Excitatory somato-sympathetic reflexes are relayed in the caudal ventrolateral medulla in the cat

The caudal ventrolateral medulla (CVLM) modulates sympathetic outflow from the rostral ventrolateral medulla (RVLM). We studied the possible role of the CVLM in the transmission of excitatory somato-sympathetic reflexes in baro- and chemoreceptor denervated chloralose-anesthetized cats. Neurotoxic doses of kainate, injected in the CVLM, caused marked increases in baseline sympathetic nerve activity (SNA) and arterial blood pressure (BP). Concomitantly, excitatory somato-sympathetic reflex responses evoked by electrical stimulation of the 4th intercostal nerve disappeared almost completely. Similar effects on SNA and BP but not on somato-sympathetic reflexes were observed when the GABA-antagonist bicuculline was injected in the RVLM. Bicuculline injected in the RVLM after kainate had no additional effects. These results suggest that in addition to a tonic GABA-ergic inhibition on the RVLM, the CVLM controls somato-sympathetic reflex transmission through interneurons located in this region.