The causes of neurodegeneration are not well understood. However, the role of environmental and endogenous toxins is receiving much attention. In this study, we compared the synthetic
neurotoxin 1-methyl-4-phenyl-pyridinium with
beta-carbolines occurring in human brain. Methylation of both nitrogens is necessary to convert a
beta-carboline into a potent inhibitor of mitochondrial complex I. The respective
beta-carboline,
2,9-dimethyl-beta-carbolinium ion is neurotoxic in rats. To investigate the underlying mechanisms, we incubated mouse
neuroblastoma 2A cells with
2,9-dimethyl-beta-carbolinium ion, and compared the findings with effects of
norharman, the precursor
beta-carboline of methylated derivatives, and with 1-methyl-4-phenyl-pyridinium.
2,9-Dimethyl-beta-carbolinium ion caused a significant increase of
reactive oxygen species (higher efficiency than 1-methyl-4-phenyl-pyridinium) and of mitochondrial membrane potential within the first minutes. After 60 min, the membrane potential dissipated. Concomitantly, the levels of
glutathione increased in
2,9-dimethyl-beta-carbolinium ion but not in 1-methyl-4-phenyl-pyridinium treated cells. After 24 h
effector caspases 3 and 7 were activated and the number of apoptotic cells increased as revealed by fluorescence-activated cell sorting cytometry. When incubated longer (48 h), cells underwent late apoptosis/secondary
necrosis as shown by fluorescence-activated cell sorting analysis and confirmed qualitatively by an electron microscopy study. The effects of
2,9-dimethyl-beta-carbolinium ion on apoptotic changes were similar to those induced by 1-methyl-4-phenyl-pyridinium(,) while
norharman showed only a weak potency at the very high doses. To investigate whether
2,9-dimethyl-beta-carbolinium ion is neurotoxic under in vivo conditions and whether only dopaminergic neurones are affected we conducted a dose-response study. Three weeks after injection of
2,9-dimethyl-beta-carbolinium ion in the substantia nigra we found a dose-dependent decrease of
dopamine and its metabolites in the striatum of rats. The levels of
5-hydroxytryptamine were diminished although the decrease was less. The levels of
noradrenaline increased after some doses. The findings strongly suggest an important role of endogenous
beta-carbolines in neurodegeneration with apoptosis as the predominant mechanism.