Light-sensitive Photoreceptors Research Articles

NAD+ and sirtuins in retinal degenerative diseases: A look at future therapies.

Retinal degenerative diseases are a major cause of morbidity in modern society because visual impairment significantly decreases the quality of life of patients. A significant challenge in treating retinal degenerative diseases is their genetic and phenotypic heterogeneity. However, despite this diversity, many of these diseases share a common endpoint involving death of light-sensitive photoreceptors. Identifying common pathogenic mechanisms that contribute to photoreceptor death in these diverse diseases may lead to a unifying therapy for multiple retinal diseases that would be highly innovative and address a great clinical need. Because the retina and photoreceptors, in particular, have immense metabolic and energetic requirements, many investigators have hypothesized that metabolic dysfunction may be a common link unifying various retinal degenerative diseases. Here, we discuss a new area of research examining the role of NAD+ and sirtuins in regulating retinal metabolism and in the pathogenesis of retinal degenerative diseases. Indeed, the results of numerous studies suggest that NAD+ intermediates or small molecules that modulate sirtuin function could enhance retinal metabolism, reduce photoreceptor death, and improve vision. Although further research is necessary to translate these findings to the bedside, they have strong potential to truly transform the standard of care for patients with retinal degenerative diseases.

The Role of the Microglial Cx3cr1 Pathway in the Postnatal Maturation of Retinal Photoreceptors.

Microglia are the resident immune cells of the CNS, and their response to infection, injury and disease is well documented. More recently, microglia have been shown to play a role in normal CNS development, with the fractalkine-Cx3cr1 signaling pathway of particular importance. This work describes the interaction between the light-sensitive photoreceptors and microglia during eye opening, a time of postnatal photoreceptor maturation. Genetic removal of Cx3cr1 (Cx3cr1GFP/GFP ) led to an early retinal dysfunction soon after eye opening [postnatal day 17 (P17)] and cone photoreceptor loss (P30 onward) in mice of either sex. This dysfunction occurred at a time when fractalkine expression was predominantly outer retinal, when there was an increased microglial presence near the photoreceptor layer and increased microglial-cone photoreceptor contacts. Photoreceptor maturation and outer segment elongation was coincident with increased opsin photopigment expression in wild-type retina, while this was aberrant in the Cx3cr1GFP/GFP retina and outer segment length was reduced. A beadchip array highlighted Cx3cr1 regulation of genes involved in the photoreceptor cilium, a key structure that is important for outer segment elongation. This was confirmed with quantitative PCR with specific cilium-related genes, Rpgr and Rpgrip1, downregulated at eye opening (P14). While the overall cilium structure was unaffected, expression of Rpgr, Rpgrip1, and centrin were restricted to more proximal regions of the transitional zone. This study highlighted a novel role for microglia in postnatal neuronal development within the retina, with loss of fractalkine-Cx3cr1 signaling leading to an altered distribution of cilium proteins, failure of outer segment elongation and ultimately cone photoreceptor loss.SIGNIFICANCE STATEMENT Microglia are involved in CNS development and disease. This work highlights the role of microglia in postnatal development of the light-detecting photoreceptor neurons within the mouse retina. Loss of the microglial Cx3cr1 signaling pathway resulted in specific alterations in the cilium, a key structure in photoreceptor outer segment elongation. The distribution of key components of the cilium transitional zone, Rpgr, Rpgrip1, and centrin, were altered in retinae lacking Cx3cr1 with reduced outer segment length and cone photoreceptor death observed at later postnatal ages. This work identifies a novel role for microglia in the postnatal maturation of retinal photoreceptors.

