Role of serotonin and catecholamines in sympathetic responses evoked by stimulation of rostral medulla.

In halothane-anesthetized, paralyzed rats, single-pulse stimulation of nucleus raphe pallidus/obscurus (NR) evoked an inhibition followed by a single wave of excitation in 1) the mass discharge of lumbar and splanchnic sympathetic nerves (modal onset latencies: LSND 210 +/- 14 ms, SSND 161 +/- 5 ms) and 2) the unit activity of lumbar sympathetic vasoconstrictor neurons (LSVNs). Stimulation of the rostral ventrolateral medulla (RVL) produced two excitatory responses in LSND, SSND, and LSVN units (modal onset latencies: RVL peak I 98 +/- 8 ms for LSND and 62 +/- 2 ms for SSND; RVL peak II 210 +/- 15 ms for LSND and 160 +/- 6 ms for SSND). Most LSVNs received excitatory inputs from both raphe and RVL. NR peak was selectively attenuated (76%) by 5,7-dihydroxytryptamine [intracisternally (ic), 2-wk] and acute intrathecal (it) methiothepine (10 micrograms, 69%) or methysergide (40 micrograms, 72%). 6-Hydroxydopamine (2-wk ic) produced no effect. Phentolamine (20 micrograms it) or prazosin (20 micrograms it) reduced RVL peak II by > 94% and attenuated NR peak (phentolamine 67%, prazosin 46%). Intrathecal kynurenic acid produced proportionately larger reduction of RVL peak I than raphe peak or RVL peak II. Our interpretations are 1) LSVNs receive convergent excitatory inputs from midline raphe, parphyramidal area, and RVL, 2) bulbospinal serotonergic neurons mediate most of the sympathoexcitation evoked from NR and a portion of RVL peak II, 3) a catecholamine probably released by C1 cells mediates most of RVL peak II and a portion of the NR response, and 4) RVL peak I is predominantly mediated by an excitatory amino acid.