Neuropeptides in the cnidaria

Abstract

This chapter reviews experimental evidence for peptides acting as transmitters or morphogens in the Cnidaria. A wide range of brain/gut peptides have been localized immunohistochemically to specific neuronal populations in Hydra. These include gastrin/CCK, substance P, neurotensin, bombesin, oxytocin/vasopressin and FMRFamide. In most cases the amino-acid sequences of the cnidarian peptides are somewhat different from their mammalian counterparts. The functions of these peptides in Hydra are unknown. A series of neuropeptides with the carboxy-terminus, Arg-Phe-amide, is ubiquitous within the phylum. Immunoreactivity to antisera against RFamide is seen in two characteristic neuronal subpopulations; a sensory and a ganglionic cell type. Two of these peptides have now been isolated and sequenced. One has the structure, pGlu-Gly-Arg-Phe-amide (Antho-RFamide) and is found in several anthozoans, a second with the sequence pGlu-Leu-Leu- Gly-Gly-Arg-Phe-amide (Poly A peptide) is found in the hydrozoan Polyorchis. Arg-Pheamide peptides have excitatory effects on both muscles and neuronal systems. In the anthozoans, RFamide peptides can increase the tone, contraction amplitude and frequency of a number of smooth muscle systems. Additionally, in the anemone Calliactis, application of Antho-RFamide can dramatically increase the firing rate in one of the ectodermal conducting systems, the SSI. In Polyorchis several RFamides produce long duration spike trains in motor neurons that may or may not be associated with membrane depolarization. A peptide called 'head activator' (pGlu-(Pro)j-(Gly)8-Ser-Lys-Val-Ile-Leu-Phe) can induce the formation of a new head when it is released at high concentrations at the cut surface of the column of Hydra. It acts by committing stem cells to become head specific neurons. © 1989 by the American Society of Zoologists.