Effects of dopamine and excitotoxicity in experimental models of Huntington's disease

Abstract: Huntington's disease (HD) is a hereditary neurodegenerative disorder caused by an expansion of a CAG repeat in the HD gene, that leads to dysfunction and death of striatal neurons. The pathogenetic mechanisms underlying HD are not known, but excitotoxicity and dopamine (DA) have been suggested to play a role. In this thesis, the effects of excitotoxicity and DA have been studied in striatal in vitro and in vivo models of HD. I examined the effect of brain-derived neurotrophic factor and ciliary neurotrophic factor on excitotoxin- and DA-induced toxicity, as well as the mechanisms involved in cell death after increased intracellular calcium in striatal neurons. I showed that two transgenic HD mice with expanded CAG repeats in exon 1 of the HD gene (R6 mice) are completely resistant to intrastriatal injections of the excitotoxin quinolinic acid (QA). However, no change in susceptibility to QA was found in transgenic HD mice expressing a 3 kb part of the HD gene, regardless of repeat length and age. Interestingly, when studying the effect of DA on R6 mice, there was an age-dependent change in susceptibility to DA-mediated toxicity. Cultures of postnatal striatal R6 neurons exhibited an increased sensitivity to DA, more cell death, production of free radicals and formation of ubiquitinated aggregates in the cytosol compared to normal neurons. When DA was injected intrastriatally into R6/1 mice, they displayed a normal sensitivity to DA at 3 weeks of age, and a partial resistance at 16 weeks of age. At the time of resistance, dopaminergic neurons from R6 mice displayed reduced soma areas in the substantia nigra pars compacta, and decreased levels of DA both at baseline and after exposure to the DA-releasing agent malonate (using microdialysis), indicating an effect of the transgene on the nigrostriatal system in these mice. Exposure of striatal neurons from R6 mice to DA induced autophagy both in vitro and in vivo. The findings in this thesis point to an important role of DA and autophagy in HD pathogenesis.