Levodopa impairs lysosomal function in sensory neurons in vitro

Abstract Parkinson’s disease (PD) is the second-most common neurodegenerative disease world-wide. Patients are diagnosed based upon movement disorders, including bradykinesia, tremor and stiffness of movement. However, non-motor signs, including constipation, rapid eye movement sleep behavior disorder, smell deficits and pain are well recognized. Peripheral neuropathy is also increasingly recognized, as the vast majority of patients show reduced intraepidermal nerve fibers, and sensory nerve conduction and sensory function is also impaired. Many case studies in the literature show that high-dose levodopa, the primary drug used in the treatment of PD, may exacerbate neuropathy, thought to involve levodopa’s metabolism to homocysteine. Here, we treated primary cultures of dorsal root ganglia and a sensory neuronal cell line with levodopa to examine effects on cell morphology, mitochondrial content and physiology, and lysosomal function. High-dose levo-dopa reduced mitochondrial membrane potential. At concentrations observed in the patient, levo-dopa enhanced immunoreactivity to beta III tubulin. Critically, levodopa reduced lysosomal content and also reduced the proportion of lysosomes that were acidic while homocysteine tended to have the opposite effect. Levodopa is a critically important drug for the treatment of PD. However, our data suggests that at concentrations observed in the patient, it has deleterious effects on sensory neurons that are not related to homocysteine. Simple Summary Parkinson’s disease (PD) is one of the most common chronic, degenerative brain diseases worldwide. Patients are diagnosed on the basis of slowness of movement and/or tremor and/or stiffness. However, many symptoms that are not movement related are now well recognized. Patients show changes in skin sensation, and the vast majority of patients show loss of sensory neurites, which enable sensation in skin. These changes in skin sensation occur prior to diagnosis; however, sensory issues may also be exacerbated by levodopa, an important drug used in the treatment of PD. Undoubtedly, levodopa is critical for the treatment of PD, but at high doses, it has repeatedly been shown to impair sensation in PD patients. Here, we show for the first time that high-dose levodopa impairs function of sensory neurons. Importantly, we also show for the first time that lysosomes, a critical organelle involved in recycling, are impaired by levodopa concentrations observed in patients. These data are important given the well-known lysosomal dysfunction observed in PD. Our data sheds light on how levodopa, the most important drug in the treatment of PD, may contribute to sensory deficits in PD.