Raphe pallidus excites a unique class of sympathetic preganglionic neurons.

The responses of splanchnic sympathetic preganglionic neurons (SPNs) to stimulation in raphe pallidus and in rostral ventrolateral medulla (RVLM) were compared to determine the basis for the excitatory responses evoked in the whole splanchnic preganglionic nerve bundle. Most (88%) of the SPNs with a short-latency (32 ms) excitatory response to RVLM stimulation were unaffected by raphe pallidus stimulation, although 12% were excited at a long latency (123 ms). Each of the SPNs with long-latency (114 ms) excitatory responses to RVLM stimulation was also excited by raphe pallidus stimulation at latencies (106 ms) that were 7 ms (P < 0.01) shorter than those evoked from the RVLM. Antidromic activation of raphe pallidus neurons from both the T8 intermediolateral nucleus (98 ms) and from the RVLM (18 ms) indicated that their spinally projecting axons emit collaterals (mean conduction time: 12 ms) into the ventrolateral medulla. In conclusion, the short-latency (70 ms) splanchnic nerve excitation evoked by RVLM stimulation is mediated primarily by SPNs that do not respond to raphe pallidus stimulation. Similarly, the long-latency (162 ms) splanchnic excitation evoked from the raphe pallidus is mediated primarily by SPNs that do not respond to the rapidly conducting sympathoexcitatory pathway from the RVLM. The long-latency (169 ms), RVLM stimulus-evoked excitation of the splanchnic nerve may arise from action potentials conducted on the axonal branches of raphe spinal neurons.