Dark adaptation is not necessary to drive the proliferative regeneration response in the adult zebrafish retina.

Macular degeneration, retinal detachment, and retinitis pigmentosa are eye diseases which cause damage to the photoreceptors and are considered leading causes of vision loss worldwide (National Eye Institute, 2022). Thus, developing animal models to study processes related to retinal damage, specifically loss of photoreceptors and their pending degeneration/regeneration is critical. This study aims to modify current light-induced retinal degeneration (LIRD) protocols within the zebrafish model to more efficiently replicate the proliferative response that drives regeneration. The most common LIRD model incorporates a 10-14 day dark adaptation period followed by multiple days of constant intense light exposure designed to damage the retinas and induce a proliferation response (Thomas et al., 2012; Vihtelic & Hyde, 2000). Modified procedures have been published and demonstrate that shortened 24-hour dark adaptation periods still result in cellular damage via apoptosis, but the proliferation response (i.e., pending regeneration) was not examined (Khan et al., 2020). Therefore, our study aims to fully eliminate dark adaptation and will use changes in retinal morphology and proliferation as the hallmark of a successfully initiated regeneration response. Dark-adapted (DA-control fish) and non-dark-adapted (NDA-experimental fish) were taken off their housing system and exposed to constant intense light for 24, 36, and 48 hr. to ensure enough time for damage and proliferation to proceed. Retinas were cross-sectioned to include dorsal and ventral retina, immunohistochemically labeled, and the outer nuclear layer (ONL), inner nuclear layer (INL), cones, and proliferation cell nuclear antigen (PCNA)-positive cells were analyzed to determine the level of structural integrity, if cell photoreceptor numbers were altered, and if proliferation occurred in response to light exposure. Dark-adapted (DA) and non-dark-adapted (NDA) zebrafish retinas displayed similar damage and increased proliferation responses to the constant intense light, indicating that the 10-14-day dark adaptation period is not necessary to induce regeneration in adult zebrafish retinas, reducing experimental timing by two weeks.