Non-rod, non-cone photoreception in the vertebrates

WITHDRAWN: Reprint of: Mitochondria: Their role in ganglion cell death and survival in primary open angle glaucoma

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.exer.2010.03.008. The duplicate article has therefore been withdrawn.

A latitudinal cline in the efficacy of endogenous signals: evidence derived from retinal cone contraction in fish

Like many physiological systems synchronised to the light:dark cycle, retinomotor movements in 'lower' vertebrates are controlled by both the ambient illumination and input from endogenous circadian oscillators. In the present study, we examine the relative influence of these two signals in various species of teleost fish with different latitudes of origin. We find equatorial species show very strong endogenous control. The cones of the glowlight tetra, for example, continue to go through undiminished cycles of contraction and relaxation that mirror the previous light:dark cycle for at least two weeks in continual darkness. To quantify the relative effectiveness of the ambient light compared with endogenous signals in causing cone contraction, the degree to which seven teleost species responded to light during the dark phase of their light:dark cycle was examined. In this situation the retina receives conflicting instructions; while the light is acting directly to cause light adaptation, any endogenous signal tends to keep the retinal elements dark adapted. The further from the equator a species originated, the more its cones contracted in response to such illumination, suggesting animals from higher latitudes make little use of endogenous oscillators and rely more on ambient illumination to control behaviours. Equatorial species, however, rely on internal pacemakers to a much greater degree and are relatively insensitive to exogenous light signals. Because these data are consistent with published observations in systems as diverse as melatonin synthesis in Arctic reindeer and the behaviour of regional populations of Drosophila, latitudinal clines in the efficacy of circadian oscillators may be a common feature among animals.

Immunocytochemical study of rhodopsin-containing putative encephalic photoreceptors in house sparrow, Passer domesticus

In seasonally breeding birds, encephalic photoreceptors (EPs) play an important role in regulating photoperiodic gonadal responses. Multiple photopigments have been suggested as the putative EPs, including rhodopsin, melanopsin, VA opsin and the cryptochromes. As for rhodopsin, two potential brain sites, the lateral septum (SL) and the infundibulum (INF) have been reported to co-express rhodopsin immunoreactivity (rhodopsin-ir) with vasoactive intestinal polypeptide immunoreactivity (VIP-ir) in groups of cerebrospinal fluid-contacting (CSF) cells, hypothesized to be the EPs for gonadal responses. In order to confirm the presence of rhodopsin in seasonally breeding birds and examine whether these EPs show daily change as do the photopigments in the retina and pineal gland, the present study immunocytochemically investigated: (1) the presence of rhodopsin expression in the deep brain of the house sparrow, Passer domesticus maintained in short days, and (2) rhythmic expression of rhodopsin and VIP in both SL and INF at Zeitgeber time (ZT) 1 and ZT 17 in house sparrows. Rhodopsin-ir and VIP-ir were observed in both areas of sparrow brains as previously described in other avian species but with a novel rod-like rhodopsin-ir cell type in the INF and novel expression of rhodopsin-ir fiber close to the preoptic area. Daily changes of rhodopsin-ir and VIP-ir cell number were revealed in the INF, but not in the SL. More rhodopsin-ir and fewer VIP-ir cells were found at ZT 17 than at ZT 1. In the median eminence, rhodopsin-ir fibers were only observed at ZT 1, and the relative optic density (ROD) of VIP-ir fibers was higher at ZT 1 than ZT 17. The results indicate daily changes of EPs in the IN and ME, suggesting a role of EPs in the orchestration of photoperiodic gonadal recrudesence.

