Synaptic neuropil in nerves of the crustacean stomatogastric nervous system: an immunocytochemical and electron microscopical study

Patches of peptide-immunoreactive varicosities have been found in nerves of the stomatogastric nervous system (STNS) of decapod crustaceans. In the present study, these patches were examined in detail in the stomatogastric nerve (stn) and in the superior oesophageal nerve (son) of the crayfish Cherax destructor by using whole-mount immunocytochemical techniques combined with confocal microscopy and, in addition, electron microscopy. Double-labeling experiments with antibodies generated against the peptides allatostatin, FMRFamide and proctolin, combined with an antibody generated against the small vesicle protein synapsin, suggest that each patch contains small synaptic vesicles in addition to all three peptides. The neuropil regions of the ganglia of the STNS were also strongly stained by the synapsin antibody. Synapsin-like immunoreactivity was also studied in the crab Cancer pagurus and the lobster Homarus americanus. A similar pattern of staining was found for all three species, but the distribution within the stn varied. In H. americanus, a lightly stained weblike structure was found on the surface of nerves including the inferior oesophageal nerve, the son, and the anterior stn. By using electron microscopy, synapses were found in the core of the stn-son junction of C. destructor, in the same region where the synapsin-like and the peptide staining was localized. In addition, putative neurohemal release sites were found in the peripheral sheath of the stn. The presynaptic profiles found in the core of the stn seem to correspond to the types of presynaptic profiles found in the neuropil of the stomatogastric ganglion. These findings demonstrate that synaptic neuropil is present in the nerves of the STNS of a decapod crustacean.