Neonatal inhibition of brain estrogen synthesis alters adult neural Fos responses to mating and pheromonal stimulation in the male rat

Chronic moderate hyperglycemia does not alter sexual motivation in the female rat

The relationship between diabetes mellitus type 2 (DM2) and sexual desire in women has not been systematically studied, therefore, animal models have been used for this purpose. When streptozotocin (STZ) is administered in the neonatal stage, the rat shows moderate chronic hyperglycemia and glucose intolerance in adulthood, resembling a DM2 model. These females show less alterations of sexual behavior (a slight decreased proceptivity and loss of paced mating) than their counterpart with severe hyperglycemia. However, the motivational components of copulation in female rats in this DM2 model have not been examined. The aim of this study was to evaluate female sexual motivation in a model of DM2 in three behavioral paradigms: the partner preference (PP), the sexual incentive motivation (SIM) and the odor preference test (OPT) tests. Neonatal females (3-4 days) were administered with streptozotocin (STZ, 70 mg/kg, intraperitoneally) or citrate buffer. At week 8, a glucose tolerance test was performed, females with blood glucose levels ≥ 250 mg/dl 60 min after a sucrose load (2 g/kg) were considered for the study. Behavioral tests were conducted at week 12, when the females were in natural proestrus. For PP we registered the time in each compartment and the sexual behavior, while in the SIM test, we calculated the time the females remained in each incentive zone. In these tests a castrated male and a sexually experienced male were used as stimuli. In OPT we evaluated the time the females spent sniffing the sawdust coming from cages housing these stimuli. In the PP and OPT hyperglycemic females behave similarly than controls, i.e., they retain a preference for sexually active males. In the SIM test there was a decrease in the time the hyperglycemic females remain in the vicinity of the sexually expert male. Data are discussed on the bases of the accessibility of the females to the stimuli.

Activity of estrogen receptor β expressing neurons in the medial amygdala regulates preference toward receptive females in male mice

The processing of information regarding the sex and reproductive state of conspecific individuals is critical for successful reproduction and survival in males. Generally, male mice exhibit a preference toward the odor of sexually receptive (RF) over nonreceptive females (XF) or gonadally intact males (IM). Previous studies suggested the involvement of estrogen receptor beta (ERβ) expressed in the medial amygdala (MeA) in male preference toward RF. To further delineate the role played by ERβ in the MeA in the neuronal network regulating male preference, we developed a new ERβ-iCre mouse line using the CRISPR-Cas9 system. Fiber photometry Ca2+ imaging revealed that ERβ-expressing neurons in the postero-dorsal part of the MeA (MeApd-ERβ+ neurons) were more active during social investigation toward RF compared to copresented XF or IM mice in a preference test. Chemogenetic inhibition of MeApd-ERβ+ neuronal activity abolished a preference to RF in "RF vs. XF," but not "RF vs. IM," tests. Analysis with cre-dependent retrograde tracing viral vectors identified the principal part of the bed nucleus of stria terminalis (BNSTp) as a primary projection site of MeApd-ERβ+ neurons. Fiber photometry recording in the BNSTp during a preference test revealed that chemogenetic inhibition of MeApd-ERβ+ neurons abolished differential neuronal activity of BNSTp cells as well as a preference to RF against XF but not against IM mice. Collectively, these findings demonstrate for the first time that MeApd-ERβ+ neuronal activity is required for expression of receptivity-based preference (i.e., RF vs. XF) but not sex-based preference (i.e., RF vs. IM) in male mice.

Retracted: Sexual orientation, neuropsychiatric disorders and the neurotransmitters involved

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editor in Chief of Neuroscience and Biobehavioral Reviews after concerns were raised with respect to the phrasing of comparisons drawn between humans and animal models. These comparisons were deemed unsupportable, and thus in the best interests of publication standards the Editor has concluded it is necessary to retract the paper. The authors disagree with the reason for the retraction.

Sex differences in main olfactory system pathways involved in psychosexual function

We summarize literature from animal and human studies assessing sex differences in the ability of the main olfactory system to detect and process sex-specific olfactory signals ("pheromones") that control the expression of psychosexual functions in males and females. A case is made in non primate mammals for an obligatory role of pheromonal signaling via the main olfactory system (in addition to the vomeronasal-accessory olfactory system) in mate recognition and sexual arousal, with male-specific as well as female-specific pheromones subserving these functions in the opposite sex. Although the case for an obligatory role of pheromones in mate recognition and mating among old world primates, including humans, is weaker, we review the current literature assessing the role of putative human pheromones (eg, AND, EST, "copulin"), detected by the main olfactory system, in promoting mate choice and mating in men and women. Based on animal studies, we hypothesize that sexually dimorphic effects of putative human pheromones are mediated via main olfactory inputs to the medial amygdala which, in turn, transmits olfactory information to sites in the hypothalamus that regulate reproduction.

Cognitive Effects of Aromatase and Possible Role in Memory Disorders

Diverse cognitive functions in many vertebrate species are influenced by local conversion of androgens to 17β-estradiol (E2) by aromatase. This enzyme is highly expressed in various brain regions across species, with some inter-species variation in terms of regional brain expression. Since women with breast cancer and men and women with other disorders are often treated with aromatase inhibitors (AI), these populations might be especially vulnerable to cognitive deficits due to low neuroE2 synthesis, i.e., synthesis of E2 directly within the brain. Animal models have been useful in deciphering aromatase effects on cognitive functions. Consequences of AI administration at various life cycle stages have been assessed on auditory, song processing, and spatial memory in birds and various aspects of cognition in rodent models. Additionally, cognitive deficits have been described in aromatase knockout (ArKO) mice that systemically lack this gene throughout their lifespan. This review will consider evidence to date that AI treatment in male and female rodent models, birds, and humans results in cognitive impairments. How brain aromatase regulates cognitive function throughout the lifespan, and gaps in current knowledge will be considered, along with future directions to better define how aromatase might guide learning and memory from early development through the geriatric period. Better understanding the importance of E2 synthesis on neurobehavioral responses at various ages will likely aid in the discovery of therapeutic strategies to prevent potential cognitive deficits, including Alzheimer's Disease, in individuals treated with AI or those possessing CYP19 gene polymorphisms, as well as cognitive effects of normal aging that may be related to changes in brain aromatase activity.

