Total retinal nitric oxide production is increased in intraocular pressure-elevated rats

Nitric oxide (NO) is a well-known vaso-dilator but its regulation in the retina is unclear. This study was conducted to quantify total NO production and retinal ganglion cell (RGC) loss in an experimental glaucoma model. Three quarters of the peri-limbal/episcleral drainage vessels and anterior angle (right eyes) of Sprague-Dawley rats were thermally blocked using laser irradiation, while the left eyes served as controls. We measured the intraocular pressure (IOP) of both eyes using a digital tonometer (Tonopen) 21, 28 and 35 days after the laser treatment. After 35 days, we determined the total NO level in retinas and remaining ocular tissues for the laser-treated and control eyes using a spectro-photometric assay. The viable RGC numbers were also determined by counting the cell bodies stained retrogradely by fluoro-gold. The laser treatment significantly increased the lOP 2.0-2.6× throughout the whole period of measurements (P < 0.0001). The mean total RGC number decreased significantly from 98 725 ± 5383 (±S.E. (M.)) to 69 276 ± 5592, or 29.8% reduction, in the laser-treated eyes after 35 days (P = 0.008). The mean total NO level in the laser-treated retina was significantly increased by 2.4× compared with controls (P = 0.016), but no significant difference was found in the eyecups (P > 0.05). Laser treatment resulted in significant IOP elevation and RGC loss, suggesting that thermal coagulation of the perilimbal region may provide an alternate protocol for glaucoma study. NO level was increased by two-fold in the retina but not in other ocular tissues. Since NO is capable of producing powerful peroxynitrite anions and hydroxyl radicals, elevated level of NO has a potential role in glaucoma.