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Published on: July 25, 2011
Carbon monoxide treatment reduces microglial activation in the ischemic rat retina
Felix Ulbrich1,2, Ulrich Goebel1,2, Daniel Böhringer3,2
1Department of Anesthesiology and Intensive Care, Medical Center, University of Freiburg, Hugstetter Strasse 55, Freiburg im Breisgau, 79106, Germany.
Purpose:
Ischemia and reperfusion (I/R) injury damages retinal neurons. Retinal injury is accompanied by activation of microglia, which scavenge the dead or dying neurons, but increasing evidence now indicates that amoeboid-shaped microglia cells activated in the brain after ischemia have neurotoxic and damaging properties in their own right. A previous study showed that postconditioning with carbon monoxide (CO) protects retinal ganglion cells (RGCs) after I/R through anti-apoptotic and anti-inflammatory mechanisms. The present study was designed to investigate and quantify the activation of retinal microglia after I/R with and without CO postconditioning.
Methods:
Adult Sprague-Dawley rats underwent retinal ischemia by increasing the ocular pressure to 120 mmHg for 1 h through a needle inserted into the anterior chamber. Reperfusion was induced by removing the needle. After I/R, one group of animals was kept in a CO (250 ppm) atmosphere for 1 h; the other group was kept in room air (Air). At 1, 2, 3, and 7 days after I/R, the eyes were enucleated and fixed. Intracardiac blood was analyzed for systemic effects of CO or I/R. Retinal cross sections were taken from the middle third of the eye and were stained with anti-Iba-1. Microglia cells were graded as amoeboid or ramified phenotypes according to morphologic criteria. Retinal thicknesses were determined.
Results:
Evaluation of retinal tissue revealed a significant reduction of amoeboid microglia cells after I/R + CO when compared to the I/R + Air group. The peak number of amoeboid microglia was observed at day 2 post-I/R + Air. This rise was attenuated by CO postconditioning (815 versus 572 cells/mm2 for I/R + Air versus I/R + CO, respectively; p = 0.005). CO reduced and further postponed the peak in the numbers of amoeboid and ramified microglia cells in ischemic eyes and prevented microglial activation in the contralateral eyes. I/R-induced leucocytosis was inhibited by CO inhalation. The reduction of retinal thickness after I/R was more serious after Air inhalation when compared to the CO group.
Conclusions:
Numerous activated microglia cells appear in the inner retina after I/R, and CO-treatment significantly attenuates this glial response. Antagonism of microglial activation may be a further neuroprotective effect of CO, apart from its direct anti-apoptotic capacity.
