Functional Properties of Photocurrents of Single Photoreceptors of the Nrl-/- Mouse

Abstract

Abstract: : Purpose: To characterize the photoresponses of the photoreceptors of the Nrl-/- mouse with single-cell recordings. Background: Nrl-/- mice have photoreceptors that do not express rod-specific proteins (*), and which may therefore be tentatively classified as "cones". Methods: Membrane currents of Nrl-/- photoreceptor outer segments were recorded with suction pipettes at 37 ^oC in Locke’s solution. The cells were stimulated with brief flashes of calibrated intensities, ranging in wavelength from 340 nm to 700 nm. Results: Saturating photocurrents ranged up to 15 pA; the activation phase of the response of Nrl-/- photoreceptors to strongly saturating flashes was substantially slower than that of rods in wildtype mice, indicating that the membrane time constant of the former is greater. Time to peak of the dim flash response was 0.21 ± 0.06 s (mean ± s.d.). All cells recorded to date were maximally sensitive at 360 nm, but exhibited a secondary spectral peak at 510 nm. Flash sensitivity, expressed as fraction of circulating current suppressed per (photon.µm²) at 360 nm, was 1.2 × 10 ^-4 ± 5 × 10 ^-5 (mean ± s.d.). The dominant time constant of recovery, estimated from the lateral shifting of saturating responses, was 0.11 ± 0.04 s (mean ± s.d.). Conclusions: Nrl-/- photoreceptors exhibit properties consistent with functional classification as cone photoreceptors. * Mears, et al. (2001) Nature Genetics, 29:447-452.

Keywords: photoreceptors • signal transduction: pharmacology/physiology • electrophysiology: non-clinical