Functional and binding studies with muscarinic M2-subtype selective antagonists

Differential effects of m1 and m2 receptor antagonists in perirhinal cortex on visual recognition memory in monkeys

Microinfusions of the nonselective muscarinic antagonist scopolamine into perirhinal cortex impairs performance on visual recognition tasks, indicating that muscarinic receptors in this region play a pivotal role in recognition memory. To assess the mnemonic effects of selective blockade in perirhinal cortex of muscarinic receptor subtypes, we locally infused either the m1-selective antagonist pirenzepine or the m2-selective antagonist methoctramine in animals performing one-trial visual recognition, and compared these scores with those following infusions of equivalent volumes of saline. Compared to these control infusions, injections of pirenzepine, but not of methoctramine, significantly impaired recognition accuracy. Further, similar doses of scopolamine and pirenzepine yielded similar deficits, suggesting that the deficits obtained earlier with scopolamine were due mainly, if not exclusively, to blockade of m1 receptors. The present findings indicate that m1 and m2 receptors have functionally dissociable roles, and that the formation of new visual memories is critically dependent on the cholinergic activation of m1 receptors located on perirhinal cells.

A fluorescence resonance energy transfer-based M2 muscarinic receptor sensor reveals rapid kinetics of allosteric modulation

Allosteric modulators have been identified for several G protein-coupled receptors, most notably muscarinic receptors. To study their mechanism of action, we made use of a recently developed technique to generate fluorescence resonance energy transfer (FRET)-based sensors to monitor G protein-coupled receptor activation. Cyan fluorescent protein was fused to the C terminus of the M(2) muscarinic receptor, and a specific binding sequence for the small fluorescent compound fluorescein arsenical hairpin binder, FlAsH, was inserted into the third intracellular loop; the latter site was labeled in intact cells by incubation with FlAsH. We then measured FRET between the donor cyan fluorescent protein and the acceptor FlAsH in intact cells and monitored its changes in real time. Agonists such as acetylcholine and carbachol induced rapid changes in FRET, indicative of agonist-induced conformational changes. Removal of the agonists or addition of an antagonist caused a reversal of this signal with rate constants between 400 and 1100 ms. The allosteric ligands gallamine and dimethyl-W84 caused no changes in FRET when given alone, but increased FRET when given in the presence of an agonist, compatible with an inactivation of the receptors. The kinetics of these effects were very rapid, with rate constants of 80-100 ms and approximately 200 ms for saturating concentrations of gallamine and dimethyl-W84, respectively. Because these speeds are significantly faster than the responses to antagonists, these data indicate that gallamine and dimethyl-W84 are allosteric ligands and actively induce a conformation of the M(2) receptor with a reduced affinity for its agonists.

Subtype-selective blockade of cardiac muscarinic receptors inhibits vagal chronotropic responses in cats

This study was designed to determine if chronotropic responses induced by neurally released acetylcholine are modified by subtype-selective blockade of cardiac muscarinic cholinoreceptors. In anesthetized cats, a single burst of vagal stimulation was generated with an incremental time delay after the P wave of the atrial electrogram (P-Stimulus interval). The slope of the relationships between P-Stimulus and P-P intervals was used to assess changes in responsiveness of cardiac pacemaker to vagal effects throughout the cardiac cycle. An increase in P-Stimulus interval over the initial portion (approximately 120 ms) of the cardiac cycle produced a significant increment in lengthening of the P-P interval. Once the maximal negative chronotropic response was achieved, a further increase in P-Stimulus interval by only approximately 25 ms resulted in profound (by 80-90%) reductions in vagal effects, thus yielding a bimodal vagal phase response curve. Antagonists of M1 (pirenzepine), M2 (methoctramine and gallamine), and M3 (4-DAMP) muscarinic cholinoreceptors produced a reduction in the magnitude of maximal lengthening of cardiac cycle as well as an increase in latency of vagal effects. However, the increment in prolongation of P-P interval induced by a given change in timing of vagal stimulation during cardiac cycle was reduced by M1 and M2 muscarinic receptor blockers, but was unaffected by 4-DAMP. None of the antagonists modified the range of P-Stimulus intervals over which the maximum-to-minimum change of vagal responses occurred. Taken together, these data suggest different contribution of various subtypes of cardiac muscarinic receptors into the negative chronotropic responses induced by brief bursts of vagal stimulation.

N-hexacosanol reverses diabetic induced muscarinic hypercontractility of ileum in the rat

Diabetic neuropathy, a major complication of diabetes mellitus, is associated with development of gastrointestinal motility dysfunction and autonomic neuropathy. N-hexacosanol has neurotrophic effects and exhibits a wide variety of biological actions. In this study, we investigated the effects of cyclohexenonic long-chain fatty alcohol (N-hexacosanol) on streptozotocin-diabetic hypercontractility in the rat ileum longitudinal muscles. Treatment with N-hexacosanol did not alter the diabetic status of the animals, i.e., body weight, serum glucose, and serum insulin levels, but significantly restored the thickness of intestine wall and ameliorated diabetes-induced hypercontractility of the rat ileum in a dose-dependent manner. Furthermore, N-hexacosanol reversed the diabetes-induced upregulation of intestinal muscarinic M(2) and M(3) receptors mRNAs in the streptozotocin-diabetic rats. These results indicate that N-hexacosanol has therapeutic effects on hypercontractility in the diabetic ileum by ameliorating overexpression of muscarinic M(2) and M(3) receptors mRNAs.

Contractile role of M2 and M3 muscarinic receptors in gastrointestinal, airway and urinary bladder smooth muscle

Both M(2) and M(3) muscarinic receptors are expressed in smooth muscle and influence contraction through distinct signaling pathways. M(3) receptors interact with G(q) to trigger phosphoinositide hydrolysis, Ca(2+) mobilization and a direct contractile response. In contrast, M(2) receptors interact with G(i) and G(o) to inhibit adenylyl cyclase and Ca(2+)-activated K(+) channels and to potentiate a Ca(2+)-dependent, nonselective cation conductance. Ultimately, these mechanisms lead to the prediction that the influence of the M(2) receptor on contraction should be conditional upon mobilization of Ca(2+) by another receptor such as the M(3). Mathematical modeling studies of these mechanisms show that the competitive antagonism of a muscarinic response mediated through activation of both M(2) and M(3) receptors should resemble the profile of the directly acting receptor (i.e., the M(3)) and not that of the conditionally acting receptor (i.e., the M(2)). Using a combination of pharmacological and genetic approaches, we have identified two mechanisms for the M(2) receptor in contraction: 1) a high potency inhibition of the relaxation elicited by agents that increase cytosolic cAMP and 2) a low potency potentiation of contractions elicited by the M(3) receptor. The latter mechanism may be involved in muscarinic agonist-mediated heterologous desensitization of smooth muscle, which requires activation of both M(2) and M(3) receptors.

Muscarinic Agonist-Mediated Heterologous Desensitization in Isolated Ileum Requires Activation of Both Muscarinic M2 and M3 Receptors

We investigated the subtypes of the muscarinic receptor mediating short-term heterologous desensitization in the isolated ileum. Treatment of the ileum from C57BL/6 mice with acetylcholine (30 microM) for 20 min caused a subsequent decrease in contractile sensitivity to both prostaglandin F2alpha (PGF2alpha) and the muscarinic agonist, oxotremorine-M. This subsensitivity was characterized by 7- and 3-fold increases in the EC50 values of the agonists, respectively, with no significant effect on the maximal response. The subsensitivity to PGF2alpha was prevented in both M2 and M3 muscarinic receptor knockout mice. Similarly, the subsensitivity to oxotremorine-M was prevented in M2 knockout mice. Acetylcholine-mediated desensitization of histamine-induced contractions in the guinea pig ileum was inhibited by both M2- and M3-selective muscarinic antagonists with high potency, although careful analysis of the data suggested behavior more consistent with an M2 antagonistic profile. Modeling studies showed that the competitive antagonism of response contingent upon activation of two receptor subtypes should exhibit a pharmacological profile similar to that of the least sensitive signaling pathway. Our results demonstrate that muscarinic agonist-mediated short-term heterologous desensitization of intestinal smooth muscle is contingent upon activation of both M2 and M3 muscarinic receptors and that activation of either receptor by itself is insufficient to cause desensitization.