Opposite-sex attraction in male mice requires testosterone-dependent regulation of adult olfactory bulb neurogenesis

Opposite-sex attraction in most mammals depends on the fine-tuned integration of pheromonal stimuli with gonadal hormones in the brain circuits underlying sexual behaviour. Neural activity in these circuits is regulated by sensory processing in the accessory olfactory bulb (AOB), the first central station of the vomeronasal system. Recent evidence indicates adult neurogenesis in the AOB is involved in sex behaviour; however, the mechanisms underlying this function are unknown. By using Semaphorin 7A knockout (Sema7A ko) mice, which show a reduced number of gonadotropin-releasing-hormone neurons, small testicles and subfertility, and wild-type males castrated during adulthood, we demonstrate that the level of circulating testosterone regulates the sex-specific control of AOB neurogenesis and the vomeronasal system activation, which influences opposite-sex cue preference/attraction in mice. Overall, these data highlight adult neurogenesis as a hub for the integration of pheromonal and hormonal cues that control sex-specific responses in brain circuits.

Non-neural androgen receptor promotes androphilic odor preference in mice

In mice, male-typical preference for female olfactory cues results largely from sexually differentiated testosterone production. It is currently unclear on which cells and tissues testosterone acts to produce male-typical preference for female olfactory cues. To further address the site of androgen action on olfactory preference, we have developed a loxP-based transgenic mouse that overexpresses androgen receptors (AR) only when activated by Cre. We used this transgene to overexpress AR globally in all tissues using a CMV-Cre driver and a Nestin-Cre driver to overexpress AR selectively in neural tissue. We then examined olfactory preference in transgenic and wildtype (Wt) littermates by simultaneously exposing animals to female-soiled, male-soiled and clean bedding. Ubiquitous overexpression of AR in CMV-AR mice increased preference for male bedding, whereas neural-specific AR overexpression in Nestin-AR transgenic mice did not differ from wildtype siblings in olfactory preference. Neural activation of olfactory brain areas in response to female-soiled bedding was also evaluated in these mice by measuring FOS immunoreactivity. This revealed a decrease in neural activity along the accessory olfactory pathway that accompanied the decrease in preference for female odors in CMV-AR males, compared to both Nestin-AR and Wt male siblings. Together, results indicate that androgens act via non-neural AR to mediate olfactory preference and neural responses to olfactory stimuli, and further suggest that AR in non-neural tissues can promote androphilic odor preferences in male mice.In mice, male-typical preference for female olfactory cues results largely from sexually differentiated testosterone production. It is currently unclear on which cells and tissues testosterone acts to produce male-typical preference for female olfactory cues. To further address the site of androgen action on olfactory preference, we have developed a loxP-based transgenic mouse that overexpresses androgen receptors (AR) only when activated by Cre. We used this transgene to overexpress AR globally in all tissues using a CMV-Cre driver and a Nestin-Cre driver to overexpress AR selectively in neural tissue. We then examined olfactory preference in transgenic and wildtype (Wt) littermates by simultaneously exposing animals to female-soiled, male-soiled and clean bedding. Ubiquitous overexpression of AR in CMV-AR mice increased preference for male bedding, whereas neural-specific AR overexpression in Nestin-AR transgenic mice did not differ from wildtype siblings in olfactory preference. Neural activation of olfactory brain areas in response to female-soiled bedding was also evaluated in these mice by measuring FOS immunoreactivity. This revealed a decrease in neural activity along the accessory olfactory pathway that accompanied the decrease in preference for female odors in CMV-AR males, compared to both Nestin-AR and Wt male siblings. Together, results indicate that androgens act via non-neural AR to mediate olfactory preference and neural responses to olfactory stimuli, and further suggest that AR in non-neural tissues can promote androphilic odor preferences in male mice.

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.

Chemosignals and hormones in the neural control of mammalian sexual behavior

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

Chemosignals, hormones and mammalian reproduction

Many mammalian species use chemosignals to coordinate reproduction by altering the physiology and behavior of both sexes. Chemosignals prime reproductive physiology so that individuals become sexually mature and active at times when mating is most probable and suppress it when it is not. Once in reproductive condition, odors produced and deposited by both males and females are used to find and select individuals for mating. The production, dissemination and appropriate responses to these cues are modulated heavily by organizational and activational effects of gonadal sex steroids and thereby intrinsically link chemical communication to the broader reproductive context. Many compounds have been identified as "pheromones" but very few have met the expectations of that term: a unitary, species-typical substance that is both necessary and sufficient for an experience-independent behavioral or physiological response. In contrast, most responses to chemosignals are dependent or heavily modulated by experience, either in adulthood or during development. Mechanistically, chemosignals are perceived by both main and accessory (vomeronasal) olfactory systems with the importance of each system tied strongly to the nature of the stimulus rather than to the response. In the central nervous system, the vast majority of responses to chemosignals are mediated by cortical and medial amygdala connections with hypothalamic and other forebrain structures. Despite the importance of chemosignals in mammals, many details of chemical communication differ even among closely related species and defy clear categorization. Although generating much research and public interest, strong evidence for the existence of a robust chemical communication among humans is lacking.

Female mice deficient in alpha-fetoprotein show female-typical neural responses to conspecific-derived pheromones

The neural mechanisms controlling sexual behavior are sexually differentiated by the perinatal actions of sex steroid hormones. We recently observed using female mice deficient in alpha-fetoprotein (AFP-KO) and which lack the protective actions of AFP against maternal estradiol, that exposure to prenatal estradiol completely defeminized the potential to show lordosis behavior in adulthood. Furthermore, AFP-KO females failed to show any male-directed mate preferences following treatment with estradiol and progesterone, indicating a reduced sexual motivation to seek out the male. In the present study, we asked whether neural responses to male- and female-derived odors are also affected in AFP-KO female mice. Therefore, we compared patterns of Fos, the protein product of the immediate early gene, c-fos, commonly used as a marker of neuronal activation, between wild-type (WT) and AFP-KO female mice following exposure to male or estrous female urine. We also tested WT males to confirm the previously observed sex differences in neural responses to male urinary odors. Interestingly, AFP-KO females showed normal, female-like Fos responses, i.e. exposure to urinary odors from male but not estrous female mice induced equivalent levels of Fos protein in the accessory olfactory pathways (e.g. the medial part of the preoptic nucleus, the bed nucleus of the stria terminalis, the amygdala, and the lateral part of the ventromedial hypothalamic nucleus) as well as in the main olfactory pathways (e.g. the piriform cortex and the anterior cortical amygdaloid nucleus), as WT females. By contrast, WT males did not show any significant induction of Fos protein in these brain areas upon exposure to either male or estrous female urinary odors. These results thus suggest that prenatal estradiol is not involved in the sexual differentiation of neural Fos responses to male-derived odors.

Rapid activation of phosphorylated mitogen-activated protein kinase after sexual stimulation in male mice

We mapped cells immunoreactive for the phosphorylated form (p44/p42) of the mitogen-activated protein kinase (pMAPK--also known as ERK1/2) in the brain of male mice after exposure to female olfactory cues or after the display of male copulatory behaviors. Exposure to soiled bedding from estrous females or the display of coital behaviors rapidly (within 10 min) induced MAPK phosphorylation in most of the brain regions known to be involved in the processing of olfactory cues (main and accessory olfactory bulbs, amygdala, and medial preoptic area) and in the control of copulatory behavior (amygdala and medial preoptic area). MAPK phosphorylation thus seems to be a useful marker to study short-term neural activation associated with the expression of specific behaviors.

