Evidence of disynaptic projections from the rostral ventrolateral medulla to the thoracic spinal cord

Sympathetic outflow is regulated by a direct pathway of the rostral ventrolateral reticular formation (rvlm) to the thoracic spinal cord. For the first time, a dual retrograde/anterograde transport technique was used to demonstrate by light microscopy, potential disynaptic pathways from the rvlm to the thoracic spinal cord in the rat. An anterograde tracer, biotinylated dextran amine (BDA) was injected into the rvlm and a retrograde tracer, FluoroGold (FG) deposited into the upper thoracic spinal cord in the same animal. Rostral ventrolateral medullary efferents labeled with BDA were apposed to thoracic reticulospinal neurons labeled with FG in the ventrolateral tegmentum, ipsilateral and contralateral to the injection site in the rvlm. Suggestive evidence was obtained of synaptic interactions with neuronal somata and proximal dendrites. The results support the idea that the rvlm projects to the thoracic cord via disynaptic, intrareticular pathways paralleling the well established monosynaptic projection.