Oestrogen Receptor β‐Immunoreactive Neurones in the Ovine Hypothalamus: Distribution and Colocalisation with Gonadotropin‐Releasing Hormone

Oestrogen powerfully affects the secretion of gonadotropin‐releasing hormone (GnRH) from the brain in all species investigated, including sheep. Until recently, it was hypothesised that such regulation occurs indirectly because few or no GnRH neurones were found to express oestrogen receptor (ER) α. The discovery of a second oestrogen receptor, ERβ, and its subsequent localisation in numerous GnRH neurones in the rat, led to a reconsideration of this hypothesis. However, colocalisation of immunoreactive ERβ protein in GnRH neurones has only been demonstrated in the rat, raising the possibility that such putative direct regulation of GnRH neurones by oestrogen may be peculiar to this species. We have previously shown that steroid receptors in the sheep brain are acutely sensitive to fixation and the full complement of immunoreactive cells can only be visualised after antigen retrieval. The aims of this study were therefore to map immunocytochemically the distribution of ERβ neurones in the ewe brain, and to determine which proportion of GnRH neurones express ERβ. Brain sections (20 µm) from four ewes killed in anestrus were subjected to high temperature antigen retrieval and immunocytochemistry. Numerous ERβ‐immunoreactive cells were located throughout the hypothalamus and, following dual‐label immunocytochemistry, over 50% of the GnRH neurones were found to express immunoreactive ERβ. The functional significance of these ERβ‐expressing GnRH neurones in the ovine brain remains to be determined.