Short and long term effects of NMDA‐induced retinal excitotoxicity on melanopsin and non‐melanopsin containing retinal ganglion cells

AbstractPurposeTo study short and long term responses of the population of RGCs expressing Brn3a (Brn3a+RGCs) and the population of intrinsically photosensitive RGCs expressing melanopsin (m+RGCs) to excitotoxicity induced by intravitreal injection of N‐methyl‐D‐Aspartate (NMDA).MethodsIn adult albino Sprague Dawley rats, the left eye received an intraocular injection of 100nM NMDA and the retinas were analyzed 3 (n=9), 7 (n=6), 14 (n=10) days (d) or 15 (n=23) months (m) later. Spectral Domain Optical Coherence Tomography (SD‐OCT; Spectralis, Heidelberg) was used to image and analyze retinal thickness longitudinally in vivo at short (3 m; n=18) and long (15 m; n=18) survival intervals. Ex vivo, retinal whole mounts were immunodetected with antibodies against Brn3a and melanopsin and the numbers of surviving Brn3a+RGCs and of m+RGCs were automatically counted and represented using isodensity or neighbour maps.ResultsIntraocular injection of 100nM NMDA causes a massive loss of Brn3a+RGCs; at 3 and 14 days Brn3a+RGCs numbers diminished to 50% and 25%respectively, but there was no further loss up to 15 months. However, m+RGCs showed 3 d after injection a transient downregulation of melanopsin that recovered shortly and by 14 d and at the end of the study (15 m) the numbers of m+RGCs were comparable to their contralateral fellow eyes. SD‐OCT examination showed important reductions of the total and inner retinal thicknesses at 3 m that progressed further up to 15 m.ConclusionThe population of Brn3a+RGCs is quite sensitive to NMDA‐induced excitotoxicity that causes rapidly the loss of approximately 75% of these neurons. In contrast, m+RGCs appear fully resistant to NMDA‐induced excitotoxicity and show only a downregulation of melanopsin expression during the first two weeks. SD‐OCT can be used to assess retinal toxicity revealed by important reductions in total and inner retinal thickness at 3 m progressing up to 15 m.