A Comparative Perspective on Extra-retinal Photoreception
Authors: Jonathan H. Pérez, Elisabetta Tolla, Ian C. Dunn, Simone L. Meddle, Tyler J. Stevenson
Journal: Trends in Endocrinology and Metabolism (2018)

Light entrains rhythmic processes from daily patterns of hormone secretion and locomotor activity to annual neuroendocrine, metabolic, and behavioral cycles. Extra-retinal photoreception, through photosensitive opsin molecules, is a widespread phenomenon in non-mammalian vertebrates. Nearly a century of research has led to the identification of many different extra-retinal opsins. Extra-retinal photoreceptors are a key component in numerous physiological, metabolic, behavioral, and morphological changes in response to light stimulation, but the precise underlying mechanisms governing physiological and metabolic oscillations are largely unknown. Emerging techniques in gene editing and silencing are providing the necessary tools to finally establish the functional roles and mechanistic connections of extra-retinal opsins. Ubiquitous in non-mammalian vertebrates, extra-retinal photoreceptors (ERPs) have been linked to an array of physiological, metabolic, behavioral, and morphological changes. However, the mechanisms and functional roles of ERPs remain one of the enduring questions of modern biology. In this review article, we use a comparative framework to identify conserved roles and distributions of ERPs, highlighting knowledge gaps. We conclude that ERP research can be divided into two largely unconnected categories: (i) identification and localization of photoreceptors and (ii) linkage of non-retinal light reception to behavioral and physiological processes, particularly endocrine systems. However, the emergence of novel gene editing and silencing techniques is enabling the unification of ERP research by allowing the bridging of this divide. Ubiquitous in non-mammalian vertebrates, extra-retinal photoreceptors (ERPs) have been linked to an array of physiological, metabolic, behavioral, and morphological changes. However, the mechanisms and functional roles of ERPs remain one of the enduring questions of modern biology. In this review article, we use a comparative framework to identify conserved roles and distributions of ERPs, highlighting knowledge gaps. We conclude that ERP research can be divided into two largely unconnected categories: (i) identification and localization of photoreceptors and (ii) linkage of non-retinal light reception to behavioral and physiological processes, particularly endocrine systems. However, the emergence of novel gene editing and silencing techniques is enabling the unification of ERP research by allowing the bridging of this divide. In the century following the identification of ERPs (see Glossary) by von Frisch [1von Frisch K. Beitrage zur Physiologie der Pigmentzellan in der Fischhaut.Pflugers Arch. 1911; 138: 319-387Crossref Scopus (172) Google Scholar], their mechanisms and functional roles have remained one of the enduring questions of modern biology. ERPs have been identified in the vast majority of extant organisms, likely representing the most basal form of light reception [2Lamb T.D. et al.Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup.Nat. Rev. Neurosci. 2007; 8: 960-976Crossref PubMed Scopus (332) Google Scholar]. ERPs are particularly common among non-mammalian vertebrates: fish, amphibians, reptiles, and birds (Table S1 in the supplemental information online). Although the precise mechanistic roles of ERPs are generally not well described, current evidence shows that these non-image-forming photoreceptors are critical, for example, in the integration of light information for movement: photokinesis in zebrafish larvae [3Fernandes A.M. et al.Deep brain photoreceptors control light-seeking behavior in zebrafish larvae.Curr. Biol. 2012; 22: 2042-2047Abstract Full Text Full Text PDF PubMed Scopus (178) Google Scholar] and patterns of locomotor activity (Xenopus tadpoles [4Currie S.P. et al.Deep-brain photoreception links luminance detection to motor output in Xenopus frog tadpoles.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: 6053-6058Crossref PubMed Scopus (18) Google Scholar], eels [5Van Veen T. et al.Light-dependent motor activity and photonegative behavior in the eel (Anguilla anguilla L.).J. Comp. Physiol. 