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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. Highlights 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. [ABSTRACT FROM AUTHOR]

Non-visual opsins mediate various light-dependent physiological events. Our previous search for non-visual opsin genes in zebrafish led to the discovery of VAL-opsin (VAL-opsinA) in deep brain cells and retinal horizontal cells of the adult fish. In this study, we report the identification and characterization of its duplicated gene, VAL-opsinB, in zebrafish. A molecular phylogenetic analysis indicates that VAL-opsinB is orthologous to a previously reported salmon gene and that the duplication of the VAL-opsin gene occurred in the teleost lineage. The recombinant protein of zebrafish VAL-opsinB forms a green-sensitive photopigment when reconstituted with 11- cis-retinal. VAL-opsinB expression was detected in a limited number of cells of the brain and the eye, and the expression pattern is distinct from that of the VAL-opsinA gene. Such a differential expression pattern suggests that VAL-opsinA and VAL-opsinB are involved in different physiological events in zebrafish. [ABSTRACT FROM AUTHOR]

The vertebrate retina contains several classes of visual pigments responsible for such diverse functions as image- and nonimage-forming vision, the entrainment of circadian cycles, and the pupilary light response. With vision being vital to the survival of many species, the elucidation of the structural and biochemical properties of visual pigments has been the focus of a large body of research that has led to rapid advances in the field of photoreception. In this review, the current understanding of the structure, function, biochemistry, and evolution of the opsins that make up the photopigments in the vertebrate retina will be reviewed. These include the rod and cone opsins, melanopsin, RGR, peropsin, and VA-opsin. The goal is to highlight important questions that have been answered and to define some of the remaining questions in the field that will provide future directions for research. [ABSTRACT FROM AUTHOR]