Effects of Bax gene deletion on social behaviors and neural response to olfactory cues in mice

Bax is a pro-death protein that plays a crucial role in developmental neuronal cell death. Bax(-/-) mice exhibit increased neuron number and lack several neural sex differences. Here we examined the effects of Bax gene deletion on social behaviors (olfactory preference, social recognition, social approach and aggression) and the neural processing of olfactory cues. Bax deletion eliminated the normal sex difference in olfactory preference behavior. In the social recognition test, both genotypes discriminated a novel conspecific, but wild-type males and Bax(-/-) animals of both sexes spent much more time than wild-type females investigating stimulus animals. Similarly, Bax(-/-) mice were more sociable than wild-type mice in a social approach test. Bax deletion had no effect on aggression in a resident/intruder paradigm where males, regardless of genotype, exhibited a shorter latency to attack. Thus, the prevention of neuronal cell death by Bax gene deletion results in greater sociability as well as the elimination of sex differences in some social behaviors. To examine olfactory processing of socially relevant cues, we counted c-Fos-immunoreactive (Fos-ir) cells in several nodes of the accessory olfactory pathway after exposure to male-soiled or control bedding. In both genotypes, exposure to male-soiled bedding increased Fos-ir cells in the posterodorsal medial amygdala, principal nucleus of the bed nucleus of the stria terminalis and medial preoptic nucleus (MPN), and the response in the MPN was greater in females than in males. However, a reduction in Fos-ir cells was seen in the anteroventral periventricular nucleus of Bax(-/-) mice.

Potential contribution of prenatal estrogens to the sexual differentiation of mate preferences in mice

The neural mechanisms controlling sexual behavior are sexually differentiated by perinatal actions of gonadal hormones. We recently observed using female mice deficient in alpha-fetoprotein (AFP-KO) and which lack the protective actions of AFP against maternal estrogens, that exposure to prenatal estrogens completely defeminized their potential to show lordosis behavior in adulthood. Therefore, we determined here whether mate preferences were also affected in female AFP-KO mice. We observed a robust preference for an estrous female over an intact male in female AFP-KO mice, which were ovariectomized in adulthood and subsequently treated with estradiol and progesterone, whereas similarly treated WT females preferred the intact male over the estrous female. Gonadally intact WT males preferred the estrous female over the male, but only when visual cues were blocked by placing stimulus animals behind opaque partitions. Furthermore, when given the choice between an intact male and a castrated male, WT females preferred the intact male, whereas AFP-KO females showed no preference. Finally when given the choice between an estrous female and an ovariectomized female, WT males preferred the estrous female whereas AFP-KO females preferred the ovariectomized female or showed no preference depending on whether they could see the stimulus animals or not. Taken together, when AFP-KO females are tested under estrous conditions, they do not show any male-directed preferences, indicating a reduced sexual motivation to seek out the male in these females. However, they do not completely resemble males in their mate preferences suggesting that the male-typical pattern of mate preferences is not solely organized by prenatal estrogens.

Androgen receptor blockade in the posterodorsal medial amygdala impairs sexual odor preference in male rats

The present study was designed to investigate the role of androgen in the medial amygdala (MeA) in the expression of sexual odor preference in male rats. Gonadally intact, sexually experienced male rats received bilateral administration of flutamide, an androgen receptor (AR) blocker, aimed at either the posterior dorsal part (MePD) or the anterior dorsal part (MeAD) of the MeA through inner cannulae inserted into the implanted guide cannulae. Prior to flutamide administration, all subjects spent longer sniffing volatile odors from an estrous female than those from a sexually active male. Experiment 1 demonstrated that the preference for the female odors over the male odors was eliminated during flutamide administration into the MePD, but not into either the MeAD or outside MePD/MeAD. This elimination of the female-directed odor preference resulted from increase of time sniffing the male odors rather than decrease of time sniffing the estrous odors. In Experiment 2, odor discrimination tests confirmed that the flutamide administration into the MePD did not induce impairment in the ability of the subjects to discriminate the estrous odors from the male odors. These results demonstrated that activation of AR in the MePD plays a critical role in the expression of the preference for estrous odors over male odors. AR blockade, however, seemed to induce a preference for male odors rather than reduce the existing preference for estrous odors, suggesting a complicated regulation of sexual odor preference by sex steroids.

Of mice and rats: key species variations in the sexual differentiation of brain and behavior

Mice and rats are important mammalian models in biomedical research. In contrast to other biomedical fields, work on sexual differentiation of brain and behavior has traditionally utilized comparative animal models. As mice are gaining in popularity, it is essential to acknowledge the differences between these two rodents. Here we review neural and behavioral sexual dimorphisms in rats and mice, which highlight species differences and experimental gaps in the literature, that are needed for direct species comparisons. Moving forward, investigators must answer fundamental questions about their chosen organism, and attend to both species and strain differences as they select the optimal animal models for their research questions.

Extracts from salivary glands stimulate aggression and inositol-1, 4, 5-triphosphate (IP3) production in the vomeronasal organ of mice

Mammals use chemical cues to coordinate social and reproductive behaviors. Chemical cues are detected by the VNO organ (VNO), which is a cartilage-encased elongated organ associated with the vomer bone in the rostral nasal cavity. The resident intruder paradigm was utilized to examine the ability of saliva and its feeder exocrine glands, the submaxillary, parotid, and sublingual glands to mediate aggression in mice. Saliva and extracts from submaxillary and parotid glands, but not extracts from sublingual glands of male CD-1 mice, induced a greater number of attacks and lower latencies to sniff and attack (p<0.05) and significantly increased IP(3) production (p<0.05) versus vehicle (PBS) in CD-1 male mice VNO. We further show that CD-1 male mouse saliva and submaxillary gland extract induced significantly more attacks and a lower latency to attack in lactating female CD-1 mice and produced significantly more inositol triphosphate (IP(3)), indicative of phospholipase C(beta) signaling which mediates pheromonal activity, in CD-1 female VNO compared to PBS. Castrated CD-1 male mouse saliva, and exocrine gland extracts induced significantly less IP(3) production in male VNO and less aggression by CD-1 males and lactating females compared to responses to normal CD-1 male mouse saliva and gland extracts. Thus, chemical cues present in saliva, submaxillary and parotid glands of CD-1 male mice are capable of stimulating aggression in male and female congenic mice which are correlated with significant production of IP(3) in the VNO. Additionally, these stimulations of aggression and IP(3) production are shown to be androgen-dependent.

