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

Role of neuroestrogens in the regulation of social behaviors - From social recognition to mating

In this mini-review, we summarize the brain distribution of aromatase, the enzyme catalyzing the synthesis of estrogens from androgens, and the mechanisms responsible for regulating estrogen production within the brain. Understanding this local synthesis of estrogens by neurons is pivotal as it profoundly influences various facets of social behavior. Neuroestrogen action spans from the initial processing of socially pertinent sensory cues to integrating this information with an individual's internal state, ultimately resulting in the manifestation of either pro-affiliative or - aggressive behaviors. We focus here in particular on aggressive and sexual behavior as the result of correct individual recognition of intruders and potential mates. The data summarized in this review clearly point out the crucial role of locally synthesized estrogens in facilitating rapid adaptation to the social environment in rodents and birds of both sexes. These observations not only shed light on the evolutionary significance but also indicate the potential implications of these findings in the realm of human health, suggesting a compelling avenue for further investigation.

Haploinsufficiency of SIX3 Abolishes Male Reproductive Behavior Through Disrupted Olfactory Development, and Impairs Female Fertility Through Disrupted GnRH Neuron Migration

Mating behavior in males and females is dependent on olfactory cues processed through both the main olfactory epithelium (MOE) and the vomeronasal organ (VNO). Signaling through the MOE is critical for the initiation of male mating behavior, and the loss of MOE signaling severely compromises this comportment. Here, we demonstrate that dosage of the homeodomain gene Six3 affects the degree of development of MOE but not the VNO. Anomalous MOE development in Six3 heterozygote mice leads to hyposmia, specifically disrupting male mounting behavior by impairing detection of volatile female estrus pheromones. Six3 is highly expressed in the MOE, main olfactory bulb (MOB), and hypothalamus; all regions essential in the proper migration of the gonadotropin-releasing hormone (GnRH) neurons, a key reproductive neuronal population that migrates along olfactory axons from the developing nose into the brain. Interestingly, we find that the reduction in Six3 expression in Six3 heterozygote mice compromises development of the MOE and MOB, resulting in mis-migration of GnRH neurons due to improper olfactory axon targeting. This reduction in the hypothalamic GnRH neuron population, by 45% in adulthood, leads to female subfertility, but does not impact male hormone levels, suggesting that male infertility is not related to GnRH neuron numbers, but exclusively linked to abnormal olfaction. We here determine that Six3 is haploinsufficient for MOE development, GnRH neuron migration, and fertility, and represents a novel candidate gene for Kallmann syndrome, a form of inherited infertility.

Non-neural androgen receptors affect sexual differentiation of brain and behaviour

Although gonadal testosterone is the principal endocrine factor that promotes masculine traits in mammals, the development of a male phenotype requires local production of both androgenic and oestrogenic signals within target tissues. Much of our knowledge concerning androgenic components of testosterone signalling in sexual differentiation comes from studies of androgen receptor (Ar) loss of function mutants. Here, we review these studies of loss of Ar function and of AR overexpression either globally or selectively in the nervous system of mice. Global and neural mutations affect socio-sexual behaviour and the neuroanatomy of these mice in a sexually differentiated manner. Some masculine traits are affected by both global and neural mutation, indicative of neural mediation, whereas other masculine traits are affected only by global mutation, indicative of an obligatory non-neural androgen target. These results support a model in which multiple sites of androgen action coordinate to produce masculine phenotypes. Furthermore, AR overexpression does not always have a phenotype opposite to that of loss of Ar function mutants, indicative of a nonlinear relationship between androgen dose and masculine phenotype in some cases. Potential mechanisms of Ar gene function in non-neural targets in producing masculine phenotypes are discussed.

