Table 3.2 Chemical changes that may occur in the CNS with advancing age.

Increased water content in the brain.

Neurotransmitter enzyme concentrations may change with age.

In the monoaminergic system there are increased levels of monoamine oxidase and decreased levels of noradrenaline, dopamine and serotonin (5-HT).

Neurotransmitter receptors may change in number with age, for example D2 dopamine receptors reduce in number in rodents and humans, whereas the Dl dopamine receptors increase in number. Serotonin receptors S1 and S2 both decrease in numbers with age.

In the cholinergic system the presynaptic marker acetylcholinesterase is reported to increase in concentration with age whereas choline acetyltransferase decreases. There is a decrease in the number of muscarinic receptors. Neuropeptide neurotransmitters show variable age-related changes in concentrations in different regions of the brain.

Vascular disease such as arteriocapillary fibrosis or endothelial proliferation may reduce blood flow to the brain and so reduce oxygen transport and nutrient supply to the nerve cells, resulting in hypoxia and accumulation of intracellular waste products leading to functional decline with or without neuronal loss.