Differences in affinities of muscarinic acetylcholine receptor antagonists for brain and heart receptors

Boronic acid adducts of technetium dioxime (BATO) complexes derived from quinuclidine benzilate (QNB) boronic acid stereoisomers: syntheses and studies of their binding to the muscarinic acetylcholine receptor

We have investigated the possibility of using BATO complexes derivatized with the muscarinic acetylcholine receptor (mAChR) antagonist, quinuclidinyl benzilate (QNB), for mAChR imaging. The BATO complexes, TcCl(DMG)3B-QNB, were prepared using QNB derivatives containing a 4'-boronic acid substituent on one of the benzilic benzene rings (QNB-boronic acid). The QNB-boronic acid molecule has two chiral centers, and all four QNB-BATO stereoisomers were made and evaluated. When studied using in vitro receptor binding assays based on tissue from rat brain caudate-putamen (which contains primarily M1 and M4 mAChR) and rat heart (M2 mAChR), the QNB-boronic acid stereoisomers had binding affinities (KA) in the range 2 x 10(5)-1 x 10(8), at least 10-fold lower than the KA for QNB (ca 2 x 10(9)). The stereochemistry of both centers had some influence on the affinity constant. When the TcCl(DMG)3B-QNB complexes were studied, none of the stereoisomeric complexes displayed measurable specific binding (KA < 10(6)), but all showed high non-specific binding. In vitro autoradiography with rat brain slices confirmed the absence of specific binding in these tracers. In vivo, the 99mTcCl(DMG)3B-QNB complexes displayed minimal brain uptake, and modest heart uptake; the latter was unlikely to be related to uptake by the mAChR. In light of these findings, we conclude that the interaction between the TcCl(DMG)3B-QNB complexes and biological membranes is dominated by the hydrophobicity of the BATO moiety. The TcCl(DMG)3B-QNB complexes, therefore, have little potential for mAChR imaging.

Novel technetium ligands with affinity for the muscarinic cholinergic receptor

The synthesis and preliminary biological characterization of two isomeric technetium labeled complexes (2,5,5,9-tetramethyl-4,7-diaza-7-(3' (R)-quinuclidinylcarboxymethyl)-2,9-decanedithiolato oxo 99/99mtechnetium(V), [99/99mTc]-1 and [99/99mTc]-2) designed to exhibit affinity to muscarinic cholinergic receptors are described. In vitro binding assays were conducted in mouse brain homogenates (whole brain-cerebellum) at 37 degrees C by the centrifugation method, where non-specific binding was defined by atropine (1 microM). The measured affinity (KD) of [99Tc]-1 for mAChR was 1.9 +/- 0.5 microM (mean +/- SEM; n = 3) and [99Tc]-2 was 4.5 +/- 0.5 microM (mean +/- SEM; n = 3). Scatchard analysis indicated that Bmax values were 10.6 +/- 0.5 and 16.9 +/- 0.5 pmol/mg tissue, respectively. In competition assays, [99Tc]-1 exhibited an apparent affinity (KI) of 16.5 microM (n = 2) against [125I] iododexetimide, whereas [99Tc]-2 exhibited an affinity (KI) of 105 microM. In vivo, 0.3% of the injected dose of [99mTc]-1 and [99mTc]-2 accumulated in the brain at 5 min after injection. These values indicate technetium analogues of neuroreceptor binding ligands can be synthesized and retain some affinity for the receptor.

Muscarinic receptor selectivities of 3-Quinuclidinyl 8-xanthenecarboxylate (QNX) in rat brain

We have determined the binding of (R)-3-Quinuclidinyl 8-xanthenecarboxylate to muscarinic acetylcholine receptor preparations from rat cortex, hippocampus, caudate/putamen, thalamus, pons and colliculate bodies. The competition curves determined with [3H]quinuclidinyl benzilate as the radioligand are well described by a two site model with a difference in affinity between the two sites of 12-fold. The proportions of high affinity site vary from 100% in the caudate/putamen to 0% in the pons/medulla. The selectivities are different from those measured by pirenzepine and are consistent with QNX exhibiting similar affinity for the M1, M3, and M4 receptors with lower affinity for the M2 receptor. This assignment was confirmed by determining the affinities of QNX for the cloned receptor subtypes.

