Assessing seasonal changes of cone photoreceptor structure in the 13-lined ground squirrel

Abstract

Purpose : To evaluate structural changes of 13-lined ground squirrel (13-LGS) cone photoreceptors throughout their annual heterothermic cycle using adaptive optics scanning light ophthalmoscopy (AOSLO) and transmission electron microscopy (TEM).

Methods : The photoreceptor mosaic of the cone-dominant 13-LGS (n=5) was imaged before, during, and after hibernation using confocal and non-confocal split-detection AOSLO. Distinct physiological states (pre-hibernation, torpor, 1 and 3 days post-hibernation, and summer euthermia) were verified by measuring body temperature and examining behavior. For each animal, montages of the same retinal region (just superior to the optic nerve head) in each physiological state were manually aligned. Cone density was calculated semi-automatically using the split-detector AOSLO image.[1] TEM was used to compare cone ultrastructure from two additional euthermic and torpid 13-LGS.

Results : Compared to euthermia, cone photoreceptor remodeling was first observed during pre-hibernation; most photoreceptors appeared dark or multi-modal in confocal AOSLO images. Despite these waveguiding changes, cones were resolvable with split-detection AOSLO in all 5 pre-hibernating 13-LGS. In contrast, cone appearance was altered during torpor in 4 of the 5 13-LGS in both confocal and split-detection AOSLO images (Figure). Cone density did not change between pre-hibernation and 1 day post-hibernation states (p = 0.179, paired t-test; 65,468 to 62,110 cell/mm²; 69,458 to 69,309 cell/mm²; 80,269 to 81,144 cell/mm²; 68,815 to 67,394 cell/mm²; and 71,424 to 69,312 cell/mm² in 5 regions sampled in 5 different 13-LGS). In addition, euthermic-like waveguiding appearance returned 1 day post-hibernation in all 5 animals. During torpor, TEM revealed fragmented outer segment discs within enlarged cone sheaths, plus a cytoplasmic gap between the distal inner segment plasma membrane and ellipsoid mitochondria (Figure).

Conclusions : Our data suggest that the 13-LGS cone mosaic is only transiently affected by any remodeling that occurs because of torpor. Pairing non-invasive imaging with ultrastructural assessment may provide insight to the biological origin of photoreceptor reflectance.

[1] PMID: 27231641

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Euthermic and torpid 13-LGS photoreceptors visualized with confocal and split-detection AOSLO in the same animal, and TEM in 2 separate animals (AOSLO scale = 10 µm; TEM scale = 2 µm).