Sexual partner preference, hypothalamic morphology and aromatase in rams

Sexual behaviour of young rams is improved and less stressful after intranasal administration of oxytocin

The aim of this study was to determine if intranasal administration of oxytocin modifies sexual behaviour and the stress response in young rams during sexual tests with ewes in oestrus. Ten rams were used in a cross-over design. At Day 0, the control group (CG, n = 5) received isotonic saline spray intranasally, and the treated group (OTG, n = 5) received oxytocin (24 IU) intranasally, 40 min before the sexual test. At Day 15, the groups were reversed. In each sexual test (20 min) with an oestrous-induced ewe, the sexual behaviour of the young rams was recorded. Serum cortisol concentrations were determined before and after the test. Less flehmen was observed in the OTG, but mounts with ejaculation were increased. The OTG presented lower serum cortisol concentration than the CG. In conclusion, intranasal administration of oxytocin modified the sexual behaviour of rams, evidenced by a decrease in flehmen behaviour and an increase in mounts with ejaculation, making sexual activity more efficacious. In addition, the treatment decreased the stress response of the rams in the sexual tests. Therefore, intranasal administration of oxytocin could be used to increase sexual activity in rams, and with less stress, providing better welfare conditions.

To What Extent are Prenatal Androgens Involved in the Development of Male Homosexuality in Humans?

Endocrine variations, including possibly reduced exposure to androgens, may contribute to the development of male homosexuality, with animal models demonstrating same-sex mate preference with altered exposure during prenatal or early postnatal development. As similar studies in humans are impossible, indirect physical and cognitive measures of androgen exposure are used. Some studies suggest that physical measures affected by prenatal androgens, including the index-to-ring finger ratio, growth indices, and facial structure, are more "feminine" in gay men. Gay men also exhibit significant childhood gender non-conformity and a "feminized" anatomical and functional brain pattern in sexual arousal, as well as domains such as language, visuospatial skills and hemispheric relationships. However, many of these results are equivocal and may be confounded by other factors. Research has also been hampered by inconsistencies in the reporting of sexual orientation and the potentially unrepresentative populations of gay men studied, while additional complexities pertaining to gender conformity and sexual role may also influence results.

Alternative Sexual Orientation in Humans: What Is Known and What Needs to Be Known Further

Since the 20th century, multiple studies have linked the variations in human sexual orientation, from heterosexuality to bisexuality or homosexuality, to a wide range of biological factors. However, a clear mechanism that leads to the development of these variations has not been established yet. This review consolidates various comprehensive studies on the possible biological factors in the fields of genetics, epigenetics, uterine environment, hormones, neuroanatomy, and neurobiology that lead to these variations. One intriguing question that 'homosexuality phenotype' faces is its ability to avoid elimination by Darwinian selection. This review tries to explain why natural selection is not eliminating the genetic factors associated with homosexuality even at the cost of the evolutionary fitness of homosexual individuals. Studies supporting certain strong candidates for alternative sexual orientation (ASO) are highlighted, which can become new research avenues for investigators in this field. Further, a novel speculation is proposed that might be contributing to the development of variation in human sexuality.

Central aromatization: A dramatic and responsive defense against threat and trauma to the vertebrate brain

Aromatase is the requisite and limiting enzyme in the production of estrogens from androgens. Estrogens synthesized centrally have more recently emerged as potent neuroprotectants in the vertebrate brain. Studies in rodents and songbirds have identified key mechanisms that underlie both; the injury-dependent induction of central aromatization, and the protective effects of centrally synthesized estrogens. Injury-induced aromatase expression in astrocytes occurs following a broad range of traumatic brain damage including excitotoxic, penetrating, and concussive injury. Responses to neural insult such as edema and inflammation involve signaling pathways the components of which are excellent candidates as inducers of this astrocytic response. Finally, estradiol from astrocytes exerts a paracrine neuroprotective influence via the potent inhibition of inflammatory pathways. Taken together, these data suggest a novel role for neural aromatization as a protective mechanism against the threat of inflammation and suggests that central estrogen provision is a wide-ranging neuroprotectant in the vertebrate brain.

