MAPK1 and major depressive disorder: In studies on intracellular signal transduction in depression, the extracellular signal-regulated kinase–cyclic adenosine monophosphate response element binding protein (ERK-CREB) pathway plays a crucial role in the pathological change and treatment of depression, thereby regulating the growth, proliferation, and differentiation of hippocampal nerve cells, meanwhile this pathway has an important learning and memory effect on the synaptic plasticity in the brain (23, 24).