Factors influencing plasma catecholamine levels in rats during immobilization

Catheterized young and old male and female rats in estrus and diestrus were immobilized for 30-min periods. Six serial blood samples were drawn over one hour and the plasma was assayed for the catecholamines (CA) norepinephrine (NE) and epinephrine (E). Baseline CA levels were higher in awake young male rats than in asleep males, and still higher in old male rats. Highest basal NE levels were found in diestrus females. Immobilization produced similar elevations in plasma CA for all groups. Peak values occurred at 1--5 min and declined slowly thereafter toward baseline. Repeated stress of young male rats caused habituation as evidenced by diminished peak CA levels during the second and third restraint. Age, time of exposure and day of the cycle in female rats had subtle, though statistically significant, effects on CA levels at various times during and after immobilization and on the total stress response.