Habituation of plasma catecholamine responses to chronic intermittent restraint stress

Adult male Sprague-Dawley rats were exposed to restraint stress (RS) for 30 min per day for 0, 5, 12, or 26 consecutive days. Each animal was then surgically prepared with an indwelling tail-artery catheter to permit direct measurements of mean arterial pressure (MAP, in mm Hg) and heart rate (HR, in beats per minute) on the day after surgery, and remote collection of blood samples during RS 2 days after surgery. Blood samples were later analyzed for plasma levels of norepinephrine and epinephrine to provide an assessment of the activity of the sympathetic-adrenal medullary system. Body-weight gain was reduced significantly in rats stressed for 12 or 26 days relative to handled controls. Baseline values of HR and of plasma levels of norepinephrine and epinephrine were similar in animals of the four groups. Baseline MAPs were lower in control rats and in those stressed for 26 days than in rats stressed for the previous 5 or 12 days. Plasma catecholamine responses to RS were reduced significantly in animals during the 6th, 13th, or 27th stress session relative to subjects restrained for the first time. These findings indicate that sympathetic-adrenal medullary responses are dampened rapidly when laboratory rats are exposed to chronic intermittent RS. In addition, the results of this experiment are consistent with our suggestion that habituation of sympathetic-adrenal medullary responsiveness occurs in animals exposed to chronic intermittent stress.