Melatonin ameliorates amyloid beta‐induced memory deficits,tau hyperphosphorylation and neurodegeneration via PI3/Akt/GSk3β pathway in the mouse hippocampus

Alzheimer's disease (AD) is the most prevalent age‐related neurodegenerative disease, pathologically characterized by the accumulation of amyloid beta (Aβ) aggregation in the brain, and is considered to be the primary cause of cognitive dysfunction. Aβ aggregates lead to synaptic disorder, tau hyperphosphorylation, and neurodegeneration. In this study, the underlying neuroprotective mechanism of melatonin against Aβ_1‐42‐induced neurotoxicity was investigated in the mice hippocampus. Intracerebroventricular (i.c.v.) Aβ_1‐42‐injection triggered memory impairment, synaptic disorder, hyperphosphorylation of tau protein, and neurodegeneration in the mice hippocampus. After 24 hr of Aβ_1‐42 injection, the mice were treated with melatonin (10 mg/kg, intraperitonially) for 3 wks, reversed the Aβ_1‐42‐induced synaptic disorder via increasing the level of presyanptic (Synaptophysin and SNAP‐25) and postsynaptic protein [PSD95, p‐GluR1 (Ser845), SNAP23, and p‐CREB (Ser133)], respectively, and attenuated the Aβ_1‐42‐induced memory impairment. Chronic melatonin treatment attenuated the hyperphosphorylation of tau protein via PI3K/Akt/GSK3β signaling by activating the p‐PI3K, p‐Akt (Ser 473) and p‐GSK3β (Ser9) in the Aβ_1‐42‐treated mice. Furthermore, melatonin decreased Aβ_1‐42‐induced apoptosis through decreasing the overexpression of caspase‐9, caspase‐3, and PARP‐1 level. Additionally, the evaluation of immunohistochemical analysis of caspase‐3, Fluorojade‐B, and Nissl staining indicated that melatonin prevented neurodegeneration in Aβ_1‐42‐treated mice. Our results demonstrated that melatonin has neuroprotective effect against Aβ_1‐42‐induced neurotoxicity through decreasing memory impairment, synaptic disorder, tau hyperphosphorylation, and neurodegeneration via PI3K/Akt/GSK3β signaling in the Aβ_1‐42‐treated mouse model of AD. On the basis of these results, we suggest that melatonin could be an effective, promising, and safe neuroprotective candidate for the treatment of progressive neurodegenerative disorders, such as AD.