Reduced photoreceptor damage after photodynamic therapy through blockade of nitric oxide synthase in a model of choroidal neovascularization

PURPOSE. To investigate the role of nitric oxide synthase (NOS) in photoreceptor degeneration associated with photodynamic therapy (PDT) in a laser-induced model of choroidal neovascularization (CNV). METHODS. PDT was performed in monkey and Brown Norway rats with laser-induced CNV. L-NAME, a NOS inhibitor, or saline was injected intraperitoneally in rats with CNV. An NO donor, or saline, was injected intravitreously into normal rats. Photoreceptor apoptosis was evaluated by TUNEL and electron microscopy. NOS, ED-1, and cleaved-caspase-3 (c-casp-3) expression were determined by immunohistochemistry. CNV lesions were examined by fluorescence angiography and choroidal flat mount. RESULTS. TUNEL and electron microscopy showed photoreceptor apoptosis after PDT. In rats, there were significantly more TUNEL-positive cells in the photoreceptors 24 hours after PDT, whereas in the CNV lesions there were more TUNEL-positive cells 6 hours after PDT. C-casp-3 was detected in the CNV lesions but not in the photoreceptors after PDT. There was no difference in the numbers of ED-1-positive macrophages before and after PDT. However, inducible NOS (iNOS) was increased after PDT in macrophages. Intravitreous injection of the NO donor without PDT also induced substantial photoreceptor apoptosis. L-NAME-treated animals had significantly fewer TUNEL-positive cells in the photoreceptors than saline-treated animals after PDT (P < 0.05). There were no differences in CNV size and leakage between L-NAME- and saline-treated groups. CONCLUSIONS. iNOS expression in macrophages contributes to PDT-induced photoreceptor degeneration. NOS inhibition reduces PDT-induced photoreceptor degeneration without compromising the treatment effect of PDT in an experimental model of CNV.