Retention of masculine sexual behavior following castration in male B6D2F1 mice

Role of membrane estrogen receptor alpha (ERα) in the rapid regulation of male sexual behavior

The activation of male sexual behavior depends on brain estrogen synthesis. Estrogens act through nuclear and membrane receptors producing effects within hours/days or seconds/minutes, respectively. In mice, estrogen receptor alpha (ERα) is the main estrogen receptor (ER) controlling the activation of male sexual behavior. Although neuroestrogens rapidly modulate mouse sexual behavior, it is not known whether these effects involve membrane ERα (mERα). This study combines two complementary approaches to address this question. C451A-ERα mice carry an ERα that cannot signal at the membrane, while estetrol (E4) is a natural estrogen acting as an agonist on nuclear ERα but as an antagonist on membrane ERα. In wild-type males, E4 decreased the number of mounts and intromissions after 10 min. In C451A-ERα males, E4 also altered sexual performance but after 30 min. E4 did not affect time spent near the female in both wild-type and C451A-ERα mice. However, regardless of genotype, the aromatase inhibitor 1,4,6-Androstatriene-3,17-dione (ATD) decreased both sexual performance and the time spent near the female after 10 and 30 min, confirming the key role of aromatization in the rapid control of sexual behavior and motivation. In conclusion, the shift in timing at which the effect of E4 is observed in mice lacking mERα suggests a role for mERα in the regulation of rapid effects of neuroestrogens on sexual performance, thus providing the first demonstration that E4 acts as an antagonist of a mER in the brain. The persisting effect of ATD on behavior in C451A-ERα mice also suggests the implication of another ER.

B6D2F1 mice that retain sexual behavior long term after castration outperform those that cease in the radial arm maze

In rodents, gonadal steroids play a critical yet variable role in behaviors such as social interaction and cognitive performance. Gonadal steroids organize sex differences observed in spatial working memory, while the absence of activational effects induced by castration generally impedes spatial learning and memory. Although male sexual behavior is typically inhibited following castration, a significant proportion of gonadectomized B6D2F1 hybrid males retains the complete repertoire of male reproductive behavior. In a prior study, amyloid precursor protein and tau, proteins involved in cognitive behavior, facilitated steroid-independent male sex behavior in B6D2F1 hybrid male mice. We used this strain to investigate the relationship between gonadal steroid-independent male sexual behavior and cognition. After identifying "maters" (animals retaining steroid-independent male sex behavior) and "non-maters," we tested spatial memory in an 8-arm radial arm maze. Although neither group demonstrated a decrease in errors as a function of time, maters committed fewer errors compared to non-maters overall (p < 0.05). Maters also completed the maze more quickly than non-maters (p < 0.05). We measured mRNA expression of APP and MAPT as well as LEPR and D2R to probe potential roles of metabolism and motivation. Uniquely among maters, increased relative expression of D2R and LEPR in the hippocampus was associated with a longer latency to complete the maze during the last 3 or across all trials, respectively. These data demonstrate that maters outperform non-maters in the radial arm maze, warranting further study of potential differences in acquisition of spatial memory tasks or learning strategy between these groups.

Strain differences in post-castration sexual and aggressive behavior in male mice

The degree to which male sexual behavior and territorial aggression are regulated by gonadal steroid hormones depends strongly on species and experience. While castration abolishes male sexual behavior in most laboratory rodents, approximately one third of B6D2F1 mice retain the full repertoire of male sexual behaviors long term ("maters"). It is not yet known whether maters retain other behaviors that typically rely on gonadal steroids to a greater extent than non-maters. In this study, we tested aggressive behavior in B6D2F1 males and males of each parental strain (C57BL/6J and DBA/2J) in the resident intruder paradigm before and after castration, as well as male sexual behavior after castration. Before castration, B6D2F1 residents displayed more attacks compared to DBA/2J males (p < 0.05). There was no difference in attack frequency between B6D2F1 and C57BL/6J males nor between DBA/2J and C57BL/6J males (p > 0.2). A greater proportion of hybrid males demonstrated intromissions and the ejaculatory reflex compared to males of either parental strain (p < 0.01). After castration, B6D2F1 residents attacked more than C57BL/6J males, but not DBA/2J males (p < 0.05; p > 0.2). There was no difference in post-castration attack frequency between maters and non-maters (p > 0.7). Finally, residents that attacked during all 3 pre-castration resident intruder tests displayed more attacks post-castration than animals that attacked during 1 pre-castration test (p < 0.05). These data suggest that strain and experience influence the expression of aggressive behavior after castration and warrant future study in experience-induced transient increases in extragonadal testosterone.

Hypothalamic cellular and molecular plasticity linked to sexual experience in male rats and mice

Male sexual behavior is subject to learning, resulting in increased efficiency of experienced males compared to naive ones. The improvement in behavioral parameters is underpinned by cellular and molecular changes in the neural circuit controlling sexual behavior, particularly in the hypothalamic medial preoptic area. This review provides an update on the mechanisms related to the sexual experience in male rodents, emphasizing the differences between rats and mice.

Access to females and early life castration individually extend maximal but not median lifespan in male mice

Investment in reproduction is predicted to accelerate ageing, but the link between reproductive investment and lifespan can be sex- and context-specific. In mammals, female reproductive costs are linked to pregnancy and lactation, but in males substantial reproductive allocation is required for a range of pre- and post-copulatory reproductive traits. Such traits include male-specific increased body size, olfactory signalling and territory defence-traits often expressed under androgen-dependent control. In this experimental study, we explored how reproduction influences lifespan in male mice, contrasting this to the established lifespan costs of reproduction in females. In a 2 × 2 factorial design, we gave either castrated or intact males (factor 1) access to a female or a male cage-mate across their entire life (factor 2). Neither castration nor access to females influenced median lifespan in male mice, but maximal lifespan was increased by either castration or reproduction when compared to intact males housed in male groups (standard male housing conditions). In females, mating significantly reduced lifespan, and while both sexes had similar lifespans in non-reproductive environments, males had a much longer lifespan when allowed mating. This data highlights the sex-specific nature of social environments and reproduction on lifespan, and the role of these conditions in promoting sexual dimorphism in ageing.

