Department of Physiology, Northwestern University Medical School, Chicago, Illinois 60611, USA
Address for correspondence: Shaun F. Morrison, Department of Physiology (M211), Northwestern University Medical School, 303 E. Chicago Avenue, Chicago, IL 60611. Voice: 312-503-5024; fax: 312-503-5101.
s-morrison2{at}northwestern.edu
The medullary premotor neurons determining the sympathetic outflow
regulating cardiac function and vasoconstriction are located
in the rostral ventrolateral medulla (RVLM). The present study
sought evidence for differential characteristics and baroreceptor
reflex sensitivities between the sympathetic nerve activity
(SNA) controlling brown adipose tissue (BAT) metabolism and
thermogenesis and cardiovascular SNA such as that controlling
mesenteric vasoconstriction via the splanchnic (SPL) nerve.
The tonic discharge of sympathetic nerves is determined by the
inputs to functionally specific sympathetic preganglionic neurons
from supraspinal populations of premotor neurons. Under normothermic
conditions, BAT SNA was nearly silent, while SPL SNA exhibited
sustained, large-amplitude bursts. Disinhibition of neurons
in the rostral raphe pallidus (RPa), a potential site of sympathetic
premotor neurons controlling BAT SNA, or icv injection of prostaglandin
E
2, a pyrogenic stimulus, elicited a dramatic increase in BAT
SNA. SPL SNA was strongly influenced by the baroreceptor reflex
as indicated by a high coherence to the arterial pressure, while
activated BAT SNA exhibited no correlation with the arterial
pressure. Since these characteristics and reflex responses in
sympathetic outflow have been shown to arise from the ongoing
or altered discharge of sympathetic premotor neurons, the marked
differences between SPL SNA and BAT SNA provide strong evidence
supporting the hypothesis that vasoconstriction and thermogenesis
(metabolism) are controlled by distinct populations of sympathetic
premotor neurons, the former in the RVLM and strongly baroreceptor-modulated
and the latter potentially in the RPa exhibiting little influence
of baroreceptor reflex activation.
