses of the genitals and the internal organs of reproduction. Therefore, the pseudorabies virus (PRV; Bartha strain), a transneuronal, viral retrograde tract tracer, was microinjected into the prostate gland to label this network. After 7 days, brains from infected animals were processed for immunohistochemical labeling of AR, ER, and PRV. The majority of PRV-ir cells exhibited either AR or ER immunoreactivity in the medial preoptic area, median preoptic nucleus, bed nucleus of stria terminalis, hypothalamic paraventricular nucleus, and zona incerta, areas known to play roles in male rat mating behavior. Other structures such as the central tegmental field/subparafascicular nucleus of the thalamus, central nucleus of the amygdala, and medial amygdala, also important in the display of male copulatory behavior, were less reliably labeled. Collectively, a steroid receptor-containing neuronal circuit, largely contained in the diencephalon, was revealed that likely is involved in the autonomic control of the prostate gland and the consummatory aspects of male rat mating behavior.

Relationship between sexual satiety and brain androgen receptors

Recently we showed that 24 h after copulation to satiety, there is a reduction in androgen receptor density (ARd) in the medial preoptic area (MPOA) and in the ventromedial hypothalamic nucleus (VMH), but not in the bed nucleus of the stria terminalis (BST). The present study was designed to analyze whether the ARd changes in these and other brain areas, such as the medial amygdala (MeA) and lateral septum, ventral part (LSV), were associated with changes in sexual behavior following sexual satiety. Males rats were sacrificed 48 h, 72 h or 7 days after sexual satiety (4 h ad libitum copulation) to determine ARd by immunocytochemistry; additionally, testosterone serum levels were measured in independent groups sacrificed at the same intervals. In another experiment, males were tested for recovery of sexual behavior 48 h, 72 h or 7 days after sexual satiety. The results showed that 48 h after sexual satiety 30% of the males displayed a single ejaculation and the remaining 70% showed a complete inhibition of sexual behavior. This reduction in sexual behavior was accompanied by an ARd decrease exclusively in the MPOA-medial part (MPOM). Seventy-two hours after sexual satiety there was a recovery of sexual activity accompanied by an increase in ARd to control levels in the MPOM and an overexpression of ARd in the LSV, BST, VMH and MeA. Serum testosterone levels were unmodified during the post-satiety period. The results are discussed on the basis of the similarities and discrepancies between ARd in specific brain areas and male sexual behavior.

Impaired male sexual behavior in activin receptor type II knockout mice

Integration of multiple hormonal and neuronal signaling pathways in the medial preoptic area (mPOA) is required for elicitation of male sexual behavior in most vertebrates. Perturbation of nitric oxide synthase (NOS) activity in the mPOA causes significant defects in male sexual behavior. Although activins and their signaling components are highly expressed throughout the brain, including the mPOA, their functional significance in the central nervous system (CNS) is unknown. Here, we demonstrate a neurophysiologic role for activin signaling in male reproductive behavior. Adult activin receptor type II null (Acvr2-/-) male mice display multiple reproductive behavioral deficits, including delayed initiation of copulation, reduced mount, and intromission frequencies, and increased mount, intromission, and ejaculation latencies. These behavioral defects in the adult mice are independent of gonadotropin-releasing hormone (GnRH) homeostasis or mating-induced changes in luteinizing hormone (LH) and testosterone levels. The impairment in behavior can be correlated to the nitric oxide content in the CNS because Acvr2-/- males have decreased NOS activity in the mPOA but not the rest of the hypothalamus or cortex. Olfactory acuity tests confirmed that Acvr2-/- mice have no defects in general odor or pheromone recognition. In addition, motor functions are not impaired and the mutants demonstrate normal neuromuscular coordination and balance. Furthermore, the penile histology in mutant mice appears normal, with no significant differences in the expression of penile differentiation marker genes compared with controls, suggesting the observed behavioral phenotypes are not due to structural defects in the penis. Our studies identify a previously unrecognized role of activin signaling in male sexual behavior and suggest that activins and/or related family members are upstream regulators of NOS activity within the mPOA of the forebrain.

