M2 receptors

CI-979 (29) is a balanced muscarinic agonist having equal affinities for cloned ml and m2 receptors (144). However, unlike prototypical muscarinic compounds such as (25), (29) increases central muscarinic tone, as indicated by behavioral and electroencephalogram (EEG) parameters, at doses lower than those requited to produce gastrointestinal effects (144). CI-979 is well tolerated in humans up to a dose of 1 mg. Dose-limiting side effects such as stomach pain and emesis were observed at a dose of 2 mg. 

FIGURE 7.7 Effect of the allosteric modulator ebumamonine on the potency of muscarinic agonists on m2 receptors. It can be seen that while no change in potency is observed for APE (arecaidine propargyl ester) pilocarpine is antagonized and arecoline is potentiated, illustrating the probe dependence of allosterism. From [1],... 

The antimuscarinic drug atropine, and its derivative ipratropiumbromide, can also be used for antiarrhyth-mic treatment. Muscarinic receptors (M2 subtype) are mainly present in supraventricular tissue and in the AV node. They inhibit adenylylcyclase via G proteins and thereby reduce intracellular cAMP. On the other hand, activation of the M2 receptor leads to opening of hyperpolarizing Ik.acii and inhibits the pacemaker current If probably via the (3y-subunit of the Gi protein associated with this receptor. The results are hyperpolarization and slower spontaneous depolarization. Muscarinic receptor antagonists like atropine lead to increased heart rate and accelerated atrioventricular conduction. There are no or only slight effects on the ventricular electrophysiology. 

Stimulation of mAChRs also results in the activation or inhibition of a large number of ion channels [5]. For example, stimulation of Mi receptors leads to the suppression of the so-called M current, a voltage-dependent Recurrent found in various neuronal tissues. M2 receptors, on the other hand, mediate the opening of cardiac Ikcacii) channels, and both M2 and M4 receptors are linked to the inhibition of voltage-sensitive calcium channels [5]. 

As distinct from the acetyl choline receptor of the neuromuscular junction, the acetyl receptors of the viscera are not blocked by nicotine but are blocked by muscarine. Moreover, based on differences in the binding of the muscarinic antagonist, pirenzapine, the muscarinic acetyl choline receptors (mAChRs), are separated into two classes, viz. high affinity mj receptors, and low affinity m2 receptors. The latter predominates in the heart, cerebellum, and smooth muscle broadly. These different receptors mediate quite different actions. 

The M2 and M4 receptors also show struetural similarities. Through G-protein (Gi) they inhibit cyclic AMP production and open K+ ehannels while activation of another G-protein (Go) closes Ca + channels. The latter effeet will cause membrane hyperpolarisation as will the Gpinduced inerease in K+ efflux. The reduction in cAMP production, although possibly leading to depolarisation, is more likely to explain the presynaptie reduction in ACh release assoeiated with the M2 receptor. 

Figure 17.7 Possible mechanism by which atypical neuroleptics with antimuscarinic activity produce few EPSs. Normally the inhibitory effects of DA released from nigrostriatal afferents on to striatal neuron D2 receptors is believed to balance the excitatory effect of ACh from intrinsic neurons acting on muscarinic (M2) receptors (a). Typical neuroleptics block the inhibitory effect of DA which leaves unopposed the excitatory effect of ACh (b) leading to the augmented activity of the striatal neurons and EPSs (see Fig. 15.2). An atypical neuroleptic with intrinsic antimuscarinic activity reduces this possibility by counteracting the excitatory effects of released ACh as well as the inhibitory effects of DA (c). Thus the control of striatal neurons remains balanced...

No overall reduction in cholinergic muscarinic receptors was found but recent studies with relatively specific ligands show a loss of presynaptic M2 receptors, in keeping with the loss of terminals, but no reduction in postsynaptic Mi receptors. Some acetylcholinesterase is found in plaques. 

Furthermore, these allosteric effects were shown to be truly subtype specific, depending on the nature of the allosteric modulating compound. Thus, alcuronium exerts positive copperativity with [3H]NMS at the M2 and M4 but not at the Mi and M3 receptors [26,27], while other neuromuscular junction blockers such as stercuronium, pancuronium, and d-tubocurarine have been shown to exhibit their effects via an allosteric mechanism specifically on the M2 receptors [28-30]. 

Datta, S., Quattrochi, J. J. Hobson, J. A. (1993). Effect of specific muscarinic M2 receptor antagonist on carbachol induced long-term REM sleep. Sleep 16, 8-14. 

