Correlations between plasma levels of amino acids and nonmotor symptoms in Parkinson's disease

Converging evidence suggests that changes in plasma levels of amino acids are involved in Parkinson's disease (PD), but their roles in nonmotor symptoms (NMS) of PD remain unclear. The aim of this study was to evaluate the correlations between plasma amino acids and NMS of PD. Plasma levels of aspartate (Asp), glutamate (Glu), glycine (Gly) and c-aminobutyric acid (GABA) were measured in 92 PD patients and 60 healthy controls. Four NMS, including depression, pain, sleep disturbances and autonomic dysfunction were assessed in enrolled subjects using the Hamilton Depression Scale, the short form of the McGill Pain Questionnaire, the Pittsburgh Sleep Quality Index and the Scale for Outcomes in Parkinson's disease for Autonomic Symptoms, respectively. Hierarchical multiple regression analysis was used to evaluate the correlations between plasma levels of amino acids and NMS. PD patients exhibited significantly higher scores of NMS scales and lower plasma levels of amino acids compared to healthy controls. Within the PD group, plasma levels of Asp and Glu were negatively associated with the severity of depression and sleep disturbances. Moreover, decreased plasma level of GABA was correlated with more severe symptoms of sleep disturbances. After controlling for gender, disease duration, severity of motor symptoms and anti-parkinsonian medications, Glu but not Asp remained significantly associated with depression, along with Asp, GABA but not Glu remained negatively associated with sleep disturbances. The altered plasma levels of amino acids may be implicated in the pathogenesis of NMS of PD.