Differences in peripheral sensory input to the olfactory bulb between male and female mice

Female mammals generally have a superior sense of smell than males, but the biological basis of this difference is unknown. Here, we demonstrate sexually dimorphic neural coding of odorants by olfactory sensory neurons (OSNs), primary sensory neurons that physically contact odor molecules in the nose and provide the initial sensory input to the brain’s olfactory bulb. We performed in vivo optical neurophysiology to visualize odorant-evoked OSN synaptic output into olfactory bub glomeruli in unmanipulated (gonad-intact) adult mice from both sexes, and found that in females odorant presentation evoked more rapid OSN signaling over a broader range of OSNs than in males. These spatiotemporal differences enhanced the contrast between the neural representations of chemically related odorants in females compared to males during stimulus presentation. Removing circulating sex hormones makes these signals slower and less discriminable in females, while in males they become faster and more discriminable, suggesting opposite roles for gonadal hormones in influencing male and female olfactory function. These results demonstrate that the famous sex difference in olfactory abilities likely originates in the primary sensory neurons, and suggest that hormonal modulation of the peripheral olfactory system could underlie differences in how males and females experience the olfactory world.

Estradiol-induced neurogenesis in the female accessory olfactory bulb is required for the learning of the male odor

Odors processed by the main and accessory olfactory bulbs (MOB, AOB) are important for sexual behavior. Interestingly, both structures continue to receive new neurons during adulthood. A role for olfactory neurogenesis in sexual behavior in female mice has recently been shown and gonadal hormones such as estradiol can modulate adult neurogenesis. Therefore, we wanted to determine the role of estradiol in learning the odors of sexual partners and in the adult neurogenesis of female aromatase knockout mice (ArKO), unable to produce estradiol. Female wild-type (WT) and ArKO mice were exposed to male odors during 7 days, and olfactory preferences, cell proliferation, cell survival and functional involvement of newborn neurons were analyzed, using BrdU injections, in combination with a marker of cell activation (Zif268) and neuronal fate (doublecortin, NeuN). Behavioral tasks indicated that both WT and ArKO females were able to discriminate between the odors of two different males, but ArKO mice failed to learn the familiar male odor. Proliferation of newborn cells was reduced in ArKO mice only in the dentate gyrus of the hippocampus. Olfactory exposure decreased cell survival in the AOB in WT females, suggesting a role for estradiol in a structure involved in sexual behavior. Finally, newborn neurons do not seem to be functionally involved in the AOB of ArKO mice compared with WT, when females were exposed to the odor of a familiar male, suggesting that estradiol-induced neurogenesis in the AOB is required for the learning of the male odor in female mice. Aromatase knockout mice (ArKO) presented deficits in olfactory preferences without affecting their olfactory discrimination abilities, and showed no functional involvement of newborn neurons in the accessory olfactory bulb (AOB) in response to the odor of a familiar male. These results suggest that estradiol-induced neurogenesis in the female AOB is required for the learning of the male odor.

Estrogen involvement in social behavior in rodents: Rapid and long-term actions

This article is part of a Special Issue ("Estradiol and cognition"). Estrogens have repeatedly been shown to influence a wide array of social behaviors, which in rodents are predominantly olfactory-mediated. Estrogens are involved in social behavior at multiple levels of processing, from the detection and integration of socially relevant olfactory information to more complex social behaviors, including social preferences, aggression and dominance, and learning and memory for social stimuli (e.g. social recognition and social learning). Three estrogen receptors (ERs), ERα, ERβ, and the G protein-coupled ER 1 (GPER1), differently affect these behaviors. Social recognition, territorial aggression, and sexual preferences and mate choice, all requiring the integration of socially related olfactory information, seem to primarily involve ERα, with ERβ playing a lesser, modulatory role. In contrast, social learning consistently responds differently to estrogen manipulations than other social behaviors. This suggests differential ER involvement in brain regions important for specific social behaviors, such as the ventromedial and medial preoptic nuclei of the hypothalamus in social preferences and aggression, the medial amygdala and hippocampus in social recognition, and the prefrontal cortex and hippocampus in social learning. While the long-term effects of ERα and ERβ on social behavior have been extensively investigated, our knowledge of the rapid, non-genomic, effects of estrogens is more limited and suggests that they may mediate some social behaviors (e.g. social learning) differently from long-term effects. Further research is required to compare ER involvement in regulating social behavior in male and female animals, and to further elucidate the roles of the more recently described G protein-coupled ERs, both the GPER1 and the Gq-mER.

