parasympathomimetica

Parasympathomimetica, or parasympathomimetics, are agents that mimic the effects of the parasympathetic nervous system by activating acetylcholine receptors, principally muscarinic receptors, or by increasing acetylcholine at synapses. They produce responses such as increased exocrine secretions, miosis, bronchoconstriction, slowed heart rate, enhanced gastrointestinal motility, and bladder contraction. The class is divided into direct-acting agents that stimulate receptors and indirect-acting agents that inhibit acetylcholinesterase, thereby prolonging acetylcholine action.

Direct-acting cholinergic agonists include bethanechol, carbachol, and pilocarpine. Bethanechol and carbachol primarily activate muscarinic receptors, with carbachol also affecting nicotinic receptors at certain doses; pilocarpine is a tertiary amine potent mainly at muscarinic receptors and is used for glaucoma and xerostomia. Indirect-acting agents inhibit acetylcholinesterase to raise acetylcholine levels; reversible inhibitors include neostigmine, physostigmine, edrophonium, and the longer-acting donepezil, rivastigmine, and galantamine. Irreversible inhibitors, such as some organophosphates, are highly toxic and historically used as pesticides or nerve agents; ophthalmic echothiophate is an example of past therapeutic use.

Clinical applications reflect the parasympathomimetic effects: glaucoma treatment with pilocarpine; postoperative ileus and urinary retention relief with bethanechol; myasthenia gravis management with neostigmine or pyridostigmine; cognitive symptoms in Alzheimer’s disease with donepezil, rivastigmine, or galantamine. Side effects are largely muscarinic (sweating, salivation, lacrimation, bradycardia, bronchoconstriction, diarrhea, abdominal cramps) and can lead to cholinergic crisis in overdose. Contraindications include asthma or COPD, peptic ulcer disease, significant bradycardia or hypotension, intestinal or urinary obstruction, and pregnancy.