Selective photoreceptor degeneration by intravitreal injection of N-methyl-N-nitrosourea

To characterize the effects of intravitreal injections of N-methyl-N-nitrosourea (MNU) in comparison to its systemic application as a measure of inducing unilateral photoreceptor degeneration. Eight-week-old male C57BL/6J mice received either intraperitoneal injections (three animals) or intravitreal injections (24 animals) of MNU in different concentrations and were observed over a period of 2 weeks using full-field electroretinography (ERG), spectral-domain optical coherence tomography (SD-OCT), and immunohistochemistry. The intraperitoneal application of MNU showed moderate systemic toxic effects, indicated by a loss of body weight of 12% within the first 2 days. In both eyes the ERG became extinguished, and SD-OCT scans showed a thinning of the retina, predominantly in the outer nuclear layer (ONL). Immunohistochemistry demonstrated the selective loss of rods and cones. Mice that received intravitreal MNU injections displayed nearly no weight loss, and no degeneration of their general welfare was observed. After 2 weeks, ERG, SD-OCT, and immunohistochemistry revealed changes identical to those seen after systemic application in the injected eye, but not in the control eye. The intraperitoneal application of MNU led to moderate systemic side effects in mice and to selective photoreceptor degeneration. Intravitreal injections of MNU also induced photoreceptor degeneration; however, no systemic side effects were observed. This tool may be helpful in larger species, where genetic models of receptor degenerations are not applicable but where the size of the eye is more suitable to study surgical or other approaches to treat blindness caused by receptor degeneration.