Differentiation of coital behavior in mammals: a comparative analysis

Changes in LH secretion in response to an estradiol challenge in male- and female-oriented rams and in ewes

Two experiments were conducted to determine whether an estradiol challenge could cause a female-type LH surge in castrated male- and female-oriented rams (MORs and FORs). Administration of 17beta-estradiol to castrated MORs and FORs and ovariectomized ewes caused an initial reduction in LH secretion followed for 12-20 h by a surge release of LH in the ewes. No surge release of LH occurred in the MORs and FORs. The pattern of changes in LH secretion within rams and ewes did not differ between the breeding and nonbreeding seasons. Treatment failed to elicit female-typical receptive sexual behaviors in the rams but did stimulate increased sexual receptivity in the ewes as determined by the measures of responsiveness to the teaser ram. Overall, no differences were found in hypothalamic-hypophyseal function in response to exogenous estradiol between MORs and FORs. These data are interpreted to suggest that in contrast to sexual attraction, the neural mechanisms controlling the LH surge and female receptivity are defeminized in MORs.

Role for estradiol in female-typical brain and behavioral sexual differentiation

The importance of estrogens in controlling brain and behavioral sexual differentiation in female rodents is an unresolved issue in the field of behavioral neuroendocrinology. Whereas, the current dogma states that the female brain develops independently of estradiol, many studies have hinted at possible roles of estrogen in female sexual differentiation. Accordingly, it has been proposed that alpha-fetoprotein, a fetal plasma protein that binds estrogens with high affinity, has more than a neuroprotective role and specifically delivers estrogens to target brain cells to ensure female differentiation. Here, we review new results obtained in aromatase and alpha-fetoprotein knockout mice showing that estrogens can have both feminizing and defeminizing effects on the developing neural mechanisms that control sexual behavior. We propose that the defeminizing action of estradiol normally occurs prenatally in males and is avoided in fetal females because of the protective actions of alpha-fetoprotein, whereas the feminizing action of estradiol normally occurs postnatally in genetic females.

Genomic imprinting and the evolution of sex differences in mammalian reproductive strategies

Two major developments have occurred that have influenced the evolution of sexually dimorphic reproductive strategies of mammals. Viviparity and development of a placenta is one such development, especially in small-brained rodent lineages, where there has been a major impact of placental hormones on the maternal brain. In the Old World primate/hominoid lineages, the massive expansion of the brain through growth of the neocortex has radically changed how reproductive strategies are determined. Genomic imprinting has played a significant part in both of these developments. Most of the imprinted genes investigated to date are expressed in the placenta and a subset are expressed in both placenta and hypothalamus. Based on phenotypes derived from targeted mutagenesis, a hypothesis is developed for the coadaptive evolution of placenta and hypothalamus, particularly in the context of neurohormonal regulation of maternalism. In small-brained mammals, maternalism places a severe restriction on sexual activity, which in the case of a female rodent is little more than several hours in a lifetime compared with the several weeks given over to maternalism. The consequent sparsity of oestrous, sexually receptive females imposes a rigorous competitive reproductive strategy in males, with the onus being on the male's ability to find oestrous females. This has resulted in a marked sex difference in the chemosensory system, particularly the VNO accessory olfactory system, for the engagement of male sexual behavior in response to oestrous females. Genomic imprinting, together with neonatal androgens, has also played a role in the developing accessory olfactory system and its role in detecting oestrous females. With the evolutionary expansion of the neocortex seen in Old World primates and hominids, reproductive strategies are complex and embedded in the social structure and hierarchies which characterize primate societies. Reproductive strategies depend far more on intelligent behavioral determinants than they do on hormonal determinants. In females, sexual activity is not restricted to oestrous periods, indeed most of the sexual activity is not reproductive. Male Old World primates continue to mate for years after castration, but loss of dominance status leads to a loss of sexual interest within days. The genetic basis for the expansion of neocortical development is complex, but those parts of the brain which have expanded are undoubtedly under the influence of imprinted genes, as studies using parthenogenetic and androgenetic chimeras and allometric analysis of brains across comparative phylogenies have shown. Sex differences in behavior owe much to social structure, social learning, and the deployment of intelligent behavioral strategies. The epigenetic effects of social learning on brain development have become equally as important as the epigenetic effects of hormones on brain development and both contribute to sex differences in behavior in large-brained primates.

The kisspeptin receptor GPR54 is required for sexual differentiation of the brain and behavior

GPR54 is a G-protein-coupled receptor, which binds kisspeptins and is widely expressed throughout the brain. Kisspeptin-GPR54 signaling has been implicated in the regulation of pubertal and adulthood gonadotropin-releasing hormone (GnRH) secretion, and mutations or deletions of GPR54 cause hypogonadotropic hypogonadism in humans and mice. Other reproductive roles for kisspeptin-GPR54 signaling, including the regulation of developmental GnRH secretion or sexual behavior in adults, have not yet been explored. Using adult wild-type (WT) and GPR54 knock-out (KO) mice, we first tested whether kisspeptin-GPR54 signaling is necessary for male and female sexual behaviors. We found that hormone-replaced gonadectomized GPR54 KO males and females displayed appropriate gender-specific adult sexual behaviors. Next, we examined whether GPR54 signaling is required for proper display of olfactory-mediated partner preference behavior. Testosterone-treated WT males preferred stimulus females rather than males, whereas similarly treated WT females and GPR54 KO males showed no preference for either sex. Because olfactory preference is sexually dimorphic and organized during development by androgens, we assessed whether GPR54 signaling is essential for sexual differentiation of other sexually dimorphic traits. Interestingly, adult testosterone-treated GPR54 KO males displayed "female-like" numbers of tyrosine hydroxylase-immunoreactive and Kiss1 mRNA-containing neurons in the anteroventral periventricular nucleus and likewise possessed fewer motoneurons in the spino-bulbocavernosus nucleus than did WT males. Our findings indicate that kisspeptin-GPR54 signaling is not required for male or female copulatory behavior, provided there is appropriate adulthood hormone replacement. However, GPR54 is necessary for proper male-like development of several sexually dimorphic traits, likely by regulating GnRH-mediated androgen secretion during "critical windows" in perinatal development.

Androgen receptor is essential for sexual differentiation of responses to olfactory cues in mice

During sexual differentiation males and females are exposed to different levels of testosterone, which promotes sex differences in the adult brain and in behavior. Testosterone can act after aromatization or reduction via a number of steroid hormone receptors. Here we provide new evidence that the androgen receptor (AR) is essential for sexual differentiation in mice. We used mice carrying the testicular feminization (Tfm) mutation of the AR. Adult Tfm males, wild-type male and female littermates were gonadectomized and given subcutaneous estradiol implants. In all sexually dimorphic traits, Tfm males had responses equivalent to females and different from males. In simultaneous choice tests, males spent significantly more time investigating female-soiled bedding, whereas females and Tfm males preferred to investigate male-soiled bedding. Tfm males and females did not have a partner preference in tests with awake stimulus animals, whereas males showed a preference for females over males. Exposure to male-soiled, but not clean, bedding produced a significant increase in c-Fos-immunoreactive cells in the medial preoptic area and bed nucleus of the stria terminalis in Tfm males and females, no increase was noted in males. Masculine sexual behavior (mounting and thrusting) was not sexually dimorphic, and all groups displayed these behaviors. Our results support data collected in humans suggesting a role for the androgen receptor in sexual differentiation of social preferences and neural responses to pheromones.

