Activation of skeletal muscle afferents evokes release of glutamate in thesubretrofacial nucleus (SRF) of cats

The subretrofacial nucleus (SRF) is a region of the rostral ventrolateral m edulla known to play a crucial role in sympathoexcitation. SRF neurons send direct projections to the intermediolateral cell columns of the spinal cor d where they form synaptic contact with preganglionic sympathetic motor neu rons. Activation of this neural pathway increases sympathetic outflow to th e heart and blood vessels affecting cardiac function and vasomotor tone. Pr evious studies utilizing electrophysiological recording techniques and c-Fo s expression have established that the activity of SRF neurons is increased during skeletal muscle contraction. However, the excitatory neurotransmitt er mediating this increased activity remains in question. In the present st udy, static contraction of the triceps surae was induced by electrical stim ulation of L7 and S1 ventral roots in anesthetized cats (n=12). Endogenous release of glutamate (Glu) from the SRF was recovered by microdialysis and measured by HPLC. Static muscle contraction for 4 min increased mean arteri al pressure (MAP) 38+/-4 mmHg from a control level of 102+/-12 mmHg (P <0.0 5). During muscle contraction the extracellular concentration of Glu recove red from the SRF increased from 623+/-117 to 1078+/-187 nM (P <0.05). To de termine the effect of muscle contraction on Glu release in the absence of s ynaptic input from other reflexogenic areas, contraction was repeated follo wing acute sinoaortic denervation and vagotomy. Following this denervation, muscle contraction increased MAP 41+/-4 mmHg (P <0.05) and Glu concentrati on from 635+/-246 to 1106+/-389 nM (P <0.05). Muscle paralysis prevented th e increases in MAP and Glu concentration during ventral root stimulation. T hese results suggest that: (i) Glu is released in the SRF during activation of contraction-sensitive skeletal muscle afferent fibers in the cat; and ( ii) synaptic input from other reflexogenic areas appears to be ineffective in modulating the release of Glu in the SRF during static muscle contractio n. (C) 2001 Elsevier Science B.V. All rights reserved.