Regulation of ERK in the hippocampus and medial prefrontal cortex of control, acute stress, CVS, CVS plus novel acute stress animals. A, Quantification and representative Western blots of phosphorylated ERK relative to total ERK in the separate hippocampus subregions of control, acutely stressed, chronic variable stressed (CVS) animals, and CVS plus novel acute stress (*, P < .05; **, P < .01; ***, P < .001; ****, P < .0001). B, Quantification and representative Western blots of phosphorylated ERK relative to total ERK in the mPFC. Chronic stress significantly increased ERK phosphorylation (*, P < .05, n = 3–4 per group), but acute stress had no effect. Data are shown as mean ± SEM.