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Sandra Kuehn, Cara Rodust, Gesa Stute, H Burkhard Dick, Stephanie C Joachim; Late optic nerve damage and long-lasting microglia-activation in an NMDA model. Invest. Ophthalmol. Vis. Sci. 2016;57(12):2236.
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© 2017 Association for Research in Vision and Ophthalmology.
In low doses glutamate is an important neurotransmitter, but large quantities lead to degeneration. The intravitreal injection of N-Methyl-D-Aspartate (NMDA), a glutamate analogue, is an established model for fast retinal degeneration. The late mechanisms were still not evaluated. Hence, we interested the late phase of this model, with a focus on optic nerve damage and glia response.
40 or 80 nmol NMDA or PBS (n=5/group) were intravitreally injected in rats. At day 3, retinal ganglion cells (RGCs, Brn-3a) and microglia (Iba1) were labelled on flatmounts. At 14 days, apoptotic (cleaved caspase 3) RGCs (Brn-3a) were stained on cross-sections. Optic nerve neurofilaments (SMI-32) and myelin fibers (LFB) were labelled on longitudinal-sections. Further apoptotic molecules, FasL and Fas, were detected in retina and optic nerve. Additionally, the total amount of microglia (Iba1) and the ones in an active state (ED1) were counted. The results were later analyzed (ANOVA, Tukey post-hoc test).
At day 3, RGCs were decreased (40 nmol: p<0.001) and microglia increased (40 nmol: p<0.001). Both, 40 (p=0.008) and 80 nmol NMDA (p<0.001), led to a decrease of RGCs, but did not change their apoptotic state (FasL, cl. caspase 3: p>0.05) at day 14. The optic nerve neurofilament degenerated in both groups (p<0.001), while the myelin fibers were not affected at this point. However, the apoptotic processes were present in the optic nerve. An increased level of co-localized FasL and Fas cells was measured in the 80 nmol group (p=0.01) and more cleaved caspase 3+ DAPI cells were noted in both NMDA groups (40 nmol: p<0.001, 80 nmol: p=0.003). Interestingly, the microglia were increased and still activated in retina and optic nerve of both groups (Iba1/ED1: p<0.01) at this advanced stage.
The effects of 40 und 80 nmol NMDA were comparable for the retina, but not really in the optic nerve. Apoptotic RGC loss is a known mechanism for this model, but the apoptotic rate is decreased in the retina at later states. However, the optic nerves degeneration still continues, especially in the 80 nmol group. Surprisingly, the microglia remained activated in both tissues at this late point. Likely, oxidative stress and degeneration of other retinal neurons lead to a continuous activation of microglia.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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