of work: Caloric restriction (CR: reducing caloric intake 30-40 percent below ad libitum levels) remains the only intervention that reproducibly extends lifespan, reduces the incidence of age-related disease, enhances stress protection, and attenuates functional decline in mammals. Although the effects of CR on aging have been widely studied in rodent models and other short-lived species, the relevance of this intervention to human aging has not been established. In 1987, the NIA began the first controlled trial of CR in a primate species. Results from these studies have thus far demonstrated that it is possible to initiate long-term CR (30 percent reduction from control levels) in primates and that physiological responses to the diet are similar to those reported in rodents. In addition, CR reduced risk factors for age-related disease, such as cardiovascular disease and diabetes. Summarized below are the most recent findings related to these ongoing studies. In the past year several studies have been completed relating to the physiology of aging in primates and rodents and effects of CR and other interventions. Additional use of MRI imaging in rhesus monkeys has repeated our earlier findings of an age-related loss in the volume of the striatum and putamen, two brain regions involved in motor performance, but no decline in overall brain volume. Analysis of learning performance in rhesus and squirrel monkeys has also demonstrated age-decline in simple learning tasks, but we found no over-all beneficial effects of CR on performance except that a simple object discrimination in squirrel monkeys was learned faster in CR monkeys compared to controls. We have also continued to monitor morbidity and mortality in our longitudinal studies of CR in rhesus and squirrel monkeys. The numbers of deaths and diagnosed diseases remain too low for conclusive statistical analyses; however, the emerging data continue to show lower morbidity and mortality in the CR monkeys in both species. In collaboration with the Laboratory of Cardiovascular Sciences, studies are underway to investigate the effects of CR on cardiovascular measures related to arterial stiffness in monkeys on diets with low and high salt content. Thus far, we have found that higher salt content increases arterial stiffness, but that this response depends on the ability of the monkeys to excrete salt in their urine. We also completed a short-term (6 mo) study to determine if CR and 2-D-deoxyglucose (2DG), acting as a CR mimetic, can protect monkeys against the neurotoxin, MPTP, used to model Parkinson's disease. We conducted extensive physiological, behavioral, neuroimaging, neuromorphological, and neurochemical analyses to support the study both before and after injection of MPTP, a neurotoxin of the dopaminergic system designed to produce symptoms of Parkinson's disease. Results demonstrated that the physiological responses of CR monkeys paralleled those observed in rats (reduced body temperature and heart rate) but 2DG did not produce these effects. Moreover, we found that CR provided some limited protection against MPTP in terms of spared motor performance and increased levels of dopamine and dopamine metabolites in the striatum of the monkeys; whereas, 2DG did not provide this protection. In rodent models, other results continued to demonstrate the effectiveness of 2DG feeding as a neuroprotective strategy. 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. Despite these encouraging results with 2DG, a recent mortality study completed in rats does not support the hypothesis that 2DG feeding will extend lifespan similar to the effects of CR. Indeed, long-term 2DG feeding appears to be toxic at certain doses that have provided neuroprotection and produces heart failure due to increased vacuolization of heart muscle. However, in a parallel study we were encouraged by the increased survival compared to controls that we observed in a group of rats treated with metformin, which is a biguanide that acts to reduce liver glucose production as well as acts as an insulin sensitizer. Known as Glucophage, it is a highly prescribed medication in the treatment of type 2 diabetes. These results indicate that alteration of insulin signaling pathways could be an effective strategy for developing a CR mimetic. In other rat studies we have shown increased insulin sensitivity appears to be related to an upregulation of adiponectin. Plasma levels of this adipokine in young rats are increased after only a couple of weeks on CR and maintained throughout life. Adiponectin has been shown to increase insulin sensitivity and could be another target as a CR mimetic.
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