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.

Periadolescent stress exposure exerts long-term effects on adult stress responding and expression of prefrontal dopamine receptors in male and female rats.

Recent research has demonstrated that experiential/environmental factors in early life can program the adult stress response in rats, and this is manifest as altered hypothalamic-pituitary-adrenocortical activity and behavior in response to a stressor. Very little work has been devoted to investigating whether the environment during adolescence plays a similar role in modulating ongoing developmental processes and how this might affect adult stress responding. Periadolescent predator odor (PPO) exposure was used here as a naturalistic model of repeated psychological stress. Behavioral and endocrine responses to PPO changed across the exposure period, and behavioral alterations persisted into adulthood. While adolescent rats showed pronounced avoidance responses upon initial PPO exposure, hyperactivity increased across the exposure period, especially in females. Corticosterone (cort) responses to stressor exposure also changed in females, with higher physiological baseline levels observed at the end of the exposure period. In adulthood, relative to rats who had received a control manipulation during adolescence, PPO-exposed rats were more fearful in a novel open field and displayed altered responses to a predator odor stress test in adulthood. Moreover, lower levels of the D2 dopamine (DA) receptor were measured in prefrontal (infralimbic and dorsopeduncular) cortices of PPO-exposed rats. These findings suggest that the adolescent period may represent a sensitive period during which developmental programming of the stress response occurs.