Endogenous inhibitor of ligand binding to the muscarinic acetylcholine receptor

Allosteric Modulation of Muscarinic Acetylcholine Receptors

An allosteric modulator is a ligand that binds to an allosteric site on the receptor and changes receptor conformation to produce increase (positive cooperativity) or decrease (negative cooperativity) in the binding or action of an orthosteric agonist (e.g., acetylcholine). Since the identification of gallamine as the first allosteric modulator of muscarinic receptors in 1976, this unique mode of receptor modulation has been intensively studied by many groups. This review summarizes over 30 years of research on the molecular mechanisms of allosteric interactions of drugs with the receptor and for new allosteric modulators of muscarinic receptors with potential therapeutic use. Identification of positive modulators of acetylcholine binding and function that enhance neurotransmission and the discovery of highly selective allosteric modulators are mile-stones on the way to novel therapeutic agents for the treatment of schizophrenia, Alzheimer’s disease and other disorders involving impaired cognitive function.

Designing human m1 muscarinic receptor-targeted hydrophobic eigenmode matched peptides as functional modulators

A new proprietary de novo peptide design technique generated ten 15-residue peptides targeting and containing the leading nontransmembrane hydrophobic autocorrelation wavelengths, "modes", of the human m(1) muscarinic cholinergic receptor, m(1)AChR. These modes were also shared by the m(4)AChR subtype (but not the m(2), m(3), or m(5) subtypes) and the three-finger snake toxins that pseudoirreversibly bind m(1)AChR. The linear decomposition of the hydrophobically transformed m(1)AChR amino acid sequence yielded ordered eigenvectors of orthogonal hydrophobic variational patterns. The weighted sum of two eigenvectors formed the peptide design template. Amino acids were iteratively assigned to template positions randomly, within hydrophobic groups. One peptide demonstrated significant functional indirect agonist activity, and five produced significant positive allosteric modulation of atropine-reversible, direct-agonist-induced cellular activation in stably m(1)AChR-transfected Chinese hamster ovary cells, reflected in integrated extracellular acidification responses. The peptide positive allosteric ligands produced left-shifts and peptide concentration-response augmentation in integrated extracellular acidification response asymptotic sigmoidal functions and concentration-response behavior in Hill number indices of positive cooperativity. Peptide mode specificity was suggested by negative crossover experiments with human m(2)ACh and D(2) dopamine receptors. Morlet wavelet transformation of the leading eigenvector-derived, m(1)AChR eigenfunctions locates seven hydrophobic transmembrane segments and suggests possible extracellular loop locations for the peptide-receptor mode-matched, modulatory hydrophobic aggregation sites.

Brain soluble fractions which modulate Na+, K+-ATPase activity likewise modify muscarinic receptor

Two brain soluble fractions, named peaks I and II, which respectively stimulate and inhibit neuronal Na+, K+-ATPase activity, have been isolated by gel filtration in Sephadex G-50. Since cholinergic transmission seems related to such enzyme activity, in this study we evaluated the effect of brain peak I, peak II, a more purified fraction II-E and commercial ouabain, on specific binding of the muscarinic antagonist [3H]quinuclidinyl benzilate to membranes from rat cerebellum, hippocampus and cerebral cortex. We found that binding was increased by peak I and decreased by peak II, II-E and ouabain, all effects proving concentration-dependent. Since the changes exerted on the muscarinic receptor followed a pattern similar to the one already described for synaptosomal membrane Na+, K+-ATPase activity, both systems seem to interact at a functional level.