1976; 111: 209-219Crossref Scopus (79) Google Scholar], lizards [6Underwood H. Menaker M. Extraretinal photoreception in lizards.Photochem. Photobiol. 1976; 23: 227-243Crossref Scopus (46) Google Scholar], ruin lizards (Podarcis sicula) [7Pasqualetti M. et al.Identification of circadian brain photoreceptors mediating photic entrainment of behavioural rhythms in lizards.Eur. J. Neurosci. 2003; 18: 364-372Crossref PubMed Scopus (18) Google Scholar]). The functionality of ERPs is most well characterized in relation to daily rhythms [8Underwood H. et al.Melatonin rhythms in the eyes, pineal bodies, and blood of Japanese quail (Coturnix coturnix japonica).Gen. Comp. Endocrinol. 1984; 56: 70-81Crossref PubMed Scopus (179) Google Scholar] and the seasonal regulation of avian reproduction [9García-Fernández J.M. et al.The hypothalamic photoreceptors regulating seasonal reproduction in birds: a prime role for VA opsin.Front. Neuroendocrinol. 2015; 37: 13-28Crossref PubMed Scopus (52) Google Scholar, 10Benoit J. Activation sexuelle obtenue chez le canard par l'éclairement artificiel pendant la période de repos génital.CR Acad. Sci. Paris. 1934; 199: 1671-1673Google Scholar, 11Oliver J. Bayle J. The involvement of the preoptic-suprachiasmatic region in the photosexual reflex in quail: effects of selective lesions and photic stimulation.J. Physiol. 1976; 72: 627-637Google Scholar]. In this review article, we synthesize our existing knowledge of ERPs across non-mammalian vertebrates, beginning with a brief background on the major types of ERPs identified and a comparison of their neuroanatomical localization across taxa. We then highlight the functional role of ERPs for the regulation of physiology, behavior, and biological rhythms including endocrine and metabolic processes. Our review article focuses on fish and birds, as the role of ERPs is best characterized in these taxa. We demonstrate that ERP research to date largely represents two distinct areas: the identification/localization of ERPs and linkage of light to behavioral/physiological outputs. However, recent developments in long-term RNAi technologies and genome editing tools are finally providing the exciting opportunity to determine the functional roles of ERPs across a diverse range of species. The first extra-retinal opsin was isolated by Vigh-Teichmann and colleagues using immunohistochemistry on thornback ray (Raja clavata) pineal glands [12Vigh-Teichmann I. et al.The pineal organ of Raja clavata: opsin immunoreactivity and ultrastructure.Cell Tissue Res. 1983; 228: 139-148Crossref PubMed Scopus (23) Google Scholar], using non-specific whole sheep anti-cattle opsin antiserum. Subsequent molecular characterization of pinopsin, isolated from chicken pineal gland [13Okano T. et al.Pinopsin is a chicken pineal photoreceptive molecule.Nature. 1994; 372: 94-97Crossref PubMed Scopus (280) Google Scholar], set the stage for the characterization of the range of known non-image-forming opsins. This identification and localization of opsins, first via nonspecific antibodies and subsequently with modern molecular techniques (e.g., in situ hybridization and RT-PCR), represents the first major focus of ERP research. The application of modern sequencing and bioinformatics techniques has further expanded the characterization of putative opsin sequences across a wide range of taxa. A recent phylogenetic analysis of annotated opsin sequences has identified five major 'super' families into which all known opsins can be placed: OPN1, OPN3, OPN4, OPN5, and retinal G protein-coupled receptor (RGR) opsin (Table 1; [14Beaudry F.E.G. et al.The non-visual opsins: eighteen in the ancestor of vertebrates, astonishing increase in ray-finned fish, and loss in amniotes.J. Exp. Zool. B Mol. Dev. Evol. 2017; 328: 685-696Crossref PubMed Scopus (21) Google Scholar]). This reductionist re-categorization of opsins replaces the current nomenclature in the literature with a robust convention based consistently on phylogenetic relationships. This approach has appropriately contextualized the evolutionary conserved neuroanatomy and functional roles of ERPs.Table 1Consolidation of Existing Opsins as Used in This Work Using Framework of Beaudry et al. 14Beaudry