Fos expression is selectively and differentially regulated by endogenous glucocorticoids in the paraventricular nucleus of the hypothalamus and the dentate gyrus.

We examined the extent to which basal levels of corticosterone, which vary in a circadian fashion, influence the pattern of Fos protein expression in the paraventricular nucleus of the hypothalamus (PVN), the hippocampal formation and three different functional cortical areas. Basal and poststress (1 h of restraint) Fos expression, as determined by immunohistochemistry, was examined in male rats with either no previous surgical manipulation or in rats 5 days after: (i) sham adrenalectomy; (ii) adrenalectomy with no corticosterone replacement; or (iii) adrenalectomy with corticosterone (25 microg/ml) in the drinking water replacement. In adrenal-intact rats, restraint produced similar patterns of Fos expression in the PVN, cortical areas and hippocampus (CA1-CA3), with peak levels of expression attained 60-90 min after restraint onset. Surprisingly, in the dentate gyrus, there was a dissociation between the two blades in the pattern of Fos expression after restraint. In the inner blade (suprapyramidal), there was a delayed induction that occurred between 60 and 90 min after restraint onset and, in the outer blade (infrapyramidal), there was a steady decline in Fos expression after restraint. Adrenalectomy had an effect on Fos expression only in the PVN and dentate gyrus, and the nature of the effect was quite different for both brain regions. In the PVN, adrenalectomy had no effect on Fos expression in unstressed rats, but resulted in an enhanced number of Fos positive cells after restraint. In the dentate gyrus, adrenalectomy resulted in an overall reduction of Fos positive cells in both blades, and this reduction was present in unstressed and stressed rats. Corticosterone replacement normalized the adrenalectomy effect on Fos expression in both brain regions. Thus, Fos expression in the rat brain displays specific patterns of dependency on the permissive effects of glucorticoids, and this dependency varies between brain regions.