The in vitro dissociation kinetics of (R,R)-[125I]4IQNB is reflected in the in vivo washout of the radioligand from rat brain

We have determined the kinetics of dissociation of (R)-3-Quinuclidinyl (R)-4-[125I]Iodobenzilate ((R,R)-[125I]4IQNB) from muscarinic acetylcholine receptor preparations from the cortex, hippocampus, caudate/putamen, thalamus, pons and colliculate bodies. The dissociation curves are well described by a biexponential function and are consistent with subtype selectivity favoring slow dissociation from the M1, M3, and M4 receptors with a 20-fold faster dissociation rate for the M2 receptor. Following intravenous injection, (R,R)-[125I]4IQNB binds to receptor in the rat brain in concentrations which reflect the receptor concentration present in a structure. We determined the extent of radioligand present at two times, 2 and 24 hrs, as an indication of the relative proportions of m-AChR which exhibits rapid vs. slow dissociation of (R,R)-[125I]4IQNB. A good correlation between in vitro and in vivo results suggests that the relative populations of receptor subtypes can be imaged using in vivo pharmacokinetics of (R,R)-[125I]4IQNB.

Effects of atropine treatment on in vitro and in vivo binding of 4-[125I]-dexetimide to central and myocardial muscarinic receptors

Upregulation of muscarinic cholinergic receptors (mAChR) after chronic atropine treatment has been described previously. The present study was designed to evaluate 4-iodine-125 dexetimide as an agent to determine changes in the number of mAChR. Rats were injected subcutaneously with atropine (500 mg/kg) either once or chronically, once daily for 10 days, and sacrificed 24 h later. In vitro binding assays with 4-[125I]-dexetimide showed significant increases in the number of mAChR in cerebra (21%) and ventricles (45%) after chronic atropine treatment but not after acute treatment. The affinity of binding to cerebral and ventricular mAChR declined after acute and chronic atropine treatment. In vivo studies were carried out involving intravenous injection of 4-[125I]-dexetimide 24 h after atropine treatment. Binding was markedly reduced in the brain and heart. Upregulation of mAChR, as seen in in vitro studies, could not be observed because of the remaining atropine. Occupancy of mAChR by atropine persisted as long as 7 days after one dose. The results of these studies indicate that 4-[125I]-dexetimide binding reflects the effects of atropine on central and peripheral muscarinic cholinergic receptors in vitro and in vivo.

Effects of cyproheptadine and pizotifen on central muscarinic receptors

The affinities of cyproheptadine, pizotifen and (+/-)-quinuclidinyl xanthane-9-carboxylate hemioxylate (QNX) were determined at muscarinic autoreceptors and postsynaptic (IP1 formation) receptors in rat hippocampal slices. The affinity values for QNX were 8.2 and 8.5 respectively. Cyproheptadine and pizotifen were less potent than QNX. Pizotifen was slightly (2-fold) less active at antagonizing IP1 formation than blocking the autoreceptors whereas cyproheptadine was equally active at antagonizing the two hippocampal muscarinic receptors.

Specificity of methoctramine in blocking muscarinic receptors which inhibit adenylate cyclase in cerebellar granule cells

In primary cultures of cerebellar granule cells, activation of muscarinic receptors stimulates both hydrolysis of phosphatidylinositol (PI) and inhibition of adenylate cyclase. The specificity of three muscarinic receptor antagonists, pirenzepine, methoctramine and (-)quinuclidinyl xanthene-9-carboxylate [(-)QNX], in blocking carbachol-stimulated hydrolysis of PI and inhibition of adenylate cyclase were determined. Pirenzepine was found to be nonspecific in blocking the carbachol-stimulated hydrolysis of PI and inhibition of adenylate cyclase, while methoctramine specifically antagonized carbachol-stimulated inhibition of adenylate cyclase with 600 times greater potency than carbachol-stimulated hydrolysis of PI. (-)Quinuclidinyl xanthene-9-carboxylate was approximately 20 times more potent in blocking the carbachol-stimulated hydrolysis of PI than inhibition of adenylate cyclase. Studies of the ability of these three antagonists to block the binding of [3H]quinuclidinyl benzilate [( 3H]QNB) to muscarinic sites on membranes from cerebellar granule cells, revealed that all three antagonists displayed binding characteristics, characteristic of two binding sites, possibly representing the two types of muscarinic receptors. However, the ratio of the affinities for each of the two binding sites was about ten for pirenzepine, 100 for methoctramine and 650 for (-)QNX. Thus, the specificity of these antagonists, in blocking the inhibition of adenylate cyclase and hydrolysis of PI did not correlate with their specificities obtained with the binding studies with [3H]QNB.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of iodine-125 labeled 3-quinuclidinyl 4'-iodobenzilate

Iodine-125 labeled 3-quinuclidinyl 4'-iodobenzilate has been prepared via the reaction of the corresponding boronic acid with sodium [125I]iodide in the presence of a mild oxidant.