Glutamate in Male and Female Sexual Behavior: Receptors, Transporters, and Steroid Independence

The survival of animal species predicates on the success of sexual reproduction. Neurotransmitters play an integral role in the expression of these sexual behaviors in the brain. Here, we review the role of glutamate in sexual behavior in rodents and non-rodent species for both males and females. These encompass the release of glutamate and correlations with glutamate receptor expression during sexual behavior. We then present the effects of glutamate on sexual behavior, as well as the effects of antagonists and agonists on different glutamate transporters and receptors. Following that, we discuss the potential role of glutamate on steroid-independent sexual behavior. Finally, we demonstrate the interaction of glutamate with other neurotransmitters to impact sexual behavior. These sexual behavior studies are crucial in the development of novel treatments of sexual dysfunction and in furthering our understanding of the complexity of sexual diversity. In the past decade, we have witnessed the burgeoning of novel techniques to study and manipulate neuron activity, to decode molecular events at the single-cell level, and to analyze behavioral data. They pose exciting avenues to gain further insight into future sexual behavior research. Taken together, this work conveys the essential role of glutamate in sexual behavior.

Mating in the absence of fertilization promotes a growth-reproduction versus lifespan trade-off in female mice

Trade-offs between growth, reproduction, and lifespan constrain animal life histories, leading to evolutionary diversification of life history cycles in different environments. In female mammals, gestation and lactation are expected to impose the major costs of reproduction, driving reproductive trade-offs, although mating also requires interactions with males that could themselves influence life history. Here we show that a male's presence by itself leads to lifelong alterations in life history in female mice. Housing C57BL/6J female mice with sterilized males early in life led to an increase in body weight, an effect that persisted across life even when females were later allowed to produce pups. We found that those females previously housed with sterile males also showed enhanced late-life offspring production when allowed to reproduce, indicating that earlier mating can influence subsequent fecundity. This effect was the opposite to that seen in females previously housed with intact males, which showed the expected trade-off between early-life and late-life reproduction. However, housing with a sterile male early in life came at a cost to lifespan, which was observed in the absence of females ever undergoing fertilization. Endocrinologically, mating also permanently reduced the concentration of circulating prolactin, a pituitary hormone influencing maternal care. Changes in hormone axes that influence reproduction could therefore help alter life history allocation in response to opposite-sex stimuli. Our results demonstrate that mating itself can increase growth and subsequent fecundity in mammals, and that responses to sexual stimuli could account for some lifespan trade-offs normally attributed to pregnancy and lactation.

Female mice seek refuge from castrated males, but not intact or vasectomized males, mitigating a socially-induced glucocorticoid response

Sexual conflict may be manifested during social interactions, shaping the costs of reproduction in sexually reproducing species. This conflict, and the physical necessity of intromission, can intensify the already costly nature of reproduction for female mammals. To identify and partition the costs that males inflict on females during mating and reproduction, we paired female mice with either other females or castrated, vasectomised, or intact (sham-vasectomised) males, thus manipulating exposure to social mating behavior and costs arising from fertilization. We also provided females with refuges where males could not enter, to test whether females show avoidance or attraction to males of different gonadal status expected to exhibit different levels of social behavior. We found that females paired with vasectomised and castrated males spent the most time in their refuge. Females housed with castrated males also had increased glucocorticoid levels, an effect that was mitigated when females could retreat from these males to a refuge. This suggests that females actively refuge from castrated males, and that housing with such males is sufficient to generate an increased glucocorticoid response. Our results show that females choose to refuge from males depending on the partner's gonadal status, choices that are linked to social induced stress responses but not exposure to male mating behaviour.

Comparing Postnatal Development of Gonadal Hormones and Associated Social Behaviors in Rats, Mice, and Humans

Postnatal development includes dramatic changes in gonadal hormones and the many social behaviors they help regulate, both in rodents and humans. Parental care-seeking is the most salient social interaction in neonates and infants, play and prosocial behaviors are commonly studied in juveniles, and the development of aggression and sexual behavior begins in peripubertal stages but continues through late adolescence into adulthood. Although parental behaviors are shown after reproductive success in adulthood, alloparenting behaviors are actually high in juveniles as well. These behaviors are sensitive to both early-life organizational effects of gonadal hormones and later-life activational regulation. However, changes in circulating gonadal hormones and the display of the previous behaviors over development differ between rats, mice, and humans. These endpoints are of interest to endocrinologist, toxicologists, and neuroscientists because of their relevance to mental health disorders and their vulnerability to effects of endocrine-disrupting chemical exposure. As such, the goal of this mini-review is to succinctly describe and relate the postnatal development of gonadal hormones and social behaviors to each other, over time, and across animal models. Ideally, this will help identify appropriate animal models and age ranges for continued study of both normative development and in contexts of environmental disruption.

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.

Changes in Sexual Behavior of Orchidectomized Rats Under Influence of Allotransplantation of Testicular Interstitial Cell Suspension

Transplantation of hormone-producing cells is an experimental endocrine dysfunction treatment. The present study investigated the effects of orchidectomy (OE) and transplantation of interstitial cell suspension (ICS) on rat sexual behavior. Adult experimental animals were divided into two populations. One of these populations had sexual experience before the experiment and the other did not. Each population was divided into three groups: control group and two orchidectomized groups. One of the orchidectomized groups was treated with ICS, and the other was treated with the vehicle. The changes in the sexual behavior were investigated on the following parameters: mount latency (ML), intromission latency (IL), ejaculation latency (EL), mount frequency (MF), intromission frequency (IF), copulatory efficacy (CE), and IF/EL ratio. The investigation of these changes lasted 4 weeks after ICS transplantation. The parameters of sexual behavior reflected a decrease in sexual function after OE at the beginning of the observation, especially for the animals that did not have a sexual experience. However, it was shown that sexual activity increased in the following 4 weeks. We have indicated that the loss of gonads attenuated the capacity to acquire sexual experience; nonetheless, it did not mean that the animals completely lost this capacity. Transplantation of ICS facilitated the maintenance of male sexual behavior after OE, fractionally enlarged the size of regressed seminal vesicles of the animals, and increased the free testosterone (T) level. These findings suggest the ICS can be considered as a temporal source of androgens, which can facilitate a restoration of sexual activity.

