Neurochemistry

It is difficult to link age-related changes in the neurochemistry of synapses to specific changes in cognitive function. Neurochemical studies of experimental animals are easier to perform and better controlled than those using postmortem human brain tissue, but they are not readily related to cognitive changes in humans. Despite such difficulties, however, there is accumulating evidence that age-related declines in transmission at cholinergic, serotonergic, and dopaminergic synapses contribute to changes in motor function, mood, and memory, respectively. Recent developments in functional brain imaging have provided significant advances in studies of the neurochemistry of synapses, changes in the aging brain, and their relationship to cognitive function. Radioactive ligands for specific neurotransmitter receptors, which can be imaged in living subjects using positron emission tomography (PET), permit investigators to visualize the activity of specific types of synapses in discrete regions of the brain. This approach permits direct comparisons of synaptic function in the brains of individuals of different ages and allows investigators to link neurochemical differences to differences in cognitive function.