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Pre- and postsynaptic effects of brimonidine on isolated rabbit iris dilator muscles

Authors Tatsui S, Ishikawa H, Shimizu K, Mashimo K

Received 18 November 2015

Accepted for publication 20 February 2016

Published 11 May 2016 Volume 2016:10 Pages 845—849


Checked for plagiarism Yes

Review by Single anonymous peer review

Peer reviewer comments 2

Editor who approved publication: Dr Scott Fraser

Sonoko Tatsui,1 Hitoshi Ishikawa,2 Kimiya Shimizu,1 Kimiyo Mashimo1

1Department of Ophthalmology, School of Medicine, Kitasato University, 2Department of Orthoptics and Visual Sciences, School of Allied Health Sciences, Kitasato University, Sagamihara, Japan

Purpose: Brimonidine is an imidazoline compound used for the treatment of glaucoma, but having very little effect on pupil diameter. Like para-aminoclonidine, most imidazoline compounds interact with postsynaptic α-adrenoceptors and cause pupil dilatation. Therefore, as part of an investigation of the mechanism of action of brimonidine on pupil diameter, the present study was initiated to measure, in vitro, the relative potency of brimonidine on the pre- and postsynaptic α-adrenoceptors of rabbit iris dilator muscle.
The contractile activity of brimonidine and its effect on twitch contraction evoked by electrical field stimulation were studied in isolated rabbit iris dilator muscles by isometric tension recording.
Brimonidine significantly inhibited the twitch contraction of the dilator muscle caused by field stimulation, without affecting the response to exogenously applied phenylephrine. Compared to phenylephrine, brimonidine caused only a small contractile response with % maximum contraction values of<10%.
Conclusion: These results suggest that brimonidine may act on nerve endings to inhibit adrenergic neurotransmission with very little effect on postsynaptic α-adrenoceptors. This may indicate that brimonidine reduced the pupil diameter just a little, thus improving night vision.

brimonidine, rabbit iris dilator, electrical field stimulation, presynaptic α2-adrenoceptor, postsynaptic α1-adrenoceptor, imidazolin

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