The cholinomimetic agent bethanechol activates IK(ACh) in feline atrial myocytes

The effect of the cholinomimetic agent, bethanechol on macroscopic membrane currents was studied in dispersed cat atrial myocytes, using the whole-cell patch-clamp technique. Bethanechol activated an inward rectifying potassium current similar to I(K(ACh)), and a delayed rectifying-like outward current, similar to I(KM3) activated by pilocarpine, choline, and tetramethylammonium, and I(KM4) activated by 4-aminopyridine. The relatively specific muscarinic receptors subtype antagonists methoctramine (M(2)), and tropicamide (M(4)) inhibited both current components induced by bethanechol, suggesting a lack of specificity of these antagonists on cat atrial myocytes. The specific antagonist of M(3) receptors, para-fluoro-hexahydro-siladifenidol did not significantly inhibit the bethanechol-induced currents. In addition, pretreatment with PTX prevented activation of the bethanechol-induced inward and outward currents, suggesting that M(3) receptors are probably not involved in the bethanechol action. The I(K(ACh)) specific blocker tertiapin inhibited both inward rectifying- and delayed rectifying-like currents. These results suggest that both current components result from activation of a single channel type, likely I(K(ACh)).

Synthesis and affinity studies of himbacine derived muscarinic receptor antagonists

A series of himbacine (1)-related analogues has been prepared featuring three different isomeric configurations with respect to the B-ring (a, b and natural c) and three different interconnecting two-carbon unsaturated units [natural (E)-ene, (Z)-ene, and yne]. The study of the binding affinities of the nine resulting compounds, including synthetic (+)-himbacine (3c), towards the M(1)-M(4) muscarine receptor subtypes revealed that analogues 3a and 5c display a promising 10-fold selectivity for the M(2) receptor as compared to the M(1) receptor.

Heterogeneous distribution of beta-adrenoceptors and muscarinic receptors in the sinoatrial node and right atrium of the dog

1. The pacemaker site is known to shift away from the sinoatrial (SA) node in response to autonomic stimuli. 2. To test whether the shifting of the impulse-initiation site that occurs during autonomic nerve stimulation can be explained by the spatial distributions of beta-adrenoceptor and muscarinic receptors, we obtained densities (B(max)) and dissociation constants (K((d)) for these receptors in three regions along the sulcus terminalis (the SA node itself and regions superior and inferior to it) and a region of the right atrial appendage in the dog. 3. The Bmax values for [(125)I]-iodocyanopindolol binding to beta-adrenoceptors were not different among the four regions. The Bmax value for [(3)H]-quinuclidinyl benzilate binding to muscarinic receptors was significantly higher in the SA node area than in any other region (P < 0.01), although there was no difference among the other three regions. 4. For each receptor, K(d) values were similar among the four regions. 5. These results suggest that spatial variations in the densities of beta-adrenoceptors and muscarinic receptors are not important for shifting the impulse-initiation site and that other factors, such as non-uniform innervation by autonomic nerve fibres, may be mainly responsible for the pacemaker shift induced by autonomic nerve stimulation.

Characterization of pre- and postsynaptic muscarinic receptors in circular muscle of pig gastric fundus

1. This study investigated the subtype of muscarinic receptors on the cholinergic neurones and smooth muscle in the circular muscle of the pig gastric fundus. 2. Muscarinic antagonists, except MT-3, concentration-dependently inhibited the contractions induced by a given concentration of acetylcholine. Concentration-response curves by acetylcholine were shifted rightwards in a parallel manner without depression of the maximum by the muscarinic antagonists, except by MT-3 that induced a leftward shift. Correlation of the pIC(50) and pA(2) values with published pK(i) values for the five muscarinic receptor subtypes suggests that the muscarinic receptors on pig gastric fundus circular muscle belong to the M(3) subtype. 3. Electrically-evoked contractions (40 V, 4 Hz, 0.25 ms, 2 min) were concentration-dependently inhibited by the muscarinic antagonists except for methoctramine and AF-DX 116, that increased the amplitude of the electrically-induced contractions in lower concentrations. MT-3 tended to increase the electrically-induced contractions. 4. The antagonists, except MT-3, concentration-dependently increased the electrically-induced tritium outflow (40 V, 4 Hz, 0.25 ms, 2 min) after incubation of the tissues with [(3)H]-choline. MT-3 (3 x 10(-8) and 10(-7) M) decreased the electrically-induced tritium release. Correlation of the pIC(50) values with published pK(i) values for the different muscarinic receptor subtypes yielded a significant and comparable correlation for M(1), M(3), M(4) and M(5) receptors. 5. These results suggest that the postsynaptic receptors in circular muscle of the pig gastric fundus belong to the M(3) subtype. However, the presynaptic receptor could not be clearly defined, although it does certainly not belong to the M(2) subtype.

Increased function of inhibitory neuronal M2 muscarinic receptors in trachea and ileum of diabetic rats

1. Release of acetylcholine from parasympathetic nerves is inhibited by neuronal M(2) muscarinic receptors. The effects of streptozotocin-induced diabetes on prejunctional M(2) and postjunctional M(3) muscarinic receptor function in rat trachea and ileum were investigated in vitro. 2. Neuronal M(2) muscarinic receptor function was tested by measuring the ability of an agonist, pilocarpine, to inhibit and an antagonist, methoctramine, to potentiate electrical field stimulation (EFS)-induced contraction of trachea and ileum. Concentration-response curves to pilocarpine and methoctramine were shifted to the left in both to a greater degree in diabetics than controls. 3. In trachea, post-junctional M(3) muscarinic receptor function was increased since maximum contractile responses to the muscarinic agonists acetylcholine and carbachol were greater in diabetics than controls. This increase offset the increased function of the inhibitory neuronal M(2) muscarinic receptors since EFS-induced, frequency-dependent contraction was equal in control and diabetic rats. 4. In contrast, post-junctional M(3) muscarinic receptor function was unchanged by diabetes since concentration-response curves to acetylcholine and carbachol were not different between groups. Thus, EFS-induced contractions of the ileum were decreased in diabetics versus controls. 5. In conclusion, inhibitory M(2) muscarinic receptors on parasympathetic nerves in the trachea and ileum are hyperfunctional in diabetic rats. The function of post-junctional M(3) muscarinic receptors in the trachea, but not the ileum, is also increased in diabetes. 6. The dysfunction of inhibitory, neuronal M(2) muscarinic receptors in the airways may protect against hyperreactivity and in the ileum may contribute to gastrointestinal dysmotility associated with diabetes.

