Effect of N-methyl-D-aspartate applied to the ventral surface of the medulla on the trachea

Structures located near the ventral surface of the medulla (VMS) affect both cardiovascular tone and respiratory activity. In addition cooling the intermediate area of the VMS blocks the increases in parasympathetic activity and tracheal tone resulting from ventilation with hypercapnic or hypoxic gas mixtures, or due to stimulation of mechanoreceptors within the lung. Since cooling the surface of the VMS may affect fibers of passage as well as cell bodies, we performed studies in which pledgets containing N-methyl-D-aspartic acid (NMDA), a synthetic excitatory amino acid, were applied to intermediate area of the VMS. The studies were performed in chloralose-anesthetized, artificially ventilated cats. Application of pledgets containing NMDA (10(-7) mol at 10(-3) M) caused increases in tracheal pressure and the onset of phasic phrenic activity, but application of 10(-8) mol at 10(-4) M of NMDA could produce tracheal constriction without the appearance of phasic phrenic activity. Applying to the entire VMS either 2-amino-5-phosphonovalerate (2-APV, 10(-6) M), a specific antagonist to NMDA, or lidocaine (2%), a local anesthetic, 60 s before the application of pledgets containing NMDA, prevented the increase in tracheal tone and phasic phrenic activity. Intravenous administration of atropine methyl nitrate 0.5 mg/kg, a cholinergic antagonist, blocked tracheal responses to local application of pledgets containing NMDA but did not affect the increase in phasic phrenic nerve activity. These findings suggest that when stimulated, neurons near the surface of the VMS in the vicinity of the intermediate area increase the activity of parasympathetic fibers to the airway.