Both nicotinic and muscarinic receptors are widespread in the CNS. Muscarinic receptors with a high affinity for pirenzepine (PZ), M, receptors, predominate in the hippocampus and cerebral cortex, whereas M2 receptors predominate in the cerebellum and brainstem, and M4 receptors are most abundant in the striatum. Central muscarinic and nicotinic receptors are targets of intense pharmacological interest for their potential roles in regulating abnormal neurological signaling in Alzheimer s disease, Parkinson s disease and certain seizure disorders. Nicotinic receptors are largely localized at prejunctional sites and control the release of neurotransmitters [10,11],... 

The selectivity in muscarinic receptor coupling is not, however, absolute. Overexpression of receptors or of particular G proteins supports interactions that may differ from those described above. For example, M2 receptors expressed in Chinese hamster ovary cells not only inhibit adenylyl cyclase but also can stimulate phosphoinositide hydrolysis through a pertussis-toxin-sensitive G protein [52] this is not seen, however, when M2 receptors are expressed in Y1 cells. These findings indicate that caution must be exercised in interpreting data obtained when receptors are expressed, often at high levels, in cells in which they normally do not function. 

Both the M2 and M4 receptors are, as indicated above, coupled to Gj pathways and appear to mediate similar responses. The M2 receptor is widely expressed in the CNS but also present in heart and smooth muscle, while M4 is preferentially expressed in the CNS, especially in forebrain. Ablation of the M2 receptor leads to complete loss of muscarinic-agonist-stimulated bradycardia [55]. In the CNS, deletion of the M2 receptor abolishes oxotremorine induced akinesia and tremors [56]. Memory and learning tasks including passive avoidance and working memory are impaired in M2-receptor knockout mice, and there is decreased LTP in hippocampal slices [12],... 

Pharmacological profiling Muscarinic acetylcholine M2 receptor Binding and functional response (bradycardia) Hsieh and Liao229... 

Although ibogaine binds to muscarinic Ml and M2 receptors in the micromolar range, its action there is uncertain (Sweetnam et al. 1995). It fails to alter the adenylyl cyclase activity of a muscarinic agonist carbachol (Rabin and Winter 1996b). 

Atropine generally increases heart rate, but it may briefly and mildly decrease it initially, due to Ml receptors on postganglionic parasympathetic neurons. Larger doses of atropine produce greater tachycardia, due to M2 receptors on the sinoatrial node pacemaker cells. There are no changes in blood pressure, but arrhythmias may occur. Scopolamine produces more bradycardia and decreases arterial pressure, whereas atropine has little effect on blood pressure (Vesalainen et al. 1997 Brown and Taylor 1996). 

To date, five subtypes of these receptors have been cloned. However, initial studies relied on the pharmacological effects of the muscarinic antagonist pirenzepine which was shown to block the effect of several muscarinic agonists. These receptors were termed Mi receptors to distinguish them from those receptors for which pirenzepine had only a low affinity and therefore failed to block the pharmacological response. These were termed M2 receptors. More recently, M3, M4 and M5 receptors have been identified which, like the Mi and M2 receptors occur in the brain. Recent studies have shown that Mi and M3 are located posts)maptically in the brain whereas the M2 and M4 receptors occur pres)maptically where they act as inhibitory autoreceptors that inhibit the release of acetylcholine. The M2 and M4 receptors are coupled to the inhibitory Gi protein which reduces the formation of cyclic adenosine monophosphate (cyclic AMP) within the neuron. By contrast, the Mi, M3 and M5 receptors are coupled to the stimulatory Gs protein which stimulates the intracellular hydrolysis of the phosphoinositide messenger within the neuron (see Figure 2.8). 

Fig. 3.13 The ACE50 method demonstrated for a mixture of ligands at 1 tM per component to the M2 receptor at 5 pM concentration. (A) NGD-3350 requires the greatest competitor concentration to be competed from the receptor, indicating that it is the highest affinity ligand. (B) Ratio plots indicate direct binding competition with atropine. (C) Select compound structures. Reprinted from [39] with permission from the American Chemical Society.

Fig. 3.17 (A) ALIS-MS results from quenching an equilibrated mixture of 2.0 iM M2 receptor plus 1.5 pM NMS by 200 pM of the isosteric ligand NMS-D3, in the presence and absence of the known allosteric ligand W-84 at 50 pM concentration. Binding by the allosteric ligand W-84 decreases the off-rate of NMS. (B) Compound structures.

Trankle, C., Andresen, I., Lambrecht, G., Mohr, K. M2 receptor binding of the selective antagonist AF-DX 384 possible involvement of the common allosteric site. Mol. Pharmacol. 1998, 53, 304-312... 

Muscarinic M2 receptor CHRM2 Agonism Vagal effects (key role in the control of heart rate and smooth muscle activity) Bradycardia. Antagonism May induce cardiac side effects (palpitations, dysrhythmia) or peripheral edema, bronchoconstriction can result from presynaptic M2 receptor antagonism if postsynaptic M3 receptors are not also blocked. 

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