Intracardiac neurons (ICNs) have a pivotal role in the regulation of heart rhythm and myocardial contractility. However, comparative data about the precise distribution of the chemically phenotypically distinct ICNs in the heart as well as their interrelations within intracardiac ganglia (ICG) are limited. In this study, we employed the whole-mount approach to characterise immunohistochemically the porcine atrial ICG. We found that almost all, 99%, of ICNs express cholinergic neuronal marker in the adult pig. Some cholinergic ICNs were simultaneously positive for adrenergic (3.6%) or nitrergic (3.3%) neuronal markers as well. These biphenotypic adrenergic ICNs were more abundant within the right atrial ganglionated subplexi (sGPi), while the biphenotypic nitrergic ICNs were more numerous in the dorsal sGPi compared with the ventral sGPi. Non-cholinergic as well as entirely adrenergic and nitrergic ICNs were seldom and compiled respectively 0.1% and 0.4% of all intracardiac neuronal population. In contrast to neuronal somata, nerve fibres observed in the adult pig atrial sGPi were also positive for vesicular glutamate transporter 2 and calcitonin gene-related peptide (CGRP). Receptor protein for CGRP, CRLR, was found on cholinergic neuronal somata within ICG as well as the putative neuro-somatic contact sites with peptidergic and adrenergic nerve fibres. Morphological analysis of neuronal somata disclosed that the largest somata were nitrergic. This study demonstrates the scanty chemical variety of ICNs in the adult porcine atrial ICG that possess mainly cholinergic neuronal somata with the few adrenergic and nitrergic ones which are patchily distributed within sGPi. Inside the ICG, cholinergic ICNs are wrapped by a complex network of chemically diverse nerve terminals, some of which contain varicosities with probable passage type synapses.
Keywords: Heart; Intracardiac neuron; Intrinsic cardiac ganglionated plexus; Phenotype; Pig.
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