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Am J Physiol Regul Integr Comp Physiol 274: R318-R323, 1998;
0363-6119/98 $5.00
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Vol. 274, Issue 2, R318-R323, February 1998

Sympathoinhibition evoked from caudal midline medulla is mediated by GABA receptors in rostral VLM

Matthew J. Coleman1 and Roger A. L. Dampney2

Departments of 1 Biomedical Sciences and 2 Physiology and Institute for Biomedical Research, University of Sydney, New South Wales 2006, Australia

The present study was performed to determine whether the powerful depressor and sympathoinhibitory response that can be evoked from neurons in the caudal midline medulla is mediated by gamma -aminobutyric acidergic (GABAergic) inhibition of sympathoexcitatory neurons in the rostral part of the ventrolateral medulla (VLM). In anesthetized barointact and barodenervated rabbits, bilateral microinjections of bicuculline into sympathoexcitatory sites in the rostral VLM resulted in a sustained increase in renal sympathetic nerve activity and abolished or reversed the depressor and sympathoinhibitory response evoked by glutamate microinjection into the caudal midline medulla. By contrast, the sympathoinhibitory response evoked from the caudal midline medulla persisted when the background level of renal sympathetic nerve activity was reflexly raised by baroreceptor unloading. The results indicate that 1) the depressor and sympathoinhibitory response evoked by stimulation of neurons in the caudal midline medulla is mediated by a GABAergic synapse in the rostral VLM and 2) there are also sympathoexcitatory neurons in the caudal midline medulla whose presence is revealed by blockade of the more powerful sympathoinhibitory response.

gamma -aminobutyric acid; ventrolateral medulla; baroreceptor reflex; raphe pallidus and obscurus; renal sympathetic nerve activity


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