Harmonics added to a flickering light can upset the balance between ON and OFF pathways to produce illusory colors

The neural signals generated by the light-sensitive photoreceptors in the human eye are substantially processed and recoded in the retina before being transmitted to the brain via the optic nerve. A key aspect of this recoding is the splitting of the signals within the two major cone-driven visual pathways into distinct ON and OFF branches that transmit information about increases and decreases in the neural signal around its mean level. While this separation is clearly important physiologically, its effect on perception is unclear. We have developed a model of the ON and OFF pathways in early color processing. Using this model as a guide, we can produce imbalances in the ON and OFF pathways by changing the shapes of time-varying stimulus waveforms and thus make reliable and predictable alterations to the perceived average color of the stimulus-although the physical mean of the waveforms does not change. The key components in the model are the early half-wave rectifying synapses that split retinal photoreceptor outputs into the ON and OFF pathways and later sigmoidal nonlinearities in each pathway. The ability to systematically vary the waveforms to change a perceptual quality by changing the balance of signals between the ON and OFF visual pathways provides a powerful psychophysical tool for disentangling and investigating the neural workings of human vision.

Expression profiling of the retina of pde6c, a zebrafish model of retinal degeneration

Retinal degeneration often affects the whole retina even though the disease-causing gene is specifically expressed in the light-sensitive photoreceptors. The molecular basis of the retinal defect can potentially be determined by gene-expression profiling of the whole retina. In this study, we measured the gene-expression profile of retinas microdissected from a zebrafish pde6cw59 (pde6c) mutant. This retinal-degeneration model not only displays cone degeneration caused by a cone-specific mutation, but also other secondary cellular changes starting from 4 days postfertilization (dpf). To capture the underlying molecular changes, we subjected pde6c and wild-type (WT) retinas at 5 dpf/ 120 h postfertilization (hpf) to RNA sequencing (RNA-Seq) on the Illumina HiSeq 2,000 platform. We also validated the RNA-Seq results by Reverse Transcription Quantitative Polymerase Chain Reaction (RT-qPCR) of seven phototransduction genes. Our analyses indicate that the RNA-Seq dataset was of high quality, and effectively captured the molecular changes in the whole pde6c retina. This dataset will facilitate the characterization of the molecular defects in the pde6c retina at the initial stage of retinal degeneration.

Vision in dim light: highlights and challenges.

On a moonless night or in the depths of the sea, where light levels are many orders of magnitude dimmer than sunlight, animals rely on their visual systems to orient and navigate, to find food and mates and to avoid predators. To see well at such low light levels is far from trivial. The paucity of light means that visual signals generated in the light-sensitive photoreceptors of the retina can easily be drowned in neural noise. Despite this, research over the past 15 years has revealed that nocturnal and deep-sea animals—even very small animals like insects with tiny eyes and brains—can have formidable visual abilities in dim light. The latest research in the field is now beginning to reveal how this visual performance is possible, and in particular which optical and neural strategies have evolved that permit reliable vision in dim light. This flurry of research is rapidly changing our understanding of both the limitations and the capabilities of animals active in very dim light. For instance, while the long-held view was that night vision allows only an impoverished, noisy and monochrome view of the world, we now know that many nocturnal animals see the world more or less in the same manner as their day-active relatives. Many are able to see colour, to use optic flow cues to control flight, and to navigate using learned visual landmarks and celestial cues such as polarized light. Much of our appreciation of the richness of the visual world seen by nocturnal animals has derived primarily from behavioural, anatomical and optical studies. More recently, enormous advances have also been made in understanding the neural basis of this performance in both single cells and circuits of cells from both nocturnal vertebrates (notably mice) and nocturnal invertebrates (notably insects). These studies indicate that the remarkable behavioural performance of these animals in dim light can only partially be explained by what we currently know of the performance of the underlying visual cells. We are thus now at an important point in the field where this gap is closing. It is thus particularly timely that this special issue brings together a unique combination of recent research on deep-sea and nocturnal animals and moreover from a wide spectrum of scientific disciplines, from ecology, evolution and quantitative visual behaviour to cellular electrophysiology, mathematical modelling and molecular biology. This landmark collection of papers is the first to exclusively address the topic of comparative vision in dim light.

Pluripotent stem cells and their utility in treating photoreceptor degenerations.

