A novel m2-selective muscarinic antagonist: binding characteristics and autoradiographic distribution in rat brain

Click Chemistry-Enabled Conjugation Strategy for Producing Dibenzodiazepinone-Type Fluorescent Probes To Target M2 Acetylcholine Receptors

The development of fluorescently labeled receptor-targeting compounds represents a powerful pharmacological tool to study and characterize ligand-receptor interactions. Despite significant advances in developing sub-type-specific antagonists for muscarinic acetylcholine receptors (mAChRs), reports on antagonists feasible for click chemistry are less common. Here, we designed and synthesized an antagonist suitable for probe attachment through click chemistry, namely, dibenzodiazepinone (DIBA)-alkyne, based on a previously reported DIBA scaffold with a high binding affinity to type-2 mAChR (M2R). To demonstrate the versatility of DIBA-alkyne as a building block for bioconjugates, we assembled DIBA-alkyne with Cyanine5 fluorophores (Cy5) and polyethylene glycol (PEG) biomolecules to obtain fluorescent DIBA antagonist (DIBA-Cy5) and fluorescent DIBA PEG derivatives. Flow cytometric analysis showed that DIBA-Cy5 possessed a high binding affinity to M2R (Kd = 1.80 nM), a two-order magnitude higher binding affinity than M1R. Fluorescent DIBA PEG derivatives maintained a potent binding to the M2R (Kd ≤ 4 nM), confirmed by confocal microscopic imaging. Additionally, DIBA-Cy5 can serve as a fluorescent ligand in the receptor-ligand competitive binding assay for other mAChR ligands, an attractive alternative to the traditional radioligand-based assay. The competitive binding mode between DIBA-Cy5 and orthosteric antagonist atropine/allosteric modulator LY2119620 indicated a dualsteric binding mode of the DIBA-type antagonist to M2R. Lastly, we demonstrated the direct staining of DIBA-Cy5 to M2R receptors in the sinoatrial node of a mouse heart. The adaptability of the clickable DIBA antagonist to a wide range of fluorophores and biomolecules can facilitate its use in various biomedical applications such as binding assays that screen compounds for M2R as the receptor target.

Dibenzodiazepinone-type muscarinic receptor antagonists conjugated to basic peptides: Impact of the linker moiety and unnatural amino acids on M2R selectivity

The family of human muscarinic acetylcholine receptors (MRs) is characterized by a high sequence homology among the five subtypes (M₁R-M₅R), being the reason for a lack of subtype selective MR ligands. In continuation of our work on dualsteric dibenzodiazepinone-type M₂R antagonists, a series of M₂R ligands containing a dibenzodiazepinone pharmacophore linked to small basic peptides was synthesized (64 compounds). The linker moiety was varied with respect to length, number of basic nitrogens (0-2) and flexibility. Besides proteinogenic basic amino acids (Lys, Arg), shorter homologues of Lys and Arg, containing three and two methylene groups, respectively, as well as D-configured amino acids were incorporated. The type of linker had a marked impact on M₂R affinity and also effected M₂R selectivity. In contrast, the structure of the basic peptide rather determined M₂R selectivity than M₂R affinity. For example, the most M₂R selective compound (UR-CG188, 89) with picomolar M₂R affinity (pK_i 9.60), exhibited a higher M₂R selectivity (ratio of K_i M₁R/M₂R/M₃R/M₄R/M₅R: 110:1:5200:55:2300) compared to the vast majority of reported M₂R preferring MR ligands. For selected ligands, M₂R antagonism was confirmed in a M₂R miniG protein recruitment assay.

Differently fluorescence-labelled dibenzodiazepinone-type muscarinic acetylcholine receptor ligands with high M2R affinity

A series of fluorescent dibenzodiazepinone-type muscarinic acetylcholine M2 receptor (M2R) ligands was synthesized using various fluorescent dyes (5-TAMRA, λ ex/λ em ≈ 547/576 nm; BODIPY 630/650, λ ex/λ em ≈ 625/640 nm; pyridinium dye Py-1, λ ex/λ em ≈ 611/665 nm and pyridinium dye Py-5, λ ex/λ em ≈ 465/732 nm). All fluorescent probes exhibited high M2R affinity (pK i (radioligand competition binding): 8.75-9.62, pK d (flow cytometry): 8.36-9.19), a very low preference for the M2R over the M1 and M4 receptors and moderate to pronounced M2R selectivity compared to the M3 and M5 receptors. The presented fluorescent ligands are considered useful molecular tools for future studies using methods such as fluorescence anisotropy and BRET based MR binding assays.