Steroid-independent male sexual behavior in B6D2F2 male mice

It is well established that male sexual behavior (MSB) is regulated by gonadal steroids; however, individual differences in MSB, independent of gonadal steroids, are prevalent across a wide range of species, and further investigation is necessary to advance our understanding of steroid-independent MSB. Studies utilizing B6D2F1 hybrid male mice in which a significant proportion retain MSB after long-term orchidectomy, identified as steroid-independent-maters (SI-maters), have begun to unravel the genetic underpinnings of steroid-independent MSB. A recent study demonstrated that steroid-independent MSB is a heritable behavioral phenotype that is mainly passed down from B6D2F1 hybrid SI-maters when crossed with C57BL6J female mice. To begin to uncover whether the strain of the dam plays a role in the inheritance of steroid-independent MSB, B6D2F1 hybrid females were crossed with B6D2F1 hybrid males. While the present study confirms the finding that steroid-independent MSB is a heritable behavioral phenotype and that SI-mater sires are more likely to pass down some components of MSB than SI-non-maters to their offspring, it also reveals that the B6D2F2 male offspring that were identified as SI-maters that displayed the full repertoire of steroid-independent MSB had the same probability of being sired from either a B6D2F1 SI-mater or SI-non-mater. These results, in conjunction with previous findings, indicate that the specific chromosomal loci pattern that codes for steroid-independent MSB in the B6D2F2 male offspring may result regardless of whether the father was a SI-mater or SI-non-mater, and that the maternal strain may be an important factor in the inheritance of steroid-independent MSB.

Inheritance of steroid-independent male sexual behavior in male offspring of B6D2F1 mice

The importance of gonadal steroids in modulating male sexual behavior is well established. Individual differences in male sexual behavior, independent of gonadal steroids, are prevalent across a wide range of species, including man. However, the genetic mechanisms underlying steroid-independent male sexual behavior are poorly understood. A high proportion of B6D2F1 hybrid male mice demonstrates steroid-independent male sexual behavior (identified as "maters"), providing a mouse model that opens up avenues of investigation into the mechanisms regulating male sexual behavior in the absence of gonadal hormones. Recent studies have revealed several proteins that play a significant factor in regulating steroid-independent male sexual behavior in B6D2F1 male mice, including amyloid precursor protein (APP), tau, and synaptophysin. The specific goals of our study were to determine whether steroid-independent male sexual behavior was a heritable trait by determining if it was dependent upon the behavioral phenotype of the B6D2F1 sire, and whether the differential expression of APP, tau, and synaptophysin in the medial preoptic area found in the B6D2F1 sires that did and did not mate after gonadectomy was similar to those found in their male offspring. After adult B6D2F1 male mice were bred with C57BL/6J female mice, they and their male offspring (BXB1) were orchidectomized and identified as either maters or "non-maters". A significant proportion of the BXB1 maters was sired only from B6D2F1 maters, indicating that the steroid-independent male sexual behavior behavioral phenotype of the B6D2F1 hybrid males, when crossed with C57BL/6J female mice, is inherited by their male offspring. Additionally, APP, tau, and synaptophysin were elevated in in the medial preoptic area in both the B6D2F1 and BXB1 maters relative to the B6D2F1 and BXB1 non-maters, respectively, suggesting a potential genetic mechanism for the inheritance of steroid-independent male sexual behavior.

Male Presence can Increase Body Mass and Induce a Stress-Response in Female Mice Independent of Costs of Offspring Production

Sexual reproduction in animals requires close interactions with the opposite sex. These interactions may generate costs of reproduction, because mates can induce detrimental physiological or physical effects on one another, due to their interest in maximising their own fitness. To understand how a male’s presence influences aspects of female physiology implicated in reproductive costs in mice, independent of offspring production, we paired females with vasectomised, castrated or intact males, or other females. Being paired with a male, irrespective of his gonadal status, increased female weight. This effect was transient in females paired with castrated males but more persistent in those with vasectomised males. Those paired with males also showed an increase in corticosterone, suggesting an increased stress response. However, this was dependent on the gonadal status of the male housing partner, since those housed with vasectomised males had lower corticosterone than those with castrated males. Altered energy metabolism was only detectable in pregnant females, and oxidative stress was not consistently affected by a female’s housing partner. These results suggest that a male’s presence alters female weight, and stresses associated with reproduction could be induced by simply the presence of a male, but reduced by mating and/or being solicited to mate.

A herbal medicine, saikokaryukotsuboreito, improves serum testosterone levels and affects sexual behavior in old male mice

Late-onset hypogonadism (LOH) is a clinical syndrome characterized with aging and declined serum testosterone levels. Sexual symptoms are also essential for the diagnosis of LOH. Testosterone replacement therapy is used widely to treat LOH. However, the side effects of it should not be ignored, such as fluid retention, hypertension and spermatogenic suppression. Therefore, alternate treatment modalities have been pursued. Herbal medicines used widely in China have achieved satisfying results with little side effects. Nonetheless, there are few pharmacological researches on them. In this study, 24-month-old mice were used as LOH animal models to explore the pharmacological effects of a herbal medicine, saikokaryukotsuboreito (SKRBT), on serum testosterone levels and sexual functions. Furthermore, the expression of steroidogenic acute regulatory (StAR) protein, a kind of rate-limiting enzyme of testosterone synthesis, was also examined. As a result, SKRBT improved the serum testosterone levels of these mice at a dose of 300 and 450 mg/kg. Multiple measures of sexual behavior were enhanced. The expression of StAR was also increased. Therefore, this study suggested that SKRBT can improve the serum testosterone levels by activating the expression of StAR and might be a viable option to treat sexual symptoms caused by LOH.

