[C-11] RTI-32 PET STUDIES OF THE DOPAMINE TRANSPORTER IN EARLY DOPA-NAIVE PARKINSONS-DISEASE - IMPLICATIONS FOR THE SYMPTOMATIC THRESHOLD

To estimate the threshold of nigrostriatal dysfunction required for sy mptomatic Parkinson's disease (PD), we employed [C-11]RTI-32 and PET t o study the dopamine transporter in striatal subdivisions of 11 L-dopa -naive patients with very early parkinsonism. As compared with the con trols (N = 10), the PD group had on the side contralateral to the maxi mal clinical symptoms, significantly reduced binding in the posterior putamen (-56%) and anterior putamen (-28%), with the reduction in caud ate (-12%) not significantly different. To the extent that dopamine tr ansporter binding accurately reflects the number of nigrostriatal dopa mine nerve terminals, these findings suggest that the clinical thresho ld for PD in the middle-age human is approximately 50% loss of dopamin ergic innervation to the posterior putamen. Our data also suggest that damage to the putamen component of the striatum is sufficient far the clinical expression of PD.