of work:
The aim of this project is to assess the effects of aging at a behavioral level of analysis, to identify neurobiological mechanisms associated with these effects, and to evaluate interventions that might alter age-related performance decrements. Rodent and nonhuman primate models are tested in a battery of sensorimotor and learning/memory tasks. Neurochemical and neurohistological assays are conducted to determine neurobiological correlates of functional losses. Interventions include various nutritional, hormonal and pharmacological treatments. Multiple genotypes and genetically modified mice are examined to determine possible genetic involvement in age-related behavioral impairment. We have identified various effective pharmacologic strategies for improving learning performance of aged rodents using manipulations of the cholinergic and glutamatergic neurotransmitter systems. The lead cholinergic compound in this effort is the cholinesterase inhibitor, phenserine, which is scheduled to begin Phase III clinical trials next year using oral dosing. New work with this compound in rats has shown that effective transdermal delivery of the drug can be accomplished. Specifically, we found that phenserine delivered through a skin patch could attenuate a learning impairment induced by a pharmacological block of cholinergic receptors. Recently we have identified the ability of novel inhibitors of glycine uptake to overcome a maze learning impairment in rats induced by a pharmacological block of N-methyl-D-aspartate (NMDA) glutamate receptors. Inhibition of glycine uptake should enhance signaling through the NMDA receptor. If further success is achieved in these studies, we will develop these leads into a single compound that delivers inhibition of both glycine uptake and cholinesterase. We believe that this strategy offers greater potential for cognitive enhancement than inhibition of either target alone. Using transgenic mice with mutations in the amyloid precursor protein (APP) gene and the presenilin-1 (PS1) gene as a model of Alzheimers disease (AD), we have noted that treatment with acetylcholinesterase and butyrylcholinesterase inhibitors reduce production of APP and its peptide, amyloid-beta (Ab); however, we did not find that long-term treatment with phenserine reduced Ab plaque formation, a pathological hallmark of AD. In the same mice and in normal aged female mice, we have noted that estradiol treatment can reduce microglia and astroglia activation following ovariectomy. Glia activation appears to be an important process in the pathogenesis of AD. In addition, the selective estrogen receptor modulator (SERM), raloxifine, can also reduce microglia activation following estrogen removal via ovariectomy. Treatment of the APPxPS1 transgenic mice with estradiol also appears to reduce Ab plaque formation as well as decrease the loss of dopamine producing neurons in the locus coeruleus. We have also investigated the effects of erythropoietin (EPO), which is kidney hormone that stimulates red blood cell production, on angiogenesis in the brain. We found that the capillary density in the corpus callosum of the APPxPS1 mice is reduced, but we did not find that EPO treatments attenuated this deficit. As a nutritional intervention in rats, we have investigated long-term calorie restriction (CR) as well as the effects of 2-deoxy-D-glucose (2DG) as a pharmacological intervention to mimic the effects of CR. We have found that amphetamine stimulated locomotor activity, which acts through a central dopaminergic mechanism, is enhanced after 3 weeks of CR and 2DG treatment and is sustained even when the treatments have been removed for several weeks. These results indicate lasting alterations on dopaminergic function induced by these treatments. Currently we are assessing memory and motor performance in rhesus monkeys on calorie restriction and diets containing 2DG. As another nutritional intervention, we have investigated the neuroprotective effects of a diet enriched with blueberries, which are high in polyphenolic compounds that can act as potent antioxidants. We found that rats maintained for 3 months on the blueberry enriched diet showed marked attenuation of the maze learning impairment induced by hippocampal injection of the neurotoxin, kainic acid (KA, which acts through glutamate receptors. As another project, we have investigated the long-term effects of learning associated with chronic chemotherapy. Rats receiving monthly injections of cyclophosphamide or 5-fluorouracil exhibited marked signs of toxicity; however, instead of impaired learning in two maze tasks, we observed that the rats undergoing chemotherapy showed superior performance. Moreover, preliminary studies indicated that these rats showed attenuated brain damage following KA injection into the hippocampus. Current studies are evaluating the role of CR as well as increased EPO production to explain these unexpected results of the chemotherapy studies.
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