Somatostatin family of peptides and its receptors in fish

Somatostatin (SRIF or SS) is a phylogenetically ancient, multigene family o f peptides. SRIF-14 is conserved with identical primary structure in specie s of all classes of vertebrates. The presence of multiple SRIF genes has be en demonstrated in a number of fish species and could extend to tetrapods. Three distinct SRIF genes have been identified in goldfish. One of these ge nes, which encodes [Pro(2)]SRIF-14, is also present in sturgeon and African lungfish, and is closely associated with amphibian [Pro(2),Met(13)]SRIF-14 gene and mammalian cortistatin gene. The post-translational processing of SRIF precursors could result in multiple forms of mature SRIF peptides, wit h differential abundance and tissue- or cell type-specific patterns. The ma in neuroendocrine role of SRIF-14 peptide that has been determined in fish is the inhibition of pituitary growth hormone secretion. The functions of S RIF-14 variant or larger forms of SRIF peptide and the regulation of SRIF g ene expression remain to be explored. Type 1 and type 2 SRIF receptors have been identified from goldfish and a type 3 SRIF receptor has been identifi ed from an electric fish. Fish SRIF receptors display considerable homology with mammalian counterparts in terms of primary structure and negative cou pling to adenylate cyclase. Although additional types of receptors remain t o be determined, identification of the multiple gene family of SRIF peptide s and multiple types of SRIF receptors opens a new avenue for the study of physiological roles of SRIF, and the molecular and cellular mechanisms of S RIF action in fish.