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Abstract In the pineal organ of zebrafish larvae, the bistable opsin parapinopsin alone generates color opponency between UV and visible light. Our previous study suggested that dark inactivation of the parapinopsin photoproduct, which activates G-proteins, is important for the regulation of the amount of the photoproduct. In turn, the photoproduct is responsible for visible light sensitivity in color opponency. Here, we found that an opsin kinase or a G-protein-coupled receptor kinase (GRK) is involved in inactivation of the active photoproduct of parapinopsin in the pineal photoreceptor cells of zebrafish larvae. We investigated inactivation of the photoproduct in the parapinopsin cells of various knockdown larvae by measuring the light responses of the cells using calcium imaging. We found that GRK7a knockdown slowed recovery of the response of parapinopsin photoreceptor cells, whereas GRK1b knockdown or GRK7b knockdown did not have a remarkable effect, suggesting that GRK7a, a cone-type GRK, is mainly responsible for inactivation of the parapinopsin photoproduct in zebrafish larvae. We also observed a similar knockdown effect on the response of the parapinopsin photoreceptor cells of mutant larvae expressing the opsin SWS1, a UV-sensitive cone opsin, instead of parapinopsin, suggesting that the parapinopsin photoproduct was inactivated in a way similar to that described for cone opsins. We confirmed the immunohistochemical distribution of GRK7a in parapinopsin photoreceptor cells by comparing the immunoreactivity to GRK7 in GRK7a-knockdown and control larvae. These findings suggest that in pineal photoreceptor cells, the cone opsin kinase GRK7a contributes greatly to the inactivation of parapinopsin, which underlies pineal color opponency.

In the pineal organ of zebrafish larvae, the bistable opsin parapinopsin alone generates color opponency between UV and visible light. Our previous study suggested that dark inactivation of the parapinopsin photoproduct, which activates G-proteins, is important for the regulation of the amount of the photoproduct. In turn, the photoproduct is responsible for visible light sensitivity in color opponency. Here, we found that an opsin kinase or a G-protein-coupled receptor kinase (GRK) is involved in inactivation of the active photoproduct of parapinopsin in the pineal photoreceptor cells of zebrafish larvae. We investigated inactivation of the photoproduct in the parapinopsin cells of various knockdown larvae by measuring the light responses of the cells using calcium imaging. We found that GRK7a knockdown slowed recovery of the response of parapinopsin photoreceptor cells, whereas GRK1b knockdown or GRK7b knockdown did not have a remarkable effect, suggesting that GRK7a, a cone-type GRK, is mainly responsible for inactivation of the parapinopsin photoproduct in zebrafish larvae. We also observed a similar knockdown effect on the response of the parapinopsin photoreceptor cells of mutant larvae expressing the opsin SWS1, a UV-sensitive cone opsin, instead of parapinopsin, suggesting that the parapinopsin photoproduct was inactivated in a way similar to that described for cone opsins. We confirmed the immunohistochemical distribution of GRK7a in parapinopsin photoreceptor cells by comparing the immunoreactivity to GRK7 in GRK7a-knockdown and control larvae. These findings suggest that in pineal photoreceptor cells, the cone opsin kinase GRK7a contributes greatly to the inactivation of parapinopsin, which underlies pineal color opponency. [ABSTRACT FROM AUTHOR]

Lower vertebrate pineal organs discriminate UV and visible light. Such color discrimination is typically considered to arise from antagonism between two or more spectrally distinct opsins, as, e.g., human cone-based color vision relies on antagonistic relationships between signals produced by red-, green-, and blue-cone opsins. Photosensitive pineal organs contain a bistable opsin (parapinopsin) that forms a signaling-active photoproduct upon UV exposure that may itself be returned to the signaling-inactive "dark" state by longer-wavelength light. Here we show the spectrally distinct parapinopsin states (with antagonistic impacts on signaling) allow this opsin alone to provide the color sensitivity of this organ. By using calcium imaging, we show that single zebrafish pineal photoreceptors held under a background light show responses of opposite signs to UV and visible light. Both such responses are deficient in zebrafish lacking parapinopsin. Expressing a UV-sensitive cone opsin in place of parapinopsin recovers UV responses but not color opponency. Changes in the spectral composition of white light toward enhanced UV or visible wavelengths respectively increased vs. decreased calcium signal in parapinopsinsufficient but not parapinopsin-deficient photoreceptors. These data reveal color opponency from a single kind of bistable opsin establishing an equilibrium-like mixture of the two states with different signaling abilities whose fractional concentrations are defined by the spectral composition of incident light. As vertebrate visual color opsins evolved from a bistable opsin, these findings suggest that color opponency involving a single kind of bistable opsinmight have been a prototype of vertebrate color opponency. [ABSTRACT FROM AUTHOR]