Hypothalamic response to the chemo-signal androstadienone in gender dysphoric children and adolescents

The odorous steroid androstadienone, a putative male chemo-signal, was previously reported to evoke sex differences in hypothalamic activation in adult heterosexual men and women. In order to investigate whether puberty modulated this sex difference in response to androstadienone, we measured the hypothalamic responsiveness to this chemo-signal in 39 pre-pubertal and 41 adolescent boys and girls by means of functional magnetic resonance imaging. We then investigated whether 36 pre-pubertal children and 38 adolescents diagnosed with gender dysphoria (GD; DSM-5) exhibited sex-atypical (in accordance with their experienced gender), rather than sex-typical (in accordance with their natal sex) hypothalamic activations during olfactory stimulation with androstadienone. We found that the sex difference in responsiveness to androstadienone was already present in pre-pubertal control children and thus likely developed during early perinatal development instead of during sexual maturation. Adolescent girls and boys with GD both responded remarkably like their experienced gender, thus sex-atypical. In contrast, pre-pubertal girls with GD showed neither a typically male nor female hypothalamic activation pattern and pre-pubertal boys with GD had hypothalamic activations in response to androstadienone that were similar to control boys, thus sex-typical. We present here a unique data set of boys and girls diagnosed with GD at two different developmental stages, showing that these children possess certain sex-atypical functional brain characteristics and may have undergone atypical sexual differentiation of the brain.

Deficits in odor-guided behaviors in the transgenic 3xTg-AD female mouse model of Alzheimer׳s disease

Alzheimer׳s disease (AD) is characterized by a number of alterations including those in cognition and olfaction. An early symptom of AD is decreased olfactory ability, which may affect odor-guided behaviors. To test this possibility we evaluated alterations in sexual incentive motivation, sexual olfactory preference, sexual olfactory discrimination, nursing-relevant olfactory preference and olfactory discrimination in female mice. We tested 3xTg-AD (a triple transgenic model, which is a "knock in" of PS1M146V, APPSwe, and tauP300L) and wild type (WT) female mice when receptive (estrous) and non-receptive (anestrous). Subjects were divided into three groups of different ages: (1) 4-5 months, (2) 10-11 months, and (3) 16-18 months. In the sexual incentive motivation task, the receptive 3xTg-AD females showed no preference for a sexually active male at any age studied, in contrast to the WT females. In the sexual olfactory preference test, the receptive WT females were able to identify sexually active male secretions at all ages, but the oldest (16-18 months old) 3xTg-AD females could not. In addition, the oldest 3xTg-AD females showed no preference for nursing-relevant odors in dam secretions and were unable to discriminate between cinnamon and strawberry odors, indicating olfactory alterations. Thus, the present study suggests that the olfactory deficits in this mouse model are associated with changes in sexual incentive motivation and discrimination of food-related odors.

Reliable sex and strain discrimination in the mouse vomeronasal organ and accessory olfactory bulb

Animals modulate their courtship and territorial behaviors in response to olfactory cues produced by other animals. In rodents, detecting these cues is the primary role of the accessory olfactory system (AOS). We sought to systematically investigate the natural stimulus coding logic and robustness in neurons of the first two stages of accessory olfactory processing, the vomeronasal organ (VNO) and accessory olfactory bulb (AOB). We show that firing rate responses of just a few well-chosen mouse VNO or AOB neurons can be used to reliably encode both sex and strain of other mice from cues contained in urine. Additionally, we show that this population code can generalize to new concentrations of stimuli and appears to represent stimulus identity in terms of diverging paths in coding space. Together, the results indicate that firing rate code on the temporal order of seconds is sufficient for accurate classification of pheromonal patterns at different concentrations and may be used by AOS neural circuitry to discriminate among naturally occurring urine stimuli.