The biological basis of sexual orientation: How hormonal, genetic, and environmental factors influence to whom we are sexually attracted

Humans develop relatively stable attractions to sexual partners during maturation and present a spectrum of sexual orientation from homosexuality to heterosexuality encompassing varying degrees of bisexuality, with some individuals also displaying asexuality. Sexual orientation represents a basic life phenomenon for humans. However, the molecular mechanisms underlying these diverse traits of sexual orientation remain highly controversial. In this review, we systematically discuss recent advancements in sexual orientation research, including those related to measurements and associated brain regions. Current findings regarding sexual orientation modulation by hormonal, genetic, maternal immune system, and environmental factors are summarized in both human and model systems. We also emphasize that future studies should recognize the differences between males and females and pay more attention to minor traits and the epigenetic regulation of sexual orientation. A comprehensive view of sexual orientation may promote our understanding of the biological basis of sex, and that of human reproduction, and evolution.

Influence of sexual behavior of Dorper rams treated with glutamate and/or testosterone on reproductive performance of anovulatory ewes

The aim of this study was to determine if exogenous administration of glutamate and (or) testosterone to male rams during the season of reproductive arrest is able to re-activate male sexual behavior and, later on, to promote through the male effect, both sexual and reproductive competence of anovulatory nulliparous ewes. Therefore, an experiment was performed under long-day photoperiods (spring; photo-reproductive arrest, 26°N). Dorper rams were randomly divided into four homogeneous experimental groups (n = 5 males each) regarding live weight (LW), body condition score (BCS), scrotal circumference (SC) and odor intensity (OI). Then, groups were treated with: i) GG (7 mg kg^-1 LW of glutamate, every 4d × 30d, im.), ii) GGT (7 mg kg^-1 LW of glutamate every 4d × 30d im + 25 mg of testosterone propionate, every 3d × 15d, im.), iii) GT (25 mg of testosterone propionate every 3d × 15d, im, and iv) GC (1 mL of saline, every 4d × 30d, im.). Thereafter, Dorper rams, (n = 4 per group) were selected and exposed to Dorper anovulatory-nulliparous ewes divided in four groups (n = 14 ewes each), and all the appetitive (ASB) and consummatory (CSB) sexual behaviors and indicators of sexual rest (ISR) were registered during the first 48 h of this male-to-females contact. Thereafter, males continued the male-to-female contact for another 8 d, in order to quantify the ewe's sexual and reproductive response through the male effect. During the sexual behavior tests, the GGT rams showed the highest ASB + CSB frequencies (P < 0.05) followed by the GG-rams with the lowest frequencies showed by the GC and GT groups. While the highest ISR behaviors were shown by the GG and GGT groups (P < 0.05) followed by the GC and GT-rams, no differences occurred regarding LW, BCS, and SC along the experimental breeding, with the largest (P < 0.05) OI shown by the GGT-rams and the lowest value observed in the control rams. Regarding the reproductive response of the nulliparous-anovulatory ewes exposed to treated rams, the glutamate + testosterone treated males induced not only an increased (P < 0.05) ovulatory activity and faster (P < 0.05) estrus response but an augmented (P < 0.05) pregnancy rate of the previously anovulatory ewes. To conclude, whereas the GG and GGT treatments generated encouraging sexual and reproductive outcomes, our results are also thought-provoking from a comparative sexual behavior perspective while may embrace potential applications to other animal industries.

GnRH-mediated olfactory and visual inputs promote mating-like behaviors in male zebrafish

The engagement of sexual behaviors is regulated by a number of factors which include gene expression, hormone circulation, and multi-sensory information integration. In zebrafish, when a male and a female are placed in the same container, they show mating-like behaviors regardless of whether they are kept together or separated by a net. No mating-like behaviors are observed when same-sex animals are put together. Through the olfacto-visual centrifugal pathway, activation of the terminalis nerve in the olfactory bulb increases GnRH signaling in the brain and triggers mating-like behaviors between males. In zebrafish mutants or wild-type fish in which the olfacto-visual centrifugal pathway is impaired or chemically ablated, in response to odor stimulation the mating-like behaviors between males are no longer evident. Together, the data suggest that the combination of olfactory and visual signals alter male zebrafish's mating-like behaviors via GnRH signaling.

Developmental and Functional Effects of Steroid Hormones on the Neuroendocrine Axis and Spinal Cord

This review highlights the principal effects of steroid hormones at central and peripheral levels in the neuroendocrine axis. The data discussed highlight the principal role of oestrogens and testosterone in hormonal programming in relation to sexual orientation, reproductive and metabolic programming, and the neuroendocrine mechanism involved in the development of polycystic ovary syndrome phenotype. Moreover, consistent with the wide range of processes in which steroid hormones take part, we discuss the protective effects of progesterone on neurodegenerative disease and the signalling mechanism involved in the genesis of oestrogen-induced pituitary prolactinomas.

