Neuropeptides are ubiquitous chemical mediators: Using the stomatogastric nervous system as a model system

The stomatogastric nervous system (STNS) controls the movements of the fore gut and the oesophagus of decapod crustaceans and is a good example for dem onstrating that peptides are ubiquitously distributed chemical mediators in the nervous system. The stomatogastric ganglion (STG), one of the four gan glia of the STNS, contains the most intensively investigated neuronal circu its. The other ganglia, including the two commissural ganglia (CoGs) and th e oesophageal ganglion (OG), are thought to be modulatory control centres, Peptides reach the STNS either as neurohormones or are released as transmit ters. Peptide neurohormones can be released either from neurohaemal organs or from local neurohaemal release zones located on the surface of nerves an d connectives. There were thought to be no peptidergic neurones with cell b odies in the STG itself. However, some have recently been described in adul ts of four species, in addition to a transient expression of peptides durin g development in two species. None of these peptidergic neurones has been i nvestigated physiologically, in contrast to peptidergic neurones that proje ct to the STG and have cell bodies in either the CoGs or the OG, It has bee n shown that neurones containing the same peptide elicit different motor pa tterns, that the peptide transmitter and the classical transmitter are not necessarily co-released and that the effect of a peptidergic neurone depend s on its firing frequency and on which other modulatory neurones are co-act ive, The activity of modulatory projection neurones can be elicited by sens ory neurones, and their activity can depend on the firing frequency of the sensory neurone. In addition to being found within the neuropile of ganglia , peptides are present in neuropile patches located within the nerves of th e STNS, suggesting that these nerves can integrate as well as transfer info rmation. Furthermore, sensory neurones and muscles exhibit peptide-like imm unoreactivity and are modulated by peptides, Bath-applied peptides elicit p eptide-specific motor patterns within the STG by targeting subsets of neuro nes, This divergence is contrasted by a convergence at the level of current s: five different peptides modulate a single current. Peptides not only ind uce motor patterns but can also switch the alliance of neurones from one ne twork to another or are able to fuse different networks, In general, peptid es are the most abundant group of modulators within the STNS; they are ubiq uitously present, indicating that they play multiple roles in the plasticit y of neural networks.