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.

Assessment of urinary pheromone discrimination, partner preference, and mating behaviors in female mice

Behavioral testing methods are described for determining whether female mice can discriminate between volatile urinary pheromones of conspecifics of the same vs. opposite sex and/or in different endocrine conditions, for determining sexual partner preference, for quantifying receptive (lordosis) behavior, and for monitoring the expression of male-typical mounting behavior in female mice.

The smell of age: perception and discrimination of body odors of different ages

Our natural body odor goes through several stages of age-dependent changes in chemical composition as we grow older. Similar changes have been reported for several animal species and are thought to facilitate age discrimination of an individual based on body odors, alone. We sought to determine whether humans are able to discriminate between body odor of humans of different ages. Body odors were sampled from three distinct age groups: Young (20–30 years old), Middle-age (45–55), and Old-age (75–95) individuals. Perceptual ratings and age discrimination performance were assessed in 41 young participants. There were significant differences in ratings of both intensity and pleasantness, where body odors from the Old-age group were rated as less intense and less unpleasant than body odors originating from Young and Middle-age donors. Participants were able to discriminate between age categories, with body odor from Old-age donors mediating the effect also after removing variance explained by intensity differences. Similarly, participants were able to correctly assign age labels to body odors originating from Old-age donors but not to body odors originating from other age groups. This experiment suggests that, akin to other animals, humans are able to discriminate age based on body odor alone and that this effect is mediated mainly by body odors emitted by individuals of old age.

Differential effect of amphetamine on c-fos expression in female aromatase knockout (ArKO) mice compared to wildtype controls

Estrogen may be involved in psychosis by an interaction with central dopaminergic activity. Aromatase knockout mice are unable to produce estrogen and have been shown to display altered behavioural responses and effects of the dopamine releaser, amphetamine. This study investigates the effect of gonadal status on amphetamine-induced c-fos expression in the brains of female aromatase knockout and wildtype mice. Six groups of mice were treated intraperitoneally with saline or 5mg/kg amphetamine. Fos immunoreactivity was assessed in the cingulate cortex, caudate putamen and nucleus accumbens. Aromatase knockout mice showed markedly reduced amphetamine-induced Fos immunoreactivity compared to wildtype mice. However, the amphetamine response was restored in aromatase-knockout mice after ovariectomy, which reduced this effect in wildtype controls. Estrogen supplementation reversed the effect of ovariectomy in wildtype mice but had no additional significant effect in aromatase-knockout mice. These results indicate that mechanisms involved in amphetamine-induced c-fos expression are altered in aromatase knockout mice and that the primary hormone involved in this effect is not estrogen, but may be another factor released from the ovaries, such as an androgen. These results provide new insight into the effect of gonadal hormones on amphetamine induced c-fos expression in this mouse model of estrogen deficiency. These results could be important for our understanding of the role of sex steroid hormones in psychosis.

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.

Effects of aromatase mutation (ArKO) on the sexual differentiation of kisspeptin neuronal numbers and their activation by same versus opposite sex urinary pheromones

Pheromones have been shown to induce sexually dimorphic responses in LH secretion. Here we asked whether the sexually dimorphic population of kisspeptin neurons in the rostral periventricular area of the third ventricle (RP3V) could relay sexually dimorphic information from the olfactory systems to the GnRH system. Furthermore, we analyzed the effects of aromatase mutation (ArKO) and thus the role of estradiol on RP3V kisspeptin neuronal numbers and on the response of these kisspeptin neurons to same- versus opposite-sex urinary pheromones. Exposure to male but not female urinary odors induced Fos protein in kisspeptin neurons in the RP3V of female wildtype (WT) mice, suggesting that these kisspeptin neurons may be part of the neural circuitry that relays information from the olfactory brain to the GnRH system in a sexually dimorphic manner. Male pheromones induced Fos in kisspeptin neurons in ArKO females, albeit significantly less compared to WT females. The sexual differentiation of kisspeptin neuronal number was lost in ArKO mice, i.e. the number of kisspeptin-immunoreactive neurons in the RP3V of ArKO females was as low as in male mice, whereas male ArKO mice had somewhat increased numbers of kisspeptin neurons. These results suggest that the sex difference in kisspeptin neuronal number in WT mice reflects an organizational action of estradiol in females. By contrast, the ability of male urinary pheromones to activate kisspeptin neurons in WT females may not depend on the organizational action of estradiol since ArKO females still showed some Fos/kisspeptin co-activation.

