ATP-induced retinal damage in cone-dominant 13-lined ground squirrel throughout euthermia

Abstract

Purpose: The 13-lined ground squirrel (13-LGS), a cone-dominant hibernator, has emerged as an accessible animal model for vision research. Induction of cone degeneration through exposure to cytotoxic chemicals has revealed variable effectiveness in the 13-LGS. Here, we sought to examine how the time of year (relative to hibernation emergence) influenced the degree of cone damage in 13-LGS following intravitreal injection of adenosine triphosphate (ATP). Methods: Eighteen (9M,9F) 13-LGS were placed into three active season experimental groups, early-season (N=6), mid-season (N=6), and late-season (N=6), to determine the time of intravitreal ATP injection. Each animal underwent baseline imaging OU using near-infrared reflectance and short wavelength autofluorescence scanning light ophthalmoscopy (SLO) as well as optical coherence tomography (OCT). Animals then received a 10µL intravitreal injection of 0.723M ATP in one eye, followed by OCT and SLO imaging at 1, 3, 10, and 21 days post-injection. Among mid-season and late-season animals where no retinal damage was observed in follow-up imaging, adaptive optics SLO (AOSLO) was performed. Retinal thickness and cone density measures from the late-season cohort were compared to values from a control group of wild-type animals (N=12 eyes, OCT; N=6 eyes, AOSLO). Results: Five animals showed retinal damage following ATP injection, including 4/6 early-season, 0/6 mid-season, & 1/6 late-season (Fisher’s exact test, p= 0.065). A two-way ANOVA showed significant differences in axial length between early-season & mid/late season cohorts (p = 0.029; p = 0.035). All animals with retinal damage displayed lesions on SLO and disrupted retinal lamination on OCT. Follow-up imaging with AOSLO on mid-season and late-season animals without observed retinal damage showed no evidence of photoreceptor disruption at any of the 97 ROIs analyzed. In late-season animals, 518 of 528 eccentricity matched locations imaged by OCT were within the normative range (mean +/- 2SD) from the control group, consistent with no ATP-induced damage. Conclusions: The 13-LGS retina may be more susceptible to retinal damage by intravitreal ATP injection during the early season. However, differences in both axial length and vitreous volume between groups may impact the effective dose. Future studies adjusting dose based on ocular biometry may help elucidate the impact of time of year on chemical response.