Progesterone treatment of adult male rats suppresses arginine vasopressin expression in the bed nucleus of the stria terminalis and the centromedial amygdala

The steroid sensitive vasopressin cells of the bed nucleus of the stria terminalis (BST) and centromedial amygdala (CMA) are involved in numerous behavioural and physiological functions. These cells are known to be greatly influenced by gonadal steroids. Castration reduces and testosterone replacement restores arginine vasopressin (AVP)-immunoreactive (-ir) labelling and AVP mRNA expression in the BST and CMA. Gonadal steroids appear to act directly in AVP-expressing cells within the BST and CMA, because the majority of AVP-ir cells in these areas contain oestrogen and androgen receptor immunoreactivity. Recently, we have localised progestin receptor immunoreactivity in virtually all of the AVP-ir cells in the BST and CMA. To understand the role played by progestin receptors in AVP cells within the BST and CMA, we treated male rats with 1 mg of progesterone or oil for 5 days, and then examined AVP immunoreactivity within the brain. We found that progesterone decreased AVP-ir labelling within the BST and CMA, as well as in two of the projection sites of these cells, the lateral septum and lateral habenula. Progesterone treatment did not alter testosterone secretion from the testes, nor did it alter adult male sexual behaviour. These data illustrate an additional mechanism by which the AVP cells in the BST and CMA can be regulated. These data also suggest that progesterone may act in the male brain to influence behaviours that are AVP-dependent.