F.E.G. et al.The non-visual opsins: eighteen in the ancestor of vertebrates, astonishing increase in ray-finned fish, and loss in amniotes.J. Exp. Zool. B Mol. Dev. Evol. 2017; 328: 685-696Crossref PubMed Scopus (21) Google ScholarFamily nameOpsins consolidatedSignaling pathwayOPN1Vertebrate ancient (VA) opsinsPinopsinsParietopsinsParapinopsinsG protein-coupled cyclic nucleotide signalingOPN3Teleost multiple tissue opsins (e.g., TMT1, TMT2, TMT3)EncephalopsinsGi/o protein-coupled cyclic nucleotide signalingOPN4MelanopsinsGq protein-coupled phosphoinositol signalingOPN5Neuropsin (OPN5)OPN6 groupOPN7 groupOPN8 groupOPN9 groupGi protein-coupled cyclic nucleotide signalingRGRRetinal G protein-coupled receptors& PeropsinsAll-trans-retinal 16Terakita A. Nagata T. Functional properties of opsins and their contribution to light-sensing physiology.Zool. Sci. 2014; 31: 653-659Crossref PubMed Scopus (44) Google Scholar Open table in a new tab In general, ERPs are comprised of a photosensitive opsin protein and a chromophore (often a 11-cis-retinal, although all-trans-retinal chromophores also exist [15Koyanagi M. et al.Amphioxus homologs of Go-coupled rhodopsin and peropsin having 11-cis-and all-trans-retinals as their chromophores.FEBS Lett. 2002; 531: 525-528Crossref PubMed Scopus (93) Google Scholar]) that signal through G protein-coupled transduction pathways (e.g., [16Terakita A. Nagata T. Functional properties of opsins and their contribution to light-sensing physiology.Zool. Sci. 2014; 31: 653-659Crossref PubMed Scopus (44) Google Scholar]). Unlike most visual opsins, the majority of non-visual opsins form bi-stable pigments that transition between light and dark states solely via light exposure, rather than requiring external input to return to the dark state [17Koyanagi M. Terakita A. Diversity of animal opsin-based pigments and their optogenetic potential.Biochim. Biophys. Acta Bioenerg. 2014; 1837: 710-716Crossref PubMed Scopus (85) Google Scholar]. Variation in opsin structure has given rise to a wide range of absorption spectra (Figure 1 and Table S2 in the supplemental information online). This variation likely reflects evolution of opsins to meet the demands of species-specific photic environments. For example, in aquatic environments blue and green wavelengths penetrate far deeper in the water column. Thus, a fish living in deep water would be under evolutionary pressure to maintain and utilize receptors sensitive to these wavelengths, while species living in shallow water would be under no such evolutionary pressure. Similarly, the mediums through which light must pass to reach the receptors, that is, water, scales, and skull for a fish versus feathers and skull in a bird, are expected to play a role in the wavelengths of light reaching the ERPs, thus altering the optimal wavelength sensitivity for ERP function. In this section, we review the current literature on ERP distributional patterns, focusing on neuroanatomical localization (for a detailed review of dermal opsins, see [18Kelley J.L. Davies W.I. The biological mechanisms and behavioral functions of opsin-based light detection by the skin.Front. Ecol. Evol. 2016; 4: 1-13Crossref Scopus (16) Google Scholar]). To synthesize this information, we have constructed a comparative brain atlas of ERP distribution (Figure 2 and Table S1 in the supplemental information online). Localization of ERP expression has been of particular interest, given the well-established function of discrete brain nuclei in controlling physiological processes, potentially allowing linkage of opsins and physiological outputs. The identification of conserved neuroanatomy is important for establishing hypothesis-driven experiments into the direct and indirect role of ERPs in controlling physiological processes (e.g., reproduction). However, care needs to be taken in assigning function to an ERP based solely on localization, as ERPs, if expressed in neurons, can easily communicate with distant brain nuclei, as demonstrated by entrainment of the mammalian central clock, in the suprachiasmatic nucleus (SCN), by melanopsin expressed in retinal ganglion cells [2Lamb T.D. et al.Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup.Nat. Rev. Neurosci. 