GPR35 is expressed both in neurons and other cells (including glia, macrophages and monocytes). KYNA has been proposed as the endogenous agonist of GPR35 (Wang et al. 2006). Kynurenic acid (KYNA) is a tryptophan metabolite (Moroni 1999). It can selectively antagonize NMDA type glutamate receptors (Perkins & Stone 1982).