Co-expression of UV- and M- Cone Opsins in Photoreceptors of Nrl-/- Mouse

Abstract

Abstract: : Purpose: To determine if Nrl-/- mouse photoreceptors express both UV- & M- cone pigments. Background: Nrl-/- mice have photoreceptors that do not express rod-specific proteins (¹), and which may therefore be tentatively classified as "cones". Methods: Single cell photocurrents in response to brief light flashes varying in wavelength from 360 nm to 650 nm were measured with suction electrodes. The a-wave component of the ERG was measured in response to 1 ms flashes varying from 350 nm to 450 nm. Results: The action spectrum of the a-wave rising phase peaked at about 360 nm and is clearly dominated by cells expressing the UV-pigment; we were unable to measure a-wave responses beyond 450 nm. All single cells recorded from were maximally sensitive in the UV, but also exhibited a distinct secondary mode of sensitivity at 500-510 nm; sensitivity at the secondary mode ranged from 0.1% to 2% that of the primary mode at 360 nm. (We can reliably detect M-pigment co-expression above 0.01% as deviation of the action spectrum from a UV-pigment template). Conclusions: Most (if not all) Nrl-/- photoreceptors co-express both the murine M-cone and UV-cone pigments, but the co-expression (in cells recorded to date) does not exceed 2%. Comparison with results of our previous ERG study (²), suggests that most "UV cones" of wildtype mice may co-express higher levels of M-cone pigment than do the UV photoreceptors of the Nrl-/- mouse. ¹ Mears, et al. (2001) Nature Genetics, 29:447-452. ² Lyubarsky et al. (1999) J. Neuroscience, 19(1): 442-455.

Keywords: color pigments and opsins • photoreceptors