Müller cells and microglia of the mouse eye react throughout the entire retina in response to the procedure of an intravitreal injection.

The animal model of N-methyl-D-aspartate (NMDA)-induced excitotoxic damage of retinal ganglion cells (RGC) is widely used to study the molecular mechanisms of RGC death and/or its prevention by neuroprotective agents. NMDA is typically applied by intravitreal injection, while contralateral control eyes are treated by the injection of PBS as vehicle. Herein we report that the procedure of an intravitreal injection alone is sufficient to cause substantial reactive changes in Müller cells and microglia throughout the entire retina. Six week old CD1 mice received a single intravitreal injection of PBS or NMDA. Immunohistochemistry showed the presence of reactive microglia and Müller cells in both NMDA- and PBS-treated eyes during the first 24 h after injection. After 7 days, the reactive changes were only present in NMDA-injected eyes, but no longer in PBS-treated eyes. Investigators using intravitreal injections in the mouse eye should be aware that vehicle-injected control eyes will undergo phenotypic changes in microglia and Müller glia, and are likely to behave differently in their biology when compared with uninjected eyes, at least within the first 24 h after experiment.