Glutamate in the nucleus of the solitary tract activates both ionotropic and metabotropic glutamate receptors

Glutamate is the proposed neurotransmitter of baroreceptor afferents at the level of the nucleus of the solitary tract (NTS). Blockade of ionotropic g lutamate receptors with kynurenic acid blocks the arterial baroreflex but, paradoxically, does not abolish the response to exogenous glutamate. This s tudy tested the hypothesis that exogenous glutamate in the NTS activates bo th ionotropic and metabotropic glutamate receptors (mGluRs). In urethan-ane sthetized rats, unilateral microinjections of glutamate into the NTS decrea sed mean arterial pressure, heart rate, and lumbar sympathetic nerve activi ty. The cardiovascular response to injection of glutamate was not altered b y NTS blockade of mGluRs with alpha-methyl-4-carboxyphenylglycine (MCPG). B lockade of ionotropic glutamate receptors with kynurenic acid attenuated th e response to glutamate injection. After combined NTS injection of MCPG and kynurenic acid, the response to glutamate was blocked. These data suggest that exogenous glutamate microinjected into the NTS acts at both ionotropic glutamate receptors and mGluRs. in addition, blockade of both classes of g lutamate receptors is required to block the cardiovascular response to micr oinjection of glutamate in the NTS.