Introduction
We present a multimodality imaging study characterizing the 6-hydroxydopamine lesion rat model for Pakinson's Disease on molecular, morphological, physiological and behavioral level. To that end we applied subsequently microSPECT and microMRI in the same animal. The co-registered microMRI, microSPECT data were spatially normalized to a microMRI rat brain template, specially designed for the application of multimodality studies. Except for high resolution anatomical imaging, MRI was used to investigate the sensitivity of MR relaxation, diffusion and perfusion parameters in detecting alterations within the affected striatum. The rats were tested behaviorally and finally sacrificed and assessed for histological correlations.
Materials & Methods
Animal model
In six wistar rats unilateral lesions were performed by injection of 24 μg 6-hydroxydopamine (6-OHDA; Sigma, St. Quentin-Fallavier, France) within the substantia nigra. Six non-lesioned rats were included as control animals.
Behavior
Amphetamine-induced asymmetric rotational behavior was investigated using an automated rotometer bowl. The analysis of amphetamine tests was based on net ipsilateral turns (defined as clockwise turning in case of a right-sided injection).
MicroSPECT
Imaging was performed on a single-head Millenium GE SPECT gamma camera with a 3 mm aperture single-pinhole collimator under Nembutal anaesthesia (60 mg/kg). 300 MBq of 123I-FP-CIT (123I-ioflupane) was applied to quantify dopamine transporter (DAT) bindingscapacity in the intact and affected striatum. Transporter binding indices were calculated as activity in the striatum divided by aspecific activity in the cerebellum minus 1, giving an indicator of DAT binding capacity.
MicroMRI
MRI was performed on a 7 T (MRRS, UK) MR system under isoflurane anaesthesia administered in a 30–70% O2-N2 mixture. Besides high resolution anatomical imaging T1, T2 and diffusion weighting imaging was performed as well as perfusion measurements. Co-registration was performed based on maximization of mutual information. Diffusion, T1, T2-and perfusion maps of basal ganglia were calculated in IDL.
Histology
Three rats have been sacrificed to assess the extent of the lesion in the substantia nigra. To that end tyrosine hydroxilase specific staining was applied.
Results & Discussion
Co-registration for all images allowed accurate delineation of the striatum on HR MRI for volume-based quantification of the microSPECT data. In a second phase, these data can be used for partial volume correction of the SPECT quantitative data. In all rats significant reduced binding capacity of DAT could be demonstrated, confirmed by histological and behavioral data demonstrating a near-complete lesion. The T2 relaxation parameter was significantly reduced in the affected striatum as compared to the contralateral intact striatum, suggestive for an increase of iron deposits within the striatum, which has been reported previously in PD patients and animal studies. The T1 relaxation time in the affected striatum was significantly increased as compared to the intact striatum and striatum of control animals. We suggest that the extracellular environment is reduced due to a loss of spines on the striatal target neurons. The apparent diffusion coefficient which reflects the extracellular space volume fraction was significantly reduced in the affected striatum. Also the resting state perfusion values were significantly decreased within the affected striatum, especially in regions corresponding to the globus pallidus.