Metabotropic glutamate receptor subtypes differentially influence neuronal recovery from in vitro hypoxia/hypoglycemia in rat hippocampal slices

Hippocampal slices were transiently exposed to an oxygen- and glucose-free environment which causes a pronounced drop of both ATP and creatine phosphate, an anoxic depolarization, and an incomplete recovery of synaptically evoked population spike in the CA1 region after 1 h (48.5 ± 3.6% of baseline values). This recovery could be markedly enhanced by the application of N-methyl- d-aspartate receptor antagonists. To examine the influence of metabotropic glutamate receptors on neuronal recovery from hypoxia/ hypoglycemia, we applied various antagonists and agonists of the metabotropic glutamate receptors to the bath during the interval from 20 min before to 10 min after hypoxia/hypoglycemia. The metabotropic glutamate receptor antagonists (+)-α-methyl-4-carboxyphenylglycine and l-2-amino-3-phosphonopropionic acid were both able to enhance the population spike recovery significantly. However, the mixed metabotropic glutamate receptor agonist 1 S,3 R-1-aminocyclopentane-1,3-dicarboxylic acid also exhibited a protective effect on population spike recovery, leaving the anoxic depolarization and N-methyl- d-aspartate responses during the hypoxia/hypoglycemia untouched. With the help of more subtype-specific agonists, we found that an activation of phospholipase C coupled (class 1) metabotropic glutamate receptors prior to hypoxia/hypoglycemia may be responsible for the protective effect seen with 1 S,3 R-1-aminocyclopentane-1,3-dicarboxylic acid, because the specific class 1 metabotropic glutamate receptor agonist trans-azetidine-2,4-dicarboxylic acid appeared to be highly protective, but only if it was applied 20 min before the hypoxia/hypoglycemia. An activation of class 2 metabotropic glutamate receptors by (2 S1′ R,2′ R,3′ R)-2-(2,3-dicarboxycycloprophyl)glycine, which inhibits adenylyl cyclase activity, led to a marked deterioration of the population spike recovery and even to a total prevention of the protective effect of the N-methyl- d-aspartate agonist d-2-amino-5-phosphonopentanoic acid. Our data suggest that prior activation of class 1 metabotropic glutamate receptors is beneficial, while their activation during hypoxia/hypoglycemia is detrimental. Furthermore, the activation of class 2 metabotropic glutamate receptors decreases the recovery from hypoxia/hypoglycemia.