Substance‐P‐related and neurokinin‐A‐related peptides from the brain of the cod and trout

An extract of the brain of the rainbow trout, Oncorhynchus mykiss contained high concentrations of both neurokinin A‐like immunoreactivity (corresponding to 90 pmol mammalian neurokinin A/g wet tissue) and substance‐P‐like immunoreactivity (corresponding to 50 pmol mammalian substance P/g wet tissue) measured by radioimmunoassay using antisera directed against the C‐terminal regions of the mammalian peptides. In contrast, an extract of the Atlantic cod, Gadus morhua contained only neurokinin‐A‐like immunoreactivity (151 pmol/g). This apparent paradox was resolved by determination of the primary structures of the fish tachykinins. Trout substance P (Lys‐Pro‐Arg‐Pro‐His‐Gln‐Phe‐Phe‐Gly‐Leu‐MetNH₂) has the same amino acid sequence in its C‐terminal region as that in the corresponding region of mammalian substance P. Cod substance P (Lys‐Pro‐Arg‐Pro‐Gln‐Gln‐Phe‐Ile‐Gly‐Leu‐MetNH₂), however, contains a substitution at position 8 (Phe → Ile) that abolishes reactivity with the antiserum to substance P but permits reactivity with the antiserum to neurokinin A. The amino acid sequence of cod and trout neurokinin A is the same (His‐Lys‐Ile‐Asn‐Ser‐Phe‐Val‐Gly‐Leu‐MetNH₂) and shows two substitutions (Thr3 → Ile and Asp4 → Asn) compared with mammalian neurokinin A. The data indicate that nervous tissue of teleost fish contain tachykinins that are analogous to the peptides found in mammalian tissues.