Hemodynamic effects elicited by microinjection of glutamatergic agonists into NTS of conscious rats

In this study, we characterized the arterial pressure, heart rate, and regi onal vascular conductance responses elicited by unilateral microinjection o f ionotropic glutamatergic agonists N-methyl-D-aspartic acid (NMDA and non- NMDA) into the nucleus of tractus solitarius (NTS) of conscious rats. Micro injections of NMDA and S-alpha -amino-3-hydroxy-5-methyl-4-isoxazolepropion ic acid (AMPA) caused changes in mean arterial pressure (MAP). Lower doses elicited decreases in MAP, whereas higher doses elicited biphasic responses (decreases followed by increases). Both agonists induced bradycardia and e licited dose-dependent vasoconstriction in the renal, mesenteric, and hindq uarter beds. AMPA elicited delayed vasodilation in the hindquarter bed but NMDA did not. Bradycardia and initial hypotension produced by each agonist were abolished by systemic administration of the muscarinic antagonist meth ylatropine. However, methylatropine did not affect either the vasoconstrict ion or the vasodilatation. The contrasting hemodynamic effects produced by NMDA and AMPA could be caused by activation of differential subsets of NTS neurons. Preferential activation of one subset could produce the NMDA-relat ed responses, whereas activation of another subset would elicit AMPA-relate d responses.