The next decade in tryptophan metabolism research

Over the last three decades, tryptophan research has progressively moved from an “obscure” to a primary fi eld of research. Interest in tryptophan metabolism and thus the number of publications has been growing concomitantly and almost exponentially. Looking at Pubmed today (November 2014), the overall number of publications about tryptophan, indoleamine 2,3 dioxygenase (IDO), and the kynurenine pathway has been rising signifi cantly (Fig. 31.1). One of the fi rst “boost” for tryptophan research was in the eighties with the identifi cation of neuroactive activities of kynurenine pathway metabolites such as quinolinic acid [1, 2] and kynurenic acid [3] and especially their actions as agonist and antagonist of the N-methyl- D -aspartate receptor. Then, the fi eld was subject to another major “hit” when IDO1 was identifi ed as a key regulator of the immune response by the work of Munn and Mellor [4]. Of course, many more studies have demonstrated the pleiotropic roles played by tryptophan and its metabolites in both physiological and pathological conditions.