The alpha-fetoprotein knock-out mouse model suggests that parental behavior is sexually differentiated under the influence of prenatal estradiol

In rodent species, sexual differentiation of the brain for many reproductive processes depends largely on estradiol. This was recently confirmed again by using the alpha-fetoprotein knockout (AFP-KO) mouse model, which lacks the protective actions of alpha-fetoprotein against maternal estradiol and as a result represents a good model to determine the contribution of prenatal estradiol to the sexual differentiation of the brain and behavior. Female AFP-KO mice were defeminized and masculinized with regard to their neuroendocrine responses as well as sexual behavior. Since parental behavior is also strongly sexually differentiated in mice, we used the AFP-KO mouse model here to ask whether parental responses are differentiated prenatally under the influence of estradiol. It was found that AFP-KO females showed longer latencies to retrieve pups to the nest and also exhibited lower levels of crouching over the pups in the nest in comparison to WT females. In fact, they resembled males (WT and AFP-KO). Other measures of maternal behavior, for example the incidence of infanticide, tended to be higher in AFP-KO females than in WT females but this increase failed to reach statistical significance. The deficits observed in parental behavior of AFP-KO females could not be explained by any changes in olfactory function, novelty recognition or anxiety. Thus our results suggest that prenatal estradiol defeminizes the parental brain in mice.

The vomeronasal organ is required for the male mouse medial amygdala response to chemical-communication signals, as assessed by immediate early gene expression

Many species use chemical signals to convey information relevant to social and reproductive status between members of the same species (conspecific), but some chemical signals may also provide information to another species (heterospecific). Both of these types of complex chemical signals may be detected by the vomeronasal organ, which sends projections to the accessory olfactory bulb and on to the medial amygdala. Previous reports in hamster and mouse suggest that the medial amygdala sorts this complex chemosensory information categorically, according to its biological relevance (salience). In the present set of experiments, male mice having undergone vomeronasal removal surgery (VNX) or a sham-operation (SHAM) were exposed to conspecific (male and female mouse urine) or heterospecific (hamster vaginal fluid and worn cat collar) chemical stimuli. Similarly to our previous report with intact male mice [Samuelsen and Meredith (2009) Brain Res 1263:33-42], SHAM mice exhibit different immediate early gene (IEG) expression patterns in the medial amygdala dependent upon the biological relevance of the chemical stimuli. However, regardless of biological relevance, vomeronasal organ removal eliminates all responses in the medial amygdala to any of the chemical stimuli. Interestingly, VNX also disrupts the avoidance of (an unfamiliar) predator odor, worn cat collar. Here we show that the medial amygdala response to the tested chemical signals is dependent upon an intact vomeronasal organ.

The main and the accessory olfactory systems interact in the control of mate recognition and sexual behavior

In the field of sensory perception, one noticeable fact regarding olfactory perception is the existence of several olfactory subsystems involved in the detection and processing of olfactory information. Indeed, the vomeronasal or accessory olfactory system is usually conceived as being involved in the processing of pheromones as it is closely connected to the hypothalamus, thereby controlling reproductive function. By contrast, the main olfactory system is considered as a general analyzer of volatile chemosignals, used in the context of social communication, for the identification of the status of conspecifics. The respective roles played by the main and the accessory olfactory systems in the control of mate recognition and sexual behavior are at present still controversial. We summarize in this review recent results showing that both the main and accessory olfactory systems are able to process partially overlapping sets of sexual chemosignals and that both systems support complimentary aspects in mate recognition and in the control of sexual behavior.

