NMDA AS WELL AS NON-NMDA RECEPTORS MEDIATE THE NEUROTRANSMISSION OF INSPIRATORY DRIVE TO PHRENIC MOTONEURONS IN THE ADULT-RAT

The neurotransmission of bulbospinal respiratory drive is believed to involve primarily non-NMDA receptors located in the phrenic motonucleu s (PMN). This conclusion is based on studies carried out mainly on in vitro brainstem-spinal cord preparations of the neonatal rat. The pres ent study was undertaken to investigate the transmitter/receptor mecha nisms in the PMN which are involved in the neurotransmission of inspir atory drive, using an in vivo adult rat model, Microinjections of glut amate, NMDA and AMPA into the PMN elicited an increase in the phrenic nerve (PN) background discharge. These injections did not alter signif icantly the frequency of spontaneously occurring PN bursts confirming that mechanisms responsible for respiratory rhythm reside in the supra spinal structures. Microinjections of an NMDA receptor blocker (AP-7), in concentrations that did not alter the responses to a non-NMDA rece ptor agonist (AMPA), reduced the PN amplitude significantly. Similarly , microinjections of a potent non-NMDA receptor blocker (NBQX), in con centrations that did not alter responses to NMDA, reduced the PN ampli tude significantly. Sequential microinjections, within an interval of 5 min, of AP-7 and NBQX into the PMN, resulted in a dramatic reduction in the spontaneous PN bursts. The reduction of PN amplitude started i mmediately after the microinjection of AP-7 and NBQX, either alone or in combination, and reached a maximum within 5-10 min. These results i ndicate that, unlike in the neonatal rat, both NMDA and non-NMDA recep tors located in the PMN play a significant role in the neurotransmissi on of the inspiratory drive in the adult rat.