Hippocampal glutamine synthetase: insensitivity to glucocorticoids and stress

Regulation of astrocyte glutamine synthetase in epilepsy

Astrocytes play a crucial role in regulating and maintaining the extracellular chemical milieu of the central nervous system under physiological conditions. Moreover, proliferation of phenotypically altered astrocytes (a.k.a. reactive astrogliosis) has been associated with many neurologic and psychiatric disorders, including mesial temporal lobe epilepsy (MTLE). Glutamine synthetase (GS), which is found in astrocytes, is the only enzyme known to date that is capable of converting glutamate and ammonia to glutamine in the mammalian brain. This reaction is important, because a continuous supply of glutamine is necessary for the synthesis of glutamate and GABA in neurons. The known stoichiometry of glutamate transport across the astrocyte plasma membrane also suggests that rapid metabolism of intracellular glutamate via GS is a prerequisite for efficient glutamate clearance from the extracellular space. Several studies have indicated that the activity of GS in astrocytes is diminished in several brain disorders, including MTLE. It has been hypothesized that the loss of GS activity in MTLE leads to increased extracellular glutamate concentrations and epileptic seizures. Understanding the mechanisms by which GS is regulated may lead to novel therapeutic approaches to MTLE, which is frequently refractory to antiepileptic drugs. This review discusses several known mechanisms by which GS expression and function are influenced, from transcriptional control to enzyme modification.

Effects of dexamethasone on K(+)-evoked glutamate release from rat hippocampal slices

Dexamethasone (DEX) at physiologically elevated (stress) concentration (1 microM) decreased K(+)-evoked glutamate release from rat hippocampal slices under superfusion in the presence of Ca2+. On the contrary 10 microM DEX increased this K(+)-evoked glutamate release while 0.1 microM DEX had no effect. The glucocorticoid antagonist for the "classic" receptor, RU 486, completely reversed the effect of 1 microM DEX. Actinomycin D had no effect. Dexamethasone at 1 microM had no effect on the Ca2(+)-independent (10 mM Mg2+ replacing 1 mM Ca2+) K(+)-evoked glutamate release. Dexamethasone at 1 microM or 10 microM had no effect on the phosphate-activated glutaminase--the key enzyme for the biosynthesis of neurotransmitter glutamate. These results suggest that the effect of DEX on K(+)-evoked glutamate release: (i) depends on its concentration; (ii) is exerted on the Ca2(+)-dependent (neurotransmitter release), at least at physiological stress concentrations; and (iii) is exerted via the classical receptor but is nongenomic.

Glucocorticoid endangerment of hippocampal neurons is NMDA-receptor dependent

The adrenal stress hormones glucocorticoids (GCs) impair the ability of hippocampal neurons to survive neurological insults, including hypoxia-ischemia and seizure. These insults are thought to be toxic via a cascade of excessive synaptic concentrations of excitatory neurotransmitters (e.g. glutamate), activation of the NMDA receptor, and pathologic mobilization of cytosolic calcium post-synaptically. We tested whether GCs exacerbate these insults by exacerbating this 'NMDA cascade'. We sought a toxin which damaged independently of the NMDA cascade, and whose toxicity was enhanced by GCs. After testing a number of neurotoxins, we found that the antimetabolite 3-acetylpyridine (3AP) fit this requirement. We then tested if blockade of the NMDA receptor blocks the ability of GCs to enhance 3AP toxicity. Hippocampi were microinfused with 160 micrograms of 3AP. Elevating circulating GC concentrations to the range seen during major stressors for a week before and after microinfusion caused a significant increase in 3AP-induced damage (when compared to adrenalectomized rats kept GC-free for the same period). Infusing the NMDA receptor blocker APV with 3AP did not alter the toxicity in adrenalectomized rats. However, APV reduced 3AP-induced damage in GC-treated rats to levels seen in adrenalectomized rats. This suggests that GCs endanger hippocampal neurons by enhancing glutamatergic signals and/or enhancing vulnerability to such signals. As a possible explanation for this observation, GCs inhibit glucose uptake into hippocampal neurons, and numerous steps in the NMDA cascade are exacerbated when neuronal energy stores are diminished.