Inhabiting the sexual landscape: toward an interpretive theory of the development of sexual orientation and identity

Building on Paula Rust's (1996) concept of a sexual landscape, we propose an interpretive theory of the development of both sexual orientation and sexual identity. We seek to reconcile human agency with active and shifting influences in social context and to recognize the inherent complexity of environmental factors while acknowledging the role that biological potential plays. We ground our model in the insights of three compatible and related theoretical perspectives: social constructionism, symbolic interactionism, and scripting theory. Within this framework, we explain how sexual orientation and sexual identities develop and potentially change.

Asexuality: what it is and why it matters

In this review article, human asexuality, a relatively understudied phenomenon, is discussed. Specifically, definitions and conceptualizations of asexuality (e.g., is it a unique category of sexual orientation?), biological and historical contexts, identity issues, discrimination against asexual people relative to other minorities, origins, and variations, including gender differences, are reviewed. Whether asexuality should be construed as a disorder is also discussed. The study of asexuality allows for a better understanding of an underrecognized sexual minority but also affords a unique opportunity to examine and better understand human sexuality.

Roles of sex and gonadal steroids in mammalian pheromonal communication

A brain circuit (the accessory olfactory system) that originates in the vomeronasal organ (VNO) and includes the accessory olfactory bulb (AOB) plus additional forebrain regions mediates many of the effects of pheromones, typically comprised of a variety of non-volatile and volatile compounds, on aspects of social behavior. A second, parallel circuit (the main olfactory system) that originates in the main olfactory epithelium (MOE) and includes the main olfactory bulb (MOB) has also been shown to detect volatile pheromones from conspecifics. Studies are reviewed that point to specific roles of several different steroids and their water-soluble metabolites as putative pheromones. Other studies are reviewed that establish an adult, 'activational' role of circulating sex hormones along with sex differences in the detection and/or processing of non-steroidal pheromones by these two olfactory circuits. Persisting questions about the role of sex steroids in pheromonal processing are posed for future investigation.

Effects of blocking developmental cell death on sexually dimorphic calbindin cell groups in the preoptic area and bed nucleus of the stria terminalis

Background

Calbindin-D28 has been used as a marker for the sexually dimorphic nucleus of the preoptic area (SDN-POA). Males have a distinct cluster of calbindin-immunoreactive (ir) cells in the medial preoptic area (CALB-SDN) that is reduced or absent in females. However, it is not clear whether the sex difference is due to the absolute number of calbindin-ir cells or to cell position (that is, spread), and the cellular mechanisms underlying the sex difference are not known. We examined the number of cells in the CALB-SDN and surrounding regions of C57Bl/6 mice and used mice lacking the pro-death gene, Bax, to test the hypothesis that observed sex differences are due to cell death.

Methods

Experiment 1 compared the number of cells in the CALB-SDN and surrounding regions in adult males, females, and females injected with estradiol benzoate on the day of birth. In experiment 2, cell number in the CALB-SDN and adjacent regions were compared in wild-type and Bax knockout mice of both sexes. In addition, calbindin-ir cells were quantified within the principal nucleus of the bed nucleus of the stria terminalis (BNSTp), a nearby region that is larger in males due to Bax-dependent cell death.

Results

Males had more cells in the CALB-SDN as well as in surrounding regions than did females, and estradiol treatment of females at birth masculinized both measures. Bax deletion had no effect on cell number in the CALB-SDN or surrounding regions but increased calbindin-ir cell number in the BNSTp.

Conclusions

The sex difference in the CALB-SDN of mice results from an estrogen-dependent difference in cell number with no evidence found for greater spread of cells in females. Blocking Bax-dependent cell death does not prevent sex differences in calbindin-ir cell number in the BNST or CALB-SDN but increases calbindin-ir cell number in the BNSTp of both sexes.

The development of male-oriented behavior in rams

The sheep offers a unique mammalian model in which to study paradoxical same-sex sexual partner preferences. Variations in sexual partner preferences occur spontaneously with as many as 8% of rams in a population exhibiting a sexual preference for other rams (male-oriented). The current review presents an overview and update of the male-oriented ram model and discusses several theories that have been invoked to explain same-sex preferences in this species. Although our understanding of the biological determinants and underlying neural substrates of sexual attraction and mate selection are far from complete, compelling evidence is discussed that supports the idea that neural substrates regulating sexual partner preferences are organized during prenatal development. The challenge for future research will be to construct an integrated picture of how hormones, genes, and experience shape sexual partner preference.

