CENTRIFUGAL FIBERS ARE THE ONLY GABAERGIC STRUCTURES OF THE RETINA OFTHE LARVAL SEA LAMPREY - AN IMMUNOCYTOCHEMICAL STUDY

The structures of the retina immunoreactive to GABA are described in l arval lamprey. Although GABAergic cells develop early in the retinas o f vertebrates, no GABA-immunoreactive perikarya were observed in the r etina 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 cen tral (photoreceptor-bearing) and lateral (no-photoreceptor) parts of t he retina. These fibers do not ascend toward the outer plexiform layer . Nerve fibers in the optic nerve and neuronal perikarya of the M5 nuc leus of the mesencephalon, which is known to project to the retina, we re also GABA-immunoreactive. The distribution of centrifugal fibers cl osely matches that of ganglion cells revealed by retrograde labelling with fluorescein-coupled dextran-amine, and the presence of biplexifor m ganglion cells in larvae is confirmed. That the ganglion cells and t he centrifugal fibers appears to be the only structures differentiated in the lateral retina of the larva suggests that the GABAergic centri fugal fibers may have a role, perhaps the neurotrophic maintenance of retinal ganglion cells, during the very long larval phase of lampreys. (C) 1998 Elsevier Science B.V.