The effect of L-dopa infusions with and without phenylalanine challenges in parkinsonian patients: Plasma and ventricular CSF L-dopa levels and clinical responses

We monitored the motor response and plasma and ventricular CSF (CSF_v) concentrations of L-dopa during IV infusions of L-dopa in two patients with advanced Parkinson's disease. Concentrations of L-dopa in CSF_V mirrored, but lagged behind, those in plasma. In the fasting state, the duration, but not the magnitude, of the motor response was greater with increasing plasma and CSFV levels of L-dopa. During IV infusions of L-dopa following oral administration of phenylalanine, a large neutral amino acid that shares a transport system into the brain with L-dopa, the duration of the motor response was markedly attenuated despite undiminished CSF_v levels of L-dopa. These observations suggest that either L-dopa entry into CSF_v and the brain are differentially affected by phenylalanine or that phenylalanine affects other steps in the motor response. These observations demonstrate that, except in the fasting state, L-dopa in CSF_V is not a reliable predictor of motor response.