Acetylcholine receptors cholinoceptors

After release from the presynaptic terminal, acetylcholine molecules may bind to and activate an acetylcholine receptor (cholinoceptor). 

After release from the presynaptic terminal, acetylcholine molecules may bind to and activate an acetylcholine receptor (cholinoceptor). Eventually (and usually very rapidly), all of the acetylcholine released will diffuse within range of an acetylcholinesterase (AChE) molecule. AChE very efficiently splits acetylcholine into choline and acetate, neither of which has significant transmitter effect, and thereby terminates the action of the transmitter (Figure 6-3). Most cholinergic synapses are richly supplied with acetylcholinesterase the half-life of acetylcholine in the synapse is therefore very short. Acetylcholinesterase is also found in other tissues, eg, red blood cells. (Another cholinesterase with a lower specificity for acetylcholine, butyrylcholinesterase [pseudocholinesterase], is found in blood plasma, liver, glia, and many other tissues.)... 

These drugs act on postsynaptic acetylcholine receptors (cholinoceptors) at all the sites in the body where acetylcholine is the effective neurotransmitter. They initially stimulate and usually later block transmission. In addition, like acetylcholine, they act on the noninnervated receptors that relax vascular smooth muscle in peripheral blood vessels. 

Insecticides derived from natural sources include nicotine, rotenone, and pyrethrum. Nicotine is obtained from the dried leaves of Nicotiana tabacum and Nicotiana rustica. It is rapidly absorbed from mucosal surfaces the free alkaloid, but not the salt, is readily absorbed from the skin. Nicotine reacts with the acetylcholine receptor of the postsynaptic membrane (sympathetic and parasympathetic ganglia, neuromuscular junction), resulting in depolarization of the membrane. Toxic doses cause stimulation rapidly followed by blockade of transmission. These actions are described in Chapter 7 Cholinoceptor-Activating Cholinesterase-Inhibiting Drugs. Treatment is directed toward maintenance of vital signs and suppression of convulsions. 

Acetylcholine receptors - muscarinic antagonists Notable active solanaceous plant alkaloids include atropine and hyoscine (scopolamine). See muscarinic cholinoceptor ANTAGONISTS. 

Acetylcholine-like drugs (cholinomimetics) are subdivided in two ways on the basis of their mode of action (ie, whether they act directly at the acetylcholine receptor or indirectly through inhibition of cholinesterase) and for those that act directly, on the basis of their spectrum of action (ie, whether they act on muscarinic or nicotinic cholinoceptors Figure 7-1). Acetylcholine may be considered the prototype that acts directly at both muscarinic and nicotinic receptors. Neostigmine is a prototype for the indirect-acting cholinesterase inhibitors. 

The major groups of cholinoceptor-activating drugs, receptors, and target tissues. ACh, acetylcholine. 

Most of the direct organ system effects of muscarinic cholinoceptor stimulants are readily predicted from a knowledge of the effects of parasympathetic nerve stimulation (see Table 6-3) and the distribution of muscarinic receptors. Effects of a typical agent such as acetylcholine are listed in Table 7-3. The effects of nicotinic agonists are similarly predictable from a knowledge of the physiology of the autonomic ganglia and skeletal muscle motor end plate. 

Effects of increasing doses of atropine on heart rate (A) and salivary flow (B) compared with muscarinic receptor occupancy in humans. The parasympathomimetic effect of low-dose atropine is attributed to blockade of prejunctional muscarinic receptors that suppress acetylcholine release. (Modified and reproduced, with permission, from Wellstein A, Pitschner HF Complex dose-response curves of atropine in man explained by different functions of Mi and M2 cholinoceptors. Naunyn Schmiedebergs Arch Pharmacol 1988 338 19.)... 

Two families of cholinoceptors, designated muscarinic and nicotinic receptors, can be distinguished from each other on the basis of their different affinities for agents that mimic the action of acetylcholine (cholinomimetic agents). 

Cholinoceptor A receptor that binds, and is activated by, acetylcholine and related drugs... 

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