Alzheimer's disease and related dementias: selective involvement of specific neuronal systems.

The brains of individuals with dementia due to Alzheimer's disease (AD), Parkinson's disease (PD), and Down's syndrome (DS) exhibit similar neuropathological features, including neuritic plaques, neurofibrillary tangles, and the loss of specific populations of neurons. In addition, these brains show similar transmitter-specific neurochemical alterations. Recent evidence indicates that several pathological and neurochemical changes are related to diseases involving projection systems innervating the telencephalon. At least two transmitter-specific circuits, cholinergic neurons in the basal forebrain and noradrenergic neurons in the locus coeruleus, are selectively affected in many patients with these disorders. This review focuses on these systems because they provide particularly useful examples of the ways in which multidisciplinary studies can provide new clinical-pathological-chemical correlations. Strategies used in these studies are applicable to a wide variety of other neurodegenerative diseases affecting specific populations of neurons. Modern neuropathological approaches to these diseases should eventually allow investigators to directly relate pathogenic processes involving transmitter-specific neuronal populations, whose projections, physiological properties, and functions are known, with the clinical manifestations occurring in human disorders of behavior and cognition.