Inhibition of field stimulation-induced contractions of rabbit vas deferens by muscarinic receptor agonists: selectivity of McN-A-343 for M1 receptors

Inhibition of the field stimulation-induced twitch responses of the rabbit vas deferens by the muscarinic receptor agonist, McN-A-343, has been attributed to presynaptic muscarinic receptors of the M1 subtype located on noradrenergic nerve terminals. Stimulation of these receptors causes inhibition of transmitter release and inhibition of the contractile response. However, the selectivity of McN-A-343 for M1 receptors has been questioned and this throws doubt on whether the prejunctional receptors of the rabbit vas deferens are of the M1 subtype. In this study we have undertaken a comprehensive re-evaluation of the inhibition of prostatic and epididymal portions of the rabbit isolated field-stimulated vas deferens by several agonists, including McN-A-343, and quantified the antagonism by M1-selective antagonists, pirenzepine and telenzepine. Prostatic and epididymal portions of vasa deferentia from New Zealand White rabbits were immersed in a low Ca2+ Krebs solution at 32+/-0.5 degrees C gassed with 5% CO2 in oxygen. Yohimbine (1.0mM) was present throughout to block prejunctional alpha2-adrenoceptors. Field stimulation was applied by repeated application of single pulses (30 V, 0.05 Hz, 0.5 ms) and isometric contractions recorded. Carbachol and oxotremorine initially potentiated the epididymal contractions but at higher concentrations there was inhibition. In the prostatic portion, oxotremorine only inhibited. McN-A-343 produced inhibitory responses only in both epididymal and prostatic portions. Pirenzepine shifted the concentration-response curves forthe inhibitory responses to oxotremorine to the right. However, the potentiation of the twitches also became more apparent with the lower concentrations of oxotremorine. Schild plots for the antagonism by pirenzepine yielded pA2 values of 7.96+/-0.004 and 7.7+/-0.02 for the epididymal and prostatic portions, respectively. The concentration-response curves for the inhibition of twitches by McN-A-343 were displaced to the right in a parallel manner by pirenzepine in both prostatic and epididymal portions with no potentiation of the twitches. The Schild plot for this antagonism generated pA2 values of 7.68+/-0.01 and 8.07+/-0.01, respectively. Telenzepine caused parallel shifts of the McN-A-343 concentration-response curves to the right in prostatic portions, the pA2 value being 8.70+/-0.13. Telenzepine (10(-7) M) abolished the inhibitory effect of carbachol to reveal only concentration-dependent potentiation of the contractions. The Schild plot for antagonism of this contractile effect yielded a pA2 value (7.07+/-0.09) that was significantly less by almost two orders of magnitude (1.70) than the value for the antagonism by telenzepine of the McN-A-343-induced inhibitory response. The pA2 values of pirenzepine and telenzepine against the inhibitory responses of the rabbit vas deferens are consistent with the involvement of M1 receptors. This leads to the conclusion that McN-A-343 causes inhibition through this receptor type. The doubts concerning the selectivity of McN-A-343 for M1 receptors are therefore unfounded. The fact that McN-A-343 does not display a selective binding profile suggests that its selectivity does not arise from affinity differences but probably resides in its intrinsic efficacy.

Characterisation of muscarinic receptor subtypes in avian smooth muscle

The identity of the muscarinic receptor subtype in the chick ileum was investigated in functional and binding studies. Preliminary studies [Choo, L.-K., Mitchelson, F., Napier, P. 1988. J. Auton. Pharmacol. 8, 259-266] suggested apparent avian and mammalian family differences in the muscarinic receptor profile of ileal smooth muscle. In the current study, further characterisation was undertaken using a greater range of antagonists exhibiting high affinity for specific muscarinic receptor subtypes. Dissociation constants from functional and binding experiments were compared with published values for antagonists at each of the five muscarinic receptor subtypes. Linear regression and correlation analyses revealed the receptor initiating the contractile response was most likely of the muscarinic M(3) receptor subtype as the slope of the linear regression was 1.01+/-0.14 and the corresponding correlation coefficient (r) was 0.95. The mammalian muscarinic M(5) receptor subtype also showed a high correlation with the data giving a slope of 0.89+/-0.27 and r value of 0.76. These findings were in direct contrast to those from binding experiments in which the single binding site detected was of the muscarinic M(2) receptor subtype. The slope of the linear regression was 1.14+/-0.24 with an r value of 0.87. Thus, these results suggest that there exists a high proportion of the muscarinic M(2) receptor subtype within the tissue that does not contribute to the functional response.

Increased density of M1 receptors in the hippocampus of juvenile rats chronically deprived of NGF

Binding studies were used to assess the changes in affinity and/or number of M1 muscarinic receptors in hippocampi from juvenile rats chronically deprived of NGF. NGF deprivation was obtained by implanting into right ventricle at postnatal day 2 (P2) hybrydoma cells secreting high levels of monoclonal antibodies against NGF (alphaD11). Parenteral myeloma cells (P3U) were used as controls. Competition experiments were used to characterise the [3H]-PNZ binding sites in membrane preparations of hippocampi from rats sacrificed at P15. [3H]-PNZ bound M1 receptors both in P3U and alphaD11 group as shown by displacing potency order of antagonists: TLZ=4-DAMP>PNZ>p-F-HHSiD>MTC. The deprivation of NGF for two weeks significantly increased the number of M1 receptors without changing the Ki values of antagonists with exception of methoctramine which showed an increase in affinity in alphaD11 group. Similar changes in binding parameters were already observed after the first week of anti-NGF treatment. In contrast, a treatment for a week with implant at postnatal day 15 failed to produce any changes in M1 binding parameters. These results provide further physiological evidence for developmentally regulated modulatory role of NGF in the cholinergic function in the hippocampus.

Muscarinic receptor activation modulates Ca2+ channels in rat intracardiac neurons via a PTX- and voltage-sensitive pathway

With use of the whole cell patch-clamp technique, effects of the potent muscarinic agonist oxotremorine methiodide (oxo-M) on voltage-activated Ca2+ channel currents were investigated in acutely dissociated adult rat intracardiac neurons. In all tested neurons oxo-M reversibly inhibited the peak Ba2+ current. Inhibition of the peak Ba2+ current by oxo-M was associated with slowing of activation kinetics and was concentration dependent. The concentration of oxo-M necessary to produce a half-maximal inhibition of current and the maximal inhibition were 40.8 nM and 75.9%, respectively. Inhibitory effect of oxo-M was completely abolished by atropine. Among different muscarinic receptor antagonists, methoctramine (100 and 300 nM) significantly antagonized the current inhibition by oxo-M, with a negative logarithm of dissociation constant of 8.3 in adult rat intracardiac neurons. Internal dialysis of neurons with guanosine 5'-(thio)triphosphate (GTPgammaS, 0.5 mM) could mimic the muscarinic inhibition of the peak Ba2+ current and significantly occlude inhibitory effects of oxo-M. In addition, the internal dialysis of guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS, 2 mM) also significantly reduced the muscarinic inhibition of the peak Ba2+ current by oxo-M. Inhibitory effects of oxo-M were significantly abolished by pertussis toxin (PTX, 200 and 400 ng/ml) but not by cholera toxin (400 ng/ml). Furthermore, the bath application of N-ethylmaleimide (50 microM) significantly reduced the inhibition of the peak Ba2+ current by oxo-M. The oxo-M shifted the activation curve derived from measurments of tail currents toward more positive potentials. A strong conditioning prepulse to +100 mV significantly relieved the muscarinic inhibition of peak Ba2+ currents by oxo-M and the GTPgammaS-induced current inhibition. In a series of experiments, changes in intracellular concentration of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid and protein kinase activities failed to mimic or occlude the current inhibition by oxo-M. The dihydropyridine antagonist nifedipine (10 microM) was not able to occlude any of the inhibitory effects of oxo-M, and oxo-M (3 microM) failed to reduce the slow tail currents induced by the L-type agonist methyl 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylate (FPL 64176; 2 microM). However, omega-conotoxin (omega-CgTX) GVIA (1 microM) significantly occluded the muscarinic inhibition of the Ba2+ currents. In the presence of omega-CgTX GVIA (1 microM) and nifedipine (10 microM), oxo-M could further inhibit approximately 20% of the total Ca2+ current. After complete removal of N-, Q-, and L-type currents with use of omega-CgTX GVIA, omega-agatoxin IVA, and nifedipine, 70% of the R-type current (approximately 6-7% of the total current) was inhibited by oxo-M (3 microM). In conclusion, the M2 muscarinic receptor activation selectively inhibits N-, Q-, and R-type Ca2+ channel currents, sparing L-type Ca2+ channel currents mainly via a PTX- and voltage-sensitive pathway in adult rat intracardiac neurons.

