Metabotropic glutamate receptors depress vagal and aortic baroreceptor signal transmission in the NTS.

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 α-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 α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA). The metabotropic glutamate receptor agonist (2 S,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 α-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.