Suppression of fetal testicular cytochrome P450 17 by maternal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin: a mechanism involving an initial effect on gonadotropin synthesis in the pituitary

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the fetal expression of testicular cytochrome P450 17 (CYP17), one of the enzymes necessary for sex steroid synthesis, was studied in Wistar rats. Fetal testicular CYP17 exhibited reduced mRNA and protein levels following exposure of the dams at gestational day 15 to 1 microg/kg TCDD. In support of this, CYP17 activity catalyzed by fetal testis homogenate was also reduced by maternal exposure to TCDD. The reduction in CYP17 expression seemed to be specific for fetal stages, because 7 day-old pups born from TCDD-treated dams did not exhibit any reduction in CYP17. In sharp contrast to the in vivo observations, TCDD failed to reduce CYP17 expression in cultured fetal testis, although CYP17 could be induced by activating cAMP-dependent signaling. To assess the role of pituitary luteinizing hormone (LH) on TCDD-induced reduction in fetal testicular CYP17, a further investigation was performed to examine whether the direct injection of LH into fetuses restores the altered CYP17 expression. The results showed that in utero injection of equine chorionic gonadotropin, an LH-mimicking hormone, completely abolishes the TCDD-produced reduction in fetal CYP17. However, neither the alpha- nor beta-subunits of LH in cultured fetal pituitary was reduced by TCDD. These results suggest that 1) maternal exposure to TCDD impairs the expression of testicular CYP17 in a fetal stage-specific manner; 2) this effect is due, at least partially, to a TCDD-produced reduction in circulating LH; and 3) TCDD exerts such an effect by affecting the upstream mechanism regulating the pituitary synthesis of LH.

Ventromedial hypothalamic nucleus lesions disrupt olfactory mate recognition and receptivity in female ferrets

Previous research showed that ferrets of both sexes rely on the perception of conspecifics' body odors to identify and motivate approach towards opposite-sex mating partners, and exposure to male body odors stimulated Fos expression in an olfactory projection circuit of female, but not male, ferrets that terminates in the ventromedial hypothalamic nucleus (VMH). We asked whether the female-typical preference of ferrets to approach male as opposed to female body odors in Y-maze tests would be disrupted by VMH lesions. Sexually experienced female ferrets were ovo-hysterectomized prior to receiving bilateral electrolytic lesions of the VMH, the preoptic area/anterior hypothalamus (POA/AH) or a sham operation. Subsequently, while receiving estradiol benzoate, females that received either complete or partial bilateral lesions of the VMH approached volatile odors from an anesthetized male on significantly fewer trials than females given POA/AH lesions or a sham operation. Both groups of ferrets with VMH lesion damage reliably discriminated between volatile anal scents as well as urinary odors from the 2 sexes in home cage habituation/dishabituation tests, suggesting that their odor-based sex discrimination remained intact. Females with complete bilateral VMH lesions showed significantly lower acceptance of neck gripping from a stimulus male (receptivity) and more aggression towards the male than all other groups of female subjects. Estrogen-sensitive neurons in the VMH appear to play a central role in female-typical neural processing of odor inputs leading to a preference to seek out a male sex partner, in addition to facilitating females' sexual receptivity.

Mammalian animal models of psychosexual differentiation: when is 'translation' to the human situation possible?

Clinical investigators have been forced primarily to use experiments of nature (e.g., cloacal exstrophy; androgen insensitivity, congenital adrenal hyperplasia) to assess the contribution of fetal sex hormone exposure to the development of male- and female-typical profiles of gender identity and role behavior as well as sexual orientation. In this review, I summarize the results of numerous correlative as well as mechanistic animal experiments that shed significant light on general neuroendocrine mechanisms controlling the differentiation of neural circuits controlling sexual partner preference (sexual orientation) in mammalian species including man. I also argue, however, that results of animal studies can, at best, provide only indirect insights into the neuroendocrine determinants of human gender identity and role behaviors.

Progesterone receptor activation signals behavioral transitions across the reproductive cycle of the female rabbit

The female rabbit is an exceptional experimental model to define mechanisms by which progesterone (P) controls the expression of reproductive behaviors. In the rabbit, the rise in P levels during pregnancy inhibits estrous scent marking ("chinning"), stimulates the excavation of a nest burrow ("digging"), and primes behaviors later used for nest construction. The pre-parturient fall of P triggers the construction of a straw nest ("straw carrying") that is lined with hair that she pulls from her own body ("hair pulling"). These behaviors can be replicated in ovariectomized (ovx) females given a schedule of estradiol (E) and P that mimics hormone levels during pregnancy (E from days 0 to 4, E + P from days 5 to 17, E from days 18 to 27). We administered PR antagonists RU486 or CDB(VA)2914 to ovx female rabbits during either the initial (days 5-11) or late (days 12-17) phases of P treatment, to determine the role of PR activation in coordinating the expression of these behaviors. Both antiprogestins attenuated the P-mediated decline in chinning and increase in digging when administered during days 5-11. When given across days 12-17, both antiprogestins triggered an early decline in digging, the onset of nest building in some Ss, and the reinstatement of chinning. These results point to a central role of PR activation for establishing and maintaining the behavioral phenotype of pregnancy, and for the behavioral transition from pregnancy to estrus.

Steroid-dependent plasticity in the medial amygdala

Behavioral sex differences have traditionally been thought to arise from gonadal steroids during a neonatal sensitive period. However, it is possible to sex-reverse certain behaviors by reversing the levels of circulating androgen in adult males and females. These results suggest that the sexually dimorphic substrates of sex behavior are subject to a high degree of plasticity, even in adulthood. I have found that circulating androgen exerts a trophic effect on the Nissl-stained morphology of an important nucleus in the control of sex behavior, namely, the posterodorsal subnucleus of the medial amygdala. First, sex-reversing the level of circulating androgen reversed the sex difference in soma size and regional volume of the posterodorsal subnucleus of the medial amygdala in adult rats. Interestingly, activation of both androgen and estrogen receptors was necessary for the post-castration maintenance of a masculine phenotype in terms of posterodorsal subnucleus of the medial amygdala cell size, whereas only estrogen receptor activity was necessary to maintain a masculine posterodorsal subnucleus of the medial amygdala volume. Then, we showed that seasonal variation in androgen was correlated with morphologic plasticity in the posterodorsal subnucleus of the medial amygdala of the Siberian hamster. However, if the experimental males were housed with females, their posterodorsal subnucleus of the medial amygdalas failed to regress in response to winter-like short daylengths. Furthermore, when male hamsters were castrated and treated with testosterone, the posterodorsal subnucleus of the medial amygdala responded to the hormone only if the animals were in summer-like photoperiods. Overall, these findings indicate that circulating androgens are critical for the maintenance of greater posterodorsal subnucleus of the medial amygdala regional volumes and soma sizes, and that environmental variables can regulate testosterone secretion and responsiveness.

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.

Alpha-fetoprotein protects the developing female mouse brain from masculinization and defeminization by estrogens

Two clearly opposing views exist on the function of alpha-fetoprotein (AFP), a fetal plasma protein that binds estrogens with high affinity, in the sexual differentiation of the rodent brain. AFP has been proposed to either prevent the entry of estrogens or to actively transport estrogens into the developing female brain. The availability of Afp mutant mice (Afp(-/-)) now finally allows us to resolve this longstanding controversy concerning the role of AFP in brain sexual differentiation, and thus to determine whether prenatal estrogens contribute to the development of the female brain. Here we show that the brain and behavior of female Afp(-/-) mice were masculinized and defeminized. However, when estrogen production was blocked by embryonic treatment with the aromatase inhibitor 1,4,6-androstatriene-3,17-dione, the feminine phenotype of these mice was rescued. These results clearly demonstrate that prenatal estrogens masculinize and defeminize the brain and that AFP protects the female brain from these effects of estrogens.

