Muscarinic receptors (mAChRs) are typical members of the G protein-coupled receptor (GPCR) family and exist in five subtypes from M1 to M5. Muscarinic receptor subtypes do not sufficiently differ in affinity to orthosteric antagonists or agonists; therefore, the analysis of receptor subtypes is complicated, and misinterpretations can occur. Usually, when researchers mainly specialized in CNS and peripheral functions aim to study mAChR involvement in behavior, learning, spinal locomotor networks, biological rhythms, cardiovascular physiology, bronchoconstriction, gastrointestinal tract functions, schizophrenia, and Parkinson's disease, they use orthosteric ligands and they do not use allosteric ligands. Moreover, they usually rely on manufacturers' claims that could be misleading. This review aimed to call the attention of researchers not deeply focused on mAChR pharmacology to this fact. Importantly, limited selective binding is not only a property of mAChRs but is a general attribute of most neurotransmitter receptors. In this review, we want to give an overview of the most common off-targets for established mAChR ligands. In this context, an important point is a mention the tremendous knowledge gap on off-targets for novel compounds compared to very well-established ligands. Therefore, we will summarize reported affinities and give an outline of strategies to investigate the subtype's function, thereby avoiding ambiguous results. Despite that, the multitargeting nature of drugs acting also on mAChR could be an advantage when treating such diseases as schizophrenia. Antipsychotics are a perfect example of a multitargeting advantage in treatment. A promising strategy is the use of allosteric ligands, although some of these ligands have also been shown to exhibit limited selectivity. Another new direction in the development of muscarinic selective ligands is functionally selective and biased agonists. The possible selective ligands, usually allosteric, will also be listed. To overcome the limited selectivity of orthosteric ligands, the recommended process is to carefully examine the presence of respective subtypes in specific tissues via knockout studies, carefully apply "specific" agonists/antagonists at appropriate concentrations and then calculate the probability of a specific subtype involvement in specific functions. This could help interested researchers aiming to study the central nervous system functions mediated by the muscarinic receptor.

• Keywords
• allosteric, multitarget, muscarinic agonist, muscarinic antagonist, muscarinic receptors, orthosteric,
• Publication type
• Journal Article MeSH
• Review MeSH

Muscarinic receptors (MR) are main cardioinhibitory receptors. We investigated the changes in gene expression, receptor number, echocardiography, muscarinic/adrenergic agonist/antagonist changes in heart rate (HR) and HR biorhythm in M(2) KO mice (mice lacking the main cardioinhibitory receptors) in the left ventricle (LV) and right ventricle (RV). We hypothesize that the disruption of M(2) MR, key players in parasympathetic bradycardia, would change the number of receptors with antagonistic effects on the heart (β(1)- and β(2)-adrenoceptors, BAR), while the function of the heart would be changed only marginally. We have found changes in LV, but not in RV: decrease in M(3) MR, β(1)- and β(2)-adrenoceptor gene expressions that were accompanied by a decrease in MR and BAR receptor binding. No changes were found both in LV systolic and diastolic function as assessed by echocardiography (e.g., similar LV end-systolic and end-diastolic diameter, fractional shortening, mitral flow characteristics, and maximal velocity in LV outflow tract). We have found only marginal changes in specific HR biorhythm parameters. The effects of isoprenaline and propranolol on HR were similar in WT and KO (but with lesser extent). Atropine was not able to increase HR in KO animals. Carbachol decreased the HR in WT but increased HR in KO, suggesting the presence of cardiostimulatory MR. Therefore, we can conclude that although the main cardioinhibitory receptors are not present in the heart, the function is not much affected. As possible mechanisms of almost normal cardiac function, the decreases of both β(1)- and β(2)-adrenoceptor gene expression and receptor binding should be considered.

• MeSH
• Atropine pharmacology   MeSH
• Receptors, Adrenergic, beta-1 metabolism   MeSH
• Receptors, Adrenergic, beta-2 metabolism   MeSH
• Bradycardia physiopathology   MeSH
• Ventricular Function, Left physiology   MeSH
• Isoproterenol pharmacology   MeSH
• Carbachol pharmacology   MeSH
• Mice, Knockout MeSH
• Mice MeSH
• Propranolol pharmacology   MeSH
• Receptor, Muscarinic M2 genetics   MeSH
• Gene Expression Regulation * MeSH
• Heart Rate physiology   MeSH
• Heart Ventricles metabolism   MeSH
• Protein Binding MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Atropine MeSH
• Receptors, Adrenergic, beta-1 MeSH
• Receptors, Adrenergic, beta-2 MeSH
• Isoproterenol MeSH
• Carbachol MeSH
• Propranolol MeSH
• Receptor, Muscarinic M2 MeSH