Sexual differentiation of pheromone processing: links to male-typical mating behavior and partner preference

Phoenix et al. (Phoenix, C., Goy, R., Gerall, A., Young, W., 1959. Organizing actions of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female guinea pig. Endocrinology 65, 369-382.) were the first to propose an essential role of fetal testosterone exposure in the sexual differentiation of the capacity of mammals to display male-typical mating behavior. In one experiment control male and female guinea pigs as well as females given fetal testosterone actually showed equivalent levels of mounting behavior when gonadectomized and given ovarian steroids prior to adult tests with a stimulus female. This finding is discussed in the context of a recent, high-profile paper by Kimchi et al. (Kimchi, T., Xu, J., Dulac, C., 2007. A functional circuit underlying male sexual behaviour in the female mouse brain. Nature 448, 1009-1014.) arguing that female rodents possess the circuits that control the expression of male-typical mating behavior and that their function is normally suppressed in this sex by pheromonal inputs that are processed via the vomeronasal organ (VNO)-accessory olfactory nervous system. In another Phoenix et al. experiment, significantly more mounting behavior was observed in male guinea pigs and in females given fetal testosterone than in control females following adult gonadectomy and treatment with testosterone. Literature is reviewed that attempts to link sex differences in the anatomy and function of the accessory versus the main olfactory projections to the amygdala and hypothalamus to parallel sex differences in courtship behaviors, including sex partner preference, as well as the capacity to display mounting behavior.

Masculinization induced by neonatal exposure to PGE(2) or estradiol alters c-fos induction by estrous odors in adult rats

Processing of relevant olfactory and pheromonal cues has long been known as an important process necessary for social and sexual behavior in rodents. Several nuclei that receive input from the vomeronasal projection pathway are involved in sexual behavior and show changes in immediate early gene expression after stimulation with a variety of sex-related stimuli. The nuclei in this pathway are sexually dimorphic due to the early patterning events induced by estradiol derived from testicular androgens, which developmentally defeminize and masculinize the brain and adult sexual behavior. Masculinization can be induced independently of estradiol via prostaglandin-E(2) (PGE(2)), and therefore assessed separately from defeminization. Here we examined the effects of brain defeminization and masculinization on neuronal response to sex-related odors using Fos, the protein product of the immediate early gene c-fos, as an indicator of activity. Female rat pups treated with a cyclooxygenase-2 inhibitor, to reduce PGE(2), plus estradiol, estradiol alone, and PGE(2) alone were exposed to estrous female odor as adults and the resulting Fos expression was examined in the medial amygdala, preoptic area, and ventromedial nucleus of the hypothalamus. Defeminized and/or masculinized females all showed patterns of Fos activity similar to control males and significantly different from control females. These results suggest that early exposure to estradiol and PGE(2) do not affect olfaction in females, but switch the activity pattern of sex-related nuclei in females to resemble that of males following exposure to sexually-relevant cues.

Evidence for a role of early oestrogens in the central processing of sexually relevant olfactory cues in female mice

We previously found that female aromatase knockout (ArKO) mice showed less investigation of socially relevant odours as well as reduced sexual behaviour. We now ask whether these behavioural deficits might be due to an inadequate processing of odours in female ArKO mice. Therefore, we exposed female ArKO mice to same- and opposite-sex urinary odours and determined the expression of the immediate early gene c-Fos along the main and accessory olfactory projection pathways. We included ArKO males in the present study as we previously observed that they show female-typical detection thresholds of urinary odours, suggesting a role for perinatal oestrogens in these behavioural responses. No sex or genotype differences were observed in the olfactory bulb after urine exposure. By contrast, sex differences in c-Fos responses were observed in wild-type (WT) mice following exposure to male urine in the more central regions of the olfactory pathway; only WT females showed a significant Fos induction in the amygdala, central medial pre-optic area and ventromedial hypothalamus. However, ArKO females did not show a c-Fos response to male odours in the ventromedial hypothalamus, suggesting that the processing of male odours is affected in ArKO females and thus that oestrogens may be necessary for the development of neural responses to sexually relevant odours in female mice. By contrast, c-Fos responses to either male or oestrous female urine were very similar between ArKO and WT males, pointing to a central role of androgen vs. oestrogen signalling in the male circuits that control olfactory investigation and preferences.

A role for the androgen receptor in the sexual differentiation of the olfactory system in mice

Olfactory signals play a central role in the identification of a mating partner in rodents, and the behavioral response to these cues varies markedly between the sexes. As several other sexually dimorphic traits, this response is thought to differentiate as a result of exposure of the developing individual to gonadal steroids, but both the identity of the specific steroid signal and the neural structures targeted for differentiation on this particular case are largely unknown. The present review summarizes results obtained in our lab using genetic males affected by the testicular feminization syndrome (Tfm) as experimental model, and that led to the identification of a role for non-aromatized gonadal steroids acting through the androgen receptor (AR) in the differentiation of olfactory cues processing in mice. The existing literature about AR-mediated sexual differentiation of the CNS in animal models is discussed, along with potential targets for the action of non-aromatized gonadal steroids in either one of the subsystems that detect and process olfactory information in rodents.

Effects of sexual experience on conspecific odor preference and male odor-induced activation of the vomeronasal projection pathway and the nucleus accumbens in female rats

In the present study in estrogen-progesterone primed ovariectomized female rats, we examined the expression of a preference for male odors and male odor-induced Fos immunoreactivity throughout the vomeronasal projection pathway and the nucleus accumbens (NAcc), using both sexually experienced and sexually naive subjects. Female rats significantly preferred airborne odors and soiled bedding from sexually active males over those from estrous females, irrespective of the presence or absence of prior sexual experience. On the other hand, the brain regions in which exposure to male-soiled bedding significantly increased Fos expression were different between sexually experienced and sexually naive subjects. Significant increment of Fos expression in the posterior-dorsal medial amygdala (MePD) and the bed nucleus of stria terminalis (BNST) in forebrain, as well as the accessory olfactory bulb, was observed in both groups of subjects. Fos expression in the anterior-dorsal medial amygdala (MeAD), the medial preoptic area (mPOA) and the NAcc core, however, was significantly increased only in the sexually experienced subjects. These results suggested that male odor-induced activations of the MePD and/or the BNST, but not of the MeAD, the mPOA and the NAcc core, are required for the expression of a male-directed odor preference in female rats.