Pubertal hormones organize the adolescent brain and behavior

Maturation of the reproductive system during puberty results in elevated levels of gonadal steroid hormones. These hormones sculpt neural circuits during adolescence, a time of dramatic rewiring of the nervous system. Here, we review the evidence that steroid-dependent organization of the adolescent brain programs a variety of adult behaviors in animals and humans. Converging lines of evidence indicate that adolescence may be a sensitive period for steroid-dependent brain organization and that variation in the timing of interactions between the hormones of puberty and the adolescent brain leads to individual differences in adult behavior and risk of sex-biased psychopathologies.

Androgen excess fetal programming of female reproduction: a developmental aetiology for polycystic ovary syndrome?

The aetiology of polycystic ovary syndrome (PCOS) remains unknown. This familial syndrome is prevalent among reproductive-aged women and its inheritance indicates a dominant regulatory gene with incomplete penetrance. However, promising candidate genes have proven unreliable as markers for the PCOS phenotype. This lack of genetic linkage may represent both extreme heterogeneity of PCOS and difficulty in establishing a universally accepted PCOS diagnosis. Nevertheless, hyperandrogenism is one of the most consistently expressed PCOS traits. Animal models that mimic fetal androgen excess may thus provide unique insight into the origins of the PCOS syndrome. Many female mammals exposed to androgen excess in utero or during early post-natal life typically show masculinized and defeminized behaviour, ovulatory dysfunction and virilized genitalia, although behavioural and ovulatory dysfunction can coexist without virilized genitalia based upon the timing of androgen excess. One animal model shows particular relevance to PCOS: the prenatally androgenized female rhesus monkey. Females exposed to androgen excess early in gestation exhibit hyperandrogenism, oligomenorrhoea and enlarged, polyfollicular ovaries, in addition to LH hypersecretion, impaired embryo development, insulin resistance accompanying abdominal obesity, impaired insulin response to glucose and hyperlipidaemia. Female monkeys exposed to androgen excess late in gestation mimic these programmed changes, except for LH and insulin secretion defects. In utero androgen excess may thus variably perturb multiple organ system programming and thereby provide a single, fetal origin for a heterogeneous adult syndrome.

Male aromatase-knockout mice exhibit normal levels of activity, anxiety and “depressive-like” symptomatology

It is well known that estradiol derived from neural aromatization of testosterone plays a crucial role in the development of the male brain and the display of sexual behaviors in adulthood. It was recently found that male aromatase knockout mice (ArKO) deficient in estradiol due to a mutation in the aromatase gene have general deficits in coital behavior and are sexually less motivated. We wondered whether these behavioral deficits of ArKO males could be related to changes in activity, exploration, anxiety and "depressive-like" symptomatology. ArKO and wild type (WT) males were subjected to open field (OF), elevated plus maze (EPM), and forced swim tests (FST), after being exposed or not to chronic mild stress (CMS). CMS was used to evaluate the impact of chronic stressful procedures and to unveil possible differences between genotypes. There was no effect of genotype on OF, EPM and FST behavioral parameters. WT and ArKO mice exposed to CMS or not exhibited the same behavioral profile during these three types of tests. However, all CMS-exposed mice (ArKO and WT) spent less time in the center of the EPM. Additionally, floating duration measured in the FST increased between two tests in both WT and ArKO mice, though that increase was less prominent in mice previously subjected to CMS than in controls. Therefore, both ArKO and WT males displayed the same behavior and had the same response to CMS however CMS exposure slightly modified the behavior displayed by mice of both genotypes in the FST and EPM paradigms. These results show that ArKO males display normal levels of activity, exploration, anxiety and "depressive-like" symptomatology and thus their deficits in sexual behavior are specific in nature and do not result indirectly from other behavioral changes.

Strategies and methods for research on sex differences in brain and behavior

Female and male brains differ. Differences begin early during development due to a combination of genetic and hormonal events and continue throughout the lifespan of an individual. Although researchers from a myriad of disciplines are beginning to appreciate the importance of considering sex differences in the design and interpretation of their studies, this is an area that is full of potential pitfalls. A female's reproductive status and ovarian cycle have to be taken into account when studying sex differences in health and disease susceptibility, in the pharmacological effects of drugs, and in the study of brain and behavior. To investigate sex differences in brain and behavior there is a logical series of questions that should be answered in a comprehensive investigation of any trait. First, it is important to determine that there is a sex difference in the trait in intact males and females, taking into consideration the reproductive cycle of the female. Then, one must consider whether the sex difference is attributable to the actions of gonadal steroids at the time of testing and/or is sexually differentiated permanently by the action of gonadal steroids during development. To answer these questions requires knowledge of how to assess and/or manipulate the hormonal condition of the subjects in the experiment appropriately. This article describes methods and procedures to assist scientists new to the field in designing and conducting experiments to investigate sex differences in research involving both laboratory animals and humans.

A previously uncharacterized role for estrogen receptor beta: defeminization of male brain and behavior

Sex differences in brain and behavior are ubiquitous in sexually reproducing species. One cause of sexual dimorphisms is developmental differences in circulating concentrations of gonadal steroids. Neonatal testes produce androgens; thus, males are exposed to both testosterone and estradiol, whereas females are not exposed to high concentrations of either hormone until puberty. Classically, the development of neural sex differences is initiated by estradiol, which activates two processes in male neonates; masculinization, the development of male-type behaviors, and defeminization, the loss of the ability to display female-type behaviors. Here, we test the hypothesis that defeminization is regulated by estrogen receptor beta (ERbeta). Adult male ERbeta knockout and WT mice were gonadectomized, treated with female priming hormones, and tested for receptive behavior. Indicative of incomplete defeminization, male ERbeta knockout mice showed significantly higher levels of female receptivity as compared with WT littermates. Testes-intact males did not differ in any aspects of their male sexual behavior, regardless of genotype. In olfactory preference tests, males of both genotypes showed equivalent preferences for female-soiled bedding. Based on these results, we hypothesize that ERbeta is involved in defeminization of brain and behavior. This aspect of ERbeta function may lead to developments in our understanding of neural-based sexually dimorphic human behaviors.

Unexpected effects of perinatal gonadal hormone manipulations on sexual differentiation of the extrahypothalamic arginine-vasopressin system in prairie voles

The sexually dimorphic extrahypothalamic arginine-vasopressin (AVP) projections from the bed nucleus of the stria terminalis to the lateral septum (LS) and lateral habenula (LHb) are denser in males than females and, in rats, require males' perinatal exposure to gonadal hormones but the absence of such exposure in females. We examined perinatal hormone effects on development of this sex difference in prairie voles (Microtus ochrogaster), which show atypical effects of hormones on sexual differentiation of some reproductive behaviors. Neonatal castration reduced the number of AVP mRNA-expressing cells in the bed nucleus of the stria terminalis and AVP immunoreactivity (ir) in the LS and LHb. Surprisingly, daily injections of 1000 microg of testosterone propionate (TP) during the first postnatal week did not maintain high levels of AVP-ir in neonatally castrated males. Furthermore, perinatal treatments with TP (75, 500, or 1000 microg), testosterone (100 microg), or dihydrotestosterone (200 microg) did not masculinize AVP-ir in the female LS or LHb. In fact, 1000 microg TP reduced it in some cases. However, 1000 microg TP lengthened anogenital distance, indicating that TP was biologically active. Neonatal estrogen receptor antagonism with tamoxifen reduced AVP-ir in the male LS, whereas treating neonatal females with the synthetic estrogen diethylstilbestrol increased septal AVP-ir. Tamoxifen and diethylstilbestrol had no effects in the LHb. Similar to rats, therefore, postnatal estrogen influences some components of the extrahypothalamic AVP system in prairie voles, but this developing system appears to be insensitive to exogenous androgens, including aromatizable androgens. Such insensitivity is atypical for a sexually dimorphic neural system in a rodent and may reflect the unusual effects of hormones on sexual differentiation of some behaviors in prairie voles.