Vertebrate ancient opsin and melanopsin: divergent irradiance detectors

Both vertebrates and invertebrates respond to light by utilising a wide-ranging array of photosensory systems, with diverse photoreceptor organs expressing a characteristic photopigment, itself consisting of an opsin apoprotein linked to a light-sensitive retinoid chromophore based on vitamin A. In the eye, the pigments expressed in both cone and rod photoreceptors have been studied in great depth and mediate contrast perception, measurement of the spectral composition of environmental light, and thus classical image forming vision. By contrast, the molecular basis for non-visual and extraocular photoreception is far less understood; however, two photopigment genes have become the focus of much study, the vertebrate ancient (va) opsin and melanopsin (opn4). In this review, we discuss the history of discovery for each gene, as well as focusing on the evolution, expression profile, functional role and broader physiological significance of each photopigment. Recently, it has been suggested independently by Arendt et al. and Lamb that an ancestral opsin bifurcated in early metazoans and evolved into two quite different photopigments, one expressed in rhabdomeric photoreceptors and the other in ciliary photoreceptors. This interpretation of the evolution of the metazoan eye has provided a powerful framework for understanding photobiological organization. Their proposal, however, does not encompass all current experimental observations that would be consistent with what we term a central "Evolution of Photosensory Opsins with Common Heredity (EPOCH)" hypothesis to explain the complexity of animal photosensory systems. Clearly, many opsin genes (e.g. va opsin) simply do not fit neatly within this scheme. Thus, the review concludes with a discussion of these anomalies and their context regarding the phylogeny of photoreceptor and photopigment development.

Visual function in mice with photoreceptor degeneration and transgenic expression of channelrhodopsin 2 in ganglion cells

The progression of rod and cone degeneration in retinally degenerate (rd) mice ultimately results in a complete loss of photoreceptors and blindness. The inner retinal neurons survive and several recent studies using genetically targeted, light activated channels have made these neurons intrinsically light sensitive. We crossbred a transgenic mouse line expressing channelrhodopsin2 (ChR2) under the control of the Thy1 promoter with the Pde6b(rd1) mouse, a model for retinal degeneration (rd1/rd1). Approximately 30-40% of the ganglion cells of the offspring expressed ChR2. Extracellular recordings from ChR2-expressing ganglion cells in degenerated retinas revealed their intrinsic light sensitivity which was approximately 7 log U less sensitive than the scotopic threshold and approximately 2 log U less sensitive than photopic responses of normal mice. All ChR2-expressing ganglion cells were excited at light ON. The visual performance of rd1/rd1 mice and ChR2 rd1/rd1 mice was compared. Behavioral tests showed that both mouse strains had a pupil light reflex and they were able to discriminate light fields from dark fields in the visual water task. Cortical activity maps were recorded with optical imaging. The ChR2rd1/rd1 mice did not show a better visual performance than rd1/rd1 mice. In both strains the residual vision was correlated with the density of cones surviving in the peripheral retina. The expression of ChR2 under the control of the Thy1 promoter in retinal ganglion cells does not rescue vision.

A neuroanatomical and physiological study of the non-image forming visual system of the cone-rod homeobox gene (Crx) knock out mouse

The anatomy and physiology of the non-image forming visual system was investigated in a visually blind cone-rod homeobox gene (Crx) knock-out mouse (Crx(-)(/)(-)), which lacks the outer segments of the photoreceptors. We show that the suprachiasmatic nuclei (SCN) in the Crx(-/-) mouse exhibit morphology as in the wild type mouse. In addition, the SCN contain vasoactive intestinal peptide-, vasopressin-, and gastrin-releasing peptide-immunoreactive neurons as present in the wild type. Anterograde in vivo tracings from the retina of the Crx(-/-) and wild type mouse showed that the retinohypothalamic projection to the SCN and the central optic pathways were similar in both animals. Telemetric monitoring of the running activity and temperature revealed that both the Crx(-/-)and wild type mouse exhibited diurnal rhythms with a 24-h period, which could be phase changed by light. However, power spectral analysis revealed that both rhythms in the Crx(-/-) mouse were less robust than those in the wild type. The normal development of the SCN and the central visual pathways in the Crx(-/-) mouse suggests that a modulatory input from the photoreceptors in the peripheral retina to the retinal melanopsin neurons or the SCN may be necessary for a normal function of the non-image forming system of the mouse. However, a change in the SCN of the Crx(-/-) mouse might also explain the observed circadian differences between the knock out mouse and wild type mouse.

Postnatal development and functional adaptations of the melanopsin photoreceptive system in the albino mouse retina

PURPOSE

To study the melanopsin system of the albino CD1 mouse retina during postnatal development.

METHODS

Pups were kept under different ambient conditions: light/dark (LD) cycles, constant light (LL), constant darkness (DD), LL followed by LD, and DD followed by LL. Using immunohistochemistry, melanopsin-expressing cells were classified as M1 or M2 according to the location of their somata and dendritic processes and were counted.

