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.     

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.     

Rapid changes in brain aromatase activity in the female quail brain following expression of sexual behaviour

In male quail, oestrogens produced in the brain (neuro‐oestrogens) exert a dual action on male sexual behaviour: they increase sexual motivation within minutes via mechanisms activated at the membrane but facilitate sexual performance by slower, presumably nuclear‐initiated, mechanisms. Recent work indicates that neuro‐oestrogens are also implicated in the control of female sexual motivation despite the presence of high circulating concentrations of oestrogens of ovarian origin. Interestingly, aromatase activity (AA) in the male brain is regulated in time domains corresponding to the slow “genomic” and faster “nongenomic” modes of action of oestrogens. Furthermore, rapid changes in brain AA are observed in males after sexual interactions with a female. In the present study, we investigated whether similar rapid changes in brain AA are observed in females allowed to interact sexually with males. A significant decrease in AA was observed in the medial preoptic nucleus after interactions that lasted 2, 5 or 10 minutes, although this decrease was no longer significant after 15 minutes of interaction. In the bed nucleus of the stria terminalis, a progressive decline of average AA was observed between 2 and 15 minutes, although it never reached statistical significance. AA in this nucleus was, however, negatively correlated with the sexual receptivity of the female. These data indicate that sexual interactions affect brain AA in females as in males in an anatomically specific manner and suggest that rapid changes in brain oestrogens production could also modulate female sexual behaviour.     

The vomeronasal organ is required for the expression of lordosis behaviour, but not sex discrimination in female mice

The role of the vomeronasal organ (VNO) in mediating neuroendocrine responses in female mice is well known; however, whether the VNO is equally important for sex discrimination is more controversial as evidence exists for a role of the main olfactory system in mate recognition. Therefore, we studied the effect of VNO removal (VNOx) on the ability of female mice to discriminate between volatile and non-volatile odours of conspecifics of the two sexes and in different endocrine states using Y-maze tests. VNOx female mice were able to reliably distinguish between male and female or male and gonadectomized (gdx) male volatile odours. However, when subjects had to discriminate between male and female or gdx male non-volatile odours, VNOx females were no longer able to discriminate between sex or different endocrine status. These results thus show that the VNO is primarily involved in the detection and processing of non-volatile odours, and that female mice can use volatile odours detected and processed by the main olfactory system for mate recognition. However, VNO inputs are needed to promote contact with the male, including facilitation of lordosis responses to his mounts. A single subcutaneous injection with gonadotropin-releasing hormone (GnRH) partially reversed the deficit in lordosis behaviour observed in VNOx females suggesting that VNO inputs may reach hypothalamic GnRH neurons to influence the display of sexual behaviour.     

Male Faces and Odours Evoke Differential Patterns of Neurochemical Release in the Mediobasal Hypothalamus of the Ewe During Oestrus: An Insight Into Sexual Motivation?

