The Critical Factor for L-Dopa Toxicity on Dopamine Neurons is Glia

Abstract

L-DOPA is toxic for dopamine neurons in culture but its toxicity has not been proven in animals or in patients with Parkinson’s disease. Since most experiments in vitro showing L-DOPA toxicity were performed in neurons cultured in the absence of glia we hypothesized that the discrepancy between the effects of L-DOPA in vivo and in vitro may be related to the presence or absence of glia, respectively. Fetal midbrain neuronal cultures were treated with L-DOPA, 200 μM, in the presence or absence of mesencephalic glia conditioned medium (GCM). In the absence of GCM, L-DOPA greatly reduced the number of tyrosine hydroxylase (TH) immunoreactive neurons and increased the levels of quinones in the medium; GCM prevented these effects of L-DOPA and increased the length and arborization of neurites of the TH immunoreactive cells. In order to characterize the compounds produced by glia and responsible for the protection of dopamine neurons the GCM was fractionated in two samples through filters of molecular size of 10 kD. Both fractions were protective against L-DOPA toxicity, through the fraction <10 kD was more effective than the fraction >10 kD. Candidate neuroprotective agents, including growth factors (BDNF, FGF, GDNF, NGF) and antioxidants (ascorbic, glutathione) were tested and found to provide protection against L-DOPA toxicity. In conclusion, glia prevents L-DOPA toxicity on dopamine neurons through the release of multiple soluble compounds to the extracellular space.