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Glucocorticoids (GCs), the adrenal steroid hormones secreted during stress, can damage the hippocampus and impair its capacity to survive coincident neurological insults. This GC endangerment of the hippocampus is energetic in nature, as it can be prevented when neurons are supplemented with additional energy substrates. This energetic endangerment might arise from the ability of GCs to inhibit glucose transport into both hippocampal neurons and astrocytes. The present study explores the GC inhibition in astrocytes. (1) GCs inhibited glucose transport approximately 15-30% in both primary and secondary hippocampal astrocyte cultures. (2) The parameters of inhibition agreed with the mechanisms of GC inhibition of glucose transport in peripheral tissues: A minimum of 4 h of GC exposure were required, and the effect was steroid specific (i.e., it was not triggered by estrogen, progesterone, or testosterone) and tissue specific (i.e., it was not triggered by GCs in cerebellar or cortical cultures). (3) Similar GC treatment caused a decrease in astrocyte survival during hypoglycemia and a decrease in the affinity of glutamate uptake. This latter observation suggests that GCs might impair the ability of astrocytes to aid neurons during times of neurologic crisis (i.e., by impairing their ability to remove damaging glutamate from the synapse).

Feedback inhibition of the adrenocortical axis by circulating glucocorticoids occurs at the pituitary and CNS sites. In the CNS, both hypothalamic and suprahypothalamic sites have been implicated as m

A classical action of glucocorticoids (GCs) is to inhibit glucose uptake into various peripheral tissues. Two recent reports suggest that GCs do the same in the brain. Because of the in vivo nature of those studies, it was impossible to determine whether this inhibition occurred at the blood-brain barrier, and/or within neurons and glia themselves. In order to answer this and other mechanistic questions, we examined the effects of GCs on glucose transport in primary brain cultures. We established that uptake of 14C-2-deoxyglucose into hippocampal cultures was linear over a 15-min period and was inhibited by D-glucose and the uptake inhibitor cytochalasin B. Using this system, we found the following. (1) Both corticosterone and dexamethasone inhibited uptake into cultures containing both neurons and glia. (2) The effect was dose-dependent; steroid concentrations in the nanomolar range inhibited uptake from 20 to 33%. The effect was time-dependent, with more than 4 h of steroid exposure needed for inhibition. (3) Non-GC steroids did not inhibit uptake. (4) The GC inhibition seemed to be mediated by the type II (glucocorticoid) corticosteroid receptor. The effect was blocked by a type II, but not a type I (mineralocorticoid) receptor antagonist. Moreover, corticosterone inhibited only at concentrations well above the Kd for the type I receptor. Finally, aldosterone inhibited transport when applied at concentrations that bound heavily to type II receptors. (5) Corticosterone did not inhibit uptake in hypothalamic, cerebellar or cortical cultures, despite the presence of corticosteroid receptors in these cultures. (6) GCs inhibited uptake in both neuron- and glia-enriched hippocampal cultures.

Glucocorticoids (GCs) can damage the hippocampus when exposure is prolonged, as well as impair the capacity of hippocampal neurons to survive various neurological insults. We have recently demonstrated that GCs impair the capacity of primary hippocampal cultures to survive many of these same insults. Using this culture system, we have characterized the features with which the GC corticosterone (CORT) impairs the capacity of these cells to survive the excitotoxin kainic acid. The GC endangerment seems to be mediated by the type II, but not type I corticosteroid receptor. As evidence for type II involvement, endangerment of cells was caused by CORT concentrations in the kilodalton range for the type II receptor, and also by the type II ligand dexamethasone; moreover, the endangerment was blocked by a type II antagonist. In contrast, a type I antagonist was not protective. Cultures contained both type I and II receptors. The effect was GC-specific, as cultures were endangered by CORT, cortisol and dexamethasone, but not by non-GC steroids. GCs did not exacerbate kainic acid damage in cerebeller or hypothalamic cultures, despite the presence of corticosteroid receptors. This agrees with the in vivo data showing that the GC exacerbation of neurological insults is either exclusive to or predominant in the hippocampus.

Glucocorticoids (GCs) disrupt the energy metabolism of neurons of the hippocampus, and thus leave them more vulnerable to a variety of damaging metabolic insults. In this manner, GCs appear to influence the rate of hippocampal neuron loss during aging in the rat, as well as the severity of hippocampal damage following hypoxia-ischemia or seizure. These GC actions could be secondary to their multitudinous peripheral actions. The present report, however, suggests that GCs directly endanger hippocampal neurons. Glucocorticoid-induced sensitization of neurons to damaging toxins was demonstrated in vitro. The viability of primary cultures of dispersed fetal rat hippocampal neurons was assessed following exposure to a variety of neurotoxins. Prior incubation of the cultures with the rodent-typical GC, corticosterone, significantly decreased neuronal viability in the face of the toxins. Such compounds included the glutaminergic excitotoxin kainic acid, the antimetabolite 3-acetylpyridine and the superoxide radical generator paraquat. As little as 10(-9) M corticosterone could potentiate damage, a concentration equivalent to low basal values in vivo. Higher concentrations of corticosterone could potentiate damage even further; these corticosterone concentrations were not themselves damaging. Administration of glucose increased neuronal viability in the face of the GC/toxin combination, without increasing viability following toxin alone. This suggests that a critical feature of the action of GCs on neurons might be the inhibition of glucose utilization (which is a hallmark of peripheral GC action).

This article finds support for two ideas that have permeated the gerontology literature. The first is that senescence is a time of decreased capacity of an organism to respond optimally to stress. Evidence is reviewed showing that aged organisms frequently are impaired in their capacity to terminate glucocorticoid secretion at the end of stress. The second idea is that frequent incidences of stress over the life-span can accelerate aspects of aging. Evidence is reviewed showing that excessive exposure to glucocorticoids (as a result of frequent stress) can have numerous deleterious consequences, including accelerated degeneration of hippocampal neurons.