Respiratory Functional and Neuroanatomical Changes in 6‐OHDA Model of Parkinson’s Disease Require Tumor Necrosis Factor Alpha Receptor 1

Abstract

Most deaths of Parkinson’s Disease (PD) patients are due to respiratory complications which appears in the late stages of this disease. Previous studies have shown brainstem degeneration which is probably linked to respiratory function impairment in a rat model of PD induced by 6‐hydroxydopamine (6‐OHDA) injection in caudate‐putamen (CPu). One of the hypotheses that explains why neurons die in this PD model is neuroinflammation mediated by a cytokine called tumor necrosis factor alpha (TNF‐α). So, this study aims to investigate the role of TNF‐α in respiratory functional and neuroanatomical deficits in the animal model of PD induced by 6‐OHDA. C57BL/6 or TNF‐α receptor 1 knockout mice (TNFR1‐/‐) were anesthetized with isoflurane 2.5% and received bilateral injection of 6‐OHDA (10 µg/µl) or vehicle in CPu through stereotaxic surgery. 20 days after surgery, mice were submitted to whole body plethysmography under normal and hypercapnic conditions (7% CO2). Animals were perfused and the brains were removed to neuroanatomical analysis. 6‐OHDA destroyed TH+ neurons in Substantia nigra (SNc) in all groups analyzed (6‐OHDA WT: 77%; 6‐OHDA TNFR1‐/‐: 79%). Degeneration was observed in preBötzinger complex (preBötC) and in retrotrapezoid nucleus (RTN) in 6‐OHDA WT group, but not in 6‐OHDA TNFR1‐/‐ group (preBötC: 6‐OHDA WT: 43%, 6‐OHDA TNFR1‐/‐: 22%; RTN: 6‐OHDA WT: 52%; 6‐OHDA TNFR1‐/‐: 15%). Astrocyte’s density in SNc is enhanced in 6‐OHDA WT group but not in 6‐OHDA TNFR1‐/‐ group (6‐OHDA WT: 122%) and is reduced in preBötC in all groups analyzed (p<0.05), while in RTN, no change was observed. Microglia analysis indicates neuroinflammation in SNc and RTN in all groups. Respiratory assessment reveals reduction in basal respiratory rate in 6‐OHDA WT group but not in 6‐OHDA TNFR1‐/‐ group (6‐OHDA WT: 25%; 6‐OHDA TNFR1‐/‐: 17%). No change was observed between groups in respiratory parameters under hypercapnia. Considering all results presented here we can state that in this model of PD, respiratory deficits are associated with neurodegeneration in respiratory brainstem areas. Abolishing TNF‐α inflammation pathway, reduction in respiratory rate and degeneration of brainstem areas involved with neural control of breathing are prevented indicating the involvement of this pathway in the respiratory changes found in the 6‐OHDA model of PD.