In the vertebrate retina, three types of photoreceptors-visual photoreceptor cones and rods and the intrinsically photosensitive retinal ganglion cells (ipRGCs)-converged through evolution to detect light and regulate image-and nonimage-forming activities such as photic entrainment of circadian rhythms, pupillary light reflexes, etc. ipRGCs express the nonvisual photopigment melanopsin (OPN4), encoded by two genes: the Xenopus (Opn4x) and mammalian (Opn4m) orthologs. In the chicken retina, both OPN4 proteins are found in ipRGCs, and Opn4x is also present in retinal horizontal cells (HCs), which connect with visual photoreceptors. Here we investigate the intrinsic photosensitivity and functioning of HCs from primary cultures of embryonic retinas at day 15 by using calcium fluorescent fluo4 imaging, pharmacological inhibitory treatments, and Opn4x knockdown. Results show that HCs are avian photoreceptors with a retinal-based OPN4X photopigment conferring intrinsic photosensitivity. Light responses in HCs appear to be driven through an ancient type of phototransduction cascade similar to that in rhabdomeric photoreceptors involving a G-protein q, the activation of phospholipase C, calcium mobilization, and the release of the inhibitory neurotransmitter GABA. Based on their intrinsic photosensitivity, HCs may have a key dual function in the retina of vertebrates, potentially regulating nonvisual tasks together with their sister cells, ipRGCs, and with visual photoreceptors, modulating lateral interactions and retinal processing.retina | light responses | horizontal cell | phototransduction | melanopsin I n the vertebrate retina, photoreceptors can be classified according to their function as canonical or noncanonical. The first group comprises specialized ciliary retinal neurons, cones, and rods in the outer retina, which participate in the image-forming processes associated with day/night vision. The second group is composed of intrinsically photosensitive retinal ganglion cells (ipRGCs) in the inner retina, which preferentially participate in the processing of photic inputs related to nonimage-forming tasks (photic synchronization of circadian rhythms, pupillary light reflexes, inhibition of pineal melatonin, etc.) (1-7). The photopigment melanopsin (OPN4) (1) confers photosensitivity to ipRGCs as clearly shown through later experiments with OPN4 knockout mice or OPN4 heterologous expression in nonretinal cells for loss or gain of function, respectively (8-11). Through evolution, OPN4 appears to have been encoded by at least two genes in vertebrates: Opn4x and Opn4m, the Xenopus and mammalian ortholog genes, respectively (12). Interestingly, the first-appearing vertebrates, nonmammals including fish, amphibians, and birds, possess both Opn4 genes whereas mammals only have Opn4m. Evidence suggests that during the course of evolution, mammals lost some visual opsins and Opn4x as they entered the nocturnal niche. In the chicken retina, different laboratories have reported the expression of Opn4 genes in outer nuclear...

The vertebrate retina contains typical photoreceptor (PR) cones and rods responsible for day/night vision, respectively, and intrinsically photosensitive retinal ganglion cells (ipRGCs) involved in the regulation of non-image-forming tasks. Rhodopsin/cone opsin photopigments in visual PRs or melanopsin (Opn4) in ipRGCs utilizes retinaldehyde as a chromophore. The retinoid regeneration process denominated as "visual cycle" involves the retinal pigment epithelium (RPE) or Müller glial cells. Opn4, on the contrary, has been characterized as a bi/tristable photopigment, in which a photon of one wavelength isomerizes 11-cis to all-trans retinal (Ral), with a second photon re-isomerizing it back. However, it is unknown how the chromophore is further metabolized in the inner retina. Nor is it yet clear whether an alternative secondary cycle occurs involving players such as the retinal G-protein-coupled receptor (RGR), a putative photoisomerase of unidentified inner retinal activity. Here, we investigated the role of RGR in retinoid photoisomerization in Opn4x (Xenopus ortholog) (+) RGC primary cultures free of RPE and other cells from chicken embryonic retinas. Opn4x (+) RGCs display significant photic responses by calcium fluorescent imaging and photoisomerize exogenous all-trans to 11-cis Ral and other retinoids. RGR was found to be expressed in developing retina and in primary cultures; when its expression was knocked down, the levels of 11-cis, all-trans Ral, and all-trans retinol in cultures exposed to light were significantly higher and those in all-trans retinyl esters lower than in dark controls. The results support a novel role for RGR in ipRGCs to modulate retinaldehyde levels in light, keeping the balance of inner retinal retinoid pools.