Parkinson's disease (PD) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity are both associated with dopaminergic neuron death in the substantia nigra (SN). Apoptosis has been implicated in this cell loss; however, whether or not it is a major component of disease pathology remains controversial. Caspases are a major class of proteases involved in the apoptotic process. To evaluate the role of caspases in PD, we analyzed caspase activation in MPTP-treated mice, in cultured dopaminergic cells, and in postmortem PD brain tissue. MPTP was found to elicit not only the activation of the effector caspase-3 but also the initiators caspase-8 and caspase-9, mitochondrial cytochrome c release, and Bid cleavage in the SN of wild-type mice. These changes were attenuated in transgenic mice neuronally expressing the general caspase inhibitor protein baculoviral p35. These mice also displayed increased resistance to the cytotoxic effects of the drug. MPTP-associated toxicity in culture was found temporally to involve cytochrome c release, activation of caspase-9, caspase-3, and caspase-8, and Bid cleavage. Caspase-9 inhibition prevented the activation of both caspase-3 and caspase-8 and also inhibited Bid cleavage, but not cytochrome c release. Activated caspase-8 and caspase-9 were immunologically detectable within MPP(+)-treated mesencephalic dopaminergic neurons, dopaminergic nigral neurons from MPTP-treated mice, and autopsied Parkinsonian tissue from late-onset sporadic cases of the disease. These data demonstrate that MPTP-mediated activation of caspase-9 via cytochrome c release results in the activation of caspase-8 and Bid cleavage, which we speculate may be involved in the amplification of caspase-mediated dopaminergic cell death. These data suggest that caspase inhibitors constitute a plausible therapeutic for PD.

Environmental exposure to the oxidant-producing herbicide paraquat has been implicated as a risk factor in Parkinson's disease. Although intraperitoneal paraquat injections in mice cause a selective loss of dopaminergic neurons in the substantia nigra pars compacta, the exact mechanism involved is still poorly understood. Our data show that paraquat induces the sequential phosphorylation of c-Jun N-terminal kinase (JNK) and c-Jun and the activation of caspase-3 and sequential neuronal death both in vitro and in vivo. These effects are diminished by the specific JNK inhibitor SP600125 and the antioxidant manganese(III) tetrakis (4-benzoic acid) porphyrin in vitro. Furthermore, JNK pathway inhibitor CEP-11004 effectively blocks paraquat-induced dopaminergic neuronal death in vivo. These results suggest that the JNK signaling cascade is a direct activator of the paraquatmediated nigral dopaminergic neuronal apoptotic machinery and provides a molecular linkage between oxidative stress and neuronal apoptosis.

Exposure of mice to the herbicide paraquat has been demonstrated to result in the selective loss of dopaminergic neurons of the substantia nigra, pars compacta (SNpc) akin to what is observed in Parkinson disease (PD). In this study, we investigate the efficacy of two synthetic superoxide dismutase/catalase mimetics (EUK-134 and EUK-189) in protecting against paraquatinduced dopaminergic cell death in both the rat dopaminergic cell line 1RB 3 AN 27 (N27) and primary mesencephalic cultures in vitro and in adult mice in vivo. Our data demonstrate that pretreatment with either EUK-134 or EUK-189 significantly attenuates paraquat-induced neurotoxicity in vitro in a concentration-dependent manner. Furthermore, systemic administration of EUK-189 decreases paraquat-mediated SNpc dopaminergic neuronal cell death in vivo. These findings support a role for oxidative stress in paraquat-induced neurotoxicity and suggest novel therapeutic approaches for neurodegenerative disorders associated with oxidative stress such as PD.

Extensive epidemiological data in humans and studies in animal models of Parkinson's disease (PD) suggest that sporadic forms of the disorder are not strictly genetic in nature but most likely because of combined environmental exposures over the period of the life-span coupled with increased genetic susceptibilities. Environmental paraquat and neonatal iron exposure have both been separately suggested as potential risk factors for sporadic forms of the disease. In this study, we demonstrate that combined environmental exposure to these two agents results in accelerated age-related degeneration of nigrostriatal dopaminergic neurons. Furthermore, pretreatment with the synthetic superoxide dismutase/catalase mimetic, EUK-189, significantly attenuated neuronal death mediated by combined paraquat and iron treatment. These findings support the notion that environmental PD risk factors may act synergistically to produce neurodegeneration associated with the disorder and that iron and paraquat may act via common oxidative stress-mediated mechanisms.

Environmental paraquat and neonatal iron exposure have both separately been suggested as potential risk factors for sporadic forms of Parkinson's disease (PD). In this study, we demonstrate that combined environmental exposure to these two agents results in modulations in microglial activation state. Apocynin, an NADPH oxidase inhibitor, was found to attenuate the release of superoxide from microglia stimulated by combined paraquat and iron and blocked paraquat-induced dopaminergic neuronal death. Furthermore, pretreatment with the synthetic superoxide dismutase/catalase mimetic, EUK-189, significantly decreased microglial activation mediated by combined paraquat and iron treatment. These findings support the notion that environmental PD risk factors may act synergetically to produce neurodegeneration associated with the disorder and that iron and paraquat may act via common oxidative stress-mediated mechanism involving microglial activation.