Characterization of a novel gene-environment-based animal model to study resilience and susceptibility to PTSD and co-morbid depression

BACKGROUND Post-traumatic stress disorder (PTSD) and co-morbid depression are frequently associated with severe symptoms, poor response to treatment and worse prognosis. Due to the absence of a suitable animal model, little is known about the biological basis of the comorbidity, severely limiting the discovery of new and more effective treatment options. The Flinders Sensitive Line rats (FSL) is a well-validated, selectively bred animal model of depression. However, several of its features, such as cognitive deficits and altered hypothalamic-pituitary-adrenal (HPA) axis response, also match symptomatic clusters of PTSD. In parallel, its resistant counterpart, the Flinders Resistant Line (FRL), is extensively used as a simple control. Still, little is known about its performance compared to the original strain, Sprague Dawley (SD), from which the FSL/FRL was originally derived. AIMS Characterizing the behavioural performance and mechanisms involved in FSL, FRL and SD rats in fear-memory paradigms. METHODS FSL, SD and FRL animals were submitted to tests assessing hippocampal-dependent and fear-related memory. Subsequently, plasticity factors and endocrine responses to stress were analysed to elucidate the molecular basis for the observed behavioural alterations. RESULTS We found that FRL animals presented intact recognition memory and innate fear responses but could not properly display conditioned responses in the Conditioned Fear Conditioning (CFC) paradigm. FSL animals, despite a poor performance in the Novel Object Recognition task (NOR), showed similar levels of conditioned responses compared to SD, but impairments in extinction learning, a feature highly related to PTSD. The behavioural alterations were accompanied by alterations in plasma corticosterone levels and hippocampal expression of the glucocorticoid receptor and FKBP51. CONCLUSION For the first time, we demonstrate an animal model of resilience and vulnerability to PTSD and co-morbid depression. The results suggest that the endophenotypes may be based on aberrant endocrine stress responses in the hippocampus. ### Competing Interest Statement The authors have declared no competing interest.