Postnatal masculinization alters the HPA axis phenotype in the adult female rat

The ability of postnatal testosterone propionate (TP) to masculinize both behaviour and gonadal cyclicity in the female rat is well documented. We have investigated whether postnatal androgen also has an organizational effect on another sexually dimorphic neuroendocrine system--the hypothalamo-pituitary-adrenal (HPA) axis. Female rats were exposed to a single injection of testosterone propionate (TP) or oil within 24 h of birth. As adults, rats were either ovariectomized and given 17beta-oestradiol replacement (OVXE2) or sham ovariectomized with cholesterol implants (SHOVX). An automated sampling system collected blood from unanaesthetized adult female rats every 10 min over a 24-h period, during a mild psychological stress (noise) and following an immunological lipopolysaccharide stress (LPS). Neonatal TP-treated SHOVX rats had a significant reduction in the number, height, frequency and amplitude of corticosterone pulses over the basal 24-h period, compared to both the neonatal oil-treated and TP-treated OVXE2 animals. The corticosterone response to both noise and LPS was also significantly decreased for the TP-treated SHOVX females. Three hours post-LPS administration, TP females had significantly lower values of paraventricular nucleus (PVN) corticotrophin releasing hormone (CRH), arginine vasopressin (AVP) and anterior pituitary proopiomelanocortin (POMC) mRNAs and greater PVN glucocorticoid receptor (GR) mRNA expression compared to the oil-treated controls. E2 replacement in adult TP rats normalized all the mRNA levels, except for PVN GR mRNA which did fall towards the levels of the oil-control animals. A single injection of TP within 24 h of birth disrupts the development of the characteristic female pattern of corticosterone secretion and the normal female HPA response to stress, resulting in a pattern similar to that seen in males. These effects can be reversed by E2 treatment in the adult TP female rat.

Restoration of male sexual behavior by adult exogenous estrogens in male aromatase knockout mice

We previously found that male aromatase knockout (ArKO) mice that carry a targeted mutation in exons 1 and 2 of the CYP19 gene and as a result cannot aromatize androgen to estrogen show impaired sexual behavior in adulthood. To determine whether this impairment was due to a lack of activation of sexual behavior by estradiol, we studied here male coital behavior as well as olfactory investigation of sexually relevant odors in male ArKO mice following adult treatment with estradiol benzoate (EB) or dihydrotestosterone propionate (DHTP). Again, we found that gonadally intact ArKO males show pronounced behavioral deficits affecting their male coital behavior as well as their olfactory investigation of volatile body odors but not that of soiled bedding. Deficits in male coital behavior were largely corrected following adult treatment with EB and the androgen DHTP, suggesting that estradiol has prominent activational effects on this behavior. By contrast, adult treatment with EB to either castrated or gonadally intact ArKO males did not stimulate olfactory investigation of volatile body odors, suggesting that this impairment may result from a lack of proper organization of this behavior during ontogeny due to the chronic lack of estrogens. In conclusion, the present studies suggest that the behavioral deficits in sexual behavior in male ArKO mice result predominantly from a lack of activation of the behavior by estrogens. This is in contrast with earlier pharmacological studies performed on rats and ferrets that have suggested strong organizational effects of estradiol on male sexual behavior.

The aromatase knockout (ArKO) mouse provides new evidence that estrogens are required for the development of the female brain

The classic view of sexual differentiation is that the male brain develops under the influence of testicular secretions, whereas the female brain develops in the absence of any hormonal stimulation. However, several studies have suggested a possible role of estradiol in female neural development, although they did not provide unequivocal evidence that estradiol is indispensable for the development of the female brain and behavior. As a result, the hypothesis subsequently languished because of the lack of a suitable animal model to test estrogen's possible contribution to female differentiation. The recent introduction of the aromatase knockout (ArKO) mouse, which is deficient in aromatase activity because of a targeted mutation in the CYP19 gene and therefore cannot aromatize androgen to estrogen, has provided a new opportunity to reopen the debate of whether estradiol contributes to the development of the female brain. Female ArKO mice showed reduced levels of lordosis behavior after adult treatment with estradiol and progesterone, suggesting that estradiol is required for the development of the neural mechanisms controlling this behavior in female mice. The neural systems affected may include the olfactory systems in that ArKO females also showed impairments in olfactory investigation of odors from conspecifics. Thus, the classic view of sexual differentiation, that is, the female brain develops in the absence of any hormonal secretion, needs to be re-examined.

Steroid receptor control of reproductive behavior

Steroid hormone receptors in the brain were thought to be only activated by steroid hormones. Once steroid binds to the receptor, it would act on DNA to regulate gene transcription. Recent data indicate that steroid receptor action is more complex. Steroid receptor activity in the brain is under the control of co-regulatory proteins, such as coactivators. It is the expression of these additional proteins that modulate the activity of steroid receptors. Furthermore, steroid receptors are not only activated by steroid, but can also be activated by neurotransmitters in the absence of steroid. For example, progestin receptors in rodent brain are sensitive to progesterone and to social cues in the environment. This review discusses these emerging mechanisms for steroid receptor control in developing and adult brain.

Sex differences in sexual partner acquisition, retention, and harassment during female homosexual consortships in Japanese macaques

Female Japanese macaques (Macaca fuscata) in certain populations are unusual in that they exhibit male-typical patterns of mounting behavior and sexual-partner preference. The goal of this study was to determine whether female Japanese macaques, from one such population, employ male-typical behavioral tactics to disrupt existing homosexual consortships, as well as to acquire and retain same-sex sexual partners. "Harassment" of homosexual consortships occurred when a sexually motivated, third-party male or female interrupted a consorting female couple by displacing or aggressing them. Sexual harassment was a male-typical strategy for disrupting existing homosexual consortships, but was rarely performed by females. "Intrusions" occurred when a male or female competitor attempted to acquire exclusive access to a female engaged in a homosexual consortship by targeting that female as the focus of competition and her partner as his/her competitor. "Sexual coercion" occurred when one individual alternately sexually solicited and aggressed another individual as part of the same behavioral sequence during an intrusion. Males employed consortship intrusions and sexual coercion when they attempted to acquire female sexual partners that were already engaged in homosexual consortships, but females rarely did so. However, females did employ male-typical patterns of aggressive competition and sexual coercion to retain same-sex sexual partners when confronted with male competitors' attempts to usurp those partners. These results indicate that female sexual activity during homosexual consortships is not uniformly "masculine" in expression, but rather is a mixture of male- and female-typical behaviors.

Effects of neonatal testosterone treatment on pacing behaviors and development of a conditioned place preference

Two experiments assessed the effects of neonatal testosterone treatment on paced mating behavior and conditioned place preference in female rats. In both experiments, females received s.c. injections of 5.0 microg testosterone propionate or oil vehicle at three days postpartum. As adults, females were ovariectomized and given s.c. injections of 10 microg estradiol benzoate and 500 microg progesterone, 48 and 4 h before mating, respectively. In Experiment 1, TP- and Oil-treated females exhibited similar high levels of lordosis responsiveness, but TP-treated females showed increased intervals between mounts and between intromissions in paced and non-paced mating conditions compared to control females. The effect was particularly pronounced during paced mating, when contact return latencies were increased approximately 2-fold by TP treatment. TP-treated females showed exaggerated pacing behavior, showing significantly greater return latencies after intromissions than Oil-treated females. In Experiment 2, TP- and Oil-treated groups were tested in a conditioned place preference paradigm to determine if the behavioral changes observed in Experiment 1 were in part a result of changes in the perceived reward produced by paced mating. TP treated and control females developed equivalent preferences for places associated with paced but not non-paced mating, indicating that neonatal TP treatment at this dosage does not disrupt or enhance the conditioned place preference induced by paced mating. The results of the two experiments demonstrate that neonatal TP treatment alters the display of pacing behavior but not the reward state induced by paced mating, and suggest that TP affects neural substrates involved in performance of paced mating without effects on those controlling lordosis or place preference conditioning.

