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Alzheimer's disease (AD) is a devastating neurodegenerative disease, the most common among the dementing illnesses. The neuropathological hallmarks of AD include extracellular β-amyloid (amyloid precursor protein (APP) deposits, intracellular neurofibrillary tangles (NFT)), dystrophic neuritis and amyloid angiopathy. The mismetabolism of APP and the defective clearance of β amyloid generate a cascade of events including hyperphosphorylated tau (τ) mediated breakdown of microtubular assembly and resultant synaptic failure which results in AD. The exact aetiopathogenesis of AD is still obscure. The preeminent hypotheses of AD include amyloid cascade hypothesis and tau hyperphosphorylation. The amyloid hypothesis states that extracellular amyloid plaques formed by aggregates of Aβ peptide generated by the proteolytic cleavages of APP are central to AD pathology. Intracellular assembly states of the oligomeric and protofibrillar species may facilitate tau hyperphosphorylation, disruption of proteasome and mitochondria function, dysregulation of calcium homeostasis, synaptic failure, and cognitive dysfunction. The tau hypothesis states that excessive or abnormal phosphorylation of tau results in the transformation of normal adult tau into PHF-tau (paired helical filament) and NFTs. Vascular hypothesis is also proposed for AD and it concludes that advancing age and the presence of vascular risk factors create a Critically Attained Threshold of Cerebral Hypoperfusion (CATCH) which leads to cellular and subcellular pathology involving protein synthesis, development of plaques, inflammatory response, and synaptic damage leading to the manifestations of AD. Multiple other aetiological and pathogenetic hypotheses have been put forward including genetics, oxidative stress, dysfunctional calcium homeostasis, hormonal, inflammatory-immunologic, and cell cycle dysregulation with the resultant neurotransmitter dysfunctions and cognitive decline. The available therapeutic agents target only the neurotransmitter dysfunction in AD and agents specifically targeting the pathogenetic mechanisms like amyloid deposition and tau hyperphosphorylation might provide a definite therapeutic edge.

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SUMMARY Alzheimer’s disease (AD) is a progressive condition that affects cognition, function, and behavior. Approximately 60–90% of patients with AD develop neuropsychiatric symptoms (NPS) such as hallucinations, delusions, agitation/aggression, dysphoria/depression, anxiety, irritability, disinhibition, euphoria, apathy, aberrant motor behavior, sleep disturbances, appetite and eating changes, or altered sexual behavior. These noncognitive behavior changes are thought to result from anatomical and biochemical changes within the brain, and have been linked, in part, to cholinergic deficiency. Cholinesterase inhibitors may reduce the emergence of NPS and have a role in their treatment. These agents may delay initiation of, or reduce the need for, other drugs such as antipsychotics. This article summarizes the effects of donepezil, a cholinesterase inhibitor, on the NPS of dementia with emphasis on AD and dementia with Lewy bodies.

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