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G.A. Williams, J.B. Calderone, G.H. Jacobs; Photoreceptors and Photopigments in a Fossorial Rodent, the Pocket Gopher (Geomys bursarius) . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4163.
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Purpose: Pocket gophers spend much of their lives in near-lightless subterranean burrows. We used structural and functional techniques to study visual adaptations associated with this extreme lifestyle. Methods: ERGs were recorded from corneal electrodes using a flicker photometric procedure. A variety of adaptation conditions were employed in an attempt to characterize signals from different receptor types. Cones were labeled using opsin antibodies (JH455, JH492) and PNA. Results: As in other cave-dwelling or burrowing animals, eye size is reduced in the pocket gopher. ERG spectral sensitivity functions reveal the presence of three spectral mechanisms: one rod-based with a spectral peak of about 497 nm, and two photopic mechanisms with respective peaks at about 360 nm (UV) and 505 nm (M). Both in terms of responsivity to lights varying in temporal frequency and in the recovery time recorded following exposure to bleaching lights the results from the pocket gopher seem quite similar to indices similarly measured in ERG recordings from the cones of other rodents. Although the gopher retina is small, having a surface area of only about 30 mm2, it contains a relative abundance of cones that can reach a density in excess of 20,000 mm2. There is only modest variation in cone density across the retina. Comparison of labeling patterns to the opsin antibodies and to PNA reveal a striking inhomogeneity in the retinal distribution of the two cone pigments that bears resemblance to the pattern earlier documented for the mouse retina. Both UV and M pigments are present in the dorsal retina of the gopher where there is widespread (if not universal) coexpression of the two pigments in individual cones. In contrast, M pigment is only very sparsely represented in the cones of the ventral retina. Conclusions: The retina of pocket gopher is relatively cone rich. There are two types of cone pigment with spectral peaks of about 360 nm and 505 nm. Many cones coexpress the two pigments but there is a distinctive regionalization of the two pigment types. It is difficult to discern the functional utility of many of these features which may more reflect the ancestry of the pocket gopher rather than adaptation to current visual demands.
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