The effects of oestrogen on hypothalamic tissue

Oestrogens exert potent influences on hypothalamic differentiation and on sexual function throughout life. During the period in which the hypothalamus differentiates, the availability of oestrogens (or aromatizable androgens) to the hypothalamus results in a male pattern of gonadotropin secretion and in male sexual behaviour in the adult. Lack of oestrogens during this period results in a cyclic schedule of gonadotropin secretion and in female sexual behaviour. Oestrogens or their metabolites determine future sex-specific activity by facilitating synapse formation in developing target circuits. The onset of puberty may also in part reflect oestrogen-stimulated synapse formation in the hypothalamus. In contrast, oestrogen given to an adult female rat causes specific multifocal lesions in the lateral arcuate nucleus. Such animals exhibit persistent oestrus and small multicystic ovaries. Since the same anovulatory syndrome can be produced by mechanical disruption of neuronal pathways from more anterior regions to the arcuate nucleus, it would appear that oestrogen could effect a 'disconnection' of the circuit responsible for cyclic drive of gonadotropin secretion. Furthermore, old female rats spontaneously exhibit the same type of reproductive derangement as well as the arcuate lesion. Thus direct effects of oestrogens on neuronal circuitry within the hypothalamus may be responsible for (1) sexual differentiation, (2) the onset of puberty and sexual activity, and (3) cessation of reproductive function in the female rat.