It has been suggested that altered KP metabolism contributes significantly to the pathophysiology of neurodegenerative and inflammatory disorders of the CNS.34 Lower levels of KYNA and the enzymes involved in its production have been found in the plasma, CSF and erythrocytes of Parkinson’s and Alzheimer’s disease sufferers.9,47,48 On the other hand, Huntington’s disease subjects exhibited elevated cortical QUIN levels in the early stages of disease onset, suggesting excitotoxicity as the cause of later neurodegeneration.49 Here, NPPA is linked to Alzheimer disease.