Heterodimerization of Dibenzodiazepinone-Type Muscarinic Acetylcholine Receptor Ligands Leads to Increased M2R Affinity and Selectivity

In search for selective ligands for the muscarinic acetylcholine receptor (MR) subtype M₂, the dimeric ligand approach, that is combining two pharmacophores in one and the same molecule, was pursued. Different types (agonists, antagonists, orthosteric, and allosteric) of monomeric MR ligands were combined by various linkers with a dibenzodiazepinone-type MR antagonist, affording five types of heterodimeric compounds ("DIBA-xanomeline," "DIBA-TBPB," "DIBA-77-LH-28-1," "DIBA-propantheline," and "DIBA-4-DAMP"), which showed high M₂R affinities (pK_i > 8.3). The heterodimeric ligand UR-SK75 (46) exhibited the highest M₂R affinity and selectivity [pK_i (M₁R-M₅R): 8.84, 10.14, 7.88, 8.59, and 7.47]. Two tritium-labeled dimeric derivatives ("DIBA-xanomeline"-type: [³H]UR-SK71 ([³H]44) and "DIBA-TBPB"-type: [³H]UR-SK59 ([³H]64)) were prepared to investigate their binding modes at hM₂R. Saturation-binding experiments showed that these compounds address the orthosteric binding site of the M₂R. The investigation of the effect of various allosteric MR modulators [gallamine (13), W84 (14), and LY2119620 (15)] on the equilibrium (13-15) or saturation (14) binding of [³H]64 suggested a competitive mechanism between [³H]64 and the investigated allosteric ligands, and consequently a dualsteric binding mode of 64 at the M₂R.

Regulation of persistent activity in hippocampal mossy cells by inhibitory synaptic potentials

The hippocampal formation receives strong cholinergic input from the septal/diagonal band complex. Although the functional effects of cholinergic activation have been extensively studied in pyramidal neurons within the hippocampus and entorhinal cortex, less is known about the role of cholinergic receptors on dentate gyrus neurons. Using intracellular recordings from rat dentate hilar neurons, we find that activation of m1-type muscarinic receptors selectively increases the excitability of glutamatergic mossy cells but not of hilar interneurons. Following brief stimuli, cholinergic modulation reveals a latent afterdepolarization response in mossy cells that can extend the duration of stimulus-evoked depolarization by >100 msec. Depolarizing stimuli also could trigger persistent firing in mossy cells exposed to carbachol or an m1 receptor agonist. Evoked IPSPs attenuated the ADP response in mossy cells. The functional effect of IPSPs was amplified during ADP responses triggered in the presence of cholinergic receptor agonists but not during slowly decaying simulated ADPs, suggesting that modulation of ADP responses by IPSPs arises from destabilization of the intrinsic currents underlying the ADP. Evoked IPSPs also could halt persistent firing triggered by depolarizing stimuli. These results show that through intrinsic properties modulated by muscarinic receptors, mossy cells can prolong depolarizing responses to excitatory input and extend the time window where multiple synaptic inputs can summate. By actively regulating the intrinsic response to synaptic input, inhibitory synaptic input can dynamically control the integration window that enables detection of coincident inputs and shape the spatial pattern of hilar cell activity.

Early-life cocaine interferes with BDNF-mediated behavioral plasticity

An important aspect of goal-directed action selection is differentiating between actions that are more or less likely to be reinforced. With repeated performance or psychostimulant exposure, however, actions can assume stimulus-elicited-or "habitual"-qualities that are resistant to change. We show that selective knockdown of prelimbic prefrontal cortical Brain-derived neurotrophic factor (Bdnf) increases sensitivity to response-outcome associations, blocking habit-like behavioral inflexibility. A history of adolescent cocaine exposure, however, occludes the "beneficial" effects of Bdnf knockdown. This finding highlights a challenge in treating addiction-that drugs of abuse may bias decision-making toward habit systems even in individuals with putative neurobiological resiliencies.

