Witnessed trauma exposure induces fear in mice through a reduction in endogenous neurosteroid synthesis

Abstract

Neurosteroids have been implicated in the pathophysiology of post-traumatic stress disorder (PTSD). Allopregnanolone is reduced in subsets of individuals with PTSD and has been explored as a novel treatment strategy. Both direct trauma exposure and witnessed trauma are risk factors for PTSD; however, the role of neurosteroids in the behavioral outcomes of these unique experiences has not been explored. Here we investigate whether observational fear is associated with a reduced capacity for endogenous neurosteroidogenesis and the relationship with behavioral outcomes. We demonstrate that both mice directly subjected to the threat (foot shocks) and those witnessing the threat have decreased plasma levels of allopregnanolone. The expression of a key enzyme involved in endogenous neurosteroid synthesis, 5-reductase type 2, is decreased in the basolateral amygdala (BLA), which is a major emotional processing hub implicated in PTSD. We demonstrate that knockdown of 5-reductase type 2 exaggerates the behavioral expression of fear in response to witnessed trauma, whereas treatment with an exogenous, synthetic neuroactive steroid GABAA receptor PAM with molecular pharmacology similar to allopregnanolone (SGE-516 [tool compound]) decreases the behavioral response to observational fear. These data implicate impaired endogenous neurosteroidogenesis in the pathophysiology of threat exposure, both direct and witnessed. Further, these data suggest that treatment with exogenous 5-reduced neurosteroids or targeting endogenous neurosteroidogenesis may be beneficial for the treatment of individuals with PTSD, whether resulting from direct or witnessed trauma.