2007; 8: 960-976Crossref PubMed Scopus (332) Google Scholar]. Conversely, expression of ERP in other neural cell types (e.g., tanycytes or astrocytes) may limit the distance over which detected photic information can be directly transmitted. The ERP brain atlas reveals that the diversity of opsin types expressed and the scope of their distribution are far higher in fish and birds than in reptiles and amphibians (Figure 2, Figure 3), suggesting possible taxonomic difference in use of light cues. However, it is currently far from clear whether this pattern represents true taxonomic differences in ERP expression and diversity or whether it is a consequence of research focus being heavily biased toward commercially important taxa (i.e., poultry and fish). Further studies are needed to determine whether amphibians and reptiles truly express fewer types of opsins in a narrower range of neural sites that do other taxa. One consistent observation across species is the high abundance of ERPs in the pineal gland. Pineal ERPs consistently include members of the OPN1 family, with OPN4 (melanopsin) found in fish and birds [3Fernandes A.M. et al.Deep brain photoreceptors control light-seeking behavior in zebrafish larvae.Curr. Biol. 2012; 22: 2042-2047Abstract Full Text Full Text PDF PubMed Scopus (178) Google Scholar, 19Chaurasia S.S. et al.Molecular cloning, localization and circadian expression of chicken melanopsin (OPN4): differential regulation of expression in pineal and retinal cell types.J. Neurochem. 2005; 92: 158-170Crossref PubMed Scopus (0) Google Scholar] and RGR opsins found in fish alone [20Koyanagi M. et al.Bistable UV pigment in the lamprey pineal.Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 6687-6691Crossref PubMed Scopus (113) Google Scholar]. In addition, uncategorized opsin expression determined by nonspecific antibodies has been reported in the pineal of fish, amphibians, and reptiles [21Foster R. et al.Opsin localization and chromophore retinoids identified within the basal brain of the lizard Anolis carolinensis.J. Comp. Physiol. A Neuroethol. Sens. Neural Behav. Physiol. 1993; 172: 33-45Crossref Scopus (99) Google Scholar, 22Vigh-Teichmann I. et al.Opsin-immunoreactive outer segments in the pineal and parapineal organs of the lamprey (Lampetra fluviatilis), the eel (Anguilla anguilla), and the rainbow trout (Salmo gairdneri).Cell Tissue Res. 1983; 230: 289-307Crossref PubMed Scopus (70) Google Scholar, 23Meyer-Rochow V.B. et al.Immunocytochemical observations on pineal organ and retina of the Antarctic teleosts Pagothenia borchgrevinki and Trematomus bernacchii.J. Neurocytol. 1999; 28: 125-130Crossref PubMed Scopus (14) Google Scholar, 24Álvarez-Viejo M. et al.Co-localization of mesotocin and opsin immunoreactivity in the hypothalamic preoptic nucleus of Xenopus laevis.Brain Res. 2003; 969: 36-43Crossref PubMed Scopus (11) Google Scholar]. Whether these represent novel opsin expression or previously identified opsins cannot be determined from the present data. However, it is clear, based on the conserved and consistent expression of opsins within the pineal complex, that extra-retinal photoreception must play a critical role in regulation of pineal activity and melatonin production and thus be critical for regulation of circadian rhythms. Another major site of conserved opsin expression is the hypothalamus, consistent with its established role as a endocrine system mediator and critical neural node for the seasonal regulation of reproduction [25Meddle S.L. Follett B. Photoperiodic activation of fos-like immunoreactive protein in neurones within the tuberal hypothalamus of Japanese quail.J. Comp. Physiol. A. 1995; 176: 79-89Crossref PubMed Scopus (64) Google Scholar, 26Meddle S.L. Follett B.K. Photoperiodically driven changes in Fos expression within the basal tuberal hypothalamus and median eminence of Japanese quail.J. Neurosci. 1997; 17: 8909-8918Crossref PubMed Google Scholar]. As with the pineal, OPN1 expression appears to be a key feature of hypothalamic opsins in all taxa examined except fish, where it is only found in the thalamus and habenula (Figure 2 and Table 1). In birds, hypothalamic OPN1 has been identified specifically as vertebrate ancient (VA)-opsin and is expressed widely throughout the hypothalamus including the preoptic area, paraventricular nucleus, bed nucleus of the stria terminalis, nucleus magnocellularis preopticus par ventralis, anterior medial hypothalamus, median eminence (ME), and nucleus supraopticus pars ventralis [9García-Fernández J.M. et al.The hypothalamic photoreceptors regulating seasonal reproduction in birds: a prime role for VA opsin.Front. Neuroendocrinol. 