The stressful quality of an experience, as perceived by rats, is believed to be largely represented by the magnitude of a hypothalamic-pituitary-adrenal (HPA) axis response. The hippocampus may be especially important for assessing the stressfulness of psychological stressors such as novel experience. If such is the case then experience-dependent immediate-early gene expression levels within the hippocampus may parallel relative levels of HPA axis activity. We examined this prospect in rats that were placed in four different novel environments (empty housing tub, circular arena, elevated pedestal or restraint tube). Restraint and pedestal produced the largest magnitude of increased ACTH and corticosterone secretion, arena an intermediate level (Experiment 2) and tub the least magnitude of increase. We saw a very similar experience-dependent pattern of relative Fos protein, c-fos mRNA and zif268 mRNA expression in the paraventricular nucleus of the hypothalamus. However, in hippocampus (and select regions of cortex), immediate-early gene expression was associated with the exploratory potential of the novel experience rather than level of HPA axis activity; pedestal and arena elicited the greatest immediate-early gene expression, tub an intermediate level and restraint the least amount of expression. We conclude that the stressfulness of psychological stressors is not represented by the amount of immediate-early gene induction elicited in hippocampus and cortex, nor does there appear to be a general enhancing or depressive influence of acute stress on immediate-early gene induction in those brain regions.
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