Mohammed T. Abou-Saleh

PET 361to control subjects, suggesting a compensatory role of this region,similar to that postulated above for AD patients.GERIATRIC DEPRESSIONDepression is a serious problem in older adults, and can occur as asymptom of a dementing illness or in the absence of dementia. Themost common neuroimaging finding in depression is reducedglucose metabolism or flow in prefrontal cortex, althoughcingulate and paralimbic regions also are involved in mooddisorders 36,37 . These regional patterns of reduction are seen inelderly as well as younger patients, although elderly depressedindividuals are more likely to have global metabolic reductions inmetabolism 38 . There is considerable evidence that the serotonergicneurotransmitter system is altered in depression 39 and ligands formeasuring these receptors have recently been developed. A fewstudies have used PET to examine levels of serotonin receptors indepressed patients, but have reported confliciting results. In oneexperiment, 5-HT 2 binding was reduced in orbitofrontal areas indepressed patients 40 , but no differences between depressed patientsand controls were found in another study 41 . In older healthyadults 5-HT 2 receptor binding is reduced compared to youngadults 42 , but is not further reduced in elderly depressed patients.However, serotonin binding is reduced in AD patients 43 , indicatingthat results from studies of geriatric depression should beviewed with caution, as they may have been influenced by coexisitingdementia, which is difficult to rule out clinically 39 .Itisclear that much work in this area remains to be done, includingthe assessment of brain function during serotonergic challenge inthe elderly.CONCLUSIONThe accumulating evidence that detectable changes in brainfunction precede structural changes in neurodegenerative conditionsand, in addition, that PET can provide a measure of severityof disordered brain function, indicates the potential value of PETin early detection and in monitoring the effects of treatmentsintended to modify the progression of these conditions. 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