From Mice to Mainframes: Experimental Models for Investigation of the Intracardiac Nervous System

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.

Molecular and functional characterization of the mouse intracardiac nervous system

Background: The intracardiac nervous system (ICNS) refers to clusters of neurons, located within the heart, that participate to the neuronal regulation of cardiac functions and are involved in the initiation of cardiac arrhythmias. Therefore, deciphering the role of the ICNS in cardiac physiology and physiopathology is mandatory. Whereas transgenic mouse models represent powerful tools to reach this goal, the mouse ICNS is still poorly characterized. Objective: The objective of the present study was to provide a phenotypic, electrophysiological and pharmacological characterization of the mouse ICNS. Methods: Global cardiac innervation and phenotypic diversity was investigated by performing immunohistochemistry on cleared murine heart and on tissue sections. Patch clamp technique was used for electrophysiological and pharmacological characterization of isolated mouse intracardiac neurons. Results: We identified the expression of 7 distinct neuronal markers within mouse intracardiac neurons demonstrating the neurochemical diversity of this network. Of note, we described for the first time in mouse, the existence of neuron expressing the calcium binding protein calbindin, the neuropeptide Y (NPY) and the cocain and amphetamine regulated transcript (CART) peptide. Electrophysiological studies also revealed the existence of two different neuronal population based on their electrical behavior. Finally, we demonstrated that these neurons can be modulated by several neuromodulators. Conclusion: This study demonstrated that mouse ICNS shares similar molecular and functional complexity to that of other species and therefore is a suitable model to decipher the role of individual neuronal subtypes in the modulation of cardiac function and in the initiation of cardiac arrhythmias.

The role of the tripartite synapse in the heart: how glial cells may contribute to the physiology and pathophysiology of the intracardiac nervous system

One of the major roles of the intracardiac nervous system (ICNS) is to act as the final site of signal integration for efferent information destined for the myocardium to enable local control of heart rate and rhythm. Multiple subtypes of neurons exist in the ICNS where they are organized into clusters termed ganglionated plexi (GP). The majority of cells in the ICNS are actually glial cells; however, despite this, ICNS glial cells have received little attention to date. In the central nervous system, where glial cell function has been widely studied, glia are no longer viewed simply as supportive cells but rather have been shown to play an active role in modulating neuronal excitability and synaptic plasticity. Pioneering studies have demonstrated that in addition to glia within the brain stem, glial cells within multiple autonomic ganglia in the peripheral nervous system, including the ICNS, can also act to modulate cardiovascular function. Clinically, patients with atrial fibrillation (AF) undergoing catheter ablation show high plasma levels of S100B, a protein produced by cardiac glial cells, correlated with decreased AF recurrence. Interestingly, S100B also alters GP neuron excitability and neurite outgrowth in the ICNS. These studies highlight the importance of understanding how glial cells can affect the heart by modulating GP neuron activity or synaptic inputs. Here, we review studies investigating glia both in the central and peripheral nervous systems to discuss the potential role of glia in controlling cardiac function in health and disease, paying particular attention to the glial cells of the ICNS.

Evidence of structural and functional plasticity occurring within the intracardiac nervous system of spontaneously hypertensive rats

We have developed intracardiac neuron whole cell recording techniques in atrial preparations from control and spontaneous hypertensive rats. This has enabled the identification of significant synaptic plasticity in the intracardiac nervous system, including enhanced postsynaptic current frequency, increased synaptic terminal density, and altered postsynaptic receptors. This increased synaptic drive together with altered cardiac neuron electrophysiology could increase intracardiac nervous system excitability and contribute to the substrate for atrial arrhythmia in hypertensive heart disease.

A Comprehensive Integrated Anatomical And Molecular Atlas Of Rodent Intrinsic Cardiac Nervous System

ABSTRACTIn this study, we developed, coordinated, and integrated several technologies including novel whole organ imaging, software development to support the very first precise 3D neuroanatomical mapping and molecular phenotyping of the intracardiac nervous system (ICN). While qualitative and gross anatomical descriptions of the anatomy of the ICN have been presented, we here bring forth the first comprehensive atlas at large scale of the entire ICN in rat at a single cell resolution. Our work for the first time provides a novel 3D model to precisely integrate anatomical, functional and molecular data in the 3D digitally reconstructed whole heart with high resolution at the micron scale. This work represents the cutting edge in a long history of attempts to understand the anatomical substrate upon which the neuronal control of cardiac function is built. To our knowledge, there has not yet been a comprehensive histological mapping to generate a neurocardiac atlas at cellular and molecular level for the whole heart of any species. We now display the full extent and the position of neuronal clusters on the base and posterior left atrium, and the distribution of molecular phenotypes in that context. In addition we display in this context distinct molecular phenotypes that are defined along the base-to-apex axis, and the present novel discovery of their phenotypical spatial gradients, have not been previously described. The development of these approaches needed to acquire these data has produced method pipelines which can not only achieve the goals of anatomical and molecular mapping of the heart, but also provide the method pipelines for mapping other organs (e.g., stomach, lung, kidney, and liver).