Evidence for two functionally distinct vasomotor subregions of rostral ventral medulla

We examined two functionally and anatomically distinct subregions of rostral ventral medulla, rostral ventrolateral medulla (RVLM, 2 mm lateral to midline) and an area we term rostral ventromedial medulla (RVMM, 1mm lateral to midline). Reducing tidal volume attenuated the depressor response to microinjection of lidocaine (200nl, 4%) into RVLM, but not into RVMM. Lidocaine reduced renal (RVR), mesenteric (MVR), and hindquarter (HQVR) vascular resistances, when injected into both sites, however HQVR was markedly reduced when RVMM was blocked. Selective stimulation with microelectrodes increased RVR, MVR, and HQVR, with RVMM producing the strongest response in HQVR and RVLM the strongest effect on RVR. Cardiovascular responses to stimulation of RVLM required the integrity of RVMM, however, the converse was not found. The integrity of RVMM, but not RVLM, was required for the cardiovascular responses produced by electrical stimulation of lateral hypothalamus. Together, these data suggest that RVLM and RVMM are both important for tonic regulation of vasomotor tone, however, these sites differ in their responses to afferent input and in the control of regional vascular resistances.