Evidence that activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors within the nucleus tractus solitarii triggers swallowing.

Swallowing is a patterned motor activity generated by neurons located within the nucleus tractus solitarii (NTS). Previous experiments have shown that administration of excitatory amino acids within the NTS induces swallowing. The present study was undertaken to identify the receptor subtypes involved in this effect. Pressure microinjections of L-glutamate (10-100 pmol), quisqualate (0.1-10 pmol) and N-methyl-D-aspartate (NMDA, 0.1-10 pmol) were performed into the NTS of decerebrate rats. Glutamate and quisqualate microinjections elicited short series of swallows while NMDA microinjections induced long-lasting, rhythmic swallowing. Pretreatment with the selective NMDA antagonist, DL-2-amino-5-phosphonovalerate (50 pmol), almost completely suppressed the response elicited by NMDA (10 pmol) but did not induce a significant modification of swallowing triggered by either glutamate (25 pmol) or quisqualate (10 pmol). Pretreatment with 6-cyano-7-nitroquinoxaline-2,3-dione (50 pmol), a selective blocker of non-NMDA receptors, suppressed the swallows elicited by glutamate and strongly inhibited the response elicited by quisqualate microinjections. The same pretreatment induced only a slight modification of the swallowing elicited by NMDA. These data demonstrate that deglutition can be triggered by activating either NMDA or non-NMDA receptors localized within the NTS, and therefore suggest that both receptor subtypes may be involved in swallowing elicited under physiological conditions.