Interaction of class III antiarrhythmic drugs with muscarinic M2 and M3 receptors: radioligand binding and functional studies

We have recently reported that class III antiarrhythmic drugs inhibit the muscarinic acetylcholine (ACh) receptor-operated K+ current (IK,ACh) in guinea-pig atrial cells by different molecular mechanisms. The data obtained from the patch-clamp study suggest that D,L-sotalol inhibits IK,ACh by blocking the muscarinic receptors, whereas MS-551 inhibits the K+ current by blocking the muscarinic receptors and depressing the function of the K+ channel itself and/or the guanine nucleotide-binding protein (G protein). This study was undertaken to determine whether the class III antiarrhythmic drugs D,L-sotalol and MS-551 interact with the muscarinic receptors of cardiac and peripheral tissues. Both drugs inhibited concentration dependently the specific [3H]N-methylscopolamine ([3H]-NMS) binding to membrane preparations obtained from guinea-pig atria and submandibular glands. The competition curves of these drugs for [3H]-NMS binding to glandular membranes were monophasic, suggesting competition with [3H]-NMS at a single site. Although the competition curve of D,L-sotalol for [3H]-NMS binding to atrial membranes was monophasic, that of MS-551 was biphasic and showed high- and low-affinity states of binding. D,L-Sotalol showed slightly, but significantly, higher affinity for cardiac-type muscarinic receptors (M2) than for glandular-type muscarinic receptors (M3). The inhibition constant (Ki) for MS-551 in glandular membranes was also slightly greater than the high-affinity Ki value for the drug in atrial membranes. In guinea-pig left atria and ilea, D,L-sotalol shifted the concentration-response curves for the negative inotropic effect and the contracting effect of carbachol in a parallel manner. The slopes of Schild plot were not significantly different from unity, suggesting competitive antagonism, and the pA2 for D,L-sotalol in left atria was slightly greater than that in ilea. MS-551 also shifted the concentration response curve for the negative inotropic effect of carbachol in atrial preparations to a greater extent than that for the contracting effect in ileal preparations, although MS-551 failed to show a pure competitive antagonism. These results suggest that both D,L-sotalol and MS-551 interact with cardiac M2 and peripheral M3 receptors, and that at high concentrations they exert anticholinergic activity in cardiac and peripheral tissues.

In vitro electro-pharmacological and autoradiographic analyses of muscarinic receptor subtypes in rat hypothalamic ventromedial nucleus: implications for cholinergic regulation of lordosis

Muscarinic agonists can act through the hypothalamic ventromedial nucleus (VMN) to facilitate lordosis. To elucidate the neuronal mechanism(s) underlying this muscarinic facilitation, effects of muscarinic agents on the single-unit activity of VMN neurons recorded in brain tissue slices of estrogen-primed female rats were analyzed. All the agonists tested, including acetylcholine (ACh), oxotremorine-M (OM), carbachol (CCh) and McN-A-343 (McN), evoked primarily excitation (80-100%), some inhibition (0-20%) and occasional biphasic responses (0-8%). By comparing the response magnitude and the effectiveness in evoking a response, the rank order for evoking excitation, the primary response, was found to be: OM > CCh > ACh approximately McN, which is consistent with that (OM > CCh > McN) for facilitating lordosis reported by others. This consistency and the frequency of its occurrence suggest that the excitatory electric action of the muscarinic agonists is related to their facilitatory behavioral effect. Experiments with antagonists selective for M1 (pirenzepine), M2 (AF-DX 116) and M3 (4-DAMP and p-F-HHSiD) indicate that muscarinic excitations are mediated by M1 and/or M3, but not M2. Since M1 receptors have been shown to be neither sufficient nor necessary to mediate the muscarinic facilitation, M3 receptor may be crucially involved in this behavioral effect. Autoradiographic assays of binding to [3H]4-DAMP with or without pirenzepine and AF-DX 116, also indicate the presence of M3 receptors in the VMN. Quantitative analyses show that the M3 binding was not affected by the in vivo estrogen priming required to permit muscarinic agonists to facilitate lordosis. Thus, while the excitation mediated by M3 is likely to be involved in muscarinic facilitation of lordosis, the regulation of M3 receptor density does not seem to be involved in the permissive

Mode of antagonism of methoctramine, AF-DX 116 and hexahydrosiladifenidol in guinea-pig left atrium and ileum: comparison of Schild and resultant analysis

1. Methoctramine, AF-DX 116 and hexahydrosiladifenidol (HHSiD) are the muscarinic antagonists most widely used to study muscarinic receptor subtypes. 2. The present study was undertaken to examine the mode of antagonism of these compounds in guinea-pig left atrium and ileum by comparison of the Schild and resultant analysis. With this method the effect of various concentrations of the test antagonist on the antagonism produced by specific concentrations of a reference antagonist was measured and the equilibrium dissociation constant of the test antagonist-receptor complex estimated. Atropine was used for comparative purposes and scopolamine as the reference antagonist. 3. At the cardiac level the affinity values obtained by Schild and resultant analysis for methoctramine and AF-DX 116, as for atropine, are very similar: these results indicate that the two cardio-selective antagonists and the non-selective antagonist, atropine, bind at a common site with the reference antagonist scopolamine. The resultant plot for the ileo-selective HHSiD has a slope considerably less than unity: this finding might indicate that this antagonist binds to a site different from that of scopolamine and it should be considered like an allosteric antagonist. 4. At the ileal level the affinity values obtained by Schild and resultant analysis are identical for the ileo-selective antagonist HHSiD as for atropine but not for methoctramine and AF-DX 116. This indicates a mutual binding site with scopolamine for HHSiD and atropine but not for the two cardio-selective antagonists. However, it is worth emphasizing that the difference between affinity values obtained by Schild and resultant analysis is seen when relatively high concentrations are required: a dual mode of interaction (both competitive and allosteric) could be involved.

In vivo modulation of histamine release by autoreceptors and muscarinic acetylcholine receptors in the rat anterior hypothalamus

The modulation of histamine release by histamine and muscarinic acetylcholine receptors was investigated by using the push-pull technique. The anterior hypothalamic area of the conscious, freely moving rat was superfused through the push-pull cannula with CSF or with CSF containing drugs and the release of endogenous histamine was determined in the superfusate. Hypothalamic superfusion with tetrodotoxin (10 mumol/l) led to a pronounced and sustained decrease in the histamine release rate. Superfusion with compound 48/80 (100 mg/l) was ineffective. Hypothalamic superfusion with the H3 agonist (R)-alpha-methylhistamine inhibited, while superfusion with the H3 antagonist thioperamide enhanced the release of histamine. The release of histamine was inhibited on hypothalamic superfusion with the muscarinic receptor agonists carbachol or oxotremorine. Histamine release was enhanced by atropine, and this release-enhancing effect was abolished by oxotremorine. The selective M1 antagonist pirenzepine (100 mumol/l) and 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 10 mumol/l), which blocks M1 and M3 receptors, also enhanced the release rate of histamine. On the other hand, 50 and 100 mumol/l methoctramine (M2 receptor antagonist) 10 and 100 mumol/l p-fluoro-hexahydro-siladifenidol (p-F-HHSiD, a M3 receptor antagonist) were ineffective. It is concluded tht histamine released in the hypothalamus originates predominantly from neurons. The release of histamine is modulated by H3 autoreceptors. The histamine release is also modulated by cholinergic neurons which modify histamine release from histaminergic neurons by stimulating M1 muscarinic acetylcholine heteroreceptors probably located on histaminergic neurons.