Age-related macular degeneration and inherited retinal degenerations represent the leading causes of blindness in industrialized countries. Despite different initiating causes, they share a common final pathophysiology, the loss of the light sensitive photoreceptors. Replacement by transplantation may offer a potential treatment strategy for both patient populations. The last decade has seen remarkable progress in our ability to generate retinal cell types, including photoreceptors, from a variety of murine and human pluripotent stem cell sources. Driven in large part by the requirement for renewable cell sources, stem cells have emerged not only as a promising source of replacement photoreceptors but also to provide in vitro systems with which to study retinal development and disease processes and to test therapeutic agents.

Live-Cell Imaging of Phagosome Motility in Primary Mouse RPE Cells.

The retinal pigment epithelium (RPE) is a post-mitotic epithelial monolayer situated between the light-sensitive photoreceptors and the choriocapillaris. Given its vital functions for healthy vision, the RPE is a primary target for insults that result in blinding diseases, including age-related macular degeneration (AMD). One such function is the phagocytosis and digestion of shed photoreceptor outer segments. In the present study, we examined the process of trafficking of outer segment disk membranes in live cultures of primary mouse RPE, using high speed spinning disk confocal microscopy. This approach has enabled us to track phagosomes, and determine parameters of their motility, which are important for their efficient degradation.

Streptophyte phytochromes exhibit an N-terminus of cyanobacterial origin and a C-terminus of proteobacterial origin.

BackgroundPhytochromes are red light-sensitive photoreceptors that control a variety of developmental processes in plants, algae, bacteria and fungi. Prototypical phytochromes exhibit an N-terminal tridomain (PGP) consisting of PAS, GAF and PHY domains and a C-terminal histidine kinase (HK).ResultsThe mode of evolution of streptophyte, fungal and diatom phytochromes from bacteria is analyzed using two programs for sequence alignment and six programs for tree construction. Our results suggest that Bacteroidetes present the most ancient types of phytochromes. We found many examples of lateral gene transfer and rearrangements of PGP and HK sequences. The PGP and HK of streptophyte phytochromes seem to have different origins. In the most likely scenario, PGP was inherited from cyanobacteria, whereas the C-terminal portion originated from a proteobacterial protein with multiple PAS domains and a C-terminal HK. The plant PhyA and PhyB lineages go back to an early gene duplication event before the diversification of streptophytes. Fungal and diatom PGPs could have a common prokaryotic origin within proteobacteria. Early gene duplication is also obvious in fungal phytochromes.ConclusionsThe dominant question of the origin of plant phytochromes is difficult to tackle because the patterns differ among phylogenetic trees. We could partially overcome this problem by combining several alignment and tree construction algorithms and comparing many trees. A rearrangement of PGP and HK can directly explain the insertion of the two PAS domains by which streptophyte phytochromes are distinguished from all other phytochromes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13104-015-1082-3) contains supplementary material, which is available to authorized users.

Cell-type specific photoreceptors and light signaling pathways in the multicellular green alga volvox carteri and their potential role in cellular differentiation

The formation of multicellular organisms requires genetically predefined signaling pathways in various cell types. Besides differences in size, energy balance and life time, cell types should be enable to modulate appropriate developmental and adaptive responses in ever-changing surrounding environment. One of the most important environmental cues is light which regulates a variety of physiological and cellular processes. During evolution, diverse light-sensitive proteins, so-called photoreceptors, and corresponding signaling pathways have evolved, in almost all kingdoms of life, to monitor light continuously and adjust their growth and development accordingly. However, considering the fact that different cell types should be enable to trigger distinct light signaling pathways according to their needs, cell-type specific light signaling pathways are required to guarantee cell type-matched modulation of cellular and developmental processes in response to different light signals. The multicellular green alga Volvox carteri, which has only 2 cell types with clear division of labor, possesses cell-type specific photoreceptors and light signaling pathways which allow differential regulation of genes involved in various cellular and metabolic pathways in response to environmental light. The existence of cell-type specific light signaling pathways in muticellular organism like Volvox reflects an early development of cell-type specific signaling mechanisms during evolution to ensure maintenance of differentiation.

