The relative selectivity of anticholinergic drugs for the M1 and M2 muscarinic receptor subtypes

Ultra-high throughput-based screening for the discovery of antiplatelet drugs affecting receptor dependent calcium signaling dynamics

Distinct platelet activation patterns are elicited by the tyrosine kinase-linked collagen receptor glycoprotein VI (GPVI) and the G-protein coupled protease-activated receptors (PAR1/4) for thrombin. This is reflected in the different platelet Ca2+ responses induced by the GPVI agonist collagen-related peptide (CRP) and the PAR1/4 agonist thrombin. Using a 96 well-plate assay with human Calcium-6-loaded platelets and a panel of 22 pharmacological inhibitors, we assessed the cytosolic Ca2+ signaling domains of these receptors and developed an automated Ca2+ curve algorithm. The algorithm was used to evaluate an ultra-high throughput (UHT) based screening of 16,635 chemically diverse small molecules with orally active physicochemical properties for effects on platelets stimulated with CRP or thrombin. Stringent agonist-specific selection criteria resulted in the identification of 151 drug-like molecules, of which three hit compounds were further characterized. The dibenzyl formamide derivative ANO61 selectively modulated thrombin-induced Ca2+ responses, whereas the aromatic sulfonyl imidazole AF299 and the phenothiazine ethopropazine affected CRP-induced responses. Platelet functional assays confirmed selectivity of these hits. Ethopropazine retained its inhibitory potential in the presence of plasma, and suppressed collagen-dependent thrombus buildup at arterial shear rate. In conclusion, targeting of platelet Ca2+ signaling dynamics in a screening campaign has the potential of identifying novel platelet-inhibiting molecules.

Unexpected scaffold rearrangement product of pirenzepine found in commercial samples

Pharmacovigilance aims at a better understanding of the molecular events triggered by medications to prevent adverse effects, which despite significant advances in our analytical repertoire plague the use of drugs until today. In this study, we find that clinically prescribed and commercially available pirenzepine may not be the correct compound. Pirenzepine can undergo an unexpected scaffold rearrangement from the pharmaceutical active ingredient (API) to a previously uncharacterized benzimidazole. The rearrangement occurs under highly acidic conditions, which were believed to favour the dihydrochloride formation of pirenzepine. The rearranged products of pirenzepine and the structurally related telenzepine have significantly decreased affinity for the muscarinic acetylcholine receptor, the pharmacological target of these compounds. Fortunately, in situ rearrangement after oral application is no safety issue, as we show that reaction kinetics in gastric acid prevent rearrangement. The research community should consider appropriate measures to perform reliable receiving inspections in the commercial supply of well described and frequently used chemicals, in particular if experiments yield unexpected results.

Repurposing Cholinesterase Inhibitors as Antidepressants? Dose and Stress-Sensitivity May Be Critical to Opening Possibilities

When stress becomes chronic it can trigger lasting brain and behavioral changes including Major Depressive Disorder (MDD). There is conflicting evidence regarding whether acetylcholinesterase inhibitors (AChEIs) may have antidepressant properties. In a recent publication, we demonstrated a strong dose-dependency of the effect of AChEIs on antidepressant-related behavior in the mouse forced swim test: whereas the AChEI donepezil indeed promotes depression-like behavior at a high dose, it has antidepressant-like properties at lower doses in the same experiment. Our data therefore suggest a Janus-faced dose-response curve for donepezil in depression-related behavior. In this review, we investigate the mood-related properties of AChEIs in greater detail, focusing on both human and rodent studies. In fact, while there have been many studies showing pro-depressant activity by AChEIs and this is a major concept in the field, a variety of other studies in both humans and rodents show antidepressant effects. Our study was one of the first to systematically vary dose to include very low concentrations while measuring behavioral effects, potentially explaining the apparent disparate findings in the field. The possibility of antidepressant roles for AChEIs in rodents may provide hope for new depression treatments. Importantly, MDD is a psychosocial stress-linked disorder, and in rodents, stress is a major experimental manipulation for studying depression mechanisms, so an important future direction will be to determine the extent to which these depression-related effects are stress-sensitive. In sum, gaining a greater understanding of the potentially therapeutic mood-related effects of low dose AChEIs, both in rodent models and in human subjects, should be a prioritized topic in ongoing translational research.

