Muscarinic receptors structure

J. C. Venter (1983). Muscarinic receptor structure. J. Biol. Chem. 258 4842-4848. 

Muscarinic Receptor Structure and Agonist/Antagonist Interactions... 

Table 4. Structures of Muscarinic Receptor Antagonists and Agonists...

Acetylcholinesterase is a component of the postsynaptic membrane of cholinergic synapses of the nervous system in both vertebrates and invertebrates. Its structure and function has been described in Chapter 10, Section 10.2.4. Its essential role in the postsynaptic membrane is hydrolysis of the neurotransmitter acetylcholine in order to terminate the stimulation of nicotinic and muscarinic receptors (Figure 16.2). Thus, inhibitors of the enzyme cause a buildup of acetylcholine in the synaptic cleft and consequent overstimulation of the receptors, leading to depolarization of the postsynaptic membrane and synaptic block. 

Figure 6.4 Schematic representation of the muscarinic receptor. All muscarinic receptors have seven transmembrane domains and the major difference between them is within the long cytoplasmic linkage connecting the fifth and sixth domains. This implies different G-protein connections and functions. Some possibilities are shown although the position of the Mi and M2 boxes is not intended to indicate their precise structural differences within the loop...

Some agonists, such as methacholine, carbachol and bethanecol are structurally very similar to ACh (Fig. 6.6). They are all more resistant to attack by cholinesterase than ACh and so longer acting, especially the non-acetylated carbamyl derivatives carbachol and bethanecol. Carbachol retains both nicotinic and muscarinic effects but the presence of a methyl (CH3) group on the p carbon of choline, as in methacholine and bethanecol, restricts activity to muscarinic receptors. Being charged lipophobic compounds they do not enter the CNS but produce powerful peripheral parasympathetic effects which are occasionally used clinically, i.e. to stimulate the gut or bladder. 

An example of an irreversible antagonist with a very long action (usually many hours) is phenoxybenzamine, which blocks a-adrenoceptors and, less potently, H,-histamine and muscarinic receptors. Its structure is shown below. Also illustrated is benzilylcholine mustard, a highly active and selective irreversible blocker of muscarinic receptors. 

ACh was first proposed as a mediator of cellular function by Hunt in 1907, and in 1914 Dale [2] pointed out that its action closely mimicked the response of parasympathetic nerve stimulation (see Ch. 10). Loewi, in 1921, provided clear evidence for ACh release by nerve stimulation. Separate receptors that explained the variety of actions of ACh became apparent in Dale s early experiments [2]. The nicotinic ACh receptor was the first transmitter receptor to be purified and to have its primary structure determined [3, 4]. The primary structures of most subtypes of both nicotinic and muscarinic receptors, the cholinesterases (ChE), choline acetyltransferase (ChAT), the choline and ACh transporters have been ascertained. Three-dimensional structures for several of these proteins or surrogates within the same protein family are also known. 

Thus, cholinergic receptor classification can be considered in terms of three stages of development. Initially, Dale [2] distinguished nicotinic and muscarinic receptor subtypes with crude alkaloids. Then, chemical synthesis and structure-activity relationships clearly revealed that nicotinic and muscarinic receptors were heterogeneous, but chemical selectivity could not come close to uncovering the true diversity of receptor subtypes. Lastly, analysis of subtypes came from molecular cloning, making possible the classification of receptors on the basis of primary structure (Fig. 11-2). 

Hill Eubank, D., Burstein, E., Spalding, T., Brauner-Osborne, H. and Brann, M. Structure of a G-protein coupling domain of a muscarinic receptor predicted by random saturation mutagenesis./. Biol. Chem. 271 3058-3065,1996. 

Fanelli, F., Menziani, M.C., Carotti, A. and De Benedetti, P.G. (1994) Theoretical quantitative structure-activity relationship analysis on three dimensional models of ligand-ml muscarinic receptor complexes. Bioorganic S. Medicinal Chemistry, 2, 195-211. 

Sir Henry Dale noticed that the different esters of choline elicited responses in isolated organ preparations which were similar to those seen following the application of either of the natural substances muscarine (from poisonous toadstools) or nicotine. This led Dale to conclude that, in the appropriate organs, acetylcholine could act on either muscarinic or nicotinic receptors. Later it was found that the effects of muscarine and nicotine could be blocked by atropine and tubocurarine, respectively. Further studies showed that these receptors differed not only in their molecular structure but also in the ways in which they brought about their physiological responses once the receptor has been stimulated by an agonist. Thus nicotinic receptors were found to be linked directly to an ion channel and their activation always caused a rapid increase in cellular permeability to sodium and potassium ions. Conversely, the responses to muscarinic receptor stimulation were slower and involved the activation of a second messenger system which was linked to the receptor by G-proteins. 

All muscarinic receptors are members of the seven transmembrane domain, G protein-coupled receptors, and they are structurally and functionally unrelated to nicotinic ACh receptors. Activation of muscarinic receptors by an agonist triggers the release of an intracellular G-protein complex that can specifically activate one or more signal transduction pathways. Fortunately, the cellular responses elicited by odd- versus even-numbered receptor subtypes can be conveniently distinguished. Activation of Ml, M3, and M5 receptors produces an inosine triphosphate (IP3) mediated release of intracellular calcium, the release of diacylglyc-erol (which can activate protein kinase C), and stimulation of adenylyl cyclase. These receptors are primarily responsible for activating calcium-dependent responses, such as secretion by glands and the contraction of smooth muscle. 

Pilocarpine is a naturally occurring cholinomimetic alkaloid that is structurally distinct from the choline esters. It is a tertiary amine that crosses membranes relatively easily. Therefore, it is rapidly absorbed by the cornea of the eye, and it can cross the blood-brain barrier. Pilocarpine is a pure muscarinic receptor agonist, and it is unaffected by cholinesterases. Muscarine is an alkaloid with no therapeutic use, but it can produce dangerous cholinomimetic stimulation following ingestion of some types of mushrooms (e.g., Inocybes). 

Even though the muscarinic receptor, which is present in postganglionic parasympathetic synapses, is much more stereospecific and structure-specific than its nicotinic counterpart, only since the 1980s have any molecular studies been undertaken to explore similarities and differences between the two classes of AChR. It has been labeled with the affinity label pH] propyl-benzilylcholine-mustard (4.5). 

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