SYNAPTIC POTENTIALS IN RESPIRATORY NEURONS DURING EVOKED PHASE SWITCHING AFTER NMDA RECEPTOR BLOCKADE IN THE CAT

1. Blockade of NMDA receptors by dizocilpine impairs the inspiratory o ff-switch (IOS) of central origin but not the IOS evoked by stimulatio n of sensory afferents. To investigate whether this difference was due to the effects of different patterns of synaptic interactions on resp iratory neurones, we stimulated electrically the superior laryngeal ne rve (SLN) or vagus nerve in decerebrate cats before and after I.V. adm inistration of dizocilpine, whilst recording intracellularly. 2. Phren ic nerve responses to ipsilateral SLN or vagal stimulation were: at mi d-inspiration, a transient inhibition often followed by a brief burst of activity; at late inspiration, an IOS; and at mid-expiration, a lat e burst of activity. 3. In all neurones (n = 16), SLN stimulation at m id-inspiration evoked an early EPSP during phase 1 (latency to the arr est of phrenic nerve activity), followed by an IPSP in inspiratory (I) neurones (n = 8) and by a wave of EPSPs in post-inspiratory (PI) neur ones (n = 8) during phase 2 (inhibition of phrenic activity). An EPSP in I neurones and an IPSP in PI neurones occurred during phase 3 (brie f phrenic burst) following phase 2. 4. Evoked IOS was associated with a fast (phase 1) activation of PI neurones, whereas during spontaneous IOS, a progressive (30-50 ms) depolarization of PI neurones preceded the arrest of phrenic activity. 5. Phase 3 PSPs were similar to those occurring during the burst of activity seen at the start of spontaneou s inspiration. 6. Dizocilpine did not suppress the evoked phrenic inhi bition and the late burst of activity. The shapes and timing of the ev oked PSPs and the changes in membrane potential in I and PI neurones d uring the phase transition were not altered. 7. We hypothesize that af ferent sensory pathways not requiring NMDA receptors (1) terminate ins piration through a premature activation of PI neurones, and (2) evoke a late buret of phrenic activity which might be the first stage of the inspiratory on-switch.