Beta-ionone activates and bleaches visual pigment in salamander photoreceptors.
Vision begins with photoisomerization of 11-cis retinal to the all-trans conformation within the chromophore-binding pocket of opsin, leading to activation of a biochemical cascade. Release of all-trans retinal from the binding pocket curtails but does not fully quench the ability of opsin to activate transducin. All-trans retinal and some other analogs, such as beta-ionone, enhance opsin's activity, presumably on binding the empty chromophore-binding pocket. By recording from isolated salamander photoreceptors and from patches of rod outer segment membrane, we now show that high concentrations of beta-ionone suppressed circulating current in dark-adapted green-sensitive rods by inhibiting the cyclic nucleotide-gated channels. There were also decreases in circulating current and flash sensitivity, and accelerated flash response kinetics in dark-adapted blue-sensitive (BS) rods and cones, and in ultraviolet-sensitive cones, at concentrations too low to inhibit the channels. These effects persisted in BS rods even after incubation with 9-cis retinal to ensure complete regeneration of their visual pigment. After long exposures to high concentrations of beta-ionone, recovery was incomplete unless 9-cis retinal was given, indicating that visual pigment had been bleached. Therefore, we propose that beta-ionone activates and bleaches some types of visual pigments, mimicking the effects of light.
Isayama
Tomoki
T
Department of Ophthalmology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Massachusetts 02114, USA. tomoki_isayama@meei.harvard.edu
McCabe England
S L
SL
Crouch
R K
RK
Zimmerman
A L
AL
Makino
C L
CL
D000818:Animals; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000037:antagonists & inhibitors; D003623:Dark Adaptation / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D066298:In Vitro Techniques; D007700:Kinetics; D007814:Larva; D008566:Membranes / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D045792:Norisoprenoids / Q000008:administration & dosage / Q000494:pharmacology*; D020419:Photoreceptor Cells, Vertebrate / Q000187:drug effects* / Q000378:metabolism* / Q000502:physiology; D012168:Retinal Pigments / Q000378:metabolism*; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000502:physiology; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D013997:Time Factors; D014562:Caudata / Q000254:growth & development / Q000378:metabolism*
D054815:Cyclic Nucleotide-Gated Cation Channels; D045792:Norisoprenoids; D012168:Retinal Pigments; C008157:beta-ionone