RESULTS

Under LD cycles an increase in the number of immunoreactive cells was observed within the first week of postnatal development. When mice were maintained in DD, the increase in the number of immunopositive cells detected was significantly higher than that in LD. On the contrary, when mice were exposed to LL within the same period, no increase was detected. To determine whether the effect of LL during the early postnatal period was reversible, the authors studied animals born in LL and subsequently maintained under LD cycles. After 3 days in LD, these animals showed a significant increase in melanopsin cell number. However, after 1 month in LD, the number was similar to that of the LD controls. Surprisingly, when mice born in DD were exposed to LL, no decrease was detected, though the immunostaining was of low intensity.

CONCLUSIONS

The amount of melanopsin protein per cell varies, depending on ambient light conditions. Periods of darkness or, more likely, the sequence of light and dark periods occurring under the daily cycles might be necessary for the normal development of the melanopsin system.

Current knowledge on the melatonin system in teleost fish

Melatonin is a much conserved feature in vertebrates that plays a central role in the entrainment of daily and annual physiological rhythms. Investigations aiming at understanding how melatonin mediates the effects of photoperiod on crucial functions and behaviors have been very active in the last decades, particularly in mammals. In fish a clear-cut picture is still missing. Here we review the available data on (i) the sites of melatonin production in fish, (ii) the mechanisms that control its daily and annual rhythms of production and (iii) the characterization of its different receptor subtypes, their location and regulation. The in vivo and in vitro data on melatonin effects on crucial neuroendocrine regulations, including reproduction, growth, feeding and behavioral responses, are also reviewed. Finally we discuss how manipulation of the photic cues impact on fish circannual clock and annual cycle of reproduction, and how this can be used for aquaculture purposes.

Current knowledge on the photoneuroendocrine regulation of reproduction in temperate fish species

Seasonality is an important adaptive trait in temperate fish species as it entrains or regulates most physiological events such as reproductive cycle, growth profile, locomotor activity and key life-stage transitions. Photoperiod is undoubtedly one of the most predictable environmental signals that can be used by most living organisms including fishes in temperate areas. This said, however, understanding of how such a simple signal can dictate the time of gonadal recruitment and spawning, for example, is a complex task. Over the past few decades, many scientists attempted to unravel the roots of photoperiodic signalling in teleosts by investigating the role of melatonin in reproduction, but without great success. In fact, the hormone melatonin is recognized as the biological time-keeping hormone in fishes mainly due to the fact that it reflects the seasonal variation in daylength across the whole animal kingdom rather than the existence of direct evidences of its role in the entrainment of reproduction in fishes. Recently, however, some new studies clearly suggested that melatonin interacts with the reproductive cascade at a number of key steps such as through the dopaminergic system in the brain or the synchronization of the final oocyte maturation in the gonad. Interestingly, in the past few years, additional pathways have become apparent in the search for a fish photoneuroendocrine system including the clock-gene network and kisspeptin signalling and although research on these topics are still in their infancy, it is moving at great pace. This review thus aims to bring together the current knowledge on the photic control of reproduction mainly focusing on seasonal temperate fish species and shape the current working hypotheses supported by recent findings obtained in teleosts or based on knowledge gathered in mammalian and avian species. Four of the main potential regulatory systems (light perception, melatonin, clock genes and kisspeptin) in fish reproduction are reviewed.

Differential light intensity and spectral sensitivities of Atlantic salmon, European sea bass and Atlantic cod pineal glands ex vivo

Photoperiod is perceived by pineal photoreceptors and transduced into rhythmic melatonin signals. These rhythms can be influenced by light intensity and spectral content. In this study we compared the light sensitivity of Atlantic salmon, European sea bass and Atlantic cod by testing ex vivo the effect of different intensities and narrow bandwidth lights on nocturnal melatonin suppression by isolated pineal glands in a flow-through culture system. Using combinations of neutral density and bandpass interference filters we tested a range of light intensities (ranging from 1.22x10(13) to 3.85x10(6) photons s(-1) cm(-2)) and three wavelengths of 80 nm width (472, 555 and 661 nm corresponding to blue, green and red, respectively). Results showed clear species specific light intensity and spectral sensitivities, with cod being from 100 to 1000 times more sensitive than sea bass and salmon. Regarding the influence of spectrum, red light was less efficient on suppressing melatonin than blue and green in salmon but results were not as clear in the two other species studied. Finally, the first evidence of relative photoreception in teleosts was obtained in cod suggesting that the definition of illuminance thresholds (day/night perception) would depend on the day intensity. Indeed, a single order of magnitude increase or decrease in day intensity was shown to elicit a significant shift in the intensity response curve of night-time melatonin suppression. Taken together, this study demonstrated species specific light intensity and spectral sensitivities within temperate teleosts.

