Somatostatin messenger RNA-containing neurons in Alzheimer's disease: an in situ hybridization study in hippocampus, parahippocampal cortex and frontal cortex.

The level of expression of somatostatin messenger RNA-containing neurons in human brain was visualized and quantified by in situ hybridization with a 35S-labelled oligonucleotide complementary to amino acids 96-111 of the preprosomatostatin complementary DNA sequence. The analysis was carried out in the frontal and parahippocampal cortices and hippocampus of six age- and post mortem delay-matched Alzheimer's disease and control brains. By northern blot analysis, in frontal cortex samples, 18S rRNA degradation was identical in control and Alzheimer brains and somatostatin messenger RNAs migrated as a single band of 1 kb. By in situ hybridization, specificity was demonstrated by abolition of the signal using either an excess of unlabelled antisense probe or using a labelled sense probe. Somatostatin messenger RNA-containing neurons displayed a similar regional and subregional distribution in control subjects and patients with Alzheimer's disease, being more abundant in the frontal cortex, followed by the hippocampus and the parahippocampal cortex. An overall reduction of labelled cell density was observed in patients with Alzheimer's disease (frontal cortex gray matter:--41%; white matter:--66%; hippocampus:--44%; parahippocampal cortex white matter:--40%). Due to a great variation between brains, this decrease only reached significance in the parahippocampal cortex (-59%, P < 0.05). A significantly lower level of expression of somatostatin messenger RNA per somatostatinergic cell was observed in the hippocampus of Alzheimer's disease patients (-47%, P < 0.05), but not in frontal cortex gray (-17%) and white (-36%) matter and parahippocampal cortex gray (-42%) and white (-29%) matter. These data are in accordance with the distribution of somatostatin cells as visualized by immunohistochemistry in human brain. They indicate that the ability of cortical cells to express somatostatin messenger RNA is partially preserved in Alzheimer disease brains and that the decrease in the amount of somatostatin messenger RNA per cell is restricted to the hippocampal formation.