Serotonin is a monoamine neurotransmitter in the CNS and an endogenous 5-HT receptor agonist. Serotonin also inhibits catechol O-methyltransferase (COMT), an enzyme that contributes to modultion the perception of pain, via non-competitive binding to the site bound by catechol substrates with a binding affinity comparable to the binding affinity of catechol itself (Ki= 44 μM). Results show that addition of 100 μM of Serotonin decreases the reaction velocity of COMT.