Morphine prevents the development of stress-enhanced fear learning

The current study investigates the pharmacotherapeutic use of morphine as a preventative treatment for stress-enhanced fear learning, an animal model that closely mimics symptoms of post-traumatic stress disorder (PTSD). PTSD is a chronic and debilitating anxiety disorder characterized by exaggerated fear and/or anxiety that may develop as a result of exposure to a traumatic event. In this model, rats are exposed to a severe stressor (15foot shocks) in one environment (Context A) and then subsequently exposed to a milder form of the same stressor (single foot shock) in a different environment (Context B). Animals that did not receive prior shock treatment exhibit fear responsiveness to Context B in line with the severity of the single shock given in this context. Animals that had received prior shock treatment in Context A exhibit an exaggerated learned fear response to Context B. Furthermore, animals receiving a single dose of morphine immediately following the severe stressor in Context A continue to show an enhanced fear response in Context B. However, animals receiving repeated morphine administration (three injections) after exposure to the severe stressor in Context A or a single dose of morphine at 48h after the severe stressor no longer exhibit an enhancement in fear learning to Context B. These results are consistent with clinical studies suggesting that morphine treatment following a severe stressor may be useful in preventing or reducing the severity of PTSD in at-risk populations. ► We utilize the stress-enhanced fear learning (SEFL) model of PTSD. ► Repeated morphine administration following trauma blocks SEFL ► Single morphine injection given immediately following trauma has no effect on SEFL. ► A single morphine injection given 48-hours after the trauma block SEFL ► Morphine may be an effective preventative therapy for populations at-risk for PTSD.