The effects of continuous stress and intermittent stress at short intervals on rat hypothalamic noradrenaline (NA) release were assessed by measuring the levels of a principal metabolite of NA, 3-methoxy-4-hydroxy-phenylethyleneglycol sulfate (MHPG-SO4) in male Wistar rats. The rats were exposed to one of five restraint stress conditions, unstressed (control), six 15 min intermittent stress periods (interspersed with 18 min non-stress periods), three 30 min intermittent stress periods (interspersed with 45 min non-stress periods), 90 min continuous stress period or 180 min continuous stress period. The 15 min intermittently stressed rats had significantly larger increases in hypothalamic MHPG-SO4 than the single 90 min and 180 min continuously stressed rats, while the 30 min intermittently stressed rats were significantly different from only the 180 min continuously stressed rats. In a comparison of the 15 min and 30 min intermittently stressed rats, which had the same total duration of stress exposure; the 15 min group had larger increases in MHPG-SO4 levels than the 30 min group. This study provides supporting evidence for the role of stress cyclicity in determining the extent of stress-induced NA release from the hypothalamus.
