A metabolite of tryptophan that has pronounced effects on neuronal signaling; broadly antagonizes ionotopic glutamate receptors; has potent effects at glutamate receptor subunit ζ-1 (Ki = 5.4 µM), GPR35 (EC50 = 39 µM), aryl hydrocarbon receptor (EC50 = 300 nM), and neuronal acetylcholine receptor α-7 (IC50 = 7 µM).