The ability of synapses to change their properties in response to environmental demands (synaptic plasticity) is essential for learning and memory. Abnormalities in synaptic plasticity are involved in Alzheimers disease and related disorders. In our continuing efforts to understand the molecular mechanisms involved in synaptic plasticity, in the contexts of aging and neurodegenerative disorders, we have made two major advances. 1) During development of the nervous system, the fate of stem cells is regulated by a cell surface receptor called Notch. Notch is also present in the adult mammalian brain;however, because Notch null mice die during embryonic development, it has proven difficult to determine the functions of Notch. Here, we used Notch antisense transgenic mice that develop and reproduce normally, but exhibit reduced levels of Notch, to demonstrate a role for Notch signaling in synaptic plasticity. Mice with reduced Notch levels exhibit impaired long-term potentiation (LTP) at hippocampal CA1 synapses. A Notch ligand enhances LTP in normal mice and corrects the defect in LTP in Notch antisense transgenic mice. Levels of basal and stimulation-induced NF-kappa B activity were significantly decreased in mice with reduced Notch levels. These findings suggest an important role for Notch signaling in a form of synaptic plasticity known to be associated with learning and memory processes. 2) Although ATP is reported to modulate synaptic plasticity, the mechanism of action of ATP on synaptic transmission is not fully understood. Here we show that ATP enhances long-term potentiation (LTP), and P2X receptor antagonists inhibit this ATP effect, but do not affect paired pulse facilitation (PPF) in rat hippocampal slices. ATP rapidly increases intracellular calcium, and P2X receptor antagonists inhibit this increase in cultured dissociated neurons. These results indicate that ATP enhances LTP via activation of postsynaptic P2X receptors. In additional studies, we have found that intermittent fasting and caloric restriction ameliorate age-related learning and memory deficits in a novel transgenic mouse model of Alzheimers disease. We also found that the antidepressant drug paroxetine was effective in suppressing amyloid pathology and preserving learning and memory ability in the same mouse model of Alzheimers disease. Other experiments have shown that diabetes impairs hippocampal neurogenesis and synaptic plasticity as the result of a chronic elevation in the level of adrenal glucocorticoids. We have found that perturbed membrane sphingolipid metabolism occurs in the brain in aging, Alzheimer's disease and HIV dementia. Studies of experimental models suggest that excessive activation of sphingomyelinases result in aberrant production of ceramides and perturbed membrane excitability and synaptic plasticity. More recently, we have discovered that several different toll-like receptors (TLRs) that were previously believed to be involved only in immune responses to infection, play important roles in synaptic plasticity and learning and memory. TLRs are therefore potential targets for the development of novel therapeutic interventions for cognitive impairment and Alzheimer's disease.
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