The Nile Rat Model of Diabetes- a Cone-Rich Retina with a Nocturnal Eye Design Enables Enhanced Resolution of the Retina

Abstract

Purpose: : The diurnally active Nile rat has a cone-rich retina and develops diabetes when fed a lab chow diet. We wished to investigate the ocular properties of this animal model, in particular whether the optical properties are similar to the human eye or to a nocturnal rat strain.

Methods: : The optical properties (including, ocular dimensions using ultrasound and frozen sections; refractions by retinoscopy and corneal curvature by keratometry) of 3 Nile rats raised on lab chow were measured, some prior to and some following euthanasia. Animals were 6 months (2 animals) and 1 year 5 months (1 animal) of age. Diabetic status was assessed via urine and plasma glucose analysis. Ocular properties were compared to published rat and human schematic eyes.

Results: : The younger rats tested normal for diabetic status and the older rat tested abnormal for both urine and plasma glucose assessments. Measurements of ocular dimensions, refractions and corneal curvatures were similar for all animals. Hyperopic refractions ranged from 8 to 12 D and corneal curvatures were not significantly different from published data of 4 month old pigmented Dark Agouti (DA) rats. Lens dimensions were also in a similar proportion in the DA and Nile rats, with the lens occupying 59-61% of the Nile rat eye length compared with 63% in the DA rat and 15% in the human eye. The almost spherical lens shape (an ellipticity between 1.17 and 1.23) were also consistent with a nocturnal eye design and was similar to the DA rat (1.14-1.23). The f numbers then expected to be similar to the DA rat (0.78) compared to the human of 2.4.

Conclusions: : The diurnal specialization of the Nile rat extends only to the retina. Optical properties are consistent with a nocturnal eye design. As such, a larger stroke adaptive optics element will be required for the correction of refraction and higher order aberrations than in human eyes. However, with this correction, the resultant resolution of retinal details will be enhanced because of the smaller f number of this eye design.

Keywords: diabetes • optical properties • refraction