Isolation and identification of a novel fish substance P and its effect onretinal cell responses

A novel bioactive peptide was isolated from the brain of the Japanese cruci an carp, Carassius auratus langsdorfii, by using the intestine of the fish as the bioassay system. The primary structure of the peptide was determined to be KPRPHQFIGLMamide. The sequence was found to be highly homologous to the fish substance P-related peptides isolated from Atlantic cod and rainbo w trout. We designated the peptide Carassius Substance P (Caa-SP). Caa-SP w as identical with the peptide whose presence was inferred from a cDNA analy sis by other investigators. Caa-SP exerted an excitatory effect on visceral muscle tissues of fish with the threshold at about 3 x 10(-9) M. The pepti de enhanced the ERG b-wave, depolarized the dark membrane potential and sli ghtly decreased the amplitude of S-potentials of the L-type horizontal cell s in the carp retina.