Involvement of NMDA receptors in the respiratory phase-transition is different in the adult guinea pig in vivo and in the isolated brainstem preparation. Morin-Surun, Marie Pierre, Eliane Boudinot, Fusao Kato, Arthur S. Foutz and Monique Denavit-Saubia. Biologie Fonctionnelle du Neurone, Institut Alfred Fessard, C.N.R.S., 91198 Gif-sur-Yvette Cedex, France.
APStracts 2:0109N, 1995.
SUMMARY AND CONCLUSIONS
1. We investigated the involvement of NMDA receptors in the respiratory pattern in an in vitro preparation of adult brainstem compared to in vivo conditions in the guinea pig. 2. In vivo, combining the NMDA channel blocker dizocilpine (MK-801) administration (3 mg kg-1) with a surgical section of the vagus nerves induced an apneustic type of respiration characterized by long inspiratory "holds" as it was shown in other species. The same effect was observed in hypothermic animals (30[acute]iC). 3. The isolated in vitro brainstems from these apneustic animals did not present a prolonged inspiratory phase. A second dose of dizocilpine (100 µM) perfused vascularly did not induce apneusis, even after increasing brainstem temperature to 35.5[acute]iC. 4. In another group of isolated brainstems of adult guinea pigs anesthetized with pentobarbital before the 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 M) or the competitive NMDA antagonist AP5 (100M and 1mM), although respiratory frequency decreased. The increase in respiratory activity produced by vascularly perfused NMDA (25-100 µM) was blocked by dizocilpine (100 M). 6. 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. 

Received 23 September 1994; accepted in final form 31 March 1995.
APS Manuscript Number J598-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  1 May 1995.