A Serotonin Story in a Toxicological Model of Parkinsonism: Real Time, In Vivo, Chemical Investigations in Mice

Abstract

Loss of midbrain dopaminergic neurons is well characterized to underlie the motor symptoms of Parkinson’s Disease (PD). There are, however, a host of non-motor, and often devastating PD symptoms and less is known about how these symptoms are mediated by neurotransmitters. Serotonin neurochemistry is linked to various aspects of the disease such as PD-associated depression, levodopa (L-Dopa) induced depression and L-Dopa induced dyskinesia (LID). Depression and LID significantly reduce quality of life for PD patients so understanding serotonin neurotransmission in PD models is of great merit for reducing these symptoms. Here we use niche, fast electrochemistry paired with mathematical modeling and machine learning to, for the first time, robustly evaluate serotonin in vivo in real time in a toxicological model of Parkinsonism, 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP). Mice treated with an acute MPTP paradigm had lower concentrations of in vivo, evoked and ambient serotonin in the hippocampus, consistent with the clinical comorbidity of depression with PD. These mice did not chemically respond to an SSRI, as strongly as did control animals, following the clinical literature showing that antidepressant success during PD is highly variable. Following levodopa (L-DOPA) administration, using a novel machine learning analysis tool, we see a dynamic shift from evoked serotonin release to dopamine release. We hypothesize that this finding shows, in real time, that serotonergic neurons uptake L-DOPA and produce dopamine at the expense of serotonin, supporting the significant clinical correlation between L-DOPA and depression. Finally, we find that this post L-DOPA dopamine release is less regulated, staying in the synapse for longer. This finding, perhaps to lack of autoreceptor control, may underlie the clinical presentation of LIDS. For the first time, this work shows the chemical intricacies of serotonin and how this modulator underlies several important non-motor deficits of PD, opening up new avenues of study for ultimately alleviating these symptoms.