Involvement of NMDA receptors in the respiratory phase transition is different in the adult guinea pig in vivo and in the isolated brain stem preparation.

1. We investigated the involvement of N-methyl-D-aspartate (NMDA) receptors in the respiratory pattern in an in vitro preparation of adult brain stem compared with in vivo conditions in the guinea pig. 2. In vivo, combining administration of the NMDA channel blocker dizocilpine (MK-801) (3 mg/kg) with a surgical section of the vagus nerves induced an apneustic type of respiration characterized by long inspiratory "holds," as has been shown in other species. The same effect was observed in hypothermic animals (30 degrees C). 3. The isolated in vitro brain stems from these apneustic animals did not present a prolonged inspiratory phase. A second dose of dizocilpine (100 microM perfused vascularly did not induce apneusis, even after increasing brain stem temperature to 35.5 degrees C. 4. In another group of isolated brain stems of adult guinea pigs anesthetized with pentobarbital sodium before decapitation, we perfused dizocilpine and NMDA through the basilar artery. The duration of periodic inspiratory motor activity recorded from the hypoglossal nerve was unaffected by dizocilpine (1-100 microM) or the competitive NMDA antagonist D- or DL-2-amino-5-phosphonopentanoic acid (100 microM and 1 mM), although respiratory frequency decreased. The increase in respiratory activity produced by vascularly perfused NMDA (25-100 microM) was blocked by dizocilpine (100 microM). 5. We conclude that the central mechanism of inspiratory termination in the vagotomized adult guinea pig requires the activation of NMDA receptors in vivo but not in vitro. This difference is not due to the hypothermic environment in vitro. Possible mechanisms for phase switching in vitro are discussed.