The effects of two chronic intermittent stressors on brain monoamines

The effects of chronic exposure (27 days) to two different stressors on brain monoaminergic activity was studied in adult male rats. The stressors used were restraint in tubes (RES) and immobilization in wooden boards (IMO). Both chronically stressed and stress naive (control) rats were subjected to 0, 15, and 60 min of the same stresser to which they were chronically exposed. Previous chronic exposure to either RES or IMO significantly reduced ACTH response to the same stressor. Monoaminergic response to these stressors was studied by measuring the levels of noradrenaline (NA), serotonin (5-HT) and their metabolites: 3-methoxy,4-hydroxyphenyletileneglycol sulfate (MHPG-SO₄) and 5-hydroxyindoleacetic acid (5-HIAA), respectively. The regions studied were: pens plus medulla, midbrain, hypothalamus, hippocampus, and frontal cortex. Previous chronic exposure to the stressors induced only few changes in the resting levels of the monoamines and their metabolites. In addition, monoaminergic response to the same stresser to which they were chronically exposed was always similar in control and chronically stressed rats. These data indicate that brain NA and 5-HT metabolism is less sensitive than ACTH to the process of habituation to a repeated stresser, at least in the gross areas of the brain analyzed in the present study.