Moreover, Kindler and colleagues reported increased TDO expression, KYN/Trp ratio (KP activation), as well as mRNA for kynurenine aminotransferases (KATI/II) and KYNA levels only in patients with higher cytokines levels, further endorsing the essential role of both immune and KP activation in schizophrenia [8]. Here, NPPA is linked to schizophrenia.