During behavioural oestrus female sheep, like females of many species, become both attracted to and sexually receptive towards males, whereas at other times they will avoid them. The mediobasal hypothalamus is the main site for the feedback action of sex steroids to induce sexual behaviour in the sheep and in previous studies we have shown that noradrenaline and serotonin are released in this region during sexual interactions with males. The current study investigated whether such changes are specific to interactions with males and if visual or olfactory cues or somatosensory stimulation during mating are critical. In vivo microdialysis sampling was carried out in the mediobasal hypothalamus of ovariectomized ewes submitted to artificial oestrous cycles. Release of monoamines and amino acid transmitters was first measured in animals during and after oestrus when they were exposed to interactions with either males or females or presentation of food. Noradrenaline concentrations only increased significantly when the females were in oestrus and interacted with males irrespective of whether intromissions were permitted. Females were then exposed to visual (faces) or odour (a home pen) cues from males or to the males themselves. Slide images of male faces increased concentrations of amines, glutamate and GABA during early oestrus, when females spent most time looking at them. During late oestrus noradrenaline, glutamate and GABA concentrations also increased in response to the male faces but no transmitter changes were seen during the luteal phase or at any time where the females were exposed to female faces, or inverted male faces. Exposure to male odour produced a lower increase in noradrenaline concentrations when females were in early oestrus but marked increases 20 and 30 min after exposure to male odours in late oestrus. No other transmitters were affected. Exposure to a male and mating with him when females were in early or late oestrus produced increased noradrenaline concentrations similar to those seen with face stimuli alone although other neurotransmitters were unaffected. These results show that noradrenaline, and to a lesser extent dopamine, serotonin, glutamate and GABA release in the mediobasal hypothalamus, can be modulated specifically in the oestrous female by sensory information coming from the male during oestrus. The differential effects of male cues during early and late oestrus suggest their involvement in (i) proceptive or anticipatory sexual responses shown by the female to male cues, and (ii) receptive sexual responses, and suggest that the mediobasal hypothalamus plays a key role in the integration of hormonal action on sexual motivation and processing of sensory information during oestrus.     

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

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

Relationships between aromatase activity in the brain and gonads and behavioural deficits in homozygous and heterozygous aromatase knockout mice

The present study was carried out to determine whether aromatase knockout (ArKO) mice are completely devoid of aromatase activity in their brain and gonads and to compare aromatase activity in wild-type and ArKO mice, as well as in heterozygous (HET) mice of both sexes that were previously shown to display a variety of reproductive behaviours at levels intermediate between wild-type and ArKO mice. Aromatase activity was extremely low, and undetectable by the tritiated water assay, in homogenates of the preoptic area-hypothalamus of adult wild-type mice, but was induced following a 12-day treatment with testosterone. The induction of aromatase activity by testosterone was significantly larger in males than in females. Even after 12 days exposure to testosterone, no aromatase activity was detected in the brain of ArKO mice of either sex whereas HET mice showed intermediate levels of activity between ArKO and wild-type. Aromatase activity was also undetectable in the ovary of adult ArKO females but was very high in the wild-type ovary and intermediate in the HET ovary. In wild-type mice, a high level of aromatase activity was detected on the day of birth even without pretreatment with testosterone. This neonatal activity was higher in males than in females, but females nevertheless appear to display a substantial level of oestrogen production in their brain. Aromatase activity was undetectable in the brain of newborn ArKO males and females and was intermediate between wild-type and ArKO in HET mice. In conclusion, the present study confirms that ArKO mice are unable to synthesize any oestrogens, thereby validating the ArKO mouse as a valuable tool in the study of the physiological roles of oestradiol. In addition, it demonstrates that the intermediate behaviour of HET mice presumably reflects the effect of gene dosage on aromatase expression and activity, that aromatase activity is sexually differentiated in mice during the neonatal period as well as in adulthood and, finally, that the neonatal female brain produces substantial amounts of oestrogens that could play a significant role in the sexual differentiation of the female brain early in life.     

Male aromatase knockout mice acquire a conditioned place preference for cocaine but not for contact with an estrous female

We have previously shown that male mice carrying a targeted mutation in the Cyp19 gene which encodes the aromatase enzyme (aromatase knockout or ArKO), showed a reduced interest to investigate volatile odors from conspecifics in a Y-maze. We asked here whether the incentive value of reproductively relevant odors is reduced in ArKO males by comparing the ability of male wild-type (WT) and ArKO mice to learn a conditioned place preference using exposure to reproductively relevant odors as incentive stimuli. When the presence of an anesthetized estrous female or soiled bedding from estrous females was used as incentive stimuli, only WT and not male ArKO mice showed conditioned place preference suggesting that the reward value of these stimuli is reduced in ArKO males. However, ArKO males showed conditioned place preference when cocaine was used as incentive stimulus, indicating that ArKO males are able to learn the conditioned place preference procedure. These results thus further confirm the important role of estradiol in sexually related behavioral responses in male mice.     

