In this study, we propose to employ a randomized, double-blind, placebo-controlled, outpatient clinical trial to test the efficacy, safety, and tolerability of a 160 mg and 40 mg challenge of the mGlu2/3 agonist pomaglumetad methionil relative to placebo in modulating fear-potentiated startle response and behavior in adults with post-traumatic stress disorder (PTSD) (N=30). Each participant will receive a single dose of the study drug (40 mg vs 160 mg vs placebo in a 1:1:1 ratio).