Study On Molecular Mechanism Of Nicotine Against The Neurotoxicity Induced By MPTP/MPP~+

Parkinson’s disease (PD) is a common neurodegenerative disease in the elderly population, prevalence rate is1-2%o in the world. The major pathological features are loss of dopaminergic neurons in the substantia nigra pars compacta (SNc) in brain, the accumulation of Lewy’s body in the neurons, leading to reduce of dopamine secretion. The clinical symptoms include postural instability, rigidity, resting tremor, bradykinesia and cognitive disorder in serious cases. Its etiology includes environmental factors, genetic factors. At present, the main treatment of PD is the replacement therapy of the L-DOPA, but its effect is not so good. The reason is the molecular mechanism on PD is still unknown.The study on the mechanism of PD depends on the establishment of the animal model and the cell model of PD.1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is commonly used in animals to produce an experiment model of PD. The1-meythyl-4-phenylpyridinium ion (MPP+), the active metabolite of MPTP, is transported in dopaminergic terminal neuron and binds complex I NADH dehydrogenase in mitochondrial membrane-bound, inhibits its activity, causes ATP depletion, and leads to the increase of reactive oxygen species (ROS), finally results in the neuronal death. Besides the mitochondrial damage and the production of ROS, the pathologic feature includes the accumulation of Lewy’s body resulting from the abnormal protein in cytoplasm. The metabolism of protein is associated with the endoplasmic reticulum. Thus, endoplasmic reticulum stress (ER stress) plays important roles in occurrence and development of PD.Thioredoxin (Trx) is a redox regulating protein with low molecular weight, regulates the balance of intracellular redox state scavenges ROS, regulates the transcription factors, inhibits apoptosis and provides the neurotrophic function.Nicotine is the main ingredient in tobacco alkaloids, which has widely biological effects. Nicotine participates in the development of the nervous system, respiratory system and the cardiovascular system, and has a close relationship with carcinogenesis. Nicotine exerts its biological effects by regulating the Nicotinic Acetylcholine Receptor (nAChR). Epidemiological studies indicate that smoking can reduce the incidence of Parkinson’s disease, and nicotine is the main ingredient of tobacco, therefore scientists indicate that nicotine may have the neuroprotective role. There were also some experimental and clinical studies showing that nicotine can prevent the Parkinson’s disease. As early as1996, studies had found that nicotine can protect the neurons by inhibiting apoptosis. Nicotine uses the Nicotinic Acetylcholine Receptor to activate the downstream signaling pathways to inhibit the cell apoptosis. Nicotine also can inhibit the apoptosis induced by opium, morphine and methadone, and nAChR antagonists block the effect of nicotine.In the present study, PD model established with MPTP/MPP+was used to explore the molecular mechanisms how nicotine protecting the neuron from MPTP/MPP+neurotoxicity.The major results were as follows:(1) Nicotine inhibited the decline of Trx-1and ER stress induced by MPP+. In the rat pheochromocytoma cells (PC12cells), pre-treatment with the nicotine could inhibit the toxicity by MPP+, and reverse the decline of Trx-1induced by MPP+Nicotine inhibited the activations of CHOP, Bip induced by MPP+. These results indicate nicotine can reverse the expression of Trx-1, inhibit the ER stress and cell toxicity induced by MPP+(2) Nicotine inhibited the neuronal apoptosis induced by MPP+through PI3K/Akt and a7nAChR. In the PC12cells, the antagonists of PI3K/Akt and ct7nAChR, LY294002and MLA, both abolished the antagonistic action of nicotine in the decline of Trx-1induced by MPP+. Besides, MLA blocked the antagonistic action of nicotine in the increase of Bip induced by MPP+. These results indicate nicotine inhibits the neuronal apoptosis induced by MPP+through PI3K/Akt and a7nAChR.(3) Nicotine inhibited the neurotoxicity induced by MPTP in vivo. In the behavioral experiment, the motor activity of mice of the MPTP group reduced significantly campared to the saline group. The pretreatment with nicotine ameliorated the dyskinesia induced by MPTP. Nicotine also ameliorated MPTP-induced impairment of limb movements scored by traction tests. These results indicate that nicotine has the ability to improve MPTP-induced functional impairments. Tyrosine hydroxylase (TH) is the maker of dopaminergic neuron. TH expression in the SNc was significantly decreased after treatment of MPTP, however pretreatment with nicotine rescued MPTP-induced down-regulation of TH. After MPTP treatment, Trx expression was decreased, and the downregulation of Trx was restored by nicotine pretreatment. Nicotine ameliorated the neuron damage induced by MPTP through the induction of Trx expression. MPTP administration increased the expressions of CHOP and Bip, resulted in the neuron apoptosis. The pretreatment with nicotine suppressed CHOP and Bip activations caused by MPTP. These data indicates that nicotine can improve functional impairment induced by MPTP, inhibit the decline of TH.Conclusion:nicotine inhibited the decline of Trx-1and ER stress induced by MPP+; nicotine inhibited the neuronal apoptosis induced by MPP+through PI3K/Akt and a7nAChR; nicotine inhibited the decline of TH induced by MPTP; nicotine ameliorated the functional impairment induced by MPTP.