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.     

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.     

DREADD‐induced silencing of the medial amygdala reduces the preference for male pheromones and the expression of lordosis in estrous female mice

Sexually naïve estrous female mice seek out male urinary pheromones; however, they initially display little receptive (lordosis) behavior in response to male mounts. Vomeronasal–accessory olfactory bulb inputs to the medial amygdala (Me) regulate courtship in female rodents. We used a reversible inhibitory chemogenetic technique (Designer Receptors Exclusively Activated by Designer Drugs; DREADDs) to assess the contribution of Me signaling to females’ preference for male pheromones and improvement in receptivity normally seen with repeated testing. Sexually naïve females received bilateral Me injections of an adeno‐associated virus carrying an inhibitory DREADD. Females were later ovariectomized, treated with ovarian hormones, and given behavioral tests following intraperitoneal injections of saline or clozapine‐N‐oxide (CNO; which hyperpolarizes infected Me neurons). CNO attenuated females’ preference to investigate male vs. female urinary odors. Repeated CNO treatment also slowed the increase in lordosis otherwise seen in females given saline. However, when saline was given to females previously treated with CNO, their lordosis quotients were as high as other females repeatedly given saline. No disruptive behavioral effects of CNO were seen in estrous females lacking DREADD infections of the Me. Finally, CNO attenuated the ability of male pheromones to stimulate Fos expression in the Me of DREADD‐infected mice but not in non‐infected females. Our results affirm the importance of Me signaling in females’ chemosensory preferences and in the acute expression of lordosis. However, they provide no indication that Me signaling is required for the increase in receptivity normally seen after repeated hormone priming and testing with a male.     

Maternal Odours Induce Fos in the Main But Not the Accessory Olfactory Bulbs of Neonatal Male and Female Ferrets

Previous research demonstrated that exposing gonadectomized adult ferrets to odours in oestrous female bedding induced nuclear Fos-immunoreactivity (Fos-IR; a marker of neuronal activity) in the main as opposed to the accessory olfactory system in a sexually dimorphic fashion, which was further augmented in both sexes by treatment with testosterone propionate. Ferrets are born in an altricial state and presumably use maternal odour cues to locate the nipples until the eyes open after postnatal (P) day 23. We investigated whether maternal odours augment neuronal Fos preferentially in the main versus accessory olfactory system of neonatal male and female ferret kits. Circulating testosterone levels peak in male ferrets on postnatal day P15, and mothers provide maximal anogenital stimulation (AGS) to males at this same age. Therefore, we assessed the ability of maternal odours to augment Fos-IR in the accessory olfactory bulb (AOB), the main olfactory bulb (MOB) and other forebrain regions of male and female ferret kits on P15 and investigated whether artificial AGS (provided with a paintbrush) would further enhance any effects of maternal odours. After separation from their mothers for 4 h, groups of male and female kits that were placed for 1.5 h with their anaesthetized mother had significantly more Fos-IR cells in the MOB granule cell layer and in the anterior-cortical amygdala, but not in the AOB cell layer, compared to control kits that were left on the heating pad. Artificial AGS failed to amplify these effects of maternal odours. Maternal odours (with or without concurrent AGS) failed to augment neuronal Fos-IR in medial amygdaloid and hypothalamic regions that are activated in adult ferrets by social odours. In neonatal ferrets of both sexes, as in adults, socially relevant odours are detected by the main olfactory epithelium and initially processed by the MOB and the anterior-cortical amygdala. In neonates, unlike adults, medial amygdaloid and hypothalamic neurones either do not respond to these inputs or respond in a manner that fails to induce Fos expression.     

