The circadian and circa-annual response of rat serum corticosterone to different stressors.

The serum corticosterone (B) response to shortlasting immobilization and the administration of ACTH was studied during the year, at the beginning of the light and dark part of the day, in SPF Wistar rats of both sexes kept under standard conditions with a 12:12 h light and dark regimen. Both sexes reacted to the stressors (mainly immobilization) by a marked increase in B at the beginning of the light period. The maximum reaction, to both immobilization and ACTH, was found in males in the spring; females reacted more evenly throughout the year, with a maximum in the winter. The reactivity of both sexes was the lowest in the summer. Circadian and circa-annual differences in the reactivity of the laboratory rat are genetically (evolutionally) fixed and must be taken into account when evaluation the results of experiments.