Cones of Wild Type & Nrl –/– Mice: Comparison of Their Functional Properties Determined From Suction Pipette Recordings With Those of WT Rods

Abstract

Abstract: : Purpose: To measure the properties of electrical responses of single WT mouse cones and Nrl ^–/– cones, and compare them with the response properties of WT rods. Methods: Photocurrents of WT and Nrl ^–/– cones and of WT rods were recorded at 35–37 ^oC with suction pipettes into which the inner segments and nuclear regions were drawn. Photoreceptors were stimulated with calibrated flashes of light of varied wavelength. In experiments with WT cones, rod activity was suppressed by steps of 500 nm light, demonstrated to saturate the circulating current of rods. Results: All recorded WT and Nrl ^–/– mouse cones were found to be maximally sensitive to near UV light (360 nm) with a secondary peak of sensitivity at ∼ 508 nm, indicating a functional co–expression of the mouse cone S– and M–pigments in each cell. For WT cones (n=12), the sensitivity S_M to midwave light (500 nm) relative to the sensitivity S_UV to UV light varied from S_M/S_UV ∼ 0 (n =1) to 0.75, with the majority in the range 0.02 – 0.06 (n=7); for Nrl ^–/– cones S_M/S_UV varied from 0.0005 to 1 (n=31) with more than half of the cells showing sensitivity below 0.01 (n=22). Complete families of light responses to UV flashes were recorded from 12 WT and 8 Nrl ^–/– cone photoreceptors and from 6 WT rods (stimulated with 500 nm flashes) in this "inner segment loose patch" recording mode. For WT cones: The maximum photocurrent was J_max = 9 pA; the average was 6 ± 1 pA (mean ± s.e.m.); the half–saturating flash intensity was Q_½ = 8,200 ± 3,300 photons µm^–2; for the dim–flash response the time to peak was t_peak = 80 ± 10 ms; for just–saturating responses, the dominant recovery time constant was τ_D = 100 ± 13 ms. For Nrl ^–/– cones: J_max = 13 ± 5 pA, Q_½ = 4,400 ± 1,400 photons µm^–2, t_peak = 91 ± 6 ms, and τ_D = 110 ± 40 ms. For WT rods: J_max = 20 ± 12 pA, Q_½ = 41 ± 14 photons µm^–2, t_peak = 210 ± 20 ms, and τ_D = 290 ± 60 ms. Conclusions: WT and Nrl ^–/– mouse cone photoreceptors have similar functional properties, including comparable absolute sensitivity, peak spectral sensitivity in the near UV light, and similar response kinetics. Cones of both strains of mice have much lower sensitivity, and faster response kinetics than WT rods. The cones of WT and Nrl ^–/– mice, however, appear to differ in their degree of M–pigment co–expression.

Keywords: electrophysiology: non-clinical • photoreceptors • transgenics/knock-outs