HNMT and major depressive disorder: A postmortem study of anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (DLPFC) tissues from individuals with depressive disorder bipolar disorder and control individuals evaluated the expression of histamine receptors and the enzymatic breakdown of histamine and histamine N-methyltransferase (HMT), indicating no change in the expression of these molecules except for a significant decrease in HMT mRNA expression in the ACC of individuals with MDD (45).