Steroid implants in the medial preoptic area or ventromedial nucleus of the hypothalamus activate female sexual behaviour in the musk shrew.

Female musk shrews are induced ovulators that do not exhibit a spontaneous behavioural oestrous cycle. Testosterone produced by the ovaries and adrenal glands, is the major steroid hormone in circulation at times of mating, and as such, regulates sexual behaviour. In the first experiment, we identified the neural site(s) of action for testosterone. Hormone implants were placed in one of three targeted brain regions. The neural sites selected were the medial anterior division of the bed nucleus of the stria terminalis (BNSTMA), medial preoptic area (mPOA) and the ventromedial nucleus of the hypothalamus (VMN). Ovariectomized females who received a unilateral testosterone propionate implant in either the mPOA or VMN, were significantly more likely to display sexual behaviour as compared to females who received an implant in the BNSTMA or any other hypothalamic nucleus. In experiments 2 and 3, we investigated whether the behavioural effects of testosterone propionate were mediated by an oestrogen receptor or the androgen receptor. Ovariectomized females that received oestradiol (E2) implants in either the mPOA or VMN were more likely to display receptivity, and had significantly shorter behavioural latencies, as compared to females implanted with either dihydrotestosterone or cholesterol. These data show that neural aromatization of testosterone to E2 in the mPOA or VMN is necessary for optimal activation of female musk shrew sexual behaviour. This finding implies a degree of neural redundancy in the networks that control the expression of sexual receptivity.

Perinatal organization of a sexually dimorphic aromatase enzyme-containing immunoreactive nucleus.

Sexual dimorphisms have been identified in the brains of many vertebrates. The adult musk shrew brain contains a sexually dimorphic nucleus of aromatase (AROM) immunoreactive (ir) neurons within the medial preoptic area (mPOA). This cell group is larger and contains more neurons in males than in females. We examined the effects of perinatal hormone manipulations on the organization of this nucleus. Administration of testosterone (T) to female pups (days 1-5 postnatal) increased the size of the AROM-ir nucleus. Males castrated on postnatal day 1 had a smaller AROM-ir nucleus as adults than did males that remained gonadally intact during development. These data show that T and/or its estrogenic metabolite act during an early critical period to organize the development of AROM-ir neurons in the mPOA.

Immunolocalization of androgen receptors and aromatase enzyme in the adult musk shrew brain.

In the brain and other tissues, estrogens are produced by aromatization of androgens. Biochemical data suggest that aromatase enzyme is regulated by the androgen receptor (AR). Neurons that contain either AR or aromatase (AROM) enzyme reside in many of the same brain regions. In this report, we examined the codistribution of AR- and AROM-enzyme-immunoreactive (-ir) neurons in several regions of the adult male and female musk shrew brain. Data were collected from the intermediate nucleus of the lateral septum (LS), medial anterior (BNSTMA) and medial posterointerior (BNSTMP) divisions of the bed nucleus of the stria terminalis, medial preoptic area (mPOA), ventromedial nucleus of the hypothalamus (VMN), medial (MeA), cortical and central nuclei of the amygdala. Males had significantly more AR-ir neurons in the BNSTMP, mPOA, VMN and LS as compared to females. With the exception of the BNSTMA and LS, males had more AROM-ir neurons in each region than females. Furthermore, males had significantly more double-labeled neurons than females in the BNSTMP, mPOA, VMN, LS and MeA. The percentage of AROM-ir neurons that also contained AR immunoreactivity ranged from 13 to 82% depending on sex and region. The highest percentage of dual-labeled neurons (79% in females and 82% in males) was found in the VMN. Taken together, these data show that there is extensive cellular colocalization of AR and AROM enzyme in specific regions of the musk shrew brain. We propose that in both sexes, androgen receptors may act as transcription factors to regulate AROM enzyme.

Induction of fos-like immunoreactivity in musk shrews after mating.

In many mammalian species the neuroendocrine regulation of male and female reproductive behavior is sexually dimorphic. By contrast, many features of female sexual behavior in the musk shrew (Suncus murinus) more closely resemble those of males than of females of other species. Female musk shrews require testosterone (T), which is neurally aromatized to estrogen, to induce sexual behavior. Aromatization occurs in the medial preoptic area (MPOA), and this region is critical for the expression of female receptivity. To compare neural responses to sexual behavior in females and males, we compared the number of Fos-like immunoreactive (Fos-ir) neurons after mating in musk shrews. In both males and females the number of Fos-ir neurons was increased by mating activity in the granule layer of the accessory olfactory bulb (gr-AOB), the bed nucleus of the stria terminalis (BNST), MPOA, the medial amygdala (MeA), and the region corresponding to the midbrain central tegmental field (CTF). Although Fos was induced by mating in several regions, this response was only dimorphic in the ventral medial nucleus of the hypothalamus (VMN), where mating significantly increased Fos-ir in females, but not in males. In both sexes, only the gr-AOB displayed an increase in Fos-ir after exposure to chemosensory cues alone. Thus, the pattern of Fos expression in the brain after mating is only sexually dimorphic in one region, the VMN. Further, in spite of past behavioral studies done in this species, which show a role for pheromones in induction of receptivity, these data show that exposure to pheromones does not induce Fos in structures caudal to the olfactory bulbs.

Co-localization of estrogen receptor and aromatase enzyme immunoreactivities in adult musk shrew brain.

Estrogens are produced by the aromatization of androgens. These steroids exert their actions after binding to their receptors. Past studies have shown that estrogen receptors (ER) and aromatase enzyme (AROM) reside in many of the same brain regions. Few studies, however, have examined the neural co-localization of these important components involved in estrogen-activated behaviors. In the present study we examined the co-localization of ER and AROM immunoreactive (ir) neurons in musk shrew (Suncus murinus) brains. Data were collected from a representative section from three neural regions, the bed nucleus of the stria terminalis (BNST), medial preoptic area (mPOA), and ventromedial nucleus of the hypothalamus (VMN). Here we report a sex difference in the number of ER-ir neurons from the analyzed section of the mPOA and BNST. Females have more ER-ir neurons in the mPOA and males have more in the BNST. In the sections we examined, males tended to have more aromatase containing neurons than females. Although there were no significant differences in the numbers of double-labeled cells, the VMN contains the greatest percentage of these cells in both males and females; followed by the mPOA and the BNST. In addition, in the mPOA of both sexes, a distinct nucleus of aromatase containing neurons which was devoid of ER immunoreactivity was noted. Area measurements of the AROM-ir nucleus showed that it was significantly larger in males than in females. Taken together, these data suggest that there is not extensive cellular co-localization of estrogen receptors and aromatase enzyme in the musk shrew brain. However, the presence of other genomic forms of ER (membrane and/or ERbeta) in AROM containing neurons has not been ruled out by this study. Thus, we hypothesize that estrogens produced in brain affect behavior by binding to ER in neurons other than those that contain aromatase enzyme.

Axon sparing lesions of the medial preoptic area block female sexual behavior.

The role of the medial preoptic area (mPOA) in regulating female musk shrew sexual behavior was assessed with excitatory neurotoxin, N-methyl-D-aspartate (NMDA) lesions. Ovariectomized, testosterone-implanted females that received lesions in the mPOA were statistically less likely to show complete sex behavior as compared to controls. These data suggest that the mPOA plays an activational role in testosterone-induced female sexual behavior.