Muscarinic receptor subtypes in the human colon: lack of evidence for atypical subtypes

Characteristics of muscarinic receptors were investigated in circular muscle from normal human colon. In saturation studies (n=18), binding of [3H]quinuclidinyl benzylate (QNB) was of high affinity (K(d) 87.3 pM) and capacity (B(max) 362+/-27 fmol/mg protein), with no differences between ascending and sigmoid colon. Kinetic studies gave a K(d) of 55 pM. Methoctramine and darifenacin displayed biphasic binding profiles, the high affinity components being compatible with a population of approximately 80+/-5% M(2) and 13+/-2% M(3) muscarinic receptors, respectively. Pirenzepine, mamba toxin 1 and mamba toxin 3 were very weak competitors, indicating negligible expression of muscarinic M(1) and M(4) receptors. Six other subtype-preferring antagonists exhibited K(i) values typical of those reported at cloned human muscarinic M(2) receptors. In the presence of methoctramine, pre-treatment with alkylating agent 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride (4-DAMP mustard) inhibited [3H]quinuclidinyl benzylate binding to 26% of sites. Following alkylation of muscarinic M(3) receptors, darifenacin bound to a single low affinity site, indicating binding to muscarinic M(2) receptors.

In vitro and in vivo m2 muscarinic subtype selectivity of some dibenzodiazepinones and pyridobenzodiazepinones

Alzheimer's disease (AD) involves selective loss of muscarinic m2, but not m1, subtype receptors in cortical and hippocampal regions of the human brain. Emission tomographic study of the loss of m2 receptors in AD has been limited by the absence of available m2-selective radioligands, which can penetrate the blood-brain barrier. We now report on the in vitro and in vivo m2 muscarinic subtype selectivity of a series of dibenzodiazepinones and pyridobenzodiazepinones determined by competition studies against (R)-3-quinuclidinyl (S)-4-iodobenzilate ((R,S)-[125I]IQNB) or [3H]QNB. Of the compounds examined, three of the 5-[[4-[(4-dialkylamino)butyl]-1-piperidinyl]acetyl]-10, 11-dihydro-5-H-dibenzo[b,e][1,4]diazepin-11-ones (including DIBA) and three of the 11-[[4-[4-(dialkylamino)butyl]-1-phenyl]acetyl]-5, 11-dihydro-6H-pyrido [2,3-b][1,4]benzodiazepin-6-ones (including PBID) exhibited both high binding affinity for the m2 subtype (</=5 nM) and high m2/m1 selectivity (>/=10). In vivo rat brain dissection studies of the competition of PBID or DIBD against (R,S)[125I]IQNB or [3H]QNB exhibited a dose-dependent preferential decrease in the binding of the radiotracer in brain regions that are enriched in the m2 muscarinic subtype. In vivo rat brain autoradiographic studies of the competition of PBID, BIBN 99, or DIBD against (R,S)[125I]IQNB exhibited an insignificant effect of BIBN 99 and confirmed the effect of PBID and DIBD in decreasing the binding of (R,S)[125I]IQNB in brain regions that are enriched in the m2 muscarinic subtype. We conclude that PBID and DIBD are potentially useful parent compounds from which in vivo m2 selective derivatives may be prepared for potential use in positron emission tomographic (PET) study of the loss of m2 receptors in AD.

The design and pharmacology of novel selective muscarinic agonists and antagonists

The muscarinic pharmacology of C1-methyl-substituted chiral compounds related to McN-A-343 and of (R)- and (S)-dimethindene has been studied. Among the McN-A-343 analogues, the (S)-enantiomers were more potent and had higher affinity than the (R)-isomers. The quaternary compound (S)-BN 228 was found to be the most potent M1-selective agonist known today (pEC50: M1/rabbit vas deferens = 7.83; M2/guinea-pig atria = 6.35; M3/guinea-pig ileum = 6.29). In both the atria and ileum the tertiary carbamate, (S)-4-F-MePyMcN, was a competitive antagonist (pA2 value = 7.39 and 6.82, respectively). In contrast, in rabbit vas deferens (S)-4-F-MePyMcN was a potent partial agonist (pEC50 = 7.22; apparent efficacy = 0.83). These results indicate that (S)-4-F-MePyMcN might be a useful tool to study M1 receptor-mediated effects involved in central cholinergic function. (S)-Dimethindene was a potent M2-selective antagonist (pA2 = 7.86/atria; pKi = 7.8/rat heart) with lower affinities for the M1 (pA2 = 6.36/rat duodenum; pKi = 7.1/NB-OK 1 cells), M3 (pA2 = 6.92/guinea-pig ileum; pKi = 6.7/rat pancreas) and M4 receptors (pKi = 7.0/rat striatum). It was more potent (up to 41-fold) than the (R)-isomer. In contrast, the stereoselectivity was inverse at ileal H1 receptors (pA2: (R)-isomer = 9.42; (S)-isomer = 7.48). Thus, (S)-dimethindene could be a valuable agent to test the hypothesis that M2 antagonists show beneficial effects in the treatment of cognitive disorders. It might also become the starting point for the development of diagnostic tools for quantifying M2 receptors in the CNS with PET imaging.

