Signal Transduction

nature of their binding, the actual points of attachment to the lining of the pocket (i.e. to particular amino acid residues) are different. The consequences of binding are alterations such as displacement of protons and water, breakage of hydrogen bonds, disturbance of van der Waals interactions and conformational changes in the receptor itself. In the case of agonists, this results in an overall increase in order (decreased entropy).5 Importantly, only agonists induce the conformational alterations that enable the receptors to communicate with the ensuing components of the signal transduction apparatus.

Acetylcholine receptors

At the molecular level, all the effects of the catecholamines, including dopamine, are mediated through members of the family of receptors that span the membrane seven times (7TM receptors, discussed later in this chapter) and subsequently they all activate or inhibit an enzyme, such as adenylyl cyclase or phospholipase C (Chapter 5). This is always mediated through a GTP-binding protein (GTPase) (Chapter 4). About 60% of the drugs used in clinical practice are directed at 7TM receptors. In contrast, acetylcholine interacts with two very distinct types of receptor that are quite unrelated to each other. These are the nicotinic receptors (ion channels) and muscarinic receptors (7TM receptors).

Acetylcholine

Although acetylcholine is a first messenger that interacts with receptors, it does not have the function of a hormone. It is confined to synapses between nerve endings and target cells, and it is the primary transmitter at the neuromuscular junction (between motor nerve and muscle end plate). In the autonomic nervous system, it is also the transmitter at preganglionic nerve endings and in most parasympathetic postganglionic nerves.

Parasympathetic stimulation through the vagus nerve causes the dilation of blood vessels, increases fluid secretion (e.g. from the pancreas and salivary glands), and slows heart rate. At the neuromuscular junction, acetylcholine is released from the presynaptic membrane. It diffuses across the junctional cleft and interacts with nicotinic receptors situated on the postsynaptic membrane. It is then removed from the synaptic cleft with ‘flashlike suddenness’ (in the words of Henry Hallett Dale). This occurs partly by diffusion, but mainly by the action of acetylcholine esterase, which converts it to choline and acetate within milliseconds. Since the affinity of nicotinic receptors for acetylcholine (KD 10 7 mol l 1) is rather moderate (at least, in comparison with some other types of receptor for their respective ligands), the rate of dissociation is sufficiently fast to allow it to detach rapidly (recall that KD koff/kon), following the steep decline in the local concentration due to the activity of the esterase.

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Receptors

Because of the extreme lability of acetylcholine , in experimental investigations it has been normal to work with stable non-hydrolysable derivatives such as carbamylcholine (carbachol). As with the natural compound, carbamycholine is an agonist at both nicotinic and muscarinic receptors.

Some of the compounds that inhibit the hydrolysis of acetylcholine are among the most toxic known (Figure 3.3). Thus, they may cause stimulation of cholinergic receptor sites throughout the central nervous system (CNS), depression at autonomic ganglia, and paralysis of skeletal muscles. This is followed by secondary depression involving irreversible receptor desensitization, discussed below. In addition to these nicotinic functions, muscarinic responses to acetylcholine also tend to persist. On the other hand, some cholinesterase inhibitors are less toxic and have found their way into clinical practice as in the treatment of myasthenia gravis and Alzheimer’s disease.

The first synthesis of an irreversible inhibitor of cholinesterase was reported as early as 1854 by Clermont, predating the isolation of the alkaloid physostigmine (eserine) from Calabar beans by about 10 years (see Chapter 1). This was tetraethyl pyrophosphate. In his dedicated pursuit of science,

Fig 3.3  Some inhibitors of serine esterases.

Organophosphate neurotoxins are shown in the upper panel. The P=O double bond is essential for toxicity. Substitution of the oxygen with sulfur, as in parathion, results in a much less toxic derivative, though the use of this class of compounds as insecticides has been banned in many countries. Also illustrated are the parasympathomimetic agents, reversible choline esterase inhibitors, physostigmine (eserine) isolated from Calabar beans and a derivative, neostigmine. Both these compounds

have found wide application in clinical practice.

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