Differential regulation of corticotropin-releasing factor mRNA in rat brain regions by glucocorticoids and stress

Abstract

The regulation of corticotropin-releasing factor (CRF) mRNA expression in the rat brain by glucocorticoids and stress was examined by Northern blot analysis and in situ hybridization histochemistry. Rats either were exposed to a single electrical footshock session and killed 2, 4, 12, or 24 hr later (acute stress), or were subjected to the same regimen twice daily for 3 or 7 d and killed on the day following the last session (chronic stress). Rats placed in the experimental chamber but not administered shock comprised a "shamhandling" group. Chronic (7 d) intermittent footshock stress resulted in an 84 ± 26% (P < 0.05) increase in CRF mRNA levels in the whole hypothalamus as detected by Northern blot analysis and a 97 ± 29% (P < 0.05) increase in the paraventricular nucleus (PVN) as detected using In situ hybridization. No significant change in CRF mRNA levels was observed in the hypothalamus at any time up to 24 hr after a single exposure to footshock stress. A different pattern of results was obtained in other CRF-expressing cell groups. In Barrington's nucleus (a pontine micturition center), both acute and chronic stress produced significant increases in CRF mRNA, while in the olfactory bulb, both paradigms resulted in decreased levels. By Northem blot analysis, CRF mRNA in the olfactory bulb declined steadily, beginning at 4 hr after acute stress, and reached significance at 24 hr (69.2 ± 1.9% of control, P < 0.05). Levels from chronically (7 d) stressed animals declined to 54.1 ± 5.1 % of control value (P < 0.05). Analysis of hybridization histochemical material revealed that both the number of positively hybridized cells and the number of silver grains per cell in the mitral and external plexiform layers of the bulb decreased following acute and chronic stress. However, CRF mRNA levels in the olfactory bulb were decreased to a comparable extent in the sham-handling group, suggesting that exposure to a novel environment can effect a decrease in CRF mRNA levels in the olfactory bulb. To provide comparisons with the effects of manipulation of glucocorticoid status, comparable analyses were carried out in separate groups of animals following adrenalectomy (ADX) with and without corticosteroid replacement. After ADX, CRF mRNA levels in the whole hypothalamus increased 60 ± 5% (P < 0.05) and were normalized following dexamethasone replacement. In contrast to the hypothalamus, no effects of steroid manipulation on CRF mRNA levels in the olfactory bulb, midbrain, cerebral cortex, or brain stem were detected. In situ hybridization revealed that ADX induced a significant increase (285 ± 49% of control, P < 0.05) in CRF mRNA levels in the PVN, while having no effect on levels in Barrington's nucleus. These results provide evidence for pronounced regional differences in the regulation of CRF mRNA by glucocorticoids and stress. In line with recent reports, CRF mRNA in the PVN is negatively regulated by glucocorticoids and positively regulated by prolonged exposure to footshock stress, though in this paradigm, at least, a single episode of acute stress was not sufficient to up-regulate CRF mRNA levels in the PVN. The expression of CRF mRNA in Barrington's nucleus is also positively regulated by stress, suggesting a possible role for CRF in stress-induced micturition. In the olfactory bulb, stress and/or factors related to novel environments down-regulate CRF mRNA. Glucocorticoids appear not to be a major factor influencing CRF mRNA expression in Barrington's nucleus or the olfactory bulb.