Neurotransmitters and neuropeptides are important signalling molecules in the brain and alterations in their expression levels have been linked to neurological disorders such as Parkinson’s disease (PD). Here we used ultrahigh mass resolution Fourier-transform ion cyclotron resonance (FTICR) MALDI-MSI for the comprehensive mapping of neurotransmitter networks and neuropeptides in specific brain regions. Our new reactive MALDI matrix (FMP-10) facilitated the covalent charge-tagging of neurotransmitters and metabolites containing phenolic hydroxyl and/or primary or secondary amine groups, including dopaminergic and serotonergic neurotransmitters and their associated metabolites. We illustrate the capabilities of the developed method on brain samples from an experimental PD model (MPTP), including L-Dopa-induced dyskinesia (LID) in PD.
We imaged the metabolism of L-Dopa and the catecholaminergic pathway in brains from LID and non-dyskinetic animals, chronically treated with L-Dopa. L-Dopa and the L-Dopa metabolite 3-O-methyldopa were abnormally highly elevated in the whole brain of dyskinetic animals resulting in significant increases in dopamine and downstream metabolites in all brain regions, except putamen and caudate. Dopamine formation was correlated with serotonin in specific layers of hippocampus and cortex in LID but not in the putamen. Furthermore, we found that dyskinesia severity correlated with the levels of some abnormally processed peptides, notably, destyrosine dynorphins, substance P (1-7), and substance P (1-9) in multiple brain regions. Our results demonstrate that the abundance of selected active neuropeptides is associated with L-DOPA concentrations in the putamen, emphasizing their sensitivity to L-DOPA. Additionally, levels of truncated neuropeptides (which generally exhibit reduced or altered receptor affinity) correlate with dyskinesia severity, particularly for peptides associated with the direct pathway (i.e., dynorphins and tachykinins). The increases in tone of the tachykinin, enkephalin, and dynorphin neuropeptides in LID result in abnormal processing of neuropeptides with different biological activity and may constitute a functional compensatory mechanism for balancing the increased L-DOPA levels across the whole basal ganglia. This study highlights the capacity of MALDI-MSI to elucidate the dynamics of neurotransmitter and neuropeptide signalling during PD and its treatment.