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 patien ts 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 greate r with increasing plasma and CSF(v) levels Of L-dopa. During IV infusi ons 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 des pite 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 th e fasting state, L-dopa in CSF(v) is not a reliable predictor of motor response.