Melanopsin⁺RGCs Are Fully Resistant to NMDA-Induced Excitotoxicity

Abstract

We studied short- and long-term effects of intravitreal injection of N-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 SD-OCT.  Ex vivo analyses were done at 3, 7, 14 days or 15 months after damage. Whole-mounted retinas were immunolabelled for Brn3a and melanopsin, the total number of Brn3a+RGCs and m+RGCs were quantified and their topography represented. In control retinas, the mean total numbers of Brn3a+RGCs and m+RGCs were 78,903±3,572 and 2,358±144 (mean ± SD; n=10), respectively. In the NMDA injected retinas, Brn3a+RGCs numbers diminished to 50% and 25%, at 3 and 14 days, respectively, but there was no further loss up to 15 months. The number of immunoidentified m+RGCs decreased significantly at 3 days, recovered between 3-7 days and was back to normal thereafter. OCT measurements revealed a significant thinning of the left retinas at 3 and 15 months. Intravitreal injections of NMDA induce a rapid loss of 75% of Brn3a+RGCs, a transient downregulation of melanopsin expression but not m+RGC death, and a thinning of the inner retinal layers.

Alpha retinal ganglion cells in pigmented mice retina: number and distribution

“…It has been documented that ONs-αRGCs are a type of intrinsically photosensitive ganglion cells, the M4-ipRGCs (Ecker et al, 2010;Schmidt et al, 2014;Sonoda et al, 2020). However, these cells are not easy to identify with the classical OPN4 antibody against melanopsin, which identifies M1, M2, and M3 ipRGCs, as they do not express sufficient melanopsin photopigment (Vidal-Villegas et al, 2019, 2021a, although their functional dependence on pigment has been characterized (Estevez et al, 2012), which has led to a lack of global consensus on the overall identification of the total M4-ipRGC population and the number of cells per retina in rodents. It has recently been reported that cells expressing the photopigment also express the transcription factor T brain 2 (Tbr2), which is crucial for the maintenance of these cells (Berg et al, 2019;Tran et al, 2019;Chen et al, 2021).…”