Neuroprotective and neurotoxic phenotypes of activated microglia in neonatal mice with respective MPTP- and ethanol-induced brain injury.

Activated microglia play important roles in the inflammatory process and progression in Parkinson's disease. These cells produce various cytokines, nitric oxide, and neurotrophins, which are pleiotropic in their action, i.e., neuroprotective or neurotoxic. In an in vivo study on a mouse model with nigrostriatal lesions produced by the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), microglia activated by systemic lipopolysaccharide (LPS) were neurotoxic toward dopamine neurons in aged mice, but unexpectedly, neuroprotective in neonatal mice. In contrast to microglia in the MPTP model, LPS-activated microglia in neonatal mice in a model made by the stereotaxic injection of ethanol into the striatum were neurotoxic, and systemic LPS administration in the ethanol-injury model caused marked increases both in the volume of necrotic lesions and in the number of degenerative neurons in the striatum. Thus, activated microglia in the neonatal mouse brain play either a neuroprotective or neurotoxic role depending on the type of brain injury.