Prolonged toxicokinetics and toxicodynamics of paraquat in mouse brain.

BACKGROUND: Paraquat (PQ) has been implicated as a risk factor for the Parkinson disease phenotype (PDP) in humans and mice using epidemiologic or experimental approaches. The toxicokinetics (TK) and toxicodynamics (TD) of PQ in the brain are not well understood. OBJECTIVES: The TK and TD of PQ in brain were measured after single or repeated doses. METHODS: Brain regions were analyzed for PQ levels, amount of lipid peroxidation, and functional activity of the 20S proteasome. RESULTS: Paraquat (10 mg/kg, ip) was found to be persistent in mouse ventral midbrain (VM) with an apparent half-life of approximately 28 days and was cumulative with a linear pattern between one and five doses. PQ was also absorbed orally with a concentration in brain rising linearly after single doses between 10 and 50 mg/kg. The level of tissue lipid peroxides (LPO) was differentially elevated in three regions, being highest in VM, lower in striatum (STR), and least in frontal cortex (FCtx), with the earliest significant elevation detected at 1 day. An elevated level of LPO was still present in VM after 28 days. Despite the cumulative tissue levels of PQ after one, three, and five doses, the level of LPO was not further increased. The activity of the 20S proteasome in the striatum was altered after a single dose and reduced after five doses. CONCLUSIONS: These data have implications for PQ as a risk factor in humans and in rodent models of the PDP.

Proteasome dysfunction in aged human alpha-synuclein transgenic mice.

A deficit in proteasome function in Parkinson's disease has been speculated. We characterized the ubiquitin-proteasome system in three regions of brain from transgenic and nontransgenic littermates. Mice expressing a doubly mutated form of human alpha-synuclein had significant impairments whereas mice expressing the wild-type gene had lesser changes compared to nontransgenic littermates. Significant abnormalities in line hm2 alpha-SYN-39 included declines in 20S-mediated proteolytic activity, the level of the 19S proteasome subunits Rpt1 and Rpn2, and the level of soluble total high MW ubiquitin cross-reacting proteins. Line hw alpha-SYN-5 had significant, but restricted proteasome abnormalities. The severity of impairment was proportional to the substantia nigra dopaminergic neuronal loss previously identified. There were significant correlations between the level of Rpn2 with the level of Rpt1, the activity of the 20S proteasome, and the level of soluble high MW ubiquitin cross-reacting proteins. These abnormalities in symptomatic line hm2 alpha-SYN-39 mice are consistent with abnormalities identified in tissue from patients with Parkinson's disease.

Increased synaptosomal dopamine content and brain concentration of paraquat produced by selective dithiocarbamates.

Exposure to pesticides may be a risk factor for Parkinson's disease based on epidemiologic data in humans, animal models and in vitro studies. Different dithiocarbamate pesticides potentiate the toxicity of both 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and paraquat in mouse models of Parkinsonism by an unknown mechanism. This study examined the effects of commercially used dithiocarbamates on [3H]dopamine transport in striatal synaptosomal vesicles and on the concentration of [14C]paraquat in vivo in mice. Different ethylenebis-dithiocarbamates and diethyl-dithiocarbamate increased dopamine accumulation in synaptosomes, whereas dimethyl-dithiocarbamate and methyl-dithiocarbamate did not. Increased dopamine accumulation in synaptosomes was dose dependent and was related to the carbon backbone of these molecules. The dithiocarbamates that increased accumulation of dopamine did not alter the influx of dopamine, but rather delayed the efflux out of synaptosomes. These same dithiocarbamates also increased the tissue content of [14C]paraquat in vivo by a mechanism that appeared to be distinct from the dopamine transporter. There was a consistent relationship between the dithiocarbamates that increased synaptosomal accumulation of dopamine and tissue content of paraquat, with those previously demonstrated to enhance paraquat toxicity in vivo. These results suggest that selective dithiocarbamates may alter the kinetics of different endogenous and exogenous compounds to enhance their neurotoxicity.

Behavioral and neurochemical effects of wild-type and mutated human alpha-synuclein in transgenic mice.

Human alpha-synuclein (halpha-SYN) is implicated in the Parkinson's disease phenotype (PDP) based on a variety of studies in man, animal models, and in vitro studies. The normal function of halpha-SYN and the mechanism by which it contributes to the PDP remains unclear. We created transgenic mice expressing either wild-type (hwalpha-SYN) or a doubly mutated (hm2alpha-SYN) form of halpha-SYN under control of the 9-kb rat tyrosine hydroxylase promoter. These mice expressed halpha-SYN in cell bodies, axons, and terminals of the nigrostriatal system. The expression of halpha-SYN in nigrostriatal terminals produced effects in both constructs resulting in increased density of the dopamine transporter and enhanced toxicity to the neurotoxin MPTP. Expression of hm2alpha-SYN reduced locomotor responses to repeated doses of amphetamine and blocked the development of sensitization. Adult hwalpha-SYN-5 transgenic mice had unremarkable dopaminergic axons and terminals, normal age-related measures on two motor coordination screens, and normal age-related measures of dopamine (DA) and its metabolites. Adult hm2alpha-SYN-39 transgenic mice had abnormal axons and terminals, age-related impairments in motor coordination, and age-related reductions in DA and its metabolites. Expression of hm2alpha-SYN adversely affects the integrity of dopaminergic terminals and leads to age-related declines in motor coordination and dopaminergic markers.