Serotonin-13C2,15N hydrochloride is the 13C- and 15N-labeled Serotonin hydrochloride (HY-B1473). Serotonin hydrochloride is a monoamine neurotransmitter in the CNS and an endogenous 5-HT receptor agonist. Serotonin hydrochloride is also a catechol O-methyltransferase (COMT) inhibitor with a Ki of 44 µM[1][2].