A novel adolescent chronic social defeat model: reverse-Resident-Intruder Paradigm (rRIP) in male rats

Psychosocial stress is linked to the etiology of several neuropsychiatric disorders, including Major Depressive Disorder and Post-Traumatic-Stress-Disorder. Adolescence is a critical neurobehavioral developmental period wherein the maturing nervous system is sensitive to stress-related psychosocial events. The effects of social defeat stress, an animal model of psychosocial stress, on adolescent neurobehavioral phenomena are not well explored. Using the standard Resident-Intruder-Paradigm (RIP), adolescent Long-Evans (LE, residents, n = 100) and Sprague–Dawley (SD, intruders, n = 100) rats interacted for five days to invoke chronic social stress. Tests of depressive behavior (forced-swim-test (FST)), fear conditioning, and long-term synaptic plasticity are affected in various adult rodent chronic stress models, thus we hypothesized that these phenomena would be similarly affected in adolescent rats. Serendipitously, we observed the Intruders became the dominant rats and the Residents were the defeated/submissive rats. This robust and reliable role-reversal resulted in defeated LE-Residents showing a depressive-like state (increased time spent immobile in the FST), enhanced fear conditioning in both hippocampal-dependent and hippocampal-independent fear paradigms and altered hippocampal long-term synaptic plasticity, measured electrophysiologically in vitro in hippocampal slices. Importantly, SD-Intruders, SD and LE controls did not significantly differ from each other in any of these assessments. This reverse-Resident-Intruder-Paradigm (rRIP) represents a novel animal model to study the effects of stress on adolescent neurobehavioral phenomenon.