Selectivity of oxomemazine for the M1 muscarinic receptors

The binding characteristics of pirenzepine and oxomemazine to muscarinic receptor were studied to evaluate the selectivity of oxomemazine for the muscarinic receptor subtypes in rat cerebral microsomes. Equilibrium dissociation constant (KD) of (-)-[3H]quinuclidinyl benzilate([3H]QNB) determined from saturation isotherms was 64 pM. Analysis of the pirenzepine inhibition curve of [3H]QNB binding to cerebral microsome indicated the presence of two receptor subtypes with high (Ki = 16 nM, M1 receptor) and low (Ki = 400 nM, M3 receptor) affinity for pirenzepine. Oxomemazine also identified two receptor subtypes with about 20-fold difference in the affinity for high (Ki = 84 nM, OH receptor) and low (Ki = 1.65 microM, OL receptor) affinity sites. The percentage populations of M1 and M3 receptors to the total receptors were 61:39, and those of OH and OL receptors 39:61, respectively. Both pirenzepine and oxomemazine increased the KD value for [3H]QNB without affecting the binding site concentrations and Hill coefficient for the [3H]QNB binding. Oxomemazine had a 10-fold higher affinity at M1 receptors than at M3 receptors, and pirenzepine a 8-fold higher affinity at OH receptors than at OL receptors. Analysis of the shallow competition binding curves of oxomemazine for M1 receptors and pirenzepine for OL receptors yielded that 69% of M1 receptors were of OH receptors and the remaining 31% of OL receptors, and that 29% of OL receptors were of M1 receptors and 71% of M3 receptors. However, M3 for oxomemazine and OH for pirenzepine were composed of a uniform population. These results suggest that oxomemazine could be classified as a selective drug for M1 receptors and also demonstrate that rat cerebral microsomes contain three different subtypes of M1, M3 and the other site which is different from M1, M2 and M3 receptors.

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)

Pharmacological profile of selective muscarinic receptor antagonists on guinea-pig ileal smooth muscle

The present study examined the effects of a series of tricyclic muscarinic receptor antagonists on muscarinic receptors present in the guinea-pig ileum, both in vitro and in vivo. The selectivity profiles of these antagonists and that of atropine were determined by their affinity for cortical muscarinic M1, cardiac M2 and submandibular M3 receptors and for m4 receptors expressed in CHO cells. The compounds pirenzepine, UH-AH 37, AQ-RA 391 and AQ-RA 618 possessed high affinity (pKi 7.94-8.22) for muscarinic M1 receptors. Pirenzepine exhibited the most pronounced muscarinic M1 selectivity. AF-DX 384 and AQ-RA 741 possessed an approximately 10-fold higher affinity for the cardiac muscarinic M2 receptor than AF-DX 116. However, both compounds also exhibited high affinity for muscarinic m4 receptors. High affinity for muscarinic M3 and m4 receptors was observed for UH-AH 37, AQ-RA 391 and AQ-RA 681. The antagonists were then tested for their interaction with the muscarinic receptors which are responsible for the methacholine-induced contraction of longitudinal muscle in vitro, circular muscle in vivo and muscarinic receptors which mediate the distension-evoked ascending reflex contraction of circular muscle in vitro. Compounds showing high affinity for muscarinic M3 receptors (e.g. AQ-RA 618) were the most potent antagonists in the functional experiments. Comparison of the binding displacement data with the functional results indicates that the effects of methacholine on the longitudinal and circular muscle of the guinea-pig ileum were predominantly mediated by muscarinic M3-type receptors. In contrast, the correlation between muscarinic M2 receptor affinity and antagonism of muscarinic receptors in the ileum was very weak.

The involvement of muscarinic receptor subtypes in the mediation of hypothermia, tremor, and salivation in male mice

The potency of centrally administered non-selective (atropine and N-methyl scopolamine) and putatively selective muscarinic antagonists (pirenzepine, AF-DX 116 and 4-DAMP) in inhibition of oxotremorine-induced hypothermia, tremor and salivation in male mice has been compared with their potency in vitro in three functional systems, where muscarinic effects are mediated preferentially by M1 (i.e. superior cervical ganglion), M2 (i.e. atrium), and M3 (i.e. ileum) receptors. Atropine, N-methyl scopolamine and 4-DAMP potently abolished the effects of oxotremorine. Pirenzepine abolished tremor and salivation, whereas hypothermia was antagonized partially only. AF-DX 116 had but weak antagonistic effects. Atropine and N-methyl scopolamine were potent antagonists in all three in vitro test systems. High potency was also seen with 4-DAMP, in particular in the ileum preparation. Pirenzepine showed its highest potency in the ganglion preparation. AF-DX 116 was a weak and non-selective antagonist in all three in vitro preparations. Our studies indicate that the muscarinic induction of tremor and salivation may be preferentially mediated by M3 receptors whereas both M2 and M3 receptors may be involved in the mediation of hypothermia. However, the overall conclusion is that compounds with higher receptor subtype selectivity are needed.

Sialogogic activities of SNI-2011 compared with those of pilocarpine and McN-A-343 in rat salivary glands: identification of a potential therapeutic agent for treatment of Sjörgen's syndrome

1. We examined the sialogogic activities in rat major salivary glands of SNI-2011, in comparison with those of pilocarpine and McN-A-343, and we characterized the subtypes of muscarine receptors that are involved in the sialogogic responses to SNI-2011 and McN-A-343. 2. SNI-2011 at doses ranging from 1 to 10 mg/kg (i.v.) increased the secretion of saliva in a dose-dependent manner. The dose-response curves for SNI-2011 were approximately parallel to curves for pilocarpine but the potency of SNI-2011 was about 25-fold lower than that of pilocarpine. 3. The total volume of saliva secreted in response to McN-A-343 was very much less than that secreted in response to SNI-2011. 4. The salivation induced by SNI-2011 and by McN-A-343 was inhibited by various antagonists with the following rank order of potency: 4-DAMP >> pirenzepine >> AF-DX 116. 5. Our results suggest that the sialogogic effects of SNI-2011 and McN-A-343 are mediated by direct stimulation of M3 receptors in salivary glands and that SNI-2011 may prove useful in the management of xerostomia in patients with Sjögren's syndrome.

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.

Rodent biodistribution and metabolism of tritiated 4-DAMP, a M3 subtype-selective cholinoceptor ligand

The biodistribution of [3H]4-DAMP (a M3-selective cholinoceptor antagonist) was studied in rats which had received either saline or saline containing atropine (to block cholinoceptors). Specific binding of the radioligand was observed in the urinary bladder, ileum, pancreas, stomach, submandibular gland and trachea. Maximal ratios of total-to-non-specific uptake reached values of 1.8 (trachea), 3.2 (bladder), 4.0 (stomach), 4.8 (ileum), 6.6 (pancreas) and 6.9 (submandibular gland) at 5-10 min post-injection; this rank order reflects the tissue densities of M3 cholinoceptors, 4-DAMP did not bind to blood cells and it was rapidly cleared from the circulation (> 90% with a half-life of 0.2 min, the remainder with a half-life of 9.4 min). Labelled metabolites appeared within 5 min in plasma, but metabolite uptake by the target organs was low (< 15% of total radioactivity 40 min post-injection). Although 4-DAMP binds to M3-cholinoceptors in vivo, its potential use as a radiopharmaceutical appears limited since the compound does not cross the blood-brain barrier and it does not show measurable specific binding in airways.

Characterization of muscarinic receptors in guinea-pig uterus

To characterize the muscarinic receptor present in guinea-pig uterus smooth muscle the affinities of a series of 27 muscarinic receptor antagonists for M1 (rat cortex), M2 (rat heart), M3 (rat submandibular gland), m4 (transfected in CHO cells) and muscarinic binding sites in guinea-pig uterus smooth muscle were determined in radioligand binding studies. In addition, functional experiments were performed to assess pKB values of the antagonist for muscarinic receptors in guinea-pig atrium and uterus. The results obtained are consistent with the presence of M2 receptors in the uterus through which the functional contractile response is mediated. Correlation coefficients of 0.98, 0.91 and 0.91 were calculated for the following linear regressions: pKi uterus vs. pKi M2, pKB uterus vs. pKi M2 and pKB uterus vs. pKB atrium. This study also revealed that the compounds dicyclomine, DAU 5884, DAU 6202 as well as AQ-RA 721 could distinguish m4 from M2 sites and are therefore important tools to characterize muscarinic receptor subtypes. In addition, DAU 5884 and DAU 6202 have been identified as highly potent M1 selective antagonists.

