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Scott H. Greenwald, James A. Kuchenbecker, Dan K. Roberson, Maureen Neitz, Jay Neitz; S-opsin Stimulation in Dually Expressing Cone Photoreceptors of Mice Suppresses ERG Responses to M Cone Pigment Stimuli. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3909.
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© ARVO (1962-2015); The Authors (2016-present)
Adult human cone photoreceptors express only a single class of opsin, but many murine cones express both S- and M-opsin. This feature of mouse cones, together with the ability to genetically manipulate them, affords opportunities to answer basic questions about the mechanisms responsible for cone opsin expression and function.
Targeted gene replacement technology was used to delete exons 1 through 4 of the mouse Opn1sw gene on chromosome 6. Full field, photopic 1 Hz ON-OFF, cone isolating ERGs were performed on anesthetized, six-month-old wildtype mice (n=9) and age-matched Opn1sw(-/-) mice (n=10), as well as on two adult human subjects. A 520 nm light isolated M cone responses. To isolate S/UV cones a 365 nm light (414 nm for human) was alternated with a 520 nm light (50% duty cycle) in a silent substitution paradigm.
In wildtype mice, the ERG b-wave generated in response to an M-isolating light was suppressed by 40% (p=0.0005) after exposure to an S cone stimulus; however, this was not observed in knockout mice (-5.2%; p=0.57) or in humans (-5.8%; p = 0.35). The time course of recovery after a 20 sec exposure to light that stimulated UV/S cones was 10-15 minutes. When the S cone stimulation was increased in duration, the M response was further suppressed and the recovery time was greatly extended. Pre-exposing a wildtype mouse to 520 nm light in the absence of the S cone stimulation did not result in a decreased M cone ERG. Interestingly, in the absence of S-opsin stimulating pre-exposure, the trend of wildtype mice was toward having larger b-wave responses to 520 nm lights as compared to knockout mice (233µV ± 13 vs 193µV ± 13; p=0.06). This result is the opposite of what would have been expected if, in the knockout, additional M-opsin replaced S-opsin in cones that would have co-expressed the two opsins in the wildtype. Anatomical results confirm that M-opsin did not replace S-opsin in Opn1sw(-/-) mice.
S-opsin activation suppresses ERG responses to middle wavelength light absorbed exclusively by M pigment when S- and M-opsins are dually expressed within single cones.
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