Function and evolutionary origin of unicellular camera-type eye structure.

The ocelloid is an extraordinary eyespot organelle found only in the dinoflagellate family Warnowiaceae. It contains retina- and lens-like structures called the retinal body and the hyalosome. The ocelloid has been an evolutionary enigma because of its remarkable resemblance to the multicellular camera-type eye. To determine if the ocelloid is functionally photoreceptive, we investigated the warnowiid dinoflagellate Erythropsidinium. Here, we show that the morphology of the retinal body changed depending on different illumination conditions and the hyalosome manifests the refractile nature. Identifying a rhodopsin gene fragment in Erythropsidinium ESTs that is expressed in the retinal body by in situ hybridization, we also show that ocelloids are actually light sensitive photoreceptors. The rhodopsin gene identified is most closely related to bacterial rhodopsins. Taken together, we suggest that the ocelloid is an intracellular camera-type eye, which might be originated from endosymbiotic origin.

Characterizing the Effect of Static Magnetic Fields on <i>C. elegans</i> Using Microfluidics

In nature, several organisms possess a magnetic compass to navigate or migrate them to desired locations. It is thought that these organisms may use biogenic magnetic matter or light-sensitive photoreceptors to sense and orient themselves in magnetic fields. To unravel the underlying principles of magnetosensitivity and magnetoreception, previous experiments have been conducted on bacteria, vertebrates, crustaceans, and insects. In this study, the model organism, C. elegans, is used to test their response and sensitivity to static magnetic fields in the range of 5 milli Tesla to 120 milli Tesla. Single wild-type C. elegans are put in microfluidic channels and exposed to permanent magnets for five cycles of thirty-second time intervals. The worm movement is recorded and analyzed with custom software to calculate the average velocity and the percentage of turning and curling. Contrary to some published studies, our results did not show a significant difference compared to control experiments. This suggests that C. elegans may not sense static magnetic fields in the range of field strengths that we tested.

Effects of Light and Wounding on Jasmonates in Rice phyAphyC Mutants.

Jasmonates (JA) are lipid-derived plant hormones. They have been shown to be important regulators of photomorphogenesis, a developmental program in plants, which is activated by light through different red and blue light sensitive photoreceptors. In rice, inhibition of coleoptile growth by light is a central event in photomorphogenesis. This growth inhibition is impaired, when jasmonate biosynthesis is knocked out. Previously, we found that JASMONATE RESISTANT 1 (OsJAR1) transcripts were not induced in the phytochrome (phy) mutant phyAphyC. Therefore, in the current study we investigated the regulation of JA and its highly bioactive derivative (+)-7-iso-jasmonoyl-l-isoleucine (JA-Ile), as well as the transcriptional regulation of several JA-dependent genes both in wild type and phyAphyC mutant. JA and JA-Ile levels increased in the mutant seedlings in response to blue light. However, in phyAphyC mutant leaves, which were continuously wounded, JA and JA-Ile levels were lower compared to those in the wild type. Hence, the mutation of phyA and phyC has differential effects on jasmonate levels depending on the tissue and developmental stage. Our results suggest that the contribution of JA-Ile to signaling during photomorphogenesis of rice is minor, as coleoptile phenotypes of phyAphyC mutants resemble those of jasmonate-deficient mutants despite the fact that induction by blue light leads to higher levels of JA-Ile compared to the wild type. We postulate that phyA and phyC could control the activity of specific enzymes metabolizing JA to active derivatives.

Opsins in Limulus eyes: characterization of three visible light-sensitive opsins unique to and co-expressed in median eye photoreceptors and a peropsin/RGR that is expressed in all eyes.

