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Cholinergic Receptor Antagonists and Agonists
 | | | | | | | A large number of autonomic neurons are cholinergic in nature. They synthesize and release acetylcholine (ACh). Cholinergic terminals contain a large number of small ACh-containing, membranebound vesicles concentrated near the synaptic end. After release from the presynaptic terminal, ACh molecules activate cholinoreceptors on the post-synaptic membrane. They are eventually hydrolyzed by acetylcholinesterase (AChE) to form choline and acetic acid. Acetylcholine receptors are classified as muscarinic or nicotinic. Muscarinic receptors are found in neuroeffector junctions and are coupled to G-proteins. On the other hand, nicotinic receptors are found in the autonomic ganglia and neuromuscular junctions. They are involved in Na+ and K+ channel openings. Pharmacological agents that stimulate cholinoreceptors may be classified into two major categories based on their action: direct and indirect. Direct-acting agents bind to and activate muscarinic or nicotinic receptors. Indirect-acting agents produce their primary effect by inhibiting the action of AChE. Hence, they indirectly increase levels of ACh in the synaptic region. | | | | Receptors | Location | Pharmalogical/Physiological Effect of Ligand Binding | | Muscarinic M1 | CNS, sympathetic post-ganglionic neurons | Stimulation of phospholipase C leading to IP3+ DAG formation, increase in intracellular Ca2+. | | Muscarinic M2 | Myocardium, smooth muscles, some presynaptic sites | Opening of K+ channels, inhibition of adenylate cyclase. | | Muscarinic M3 | Glandular tissue, vascular smooth muscles | Stimulation of phospholipase C leading to IP3+ DAG formation, increase in intracellular Ca2+. | | Muscarinic M4 | CNS | Inhibition of adenylate cyclase. | | Muscarinic M5 | CNS | IP3+ DAG formation, increase in intracellular Ca2+. | | Nicotinic | Autonomic ganglia, skeletal muscle, adrenal medulla | Opening of Na+-K+ channels, depolarization. |
| | | | | | | Cholinergic Receptor Antagonists |
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| Atropine Sulfate, Atropa belladonna L. |
189360 |
| Methyllycaconitine Perchlorate, Delphinium sp. |
462040 |
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| | | | Cholinergic Receptor Agonists |
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| | | | | Cholinesterase Inhibitors | | | | | |
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