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08-SEPTEMBER-2008 07:42:01 - 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 Formula C7H16NO2 Mol. mass 146.21 g/mol SMILES eMolecules PubChem Pharmacokinetic data Bioavailability ? Metabolism ? Half life approximately 2 minutes Excretion ? Therapeutic considerations Pregnancy cat. ? Legal status legal with license Routes ? The chemical compound acetylcholine often abbreviated ACh is a 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. Contents 1 History 2 Chemistry 3 Function 3.1 In PNS 3.2 In CNS 3.2.1 Structure 3.2.2 Plasticity 3.2.3 Excitability 4 Synthesis and Degradation 5 Receptors 5.1 Myasthenia gravis 5.2 Nicotinic 5.3 Muscarinic 6 Drugs Acting on the ACh System 6.1 ACh Receptor Agonists 6.1.1 Associated disorders 6.1.1.1 Alzheimer's disease 6.1.2 Direct Acting 6.1.3 Cholinesterase inhibitors 6.1.3.1 Reversible 6.1.3.2 Irreversible 6.1.4 Reactivation of Acetylcholine Esterase 6.2 ACh Receptor Antagonists 6.2.1 Antimuscarinic Agents 6.2.2 Ganglionic Blockers 6.2.3 Neuromuscular Blockers 6.2.4 Synthesis inhibitors 6.2.5 Release inhibitors 6.2.6 Other / Uncategorized / Unknown 7 References 8 External links History Acetylcholine ACh was first identified in 1914 by Henry Hallett Dale for its actions on heart tissue. It was confirmed as a neurotransmitter by Otto Loewi who initially gave it the name vagusstoff because it was released from the vagus nerve. Both received the 1936 Nobel Prize in Physiology or Medicine for their work. Acetylcholine was also the first neurotransmitter to be identified. Chemistry Acetylcholine is an ester of acetic acid and choline with chemical formula CH3COOCH2CH2N+CH33. 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 peripheral nervous system, 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 epinephrine and norepinephrine 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 NMDA receptors or indirectly by suppressing adaptation. The suppression of adaptation has been shown in brain slices of regions CA1, cingulate cortex, and piriform cortex, as well as in vivo in cat somatosensory and motor cortex by decreasing the conductance of voltage-dependent M currents and Ca2+-dependent 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 enzyme choline acetyltransferase from the compounds choline and acetyl-CoA. The enzyme acetylcholinesterase converts acetylcholine into the inactive metabolites choline and 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, nicotinic acetylcholine receptors nAChR and muscarinic acetylcholine receptors mAChR. They are named for the ligands used to activate the receptors. Myasthenia gravis The disease myasthenia gravis, characterized by muscle weakness and fatigue, occurs when the body inappropriately produces antibodies against acetylcholine nicotinic receptors, and thus inhibits proper acetylcholine signal transmission. Over time, the motor end plate is destroyed. Drugs that competitively inhibit acetylcholinesterase e.g., neostigmine, physostigmine, or primarily mestinon 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. Nicotinic Nicotinic AChRs are ionotropic receptors permeable to sodium, potassium, and chloride ions. They are stimulated by nicotine and acetylcholine. They are of two main types, muscle type and neuronal type. The former can be selectively blocked by curare and the latter by 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 metabotropic, and affect neurons over a longer time frame. They are stimulated by muscarine and acetylcholine, and blocked by atropine. Muscarinic receptors are found in both the central nervous system and the peripheral nervous system, in heart, lungs, upper GI tract and sweat glands. Extracts from the plant Deadly nightshade included this compound atropine, and the blocking of the muscarinic AChRs increases pupil size as used for attractiveness in many European cultures in the past. Now, ACh is sometimes used during cataract surgery to produce rapid constriction of the pupil. It must be administered intraocularly because corneal 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. 