Increased dendritic spine density and tau expression are associated with individual differences in steroidal regulation of male sexual behavior

Male sexual behavior (MSB) is modulated by gonadal steroids, yet this relationship is highly variable across species and between individuals. A significant percentage (~30%) of B6D2F1 hybrid male mice demonstrate MSB after long-term orchidectomy (herein after referred to as "maters"), providing an opportunity to examine the mechanisms that underlie individual differences in steroidal regulation of MSB. Use of gene expression arrays comparing maters and non-maters has provided a first pass look at the genetic underpinnings of steroid-independent MSB. Surprisingly, of the ~500 genes in the medial preoptic area (MPOA) that differed between maters and non-maters, no steroid hormone or receptor genes were differentially expressed between the two groups. Interestingly, best known for their association with Alzheimer's disease, amyloid precursor protein (APP) and the microtubule-associated protein tau (MAPT) were elevated in maters. Increased levels of their protein products (APP and tau) in their non-pathological states have been implicated in cell survival, neuroprotection, and supporting synaptic integrity. Here we tested transgenic mice that overexpress tau and found facilitated mounting and intromission behavior after long-term orchidectomy relative to littermate controls. In addition, levels of synaptophysin and spinophilin, proteins generally enriched in synapses and dendritic spines respectively, were elevated in the MPOA of maters. Dendritic morphology was also assessed in Golgi-impregnated brains of orchidectomized B6D2F1 males, and hybrid maters exhibited greater dendritic spine density in MPOA neurons. In sum, we show for the first time that retention of MSB in the absence of steroids is correlated with morphological differences in neurons.

Update on corpus cavernosum smooth muscle contractile pathways in erectile function: a role for testosterone?

INTRODUCTION

Normal erectile function (EF) involves a coordinated relaxation of the arteries that supply the penis and the corpus cavernosum smooth muscle (CCSM), resulting in expansion of the sinusoids and increased intracavernous pressure. But the CCSM spends the majority of its time in the contracted state which is mediated by norepinephrine released from nerve endings and other vasoconstrictors like endothelins released from the endothelium. These agents cause smooth muscle myosin (SMM) phosphorylation by elevating intracellular calcium. When calcium returns to basal levels, the calcium sensitivity increases and prevents myosin dephosphorylation, which involves the RhoA/Rho-kinase (ROK) mechanism, thus maintaining force. Although mounting evidences demonstrate that androgens have a major influence on EF that is not just centrally mediated, this notion remains quite controversial.

AIM

To summarize the current knowledge on CCSM contractile pathways, the role they play in modulating EF, and the influence of androgens.

METHODS

The article reviews the literature and contains some previously unpublished data on CCSM contraction signaling including the role that androgens are known to play in modulating these pathways.

MAIN OUTCOME MEASURES

Data from peer-reviewed publications and previously unpublished observations.

RESULTS

In addition to downregulation of many pro-erectile molecular mechanisms, decreased testosterone (T) levels upregulate CCSM contractility, including hyperresponsiveness to α-adrenergic agonists, increased SMM phosphorylation, alteration of SMM isoform composition, activation of RhoA/ROK signaling and modulation of sphingosine-1-phosphate regulation of CCSM tone.

CONCLUSIONS

Decreased T levels upregulate CCSM contractile signaling. Meanwhile, it downregulates CCSM relaxation pathways synergizing to produce erectile dysfunction (ED). Although some urologists and researchers are still skeptical of the influence of androgens on penile erection, understanding these molecular control mechanisms as well as the influence that androgens have on these pathways should provide new evidence supporting the roles of androgens in EF and enhance the discovery of novel targets for drug development to treat ED.

Male sexual behavior does not require elevated testosterone in a lizard (Coleonyx elegans, Eublepharidae)

Male sexual behavior depends on gonadal androgens in species of all major vertebrate lineages, including reptiles. However, male sexual behavior includes distinct appetitive and consummatory phases, typically denoted as courtship and mounting, with potentially different hormonal control. Different proximate controls of courtship versus mounting could enable disconnected evolutionary losses and gains of various aspects of male sexual behavior. Male courtship display, which is activated by testosterone (T) in many species, is an ancestral trait in the lizard family Eublepharidae. However, Coleonyx elegans (Yucatan Banded Gecko) lost the courtship display, while retaining a highly simplified male sexual behavior that involves only mounting for copulation. We performed surgical manipulations (castration with and without T replacement in adult males; implantation of adult females with exogenous T) to investigate hormonal mechanisms involved in this evolutionary novelty. Our results indicate that the expression of simplified sexual behavior in C. elegans does not require elevated circulating levels of T, a finding that is previously unreported in lizards. In females, however, exogenous T induced male-like mounting. Thus, the mounting phase of sexual behavior is not activated by T in the traditional sense of this term but probably requires post-natal, maturational organization (if not periodic reorganization) by androgens. We conclude that the simplification of male sexual behavior and its independence from elevated levels of circulating androgens in C. elegans evolved via 1) evolutionary loss of the androgen-activated courtship display and 2) retention of the mounting phase, which has a longer "functional memory" for the effects of androgenic steroids.

Amyloid beta precursor protein regulates male sexual behavior

Sexual behavior is variable between individuals, ranging from celibacy to sexual addictions. Within normal populations of individual men, ranging from young to middle aged, testosterone levels do not correlate with libido. To study the genetic mechanisms that contribute to individual differences in male sexual behavior, we used hybrid B6D2F1 male mice, which are a cross between two common inbred strains (C57BL/6J and DBA/2J). Unlike most laboratory rodent species in which male sexual behavior is highly dependent upon gonadal steroids, sexual behavior in a large proportion of these hybrid male mice after castration is independent of gonadal steroid hormones and their receptors; thus, we have the ability to discover novel genes involved in this behavior. Gene expression arrays, validation of gene candidates, and transgenic mice that overexpress one of the genes of interest were used to reveal genes involved in maintenance of male sexual behavior. Several genes related to neuroprotection and neurodegeneration were differentially expressed in the hypothalamus of males that continued to mate after castration. Male mice overexpressing the human form of one of these candidate genes, amyloid beta precursor protein (APP), displayed enhanced sexual behavior before castration and maintained sexual activity for a longer duration after castration compared with controls. Our results reveal a novel and unexpected relationship between APP and male sexual behavior. We speculate that declining APP during normal aging in males may contribute to the loss of sexual function.