Detailed analysis of the male copulatory motor pattern in mammals: hormonal bases

Data obtained, using a polygraphic technique, on the characteristics of the motor and genital copulatory responses of male rabbits, rats, mice, hamsters, and guinea pigs are reviewed. This methodology provided detailed information, not accessible to other analyses, on the frequency and dynamic organization of copulatory pelvic thrusting trains of the species studied. This comparative analysis showed that: (1) The male rat may display two types of ejaculatory responses, differing in the dynamic organization of the pelvic thrusting train, and in the duration of the intravaginal thrusting period preceding ejaculation. (2) In the guinea pigs and small rodents, but not in rabbits, pelvic thrusting at ejaculatory responses persists during intromission, and a period of fast intravaginal thrusting is associated with ejaculation. (3) The motor copulatory pattern of the rabbit, but not of the rat, hamster, or guinea pig, is affected by castration and hormone treatment, suggesting that, in rabbits, androgen acts both on motivation and on the spinal neural systems related to copulation.

Sexual differentiation of the neuroendocrine mechanisms regulating mate recognition in mammals

When in breeding condition, male and female mammals seek out and mate with opposite-sex conspecifics. The neural mechanisms controlling mate recognition and heterosexual partner preference are sexually differentiated by the perinatal actions of sex steroid hormones. Many mammalian species use odours to identify potential mates. Thus, sex differences in partner preference may actually reflect sex differences in how male and female mammals perceive socially relevant odours. Two olfactory systems have evolved in vertebrates that differ considerably in their anatomy and function. It is generally believed that the main olfactory system is used to detect a wide variety of volatile odours derived from food prey among many sources, whereas the accessory olfactory system has evolved to detect and process primarily nonvolatile odours shown to influence reproductive behaviours and neuroendocrine functions. Some recent results obtained in oestradiol-deficient aromatase knockout (ArKO) mice that provide evidence for a developmental role of oestradiol in olfactory investigation of volatile body odours are discussed, suggesting that: (i) oestrogens contribute to the development of the main olfactory system and (ii) mate recognition is mediated by the main as opposed to the accessory olfactory system. Thus, sex differences in mate recognition and sexual partner preference may reflect sex differences in the perception of odours by the main olfactory system.

Sex differences in the developing brain: crossroads in the phosphorylation of cAMP response element binding protein

Although it is widely known that steroid hormones differentiate the brain, little is known about the signal transduction pathways that are influenced by steroid hormones during development. This review focuses on divergence in the phosphorylation of cAMP response element binding protein (CREB) in the developing male and female rat brain. At birth, males have an increased phosphorylation of CREB compared to females. As CREB mediates changes in cellular morphology, function and survival rates, its activation may underlie an important event in steroid-mediated sexual differentiation of the brain. The importance of CREB is further supported by a sex difference in the expression of the nuclear receptor coactivator, CREB-binding protein, a critical factor involved in the genomic actions of CREB. This suggests that the developing male brain may be in a hyper-responsive state to factors that lead to increased phosphorylation of CREB, resulting in divergent responses in males versus females. An example of this divergence is the response to GABA. In the male rat brain, GABA action leads to increased phosphorylation of CREB; whereas GABA action in the female brain leads to decreased phosphorylation of CREB. The potential consequences of this divergence in the regulation of CREB are discussed in relation to adult sexually dimorphic brain morphology, physiology and behaviour.

Induction of Fos in the accessory olfactory system by male odors persists in female mice with a null mutation of the aromatase (cyp19) gene

The ability of odors from soiled male bedding to induce neuronal Fos-immunoreactivity (IR) in sensory neurons located in both the apical and basal zones of the vomeronasal organ (VNO) and in two segments of the VNO-projection pathway, the anterior nucleus of the medial amygdala and the bed nucleus of the stria terminalis (BNST), was significantly reduced in adult, ovariectomized, estrogen-treated female mice with a homozygous null mutation of the cyp19 gene (ArKO) which encodes the estrogen biosynthetic P450 enzyme, aromatase. However, a significant odor-induced activation of Fos-IR was seen in other segments of the VNO-projection pathway of ArKO females, including the accessory olfactory bulb (AOB) granule cell layer, the posterior-dorsal medial amygdala (MePD), and the medial preoptic area (MPA). These results suggest that the VNO/accessory olfactory pathway to the hypothalamus was functional in ArKO females even though they had presumably been exposed to less estrogenic stimulation than wild-type (WT) control females throughout development and until the time that estrogen treatment was begun in adulthood. Thus, the hypothesis of Toran-Allerand [Prog. Brain Res. 61 (1984) 63] that female-typical features of neuroendocrine and behavioral function require perinatal exposure to estrogen was not supported, at least for the VNO/accessory olfactory system.

The aromatase knock-out mouse provides new evidence that estradiol is required during development in the female for the expression of sociosexual behaviors in adulthood

We used estrogen-deficient aromatase knock-out (ArKO) mice to determine whether estrogens contribute to the development of the brain and behavior in females. Female mice of three different genotypes [i.e., wild type (WT), heterozygous (HET), and homozygous (ArKO)] were ovariectomized in adulthood and subsequently tested for odor preferences (choice: intact male vs estrous female) in a Y-maze. When treated with testosterone, ArKO females spent significantly less time sniffing odors (both volatile and nonvolatile) from either male or female stimuli compared with WT and HET females. When given direct access to anesthetized stimulus animals or when given a choice between odor and visual cues from both stimulus animals, ArKO females continued to spend less time investigating the stimuli compared with WT and HET females. These defects in olfactory investigation of ArKO females were partially corrected with estradiol treatment in adulthood. Estradiol-treated ArKO females no longer differed from WT and HET females in the time spent investigating either nonvolatile odors or the anogenital region of anesthetized animals. However, ArKO females still investigated volatile odors and/or visual cues less than WT and HET females. Sexual receptivity was severely impaired in ArKO females after treatments with estradiol and progesterone that successfully induced receptivity in WT and HET females. Furthermore, ArKO females showed diminished levels of male sexual behaviors, whereas WT and HET females readily mounted an estrous female. Together, these findings demonstrate that estrogen is required for normal female development. The concept that the female brain develops in the absence of any hormonal stimulation should therefore be reconsidered.

