Neuroinflammation, mitochondrial dysfunction and selective loss of dopaminergic neurons have been consistently observed in the substantia nigra of PD patients 
. However, the correlation among these three in PD remains unclear. Here, we showed that neuroinflammation is able to mediate mitochondrial impairment by S-nitrosylation/nitration of mitochondrial proteins. This is followed by progressive dopaminergic neurodegeneration in the nigrostriatal system.
The loss of the dopaminergic neurons might be attributable to the intrinsic sensitivity of dopaminergic neurons to compromised mitochondrial function. Greenamyre and colleagues reported that systemic administration of a complex I inhibitor, rotenone to rats produced a selective degeneration of dopaminergic neurons in the substantia nigra 
. Consistent with their report, we recently found that trichloroethylene causes selective loss of the nigral dopaminergic neurons in rats via complex I inhibition, and long term exposure to the chemical may be related to the development of parkinsonism in a group of factory workers 
. Furthermore, mitochondrial dysfunction alone is sufficient to initiate parkinsonism in conditional knock-out mice by disrupting the gene for mitochondrial transcription factor A in the nigral dopamine neurons 
. These observations are in agreement with the fact that rare familial forms of PD are related to mutations in the gene encoding PINK1 or DJ1, which both regulate mitochondrial function 
. Thus, it can be questioned, what makes the nigral dopaminergic neurons substantially susceptible to mitochondrial dysfunction. A recent study showed that nigral dopamine neurons unusually rely on L-type voltage-gated calcium ion channels for basal activity, and the reliance increases with age 
. The high dependency on the calcium channel leads to sustained elevation in cytosolic calcium concentration, which enhances mitochondrial respiration, reactive oxygen species generation, and ATP demand 
. Therefore, the nigral dopaminergic neurons can be devastated by mitochondrial insults, which are tolerable to the other populations of neurons.
The implication of NO in PD pathogenesis is supported by the observations that iNOS expression is upregulated in activated microglia 
, the immunoreactivity of nitrated α-synuclein is prominently positive in the Lewy bodies 
, and several enzymes including parkin, peroxiredoxin-2, and protein-disulphide isomerase lose their function by S-nitrosylation in PD brains 
. Although association of NO with mitochondrial dysfunction in PD is unclear, NO and its metabolite, peroxynitrite can compromise mitochondrial biomolecules to induce impairment in mitochondrial respiration. It has been well known that NO reversibly inhibit cytochrome c oxidase activity and depletes mitochondrial antioxidants such as glutathione 
. Importantly, peroxynitrite and high level of NO irreversibly inactivates mitochondrial complex I activity by tyrosine nitration, S-nitrosylation, oxidation of residues and damage of iron sulfur center 
. These notions are in agreement with our results that intrastriatal LPS injection leads to complex I nitration and S-nitrosylation, and is accompanied decrease in mitochondrial respiration rate. This evidence strongly supports the hypothesis that excessive production of NO in the brain might contribute to mitochondrial dysfunction and subsequent neuronal energy deficiency observed in PD.
Inflammatory activation of microglia is related to increases in release of glutamate and consequent excitotoxicity 
. It has been reported that neuronal NOS (nNOS)-derived NO accounts, at least in part, for glutamate-induced excitotoxicity 
. We found that activated microglia appears one week after LPS challenge, and the microgliosis were sustained for four weeks in the substantia nigra. In addition, we observed that treatment of L-N(G)-nitro-arginine, an inhibitor of nNOS, was also neuroprotective against intrastriatal LPS in mice, suggesting the possible contribution of nNOS to the dopamine neurodegeneration 
. This is further supported by our observation of cytoplasmic accumulation of α-synuclein and ubiquitin in the nigral dopamine-producing neurons, as extensive nitrosative/oxidative stress might cause impairment of the ubiquitin proteasome system resulting in accumulation of misfolded proteins 
. Support to the notion comes from the result that the accumulation of α-synuclein was attenuated by treatment with L-NIL (unpublished data). Hence, our results might be relevant for the molecular pathway for Lewy body formation and PD pathogenesis.
It is interesting that loss of a relatively small population of nigral dopamine neurons caused a decrease in the striatal dopamine level and behavioral impairment in the LPS-treated animals. The behavioral deficit began even at early time-point like one week following LPS injection. This phenomenon may suggest that the striatal inflammation antagonizes the function of the dopaminergic nigrostriatal pathway via altering synthesis and/or release of dopamine, or by modulating dopamine-mediated signal transduction before a significant demise of the dopaminergic neurons occurs. In support of this suggestion, it was shown that MPTP treatment induces striatal TH nitration, which was related to inactivation of the enzyme and a subsequent greater decline in dopamine level, compared to the loss of the dopaminergic neurons 
. In addition, direct injection of proinflammatory mediators into the striatum such as prostaglandin D2 and the thromboxane A2 agonist induced impairment of motor behavior; although, the specific mechanism was not elucidated 
. Therefore, further study is required to fully illuminate the mechanism by which neurochemical alterations and behavioral deficit occur following intrastriatal LPS injections.
Taken together, results of the present study provide strong support to the hypothesis that neuroinflammation may significantly contribute to PD pathogenesis. This animal model may be useful for studying neuroinflammatory mechanisms by which nigral dopamine neurons degenerate in PD.