Tachykinin-related peptides in invertebrates: a review

Peptides with sequence similarities to members of the tachykinin family hav e been identified in a number of invertebrates belonging to the mollusca, e chiuridea, insecta and crustacea. These peptides have been designated tachy kinin-related peptides (TRPs) and are characterized by the preserved C-term inal pentapeptide FX(1)GX(2)Ramide (X-1 and X-2 are variable residues). All invertebrate TRPs are myostimulatory on insect hindgut muscle, but also ha ve a variety of additional actions: they can induce contractions in cockroa ch foregut and oviduct and in moth heart muscle, trigger a motor rhythm in the crab stomatogastric ganglion, depolarize or hyperpolarize identified in terneurons of locust and the snail Helix and induce release of adipokinetic hormone from the locust corpora cardiaca. Two putative TRP receptors have been cloned from Drosophila; both are G-protein coupled and expressed in th e nervous system. The invertebrate TRPs are distributed in interneurons of the CNS of Limulus, crustaceans and insects. In the latter two groups TRPs are also present in the stomatogastric nervous system and in insects endocr ine cells of the midgut display TRP-immunoreactivity. In arthropods the dis tribution of TRPs in neuronal processes of the brain displays similar patte rns. Also in coelenterates, flatworms and molluscs TRPs have been demonstra ted in neurons. The activity of different TRPs has been explored in several assays and it appears that an amidated C-terminal hexapeptide (or longer) is required for bioactivity. In many invertebrate assays the first generati on substance P antagonist spantide 1 is a potent antagonist of invertebrate TRPs and substance P. Locustatachykinins stimulate adenylate cyclase in lo cust interneurons and glandular cells of the corpora cardiaca, but in other tissues the putative second messenger systems have not yet been identified . The heterologously expressed Drosophila TRP receptors coupled to the phos pholipase C pathway and could induce elevations of inositol triphosphate. T he structures, distributions and actions of TRPs in various invertebrates a re compared and it is concluded that the TRPs are multifunctional peptides with targets both in the central and peripheral nervous system and other ti ssues, similar to vertebrate tachykinins. Invertebrate TRPs may also be inv olved in developmental processes. (C) 1999 by Elsevier Science Inc.