Excitatory amino acid receptors in the nucleus tractus solitarius mediate the responses to the stimulation of cardio-pulmonary vagal afferent C fiber endings.

Stimulation of cardio-pulmonary vagal afferent C fiber endings by right atrial injections of phenylbiguanide (PBG, 40 micrograms/kg) elicited apnea, bradycardia and hypotension. These responses were abolished by bilateral vagotomy low in the neck. Stimulation of neurons in a specific nucleus 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 stimulation of cardio-pulmonary vagal afferent C fiber endings by PBG. Inhibition of neurons in this NTS site by injections of muscimol abolished the responses to PBG. Specific blockade of NMDA receptors by microinjections of AP-7 (100 pmol) or non-NMDA receptors (KA and AMPA) by injections 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 site blocked the responses to right atrial injections of PBG. These results indicate that: (1) blockade of either NMDA receptors or non-NMDA receptors (KA and AMPA) alone in the above-mentioned NTS site does not abolish the responses to the stimulation of cardio-pulmonary vagal afferent 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 stimulation of these nerve endings.