Exposure to oestrogen prenatally does not interfere with the normal female-typical development of odour preferences

The neural mechanisms controlling mate recognition and heterosexual partner preference are sexually differentiated by perinatal actions of sex steroid hormones. We previously showed that the most important action of oestrogen during prenatal development is to defeminise and, to some extent, masculinise brain and behaviour in mice. Female mice deficient in alpha-foetoprotein (AFP) due to a targeted mutation in the Afp gene (AFP-KO) do not show any female sexual behaviour when paired with an active male because they lack the protective action of AFP against maternal oestrogens. In the present study, we investigated whether odour preferences, another sexually differentiated trait in mice, are also defeminised and/or masculinised in AFP-KO females due to their prenatal exposure to oestrogens. AFP-KO females of two background strains (CD1 and C57Bl/6j) preferred to investigate male over female odours when given the choice between these two odour stimuli in a Y-maze, and thus remained very female-like in this regard. Thus, the absence of lordosis behaviour in these females cannot be explained by a reduced motivation of AFP-KO females to investigate male-derived odours. Furthermore, the presence of a strong male-directed odour preference in AFP-KO females suggests a postnatal contribution of oestrogens to the development of preferences to investigate opposite-sex odours.

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.

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.

Cellular activation patterns of the main olfactory bulb and accessory olfactory bulb following exposure to beddings soiled by same- or opposite-sex conspecifics in Mandarin voles (Microtus mandarinus)

Using induction of Fos as an index, we studied cellular activation patterns in the accessory olfactory bulb (AOB) and main olfactory bulb (MOB) of Mandarin voles ( Microtus mandarinus (Milne-Edwards, 1871)) following their exposure to beddings soiled by different sexes. Male and female Mandarin voles that were exposed to beddings soiled by the opposite sex produced significantly more Fos-immunoreactive (Fos-ir) neurons in the anterior portion of the AOB and significantly fewer Fos-ir neurons in the posterior portion of the AOB than voles exposed to beddings soiled by the same sex. Furthermore, male and female Mandarin voles exposed to bedding soiled by different sex produced different numbers of Fos-ir cells in MOB. Mandarin voles exposed to beddings soiled by the opposite sex produced significantly more Fos-ir neurons in MOB than voles exposed to beddings soiled by the same sex. Our results establish that Mandarin voles of each sex showed different cellular activation patterns in AOB and MOB following exposures to sex-specific beddings. We suggest that both AOB and MOB were involved in sexual activities induced by chemosensory signals.

Effect of vomeronasal organ removal from male mice on their preference for and neural Fos responses to female urinary odors

Four experiments were conducted to determine whether vomeronasal organ (VNO) inputs in male mice mediate the rewarding properties of estrous female urinary odors. Sexually naive male mice with either an intact (VNOi) or lesioned (VNOx) VNO preferred to investigate female urine over water in Y-maze tests. Subsequently, VNOi males ran significantly more quickly and remained in nasal contact longer with estrous female urine than with male urine, whereas VNOx males investigated these odors equally. In home-cage habituation-dishabituation tests, VNOi males also investigated female urine significantly longer than did VNOx males, although both groups investigated female urine longer than other non-body odors. Finally, female urinary odors induced Fos in the nucleus accumbens core of VNOi males but not of VNOx males. Our results suggest that female urinary odors retain some incentive value in VNOx males. However, once direct nasal contact is made with female urine, VNO inputs further activate forebrain mechanisms that amplify the reward salience of this stimulus for the male mouse.

Attraction thresholds and sex discrimination of urinary odorants in male and female aromatase knockout (ArKO) mice

We previously found that both male and female aromatase knockout (ArKO) mice, which cannot synthesize estrogens due to a targeted mutation of the aromatase gene, showed less investigation of volatile body odors from anesthetized conspecifics of both sexes in Y-maze tests. We now ask whether ArKO mice are in fact capable of discriminating between and/or responding to volatile odors. Using habituation/dishabituation tests, we found that gonadectomized ArKO and wild-type (WT) mice of both sexes, which were tested without any sex hormone replacement, reliably distinguished between undiluted volatile urinary odors of either adult males or estrous females versus deionized water as well as between these two urinary odors themselves. However, ArKO mice of both sexes were less motivated than WT controls to investigate same-sex odors when they were presented last in the sequence of stimuli. In a second experiment, we compared the ability of ArKO and WT mice to respond to decreasing concentrations of either male or female urinary odors. We found a clear-cut sex difference in urinary odor attraction thresholds among WT mice: WT males failed to respond to urine dilutions higher than 1:20 by volume, whereas WT females continued to respond to urine dilutions up to 1:80. Male ArKO mice resembled WT females in their ability to respond to lower concentrations of urinary odors, raising the possibility that the observed sex difference among WT mice in urine attraction thresholds results from the perinatal actions of estrogen in the male nervous system. Female ArKO mice failed to show significant dishabituation responses to two (1:20 and 1:80) dilutions of female urine, perhaps, again, because of a reduced motivation to investigate less salient, same-sex urinary odors. Previously observed deficits in the preference of ArKO male and female mice to approach volatile body odors from conspecifics of either sex cannot be attributed to an inability of ArKO subjects to discriminate these odors according to sex but instead may reflect a deficient motivation to approach same-sex odors, especially when their concentration is low.

Effects of sexual experience on conspecific odor preference and estrous odor-induced activation of the vomeronasal projection pathway and the nucleus accumbens in male rats

Male rats prefer odors from estrous females to those from sexually active males. Several studies, however, have demonstrated that prior sexual experience was required to develop the preference for estrous odor. Immunohistological methods for visualizing Fos protein have been shown that in sexually experienced male rats, estrous odors activate brain areas throughout the vomeronasal projection pathway (VN pathway) and the nucleus accumbens (NAcc). In the present study, we examined the contribution of prior sexual experience to the estrous odor-induced neuronal activation of these brain areas in relation to the development of the preference for estrous odor. Sexually experienced testosterone-implanted castrates showed the preference for the odor from an estrous female as opposed to the odor from a sexually active male. In these subjects, significant increment of Fos-like immunoreactivity (Fos-Li) after exposure to estrous female soiled bedding was observed in all brain regions examined, confirming the results of previous studies. Sexually naïve subjects, on the other hand, did not show the preference for estrous odor and the significant increment of Fos-Li was observed only in the accessory olfactory bulb (AOB) and the posterior-dorsal medial amygdala (MePD) of the VN pathway. These results suggested that sexual experience is required for the estrous odor-induced activation of more central portions of the VN pathway, such as the medial preoptic area (mPOA) and the bed nucleus of the stria terminalis (BNST), and the NAcc. The activation of some of these brain regions, therefore, is probably involved in the development of the preference for estrous odor.

