Photoreceptors of Nrl -/- mice coexpress functional S- and M-cone opsins having distinct inactivation mechanisms.

While the deletion of Grk1 slowed the recovery of responses driven by both pigments, the effect on the M-pigment response was much larger; thus, in the absence of Grk1, the average Δ T 50 of the response to the M-pigment increased 7.5-fold (from 0.33 to 2.44 s), while the Δ T 50 of the UV-pigment response increased 2.8-fold (from 0.25 to 0.69 s); both these differences in Δ T 50 are highly significant (P < 0.0004), arguing strongly that Grk1 is necessary for normal inactivation of both pigments.

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