Development of an acute excitotoxic model of Huntington's disease in sheep

Abstract

Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder. The earliest and most severe neuropathological change in HD occurs within the striatum. Exogenous excitotoxic lesioning of the rodent and non-human primate (NHP) striatum is used to model HD. Apart from NHPs, no other excitotoxic large animal model of HD has been established. Sheep have the potential to be an important species for modelling neurodegenerative disease, primarily because of neuroanatomical similarities between the sheep and human brain. This thesis describes the development of an excitotoxic sheep model of HD using the excitotoxin, quinolinic acid (QA). QA is an N-methyl-D-aspartate (NMDA) glutamate receptor agonist that produces pathological changes within the striatum that resemble those seen in HD. Sixteen castrated-male, 18 month old, Merino-Border Leicester cross sheep underwent two surgical procedures, four weeks apart, to infuse 75 μl of 180 mM QA (experimental group) or 75 μl of saline (control group) into the left (first surgery) and then the right (second surgery) caudate nucleus of the striatum. Longitudinal magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS) and diffusion tensor imaging (DTI) of the brains of the sheep was performed on a 3-Tesla scanner pre-surgically, one week after the first surgery, five weeks after the first surgery and sixteen weeks after the first surgery to investigate the neuropathological changes that occur in vivo after QA lesioning of the sheep striatum. The phenotypic consequences of lesioning the sheep striatum with QA were investigated using a veterinary neurological examination, dopamine agonist induced rotation and a two-choice discrimination task. The author / investigator was blind to the treatment group. MRI revealed QA-lesion hyperintensity and dilation of the lateral ventricles, consistent with atrophy of the caudate nucleus. MRS and DTI revealed a significant decrease in the neuronal marker N-acetylaspartate (NAA), and in fractional anisotropy (FA) in the acutely-lesioned (one week after surgery) striatae of the QA-lesioned sheep, followed by recovery in NAA and a significant increase in FA in the chronic (five to sixteen weeks) QA-lesioned striatae. NAA and FA changes are consistent with neuronal loss and structural disruption in the acute lesion, followed by recovery of reversibly impaired neurons, structural reorganisation and gliosis in the chronic lesion. Heterogeneous neuronal loss and damage and gliosis were visible on histological analysis of the QA-lesioned sheep striatae, supporting the in vivo MRS and DTI detected changes. Neurological examination of the sheep revealed evidence of laterality and mild hind limb motor paresis in seven out of eight of the QA-lesioned sheep, however the examination was not informative of lesion characteristics. A directional bias was evident in the QA-lesioned sheep during rotation studies. However, the direction and magnitude of bias in individual sheep at any one timepoint varied markedly, making identification of QA-lesioned individuals difficult. There was no difference between the QA-lesioned and saline-treated sheep in performance of the acquisition and reversal phases of the two-choice discrimination task. The behavioural studies described in this thesis were not suitable for comprehensive identification and characterisation of QA lesions in the striatum of sheep. This is the first description of the development of an acute excitotoxic sheep model of HD. The experiments demonstrate that longitudinal analysis of the neuropathological changes in the QA-lesioned sheep striatum is possible using advanced magnetic resonance modalities performed on a clinically relevant 3-Tesla scanner and that neuropathological changes are consistent with HD-like pathology in other species. Furthermore, phenotypic investigation of the QA-lesioned sheep is possible, however more refined methods than those described need to be utilised. The excitotoxic sheep model of HD is clinically relevant HD model with potential for use in disease mechanism and therapy investigations.