Purpose: To investigate the effects of laser photocoagulation (LP)-induced ocular hypertension (OHT) on the survival and retrograde axonal transport of retinal ganglion cells (RGC), as well as on the function of retinal layers.
Methods: Adult albino Swiss mice (35-45 g) received laser photocoagulation of limbal and episcleral veins in the left eye. Mice were sacrificed at 8, 17, 35, and 63 days. Intraocular pressure (IOP) in both eyes was measured with a Tono-Lab before LP and at various days after LP. Flash electroretinogram (ERG) scotopic threshold response (STR) and a- and b-wave amplitudes were recorded before LP and at various times after LP. RGCs were labeled with 10% hydroxystilbamidine methanesulfonate (OHSt) applied to both superior colliculi before sacrifice and in some mice, with dextran tetramethylrhodamine (DTMR) applied to the ocular stump of the intraorbitally transected optic nerve. Retinas were immunostained for RT97 or Brn3a. Retinas were prepared as whole-mounts and photographed under a fluorescence microscope. Labeled RGCs were counted using image analysis software, and an isodensity contour plot was generated for each retina.
Results: IOP increased to twice its basal values by 24 h and was maintained until day 5, after which IOP gradually declined to reach basal values by 1 wk. Similar IOP increases were observed in all groups. The mean total number of OHSt(+) RGCs was 13,428+/-6,295 (n=12), 10,456+/-14,301 (n=13), 12,622+/-14,174 (n=21), and 10,451+/-13,949 (n=13) for groups I, II, III, and IV, respectively; these values represented 28%, 23%, 26%, and 22% of the values found in their contralateral fellow retinas. The mean total population of Brn3a(+) RGCs was 24,343+/-5,739 (n=12) and 10,219+/-8,887 (n=9), respectively, for groups I and III; these values represented 49% and 20%, respectively, of the values found in their fellow eyes. OHT retinas showed an absence of OHSt(+) and DTMR(+) RGCs in both focal wedge-shaped and diffuse regions of the retina. By 1 wk, there was a discrepancy between the total number of surviving OHSt(+) RGCs and Brn3a(+) RGCs, suggesting that a large proportion of RGCs had impaired retrograde axonal transport. In the retinal areas lacking backlabeled RGCs, neurofibrillar staining revealed aberrant expression of RT97 within axons and RGC bodies characteristic of axotomy. Elevated IOP induced significant reductions in the registered ERG waves, including positive STR, a- and b-waves, that were observed by 24 h and remained throughout the period of study for the three groups analyzed.
Conclusions: LP of the perilimbal and episcleral veins resulted in OHT leading to a lack of retrograde axonal transport in approximately 75% of the original RGC population. This lack did not progress further between 8 and 63 days, and it was both focal (in sectors with the apex located in the optic disc) and diffuse within the retina. In addition, severe amplitude diminutions of the STR and a- and b-waves of the ERG appeared as early as 24 h after lasering and did not recover throughout the period of study, indicating that increased IOP results in severe damage to the innermost, inner nuclear, and outer nuclear layers of the retina.