Imaging neurochemical changes associated with Parkinson´s disease and L-DOPA-induced dyskinesia using mass spectrometry

Abstract: Parkinson’s disease (PD), caused by a loss of midbrain dopamine neurons, is the second most common neurodegenerative disease worldwide after Alzheimer’s disease. The primary treatment choice for PD is L-DOPA, the precursor for dopamine, which only affects symptoms and does not inhibit disease progression. Most patients develop motor complications during long-term L-DOPA treatment called L-DOPA-induced dyskinesia (LID), which are abnormal involuntary movements. LID has been associated with biochemical alterations in a number of signalling systems in the basal ganglia, including the dopaminergic, serotonergic, cholinergic and opioidergic systems, among others. Defining region-specific alterations of these signalling molecules and comprehensive metabolic pathways in the brain will help to improve our understanding of their involvement in LID. In the work upon which this thesis is based, we exploited the advantages of mass spectrometry imaging (MSI) to perform on-tissue mapping of a large number of molecules in a single experiment for investigating biochemical changes associated with LID. A novel matrix-assisted laser desorption/ionisation (MALDI) MSI on-tissue chemical derivatisation approach was developed that enabled imaging of primary amine and phenolic hydroxyl group containing neurotransmitters and their comprehensive metabolic pathways. In addition, a tissue clean-up protocol which improved the limit of detection of multiple neuropeptides involved in basal ganglia signalling was established. These methods were applied to neurotoxin-based animal models of PD and LID, including the gold-standard model, namely the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administered non-human primate model. LID was found to be associated with extremely high levels of L-DOPA throughout the brain, but no significant increase in striatal dopamine was observed, contradicting the widely accepted hypothesis that LID is induced by elevated striatal dopamine levels. Furthermore, LID was associated with increased levels of signalling neuropeptides throughout the basal ganglia, where abnormally processed neuropeptides correlated with LID severity. Untargeted multivariate analysis revealed that LID was associated with increased abundance of the vasculature marker heme B in the striatum, suggesting angiogenesis and increased blood flow to this region. Moreover, important methyl donors, including S-adenosylmethionine, betaine and α-glycerophosphocholine were affected by MPTP exposure and LID. In conclusion, the studies included in this thesis provide methods for investigating multiple signalling molecules in single tissue sections and novel and comprehensive insights into the biochemical changes that occur in LID.