In vivo muscarinic binding of 3-(alkylthio)-3-thiadiazolyl tetrahydropyridines

Based on encouraging in vitro data indicating M2 subtype selectivity, we synthesized, radiolabeled with 18F, and evaluated 3-(3-(2-fluoroethylthio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetr ahydro-1-methylpyridine [FE-TZTP], and 3-(3-(3-fluoropropylthio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tet rahydro-1-methylpyridine [FP-TZTP] for muscarinic subtype selectivity in vivo. [18F]FE-TZTP displays high uptake in vivo but is inhibited only weakly by coinjecting unlabeled P-TZTP. Contrarily, [18F]FP-TZTP shows significant inhibition of uptake by coinjecting unlabeled P-TZTP or the muscarinic agonist L-687,306 (3-(3-cyclopropyl-1,2,4-oxadiazol-5-yl)-1-azabicyclo[2.2.1]heptane ). Using in vivo autoradiography, [18F]FP-TZTP displays regional distribution consistent with M2 subtype distribution. In addition, [18F]FP-TZTP shows specific uptake in the heart at 5 min. Analysis of metabolites in the awake rat brain revealed that the parent compound represents >95% of the extractable activity at 30 min. In vivo studies in rhesus monkeys revealed rapid brain uptake of [18F]FP-TZTP, with clearance sustained over 2 h. Administration of P-TZTP or FP-TZTP (80 nmol/kg) at 60 min after injection of [18F]FP-TZTP results in a significant displacement of brain activity in all regions. Metabolite analysis in monkey plasma shows that parent compound represents 20% of the extractable radioactivity at 40 min postinjection. One metabolite, which increases with time, has similar lipophilicity to the parent. However, based on metabolism in rat we believe metabolites are not in the brain to any significant extent in monkeys during the time of imaging studies. Regional uptake, autoradiographic distribution, and clearance rates in the brain are consistent with the hypothesis that [18F]FP-TZTP is M2 selective in vivo.

In vivo muscarinic binding selectivity of (R,S)- and (R,R)-[18F]-fluoromethyl QNB

We have developed a multistep radiochemical synthesis of two diastereomers of quinuclidinyl-4-[18F]-fluoromethyl-benzilate ([18F]-FMeQNB), a high-affinity ligand for muscarinic acetylcholine receptors. Previously, we have shown that the nonradioactive (R,R)-diastereomer displays an eightfold selectivity for M1 over M2 while the nonradioactive (R,S)-diastereomer displays a sevenfold selectivity for M2 over M1 in vitro. This paper reports the results of in vivo comparison studies. In the rat, uptake of (R,S)-[18F]-FMeQNB was nearly uniform in all brain regions following the concentration of M2 subtype. The uptake was reduced by 36-54% in all brain regions on coinjection with 50 nmol of unlabeled ligand. An injection of (R,S)-[18F]-FMeQNB followed at 60 min by injection of unlabeled ligand and subsequent sacrifice at 120 min displaced 30-50% of radioactivity in the pons, medulla, and cerebellum, which contain a high proportion of M2 subtype. The most dramatic displacement and inhibition of uptake on coinjection of (R,S)-[18F]-FMeQNB was observed in the heart. In rhesus monkey, the compound showed prolonged uptake and retention in the brain. In the blood, the parent compound degraded rapidly to a single radiolabeled polar metabolite believed to be fluoride. Within 30 min the parent compound represented less than 5% of the plasma activity. Displacement with (R)-QNB was generally slow, but was more rapid from those tissues which contain a higher proportion of M2 subtype. The results are consistent with the hypothesis that (R,S)-[18F]-FMeQNB is M2 selective in vivo. On the other hand, (R,R)-[18F]-FMeQNB showed higher uptake in those brain regions containing a higher concentration of M1 subtype. Uptake in the heart at 60 min was much lower than that observed with the (R,S)-diastereomer. Inhibition of uptake on coinjection with unlabeled (R,S)-FMeQNB is only significant in the heart, thalamus, and pons. Inhibition of uptake on coinjection with unlabeled (R,R)-FMeQNB is quite uniform in all brain regions. Displacement with (R)-QNB shows a more varying amount displaced. These results are consistent with (R,R)-[18F]-FMeQNB being M1 selective in vivo.

