Expression of nicotinic acetylcholine receptor subunits in the cerebral cortex in Alzheimer's disease: Histotopographical correlation with amyloid plaques and hyperphosphorylated-tau protein

Impairment of cholinergic transmission and decreased numbers of nicotinic binding sites are well-known features accompanying the cognitive dysfunction seen in Alzheimer's disease (AD). In order to elucidate the underlying cause of this cholinoceptive dysfunction, the expression of two pharmacologically different nicotinic acetylcholine receptor (nAChR) subunits (α4, α7) was studied in the cerebral cortex of Alzheimer patients as compared to controls. Patch-clamp recordings of 14 dissociated neurons of control cortices showed responses suggesting the existence of α4- and α7-containing functional nAChRs in the human cortex. In cortices of Alzheimer patients and controls, the pattern of distribution and the number of α4 and α7 mRNA-expressing neurons were similar, whereas at the protein level a decrease in the density of α4- and α7-expressing neurons of =≃30% was observed in Alzheimer patients. The histotopographical correlation of nAChR expression with accompanying pathological changes, e.g. accumulation of hyperphosphorylated-tau (HP-tau) protein and β-amyloid showed that neurons in the vicinity of β-amyloid plaques bore both nAChR transcripts. Neurons heavily labelled for HP-tau, however, expressed little or no α4 and α7 mRNA. These results point to an impaired synthesis of nAChRs on the protein level as a possible cause of the cholinoceptive deficit in AD. Further investigations need to elucidate whether interactions of HP-tau with nAChR mRNA, or alterations in the quality of α4 and α7 transcripts give rise to decreased protein expression at the level of individual neurons.