- We have explored the structure and actions of terminal nerve (TN) fibers in the teleostean retina, the most accessible of TN projections. Using immunocytochemistry we have shown that the goldfish TN contains neuropeptides related to the molluscan cardioexcitatory peptide (FMRFamide) as well as luteinizing hormone-releasing hormone (LHRH). Retinal TN terminals were found upon major dendrites in the distal inner plexiform layer and neuronal cell bodies in the amacrine cell layer. Electron-microscopic double-labeling revealed TN terminals applied to the surface of [3H]-dopamine-, glycine-, and gamma-aminobutyric acid (GABA)-accumulating cells. Synthetic LHRH and FMRFamide at less than 1 microM modified spontaneous and light-evoked activity of ganglion cells in isolated superfused goldfish retina, especially during the active breeding season. Salmon(I)-LHRH was 10-30 times as potent as mammalian LHRH and caused rapid, prolonged desensitization. We conclude that LHRH- and FMRFamide-like peptides may be released by retinal TN endings, probably in concert with reproductive activity, and that they act independently through horizontal and/or amacrine cell pathways to modify visual information processing in the retina.