2015; 37: 13-28Crossref PubMed Scopus (52) Google Scholar, 27Halford S. et al.VA opsin-based photoreceptors in the hypothalamus of birds.Curr. Biol. 2009; 19: 1396-1402Abstract Full Text Full Text PDF PubMed Scopus (128) Google Scholar]. OPN4 has also been identified in the hypothalamus of amphibians (medial preoptic nucleus and SCN; [28Provencio I. et al.Melanopsin: an opsin in melanophores, brain, and eye.Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 340-345Crossref PubMed Scopus (753) Google Scholar]), birds (lateral septal organ [29Chaurasia S. et al.Molecular cloning, localization and circadian expression of chicken melanopsin (OPN4): differential regulation of expression in pineal and retinal cell types.J. Neurochem. 2005; 92: 158-170Crossref PubMed Scopus (171) Google Scholar], premamillary nucleus (PMM) [30Kang S. et al.Melanopsin expression in dopamine-melatonin neurons of the premammillary nucleus of the hypothalamus and seasonal reproduction in birds.Neuroscience. 2010; 170: 200-213Crossref PubMed Scopus (63) Google Scholar, 31Kosonsiriluk S. et al.Photoreceptive oscillators within neurons of the premammillary nucleus (PMM) and seasonal reproduction in temperate zone birds.Gen. Comp. Endocrinol. 2013; 190: 149-155Crossref PubMed Scopus (24) Google Scholar, 32Haas R. et al.Expression of deep brain photoreceptors in the Pekin drake: a possible role in the maintenance of testicular function.Poult. Sci. 2017; 96: 2908-2919Crossref PubMed Scopus (14) Google Scholar]), and fish (SCN and lateral tuberal nucleus [33Sandbakken M. et al.Isolation and characterization of melanopsin photoreceptors of Atlantic salmon (Salmo salar).J. Comp. Neurol. 2012; 520: 3727-3744Crossref PubMed Scopus (27) Google Scholar]). Although identified, reptilian OPN4 has only been isolated from joint telencephalon and diencephalon tissue preparations, preventing further localization [34Frigato E. et al.Isolation and characterization of melanopsin and pinopsin expression within photoreceptive sites of reptiles.Naturwissenschaften. 2006; 93: 379-385Crossref PubMed Scopus (39) Google Scholar]. OPN5 (neuropsin) represents the final major hypothalamic opsin; mRNA expression has been identified in the medial lateral septal organ [35Kang S.W. Kuenzel W.J. Deep-brain photoreceptors (DBPs) involved in the photoperiodic gonadal response in an avian species, Gallus gallus.Gen. Comp. Endocrinol. 2015; 211: 106-113Crossref PubMed Scopus (28) Google Scholar, 36Kuenzel W.J. et al.Exploring avian deep-brain photoreceptors and their role in activating the neuroendocrine regulation of gonadal development1.Poult. Sci. 2015; 94: 786-798Crossref PubMed Scopus (40) Google Scholar] and in the paraventricular organ (PVO) [37Stevenson T.J. Ball G.F. Disruption of neuropsin mRNA expression via RNA interference facilitates the photoinduced increase in thyrotropin-stimulating subunit β in birds.Eur. J. Neurosci. 2012; 36: 2859-2865Crossref PubMed Scopus (36) Google Scholar, 38Nakane Y. et al.A mammalian neural tissue opsin (Opsin 5) is a deep brain photoreceptor in birds.Proc. Natl. Acad. Sci. U. S. A. 2010; 107: 15264-15268Crossref PubMed Scopus (222) Google Scholar]. OPN5 expression has been identified in cerebrospinal fluid (CSF)-contacting neurons of the PVO that project to the ME in birds [38Nakane Y. et al.A mammalian neural tissue opsin (Opsin 5) is a deep brain photoreceptor in birds.Proc. Natl. Acad. Sci. U. S. A. 2010; 107: 15264-15268Crossref PubMed Scopus (222) Google Scholar] and the PVO of fish via in situ hybridization [39Sato K. et al.Two UV-sensitive photoreceptor proteins, OPN5 m and OPN5m2 in ray-finned fish with distinct molecular properties and broad distribution in the retina and brain.PLoS One. 2016; 11e0155339Crossref PubMed Scopus (23) Google Scholar]. Finally, OPN5 protein has also been detected in the periventricular area of the hypothalamus in Xenopus tadpoles [4Currie S.P. et al.Deep-brain photoreception links luminance detection to motor output in Xenopus frog tadpoles.