The protective role of levodopa in the human substantia nigra.

Laboratory studies and studies of animal models of parkinsonism have produced contradictory evidence, but there is no evidence that LD is toxic to normal substantia nigra in animals. We have studied human subjects longitudinally to address that issue. A cumulative LD dose up to 24 kg was not toxic in one autopsied essential tremor (ET) case. Two other ET patients did not develop parkinsonism on 8.5 kg and 21 kg cumulative LD dose, respectively. One DOPA-responsive dystonia (DRD) case has no evidence of parkinsonism after 29 years and more than 17 kg of LD. A DRD patient who received 3 kg over 11 years had normal SN neuronal complement at autopsy. One patient who has clinical and laboratory evidence of nigral pathology as the basis of parkinsonism when untreated for 18 years had progressive disability. While on LD he has excellent symptomatic benefit and virtual cessation of progression of the disease. Epidemiologic studies indicate that those prescribed LD at an early stage of illness and hence given larger cumulative lifetime doses have longer survival than those in whom LD is started late and who receive smaller total doses. Our observations and the available literature support that levodopa is not toxic to normal or diseased substantia nigra in human beings. Evidence presented here indicates that levodopa has a protective effect on the human substantia nigra neurons.