We investigated the role of beta3-adrenoceptors (AR) in cold stress (1 or 7 days in cold) in animals lacking main cardioinhibitive receptors-M2 muscarinic receptors (M(2)KO). There was no change in receptor number in the right ventricles. In the left ventricles, there was decrease in binding to all cardiostimulative receptors (beta1-, and beta2-AR) and increase in cardiodepressive receptors (beta3-AR) in unstressed KO in comparison to WT. The cold stress in WT animals resulted in decrease in binding to beta1- and beta2-AR (to 37%/35% after 1 day in cold and to 27%/28% after 7 days in cold) while beta3-AR were increased (to 216% of control) when 7 days cold was applied. MR were reduced to 46% and 58%, respectively. Gene expression of M2 MR in WT was not changed due to stress, while M3 was changed. The reaction of beta1- and beta2-AR (binding) to cold was similar in KO and WT animals, and beta3-AR in stressed KO animals did not change. Adenylyl cyclase activity was affected by beta3-agonist CL316243 in cold stressed WT animals but CL316243 had almost no effects on adenylyl cyclase activity in stressed KO. Nitric oxide activity (NOS) was not affected by BRL37344 (beta3-agonist) both in WT and KO animals. Similarly, the stress had no effects on NOS activity in WT animals and in KO animals. We conclude that the function of M2 MR is substituted by beta3-AR and that these effects are mediated via adenylyl cyclase rather than NOS.

• MeSH
• Adenylyl Cyclases metabolism   MeSH
• Receptors, Adrenergic, beta-3 metabolism   MeSH
• Adaptation, Physiological * genetics   MeSH
• Stress, Physiological * genetics   MeSH
• Catecholamines biosynthesis   MeSH
• Mice MeSH
• Cold Temperature * MeSH
• Receptor, Muscarinic M2 deficiency   genetics   metabolism   MeSH
• Gene Expression Regulation MeSH
• Heart physiopathology   MeSH
• Heart Ventricles enzymology   pathology   physiopathology   MeSH
• Nitric Oxide Synthase metabolism   MeSH
• Protein Binding MeSH
• Binding Sites MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Adenylyl Cyclases MeSH
• Receptors, Adrenergic, beta-3 MeSH
• Catecholamines MeSH
• Receptor, Muscarinic M2 MeSH
• Nitric Oxide Synthase MeSH

Glucocorticoids affect the expression and density of neurotransmitter receptors in many tissues but data concerning the heart are contradictory and incomplete. We injected rats with hydrocortisone for 1-12 days and measured the densities of cardiac muscarinic receptors, alpha(1)-, beta(1)- and beta(2)-adrenoceptors and propranolol-resistant binding sites (formerly assumed to be the putative beta(4)-adrenoceptor). Some aspects of intracellular signalling were also evaluated: we measured adenylyl cyclase activity (basal, isoprenaline- and forskolin-stimulated and carbachol-inhibited), the coupling between muscarinic receptors and G proteins and basal and isoprenaline-stimulated heart rate. The density of cardiac muscarinic receptors increased (in both the atria and the ventricles). The density of beta(1)-adrenoceptors increased in the atria and was little changed in the ventricles. The density of beta(2)-adrenoceptors increased in both the atria and the ventricles. The number of alpha(1)-adrenoceptors decreased initially, followed by a transient increase in the atria and did not change in the ventricles. The density of propranolol-resistant binding sites first increased and then diminished in the atria and did not change in the ventricles. Although there were noticeable changes in receptor densities, the stimulatory and inhibitory effects on adenylyl cyclase, basal and isoprenaline-stimulated heart rate and the coupling between muscarinic receptors and G proteins were not significantly altered. This may indicate that changes in receptor densities might be one of the mechanisms maintaining stable functional output.

• MeSH
• Receptors, Adrenergic drug effects   metabolism   physiology   MeSH
• Receptors, Adrenergic, alpha-1 drug effects   metabolism   physiology   MeSH
• Adrenergic beta-Antagonists pharmacology   MeSH
• Receptors, Adrenergic, beta-1 drug effects   metabolism   physiology   MeSH
• Receptors, Adrenergic, beta-2 drug effects   metabolism   physiology   MeSH
• Receptors, Adrenergic, beta drug effects   metabolism   physiology   MeSH
• Glucocorticoids administration & dosage   pharmacology   MeSH
• Hydrocortisone administration & dosage   pharmacology   MeSH
• Injections, Subcutaneous MeSH
• Rats MeSH
• Myocardium metabolism   MeSH
• Rats, Wistar MeSH
• Propanolamines pharmacology   MeSH
• Propranolol pharmacology   MeSH
• Radioligand Assay MeSH
• Receptors, Muscarinic drug effects   metabolism   physiology   MeSH
• Signal Transduction drug effects   MeSH
• Heart Ventricles drug effects   MeSH
• Binding Sites MeSH
• Animals MeSH
• Check Tag
• Rats MeSH
• Male MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Comparative Study MeSH
• Names of Substances
• adrenergic beta-4 receptor MeSH Browser
• Receptors, Adrenergic MeSH
• Receptors, Adrenergic, alpha-1 MeSH
• Adrenergic beta-Antagonists MeSH
• Receptors, Adrenergic, beta-1 MeSH
• Receptors, Adrenergic, beta-2 MeSH
• Receptors, Adrenergic, beta MeSH
• CGP 12177 MeSH Browser
• Glucocorticoids MeSH
• Hydrocortisone MeSH
• Propanolamines MeSH
• Propranolol MeSH
• Receptors, Muscarinic MeSH