Retinal remodeling throughout hibernation in the 13-lined ground squirrel

Abstract

Purpose : To evaluate structural changes in the ground squirrel retina during distinct metabolic states of hibernation.

Methods : Retinas of 13-lined ground squirrels (13LGS; n=18) were imaged during four natural metabolic states (euthermia, pre-hibernation, torpor, or induced arousal) using a Bioptigen Envisu R2200 optical coherence tomographer (OCT). Each state was verified by examining body temperature (T_b) and behavioral phenotype. Layer thickness was measured at the visual streak (VS) from logarithmic vertical B-scans that were registered and averaged using custom software. Longitudinal reflectance profiles (LRP) from linear gray-scale images were averaged from three locations (VS, 500µm inferior to VS, and 500µm superior to VS) to assess morphological changes between metabolic states.

Results : Consistent metabolic state-dependent changes were detected (see table and figure). The inner nuclear layer (INL) was thickened in all states relative to euthermia. The distance between the interdigitation zone and Bruch’s membrane increased during torpor. Choroidal thinning was seen during torpor which returned to euthermic levels 60 minutes into arousal. Hyper-reflectivity was seen throughout the retina (most prominently in the INL) during torpor. Hyper-reflective blood was seen in all retinal vasculature during pre-hibernation when T_b is falling to room temperature. The ellipsoid zone, interdigitation zone¹ (also called inner segment/outer segment junction and photoreceptor outer segment tips,² respectively), and Bruch’s membrane all had decreased reflectance in torpor and arousal when compared to euthermia.

Conclusions : The source of outer retinal band reflectivity is controversial yet critical to interpretation of OCT images in any species. Metabolic changes in 13LGS retina across the hibernation cycle provide a natural model of reversible retinal remodeling that can help explain the origin of OCT reflectance and may be informative for therapeutic neuroprotection.

¹Staurenghi PMID:24755005
²Jonnal PMID:25324288

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.

 

Retinal layer thickness and reflectance data for each metabolic condition (mean±SD). Significant differences are shown relative to euthermia (t-test, *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001, ****p ≤ 0.0001).

 

Representative 13LGS visual streak OCTs from the four metabolic states. Below are magnified linear gray scale images and example LRPs from each state (orange line denotes ELM location, green box denotes LRP sampling region).