MAPK3 and major depressive disorder: ERK1/2, which is downstream of several receptors and a regulator of the transcription of many targets, has been implicated in the depression-like symptoms elicited by stress-related insults, and antidepressants have been shown to restore stress-related reductions in hippocampal phosphorylated ERK1/2 (p-ERK1/2; the activated form of ERK1/2) [14].