Characterization of BIBN 99: a lipophilic and selective muscarinic M2 receptor antagonist

The present study was designed to characterize the receptor selectivity profile of the novel muscarinic M2 receptor antagonist BIBN 99 (5,11-dihydro-8-chloro-11-[[4-[3-[(2,2-dimethyl-1- oxopentyl)ethylamino]propyl]-1-piperidinyl]acetyl]-6H- pyrido[2,3-b][1,4]benzodiazepin-6-one). In radioligand binding studies BIBN 99 showed high affinity for m2/M2 sites (pKi = 7.52/7.57), intermediate affinity for m4 sites (pKi = 6.76) and low affinity for m1/M1 (pKi = 5.97/6.17), m3/M3 (pKi = 6.11/6.04) and m5 sites (pKi = 5.84). Functional studies in vitro showed BIBN 99 to be a competitive antagonist and to have an 11- to 25-fold higher affinity for M2 receptors than for putative M1 receptors in the rabbit vas deferens or M3 receptors in guinea-pig trachea. In vivo studies revealed that BIBN 99 is able to cross the blood-brain barrier, and although showing an approximately 3-fold higher affinity for M2 binding sites BIBN 99 appeared to be 7- to 18-fold less potent than AF-DX 116 in inhibiting muscarinic agonist or vagally induced bradycardia in rats and guinea-pigs. The results show that BIBN 99 is the first lipophilic muscarinic M2 receptor antagonist to have remarkable M2 versus M1 selectivity (30-fold). In addition, BIBN 99 possesses central nervous system activity and only minor peripheral cardiac effects.

The interaction of McN-A-343 with muscarine receptors in cardiac and smooth muscle

The interaction of the muscarine receptor partial agonist (4-m-chlorophenylcarbamoyloxy)-2-butynyltrimethylammonium chloride (McN-A-343) was investigated at muscarine receptors in the atria and taenia caeci of the guinea-pig to compare its interaction at the muscarine M2 receptor in the two tissues. In the smooth muscle, the muscarine M3 receptor subtype is responsible for the contractile response but the major subtype detected in binding or antibody experiments is the M2 subtype. In guinea pig atria the dissociation constant of McN-A-343 at muscarine receptors was 15.2 microM determined in functional experiments on left atria in McEwen's solution or 14.8 microM in binding experiments with [3H]-(-)-quinuclidinyl benzilate ([3H]QNB) in the same medium containing 5'-guanylylimododiphosphate (50 microM). In the taenia caeci, the dissociation constant estimated for McN-A-343 at the M3 receptor from functional experiments based on the contractile response to the agonist in McEwen's solution was 4.6 microM. This value was similar to the dissociation constant (6.2 microM) estimated from binding studies versus [3H]QNB conducted in the same medium although studies with 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepine 6-one (AF-DX 116) versus [3H]-(-)-N-methylscopolamine suggested that 70% of the receptors were the M2 subtype. The presence of the M2 subtype in the taenia caeci was also confirmed by the ability of oxotremorine to inhibit the increase in cAMP produced by isoprenaline (10 microM) since apparent pKB values for AF-DX 116 and hexahydrosiladiphenidol were 6.95 and 6.75, respectively. McN-A-343 (100 microM) failed to inhibit the response to isoprenaline and did not antagonize the inhibitory response to oxotremorine. It is concluded that the apparent affinity of McN-A-343 for muscarine M2 receptors in the atria and the taenia caeci differs and a number of explanations are discussed.

Characterization of muscarinic receptors mediating vasodilation in rat perfused kidney

The muscarinic receptor mediating vasodilation of resistance vessels in the rat isolated, constant-pressure perfused kidney (preconstriction by 10(-7) M cirazoline) was characterized by subtype-preferring agonists and selective antagonists. The agonists produced vasodilation with the following rank order of potency: arecaidine propargyl ester (APE) > 5-methylfurtrethonium = methacholine = oxotremorine > (S)-aceclidine > arecaidine 2-butyne-1,4-diyl bisester > 4-Cl-McN-A-343 = (R)-nipecotic acid ethyl ester = N-ethyl-guvacine propargyl ester approximately (R)-aceclidine = (S)-nipecotic acid ethyl ester > McN-A-343. Agonist-induced vasodilation disappeared after destruction of the endothelium with detergent. Highly significant correlations of agonist potencies for vasodilation were found between rat kidney and guinea-pig ileum submucosal arterioles as well as agonist potencies at smooth muscle muscarinic M3 receptors of the guinea-pig ileum. The rank order of antagonist potencies (4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > (R)-hexahydro-difenidol approximately hexahydro-sila-difenidol > pirenzepine approximately p-fluoro-hexahydro-sila-difenidol approximately himbacine approximately AF-DX 384 approximately AQ-RA 741 > (S)-hexahydro-difenidol) to attenuate vasodilation to APE in rat kidney, correlated significantly with affinities at M3 receptors in submucosal arterioles and in smooth muscle of the guinea-pig ileum, but differed from those at M1 and M2 receptors in rabbit vas deferens. The agonist and antagonist potencies suggest that vasodilation elicited by muscarinic stimuli in endothelium-intact rat renal vasculature is mediated by functional muscarinic M3 receptors.

M2 muscarinic receptor-mediated inhibition of the Ca2+ current in rat magnocellular cholinergic basal forebrain neurones

1. The actions of muscarinic agonists and antagonists upon the Ca2+ current (ICa) in acutely dissociated magnocellular cholinergic basal forebrain neurones from 11 to 14-day-old postnatal rats were studied using the whole-cell patch-clamp technique. 2. In all cells studied, muscarinic agonists inhibited a transient component of high-voltage-activated (HVA) current, but had no effect upon the low-voltage-activated (LVA) current. The mean IC50 values for ACh and oxotremorine methiodide (oxo-M), obtained from non-cumulative dose-response curves, were 204 and 363 nM respectively. Superfusion with the K+ channel blocker, tetraethylammonium chloride (TEA; 30 mM) shifted the ACh dose-response curve to the right giving an IC50 value of 22:9 microM. 3. Pirenzepine (0.1-1 microM) and methoctramine (0.03-0.3 microM) produced parallel shifts to the right in the agonist dose-response curves. Schild analysis of the agonist dose ratios yielded pA2 (negative log of the apparent dissociation constant KB) values for pirenzepine and methoctramine of 6.8 and 8.2 respectively, indicating the involvement of an M2 receptor subtype. 4. In the presence of GTP-gamma-S (10-500 microM) in the patch pipette, the agonist-induced inhibition of ICa became irreversible. Dialysis with GDP-beta-S (1 mM) abolished all agonist-induced inhibition of the Ca2+ current. The agonist-induced inhibition of ICa was totally blocked by pretreatment with pertussis toxin (500 ng ml-1) but unaffected by preincubation with cholera toxin (500 ng ml-1). Superfusion with 8-bromo cAMP (0.5-1 mM) did not mimic or prevent the effect of agonist application. 5. Inhibition of the Ca2+ current by muscarinic agonists was only partially blocked by omega-conotoxin GVIA (omega-CgTX GVIA), with approximately 46% of the agonist-sensitive current being resistant to omega-CgTX GVIA. Both the agonist- and omega-CgTX GVIA-sensitive components of the current were abolished following maximal inhibition of ICa by GTP-gamma-S. 6. These results indicate that inhibition of the Ca2+ current by muscarinic agonists is mediated via an M2 muscarinic receptor coupled to a pertussis toxin-sensitive G-protein. The possible modulation of multiple HVA Ca2+ channels by muscarinic agonists and the role that these receptors may play in presynaptic modulation of neurotransmitter release are discussed.

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.

