Amyloid Peptide-a7 Nicotinic Acetylcholine Receptor Interactions: Implications For Cytoprotection In Vitro

Abstract

Brain deposition of (3-amyloid peptide 1-42 (A(31 -42)-containing senile plaques has been a consistent finding in Alzheimer’s disease (AD). However, the link between Apl-42 and neuronal degeneration remains unclear. It has been reported that AP peptides bind with selectivity to a l nicotinic acetylcholine receptors (a7nAChRs), in both healthy and Alzheimer’s Diseased brain tissues. The goal of this study was to demonstrate the functional inhibition of oc7nAChRs induced by Api-42, both in systems in vitro and in vivo. Initially, differentiated PC-12 cells were preloaded with fura 2-AM and intracellular free Ca2+ levels were determined by fluorescent imaging. Nicotine-induced Ca2+ signals were inhibited by pretreatment with the a7nAChR-selective antagonists, abungarotoxin (BTX) and methyllycaconitine (MLA). Nicotine induced Ca2+ influx was also blocked by pretreatment with 100 nM Api-42. In the same model, nicotine produced a concentration-dependent increase in cell viability in differentiated PC-12 cells that underwent nerve growth factor (NGF) withdrawal for 24 hr. The cytoprotective action of nicotine was efficiently antagonized by co-treatment with a7nAChR antagonists. A concentration-dependent inhibition of the cytoprotective action of nicotine also was produced by co-treatment with Apl-42 (1-100 nM). Also in differentiated PC-12 cells, nicotine induced a concentration-dependent increase in cell surface Trk A receptor expression. This increase was almost completely reversed by a7receptor-selective antagonists, and by co-treatment with Api-42. In in vivo studies with rats, intracerebroventricular (icv) injection of choline, a selective a7nAChR agonist, produced transient, but dose-dependent pressor responses and prolonged decreases in heart rate. Icv pretreatment with BTX and MLA significantly inhibited the cardiovascular responses to subsequent injection of choline. Pretreatment with the Api-42 also significantly inhibited the choline-induced cardiovascular changes suggesting that the peptide can block an oc7nAChR-mediate response in vivo. Nicotine also was administered to rats by direct injection into a lateral cerebral ventricle. Estimation of Trk A expression in necropsied brain tissues revealed significant increases in hippocampus and entorhinal cortex. These increases were significantly inhibited in rats co-treated with a-bungarotoxin or with Api-42. The data derived from these in vitro and in vivo experiments support the hypothesis that low physiological concentrations of AP peptides inhibit the function of a7nAChRs, thereby contributing to the loss in neuronal viability that accompanies Alzheimer’s disease.