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, loc omotion, rearing) that were shown by toy cat-exposed controls. The cro uching of cat-exposed rats declined slightly but reliably with increas ing time within daily 60-min exposure sessions. However, the lack of a reliable cat-exposure x days interaction for crouching over the 20 da ys of resting indicated minimal habituation of the rats' defensive res ponse to the cat over this exposure schedule, although rat and cat wer e separated by a wire mesh screen, precluding contact and pain. Follow ing the 20th day of exposure, cat-exposed rats showed reliably higher basal plasma corticosterone levels, suggesting a lack of habituation o f this stress-linked response as well. Adrenal weights were also highe r and thymus weights lower in these animals compared with controls, wh ile spleen and testes weights and testosterone levels were not reliabl y different. Of the 13 cat-exposed subjects, 6 (and a single control) failed to show a 10 mu g/mL corticosterone (CORT) increase in response to an acute restraint stressor. Ln 3 of these 6 cat-exposed rats, the failure to meet this criterion was attributable to a low level of COR T 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 i ntense, chronic stressors such as subordination, and suggest that repe ated predator exposure produces a pattern of intense behavioral and en docrine response that is very slow to habituate. Because it is a natur al stressor for both male and female subjects, and one for which pain and even handling of the subject is unnecessary, cat exposure may prov ide a particularly relevant and adaptable paradigm for research involv ing analysis of gender effects on the stress response. (C) 1998 Elsevi er Science Inc.