The purpose of this study is to examine the retinal expression of
intrinsic cell survival molecules and to elucidate the effect of an
α2-adrenergic receptor agonist in the chronic ocular hypertensive rat
model. Chronic ocular hypertension was induced in both eyes of each rat by
episcleral vein cauterization. Two five-microliter drops of the selective
α2-adrenoceptor agonist brimonidine 0.2% (Alphagan; Allergan Inc.,
Irvine, CA, USA) were topically administered twice daily for up to eight
weeks in one eye. The fellow eye received balanced salt solution as a
control. Protein and mRNA expression were evaluated at 1, 4, and 8 weeks
after injury. Retinal expression of BDNF, Akt, and GFAP was assessed using
immunohistochemistry. Retinal levels of mRNA for BDNF, bcl-2, and bcl-xL
were determined using semi-quantitative RT-PCR. Retinal ganglion cell
(RGC) density was evaluated after retrograde labeling with 4-Di-10-ASP
(DiA). A significant decrease in RGC density was observed in ocular
hypertensive eyes. Cauterized eyes showed an increase in GFAP expression
from one week after injury, and the expression of bcl-2, bcl-xL, and BDNF
mRNA was also increased. Treatment of ocular hypertensive eyes with
brimonidine resulted in a reduction in RGC loss, a decrease in the level
of GFAP immunoreactivity, and an increment in BDNF mRNA and p-Akt
expression. Brimonidine appears to protect RGCs from neurodegeneration
through mechanisms involving α2-adrenergic receptor mediated survival
signal activation and up-regulation of endogenous neurotrophic factor
expression in the chronic ocular hypertensive rat retina.