Endogenous Alzheimer's brain factor and oxidized glutathione inhibit antagonist binding to the muscarinic receptor

An endogenous inhibitor (< 3,500 Da) of antagonist binding to the muscarinic acetylcholine receptor has been extracted from Alzheimer's disease (AD) brain with trifluoracetic acid. Oxidized glutathione, (GSSG) has also been found to inhibit antagonist binding to the receptor. However, in its reduced form, glutathione (GSH) like other reducing agents, markedly enhances the inhibitory effect of both GSSG and the endogenous AD inhibitor. EDTA and the free radical scavengers Mn(2+) and Trolox, a vitamin E analog, block the action of the endogenous AD inhibitor but not of GSSG in the presence of GSH. Further, while GSSG inhibition is reversible, the action of the endogenous AD inhibitor is irreversible, consistent with a free radical mechanism. The enhancement of endogenous AD inhibitor activity by GSH suggested that GSH may be involved in formation of the free radical generated by the inhibitor. The glutathione thiyl radical is shown to inhibit antagonist binding to the receptor and is, therefore, a good candidate for the free radical formed by the endogenous AD inhibitor. The ability of Trolox to block the reduction in muscarinic receptor binding caused by the endogenous AD inhibitor is encouraging and suggests that free radical scavengers, such as vitamin E, may have a potential therapeutic role in AD by protecting the integrity of the muscarinic receptor.

Allosteric modulation of muscarinic acetylcholine receptors

Five subtypes of muscarinic acetylcholine receptors have been identified in mammalian tissues, but the selectivity of ligands that are active at these receptors is low. It is possible, however, that selective compounds may be developed by targeting their allosteric site(s). Important new insights into the mechanism of allosteric control of muscarinic receptors have been obtained recently in investigations of the allosteric effects of neuromuscular blockers, and competition between ligands for the allosteric binding site has now been demonstrated. It is now apparent that the binding site for most allosteric ligands is close to the binding site for acetylcholine but that it is located at a more extracellular position. Stanislav Tucek and Jan Proska discuss the pharmacological implications of ligand interaction at these two sites and the therapeutic possibilities.

Serum anticholinergic activity in hospitalized older persons with delirium: a preliminary study

OBJECTIVE

To evaluate the relationship between total serum anticholinergic activity (SAA) and the presence or absence of delirium in older hospitalized persons on general medical wards.

DESIGN

Case-control study and within-subjects repeated-measures in recovered delirious patients.

SETTING

Minneapolis Veterans Affairs Medical Center medical wards.

PARTICIPANTS

Eleven male delirious patients (DSM-III-R criteria) aged 60 or older and 11 comparably aged male nondelirious controls.

MEASUREMENTS

Radioreceptor bioassay of total SAA using tritiated quinuclidinyl benzilate (QNB) binding to muscarinic receptors. Results are expressed in terms of atropine equivalents (nM).

MAIN RESULTS

Mean SAA was significantly elevated in the delirious group (mean +/- SD = 6.05 +/- 2.97 nM atropine equivalents) compared with the controls (3.38 +/- 2.49; t(20) = 2.28, P < .05). At study entry, mean SAA was significantly higher in delirious subjects whose symptoms eventually resolved completely (mean +/- SD = 7.77 +/- 2.37) compared with subjects whose delirious symptoms persisted (3.99 +/- 2.30; t(9) = 2.68, P < .05). All six patients in whom delirium resolved completely had a decrease in serum anticholinergic activity when measured during delirium (7.77 +/- 2.37) and after symptom resolution (3.92 +/- 2.61; t(5) = 3.29, P < .05).

CONCLUSIONS

Our findings suggest that serum anticholinergic activity may play a role in delirium in medical inpatients. The relationships between SAA and delirium in medical patients and between total SAA and medication use warrant further study.

