Short-term effect of oestradiol on neurokinin B mRNA expression in the infundibular nucleus of ewes

In sheep, essentially all the neurokinin B (NKB) neurones of the infundibular nucleus express oestradiol receptor alpha, and analysis of female and male brains has revealed an exceptionally marked female-dominant sex difference in the numbers of NKB neurones in the infundibular nucleus. This neuronal population is located in an oestradiol-sensitive brain area involved in the control of gonadotropin-releasing hormone (GnRH) secretion and oestrous behaviour, but its physiological role is poorly documented. The aim of the present study was to analyse NKB mRNA expression at a crucial time when the steroid has stimulated the pathways leading to the induction of these two events. After cloning a specific ovine NKB antisense riboprobe, we examined the effects of a short oestradiol treatment (4 h subcutaneously) on the expression of NKB mRNA in the caudal part of the infundibular nucleus of progesterone-primed ovariectomized ewes. We demonstrated that oestradiol decreased both the level of NKB mRNA expression (34%) and the number of cells containing NKB mRNA (43%). Oestradiol acts strongly on these NKB cells in the short term. We suggest that this early change in NKB mRNA expression during the preovulatory period might be involved in the control of the induction of GnRH secretion or oestrous behaviour.