Thiochrome Enhances Acetylcholine Affinity at Muscarinic M4 Receptors: Receptor Subtype Selectivity via Cooperativity Rather than Affinity

Thiochrome (2,7-dimethyl-5H-thiachromine-8-ethanol), an oxidation product and metabolite of thiamine, has little effect on the equilibrium binding of l-[3H]N-methyl scopolamine ([3H]NMS) to the five human muscarinic receptor subtypes (M1-M5) at concentrations up to 0.3 mM. In contrast, it inhibits [3H]NMS dissociation from M1 to M4 receptors at submillimolar concentrations and from M5 receptors at 1 mM. These results suggest that thiochrome binds allosterically to muscarinic receptors and has approximately neutral cooperativity with [3H]NMS at M1 to M4 and possibly M5 receptors. Thiochrome increases the affinity of acetylcholine (ACh) 3- to 5-fold for inhibiting [3H]NMS binding to M4 receptors but has no effect on ACh affinity at M1 to M3 or M5 receptors. Thiochrome (0.1 mM) also increases the direct binding of [3H]ACh to M4 receptors but decreases it slightly at M2 receptors. In agreement with the binding data, thiochrome does not affect the potency of ACh for stimulating the binding of guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS) to membranes containing M1 to M3 receptors, but it increases ACh potency 3.5-fold at M4 receptors. It also selectively reduces the release of [3H]ACh from potassium-stimulated slices of rat striatum, which contain autoinhibitory presynaptic M4 receptors, but not from hippocampal slices, which contain presynaptic M2 receptors. We conclude that thiochrome is a selective M4 muscarinic receptor enhancer of ACh affinity and has neutral cooperativity with ACh at M1 to M3 receptors; it therefore demonstrates a powerful new form of selectivity, "absolute subtype selectivity", which is derived from cooperativity rather than from affinity.

Analogs of WIN 62,577 define a second allosteric site on muscarinic receptors

WIN 51,708 (17-beta-hydroxy-17-alpha-ethynyl-5-alpha-androstano[3,2-b]pyrimido[1,2-a]benzimidazole) and WIN 62,577 (17-beta-hydroxy- 17-alpha-ethynyl-delta(4)-androstano[3,2-b]pyrimido[1,2-a]benzimidazole) are potent and centrally active antagonists at rat, but not human, NK(1) receptors. The interactions of these compounds and some analogs with [(3)H]N-methyl scopolamine ([(3)H]NMS) and unlabeled acetylcholine (ACh) at M(1)-M(4) muscarinic receptors have been studied using equilibrium and nonequilibrium radioligand binding methods. The results are consistent with the predictions of the allosteric ternary complex model. The WIN compounds have log affinities for the unliganded receptor in the range 5 to 6.7, and exhibit positive, negative, or neutral cooperativity with [(3)H]NMS and ACh, depending on the receptor subtype and nature of the interacting ligands. WIN 62,577 is an allosteric enhancer of ACh affinity at M(3) receptors. Although interacting allosterically, WIN 62,577 and WIN 51,708 do not affect [(3)H]NMS dissociation from M(3) receptors. Certain analogs have higher affinities than WIN 62,577, and truncated forms of WIN 62,577, including steroids, also act allosterically. One analog, 17-beta-hydroxy-17-alpha-Delta(4)-androstano[3,2-b]pyrido[2,3-b]indole (PG987), has the unique effect of speeding [(3)H]NMS dissociation; its largest effect, 2.5-fold, is at M(3) receptors. The interaction between PG987 and other allosteric agents on [(3)H]NMS dissociation from M(3) receptors indicate that PG987 binds reversibly to a site distinct from that to which gallamine and strychnine bind: in contrast, PG987 seems to bind to the same site on M(3) receptors as KT5720, staurosporine, and WIN 51,708. Therefore, in addition to the allosteric site that binds strychnine (and probably chloromethyl brucine, another allosteric enhancer) there is a second, nonoverlapping, pharmacologically distinct allosteric site on M(3) receptors that also supports positive cooperativity with ACh.

