Early induction of c-fos precedes increased expression of corticotropin-releasing factor messenger ribonucleic acid in the paraventricular nucleus after immobilization stress.

CRF plays a role in coordinating endocrine, physiological, and behavioral responses to stressful stimuli. Several kinds of stressors have been reported to induce an increase in CRF mRNA expression in the paraventricular nucleus of the hypothalamus (PVN). Recently, the expression of c-fos mRNA has shown promise as a useful tool for metabolic mapping at the cellular level, because various types of stimulation induce c-fos mRNA expression in specific neuron populations in various brain regions. The aim of the present study is to clarify a possible anatomical-temporal correlation between the early induction of c-fos and the enhanced expression of CRF mRNA after stress. Wistar male rats were exposed to immobilization stress for 60 min and killed before and 15, 30, 60, 90, 120, and 180 min after the beginning of immobilization. In situ hybridization was performed by hybridizing sections with 35S-labeled prepro-CRF and c-fos cRNA probes. Relative levels of CRF and c-fos mRNA were compared by estimating the number of grains over the PVN in emulsion-dipped autoradiograms. Rapid induction (within 15 min) of c-fos mRNA was noted in the parvocellular division of the PVN after immobilization stress. The level of c-fos mRNA peaked at 30 min, then gradually declined to the control level within 90 min after the beginning of stress [the number of grains over the PVN: control, 326 +/- 180; 15 min, 2091 +/- 680 (P less than 0.05 vs. control); 30 min, 3385 +/- 239 (P less than 0.05 vs. control)]. The distribution of c-fos mRNA was almost identical to that of CRF mRNA in the PVN. On the other hand, the time course of CRF mRNA induction was delayed to the c-fos mRNA expression. A significant increase in CRF mRNA levels was noted only 120 and 180 min after stress [the number of grains over PVN: control, 3868 +/- 221; 120 min, 5957 +/- 677 (P less than 0.05 vs. control); 180 min, 6600 +/- 450 (P less than 0.05 vs. control)]. The results demonstrate that increased expression of CRF mRNA is preceded by c-fos mRNA induction in the PVN after stress suggesting a role of c-fos in the activation of CRF gene expression.