Responsiveness of the hypothalamic-pituitary-adrenal axis to different novel environments is a consistent individual trait in adult male outbred rats

Susceptibility to some stress-induced pathologies may be strongly related to individual differences in the responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to stressors. However, there have been few attempts in rodents to study the reliability of the individual differences in the responsiveness of the HPA to stressors and the relationship to resting corticosterone levels. In the present work, we used a normal population of Sprague-Dawley rats, with a within-subject design. Our objectives were to study: (a) the reliability of the ACTH and corticosterone response to three different novel environments widely used in psychopharmacology and (b) the relationship between stress levels of HPA hormones and the daily pattern of corticosterone secretion (six samples over a 24-h-period). Animals were repeatedly sampled using tail-nick procedure. The novel environments were the elevated plus-maze, the hole-board and the circular corridor. Animals were sampled just after 15 min exposure to the tests and again at 15 and 30 min after the termination of exposure to them (post-tests). The hormonal levels just after the tests indicate that the hole-board seems to be more stressful than the circular corridor and the elevated plus-maze, the latter being characterized by the lowest defecation rate. Correlational analysis revealed that daily pattern of resting plasma corticosterone levels did not correlate to HPA responsiveness to the tests, suggesting no relationship between resting and stress levels of HPA hormones. In contrast, the present study demonstrates, for the first time, a good within-subject reliability of the ACTH and corticosterone responses to the three environments, suggesting that HPA responsiveness to these kind of stressors is a consistent individual trait in adult rats, despite differences in the physical characteristics of the novel environments.