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Division of Cardiovascular Medicine and Department of Pharmacology, University of California, Davis, California 95616
We sought to determine whether metabotropic
glutamate receptors contribute to frequency-dependent depression of
vagal and aortic baroreceptor signal transmission in the nucleus of the solitary tract (NTS) in vivo. In 
-chloralose-anesthetized rabbits, we determined the number of extracellular action potentials
synaptically evoked by low (1 Hz)- or high-frequency vagal (3-20
Hz) or aortic depressor nerve (ADN) (6-80 Hz) stimulation and
postsynaptically evoked by the ionotropic glutamate receptor agonist
-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). The
metabotropic glutamate receptor agonist
(2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) attenuated NTS responses monosynaptically evoked by 1-Hz
vagus stimulation by 34% (n = 25;
P = 0.011), while augmenting AMPA-evoked responses by 64% (n = 17;
P = 0.026). The metabotropic glutamate
receptor antagonist -methyl-4-phosphonophenylglycine (MPPG) did not
affect NTS responses to low-frequency vagal stimulation (n = 11) or AMPA
(n = 10) but augmented responses to
high-frequency stimulation by 50% (n = 25; P = 0.0001). MPPG also augmented
NTS responses to high-frequency ADN stimulation by 35%
(n = 9;
P = 0.048) but did not affect
responses to low-frequency stimulation (n = 9) or AMPA
(n = 7). The results suggest that
metabotropic glutamate receptors, presumably at presynaptic sites,
contribute to frequency-dependent depression of vagal and aortic
baroreceptor signal transmission in NTS.
frequency; synaptic transmission; autoreceptors
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