Interactions between the rostral ventral medulla and other central sites involved in vasomotor regulation.

We showed earlier that the rostral ventral medulla can be functionally and anatomically divided into the rostral ventrolateral medulla (RVLM) and the rostral ventromedial medulla (RVMM). In this study, we examined the relationship between the lateral hypothalamus and these two medullary sites. Electrical stimulation of the lateral hypothalamus produced a pressor response mediated by increases in renal and mesenteric vascular resistance. Inactivating RVLM had no effect on this response whereas inactivating RVMM significantly blunted the pressor response and the increases in renal and mesenteric resistance. In other studies, inactivating dorsomedullary sites significantly reduced arterial pressure. Unlike RVLM, these depressor responses were not significantly altered by reducing tidal volume. Inactivating sites 0.5 mm medial and lateral to the RVMM, as well as a site 0.5 mm caudal to the RVLM, resulted in depressor responses that were unaffected by reducing tidal volume. However, inactivation of a site 1.0 mm caudal to the RVLM but still within the ventrolateral medullary pressor area, resulted in a depressor response that was enhanced by reduced tidal volume. Together, these data demonstrate a functional differentiation of medullary sites controlling vasomotor outflow.