Neurochemical identity of neurones expressing oestrogen and androgen receptors in sheep hypothalamus.

Gonadal steroids exert important feedback influences on hypothalamic neurones involved in regulating reproductive behaviour and pituitary hormone secretion. The recent development of antibodies specific for individual gonadal steroid receptors has been of great use in determining precisely which cells in the hypothalamus express androgen, oestrogen and progesterone receptors. In the sheep brain, both oestrogen and androgen receptor antibodies have been used successfully and the distribution of cells expressing both receptors has now been determined in ewes and rams, respectively. In addition, the predominantly nuclear localization of the steroid receptors has enabled double-labelling immunocytochemical procedures to determine the neurochemical phenotype of neurones expressing the steroid receptor. Work in the sheep hypothalamus shows that gonadotrophin-releasing hormone neurones do not possess oestrogen or androgen receptors. However, substantial numbers of cells containing oestrogen receptors in the preoptic area of ewes contain the inhibitory neurotransmitter gamma aminobutyric acid, while most oestrogen receptor-immunoreactive neurones in the ventromedial nucleus synthesize the inhibitory neuropeptide somatostatin. Androgen receptors have been detected in many of the ventromedial somatostatin neurones in rams. In contrast, the neurochemical phenotype of the great majority of oestrogen and androgen receptor-immunoreactive cells in the arcuate nucleus remains unknown. The identification of the neurotransmitters and neuropeptides synthesized by neurones possessing androgen and oestrogen receptors in different regions of the ovine hypothalamus provides a neuroanatomical basis for understanding the mechanisms by which gonadal steroids regulate reproductive function.