Structural and Functional Reorganization of the Rat Brain in the 6-OHDA Model of Parkinson’s Disease
Vincent Perlbarg1,2, Benjamin Butler3, Justine Lambert3, Romain Valabrègue3, Anne-Laure Privat3, Chantal François3, Stéphane Lehéricy4,5, and Alexandra Petiet4,5

1Bioinformatics and Biostatistics Platform, Brain and Spine Institute, Paris, France, 2UPMC/Inserm UMRS1146 / CNRS UMR7371, Paris, France, 3Brain and Spine Institute, Paris, France, 4Center for Neuroimaging Research, Brain and Spine Institute, Paris, France, 5UPMC/Inserm UMRS1127 / CNRS UMR7225, Paris, France

Synopsis

Parkinson’s disease (PD) is characterized by neurodegeneration of the dopaminergic neurons in the substantia nigra pars compacta (SNc), which can be recapitulated in the 6-hydroxydopamine (6-OHDA) rat model. To evaluate structural and functional cerebral reorganization after induction of the lesion, we performed a longitudinal study up to 6 weeks using diffusion and resting-state functional MRI. Our results showed increased fractional anisotropy in the striatum ipsilateral to the lesion and increased bilateral functional connectivity between the striatum, the globus pallidus and the sensorimotor cortex in the 6-OHDA group. These results will help improve our understanding of cerebral alterations and reorganization in PD pathology.

Purpose

Parkinson’s disease (PD) is characterized by neurodegeneration of the dopaminergic neurons in the substantia nigra pars compacta (SNc). MRI has been used to study neurodegeneration in the nigro-striatal system in the 6-hydroxydopamine (6-OHDA) rat model but with diffusion MRI1,2 or SPECT imaging1. Here we evaluated the nigro-striatal pathway degeneration using both diffusion and functional MRI We hypothesized that the neurotoxin would induce structural and functional alterations in the SN system.

Materials and Methods

Fifteen Sprague Dawley male rats (300-450g) were stereotactically injected in the right striatum (STR) with 6-OHDA (N=10) or saline (sham condition) (N=5). Imaging was performed 3 weeks later (peak of the lesion3) at 11.7T (Bruker Biospec 117/16 USR horizontal bore, 750mT/m gradients, Paravision 5.1, Ettlingen, Germany) with a rat surface head coil for signal reception and a 72-mm resonator for signal emission. All animals were anesthetized with 1.5% Isoflurane. Local first and second order shimming was performed using a Bruker MAPSHIM macro based on a fieldmap acquisition (500μm isotropic resolution; first-TE=1.4ms; delta-TE=3.4ms). Anatomical T2-weighted (T2w) images were acquired with a multi-slice multi-echo (MSME) sequence; TR=8000ms; TE=13.5ms; Matrix (Mtx)=384x256; field-of-view (FOV)=3.84x2.56cm2; resolution(res)=100x100μm2; slice thickness=200μm; 128 slices; number of excitations (Nex)=1; acquisition time (Tacq)=34min. For resting-state fMRI, multi-slice echo-planar images (EPI) were acquired with TR=3000ms; TE=17.5ms; receiver bandwidth=400kHz; Mtx=96x96; FOV=3.84x3.84cm2; res=400x400μm2; slice thickness=400μm; 52 slices; 110 repetitions; 4 dummy scans to allow T1 steady-state; Tacq=5min30s. For diffusion imaging, a 3D EPI-based sequence was used with TR=300ms; TE=24.2ms; Mtx=128x96x96; FOV=3.2x2.4x2.4cm3; res=250μm isotropic; b-value=1500s/mm2; 81 directions; Tacq=42min. A fieldmap acquisition was used to correct EPI geometric distortions. Functional time-series were motion-corrected by rigid-body realigment to the first volume, spatially smoothed (gaussian kernel with FWHM=500μm) and corrected from physiological noise by using compCorr algorithm. DWI data were preprocessed with standard tools of fsl 5.07: eddy_cor for eddy curent corection and fugue for epi distortion correction with the field map acquisition. we then used dtifit to compute the tensor metrics. T2w brain template was created from the 11 individual images. To do so, for each rat, T2w was coregistered to the Karolinska rat brain template4 by using Ants software and averaged across the 11 rats. Diffusion and functional images were coregistered to our template by combining linear transformation of functional/diffusion images to the individual T2w and a non-linear transformation of the individual T2w to our T2 template. A total of six 6-OHDA rats and five sham rats were included in the data analysis (4 6-OHDA rats were excluded for misregistration issues). Regions of interest (ROI) were imported from the WHS rat brain atlas5, co-registered to our template and manually adjusted when necessary (e.g. to avoid the injection line) in both hemispheres in the STR, the SN, the internal capsule (IC), the globus pallidus (GP), the thalamus (TH), the motor cortex (M1M2), and the somatosensory cortex (S1S2). For each rat, and each pair of ROIs, functional connectivity (FC) was evaluated by Paerson correlation coefficient between the averaged functional time-series within each ROI. Finally, averaged Fractional Anisotropy were calculated within each ROI for each rat. Group differences between FC and FA measures were tested by using non-parametric Kruskall-Wallis rank test and considered significant if p<0.05.

