HDAC9 and depressive disorder: Sodium butyrate, an effective HDAC inhibitor, can improve cognitive impairment in depression patients, possibly by upregulating thyroxine receptor (Ttr), downregulating serotonin 2A receptor (Htr2a), and upregulating neurotrophic factors (Yamawaki et al., 2012; Valvassori et al., 2014).