Plasma Corticosterone Concentrations Sensitively Reflect Levels of Stimulus Intensity in the Rat

In Experiment 1 plasma concentrations of both corticosterone and adrenocorticotrophic hormone (ACTH) in rats were found to be greater than resting levels following 10 min, but not 2.5 min of exposure to an unfamiliar environment. In Experiment 2, rats exposed to 3 increasingly unfamiliar environments for 15 min showed 3 corresponding elevations in mean plasma corticosterone level. Rats receiving handling similar to that of the experimental animals showed plasma concentrations of corticosterone which were greater than those seen in undisturbed controls, but less than those observed in rats exposed to the least unfamiliar environment. Plasma concentrations of ACTH, however, did not accurately reflect treatment differences, indicating that either the 15 min time point was not optimal for detecting differences in circulating ACTH or, that unlike corticosterone, ACTH levels did not vary systematically in response to variations in the degree of environmental change experienced by the animals. Overall these results extend previous findings and provide additional support for the use of fluctuations in corticoid levels to measure changes in emotional state