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Acetylcholine is an organic cation that acts as a neurotransmitter in many organisms, including humans. Acetylcholine is also the principal neurotransmitter in all autonomic ganglia. In cardiac tissue, acetylcholine neurotransmission has an inhibitory effect, which lowers heart rate. However, acetylcholine also behaves as an excitatory neurotransmitter at neuromuscular junctions in skeletal muscle.
It is an ester of acetic acid and choline. The molecule is a polyatomic cation, meaning that it is a positive ion comprising of more than one atom. It contains the quaternary ammonium cation. Acetylcholine does not penetrate lipid membranes, this is because of the charged ammonium group which gives the substance a highly polar molecule. As a consequence of this, if the substance is introduced externally it won't penetrate the blood-brain barrier (BBB), it will then remain in the extracellular space waiting to get broken down in the body and excreted.
The cholinergic system
Acetylcholine acts as a neurotransmitter in both the peripheral nervous system (PNS) and central nervous system (CNS). Acetylcholine is also one of many neurotransmitters in the autonomic nervous system (ANS) and is the only neurotransmitter used in the motor division of the somatic nervous system (sensory neurons use glutamate and various peptides at their synapses). It functions as both a neurotransmitter and a neuromodulator. The cholinergic areas of the brain have distinct critical functions which some of them are; attention, memory (long- and short term), motivation, and arousal.
Drugs targeting the cholinergic system
An acetylcholine receptor agonist works by increasing the level of receptor activation, either directly or indirectly. Direct agonists work by imitating the neurotransmitter. Muscarine, an alkaloid found in a variety of mushrooms including Amanita muscaria, acts as a direct agonist of muscarinic acetylcholine receptors. Nicotine, an alkaloid found in Nicotiana tabacum, acts as a direct agonist of nicotinic acetylcholine receptors. Indirect agonists most often work by inhibiting the enzyme acetylcholinesterase; this enzyme breaks down acetylcholine, and therefore inhibition of it increases activation of cholinergic receptors. The stimulant drug caffeine is an indirect agonist, as is THC. More potent agents include donepezil and galantamine, both of which are used in the treatment of Alzheimer's disease. These work by blocking acetylcholinesterase, which is an enzyme that breaks down acetylcholine, leading to an increased build up over time. These medications are non-competitive, reversible inhibitors of acetylcholinesterase. Nerve agents like sarin irreversibly inhibit the enzyme, allowing a potentially lethal accumulation of acetylcholine to occur.
Precursors such as choline, citicoline and alpha-GPC are direct building blocks for acetylcholine and supplementing them may allow it to accumulate at higher levels than that which it otherwise would.
An acetylcholine receptor antagonist works by attaching to acetylcholine receptors to prevent agonists binding. The deliriant drugs atropine, scopolamine (both found in Datura) and diphenhydramine all act as antagonists upon muscarinic acetylcholine receptors, as does the chemical incapacitating agent 3-Quinuclidinyl benzilate, better known as BZ.
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- Kapalka, George M. (2010). "Substances Involved in Neurotransmission". Nutritional and Herbal Therapies for Children and Adolescents. https://doi.org/10.1016%2Fb978-0-12-374927-7.00004-2
- Zeisel, S. H., & Da Costa, K. A. (2009). Choline: An essential nutrient for public health. Nutrition Reviews, 67(11), 615–623. https://doi.org/10.1111/j.1753-4887.2009.00246.x