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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
tractus solitarius (NTS). Exogenous glutamate in the NTS activates
neurons through ionotropic and metabotropic glutamate receptors
(mGluRs). This study tested the hypothesis that group I mGluRs in the
NTS produce depressor, bradycardic, and sympathoinhibitory responses.
In urethan-anesthetized rats, unilateral 30-nl microinjections of the
group I-selective mGluR agonist 3,5-dihydroxyphenylglycine (DHPG) into
the NTS decreased mean arterial pressure, heart rate, and lumbar
sympathetic nerve activity. The dose of drug that produced 50% of the
maximal response (ED50) was 50-100 µM. The response
to microinjection of equal concentrations of DHPG or the general mGluR
agonist
1-aminocyclopentane-1S,3R-dicarboxylic acid (ACPD) produced similar cardiovascular effects. The cardiovascular response to injection of DHPG or ACPD was abolished by NTS blockade of
mGluRs with 
-methyl-4-carboxyphenylglycine (MCPG). Blockade of
ionotropic glutamate receptors with kynurenic acid did not attenuate
the response to DHPG or ACPD injection. These data suggest that DHPG
and ACPD activate mGluRs in the NTS and do not require ionotropic
glutamate receptors to produce their cardiovascular response. In the
NTS the group I mGluRs produce responses that are consistent with
excitation of neurons involved in reducing sympathetic outflow, heart
rate, and arterial pressure.
sympathetic nerve activity; arterial baroreflex; 3,5-dihydroxyphenylglycine; 1-aminocyclopentane-1S,3R-dicarboxylic acid
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