Synthesis and structure-activity relationships of new muscarinic antagonists

In an attempt to develop more selective muscarinic acetylcholine receptor (m-AcChR) antagonists, (R)-1-azabicyclo[2.2.2]oct-3-yl-thioxanthene-9-carboxylate, (R,S)-thiochromane-4-carboxylate, and (R,S)-chromane-4-carboxylate were synthesized. Evaluation of the binding affinities of these compounds to muscarinic receptors indicates that replacing the oxygen by sulfur in the xanthenyl and chromanyl moieties does not significantly change selectivity, but does reduce the affinity of 5 and enhance the affinity of 9a.

Comparison of two radiolabeled quinuclidinyl benzilate ligands for the characterization of the human peripheral lung muscarinic receptor

Quinuclidinyl benzilate, a muscarinic antagonist, has previously been used in its tritiated form ([3H]-QNB) to study the lung muscarinic receptor. We investigated whether a newer iodinated form of QNB ([125I]-QNB) of higher specific activity would be an appropriate ligand to study the human peripheral lung muscarinic receptor. Both the tritiated and iodinated ligands bound specifically to human lung at 23 degrees C. At 37 degrees C the specific binding of [3H]-QNB increased slightly, but no specific binding of [125I]-QNB was found. The data from multiple equilibrium binding experiments covering a wide range of radiolabeled QNB concentrations were combined and analyzed using the computer modeling program, LIGAND. The tritiated QNB identified a single affinity human lung binding site with a Kd of 46 +/- 9 pM and a receptor concentration of 34 +/- 3 fmol/mg protein. The iodinated QNB identified a single higher affinity human lung binding site (Kd = 0.27 +/- 0.32 pM) of much smaller quantity (0.62 +/- 0.06 fmol/mg protein). Competition studies comparing the binding of unlabeled QNB relative to labeled QNB indicated that unlabeled QNB had the same Kd as that measured for [3H]-QNB, but a 5 log greater Kd than that measured for [125I]-QNB. Other muscarinic receptor agonists and antagonists competed with [3H]-QNB, but not [125I]-QNB for binding to muscarinic receptors with the expected magnitude and rank order of potency. We conclude that of the 2 radiolabeled forms of QNB available, only the tritiated form should be used to study the human peripheral lung muscarinic receptor.

Propylbenzilylcholine mustard is selective for rat heart muscarinic receptors having a low affinity for agonists

Propylbenzilylcholine mustard (PrBCM), an irreversible muscarinic antagonist, inactivated receptors with a low affinity for agonists faster than those with a high affinity in rat heart membranes. This result was obtained using either: (a) a low ionic strength buffer (allowing heterogeneity among antagonist binding sites, (b) the same buffer enriched with GTP, or (c) a high ionic strength buffer (where antagonists showed similar binding characteristics to all receptors). These data suggest either that PrBCM is a 'selective' antagonist which detects conformational differences between low and high affinity receptors, or that the agonist affinity of cardiac muscarinic receptors is determined, in part, by the relative concentrations of receptor and GTP binding protein.

Biochemical characteristics of muscarinic cholinoreceptors in swine tracheal smooth muscle

