The intracardiac nervous system (IcNS), sometimes referred to as the “little brain” of the heart, is involved in modulating many aspects of cardiac physiology. In recent years our fundamental understanding of autonomic control of the heart has drastically improved, and the IcNS is increasingly being viewed as a therapeutic target in cardiovascular disease. However, investigations of the physiology and specific roles of intracardiac neurons within the neural circuitry mediating cardiac control has been hampered by an incomplete knowledge of the anatomical organisation of the IcNS. A more thorough understanding of the IcNS is hoped to promote the development of new, highly targeted therapies to modulate IcNS activity in cardiovascular disease. In this paper, we first provide an overview of IcNS anatomy and function derived from experiments in mammals. We then provide descriptions of alternate experimental models for investigation of the IcNS, focusing on a non-mammalian model (zebrafish), neuron-cardiomyocyte co-cultures, and computational models to demonstrate how the similarity of the relevant processes in each model can help to further our understanding of the IcNS in health and disease.

Popeye domain-containing (POPDC) proteins selectively bind cAMP and mediate cellular responses to sympathetic nervous system (SNS) stimulation. The first discovered human genetic variant (POPDC1^S201F) is associated with atrioventricular (AV) block, which is exacerbated by increased SNS activity. Zebrafish carrying the homologous mutation (popdc1^S191F) display a similar phenotype to humans. To investigate the impact of POPDC1 dysfunction on cardiac electrophysiology and intracellular calcium handling, homozygous popdc1^S191F and popdc1 knock-out (popdc1^KO) zebrafish larvae and adult isolated popdc1^S191F hearts were studied by functional fluorescent analysis. It was found that in popdc1^S191F and popdc1^KO larvae, heart rate (HR), AV delay, action potential (AP) and calcium transient (CaT) upstroke speed, and AP duration were less than in wild-type larvae, whereas CaT duration was greater. SNS stress by β-adrenergic receptor stimulation with isoproterenol increased HR, lengthened AV delay, slowed AP and CaT upstroke speed, and shortened AP and CaT duration, yet did not result in arrhythmias. In adult popdc1^S191F zebrafish hearts, there was a higher incidence of AV block, slower AP upstroke speed, and longer AP duration compared to wild-type hearts, with no differences in CaT. SNS stress increased AV delay and led to further AV block in popdc1^S191F hearts while decreasing AP and CaT duration. Overall, we have revealed that arrhythmogenic effects of POPDC1 dysfunction on cardiac electrophysiology and intracellular calcium handling in zebrafish are varied, but already present in early development, and that AV node dysfunction may underlie SNS-induced arrhythmogenesis associated with popdc1 mutation in adults.

SUMMARY PARAGRAPH: Maintenance of cardiac function involves a regulatory loop in which electrical excitation causes the heart to contract through excitation-contraction coupling (ECC), ¹ and the mechanical state of the heart directly affects its electrical activity through mechano-electric coupling (MEC). ² However, in pathological states such as acute ischaemia that alter early or late electro-mechanical coordination ( i.e. , disturbances in ECC or repolarisation-relaxation coupling, RRC), MEC may contribute to the initiation and / or sustenance of arrhythmias (mechano-arrhythmogenesis). ³ The molecular identity of specific factor(s) underlying mechano-arrhythmogenesis in acute ischaemia, however, remain undefined. ⁴ By rapid stretch of rabbit single left ventricular cardiomyocytes, we show that upon ATP-sensitive potassium channel-induced alterations of RRC, overall vulnerability to mechano-arrhythmogenesis is increased, with mechano-sensitive ^5–11 transient receptor potential kinase ankyrin 1 (TRPA1) channels ¹² acting as the molecular driver through a Ca ²⁺ -mediated mechanism. Specifically, TRPA1 activation drives stretch-induced excitation and creates a substrate for self-sustained arrhythmias, which are maintained by increased cytosolic free Ca ²⁺ concentration ([Ca ²⁺ ] _i ) and spontaneous [Ca ²⁺ ] _i oscillations. This TRPA1-dependent mechano-arrhythmogenesis involves microtubules, and can be prevented by block of TRPA1 or buffering of [Ca ²⁺ ] _i . Thus, in cardiac pathologies with disturbed RRC dynamics and / or augmented TRPA1 activity, TRPA1 may represent an anti-arrhythmic target with untapped therapeutic potential. ^13–17