Involvement of N-methyl-D-aspartate receptors in post-hypoxic depression of the dorsomedial medulla in cats.

Pressor and sympathoexcitatory responses induced by microinjection of glutamate (Glu) into the dorsomedial medulla (DM) were depressed after hypoxia in anesthetized cats. This study was undertaken to investigate which Glu receptor subtypes would be involved in the post-hypoxic depression. Hypoxia was induced by inhalation of a 5% O(2) and 95% N(2) gas mixture. The pressor and sympathoexcitatory responses to microinjections of N-methyl-D-aspartic acid (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) into the DM were significantly depressed after hypoxia, suggesting the involvement of both NMDA and AMPA receptors. However, pretreatment with kynurenic acid or DL-2-amino-5-phosphonopentanoic acid, but not 6-cyano-7-nitroquinoxaline-2,3-dione, 5 min before hypoxia could effectively prevent the post-hypoxic depression of Glu-induced responses. These results further suggest that post-hypoxic depression of Glu-induced responses in the DM was predominately mediated by NMDA receptors.