Glaucoma-induced vision loss is the result of degeneration of
ganglion cells and their axons. Although it is well established that
elevated IOP is one of the risk factors in this disease process the
mechanism of degeneration is unclear. The continued loss of vision in
eyes with glaucoma, despite control of IOP, emphasizes the need for
neuroprotective therapy. The development of a chronic ocular
hypertension model in the rat provides a valuable means of evaluating
the mechanism and process of injury and can be used to evaluate
compounds with neuroprotective activity.
4 10 15 16 17 18 In
this study, the neuroprotective effect of brimonidine was examined
using rats with laser-induced chronic ocular hypertension. All the rat
models reported by others and the one described in this study have
shown that elevation of IOP causes ganglion cell or optic nerve fiber
loss.
4 10 15 16 17 18 It has also been shown that the level of
IOP obtained determines the extent of ganglion cell loss.
4 In this study, IOP elevation of twofold caused ganglion cell loss of
44% after 2 months, averaging 5.5% per week. This result is
comparable to previous reports showing that cell losses of
approximately 37% and 47% were obtained after 6 and 10 weeks,
averaging 6% and 5% per week, respectively.
4 15 Other
investigators have observed cell loss at a slower rate of 1.4% per
week when the average cell loss was calculated over a longer period of
6 months and the increase in IOP was 1.6-fold or less.
17 We noted a biphasic rate of ganglion cell loss with a fast rate of 12%
per week for the first 3 weeks of IOP elevation, followed by a slower
rate of 2% per week for the remainder of the experimental period. The
rate of ganglion cell loss in the other studies is
linear.
4 15 17
In prior studies cautery was used once to occlude two or three major
episcleral veins, 2 to 3 mm from the limbus, whereas laser treatment in
the present study was performed twice, on several veins within 1 mm
from the limbus. Thus, in addition to a difference in the technique of
vessel occlusion, the discrepancy in rate of ganglion cell loss could
also be due to the rate at which the target IOP was reached and
maintained. In this study, target IOP was reached by two laser
treatments within 1 week, which may have initiated a faster rate of
initial ganglion cell loss. The consistent and significant loss of
ganglion cells after 3 weeks of elevated IOP provided us with an
opportunity to evaluate neuroprotective activity of pharmacologic
agents within a relatively short period.
In this study, we provide evidence that theα
2-adrenergic agonist brimonidine is
neuroprotective of ganglion cells in eyes with laser-induced chronic
ocular hypertension. Brimonidine is currently used to lower IOP in the
treatment of open-angle glaucoma.
19 20 21 The
neuroprotective effect of brimonidine was evaluated in two different
ways. When brimonidine was applied at the time of IOP elevation, before
pressure-induced ganglion cell injury began, it attenuated ganglion
cell loss by 50% (cell loss with vehicle 33% and with brimonidine
15%), in spite of the sustained increase in IOP. More interesting was
that when brimonidine was administered 10 days after IOP was elevated,
it prevented any further loss of ganglion cells. In both treatment
paradigms systemically applied brimonidine (400 μg/d or 17 μg/h to
each rat) reached the retina in sufficient amount to exert
neuroprotection but had little or no effect on IOP. However, topically
applied brimonidine (0.2%, 10 μg/5 μl) was effective in lowering
IOP by 40% (data not shown) and was neuroprotective of retinal neurons
in a rat acute retinal ischemia–reperfusion injury
model.
22 Moreover a recent study has shown that vitreal
levels of brimonidine after topical application of a 0.2% dose in
phakic eyes with planned vitrectomy was 9.3 ± 8
nM.
23 This concentration is more than adequate to activate
the α
2 receptors in the retina. The median
effective dose (EC
50) of brimonidine for
activating α
2 receptors is approximately 2
nM.
24
The neuroprotective effect of brimonidine was compared with that of
timolol. Both brimonidine and timolol are currently used for the
treatment of glaucoma, and they lower IOP significantly. It is thought
that lowering of IOP reduces injury and preserves the visual field. In
this study, brimonidine, but not timolol, was neuroprotective. Similar
to brimonidine, topical timolol (0.5%, 5 μl) caused a decrease in
IOP by 27% (data not shown). Similarly, the amount of timolol that
reached the eye when delivered by osmotic pump appeared to be lower
than required to decrease IOP. This emphasizes the direct
neuroprotective effect of brimonidine.
In several models of retinal injury, stress, or degeneration,
expression of GFAP increases in Müller
cells.
13 25 26 The increase in immunoreactivity of GFAP in
this ocular hypertension model may be a response to the stress and
pathologic process that leads to degeneration of ganglion cells. The
decrease in GFAP immunoreactivity by brimonidine suggests that
activation of the selective α
2-adrenergic
receptors reduces this injury in the retina.
Several mechanisms underlying the neuroprotective activity ofα
2-adrenergic agonists have been proposed.
Brimonidine has been shown to increase neurotrophic
factors
7 and thus may be neuroprotective by enhancing the
survival of ganglion cells in this hostile environment. In addition,
activation of presynaptic α
2-receptors results
in inhibition of transmitter release.
27 28 It is possible
that brimonidine treatment attenuated the release of glutamate in the
eyes with elevated IOP. Increase in vitreal glutamate has been
implicated in excitotoxicity of ganglion cells in
glaucoma.
29 Although we did not measure vitreal glutamate
in this study, there is a report showing that that brimonidine inhibits
accumulation of glutamate in the vitreous after acute retinal
ischemia.
9
In summary, we have shown that laser photocoagulation of limbal and
episcleral veins in the rat caused chronically elevated IOP, resulting
in significant loss of ganglion cells. Systemic treatment with theα 2 agonist brimonidine provided neuroprotection
to ganglion cells in this model.
The authors thank James Burke and Peter Baciu for critical comments
on the manuscript.