Isolation of neurogenic and myogenic activities by Joro spider toxin in the adult heart of the isopod crustacean Ligia exotica

The effects of Joro spider toxin (JSTX), a specific glutamate antagonist, o n the adult heart of the isopod crustacean Ligia exotica were examined. By application of JSTX, excitatory junctional potentials (EJPs) caused by the cardiac ganglion activity in the myocardium were gradually abolished. Subse quently, the cardiac ganglion and myocardium exhibited independent activiti es with their respective rhythms. In saline containing JSTX, no changes wer e observed in the muscle activity when the ganglionic activity was changed by current injection into the cardiac ganglion neuron. These results indica te that two pacemaker sites, the cardiac ganglion and cardiac muscle, are p resent in the adult heart of Ligia exotica and suggest glutamatergic neurom uscular transmission between them.