Defense changes in stress nonresponsive subordinate males in a visible burrow system.

Dominant and subordinate male rats housed in a visible burrow system (VBS), and male controls each housed with a female, were run in a series of tests evaluating their response to threatening stimuli. Subordinates were later assessed as stress responsive (SRS) or nonresponsive (NRS) on the basis of plasma corticosterone response to restraint stress. In the tests using mildly threatening stimuli (handling, open field), NRS had longer latencies to show a righting response and reduced activity compared to SRS or (handling, open field) to dominants. There were no differences among the VBS groups in tests with more intense threat stimuli (cat odor, cat presentation). These results suggest that stressful social experience may produce a shift toward more passive and immobile forms of defense and that the experience of subordination may interact with individual differences characteristics of rats to exacerbate this shift. The hypothalamic-pituitary-adrenal (HPA) axis changes of NRS, manifested as reduced plasma corticosterone response to restraint, and previous findings of sharply reduced testosterone in these animals, along with their passive defensive behaviors, form a pattern that is suggestive of a biobehavioral "reactive" (as opposed to "proactive") coping style, and suggest that these behavioral and endocrine variables may show related changes even when altered by individual experience.