Amyloid β-peptide disrupts carbachol-induced muscarinic holinergic signal transduction in cortical neurons

Cholinergic pathways serve important functions in learning and memory processes, and deficits in cholinergic transmission occur in Alzheimer disease (AD). A subset of muscarinic cholinergic receptors are linked to G-proteins that activate phospholipase C, resulting in the liberation of inositol trisphosphate and Ca²⁺ release from intracellular stores. We now report that amyloid β-peptide (Aβ), which forms plaques in the brain in AD, impairs muscarinic receptor activation of G proteins in cultured rat cortical neurons. Exposure of rodent fetal cortical neurons to Aβ25-35 and Aβ1-40 resulted in a concentration and time-dependent attenuation of carbachol-induced GTPase activity without affecting muscarinic receptor ligand binding parameters. Downstream events in the signal transduction cascade were similarly attenuated by Aβ. Carbachol-induced accumulation of inositol phosphates (IP, IP₂, IP₃, and IP₄) was decreased and calcium imaging studies revealed that carbachol-induced release of calcium was severely impaired in neurons pretreated with Aβ. Muscarinic cholinergic signal transduction was disrupted with subtoxic levels of exposure to Aβ. The effects of Aβ on carbachol-induced GTPase activity and calcium release were attenuated by antioxidants, implicating free radicals in the mechanism whereby Aβ induced uncoupling of muscarinic receptors. These data demonstrate that Aβ disrupts muscarinic receptor coupling to G proteins that mediate induction of phosphoinositide accumulation and calcium release, findings that implicate Aβ in the impairment of cholinergic transmission that occurs in AD.