14-17 Good evidence now exists for oxidative damage to the AD brain.18-21 A corollary to the oxidative injury hypothesis is that nitric oxide (NO) and/or its highly reactive derivative peroxynitrite also play a role in cell injury or death in AD.22,23 Peroxynitrite is currently thought to be a principal means whereby NO expression can result, in cytotoxicity.24 Evidence for peroxynitrite-induced nitration of neuronal proteins has been found in the AD brain.25,26 Reactive oxygen species (ROS) and reactive nitrogen species are hypothesized

in AD to be both extrinsic to neurons (generated by glial cells)27 and intrinsic (generated by neurons themselves under conditions Inhibitors,research,lifescience,medical of oxidative stress, such as β-AP toxicity).28 Inhibitors,research,lifescience,medical Microglia, which are found in and around neuritic plaques in AD, have pivotal roles in the inflammatory, oxidative, and reactive

nitrogen hypotheses of neuronal injury in AD. As intrinsic immune effector cells of the brain, in a variety of diseases or disease models microglia secrete and respond to inflammatory Inhibitors,research,lifescience,medical cytokines, present antigen, secrete complement and express complement receptors, are phagocytic, show a respiratory burst resulting in production of oxygen free radicals, produce large amounts of reactive nitrogen species, and have a variety of other immune -related functions.29,30 β-AP is thought to be neurotoxic and to play a key role in the pathophysiology of AD.31-33 Significantly, β-AP induces cultured microglia to produce many agents with the potential to directly or indirectly injure neurons, including Inhibitors,research,lifescience,medical inflammatory and chemotactic cytokines,34,35 NO,27,36,37 and ROS.36,38 However, β-AP-induced increases in microglial production Inhibitors,research,lifescience,medical of these factors have been disappointingly modest, on the order of only two to three times control levels. Studies using microglial-neuronal cocultures suggest that microglial activity may be important in β-AP-mediated neurotoxicity in AD, but data are conflicting as to the mechanism.

Endotoxin-, cytokine-, or phorbol-ester-stimulated rodent microglia have been convincingly shown to be neurotoxic through NO or ROS mechanisms.39-42 More relevant to AD, Meda27 found that β-AP 25-35 induced neurotoxicity in microglial-neuronal cocultures, which was attributed Batimastat to microglial TNF-α and reactive nitrogen intermediates. McMillian43 used β-AP-stimulated mixed astrocyte/microglial/neuronal cultures and found that a nonspecific nitric oxide synthase (NOS) inhibitor blocked neurotoxicity; Ii et al obtained similar results.44 In contrast, Giulian45 also induced neurotoxicity with β-AP in microglial-neuronal cocultures, but found no evidence of involvement, of NO or other free radicals. Van Muiswinkel38 found that β-AP increased superoxide production by phorbol-esterprimed microglia, but had no effect on NO production (neurotoxicity was not tested).