Influence of age or circadian time on bcl-2 and bax mRNA expression in the rat hippocampus after corticosterone exposure

A rapid elevation in the level of endogenous corticosterone (CORT) functions in the stress response associated with the hypothalamus–pituitary–adrenal axis, and it has been well documented that high levels of CORT play neurotoxic roles in the hippocampus. Both aging and the circadian rhythm possibly affect the sensitivity to CORT, although their endogenous modifications in the CORT-mediated events remain unclear. To explore the influence of age or circadian time on hippocampal vulnerability to excess CORT, we examined the relative mRNA expression of bcl-2 and bax in the dentate gyrus (DG) and the CA1 subfield, compared with the CA3 as an internal standard, after acute CORT administration using in situ RT-PCR. Male rats aged 10 weeks (young) or 6 months (adult) were treated with CORT at 0800 or 2000 h. The bcl-2 to bax mRNA ratio in the dentate gyrus (DG) was significantly decreased 2 h after CORT exposure in the young rats treated at 0800 or 2000 h. In the adult rats, the treatment with CORT at 0800 h significantly decreased the bcl-2 to bax ratio, whereas the treatment at 2000 h was ineffective; the discrepancy between the treatment time points was apparent in adult rats, but not in young rats. Our results emphasize the importance of circadian time as well as age as a factor influencing the stress paradigm.