Photoreceptor synapses degenerate early in experimental choroidal neovascularization

Severe visual loss in patients with age-related macular degeneration is associated with the development of choroidal neovascularization (CNV). The pathogenic mechanisms for CNV formation have been extensively investigated, but remarkably little research has addressed the mechanisms for dysfunction of the retina in CNV. Using laser-induced CNV in mice, we evaluated the mechanisms of retinal dysfunction. At 3 days, 1 week, 2 weeks, and 4 weeks after laser application, retinas under experimental CNV were characterized physiologically (ERG recordings, synaptic uptake of the exocytotic marker FM1-43, and light-induced translocation of transducin), histologically, and immunohistochemically. ERG amplitudes were reduced by 20% at 1 week after CNV. Depolarization-induced FM1-43 uptake in photoreceptor synapses was selectively reduced by 45% at 1 week after CNV. Although photoreceptor outer segments were shortened by 36%, light adaptation as measured by transducin translocation was mostly preserved. Early in CNV (3 days to 1 week), Muller cells demonstrated induction of c-fos and pERK expression. Also, the density of macrophage-like, F4/80 immunoreactive cells increased ∼3-fold. Minimal photoreceptor death occurred during the first week, and was variable thereafter. At later times in CNV formation (≥2 weeks), expression of photoreceptor synaptic markers was reduced in the outer plexiform layer, indicating loss of photoreceptor synaptic terminals. ERG amplitudes, synaptic uptake of FM1-43, and the induction of c-fos and pERK in Muller cells were altered within 1 week of experimental CNV, suggesting that during CNV formation, deficits in retinal function, in particular photoreceptor synaptic function, precede degeneration of photoreceptor terminals and photoreceptor cell death.