Nitric oxide-producing cells project from retinal grafts to the inner plexiform layer of the host retina

Amacrine cells expressing nitric oxide synthase (NOS) are seen in normal retinas and retinal grafts to extend long processes, which can be followed for long distances. Taking advantage of the morphologic features of these cells, the present study examined whether graft-host connections involve cells capable of producing nitric oxide, a recognized retinal neuromodulatory compound.

Embryonic day 15 rabbit retinas were transplanted to the subretinal space of adult rabbits. The localization of the neuronal form of NOS was assessed by immunocytochemistry in grafts that had reached the equivalent ages of postnatal days 5, 12, 20, 45, 90, and 102.

NOS-containing cells and processes were seen in all the transplants. Processes were found to project mainly toward areas within the graft. Yet, at all survival times examined, single immunolabeled fibers could be seen to cross the graft- host border. In fortuitous cases, it was possible to establish that the bridging fiber originated in the graft. Further, bridging fibers were seen to reach the NOS-immunolabeled host inner plexiform layer.

Graft NOS-containing cells are not only capable of projecting into the host but also of reaching the appropriate target for NOS-containing fibers within the host retina. This indicates that at least some graft-host connections are established by graft cells that retain their ability to synthesize a modulatory compound and which potentially could contact their partner cells in the host retina.