The eyes of the horseshoe crab Limulus polyphemus have long been used for studies of basic mechanisms of vision, and the structure and physiology of Limulus photoreceptors have been examined in detail. Less is known about the opsins Limulus photoreceptors express. We previously characterized a UV opsin (LpUVOps1) that is expressed in all three types of Limulus eyes (lateral compound eyes, median ocelli and larval eyes) and three visible light-sensitive rhabdomeric opsins (LpOps1, -2 and -5) that are expressed in Limulus lateral compound and larval eyes. Physiological studies showed that visible light-sensitive photoreceptors are also present in median ocelli, but the visible light-sensitive opsins they express were unknown. In the current study we characterize three newly identified, visible light-sensitive rhabdomeric opsins (LpOps6, -7 and -8) that are expressed in median ocelli. We show that they are ocellar specific and that all three are co-expressed in photoreceptors distinct from those expressing LpUVOps1. Our current findings show that the pattern of opsin expression in Limulus eyes is much more complex than previously thought and extend our previous observations of opsin co-expression in visible light-sensitive Limulus photoreceptors. We also characterize a Limulus peropsin/RGR (LpPerOps1). We examine the phylogenetic relationship of LpPerOps1 with other peropsins and RGRs, demonstrate that LpPerOps1 transcripts are expressed in each of the three types of Limulus eyes and show that the encoded protein is expressed in membranes of cells closely associated with photoreceptors in each eye type. These finding suggest that peropsin was in the opsin repertoire of euchelicerates.

Regulation of heart rate in zebrafish embryos

Although zebrafish has emerged as model for studying cardiovascular function, still very little is known about heart rate control in these animals. Here we report that known drugs that modify heart rate in mammals have similar effects in 2 days post fertilization zebrafish embryos. Injection of 0.5mM carbachol or propranolol into the blood stream reduced the heart rate by >25%, while norepinephrine resulted in a ~10% increase. We further discovered that brief exposure of embryos (5 sec) to UV light similarly increased the heart rate, up to 50%. Surprisingly, UV light exposure restored the charbacol‐induced drop in heart rate to normal levels. To determine whether UV light sensitive photoreceptors in the eyes or pineal gland (PG) mediated this effect, we ablated either one or both organs in GFP‐expressing transgenic embryos using a two‐photon laser. Ablation of both eyes and PG reduced the UV light‐mediated heart rate increase by ~30%, whereas ablation of either one led to a partial reduction. Nevertheless, we still observed a small but persistent increase, corresponding to a yet unknown source. Our observations suggest a novel mechanism for the regulation of heart rate by UV light, independent of autonomic function.

The Role of the Chromophore in the Biological Photoreceptor Phytochrome: An Approach Using Chemically Synthesized Tetrapyrroles

In plants and bacteria, phytochromes serve as light-inducible, red-/far-red light sensitive photoreceptors that control a wide range of photomorphogenetic processes. Phytochromes comprise a protein moiety and a covalently bound bilin chromophore. Bilins are open-chain tetrapyrrole compounds that derive biosynthetically from ubiquitous porphyrins. The investigations of phytochromes reveal that precise interactions between the protein moiety and its bilin chromophore are essential for the proper functioning of this photoreceptor; accordingly, synthetic manipulation of the parts is an important method for studying the whole. Although variations in the protein structure are readily accomplished by routine mutagenesis protocols, the generation of structurally modified bilins is a laborious, multistep process. Recent improvement in the synthesis of open-chain tetrapyrroles now permits the generation of novel, structurally modified (and even selectively isotope-labeled) chromophores. Furthermore, by using the capability of recombinant apo-phytochrome to bind the chromophore autocatalytically, researchers can now generate novel chromoproteins with modified functions. In the protein-bound state, the phytochrome chromophore is photoisomerized at one double bond, in the bridge between the last two of the four pyrrole rings (the C and D rings), generating the thermally stable, physiologically active P(fr) form. This conversion--photoisomerization from the form absorbing red light (P(r)) to the form absorbing far-red light (P(fr))--covers 12 orders of magnitude, from subpicoseconds to seconds. Such spectroscopic and kinetic studies yield a wealth of time-resolved spectral data, even more so, if proteins with changed sequence or chromophore structure are utilized. In particular, bilins with a changed substitution pattern at the photoisomerizing ring D have shed light on the chromophore-protein interactions during the photoisomerization. The mechanisms generating and stabilizing the light-induced P(fr) form of phytochromes are now seen in greater detail. On the other hand, the use of bilins with selective incorporation of stable isotopes identify light-induced conformational motions when studied by vibrational (FTIR and Raman) and NMR spectroscopy. In this Account, we present spectroscopic investigations that provide structural details in these biological photoreceptors with great precision and document the dynamics elicited by light excitation. This approach yields important information that complements the data deduced from crystal structure.

