Abstract
The gut-brain axis plays an important role in Parkinson’s disease (PD) by acting as a route for vagal propagation of aggregated α-synuclein in the gut-first endophenotype and as a mediator of gastrointestinal dyshomeostasis via the nigro-vagal pathway in the brain-first endophenotype of the disease. One important mechanism by which the gut-brain axis may promote PD is by regulating gastrointestinal redox homeostasis as overwhelming evidence suggests that oxidative stress plays a key role in the etiopathogenesis and progression of PD and the gastrointestinal tract maintains redox homeostasis of the organism by acting as a critical barrier to environmental and microbiological electrophilic challenges. The present aim was to utilize the bilateral intrastriatal 6-hydroxydopamine (6-OHDA) brain-first PD model to study the effects of isolated central pathology on redox homeostasis of the gastrointestinal tract. Three-month-old male Wistar rats were either not treated (intact controls; CTR) or treated bilaterally intrastriatally with vehicle (CIS) or 6-OHDA (6-OHDA). Motor deficits were assessed with the rotarod performance test, and the duodenum, ileum, and colon were dissected for biochemical analyses 12 weeks after the treatment. Lipid peroxidation, total antioxidant capacity, low-molecular-weight thiols, and protein sulfhydryls, the activity of total and Mn/Fe superoxide dismutases, and total and azide-insensitive catalase/peroxidase were measured. Both univariate and multivariate models analyzing redox biomarkers indicate that significant disturbances in gastrointestinal redox balance are not present. The findings demonstrate that motor impairment observed in the brain-first 6-OHDA model of PD can occur without concurrent redox imbalances in the gastrointestinal system.
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Introduction
Parkinson’s disease (PD) is a chronic progressive neurodegenerative condition characterized by the degeneration of dopaminergic neurons in substantia nigra (SN) pars compacta that results in the development of bradykinesia, tremor at rest, rigidity, and postural instability [1]. Although the etiopathogenesis of the disease remains to be elucidated accumulating evidence points to the involvement of the gastrointestinal tract as (i) the prodromal non-motor symptoms affecting the gastrointestinal tract (e.g., dysphagia, delayed gastric emptying, and constipation) are prevalent and precede the motor phase of PD (sometimes by decades) [2,3,4]; (ii) a stereotypical spreading pattern of α-synuclein pathology [5, 6] supports the hypothesis that misfolded α-synuclein may originate from the gut [7]; and (iii) mechanistic animal studies clearly demonstrate that pathophysiological events in the gastrointestinal tract are sufficient to trigger and promote the development of the central nervous system (CNS) pathology resembling PD (e.g., [8,9,10,11]). Based on the aforementioned evidence a body-first hypothesis of PD has been proposed with the gastrointestinal tract considered the most likely site of early molecular pathophysiological events [12].
In contrast, some studies suggest that in a considerable proportion of patients, PD does not propagate in concordance with the Braak staging system [13, 14]. Furthermore, although highly prevalent, gastrointestinal symptoms are not present in all patients diagnosed with PD, and they do not always appear before the onset of motor symptoms [2, 15]. Consequently, it is evident that in some patients a brain-first hypothesis provides a more accurate explanation of the PD progression.
Based on the aforementioned data, a working model has been proposed which recognizes PD as a complex disease composed of at least two clusters of phenotypes (brain-first and gut-first) [12, 16]. The gut-brain axis plays an important role in both subtypes acting as a route for vagal propagation of aggregated α-synuclein in the gut-first phenotype and as a mediator of gastrointestinal dyshomeostasis via the nigro-vagal pathway in the brain-first phenotype (e.g., [17]). Nevertheless, the mechanisms by which the gut-brain axis may contribute to the propagation of the disease and the appearance of gastrointestinal symptoms remain poorly understood and challenging to study due to overlap** brain and gut pathology in animal models.
In this context, the CNS-targeted 6-hydroxydopamine (6-OHDA) rodent models provide a unique way to study the effects of the brain-first predominant subtype of the disease on the pathophysiological alterations in the gut as the toxin cannot cross the blood-brain barrier. The central 6-OHDA administration model was first introduced by Ungerstedt following the idea that high selectivity of the toxin towards the dopamine uptake sites may result in a specific nigrostriatal dopaminergic lesion [18]. Since its introduction, the model was widely used for investigating many aspects of PD as it successfully recapitulates several important features observed in patients suffering from the idiopathic form of the disease: (i) administration of 6-OHDA mimics increased oxidative stress in dopaminergic neurons found in PD [19, 20]; (ii) SN pars compacta that shows the greatest susceptibility to 6-OHDA-induced injury is also the most affected area in PD patients [21]; (iii) 6-OHDA toxicity can be facilitated with iron [22] and iron dyshomeostasis plays an important role in the etiopathogenesis and progression of PD [23,24,25,26,27,28]; (iv) 6-OHDA is produced in physiological conditions upon oxidation of dopamine and it is present in the urine of patients suffering from PD. Consequently, it is possible that endogenous 6-OHDA may be involved in the etiopathogenesis of PD in humans [29, 30].
Some groups already utilized the model to study the mechanisms of gastrointestinal dyshomeostasis in the context of the brain-first PD-like nigrostriatal lesion primarily with a focus on gastrointestinal motility (e.g., [31,32,85,86,87] may have provided a different insight.
Data Availability
Raw data can be obtained from the corresponding author. The manuscript has been preprinted on bioRxiv [88].
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Open Access funding enabled and organized by Projekt DEAL. This work was funded by the Croatian Science Foundation (IP-2018-01-8938). The research was co-financed by the Scientific Centre of Excellence for Basic, Clinical, and Translational Neuroscience (project “Experimental and clinical research of hypoxic-ischemic damage in perinatal and adult brain”; GA KK01.1.1.01.0007 funded by the European Union through the European Regional Development Fund).
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JH, ABP, AK, and JOB—in vivo part of the experiment, tissue collection. JH, MJ, GG, ES—biochemical measurements. JH—data curation, data analysis, writing the first draft of the manuscript. MJ, GG, ES, DV, ABP, AK, JOB, and MSP—critical revision of the manuscript. MSP—funding, supervision.
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Homolak, J., Joja, M., Grabaric, G. et al. The Absence of Gastrointestinal Redox Dyshomeostasis in the Brain-First Rat Model of Parkinson’s Disease Induced by Bilateral Intrastriatal 6-Hydroxydopamine. Mol Neurobiol (2024). https://doi.org/10.1007/s12035-023-03906-7
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DOI: https://doi.org/10.1007/s12035-023-03906-7