Results

FA measurements showed significantly increase FA in the ipsilateral STR (p=0.03) and a trend for increased FA in the contralateral STR (p=0.07) in the 6-OHDA group compared to the sham group (Figure1). This increase is mainly due to the decrease of radial diffusivity and the relative preservation of axial diffusivity suggesting neuronal loss. In parallel, FC increases significantly between contralateral M1/2 and contralateral GP (p=0.02), between ipsilateral STR and ipsilateral GP (p=0.01) and between contralateral STR and ipsilateral GP (p=0.04) (Figure 2).

Discussion and Conclusion

This work showed significant alterations of functional connectivity and diffusion metrics in cerebral regions involved in the nigro-striatal system (STR, GP, TH, M1M2). Significant FA increase in STR, related to local neuronal loss, is associated with an increase of FC, suggesting a compensatory effect. These results will help improve our understanding of cerebral alterations and reorganization in PD pathology.As a perspective, we plan (i) to reconstruct the nigro-striatal pathway with diffusion-based tractography and to evaluate structural connectivity alterations; and (ii) to evaluate dopaminergic neuron depletion with histological markers and to correlate them with our MRI findings.

Acknowledgements

This work was supported by the France Parkinson Association and by the “Investissements d'Avenir”, IHU-A-ICM, Paris Institute of Translational neuroscience ANR-10-IAIHU-06.

References

1. Van Camp N, Vreys R, Van Laere K, et al. Morphologic and functional changes in the unilateral 6-hydroxydopamine lesion rat model for Parkinson’s disease discerned with μSPECT and quantitative MRI. Magn Reson Mater Phy. 2010;23:65–75. 2. Soria G, Aguilar E, Tudela R, et al. Eur J Neurosci. 2011;33(8):1551-60. 3. Debeir T, Ginestet L, François C, et al. Effect of intrastriatal 6-OHDA lesion on dopaminergic innervation of the rat cortex and globus pallidus. Experimental Neurol 2005;193:444-54. 4. Schweinhardt P, et al. A template for spatial normalization of MR images of the rat brain. J Neurosci Methods. 2003 129:105-13. 5. Papp EA, Leergaard TB, Calabrese E, et al. Waxholm Space atlas of the Sprague Dawley rat brain. Neuroimage. 2014;97:374-86.

Figures

Mean FA values measured in the ipsilateral STR (right) and contralateral STR (left) of the 6-OHDA and the sham groups. *p=0.03. Error bars represent standard mean errors.

FC measures between contralateral M1/2 and contralateral GP (left), between ipsilateral STR and ipsilateral GP (middle) and between contralateral STR and ipsilateral GP (right). *p<0.05. Error bars represent standard mean errors.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
1250