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Department of Veterinary Biomedical Sciences, Dalton Cardiovascular Research Center, University of Missouri-Columbia, Columbia, Missouri 65211
Glutamate
is the proposed neurotransmitter of baroreceptor afferents at the level
of the nucleus of the solitary tract (NTS). Blockade of ionotropic
glutamate receptors with kynurenic acid blocks the arterial baroreflex
but, paradoxically, does not abolish the response to exogenous
glutamate. This study tested the hypothesis that exogenous glutamate in
the NTS activates both ionotropic and metabotropic glutamate receptors
(mGluRs). In urethan-anesthetized rats, unilateral microinjections of
glutamate into the NTS decreased mean arterial pressure, heart rate,
and lumbar sympathetic nerve activity. The cardiovascular response to
injection of glutamate was not altered by NTS blockade of mGluRs with
-methyl-4-carboxyphenylglycine (MCPG). Blockade of ionotropic
glutamate receptors with kynurenic acid attenuated the response to
glutamate injection. After combined NTS injection of MCPG and kynurenic
acid, the response to glutamate was blocked. These data suggest that
exogenous glutamate microinjected into the NTS acts at both ionotropic
glutamate receptors and mGluRs. In addition, blockade of both classes
of glutamate receptors is required to block the cardiovascular response
to microinjection of glutamate in the NTS.
sympathetic nerve activity; blood pressure; arterial baroreflex; rat
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