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Direct-Acting Cholinergic Agonists: Pharmacological Actions

Direct-Acting Cholinergic Agonists: Pharmacological Actions

Direct-acting cholinergic agonists exert their pharmacological actions by mimicking the effects of acetylcholine on postsynaptic muscarinic receptors to generate parasympathetic responses. These agents elicit a range of physiological responses, including cardiovascular effects. For example, activation of muscarinic receptors induces bradycardia, decreased cardiac output, reduced peripheral resistance, and consequent hypotension. In the eye, stimulation of M3 receptors leads to smooth muscle...

Direct-Acting Cholinergic Agonists: Therapeutic Uses

Direct-Acting Cholinergic Agonists: Therapeutic Uses

Direct-acting cholinergic agonists have many therapeutic uses in various medical fields. Choline esters, including acetylcholine, have limited clinical utility due to their non-selectivity and short duration of action. Still, acetylcholine and carbachol are applied topically during ophthalmologic surgery to induce miosis. Pilocarpine, a muscarinic and ganglionic stimulator, effectively treats open-angle glaucoma and alleviates xerostomia and dry mouth caused by radiotherapy or Sjögren syndrome.

Cholinergic Antagonists: Pharmacokinetics

Cholinergic Antagonists: Pharmacokinetics

Cholinergic antagonists—such as antimuscarinics—are available in oral, topical, ocular, parenteral, and inhalational formulations. Most antimuscarinics are oral formulations, while scopolamine is available as a topical patch, and ipratropium and tiotropium are available as inhalation aerosols or powders. Atropine, tropicamide, and cyclopentolate are topically instilled in the eye. Most antimuscarinics are lipid-soluble and readily absorbed from the gastrointestinal tract and the conjunctiva.

Open Angle Glaucoma: Treatment

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In open-angle glaucoma, the iridocorneal angle remains open, but the trabecular meshwork becomes stiff, slowing down the outflow of aqueous humor. This causes a buildup of aqueous humor in the anterior chamber, leading to a sudden increase in intraocular pressure. The treatment for open-angle glaucoma focuses on reducing the elevated intraocular pressure by either decreasing the secretion of aqueous humor or increasing its outflow.
Drugs such as carbonic anhydrase inhibitors, α2- and...

Cholinergic Antagonists: Pharmacological Actions

Cholinergic Antagonists: Pharmacological Actions

Antimuscarinic drugs block muscarinic receptors in multiple systems, including the gut, eye, smooth muscles, respiratory tract, cardiovascular, and central nervous systems. They produce similar effects with varying selectivity depending on the specific agent and tissue. Here are the key pharmacological actions of antimuscarinics:
Gastrointestinal Effects: Antimuscarinics reduce gut contractions, increase gastric emptying, and slow intestinal transit. They partly inhibit gastric acid secretion...

Cholinergic Antagonists: Therapeutic Uses

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Antimuscarinic drugs have various therapeutic applications by inhibiting parasympathetic stimulation in different systems. Here are the key therapeutic uses of antimuscarinics:
Respiratory Tract: Ipratropium, aclidinium, and tiotropium treat asthma, chronic bronchitis, and chronic obstructive pulmonary disease (COPD). They protect against bronchoconstriction caused by irritants like cigarette smoke, sulfur dioxide, and ozone. They also help reduce nasopharyngeal secretions in common...

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Related Experiment Video

Updated: Jun 3, 2026

Assessing Pupil-linked Changes in Locus Coeruleus-mediated Arousal Elicited by Trigeminal Stimulation

Assessing Pupil-linked Changes in Locus Coeruleus-mediated Arousal Elicited by Trigeminal Stimulation

Published on: November 26, 2019

Apraclonidine and my pupil.

Melissa Cambron¹, Heidi Maertens, Luc Crevits

¹Ghent University, and Department of Neurology, Ghent University Hospital, Ghent, Belgium. Melissa.cambron@vub.ac.be

Clinical Autonomic Research : Official Journal of the Clinical Autonomic Research Society

|March 9, 2011

Summary

Apraclonidine 1% causes miosis in healthy eyes, peaking 30-60 minutes post-instillation. This effect differs from its use in diagnosing Horner

Area of Science:

Ophthalmology
Pharmacology

Background:

Apraclonidine 1% is used to diagnose Horner's syndrome by causing mydriasis in affected eyes due to denervation supersensitivity.
However, recent studies suggest apraclonidine may have a mild miotic effect in healthy eyes.

Purpose of the Study:

To investigate the effect of apraclonidine on pupillary parameters in healthy eyes.
To determine the optimal time for evaluating apraclonidine's effect on the pupil.

Main Methods:

Infrared pupillography was used to measure pupil diameter and light response.
Measurements were taken at various intervals up to 360 minutes after instilling apraclonidine 1% in one eye of 14 healthy volunteers.

Main Results:

Apraclonidine 1% caused relative miosis in the treated eye, most significantly at 30 and 60 minutes post-instillation.

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The amplitude of light-induced pupil constriction increased in the apraclonidine-treated eye.

No significant changes were observed in pupil latency or constriction/redilation velocity.

Conclusions:

Apraclonidine 1% induces relative miosis in healthy subjects, with the effect most pronounced between 30-60 minutes.
This miotic effect is attributed to a stronger alpha-2 agonistic effect than alpha-1 agonistic effect in healthy eyes.
In contrast, alpha-1 agonistic effects dominate in Horner's syndrome patients due to receptor supersensitivity, causing mydriasis. This highlights the importance of instilling apraclonidine in the unaffected eye for accurate Horner's syndrome diagnosis.