Table 1.

Some neuropsychoimmunological implications of tryptophan metabolism

Tryptophan and the downstream metabolites kynurenine and 3-hydroxykynurenine can cross the blood-brain barrier [17]

Tryptophan can also be metabolized by brain-resident immune cells, microglia, astrocytes and neurons, although downstream metabolism differs between the cell types [13]

Downstream metabolites can negatively affect neurological functions, e.g. quinolinic acid [17], or act as neuroprotectants, such as kynurenic acid [18, 19]

Serotonin (5-hydroxytryptamine) is formed from 5-hydroxytryptophan [13], and a low concentration of this neurotransmitter may precipitate neuropsychiatric symptoms like depression or other mood disorders [20]