Stressor categorization: acute physical and psychological stressors elicitdistinctive recruitment patterns in the amygdala and in medullary noradrenergic cell groups

It has been hypothesized that the brain categorizes stressors and utilizes neural response pathways that vary in accordance with the assigned category . If this is true, stressors should elicit patterns of neuronal activation within the brain that are category-specific. Data from previous Immediate-e arly gene expression mapping studies have hinted that this is the case, but interstudy differences in methodology render conclusions tenuous. In the p resent study, immunolabelling for the expression of c-fos was used as a mar ker of neuronal activity elicited in the rat brain by haemorrhage, immune c hallenge, noise, restraint and forced swim. All stressors elicited c-fos ex pression in 25-30% of hypothalamic paraventricular nucleus corticotrophin-r eleasing-factor cells, suggesting that these stimuli were of comparable str ength, at least with regard to their ability to activate the hypothalamic-p ituitary-ad renal axis. In the amygdala, haemorrhage and immune challenge b oth elicited c-fos expression in a large number of neurons in the central n ucleus of the amygdala, whereas noise, restraint and forced swim primarily elicited recruitment of cells within the medial nucleus of the amygdala. In the medulla, all stressors recruited similar numbers of noradrenergic (A1 and A2) and adrenergic (C1 and C2) cells. However, haemorrhage and immune c hallenge elicited c-fos expression In subpopulations of A1 and A2 noradrene rgic cells that were significantly more rostral than those recruited by noi se, restraint or forced swim. The present data support the suggestion that the brain recognizes at least two major categories of stressor, which we ha ve referred to as 'physical' and 'psychological'. Moreover, the present dat a suggest that the neural activation footprint that is left in the brain by stressors can be used to determine the category to which they have been as signed by the brain.