BDNF and major depressive disorder: The first module controls the downstream GRIA1, GRIN2A, GSK3A, CREB3, BDNF, FOS, ATF2, MAPK8, JUND and other genes to treat depression through the cascade reaction of ADCYAP1--ADCYAP1R1--GNAS--ADCY1--cAMP--PRKACA.