Transneuronal tracing of neural pathways regulating muscle blood flow in the rat

Blood flow to skeletal muscles increases during exercise due to a variety of factors, including alterations in the activity of sympathetic efferents innervating muscle arterioles. The present study used the transneuronal transport of pseudorabies virus (PRV) to map the descending pathways that regulate sympathetic outflow to the gastrocnemius (GS) muscle in rats. To prevent PRV from being transmitted to the brainstem via motor circuitry, a spinal transection was performed just below the L2 level, which is caudal to the majority of sympathetic preganglionic neurons innervating the muscle but rostral to the GS motoneurons. Immunohistochemical detection of the enzyme tyrosine hydroxylase (TH) was also incorporated into the experiments, so it could be ascertained whether brainstem neurons infected by PRV injections were catecholaminergic. Infected neurons were present in all the areas of the brain that have previously been shown to contribute to regulating sympathetic outflow: the medullary raphe nuclei, rostral ventrolateral medulla (RVLM), rostral ventromedial medulla, A5 region, locus coeruleus (LC), nucleus subcoeruleus (SC), and the hypothalamic paraventricular nucleus (PVN). The RVLM contained neurons infected following the shortest survival times, indicating that this region might play an especially large role in regulating sympathetic outflow to muscle. Approximately half of the virus‐infected cells in RVLM, A5 and SC were catecholaminergic. These data show that a variety of brainstem areas participate in regulating sympathetic outflow to muscle.