Sexual partner preference requires a functional aromatase (cyp19) gene in male mice

Sexual motivation, sexual partner preference, and sexual performance represent three different aspects of sexual behavior that are critical in determining the reproductive success of a species. Although the display of sexual behavior is under strict hormonal control in both sexes, the relative roles of androgen and estrogen receptors in activating the various components of male sexual behavior are still largely unknown. A recently developed mouse model that is deficient in estradiol due to targeted disruption of exons 1 and 2 of the Cyp19 gene (aromatase knockout (ArKO) mice) was used here to analyze the role of estradiol in the control of all three aspects of male sexual behavior. When tested in a Y-maze providing volatile olfactory cues, male ArKO mice did not show a preference for the odors from an estrous female over those from an intact male, whereas wild-type (WT) and heterozygous (HET) males clearly preferred to sniff estrous odors. When provided with visual and olfactory cues, male ArKO mice also failed to show a preference for an estrous female when given a choice between an estrous female and an empty arm. However, sexual partner preferences of male ArKO mice were not sex-reversed: they did not prefer to investigate an intact male over an estrous female or empty arm. Thus, male ArKO mice seemed to have general deficits in discriminating between conspecifics by using olfactory and visual cues. Male coital behavior was also severely impaired in male ArKO mice: they displayed significantly fewer mounts, intromissions, and ejaculations than WT and HET males. Latencies to first mount or intromission were also significantly longer in ArKO males compared to WT and HET males, in addition to them showing less interest in investigating olfactory and visual cues in a Y-maze, suggesting that they were sexually less motivated. However, three out of seven male ArKO mice were capable of siring litters provided they were housed with a female for a prolonged period of time. In conclusion, aromatization of testosterone to estradiol appears to be essential for sexual motivation and sexual partner preference. By contrast, estradiol may play only a limited role in the expression of male coital behaviors.

Expression of the nuclear receptor coactivator, cAMP response element-binding protein, is sexually dimorphic and modulates sexual differentiation of neonatal rat brain

Recent studies indicate that the transcriptional activity of steroid receptors is governed by proteins called nuclear receptor coactivators. Using immunocytochemistry, we found that on the day of birth (postnatal d 0) males express higher levels of the nuclear receptor coactivator, cAMP response element binding protein-binding protein (CBP), within the ventromedial hypothalamus, medial preoptic area, and arcuate nucleus. Using Western immunoblots, we confirmed that males have higher levels of CBP on postnatal d 0, 1, and 5; however, there was no sex difference on postnatal d 11. To examine the functional role of CBP, we infused oligodeoxynucleotides that were antisense to CBP mRNA or a scrambled sequence as a control into the hypothalamus of female rats on postnatal d 0, 1, and 2. On postnatal d 1, all rats were injected with 100 microg testosterone propionate to both masculinize (increase male) and defeminize (decrease female) sexual behavior. Rats were ovariectomized in adulthood and tested for adult sexual behavior. Neonatal CBP antisense oligodeoxynucleotides treatment interfered with the defeminizing, but not the masculinizing, actions of testosterone. These results indicate that CBP expression in developing rat brain is sexually dimorphic and an important modulator for steroid hormone action.

Effect of sex steroids and coital experience on ferrets' preference for the smell, sight and sound of conspecifics

Previous research showed that ferrets of both sexes recognize potential opposite-sex mates on the basis of volatile body odors. We compared the ability of estrogen and androgen treatments to activate a preference in gonadectomized male and female ferrets for distal cues (volatile body odors alone or volatile odors+sight+sounds) from male versus female stimulus ferrets using an airtight Y-maze and a "stimulus proximity" or a "discrete trials" testing paradigm. Sexually naive, gonadectomized male and female ferrets that received either testosterone propionate (TP) or estradiol benzoate (EB) spent equal time in proximity to goal boxes that provided either volatile odors alone or odors+sight+sounds of male and female stimulus animals. After they received coital experience, male and female subjects (treated with either EB or TP) showed a significant preference for both types of opposite-sex stimuli. When discrete trials tests were given to these ferrets prior to receiving coital experience, EB-treated females preferred to approach odor only cues, as well as odors+sight+sounds of stimulus males, and this preference was further strengthened after coital experience. Sexually naive, TP-treated males preferred to approach volatile odor cues from stimulus females; however, these animals showed an equal preference for odors+sight+sounds of stimulus females and males. Again, after coital experience, males' preference for both sets of cues from stimulus females was significantly enhanced. Thus, in sexually naive ferrets, discrete trials, but not stimulus proximity tests, revealed a preference for distal cues (body odors with or without concurrent sight and sounds) from opposite-sex conspecifics in subjects of both sexes. Coital experience significantly enhanced these preferences for heterosexual distal cues under both testing paradigms.

Parental responsiveness is feminized after neonatal castration in virgin male prairie voles, but is not masculinized by perinatal testosterone in virgin females

We previously found a large sex difference in the parental responsiveness of adult virgin prairie voles (Microtus ochrogaster) such that most males are spontaneously parental, whereas most females are not. Because this sex difference is independent of the gonadal hormones normally circulating in adult virgin voles, the present study examined whether perinatal hormones influence the development of this sex difference. Males were treated prenatally (via their pregnant dam) with both the androgen receptor blocker flutamide (5 mg/day/dam) and the aromatase inhibitor ATD (1 mg/day/dam), or oil, for the last 2 weeks of gestation. Half of the subjects from each group were castrated on the day of birth and the other half received a sham surgery. As adults, intact males were castrated and all males received a silastic capsule filled with testosterone. Prenatal treatment with flutamide and ATD had no effect on males' behavior toward pups, but neonatal castration significantly reduced the percentage of males acting parentally. In a second experiment, females were exposed to testosterone propionate (TP; 50 microg/day/dam) or oil via their dam during the last 2 weeks of gestation. For the first neonatal week, half of the females from each group were injected with TP (1 mg/day) and the other half oil. As adults, females were ovariectomized and half from each group received a testosterone-filled capsule and the other half received an empty capsule. None of the perinatal TP treatments increased females' parental responsiveness, although females from all groups that received testosterone capsules as adults were highly parental. Therefore, although postnatal testicular hormones are necessary for high parental responsiveness in males, the behavior of females is not influenced by perinatal exposure to testosterone.

Low-sexually performing rams but not male-oriented rams can be discriminated by cell size in the amygdala and preoptic area: a morphometric study

Brain regions of male sheep behaviorally classified as high-sexually performing (n=10), low-sexually performing (n=8) or male-oriented (n=9) were examined to determine if differences in reproductive behavior were associated with differences in density or sizes of neurons. High-sexually performing rams actively mounted estrous ewes, low-sexually performing rams failed to mount or had long latencies to mounting estrous ewes, and male-oriented rams mounted other rams in preference to ewes in estrus. Cell densities and sizes were quantified in Nissl stained sections through the medial amygdala (meAMY), preoptic area (POA), bed nucleus of the stria terminalis (BNST), ventromedial hypothalamic nucleus (VMH), lateral geniculate nucleus (LG) and medial geniculate nucleus (MG). Multivariate discriminant analysis based on soma sizes within nuclei of known importance for reproductive behavior and/or gonadotropin release (meAMY, POA, BNST and VMH) discriminated (Wilks Lambda P<0.05) low-performing rams from high-performing and male-oriented rams, but did not discriminate (Wilks Lambda P=0.14) between high-performing and male-oriented rams. Cell size in the parvocellular and magnocellular layers of the LG along with cells of the MG, structures without a specific role in reproduction, did not discriminate any of the three behaviorally defined groups of rams (Wilks Lambda P=0.57). Density of cells present in structures important for the display of reproductive behavior (POA, meAMY, BNST) and/or gonadotropin release (POA, VMH) had no discriminating power nor did density of cells in structures important for the processing of visual (LG) or auditory (MG) stimuli. In conclusion, significant differences in sizes of cells located within nuclei that are specifically important for the display of male reproductive behavior were found in low-sexually performing rams compared to high-sexually performing and male-oriented rams. These differences may result from neuron development in utero or occur later as a consequence of endocrine factors or behavioral experience. Neuronal cell size is a critical variable that determines excitability to synaptic inputs because cell surface area varies exponentially with cell diameter. Relatively small differences in neuron diameter could relate to functionally important differences in neuronal excitability.

