Different neurons of the rostral ventrolateral medulla (RVL) regulate sympathetic activity and regional cerebral blood flow (RCBF) and EEG

Stimulation of the RVL electrically, chemically or by histotoxic hypoxia with NaCN elevates arterial pressure (AP), increases global rCBF and synchronizes the electroencephalogram (EEG) (Golanov and Reis, Am. J. Physiol. 268:R454u, 1994). We sought to determine whether the same neurons in RVL mediate the sympathetic, cerebrovascular, and EEG responses. Rats (n=32) were anesthetized, spinalized, ventilated, and instrumented to record AP, cortical rCBF (laser-Doppler flowmetry) and EEG and the medulla systematically explored by microstimulation with 5 sec trains (100 mA, 50 Hz). Stimulation within the sympathoexcitatory region of the rostral RVL (rRVL) increased rCBF within 2 sec by 20+3% and increased the power of the 4-6 Hz EEG rhythm. However, the largest elevations of rCBF (by 34+8%, p<0.05, compared with rRVL) were elicited from a more caudal vasopressor region (cRVL) which also synchronized the EEG. Comparable results were obtained by microinjection of L-glutamate (5 nM, 40 nL) into rRVL and cRVL. Inactivation of the cRVL (procaine or lesions) reduced the increase in rCBF (by 70%, p<0.05) and EEG synchronization evoked from rRVL. In contrast, inactivation of rRVL minimally changed responses to stimulation of cRVL. NaCN elevated rCBF only from rRVL. We conclude: (a) regions of RVL exciting spinal sympathetic neurons and modifying cerebral cortical blood flow and EEG are anatomically distinct; (b) only rRVL neurons are excited by hypoxia increasing AP over a spinosympathetic pathway and elevating rCBF and synchronizing the EEG indirectly by a projection to cRVL.