Visual prosthesis : Artificial vision

In the normal visual pathway, light travels through the tear film, cornea, aqueous, pupil, lens, and vitreous, to activate the light sensitive photoreceptors and set up the trans-synaptic connections of the retina (Fig. 1). In cases where there is gross destruction of the eye like phthisis bulbi, restoration of vision is not possible. But in conditions where blindness is due to a disease in the photoreceptors in the retina like Retinitis Pigmentosa (RP) (Fig. 2) or Age Related Macular Degeneration (AMD) (Fig. 3 a,b) wherein the neural connections are intact, in cortical tumours and lesions involving the visual pathways or occipital cortex wherein the photoreceptors are intact, there exists a scientific possibility wherein a device can be implanted at any location in this pathway to set up a neuronal electrical stimulation. Such a device is known as a visual prosthesis or bionic eye and the vision created is known as artificial vision. Incidentally, RP is the leading inherited cause of blindness, with 1.5 million people affected worldwide [2]. AMD seen in adults over 65 years is the leading cause of visual loss with 700,000 new cases diagnosed annually in the USA and 10 % of whom become legally blind each year. Moreover, these are a group of blind people who have had good vision all their life and have become blind at a time when all faculties are on the decline and daily survival requires vision [3]. Fig. 1 Human visual system [3] Fig. 2 Retinitis Pigmentosa (RP) Fig. 3 (a) Age-related macular degeneration (AMD) and (b) corresponding fundus fluorescein angiogram of the case Depending upon location along the visual pathway this prosthetic device could be in the visual cortex, on the optic nerve or at the retina.

UVB-Based Mate-Choice Cues Used by Females of the Jumping Spider Phintella vittata

Although there are numerous examples of animals having photoreceptors sensitive to UVA (315-400 nm) [1] and relying on UVA-based mate-choice cues [2-5], here we provide the first evidence of an animal using UVB (280-315 nm) for intraspecific communication. An earlier study showed that Phintella vittata, a jumping spider (Salticidae) from China, reflects UVB [6]. By performing six series of binary mate-choice experiments in which we varied lighting conditions with filters (UVB+ [no filter] versus UVB-, UVB+ versus ND1, UVB+ versus ND2, UVB- versus ND1, UVB- versus ND2, and UVB- versus UVA-), we show that significantly more UVB + males than UVB- males are chosen by females as preferred mates. Female preference for UVB-reflective males is not affected by differences in brightness or by UVA.

Septate junctions are required for ommatidial integrity and blood–eye barrier function in Drosophila

The anatomical organization of the Drosophila ommatidia is achieved by specification and contextual placement of photoreceptors, cone and pigment cells. The photoreceptors must be sealed from high ionic concentrations of the hemolymph by a barrier to allow phototransduction. In vertebrates, a blood–retinal barrier (BRB) is established by tight junctions (TJs) present in the retinal pigment epithelium and endothelial membrane of the retinal vessels. In Drosophila ommatidia, the junctional organization and barrier formation is poorly understood. Here we report that septate junctions (SJs), the vertebrate analogs of TJs, are present in the adult ommatidia and are formed between and among the cone and pigment cells. We show that the localization of Neurexin IV (Nrx IV), a SJ-specific protein, coincides with the location of SJs in the cone and pigment cells. Somatic mosaic analysis of nrx IV null mutants shows that loss of Nrx IV leads to defects in ommatidial morphology and integrity. nrx IV hypomorphic allelic combinations generated viable adults with defective SJs and displayed a compromised blood–eye barrier (BEB) function. These findings establish that SJs are essential for ommatidial integrity and in creating a BEB around the ion and light sensitive photoreceptors. Our studies may provide clues towards understanding the vertebrate BEB formation and function.

