Acetylcholine

Description

Acetylcholine is stored in vesicles in the presynaptic neuron. These fuse with presynaptic membrane upon stimulation by a nerve signal, thus, generating a pulse of neurotransmitter, which diffuses across the membrane. Acetylcholine may either bind reversibly to one of two different types of acetylcholine receptors on the postsynaptic membrane or be destroyed by the acetylcholine-hydrolyzing enzyme, acetylcholinesterase.

Description

Acetylcholine is a choline molecule that has been acetylated at the oxygen atom. Because of the presence of a highly polar, charged ammonium group, acetylcholine does not penetrate lipid membranes. Because of this, when the drug is introduced externally, it remains in the extracellular space and does not pass through the blood–brain barrier.

Uses

neurotransmitter (ester of choline and acetic acid)

Uses

Acetylcholine does not have therapeutic value as a drug for intravenous administration because of its multi-faceted action and rapid inactivation by cholinesterase. Likewise, it is possible for a collaptoid state to develop, and arterial pressure can rapidly fall and the heart can stop. However, it is used in the form of eye drops to cause miosis during cataract surgery, which makes it advantageous because it facilitates quick post-operational recovery.

Biological Functions

The discovery that ACh was a transmitter in the peripheral nervous system formed the basis for the theory of neurotransmission. ACh is also a neurotransmitter in the mammalian brain; however, only a few cholinergic tracts have been clearly delineated.ACh is an excitatory neurotransmitter in the mammalian CNS.There is good evidence that ACh (among other neurotransmitters) is decreased in certain cognitive disorders, such as Alzheimer’s disease.

Mechanism of action

Acetylcholine functions primarily as a chemical neurotransmitter in the nervous systems of all animals. When a cholinergic neuron is excited, it releases transmitter into the synaptic cleft where it can bind to a number of different receptor proteins. The receptors for acetylcholine can be classified into two general categories based primarily on the actions of different plant alkaloids that affect their function: nicotinic (nicotine binding) or muscarinic (muscarine binding). Several different subtypes for each of these general receptor classes have been characterized. The receptor binding event can be transduced into opening of cationic or anionic ion channels or coupled to some other metabolic signal such as phospholipid turnover rates or activation of second-messenger systems. Both inhibitory or, more commonly, excitatory responses are induced in the neurons or effector cells which receive the neurotransmitter signal, making acetylcholine-mediated neurotransmission particularly versatile.
In addition to the ubiquitous presence of acetylcholine in the nervous systems of all animals, it is also found in a limited number of plants, bacteria, fungi and protozoa. This widespread distribution in a variety of species most likely indicates the appearance of acetylcholine-metabolizing proteins fairly early in evolutionary history. In vertebrates, acetylcholine is also found in non-neuronal tissues such as primate placenta and sperm where its functional role, if any, remains unknown.

Clinical Use

The cholinergic system was the first neurotransmitter system shown to have a role in wakefulness and initiation of REM sleep. Because of the poor penetration of the cholinergic drugs into the CNS, the role of this system in sleep has relied on animal studies using microinjection into the brain, primarily in the area of the dorsal pontine tegmentum. Acetylcholine, cholinergic agonists (e.g., arecoline or bethanechol), and cholinesterase inhibitors are effective in the initiation of REM sleep from NREM sleep after microinjection. Conversely, administration of anticholinergic drugs (e.g., atropine or scopolamine) hinders the transition to REM sleep. Increase in the rate of discharge of these cholinergic cells (that activate the thalamus, cerebral cortex, and hippocampus) during REM sleep parallel the same pattern seen with arousal and alertness.

Synthesis

Acetylcholine, 2-acetoxy-N,N,N-trimethylethyl ammonium chloride (13.1.2), is easily synthesized in a number of different ways. For example, 2-chloroethanol is reacted with trimethylamine, and the resulting N,N,N-trimethylethyl-2-ethanolamine hydrochloride (13.1.1), also called choline, is acetylated by acetic acid anhydride or acetylchloride, giving acetylcholine (13.1.2). A second method consists of reacting trimethylamine with ethylene oxide, giving N,N,N-trimethylethyl-2-ethanolamine hydroxide (13.1.3), which upon reaction with hydrogen chloride changes into the hydrochloride (13.1.1), which is further acetylated in the manner described above. Finally, acetylcholine is also formed by reacting 2-chloroethanol acetate with trimethylamine [1¨C7].

Environmental Fate

Cholinergic agents can increase the acetylcholine level at the synaptic junction and cause rapid firing of the postsynaptic membrane. Antiacetylcholinesterase agents block the acetylcholinesterase enzyme and thus increase the acetylcholine level in the synapse causing rapid firing of the postsynaptic membrane.

Metabolism

Acetylcholine in the synapse can bind with cholinergic receptors on the postsynaptic or presynaptic membranes to produce a response. Free acetylcholine that is not bound to a receptor is hydrolyzed by AChE. This hydrolysis is the physiologic mechanism for terminating the action of acetylcholine. Enough AChE is present in the synapse to hydrolyze approximately 3 × 108 molecules of acetylcholine in 1 millisecond; thus, adequate enzyme activity exists to hydrolyze all the acetylcholine (~3 × 106 molecules) released by one action potential. A number of useful therapeutic cholinomimetic agents have been developed based on the ability of the compounds to inhibit AChE; these agents are addressed later in this chapter.

Raw materials

Preparation Products