Expression of aggression attenuates stress-induced increases in rat brain noradrenaline turnover

This experiment determined whether or not an aggressive biting response could attenuate stress-induced increases in brain noradrenaline (NA) turnover, by measuring contents of NA and its major metabolite, 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MHPG-SO4), in discrete brain regions of male Wistar rats. Rats were exposed to a 10 min supine restraint stress with or without being allowed to bite a wooden stick. In each group, the animals were sacrificed by decapitation either 0 min or 50 min after release from stress. NA and MHPG-SO4 levels were unaffected in both biting and non-biting groups immediately after stress, as compared to controls. Fifty min after release from stress, increases in plasma corticosterone levels induced by stress recovered in the biting group but remained high in the non-biting group. MHPG-SO4 levels significantly increased in the hypothalamus, amygdala, thalamus, midbrain, basal ganglia, hippocampus and cerebral cortex in both stressed groups, however the increases in the non-biting group were significantly higher than those in the biting group in the first 5 of these regions. These findings clearly show that giving rats an opportunity to express aggression during stress exposure results in a significant attenuation of stress-induced increases in NA turnover in specific brain regions, such as the hypothalamus and limbic areas. The present experiment provides a possible neurochemical basis for clinical studies showing that suppression of anger in a stressful, frustrating situation leads to pathological consequences in humans.