Laser–Induced Ocular Hypertension Causes an Alteration of the Retrograde Axoplasmic Transport in Adult Rats

Abstract

Abstract: : Purpose: To study the effects of chronic ocular hypertension, induced by laser photocoagulation of episcleral and limbal veins, on the retrograde axonal transport in adult rat retinal ganglion cells (RGCs). Methods: In adult Sprague–Dawley rats, the intraocular pressure of the left eye was elevated by laser photocoagulation (532 nm wavelength, 500 mW power, 0.25 second duration, 100 µm diameter spot size) of limbar and episcleral veins. Intraocular pressure in both eyes was measured with a Tono–Pen prior to the procedure and 1 and 2 weeks after lasering. Retrograde axonal transport was investigated by examining the location and numbers of RGCs labeled with Fluoro–Gold (FG) applied to both superior colliculi 1 week before sacrifice and with dextran tetramethylrhodamine (DTMR, molecular weight 3000) applied intraorbitally to the ocular stump of the transected left optic nerve 5 days after FG application and 2 days prior to sacrifice. The nerve fiber layer was examined with one antibody against the 200 kD subunit of the neurofilaments (RT97). Results: Laser treatment increased approximately 30% the baseline levels of the intraocular pressure, and this elevation was maintained throughout the period of study. Three weeks after lasering, there were pie–shaped sectors of the retina devoid of FG–labeled RGCs and of DMTR–labeled axons. However, these sectors contained RGC axons labeled with RT97 and several axons showed the typical features of axonal degeneration (axonal beading). These sectors were more frequent in the superior retina and their size varied between animals, although often occupied as much as one quadrant of the retina. Conclusions: In adult rats, elevation of the intraocular pressure causes 3 weeks later an alteration of the retrograde axonal transport and possibly a secondary RGC degeneration in triangular sectors of the retina.

Keywords: intraocular pressure • ganglion cells • neuro-ophthalmology: optic nerve