Toxic Effect of Parkinson’s Drug on Brain Cells Studied New approach for analysis of levodopa side effect (dyskinesia)

Levodopa is considered one of the most important drugs for the treatment of Parkinson’s disease. In the past, it was often reported that treatment with the substance could be harmful to neurons because it leads to increased formation of oxygen radicals. A team of scientists at the Technische Universität Braunschweig in cooperation with the laboratory of Prof. Marcel Leist at the University of Konstanz has now been able to show that in almost all of these studies it was a matter of artifacts caused by too much oxygen in the cell culture experiments. Instead, the influence of levodopa could be studied for the first time without interfering oxygen radicals and the effect of the drug on the metabolism of neurons and their mitochondria could also be determined. It looks as if levodopa interferes with the energy supply of neurons and this could also be the cause of levodopa side effects. The results are published by the journal “Cell Death Discovery”.

In the brain, certain neurons produce dopamine, a neurotransmitter from the catecholamine group that is important for certain motor function. These dopamine-producing neurons are located in the midbrain; degeneration of these neurons is the cause of Parkinson’s disease. The first motor symptoms appear when about half of these dopaminergic neurons have died. Unfortunately, there is no cure for Parkinson’s yet, but the symptoms can be treated. The main drug for this is levodopa (L-dihydroxyphenylalanine / L-DOPA), a precursor of dopamine that can cross the blood-brain barrier and is then converted to dopamine in the brain.

Direct effects of levodopa on brain cells studied

Since Parkinson’s patients are sometimes treated for decades with very high concentrations of levodopa, the question arises as to how this substance affects brain cells. Since it is difficult to gain direct access to human neuronal cells, many experiments have been carried out in cell culture. Levodopa has been found to be highly reactive and unstable, rapidly degrading to neuromelanin and producing toxic intermediates that exert high oxidative stress on cells. In most cell culture studies, significant toxicity was observed in this process. All biochemical and molecular effects exerted by this drug on cells were attributed to the formation of these radical oxygen species.

In the previous published cell culture studies, neuronal cells often die at doses that may also occur in patients after treatment. Thus, there has been much speculation as to whether L-DOPA treatment may cause further damage to the brain, especially at high doses for prolonged periods. Interestingly, L-DOPA has been shown to have no direct toxicity in both animal and human studies, and the molecular observations from cell culture could not been confirmed.

“Our goal was to investigate what effects levodopa has on the metabolism of neuronal cells,” says Prof. Karsten Hiller from the Institute of Biochemistry, Biotechnology and Bioinformatics at TU Braunschweig. “Since cellular metabolism strongly depends on the oxygen concentration, we had decided to perform our cell culture experiments under conditions with little oxygen (2%, hypoxia). This sounds surprising at first, but it corresponds to the conditions that the cells also find in the brain.” The first completely unexpected observation was that under these conditions, L-DOPA had little toxic effect on the cells. As the study progressed, the researchers were able to show that the reduced oxygen concentration meant that levodopa no longer decayed as quickly, and far fewer of the toxic oxygen radicals were formed in the process. “Ironically, we also observed that stabilized levodopa under hypoxic conditions can even act as an antioxidant and scavenge oxygen radicals. So, depending on the oxygen concentration, levodopa is, excitingly, a double-edged sword in molecular terms,” Prof. Hiller added.

Another finding of the study is that although levodopa is not toxic to cells, it nevertheless has far-reaching effects on neurons. According to the study, levodopa strongly interferes with mitochondrial metabolism. Mitochondria are the power plants of our cells and supply them with energy. To this end, cellular respiration, among other things, also takes place in the mitochondria. The researchers were able to show that levodopa reduces cellular respiration and thus also influences the energy supply of the cells. These metabolic changes are independent of reactive oxygen radicals and could thus also occur in the brains of Parkinson’s patients receiving levodopa treatment.

Research approach: Possible association with levodopa side effects

Parkinson’s patients who are treated with levodopa over long periods of time can develop dyskinesia. This is a very unpleasant side effect that causes severe disturbances in movement. These involuntary movements occur in about 90 percent of affected individuals after many years of treatment – often when blood concentrations are highest after levodopa administration. So far, it has not been conclusively clarified how these side effects might develop. “One aspect could be that mitochondrial activity is reduced after a levodopa dose, leaving the neuronal cell with less energy to coordinate movement. We are now working together with the group of Norbert Brüggemann and Christine Klein from the University of Lübeck to validate these observations in patients.”