Acetylcholine



Acetylcholine
Systematic (IUPAC) name
2-acetoxy-N,N,N-trimethylethanaminium
Identifiers
CAS number 51-84-3
ATC code S01EB09
PubChem 187
DrugBank EXPT00412
Chemical data
O2 
Mol. mass 146.21 g/mol
SMILES search in eMolecules, PubChem
Pharmacokinetic data
Bioavailability  ?
Metabolism  ?
Half life approximately 2 minutes
Excretion  ?
Therapeutic considerations
Pregnancy cat.

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Legal status

legal with license

Routes  ?

The neurotransmitter in both the peripheral nervous system (PNS) and central nervous system (CNS) in many organisms including humans. Acetylcholine is one of many neurotransmitters in the autonomic nervous system (ANS) and the only neurotransmitter used in the somatic nervous system. It is also the neurotransmitter in all autonomic ganglia.

Chemistry

Acetylcholine is an N+(CH3)3. This structure is reflected in the systematic name, 2-acetoxy-N,N,N-trimethylethanaminium.

Function

Acetylcholine has functions both in the peripheral nervous system (PNS) and in the central nervous system (CNS) as a neuromodulator.

In the PNS, acetylcholine activates muscles, and is a major neurotransmitter in the autonomic nervous system.

In the CNS, acetylcholine and the associated neurons form a neurotransmitter system, the cholinergic system, which tends to cause excitatory actions.

In PNS

In the PNS, acetylcholine activates muscles, and is a major neurotransmitter in the autonomic nervous system.

When acetylcholine binds to acetylcholine receptors on skeletal muscle fibers, it opens ligand gated sodium channels in the cell membrane. Sodium ions then enter the muscle cell, stimulating muscle contraction. Acetylcholine, while inducing contraction of skeletal muscles, instead induces decreased contraction in cardiac muscle fibers. This distinction is attributed to differences in receptor structure between skeletal and cardiac fibers.

In the autonomic nervous system, acetylcholine is released in the following sites:

  • all pre- and post-ganglionic parasympathetic neurons
  • all preganglionic sympathetic neurons
    • preganglionic sympathetic fibers to suprarenal medulla, the modified sympathetic ganglion; on stimulation by acetylcholine, the suprarenal medulla releases noradrenaline
  • some postganglionic sympathetic fibers
    • sudomotor neurons to sweat glands.

In CNS

In the central nervous system, ACh has a variety of effects as a neuromodulator, e.g., for plasticity and excitability. Other effects are arousal and reward.

Structure

Acetylcholine and the associated neurons form a neurotransmitter system, the cholinergic system. It originates mainly in pontomesencephalotegmental complex, basal optic nucleus of Meynert and medial septal nucleus, and projects axons to vast areas of the brain:

  • The pontomesencephalotegmental complex acts mainly on M1 receptors in the brainstem .
  • Basal optic nucleus of Meynert acts mainly on M1 receptors in the neocortex.
  • Medial septal nucleus acts mainly on M1 receptors in the hippocampus and neocortex.

Plasticity

ACh is involved with synaptic plasticity, specifically in learning and short-term memory.

Acetylcholine has been shown to enhance the amplitude of synaptic potentials following long-term potentiation in many regions, including the dentate gyrus, CA1, piriform cortex, and neocortex. This effect most likely occurs either through enhancing currents through K+ currents.

Excitability

Acetylcholine also has other effects on excitability of neurons. Its presence causes a slow depolarization by blocking a tonically-active K+ current, which increases neuronal excitability. It appears to be a paradox, however, that ACh increases spiking activity in inhibitory interneurons while decreasing strength of synaptic transmission from those cells. This decrease in synaptic transmission also occurs selectively at some excitatory cells: For instance, it has an effect on intrinsic and associational fibers in layer Ib of piriform cortex, but has no effect on afferent fibers in layer Ia. Similar laminar selectivity has been shown in dentate gyrus and region CA1 of the hippocampus. One theory to explain this paradox interprets acetylcholine neuromodulation in the neocortex as modulating the estimate of expected uncertainty, acting counter to norepinephrine (NE) signals for unexpected uncertainty. Both would then decrease synaptic transition strength, but ACh would then be needed to counter the effects of NE in learning, a signal understood to be 'noisy'.

Synthesis and Degradation

Acetylcholine is synthesized in certain neurons by the acetyl-CoA.

The enzyme acetylcholinesterase converts acetylcholine into the inactive acetate. This enzyme is abundant in the synaptic cleft, and its role in rapidly clearing free acetylcholine from the synapse is essential for proper muscle function.

Receptors

Main article: Acetylcholine receptor

There are two main classes of acetylcholine receptor (AChR), ligands used to activate the receptors.

