ESTROGEN UNCOUPLES NORADRENERGIC ACTIVATION OF FOS EXPRESSION IN THE FEMALE RAT PREOPTIC AREA

The preoptic area of the rat brain is a site at which gonadal steroids act to regulate sexual behaviour and gonadotrophin secretion. The exp ression of the immediate-early gene product, Fos, in the preoptic area was investigated in conscious ovariectomised, vehicle and estrogen-tr eated animals which had received an intracerebroventricular (i.c.v.) i nfusion of noradrenaline, and also in anaesthetised proestrous and ova riectomised rats following electrical stimulation of the brainstem Al or A2 noradrenergic cell groups. In ovariectomised oil-treated rats, a third ventricular infusion of noradrenaline (45 mu g) resulted in a s ignificant (P < 0.05) increase in the numbers of Fos-immunoreactive ce ll nuclei throughout the preoptic area, compared to vehicle controls. In contrast, Fos expression in animals which had received estrogen rep lacement showed no change in response to i.c.v. noradrenaline compared with saline-treated controls. In anaesthetised, ovariectomised animal s electrical stimulation of the Al cell group resulted in a significan t increase (P < 0.05) in Fos-like immunoreactivity compared with sham controls, specifically within the ventral preoptic area whilst stimula tion of the A2 cell group had no significant effect. In anaesthetised, proestrous rats receiving electrical stimulation no significant chang es in Fos-like immunoreactivity were detected within the preoptic area after either A1 or A2 stimulation compared with paired controls. Thes e results show that noradrenaline-induced Fos expression in the preopt ic area is dependent on estrogen status and suggest that the estrogeni c regulation of reproductive functions may thus involve altered respon ses to noradrenaline in sub-populations of preoptic neurones.