Down-regulation of microglial activity attenuates axotomized nigral dopaminergic neuronal cell loss.

BACKGROUND: There is growing evidence that inflammatory processes of activated microglia could play an important role in the progression of nerve cell damage in neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease which harbor features of chronic microglial activation, though the precise mechanism is unknown. In this study, we presented in vivo and ex vivo experimental evidences indicating that activated microglia could exacerbate the survival of axotomized dopaminergic neurons and that appropriate inactivation of microglia could be neuroprotective. RESULTS: The transection of medial forebrain bundle (MFB) of a rat induced loss of dopaminergic neurons in a time-dependent manner and accompanied with microglial activation. Along with microglial activation, production of reactive oxygen species (ROS) was upregulated and TH/OX6/hydroethidine triple-immunofluorescence showed that the microglia mainly produced ROS. When the activated microglial cells that were isolated from the substantia nigra of the MFB axotomized animal, were transplanted into the substantia nigra of which MFB had been transected at 7 days ago, the survival rate of axotomized dopaminergic neurons was significantly reduced as compared with sham control. Meanwhile, when the microglial activation was attenuated by administration of tuftsin fragment 1-3 (microglia inhibitory factor) into the lateral ventricle using mini-osmotic pump, the survival rate of axotomized dopaminergic neurons was increased. CONCLUSION: The present study suggests that activated microglia could actively produce and secrete unfavorable toxic substances, such as ROS, which could accelerate dopaminergic neuronal cell loss. So, well-controlled blockade of microglial activation might be neuroprotective in some neuropathological conditions.

Co-localization of activating transcription factor 3 and phosphorylated c-Jun in axotomized facial motoneurons.

Activating transcription factor 3 (ATF3) and c-Jun play key roles in either cell death or cell survival, depending on the cellular background. To evaluate the functional significance of ATF3/c-Jun in the peripheral nervous system, we examined neuronal cell death, activation of ATF3/c-Jun, and microglial responses in facial motor nuclei up to 24 weeks after an extracranial facial nerve axotomy in adult rats. Following the axotomy, neuronal survival rate was progressively but significantly reduced to 79.1% at 16 weeks post-lesion (wpl) and to 65.2% at 24 wpl. ATF3 and phosphorylated c-Jun (pc-Jun) were detected in the majority of ipsilateral facial motoneurons with normal size and morphology during the early stage of degeneration (1-2 wpl). Thereafter, the number of facial motoneurons decreased gradually, and both ATF3 and pc-Jun were identified in degenerating neurons only. ATF3 and pc-Jun were co-localized in most cases. Additionally, a large number of activated microglia, recognized by OX6 (rat MHC II marker) and ED1 (phagocytic marker), gathered in the ipsilateral facial motor nuclei. Importantly, numerous OX6- and ED1-positive, phagocytic microglia closely surrounded and ingested pc-Jun-positive, degenerating neurons. Taken together, our results indicate that long-lasting co-localization of ATF3 and pc-Jun in axotomized facial motoneurons may be related to degenerative cascades provoked by an extracranial facial nerve axotomy.