A centrifugal pathway to the mouse accessory olfactory bulb from the medial amygdala conveys gender-specific volatile pheromonal signals

We previously found that female mice exhibited Fos responses in the accessory olfactory bulb (AOB) after exposure to volatile opposite-sex, but not same-sex, urinary odours. This effect was eliminated by lesioning the main olfactory epithelium, raising the possibility that the AOB receives information about gender via centrifugal inputs originating in the main olfactory system instead of from the vomeronasal organ. We asked which main olfactory forebrain targets send axonal projections to the AOB, and whether these input neurons express Fos in response to opposite-sex urinary volatiles. Female mice received bilateral injections of the retrograde tracer cholera toxin B (CTB), into the AOB, and were exposed to either same- or opposite-sex volatile urinary odours 1 week later. We found CTB-labeled cell bodies in several forebrain sites including the bed nucleus of the accessory olfactory tract, the rostral portion of the medial amygdala (MeA) and the posteromedial cortical nucleus of the amygdala. A significant increase in the percentage of CTB/Fos co-labeled cells was seen only in the MeA of female subjects exposed to male but not to female urinary volatiles. In Experiment 2, CTB-injected females were later exposed to volatile odours from male mouse urine, food, or cat urine. Again, a significant increase in the percentage of CTB/Fos co-labeled cells was seen in the MeA of females exposed to male mouse urinary volatiles but not to food or predator odours. Main olfactory-MeA-AOB signaling may motivate approach behaviour to opposite-sex pheromonal signals that ensure successful reproduction.

Sexually dimorphic enhancement by estradiol of male urinary odor detection thresholds in mice

We asked whether sex and adult estrogen exposure influence the detection thresholds for urinary odors used by mice to guide their social behaviors. Gonadectomized (GDX) male and female mice were trained on a two-choice food-motivated task to determine detection thresholds for male urinary odors. There was no significant sex difference in the detection of these odors by GDX subjects without hormone replacement. However, during treatment with estradiol benzoate (EB), GDX females, but not GDX males, showed an enhanced ability to detect these odors. To investigate a possible mechanism for this effect, the authors measured GDX females' odor-sampling behavior (sniffing) by monitoring intranasal pressure transients during performance of the urinary odor detection task with and without EB treatment. Under both hormone conditions, females decreased their sniffing frequency as the urinary odor concentration decreased, with this decrease being significantly greater while GDX females received EB. Thus, estradiol enhanced detection thresholds for male urine in a sex-specific manner, and this enhanced sensitivity in females was correlated with altered odor-sampling behavior.

Activational effects of estradiol and dihydrotestosterone on social recognition and the arginine-vasopressin immunoreactive system in male mice lacking a functional aromatase gene

In rodents, parts of the arginine-vasopressin (AVP) neuronal system are sexually dimorphic with males having more AVP-immunoreactive cells/fibers than females. This neuropeptide neuronal system is highly sensitive to steroids and has been proposed to play an important role in the processing of olfactory cues critical to the establishment of a social memory. We demonstrate here that gonadally intact male aromatase knockout (ArKO) mice, which cannot aromatize androgens into estrogens due to a targeted mutation in the aromatase gene, showed severe deficits in social recognition as well as a reduced AVP-immunoreactivity in several brain regions. To determine whether this reduction is due to a lack of organizational or activational effects of estrogens, we assessed social recognition abilities and AVP-immunoreactivity in male ArKO and wild-type (WT) mice when treated with estradiol benzoate (EB) in association with dihydrotestosterone propionate (DHTP) in adulthood. Adult treatment with EB and DHTP restored social recognition abilities in castrated ArKO males since they showed normal female-oriented ultrasonic vocalizations and were able to recognize an unfamiliar female using a habituation-dishabituation paradigm. Furthermore, adult treatment also restored AVP-immunoreactivity in the lateral septum of ArKO males to levels observed in intact WT males. These results suggest that social recognition in adulthood and stimulation of AVP expression in the adult mouse forebrain depend predominantly on the estrogenic metabolite of testosterone. Furthermore, our results are in line with the idea that the organization of the AVP system may depend on androgen or sex chromosomes rather than estrogens.

