Intragastric Administration of Low-Dose Rotenone Post-Colitis Exacerbates Damage to the Nigrostriatal Dopaminergic System in Parkinson’s Disease: the Pace Accelerates Even More

Background The contribution of gastrointestinal (GI) inflammation and local exposure to neurotoxins in the gut offers the most in-depth explanation of Parkinson’s disease (PD) etiopathogenesis through abnormal accumulation and spreading of alpha-synuclein (α-syn) aggregates from the gut to the brain. Objectives This study was designed to investigate whether dextran sodium sulfate (DSS)-mediated colitis may have lasting effects on dopaminergic pathways in the brain and whether or not colitis exacerbated susceptibility to later exposure to the neurotoxin rotenone. Methods To induce chronic colitis, 10 months old C57BL/6 mice were pre-exposed to 3 cycles of 7 days of 1% (w/v) DSS administration in drinking water followed by 14 days of regular drinking water. After colitis-induction, animals received a low dose of intragastric rotenone for the next 8 weeks, followed by testing for Parkinsonian behavior and GI phenotypes of inflammation. At the end of the 8th week after colitis, colon, brain stem, and midbrain tissue were isolated and analyzed for α-syn, inflammatory markers, and dopaminergic neuronal loss. Gut microbial composition was assessed by 16S rRNA sequencing analysis. Results We found that local rotenone exposure for 8 weeks did not affect colitis severity and colonic tight junction(TJ) protein expression (ZO-1, Occludin, and Claudin-1). On the other hand, we found that while eight weeks of chronic rotenone administration led to an increase in inflammatory markers, the presence of pre-existing colitis resulted in a considerable change in gut microbiota composition and a decrease in TJ’s protein expression. In addition, the administration of rotenone in mice post-colitis caused gastrointestinal function impairment and poor behavioral performances. Itworsened rotenone-induced α-syn pathology in the colon, which extended upward and resulted in severe dopaminergic neuron loss and significant astroglia activation in the dorsal motor nucleus of the vagus (DMV), locus coeruleus, substantia nigra as well as in striatum. Interestingly, in the case of rotenone alone, we found that α-syn induced ChAT+ neuronal death is restricted to the DMV. These findings indicate that long-term rotenone exposure in conjunction with early inflammatory intestinal milieu exacerbates the progression of α-syn pathology and aggravates neurodegeneration in the intragastric mouse PD model. Conclusions This work provides detailed insight into the involvement of GI inflammation triggered after a neurotoxic insult in the colon and explores their potential to impact central dopaminergic degeneration in PD. This way, we can identify potential therapeutic targets that stop the enteric inflammatory processes involved in progressing PD. ![Figure][1] Highlights ### Competing Interest Statement The authors have declared no competing interest. * α-syn : Alpha synuclein ANOVA : One-way analysis of variance BCA : Bicinchoninic acid CMC : Carboxymethylcellulose CNS : Central nervous systems ChAT : Choline acetyl transferase CPCSEA : Committee for the Purpose of Control and Supervision of Experiments on Animals CCl2 : C-C chemokine ligand 2 DAI : Disease Activity Index DAergic : Dopaminergic DAB : 3,3′-diaminobenzidine DAPI : 4′,6-diamidino-2-phenylindole DSS : Dextran sodium sulphate DMV : Dorsal motor nucleus of vagus ENS : Enteric nervous system EGC : Enteric glial cells GI : Gastrointestinal HRP : Horseradish peroxidase IBD : Inflammatory Bowel Disease LRRK2 : Leucine rich repeat kinase 2 LC-MS : Liquid chromatography mass spectroscopy LC : Locus coeruleus OD : Optical Density p-syn : Phosphorylated S129 synuclein PD : Parkinson’s disease PBS : Phosphate buffer saline PB : Phosphate buffer PFA : Paraformaldehyde PMSF : Phenylmethyl sulfonyl fluoride RA : Relative Abundance PVDF : Polyvinylidene difluoride RBW : Round beam walk SDS : Sodium dodecyl sulfate SNpc : Substantia nigra pars compacta Tg : Transgenic TH : Tyrosine hydroxylase ZO-1). : Zona Occludens [1]: pending:yes