MelanopsinRGCs Are fully Resistant to NMDA-Induced Excitotoxicity.

We studied short- and long-term effects of intravitreal injection of -methyl-d-aspartate (NMDA) on melanopsin-containing (m) and non-melanopsin-containing (Brn3a) retinal ganglion cells (RGCs). In adult SD-rats, the left eye received a single intravitreal injection of 5µL of 100nM NMDA. At 3 and 15 months, retinal thickness was measured in vivo using Spectral Domain-Optical Coherence Tomography (SD-OCT). Ex vivo analyses were done at 3, 7, or 14 days or 15 months after damage. Whole-mounted retinas were immunolabelled for brain-specific homeobox/POU domain protein 3A (Brn3a) and melanopsin (m), the total number of Brn3aRGCs and mRGCs were quantified, and their topography represented. In control retinas, the mean total numbers of Brn3aRGCs and mRGCs were 78,903 ± 3572 and 2358 ± 144 (mean ± SD; = 10), respectively. In the NMDA injected retinas, Brn3aRGCs numbers diminished to 49%, 28%, 24%, and 19%, at 3, 7, 14 days, and 15 months, respectively. There was no further loss between 7 days and 15 months. The number of immunoidentified mRGCs decreased significantly at 3 days, recovered between 3 and 7 days, and were back to normal thereafter. OCT measurements revealed a significant thinning of the left retinas at 3 and 15 months. Intravitreal injections of NMDA induced within a week a rapid loss of 72% of Brn3aRGCs, a transient downregulation of melanopsin expression (but not mRGC death), and a thinning of the inner retinal layers.