Photoimmunomodulation and melatonin

The seasons, and daily physical rhythms can have a profound effect on the physiology of the living organism, which includes immune status. The immune system can be influenced by a variety of signals and one of them is photic stimulus. Light may regulate the immunity through the neuroendocrine system leading to the most recent branch of research the "Photoimmunomodulation". Mammals perceive visible light (400-700 nm) through some specialized photoreceptors located in retina like retinal ganglion cells (RGC). This photic signal is then delivered to the visual cortex from there to the suprachiasmatic nucleus (SCN) of the hypothalamic region. Melatonin--one of the universally accepted chronobiotic molecule secreted by the pineal gland is now emerging as one of the most effective immunostimulatory compound in rodents and as oncostatic molecule at least in human. Its synthesis decreases with light activation along with norepinephrine and acetylcholine. The changes in level of melatonin may lead to alterations (stimulatory/inhibitory) in immune system. The evidences for the presence of melatonin receptor subtypes on lymphoid tissues heralded the research area about mechanism of action for melatonin. Further, melatonin receptor subtypes-MT1 and MT2 was noted on pars tuberalis, SCN and on lymphatic tissues suggesting a direct action of melatonin in modulation of immunity by photoperiod as well. The nuclear receptors (ROR, RZR etc.) of melatonin are known for its free radical scavenging actions and might be indirectly controlling the immune function.

Evolution and the origin of the visual retinoid cycle in vertebrates

Absorption of a photon by visual pigments induces isomerization of 11-cis-retinaldehyde (RAL) chromophore to all-trans-RAL. Since the opsins lacking 11-cis-RAL lose light sensitivity, sustained vision requires continuous regeneration of 11-cis-RAL via the process called 'visual cycle'. Protostomes and vertebrates use essentially different machinery of visual pigment regeneration, and the origin and early evolution of the vertebrate visual cycle is an unsolved mystery. Here we compare visual retinoid cycles between different photoreceptors of vertebrates, including rods, cones and non-visual photoreceptors, as well as between vertebrates and invertebrates. The visual cycle systems in ascidians, the closest living relatives of vertebrates, show an intermediate state between vertebrates and non-chordate invertebrates. The ascidian larva may use retinochrome-like opsin as the major isomerase. The entire process of the visual cycle can occur inside the photoreceptor cells with distinct subcellular compartmentalization, although the visual cycle components are also present in surrounding non-photoreceptor cells. The adult ascidian probably uses RPE65 isomerase, and trans-to-cis isomerization may occur in distinct cellular compartments, which is similar to the vertebrate situation. The complete transition to the sophisticated retinoid cycle of vertebrates may have required acquisition of new genes, such as interphotoreceptor retinoid-binding protein, and functional evolution of the visual cycle genes.

N-linked deglycosylated melanopsin retains its responsiveness to light

Melanopsin is an opsin expressed in the plasma membrane of retinal ganglion cells that mainly project to the circadian clock and thus is important for nonvisual responses to light. Rat melanopsin contains two potential sites (Asn31 and Asn35) for N-linked glycosylation in the N-terminal extracellular part. To investigate if melanopsin is N-linked glycosylated and whether N-bound glycans influence the response of melanopsin to light as evidenced by Fos mRNA induction, we transfected PC12 cells to stably express rat wild-type melanopsin or mutant melanopsin lacking both N-linked glycosylation sites. Immunoblotting for membrane-bound melanopsin from the PC12 cells transfected to express wild-type melanopsin disclosed two immunoreactive bands of 62 and 49 kDa. Removal of N-linked glycosylation by tunicamycin or PNGase F changed the 62 kDa band to a 55 kDa band, while the 49 kDa band corresponding to the core melanopsin protein was unaffected. Likewise, mutation of the two extracellular N-linked glycosylation sites gave a melanopsin size comparable to that of PNGase F or tunicamycin treatment (55 kDa). Further in vitro O-linked deglycosylation of wild-type or mutant melanopsin with O-glycosidase and neuraminidase converted the 55 kDa band to a 49 kDa band. Finally, neither in vivo N-linked deglycosylation nor mutations of the two N-linked glycosylation sites significantly affected melanopsin function measured by Fos induction after light stimulation. In conclusion, we have shown that heterologously expressed rat melanopsin is both N-linked and O-linked glycosylated and that N-linked glycosylation is not crucial for the melanopsin response to light.

