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Chronic psychosocial stress enhances long-term depression in a subthreshold amyloid-beta rat model of Alzheimer's disease Tran, Trinh, Srivareerat,et al. Khalil Khlifi Pharm D. Candidate 2012 University of Georgia, College of Pharmacy Alzheimer's Disease • Progressive, irreversible brain disorder that gradually destroys memory and cognitive abilities • Clinically, patient's are seen to have marked memory loss and impairment of social and occupational functions • Pathologically, it is characterized by the accumulation of amyloidbeta peptides and tau protein o Accumulation is caused by abnormally folded amyloid-beta and tau proteins o The plaques are made up of small peptides 39-43 amino acids in length and deposit outside neurons o Tau is used to stabilize the microtubules and a chemical change causes it to be hyperphosphorylated and thereby creates neurofibrillary tangles and disintegrates the neuronal transport system Stress and AD • Genetic factors have a very definitive role on the progression of the disease but what does the effect of non-genetic factors have on progression of these patients? o Exposure to severe and/or prolonged stress challenges homeostasis mechanisms and may cause over-activation and dysregulation of stress-activated systems o These may lead to subsequent negative changes in the brain • The hippocampus is particularly susceptible to stress functions and is one of the first regions to be affected by AD o The hippocampus is involved in learning and memory processes Methods and Materials • Adult male Wistar rats weighing 200-225g at the beginning of the experiment • Housed in groups of 6 with free access to food and water in a temperature and light controlled environment • 4 Treatment groups were designated in this study o Control o Stress o Sub-amyloid-beta (SAB) o Stress and sub-amyloid-beta (SSAB) • The SAB and SSAB groups were infused with subpathogenic doses of AB1-42 • The Control and Stress groups were infused with an inactive reverse peptide AB42-1 Methods and Materials • Both SAB and SSAB were subjected to chronic psychosocial stress using a form of "intruder" stress o Rats were kept in the same cages for 1 week then 2 rats were swapped in each cage daily for 6 weeks total Stress was measured by marked increases in corticosterone levels • Hippocampal Dissection o Animals were killed under urethane anaesthesia and the right hippocampi was removed and the dorsal and vental sides of area CA1 were separated and placed in microcentrifuge tubes and stored at -80C until processed Method and Materials • Immunoblotting o Hippocampal tissue was placed in an isotonic buffered cocktail containing protease and phosphatase inhibitors o Following antibodies were used in the study Amyloid precursor protein (APP) Beta-site amyloid precursor protein cleaving enzyme (BACE) p-CaMKII CaMKII Calcineurin Brain-derived neutrotrophic fator GAPDH antibody Statistics • Carried out with a two-way analysis of variance (ANOVA) followed by Tukey's post hoc test • Minimum significance of p<0.05 ANOVA • Provides statistical test of whether or not the means of several groups are all equal and therefore generalizes t-test to more than two groups o Doing multiple t-tests between 2 groups would result in an increased chance of committing a type I error (rejects a true null hypothesis) Results • Effects on chronic stress on long term depression (LTD) in subAB rats o Magnitude of LTD was similar in both control and subAB rats however it was significantly enhanced in the stress compared with the control and subAB rats as shown as a decline in fEPSP (field exictatory post-synaptic potential) slope Control 78.38 +/- 1.97% Stress 62.93 +/- 4.35% SubAB 75.98 +/- 3.41% o The slope of fEPSP was significantly (<0.001) lower in SSAB than those in control, stress, and SAB groups Results • Levels of calcineurin during expression of LTD o The levels of calcineurin were markedly increased by 30 +/- 6% in stimulated stress group and 26 +/- 3% in SSAB compared to unstimulated control group • Levels of p-CaMkII and total CaMKII during expression of LTD o Levels of p-CaMKII were significantly increased by 116+/31% in SAB and 125+/-47% in SSAB rats as compared to unstimulated control group o Significant increases in levels of CaMKII in all the stimulated groups as compared to the unstimulated groups • Basal levels of APP o There was no significant change in the APP levels across all groups Discussion • Human studies have never revealed an association between the number of stressful or negative life events and the effects on cognitive decline • However, studies involving those with AD have shown that increased cortisol levels with one APOE-4 allele showed poorer coginitive decline o The researchers agree with the above statement based on the data in this study that points towards accelerated cognitive impairment in chronic stress situations • Chronic stress compounded with amyloid-beta causes an enhanced LTD shown in the SSAB group • Total CaMKII was increased in all groups whereas p-CaMKII was only increased in the stress groups and the normal brain both work to reverse LTD but in the SAB and SSAB group it potentiated LTD Discussion • As the great majority of the cases of AD are of the late onset sporadic type, it is presumed that it may be triggered or accelerated by non-genetic risk factors, such as chronic stress • Exposure to stress lead to an increase in LTD but was enhanced with the addition of amyloid-beta • The exact mechanism of acceleration is still to be explained but there are currently 2 ideas o Excessive secretion or prolonged exposure to glucocorticoids increases neuronal vulnerability to age-related disease o Alter the processing and production of various AD-related proteins resulting in negative alterations in cognition and synaptic plasticity Author's Conclusions Resources 1. "Analysis of Variance." Wikipedia, the Free Encyclopedia. Web. 08 Feb. 2012. <http://en.wikipedia.org/wiki/ANOVA>. 2. Alzheimer, Alois. "Alzheimer's Disease." Wikipedia, the Free Encyclopedia. Web. 08 Feb. 2012. <http://en.wikipedia.org/wiki/Alzheimer>.