Sex differences in expression of oestrogen receptor α but not androgen receptor mRNAs in the foetal lamb brain.

Gonadal steroid hormones play important roles during critical periods of development to organise brain structures that control sexually dimorphic neuroendocrine responses and behaviours. Specific receptors for androgens and oestrogens must be expressed at appropriate times during development to mediate these processes. The present study was performed to test for sex differences in the relative expression of oestrogen receptor (ER)α and androgen receptor (AR) mRNA during the window of time in gestation that is critical for behavioural masculinisation and differentiation of the ovine sexually dimorphic nucleus (oSDN) in the sheep. In addition, we examined whether ERα and AR mRNA expression is localised within the nascent oSDN and could be involved in its development. Using the quantitative real-time polymerase chain reaction, we found that females expressed more ERα mRNA than males in medial preoptic area and medial basal hypothalamus during the mid-gestational critical period for brain sexual differentiation. No sex differences were found for AR mRNA in any tissue examined or for ERα in amygdala and frontal cortex. Using radioactive in situ hybridisation, we found that the distributions of ERα and AR mRNA overlapped with aromatase mRNA, which delineates the boundaries of the developing oSDN and identifies this nucleus as a target for both androgens and oestrogens. These data demonstrate that the transcriptional machinery for synthesising gonadal steroid receptors is functional in the foetal lamb brain during the critical period for sexual differentiation and suggest that possible mechanisms for establishing dimorphisms controlled by gonadal steroids may exist at the level of steroid hormone receptor expression.