Neonatal exposure of female ferrets to testosterone alters sociosexual preferences in adulthood

Sociosexual preferences in adult female ferrets, as measured in a choice arena, were altered by neonatal exposure to exogenous testosterone. Adult female ferrets showed a preference for males which did not depend on the presence of gonadal steroids, because gonadectomized and gonadectomized estrogen-treated females showed identical preferences for males. Adult castrated males showed no preference for females unless these males were treated with testosterone. A similar no-preference pattern was found in adult females that had received testosterone neonatally. Females exposed neonatally to dihydrotestosterone or estradiol exhibited the normal females' male-oriented preference. These results indicate that testosterone secreted by the testes in the developing male may interrupt the phenotypic female development pattern and hence prevent the emergence of a homosexual preference in adulthood in the male ferret.     

Central forebrain Fos responses to familiar male odours are attenuated in recently mated female mice

Exposure of recently mated female mice to the urinary odours of an unfamiliar male blocks pregnancy (the Bruce effect). The absence of a pregnancy block in response to the stud male's familiar odours depends on an olfactory memory that is formed in the accessory olfactory bulb (AOB) in response to vomeronasal organ (VNO) inputs during mating. Sexually naive Balb/c female mice in pro-oestrus/oestrus were either placed onto soiled bedding ('bedding-only' females) from, or allowed to mate with, a Balb/c male ('recently mated' females). After 42 h, females were placed for 90 min onto clean bedding (controls) or onto soiled bedding from either a C57BL/6 male (unfamiliar bedding) or a Balb/c male (familiar bedding). Significant increases in Fos-immunoreactivity (Fos-IR, a marker of neuronal activation) occurred in the medial amygdala and the medial preoptic area (MPA) of 'bedding only' females exposed to either unfamiliar or familiar bedding and in 'recently mated' females exposed to unfamiliar bedding but not to familiar bedding. This suggests that a mating-induced memory prevents the later activation by the familiar stud male's odours of neurons in forebrain regions that receive inputs from the VNO--AOB. 'Bedding-only' females later exposed to either familiar or unfamiliar bedding had increased Fos-IR in the G alpha(o) protein-expressing basal zone of the VNO whereas no such effect occurred in 'recently mated' females. Familiar, as well as unfamiliar, male odours augmented Fos-IR in significantly more rostral than caudal AOB granule cells in all groups, with the effect being strongest in 'recently mated' females exposed to familiar male bedding. This outcome is consistent with the absence of odour-induced Fos-IR in forebrain regions of these females and, presumably, the absence of a pregnancy block.     

Corticosterone rapidly reduces male odor preferences in female mice

There is accumulating evidence for rapid, non-genomic behavioral effects of various steroids including that of the glucocorticoid, corticosterone. Using an odor preference test, the responses of which are indicative of mate preferences and sexual interest, we examined the effects of acute corticosterone on the responses of oestrous female mice to male odors. Control female mice displayed an overwhelming preference for the odors of male mice. Peripheral administration of corticosterone elicited a significant dose-related (1.0-5.0 mg/kg) decrease in female preference for male odors at 10 min, but not at 60 min, after administration. These inhibitory effects of corticosterone on odor preferences were significantly reduced by the competitive NMDA antagonist, NPC 12626, and enhanced by the GABA antagonist bicuculline. This indicates that corticosterone has rapid inhibitory effects on olfactory mediated female mate preferences and responses to male odor that in part involve interactions with NMDA and GABA receptor mechanisms.     

Lesions of the medial preoptic area/anterior hypothalamus (MPOA/AH) modify partner preference in male rats

