Cortical M1 muscarinic receptor-G-protein coupling, high-affinity, guanine nucleotide-sensitive agonist binding (Flynn et al., 1991; Warpman et al., 1993) and muscarinic receptor-stimulated [3H]PIP2 hydrolysis (Ferrari-DiLeo and Flynn, 1993) are known to be defective in Alzheimer disease. Whether this defect reflects an alteration in the M1 muscarinic receptor, its respective guanine nucleotide binding (G) protein or both is not known. This study compares the number and both basal and muscarinic receptor-mediated function of G-proteins in synaptosomal membranes from cerebral cortical samples of age-matched control subjects and Alzheimer disease patients. Immunoblotting with anti-G alpha q/11 and anti-G beta antibodies demonstrated no alteration in the number of these G-protein subunits in Alzheimer disease. Basal [35S]GTP gamma S binding and hydrolysis of [gamma-32P]GTP by high-affinity GTPase also were not significantly altered in Alzheimer disease compared to control membrane samples. However, muscarinic agonist-stimulated GTP gamma S binding and GTP hydrolysis were significantly reduced (80-100%) in Alzheimer disease cortical samples. Diminished agonist-stimulated GTP gamma S binding and GTP hydrolysis correlated with the loss of guanine nucleotide-sensitive, high-affinity agonist binding (KL/KH) ratio) to the M1 receptor subtype. These data provide further evidence for the loss of muscarinic receptor-G protein coupling in Alzheimer disease and support the hypothesis that muscarinic receptor-mediated cortical activation may be compromised in Alzheimer disease.