Glutamate in synaptic terminals is reduced by lack of glucose but not hypoxia in rat hippocampal slices.

Although excessive release of the neurotransmitter glutamate contributes to ischemic neuronal damage, immunocytochemical studies have not found a loss of glutamate from ischemic axon terminals. We examined the effects of two components of ischemia, hypoxia and hypoglycemia, on glutamate loss from rat hippocampal slices. In vitro hypoglycemia induced by incubation for 1 h without glucose depleted 50% of glutamate from slices when ATP levels were about 5 nmol/mg protein. Hypoxic slices aerated with N2 reached similar ATP levels without significant glutamate depletion. To induce 50% glutamate losses with chemical hypoxia, ATP had to be depleted to < 1 nmol/mg protein. Immunocytochemical staining indicated that glutamate-like immunoreactivity was reduced throughout slices by hypoglycemia. Hypoxia decreased glutamate-like immunoreactivity in neuronal perikarya and dendrites of pyramidal cells and granule cells. However, in contrast to hypoglycemia, hypoxia maintained or increased glutamate-like immunoreactivity in many terminals. Hypoxia and hypoglycemia induced similar, ATP-dependent releases of glutamate into supernatants, which could account for only part of the hypoglycemic losses. The additional hypoglycemic losses were consistent with increased catabolism of glutamate. Glutamate losses from hypoglycemic terminals were reduced by blockade of aspartate aminotransferase or the tricarboxylic acid cycle. Exogenous glutamate increased glutamate in hypoglycemic slices to hypoxic levels and returned glutamate-like immunoreactivity to terminals, suggesting that terminals maintained glutamate uptake during metabolic insults. Hypoglycemia induces a large loss of glutamate that does not occur during hypoxia. The greater loss of glutamate from terminals during hypoglycemia is consistent with increased metabolism of glutamate via aspartate aminotransferase and not increased release of glutamate. Continued uptake of glutamate by hypoxic terminals may help to maintain their levels of glutamate. Hypoglycemic metabolism of glutamate may decrease pathologic glutamate release and contribute to the prolonged neurologic abnormalities associated with recovery from hypoglycemia.