Contribution of M1 and M2 muscarinic receptor subtypes to convulsions in fasted mice treated with scopolamine and given food

Treatment of fasted mice and rats with the nonselective muscarinic antagonist, scopolamine or atropine, causes convulsions after food intake. This study evaluated the effect of fasting on the expression of M₁ and M₂ muscarinic receptors in the brain regions, the relationship between receptor expression and seizure stages, and the muscarinic receptor subtype which plays a role in the occurrence of convulsions. Mice were grouped as allowed to eat ad lib (fed) and deprived of food for 24h (fasted). Fasted animals developed convulsions after being treated with scopolamine (60%) or the selective M₁ receptor antagonist pirenzepine (10mg/kg; 20% and 60mg/kg; 70%) and given food. Fasting increased expression of M₁ receptors in the frontal cortex and M₂ receptors in the hippocampus, but produced no change in the expression of both receptors in the amygdaloid complex. Food intake after fasting decreased M₁ receptor expression in the frontal cortex and M₁ and M₂ receptor expression in the hippocampus. Seizure severity was uncorrelated with muscarinic receptor expression in the brain regions. Taken together, these findings provide evidence for the role of M₁ muscarinic receptor antagonism and fasting-induced increases in M₁ and M₂ expression possible underlying mechanism in the occurrence of convulsions in fasted animals.

Cholinergic modulation of spatial learning, memory and navigation

Spatial learning, including encoding and retrieval of spatial memories as well as holding spatial information in working memory generally serving navigation under a broad range of circumstances, relies on a network of structures. While central to this network are medial temporal lobe structures with a widely appreciated crucial function of the hippocampus, neocortical areas such as the posterior parietal cortex and the retrosplenial cortex also play essential roles. Since the hippocampus receives its main subcortical input from the medial septum of the basal forebrain (BF) cholinergic system, it is not surprising that the potential role of the septo-hippocampal pathway in spatial navigation has been investigated in many studies. Much less is known of the involvement in spatial cognition of the parallel projection system linking the posterior BF with neocortical areas. Here we review the current state of the art of the division of labour within this complex 'navigation system', with special focus on how subcortical cholinergic inputs may regulate various aspects of spatial learning, memory and navigation.

Convergent Mechanisms Underlying Rapid Antidepressant Action

Traditional pharmacological treatments for depression have a delayed therapeutic onset, ranging from several weeks to months, and there is a high percentage of individuals who never respond to treatment. In contrast, ketamine produces rapid-onset antidepressant, anti-suicidal, and anti-anhedonic actions following a single administration to patients with depression. Proposed mechanisms of the antidepressant action of ketamine include N-methyl-D-aspartate receptor (NMDAR) modulation, gamma aminobutyric acid (GABA)-ergic interneuron disinhibition, and direct actions of its hydroxynorketamine (HNK) metabolites. Downstream actions include activation of the mechanistic target of rapamycin (mTOR), deactivation of glycogen synthase kinase-3 and eukaryotic elongation factor 2 (eEF2), enhanced brain-derived neurotrophic factor (BDNF) signaling, and activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs). These putative mechanisms of ketamine action are not mutually exclusive and may complement each other to induce potentiation of excitatory synapses in affective-regulating brain circuits, which results in amelioration of depression symptoms. We review these proposed mechanisms of ketamine action in the context of how such mechanisms are informing the development of novel putative rapid-acting antidepressant drugs. Such drugs that have undergone pre-clinical, and in some cases clinical, testing include the muscarinic acetylcholine receptor antagonist scopolamine, GluN2B-NMDAR antagonists (i.e., CP-101,606, MK-0657), (2R,6R)-HNK, NMDAR glycine site modulators (i.e., 4-chlorokynurenine, pro-drug of the glycineB NMDAR antagonist 7-chlorokynurenic acid), NMDAR agonists [i.e., GLYX-13 (rapastinel)], metabotropic glutamate receptor 2/3 (mGluR2/3) antagonists, GABAA receptor modulators, and drugs acting on various serotonin receptor subtypes. These ongoing studies suggest that the future acute treatment of depression will typically occur within hours, rather than months, of treatment initiation.

