Dorsomedial medulla is more susceptible than rostral ventrolateral medulla to hypoxic insult in cats

Inhibitory actions of serotonin on glutamate release in dorsal medulla suppress systemic arterial pressure of cats

The role of serotonin and glutamate release in dorsal medulla (DM) for regulation of systemic arterial pressure (SAP) was examined with microdialysis and high performance liquid chromatograph in anesthetized cats. KCl-perfusion in DM increased serotonin and glutamate concentrations in DM. Perfusion of serotonin resulted in decreases in glutamate concentration and SAP. Perfusion of alaproclate, a serotonin reuptake inhibitor that produced an increase in serotonin concentration in DM, had the same results as perfusion of serotonin. In conclusion, serotonin and glutamate appeared to be tonically and endogenously released from nerve terminals in DM, and the decrease in SAP could be attributed to the decreased glutamate release resulting from inhibitory action of serotonin in DM. The putative roles of serotonin and glutamate in DM may be important in SAP regulation.

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.