Regulation of Melanopsin Expression

Circadian rhythms in mammals are adjusted daily to the environmental day/night cycle by photic input via the retinohypothalamic tract (RHT). Retinal ganglion cells (RGCs) of the RHT constitute a separate light-detecting system in the mammalian retina used for irradiance detection and for transmission to the circadian system and other non-imaging forming processes in the brain. The RGCs of the RHT are intrinsically photosensitive due to the expression of melanopsin, an opsin-like photopigment. This notion is based on anatomical and functional data and on studies of mice lacking melanopsin. Furthermore, heterologous expression of melanopsin in non-neuronal mammalian cell lines was found sufficient to render these cells photosensitive. Even though solid evidence regarding the function of melanopsin exists, little is known about the regulation of melanopsin gene expression. Studies in albino Wistar rats showed that the expression of melanopsin is diurnal at both the mRNA and protein levels. The diurnal changes in melanopsin expression seem, however, to be overridden by prolonged exposure to light or darkness. Significant increase in melanopsin expression was observed from the first day in constant darkness and the expression continued to increase during prolonged exposure in constant darkness. Prolonged exposure to constant light, on the other hand, decreased melanopsin expression to an almost undetectable level after 5 days of constant light. The induction of melanopsin by darkness was even more pronounced if darkness was preceded by light suppression for 5 days. These observations show that dual mechanisms regulate melanopsin gene expression and that the intrinsic light-responsive RGCs in the albino Wistar rat adapt their expression of melanopsin to environmental light and darkness.

Chronic ethanol attenuates circadian photic phase resetting and alters nocturnal activity patterns in the hamster

Acute ethanol (EtOH) administration impairs circadian clock phase resetting, suggesting a mode for the disruptive effect of alcohol abuse on human circadian rhythms. Here, we extend this research by characterizing the chronobiological effects of chronic alcohol consumption. First, daily profiles of EtOH were measured in the suprachiasmatic nucleus (SCN) and subcutaneously using microdialysis in hamsters drinking EtOH. In both cases, EtOH peaked near lights-off and declined throughout the dark-phase to low day-time levels. Drinking bouts preceded EtOH peaks by approximately 20 min. Second, hamsters chronically drinking EtOH received a light pulse during the late dark phase [Zeitgeber time (ZT) 18.5] to induce photic phase advances. Water controls had shifts of 1.2 +/- 0.2 h, whereas those drinking 10% and 20% EtOH had much reduced shifts (0.5 +/- 0.1 and 0.3 +/- 0.1 h, respectively; P < 0.001 vs. controls). Third, incremental decreases in light intensity (270 lux to 0.5 lux) were used to explore chronic EtOH effects on photic entrainment and rhythm stability. Activity onset was unaffected by 20% EtOH at all light intensities. Conversely, the 24-h pattern of activity bouts was disrupted by EtOH under all light intensities. Finally, replacement of chronic EtOH with water was used to examine withdrawal effects. Water controls had photic phase advances of 1.1 +/- 0.3 h, while hamsters deprived of EtOH for 2-3 days showed enhanced shifts (2.1 +/- 0.3 h; P < 0.05 vs. controls). Thus, in chronically drinking hamsters, brain EtOH levels are sufficient to inhibit photic phase resetting and disrupt circadian activity. Chronic EtOH did not impair photic entrainment; however, its replacement with water potentiated photic phase resetting.