Caloric restriction retards age-related changes in rat retina

The neural retina of mammals consists of light sensitive photoreceptors and connecting neural cells that receive and send visual signal to the brain. Dietary caloric restriction (CR) is the only experimental intervention that can reliably retard the age-related degeneration of the retina in a normal mammalian model. Here, we studied the effect of CR on various biochemical parameters in the retina of male Brown Norway rats at different ages. We found that CR slowed the age-dependent protein insolubilization, blunted the declines in the total soluble thiols, and reduced glutathione and ascorbic acid levels in neural retina. We also observed that CR retarded the age-related decline in the levels of taurine, a vital amino acid in neural retina. These data are the first to implicate that CR may retard the age-related degeneration of retina by attenuating the oxidative stress and/or by sustaining the pool of protective factors in the neural retina.

TICK'S AFTER DARK ANTICS

For a small creature, ticks have a big reputation. Epidemiologists like Lise Gern know that their tiny bites are far from harmless, infecting innocent victims with one of several unpleasant diseases. However, Patrick Guerin is fascinated by the infamous arthropod's metabolic budgeting. Ticks only feed three times in the course of their lives. They can live for years in the laboratory on just a few drops of blood, but they must use their energy wisely. Virtually nothing was known about Ixodes ricinus' daily movements and the compromises they make to get the most from their three meals. Working with Jean-Luc Perret in Neuchâtel Switzerland, he simulated the tick's environment in the lab while recording their movements,and discovered that ticks naturally limit their activities until after dark(p.1809). And when they took a closer look at the tiny animals, they discovered that ticks have simple light receptor cells, which help coordinate their nocturnal movements.After waiting to ambush a passing victim, perched on the top of a plant in the desiccating sun and wind, ticks need to rehydrate. But they can't drink from pools of water trapped in the undergrowth, in case they pick up a fatal infection. Instead, a tick descends from its dry plant-top perch and returns to the humid plant litter on the ground to rehydrate by water sorption from the air; but at an energetic price. Guerin and Perret wondered whether the arthropod had any strategies for minimising the physical impact of its search for water.Perret went clambering through the forest, collecting the waiting creatures by dragged a towel through the undergrowth. Back in the lab he provided each tick with a vertical plastic tube, plugged at the bottom with damp cotton, so that they could perch at the top as if hunting for victims, and descend to the cotton plug when they needed to rehydrate. But if Perret were going to capture the tick's spontaneous behaviour, he would have to record their activity as he simulated day and night, while making sure that there were no tasty scientists around to distract the ticks. He adapted a webcam to detect infrared radiation and recorded the tick's antics as they roamed undisturbed for ten days.Amazingly, the tiny arthropods only moved after dark. Perret explains that insects lose water through their spiracles when they are physically active and respiring. By restricting their movements to after dark, ticks could limit their water losses by moving when there are `less severe desiccating conditions in the field'.But how did the ticks know when it was dark? Although some ticks have simple eye structures, I. ricinus didn't. He teamed up with Peter-Allan Diehl and Michèle Vlimant to take a closer look at the tiny arthropods, and discovered that they had two sets of light sensitive photoreceptors arranged down both sides. And the nerves from each light receptor connected directly to the tick's neural centre, the synganglion. Guerin adds that although it isn't clear whether the tick's newly discovered visual system contributes to its hunting tactics, it certainly helps it to decide when to conserve energy by only moving after dark. Which could prove to be vital intelligence in the epidemiologist's battle to protect us from the tiny arthropod's appetite for blood.