Wired on steroids: sexual differentiation of the brain and its role in the expression of sexual partner preferences

The preference to seek out a sexual partner of the opposite sex is robust and ensures reproduction and survival of the species. Development of female-directed partner preference in the male is dependent on exposure of the developing brain to gonadal steroids synthesized during critical periods of sexual differentiation of the central nervous system. In the absence of androgen exposure, a male-directed partner preference develops. The development and expression of sexual partner preference has been extensively studied in rat, ferret, and sheep model systems. From these models it is clear that gonadal testosterone, often through estrogenic metabolites, cause both masculinization and defeminization of behavior during critical periods of brain development. Changes in the steroid environment during these critical periods result in atypical sexual partner preference. In this manuscript, we review the major findings which support the hypothesis that the organizational actions of sex steroids are responsible for sexual differentiation of sexual partner preferences in select non-human species. We also explore how this information has helped to frame our understanding of the biological influences on human sexual orientation and gender identity.

History of aromatase: saga of an important biological mediator and therapeutic target

Aromatase is the enzyme that catalyzes the conversion of androgens to estrogens. Initial studies of its enzymatic activity and function took place in an environment focused on estrogen as a component of the birth control pill. At an early stage, investigators recognized that inhibition of this enzyme could have major practical applications for treatment of hormone-dependent breast cancer, alterations of ovarian and endometrial function, and treatment of benign disorders such as gynecomastia. Two general approaches ultimately led to the development of potent and selective aromatase inhibitors. One targeted the enzyme using analogs of natural steroidal substrates to work out the relationships between structure and function. The other approach initially sought to block adrenal function as a treatment for breast cancer but led to the serendipitous finding that a nonsteroidal P450 steroidogenesis inhibitor, aminoglutethimide, served as a potent but nonselective aromatase inhibitor. Proof of the therapeutic concept of aromatase inhibition involved a variety of studies with aminoglutethimide and the selective steroidal inhibitor, formestane. The requirement for even more potent and selective inhibitors led to intensive molecular studies to identify the structure of aromatase, to development of high-sensitivity estrogen assays, and to "mega" clinical trials of the third-generation aromatase inhibitors, letrozole, anastrozole, and exemestane, which are now in clinical use in breast cancer. During these studies, unexpected findings led investigators to appreciate the important role of estrogens in males as well as in females and in multiple organs, particularly the bone and brain. These studies identified the important regulatory properties of aromatase acting in an autocrine, paracrine, intracrine, neurocrine, and juxtacrine fashion and the organ-specific enhancers and promoters controlling its transcription. The saga of these studies of aromatase and the ultimate utilization of inhibitors as highly effective treatments of breast cancer and for use in reproductive disorders serves as the basis for this first Endocrine Reviews history manuscript.

The neurobiology of sexual partner preferences in rams

The question of what causes a male animal to seek out and choose a female as opposed to another male mating partner is unresolved and remains an issue of considerable debate. The most developed biologic theory is the perinatal organizational hypothesis, which states that perinatal hormone exposure mediates sexual differentiation of the brain. Numerous animal experiments have assessed the contribution of perinatal testosterone and/or estradiol exposure to the development of a male-typical mate preference, but almost all have used hormonally manipulated animals. In contrast, variations in sexual partner preferences occur spontaneously in domestic rams, with as many as 8% of the population exhibiting a preference for same-sex mating partners (male-oriented rams). Thus, the domestic ram is an excellent experimental model to study possible links between fetal neuroendocrine programming of neural mechanisms and adult sexual partner preferences. In this review, we present an overview of sexual differentiation in relation to sexual partner preferences. We then summarize results that test the relevance of the organizational hypothesis to expression of same-sex sexual partner preferences in rams. Finally, we demonstrate that the sexual differentiation of brain and behavior in sheep does not depend critically on aromatization of testosterone to estradiol.

