Dexamethasone Potentiate Aβ-Induced Learning And Memory Impairment And Its Related Mechanisms

Alzheimer's disease(AD) is the major neurodegenerative disorder of the elderly. The main pathological hallmarks of AD is extracellular accumulation of selective insults of neuronal synapsis, neurofibrillary tangle(NFT) and senile plaques (SP) in vulnerable brain regions, composed primarily of aggregatedβ-amyloid peptide(Aβ). The hippocampus, among other areas, is the principal target tissue in the brain for the hormone. The etiology of the more common sporadic cases remains unknown, and up to now, there is no reliable methods to prevent and treat the disease. It is generally considered that Aβplays a pivotal role in the pathogenesis of AD. Aβis a heterogeneous 39-42-amino acid peptide. The hippocampus is closely related to memory and learning impairment which involved much glucocoricoids receptor(GR) , and it is the principal target tissue of which glucocorticoids(GCs) affect. GCs are important adrenal steroids that affect numerous physiological processes in the brain, and widely used in medicine. Previous studies showed that increased plasma levels of DEX can potentiate the loss of hippocampal synapsis and aggravate the apotosis of the dentate gyrus,accompanied with atrophy of hippocampus. Many studies in the world are focusing on the relationship between the GCs, Aβand pathogenesis of AD, but whether GCs could enhance Aβ-inducing hippocampal neuronal injury is not well determined. Therefore, in this study we investigate whether Dexamethasone(DEX) could potentiate Aβ-induced learning and memory impairment and its relative mechanism in SD rats in vivo. On the other hand, we also examine whether DEX could enhance Aβ-induced cell death in hippocampal neurons in vitro, and, if so, what is the underlying mechanism. Objective The objectives of this experiments were to explore the mechanisms responsible for dexamethasone enhanced Aβ-induced cell death in hippocampal neurons.Methods Aβ25–35 was injected into the CA1 field of hippocampus to establish the Alzheimer's Disease rat model. Morris water maze test was used to investigate whether DEX(1 mg/kg/d,5 mg/kg/d) could potentiate Aβ(5μg, each CA1)-induced learning and memory impairment in SD rats in vivo, and the histopathologic changes in CA1 field of hippocampus was examined under a light microscope. The effect of DEX on Aβ25-35-induced phospho-tau was evalued by immunohistechemical. Primitive hippocampal neurons derived from 18 day embryonic rat were cultured. Cultured cells were treated with DEX alone at 10μM for 48 h or Aβ25-35 alone at 5μM for 24 h, or cultured cells were pretreated with DEX at 10μM for 24 h followed by Aβ25-35 at 5μM for various time, and then the cell viability was measured by colorimetric MTT assay, p53mRNA was assessed using RT-PCR, and the effect of DEX on Aβ25-35-induced nuclear factorκB (NF-κB), phospho-tau and p53 protein was evalued by western blot.Results1. Microinjection of Aβ25-35(5μg, each CA1) bilateralis into the CA1 region of SD rats and DEX(5 mg/kg/d, sc×7d)could slightly increase the escape latency and the swim distances in SD rats during training session in the Morris water maze test, but there was no statistic significant compared with vehicle-treated control. DEX(1 mg/kg/d, sc, 7d) did not decreased the escape latency and the swim distances in SD rats during training session in the Morris water maze test, but DEX(1 mg/kg/d ,5 mg/kg/d, sc×7d) could potentiate Aβ(5μg, each CA1)-induced learning and memory impairment in SD rats. Severe histological damage was observed in the CA1 cell fields of the hippocampus in DEX plus Aβ-treated group. These neuropathological changes were characterized by decreased cell number, soma shrinkage and condensation, or nuclear pyknosis.2. Immunohistechemical results showed that a significant increase of tau(231) positive cells were also detected in DEX(5 mg/kg/d) as well as Aβ,DEX plus Aβtreated group in the hippocampal tissue of rats. The expressions of p-tau in DEX plus Aβtreated group is increased compared with DEX or Aβalone treated group.3. Treatment for 48 h with DEX(0.01,0.1,1,10μM) alone did not cause a significant reduction in MTT compared with vehicle-treated control cultures. Treatment with aggregated Aβ25-35 alone decreased cell viability in a concentration-dependent manner. A 24-h preincubation with DEX(1 or 10μM)further decreased the viability of hippocampal neuron induced by Aβ25-35(1 or 5μM).4. Aβ25-35(5μM) causes a time-dependent increase in the level of nuclear NF-κB p65 proteins, whereas maximal NF-κB p65 was obtained with Aβ25-35 at 4 h. A 24 h preincubation with DEX(10μM) could down-regulate the elevated level of nuclear NF-κB p65 proteins induced by Aβ25-35. Aβ25-35(5μM) alone could decrease the cytoplasmic level of IκBαprotein 24 h after Aβ25-35 was added to the culture. Increased levels of cytoplasmic level of IκBαprotein were observed in hippocampal neurons 24h after incubation with DEX(10μM).5. Aβ25-35 (5μM) treatment resulted in significant increases in the level of total protein of p53 24 h after Aβwas added to the culture. Treatment with DEX(10μM) alone for 48 h did not increase the levels of total protein of p53. Pretreatment with DEX for 24 h didn't promote the increased total protein of p53 induced by Aβ25-35..6. Aβ25-35(0-20μM) dose-dependently induced phosphorylation of tau at Thr-231 in primary hippocampal neurons cocultured for 1 h with Aβ25-35, whereas maximal phosphorylation was obtained with Aβ25-35 at 10μM. Treatment of neurons with DEX in primary culture for 24 h could results in an slight elevation in tau phosphorylation at Thr-231, and pretreated of neuron with DEX at 10μM for 24 h could promote the increased level of phospho-tau at Thr-231 induced by Aβ25-35(5μM).7. Aβ25-35(5μM) treatment resulted in small but significant increases in the level of total protein protein of p53mRNA 18 h after Aβwas added to the culture. Treatment with DEX(10μM) alone did not increase the levels of total protein of p53mRNA. Pretreatment with DEX for 24 h didn't promote the increased total protein of p53mRNA induced by Aβ25-35 as well..ConclusionsIn vivo, DEX could potentiate Aβ-induced learning and memory impairment in SD rats. These neuropathological changes were characterized by decreased cell number, soma shrinkage and condensation, or nuclear pyknosis. These results suggested that DEX could enhance Aβ-induced cell death in hippocampal neurons and dysfunction of learing and memory which may involve the decrease of the numbers of neurons and increase of abnormal phosphorylation of tau. In vitro, DEX could potentiate the neurotoxic action of Aβmediated by further increasing the level of phospho-tau at Thr-231, down-regulating the level of nuclear NF-κB protein. Pretreatment of hippocampal neurons with DEX did not influnce p53 proteins level induced by Aβ25-35, which suggested that DEX could enhance Aβ-induced cell death in hippocampal neurons was not possibly carried out by this means. Since Aβand GCs increases in the brain with aging. GCs potentiate the neurotoxic action of Aβmaybe one of the mechanisms responsible for AD.