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T. Isayama, S. L. McCabe, R. K. Crouch, A. L. Zimmerman, C. L. Makino; Dark Activation of Salamander Visual Pigments. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5830. doi: https://doi.org/.
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The truncated retinoid, β-ionone, enhances opsin activity in all partially-bleached salamander photoreceptors except red-sensitive cones leading to decreased circulating current and sensitivity to flashes, and accelerated response kinetics (Isayama et al., 2006). Here, we demonstrate that β-ionone also activates visual pigments (opsin bound to 11-cis retinal) in some dark-adapted rods and cones.
β-Ionone or 9-cis retinal was applied to the outer segment of single photoreceptors of salamander (A. tigrinum), and absorbance measured with a microspectrophotometer or currents recorded using a suction electrode. Patch clamp recordings of Xenopus oocytes expressing homomeric rod cyclic nucleotide-gated (CNG) channels were performed to see if the reduction in circulating current resulted from retinoid-mediated inhibition of the channels.
β-Ionone applied at low concentrations (10-20 µM) was taken up into outer segments of dark-adapted photoreceptors and activated reversibly the visual pigments of blue-sensitive (BS) rods and cones, and UV-sensitive cones, while the chromophore-binding pocket was occupied by 11-cis retinal. The effect persisted in BS rods even after pre-exposure to 9-cis retinal to ensure complete pigment regeneration. Patch clamp experiments ruled out direct inhibition of the CNG channels by 10 µM β-ionone. After extended exposure to higher concentrations of β-ionone, BS rods did not fully recover circulating current, sensitivity or flash response kinetics, suggesting that some pigment had been bleached. Treatment of one BS rod afterward with 9-cis retinal to regenerate pigment enabled full recovery. Other BS rods receiving a background light that initially suppressed the same fraction of circulating current as did the β-ionone exposure, had a greater recovery after the light was turned off probably because less pigment was bleached by the light.
β-ionone activates some visual pigments and causes bleaching, but it is not yet clear which action occurs first.
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