Porcine hypothalamic aromatase cytochrome P450: isoform characterization, sex-dependent activity, regional expression, and regulation by enzyme inhibition in neonatal boars

Domestic pigs have three CYP19 genes encoding functional paralogues of the enzyme aromatase cytochrome P450 (P450arom) that are expressed in the gonads, placenta, and preimplantation blastocyst. All catalyze estrogen synthesis, but the gonadal-type enzyme is unique in also synthesizing a nonaromatizable biopotent testosterone metabolite, 1OH-testosterone (1OH-T). P450arom is expressed in the vertebrate brain, is higher in males than females, but has not been investigated in pigs, to our knowledge. Therefore, these studies defined which of the porcine CYP19 genes was expressed, and at what level, in adult male and female hypothalamus. Regional expression was examined in mature boars, and regulation of P450arom expression in neonatal boars was investigated by inhibition of P450arom with letrozole, which is known to reprogram testicular expression. Pig hypothalami expressed the gonadal form of P450arom (redesignated the "gonadal/hypothalamic" porcine CYP19 gene and paralogue) based on functional analysis confirmed by cloning and sequencing transcripts. Hypothalamic tissue synthesized 1OH-T and was sensitive to the selective P450arom inhibitor etomidate. Levels were 4-fold higher in male than female hypothalami, with expression in the medial preoptic area and lateral borders of the ventromedial hypothalamus of boars. In vivo, letrozole-treated neonates had increased aromatase activity in hypothalami but decreased activity in testes. Therefore, although the same CYP19 gene is expressed in both tissues, expression is regulated differently in the hypothalamus than testis. These investigations, the first such studies in pig brain to our knowledge, demonstrate unusual aspects of P450arom expression and regulation in the hypothalamus, offering promise of gaining better insight into roles of P450arom in reproductive function.

Prenatal programming of sexual partner preference: the ram model

In our laboratory, the domestic ram is used as an experimental model to study the early programming of neural mechanisms underlying same-sex partner preference. This interest developed from the observation that approximately 8% of domestic rams are sexually attracted to other rams (male-oriented) in contrast to the majority of rams that are attracted to oestrous ewes (female-oriented). One prominent feature of sexual differentiation in many species is the presence of a sexually dimorphic nucleus (SDN) in the preoptic/anterior hypothalamus that is larger in males than in females. Lesion studies in rats and ferrets implicate the SDN in the expression of sexual preferences. We discovered an ovine SDN (oSDN) in the preoptic/anterior hypothalamus that is smaller in male- than in female-oriented rams and similar in size to the oSDN of ewes. Neurones of the oSDN show abundant aromatase expression that is also reduced in male-oriented compared to female-oriented rams. This observation suggests that sexual partner preferences are neurologically hard-wired and could be influenced by hormones. Aromatase-containing neurones constitute a nascent oSDN as early as day 60 of gestation, which becomes sexually dimorphic by day 135 of gestation when it is two-fold larger in males than in females. Exposure of fetal female lambs to exogenous testosterone from days 30-90 of gestation resulted in a masculinised oSDN. These data demonstrate that the oSDN develops prenatally and may influence adult sexual preferences. Surprisingly, inhibition of aromatase activity in the brain of ram foetuses during the critical period did not interfere with defeminisation of adult sexual partner preference or oSDN volume. These results fail to support an essential role for neural aromatase in the sexual differentiation of sheep brain and behaviour. Thus, we propose that oSDN morphology and male-typical partner preferences may instead be programmed through an androgen receptor mechanism not involving aromatisation.

Effect of prenatal androgens on click-evoked otoacoustic emissions in male and female sheep (Ovis aries)

Otoacoustic emissions (OAEs) were measured in male and female Suffolk sheep (Ovis aries). Some sheep had been administered androgens or estrogens during prenatal development, some were gonadectomized after birth, and some were allowed to develop normally. As previously reported for spotted hyenas, gonadectomy did not alter the OAEs for either sex; accordingly, the untreated/intact and the untreated/gonadectomized animals were pooled to form the control groups. The click-evoked otoacoustic emissions (CEOAEs) exhibited by the female control group (N=12) were slightly stronger (effect size=0.42) than those in the male control group (N=15), which is the same direction of effect reported for humans and rhesus monkeys. Females administered testosterone prenatally (N=16) had substantially weaker (masculinized) CEOAEs than control females (effect size=1.15). Both of these outcomes are in accord with the idea that prenatal exposure to androgens weakens the cochlear mechanisms that underlie the production of OAEs. The CEOAEs of males administered testosterone prenatally (N=5) were not different from those of control males, an outcome also seen in similarly treated rhesus monkeys. Males administered dihydrotestosterone (DHT) prenatally (N=3) had slightly stronger (hypo-masculinized) CEOAEs than control males. No spontaneous otoacoustic emissions (SOAEs) were found in any ears, a common finding in non-human species. To our knowledge, this is the first ruminant species measured for OAEs.

