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Parkinson's disease (PD) is associated with a reduction in 26/20S proteasome and mitochondrial function and depletion of dopamine. Activation of mitochondrial function with the NAD+ precursor nicotinamide riboside (NR) is a potential therapeutic for PD. However, despite recently started clinical trials, analysis of NR in mammalian animal PD models is lacking and data in simpler PD models is limited. We analyzed the effect of NR in C. elegans and in mouse 26/20S proteasome inhibition models of PD. In C. elegans, NR rescued α-synuclein overexpression induced phenotypes likely by activating the mitochondrial unfolded protein response. However, in a proteasome inhibitor-induced mouse model of PD, NR first partially rescued behavioural dysfunction, but later resulted in decrease in dopamine and its related gene expression in the substantia nigra. Our results suggest that reduction in 26/20S function with long term NR treatment may increase risk for developing reduced nigrostriatal DA function.

The increase in presynaptic striatal dopamine is the main dopaminergic abnormality in schizophrenia (SCZ). SCZ is primarily treated by modulating the activity of monoamine systems, with a focus on dopamine and serotonin receptors. Glial cell line-derived neurotrophic factor (GDNF) is a strong dopaminergic factor, that recently was shown to correlate with SCZ in human CSF and in striatal tissue. A 2-3-fold increase in GDNF in the brain was sufficient to induce SCZ-like dopaminergic and behavioural changes in mice. Here, we analysed the effect of acute, chronic, and embryonic methamphetamine, a drug known to enhance the risk of psychosis, on Gdnf and its receptors, Gfra1 and Ret, as well as on monoamine metabolism-related gene expression in the mouse brain. We found that acute methamphetamine application increases Gdnf expression in the striatum and chronic methamphetamine decreases the striatal expression of GDNF receptors Gfra1 and Ret. Both chronic and acute methamphetamine treatment upregulated the expression of genes related to dopamine and serotonin metabolism in the striatum, prefrontal cortex, and substantia nigra. Our results suggest a potential mechanism as to how methamphetamine elicits individual psychosis risk in young adults—variation in initial striatal GDNF induction and subsequent GFRα1 and RET downregulation may determine individual susceptibility to psychosis. Our results may guide future experiments and precision medicine development for methamphetamine-induced psychosis using GDNF/GFRa1/RET antagonists.