Hydroxycinnamic acid amides from Scopolia tangutica inhibit the activity of M1 muscarinic acetylcholine receptor in vitro

Scopolia tangutica Maxim (S. tangutica) extracts have been traditionally used as antispasmodic, sedative, and analgesic agents in Tibet and in the Qinghai province of China. Their active compositions are however poorly understood. We have recently isolated five new hydroxycinnamic acid (HCA) amides along with two known HCA amides, one cinnamic acid amide from these extracts. In this study, we evaluate their abilities to inhibit carbacol-induced activity of M1 muscarinic acetylcholine receptor along with the crude extracts. Chinese hamster ovary cells stably expressing the recombinant human M1 receptor (CHO-M1 cells) were employed to evaluate the anticholinergic potentials. Intracellular Ca(2+) changes were monitored using the FLIPR system. Five HCA amides as well as the crude S. tangutica extract displayed dose-dependent inhibitory effects against M1 receptor. These findings demonstrate that HCA amides are part of the M1 receptor-inhibiting principles of S. tangutica. Since blockade of parasympathetic nerve impulse transmission through the inhibition of the M1 receptor lessens smooth muscle spasms, our findings provided a molecular explanation for the traditional use of S. tangutica against spasm.

Functional activation of G-proteins coupled with muscarinic acetylcholine receptors in rat brain membranes

The functional activation of Gi/o proteins coupled to muscarinic acetylcholine receptors (mAChRs) was investigated with the conventional guanosine-5'-O-(3-[(35)S]thio) triphosphate ([(35)S]GTPγS) binding assay in rat brain membranes. The most efficacious stimulation elicited by acetylcholine or carbachol (CCh) was obtained in striatal membranes. The pharmacological properties of mAChR-mediated [(35)S]GTPγS binding determined with a series of muscarinic agonists and antagonists were almost identical among the three brain regions investigated, i.e., cerebral cortex, hippocampus, and striatum, except for the apparent partial agonist effects of (αR)-α-cyclopentyl-α-hydroxy-N-[1-(4-methyl-3-pentenyl)-4-piperidinyl]benzeneacetamide fumarate (J 104129) observed only in the hippocampus, but not in the other two regions. Among the muscarinic toxins investigated, only MT3 attenuated CCh-stimulated [(35)S] GTPγS binding. The highly selective allosteric potentiator at the M4 mAChR subtype, 3-amino-N-[(4-chlorophenyl)methyl]-4,6-dimethylthieno[2,3-b]pyridine-2-carboxamide (VU 10010), shifted the concentration-response curve for CCh leftwards as well as upwards. On the other hand, neither thiochrome nor brucine N-oxide was effective. The increases induced by CCh and 5-HT were essentially additive, though not completely, indicating that the mAChRs and 5-HT1A receptors were coupled independently to distinct pools of Gi/o proteins. Collectively, all of the data suggest that functional activation of Gi/o proteins coupled to mAChRs, especially the M4 subtype, is detectable by means of CCh-stimulated [(35)S]GTPγS binding assay in rat discrete brain regions.

Synthesis and SAR of N-(4-(4-alklylpiperazin-1-yl)phenyl)benzamides as muscarinic acetylcholine receptor subtype 1 (M1) anatgonists

This Letter describes the synthesis and SAR, developed through an iterative analog library approach, of a novel series of selective M(1) mAChR antagonists, based on an N-(4-(4-alkylpiperazin-1-yl)phenyl)benzamide scaffold for the potential treatment of Parkinson's disease, dystonia and other movement disorders. Compounds in this series possess M(1) antagonist IC(50)s in the 350 nM to >10 microM range with varying degrees of functional selectivity versus M(2)-M(5).

TorsinA and dystonia: from nuclear envelope to synapse

A GAG deletion in the DYT1 gene is responsible for the autosomal dominant movement disorder, early onset primary torsion dystonia, which is characterised by involuntary sustained muscle contractions and abnormal posturing of the limbs. The mutation leads to deletion of a single glutamate residue in the C-terminus of the protein torsinA, a member of the AAA+ ATPase family of proteins with multiple functions. Since no evidence of neurodegeneration has been found in DYT1 patients, the dystonic phenotype is likely to be the result of neuronal functional defect(s), the nature of which is only partially understood. Biochemical, structural and cell biological studies have been performed in order to characterise torsinA. These studies, together with the generation of several animal models, have contributed to identify cellular compartments and pathways, including the cytoskeleton and the nuclear envelope, the secretory pathway and the synaptic vesicle machinery where torsinA function may be crucial. However, the role of torsinA and the correlation between the dysfunction caused by the mutation and the dystonic phenotype remain unclear. This review provides an overview of the findings of the last ten years of research on torsinA, a critical evaluation of the different models proposed and insights towards future avenues of research.