The tritiated muscarinic cholinoreceptor antagonist quinuclidinyl benzilate, [3H]QNB, was used to characterize the muscarinic receptors associated with homogenized membrane of the smooth muscle from swine trachea. Based on receptor binding assays, the homogenate had specific, saturable, high-affinity receptors for [3H]QNB. Specific binding was time- and temperature-dependent. The association of [3H]QNB with the muscarinic receptor reached equilibrium much sooner at 37 degrees C than 25 degrees C at a [3H]QNB concentration of 180 pM (30 min and 2 h, respectively). Equilibrium at both temperatures was attained within 5 min at a [3H]QNB concentration of 1800 pM. All remaining experiments were performed at 37 degrees C. Binding was saturable with respect to [3H]QNB and tissue concentrations. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 51 +/- 20 pM and a maximum receptor density (Bmax) of 2.17 +/- 0.27 pmole/mg protein. The Hill coefficient for [3H]QNB binding was 1.07 +/- 0.16. The association (K1) and dissociation (K-1) rate constants were determined to be (5.51 +/- 0.16) X 10(8) M-1 min-1 and (1.41 +/- 0.18) X 10(-2) min-1, respectively. KD calculated from the ratio of K1 and K-1 was 26.3 +/- 3.8 pM; this value is close to the value of KD calculated from Scatchard plots of binding isotherms. The density of muscarinic receptor binding sites was 10-fold greater in tracheal smooth muscle than in tracheal epithelium (0.20 +/- 0.03 pmole/mg protein). There is no difference between weanling and young adult swine in the density of muscarinic receptors in tracheal smooth muscle. The nonselective muscarinic antagonists atropine, scopolamine and quinuclidinyl benzilate (QNB) competitively inhibited [3H]QNB binding to the homogenate with Hill coefficients of 0.9-1.0 and inhibition constants (Ki) of nanomolar range. Competition with selective muscarinic antagonists pirenzepine and 3-quinuclidinyl xanthene-9-carboxylate (QNX) gave Ki values, 0.26 M and 0.78 nM, respectively, and Hill coefficients of approximately 1. There was a single population of [3H]QNB binding sites of the M2 subtype for all tested muscarinic antagonists. Competition with selective muscarinic agonists pilocarpine and carbachol yielded Ki values of micromolar range, Hill coefficients of less than 1, and revealed the existence of two binding sites (P less than 0.01).

Membrane fluidization increases low-affinity muscarinic receptor binding in brain: changes with aging

Specific cholinergic muscarinic receptor binding was determined with L-[3H]quinuclidinyl benzilate ([3H]QNB) in homogenates from crude synaptosomal pellets prepared from mouse whole-brain homogenates. Specific total (high- and low-affinity) binding was determined in the absence of the agonist carbachol and low-affinity binding in its presence. These membrane preparations were fluidized by adding in vitro aliphatic alcohols ranging from ethanol to hexanol and by increasing the incubation temperatures. At 23 degrees C hexanol (14.7 mM) nearly doubled the low-affinity binding in the presence of carbachol (0.32 mM) and decreased high-affinity binding by the same amount. This suggested a change of muscarinic receptors from high- to low-affinity conformation. Increase of incubation temperature from 24 degrees C to 37 degrees C nearly tripled low-affinity binding. Brain homogenates from female C57BL/6J mice, ages 6, 12, 18, and 30 months, showed a progressively lower stimulation by hexanol of low-affinity [3H]QNB binding in the presence of carbachol. We postulate that this diminished change with age of [3H]QNB-receptor binding in response to alcohols may be a result of increasing membrane rigidity with advancing age. Rigidity of membranes may link aging at the membrane level, synaptic receptors, and impaired learning behavior.

Subclassification of muscarinic receptors in the heart, urinary bladder and sympathetic ganglia in the pithed rat. Selectivity of some classical agonists

In pithed normotensive rats muscarinic receptors were characterized in heart, urinary bladder and sympathetic ganglia; the selectivity of some classical muscarinic agents for these subtypes was investigated. The potencies in decreasing heart rate, increasing bladder pressure and increasing diastolic blood pressure were measured for the following, intraarterially administered cholinergic agonists: McN-A-343 ([4-m-chlorophenylcarbamoyloxy]-2-butynyltrimethylammonium), pilocarpine, carbachol, oxotremorine, arecoline, acetyl-beta-methylcholine and acetylcholine. The selective M1-antagonist pirenzepine, the mixed M1/M2-antagonist dexetimide and the cardioselective M2-antagonist gallamine were used as tools for identification of the receptors. All data were obtained after intravenous pretreatment with a high dose of atenolol to eliminate tachycardia induced by stimulating sympathetic ganglionic muscarinic receptors. Dexetimide strongly antagonized the bradycardia as well as the increase in bladder pressure induced by pilocarpine, carbachol, oxotremorine, arecoline, acetyl-beta-methylcholine and acetylcholine, whereas pirenzepine was much less effective. Gallamine antagonized the bradycardia, whereas no influence was found on the bladder contraction. Pilocarpine acted as a partial agonist in reducing heart rate as well as in increasing bladder pressure, whereas McN-A-343 was almost ineffective in doses up to 1 mg/kg. The hypertensive response to pilocarpine and carbachol was less pronounced than that produced by McN-A-343. Pirenzepine and dexetimide significantly antagonized the hypertensive response to McN-A-343 and pilocarpine, whereas gallamine was much less effective. The hypertensive response induced by carbachol was totally blocked by hexamethonium. The other agonists used in this study did not produce a significant increase in diastolic blood pressure in doses that produced a maximal effect on heart rate and urinary bladder pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity of muscarinic receptors in the guinea pig esophageal muscularis mucosae and ileal longitudinal muscle

