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Anticholinergics are a class of medications that inhibit parasympathetic nerve impulses by selectively blocking the binding of the neurotransmitter acetylcholine to its receptor in nerve cells. The nerve fibers of the parasympathetic system are responsible for the involuntary movements of smooth muscles present in the gastrointestinal tract, urinary tract, lungs, etc. Anticholinergics are divided into three categories in accordance with their specific targets in the central and/or peripheral nervous system: antimuscarinic agents, ganglionic blockers, and neuromuscular blockers.
Anticholinergic drugs are used to treat a variety of disorders such as gastrointestinal cramps, urinary bladder spasm, asthma, motion sickness, muscular spasms, poisoning with certain toxic compounds, and as an aid to anesthesia.
Antimuscarinic agents are so called because they block muscarine, a poisonous substance found in the Amanita muscaria, a nonedible mushroom species. Muscarine is a toxic compound that competes with acetylcholine for the same cholinoreceptors. Antimuscarinic agents are atropine, scopolamine, and ipratropium bromide. Atropine and scopolamine are alkaloids naturally occurring in Atropa belladonna and Datura stramonium plants, whereas ipratropium bromide is a derivative of atropine used to treat asthma.
Under the form of atropine sulfate, atropine is used in the treatment of gastrointestinal and bladder spasm, cardiac arrhythmias, and poisoning by cholinergic toxins such as organophosphates or muscarine. Atropine is used in ophthalmology as well when the measurement of eye refractive errors (i.e., cyclopegia) is required, due to its papillary dilation properties. Scopolamine shows an effect in the peripheral nervous system similar to those of atropine. However, scopolamine is a central nervous system (CNS) depressant and constitutes a highly effective treatment to prevent motion sickness, although at high doses it causes CNS excitement with side effects similar to those caused by high doses of atropine. Its use in ophthalmology is identical in purpose to that of atropine. The main use of ipratropium is for asthma treatment. Ipratropium is also administered to patients with chronic obstructive pulmonary disease.
Benapryzine, benzhexol, orphenadrine, and bornaprine are other examples of anticholinergic drugs used alone or in combination with other medications in Parkinson's disease to improve motor function. Disturbances in dopaminergic transmissions are associated with the symptoms observed in Parkinson's disease. The beneficial effects of anticholinergics in this disease are due to the resulting imbalance between dopamine and acetylcholine ratio in neurons (e.g., levels of acetylcholine lower than dopamine levels). These anticholinergic agents may interfere with mood and also decrease gastrointestinal movements, causing constipation; and the positive effects on motor functions vary among patients. Other classes of drugs available today that act on the pathways of dopamine and its receptors to treat Parkinson's disease, such as levodopa, tolcapone, and pramipexol, effectively increase the levels of dopamine at dopaminergic receptors in neurons.
Ganglionic blockers are anticholinergic agents that target nicotinic receptors in nerve cells of either sympathetic or parasympathetic systems. The most used ganglionic blockers are trimethaphan and mecamylamine. Trimethaphan is administered by intravenous infusion for the emergency short-term control of extreme high blood pressure caused by pulmonary edema, or in surgeries that require a controlled lower blood pressure, such as the repair of an aortic aneurysm. Mecamylamine is used to treat moderately severe and severe hypertension (high blood pressure), as the drug is easily absorbed when taken orally.
Neuromuscular anticholinergic agents act on motornerve cholinoreceptors. They prevent the transmission of signals from motor nerves to neuromuscular structures of the skeletal muscle. Neuromuscular blockers are very useful as muscle relaxants in several surgical procedures, either as an adjuvant to anesthesia or as a pre-anesthetic. Their main therapeutic use is in surgical procedures. Examples of the first group are mivacurium, tubocurarine,
Author Info: Sandra Galeotti MS, The Gale Group Inc., Gale, Detroit, Gale Encyclopedia of Neurological Disorders, 2005
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