Synthesis and SAR of selective muscarinic acetylcholine receptor subtype 1 (M1 mAChR) antagonists

This Letter describes the synthesis and SAR, developed through an iterative analogue library approach, of a novel series of selective M1 mAChR antagonists for the potential treatment of Parkinson's disease, dystonia and other movement disorders. Compounds in this series possess M1 antagonist IC(50)s in the 441nM-19microM range with 8- to >340-fold functional selectivity versus rM2-rM5.

The evaluation of antimuscarinic-induced convulsions in fasted rats after food intake

The present study was performed to evaluate convulsions after food intake in fasted rats pretreated with scopolamine or atropine and to determine whether these convulsions respond to drugs found effective in fasted mice. Scopolamine (2.4 mg/kg) and atropine (2.4 mg/kg) were given intraperitoneally (i.p.) to rats fasted for 52h. Both drugs induced convulsions after animals were allowed to eat ad lib. Another group of fasted rats pretreated with saline, MK-801 (0.1mg/kg), clonidine (0.1mg/kg), chlorpromazine (2 and 4 mg/kg), valproate (200mg/kg), diazepam (1.5 and 2mg/kg) or gabapentin (50mg/kg) were treated i.p. with saline or scopolamine (2.4 mg/kg) and were allowed to eat ad lib. Clonidine, MK-801, chlorpromazine (4 mg/kg) and diazepam (2 mg/kg) reduced the incidence of scopolamine-induced convulsions in fasted rats. Gabapentin could only prolong the onset of convulsions. Neither treatment was effective against myoclonus of hindlimbs. Present results showed that fasted rats also develop antimuscarinic-induced convulsions which do not completely respond to treatments found effective in convulsions of fasted mice.

Modulatory action of metabotropic glutamate receptor (mGluR) 5 on mGluR1 function in striatal cholinergic interneurons

Within basal ganglia, group I metabotropic glutamate receptor subtypes (mGluR1 and 5) frequently co-localize in the same neuron. However, little is known about how these receptors functionally interact. We addressed this issue by means of electrophysiological recordings of striatal cholinergic interneurons, a neuronal subtype that co-express both group I mGluRs. The group I non-selective agonist 3,5-DHPG induced a membrane depolarization/inward current that was prevented by co-application of LY 367385, a selective mGluR1 antagonist, and SIB 1757 or MPEP, blockers of mGluR5 subtype. The reversal potential for the response to 3,5-DHPG was close to the equilibrium potential for potassium channels. Repeated bath or focal applications of 3,5-DHPG induced a progressive decline in the amplitude of the membrane depolarization, suggesting that group I mGluRs undergo receptor desensitization. Interestingly, in the presence of the mGluR5 blocker, SIB 1757, this event was not observed, whereas it occurred in LY 367385. PKC blockers chelerythrine and calphostin C mimicked the inhibitory effect of SIB 1757. In a subset of interneurons, in MPEP or SIB 1757, 3,5-DHPG induced a 0.5-1 Hz oscillatory response, that was prevented by L-type Ca2+ channel blockers, and by the tyrosine kinase inhibitors genistein and lavendustin. Together, these data suggest that mGluR5 modulates mGluR1 activity to shape cell excitability.

TorsinA protein and neuropathology in early onset generalized dystonia with GAG deletion

Familial, early onset, generalized torsion dystonia is the most common and severe primary dystonia. Most cases are caused by a 3-bp deletion (GAG) in the coding region of the TOR1A (DYT1) gene, which is widely expressed in human brain and encodes the protein torsinA. This study compares neuropathology and torsinA expression in the normal human brain with that in dystonia cases with and without the GAG deletion. TorsinA-like protein was expressed in neuronal cytoplasm throughout the human brain, including cerebellum, substantia nigra, hippocampus, and neostriatum, with higher levels in specific neurons. This immunostaining pattern was not discernibly different in dystonia and normal brains in midbrain and neostriatal regions. However, nigral dopaminergic neurons appeared to be larger in both GAG-deletion and non-GAG-deletion dystonia brains compared to normal, and may be more closely spaced in GAG-deletion brains. Beyond these apparent changes in neuronal size and spacing in dystonia brains, there was no indication of neuron loss, inflammation, DNA strand breaks, or altered distribution of torsin-like immunoreactivity, supporting a functional rather than degenerative etiology of early onset torsion dystonia.