The androgen receptor is selectively involved in organization of sexually dimorphic social behaviors in mice

It is well established that sexually dimorphic neural regions are organized by steroid hormones during development. In many species, neonatal males are exposed to more testosterone than their female littermates, and ultimately it is the estradiol, produced by aromatization of testosterone, that affects sexual differentiation. However, the androgen receptor also plays an important role in the masculinization of brain and behavior. Here we tested the hypothesis that sexually dimorphic social and odor preference behaviors can be differentiated by a nonaromatizable androgen during development by treating female mice on the day of birth (PN0) with dihydrotestosterone (DHT). Control mice received a single vehicle injection on PN0. Adults were gonadectomized, treated with estradiol, and tested for social behaviors. In contrast with control females, females treated on PN0 with DHT, like male controls, exhibited a preference for female-soiled vs. male-soiled bedding, a preference to investigate a female vs. a male and reduced c-Fos-immunoreactivity (ir) in several neural areas after exposure to male-soiled bedding. However, females treated with DHT on PN0 had normal female-typical sexual behavior. The number of calbindin-ir cells in the preoptic area is sexually dimorphic (males more than females), but females given DHT on PN0 had intermediate numbers of calbindin-ir neurons, not significantly different from control males or females. Our data demonstrate that organization of social and olfactory preferences in mice can be affected by perinatal DHT and lends support to the role of androgen receptor in organization of sexual differentiation of brain and behaviors.

Sex and seasonal differences in morphology of limbic forebrain nuclei in the green anole lizard

Sex and seasonal differences in the brain occur in many species and are often related to behavioral expression. For example, morphology of limbic regions involved in male sex behavior are larger in males than in females, and sometimes are larger in the breeding than non-breeding season. Morphology can often be altered in adulthood by manipulating levels of steroid hormones. In untreated green anole lizards, previous work indicated that neuron soma size and density did not differ between the sexes in the preoptic area (POA) or ventromedial nucleus of the amygdala (AMY), two brain regions involved in the control of male reproductive behaviors [O'Bryant, E.L., Wade, J., 2002. Seasonal and sexual dimorphisms in the green anole forebrain. Horm. Behav. 41, 384-395.]. However, soma size was larger in both areas in breeding than non-breeding animals. The current study examined sex and seasonal differences in estimated brain region volume and total neuron number in the POA, AMY, and the ventromedial hypothalamus (VMH), a region typically involved in female reproductive behaviors. The volume of the POA was larger in males, and the POA and VMH of breeding animals were larger than those of non-breeding individuals. Differences in cell number did not exist in either of these two regions. In contrast, neuron counts in the AMY were greater in non-breeding than breeding animals, but the volume did not differ between the seasons. These data suggest that the structure of limbic brain regions is dynamic in adulthood and that parallels between morphology and the expression of masculine behavior exist for the POA, whereas other relationships are more complicated.

Central pattern generators for social vocalization: androgen-dependent neurophysiological mechanisms

Historically, most studies of vertebrate central pattern generators (CPGs) have focused on mechanisms for locomotion and respiration. Here, we highlight new results for ectothermic vertebrates, namely teleost fish and amphibians, showing how androgenic steroids can influence the temporal patterning of CPGs for social vocalization. Investigations of vocalizing teleosts show how androgens can rapidly (within minutes) modulate the neurophysiological output of the vocal CPG (fictive vocalizations that mimic the temporal properties of natural vocalizations) inclusive of their divergent actions between species, as well as intraspecific differences between male reproductive morphs. Studies of anuran amphibians (frogs) demonstrate that long-term steroid treatments (wks) can masculinize the fictive vocalizations of females, inclusive of its sensitivity to rapid modulation by serotonin. Given the conserved organization of vocal control systems across vertebrate groups, the vocal CPGs of fish and amphibians provide tractable models for identifying androgen-dependent events that are fundamental to the mechanisms of vocal motor patterning. These basic mechanisms can also inform our understanding of the more complex CPGs for vocalization, and social behaviors in general, that have evolved among birds and mammals.

Steroid-dependent plasticity of vocal motor systems: Novel insights from teleost fish

Vocal communication is a trait shared by most vertebrates. Non-mammalian model systems have provided exquisite examples of how motor and sensory systems, respectively, produce and encode the physical attributes of acoustic communication signals that play essential roles in mediating the dynamics of social behavior. These same models, mainly developed for a few species of fish, amphibians and birds, have proven to be equally important for demonstrating how steroids and other hormones shape the neural mechanisms of vocal communication. This review mainly considers recent studies in teleost fish demonstrating the role of steroids in the rapid modulation of the firing properties of a central pattern generator for vocalization. Thus, steroids, like other classes of neurochemicals, can play an instrumental role in reshaping the neurophysiological coding of motor patterning, in this case for social signaling behavior.

