Antagonism of glutamate receptors in the intermediate and caudal NTS of awake rats produced no changes in the hypertensive response to chemoreflex activation

Abstract

The role of excitatory amino acid (EAA) receptors in the neurotransmission of the sympathoexcitatory component of the chemoreflex (pressor response) in the intermediate and caudal aspects of the commissural nucleus tractus solitarii (NTS) of awake rats was evaluated. Microinjection of kynurenic acid, a non-selective antagonist of EAA receptors, into the intermediate and caudal commissural NTS produced a large increase in the baseline mean arterial pressure (MAP), which may reduce the magnitude of the pressor response to chemoreflex activation. To avoid this problem sodium nitroprusside (SNP) was infused (i.v.) after microinjections of kynurenic acid (2 nmol/50 nl) into the NTS, in order to normalize the MAP and then the chemoreflex was activated and the magnitude of the pressor response evaluated. Microinjection of kynurenic acid into the intermediate (bilaterally) and caudal (midline) commissural NTS ( n=6) produced a significant increase in baseline MAP (103±5 vs. 137±6 mm Hg) normalized by SNP infusion (107±4 mm Hg) and under this experimental condition the pressor response to chemoreflex activation was not statistically different in relation to the control (37±7 vs. 44±6 mm Hg). Bilateral microinjections of kynurenic acid into the caudal NTS ( n=8) also produced a significant increase in baseline MAP (109±4 vs. 145±6 mm Hg) normalized by SNP infusion (109±6 mm Hg). After normalization of MAP, the pressor response to chemoreflex activation at 3 (34±6 mm Hg) and 10 min (37±6 mm Hg) was also not different in relation to the control (46±5 mm Hg). These data indicate that the antagonism of EAA receptors simultaneously in the intermediate (bilateral) and caudal (midline) commissural NTS or only in the caudal commissural NTS (bilateral) of awake rats had no effect on the hypertensive response to chemoreflex activation. We suggest that neurotransmitter other than l-glutamate may take part in the neurotransmission of the sympathoexcitatory component of the chemoreflex at the NTS level.