Purpose: About 6.4% of the U.S. population suffers from posttraumatic stress disorder (PTSD). Trauma-focused psychotherapies are generally effective in PTSD, but responses vary greatly across individuals and PTSD subpopulations. Neurobiological factors impacted by life experiences, stress, and genetics can affect treatment responses. These factors can alter brain capacities needed to reprocess traumatic memories prevent them from triggering intensely distressing, disruptive, out-of-place responses. For example, during psychotherapy for PTSD, trauma memory activation engages two competing brain processes that affect recovery: extinction versus reconsolidation of trauma-related emotional, physiological, and behavioral responses. This study tests whether a single intravenous (IV) dose of allopregnanolone (Allo) compared to placebo (which is non-active): 1. promotes consolidation of extinction learning (sub-study 1) or 2. blocks reconsolidation physiological responses triggered by aversive memories (sub-study 2). The study also tests whether Allo compared to placebo affects retention of non-aversive memories.