Long-term exposure to high levels of corticosterone aggravates AF64A-induced cholinergic hypofunction in rat hippocampus in vivo

Concentration dependent actions of glucocorticoids on neuronal viability and survival

A growing body of evidence based on experimental data demonstrates that glucocorticoids (GCs) can play a potent role in the survival and death of neurons. However, these observations reflect paradoxical features of GCs, since these adrenal stress hormones are heavily involved in both neurodegenerative and neuroprotective processes. The actual level of GCs appears to have an essential impact in this bimodal action. In the present short review we aim to show the importance of concentration dependent action of GCs on neuronal cell viability and cell survival in the brain. Additionally, we will summarize the possible GC-induced cellular mechanisms at different GC concentrations providing a background for their effect on the fate of nerve cells in conditions that are a challenge to their survival.

Sex differences in spatial and non-spatial Y-maze performance after chronic stress

Chronic restraint is known to alter hippocampal CA3 dendritic morphology and spatial memory in male rats. The present study examined whether female rats, which exhibit different anatomical adaptations to chronic stress than those of males, would also show spatial memory impairments. Male and female Sprague-Dawley rats were restrained for 6 h/day for 21 days, a time frame previously demonstrated to cause hippocampal CA3 dendritic atrophy. The day after the last restraint session, rats were tested on a Y-maze, a habituation task that can be used to assess spatial memory. Chronic stress impaired Y-maze performance in both sexes without affecting levels of locomotion as measured by total arm entries in the first minute. However, Y-maze performance of stressed females improved in 2-5 min when chronically stressed males continued to show poor Y-maze performance. The enhanced Y-maze performance of chronically stressed females occurred when total arm entries were higher compared to the entries made by males. Therefore, correlations were performed between total arm entries and spatial memory in 1 and 2-5 min. In the first minute when control females demonstrated functional spatial memory, female controls with the lowest locomotor levels exhibited the best performance. The correlations for stressed females were not significant, and neither were the correlations for any group in 2-5 min. Overall, these results show important sex differences in response to chronic stress with females exhibiting an ability to recover quickly from deficits in Y-maze performance.

Action of glucocorticoids on survival of nerve cells: promoting neurodegeneration or neuroprotection?

Extensive studies during the past decades provided compelling evidence that glucocorticoids (GCs) have the potential to affect the development, survival and death of neurones. These observations, however, reflect paradoxical features of GCs, as they may be critically involved in both neurodegenerative and neuroprotective processes. Hence, we first address different aspects of the complex role of GCs in neurodegeneration and neuroprotection, such as concentration dependent actions of GCs on neuronal viability, anatomical diversity of GC-mediated mechanisms in the brain and species and strain differences in GC-induced neurodegeneration. Second, the modulatory action of GCs during development and ageing of the central nervous system, as well as the contribution of altered GC balance to the pathogenesis of neurodegenerative disorders is considered. In addition, we survey recent data as to the possible mechanisms underlying the neurodegenerative and neuroprotective actions of GCs. As such, two major aspects will be discerned: (i) GC-dependent offensive events, such as GC-induced inhibition of glucose uptake, increased extracellular glutamate concentration and concomitant elevation of intracellular Ca(2+), decrease in GABAergic signalling and regulation of local GC concentrations by 11 beta-hydroxysteroid dehydrogenases; and (ii) GC-related cellular defence mechanisms, such as decrease in after-hyperpolarization, increased synthesis and release of neurotrophic factors and lipocortin-1, feedback regulation of Ca(2+) currents and induction of antioxidant enzymes. The particular relevance of these mechanisms to the neurodegenerative and neuroprotective effects of GCs in the brain is discussed.

Recovery of the brain choline level in treated Cushing's patients as monitored by proton magnetic resonance spectroscopy

In a previous study from our group [A. Khiat, C. Bard, A. Lacroix, J. Rousseau, Y. Boulanger, Brain metabolic alterations in Cushing's syndrome as monitored by proton magnetic resonance spectroscopy, NMR Biomed. 12 (1999) 357-363], proton magnetic resonance spectroscopy (1H MRS) was used to evaluate changes in cerebral metabolites in patients with Cushing's syndrome as compared to normal subjects. Data recorded in the frontal, thalamic and temporal areas demonstrated statistically significant decreases of the Cho/Cr ratios in the frontal and thalamic areas but not in the temporal area for Cushing's syndrome patients. No statistically significant changes in the NAA/Cr ratios were measured in any of the areas studied. In this follow-up study, MRS data are reported for ten patients after correction of hypercortisolism which demonstrate a statistically significant recovery of the choline levels in the frontal and thalamic areas. No variation in the NAA, Cr and mI metabolite ratios relative to H(2)O could be measured. Results are interpreted as an inhibition of the phosphatidylcholine degrading phospholipases by glucocorticoids which disappears after correction of hypercortisolism.