Neuroendocrinology and sexual differentiation in eusocial mammals

Sexual differentiation of the mammalian nervous system has been studied intensively for over 25 years. Most of what we know, however, comes from work on relatively non-social species in which direct reproduction (i.e., production of offspring) is virtually the only route to reproductive success. In social species, an individual's inclusive fitness may include contributions to the gene pool that are achieved by supporting the reproductive efforts of close relatives; this feature is most evident in eusocial organisms. Here, we review what is known about neuroendocrine mechanisms, sexual differentiation, and effects of social status on the brain and spinal cord in two eusocial mammals: the naked mole-rat and Damaraland mole-rat. These small rodents exhibit the most rigidly organized reproductive hierarchy among mammals, with reproduction suppressed in a majority of individuals. Our findings suggest that eusociality may be associated with a relative lack of sex differences and a reduced influence of gonadal hormones on some functions to which these hormones are usually tightly linked. We also identify neural changes accompanying a change in social and reproductive status, and discuss the implications of our findings for understanding the evolution of sex differences and the neuroendocrinology of reproductive suppression.

Androgen- and estrogen-independent regulation of copulatory behavior following castration in male B6D2F1 mice

Male reproductive behavior is highly dependent upon gonadal steroids. However, between individuals and across species, the role of gonadal steroids in male reproductive behavior is highly variable. In male B6D2F1 hybrid mice, a large proportion (about 30%) of animals demonstrate the persistence of the ejaculatory reflex long after castration. This provides a model to investigate the basis of gonadal steroid-independent male sexual behavior. Here we assessed whether non-gonadal steroids promote mating behavior in castrated mice. Castrated B6D2F1 hybrids that persisted in copulating (persistent copulators) were treated with the androgen receptor blocker, flutamide, and the aromatase enzyme inhibitor, letrozole, for 8 weeks. Other animals were treated with the estrogen receptor blocker, ICI 182,780, via continual intraventricular infusion for 2 weeks. None of these treatments eliminated persistent copulation. A motivational aspect of male sexual behavior, the preference for a receptive female over another male, was also assessed. This preference persisted after long-term castration in persistent copulators, and administration of ICI 182,780 did not influence partner preference. To assess the possibility of elevated sensitivity to sex steroids in brains of persistent copulators, we measured mRNA levels for genes that code for the estrogen receptor-alpha, androgen receptor, and aromatase enzyme in the medial preoptic area and bed nucleus of the stria terminalis. No differences in mRNA of these genes were noted in brains of persistent versus non-persistent copulators. Taken together our results suggest that non-gonadal androgens and estrogens do not maintain copulatory behavior in B6D2F1 mice which display copulatory behavior after castration.

Effects of sex chromosome aneuploidy on male sexual behavior

Incidence of sex chromosome aneuploidy in men is as high as 1:500. The predominant conditions are an additional Y chromosome (47,XYY) or an additional X chromosome (47,XXY). Behavioral studies using animal models of these conditions are rare. To assess the role of sex chromosome aneuploidy on sexual behavior, we used mice with a spontaneous mutation on the Y chromosome in which the testis-determining gene Sry is deleted (referred to as Y(-)) and insertion of a Sry transgene on an autosome. Dams were aneuploid (XXY(-)) and the sires had an inserted Sry transgene (XYSry). Litters contained six male genotypes, XY, XYY(-), XXSry, XXY(-)Sry, XYSry and XYY(-)Sry. In order to eliminate possible differences in levels of testosterone, all of the subjects were castrated and received testosterone implants prior to tests for male sex behavior. Mice with an additional copy of the Y(-) chromosome (XYY(-)) had shorter latencies to intromit and achieve ejaculations than XY males. In a comparison of the four genotypes bearing the Sry transgene, males with two copies of the X chromosome (XXSry and XXY(-)Sry) had longer latencies to mount and thrust than males with only one copy of the X chromosome (XYSry and XYY(-)Sry) and decreased frequencies of mounts and intromissions as compared with XYSry males. The results implicate novel roles for sex chromosome genes in sexual behaviors.

The spinal nucleus of the bulbocavernosus: firsts in androgen-dependent neural sex differences

Cell number in the spinal nucleus of the bulbocavernosus (SNB) of rats was the first neural sex difference shown to differentiate under the control of androgens, acting via classical intracellular androgen receptors. SNB motoneurons reside in the lumbar spinal cord and innervate striated muscles involved in copulation, including the bulbocavernosus (BC) and levator ani (LA). SNB cells are much larger and more numerous in males than in females, and the BC/LA target muscles are reduced or absent in females. The relative simplicity of this neuromuscular system has allowed for considerable progress in pinpointing sites of hormone action, and identifying the cellular bases for androgenic effects. It is now clear that androgens act at virtually every level of the SNB system, in development and throughout adult life. In this review we focus on effects of androgens on developmental cell death of SNB motoneurons and BC/LA muscles; the establishment and maintenance of SNB motoneuron soma size and dendritic length; BC/LA muscle morphology and physiology; and behaviors controlled by the SNB system. We also describe new data on neurotherapeutic effects of androgens on SNB motoneurons after injury in adulthood.

Androgen-sensitivity of somata and dendrites of spinal nucleus of the bulbocavernosus (SNB) motoneurons in male C57BL6J mice

In rats, androgens in adulthood regulate the morphology of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), including the size of their somata and the length of their dendrites. There are conflicting reports about whether androgens exert similar influences on SNB motoneurons in mice. We castrated or sham-operated C57BL6J mice at 90 days of age and, thirty days later, injected cholera toxin conjugated horseradish peroxidase into the bulbocavernosus muscle (to label SNB motoneurons) on one side, and into intrinsic foot muscles contralaterally (to label motoneurons of the retrodorsolateral nucleus (RDLN)). Castrated mice had significantly smaller SNB somas compared to sham-operated mice while there were no differences in soma size of RDLN motoneurons. Dendritic length in C57BL6J mice, estimated in 3-dimensions, also decreased significantly after adult castration. In rats, androgens act directly through androgen receptors (AR) in SNB motoneurons to control soma size and nearly all SNB motoneurons contain AR. Since SNB somata in C57BL6J mice shrank after adult castration, we used immunocytochemistry to characterize AR expression in SNB cells as well as motoneurons in the RDLN and dorsolateral nucleus (DLN). A pattern of labeling matched that seen previously in rats: the highest percentage of AR-immunoreactive motoneurons are in the SNB (98%), the lowest in the RDLN (25%) and an intermediate number in the DLN (78%). This pattern of AR labeling is consistent with the possibility that androgens also act directly on SNB motoneurons in mice to regulate soma size in mice.

