Neurotransmitter deficits in Alzheimer's disease and in other dementing disorders.

The evidence for deficiencies in neurotransmitters in Alzheimer's disease is reviewed. Major losses occur in the subcortical afferent projection systems based on acetylcholine, noradrenaline and serotonin. Within the cortex, somatostatin containing neurones and the large pyramidal cells, presumed to use glutamate/aspartate as transmitters, are the most severely damaged cells. The anatomical distribution of cell loss is explainable if the primary site of damage lies within the cortex; nerve cells are damaged by virtue of their presence within or their connections to this region. The senile plaque may represent the site of this damage and neurofibrillary tangle formation and accumulation may lead to cell death. In patients with Down's syndrome who live past 40 years, changes in transmitters apparently identical to those in Alzheimer's disease occur. The dementia of Parkinson's disease appears related to damage to cholinergic, noradrenergic and dopaminergic systems and may reflect a failure of these subcortical regions to sufficiently "activate" an otherwise undamaged cortex.