Brain-derived neurotrophic factor prevents neuronal cell death induced by corticosterone

Corticosterone (CORT), one of the glucocorticoids, causes neuronal damage in the hippocampus, but the mechanism(s) of action underlying its effects remains unknown. Brain-derived neurotrophic factor (BDNF) is a neurotrophic factor that belongs to the neurotrophin family, affects the survival and/or differentiation of various types of neurons in vitro, and is able to antagonize neuronal death induced by various brain insults or neurotoxins in vivo. In this study, the effects of CORT on BDNF protein contents and mRNA expression were investigated in relation to neuronal survival/death of cultured rat hippocampal neurons, because the colocalization of BDNF with its receptor, TrkB, suggests that BDNF may exert its putative protective and trophic effects through an autocrine mechanism in the hippocampus. Administration of CORT accelerated the neuronal death that proceeds after serum deprivation, and simultaneously reduced the levels of BDNF mRNA and intracellular BDNF content. Exogenously added BDNF actually attenuated CORT-induced neuronal death, but not in the presence of K252a, an inhibitor of the tyrosine kinase activity of Trk family receptors. These observations suggest that CORT induces damage to hippocampal neurons, at least partly, via reducing their BDNF synthesis.

Inhibitory avoidance and appetitive learning in aged normal mice: comparison with transgenic mice having elevated plasma growth hormone levels

Groups of 25-month-old ("old") B6C3 hybrid male mice, 6-month-old ("young") normal males, and their age-matched transgenic (TG) siblings overexpressing the bovine growth hormone gene were given an inhibitory avoidance training trial (0.20-mA electric shock, 1.0-s duration). The old B6C3 hybrids and the young TG mice displayed poorer retention (shorter latencies to enter the shock compartment) 24 h and 42 days after training than did the young normal mice. In a subsequent multiple-trial acquisition test, young TG and old normal mice required more trials to reach the criterion of complete inhibition of step-through responding for 300 s than did young normal mice. Young normal and young TG mice did not differ in trials to extinction, but TG mice met the extinction criterion sooner than did old normal mice, suggesting poorer longterm retention. In tests of T-maze appetitive learning, young normal, old normal, and young TG mice did not differ in acquisition or 24-h retention. Contrary to expectation, TG mice acquired T-maze reversal learning in fewer trials than did young normal or old normal mice. The TG and young normal mice did not differ in retention when retested 44 days after initial training, but old normal mice showed poorer retention than did the young normals. Results of locomotor activity and shock response tests suggested that learning impairments were not due to differences in locomotor activity or shock response thresholds in these animals. Tests in an elevated plus maze indicated that young TG mice were less anxious in a novel environment than their normal siblings, which may contribute to their impaired inhibitory avoidance learning. These findings suggest that 6-month-old TG mice overexpressing the bovine growth hormone gene display alterations in inhibitory avoidance (but not appetitive) learning similar to those occurring in 25-month-old normal mice. The neurobiological mechanisms mediating inhibitory avoidance and T-maze appetitive learning in these animals may be largely dissociated.

Stress, Glucocorticoids, and Damage to the Nervous System: The Current State of Confusion

An extensive literature demonstrates that glucocorticoids (GCs), the adrenal steroids secreted during stress, can have a broad range of deleterious effects in the brain. The actions occur predominately, but not exclusively, in the hippocampus, a structure rich in corticosteroid receptors and particularly sensitive to GCs. The first half of this review considers three types of GC effects: a) GC-induced atrophy, in which a few weeks' exposure to high GC concentrations or to stress causes reversible atrophy of dendritic processes in the hippocampus; b) GC neurotoxicity where, over the course of months, GC exposure kills hippocampal neurons; c) GC neuroendangerment, in which elevated GC concentrations at the time of a neurological insult such as a stroke or seizure impairs the ability of neurons to survive the insult. The second half considers the rather confusing literature as to the possible mechanisms underlying these deleterious GC actions. Five broad themes are discerned: a) that GCs induce a metabolic vulnerability in neurons due to inhibition of glucose uptake; b) that GCs exacerbate various steps in a damaging cascade of glutamate excess, calcium mobilization and oxygen radical generation. In a review a number of years ago, I concluded that these two components accounted for the deleterious GC effects. Specifically, the energetic vulnerability induced by GCs left neurons metabolically compromised, and less able to carry out the costly task of containing glutamate, calcium and oxygen radicals. More recent work has shown this conclusion to be simplistic, and GC actions are shown to probably involve at least three additional components: c) that GCs impair a variety of neuronal defenses against neurologic insults; d) that GCs disrupt the mobilization of neurotrophins; e) that GCs have a variety of electrophysiological effects which can damage neurons. The relevance of each of those mechanisms to GC-induced atrophy, neurotoxicity and neuroendangerment is considered, as are the likely interactions among them.

The AF64A model of cholinergic hypofunction: an update

Based on numerous reports in the literature since 1980, one can now conclude that ethylcholine aziridinium (AF64A) is selective for the cholinergic system in vivo, and that the effect is both dose- and site-dependent. Thus, AF64A treatment, under the correct conditions of dose and time will result in selective reductions in levels of ACh, AChE, ChAT, HAChT, and K(+)- and ouabain-stimulated release of ACh. While other neurotransmitters may also be affected in brains of AF64A treated rats, the effect is only transient and is most probably secondary to the initial cholinergic deficit-induced by AF64A, reflecting an adaptive reaction of these neurotransmitter systems, which are normally integrated with cholinergic interconnections, to the cholinergic deficiency induced by AF64A. This paper provides a historical perspective for the development of AF64A as a selective cholinotoxin, and surveys its potential mechanisms of action at the neurochemical and molecular levels. Moreover, the availability of an animal model such as the AF64A-treated rat, in which the cholinergic system has been compromised selectively for an extended period of time, has allowed investigators to study a wide variety of questions that relate to factors controlling cholinergic function in vivo. Several key illustrations are presented at the end of this paper.