Abstract
Purpose: :
Previously (ARVO 08, #2065) we reported a model of "glaucoma tolerance" wherein repetitive hypoxic preconditioning (RHP) completed prior to a 3-wk period of elevated IOP secondary to episcleral vein ligation completely prevented retinal ganglion cell (RGC) somal (brn3 in flat mounts) and axonal (optic nerve [ON] axon density) loss. In the present study, we extended the model to 10 wks of sustained IOP elevation, and examined morphological changes in neurofilament heavy chains (NfH) and astrocytes in retina and ON, and whether RHP-induced protection was still present.
Methods: :
We ligated the episcleral veins of one eye of adult male C57Bl/6J mice; newly patent veins were religated weekly (based on IOP magnitude) for 6 wks. A subgroup received RHP (a total of six 2-h exposures to systemic hypoxia [11% O2] every other day), completed 3 days prior to initial vein ligation. After 10 wks, flat-mounted retinae and cross-sections of nonmyelinated, post-laminar ON from both groups (GL only and RHP+GL) were examined by immunofluorescent confocal microscopy for brn3 (RGC antigen), SMI32 (dephosphorylated NfH), SMI34 (hyper-phosphorylated NfH), and GFAP (astrocytes); fellow eyes served as respective controls.
Results: :
A rapid (24 h) and sustained (measured weekly) increase in IOP was recorded in GL eyes (from 10.6±0.3 mmHg [baseline] to a 10-wk average of 18.4±0.5 mmHg; p=0.004) relative to fellow eyes (10-wk average=11.3±0.4 mmHg; n=10-15). The 32% decrease (p<0.001) in Brn3+ cell density in peripheral retina at 10 wks in GL mice (n=7) was completely attenuated (94% protection; p=0.006) in RHP+GL-treated mice (n=4). In flat mounts, the extent of reductions in SMI32+ axon number and soma density in GL mice, was much less in RHP+GL mice. Increases in SMI34+ soma staining number and intensity in GL mice, were not seen in RHP+GL mice. In the ON, SMI32+ axons were also reduced in number in GL mice, relative to the preservation of SMI32+ axons in the ON of RHP+GL mice. SMI34+ axons increased in GL, but not in RHP+GL, mice. GFAP expression, both arborization and intensity, increased in the ON of both GL and RHP+GL mice.
Conclusions: :
The activation of endogenous protective mechanisms by prior RHP robustly protects against RGC somal and axonal injury and dropout over 10-wks of sustained intraocular hypertension. Enhanced survival of RGCs was associated with preservation of axonal integrity and the ongoing activation of ON astrocytes. This novel capacity for sustained RGC self-preservation in the face of sustained IOP elevation is ripe for further investigation.
Keywords: ganglion cells • protective mechanisms • optic nerve