Distribution of androgen receptor immunoreactivity in the brainstem of male rats

Gonadal steroids such as testosterone and estrogen are necessary for the normal activation of male rat sexual behavior. The medial preoptic area (MPOA), an important neural substrate regulating mating, accumulates steroids and also expresses functional androgen receptors (AR). The MPOA is intimately connected with other regions implicated in copulation, such as the bed nucleus of the stria terminalis and medial amygdala. Inputs to the MPOA arise from several areas within the brainstem, synapsing preferentially onto steroid sensitive MPOA cells which are activated during sexual activity. Given that little is known about the distribution of AR protein in the brainstem of male rats, we mapped the distribution of AR expressing cells in the pons and medulla using immunocytochemistry. In agreement with previous reports, AR immunoreactivity (AR-ir) was detected in ventral spinal motoneurons and interneurons. In addition, AR-ir was detected in areas corresponding to the solitary tract, lateral paragigantocellular and alpha and ventral divisions of the gigantocellular reticular nuclei, area postrema, raphe pallidus, ambiguus nucleus, and intermediate reticular nucleus. Several regions within the pons contained AR-ir, such as the tegmental and central gray, parabrachial nucleus, locus coeruleus, Barrington's nucleus, periaqueductal gray, and dorsal raphe. In contrast with in situ hybridization studies, auditory and somatosensory areas were AR-ir negative, and, except for very light staining in the prepositus nucleus, areas carrying vestibular information did not display AR-ir. Additionally, cranial nerve motoneurons of the hypoglossal, facial, dorsal vagus, and spinal trigeminal did not display AR-ir in contrast to previous reports. The data presented here indicate that androgens may influence numerous cell groups within the brainstem. Some of these probably constitute a steroid sensitive circuit linking the MPOA to motoneurons in the spinal cord via androgen responsive cells in the caudal ventral medulla.

Androgen receptor blockade in the MPOA or VMN: effects on male sociosexual behaviors

Previously, our laboratory has shown that androgen receptors in the medial preoptic area (MPOA) and ventromedial nucleus (VMN) are necessary for copulation in male rats. The present study examined whether these receptors are required for other sociosexual behaviors. In Experiment 1, different regions of the VMN were implanted with the antiandrogen hydroxyflutamide (OHF). We found that implants located in anterodorsal portions of the VMN were more effective at inhibiting the restoration of copulation than implants in the posteroventral VMN. In Experiment 2, a second set of male rats was pretested for copulation and other sociosexual behaviors and was castrated. Experimental animals then received Silastic capsules filled with testosterone (T) plus intracranial (IC) implants filled with OHF to selectively block androgen receptors in either the MPOA or VMN. We found that androgen receptor blockade in the MPOA inhibited the restoration of copulation but had no effect on other sociosexual behaviors. OHF directed at the VMN inhibited the restoration of copulation and 50-kHz vocalizations but had no effect on scent marking. Two tests were used to assay sexual motivation: partner preference and conditioned place preference (CPP). Both methods revealed impairments in sexual motivation in the VMN group but not in animals receiving OHF in the MPOA. Taken together, these data suggest that androgen receptors in the MPOA are essential for copulatory performance, while androgen receptors in the VMN are important for copulation, sexual motivation, and androgen-dependent vocalizations.

Preclinical models of sexual desire: conceptual and behavioral analyses

The epidemiology, etiology and proposed treatments for the sexual desire disorders are briefly reviewed before turning to an analysis of preclinical models. We suggest that the concept of sexual desire in the human is equivalent to sexual motivation as employed in the scientific literature. Many animal tests for sexual motivation have been described over the years. Most of them are based on the evaluation of the rate or speed of performing learned operant responses. These are not ideal measures for inferring the intensity of sexual motivation. We present a test for sexual incentive motivation, which has been used in male and female rats. No learning is involved, and the test is rather insensitive to variations in ambulatory activity and it does not employ rate measures. A procedure that recently has attracted much attention, paced-mating behavior in the female, does not seem to be as useful as could be expected. In fact, it does not appear to be superior to tests for sexual receptivity (lordosis). The lack of established, clinically efficient treatments for sexual desire disorders makes it difficult to evaluate if any model has predictive validity. However, the model proposed here may be isomorphic and homologous to the human condition.

Sexual and olfactory preference in noncopulating male rats

In some species including rats, mice, gerbils, and rams, apparently normal males fail to copulate when repeatedly tested with receptive females. These animals are called "noncopulators (NC)," and the cause of this behavioral deficit is unknown. It has been shown that NC rats do not have hormonal alterations or deficits in the mechanisms that control penile function. The present study was designed to examine (Experiment 1) whether NC male rats prefer receptive females to sexually active males. In addition, the olfactory preference for bedding soiled from estrous or for anestrous bedding was investigated. These tests were performed in NC and copulating (C) male rats when the subjects were intact, gonadectomized (GDX), or GDX and treated with high doses of testosterone propionate (TP). Our results demonstrate that NC rats do not display sexual behavior even after high TP treatment. While C male rats have a clear preference for receptive females as opposed to a sexually active male, NC rats do not. In all hormonal conditions, the preference shown by NC rats for estrous bedding was significantly reduced in comparison to that seen in C rats. TP treatment in NC rats did not modify either partner or odor preference. In Experiment 2, we evaluated if NC rats are feminized and if it could be easier to induce feminine-like behavior by hormone treatment with estradiol benzoate (EB) or with EB plus progesterone (P) (EB+P). Odor preference for estrous or male bedding under these hormonal conditions was also compared. No differences between NC and C rats were found in feminine sexual behavior. In the olfactory test, we found that NC rats prefer odors from receptive females as opposed to male odors, but this preference is reduced compared to that of C rats. Males treated with EB or EB+P show no preference for female odors. These results demonstrate that treatment with EB or EB+P does not increase feminine sexual behavior in NC rats.