Ventrolateral medullary neurons relay cardiovascular inputs to the paraventricular nucleus

Horseradish peroxidase (HRP) and single-unit recording experiments were done in cats to identify neurons in ventrolateral medulla (VLM) that project directly to the paraventricular nucleus (PVH) and relay cardiovascular information from carotid sinus (CSN) and aortic depressor (ADN) nerves. After diffusion of HRP into the PVH, retrogradely labeled neurons were observed in the VLM. The region of the VLM containing HRP-labeled neurons was then explored for single units antidromically activated by stimulation of the PVH in chloralosed, paralyzed, and artificially ventilated cats. These units were then tested for their responses to stimulation of the CSN and ADN. Antidromic potentials were recorded from 100 units in the VLM. Of these units, 65% were orthodromically excited by stimulation of buffer nerves; 28 by only CSN, 19 by only ADN, and 18 by both CSN and ADN. The axons of antidromically activated units responding to buffer nerves conducted at slower velocities than those of nonresponsive units. These data demonstrate that VLM neurons projecting directly to PVH integrate cardiovascular afferent information and suggest that these VLM neurons may be involved in the control of the activity of magnocellular neurosecretory neurons in the PVH during activation of baroreceptor and chemoreceptor afferent fibers.