Post-castration retention of reproductive behavior and olfactory preferences in male Siberian hamsters: role of prior experience

Reproductive behavior of virtually all adult male rodents is dependent on concurrent availability of gonadal steroids. The ejaculatory reflex is incompatible with long-term absence of testicular steroids and typically disappears within 3 weeks after castration. Male Siberian hamsters are an exception to this rule; mating culminating in the ejaculatory reflex occurs as many as 6 months after castration (persistent copulation). The emergence of persistent copulation many weeks after gonadectomy is here shown not to require repeated post-castration sexual experience. Preoperative sexual experience, on the other hand, significantly increases the percent of males that copulate after gonadectomy, but is not required for the emergence of this trait in 25% of males. Castration prior to puberty prevents persistent copulation in all individuals in adulthood. Persistent copulators, unlike males that cease mating activity after castration, prefer the odors of estrous over non-estrous females when tested 4 months after castration and 7 weeks after the last mating test. Neural circuits of persistent copulators retain the ability to mediate male sex behavior and preferences for female odors in the complete absence of gonadal steroids; they are influenced by preoperative sexual experience and organizational effects of gonadal hormones at the time of puberty.

Exogenous testosterone reverses age-related atrophy in a spinal neuromuscular system

Aging is associated with a variety of pathologies, including motor dysfunctions and reductions in sexual behavior. In male rats, declines in sexual behavior during the aging process may be caused in part by the loss of the lumbar spinal cord motoneurons that innervate the penile musculature. Alternatively, declining sexual behavior may be caused by the precipitous reductions in circulating testosterone that occur during aging. In this paper, we report two experiments examining these issues. In Experiment 1, we counted motoneurons in the lumbar motor nuclei and measured several androgen-sensitive morphological properties of the penile muscles and their innervating motoneurons at several time points during the aging process. Motoneuron number in the lumbar nuclei did not change over time, even with very advanced age. In contrast, the penile muscles and their innervating motoneurons underwent profound atrophy, with muscle weight and motoneuron dendritic length declining to less than 50% of young adult levels. In Experiment 2, we treated aged animals with exogenous testosterone, and then examined their penile neuromuscular systems for morphological changes. Testosterone treatment, both acute and chronic, completely reversed age-related declines in the weight of the penile muscles and in the soma size and dendritic length of their innervating motoneurons. Together, these data suggest that reductions in male sexual behavior during the aging process are caused primarily by declines in testosterone levels rather than motoneuron loss. Furthermore, they raise the possibility that testosterone treatment could play an important role in maintaining neuronal connectivity in the aging body.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Of mice and missing data: what we know (and need to learn) about male sexual behavior

With recent advances in molecular genetics, the popularity of mice as subjects for behavioral neuroscience is increasing at an exponential rate. Unfortunately, the existing body of knowledge on sexual behavior in male mice is not large and many basic gaps exist. The assumption that what is true of rats is also true of mice is a dangerous one that can misdirect and, in the worst case, impede progress. We summarize the current knowledge about the sexual behavior of male mice, with an emphasis on hormonal bases of these behaviors. Behavioral differences between strains, developmental actions of steroids, activational actions of steroids given peripherally and in the brain, and data generated in various receptor knockout and related mice are discussed. In addition, suggestions are made for the standardization of experimental protocols used in investigations of the sexual physiology and behavior of male mice in order to facilitate between-experiment and between-laboratory comparisons and to expedite the growth of knowledge in this area.

Background matters: the effects of estrogen receptor alpha gene disruption on male sexual behavior are modified by background strain

One approach to study interactions between behavior and genetics is to use inbred mice with different genetic backgrounds. To examine the effect of background on a specific gene, we conducted a series of experiments with a well-characterized knockout (KO) mouse, the estrogen receptor alpha KO (ERalphaKO). The ERalphaKO mouse has so far been examined in one inbred line, C57BL/6J. Here, we examined the behavior of ERalphaKO mice within three different backgrounds mixed with C57BL/6J; DBA/2J, BALB/c, and A/J. First, we assessed masculine sexual behavior in both intact male and testosterone-treated female offspring. More ERalphaKO males in the DBA/2J (5/12) and BALB/c (5/13) backcrosses displayed intromissions and many ejaculated as compared with males in a C57BL/6J and A/J mixed background. Many fewer ERalphaKO females than males displayed masculine sexual behavior in any of the three hybrid crosses. We assessed fertility in males from the C57BL/6J by DBA/2J cross and found that one of 12 ERalphaKO males sired a litter. Several other characteristics of sexual behavior and physiology were unaffected by genetic background in ERalphaKO mice. Our data suggest that genetic background has dramatic effects on male sexual behavior and its dependence on the ERalpha gene.

Long-term persistence of male copulatory behavior in castrated and photo-inhibited Siberian hamsters

Gonadal steroids are essential for the long-term maintenance of the full repertoire of sexual behavior in male rodents. Typically, all individuals of several species cease to display the ejaculatory reflex within a few weeks of castration. The present study documents the persistence of the ejaculatory reflex 19 weeks after orchidectomy in 40% of male Siberian hamsters maintained in long or short day lengths; testosterone was undetectable in the circulation of these animals. Intact hamsters transferred from a long to a short photoperiod underwent gonadal regression: 50% of these animals continued to display mating behavior culminating in ejaculation throughout 25 weeks of testing. The remaining animals failed to ejaculate after approximately 11 weeks of short day treatment but resumed mating coincident with spontaneous gonadal recrudescence. Activation of sex behavior in the latter cohort appears to depend on gonadal steroids and is in contrast to the copulatory behavior of the substantial proportion of the study population that sustains the full sexual repertoire in the long-term absence of gonadal steroids. Sex behavior of the latter animals may be dependent on nongonadal steroids or mediation by steroid-independent mechanisms.

