Examining structural plasticity within human cardiac Ganglionated Plexi in the context of Atrial Fibrillation

Abstract

Atrial fibrillation (AF) is the most prevalent pathological atrial arrhythmia in which disorganised rapid atrial contractions significantly raise the risk of stroke, heart failure and dementia. The autonomic nervous system has been implicated in the arrythmogenesis of AF, particularly the autonomic intracardiac nervous system which is a dense network of postganglionic neuronal clusters known as ganglionated plexi (GP) located in the epicardial adipose tissue on the myocardium. How GP are linked to AF is currently unknown. The aim of this thesis was to examine structural changes occurring in GP with AF. Epicardial adipose tissue samples containing the right atrial ganglionated plexus (RAGP) were resected with consent from AF and non-AF patients undergoing open heart surgery. Samples were fixed for large tissue custom line scanning stage confocal imaging to enable visualisation of entire human GP for the first time. A combination of CUBIC, hydrophilic, and hydrophobic clearing methods were used to render samples optically transparent, followed by immunohistochemistry to visualise GP neuron morphology, phenotypes, and synapses. Structural heterogeneity was observed at the tissue, cellular and subcellular levels, with GP varying greatly in volume and number of neurons. GP neurons from AF and non-AF patients could be distinguished by the differential distribution of both neuronal and synapse area, suggesting changes in synaptic structure and cell morphology with AF that could reflect differences in neuron communication both within and between GP. Multiple GP cellular phenotypes were also detected including predominant cholinergic neurons, noradrenergic neurons, small intensely fluorescent cells, and a novel sensory neuron population. Data to date suggest some phenotypic changes occur with AF, and this requires further investigation. Preliminary data also suggest involvement of human GP neurons in a AF neurostimulation paradigm therapy. Together these data identify structural plasticity occurs in GP neurons in AF patients that could link to potential causation pathways.