Figure 1.

The metabolic pathway converting serotonin to melatonin involves the enzymes AANAT and ASMT (previously known as HIOMT). Although AANAT has been believed to control the rate-limiting step, recent studies [13] imply an effect of ASMT upon the metabolic conversion of serotonin to melatonin. The activity of both AANAT and ASMT might influence the concentration of N-acetylserotonin, which influences expression of TRKB (a BDNF receptor).