In parallel to the MRI aging studies of the human brain, there have been similar monkey-based reports of global volumetric changes with age, with decreases in gray matter, compensatory increases in CSF, but stability in white matter volume (Andersen et al., 1999). Age-related changes in the monkey brain were analyzed in another MRI-based study and linked cognitive deficits to white matter loss (Lai et al., 1995; reviewed in Peters et al., 1996; Wisco et al., 2001). A more recent study on the effects of aging in squirrel monkeys also showed cognitive deficits with age, but with an increase in white matter content in the anterior half of the brain (Lyons et al., 2004). However, it is important to note that the other studies cited above used rhesus monkeys and examined a wider range of older animals. Evaluations of subcortical structures have also been published, such as the report on the decrease in overall volume with age in the male rhesus monkey striatum (Matochik et al., 2000). This age-related striatal change was recently confirmed in both sexes, although it was noted that only the aged male rhesus monkeys performed worse than young controls on fine motor tasks (Lacreuse et al., 2005). In the monkey hippocampus, cerebral blood volume, as assessed with MRI, suffered from an age-related decline that correlated with cognitive deficits (Small et al., 2004). Neurochemical studies in monkeys using proton magnetic resonance spectroscopy are limited (Herndon et al., 1998), but have disclosed effects of aging on various neurochemical parameters of neuronal function.
Few manipulations on monkeys as they relate to MRI-based brain studies have been performed. A recently published study (Matochik et al., 2004) examined the effects of age and calorie restriction (CR) on the caudate and putamen of monkeys, but revealed differential effects of treatment on these subregions. Indeed, acute CR was found not to globally protect against the age-related volumetric loss in male rhesus brain. Additional research will address whether long-term CR is protective against both the structural changes in the brain with age and if cognitive function is preserved in a parallel manner.
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