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 THC is one such drug, effective at reducing the formation of characteristic neurofibrillary tangles and amyloid beta plaques2. Direct Acting Acetylcholine Bethanechol Carbachol Cevimeline Pilocarpine Suberylcholine Nicotine in small doses Cholinesterase inhibitors Main article: 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 enzyme inhibitors, and increase the action of acetylcholine by delaying its degradation; some have been used as nerve agents Sarin and VX nerve gas or pesticides organophosphates and the carbamates. In clinical use, they are administered to reverse the action of muscle relaxants, to treat myasthenia gravis, and to treat symptoms of Alzheimer's disease 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. Many medications in Alzheimer's disease Donepezil Galantamine Rivastigmine Tacrine THC Edrophonium differs myasthenic and cholinergic crisis Neostigmine in myasthenia gravis Physostigmine in glaucoma and anticholinergic drug overdoses Pyridostigmine in myasthenia gravis Carbamate insecticides e.g., Aldicarb Huperzine A Irreversible Semi-permanently inhibit the enzyme acetylcholinesterase. Echothiophate Isofluorophate Organophosphate Insecticides Malathion, Parathion, Azinphos Methyl, Chlorpyrifos, among others Organophosphate-containing nerve agents e.g., Sarin, VX Victims of organophosphate-containing nerve agents commonly die of suffocation as they cannot relax their diaphragm. Reactivation of Acetylcholine Esterase Pralidoxime ACh Receptor Antagonists Antimuscarinic Agents Atropine Ipratropium Scopolamine Tiotropium Ganglionic Blockers Mecamylamine Hexamethonium Nicotine in high doses Trimethaphan Neuromuscular Blockers Atracurium Cisatracurium Doxacurium Metocurine Mivacurium Pancuronium Rocuronium Succinylcholine Tubocurarine Vecuronium HemiCholine dupogimine 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 alpha-latrotoxin has the reverse effect. Other / Uncategorized / Unknown surugatoxin suxamethonium References ^ Katzung, B.G. 2003. Basic and Clinical Pharmacology 9th ed.. McGraw-Hill Medical. ISBN 0-07-141092-9 ^ 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; 36:773-7. 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; 671:1-27. PMID 7748496 Yu, AJ Dayan, P. Uncertainty, neuromodulation, and attention. Neuron. 2005 May 19; 464:681-92. PMID 15944135 External links Washington University St. Louis writeup v d e Neurotransmitter systems Acetylcholine Basal optic nucleus of Meynert - Hippocampus BA/M Dopamine Mesocortical pathway - Mesolimbic pathway Pars compacta - Nigrostriatal pathway - Striatum Tuberoinfundibular pathway Norepinephrine Locus ceruleus Serotonin Raphe nuclei AA Aspartate Climbing fibers GABA Globus pallidus Glycine Renshaw cells Glutamate Thalamus - Subthalamic nucleus - Globus pallidus v d e Ophthalmologicals: antiglaucoma preparations and miotics S01E Sympathomimetics Apraclonidine Brimonidine Clonidine Dipivefrine Epinephrine Parasympathomimetics M: Aceclidine Pilocarpine M/N: Acetylcholine Carbachol AI: Demecarium Ecothiopate Stigmine Fluostigmine, Neostigmine, Physostigmine Paraoxon Carbonic anhydrase inhibitors Acetazolamide Brinzolamide Diclofenamide Dorzolamide Methazolamide Beta blocking agents Befunolol Betaxolol Carteolol Levobunolol Metipranolol Timolol Prostaglandin analogues Bimatoprost Latanoprost Travoprost Unoprostone Other agents Dapiprazole Guanethidine Retrieved from http://en..org/wiki/Acetylcholine Categories: Choline esters | Acetates | Neurotransmitters | Quaternary ammonium compounds Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages العربية Cebuano ÄŒesky Dansk Deutsch Ελληνικά Español Français Hrvatski Italiano עברית Lietuvių Magyar Nederlands 日本語 ‪Norsk bokmÃ¥l‬ Polski Português РуÑ?Ñ?кий SlovenÄ?ina SlovenÅ¡Ä?ina СрпÑ?ки / Srpski Suomi Svenska Tagalog Türkçe УкраїнÑ?ька اردو ä¸æ–‡ This page was last modified on 28 August 2008, at 02:16
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