Rostral ventrolateral medulla and sympathorespiratory integration in rats.

The respiratory modulation of the lumbar sympathetic nerve discharge (LSND) was examined in halothane-anesthetized, paralyzed, and vagotomized rats by means of phrenic nerve discharge (PND)-triggered histograms. The respiratory modulation was 1) proportional to PND amplitude during chemoreceptor activation with CO2 and 2) reduced at elevated arterial pressure. Bilateral injections of bicuculline [gamma-aminobutyric acid (GABA)A receptor antagonist, n = 5] into the rostral ventrolateral medulla (RVLM), but not into medullary raphe, reversibly increased mean arterial pressure (MAP) and resting LSND, decreased the baroreflex, reduced PND amplitude and central respiratory rate, and greatly magnified the respiratory modulation of LSND. Injections of strychnine (glycine receptor antagonist, n = 5) or phaclofen (GABAB receptor antagonist, n = 2) into RVLM were without effect. Injections of kynurenic acid (excitatory amino acid receptor antagonist) into RVLM (n = 8), but not raphe (n = 3), reduced PND amplitude, increased central respiratory rate, reduced MAP, elevated resting LSND slightly, and greatly reduced the respiratory modulation of LSND. These data suggest that the rostral tip of the ventrolateral medulla represents a critical link between the central respiratory rhythm generator and the vasomotor outflow. Also, it indicates that the respiratory modulation of SND does not involve a gating of the activity of the medullary neurons that convey baroreceptor information to RVLM sympathoexcitatory neurons.