Graded neuromuscular transmission in the heart of the isopod crustacean Ligia exotica

We present several lines of evidence for the occurrence of graded synaptic transmission in addition to impulse-mediated transmission at the neuromuscu lar junction between cardiac ganglion (CG) neurones and the myocardium in t he isopod crustacean Ligia exotica, In the heart of adult Ligia exotica, th e CG acts as a primary pacemaker for the heartbeat by generating periodic b ursts of impulses and entrains the myogenicity of the myocardium via impuls e-mediated excitatory junctional potentials, When impulse generation was bl ocked by tetrodotoxin (TTX; 50 nmol l(-1)), the CG neurones and the myocard ium periodically exhibited synchronized slow depolarizing potentials. The a ssociation between the slow depolarizing potentials in the neurone and the myocardium was eliminated by application of Joro spider toxin (JSTX), a spe cific glutamate antagonist. When the CG neurone was made quiescent by a hig her dose of TTX (1.0 mu mol l(-1)), sinusoidal current injected into the CG neurone induced similar sinusoidal membrane potential responses in the myo cardium. The sinusoidal muscle responses were eliminated by application of either JSTX or low-Ca2+ saline. Under voltage-clamp conditions, the myocard ium exhibited periodic inward current responses to sinusoidal current stimu li applied to the CG neurone. The reversal potential for the current respon se of the myocardium was similar to that of the impulse-mediated excitatory junctional current (EJC). Extracellular macropatch recordings of EJCs made at the neuromuscular junctional site revealed the spontaneous appearance o f miniature EJCs asynchronous with the CC spikes in addition to large spike -evoked EJCs, The miniature EJCs were present in saline containing TTX, and their frequency was strongly affected by the slow membrane potential chang e in the CG neurone. These results suggest that the CG neurones drive the m yocardium by graded neuromuscular transmission in addition to impulse-media ted transmission in the heart of Ligia exotica.