Axonal Transport Disruption in Retinal Ganglion Cells Following Transient Increase in Intraocular Pressure

Abstract

 

Purpose

 

Glial activation, changes in optic nerve head (ONH) structure, and disruption of axonal transport (AT) have all been associated with retinal ganglion cell (RGC) loss. The time-course of such changes is not fully understood. We examined AT following a transient ischemic injury induced by elevated intraocular pressure (IOP).

 

Methods

 

The right eye of Brown Norway rats was cannulated with a 30-gauge needle attached to a saline reservoir, raised to induce IOP of 120 mmHg. The left eye served as control. Elevated IOP was maintained for 90 (known to cause RGC loss) or 30 (known not to cause RGC loss) minutes. Retrograde and anterograde AT was tracked via injections of cholera-toxin β-subunit (CTB) Alexa488 conjugate immediately prior to IOP increase into the superior colliculus or vitreous, respectively. Rats were sacrificed following IOP increase after 3, 6 or 24 hours for anterograde AT analysis, and 3, 7 or 14 days for retrograde AT analysis. Average fluorescence of each experimental and control optic nerve (ON) was measured from confocal images of longitudinal sections along its entire length (see Figure). Data were analysed with a linear mixed-effects model. Loss of RGCs was quantified in retinal wholemounts with the RGC marker, Brn3a.

 

Results

 

A 90-minute increase in IOP caused a mean 98% (N=2) loss of RGCs at 7 days post high IOP, and retrograde accumulation of CTB in the ONH. In experimental ONs there was a mean 28% (N=12, P<0.001) decrease in retrograde AT and a mean 56% (N=12, P<0.001) decrease in anterograde AT compared to control ONs, with no significant effect of recovery time. Following 30-minutes IOP elevation, RGC loss was negligible (1.2%, N=2) and there was no evidence of blockade at the ONH. However, there was a mean 13% (N=12, P<0.001) decrease in retrograde AT and a mean 8% (N=12, P<0.001) decrease in anterograde AT. Like the 90-minute injury, changes in AT following 30-minutes IOP elevation showed no significant effect of recovery time.

 

Conclusions

 

Despite no meaningful RGC loss following 30-minutes of elevated IOP, there was significant blockade of both retrograde and anterograde AT evident in the ON. It is possible that AT blockade may not be the primary cause for RGC loss, or that the induced AT blockade did not reach the threshold for RGC death.

 

 

Keywords: 531 ganglion cells • 568 intraocular pressure • 629 optic nerve