Almost all degenerative diseases of the CNS are associated with chronic inflammation. A central step in this process is the activation of brain mononuclear phagocyte cells, called microglia. While it is recognized that healthy neurons and astrocytes regulate the magnitude of microglia-mediated innate immune responses and limit excessive CNS inflammation, the endogenous signals governing this process are not fully understood. In the peripheral nervous system, recent studies suggest that an endogenous 'cholinergic anti-inflammatory pathway' regulates systemic inflammatory responses via a7 nicotinic acetylcholinergic receptors (nAChR) found on blood-borne macrophages. These data led us to investigate whether a similar cholinergic pathway exists in the brain that could regulate microglial activation. Here we report for the first time that cultured microglial cells express a7 nAChR subunit as determined by RT-PCR, western blot, immunofluorescent, and immunochistochemistry analyses. Acetylcholine and nicotine pre-treatment inhibit lipopolysaccharide (LPS)-induced TNF-a release in murine-derived microglial cells, an effect attenuated by a7 selective nicotinic antagonist, a-bungarotoxin. Furthermore, this inhibition appears to be mediated by a reduction in phosphorylation of p44/42 and p38 mitogen-activated protein kinase (MAPK). Though preliminary, our findings suggest the existence of a brain cholinergic pathway that regulates microglial activation through a7 nicotinic receptors. Negative regulation of microglia activation may also represent additional mechanism underlying nicotine's reported neuroprotective properties.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder pathologically characterized by deposition of ␤-amyloid (A␤) peptides as senile plaques in the brain. Recent studies suggest that green tea flavonoids may be used for the prevention and treatment of a variety of neurodegenerative diseases. Here, we report that (-)-epigallocatechin-3-gallate (EGCG), the main polyphenolic constituent of green tea, reduces A␤ generation in both murine neuron-like cells (N2a) transfected with the human "Swedish" mutant amyloid precursor protein (APP) and in primary neurons derived from Swedish mutant APP-overexpressing mice (Tg APP sw line 2576). In concert with these observations, we find that EGCG markedly promotes cleavage of the ␣-C-terminal fragment of APP and elevates the N-terminal APP cleavage product, soluble APP-␣. These cleavage events are associated with elevated ␣-secretase activity and enhanced hydrolysis of tumor necrosis factor ␣-converting enzyme, a primary candidate ␣-secretase. As a validation of these findings in vivo, we treated Tg APP sw transgenic mice overproducing A␤ with EGCG and found decreased A␤ levels and plaques associated with promotion of the nonamyloidogenic ␣-secretase proteolytic pathway. These data raise the possibility that EGCG dietary supplementation may provide effective prophylaxis for AD.

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