Toxicity of the specific antimuscarinic agent methoctramine and other non-specific anticholinergic drugs in human neuroblastoma cell lines in vitro

The highly selective cardiac-M(2) muscarinic acetylcholine receptor (mAChR) antagonist methoctramine shows a number of concentration-dependent biochemical responses. At micromolar concentrations it interacts allosterically with the mAChR and has 'agonist-like' effects on the phosphoinositide and cyclic AMP second messenger systems. Direct stimulation or inhibition of second messenger systems has been reported to modulate cellular homoeostasis and differentiation. This study showed that methoctramine was toxic, in micromolar concentrations, to the human neuroblastoma cell lines SK-N-SH, LAN-5 and SH-EP1, the last being a clone that does not contain muscarinic receptors. The selective M2 mAChR antagonists 11-{2-[(diethylamino)methyl]-1-piperidinyl}-5,11-dihydro-6H-pyrido(2,3-6)(1-4)benzodiazepine-6-on (AF-DX 116) and gallamine, as well as the selective M1 and M3 antagonists pirenzepine and 4-diphenylacetoxy-n-methylpiperidine (4-DAMP), had no toxic effects. Lithium provided significant protection against methoctramine toxicity, whereas carbamylcholine, pertussis toxin and forskolin had no influence on its toxicity. At micromolar concentrations, the clinically used, non-selective mAChR antagonists ethopropazine, benztropine, trihexyphenidyl and orphenadrine displayed toxicity similar to that of methoctramine. Methoctramine, ethopropazine, benztropine and trihexyphenidyl enhanced significantly [(3)H]thymidine uptake at subtoxic concentrations. These results demonstrate that (a) the toxicity of methoctramine is by way of non-muscarinic mechanism, (b) some anticholinergic drugs commonly used in clinical medicine have toxic properties similar to those of methoctramine and (c) at subtoxic micromolar concentrations anti-muscarinic drugs have some trophic properties.

The effects of thyrotropin-releasing hormone and scopolamine in Alzheimer's disease and normal volunteers

Thyrotropin-releasing hormone (TRH), a neuromodulator and possibly a neurotransmitter in the central nervous system, was shown in a prior study of young normal volunteers to attenuate the memory impairment induced by the anticholinergic drug scopolamine. In the present study, the cognitive, behavioral and physiologic effects of high dose TRH (0.5 mg/kg), both alone and following administration of scopolamine, were examined in 10 Alzheimer's disease (AD) patients (mean age±SD=63.5 years) and 12 older normal volunteers (mean age=64.9±8.8 years). On the day AD subjects received TRH alone, modest but statistically significant improvement from baseline performance was documented on some tests of learning and memory, especially in those with mild dementia severity. In comparing cognitive test performance between the scopolamine alone and scopolamine+TRH conditions, only two test scores were significantly higher in the latter condition. In the group of older volunteers, TRH did not attenuate scopolamine-induced cognitive impairment, contrary to prior findings in a group of younger controls. In fact, older subjects performed worse after receiving scopolamine followed by TRH than after receiving scopolamine alone. In addition, no change from baseline cognitive performance was detected after subjects received TRH alone. These findings raise several questions and speculations on possible age-related changes in the cholinergic system, as well as on the mechanism of the interaction of TRH with the cholinergic system.

NMDA antagonist neurotoxicity: mechanism and prevention

Antagonists of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor, including phencyclidine (PCP) and ketamine, protect against brain damage in neurological disorders such as stroke. However, these agents have psychotomimetic properties in humans and morphologically damage neurons in the cerebral cortex of rats. It is now shown that the morphological damage can be prevented by certain anticholinergic drugs or by diazepam and barbiturates, which act at the gamma-aminobutyric acid (GABA) receptor-channel complex and are known to suppress the psychotomimetic symptoms caused by ketamine. Thus, it may be possible to prevent the unwanted side effects of NMDA antagonists, thereby enhancing their utility as neuroprotective drugs.

