Alzheimer's disease (AD) is associated with decreased brain energy metabolism. Hypometabolism in the posterior cingulate cortex (PCC) occurs before the onset of memory deficits in subjects at genetic risk for AD who are not yet cognitively impaired. There is a specific inhibition in cytochrome oxidase (C.O.) in the PCC, an area involved in spatial navigation. Creating an animal model that exhibits the early pathophysiology of AD is important for developing and testing drugs that could reverse memory problems associated with such deficits. Methylene blue (MB) is a compound that improves C.O. activity and memory retention in rats. This dissertation had three specific aims: 1) to examine if isolated PCC hypometabolism causes spatial memory deficits in rats; 2) to find a dose of MB that improves memory without nonspecific behavioral effects; and 3) to prevent memory deficits from PCC hypometabolism with low dose MB. PCC hypometabolism was produced by focal administration of sodium azide, an inhibitor of C.O. activity. PCC hypometabolism resulted in impaired spatial memory in a hole board food-search task, increased oxidative damage, and neurotoxicity in the PCC. In addition, PCC hypometabolism resulted in reduced inter-regional correlations in brain activity. Our second set of studies examined the dose-response effects of MB. Our findings demonstrated that a low dose of MB: 1) enhanced memory in open field habituation and object recognition tasks; 2) did not affect general locomotor activity, exploration, motivation, or anxiety; and 3) increased brain oxygen consumption 24 hr after in vivo administration. Finally, our last study found that low dose MB prevented the deficits caused by PCC hypometabolism. MB did not prevent PCC inhibition or cell loss caused by sodium azide. Inter-regional correlations of brain metabolic activity suggested that rats treated with MB were using a different, but equally efficient, strategy for memory retrieval. This animal model of C.O. hypometabolism in the PCC can provide information to understand the mechanisms that regulate early pathological degeneration and reveal new therapeutic strategies aimed at reducing or preventing cognitive decline. Studies of low dose MB in humans are needed to examine its effects in AD patients.

Current approaches to treat neurodegenerative disease provide only mild symptomatic relief but do not modify the natural history of these conditions. A large body of evidence suggests that mitochondrial dysfunction is a key event in the pathophysiology of neurodegeneration. Supporting and improving mitochondrial function has a big potential as a strategy for neuroprotection. The goal of this dissertation was to test whether interventions that target mitochondrial function are effective at preventing neurodegeneration induced by mitochondrial failure in vivo. A rodent model of optic neuropathy induced by the mitochondrial toxin rotenone was used to test the neuroprotective effects of methylene blue (MB) and near-infrared light (NIL), two interventions with mechanisms of action localized to mitochondria. This work also tested the effects of memantine, an NMDA receptor blocker, to further characterize the relationship between excitotoxicity and mitochondrial dysfunction. Neuroprotective effects were evaluated via behavioral testing of visual function and histopathological analysis of the retina. The neurochemical effects of MB, NIL and memantine were analyzed in vitro and in vivo with indicators of oxidative stress, cell respiration and catalytic activity of respiratory enzymes, including NADH dehydrogenase and cytochrome oxidase. MB, a diaminophenothiazine with potent antioxidant and unique redox properties, prevented the changes in visual function and the retinal histopathology induced by rotenone. In vitro, MB increased oxygen consumption and prevented the increases in oxidative stress in brain tissue induced by rotenone. NIL prevented the behavioral impairment and the decrease in retinal and visual pathway metabolic activity, retinal nerve fiber layer thickness and ganglion cell layer cell density induced by rotenone in a dose-dependent manner. Whole-brain cytochrome oxidase and superoxide dismutase activities were also increased in NIL-treated subjects in a dose-dependent manner, suggesting an in vivo transcranial effect of NIL. Finally, uncompetitive NMDA receptor blockade with memantine displayed neuroprotection against rotenone-induced neurodegeneration in a dose-response manner, and this effect was associated with a decrease in retinal oxidative stress and a long-term increase in neuronal energy metabolism capacity. These data constitute a proof-of-principle that interventions that target the mitochondria and support the function of the respiratory chain are effective at preventing neurodegeneration in vivo. / text