Responses of sympathetic preganglionic neurons to rostral ventrolateral medullary stimulation

Sympathoexcitatory pathways from the rostral ventrolateral medulla (RVL) play a significant role in maintaining the tonic discharge of sympathetic preganglionic neurons (SPNs) and, in turn, resting arterial pressure. We examined the responses of antidromically identified, splanchnic SPNs to single stimuli applied to the RVL. Four characteristic response patterns were observed. Forty-five percent (group 1) of responsive SPNs were excited with latencies between 15 and 41 ms (mean 27 ms). The spontaneous activity of these SPNs (mean 3.3 Hz) was strongly modulated by the baroreceptor reflex. Twenty-three percent (group 2) of SPNs were excited with a significantly longer latency (mean 108 ms) and frequently exhibited little spontaneous discharge. Twenty-one percent (group 3) were excited at both short (mean 29 ms) and long (mean 117 ms) latencies. Eleven percent (group 4) of responsive splanchnic SPNs had a biphasic response to RVL stimulation consisting of an early inhibition followed by a late, longer lasting increase in discharge probability. Spontaneous discharge of group 4 SPNs (mean 6.2 Hz) was significantly higher than other groups and was not related to the cardiac cycle (mean arterial pressure 105 mmHg). Microinjection of kainic acid into the RVL greatly reduced the short- and long-latency splanchnic nerve excitations evoked by RVL stimulation. The heterogeneity of the RVL stimulus-evoked responses in splanchnic SPNs may reflect specificity in the descending regulation of functionally distinct subsets of SPNs regulating the various tissues in the splanchnic distribution.