Autoradiographic evidence that (R)-3-quinuclidinyl (S)-4-fluoromethylbenzilate ((R,S)-FMeQNB) displays in vivo selectivity for the muscarinic m2 subtype

Alzheimer's disease (AD) involves selective loss of muscarinic m2, but not m1, subtype neuroreceptors in cortical and hippocampal regions of the human brain. Until recently, emission tomographic study of the loss of m2 receptors in AD has been limited by the absence of available m2-selective radioligands that can penetrate the blood-brain barrier. We now demonstrate the in vivo m2 selectivity of a fluorinated derivative of QNB, (R)-3-quinuclidinyl (S)-4-fluoromethylbenzilate ((R,S)-FMeQNB), by studying autoradiographically the in vivo inhibition of radioiodinated (R)-3-quinuclidinyl (S)-4-iodobenzilate ((R,S)-[125I]IQNB) binding by unlabelled (R,S)-FMeQNB. In the absence of (R,S)-FMeQNB, (R,S)-[125I]IQNB labels brain regions in proportion to the total muscarinic receptor concentration; in the presence of 75 nmol of (R,S)-FMeQNB, (R,S)-[125I]IQNB labelling in those brain regions containing predominantly m2 subtype is reduced to background levels. We conclude that (R,S)-FMeQNB is m2-selective in vivo, and that (R,S)-[18F]FMeQNB may be of potential use in positron emission tomographic (PET) study of the loss of m2 receptors in AD.

The (S)-(+)-enantiomer of dimethindene: a novel M2-selective muscarinic receptor antagonist

The present study was designed to determine to determine the in vitro affinity profile of (R)-(-)-dimethindene and (S)-(+)-dimethindene at muscarinic receptor subtypes using both functional and binding assays. In addition, the racemate was investigated in functional studies. The functional muscarinic receptors studied were putative M1 receptors in rabbit vas deferens and rat duodenum, M2 receptors in guinea-pig left atria and rabbit vas deferens, as well as M3 receptors in guinea-pig ileum and trachea. Furthermore, the histamine H1 antagonism by (R)-(-)- and (S)-(+)-dimethindene has been examined in guinea-pig ileum. Muscarinic binding selectivity was assessed in homogenates from human neuroblastoma NB-OF 1 cells (M1), rat heart (M2), pancreas (3) and striatum (M4). The results demonstrate that (S)-(+)-dimethindene is a potent M2-selective muscarinic receptor antagonist (pA2 = 7.86/7.74; pKi = 7.78) with lower affinities for the muscarinic M1 (pA2 = 6.83/6.36; pKi = 7.08), the M3 (pA2 = 6.92/6.96; pKi = 6.70) and the M4 receptors (pKi = 7.00), respectively. The (S)-(+)-enantiomer was more potent (up to 41-fold) than the (R)-(-)-enantiomer in all muscarinic assays. In contrast, the stereoselectivity was inverse at histamine H1 receptors, the (R)-(-)-enantiomer being the eutomer (pA2 = 9.42; pA2/(S)-isomer = 7.48). In conclusion, (S)-(+)-dimethindene is a useful tool to investigate muscarinic receptor heterogeneity. In addition, this lipophilic compound might become the starting point for the development of M2-selective muscarinic receptor antagonists useful as diagnostic tools for quantifying muscarinic M2 receptors in the central nervous system with positron emission tomography imaging, and to test the hypothesis that muscarinic M2 receptor antagonists show beneficial effects in the treatment of cognitive disorders.