Sexual behavior reduces hypothalamic androgen receptor immunoreactivity

Male sexual behavior is regulated by limbic areas like the medial preoptic nucleus (MPN), the bed nucleus of the stria terminalis (BST), the nucleus accumbens (nAcc) and the ventromedial hypothalamic nucleus (VMN). Neurons in these brain areas are rich in androgen receptors (AR) and express FOS-immunoreactivity in response to mating. In many species sexual satiation, a state of sexual behavior inhibition, is attained after multiple ejaculations. The mechanisms underlying sexual satiation are largely unknown. In this study we show that sexual activity reduces androgen receptor immunoreactivity (AR-ir) in some of the brain areas associated with the control of male sexual behavior, but not in others. Thus, one ejaculation reduced the AR-ir in the MPN and nAcc, but not in the BST and VMN. Copulation to satiation, on the other hand, reduced AR-ir in the MPN, nAcc and VMN, and not in the BST. The AR-ir reduction observed in the MPN of sexually satiated rats was drastic when compared to that of animals ejaculating once. Serum androgen levels did not vary after one ejaculation or copulation to exhaustion. These data reveal that sexual activity reduces AR in specific brain areas and suggest the possibility that such a reduction underlies the sexual inhibition that characterizes sexual satiety.

Castration reduces motoneuron soma size but not dendritic length in the spinal nucleus of the bulbocavernosus of wild-type and BCL-2 overexpressing mice

Motoneurons in the spinal nucleus of the bulbocavernosus (SNB) and their target muscles, bulbocavernosus and levator ani (BC/LA), constitute an androgen-sensitive neuromuscular system. Testosterone regulates SNB soma size, SNB dendritic length, and BC/LA muscle mass in adult male rats. Recent evidence indicates that the cell death-regulatory protein, Bcl-2, may also play a role in adult neural plasticity. The present study examined whether gonadal hormones and/or the Bcl-2 protein influence the morphology of the SNB neuromuscular system in adult B6D2F1 mice. In Experiment 1, adult wild-type and Bcl-2 overexpressing males were castrated and implanted with silastic capsules containing testosterone or left blank. Six weeks after castration, cholera toxin-horseradish peroxidase was injected into the BC muscle to label SNB dendrites. Animals were killed 48 h later, and BC/LA muscle mass, SNB soma size, and SNB dendritic arbors were examined. In Experiment 2, wild-type and Bcl-2 overexpressing males were castrated or sham castrated, implanted with testosterone-filled or blank capsules, and examined 12 weeks later. In both experiments, BC/LA muscle mass and SNB soma size were significantly reduced in castrates receiving blank capsules. Surprisingly, however, there was no effect of hormone manipulation on any of several measures of dendritic length. Thus, the dendritic morphology of SNB motoneurons appears to be relatively insensitive to circulating androgen levels in B6D2F1 mice. Bcl-2 overexpression did not influence BC/LA muscle mass, SNB soma size, or SNB dendritic length, indicating that the morphology of this neuromuscular system and the response to castration are not altered by forced expression of the Bcl-2 protein.

Participation of 5-HT(1B) receptors in the inhibitory actions of serotonin on masculine sexual behaviour of mice: pharmacological analysis in 5-HT(1B) receptor knockout mice

1 The role of the 5-Hydroxytryptamine(1B) (5-HT(1B)) receptor subtype in masculine sexual behaviour in mice was analysed in both 5-HT(1B) receptor knockout (KO(1B)) and wild-type (WT) animals. 2 Comparison of male copulatory behaviour of WT and KO(1B) strains revealed that KO(1B) mice become interested earlier in sexual behaviour, but require more stimulation to achieve ejaculation than its corresponding WT strain. 3 The pharmacological manipulation of male sexual activity in the WT strain showed that the serotonin precursor 5-Hydroxytryptophan (5-HTP), the 5-HT(1B) agonist (1-(m-trifluoromethylphenyl) piperazine (TFMPP) and the 5-Hydroxytryptamine(1A) (5-HT(1A)) receptor agonist 8-hydroxy-2-di-n-propylamino-tetralin (8-OH-DPAT) all inhibited male copulatory behaviour in mice. 4 In KO(1B) mice, TFMPP lacked an effect, 5-HTP exerted a mild inhibitory effect while 8-OH-DPAT provoked only a tendency towards a reduction in the percentage of animals that achieved ejaculation. In general, KO(1B) mice were less sensitive to the inhibitory actions of 5-HTP and 8-OH-DPAT than the WT strain. 5 Based on these results, we can suggest that serotonin plays a general inhibitory role in the sexual behaviour of male mice and that both 5-HT(1B) and 5-HT(1A) receptor subtypes participate in the inhibitory actions of this neurotransmitter. 6 The absence of the 5-HT(1B) receptor subtype affected both components of mouse masculine sexual behaviour, motivation and execution, further confirming the involvement of this receptor subtype in the control of this behaviour. In addition, the diminished sensitivity to serotonergic stimulation exhibited by KO(1B) mice suggests the occurrence of compensatory changes as a consequence of the absence of the 5-HT(1B) receptor subtype.

Castration increases pulsatile luteinizing hormone release, but fails to diminish mounting behavior in sexually experienced bulls

We tested the hypothesis that mounting and chemoinvestigatory behaviors are testosterone-dependent in bulls. Eighteen bulls were divided into three treatment groups: intact (I), castrated (C) and castrated+testosterone (T). Sexual behaviors of all bulls were tested with an unrestrained receptive female 1 week prior to and weekly for 4 weeks after castration. Mounts with intromissions, aborted mounts and flehmen responses were quantified for each test period. In addition, patterns of LH and testosterone secretion were assessed at these times. Neither mounts with intromissions nor aborted mounts were affected by treatment. In contrast, numbers of flehmen responses were lower in C bulls than in the other groups following castration. Before castration, concentrations of LH were not different among groups and LH pulse frequency was approximately one pulse per hour. Castration resulted in a 2-fold increase in mean concentrations of LH and a 6-fold increase in LH pulse frequency. Neither mean concentration of LH nor LH pulse frequency changed in I or T bulls. The data fail to support the hypothesis that mounting behavior is T-dependent, but supports the hypothesis that this steroid hormone regulates flehmen behavior in sexually experienced bulls.

