Quantitative Mapping of Cardiac Sympathetic Innervation in the Mouse Atria

Zhang Y, Bizanti A, Harden SW, Chen J, Bendowski K, Hoover DB, Gozal D, Shivkumar K, Heal M, Tappan S, Cheng ZJ. Topographical mapping of catecholaminergic axon innervation in the flat-mounts of the mouse atria: a quantitative analysis. Sci Rep. 2023 Apr 7;13(1):4850. doi: 10.1038/s41598-023-27727-9.

Background: The sympathetic nervous system (SNS) regulates cardiac functions such as heart rate, contractility and conduction velocity. However, a comprehensive three-dimensional (3D) map of cardiac sympathetic innervation remains lacking. Earlier studies using sectioned or partial heart preparations disrupted axonal continuity and prevented large-scale morphological analysis. Therefore, a detailed topographical description of catecholaminergic innervation in the mouse atria is essential for understanding sympathetic regulation of cardiac physiology and remodeling.

Hypothesis: This study hypothesized that sympathetic postganglionic catecholaminergic axons display distinct and quantifiable topographical distributions across different atrial regions, with region-specific variations in axon density and organization.

Methods: The authors used whole-mount atrial preparations from C57BL/6J mice. Tissues were processed for tyrosine hydroxylase (TH) immunolabeling, imaged with confocal microscopy and analyzed using Neurolucida 360 for 3D tracing and digitization of catecholaminergic axons. Neurolucida Explorer was employed for morphometric and density analyses across defined atrial regions.

Results: Four to five major TH-immunoreactive bundles entered the atria at consistent sites and branched into overlapping projection fields. Axon density was greatest near the sinoatrial node in the right atrium and at the left atrium–pulmonary vein junction. Approximately 18–30% of intrinsic cardiac ganglion neurons were TH-positive, but few showed direct axonal innervation. Dense sympathetic fibers also surrounded blood vessels and adipocytes.

Conclusions: This study provided the first comprehensive quantitative map of catecholaminergic innervation in mouse atria, revealing pronounced regional asymmetry and preferential right- and left-sided projections. These findings establish an anatomical foundation for future functional and pathological analyses of cardiac sympathetic control.