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. 2021 Apr 1;15:662453. doi: 10.3389/fncel.2021.662453

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Copyright © 2021 Abbas and Vinberg.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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The visual pigment cycle reactions in vertebrate and invertebrate photoreceptors: (A) Pathways for visual pigment regeneration in the vertebrate retina, including both the classical and non-classical pathways. A cone photoreceptor (dark blue) and its' interactions with the muller glial cells (magenta) and Retinal pigment epithelium (RPE, light blue). Red arrows with red text annotation indicate enzyme mediated reactions and their respective enzymes. The classical pathway occurs via the RPE, all-trans-retinol is taken up by IRBP into the RPE, where LRAT, RPE65, and 11-cis-RDH mediates its conversion to 11-cis-retinal which is then taken up by the photoreceptor via IRBP again, which reassociated with the apo-opsin. The intrinsic light mediated pathway combining all-trans-RAL with phosphatidylethanolamine (PE) to form all-trans-N-retinyl-PE (N-retPE), which is then converted to 11-cis-retinal via a blue photon. Light mediated pigment regeneration can occur both in Müller cells and RPE, whereby all-trans retinal is converted to 11-cis retinal mediated by RGR opsin and the absorption of a photon. Stores of 11-cis-retinoids as well as all-trans-RE are present in both the RPE and Müller cells. (B) Summary of the visual pigment cycle in invertebrates. The invertebrate visual pigment, 3-OH 11-cis retinal is a bistable pigment, and remains associated to the opsin, upon absorption of a second photon, it can convert back from 3-OH all-trans retinal to 3-OH 11-cis retinal. An external pathway is only required in conditions in which the animal is calorie restricted. This pathway is mediated by the retinal pigment cells that surround the rhabdomeres and involves the Photoreceptor retinol dehydrogenase (PDH) and retinol dehydrogenase B (RDHB) enzymes.