Enhancement of Muscle Contraction in the Stomach of the Crab Cancer borealis: a Possible Hormonal Role for GABA

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 μmol 1−1. 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 μmol 1−1, 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.