Sexual behavior activates the expression of the immediate early genes c-fos and Zenk (egr-1) in catecholaminergic neurons of male Japanese quail

We analyzed the expression of the immediate early genes c-fos and Zenk (egr-1) in the brain of male quail that were gonadally intact (I) or castrated and treated (CX+T) or not (CX) with testosterone and had been exposed for 60 min either to a sexually mature female (F), or to an empty arena (EA) or were left in their home cage (HC). Alternate sections in the brains collected 90 min after the start of behavioral interactions were stained by immunocytochemistry for the proteins FOS or ZENK alone or in association with tyrosine hydroxylase (TH), a marker of catecholaminergic neurons. C-fos and Zenk expression was statistically increased in six brain areas of sexually active birds (I+F, CX+T+F) compared with controls (CX+F, CX+T+EA, CX+T+HC), i.e. the preoptic area, bed nucleus striae terminalis, arcopallium, nucleus intercollicularis, periaqueductal gray and the ventral tegmental area. Interestingly, c-fos and Zenk expression was high in the nucleus intercollicularis, a midbrain vocal control nucleus, of I+F and CX+T+F birds that displayed copulatory behavior but emitted few crows but not in the nucleus intercollicularis of CX+T+EA birds that crowed frequently. Increases in c-fos expression were observed in TH-immunoreactive cells in the periaqueductal gray and ventral tegmental area, but not in the substantia nigra, of I+F and CX+T+F birds indicating the activation of dopaminergic neurons during sexual behavior. Together, these data confirm the implication of the steroid-sensitive preoptic area and bed nucleus striae terminalis in the control of copulation and support the notion that dopamine is involved in its control.

Importance of olfactory and vomeronasal systems for male sexual function

Chemosensory cues stimulate male sexual arousal and behavior. The main olfactory system has an important role in attracting males to estrous females, and the vomeronasal receptors are important for activating accessory olfactory pathways that engage mating behavior in a sexually dimorphic manner. The gonadotropin releasing hormone (GnRH) neurons like the vomeronasal organ (VNO) neurons take their origin in the olfactory placode and migrate to the basal forebrain along pathfinder axons that take their origin in the developing VNO. The maturation of both systems is synchronized in time such that the early postnatal testosterone surge masculinizes the VNO neural relay en route to the medio preoptic area (MPOA). Although VNO slices and VNO receptor neurons in culture respond to volatile odors, in vivo electrophysiological recordings at the first relay in the accessory olfactory bulb (AOB) are silent until the male makes active nuzzling investigations of the female. The VNO neurons may therefore respond to volatiles that are transported into the organ on carrier peptides that themselves may play a part in receptor activation. In the context of modern molecular phylogenetic studies, it is becoming less likely that pheromones acting via the VNO have any part to play in human sexual behavior, but the possibility exists for conserved VNO genes influencing human reproduction via fertilization.

Sexual partner preference, hypothalamic morphology and aromatase in rams

The male-oriented ram is a unique and valuable animal model for the study of hormonal, developmental and genetic contributions to sexual partner preference. Unlike most other mammalian models that are in use currently, variations in sexual attraction occur spontaneously in domestic ram populations. It is estimated that as many as 8-10% of rams exhibit a sexual partner preference for other males, classifying them as male-oriented rams. Studies have failed to identify any compelling social factors that can predict or explain the variations in sexual partner preferences of rams. Nor is there consensus on the endocrine and sensory responsiveness of male-oriented rams to other rams. However, a number of studies have reported differences in brain structure and function between male-oriented and female-oriented rams, suggesting that sexual partner preferences are neurologically hard-wired. Recently, we identified a sexually dimorphic nucleus (oSDN) in the sheep preoptic area-anterior hypothalamus. The oSDN is larger in female-oriented rams than in male-oriented rams and similar in size in male-oriented rams and ewes. In addition, mRNA levels for aromatase in the oSDN were higher in males than in females and were higher in female-oriented rams than in male-oriented rams. These results suggest a relationship between steroid hormones, specifically estrogens and oSDN morphology. In this review, we provide an overview of sexual behavior in rams and discuss the multiple factors that may contribute to the development and adult expression of same-sex partner preferences in rams.

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.

Forebrain Fos responses to reproductively related chemosensory cues in aromatase knockout mice

Sexually relevant pheromonal cues are detected by the vomeronasal system which includes the posterodorsal part of the medial amygdala, the posteromedial part of the bed nucleus of the stria terminalis and the medial preoptic area. Copulatory behavior is impaired in mice lacking functional aromatase, the enzyme converting testosterone into estradiol. In this study, we used male aromatase knockout (ArKO) mice to investigate the role of aromatase in the differentiation and activation of preference for male- or female-related odorants. Moreover, using Fos immunoreactivity as a marker of neuronal activation we investigated the ability of sex-related pheromonal cues to activate the vomeronasal system. Both gonadally intact wild-type and ArKO mice preferred to investigate urine from females. The lack of estrogens did not reverse odor preferences, i.e. male ArKO mice did not show a preference for male odors. Exposure to soiled bedding from females induced Fos-protein in the posterodorsal part of the medial amygdala, in the posteromedial part of the bed nucleus of the stria terminalis, and in the periventricular part of the medial preoptic area of both the genotypes. Exposure to soiled bedding from intact males induced Fos in the posterodorsal part of the medial amygdala in wild-type mice and in the periventricular medial preoptic area in wild-type and ArKO mice. These results suggest that preference for female-related odors and the Fos-mediated activation of the vomeronasal system do not rely on estradiol. Furthermore, sensitivity to female chemosensory cues and copulatory behavior are uncoupled in this knockout model.

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.

Facilitation of male-like coital behavior in female rats by kindling

Kindling is a model of epilepsy and brain plasticity. When applied to the medial preoptic area (MPOA) of non-copulating male rats kindling induces masculine sexual behavior. In order to test if kindling could facilitate male-like coital behavior in female rats, sexually naive females were ovariectomized and kindled in the amygdala (AMG) or the MPOA until an intermediate stage (between 1 and 3, MPOA1-3) or until stage 5 (MPOA5 group). Once kindling was established, females were treated with 2.5 mg/Kg of testosterone propionate (TP) for 15 days. Male-like coital behavior was evaluated on days 5, 10 and 15 of treatment. Subjects were then injected with a TP dose of 5 mg/kg for 15 days and tested in the same way as with the lower dose. The number of mounts was significantly increased and the mount latency was significantly reduced in the MPOA1-3 group when tested 5 days after treatment with the low dose of TP. The same facilitation was observed in MPOA1-3 and MPOA5 groups on day 10 of treatment with the low dose of TP. When the animals were under the high dose of TP treatment, the number of intromissions was increased in all experimental groups (including the AMG kindled group) in comparison to sham animals. In a second experiment we evaluated if the facilitation of male-like coital behavior induced by kindling was produced by a modification of the response of the vomeronasal system to sexually relevant cues. Ovariectomized females were stimulated until they reached kindling stage 2, then they were treated with 2.5 mg/kg of TP for 5 days. After animals were exposed for 90 min to clean sawdust or sawdust soiled by estrous females they were perfused. Fos was detected by immunocytochemistry along the vomeronasal pathway. No differences were found in Fos responses between sham and MPOA kindled females. The facilitation of masculine sexual behavior observed in AMG kindled females may be a consequence of the propagation of the AD to other brain regions involved in the expression of masculine sexual behavior. We propose that masculine sexual behavior is facilitated in MPOA kindled female rats by local neural changes produced by this kind of stimulation without modifying the response of the vomeronasal system to sexually relevant cues.

