Asymmetric blood flow within a shared capillary network links the nucleus of the solitary tract and area postrema

The nucleus of the solitary tract (NTS) and the area postrema (AP) form a tightly coupled dorsal medullary complex that integrates visceral and humoral signals governing autonomic and cardiometabolic regulation. While their neural interconnections are well characterized, the organization and functional significance of their shared vascular network remain poorly understood. Here, we used in vivo two-photon microscopy in rats, combined with fluorescent vascular labeling and retrograde neuronal tracing, to visualize and quantify blood flow within the AP-NTS microcirculation. We identified direct capillary connections between the two regions, confirming a continuous vascular network previously inferred from ex vivo studies. Analysis of red blood cell trajectories revealed that blood flow across these junctions is predominantly unidirectional, from the NTS toward the AP. Morphometric measurements showed that AP capillaries are nearly twice the diameter of those in the NTS, implying a lower local vascular resistance and providing a structural basis for this directionality. Despite these geometric differences, capillary flow velocities were similar between regions, consistent with active regulation that maintains stable perfusion dynamics. Together, these findings uncover a previously unrecognized, functionally asymmetric vascular pathway within the dorsal medulla. By enabling the directed transfer of diffusible neurohumoral signals from the NTS to the AP, this specialized microvascular network adds a novel layer of communication between two key brainstem autonomic centers and may represent an additional mechanism for the integration of visceral and circulating information.