Behavioral and endocrine change following chronic predatory stress.

Adult male rats showed very high levels of crouching when exposed to a cat, with suppression of the nondefensive behaviors (e.g., lying, locomotion, rearing) that were shown by toy cat-exposed controls. The crouching of cat-exposed rats declined slightly but reliably with increasing time within daily 60-min exposure sessions. However, the lack of a reliable cat-exposure x days interaction for crouching over the 20 days of testing indicated minimal habituation of the rats' defensive response to the cat over this exposure schedule, although rat and cat were separated by a wire mesh screen, precluding contact and pain. Following the 20th day of exposure, cat-exposed rats showed reliably higher basal plasma corticosterone levels, suggesting a lack of habituation of this stress-linked response as well. Adrenal weights were also higher and thymus weights lower in these animals compared with controls, while spleen and testes weights and testosterone levels were not reliably different. Of the 13 cat-exposed subjects, 6 (and a single control) failed to show a 10 microg/mL corticosterone (CORT) increase in response to an acute restraint stressor. In 3 of these 6 cat-exposed rats, the failure to meet this criterion was attributable to a low level of CORT following restraint, suggesting failure of the normal CORT surge to the acute restraint stressor. These findings of organ weight changes, enhanced basal CORT, and reduced CORT response to stress in a subgroup of animals are similar to many of the phenomena obtained with other intense, chronic stressors such as subordination, and suggest that repeated predator exposure produces a pattern of intense behavioral and endocrine response that is very slow to habituate. Because it is a natural stressor for both male and female subjects, and one for which pain and even handling of the subject is unnecessary, cat exposure may provide a particularly relevant and adaptable paradigm for research involving analysis of gender effects on the stress response.

Chronic social stress alters levels of corticotropin-releasing factor and arginine vasopressin mRNA in rat brain.

In the visible burrow system model of chronic social stress, male rats housed in mixed-sex groups quickly form a dominance hierarchy in which the subordinates appear to be severely stressed. A subgroup of subordinates have an impaired corticosterone response after presentation of a novel restraint stressor, leading to their designation as nonresponsive subordinates. To examine the mechanism underlying the blunted corticosterone response in these animals, in situ hybridization histochemistry was used to quantify corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) mRNA expression in the brain. In two separate visible burrow system experiments, the nonresponsive subordinates expressed a significantly lower average number of CRF mRNA grains per cell in the paraventricular hypothalamic nucleus compared with stress-responsive subordinates, dominants (DOM), or cage-housed control (CON) rats. The number of CRF mRNA labeled cells was also significantly lower in nonresponders than in responsive subordinates or DOM. In the central amygdala, CRF mRNA levels were increased in both groups of subordinates compared with CON rats, whereas responsive subordinates exhibited higher levels than the DOM rats as well. AVP mRNA levels did not vary with behavioral rank in any subdivision of the paraventricular hypothalamic nucleus. In the medial amygdala, the number of cells expressing AVP mRNA was significantly greater in CON rats compared with both groups of subordinates, although the average number of AVP mRNA grains per cell did not vary with rank. In addition, the number of AVP-positive cells significantly correlated with plasma testosterone level.