Circadian System Functionality, Hippocampal Oxidative Stress, and Spatial Memory in the APPswe/PS1dE9 Transgenic Model of Alzheimer Disease: Effects of Melatonin or Ramelteon

Jul 25, 2012Chronobiology international

Circadian system function, brain cell stress, and memory in an Alzheimer's model: effects of melatonin or ramelteon

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Abstract

Melatonin treatment did not significantly affect circadian and behavioral parameters in an Alzheimer mouse model.

Alzheimer disease (AD) is associated with severe disruptions in circadian rhythms and reduced melatonin levels.
In the APPswe/PS1 double transgenic mouse model, melatonin treatment showed no significant effects on oxidative stress markers or circadian behavior.
Ramelteon, a selective MT1/MT2 receptor agonist, significantly reduced circadian body temperature rhythm amplitude in AD mice.
Tg mice treated with ramelteon displayed increased circadian rhythm fragmentation compared to untreated controls.
Hippocampal protein oxidation was significantly reduced in Tg mice treated with melatonin and ramelteon compared to untreated Tg controls.
Behavioral tests indicated that Tg controls explored novel objects more than wild-type controls, suggesting preserved exploratory behavior.

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Alzheimer disease (AD) is a neurodegenerative disorder that primarily causes β-amyloid accumulation in the brain, resulting in cognitive and behavioral deficits. AD patients, however, also suffer from severe circadian rhythm disruptions, and the underlying causes are still not fully known. Patients with AD show reduced systemic melatonin levels. This may contribute to their symptoms, since melatonin is an effective chronobiotic and antioxidant with neuroprotective properties. Here, the authors critically assessed the effects of long-term melatonin treatment on circadian system function, hippocampal oxidative stress, and spatial memory performance in the APPswe/PS1 double transgenic (Tg) mouse model of AD. To test if melatonin MT1/MT2 receptor activation, alone, was involved, the authors chronically treated some mice with the selective MT1/MT2 receptor agonist ramelteon. The results indicate that many of the circadian and behavioral parameters measured, including oxidative stress markers, were not significantly affected in these AD mice. During the day, though, Tg controls (Tg-CON) showed significantly higher mean activity and body temperature (BT) than wild-type (WT) mice. Overall, BT rhythm amplitude was significantly lower in Tg than in WT mice. Although melatonin treatment had no effect, ramelteon significantly reduced the amplitude of the BT rhythm in Tg mice. Towards the end of the experiment, Tg mice treated with ramelteon (Tg-RAM) showed significantly higher circadian rhythm fragmentation than Tg-CON and reduced circadian BT rhythm strength. The free-running period (τ) for the BT and locomotor activity (LA) rhythms of Tg-CON was <24 h. Whereas melatonin maintained τ at 24 h for BT and LA in both genotypes, ramelteon treatment had no effect. In the behavioral tests, the number of approaches and time spent exploring novel objects were significantly higher in Tg-CON than WT controls. Brain tissue analysis revealed significant reduction in hippocampal protein oxidation in Tg-MEL and Tg-RAM compared with Tg-CON animals. These results suggest that not all aspects of the circadian system are affected in the APPswe/PS1 mice. Therefore, care should be taken when extending the results obtained in Tg mice to develop new therapies in humans. This study also revealed the complexity in the therapeutic actions of melatonin and ramelteon in this mouse model of AD.

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Circadian System Functionality, Hippocampal Oxidative Stress, and Spatial Memory in the APPswe/PS1dE9 Transgenic Model of Alzheimer Disease: Effects of Melatonin or Ramelteon

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