The retino–retinal projection: Tracing retinal ganglion cells projecting to the contralateral retina

•A direct retino–retinal projection between the eyes via the optic chiasm, as evidenced by retino–retinal ganglion cells, persists into adulthood in rodents.•Optic nerve stump labeling yields greater numbers of retino–retinal ganglion cells.•Retino–retinal ganglion cells can have collateral axon projections to the brain. We investigated the presence of a direct retino–retinal (R–R) projection between the two eyes via the optic chiasm of retinal ganglion cells (RGCs) in adult Long-Evans rats. We also explored the presence of collateral projections originating from these cells to the brain. In the first group of animals, right optic nerves (ONs) were orbitally transected approximately 2mm behind the globe followed by application of fluorochrome (2% Fluorogold [FG]) to the optic nerve stump to retrogradely label the R–R projection RGCs (R–RGCs) on the contralateral side. Animals were then sacrificed after 3, 5, 7, or 21 days. Contralateral retinas were fixed, whole-mounted, and imaged for R–RGCs. In a second group of animals, RGCs were retrogradely labeled with 15% rhodamine-β-isothiocynate (RITC) at the superior colliculi, where approximately 96% of rat RGCs synapse. Seven days later, the right ONs were transected and 2% FG applied to the proximal and distal ON stumps. Animals were then sacrificed after 5 days. Contralateral retinas were examined for co-labeled (RITC/FG) RGCs. Control rats underwent the same procedures excluding fluorescent tracer application. In the first group of animals, the number of R–RGCs in the contralateral eye ranged from 3 to 25 and did not depend on survival time. The second group of animals revealed evidence of co-labeled contralateral RGCs. Results suggest that a greater number of R–RGCs persist into adulthood than previously reported [M. Müller, H. Holländer, 1988]. Furthermore, the presence of co-labeled RGCs in the contralateral eye indicates that in adult rodents some R–R projections have a collateral projection to the brain, whereas previous reports had only found collateral projections in newborns.