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Editors Selection IGR 22-3

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Comment by Makoto Aihara on:

22579 Effect of brimonidine on retinal ganglion cell survival in an optic nerve crush model, Ma K; Xu L; Zhang H et al., American Journal of Ophthalmology, 2009; 147: 326-331

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Ma et al. determined the loss of retinal ganglion cells in rats twentythree days after their optic nerves were given a 60-second crush with a 40~g power microclip. During this time the rats received daily intraperitoneal injections of brimonidine (1 mg/Kg) or saline. Ma and collaborators concluded that ganglion cell death was greatest in the saline treated group of rats and that brimonidine significantly enhanced their survival. Standard experimental procedures were used where it was necessary to precisely repeat various steps in a number of rats of presumed same age and sex. This included placement of a microclip to the optic nerve, injection of flurogold into the middle of superior colliculi and the quantification of numbers of labeled ganglion cells in repeatedly defined areas of retinas. Ma et al. conclude that brimonidine reduces the impact of optic nerve crush to the retinal ganglion cells which is consistent with other studies where insults of a variety of types were delivered to retinal ganglion cells in situ and where brimonidine was administered by different regimes. The authors sensibly discuss the relevance of their data related to the clinical use of brimonidine for the treatment of glaucoma. Some of the known physiological influences of α₂-agonists such as brimonidine are that it causes vasoconstriction and hypotension. This raises the question whether, in the present studies, enhancement of retinal ganglion cell survival is due to a direct or indirect effect of intraperitoneal injections of brimonidine. A single publication has shown α₂A- receptors to be associated with retinal ganglion cells demonstrating that a direct action is possible. Interestingly, few data exist which describe the physiological role(s) of retinal ganglion cell α₂-receptors or indeed the role(s) of their natural ligands (adrenaline, noradrenaline) in retinal physiology.

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