Melatonin reduces β-amyloid accumulation and improves short-term memory in streptozotocin-induced sporadic Alzheimer's disease model

Abstract

Melatonin is a hormone secreted by the pineal gland, it can be associated with circadian rhythms, aging and neuroprotection. Melatonin levels are decreased in sporadic Alzheimer's disease (sAD) patients, which suggests a relationship between the melatonergic system and sAD. Melatonin may reduce inflammation, oxidative stress, TAU protein hyperphosphorylation, and the formation of β-amyloid (Aβ) aggregates. Therefore, the objective of this work was to investigate the impact of treatment with 10 mg/kg of melatonin (i.p) in the animal model of sAD induced by the intracerebroventricular (ICV) infusion of 3 mg/kg of streptozotocin (STZ). ICV-STZ causes changes in the brain of rats similar to those found in patients with sAD. These changes include; progressive memory decline, the formation of neurofibrillary tangles, senile plaques, disturbances in glucose metabolism, insulin resistance and even reactive astrogliosis characterized by the upregulation of glucose levels and glial fibrillary acidic protein (GFAP). The results show that ICV-STZ caused short-term spatial memory impairment in rats after 30 days of STZ infusion without locomotor impairment which was evaluated on day 27 post-injury. Furthermore, we observed that a prolonged 30-day treatment with melatonin can improve the cognitive impairment of animals in the Y-maze test, but not in the object location test. Finally, we demonstrated that animals receiving ICV-STZ have high levels of Aβ and GFAP in the hippocampus and that treatment with melatonin reduces Aβ levels but does not reduce GFAP levels, concluding that melatonin may be useful to control the progression of amyloid pathology in the brain.

Keywords: AD, Alzheimer Disease; APP, Amyloid precursor protein; Alzheimer's disease; Aβ, β-amyloid; GFAP; GFAP, Glial fibrillary acidic protein; ICV-STZ, Intracerebroventricular injection of streptozotocin; MEL, Melatonin; MT1, Melatonin Receptor 1; MT2, Melatonin Receptor 2; Melatonin; OLT, Object location test; STZ, Streptozotocin; Short-term memory; Streptozotocin; TNF-α, Tumor Necrosis factor alpha; Y maze; sAD, Sporadic Alzheimer disease; β-amyloid.

Fig. 1

Effects of prolonged melatonin treatment in the memory test in STZ-lesioned rats. Evaluated short-term spatial memory expressed by the object discrimination index in the object location test (A) and by the time spent in the new arm of the Y-maze (B). (C) Representative image of the route traveled by an animal from each treatment group in the Y-maze. The arm skirted with the color black indicates the novel arm of the apparatus. The image illustrates the exploration of the apparatus by the rats of these groups, and the STZ+VEH group explored less of the novel arm. The data are expressed as mean ± SEM and were analyzed by one-way (ANOVA) with Tukey’s multiple comparison test (* p < 0.05, ** p < 0.01). SHAM+VEH (n = 8), SHAM+MEL (n = 8), STZ+VEH (n = 6), STZ+MEL (n = 8).

Fig. 2

Effects of a 27-day treatment with 10 mg/kg melatonin (i.p) or vehicle on spontaneous locomotor activity in ICV-STZ or SHAM animals in the open field test. The data are expressed as mean ± SEM and were analyzed by one-way (ANOVA) with Tukey’s multiple comparison test (*p < 0.05). SHAM+VEH (n = 8), SHAM+MEL (n = 8), STZ+VEH (n = 6), STZ+MEL (n = 9).

Fig. 3

ICV-STZ at a dose of 3 mg/kg did not cause changes in APP levels while melatonin significantly reduced the elevation of Aβ protein levels induced by ICV-STZ. (A) Quantification of APP levels (B) Quantification of Aβ protein levels. (C) Representative image of Western blotting for APP. (C) Representative Western blotting image for Aβ protein. Data are expressed as mean ± SEM (n = 4 per group) and were analyzed by one-way analysis (ANOVA) with Tukey's multiple comparison test (**p < 0.01, ***p < 0.001, ** **p < 0.0001).

Fig. 4

30-day treatment with 10 mg/kg melatonin did not reduce an increase of GFAP in the hippocampus of the rats induced by STZ. (A) Quantification of GFAP levels. (B) Representative western blot image for GFAP. The data are expressed as mean ± SEM (n = 4 per group) and were analyzed by one-way (ANOVA) with Tukey’s multiple comparison test (* p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001).

Fig. 5

Correlations between the percentage of time spent in the new arm (Y-maze), Aβ and the hippocampal GFAP in the animal model of sporadic Alzheimer's disease induced by ICV-STZ. (A) % of time spent in the new arm and Aβ levels. (B) % of time spent in the new arm and GFAP levels. (C) GFAP levels and Aβ levels. (D) Legend representing the symbols and experimental groups respectively. Pearson correlation coefficient, (* p < 0.05, ** p < 0.01).