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: 6053-6058Crossref PubMed Scopus (18) Google Scholar]. OPN5 orthologs have been identified in reptilian sequence data [14Beaudry F.E.G. et al.The non-visual opsins: eighteen in the ancestor of vertebrates, astonishing increase in ray-finned fish, and loss in amniotes.J. Exp. Zool. B Mol. Dev. Evol. 2017; 328: 685-696Crossref PubMed Scopus (21) Google Scholar], but they have yet to be localized. The shared hypothalamic distribution of these three opsins places them in close proximity to neuroendocrine cells that mediate major endocrine axes, suggesting a role in their regulation. This is supported by the colocalization of OPN1 with arginine vasotocin and gonadotropin-releasing hormone (GnRH)-expressing neurons in the median eminence of birds [9García-Fernández J.M. et al.The hypothalamic photoreceptors regulating seasonal reproduction in birds: a prime role for VA opsin.Front. Neuroendocrinol. 2015; 37: 13-28Crossref PubMed Scopus (52) Google Scholar]. The non-conserved regions of ERP expression, particularly those representing brain regions that have been assumed to be non-photosensitive, are also of interest. For instance, the olfactory bulbs of fish contain OPN1 [40Masuda T. et al.Retina-type rhodopsin gene expressed in the brain of a teleost, ayu (Plecoglossus altivelis).Zool. Sci. 2003; 20: 989-997Crossref PubMed Scopus (19) Google Scholar] and OPN3 [41Fischer R.M. et al.Co-expression of VAL- and TMT-opsins uncovers ancient photosensory interneurons and motorneurons in the vertebrate brain.PLOS Biol. 2013; 11e1001585Crossref PubMed Scopus (43) Google Scholar], but this region has not been examined in any other taxa. Similarly, both OPN1 and OPN3 family opsins have been isolated in the hindbrain region of fish, an area that was linked to the photomotor response [41Fischer R.M. et al.Co-expression of VAL- and TMT-opsins uncovers ancient photosensory interneurons and motorneurons in the vertebrate brain.PLOS Biol. 2013; 11e1001585Crossref PubMed Scopus (43) Google Scholar, 42Hang C.Y. et al.Localization and characterization of val-opsin isoform-expressing cells in the brain of adult zebrafish.J. Comp. Neurol. 2014; 522: 3847-3860Crossref PubMed Scopus (11) Google Scholar], and an unknown opsin was linked to the amphibian brainstem [4Currie S.P. et al.Deep-brain photoreception links luminance detection to motor output in Xenopus frog tadpoles.Proc. Natl. Acad. Sci. U. S. A. 2016; 113: 6053-6058Crossref PubMed Scopus (18) Google Scholar]. Other regions of unique expression include the fish optic tectum [39Sato K. et al.Two UV-sensitive photoreceptor proteins, OPN5 m and OPN5m2 in ray-finned fish with distinct molecular properties and broad distribution in the retina and brain.PLoS One. 2016; 11e0155339Crossref PubMed Scopus (23) Google Scholar, 41Fischer R.M. et al.Co-expression of VAL- and TMT-opsins uncovers ancient photosensory interneurons and motorneurons in the vertebrate brain.PLOS Biol. 2013; 11e1001585Crossref PubMed Scopus (43) Google Scholar] and the avian cerebellum [43Kato M. et al.Two opsin 3-related proteins in the chicken retina and brain: a TMT-type opsin 3 is a blue-light sensor in retinal horizontal cells, hypothalamus, and cerebellum.PLoS One. 2016; 11e0163925Crossref PubMed Scopus (23) Google Scholar]. A lack of data from other species leaves open whether these patterns are unique. Furthermore, it is unclear whether these brain regions (e.g., brainstem, cerebellum) are themselves detecting and integrating photoperiodic information locally. One possibility is that these opsins have classical light detection functions, but act to signal distant brain regions (e.g., SCN, pineal gland, or hypothalamus) similar to transmission of light information from mammalian OPN4 in retinal ganglion cells [2Lamb T.D. et al.Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup.Nat. Rev. Neurosci. 