Regulation of male sexual behavior by progesterone receptor, sexual experience, and androgen

Recent studies have demonstrated that physiological doses of progesterone may facilitate the androgen-dependent display of male sexual behavior in laboratory rats and three species of lizard. We used mice with a targeted disruption of the progesterone receptor to investigate whether such interactions exist in male mice and whether they may be modified by sexual experience. We found that naive intact male progesterone receptor knockout (PRKO) mice exhibit reduced mount frequencies compared to wild-type (WT) mice. Also unlike WT mice, sexually experienced PRKO males show profound losses in many measures of sexual behavior following castration. In a second experiment, we tested whether male mice heterozygous for a null mutation at the progesterone receptor locus were responsive to testosterone and progesterone treatment. We found that heterozygous males showed a reduced response to testosterone. The data are consistent with experiments indicating that the progesterone receptor is able to facilitate male-typical sex behaviors in other species and suggest novel mechanisms underlying the interaction of androgens and experience.

Calcitonin gene-related peptide-like immunoreactivity in spinal motoneurons of the male mouse is affected by castration and genotype

Calcitonin gene-related peptide (CGRP) is found in motoneurons of the mammalian spinal cord, including motoneurons of the androgen-dependent spinal nucleus of the bulbocavernosus (SNB) of the mouse. Motoneurons of the SNB innervate the bulbocavernosus (BC), a striated muscle involved in penile reflexes. CGRP is though to be a trophic factor produced by motoneurons to regulate the expression of the acetylcholine receptor at the neuromuscular junction. In rats, the number of SNB motoneurons containing CGRP is increased by gonadal steroids. This regulation appears to rely on an activity-dependent factor produced by the BC muscle. The purpose of the present study was to examine, using immunohistochemistry, the steroid dependence of CGRP in the SNB of male house mice. Genotypic differences in the steroid regulation of CGRP immunoreactivity were examined in three strains of mice that differ in their behavioral sensitivity to castration. The results demonstrate that castration reduces the number of CGRP-positive SNB motoneurons in mice. The magnitude of the change in CGRP in response to castration and the length of time required following castration to alter CGRP were dependent on genotype. Interestingly, the effect of castration in mice, to reduce the number of CGRP-immunoreactive SNB motoneurons, is opposite in direction from the increase in CGRP SNB motoneurons observed in rats observed following castration. These experiments suggest that androgens may alter neuromuscular junction function of mouse SNB by regulating the production of CGRP in a species-specific, genotypically dependent fashion.

Sexual activity and the morphology of steroid-sensitive rat spinal motoneurons

In adult male rats, differential sexual experience has been shown to alter plasma androgen titers. Alterations in androgen levels have also been shown to significantly alter the structure of neurons in the spinal nucleus of the bulbocavernosus (SNB), a spinal motor nucleus that innervates perineal muscles involved in copulatory behavior. To determine if experientially induced alterations in androgen levels might alter SNB structure, male rats were assigned to one of three groups: sexually active, sexually inactive but exposed to inaccessible cycling females, and isolated. After 4 weeks of differential sexual experience, the soma size, total arbor, density, maximal fiber length, and frequency distribution of lengths of dendrites were examined in SNB motoneurons. No differences in motoneuron morphology were observed, suggesting that these motoneurons are not sensitive to socially induced alterations in androgen titers.

Environmental influences on masculine sexual behavior in mice

Retention of masculine copulatory behaviors following castration varies among B6D2F1 male mice. In the present study, we examined the effect of test environment and the amount of behavioral testing after castration, on retention of copulatory responses in castrated B6D2F1 male mice. Results showed that weekly behavioral testing after castration was not necessary for the retention of ejaculatory reflexes. However, the test environment had a major effect. Following castration, 26% of the males completely stopped showing ejaculatory responses when tested in the test arenas. When these males were tested in their home cages, 75% achieved at least one ejaculatory response in four home cage tests. Castrated males that continued to copulate in the test arena situation also achieved ejaculatory reflexes in their home cages. These results indicate that for some B6D2F1 males, the retention of sexual behaviors after castration is influenced by environmental factors.

Anatomical organization of the sexually dimorphic perineal neuromuscular system in the house mouse

The anatomy of the sexually dimorphic motoneuron nuclei of the bulbocavernosus (BC) and ischiocavernosus (IC) muscles, as well as the non-sexually dimorphic external and sphincter (EAS), was examined in hybrid B6D2F1 mice using the retrograde tracer, cholera toxin-bound horseradish peroxidase. Motoneurons innervating the BC were located in the dorsomedial nucleus (DM), as well as in the ventral nucleus (V) and in the mid-region of the ventral horn (MVH). Following injections restricted to the IC, labelled neurons were found in the dorsolateral nucleus (DL), as well as in V, DM and MVH. Cells innervating the EAS in both males and females were located in the DM, as well as in the V and MVH. An elaborate network of dendrites extended between all labelled nuclei. The present results demonstrate that the anatomical specificity of the sexually dimorphic neuromuscular system of the mouse differs from that observed in other species.

Testicular hormones reduce individual differences in the aggressive behavior of male mice: a theory of hormone action

A chronology of theoretical development in studies of the role of testicular hormones in murine aggression is presented. Evidence which brings into question the generality of current theory is reviewed, and the implications of this evidence for the direction of future research are discussed. A new method by which to characterize individual differences in aggressive behavior is described, and recent data which provide the basis for the development of a new theory are presented. It is theorized that testicular hormones reduce individual differences in the aggressive behavior of male mice, and that this behavioral "homogenization" is mediated by a testicular-hormone regulated "discrimination mechanism," possibly localized within the mouse olfactory system. The generality of this theory and the implications for other hormone/behavior systems are also discussed.

Hormonal restoration of masculine sexual behavior in long-term castrated B6D2F1 mice

In contrast to the facilitative effects reported for other rodents, testosterone treatment at the time of castration previously was reported to inhibit masculine sexual behavior in male B6D2F1 mice. Males of this genotype vary in their behavioral response to castration. Some castrates retain sexual behaviors for many weeks after surgery, whereas others do not. In the present study, we sought to determine the effects of exogenous steroid hormone treatment on castrated B6D2F1 mice that had ceased to show copulatory behavior. Testosterone propionate and estradiol benzoate restored copulatory behavior to precastration levels in B6D2F1 males that did not retain sexual behaviors after castration.