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.

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.

Sexually dimorphic activation of the accessory, but not the main, olfactory bulb in mice by urinary volatiles

Previous research suggests that volatile body odourants detected by the main olfactory epithelium (MOE) are processed mainly by the main olfactory bulb (MOB) whereas nonvolatile body odourants detected by the vomeronasal organ (VNO) are processed via the accessory olfactory bulb (AOB). We asked whether urinary volatiles from males and females differentially activate the AOB in addition to the MOB in gonadectomized mice of either sex. Exposure to urinary volatiles from opposite-sex but not same-sex conspecifics augmented the number of Fos-immunoreactive mitral and granule cells in the AOB. Volatile urinary odours from male as well as female mice also stimulated Fos expression in distinct clusters of MOB glomeruli in both sexes. Intranasal administration of ZnSO(4), intended to disrupt MOE function, eliminated the ability of volatile urinary odours to stimulate Fos in both the MOB and AOB. In ovariectomized, ZnSO(4)-treated females a significant, though attenuated, AOB Fos response occurred after direct nasal exposure to male urine plus soiled bedding, suggesting that VNO signaling remained partially functional in these mice. Future studies will determine whether MOE or VNO signaling, or both types of input, drive the sexually dimorphic response of the AOB to volatile opposite-sex odours and whether this AOB response contributes to reproductive success.

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.

Sex differences in catechol contents in the olfactory bulb of control and unilaterally deprived rats

The dopaminergic system plays important roles in the modulation of olfactory transmission. The present study examines the distribution of dopaminergic cells and the content of dopamine (DA) and its metabolites in control and deprived olfactory bulbs (OB), focusing on the differences between sexes. The content of DA and of its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were measured by HPLC. The morphology and distribution of dopaminergic neurons were studied using tyrosine hydroxylase (TH) immunohistochemistry. Cells were typified with TH-parvalbumin, TH-cholecystokinin or TH-neurocalcin double-immunofluorescence assays. Biochemical analyses revealed sex differences in the content of DA and of its metabolites. In normal conditions, the OBs of male rats had higher concentrations of DA, DOPAC and HVA than the OBs of females. The immunohistochemical data pointed to sex differences in the number of TH-immunopositive cells (higher in male than in female rats). Colocalization analyses revealed that dopaminergic cells constitute a different cell subpopulation from those labelled after parvalbumin, cholecystokinin or neurocalcin immunostaining. Unilateral olfactory deprivation caused dramatic alterations in the dopaminergic system. The DA content and the density of dopaminergic cells decreased, the contents of DA and DOPAC as well as TH immunoreactivity were similar in deprived males and females and, finally, the metabolite/neurotransmitter ratio increased. Our results show that the dopaminergic modulation of olfactory transmission seems to differ between males and females and that it is regulated by peripheral olfactory activity. A possible role of the dopaminergic system in the sexually different olfactory sensitivity, discrimination and memory is discussed.

