Investigation of sex differences in behavioural, endocrine, and neural measures following repeated psychological stressor exposure

Animal models of repeated stressor exposure have generally been limited to physical stressors, despite the fact that the purpose of such models is to represent repeated stress in humans, which is usually psychological in nature. The present study was undertaken to investigate the behavioural, endocrine, and neural responses to a repeated psychological stressor exposure in male and female rats. Long-Evans rats were exposed to cat odour or a control condition for 1 h each day from Day 1 to Day 22. Every fourth day, defensive (e.g. hiding), and non-defensive (e.g. grooming) behaviour was quantified, during both the initial and the final 10 min of the hour. Defensive behaviours in cat odour-exposed animals remained vigorous during the initial 10 min of exposure across 22 exposure days. Non-defensive behaviours were suppressed during early exposures, but this suppression habituated across repeated exposures. Overall, the pattern of behavioural results indicated enhanced responses to novelty and to repeated cat odour exposure, in females, relative to males. Plasma corticosterone (CORT) levels were higher in females relative to males overall. However, males, but not females, exposed to cat odour had higher levels of CORT following exposure on Days 1 and 22, relative to controls. Finally, mRNA levels of glucocorticoid receptor, mineralocorticoid receptor, and brain-derived neurotrophic factor, all of which are modulated by CORT. were examined in hippocampus at the completion of stressor exposure, but none was affected by repeated stressor exposure. Results are discussed within the context of potential differences in effects of repeated psychological versus physical stressors. (C) 2007 Elsevier B.V. All rights reserved.