Differential roles for glutamate receptor subtypes within commissural NTS in cardiac-sympathetic reflex

Ischemic stimulation of cardiac receptors evokes excitatory sympathetic ref lexes, Although the nucleus of the solitary tract (NTS) is an important sit e for integration of visceral afferents, its involvement in the cardiac-ren al sympathetic reflex remains to be fully defined. This study examined the role of glutamate receptor subtypes in the commissural NTS in the sympathet ic responses to stimulation of cardiac receptors. Renal sympathetic nerve a ctivity (RSNA) was recorded in anesthetized rats. Cardiac receptors were st imulated by epicardial application of bradykinin (BK; 10 mug/ml). Applicati on of BK significantly increased the mean arterial pressure from 78.2 +/-2. 2 to 97.5 +/-2.9 mmHg and augmented RSNA by 38.5 +/-2.5% (P<0.05). Bilatera l microinjection of 10 pmol of 6-cyano-7-nitroquinoxaline-2,3-dione, a non- N-methyl-D-aspartate (NMDA) antagonist, into the commissural NTS eliminated the pressor and RSNA responses to BK application in 10 rats. However, micr oinjection of 2-amino-5-phosphonopentanoic acid (0.1 and 1 nmol, n=8), an N MDA-receptor antagonist, or <alpha>-methyl-4-carboxyphenylglycine (0.1 and 1 nmol, n=5), a glutamate metabotropic receptor antagonist, failed to atten uate significantly the pressor and RSNA responses to stimulation of cardiac receptors with BK. Thus this study suggests that non-NMDA, but not NMDA an d glutamate metabotropic, receptors in the commissural NTS play an importan t role in the sympathoexcitatory reflex response to activation of cardiac r eceptors during myocardial ischemia.