Interaction of environmental with genetic factors is assumed to play a role in the cause of sporadic Parkinson's disease (PD). Manganese (Mn) can produce a neurological syndrome similar to PD, and mutations and multiplications of alpha-synuclein (alpha-syn) cause familial parkinsonian syndromes. Also, genome-wide association studies reveal an influence of alpha-syn polymorphisms on the susceptibility to PD. We therefore exposed male transgenic C57/Bl6 mice expressing either wild-type or a doubly mutated (A53T and A30P) form of human alpha-syn under control of the 9-kb rat tyrosine hydroxylase (TH) promoter and non transgenic littermates to MnCl2 enriched (1%) or control food starting at the age of 4 months. Locomotor activity was repeatedly quantified using automated activity chambers and found to be increased by Mn without significant effect of the transgenes. Mice were sacrificed at the age of 7 or 20 months. Striatal Mn content was significantly increased about threefold. The quantification of TH positive cells in the substantia nigra pars compacta (SN p.c.) showed a significant reduction in aged mice (-10%) but no effect of Mn or transgenes (3-way analysis of variances (ANOVA) with the factors gene, Mn and age).
Neurochemical analysis of the striatum and frontal cortex in the 7 months old mice for the neurotransmitters norepinephrine (NE), dopamine (DA) and serotonin (5-HT) and the metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA), as well as the amino acids aspartate, glutamate, taurine and gamma- aminobutyric acid (GABA) revealed a significant interaction of gene and Mn only for the ratio HVA/DA and for aspartate in the striatum (2-way ANOVA with factors gene and Mn). The HVA/DA ratio and aspartate levels, which were significantly increased in human alpha-syn as compared to non transgenic mice (17 and 11%, respectively), were significantly reduced after Mn exposure by 20 and 13%, respectively, in human alpha-syn expressing, but not in non transgenic and mutated human alpha-syn mice. Analysis of the same parameters in the 20 months old mice did not give any significant changes. A significant correlation of the single values of HVA/DA with the single values of aspartate over all treatment groups (9-10 mice per group) suggest a causal relationship and are in agreement with findings on inhibition of striatal aspartate release by DA in the literature. In conclusion, under our experimental conditions, Mn and alpha-syn, wild-type and doubly mutated, did not induce signs of neurodegeneration, neither separately nor in interaction. However, Mn affected striatal DA neurotransmission through human alpha-syn, an effect which was lost by the two mutations which are pathogenetic in PD.