EXCITATORY AMINO-ACID RECEPTORS IN THE NUCLEUS-TRACTUS-SOLITARIUS MEDIATE THE RESPONSES TO THE STIMULATION OF CARDIOPULMONARY VAGAL AFFERENT C-FIBER ENDINGS

Stimulation of cardio-pulmonary vagal afferent C fiber endings by righ t atrial injections of phenylbiguanide (PBG, 40 mug/kg) elicited apnea , bradycardia and hypotension. These responses were abolished by bilat eral vagotomy low in the neck. Stimulation of neurons in a specific nu cleus tractus solitarius site (0.5 mm rostral, 0.5 mm lateral and 0.5 mm deep with reference to the calamus scriptorius) by injections of L- glutamate produced responses similar to those following the stimulatio n of cardio-pulmonary vagal afferent C fiber endings by PBG. Inhibitio n of neurons in this NTS site by injections of muscimol abolished the responses to PBG. Specific blockade of NMDA receptors by microinjectio ns of AP-7 (100 pmol) or non-NMDA receptors (KA and AMPA) by injection s of DNQX (10-25 pmol) into this NTS site did not block the responses to PBG. Microinjections of kynurenic acid (1-4 nmol) into this NTS sit e blocked the responses to right atrial injections of PBG. These resul ts indicate that: (1) blockade of either NMDA receptors or non-NMDA re ceptors (KA and AMPA) alone in the above-mentioned NTS site does not a bolish the responses to the stimulation of cardio-pulmonary vagal affe rent C fiber endings; (2) it is necessary to block NMDA as well as non -NMDA receptors in this NTS site for abolishing the responses to the s timulation of these nerve endings.