Long-Term Effect of Optic Nerve Axotomy on the Retinal Ganglion Cell Layer.

Purpose: To analyze the long-term effect of optic nerve injury on retinal ganglion cells (RGCs) and melanopsin+RGCs orthotopic and displaced, and on the rest of the ganglion cell layer (GCL) cells.
Methods: In adult albino rats, the left optic nerve was crushed (ONC) or transected (ONT). Injured and contralateral retinas were analyzed at increasing survival intervals (up to 15 months). To study all GCL cells and RGCs, retinas were immunodetected with Brn3a and melanopsin to identify the general RGC population (Brn3a+) and m+RGCs, and counter-stained with 4',6-diamidino-2-phenylindole (DAPI). Brn3a+RGCs and m+RGCs displaced to the inner nuclear layer were analyzed as well. In additional retinas, glial cells in the GCL were identified with glial fibrillary acidic protein (GFAP) or Iba1, and in some retinas, Brn3a, calretinin, and γ-synuclein were immunodetected.
Results: Orthotopic and displaced RGCs behave similarly within the RGC and m+RGC populations. Both lesions cause an exponential loss of RGCs (4%-1% survival at 6 months after ONC or ONT), but not of m+RGCs, whose number remains stable from 1 to 15 months (34%-44% of the initial population). γ-synuclein is expressed by RGCs and displaced amacrine cells (dACs), allowing us to confirm that axotomy does not affect the latter, and to determine that out of the approximately 217,406 cells that compose the GCL (excluding endothelia), 10% are glial cells, 50% dACs, and the remaining 40% are RGCs.
Conclusions: In the GCL, only RGCs are lost after axotomy, and there are important differences in the course of loss and rate of survival between melanopsin+RGCs and the rest of RGCs.