Evaluation of stereoisomers of 4-fluoroalkyl analogues of 3-quinuclidinyl benzilate in in vivo competition studies for the M1, M2, and M3 muscarinic receptor subtypes in brain

To develop a subtype selective muscarinic acetylcholine receptor (mAChR) antagonist for PET, fluorine-19 labeled alkyl analogues of quinuclidinyl benzilate (QNB) were synthesized by stereoselective reactions. To investigate these analogues for tissue subtype specificity, in vivo competitive binding studies were performed in rat brain using (R)-3-quinuclidinyl (R)-4-[125I]iodobenzilate (IQNB). Five, fifty, or five-hundred nmol of the non-radioactive ligands were coinjected intravenously with 8 pmol of the radioligand, Cold (R,R)-IQNB blocked (R,R)-[125I]IQNB in a dose-dependent manner, without showing regional specificity. For the (R,S)-fluoromethyl, -fluoroethyl and -fluoropropyl derivatives, a higher percent blockade was seen at 5 and 50 mmol levels in M2 predominant tissues (medulla, pons, and cerebellum) than in M1 predominant tissues (cortex, striatum and hippocampus). The blockade pattern of the radioligand also correlated qualitatively with the percentage of M2 receptors in the region. The S-quinuclidinyl analogues showed M2 selectivity but less efficient blockade of the radioligand, indicating lower affinities. Radioligand bound to the medulla was inversely correlated to the M2 relative binding affinity of the fluoroalkyl analogues. These results indicate that the nonradioactive ligand blocks the radioligand based on the affinity of the nonradioactive ligand for a particular receptor subtype compared to the affinity of the radioligand for the same receptor subtype. Of the seven compounds evaluated, (R,S)-fluoromethyl-QNB appears to show the most selectivity for the M2 subtypes in competition studies in vivo.

Muscarinic receptor modulation of acetylcholine release from rat cerebral cortex and hippocampus

An attempt to identify the muscarinic receptor subtypes involved in presynaptic modulation of acetylcholine (ACh) release from cortical and hippocampal slices was made by means of several muscarinic antagonists. Cortical and hippocampal slices prepared from adult rats were superfused with Krebs solution containing physostigmine; ACh content of the superfusate at rest and after electrical stimulation (1 Hz) was quantified by high performance liquid chromatography. The antagonists were added to the Krebs at the concentration of 1 microM. ACh release at rest was enhanced only in the cortex by (+/-)-5,11-dihydro-11-([(2-[2-[(dipropylamino)methyl]-1- piperidinyl)ethyl)amino]carbonyl)-6H-pyrido[2,3-b](1,4)- benzodiazepine-6-one (AFDX384), an M2/M4 selective antagonist. The evoked ACh release from the cerebral cortex was significantly increased by AFDX384, methoctramine, pirenzepine, M2/M4, M2 and M1 selective antagonists, respectively, and scopolamine. This finding suggests that M1, M2 and M4 presynaptic receptor subtypes could regulate evoked ACh release in the cortex. In hippocampal slices, the evoked ACh release was enhanced by AFDX384, pirenzepine and scopolamine but not by methoctramine. In this region ACh release seems therefore regulated only by M1 and M4 receptor subtypes. The M3 antagonist (+/-)-p-fluorohexahydro-sila-difenidol hydrochloride did not affect ACh release.

Functional characterization of a muscarinic receptor stimulating gastrin release from rabbit antral G-cells in primary culture

In previous studies carbachol-induced stimulation of gastrin release from antral G-cells in primary culture suggested the presence of muscarinic acetylcholine receptors on this cell type. Therefore, we attempted to pharmacologically characterize the muscarinic acetylcholine receptor subtype involved. Enzymatically isolated rabbit antral mucosal cells (0.8% G-cells) were separated by counterflow elutriation yielding a fraction (1.7% G-cells) that was placed in culture on collagen-coated well plates. After 24-36 h of culture 13.0 +/- 2.4% of total adherent cells were immunoreactive for gastrin as shown by immunocytochemical staining using the avidin-biotin complex method. In this preparation basal gastrin release ranged from 3.3 +/- 0.3 to 4.1 +/- 0.3% of total cellular content. Maximal gastrin release in response to the acetylcholine receptor agonist carbachol (10(-4) M) or the selective muscarinic receptor agonist arecaidine propargyl ester (10(-4) M) was 8.5 +/- 0.4% and 7.6 +/- 0.4% of total cellular content, respectively. The EC50 values were 3.7 +/- 0.5 x 10(-6) M carbachol and 1.8 +/- 0.4 x 10(-6) M arecaidine propargyl ester. At a concentration of 10(-6) M the non-selective muscarinic receptor antagonist atropine and the muscarinic M3 receptor preferring antagonist hexahydro-sila-difenidol (HHSiD; M3 > or = M1 > M2) completely inhibited gastrin release in response to carbachol (Ki values: 52 x 10(-9) M atropine and 29 x 10(-9) M HHSiD) and arecaidine propargyl ester (Ki values: 11 x 10(-9) M atropine and 13 x 10(-9) M HHSiD).(ABSTRACT TRUNCATED AT 250 WORDS)

