Postnatal Rats Exposed To Subthreshold Doses Of Chlorpyrifos Induce Dopaminergic Neuronal Injury In The Substantia Nigra And Neural Behavioral Changes

Objective: One of the important questions in our country's public health system is whether "safety dose" of the organophosphorus pesticide is really "safe". The present study is design to explore the effects of prenatal animals exposed to doses subthreshold for overt systemic toxicity (thereafter refer as subthreshold dose) organophosphorus pesticide, chlorpyrifos (CPF) on young adult dopaminergic neurons in the midbrain substantia nigra and neural behavioral Changes.Methods: Postnatal 11 days SD rats (n＝138) were randomly divided in to CPF group (n＝46) and control group (n＝92), and control group then further divided into dimethysulfoxide (DMSO, n＝46) group and saline group (n＝46)。In order to produce animal model for CPS toxicity, postnatal 11-14 days rats were injected with CPF at subthreshold doses for overt systemic toxicity (5 mg/kg/day in 1 ml DMSO, s.c.). The control groups were injected the same doses of DMSO or saline respectively. The acute symptoms of organophosphorus pesticide intoxicates were closely observed for 12 hours after each injection and the animals were sacrificed at 15, 20, 30, and 60 days. Transmission electronic microscope was used to examine ultrastructural changes after exposure to CPE. CPF effects on midbrain dopaminergic neurons and hippocampal astrocytes were examined with immunohistochemical straining for tyrosine hydroxylase (TH), and glial fibrillary acidic protein (GFAP) alternatively. Semi-quantitative RT-PCR was used to evaluate mRNA levels of microglial surface marker (CD11b) and glial derived neural trophic factor (GDNF). Protein levels of TNF-αand IL-1βwere measured with ELISA. Elevated plus maze was used to examine the levels of animal anxiety. All data were analyzed using statistic software SPSS v12.0. All values are expressed as the mean±s.e.m. Differences among means were analyzed using one way analysis of variance (ANOVA) with time, treatment as the independent factors. When A.NOVA showed significant differences, least significant difference, LSD) t test was used. A p-value of less than 0.05 was taken as statistically significant.Results: The Prenatal exposure to subthreshold CPF produced the following significant findings in young adult rats:1. CPF induced a rapid microglial activation. The microglia under the ultrastructural changed from a resting to a phagocytotic activated microglia. Increased CD11b mRNA levels were pronounced in early CPF exposure (postnatal day 15) than later (postnatal day 20) (P＜0.05) CPF induce increased the proinflammatory cytokine TNF-α(p＜0.01) but not IL-1β(P＞0.05) in the substantia nigra.2. CPF exposure decreased level of GDNF mRNA and TH immunoreactivity (postnatal days 30-60) (P＜0.05) in the substantia nigra. 3. Prenatal CPF exposure induced astrocyte activation (P＜0.05) in the hippocampus and increased young adult rat (postnatal day 60) anxiety levels (P＜0.01).4. DMSO treatment suppressed microglial activation and increase GDNF mRNA level in the rat brain compared to saline control (P＜0.01).Significance: Our study, for the first time, demonstrated that prenatal animals exposed subthreshold does of organophosphorus pesticide, CPF, induced dopaminergic neuronal injury in young adult animals by activation of microglial cells, increase proinflammatory cytokines and decrease neurotrophic factor in the substantia nigra. This finding may shed a light on etiology of neurodegenerative diseases, such as Parkinson's disease. In addition, we found that young adult animals increase the anxiety levels after prenatal exposed to subthreshold CPF in accompany with increased astrocyte activation in the hippocampus. To our surprise, we also found that a common compound; DMSO can protect dopaminergic neurons by increase neurotrophic factor, GDNF and decreased microglial activation the substantia nigra. This finding may have potential therapeutic applications for neurodegenerative diseases, especially for Parkinson's disease.