Centrifugal fibers are the only GABAergic structures of the retina of the larval sea lamprey: an immunocytochemical study.

The structures of the retina immunoreactive to GABA are described in larval lamprey. Although GABAergic cells develop early in the retinas of vertebrates, no GABA-immunoreactive perikarya were observed in the retina of lamprey larvae. The only GABA-immunoreactive structures were beaded fibers of the centrifugal system, which produced a dense plexus at the level of the optic fiber/inner plexiform layer in both the central (photoreceptor-bearing) and lateral (no-photoreceptor) parts of the retina. These fibers do not ascend toward the outer plexiform layer. Nerve fibers in the optic nerve and neuronal perikarya of the M5 nucleus of the mesencephalon, which is known to project to the retina, were also GABA-immunoreactive. The distribution of centrifugal fibers closely matches that of ganglion cells revealed by retrograde labelling with fluorescein-coupled dextran-amine, and the presence of biplexiform ganglion cells in larvae is confirmed. That the ganglion cells and the centrifugal fibers appears to be the only structures differentiated in the lateral retina of the larva suggests that the GABAergic centrifugal fibers may have a role, perhaps the neurotrophic maintenance of retinal ganglion cells, during the very long larval phase of lampreys.