Multiple allosteric sites on muscarinic receptors

Proteins and small molecules are capable of regulating the agonist binding and function of G-protein coupled receptors by multiple allosteric mechanisms. In the case of muscarinic receptors, there is the well-characterised allosteric site that binds, for example, gallamine and brucine. The protein kinase inhibitor, KT5720, has now been shown to bind to a second allosteric site and to regulate agonist and antagonist binding. The binding of brucine and gallamine does not affect KT5720 binding nor its effects on the dissociation of [3H]-N-methylscopolamine from M1 receptors. Therefore it is possible to have a muscarinic receptor with three small ligands bound simultaneously. A model of the M1 receptor, based on the recently determined structure of rhodopsin, has the residues that have been shown to be important for gallamine binding clustered within and to one side of a cleft in the extracellular face of the receptor. This cleft may represent the access route of acetylcholine to its binding site.

Allosteric interactions of staurosporine and other indolocarbazoles with N-[methyl-(3)H]scopolamine and acetylcholine at muscarinic receptor subtypes: identification of a second allosteric site

We have studied the interactions of five indolocarbazoles with N-[methyl-(3)H]scopolamine (NMS) and unlabeled acetylcholine at M(1)-M(4) muscarinic receptors, using equilibrium and nonequilibrium radioligand binding studies. The results are consistent with an allosteric model in which the primary and allosteric ligands bind simultaneously to the receptor and modify each other's affinities. The compounds were generally most active at M(1) receptors. [(3)H]NMS binding was enhanced by staurosporine, KT5720, and KT5823 at M(1) and M(2) receptors, and by K-252a at M(1) receptors. Gö 7874 reduced [(3)H]NMS affinity by up to threefold for all subtypes. A range of cooperative effects with acetylcholine was seen, and, at the M(1) receptor, KT5720 had a log affinity of 6.4 and enhanced acetylcholine affinity by 40%. The compounds inhibited the dissociation of [(3)H]NMS to different extents across the receptor subtypes, with the largest effects at M(1) receptors. In equilibrium binding studies the inhibitory potency of gallamine at M(1) receptors was not affected by KT5720, indicating that these agents bind to two distinct allosteric sites and have neutral cooperativity with each other. In contrast, gallamine and staurosporine had a negatively cooperative or competitive interaction at M(1) receptors. Similarly, the potency and relative effectiveness of KT5720 for inhibiting [(3)H]NMS dissociation from M(1) receptors were not affected by gallamine or brucine, but were affected in a complex manner by staurosporine. These results demonstrate that there are at least two distinct allosteric sites on the M(1) receptor, both of which can support positive cooperativity with acetylcholine.

Allosteric effects of four stereoisomers of a fused indole ring system with 3H-N-methylscopolamine and acetylcholine at M1-M4 muscarinic receptors

We previously demonstrated that brucine and some analogues allosterically enhance the affinity of ACh at muscarinic receptor subtypes M1, M3 or M4. Here we describe allosteric effects at human M1-M4 receptors of four stereoisomers of a pentacyclic structure containing features of the ring structure of brucine. All compounds inhibited 3H-NMS dissociation almost completely at all subtypes with slopes of 1, with similar affinity values at the 3H-NMS-occupied receptor to those estimated from equilibrium assays, consistent with the ternary complex allosteric model. Compound 1a showed positive cooperativity with H-NMS and small negative or neutral cooperativity with ACh at all subtypes. Its stereoisomer, 1b, showed strong negative cooperativity with both 3H-NMS and ACh across the subtypes. Compound 2a was positive with 3H-NMS at M2 and M4 receptors, neutral at M3 and negative at M1 receptors; it was negatively cooperative with ACh at all subtypes. Its stereoisomer, 2b, was neutral with 3H-NMS at M1 receptors and positive at the other subtypes; 2b was negatively cooperative with ACh at M1, M3 and M4 receptors but showed 3-fold positive cooperativity with ACh at M2 receptors. This latter result was confirmed with further 3H-NMS and 3H-ACh radioligand binding assays and with functional assays of ACh-stimulated 35S-GTPgammaS binding. These results provide the first well characterised instance of a positive enhancer of ACh at M2 receptors, and illustrate the difficulty of predicting such an effect.