When given the choice, male rats will interact with a receptive female while female rats will interact with a sexually active male. In the present experiment partner preference was tested in male and female rats before and after lesions of the medial preoptic area of the anterior hypothalamus (MPOA/HA). Subjects were gonadectomized, treated for 10 days with 5 μg/kg of estradiol benzoate (EB) and tested for male coital behavior with receptive females and for partner preference in a three compartment box with free access to either a sexually receptive female or a sexually active male. The same tests were repeated after 10 days of treatment with 5 mg/kg of testosterone propionate (TP). The subjects then received a bilateral electrolytic lesion aimed at the MPOA/AH. Two weeks after the lesion the hormonal treatments and behavioral tests were repeated in the same sequence. Prior to the lesion, females showed a clear preference to interact with the stimulus male while male subjects showed a preference to interact with the receptive female regardless of the hormonal treatment they received. After lesions the females preference for the opposite sex was not modified, they spent more time in the chamber with the stimulus male regardless of whether they had an extensive bilateral destruction of the MPOA/AH or a sham lesion. Males with bilateral destruction of the MPOA/AH changed their partner preference after the lesion. They spent significantly more time in the chamber with the stimulus male than in the chamber with the receptive female. As well, the coital behavior of males with bilateral destruction of the MPOA/AH was significantly reduced after the lesion. The change of preference was observed when the lesioned animals were treated either with EB or TP. The results of the present experiment further support the notion that the MPOA/AH is a crucial structure in the integration of sensory cues that determine partner preference.     

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.     

Refining the dual olfactory hypothesis: pheromone reward and odour experience

In rodents, sexual advertisement and gender recognition are mostly (if not exclusively) mediated by chemosignals. Specifically, there is ample evidence indicating that female mice are ‘innately’ attracted by male sexual pheromones that have critical non-volatile components and are detected by the vomeronasal organ. These pheromones can only get access to the vomeronasal organ by active pumping mechanisms that require close contact with the source of the stimulus (e.g. urine marks) during chemoinvestigation.We have hypothesised that male sexual pheromones are rewarding to female mice. Indeed, male-soiled bedding can be used as a reinforcer to induce conditioned place preference, provided contact with the bedding is allowed. The neural mechanisms of pheromone reward seem, however, different from those employed by other natural reinforcers, such as the sweetness or postingestive effects of sucrose.In contrast to vomeronasal-detected male sexual pheromones, male-derived olfactory stimuli (volatiles) are not intrinsically attractive to female mice. However, after repeated exposure to male-soiled bedding, intact female mice develop an acquired preference for male odours. On the contrary, in females whose accessory olfactory bulbs have been lesioned, exposure to male-soiled bedding induces aversion to male odorants. These considerations, together with data on the different properties of olfactory and vomeronasal receptors, lead us to make a proposal for the complementary roles that the olfactory and vomeronasal systems play in intersexual attraction and in other forms of intra- or inter-species communication.     

Brain‐synthesized oestrogens regulate cortical migration in a sexually divergent manner

Oestrogens play an important role in brain development where they have been implicated in controlling various cellular processes. Several lines of evidence have been presented showing that oestrogens can be synthesized locally within the brain. Studies have demonstrated that aromatase, the enzyme responsible for the conversion of androgens to oestrogens, is expressed during early development in both male and female cortices. Furthermore, 17β‐oestradiol has been measured in foetal brain tissue from multiple species. 17β‐oestradiol regulates neural progenitor proliferation as well as the development of early neuronal morphology. However, what role locally derived oestrogens play in regulating cortical migration and, moreover, whether these effects are the same in males and females are unknown. Here, we investigated the impact of knockdown expression of Cyp19a1, which encodes aromatase, between embryonic day (E) 14.5 and postnatal day 0 (P0) had on neural migration within the cortex. Aromatase was expressed in the developing cortex of both sexes, but at significantly higher levels in male than female mice. Under basal conditions, no obvious differences in cortical migration between male and female mice were observed. However, knockdown of Cyp19a1 resulted in an increase in cells within the cortical plate, and a concurrent decrease in the subventricular zone/ventricular zone in P0 male mice. Interestingly, the opposite effect was observed in females, who displayed a significant reduction in cells migrating to the cortical plate. Together, these findings indicate that brain‐derived oestrogens regulate radial migration through distinct mechanisms in males and females.     

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.