Social defeat-induced contextual conditioning differentially imprints behavioral and adrenal reactivity: A time-course study in the rat

The present experiments were based on the rat resident-intruder paradigm and aimed at better understanding the long-term conditioning properties of this social stress model. Intruders were exposed to aggressive conspecifics residents. During 3 daily encounters, intruders were either defeated or threatened by residents, providing the defeated-threatened (DT) and threatened-threatened (TT) groups respectively, or exposed to a novel empty cage (EC). The effect of such exposures was assessed in 3 separate experiments 8, 14, or 21 days following the last session on both behavior and hypothalamus-pituitary-adrenal (HPA) axis parameters. A specific and persistent behavioral conditioning due to social defeat but also to the sole social threat experience was observed as defensive behaviors and anxiety-like behaviors were observed respectively in DT and TT rats, highlighting a lack of habituation for the conditioning properties of this social stressor. On the other hand, at the earlier time points examined a less specific activation of the HPA axis parameters was found, starting to show habituation at day 21 in EC but not in DT or TT rats. These data give further support to the lasting effects of this social stress model, bestowing a special emphasis upon the impact of its psychological component and upon the relevance of its development and maintenance over time. (C) 2007 Elsevier Inc. All rights reserved.