Allosteric interactions of quaternary strychnine and brucine derivatives with muscarinic acetylcholine receptors

The affinity and allosteric properties of 22 quaternary derivatives of strychnine and brucine at the m1-m4 subtypes of muscarinic receptors have been analyzed and compared. The subtype selectivity, in terms of affinity, was in general m2 > m4 > m1 > m3. The highest affinities were found for N-benzyl, N-2-naphthylmethyl, and N-4-biphenylylmethyl strychnine (13, 14, and 18, respectively). All the strychnine and brucine derivatives were positively cooperative with the antagonist, N-methylscopolamine, at m2 receptors and, in the case of the strychnine analogues, were positively cooperative with N-methylscopolamine at least at one other subtype. The strychnine analogues were negatively cooperative with the neurotransmitter, acetylcholine, at all subtypes whereas brucine and five of the six derivatives examined were positively cooperative with acetylcholine at one or more subtypes (m1-m5) and exhibited different patterns of subtype selectivity. The ability to generate subtype-selective allosteric enhancers of acetylcholine binding and function may be of use in the development of drugs for the treatment of Alzheimer's disease.

Muscarinic interactions of bisindolylmaleimide analogues

We have used radioligand binding studies to determine the affinities of seven bisindolylmaleimide analogues, six of which are selective inhibitors of protein kinase C, at human muscarinic M1-M4 receptors. The compounds were most potent at M1 receptors, and Ro-31-8220 was the most potent analogue, with a Kd of 0.6 microM at M1 receptors. The weakest compounds, bisindolylmaleimide IV and bisindolylmaleimide V, had Kd values of 100 microM. If it is necessary to use protein kinase C inhibitors at concentrations of 10 microM or more in studies involving muscarinic receptors then bisindolylmaleimide IV may be the most appropriate inhibitor to use.

Subtype-selective positive cooperative interactions between brucine analogues and acetylcholine at muscarinic receptors: radioligand binding studies

We studied the interactions of strychnine, brucine, and three of the N-substituted analogues of brucine with [3H]N-methylscopolamine (NMS) and unlabeled acetylcholine at m1-m5 muscarinic receptors using equilibrium and nonequilibrium radioligand binding studies. The results were consistent with a ternary allosteric model in which both the primary and allosteric ligands bind simultaneously to the receptor and modify the affinities of each other. The compounds had Kd values in the submillimolar range, inhibited [3H]NMS dissociation, and showed various patterns of positive, neutral, and negative cooperativity with [3H]NMS and acetylcholine, but there was no predictive relationship between the effects. Acetylcholine affinity was increased approximately 2-fold by brucine at m1 receptors, approximately 3-fold by N-chloromethyl brucine at m3 receptors, and approximately 1.5-fold by brucine-N-oxide at m4 receptors. The existence of neutral cooperativity, in which the compound bound to the receptor but did not modify the affinity of acetylcholine, provides the opportunity for a novel form of drug selectivity that we refer to as absolute subtype selectivity: an agent showing positive or negative cooperativity with the endogenous ligand at one receptor subtype and neutral cooperativity at the other subtypes would exert functional effects at only the one subtype, regardless of the concentration of agent or its affinities for the subtypes. Our results demonstrate the potential for developing allosteric enhancers of acetylcholine affinity at individual subtypes of muscarinic receptor and suggest that minor modification of a compound showing positive, neutral, or low negative cooperativity with acetylcholine may yield compounds with various patterns of cooperativity across the receptor subtypes.

Selective allosteric enhancement of the binding and actions of acetylcholine at muscarinic receptor subtypes

The ternary allosteric model predicts the possibility of discovering molecules with novel and highly subtype-selective modes of action. This approach has been applied to muscarinic receptors. The alkaloid brucine is capable of selectively enhancing by an allosteric mechanism the effects of low but not high concentrations of acetylcholine at only the m1 subtype of muscarinic receptors. A simple derivative of brucine, N-chloromethylbrucine, enhances acetylcholine actions selectively at only m3 receptors. In addition it binds to, but does not affect, the properties of m4 receptors, thereby demonstrating neutral cooperativity and an 'absolute' selectivity of action at m3 receptors over m4 receptors. Brucine N-oxide enhances acetylcholine binding at m3 and m4 receptors and is neutral at m1 and m5 receptors. These findings allow the possibility of developing muscarinic agents that have a novel and highly targeted mode of action; they may act only on a single muscarinic receptor subtype which is functioning sub-optimally and therefore be of use therapeutically in the early stages of Alzheimer's Disease.