Organizational and activational effects of gonadal steroid hormones on the EEG of male and female rats

To analyze organizational and activational effects of sex steroids on adult rat electroencephalographic activity (recorded at postnatal day 100), seven groups were included: males (48)-intact, neonatally or adult castrated; females (64)-intact, ovariectomized and exposed pre- or neonatally to testosterone propionate. In males, neonatal orchidectomy increased beta relative power, whereas both neonatal and adult castration reduced interparietal correlation. In females, prenatal testosterone administration produced higher theta absolute power; theta relative power was higher in all experimental groups, whereas beta1 and beta2 were decreased by prenatal and increased by neonatal virilization; prenatal virilization enhanced, while neonatal virilization and adult ovariectomy decreased interparietal correlation. These data indicate that females are more sensitive to early prenatal than to neonatal organizational effects of sex steroids, and some electroencephalographic features are feminized in castrated males and virilized in perinatally androgenized females.

Influence of gonadal hormones on the development of parental behavior in adult virgin prairie voles (Microtus ochrogaster)

Prairie voles (Microtus ochrogaster) are a socially monogamous species and both sexes are parental after the birth of pups. In contrast, sexually inexperienced adult prairie voles differ in their behavior towards pups such that virgin males are paternal whereas virgin females are often infanticidal. To test whether there exists a discrete perinatal 'sensitive period' during which gonadal hormones influence this behavior, and to distinguish between the relative contributions of estrogenic and androgenic mechanisms to this influence, prairie voles were exposed to testosterone propionate (TP), the anti-androgen flutamide, or the aromatase inhibitor 1,4,6-androstatriene-3,17-doine (ATD) either prenatally via their pregnant dam for the last 15-19 days of the 22-day gestational period or postnatally on days 1-7. None of the treatments altered the high paternal responsiveness of males or the high infanticide rate in females when compared with controls. Females exposed prenatally to ATD, however, had levels of parental behavior that were significantly higher than the lowest levels observed in prenatally TP-treated females. These results suggest that sex differences in the parental behavior of adult virgin prairie voles are not generated exclusively by androgenic or estrogenic mechanisms during a restricted prenatal or early postnatal 'sensitive period' and that the parental behavior of virgin females may be more susceptible to any influence of gonadal hormones during development than males.

Dopamine activates masculine sexual behavior independent of the estrogen receptor alpha

Estrogen receptor alpha (ERalpha) is believed to be a critical part of the regulatory processes involved in normal reproduction and sexual behavior. However, in this study we show the ERalpha is not required for display of masculine sexual behavior. Male and female, ERalpha knock-out (ERalphaKO) and wild-type mice were gonadectomized and implanted with testosterone. Sexual behavior and social preferences were tested after injection of the dopamine agonist, apomorphine (APO), or vehicle. All wild-type mice showed normal masculine behavior, including mounts and pelvic thrusts in females, and ejaculation in males. In agreement with past reports, ERalphaKO mice, given vehicle, failed to show mating behavior. Yet, ERalphaKO males given APO showed masculine copulatory behavior and chemoinvestigatory behavior directed at females. ERalphaKO females, treated with APO, mounted and thrusted when tested with receptive females. HPLC revealed that wild-type and ERalphaKO mice had equivalent catecholamine content in brain regions associated with masculine sexual behavior. These data show that the ERalpha is not essential during development or adulthood for the expression of masculine sexual behavior in mice. Moreover, dopamine can activate sexual behavior via a mechanism that either acts on an ER other than ERalpha or via an estrogen-independent pathway.

Anatomy of the baculum-corpus cavernosum interface in the norway rat (Rattus norvegicus), and implications for force transfer during copulation

The baculum is a nonappendicular bone found in the glans tissue of members of five orders of mammals. Its function during copulation is unknown. Anatomical examination of the baculum and corpus cavernosum in the Norway rat (Rattus norvegicus) shows that the two structures are connected by a layer of fibrocartilage, and that the distal tip of the corpus cavernosum swells during erection to surround the proximal end of the baculum. Microradiographs of bacula from sexually experienced males show that regions of the bone may be remodeling; these data suggest that the baculum is load-bearing. On the basis of this anatomy, I propose that the baculum increases the overall flexural stiffness of the penis during copulation by transferring bending and compressive forces from the distal end of the glans to the tensile wall of the corpus cavernosum. Forces on the distal end of the penis during copulation press the baculum against the corpus cavernosum, reducing its internal volume and increasing intracavernosal pressure and corpus cavernosum wall strains. Because the wall of the erect corpus cavernosum is reinforced with inextensible collagen fibers, an increase in wall strain will also increase wall tissue stiffness, and thereby increase the flexural stiffness of the corpus cavernosum.

Vaginocervical stimulation inhibits female-female mounting in laboratory rats

The objective of this study was to examine how vaginocervical stimulation (VCS) affects female-female mounting in laboratory rats. After receiving sexual stimulation from the male, the latency for the female to mount another female was significantly longer than that of control females. In the absence of any copulatory stimulation, the latency to initiate mounting of another female was about 3 min. However, following three mounts or three intromissions by a male, the latency for the experimental female to initiate mounting increased to about 10 min, and ejaculation abolished mounting for almost 2 h. Once females began mounting, regardless of the copulatory stimulation they received prior to testing, their mounting rate (mounting frequency/2 h) did not differ from stimulus control females. Artificial VCS also inhibited female mounting and anesthetization of the vaginocervical area diminished the inhibiting effect of ejaculation. Taken together, the present results provide evidence that VCS can temporarily inhibit female mounting, and that the duration of the inhibition is related to the amount of VCS received. These data are interpreted within the perspective that female mounting behavior is not a sexual behavior and is consequently suppressed within the context of normal copulation.

Estrogen receptor null mice: what have we learned and where will they lead us?

All scientific investigations begin with distinct objectives: first is the hypothesis upon which studies are undertaken to disprove, and second is the overall aim of obtaining further information, from which future and more precise hypotheses may be drawn. Studies focusing on the generation and use of gene-targeted animal models also apply these goals and may be loosely categorized into sequential phases that become apparent as the use of the model progresses. Initial studies of knockout models often focus on the plausibility of the model based on prior knowledge and whether the generation of an animal lacking the particular gene will prove lethal or not. Upon the successful generation of a knockout, confirmatory studies are undertaken to corroborate previously established hypotheses of the function of the disrupted gene product. As these studies continue, observations of unpredicted phenotypes or, more likely, the lack of a phenotype that was expected based on models put forth from past investigations are noted. Often the surprising phenotype is due to the loss of a gene product that is downstream from the functions of the disrupted gene, whereas the lack of an expected phenotype may be due to compensatory roles filled by alternate mechanisms. As the descriptive studies of the knockout continue, use of the model is often shifted to the role as a unique research reagent, to be used in studies that 1) were not previously possible in a wild-type model; 2) aimed at finding related proteins or pathways whose existence or functions were previously masked; or 3) the subsequent effects of the gene disruption on related physiological and biochemical systems. The alpha ERKO mice continue to satisfy the confirmatory role of a knockout quite well. As summarized in Table 4, the phenotypes observed in the alpha ERKO due to estrogen insensitivity have definitively illustrated several roles that were previously believed to be dependent on functional ER alpha, including 1) the proliferative and differentiative actions critical to the function of the adult female reproductive tract and mammary gland; 2) as an obligatory component in growth factor signaling in the uterus and mammary gland; 3) as the principal steroid involved in negative regulation of gonadotropin gene transcription and LH levels in the hypothalamic-pituitary axis; 4) as a positive regulator of PR expression in several tissues; 5) in the positive regulation of PRL synthesis and secretion from the pituitary; 6) as a promotional factor in oncogene-induced mammary neoplasia; and 7) as a crucial component in the differentiation and activation of several behaviors in both the female and male. The list of unpredictable phenotypes in the alpha ERKO must begin with the observation that generation of an animal lacking a functional ER alpha gene was successful and produced animals of both sexes that exhibit a life span comparable to wild-type. The successful generation of beta ERKO mice suggests that this receptor is also not essential to survival and was most likely not a compensatory factor in the survival of the alpha ERKO. In support of this is our recent successful generation of double knockout, or alpha beta ERKO mice of both sexes. The precise defects in certain components of male reproduction, including the production of abnormal sperm and the loss of intromission and ejaculatory responses that were observed in the alpha ERKO, were quite surprising. In turn, certain estrogen pathways in the alpha ERKO female appear intact or unaffected, such as the ability of the uterus to successfully exhibit a progesterone-induced decidualization response, and the possible maintenance of an LH surge system in the hypothalamus. [ABSTRACT TRUNCATED]

