Differential drives from rostral ventrolateral medullary neurons to three identified sympathetic outflows

Simultaneous recordings were made in chloralose-anesthetized cats from muscle vasoconstrictor (MVC), visceral vasoconstrictor (VVC), and renal (RSN) sympathetic fibers. Their barosensitivity was demonstrated, after which all baroreceptor nerves were cut. Multiple microinjections of sodium glutamate (5 nl, 0.1 M) were made in a grid pattern covering the rostral ventrolateral medulla (RVLM) pressor area on seven sides of four animals. Injections increased blood pressure by up to 87 +/- 22 mmHg and MVC, VVC, and RSN activities by up to 169 +/- 55, 62 +/- 23, and 67 +/- 36%, respectively. The relative responses between nerves (taken in pairs) showed significant inhomogeneity across injections into 5/7, 6/7, and 1/7 RVLMs for MVC vs. VVC, MVC vs. RSN, and VVC vs. RSN, respectively. Calculation showed that overall, less than half the response spikes due to RVLM glutamate injections could be accounted for by a hypothetical population of neurons that excited MVC, VVC, and RSN outflows in fixed proportion. Sites that exclusively drove MVC, VVC, or RSN outflows were found in 7/7, 3/7, and 5/7 RVLMs, respectively. Sites that selectively drove either nerve of a given pair were ubiquitous. The RVLM territories from which the three outflows could be activated overlapped but showed clear differences. Their shapes and sizes differed between animals and even between sides. They covered the subretrofacial (SRF) nucleus plus a variable rostromedial extension. The data support the view that the RVLM neurons driving these three sympathetic outflows are largely, perhaps entirely, separate populations.