RecoveryTail–Phase Phenotype of Single Photoreceptors of the Nrl/Grk1 Double–Knockout Mouse

Abstract

Abstract: : Purpose: To investigate the role of Grk1 in the recovery of photoresponses of Nrl ^–/– mouse photoreceptors, putative cone cells. Background:In the absence of the Neural Retina Leucine Zipper Protein (Nrl) the mouse retina develops no rods and has an enhanced S–cone phenotype. Grk1 is the only known opsin kinase present in mouse¹ and is necessary for fast recovery of cone–driven ERG responses² and for light–dependent phosphorylation of S opsin³. Methods: The responses to brief flashes of UV and midwave light of single photoreceptors of Nrl ^–/– and Nrl ^–/– Grk1 ^–/– mice were recorded with suction electrodes and analyzed. Results: Comparisons of the rising phase of the responses indicate that amplification is comparable in Nrl ^–/– and Nrl ^–/– Grk1 ^–/– cells, but is ∼ 15–fold lower in rods of WT mice. Cells of Nrl ^–/– Grk1 ^–/– mice have a greatly slowed recovery of circulating current after saturating UV flashes, but only slightly slowed recovery after dim UV flashes. For cells of both Nrl ^–/– and Nrl ^–/– Grk1 ^–/– mice, the tail phase of the responses decayed approximately exponentially. The time constant (τ_tail) increased about 3 to 4–fold per decade increase in intensity for both cell types, but was ∼ 3–fold slower at each intensity for Nrl ^–/– Grk1 ^–/– cells. All cells functionally co–express both UV(S) and M–pigments. For Nrl ^–/– cells dim–flash responses to M stimulation recovered on avg. 20% more slowly (at FWHM) than to UV stimulation. For Nrl ^–/– Grk1 ^–/– cells, dim flash responses to M–stimulation recovered on avg. 136% more slowly than to UV. Conclusions: The data suggest that Grk1 is essential for normal recovery kinetics of the circulating current of Nrl ^–/– photoreceptors. The recovery tail phase phenotype depended on both intensity and wavelength in single cells, suggesting that UV– and M–pigments are not inactivated by the same mechanism. In particular, it appears that the UV pigment may have a Grk1–independent inactivation mechanism. ¹ Chen, et al. (2001) Mol. Vision, 7:305–13. ² Lyubarsky, et al. (2000) J Neurosci., 20(6):2209–17. ³ Zhu, et al. (2003) J. Neurosci., 23(14):6152– 6160.

Keywords: photoreceptors • color pigments and opsins • retina: distal (photoreceptors, horizontal cells, bipolar cells)