Immunocytochemical and morphological evidence for a retinopetal projection in anuran amphibians

The centrifugal projection to the retina in anuran amphibians (Rana catesbeiana and Xenopus laevis) has been investigated by immunocytochemistry, HRP transport, and optic nerve lesionings. FMRFamide- and N-terminal substance P-immunoreactive (Fa-ir and SP(3-7)-ir) fibers were abundant in the normal retina and optic nerve but almost absent distal to an optic nerve section or crush after 7-14 days survival. Fa-ir and SP(3-7)-ir fibers were traced to the optic nerve from the lamina terminalis (or the septopreoptic junctional area), where there are many Fa-ir and SP(3-7)-ir perikarya. After application of HRP to the optic nerve and survival for 9-10 days, retrogradely labeled neurons were observed in the lamina terminalis. Conversely, following HRP injection into the septal and preoptic area, labeled fibers were observed in the optic nerve. These results suggest that Fa-ir and SP(3-7)-ir efferent fibers project from the lamina terminalis to the retina. But in anurans, unlike teleosts, these fibers are not gonadotropin-releasing-hormone (GnRH)-ir. The morphological relations of this retinopetal pathway with the GnRH-ir nervus terminalis are discussed.