Interaction of selective compounds with muscarinic receptors at dispersed intestinal smooth muscle cells

1. The characterization of muscarinic receptors on single cells of the guinea-pig ileum longitudinal smooth muscle, devoid of neuronal elements, was functionally studied by estimating the affinities of muscarinic antagonists on acetylcholine-induced contractions. 2. Atropine (5 x 10(-11) to 5 x 10(-6) M), 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP, 5 x 10(-8) to 5 x 10(-6) M), cyclohexyl(4-fluoro-phenyl) (3-piperidinopropyl) silanol (pFHHSiD, 5 x 10(-7) to 5 x 10(-5) M) as well as pirenzepine (5 x 10(-7) to 5 x 10(-5) M) competitively antagonized the acetylcholine-dependent contractions with different affinities (atropine > 4-DAMP > pFHHSiD > pirenzepine). 3. Methoctramine (5 x 10(-7) to 5 x 10(-5) M), and AF-DX 116 (5 x 10(-6) and 5 x 10(-5) M) also showed antagonist properties but these deviated from simple competition. These compounds, which discriminate between M2 and M3 receptors, showed a potency lower than that of pirenzepine, the rank order of potencies being pirenzepine > methoctramine > AF-DX 116. When concentrations of AF-DX 116, methoctramine and pirenzepine were increased an unspecific contractile effect occurred. 4. McN-A-343, a partial agonist on intact guinea-pig longitudinal smooth muscle strips, on this preparation induced a weak contraction (about 7% in comparison to control) that was not reversed by antimuscarinic agents. 5. These data indicate that M3 rather than M2 receptor sites are present on this tissue.

Quest for agonist and antagonist selectivity at muscarinic receptors in guinea-pig smooth muscles and cardiac atria

Potencies of 11 muscarinic agonists in eliciting contraction of smooth muscle in guinea-pig ileum, trachea, urinary bladder and uterus and in inhibiting the rate of contractions of cardiac atria were compared. While acetylcholine (ACh) was the most potent agonist on the ileum, uterus and cardiac atria, cis-L(+)-dioxolane was equally as potent as ACh on the ileum and more potent on the urinary bladder and trachea. Compared to ACh, methylfurmethide, oxotremorine, acetoxybut-2-inyl-trimethylammonium and cis-L(+)-dioxolane acted weakly on the atria. Aceclidine, arecoline and acetyl-beta-methylcholine displayed selectivity for the urinary bladder and pilocarpine for the tracheal and urinary bladder smooth muscles. Oxotremorine had very low activity on the uterus. The stereoselectivity of muscarinic ACh receptors (mAChRs) for cis-L(+)-and cis-D(-)-dioxolane was low in the urinary bladder and uterus and high in the ileum and trachea. Most antagonists showed little selectivity between different organs, but S(-)-phenylcyclohexylglycoloyl choline was 6 times more active on the urinary bladder than on the ileum and AF-DX 116 was 12-30 times more active on the atria than on the smooth muscles. Among the N-alkyl derivatives of benzilylcholine, the octyl derivative as 400 times more active on the ileum than on the atria, while among the N-alkyl derivatives of QNB, the N-decyl derivative was 41 times more active on the ileum. The observed differences in the potency of various agonists and their stereoisomers on different smooth muscles cannot be explained by differences in the accessibility of receptors or in receptor reserve.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereoselective interaction of procyclidine, hexahydro-difenidol, hexbutinol and oxyphencyclimine, and of related antagonists, with four muscarinic receptors

We investigated the binding properties of the (R)- and (S)-enantiomers of the muscarinic antagonists trihexyphenidyl, procyclidine, hexahydro-difenidol, p-fluoro-hexahydro-difenidol, hexbutinol, p-fluoro-hexbutinol, and their corresponding methiodides at muscarinic M1, M2, M3 and M4 receptor subtypes. In addition, binding properties of the (R)- and (S)-enantiomers of oxyphencyclimine were studied. The (R)- enantiomers (eutomers) of all the compounds had a greater affinity than the (S)-isomers for the four muscarinic receptor subtypes. The binding patterns of the (R)- and (S)-enantiomers were generally different. We did not observe any general correlation between the potency of the high-affinity enantiomer and the affinity ratio (eudismic ratio) of the two enantiomers. The results are discussed in terms of a 'four subsites' binding model.

Stereoisomerism and muscarinic receptor agonists: synthesis and effects of the stereoisomers of 3-[5-(3-amino-1,2,4-oxadiazol)yl]-1- azabicyclo[2.2.1]heptane

The preparation and the biological activities of the four stereoisomers of 3-[5-(3-amino-1,2,4-oxadiazol)yl]-1-azabicyclo[2.2.1]heptane are described. The most potent stereoisomer, 3a, has the 3R,4R configuration, and in vitro activities in (pD2(% efficacy): ileum 8.8 (87%), hippocampus 9.8 (116%) and ganglion 10.2 (36%)). 3b (3S,4S) was weaker (ileum 8.1 (121%), hippocampus 8.5 (107%), ganglion 9.0 (63%)). The other two stereoisomers, 4a (3S,4R; ileum 7.1 (108%), hippocampus 8.2 (116%), ganglion 7.3 (31%)) and 4b (3R,4S; ileum 7.0 (100%), hippocampus 7.0 (120%), ganglion 7.2 (67%)) are of comparable activity, with an analogous profile to that of the more potent stereoisomers. Thus, compounds 3a and 4a, possessing the 4R stereochemistry, showed selectivity for the hippocampus over the ileum. Compound 3a was, however, more potent in the ganglion than in the hippocampus. All four stereoisomers were full agonists in the hippocampus, indicating M1 activity; however, they were partial agonists in the depolarisation of the rat superior cervical ganglion, another M1-mediated response. This may be due to M2-mediated hyperpolarization. With 3a (0.01 mg/kg i.p.), expression of c-fos mRNA was observed in the hypothalamus and in brain areas involved in sensory processing; these effects were totally blocked by pretreatment with 2 mg/kg scopolamine. In particular, activation of the superior colliculus is consistent with potent M2 activity.

Muscarinic M2-selective ligands also recognize M4 receptors in the rat brain: evidence from combined in situ hybridization and receptor autoradiography

We have used autoradiographic techniques to examine the characteristics and distribution of the binding of reported selective M2 muscarinic ligands and compared them with the distribution of cells expressing mRNAs for the different subtypes of muscarinic receptors. Our results suggest that the M2 ligands used in the present study ([3H]OXO-M, ([3H]OXO-M,[3H]AF-DX384,AF-DX116, methoctramine) also recognize M4 receptors present in regions such as the striatum and olfactory tubercle. This is supported by 1) relative abundances of the different transcripts, with m2 mRNA being very scarce and m4 mRNA very abundant in these regions; 2) comparison of the pharmacological characteristics of M2-ligand binding sites in brain areas selected by their exclusive expression of M2 receptors versus areas enriched in M4 receptors. An important conclusion of these studies is that none of the muscarinic radioligands available at the present time appears to label specifically a single muscarinic receptor subtype population. Areas are suggested where autoradiographic techniques can be helpful in elucidating the subtype selectivity of existing and new ligands.

The potential for muscarinic receptor subtype-specific pharmacotherapy for Alzheimer's disease

In several neurodegenerative disorders, including Alzheimer's disease, a loss of the cholinergic projections of the basal forebrain to the cerebral cortex and hippocampus occurs. Studies of the anatomic and physiologic characteristics of these ascending cholinergic systems suggest that they are important in processing information and in memory function. Muscarinic receptors are situated at various critical control points in these pathways. Activation of postsynaptic muscarinic receptors often increases the excitability of neurons; thus, the signal-to-noise ratio for sensory processing is enhanced. In addition, muscarinic receptors negatively control cholinergic tone at presynaptic sites. Molecular biologic methods have disclosed the existence of five muscarinic receptors, which are coupled to different second messenger systems. The evidence reviewed suggests that at least four of the five muscarinic receptor genes are expressed as functional receptor proteins in the neocortex and hippocampal formation. On the basis of the current information about their pharmacologic properties and coupling mechanisms in nervous tissue, drugs that selectively affect subtypes of muscarinic receptors could enhance cortical cholinergic function and thereby ameliorate certain cognitive impairments in Alzheimer's disease.

