Skeletal muscle has the ability to take up a compound known as kynurenine and convert it into kynurenic acid ... This essentially reduces the amount of kynurenine available to then be transformed into other harmful metabolites, such as quinolinic acid, in the brain. So, quinolinic acid is a neurotoxin that plays a role in depression. It plays a role in schizophrenia and neurodegenerative disease.