3-alpha-Chlorimperialine: an M2-selective muscarinic receptor antagonist that penetrates into brain

Muscarinic M2 receptors have been found to be severely depleted in post-mortem brains of Alzheimer's patients. This loss of receptor may represent a useful diagnostic marker, if it could be quantitatively imaged with single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. In order to develop a radioligand with selectivity for muscarinic M2 receptors, we now report that 3-alpha-chlorimperialine is a potent M2 receptor antagonist with a Ki of 0.32 nM at M2 receptors, a 12-fold selectivity for M2 over M1 receptors, and a 5-fold selectivity for M2 over M4 receptors. Furthermore, 2% of the injected dose of 3-alpha-chlorimperialine per gram tissue penetrates into brain within 30 min, then washes out gradually. Taken together, these studies demonstrate that 3-alpha-chlorimperialine is a potent M2-selective muscarinic antagonist that penetrates into brain and may be a useful substrate for radioiodination and subsequent imaging of brain muscarinic M2 receptors.

[3H]QNB displays in vivo selectivity for the m2 subtype

Alzheimer's disease (AD) involves selective loss of muscarinic m2, but not m1, subtype neuroreceptors in the posterior parietal cortex of the human brain. Emission tomographic study of the loss of m2 receptors in AD is limited by the fact that there is currently no available m2-selective radioligand which can penetrate the blood-brain barrier. [3H](R)-3-quinuclidinylbenzilate ([3H]QNB) is commonly used for performing in vitro studies of the muscarinic acetylcholine receptor (mAChR), either with membrane homogenates or with autoradiographic slices, in which [3H]QNB is nonsubtype-selective. We report here the results of in vivo studies, using both carrier-free and low specific activity [3H]QNB, which show that [3H]QNB exhibits a substantial in vivo m2-selectivity. Previously reported in vivo (R)-3-quinuclidinyl (R)-4-iodobenzilate ((R,R)-[125I]IQNB) binding appears to be nonsubtype-selective. Apparently the bulky iodine substitution in the 4 position reduces the subtype selectivity of QNB. It is possible that a less bulky fluorine substitution might permit retention of the selectivity exhibited by QNB itself. We conclude that a suitably radiolabeled derivative of QNB, possibly labeled with 18F, may be of potential use in positron emission tomographic (PET) study of the loss of m2 receptors in AD.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

A novel muscarinic receptor ligand which penetrates the blood brain barrier and displays in vivo selectivity for the m2 subtype

Alzheimer's disease (AD) involves selective loss of muscarinic m2, but not m1, subtype neuroreceptors in the posterior parietal cortex of the human brain. Emission tomographic study of the loss of m2 receptors in AD is limited by the fact that there is currently no available m2-selective radioligand which can penetrate the blood-brain barrier. In our efforts to prepare such a radioligand, we have used competition studies against currently existing muscarinic receptor radioligands to infer the in vitro and in vivo properties of a novel muscarinic receptor ligand, 5-[[4-[4-(diisobutylamino)butyl]-1-phenyl]acetyl]-10,11-dihydro-5H - -dibenzo [b,e][1,4]diazepin-11-one (DIBD). In vitro competition studies against [3H](R)-3-quinuclidinylbenzilate ([3H]QNB) and [3H]N-methylscopolamine ([3H]NMS), using membranes derived from transfected cells expressing only m1, m2, m3, or m4 receptor subtypes, indicate that DIBD is selective for m2/m4 over m1/m3. In vivo competition studies against (R,R)-[125I]IQNB indicate that DIBD crosses the blood brain barrier (BBB). The relationship of the regional percentage decrease in (R,R)-[125I]IQNB versus the percentage of each of the receptor subtypes indicates that DIBD competes more effectively in those brain regions which are known to be enriched in the m2, relative to the m1, m3, and m4, receptor subtype; however, analysis of the data using a mathematical model shows that caution is required when interpreting the in vivo results. We conclude that a suitably radiolabeled derivative of DIBD may be of potential use in emission tomographic study of changes in m2 receptors in the central nervous system.