2007; 8: 960-976Crossref PubMed Scopus (332) Google Scholar]. Alternately, it is possible that these opsins have been co-opted into physiological roles independent of light detection, similar to the involvement of visual rhodopsins in the mechanotransduction of sound in the auditory pathway of Drosophila [44Senthilan P.R. et al.Drosophila auditory organ genes and genetic hearing defects.Cell. 2012; 150: 1042-1054Abstract Full Text Full Text PDF PubMed Scopus (134) Google Scholar]. Future research should focus on determining whether these patterns of expression are in fact conserved and establish the functional role of opsins, light sensing or otherwise, in these presumptively non-photosensitive brain regions. In this section, we outline the second major focus of ERP research: the linking of extra-retinal light detection to the control of a myriad of physiological and behavioral phenomena (Figure 4, Key Figure) from movement behavior to seasonal and circadian rhythms (Box 1; [45Biological Timekeeping: Clocks, Rhythms and Behaviour.in: Kumar V. Springer, 2017Crossref Scopus (5) Google Scholar]). We highlight studies that have experimentally linked a specific opsin to its output, bridging the classic divide between ERP localization and regulatory roles, such as the use of mutant zebrafish lacking OPN4 expression in behavioral studies or the ablation of a specific population of opsin-containing cells.Box 1Circadian Rhythms of Physiology and Endocrinology The adaptation of body color change to the coloration of the surrounding environment was the first phenomenon to be linked to ERP photoreception. von Frisch [1von Frisch K. Beitrage zur Physiologie der Pigmentzellan in der Fischhaut.Pflugers Arch. 1911; 138: 319-387Crossref Scopus (172) Google Scholar] demonstrated that light-induced skin pigmentation change in European minnows (Phoxinus phoxinus) was dependent upon deep-brain photoreception. He observed that minnows lacking eyes were still able to vary their skin color, but not those whose diencephalon was damaged [1von Frisch K. Beitrage zur Physiologie der Pigmentzellan in der Fischhaut.Pflugers Arch. 1911; 138: 319-387Crossref Scopus (172) Google Scholar]. The identity of these ERPs remains unresolved as OPN1, OPN3, OPN4, and OPN5 have all been identified in the diencephalon of fish (Figure 2 and Table S1 in the supplemental information online). The neural pathways and signaling mediators by which photic information detected by deep-brain ERPs is transduced into color change at the skin level remain poorly described. However, control of color change is not limited to brain-based opsins. Blinded chameleons (species unspecified) have been shown to retain their ability to change body color in response to dermal photostimulation, implying a direct detection of light in the skin [46Zoond A. Bokenham N. Studies in reptilian colour response: II: The role of retinal and dermal photoreceptors in the pigmentary activity of the chameleon.J. Exp. Biol. 1935; 12: 39-43Google Scholar]. Similarly, body color change has also been linked to dermal ERPs in Xenopus, wherein changes in light exposure trigger changes in dermal melanosome aggregation [47Miyashita Y. et al.The photoreceptor molecules in Xenopus tadpole tail fin, in which melanophores exist.Zool. Sci. 2001; 18: 671-674Crossref Scopus (22) Google Scholar]. RT-PCR and nested PCR studies of tadpole tails have also identified expression of OPN1 and OPN4 mRNA in the dermal melanophores, suggesting localized control of color change [47Miyashita Y. et al.The photoreceptor molecules in Xenopus tadpole tail fin, in which melanophores exist.Zool. Sci. 2001; 18: 671-674Crossref Scopus (22) Google Scholar]. RT-PCR studies have identified both OPN1 and visual cone type opsins in the skin of the neon tetra (Paracheirodon innesi) as potential regulators of color change [48Kasai A. Oshima N. Light-sensitive motile iridophores and visual pigments in the neon tetra, Paracheirodon innesi.Zool. Sci. 2006; 23: 815-819Crossref PubMed Scopus (29) Google Scholar]. Gene expression profiling in zebrafish has identified more than 25 opsin gene variants expressed in the skin [18Kelley J.L. Davies W.I. The biological mechanisms and behavioral functions of opsin-based light detection by the skin.Front. Ecol. Evol. 2016; 4: 1-13Crossref Scopus (16) Google Scholar]. The mechanisms and molecular pathways linking skin and neural opsin photoreception to color change remain an active and open area of robust research (for review, see [18Kelley J.L. Davies W.I. The biological mechanisms and behavioral functions of op