Carassius RFamide, a novel FMRFa-related peptide, is produced within the retina and involved in retinal information processing in cyprinid fish

Carassius RFamide (C-RFa) is a novel peptide, isolated originally from the brain of the Japanese crucian carp and sharing homologies with mammalian prolactin-releasing peptide (PrRP). It has been demonstrated previously that C-RFa mRNA is abundant in the proximal half (fundus) of the Japanese crucian carp eye. In the present work, we localized C-RFa by immunohistochemistry mainly to perikarya, in the proximal half of the inner nuclear layer (amacrine cell layer). This distribution is different from that of FMRFamide, which is confined to axon terminals of terminal nerve efferent fibers in the inner plexiform layer. Electrophysiological recording revealed that C-RFa depolarized some amacrine cells and hyperpolarized L-type horizontal cells in the carp. These results suggest that C-RFa is produced within the cyprinid retina and functions as a transmitter or neuromodulator in retinal image processing.