Psychological stress-induced increases in noradrenaline release in rat brain regions are attenuated by diazepam, but not by morphine.

By measuring levels of noradrenaline (NA) and its major metabolite, 3-methoxy-4-hydroxyphenylethyleneglycol sulfate (MHPG-SO4) in the hypothalamus, amygdala and locus coeruleus region, we investigated the effects of diazepam 5.0 mg/kg, morphine 6.0 mg/kg, or naloxone at 5.0 or 10 mg/kg injected SC immediately before stress exposure, on increases in NA release caused by psychological stress. Psychological stress, wherein rats were exposed to emotional responses which were displayed by other electrically shocked rats, significantly increased MHPG-SO4 levels in the three brain regions examined and elevated plasma corticosterone levels. Both increases in brain MHPG-SO4 levels and elevations of plasma corticosterone levels induced by stress were attenuated significantly by diazepam but neither by morphine nor by naloxone. MHPG-SO4 levels in the hypothalamus and amygdala in the morphine-stress group were significantly higher than those in the saline-stress group. These findings suggest that psychological stress, in which an emotional factor is predominantly involved, causes increases in NA release in these brain regions examined and that these increases are attenuated only by diazepam, in contrast to the previous report, where increases in brain NA release caused by immobilization stress are attenuated not only by diazepam but also by morphine and are enhanced by naloxone.