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Methamphetamine neurotoxicity involves vacuolation of endocytic organelles and dopamine-dependent intracellular
J F Cubells1, S Rayport, G Rajendran
1Department of Psychiatry, Columbia University, New York, New York 10032.
Abstract:
Methamphetamine (MA) produces selective degeneration of dopamine (DA) neuron terminals without cell body loss. While excitatory amino acids (EAAs) contribute to MA toxicity, terminal loss is not characteristic of excitotoxic lesions nor is excitotoxicity selective for DA fibers; rather, EAAs may modulate MA-induced DA turnover, suggesting that DA-dependent events play a key role in MA neurotoxicity. To examine this possibility, we used postnatal ventral midbrain DA neuron cultures maintained under continuous EAA blockade. As in vivo, MA caused neurite degeneration but minimal cell death. We found that MA is a vacuologenic weak base that induces swelling of endocytic compartments; MA also induces blebbing of the plasma membrane. However, these morphological changes occurred in MA-treated cultures lacking DA neurons. Therefore, while collapse of endosomal and lysosomal pH gradients and vacuolation may contribute to MA neurotoxicity, this does not explain selective DA terminal degeneration. Alternatively, MA could exert its neurotoxic effects by collapsing synaptic vesicle proton gradients and redistributing DA from synaptic vesicles to the cytoplasm. This could cause the formation of DA-derived free radicals and reactive metabolites. To test whether MA induces oxidative stress within living DA neurons, we used 2,7-dichlorofluorescin diacetate (DCF), an indicator of intracellular hydroperoxide production. MA dramatically increased the number of DCF-labeled cells in ventral midbrain cultures, which contain about 30% DA neurons, but not in nucleus accumbens cultures, which do not contain DA neurons. In the DA neuron cultures, intracellular DDF labeling was localized to axonal varicosities, blebs, and endocytic organelles. These results suggest that MA redistributes DA from the reducing environment within synaptic vesicles to extravesicular oxidizing environments, thus generating oxygen radicals and reactive metabolites within DA neurons that may trigger selective DA terminal loss.
Insights
Methamphetamine causes selective dopamine terminal damage by disrupting dopamine storage, leading to oxidative stress and neurotoxicity. This research clarifies the mechanism behind methamphetamine's impact on dopamine neurons.
Area of Science:
- Neuroscience
- Neuropharmacology
- Toxicology
Background:
- Methamphetamine (MA) causes selective degeneration of dopamine (DA) neuron terminals without cell body loss.
- Excitatory amino acids (EAAs) contribute to MA toxicity, but terminal loss is not characteristic of excitotoxicity, suggesting DA-dependent mechanisms are key.
- Previous findings indicate MA causes neurite degeneration and minimal cell death in DA neuron cultures, with morphological changes observed even in DA-neuron-absent cultures.
Purpose of the Study:
- To investigate the role of dopamine-dependent events in methamphetamine neurotoxicity.
- To determine if collapsing synaptic vesicle proton gradients and redistributing DA contributes to MA-induced oxidative stress.
- To examine the generation of DA-derived free radicals and reactive metabolites within DA neurons.
Main Methods:
- Utilized postnatal ventral midbrain DA neuron cultures under continuous EAA blockade.
- Assessed MA-induced morphological changes, including vacuolation and plasma membrane blebbing.
- Employed 2,7-dichlorofluorescin diacetate (DCF) to detect intracellular hydroperoxide production as an indicator of oxidative stress.
Main Results:
- MA induced vacuologenic swelling of endocytic compartments and plasma membrane blebbing, but these effects were not exclusive to DA neurons.
- MA significantly increased DCF-labeled cells in DA neuron cultures, indicating increased oxidative stress, but not in nucleus accumbens cultures lacking DA neurons.
- Intracellular DCF labeling in DA neuron cultures was localized to axonal varicosities, blebs, and endocytic organelles.
Conclusions:
- MA-induced vacuolation and pH gradient collapse may contribute to neurotoxicity but do not explain selective DA terminal degeneration.
- MA redistributes DA from synaptic vesicles to the cytoplasm, creating an oxidizing environment that generates oxygen radicals and reactive metabolites.
- This oxidative stress within DA neurons is suggested to be the primary mechanism triggering selective DA terminal loss.