Choline esters: central effects, interactions and selected uses
THE CONTEXT
Acetylcholine has route-dependent central effects. The passage also describes interactions of choline esters, bethanechol use and the actions of pilocarpine.
Central effects of acetylcholine
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Intravenous acetylcholine does not cross the blood-brain barrier and produces no central effects.
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Direct injection into the brain produces an arousal response followed by depression.
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Cholinergic drugs that enter the brain produce complex behavioural and neurological effects.
Interactions
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Anticholinesterases markedly potentiate acetylcholine and potentiate methacholine to a lesser extent.
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Their interaction with carbachol or bethanechol is only additive; the difference depends on the role of cholinesterase in terminating each ester’s action.
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Atropine and related drugs competitively antagonize muscarinic actions. Adrenaline is a physiological antagonist.
Clinical use and bethanechol
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Choline esters are rarely used clinically. Acetylcholine is not used because its action is evanescent and nonselective.
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Bethanechol has been used to promote urination in nonobstructive urinary retention and neurogenic bladder.
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Bethanechol may provide symptomatic relief in congenital megacolon and gastroesophageal reflux, but is rarely used for these conditions.
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Prominent adverse effects include belching, colic, involuntary urination or defecation, flushing, sweating, a fall in blood pressure and bronchospasm.
Pilocarpine
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Pilocarpine has prominent muscarinic actions and also stimulates ganglia. It causes marked sweating, salivation and increased other secretions.
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Small doses generally lower blood pressure. Higher doses may cause hypertension and tachycardia, probably through ganglionic muscarinic stimulation.
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Ocular application penetrates the cornea and promptly causes miosis, ciliary muscle contraction and a reduction in intraocular tension lasting 4–8 hours.
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Pilocarpine is used as 0.5–4% eye drops and is described as a third-line drug for open-angle glaucoma.