Neuropeptides in the crayfish stomatogastric nervous system

Neuropeptides are peptides with profound effects on the nervous system. The function of neuropeptides can be studied in detail in the stomatogastric nervous system (STNS). Neuropeptides are ubiquitously distributed in the STNS and it contains well-studied neural circuits that are strongly modulated by neuropeptides. The STNS controls the movements of the foregut in crustaceans and has been studied intensively in a variety of decapod crustaceans including crayfish. This article reviews our knowledge of neuropeptides in the crayfish STNS. Within crayfish, peptides reach the circuits of the STNS as neurohormones released by neurohaemal organs or by putative neurohemal zones located within the STNS. As transmitters, neuropeptides are present in identified motoneurons, interneurons, and sensory neurons (mainly shown by immunocytochemistry), indicating a multiple role of peptides in the plasticity of neural networks. Neuropeptides are not only present in varicosities within the neuropil of ganglia, but also in varicosities on muscles and within small neuropil patches along nerves. This suggests that the muscles of the stomach are under a more direct modulatory control than previously thought, and that information processing can also occur within nerves. In addition to anatomical studies, biochemical and electrophysiological methods were used. For example, MALDI-TOF MS (matrix-assisted laser desorption ionization time of flight mass spectrometry) revealed the presence of four different peptides of the orcokinin family within a single neuron, and electrophysiological experiments demonstrated that the networks of the STNS are not only under excitatory but also inhibitory peptidergic influence. Comparing the similarities and differences between the STNS of crayfish and that of other decapod crustaceans has already contributed to our knowledge about peptides and will further help to unravel peptide function in the plasticity of neural circuits. For example, the identified neurons in the STNS can be used to study co-transmission because neuropeptides are co-localized with classical transmitters, biogenic amines, or other peptides in these neurons.