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 adrena l glands, is the major steroid hormone in circulation at times of mating, a nd as such, regulates sexual behaviour. In the first experiment, we identif ied the neural site(s) of action for testosterone. Hormone implants were pl aced in one of three targeted brain regions. The neural sites selected were the medial anterior division of the bed nucleus of the stria terminalis (B NSTMA), medial preoptic area (mPOA) and the ventromedial nucleus of the hyp othalamus (VMN). Ovariectomized females who received a unilateral testoster one propionate implant in either the mPOA or VMN, were significantly more l ikely to display sexual behaviour as compared to females who received an im plant in the BNSTMA or any other hypothalamic nucleus. In experiments 2 and 3, we investigated whether the behavioural effects of testosterone propion ate were mediated by an oestrogen receptor or the androgen receptor. Ovarie ctomized females that received oestradiol (E-2) implants in either the mPOA or VMN were more likely to display receptivity, and had significantly shor ter behavioural latencies, as compared to females implanted with either dih ydrotestosterone or cholesterol. These data show that neural aromatization of testosterone to E-2 in the mPOA or VMN is necessary for optimal activati on of female musk shrew sexual behaviour. This finding implies a degree of neural redundancy in the networks that control the expression of sexual rec eptivity.