MAPK3 and major depressive disorder: Treatment with paeoniflorin at a dose of 30 or 60 mg/kg (i.p., 5 weeks, once daily) or 10 mg/kg (oral administration, 2 weeks) can also reverse CUS-induced depression-like behaviors in rats, by reducing serum corticosterone and adrenocorticotropic hormone levels and 5-HT2A receptor expression levels in the hippocampus and increasing brain noradrenaline and serotonin levels, as well as hippocampal expression levels of ERK1/2, CREB, and the 5-HT1A receptor (Qiu et al., 2013; Huang et al., 2015; Zhong et al., 2019) (Figure 6).