Evidence for cholinergic inhibitory and serotonergic excitatory neuromuscular transmission in the heart of the bivalve Mercenaria mercenaria

The heart of Mercenaria mercenaria is innervated bilaterally at the atria. A pair of cardiac nerves arise as a branch of the cerebro-visceral connective and run to the posterior end of the junction between each atrium and its efferent branchial vessel. Innervation evidently spreads over the heart, since both inhibitory and excitatory junctional potentials (IJPs and EJPs) can be recorded from the atria, the atrio-ventricular (AV) valve or the ventricle. The cardiac nerves contain inhibitory and excitatory axons. Neural stimulation can cause increases in the frequency or amplitude of beating, depending on the strength and frequency of stimulation. Electrical stimulation of the nerves to the incurrent and excurrent siphons causes bradycardia or tachycardia even after cutting the cerebro-visceral connective at a site anterior to the origin of the cardiac nerves. This may indicate a reflex pathway involving neurons whose cell bodies are located in the visceral ganglion. Neural depression of myocardial action potentials is mediated by discrete IJPs, which follow nerve stimuli one-to-one. IJPs can be recorded from the atria, the AV valve or the ventricle. A long-lasting hyperpolarization follows cessation of excitatory stimulation of the cardiac nerve. IJPs may be produced by cholinergic nerves and are mediated primarily by Cl-. They are blocked by Mytolon and by d-tubocurarine (dTC), but not by methylxylocholine. In low-[Cl-] solution, IJPs are inverted into depolarizing junctional potentials, which are blocked by Mytolon and dTC. Neurally induced tachycardia is mediated by discrete EJPs, which also follow stimuli applied to the cardiac nerve in a one-to-one manner. EJPs can be recorded from the atria, the AV valve and the ventricle. The myocardium and the AV valve were excited by application of serotonin. EJPs recorded from these sites were reduced in amplitude by methysergide (1-methyl-d-lysergic acid butanolamide), suggesting that the EJPs may be serotonergic. Just after entering the heart, at the posterior end of the junction with the efferent branchial vessel, the cardiac nerves contain thick processes which show serotonin-like immunoreactivity. These processes spread along the ramifications of the nerves, which extend to the atrium, the AV valve, the ventricle and even into the wall of the aorta. This study provides direct electrophysiological evidence for serotonergic EJPs and cholinergic IJPs, plus immunocytochemical evidence for neural processes containing serotonin, in the myocardium.