Inhibitor of antagonist binding to the muscarinic receptor is elevated in Alzheimer's brain

The 100,000 x g supernatant fraction of human brain contains endogenous inhibitors of antagonist binding to the muscarinic receptor. Significantly greater inhibition was observed with Alzheimer's than non-demented control supernatant fractions. Low molecular weight inhibitor was separated from larger inhibitor species by membrane dialysis (3,500 dalton cut-off). The activity of low molecular weight inhibitor was greatly increased by sulfhydryl reducing agents. While the low molecular weight inhibitor was stable to heat, acid and base for short time periods (< 20 min), it was inactivated by acid hydrolysis (50% loss after 16 h, 100% loss after 96 h). The low molecular weight inhibitor activity is elevated approximately three-fold in Alzheimer's brain. The low molecular weight inhibitor from Alzheimer's brain was found to be a non-competitive inhibitor. This is the first report of endogenous inhibitors in human brain of ligand binding to the muscarinic receptor and of increased inhibitor activity in Alzheimer's disease.

Reduction in the numbers of muscarinic receptors by an endogenous protein

The soluble fraction from the ileal longitudinal muscle of guinea pigs was examined for the presence of an endogenous modulator of muscarinic receptors. In the presence of the soluble fraction, the binding of [3H]quinuclidinyl benzilate to the membranes from the tissue was inhibited in a concentration-dependent manner. The inhibitory activity in the soluble fraction was heat stable, but was inactivated by trypsin treatment. Several protease inhibitors had no effect on the inhibitory activity. These results suggest the existence of an endogenous protein that inhibits the binding of the muscarinic ligand to the receptor. Ultrafiltration demonstrated that the protein factor had a molecular mass of more than 10,000 Da. Saturation binding and dissociation kinetic experiments indicate neither a competitive nor allosteric mode of inhibitory action and suggest that an irreversible block or internalization of muscarinic receptors is induced by the endogenous protein.

An endogenous factor induces heterogeneity of binding sites of selective muscarinic receptor antagonists in rat heart

According to molecular biological and pharmacological criteria, rat heart membranes normally express only one muscarinic receptor subtype. The selective antagonists pirenzepine and AF-DX 116 bind to this receptor with a single affinity: low and high, respectively. We report here that an endogenous, intracellular factor alters the affinity of selective antagonists for muscarinic receptors in the heart. Thus, when the intracellular fluid is added back to rat heart membranes, both pirenzepine and AF-DX 116 bind to two receptor sites. Approximately 30% of the receptors bind pirenzepine with high affinity and AF-DX 116 with low affinity. Thus, while cardiac muscarinic receptors are coded for by a single mRNA and are therefore genetically homogeneous, the resulting receptor protein might behave like a mixture of receptor subtypes in intact tissues due to the influence of intracellular factors on receptor conformation.

Reduction of muscarinic acetylcholine receptor number and affinity by an endogenous substance

We examined the soluble fraction from homogenates of 12-day embryonic chick heart for the presence of an endogenous modulator of muscarinic acetylcholine receptors (mAChR). Homogenates were separated into 100,000 g soluble and crude membrane fractions by differential centrifugation. Aliquots of membranes were incubated in the presence or absence of the soluble fraction and the muscarinic antagonist, [3H]quinuclidinyl benzilate ( [3H]QNB), and the data subjected to Scatchard analysis. In the presence of the soluble fraction, mAChR number decreased up to 70% and the affinity for [3H]QNB decreased six- to eightfold. These results suggested that an endogenous soluble factor (ESF) affected cholinergic ligand binding to the receptor. The amount of ESF extracted from less than 10 mg of brain was sufficient to reduce by 50% [3H]QNB binding to 50 fmol mAChR. ESF activity was partially purified by heat and acid treatment. The loss of receptors was dependent upon the amount of ESF added and was time dependent. QNB protected some receptors from loss due to ESF. The change in mAChR affinity for [3H]QNB was observed only if ESF was present continuously during the [3H]QNB binding assay. Ultrafiltration and gel filtration showed that ESF was less than 10,000 daltons and probably less than 700 daltons. ESF activity was blocked by EDTA. However, ESF was not a divalent cation since it was base labile, and removal of divalent cations with Chelex-100 did not inhibit ESF activity. ESF activity was also blocked by catechol, catecholamines, ascorbate, and dithiothreitol. ESF was present in embryonic but not in adult heart.