BDNF and post-traumatic stress disorder: In a study using footshock paired with white noise in C57BL/6 mice (a model for PTSD and anxiety), animals that displayed fear extinction where the induced fear behavior such as freezing is lost by extinction training had increased levels of histone H4 acetylation around the promoter of Bdnf exon IV, compared with both naive and fear-conditioned animals without extinction controls.