Corticosterone responses of adult Lewis and Fischer rats

Indwelling catheters were implanted into the inferior vena cava of adult male and female Lewis/N and Fischer 344 rats. Each animal was exposed to ACTH, novelty stimulation, nicotine, lipopolysaccharide (LPS), and saline on 5 consecutive days. Blood was withdrawn before (baseline) and at several time points after the stimulus on each day. There were no differences in baseline corticosterone levels nor in responses to saline in any group. In general, responses to stimulation peaked at 15-30 min and returned to baseline by 60-90 min. Corticosterone responses to LPS showed a different time course; maximal responses occur at 1-2 h and return to baseline by 24 h. Fischer animals showed higher corticosterone levels than Lewis rats during the response to stimulation, but returned to baseline at the same times. Females of each strain showed higher corticosterone responses than males at 15, 30, and 45 min after ACTH, but the sexes did not differ in response to the other stimuli. For individual rats, the maximum response to ACTH was slightly correlated with the maximum response to novelty stimulation, nicotine, and saline but was not correlated with the response to LPS.