Brain aromatase: roles in reproduction and neuroprotection

It is well established that aromatization constitutes an essential part of testosterone's signaling pathway in brain and that estrogen metabolites, often together with testosterone, organize and activate masculine neural circuits. This paper summarizes the current understanding regarding the distribution, regulation and function of brain aromatase in mammals. Data from our laboratory are presented that highlight the important function of aromatase in the regulation of androgen feedback sensitivity in non-human primates and the possible role that aromatase plays in determining the brain structure and sexual partner preferences of rams. In addition, new data is presented indicating that the capacity for aromatization in cortical astrocytes is associated with cell survival and may be important for neuroprotection. It is anticipated that a better appreciation of the physiological and pathophysiological functions of aromatase will lead to important clinical insights.

The ram as a model for behavioral neuroendocrinology

The sheep offers a unique model to study male sexual behavior and sexual partner preference. Rams are seasonal breeders and show the greatest libido during short days coincident with the resumption of ovarian cyclicity in the ewe. Threshold concentrations of testosterone are required for the acquisition and display of adult sexual behavior. In addition, estrogens produced from circulating testosterone by cytochrome P450 aromatase in the preoptic area are critical for the maintenance of sexual behaviors in rams. Sex differences in adult reproductive behaviors and hormone responsiveness are the result of permanent organizational effects exerted by testosterone and its metabolites on brain development. Early exposure to ewes enhances ram sexual performance, but cannot prevent some rams from exhibiting male-oriented sexual partner preferences. Neurochemical and neuroanatomical studies suggest that male-oriented ram behavior may be a consequence of individual variations in brain sexual differentiation.

Plasticity in brain sexuality is revealed by the rapid actions of steroid hormones

Divergent steroid hormone profiles can shape the development of male versus female neural phenotypes, but whether they also determine differences in the short-term, neurophysiological patterning of behavior is unknown. We now show that steroid hormone-specific modulation of a vocal pattern generator (VPG) diverges between reproductive morphs in a teleost fish. Only type I male midshipman acoustically court females, whereas type II males steal fertilizations from type I males and, like females, generate only agonistic calls. The androgen 11-ketotestosterone (11kT), but not testosterone (T), rapidly (within 5 min) increases type I VPG output. As now shown, T, but not 11kT, rapidly increases VPG output in type II males and females, consistent with the predominant circulating androgen in type II males and females (T) versus type Is (11kT). Receptor and enzyme antagonists reveal an unexpected divergence in androgen- versus estrogen-dependent mechanisms in, respectively, type II males versus females. Cortisol, the main circulating glucocorticoid, also has divergent actions: suppressing versus increasing VPG output in, respectively, type II males and females versus type Is. In summary, rapid steroid action on VPG activity is uncoupled from gonadal phenotype (convergent between type II males and females), whereas the receptor-mediated mechanisms of androgen action are predicted by gonadal phenotype (both male morphs are sensitive to androgen receptor blockade, whereas females are not). A comparable mix of neuroendocrine traits may explain the widespread distribution of intrasexual behavioral phenotypes among teleosts and vertebrates in general. Moreover, the fundamental organization/activation principles that predict the steroid-dependent expression of "maleness" and "femaleness" may now include rapid steroid actions on the neurophysiological patterning of behavior.

Bilateral damage to the sexually dimorphic medial preoptic area/anterior hypothalamus of male ferrets causes a female-typical preference for and a hypothalamic Fos response to male body odors

