Studies comparing the expression of the amyloid beta protein precursor (APP) gene in brains of patients with Alzheimer's disease (AD) and control individuals have resulted in contradictory findings indicative of selective reductions and relative increases of APP alternative transcripts in AD brain. It has been suggested that changes in APP expression in relation to AD neuropathology may represent highly specific and localized events involving only select populations of cells in particular brain regions. For example, reported AD-related alterations in ratios of APP alternative transcripts could be attributed to changes in expression of APP in specific neuronal subpopulations, and/or reactive astrocytes and/or microglia. To address this question, we have employed in situ hybridization using biotinylated oligonucleotide probes designed to localize specific APP mRNA transcripts in the hippocampal formation of AD patients and age-matched controls since this method allows a clear distinction of the classes of neurons and glia containing a particular message.(ABSTRACT TRUNCATED AT 250 WORDS)