RESUMO
BACKGROUND: Alzheimer's disease (AD) underlies dementia for millions of people worldwide, and its occurrence is set to double in the next 20 years. Currently, approved drugs for treating AD only marginally ameliorate cognitive deficits, and provide limited symptomatic relief, while newer substances under therapeutic development are potentially years away from benefiting patients. Melatonin (MEL) for insomnia has been proven safe with >15 years of over-the-counter access in the US. MEL exerts multiple complementary mechanisms of action against AD in animal models; thus it may be an excellent disease-modifying therapeutic. While presumed to provide neuroprotection via activation of known G-protein-coupled melatonin receptors (MTNRs), some data indicate MEL acts intracellularly to protect mitochondria and neurons by scavenging reactive oxygen species and reducing free radical formation. We examined whether genetic deletion of MTNRs abolishes MEL's neuroprotective actions in the AßPP(swe)/PSEN1dE9 mouse model of AD (2xAD). Beginning at 4 months of age, both AD and control mice either with or without both MTNRs were administered either MEL or vehicle in drinking water for 12 months. RESULTS: Behavioral and cognitive assessments of 15-month-old AD mice revealed receptor-dependent effects of MEL on spatial learning and memory (Barnes maze, Morris Water Maze), but receptor-independent neuroprotective actions of MEL on non-spatial cognitive performance (Novel Object Recognition Test). Similarly, amyloid plaque loads in hippocampus and frontal cortex, as well as plasma Aß1-42 levels, were significantly reduced by MEL in a receptor-independent manner, in contrast to MEL's efficacy in reducing cortical antioxidant gene expression (Catalase, SOD1, Glutathione Peroxidase-1, Nrf2) only when receptors were present. Increased cytochrome c oxidase activity was seen in 16 mo AD mice as compared to non-AD control mice. This increase was completely prevented by MEL treatment of 2xAD/MTNR+ mice, but only partially prevented in 2xAD/MTNR- mice, consistent with mixed receptor-dependent and independent effects of MEL on this measure of mitochondrial function. CONCLUSIONS: These findings demonstrate that prophylactic MEL significantly reduces AD neuropathology and associated cognitive deficits in a manner that is independent of antioxidant pathways. Future identification of direct molecular targets for MEL action in the brain should open new vistas for development of better AD therapeutics.
Assuntos
Doença de Alzheimer/tratamento farmacológico , Melatonina/uso terapêutico , Fármacos Neuroprotetores/uso terapêutico , Receptores de Melatonina/fisiologia , Doença de Alzheimer/genética , Doença de Alzheimer/patologia , Doença de Alzheimer/psicologia , Peptídeos beta-Amiloides/sangue , Precursor de Proteína beta-Amiloide/genética , Animais , Ansiedade/genética , Modelos Animais de Doenças , Avaliação Pré-Clínica de Medicamentos , Comportamento Exploratório , Lobo Frontal/metabolismo , Lobo Frontal/patologia , Glutationa Peroxidase/biossíntese , Glutationa Peroxidase/genética , Hipocampo/metabolismo , Hipocampo/patologia , Aprendizagem em Labirinto , Transtornos da Memória/genética , Camundongos , Camundongos Endogâmicos C3H , Camundongos Endogâmicos C57BL , Camundongos Knockout , Mitocôndrias/efeitos dos fármacos , Mitocôndrias/fisiologia , Mutação , Fator 2 Relacionado a NF-E2/biossíntese , Fator 2 Relacionado a NF-E2/genética , Proteínas do Tecido Nervoso/biossíntese , Proteínas do Tecido Nervoso/genética , Fragmentos de Peptídeos/sangue , Placa Amiloide/patologia , Presenilina-1/genética , Receptores de Melatonina/deficiência , Receptores de Melatonina/genética , Superóxido Dismutase/biossíntese , Superóxido Dismutase/genética , Superóxido Dismutase-1 , Glutationa Peroxidase GPX1RESUMO
Melatonin, an indoleamine hormone secreted into circulation at night primarily by the brain's pineal gland, has been shown to have a wide variety of actions on the development and physiology of neurons in the CNS. Acting via two G-protein-coupled membrane receptors (MT1 and MT2), melatonin modulates neurogenesis, synaptic functions, neuronal cytoskeleton and gene expression. In the present studies, we sought to characterize the behavior and neuronal biology of transgenic mice lacking both of these melatonin receptors as a way to understand the hormone's receptor versus non-receptor-mediated actions in CNS-dependent activities, such as learning and memory, anxiety, general motor performance and circadian rhythmicity. Assessment of these behaviors was complemented by molecular analyses of gene expression in the brain. Our results demonstrate mild behavioral hyperactivity and a lengthened circadian period of free-running motor activity in melatonin receptor-deficient mice as compared to receptor-intact control mice beginning at an early age. Significant improvement in cognitive performance was found using the Barnes Maze and the Y-Maze. No behavioral changes in anxiety levels were found. Electrophysiological measures in hippocampal slices revealed a clear enhancement of long-term potentiation in mice lacking melatonin receptors with no significant differences in paired-pulse facilitation. Quantitative analysis of brain protein expression levels of phosphoCREB and phosphoERK1/2 and key markers of synaptic activity (synapsin, glutamate receptor 1, spinophilin, and glutamic acid decarboxylase 1) revealed significant differences between the double-knockout and wild-type animals, consistent with the behavioral findings. Thus, genetic deletion of melatonin receptors produces mice with enhanced cognitive and motor performance, supporting the view that these receptors play an important role in neurobehavioral development.
