Agonist binding to M1 muscarinic receptors is sensitive to guanine nucleotides

Spatial memory in aged rats is related to PKCgamma-dependent G-protein coupling of the M1 receptor

In the present study, individual differences in spatial memory in aged Fischer 344 (F344) rats were associated with the extent of G-protein coupling of the M1 muscarinic receptor and the dendritic-to-somal ratio of hippocampal PKCgamma (d/sPKCgamma) immunogenicity. Following testing in the eight-arm radial maze task, 7 young and 13 aged rat brains were sectioned through the dorsal hippocampal formation (HF). G-protein coupling of the M1 receptor was assessed autoradiographically using competition binding studies in the presence and absence of a G-protein uncoupler to determine high (K(H)) and low (K(L)) affinity states for agonist in the HF, neocortex, and amygdala. In aged animals, a relationship between choice accuracy in the maze and K(H), a measure of M1 receptor-G-protein coupling was seen in the dentate gyrus, CA3, CA1, and neocortex. Furthermore, choice accuracy and d/sPKCgamma immunogenicity showed a significant relationship in CA1. Lastly, a correlation was seen in the CA1 of aged animals between K(H) and d/sPKCgamma. These relationships did not hold for the amygdala. Thus, individual differences in a naturally occurring age-dependent disruption of cholinergic-PKCgamma signal transduction is associated with spatial memory dysfunction.

Attenuation of muscarinic receptor-G-protein interaction in Alzheimer disease

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.

Loss of high-affinity agonist binding to M1 muscarinic receptors in Alzheimer's disease: implications for the failure of cholinergic replacement therapies

Cholinergic replacement therapies have yielded little or no clinical improvement in Alzheimer's disease (AD). Since the number of postsynaptic muscarinic receptors remains unchanged in the cerebral cortex, the involvement of other neurotransmitter systems may account for this limited efficacy. Alternatively, there may be a defective coupling of the muscarinic receptor with its nucleotide-binding protein in AD, which would severely limit the ability of cholinergic agonists to activate intracortical second messengers. To address this possibility, we assessed the ability of the putative M1 muscarinic receptor to form high-affinity agonist-receptor complexes with guanine nucleotide regulatory proteins in postmortem frontal cortex. Agonist affinity states of the M1 muscarinic receptor were measured by carbachol/[3H]-pirenzepine competition. M1 muscarinic receptors exhibited both high (KH) and low (KL) affinities for the agonist carbachol. High-affinity agonist binding to M1 receptors in postmortem frontal cortex samples from subjects with AD was reduced, demonstrated by an increase in the KH value. Low-affinity agonist binding (KL value) was unchanged in AD and was not significantly different from the KL value for the uncoupled receptor determined in the presence of guanine nucleotides. The increase in the KH value resulted in a 70% decrease in the average KL/KH ratio for AD as compared to control samples. Choline acetyltransferase activities correlated significantly with the KL/KH ratios (r = 0.73, p less than 0.001). These data suggest that the KL/KH ratio for muscarinic agonists may serve as a neurochemical marker of disease severity. The reduced ability of the M1 receptor subtype to form a high-affinity agonist state in AD may account for the failure of cholinergic replacement therapies to improve specific features of memory and cognition.