Reduced high-affinity agonist binding at the M1 muscarinic receptor in Alzheimer's disease brain:: Differential sensitivity to agonists and divalent cations

M-1 muscarinic acetylcholine receptor (M(1)AchR)-G protein coupling, as measured by high-affinity agonist binding, was examined in membranes prepared from postmortem human temporal cortex (Brodmann area 38) from individuals with Alzheimer's disease (AD, n = 8) and age-matched controls (n = 6). Binding competitions between the M(1)AchR-selective antagonist [H-3]pirenzepine ([H-3]PZ) and muscarinic agonists carbachol, acetylcholine, oxotremorine, and oxotremorine M were conducted. In the presence of 1 mM. MgCl2, the inhibition of [H-3]PZ binding by carbachol, acetylcholine, or oxotremorine M was best described by a two-affinity state model for control and AD cases, while oxotremorine binding affinity was best fit to a single-state model. Although both control and AD groups had similar K-D values for the high- and low-affinity agonist binding sites, the proportion of M(1)AchRs exhibiting high affinity for carbachol and acetylcholine was reduced by 48 and 33%, respectively, in AD membranes relative to controls (P < 0.05). No changes in the binding of the oxotremorine M or oxotremorine were noted. The nonhydrolyzable guanine nucleotide Gp-pNHp (100 mu M) reduced the proportion of M(1)AchRs with high affinity for agonists in both control and AD membranes. Substitution of 1 mM. MnCl2 for MgCl2 restored high-affinity carbachol binding at the M(1)AchR in AD membranes similar to that seen in controls. In the presence of 1 mM MnCl2, agonist binding in controls did not differ from 1 mM MgCl2. In the absence of cations (1 mM EDTA), no differences between control and AD M(1)AchR carbachol binding were observed. Thus, the loss of high-affinity agonist binding at the M(1)AchR in AD is dependent on the agonist and cation studied, (C) 1999 Academic Press.