Investigating the effects of inflammation on dopaminergic neurons in the lipopolysaccharide model of Parkinson’s disease

Parkinson’s disease is a common neurodegenerative disease that particularly targets the dopaminergic neurons of the substantia nigra pars compacta. While the specific aetiology of Parkinson’s disease is unknown, neuroinflammation is thought to have a significant role in the development of the disease. The work in this thesis examines the molecular events that occur in the brain in the peripheral lipopolysaccharide injection model of Parkinson’s disease. In this model, lipopolysaccharide is injected into the intraperitoneal region. The focus of the work in this thesis was on the pre-degenerative (prodromal) phase of the disease process and so the molecular events that occurred one month after induction of inflammation by lipopolysaccharide injection were studied. The effect of lipopolysaccharide induced inflammation in the brain on the oxidative stress markers 4-hyroxynonenal and 3-nitrotyrosine were determined. No changes for 4-hyroxynonenal were observed in either male or female rats, in the substantia nigra or the ventral tegmental area. For 3-nitrotyrosine, no differences were observed in ether the substantia nigra or the ventral tegmental area for female rats. However, there was a significant decrease in the levels of 3-nitrotyrosine in the substantia nigra and a significant increase in the levels of 3-nitrotyrosine in the ventral tegmental area in the male rats. The expression of 79 genes that are implicated in Parkinson’s disease was assessed in male and female rats in both the substantia nigra and ventral tegmental area. There were six genes in the substantia nigra of male rats that were differentially expressed with each showing a downregulation of two-fold or more; including, Brain acid soluble protein, Synaptogyrin 3, Transcription factor 7 like 2, Calcium dependent protein secretion activator 2, and Vesciular monoamine transporter 2. In the ventral tegmental area of male rats, there were seven genes that were differentially expressed, with Neurofilament light chain and S-phase kinase protein 1 showing downregulation of two-fold or more. The analysis of gene expression in the females showed that apart from a 1.3-fold upregulation of Synaptogyrin 3, none of the other 78 genes studied showed differential expression in either the substantia nigra or the ventral tegmental area. The protein levels of Brain acid soluble protein, Synaptogyrin 3, and Vesicular monoamine transporter 2 were determined in the substantia nigra and ventral tegmental area regions in both male and female rats, one month after LPS injection. There were no significant differences observed for Synaptogyrin 3 in either the substantia nigra or the ventral tegmental area of male and female rats. For Vesciular monoamine transporter 2, no significant differences were observed in the substantia nigra and ventral tegmental area of male rats and in the ventral tegmental area of female rats; however, an increase in the levels of Vesicular monoamine transporter 2 was observed in the substantia nigra of female rats. In respect to Brain acid soluble protein, no significant differences were observed in the substantia nigra of male and female rats and in the ventral tegmental area of female rats; however, an increase in the levels of Brain acid soluble protein was observed in the ventral tegmental area of male rats. These results suggest that the effects of lipopolysaccharide induced inflammation in the substantia nigra and the ventral tegmental area are mutually exclusive and that they respond differently to lipopolysaccharide. In addition, the data also suggest that there is sexual dimorphic response to lipopolysaccharide induced inflammation. The effects of lipopolysaccharide induced inflammation in the brain were further studied by examining the effect of induction of inflammation on the whole transcriptome of the substantia nigra in male rats. The results showed that less than three percent of the genes in the substantia nigra were differentially expressed in the lipopolysaccharide treated rats. The results from the transcriptome study were subjected to bioinformatics analysis to delineate the pattern of changes of gene expression. Analysis of the mRNA expression of target genes showed that complement component 3 mRNA was upregulated in both the substantia nigra and the ventral tegmental area of male rats. As complement component 3 is a key intersection point in the complement pathway this suggests that complement activation is a key part of the chronic changes that occur the brain prior to the degeneration of the dopaminergic neurons in this Parkinson’s disease model.