Sexual experience does not compensate for the disruptive effects of zinc sulfate--lesioning of the main olfactory epithelium on sexual behavior in male mice

Recent studies point to an important role for the main olfactory epithelium (MOE) in regulating sexual behavior in male mice. We asked whether sexual experience could compensate for the disruptive effects of lesioning the MOE on sexual behavior in male mice. Male mice, which were either sexually naive or experienced, received an intranasal irrigation of either a zinc sulfate solution to destroy the MOE or saline. Sexual behavior in mating tests with an estrous female was completely abolished in zinc sulfate-treated male mice regardless of whether subjects were sexually experienced or not before the treatment. Furthermore, zinc sulfate treatment clearly disrupted olfactory investigation of both volatile and nonvolatile odors. Destruction of the MOE by zinc sulfate treatment was confirmed by a significant reduction in the expression of Fos protein in the main olfactory bulb following exposure to estrous female urine. By contrast, vomeronasal function did not seem to be affected by zinc sulfate treatment: nasal application of estrous female urine induced similar levels of Fos protein in the mitral and granule cells of the accessory olfactory bulb (AOB) of zinc sulfate- and saline-treated males. Likewise, the expression of soybean agglutinin, which stains the axons of vomeronasal organ neurons projecting to the glomerular layer of the AOB, was similar in zinc sulfate- and saline-treated male mice. These results show that the main olfactory system is essential for the expression of sexual behavior in male mice and that sexual experience does not overcome the disruptive effects of MOE lesioning on this behavior.

Enhanced urinary odor discrimination in female aromatase knockout (ArKO) mice

We asked whether odor discrimination abilities are sexually dimorphic in mice and, if so, whether the perinatal actions of estradiol contribute to these sex differences. The ability to discriminate different types of urinary odors was compared in male and female wild-type (WT) subjects and in mice with a homozygous-null mutation of the estrogen synthetic enzyme, aromatase (aromatase knockout; ArKO). Olfactory discrimination was assessed in WT and ArKO male and female mice after they were gonadectomized in adulthood and subsequently treated with estradiol benzoate. A liquid olfactometer was used to assess food-motivated olfactory discrimination capacity. All animals eventually learned to distinguish between urinary odors collected from gonadally intact males and estrous females; however, WT males as well as ArKO mice of both sexes learned this discrimination significantly more rapidly than WT females. Similar group differences were obtained when mice discriminated between urinary odors collected from gonadally intact vs. castrated males or between two non-social odorants, amyl and butyl acetate. When subjects had to discriminate volatile urinary odors from ovariectomized female mice treated with estradiol sequenced with progesterone versus estradiol alone, ArKO females quickly acquired the task whereas WT males and females as well as ArKO males failed to do so. These results demonstrated a strong sex dimorphism in olfactory discrimination ability, with WT males performing better than females. Furthermore, female ArKO mice showed an enhanced ability to discriminate very similar urinary odorants, perhaps due to an increased sensitivity of the main olfactory nervous system to adult estradiol treatment as a result of perinatal estrogen deprivation.

Destruction of the main olfactory epithelium reduces female sexual behavior and olfactory investigation in female mice

We studied the contribution of the main olfactory system to mate recognition and sexual behavior in female mice. Female mice received an intranasal irrigation of either a zinc sulfate (ZnSO4) solution to destroy the main olfactory epithelium (MOE) or saline (SAL) to serve as control. ZnSO4-treated female mice were no longer able to reliably distinguish between volatile as well as nonvolatile odors from an intact versus a castrated male. Furthermore, sexual behavior in mating tests with a sexually experienced male was significantly reduced in ZnSO4-treated female mice. Vomeronasal function did not seem to be affected by ZnSO4 treatment: nasal application of male urine induced similar levels of Fos protein in the mitral and granule cells of the accessory olfactory bulb (AOB) of ZnSO4 as well as SAL-treated female mice. Likewise, soybean agglutinin staining, which stains the axons of vomeronasal neurons projecting to the glomerular layer of the AOB was similar in ZnSO4-treated female mice compared to SAL-treated female mice. By contrast, a significant reduction of Fos in the main olfactory bulb was observed in ZnSO4-treated females in comparison to SAL-treated animals, confirming a substantial destruction of the MOE. These results show that the MOE is primarily involved in the detection and processing of odors that are used to localize and identify the sex and endocrine status of conspecifics. By contrast, both the main and accessory olfactory systems contribute to female sexual receptivity in female mice.