Tonic influences from the rostral medulla affect sympathetic nerves differentially.

Tonically active neurons in the rostral ventrolateral medulla (RVLM) that project to the autonomic regions of the spinal cord are essential for maintenance of arterial blood pressure at normal levels. Microinjection of glycine into the RVLM in anesthetized cats to inhibit the tonic discharge of these neurons caused variable initial responses in renal and mesenteric nerve discharge and arterial blood pressure. These initial responses were consistently followed by more prolonged decreases in renal and mesenteric nerve discharge and decreases in arterial blood pressure. The tonic influences of neurons in the RVLM were found to be distributed unequally to sympathetic nerves because activity of renal nerves was decreased significantly more than that of mesenteric nerves. The variable nerve and cardiovascular responses during the first 1-3 min after glycine injection were not solely due to loading or unloading of baroreceptors because similar initial responses were seen in vagotomized and sinoaortic denervated cats. Additionally, when muscimol was microinjected into the same sites, only consistent and prolonged decreases in nerve discharge and blood pressure occurred. The inhibitory actions of muscimol on RVLM neurons caused significantly greater decreases in renal than mesenteric nerve activity. Together, these findings demonstrate that the tonic discharge of neurons in the RVLM has unequal influences on renal and mesenteric nerves.