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

The stomatogastric nervous system (STNS) controls the movements of the foregut and the oesophagus of decapod crustaceans and is a good example for demonstrating that peptides are ubiquitously distributed chemical mediators in the nervous system. The stomatogastric ganglion (STG), one of the four ganglia of the STNS, contains the most intensively investigated neuronal circuits. The other ganglia, including the two commissural ganglia (CoGs) and the oesophageal ganglion (OG), are thought to be modulatory control centres. Peptides reach the STNS either as neurohormones or are released as transmitters. Peptide neurohormones can be released either from neurohaemal organs or from local neurohaemal release zones located on the surface of nerves and connectives. There were thought to be no peptidergic neurones with cell bodies in the STG itself. However, some have recently been described in adults of four species, in addition to a transient expression of peptides during development in two species. None of these peptidergic neurones has been investigated physiologically, in contrast to peptidergic neurones that project to the STG and have cell bodies in either the CoGs or the OG. It has been shown that neurones containing the same peptide elicit different motor patterns, that the peptide transmitter and the classical transmitter are not necessarily co-released and that the effect of a peptidergic neurone depends on its firing frequency and on which other modulatory neurones are co-active. The activity of modulatory projection neurones can be elicited by sensory 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 the STNS, suggesting that these nerves can integrate as well as transfer information. Furthermore, sensory neurones and muscles exhibit peptide-like immunoreactivity and are modulated by peptides. Bath-applied peptides elicit peptide-specific motor patterns within the STG by targeting subsets of neurones. This divergence is contrasted by a convergence at the level of currents: five different peptides modulate a single current. Peptides not only induce motor patterns but can also switch the alliance of neurones from one network to another or are able to fuse different networks. In general, peptides are the most abundant group of modulators within the STNS; they are ubiquitously present, indicating that they play multiple roles in the plasticity of neural networks.