Pharmacologic characteristics of muscarinic receptors in the muscularis mucosae of the guinea pig esophagus were examined in vitro and compared with those of the longitudinal muscle of the guinea pig ileum. All cholinomimetics tested produced a sustained contraction of the muscularis mucosae, in a concentration-dependent manner, which was associated with a small biphasic change in membrane potential, initially a hyperpolarization followed by a depolarization. The contractile response was hardly modified by verapamil, but was depressed by calcium removal from the bathing medium. Both atropine and pirenzepine antagonized the contractile response in a competitive manner, with higher pA2 values than those in the ileum. These results indicate that the muscarinic receptors of the muscularis mucosae of the guinea pig esophagus mainly link with calcium ion channels which are independent of changes in membrane potential and that their subtype populations are probably pharmacologically distinct from those in the ileal longitudinal muscle.

Cholinergic systems and multiple cholinergic receptors in ocular tissues

Acetylcholine (ACh), choline acetyltransferases and cholinesterases occur in cornea, iris-ciliary body complex and retina of several vertebrates. In cornea, ACh may serve as a sensory transmitter as well as a local hormone, the function of which is not delineated. The function of ACh as the parasympathetic neurotransmitter at the iris and ciliary body is well established. The muscarinic receptors on the iris smooth muscle are similar to the muscarinic receptors (M2 type in two way classification) at other smooth muscles towards their interaction with agonists and antagonists. Binding studies using radiolabeled antagonists and their displacement by agonists indicate that muscarinic receptors in membranes of iris-ciliary body complex are heterogeneous indicating more than one subtype of muscarinic receptors. A subtype other than M2 receptors may occur at the presynaptic sites of parasympathetic nerves, which have yet to be investigated using specific agonists and antagonists. Cholinergic markers, choline acetyltransferase and acetylcholinesterase, differ quantitatively and qualitatively in retinas of different species. However, amacrine cells are cholinergic in all vertebrate species. Although they make up 1% of retinal neurons, they influence the activity of a majority of ganglion cells. Cholinergic effects in ganglia are mediated through nicotinic and muscarinic receptors. Both of these types of cholinergic receptors are heterogeneous. They have yet to be investigated for their subtypes using specific agonists and antagonists. Although the role of cholinergic retinal neurons in the processing of visual information is not known, their input to ganglion cells generally increases the rate of spontaneous activity or the number of action potentials in light-evoked responses. Thus, the cholinergic input seems to modify the overall neuronal input to the ganglion cells from the receptive fields. Endothelial cells of blood vessels contain muscarinic receptors, which are activated by ACh to cause relaxation. Although retinal blood vessels provide recognizable characteristic signs in diabetes mellitus and hypertensive disease, no information is available on the muscarinic receptors of these vessels.

[125I] 3-quinuclidinyl 4-iodobenzilate: a high affinity, high specific activity radioligand for the M1 and M2-acetylcholine receptors

We have prepared a radioiodinated ligand which binds with high affinity to the muscarinic acetylcholine receptor (m-AChR). A derivative of 3-quinuclidinyl benzilate, [125I] labeled (R) 1-aza-bicyclo(2.2.2)oct-3-yl (R,S)-alpha-hydroxy-alpha-(4-[125I]iodophenyl)phenyl acetate (4- IQNB ) exhibits an affinity for the m-AChR from corpus striatum higher than that of (R) [3H] QNB. Additionally, [125I] 4- IQNB exhibits receptor selectivity for the M1 receptor since the affinity for the receptor from dog and rat heart is lower than that using dog or rat corpus striatum.