Spring B, Hitsman B, Pingitore R, McChargue DE, Gunnarsdottir D, Corsica J, Pergadia M, Doran N, Crayton JW, Baruah S, Hedeker D. Effect of tryptophan depletion on smokers and nonsmokers with and without history of major depression. Biol Psychiatry. 2007 Jan 01; 61(1):70-7.