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.

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.

No abstract

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.

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.