The response to microinjection of glutamate in the nucleus of the solitary tract is dependent on ionotropic and metabotropic receptors

We hypothesized that glutamate (Glu) microinjected into the nucleus of the solitary tract (NTS) acts on both ionotropic and metabotropic Glu receptors (mGluRs). Urethane anesthetized rats were instrumented with catheters, electrodes for lumbar sympathetic nerve activity (LSNA), and placed in a stereotaxic apparatus. Multibarrel glass pipettes were placed unilaterally into the NTS. Control Glu responses (10 mM, 30 nl) were compared to Glu responses recorded during microinjection (30 nl/10 sec) of 1) kynurenic acid (KYN, 40 mM) to block ionotropic receptors, 2) α-methyl-4-carboxyphenylglycine (MCPG, 10 mM) to block mGluRs, or 3) both KYN and MCPG. Control Glu injections decreased MAP (19±3 mmHg), HR (10±4 bpm), and LSNA (22±4 %). During microinjection of KYN, the MAP, HR, and LSNA response to Glu was partially inhibited (31±22%, 43±13%, 52±14%, respectively). MCPG also partially inhibited the MAP and LSNA response to Glu injection (44±20%, 27±19%, respectively). Combination of ionotropic and mGluR blockade produced a greater inhibition than either antagonist alone-MAP (77±4%), HR (44±16%), LSNA (84±5%). Data suggest that glutamate injected into the NTS acts on both ionotropic and mGluRs. Blockade of both types of receptors is required to block the response to microinjection of Glu into the NTS.