1-Methylhistamine is a major metabolite of histamine by histamine N-methyltransferase in the pathway of histidine metabolism.[1] It has been used as a biomarker of histaminergic system activity in the brains of Alzheimer’s disease patients as well as those with hypersomnia and other neurological conditions.[2] [3]

Reference:
[1]. Lindell, S.E., and Schayer, R.W. Metabolism of injected [14C] histamine in the kidney of the dog. Br.J.Pharmacol.Chemother. 13(1), 52-53 (1958).
[2]. Motawaj, M., Peoc'h, K., Callebert, J., et al. CSF levels of the histamine metabolite tele-methylhistamine are only slightly decreased in Alzheimer’s disease. J.Alzheimers Dis. 22(3), 861-871 (2010).
[3]. Dauvilliers, Y., Delallée, N., Jaussent, I., et al. Normal cerebrospinal fluid histamine and tele-methylhistamine levels in hypersomnia conditions. Sleep 35(10), 1359-1366 (2016).