Anticonvulsant actions of anticholinergic drugs in soman poisoning

The acute effects of the organophosphorus cholinesterase inhibitor soman include hypersecretions, convulsions, and death. The purpose of this study was to evaluate the anticholinergic compounds aprophen, atropine sulfate, azaprophen, benactyzine, benztropine, biperiden, scopolamine HBr, and trihexyphenidyl for their efficacy in preventing soman-induced hypersecretions and convulsions. Male rats were injected with the oxime HI-6 (125 mg/kg, i.p.), to increase survival time, along with various intramuscular doses of the anticholinergics 30 min prior to a dose of soman (180 micrograms/kg, s.c.; equivalent to 1.6 x the median lethal dose) that produced 100% convulsions. Signs of intoxication as well as the time-to-onset of convulsions were observed. The calculated anticonvulsant median effective dose values were 0.18, 0.33, 0.36, 0.55, 2.17, 2.30, 2.45, and 31.09 mumol/kg for scopolamine HBr, biperiden, trihexyphenidyl, benactyzine, benztropine, azaprophen, aprophen, and atropine sulfate, respectively. The same rank order of potency for inhibition of hypersecretions among these compounds was observed. Parallel studies with quaternary analogs of atropine sulfate and scopolamine HBr demonstrated, however, that these charged compounds afford no protection against soman-induced hypersecretions and convulsions. The results indicate that tertiary anticholinergic compounds afford protection against soman-induced convulsions and hypersecretions and that the beneficial anticonvulsant effects are mediated through the central cholinergic system. Excitatory amino acid neurotransmitter systems may be involved in the effectiveness of these compounds.

Anticonvulsants for poisoning by the organophosphorus compound soman: pharmacological mechanisms

Exposure to high doses of organophosphorus nerve agents such as soman, even with carbamate pretreatment, produces a variety of toxic cholinergic signs, including secretions, convulsions and death. Evidence suggests that soman-induced convulsions may be associated with postexposure brain neuropathology. The purpose of this study was to investigate the pharmacologic mechanism of action of soman-induced convulsions and of anticonvulsant drugs. Various classes of compounds were evaluated for their efficacy in preventing soman-induced convulsions in rats pretreated with the oxime HI-6 to increase survival time, along with various doses of the test compounds (IM) either in the absence or presence of atropine sulfate (16 mg/kg, IM) 30 minutes prior to a soman challenge dose (180 micrograms/kg, SC; equivalent to 1.6 x LD50) that produced 100% convulsions. Without atropine sulfate, only tertiary anticholinergics (scopolamine, trihexyphenidyl, biperiden, benactyzine, benztropine, azaprophen and aprophen), caramiphen, carbetapentane and MK-801 were effective anticonvulsants. In the presence of atropine sulfate, the benzodiazepines (diazepam, midazolam, clonazepam, loprazolam and alprazolam), mecamylamine, flunarizine, diphenylhydantoin, clonidine, CGS 19755 and Organon 6370 studied were effective. We have examined the possibility that diazepam may exert some of its anticonvulsant effects through cholinergic mechanisms and found that a reduced release of ACh into synapses after diazepam and atropine treatment may account for diazepam's anticonvulsant activity against soman. We also found that at anticonvulsant doses biperiden and trihexyphenidyl each significantly reversed the effects of soman on striatal levels of DOPAC and HVA, the metabolites of dopamine, and have concluded that in addition to actions on muscarinic receptors, the anticonvulsant effects of these anticholinergics in soman poisoning may be partially related to their actions on the striatal dopaminergic system. These findings allow us to postulate that central muscarinic cholinergic mechanisms are primarily involved in eliciting the convulsions following exposure to soman and that subsequent recruitment of other excitatory neurotransmitter systems and loss of inhibitory control may be responsible for sustaining the convulsions and for producing the subsequent brain damage. Future studies to confirm these neuropharmacological mechanisms are proposed.

Affinity and selectivity of biperiden enantiomers for muscarinic receptor subtypes

The affinity of both the (+)- and the (-)-stereoisomer of biperiden for different muscarinic receptor subtypes was investigated in vitro in functional studies with field-stimulated rabbit vas deferens (M1-receptor), guinea-pig ileum (smooth muscle M2 beta-receptor) and rat left atrium (cardiac M2 alpha-receptor). (+)-Biperiden had its highest affinity to M1-receptors (pA2 = 9.07), had low affinity to cardiac M2 alpha-receptors (pA2 = 7.25) and intermediate affinity to ileal M2 beta-receptors (pA2 = 8.27). The ability of (+)-biperiden to discriminate between ileal M2 beta- and cardiac M2 alpha-receptors (factor = 10) was similar to that of 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, factor = 9). In contrast, (-)-biperiden displayed low but nearly undistinguishable affinity for all muscarinic receptor subtypes studied (pA2 = 5.59 +/- 6.38). (+)-Biperiden discriminated strongly between M1- and cardiac M2 alpha-receptors (factor 66), thus being even more selective than pirenzepine (factor 28) which makes it one of the most M1-/cardiac M2 alpha-selective antimuscarinic drugs now available. These results indicate that (+)-biperiden could represent a further valuable tool for the characterization of muscarinic receptor subtypes.