Age-related changes in plasma catecholamine responses to chronic intermittent stress.

We examined habituation and sensitization of plasma catecholamine responses to stressful stimulation in young adult (3 months) and aged (22 months) Fischer 344 (F-344) male rats. Aged rats had greater elevations in plasma levels of norepinephrine (NE) and epinephrine (EPI) following exposure to restraint stress compared to young adult controls. Within ages, plasma catecholamine responses were similar in rats stressed for the first time compared to those stressed for the 27th time. When chronically stressed young adult and aged F-344 rats were exposed to a novel stressor, swim stress at 25 degrees C, plasma catecholamine responses were significantly greater than for age-matched handled controls. The magnitude of sensitization of plasma catecholamine responses to the novel stressor was similar for young adult and aged F-344 rats. These results indicate that aged rats have enhanced plasma catecholamine responses to acute restraint stress compared to young adults. In addition, rats of both ages displayed comparable levels of sensitization of plasma catecholamine responses to a novel stressor. These findings emphasize that aged rats differ from young adult rats in some but not all aspects of sympathetic-adrenal medullary regulation. Further, these age-related differences in sympathetic-adrenal medullary responses are unmasked when animals are exposed to stressful stimulation.