Factors which abolish hypoglycemic seizures do not increase cerebral glycogen content in vitro

The brain is heavily dependant on glucose for its function and survival. Hypoglycemia can have severe, irreversible consequences, including seizures, coma and death. However, the in vivo content of brain glycogen, the storage form of glucose, is meager and is a function of both neuronal activity and glucose concentration. In the intact in vitro hippocampus isolated from mice aged postnatal days 8-13, we have recently characterized a novel model of hypoglycemic seizures, wherein seizures were abolished by various neuroprotective strategies. We had hypothesized that these strategies might act, in part, by increasing cerebral glycogen content. In the present experiments, it was found that neither decreasing temperature nor increasing glucose concentrations (above 2 mM) significantly increased hippocampal glycogen content. Preparations of isolated frontal neocortex in vitro do not produce hypoglycemic seizures yet it was found they contained significantly lower glycogen content as compared to the isolated intact hippocampus. Further, the application of either TTX, or a cocktail containing APV, CNQX and gabazine, to block synaptic activity, did not increase, but paradoxically decreased, hippocampal glycogen content in the isolated intact hippocampus. Significant decreases in glycogen were noted when neuronal activity was increased via incubation with l-aspartate (500 muM) or low Mg(2+). Lastly, we examined the incidence of hypoglycemic seizures in hippocampi isolated from mice aged 15-19 and 22-24 days, and compared it to the incidence of hypoglycemic seizures of hippocampi isolated from mice aged 8-13 days described previously (Abdelmalik et al., 2007 Neurobiol Dis 26(3):646-660). It was noted that hypoglycemic seizures were generated less frequently, and had less impact on synaptic transmission in hippocmpi from PD 22-24 as compared to hippocampi from mice PD 15-19 or PD 8-13. However, hippocampi from 8- to 13-day-old mice had significantly more glycogen than the other two age groups. The present data suggest that none of the interventions which abolish hypoglycemic seizures increases glycogen content, and that low glycogen content, per se, may not predispose to the generation of hypoglycemic seizures.