TESTOSTERONE IMPLANTS IN SPECIFIC NEURAL SITES ACTIVATE FEMALE SEXUAL-BEHAVIOR

Sexual behaviour in most female mammals is regulated by oestrogen, oft en acting synergistically with progesterone. Moreover, the most import ant neural site of action for oestradiol is the ventromedial nucleus. In the female musk shrew, Suncus murinus, testosterone (T) activates s exual behaviour. Virgin females first engage in copulatory behaviour m any hours in advance of follicular development and ovulation, when pla sma oestradiol levels are very low. Testosterone, produced by the ovar ies and the adrenal glands, must be aromatized centrally to oestradiol to initiate sexual behaviour. To identify the neural sites of action for T, ovariectomized females received unilateral hormone implants con taining testosterone propionate. Hormone pellets were placed in 1 of s everal brain sites including the medial preoptic area and the dorsalme dial hypothalamus (DMH). Implants in either of these 2 sites, but not in the lateral preoptic area, internal capsule, nor anterior hypothala mus stimulated the induction of sexual behaviour. Hormone implants in the ventrolateral hypothalamus resulted in partially receptive animals . Immunocytochemistry was employed to determine which steroid receptor s were present in the 2 behaviourally active sites. The medial preopti c area (MPO) and the dorsal and ventromedial hypothalamus both contain many cells that express oestrogen receptor immunoreactivity. A smalle r subset of neurons in these regions are immunoreactive for androgen r eceptors. In summary, testosterone can act specifically in either the MPO or the DMH to induce female sexual behaviour. Both sites contain c ells that express oestrogen and androgen receptors. Thus, testosterone may work via one or both of these steroid receptors to regulate behav iour. The results of these studies, and data collected previously, sho w that female sexual behaviour in this species is regulated by T and t hat this steroid can induce full sexual behaviour in 2 specific neural sites. Both of the sites of action identified in this experiment are critical for the expression of sexual behaviour in other species. The MPO has been shown to regulate male sexual behaviour via the aromatiza tion of T to oestradiol in rodents and birds. The DMH is just dorsal t o the ventromedial nucleus (VMN) which is essential for the regulation of sexual behaviour in female rodents. Further work is needed to dete rmine if the neural circuitry for sexual behaviour in the musk shrew i nvolves either the MPO or the VMN, if both areas are essential, or per haps, the 2 regions regulate different aspects of female sexual behavi our.