Identification of Spinal Afferent Innervation in the Rat Heart: Atria and Ventricles: Anterograde Tracing

The spinal afferent innervation of the heart regulates cardiac functions by sending sensory information from the dorsal root ganglia (DRG) to the brain. However, the distribution and morphology of spinal afferents in the whole heart are not well characterized. The challenges here are: 1) diffculty in surgically accessing the upper thoracic DRG to label only cardiac spinal afferents. 2) handling flat-mounts of whole atria and ventricles (thickness ~850 μm) without using sections or partial tissues that could disrupt the continuity of axonal structures. To overcome these challenges, we injected anterograde tracer dextran biotin (DB) into the left DRG (C8-T3) of male SD rats (3-5 months) and allowed them to recover for 14 days for tracer transportation. Flat-mounts of the left and right atria and ventricles were prepared, and DAB (3,3′-Diaminobenzidine) stained, followed by imaging, tracing, and digitization using the Neurolucida system. Our findings are: In the atria, the DB-labeled axons entered the atria potentially through the left precaval vein (LPCV), with predominant innervation of the left side of the atrial wall and extended their projection towards the auricles, middle region of the atrium, and pulmonary veins. Most of the spinal afferent axons were present in the myocardial layer. Varicose DB-labeled axons were also observed on cardiac targets at different regions, and distinct morphological structures were found. In the cardiac muscle, DB-labeled axons ran mainly in the direction of muscle fibers. Also, varicose axons ran along the small vasculature on the atria. Spinal afferents formed varicose contacts with individual principal neurons within some intrinsic cardiac ganglia (ICG). In the ventricles, DB-labeled axons were present in the left and right ventricles, which ramified and expressed spherical-like varicosities on the cardiac muscle. In both the atria and ventricles, spinal afferent axons formed simple, branching, and complex terminal structures. Thus, our work, for the first time, successfully labeled, traced, and digitized the spinal cardiac afferents in the flat-mounts of whole atria and ventricles, which will provide a foundation for specific labeling of cardiac spinal afferent fibers. In addition, it will lead to the future study of topographical cardiac spinal afferent innervation, functional studies, and their remodeling in pathological conditions. This study was supported by NIH HEAL/SPARC U01 NS113867-01, NIH R15 R15HL137143-01A1, and UCF dissertation completion Fellowships. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.