Previous studies showed that bilateral lesions of the male ferret's preoptic area/anterior hypothalamus (POA/AH), centered in the sexually dimorphic nuclei present in this region, caused subjects to seek out a same-sex male, as opposed to a female conspecific. Male subjects with POA/AH lesions (which were also castrated and given estradiol) displayed female-typical receptive behavior in response to neck gripping by a stimulus male, implying that subjects' approaches to a same-sex conspecific were sexually motivated. We asked whether the effect of POA/AH lesions on males' partner preference reflects a shift in the central processing of body odorant cues so that males come to display a female-typical preference to approach male body odorants. Sexually experienced male ferrets in which electrolytic lesions of the POA/AH caused bilateral damage to the sexually dimorphic male nucleus (MN) resembled sham-operated females by preferring to approach body odors emitted from anesthetized male as opposed to female stimulus ferrets confined in the goal boxes of a Y-maze. This lesion-induced shift in odor preference was correlated with a significant increase in the ability of soiled male bedding to induce a Fos response in the medial POA of males with bilateral damage to the MN-POA/AH. No such partner preference or neural Fos responses were seen in sham-operated males or in other groups of males with POA/AH lesions that either caused unilateral damage or no damage to the MN-POA/AH. Male-typical hypothalamic processing of conspecifics' body odorants may determine males' normal preference to seek out odors emitted by female conspecifics, leading to mating and successful reproduction.

Functional significance of the rapid regulation of brain estrogen action: where do the estrogens come from?

Estrogens exert a wide variety of actions on reproductive and non-reproductive functions. These effects are mediated by slow and long lasting genomic as well as rapid and transient non-genomic mechanisms. Besides the host of studies demonstrating the role of genomic actions at the physiological and behavioral level, mounting evidence highlights the functional significance of non-genomic effects. However, the source of the rapid changes in estrogen availability that are necessary to sustain their fast actions is rarely questioned. For example, the rise of plasma estrogens at pro-estrus that represents one of the fastest documented changes in plasma estrogen concentration appears too slow to explain these actions. Alternatively, estrogen can be synthesized in the brain by the enzyme aromatase providing a source of locally high concentrations of the steroid. Furthermore, recent studies demonstrate that brain aromatase can be rapidly modulated by afferent inputs, including glutamatergic afferents. A role for rapid changes in estrogen production in the central nervous system is supported by experiments showing that acute aromatase inhibition affects nociception as well as male sexual behavior and that preoptic aromatase activity is rapidly (within min) modulated following mating. Such mechanisms thus fulfill the gap existing between the fast actions of estrogen and their mode of production and open new avenues for the understanding of estrogenic effects on the brain.

Roles of estrogen receptors α and β in differentiation of mouse sexual behavior

Sex differences in brain and behavior are ubiquitous in sexually reproducing species. Developmental differences in circulating concentrations of gonadal steroids underlie many sexual dimorphisms. During the late embryonic and early perinatal periods, the testes produce androgens, thus, male brains are exposed to testosterone, and in situ testosterone is aromatized to estradiol. In contrast, females are not exposed to high concentrations of testosterone or estradiol until puberty. In many species, neural sex differences and sexually dimorphic behaviors in adults are initiated primarily by estradiol exposure during early development. In brain, estradiol activates two independent processes: masculinization of neural circuits and networks that are essential for expression of male-typical adult behaviors, and defeminization, the loss of the ability to display adult female-typical behaviors. Here, data for the roles of each of the known estrogen receptors (estrogen receptor alpha and estrogen receptor beta) in these two processes are reviewed. Based on work done primarily in knockout mouse models, separate roles for the two estrogen receptors are suggested. Estrogen receptor alpha is primarily involved in masculinization, while estrogen receptor beta has a major role in defeminization of sexual behaviors. In sum, estradiol can have selective effects on distinct behavioral processes via selective interactions with its two receptors, estrogen receptor alpha and estrogen receptor beta.

5alpha-reductase 2 inhibition impairs brain defeminization of male rats: reproductive aspects

The present study was carried out to determine whether 5alpha-reductase 2 (5alpha-R2) metabolic pathway plays a key role in brain sexual differentiation. The inhibition of 5alpha-R2 by finasteride (20 mg/kg/day) from gestational day 19 to postnatal day 5 has long-term effects on sexual behavior and reproductive physiology detected only in adult life. Sexual maturation assessed by timing of preputial separation was unchanged. Finasteride-treated males were able to mate with untreated females which became pregnant but exhibited increased rate of pre-implantation loss. The subfertility observed was probably due to abnormally shaped sperm, since the sperm number was not altered. While plasma testosterone was enhanced, LH levels were not changed. The copulatory potential was not affected and all finasteride-treated rats presented male sexual behavior. Despite this, 53% of them showed homosexual behavior when pretreated with estradiol, suggesting an incomplete brain defeminization. These results indicate that 5alpha-R2 acts in brain sexual differentiation of male rats. Moreover, we suggest that 5alpha-R2 not only produces essential metabolites that act together with estradiol in brain sexual differentiation but also protects the brain from the damaging effects of estradiol excess.