phototransduction machinery in the rod outer segment has a strong efficacy gradient

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-05, Vol.112 (20), p.E2715-E2724
• Main Authors: Mazzolini, Monica, Facchetti, Giuseppe, Andolfi, Laura, Zaccaria, Remo Proietti, Tuccio, Salvatore, Treu, Johannes, Altafini, Claudio, Di Fabrizio, Enzo M., Lazzarino, Marco, Rapp, Gert, Torre, Vincent
• Format: Article
• Language: English
• Subjects: Animals; Atoms & subatomic particles; Biochemistry; Biological Sciences; Computer Simulation; Fibers; Lasers; Light; Light intensity; Male; Models, Neurological; Optics; Patch-Clamp Techniques; Phosphoric Diester Hydrolases - metabolism; Photic Stimulation; PNAS Plus; Rod Cell Outer Segment - enzymology; Rod Cell Outer Segment - physiology; Signal transduction; Vision, Ocular - physiology; Xenopus laevis
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1423162112

Rod photoreceptors consist of an outer segment (OS) and an inner segment. Inside the OS a biochemical machinery transforms the rhodopsin photoisomerization into electrical signal. This machinery has been treated as and is thought to be homogenous with marginal inhomogeneities. To verify this assumption, we developed a methodology based on special tapered optical fibers (TOFs) to deliver highly localized light stimulations. By using these TOFs, specific regions of the rod OS could be stimulated with spots of light highly confined in space. As the TOF is moved from the OS base toward its tip, the amplitude of saturating and single photon responses decreases, demonstrating that the efficacy of the transduction machinery is not uniform and is 5–10 times higher at the base than at the tip. This gradient of efficacy of the transduction machinery is attributed to a progressive depletion of the phosphodiesterase along the rod OS. Moreover we demonstrate that, using restricted spots of light, the duration of the photoresponse along the OS does not increase linearly with the light intensity as with diffuse light. Significance Phototransduction is now considered to be a quite thoroughly understood phenomenon. It is well known that new discs are continuously generated at the base of the outer segments (OSs) and old discs are shed at their tip, but the rod OSs are considered a well-stirred compartment with minor inhomogeneities. To verify this assumption and to better understand the machinery within the OS, we developed a new methodology to deliver highly localized lights. We found that, as the light stimulus is moved from the OS base toward its tip, the amplitude of saturating and single photon responses decreased by 5–10 times. This gradient of efficacy is attributed to a progressive loss of phosphodiesterase. Therefore, OSs are highly inhomogeneous compartments.