Muscarine, structure

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

Muscarine is a poisonous substance present in the mushroom Amanita muscaria. Its structure is represented by the constitution shown here. 

Muscarinic acetylcholine receptors (mAChRs) form a class of cell surface receptors that are activated upon binding of the neurotransmitter, acetylcholine. Structurally and functionally, mAChRs are prototypical members of the superfamily of G protein-coupled receptors. Following acetylcholine binding, the activated mAChRs interact with distinct classes of heterotrimeric G proteins resulting in the activation or inhibition of distinct downstream signaling cascades. 

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. 

Matsuyama, T., Luiten, P.G.M., Spencer, J. and Strosberg, A.D. (1988) Ultra-structural localisation of immunoreactive sites for muscarinic acetylcholine receptor proteins in the rat cerebral cortex. Neuroscience Research Communications 2, 69-76. 

The authors also identified the most common structural motifs unique to ligands of individual classes of GPCRs such as the adrenergic, dopamine, histamine, muscarinic, and serotonin receptors as shown in Table 1. 

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. 

Acetylcholine receptors have been classified into sub-types based on early studies of pharmacologic selectivity. Long before structures were known, two crude alkaloid fractions, containing nicotine and muscarine (Fig. 11-2), were used to subclassify receptors in the cholinergic nervous system (Fig. 11-3). The greatly different... 

FIGURE 11-3 Structure of compounds important to the classification of receptor subtypes at cholinergic synapses. Compounds are subdivided as nicotinic (N) and muscarinic (Ad). The compounds interacting with nicotinic receptors are subdivided further according to whether they are neuromuscular (N,) or ganglionic (N2). Compounds with muscarinic subtype selectivity (M M2, M3, M4) are also noted. 

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. 

Wolf-Pflugmann M, Lambrecht G, Wess J, Mutschler E. (1989). Synthesis and muscarinic activity of a series of tertiary and quaternary N-substituted guvacine esters structurally related to arecoline and arecaidine propargyl ester. Arzneimittelforschung. 39(5) 539-44. 

Muscarine is a tetrahydrofuran derivative with the structure shown in Figure 3.7a. Because of the three chirality centers present in the molecule, muscarine exists in eight isomers, of which only one, L-(- -)-muscarine, is active. The remaining isomers also have been detected in toxic fungi, but because of their low biological activity and low concentration they do not contribute to toxicity. 

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