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1From the F. M. Kirby Center for Molecular Ophthalmology, Department of Ophthalmology, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; the 2Departments of Pharmacology and 3Neuroscience, School of Medicine, University of California, San Diego, California; 4Department of Pathology, Anatomy and Cell Biology, Thomas Jefferson University, Philadelphia, Pennsylvania; and the 5Departments of Ophthalmology and Visual Sciences and 7Human Genetics, University of Michigan, Ann Arbor, Michigan.
PURPOSE. To test the hypothesis that Nrl–/– photoreceptors are cones, by comparing them with WT rods and cones using morphological, molecular, histochemical, and electrophysiological criteria.
METHODS. The photoreceptor layer of fixed retinal tissue of 4- to 6-week-old mice was examined in plastic sections by electron microscopy, and by confocal microscopy in frozen sections immunolabeled for the mouse UV-cone pigment and colabeled with PNA. Quantitative immunoblot analysis was used to determine the levels of expression of key cone-specific proteins. Single- and paired-flash methods were used to extract the spectral sensitivity, kinetics, and amplification of the a-wave of the ERG.
RESULTS. Outer segments of Nrl–/– photoreceptors (
7 µm) are shorter than those of wild-type (WT) rods (25 µm) and cones (15 µm); but, like WT cones, they have 25 or more basal discs open to the extracellular space, extracellular matrix sheaths stained by PNA, chromatin "clumping" in their nuclei, and mitochondria two times shorter than rods. Nrl–/– photoreceptors express the mouse UV cone pigment, cone transducin, and cone arrestin in amounts expected, given the relative size and density of cones in the two retinas. The ERG a-wave was used to assay the properties of the photocurrent response. The sensitivity of the Nrl–/– a-wave is at its maximum at 360 nm, with a secondary mode at 510 nm having approximately one-tenth the maximum sensitivity. These wavelengths are the 
max of the two mouse cone pigments. The time to peak of the dim-flash photocurrent response was 50 ms, more than two times faster than that of rods.
CONCLUSIONS. Many morphological, molecular, and electrophysiological features of the Nrl–/– photoreceptors are cone-like, and strongly distinguish these cells from rods. This retina provides a model for the investigation of cone function and cone-specific genetic disease.
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