Intrinsic cardiac neurons (ICNs) are crucial cells in the neural regulation of heart rhythm, myocardial contractility, and coronary blood flow. Intrinsic cardiac neurons exhibit diversity in their morphology and neurotransmitters that probably are age-dependent. Therefore, neuroanatomical heart studies have been currently focused on the identification of chemical phenotypes of ICNs to disclose their possible functions in heart neural regulation. Employing whole-mount immunohistochemistry, we examined ICNs from atria of the newborn pigs (Sus scrofa domesticus) as ICNs at this stage of development have never been neurochemically characterized so far. We found that the majority of the examined ICNs (> 60 %) were of cholinergic phenotype. Biphenotypic neuronal somata (NS), i.e. simultaneously positive for two neuronal markers, were also rather common and distributed evenly within the sampled ganglia. Simultaneous positivity for cholinergic and adrenergic neuromarkers was specific in 16.4 %, for cholinergic and nitrergic – in 3.5% of the examined NS. Purely either adrenergic or nitrergic ICNs were were observed at 13% and 3.1%, correspondingly. Purely adrenergic and nitrergic NS were the most frequent in the ventral left atrial subplexus. Similarly to neuronal phenotype, sizes of NS also varied depending on the atrial region providing insights into their functional implications. Axons, but not NS, positive for classic sensory neuronal markers (vesicular glutamate transporter 2 and calcitonin gene-related peptide) were identified within epicardiac nerves and ganglia. Moreover, a substantial number of ICNs could not be attributed to any phenotype as they were not immunoreactive for antisera used in this study. Numerous dendrites with putative peptidergic and adrenergic contacts on cholinergic NS contributed to neuropil of ganglia. Our observations demonstrate that intrinsic cardiac ganglionated plexus is not fully developed in the newborn pig despite of dense network of neuronal processes and numerous signs of neural contacts within ganglia.

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