HTR5A and major depressive disorder: In addition, lesions in the LHb improves depression-like behaviors in rats by increasing the levels of serotonin (5-HT) in the dorsal raphe nucleus (DRN; Yang et al., 2008), since reduced levels of 5-HT in the DRN play an important role in the pathogenesis of depression (Stockmeier, 2003; Oquendo et al., 2007; Gao et al., 2008).