Cells of the arcuate nucleus and ventromedial nucleus of the ovariectomized ewe that respond to oestrogen: A study using Fos immunohistochemistry

Oestrogen produces a positive feedback effect on the secretion of gonadotro pin releasing hormone (GnRH) and luteinizing hormone (LH) when implanted in to the ventromedial/arcuate nucleus of the ovariectomized (OVX) ewe. This h as led to the belief that it is in this area of the hypothalamus that oestr ogen causes the preovulatory surge in GnRH/LH. To date, however, the cell t ypes that are integral to this response have not been identified. The prese nt study aimed to examine cellular responsiveness to oestrogen in this regi on of the brain using Fos immunohistochemistry and further aimed to determi ne the cell type that shows an acute response to oestrogen. OVX ewes (n=4-6 per group) were given i.m. injections of oestradiol benzoate or oil (vehic le) and were killed 1-6 h later. Brains were perfused for immunohistochemis try. The number of cells in the arcuate nucleus which were immunopositive f or Fos was greater (two- to fourfold) in the oestradiol benzoate-treated OV X ewes (n=5) 1 h after injection. The number of Fos-positive cells in the v entromedial hypothalamic nucleus was 10-fold greater in the oestradiol benz oate-treated ewes 1 h after injection. Because there were high levels of Fo s-immunoreactive cells in oil-treated ewes, we repeated the experiment with i.v. injection of 50 mug oestrogen or vehicle (n=5). With this latter proc edure, we found that oestrogen injection caused a significant increase in t he number of Fos immunoreactive cells in the arcuate nucleus within 1 h, bu t there was no response in the ventromedial hypothalamus. To further charac terize the types of cells that might respond to oestrogen, we double-labell ed cells for Fos and either adrenocorticotropin hormone, neuropeptide Y or tyrosine hydroxylase (a marker for dopaminergic cells). These cell types co uld account for less than 30% of the total number of cells that were Fos-po sitive and oestrogen treatment did not cause an increase in the Fos labelli ng of any of these types of cell. These data show that oestrogen activates cells of the arcuate/ventromedial hypothalamus within 1 h of injection and that this response could relate to the feedback effects of this gonadal hor mone. The majority of cells that produce Fos following oestrogen injection are of unknown phenotype. The data further suggest that induction of cells of the ventromedial hypothalamic nucleus require more prolonged oestrogen s timulus than cells of the arcuate nucelus.