Metabolite of tryptophan, q.v. Prepn: S. Hoogewerff, W. A. van Dorp, Ber. 12, 747 (1879); W. Koenigs, ibid. 983; O. Fischer, E. Renouf, ibid. 17, 755 (1884); E. Sucharda, ibid. 58B, 1727 (1925); V. Yu. Stiks, S. A. Bulgach, ibid. 65B, 11 (1932); A. F. Lindenstruth, C. A. VanderWerf, J. Am. Chem. Soc. 71, 3020 (1949). Metabolism studies: L. M. Henderson et al., J. Biol. Chem. 181, 667, 677, 687, 731 (1949); H. P. Sarett, ibid. 193, 627 (1951). Neuroexcitatory activity and possible role in neurodegenerative disorders: R. Schwarcz et al., Science 219, 316 (1983).