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S. Bonfield, N. Crowder, G. Gilmour, S. Mema, F. Gaillard, W. K. Stell, Y. Sauvé; Retina Function and Spatial Vision in a Diurnal Cone-Rich Laboratory Rodent, the Nile Grass Rat (Arvicanthis Niloticus). Invest. Ophthalmol. Vis. Sci. 2008;49(13):3838.
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Research in retinal disorders has come to rely on cone-poor rodent models such as mice or rats, ~3% of whose photoreceptors are cones. These species are primarily nocturnal or crepuscular. Here we report on visual function in Arvicanthis niloticus (Nile grass rat), a diurnal laboratory rodent with 40% cones.
Optokinetic responses (OKR) to moving gratings were tested using OptoMotryTM (Cerebral Mechanics, Lethbridge, AB, Canada). Nile grass rats, aged 101-104 days (n = 4), were presented with horizontally drifting sine-wave gratings of various spatial frequencies (SF = 0.025 c/d - 1.4 c/d) and drift speeds (6, 12, 24 d/s), rotating to either left or right. Full-field flash cone-driven electroretinogram (ERG) was also recorded.
OKR: The contrast sensitivity (CS) function appeared nearly flat from SF = 0.025 - 0.8 c/d, with a decline towards higher SFs. CS functions at velocities (V) = 6 and 12 d/s were indistinguishable, whereas CS remained elevated at lower SFs at V = 24 d/s. The maximal CS was very high (36.5±17.6 at 0.2 c/d, 52.7 ± 37.0 at 0.4 c/d, V = 6 d/s; 42.2 ± 16.3 at 0.2 c/d and 52.9±32.7 at 0.4 c/d, V = 12 c/d), and CS was still rather high at 0.025 c/d (9.3±1.5 at 6 d/s; 10.1±2.5 at 12 d/s). The highest SF to which all animals responded at 100% contrast (="acuity") was 1.0 c/d. ERG: Several cone ERG features, absent in laboratory rats and mice, were typical in the Nile grass rat: a) large a-waves (81±16 µV, n =16); b) a photopic Hill effect; and c) maximal flicker amplitudes maintained from 5 to 35 Hz without fusing, even at the highest frequency tested, 60 Hz.
The photopic visual responses of the Nile grass rat are robust, in keeping with its having a cone-rich retina. The OKR contrast sensitivity and acuity are more than twice as high as in laboratory rats and mice, and several photopic ERG features are unique to Nile rats. The Nile grass rat is a distinctive species that shows promise as a model for understanding retinal disorders, particularly those due to cone pathologies.
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