Noradrenergic projections from the A1 field to the preoptic area in the brain of the ewe and Fos responses to oestrogen in the A1 cells.

Previous studies have shown that there is a population of noradrenergic cells in the caudal A1 field of the brainstem of the ewe that contain oestrogen receptors and project to the preoptic area, where gonadotrophin releasing hormone (GnRH) neurones are located. There are some discrepancies in the literature regarding the extent of this projection and the location of the cells in the A1 region. The issue has been a focus of attention because the positive feedback response to oestrogen that causes the ovulatory GnRH/luteinizing hormone surge may originate from this brainstem region. The aim of the present study was to determine the extent of the projections to the preoptic area and to determine whether the caudal A1 cells are activated by oestrogen. Eleven ovariectomized ewes received an injection of the retrograde tracer FluoroGold into the preoptic hypothalamus and four of these also received an i.m. injection of oestrogen 2 h before tissue collection. A further three sheep received i.m. oil injections to act as controls for those receiving oestrogen. Dopamine-beta-hydroxylase (DBH)-positive, retrogradely labelled cells were found within the A1 field in sheep that received preoptic FluoroGold injections. Cells in the vicinity of the A2 and A6 fields, that were retrogradely labelled with FluoroGold, were not DBH-positive. Thus, cells in the A1 field provide a direct noradrenergic projection to the preoptic area and may be involved in the control of the secretion of GnRH in this species. Cells that project to the preoptic hypothalamus from more rostrally located areas of the brainstem are not noradrenergic. In the animals that received oestrogen, double-labelling immunohistochemistry was performed throughout the A1 field for FluoroGold, DBH and Fos. DBH cells of the A1 field expressed Fos only in the oestrogen-treated animals and not in the oil-treated animals. There was a decline in the number of DBH cells that were retrogradely labelled from the caudal region of A1 towards obex. There was a similar gradient in the number of cells that were double-labelled for Fos and FluoroGold. We conclude that there is a population of noradrenergic cells in the caudal A1 field that project to the preoptic area; this is a larger group of cells than previously reported. Oestrogen elicits an acute Fos response in these cells, which may be involved in the time-delayed positive feedback response on GnRH cells. The caudal-to-rostral gradient in the labelling with FluoroGold and Fos in DBH-positive cells is similar to that seen previously for oestrogen receptor in DBH-positive cells in the A1 field.