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Reduction of capsaicin-induced ocular pain and neurogenic inflammation by calcium antagonists
G G Gonzalez1, P Garcia de la Rubia, J Gallar
1Department of Physiology, University of Alicante, Spain.
Purpose:
To examine whether blockade of chemosensitivity of corneal nociceptors by Ca2+ antagonists decreases pain and irritation induced by capsaicin.
Methods:
In adult rabbits, the number of lid-squeezing movements and the degree of palpebral opening, miotic response, and conjunctival vasodilation evoked by a bilateral instillation of 30 microliters of capsaicin (33 mM) were measured at different times (up to 5 hours) after the drug. Irritative responses to capsaicin in eyes pretreated with diltiazem, verapamil, or nifedipine were compared with those that received only the vehicle. Protein content in aqueous humor was also measured at the end of the experiment.
Results:
Diltiazem at doses of 1 to 28 mM, administered 15 minutes before the application of capsaicin, significantly decreased scratching movements, conjunctival hyperemia, closure of the eye, and elevated aqueous protein concentration induced by capsaicin; however, it did not significantly reduce miosis. Nifedipine (2.8 and 10 mM) diminished the number of scratching movements but not other inflammatory parameters, whereas verapamil (2.8 and 10 mM) was totally ineffective in attenuating ocular signs of irritation produced by capsaicin.
Conclusions:
These results suggest that by lowering capsaicin-induced neural activity in nociceptive terminals, diltiazem decreases pain and neurogenic inflammation and may be useful as both an analgesic and an antiinflammatory agent in the eye.
Insights
Calcium channel blockers like diltiazem may reduce eye irritation and pain. Diltiazem effectively decreased capsaicin-induced ocular discomfort and inflammation in rabbits.
Area of Science:
- Ophthalmology
- Neuroscience
- Pharmacology
Background:
- Corneal nociceptors are sensitive to chemical stimuli like capsaicin.
- Capsaicin induces pain and neurogenic inflammation in the eye.
Purpose of the Study:
- To investigate if calcium (Ca2+) antagonists can block chemosensitivity in corneal nociceptors.
- To determine if Ca2+ antagonists reduce capsaicin-induced ocular pain and irritation.
Main Methods:
- Adult rabbits received capsaicin instillation in both eyes.
- Ocular responses including lid squeezing, eye closure, miosis, and conjunctival vasodilation were measured.
- Eyes were pretreated with diltiazem, verapamil, or nifedipine, and compared to vehicle-treated eyes.
Main Results:
- Diltiazem significantly reduced scratching, conjunctival hyperemia, eye closure, and aqueous protein concentration.
- Nifedipine decreased scratching but not other inflammatory signs.
- Verapamil showed no significant effect on capsaicin-induced ocular irritation.
Conclusions:
- Diltiazem lowers capsaicin-induced neural activity in corneal nociceptors.
- Diltiazem effectively reduces ocular pain and neurogenic inflammation.
- Diltiazem shows potential as an ophthalmic analgesic and anti-inflammatory agent.