Expression and regulation of the immediate-early gene product Arc in the accessory olfactory bulb after mating in male rat

Recent studies of the accessory olfactory bulb have shown that the expression of immediate-early genes, e.g., c-fos, c-jun and egr-1, can be used as a marker of neuronal activity in response to pheromonal cues. In this study, we analyzed the expression pattern, in response to mating, of the novel immediate-early gene product Arc (an activity-regulated cytoskeleton-associated protein). Arc is hypothesized to play a role in activity-dependent neuronal plasticity in the hippocampus. In a control group of male rats, only a small number of Arc-immunoreactive cells were observed in the accessory olfactory bulb. In a mating group, however, a marked increase in the number of Arc-immunoreactive cells was observed only in the granule cell layer of the accessory olfactory bulb. The increase in the number of Arc-immunoreactive cells after mating was similar to that observed for other immediate-early genes. However, for the mating group, the increase in Arc-positive cells was limited to the granule cell layer. Granule cells have been shown to exhibit a strong synaptic plasticity in response to pheromonal stimulation. From these findings we suggest that Arc plays an important role in neuronal plasticity in the accessory olfactory bulb.

Glial fibrillary acidic protein immunodetection and immunoreactivity in the anterior and posterior medial amygdala of male and female rats

The medial amygdala (MeA) has receptors for gonadal hormones and modulates reproductive behaviors in rats. Adult male and female rats were used for the immunodetection, a less accurate technique, and the immunohistochemistry for the astrocytic marker glial fibrillary acidic protein (GFAP) in the anterior and posterior MeA. Both procedures were done using polyclonal anti-GFAP and were quantified by densitometry. The first technique provided no evidence for a difference between sexes in the immunocontent of GFAP in any region of the MeA (p > 0.1). Nevertheless, the measure of the intensity of GFAP immunoreactivity (GFAP-IR) showed that females had a higher GFAP-IR in the posterodorsal (p < 0.01) and in the posteroventral subregions of the MeA (p < 0.01) than males. No sex difference was found in its anterodorsal part (p > 0.1). The present results point out the differences between these two above-mentioned techniques but add a new finding to the previously described sexual dimorphism in the MeA, i.e., the GFAP-IR. Data also suggest that probably astrocytes can be affected by sex steroids in this brain area. It is likely that this regionally specific difference in the GFAP-IR may contribute to the distinct functional roles that the MeA subregions have in male and female rats.

17beta-Estradiol increases, aging decreases, c-Fos expression in the rat accessory olfactory bulb

In the present paper we investigated the c-Fos immunoreactivity in the accessory olfactory bulb (AOB) of juvenile, adult and old rats of both sexes, as well as the effect of 17beta-estradiol on the expression of this immediate early gene. Basal c-Fos expression in the olfactory bulb decreased with age, and estrogen treatment caused an increase in the number of neurons expressing c-Fos in the AOB. The results indicate that both aging and estrogen have roles in the ability of neurons to co-ordinate genetic activity. Our observations may explain the decrease in age-related changes of brain plasticity, and provide data for the understanding of hormonally regulated neuronal plasticity.

Conditioning and sexual behavior: a review

Sexual behavior is directed by a sophisticated interplay between steroid hormone actions in the brain that give rise to sexual arousability and experience with sexual reward that gives rise to expectations of competent sexual activity, sexual desire, arousal, and performance. Sexual experience allows animals to form instrumental associations between internal or external stimuli and behaviors that lead to different sexual rewards. Furthermore, Pavlovian associations between internal and external stimuli allow animals to predict sexual outcomes. These two types of learning build upon instinctual mechanisms to create distinctive, and seemingly "automated," patterns of sexual response. This article reviews the literature on conditioning and sexual behavior with a particular emphasis on incentive sequences of sexual behavior that move animals from distal to proximal with regard to sexual stimuli during appetitive phases of behavior and ultimately result in copulatory interaction and mating during consummatory phases of behavior. Accordingly, the role of learning in sexual excitement, in behaviors that bring about the opportunity to mate, in courtship and solicitation displays, in sexual arousal and copulatory behaviors, in sexual partner preferences, and the short- and long-term influence of copulatory experience on sexual and reproductive function is examined. Although hormone actions set the stage for sexual activity by generating the ability of animals to become sexually excited and aroused, it is each animal's unique experience with sexual behavior and sexual reward that molds the strength of responses made toward sexual incentives.

Sex difference and steroid modulation of pheromone-induced immediate early genes in the two zones of the mouse accessory olfactory system

Two anatomically and neurochemically distinct zones within the vomeronasal organ (VNO) and accessory olfactory bulb (AOB) have been identified that are responsible for the detection of pheromones. Using markers to distinguish between apical and basal neurons of the VNO neuroepithelium and rostral versus caudal AOB glomeruli, we examined immediate early gene immunoreactivity (IEG-IR) in gonadectomized, steroid-treated mice in response to pheromones of male and female conspecifics. After exposure of estradiol-treated females to soiled male bedding, more VNO neurons in the basal than the apical layer exhibited IEG-IR compared with VNO neurons of estradiol-treated males. Conversely, whereas soiled female bedding failed to induce IEG-IR in VNO neurons of estradiol-treated males or females, both apical and basal neurons were activated in testosterone-treated males. Male and female pheromones also activated mitral and granule cells in the AOBs of all subjects, but responses to different pheromones were distributed across the boundary of the rostral and caudal regions. These data show that differences in the response of males and females to the same pheromonal stimulus are found in the sensory neurons of the VNO. We propose that centrifugal, noradrenergic inputs to VNO neurons, which may differ in the two sexes and respond differently to adult sex steroids, modulate sensitivity to pheromonal stimulation.