Nicotinic

Nicotinic AChRs are hexamethonium. The main location of nicotinic AChRs is on muscle end plates, autonomic ganglia (both sympathetic and parasympathetic), and in the CNS.[1]

Muscarinic

Muscarinic receptors are cholinesterase metabolizes topically-administered ACh before it can diffuse into the eye. It is sold by the trade name Miochol-E (CIBA Vision). Similar drugs are used to induce mydriasis (dilation of the pupil) in cardiopulmonary resuscitation and many other situations.

Drugs Acting on the ACh System

Blocking, hindering or mimicking the action of acetylcholine has many uses in medicine. Drugs acting on the acetylcholine system are either agonists to the receptors, stimulating the system, or antagonists, inhibiting it.

ACh Receptor Agonists

Acetylcholine receptor agonists can either have an effect directly on the receptors or exert their effects indirectly, e.g., by affecting the enzyme acetylcholinesterase, which degrades the receptor ligand.

Associated disorders

ACh Receptor Agonists are used to treat myasthenia gravis and Alzheimer's disease.

Myasthenia gravis

The disease myasthenia gravis, characterized by muscle weakness and fatigue, occurs when the body inappropriately produces antibodies against acetylcholine receptors, and thus inhibits proper acetylcholine signal transmission. Over time, the motor end plate is destroyed. Drugs that competitively inhibit acetylcholinesterase (e.g., neostigmine or physostigmine) are effective in treating this disorder. They allow endogenously-released acetylcholine more time to interact with its respective receptor before being inactivated by acetylcholinesterase in the gap junction.

Alzheimer's disease

Since a shortage of acetylcholine in the brain has been associated with Alzheimer's disease, some drugs that inhibit acetylcholinesterase are used in the treatment of that disease. A recent study has shown that amyloid beta plaques[2].

Direct Acting

Cholinesterase inhibitors

Most indirect acting ACh receptor agonists work by inhibiting the enzyme acetylcholinesterase. The resulting accumulation of acetylcholine causes continuous stimulation of the muscles, glands, and central nervous system.

They are examples of rivastigmine, which increases cholinergic activity in the brain).

Reversible

The following substances reversibly inhibit the enzyme acetylcholinesterase (which breaks down acetylcholine), thereby increasing acetylcholine levels.

Irreversible

Semi-permanently inhibit the enzyme acetylcholinesterase.

Victims of sarin gas, commonly die of suffocation as they cannot relax their diaphragm.

Reactivation of Acetylcholine Esterase

ACh Receptor Antagonists

Antimuscarinic Agents

Ganglionic Blockers

Neuromuscular Blockers

Synthesis inhibitors

Organic mercurial compounds have a high affinity for sulfhydryl groups, which causes dysfunction of the enzyme choline acetyltransferase. This inhibition may lead to acetylcholine deficiency, and can have consequences on motor function.

Release inhibitors

Botulin acts by suppressing the release of acetylcholine; where the venom from a black widow spider has the reverse effect.

Other / Uncategorized / Unknown

  • surugatoxin

History

Acetylcholine (ACh) was first identified in 1914 by vagusstoff because it was released from the vagus nerve. Both received the 1936 Nobel Prize in Physiology or Medicine for their work.

Acetylcholine is the first neurotransmitter to be identified.

References

  1. ^ Katzung, B.G. (2003). Basic and Clinical Pharmacology (9th ed.). McGraw-Hill Medical. ISBN 0-07-141092-9
  2. ^ Eubanks LM, Rogers CJ, Beuscher AE 4th, Koob GF, Olson AJ, Dickerson TJ, Janda KD. "A molecular link between the active component of marijuana and Alzheimer's disease pathology." Molecular Pharmaceutics. 2006 Nov-Dec; 3(6):773-7. PMID 17140265
  • Brenner, G. M. and Stevens, C. W. (2006). Pharmacology (2nd ed.). Philadelphia, PA: W.B. Saunders Company (Elsevier). ISBN 1-4160-2984-2
  • Canadian Pharmacists Association (2000). Compendium of Pharmaceuticals and Specialties (25th ed.). Toronto, ON: Webcom. ISBN 0-919115-76-4
  • Carlson, NR (2001). Physiology of Behavior (7th ed.). Needham Heights, MA: Allyn and Bacon. ISBN 0-205-30840-6
  • Gershon, Michael D. (1998). The Second Brain. New York, NY: HarperCollins. ISBN 0-06-018252-0
  • Hasselmo, ME. "Neuromodulation and cortical function: Modeling the physiological basis of behavior." Behavioral Brain Research. 1995 Feb; 67(1):1-27. PMID 7748496
  • Yu, AJ & Dayan, P. "Uncertainty, neuromodulation, and attention." Neuron. 2005 May 19; 46(4):681-92. PMID 15944135
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Acetylcholine". A list of authors is available in Wikipedia.