The application notes include recommended starting dilutions; optimal dilutions/concentrations should be determined by the end user.
1/1000 - 1/5000.
5-methoxytryptamine (5-MT) is a derivative of serotonin (or 5HT) and also a metabolite and precursor of melatonin. This potent serotonergic neuromodulator is involved in numerous physiological systems, e.g an article published in Pharmacol Biochem Behav. 2004 Feb;77(2):275-9 suggests a role melatoinin metabolites in nociception.