Mating induces gonadotropin-releasing hormone neuronal activation in anosmic female ferrets

In females of both spontaneously and induced ovulating species, pheromones from male conspecifics can directly stimulate GnRH neuronal activity, thereby inducing pituitary LH secretion and stimulating the onset of estrus. However, whether pheromones contribute to the steroid- or mating-induced preovulatory activation of GnRH neurons is less clear. Previous studies in the ferret, an induced ovulator, raised the possibility that olfactory cues contribute to the ability of genital-somatosensory stimulation to activate GnRH neurons in the mediobasal hypothalamus (MBH). In the present study the percentage of GnRH neurons colabeled with Fos-immunoreactivity (IR), used as a marker for neuronal activation, was investigated in the MBH of mated gonadectomized, estradiol-treated female ferrets in which both nares were occluded. In addition, the percentage of GnRH neurons colabeled with Fos-IR was examined in the MBH of gonadectomized, estradiol-treated female ferrets exposed to male bedding. Bilateral nares occlusion successfully blocked mating or odor-induced increments in Fos-IR in central olfactory regions, including the cortical and medial amygdala. By contrast, the percentage of GnRH neurons expressing Fos-IR did not differ between mated nares- and sham-occluded females. Exposure to male bedding alone failed to induce Fos-IR in MBH GnRH neurons. Thus, the mating-induced preovulatory activation of GnRH neurons in the female ferret's MBH appears to rely solely on genital-somatosensory as opposed to olfactory inputs.

Oestrogen receptor alpha is essential for female-directed chemo-investigatory behaviour but is not required for the pheromone-induced luteinizing hormone surge in male mice

The expression of normal masculine sexual behaviour requires testosterone. Testosterone can bind to androgen receptors, either in its native form, or after reduction to other androgen metabolites. In addition, testosterone can be aromatized to oestrogen, and bind to oestrogen receptor alpha and/or beta. Male copulatory behaviour is deficient in mice lacking functional oestrogen receptor alpha gene (ERalphaKO mice). We sought to determine which aspect(s) of masculine sexual behaviour is compromised in the ERalphaKOs. Specifically, we asked whether ERalphaKO males have reduced motivation and/or an inability to recognize oestrous females. We found significant differences between mice of different genotypes in the amount of chemo-investigatory behaviour displayed and in the target of their investigation. Wild-type males spent more time investigating ovariectomized, oestradiol-treated females, than either males, or ovariectomized females that had not received hormone priming. ERalphaKO males spent little time investigating any of the stimulus mice and showed no preferences. To test the hypothesis that this lack of chemo-investigatory behaviour is due to the inability of ERalphaKO males to detect and respond to female pheromones, we exposed males to chemosensory cues (soiled bedding) from females. Males resided in clean, or female-soiled, cage bedding for 60 min. Next, blood was collected and plasma luteinizing hormone (LH) assayed. We also assessed Fos-like immunoreactivity (Fos-ir) in several neural regions involved in processing chemosensory cues. Despite the fact that male ERalphaKOs spend little time engaged in chemo-investigation of females, their neuroendocrine responses to female-soiled bedding were similar to those seen in wild-type males. Our data suggest that the normal coupling between the neuroendocrine response to females and the generation of sexual behaviour is disrupted in ERalphaKO mice. Responses to female pheromones do not require ERalpha. However, normal male sexual performance requires the ERalpha gene.

Cell-body lesions of the posterodorsal preoptic nucleus or posterodorsal medial amygdala, but not the parvicellular subparafascicular thalamus, disrupt mating in male gerbils

In gerbils, the posterodorsal preoptic nucleus (PdPN) and the lateral part of the posterodorsal medial amygdala (MeApd) express Fos with ejaculation. In contrast, the medial/central part of the MeApd expresses Fos when a sexually experienced male reenters the environment associated with mating. The parvicellular part of the subparafascicular thalamic nucleus (SPFp) of gerbils expresses Fos under both conditions. To study the role of the PdPN and MeApd in male sex behavior, male gerbils were tested for mating before and after these areas were bilaterally lesioned by infusions of N-methyl-D-aspartate (NMDA). Controls received the vehicle or inactive isomer, NMLA. Lesions in either area reduced mounting, but MeApd lesions, which were more complete than PdPN lesions, also delayed ejaculation when males intromitted. To determine if the MeApd and PdPN affect mating via a common pathway, they were bilaterally disconnected by lesioning them unilaterally, contralateral to each other. Other groups received ipsilateral lesions, NMLA, or bilateral lesions of the PdPN or MeApd. In addition, the SPFp was studied using bilateral lesions. MeApd and PdPN lesions again decreased mounting, and this time both lesions, which were quite complete, delayed ejaculation when males intromitted. Contralateral lesions that bilaterally disconnected these cell groups from each other mimicked both effects. Thus, the MeApd and PdPN affect mounting and ejaculation, at least in part, via their connections with each other. In contrast, SPFp lesions did not affect mating. Thus, SPFp cells activated at ejaculation may react to ejaculation rather than trigger it, possibly initiating preparations for paternity.

Neuronal Fos activation in olfactory bulb and forebrain of male rats having erections in the presence of inaccessible estrous females

Volatile odors from estrous female rats are necessary and sufficient to induce non-contact penile erections in male rats. It is not known whether these pheromones are detected by the accessory as opposed to the main olfactory system or whether they are processed by forebrain regions that receive olfactory inputs. Using nuclear Fos immunoreactivity as a marker of neuronal activation, we asked how the detection and processing of distal cues from inaccessible estrous females, which elicited non-contact penile erections, compared with the processing of sensory cues from soiled estrous bedding which did not elicit non-contact penile erections. In Experiment 1, groups of sexually experienced males were given one of five treatments. A control group was placed on clean bedding. A second group displayed non-contact penile erections when exposed to the smell, sight and sound of an estrous female restrained behind a permeable barrier. A third group was exposed to the same stimuli as the second (an estrous female) but failed to exhibit non-contact penile erections during the first hour of testing. A fourth group was placed on soiled estrous bedding, and a fifth group was allowed two ejaculations with an estrous female. All males were perfused with 4% paraformaldehyde 2 h after the onset of these respective treatments, and their brains were later processed for Fos immunoreactivity. Non-contact penile erections were observed in males that were exposed to distal cues from an estrous female but not in males exposed to soiled estrous bedding. Males that displayed non-contact penile erections or that were exposed to estrous bedding showed significantly more neuronal Fos immunoreactivity than clean-bedding controls in the nucleus accumbens core and shell, anterior and posterior medial amygdala, bed nucleus of the stria terminalis and the medial preoptic nucleus. Even greater neuronal Fos responses occurred in these regions in mated males. In Experiment 2 these same treatments were given to another cohort of sexually experienced males. Increased neuronal Fos immunoreactivity was observed in the granule and mitral cell layers of the accessory olfactory bulb of males that were either mated or exposed to estrous bedding, but not in males that displayed non-contact penile erections in response to distal cues from an estrous female. The volatile odors which presumably caused non-contact penile erections failed to stimulate significant neuronal Fos immunoreactivity in five main olfactory bulb sites examined. Even so, it seems likely that these pheromones are detected via the main olfactory system and are subsequently processed by the same projection circuit that responds to other pheromones present in estrous bedding that are incapable of eliciting non-contact penile erections.