The medial preoptic and anterior hypothalamic regions of the rhesus monkey: cytoarchitectonic comparison with the human and evidence for sexual dimorphism

Examination of thionin-stained sections through the hypothalamus of the rhesus monkey revealed nuclei that resemble the first, second and third interstitial nuclei of the anterior hypothalamus (INAH1-3) of the human. Volumetric analysis of these nuclei in a small sample of monkeys suggests that the nucleus that resembles INAH3 is larger in males than in females. INAH1-3 have each been reported to be larger in men than in women and each has been considered as a potential candidate for homology with the much-studied sexually dimorphic nucleus of the preoptic area (SDN-POA) of the rat. Positional and cytoarchitectonic criteria suggest that of these nuclei, INAH3 and its potential counterpart in the rhesus monkey are the best candidates for homology with the SDN-POA. While the criteria employed in the present study may be used to suggest homologies, they are not adequate to confirm them. Confirmation of the homologies suggested here must rely on other considerations such as connectivity, neurotransmitter and peptide content, and function. It is hoped that the present report will stimulate interest in further examinations of the rhesus hypothalamus that will test both the suggested homologies and the evidence for sexual dimorphism.

Testosterone augments neuronal Fos responses to estrous odors throughout the vomeronasal projection pathway of gonadectomized male and female rats

Pheromonal signals emanating from female rats' soiled bedding have previously been shown to attract male conspecifics and to augment the number of Fos-immunoreactive neurons present in portions of the vomeronasal projection pathway, ranging from the accessory olfactory bulb (AOB) to the medial preoptic area (mPOA) of gonadectomized, testosterone-treated male as well as female subjects. In the present study we extended these findings by showing that these neuronal Fos responses to estrous odors occurred only in gonadectomized subjects which received testosterone propionate (TP), as opposed to oil vehicle, at the time of testing. Previously, when treated with TP, all subjects had displayed mounting with an estrous female. In subsequent tests, gonadectomized male and female subjects spent significantly more time investigating soiled estrous bedding as opposed to clean or anestrous bedding, again, provided they were receiving TP. We propose that testosterone facilitates odor-induced neuronal Fos expression either via its conversion to estradiol, and the subsequent action of this steroid at estrogen response elements on the c-fos gene, or via some indirect mechanism involving centrifugal control of AOB neurotransmission. The ability of female odors to stimulate equivalent numbers of Fos-IR cells in gonadectomized, TP-treated male and female rats indicates that the functional responsiveness of neurons throughout the vomeronasal projection pathway is made more male-like by the fetal actions of testosterone in female as well as in male rats. Previous studies established that males have more neurons than females in several segments of the vomeronasal pathway; however, this morphological sexual dimorphism does not account for the observed isomorphic neuronal Fos responses to female odors.

Postnatal testicular secretions partially restore the disturbances in reproductive activity caused by prenatal hormonal manipulation

Male rats that prenatally had been exposed to an antiestrogen, nitromifene citrate (CI 628), showed evidence of impaired defeminization and masculinization in adulthood, suggesting a role of endogenous estrogen for the sexual differentiation of the male. The present study was undertaken to investigate a possible role of postnatal testicular secretions for the above behavioral effects. Male rats were exposed prenatally to CI 628 (1 mg/rat) or saline, and castrated on Day 0, Day 10, or Day 90 after birth. After treatment with gonadal hormones in adulthood, the males were tested for feminine and masculine sexual behavior and for sexual orientation, both when sexually naive and after they had acquired sexual experience. The following conclusions were drawn: 1. Permanent deficits of lordotic behavior were observed in all experimental groups, suggesting the importance of prenatal estrogen for the defeminization process. 2. Hop/darting and ear wiggling behaviors were enhanced in Day-0 and Day-10 castrates, and blocked in the Day-90 castrates. The restitution of these behaviors to normal levels in Day-90 castrates suggests that, in addition to prenatal estrogen, postnatal testicular secretions also are involved in the behavioral defeminization process. 3. Prenatal estrogen contributes to masculinization as evidenced by the impaired ejaculatory behavior observed in all experimental groups. 4. Male-typical sexual orientation toward the female seems to be facilitated by prenatal estrogen. Both the masculinization and the defeminization of male-typical sexual orientation toward a female were impaired by castration at birth and at Day 10 in the experimental animals, but a full restoration of the sexual orientation toward females was seen in Day-90 castrates, suggesting the restorative role of postnatal testicular secretions for these processes.

Quantitative estimation of estrogen and androgen receptor-immunoreactive cells in the forebrain of neonatally estrogen-deprived male rats

Using quantitative immunocytochemical procedures, the total number of estrogen and androgen receptors was estimated in a large number of hypothalamic and limbic nuclei of male rats, in which brain estrogen formation was inhibited neonatally by treatment with the aromatase inhibitor 1,4,6-androstatriene-3,17-dione. The highest densities of estrogen receptor immunoreactivity were observed in the periventricular preoptic area and the medial preoptic area. Neonatally estrogen-deprived males showed a higher estrogen receptor immunoreactivity than control males in the periventricular preoptic area and the ventrolateral portion of the ventromedial nucleus of the hypothalamus, i.e. those brain areas in which sex differences have been reported, with female rats showing a greater estrogen binding capacity than male rats. The highest densities of androgen receptor immunoreactivity were found in the septohypothalamic nucleus, the medial preoptic area, the posterior division of the bed nucleus of the stria terminalis and the posterodorsal division of the medial amygdaloid nucleus. No significant differences in distribution or total numbers of androgen receptors were found between neonatally estrogen-deprived males and control males. These findings suggest that neonatal estrogens, derived from the neural aromatization of testosterone, are involved in the sexual differentiation of the estrogen receptor system in the periventricular preoptic area and the ventromedial hypothalamus. The role of neonatal estrogens in the development of the forebrain androgen receptor system is less clear.

Understanding human sexuality--specifically homosexuality and the paraphilias--in terms of chaos theory and fetal development

This paper considers human sexual orientation, specifically homosexuality and some paraphilias, to occur as a result of intrauterine development, itself a mathematically chaotic process. Parameter space, an example of state space in the phase diagram, has been used to illustrate different phenotypes. The crossing of a bifurcation boundary by the developing fetus is proposed as a mechanism by which it may be changed from one sexual orientation to another, e.g. from heterosexuality to homosexuality. The factors which push the fetus over a bifurcation boundary, which include a Y-chromosome, specific hormone administration, the lying contiguous to an opposite-sex fetus in multiple pregnancies, maternal stress and immune factors are described. The syndromes congenital renal hyperplasia and the androgen insensitivity syndrome and their relevance to this model are also discussed. Finally, chaos theory is used to encompass the complex interactions between fetal development and cultural factors in human sexuality.