The crustacean stomatogastric ganglion, which is situated in the ophthalmic artery, can be modulated by both intrinsically released molecules and hormones. In the crab Cancer borealis, over a dozen neuroactive compounds have been identified in the input axons that project into the stomatogastric neuropil. However, little is known about the modulator content of the two major neurohemal organs, the sinus glands and the pericardial organs, in this crab. We now report the results of a series of immunocytochemical experiments designed to identify putative neurohormones in these tissues. We find that the majority of modulators present in the input axons of the stomatogastric ganglion are also present in at least one of the neurohemal organs. Specifically, allatostatin-like, buccalin-like, cholecystokinin-like, FLRFamide-like, GABA-like, locustatachykinin-like, myomodulin-like, proctolin-like, red pigment concentrating hormone-like and serotonin-like immunoreactivities are all present in both the stomatogastric neuropil and at least one of the neurohemal organs. Thus, these substances are likely to serve a dual role as both local and hormonal modulators of the stomatogastric network. Two other substances, beta-pigment dispersing hormone and crustacean cardioactive peptide, are not present in the stomatogastric neuropil, but beta-pigment dispersing hormone immunoreactivity is present in the sinus glands and crustacean cardioactive peptide immunoreactivity is present in the pericardial organs. It is likely that crustacean cardioactive peptide exerts its influence on the stomatogastric neural circuit via hormonal pathways. Double-labeling experiments show that the patterns of modulator co-localization present in the stomatogastric neuropil are different from those in the neurosecretory organs, suggesting that few rules of colocalization hold across these tissues.

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