Stomatogastric musculature from crabs in the genus Cancer provides a system in which modulatory roles of peptides from the FLRFamide family can be compared. The anterior cardiac plexus (ACP) is a neuroendocrine release site within the Cancer stomatogastric nervous system that is structurally identical in C. borealis, C. productus, and C. magister but that appears to contain FLRFamide-like peptide(s) only in C. productus. We measured the effect of TNRNFLRFamide on nerve-evoked contractions of muscles that were nearby, an intermediate distance, or far from the ACP. We found the spatial pattern of FLRFamidergic modulation of muscles in C. productus to be qualitatively different than in C. borealis or C. magister. In C. productus, muscles proximal to the ACP were more responsive than distal muscles. In C. borealis, FLRFamidergic response was less dependent on muscle location. These results suggest that functionally different roles of FLRFamides in modulating stomatogastric muscle movements may have evolved in different Cancer species.

Gamma-aminobutyric acid (GABA) is best known as an inhibitory neurotransmitter in the mammalian central nervous system. Here we show, however, that GABA has an excitatory effect on nerve-evoked contractions and on excitatory junctional potentials (EJPs) of the gastric mill 4 (gm4) muscle from the stomach of the crab Cancer borealis. The threshold concentration for these effects was between 1 and 10 micromol l(-1). Using immunohistochemical techniques, we found that GABA is colocalized with the vesicle-associated protein synapsin in nearby nerves and hence is presumably released there. However, since these nerves do not innervate the muscle directly, we conclude that these release sites are not the likely source of the GABA responsible for muscle modulation. We also extracted hemolymph from the crab pericardial cavity, which contains the pericardial organs, a major neurosecretory structure. Through reversed-phase liquid chromatography-mass spectrometry analysis we determined the concentration of GABA in the hemolymph to be 3.3 +/- 0.7 micromol l(-1), high enough to modulate the muscle. These findings suggest that the gm4 muscle could be modulated by GABA produced by and released from a distant neurohemal organ.