Idaverine, an M2- vs. M3-selective muscarinic antagonist, does not prevent motion sickness in cats

In this study, the affinity profile of idaverine for the M1- (neuronal tissue), M2- (heart) and M3- (glandular tissue/nonvascular smooth muscle) muscarinic receptors was examined by means of radioligand binding and in vitro organ bath experiments in order to use the compound for the investigation of the muscarinic receptor subtype involved in motion sickness. In the profile study a comparison was made with the muscarinic antagonists atropine, pirenzepine (M1-selective) AF-DX 116 (M2-selective) and 4-DAMP (high affinity for M1- and M3-binding sites). The affinity of idaverine appeared to be equally high for the M1- and M2-binding sites. However, the affinity for the M1-binding sites should be interpreted cautiously since the Hill slope deviated from unity. Idaverine showed a 20-fold selectivity for the M2-binding sites over the M3-binding sites, whereas it showed a small selectivity (less than 5-fold) for the M2-receptors compared to the ileal and tracheal smooth muscle receptors. Thus idaverine appears to be M2 over M3 selective. However, in contrast to AF-DX 116, it is not clear whether idaverine is also M2 over M1 selective. In experiments with cats, idaverine failed to prevent motion sickness at doses from 0.03 to 3 mg/kg. These results are interpreted to implicate M3-receptors in the motion sickness suppressant effect of antimuscarinic drugs.

Pharmacological properties of cloned muscarinic receptors expressed in A9 L cells; comparison with in vitro models

The effects of a series of muscarinic agonists and antagonists at cloned m1 and m3 muscarinic receptors expressed in mouse fibroblast A9 L cells have been compared with their effects in in vitro models of M1 (rat superior cervical ganglion) and M3 (guinea-pig ileum) muscarinic receptors. A good correlation existed between the potencies of muscarinic agonists at cloned m1 muscarinic receptors and the M1 sites in rat ganglion (r = 0.80) as well as at cloned m3 receptors and guinea-pig ileum M3 receptors (r = 0.87). However, cross correlations of potencies in rat ganglion and cloned m3 receptors as well as in guinea-pig ileum and in cloned m1 receptors also yielded relatively high correlation coefficients (0.71 and 0.91, respectively). Low correlation coefficients were found for the maximal responses of muscarinic agonists in rat ganglion and cloned m1 receptors (0.53) and in guinea-pig ileum and cloned m3 receptors (0.36). A high correlation between pA2 values of muscarinic antagonists at cloned m1 receptors and in rat ganglion (r = 0.97) and between cloned m3 receptors and guinea-pig ileum (r = 0.98) was found. Cross correlation of pA2 values in rat ganglion and cloned m3 receptors and in guinea-pig ileum and cloned m1 receptors yielded correlation coefficients of 0.82 and 0.72, respectively. The data indicate that the cloned muscarinic receptor sites seem similar to the corresponding endogenous sites. The good correlations in corresponding but also non-corresponding receptor models reflect the relatively low selectivity of the majority of the compounds investigated.

Functional characterization of the muscarinic receptor in rat lungs

The effects of various muscarinic antagonists on antigen- and acetylcholine-induced bronchoconstriction were studied. In isolated and ventilated lungs of naive rats, the pA2 values with respect to acetylcholine-induced bronchoconstriction were 9.01 (atropine), 8.39 (ipratropium bromide), 7.39 (pirenzepine), 5.94 (AF-DX 116, a M2-selective muscarinic antagonist), 6.91 (UH-AH 37, a novel muscarinic antagonist) and 9.37 (4-DAMP: 4-diphenylacetoxy-N-methylpiperidine methobromide). Except for ipratropium bromide, the slopes of the Schild plots were not significantly different from unity. None of the drugs were potent or effective in inhibiting bronchoconstriction or histamine release evoked by antigen challenge in actively sensitized rats. However, in vivo, in anesthetized spontaneously breathing rats, vagotomy and atropine (1 mg/kg) did reduce antigen-induced bronchoconstriction. It is concluded that functional muscarinic receptors in isolated rat lungs are probably of the M3 receptor subtype. With respect to antigen-induced bronchoconstriction and mediator release in a denervated model such as the isolated lung, they are of little, if any, importance. In vivo, vagotomy and atropine reduced antigen-induced bronchoconstriction, probably by blockade of a vagal reflex which is thought to play a role in antigen-evoked bronchoconstriction.

M3 cholinoceptors and P2y purinoceptors mediating relaxation of arteries in spontaneously hypertensive rats at prehypertensive stages

Muscarinic cholinoceptor and purinoceptor subtypes were determined in femoral and mesenteric artery strips from spontaneously hypertensive rats (SHR) at prehypertensive stages. Acetylcholine (ACh), carbachol and oxotremorine induced endothelium-dependent relaxations during contractions evoked by 5-hydroxytryptamine. The order of relative potency of the agonists was ACh greater than carbachol = oxotremorine in the prehypertensive SHR and Wistar Kyoto rats (WKY). Schild plot data for muscarinic cholinoceptor antagonists obtained with carbachol or oxotremorine as agonist indicated that the arteries of prehypertensive SHR and WKY possessed M3 cholinoceptors mediating relaxation. The relaxation responses of the prehypertensive SHR and WKY arteries to ATP were inhibited by reactive blue 2 but not altered after P2x purinoceptors had been desensitized by alpha,beta-methylene ATP, which suggests that the arteries possessed P2y purinoceptors mediating relaxation. The responses of the femoral and mesenteric arteries from prehypertensive SHR to ACh, carbachol and ATP were similar to those of WKY arteries, although the response of the prehypertensive SHR artery to oxotremorine was significantly increased compared to that of the WKY artery. The results suggest that M3 cholinoceptor- and P2y purinoceptor-mediated relaxations are not changed in prehypertensive SHR arteries as compared to those in WKY arteries.

Differential response of striatal dopamine and muscarinic cholinergic receptor subtypes to the loss of dopamine. I. Effects of intranigral or intracerebroventricular 6-hydroxydopamine lesions of the mesostriatal dopamine system

Quantitative autoradiography was utilized to examine the response of the dopamine (DA) and muscarinic cholinergic system within the striatum to lesions of the mesostriatal DA system following intranigral 6-hydroxydopamine (6-OHDA) injections. In addition, the response of DA system was examined in the striatum of animals treated with low, medium, or high doses of 6-OHDA made intracerebroventricularly (icv). Three weeks following removal of the mesostriatal DA fibers with intranigral 6-OHDA, there was an almost complete depletion of DA and [3H]mazindol binding throughout the striatum. The resulting increase in D2 receptors labeled with [3H]spiroperidol (27%) was most evident in the lateral striatum and topographically correlated with an increase in choline uptake sites labeled with [3H]hemicholinium-3 (20%). There was a smaller but significant decrease in D1 receptors labeled with [3H]SCH 23390 (15-18%) that was not topographically related to changes in [3H]spiroperidol or [3H]hemicholinium-3 binding. All doses of icv 6-OHDA produced a significant loss of DA and of [3H]mazindol binding as compared to vehicle injections that was more pronounced in the medial than in the lateral striatum. No increase in D1 receptors was observed with any dose of 6-OHDA and greater than 90% loss of DA and [3H]mazindol resulted in an increase in D2 receptors in the lateral striatum and a reduction in D1 receptors in the dorsal striatum. These data are consistent with the evidence that there is independent regulation of the two subtypes of the DA receptor. Moreover, the distribution and regulation of the subtypes of the muscarinic receptor were independent. Muscarinic M2 receptors ([3H]N-methylscopolamine in presence of excess pirenzepine) showed a lateral to medial gradient (highest laterally) that was related to the pattern of choline uptake sites and D2 receptors. Loss of DA resulted in a reduction in M2 receptors (24-30%) that was correlated with the increase in choline uptake sites. In contrast, M1 ([3H]pirenzepine) receptors showed a reverse gradient from the M2 receptor and a smaller reduction following loss of DA.