Spinal afferent innervation from the left dorsal root ganglia in flat-mounts of whole ventricles of rats: Anterograde tracing.

Spinal afferents relay sensory information and regulate cardioprotective reflexes. These afferent signals go to cell bodies in the dorsal root ganglia (DRG) and ultimately to the central nervous system. However, the morphology and distribution of spinal afferent terminals in the heart have not been well delineated. This is partly due to challenges, including limited surgical access to the upper thoracic DRGs, tissue thickness, and tracer options. In this study, we injected tracer dextran biotin (DB) into the left DRG (C8-T3) of male Sprague-Dawley rats (3-5 months). After 16 days, the animals were sacrificed, and the left and right ventricles were separated from the rest of the heart. Next, these samples were stained with diaminobenzidine and mounted for visualization under brightfield microscopy. DB-labeled axons were identified and digitally traced using the Neurolucida system. Our findings showed that spinal afferent neurons of the left DRG project individual axons that enter the ventricles from the base. These axons are distributed across various regions within both ventricles. While these axons are present in the epicardial and myocardial layers, they were not identified in the endocardial layer. Within the epicardial and myocardial layers, spinal afferents form extensive terminal arborizations and make presumptive contact with cardiac muscle, blood vessels, and adipose tissue. This investigation marks the first anterograde characterization of spinal afferent axon terminal distribution and morphology in flat-mount preparations of whole rat ventricles.