Neurochemical aspects on aging and diseases with cognitive impairment

The concentrations of monoamines and cholineacetyl transferase (CAT) are reduced in brains from patients with normal aging. According to findings about the dopamine (DA) system, the age‐sensitive decrease has its onset in the 60s and after that has a continuous progress. In patients with Alzheimer's disease (AD/SDAT), multiple changes indicate disturbed metabolism in the acetylcholine (ACh), DA, noradrenaline (NA), and 5‐hydroxytryptamine (5‐HT) systems. The activity of monoamine oxidase B (MAO‐B) increases significantly with age and a further‐increase has been reported to occur in Alzheimer‐afflicted brains. In the platelets, too, there is increased MAO‐B activity in AD/SDAT patients. Increased enzyme activity is considered a marker of gliosis in the brain and a marker of macrocytosis in platelets. Gangliosides are reduced in several areas of the brain in AD/SDAT patients, which indicates a reduction of nerve terminals. Significantly reduced concentrations of myelin components have also been recorded in brains from patients with AD/SDAT, which is in agreement with histopathological findings. The neurochemical changes recorded in brains from normally aged individuals are very similar to those found in AD/SDAT brains, although the latter changes are more‐severe. Thus, it has been suggested that the pathological process in AD/SDAT may be an exacerbation of the process taking place in normal aging. The multiple neurochemical changes recorded in grey, as well as in white matter must still be considered changes of secondary nature. At the present level of knowledge, it is not possible to single out any one of these changes as of special etiological importance. In the brains of patients with AD/SDAT, more fundamental disturbances must be considered, the nature of which we still do not know. It should also be emphasized that the pathological changes in the central nervous system reported in patients with AD/SDAT have, to some extent, also been found outside the brain, indicating that the pathological process in AD/SDAT is a general process to which the central nervous system is possibly especially sensitive.