Ionotropic glutamate receptors in the external lateral parabrachial nucleus participate in processing cardiac sympathoexcitatory reflexes

Abstract

Stimulation of cardiac sympathetic afferents during myocardial ischemia with metabolites such as bradykinin (BK) evokes sympathoexcitatory reflex responses and activates neurons in the external lateral parabrachial nucleus (elPBN). The present study tested the hypothesis that this region in the pons processes sympathoexcitatory cardiac reflexes through an ionotropic glutamate receptor mechanism. The ischemic metabolite BK (0.1-1 μg) was injected into the pericardial space of anesthetized and bilaterally vagotomized or intact cats. Hemodynamic and renal sympathetic nerve activity (RSNA) responses to repeated administration of BK before and after unilateral 50-nl microinjections of kynurenic acid (Kyn; 25 mM), 2-amino-5-phosphonopentanoic acid (AP5; 25 mM), and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzol(F)quinoxaline (NBQX; 10 mM) into the elPBN were recorded. Intrapericardial BK evoked significant increases in mean arterial pressure (MAP) and RSNA in seven vagotomized cats. After blockade of glutamate receptors with the nonselective glutamate receptor antagonist Kyn, the BK-evoked reflex increases in MAP (50 ± 6 vs. 29 ± 2 mmHg) and RSNA (59 ± 8.6 vs. 29 ± 4.7%, before vs. after) were significantly attenuated. The BK-evoked responses returned to pre-Kyn levels 85 min after the application of Kyn. Similarly, BK-evoked reflex responses were reversibly attenuated by blockade of glutamate N-methyl-d-aspartate (NMDA) receptors with AP5 (n = 5) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with NBQX (n = 5). In contrast, we observed that the repetitive administration of BK evoked consistent reflex responses including MAP and RSNA before and after microinjection of 50 nl of the artificial cerebrospinal fluid vehicle into the elPBN in five animals. Microinjection of glutamate receptor antagonists into regions outside the elPBN did not alter BK-induced reflex responses. Microinjection of Kyn into the elPBN reversibly attenuated BK-induced reflex responses in four vagus intact animals. These data are the first to show that NMDA and AMPA ionotropic glutamate receptors in the elPBN play an important role in processing cardiac excitatory reflex responses.

Fig. 1.

Bar graphs showing increases [changes in (Δ)] in mean arterial pressure (MAP) and integrated renal sympathetic nerve activity (RSNA) during the intrapericardial application of bradykinin (BK) before and after microinjection of 50 nl of vehicle in five animals (A and B) and kynurenic acid (Kyn; 25 mM) in seven animals (C and D) into the external lataral parabrachial nucleus (elPBN) of vagotomized animals. Values are means ± SE. *P < 0.05 compared with control (ctrl).

Fig. 2.

Original records of arterial pressure, RSNA, and integrated RSNA responses to the intrapericardial application of BK before (A) and after (B) microinjection of Kyn into the elPBN in a cat after bilateral vagotomy. A: increase in MAP by 46 mmHg and integrated RSNA by 56% to initial BK (1 μg). B: blockade of glutamate receptors with Kyn attenuated responses to BK. C: reflex responses to BK during recovery 85 min after the administration of Kyn. Bars 1–6 show representative tracings of RSNA at the times indicated by the short line above the integrated RSNA recordings.

Fig. 3.

Increases (Δ) in MAP (A) and integrated RSNA (B) evoked by intrapericardial BK stimulation before and after microinjection of 50 nl Kyn (25 mM) into the elPBN of four vagus intact animals. Values are means ± SE. *P < 0.05 compared with control.

Fig. 4.

Increases (Δ) in MAP and integrated RSNA induced by the intrapericardial application of BK before and after microinjection of 50 nl of 2-amino-5-phosphonopentanoic acid (AP5; 25 mM) in five cats (A and B) and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzol(F)quinoxaline (NBQX; 10 mM) in five separate animals (C and D) into the elPBN. Values are means ± SE. *P < 0.05 compared with control.

Fig. 5.

Changes in MAP (A) and integrated RSNA (B) induced by the intrapericardial application of BK before and after microinjection of glutamate receptor antagonists into regions outside the elPBN (n = 6). Values are means ± SE.

Fig. 6.

A and B: composite map showing sections of the pons with sites of microinjections within (*, n = 26) and outside (+, n = 6) the elPBN. V4, fourth ventricle; BC, brachium conjuctivum; KF, Kolliker-Fuse nucleus; SOM, medial nucleus of the superior olive; TB, trapezoid body; P, pyramidal tract. C: original slide of the pons showing an actual microinjection site (arrow) in the elPBN of a cat. Scale bar = 3 mm. Coronal sections in A and B were adapted from Berman's atlas (2).