Daily rhythm of melanopsin-expressing cells in the mouse retina

In addition to some other functions, melanopsin-expressing retinal ganglion cells (RGCs) constitute the principal mediators of the circadian photoentrainment, a process by which the suprachiasmatic nucleus (the central clock of mammals), adjusts daily to the external day/night cycle. In the present study these RGCs were immunohistochemically labelled using a specific polyclonal antiserum raised against mouse melanopsin. A daily oscillation in the number of immunostained cells was detected in mice kept under a light / dark (LD) cycle. One hour before the lights were on (i.e., the end of the night period) the highest number of immunopositive cells was detected while the lowest was seen 4 h later (i.e., within the first hours of the light period). This finding suggests that some of the melanopsin-expressing RGCs “turn on” and “off” during the day/night cycle. We have also detected that these daily variations already occur in the early postnatal development, when the rod/cone photoreceptor system is not yet functional. Two main melanopsin-expressing cell subpopulations could be found within the retina: M1 cells showed robust dendritic arborization within the OFF sublamina of the inner plexiform layer (IPL), whilst M2 cells had fine dendritic processes within the ON sublamina of the IPL. These two cell subpopulations also showed different daily oscillations throughout the LD cycle. In order to find out whether or not the melanopsin rhythm was endogenous, other mice were maintained in constant darkness for 6 days. Under these conditions, no defined rhythm was detected, which suggests that the daily oscillation detected either is light-dependent or is gradually lost under constant conditions. This is the first study to analyze immunohistochemically the daily oscillation of the number of melanopsin-expressing cells in the mouse retina.

Sleep and circadian rhythms in humans

During the past 50 years, converging evidence reveals that the fundamental properties of the human circadian system are shared in common with those of other organisms. Concurrent data from multiple physiological rhythms in humans revealed that under some conditions, rhythms oscillated at different periods within the same individuals and led to the conclusion 30 years ago that the human circadian system was composed of multiple oscillators organized hierarchically; this inference has recently been confirmed using molecular techniques in species ranging from unicellular marine organisms to mammals. Although humans were once thought to be insensitive to the resetting effects of light, light is now recognized as the principal circadian synchronizer in humans, capable of eliciting weak (Type 1) or strong (Type 0) resetting, depending on stimulus strength and timing. Realization that circadian photoreception could be maintained in the absence of sight was first recognized in blind humans, as was the property of adaptation of the sensitivity of circadian photoreception to prior light history. In sighted humans, the intrinsic circadian period is very tightly distributed around approximately 24.2 hours and exhibits aftereffects of prior entrainment. Phase angle of entrainment is dependent on circadian period, at least in young adults. Circadian pacemakers in humans drive daily variations in many physiologic and behavioral variables, including circadian rhythms in alertness and sleep propensity. Under entrained conditions, these rhythms interact with homeostatic regulation of the sleep/wake cycle to determine the ability to sustain vigilance during the day and to sleep at night. Quantitative understanding of the fundamental properties of the multioscillator circadian system in humans and their interaction with sleep/wake homeostasis has many applications to health and disease, including the development of treatments for circadian rhythm and sleep disorders.

Pupillary correlates of light-evoked melanopsin activity in humans

We investigated whether cones are the only photosensitive process mediating the photopic pupillary light reflex. New analyses were performed on previously published recordings, focusing on those evoked by the onset of photopically equated short- and long-wavelength stimuli. Comparisons between responses revealed contraction differences that slowly grew to a peak and gradually declined. The late contraction was associated with short wavelengths and appeared mostly at the higher stimulus intensities. We conclude that cones are not the only photoreception process mediating the photopic ON-reflex and infer that melanopsin is another. Melanopsin contributes to the steady-state pupil size in daylight illumination.

Clockwork blue: on the evolution of non-image-forming retinal photoreceptors in marine and terrestrial vertebrates

This paper presents a hypothesis that could explain why blue light appears to dominate non-image-forming (NIF) ocular photoreception in marine as well as terrestrial vertebrates. Indeed, there is more and more evidence suggesting that 'novel' retinal photoreceptors, which are sensitive to blue light and were only discovered in the 1990s, could be a feature shared by all vertebrates. In our view, blue light photoreception evolved and persisted as NIF photoreception because it has been useful in the colonisation of extensive photo-dependent oceanic habitats and facilitated the move of vertebrates from an aquatic to a terrestrial environment. Because the available scattered evidence is compatible with the validity of our hypothesis, we hope that our rationale will be followed up. Indeed, it (1) involves testable predictions, (2) provides plausible explanations for previous observations, (3) unites phenomena not previously considered related to one another and (4) suggests tests that have not been carried out before. Overall, our approach not only embraces cross-disciplinary links; it, moreover, serves as a reminder of an all-embracing evolutionary history, especially with regard to a ubiquitous photoreceptive 'clockwork-blue' in marine and terrestrial vertebrates.

The trivial function of sleep

Rest in poikilothermic animals is an adaptation of the organism to adjust to the geophysical cycles, a doubtless valuable function for all animals. In this review, we argue that the function of sleep could be trivial for mammals and birds because sleep does not provide additional advantages over simple rest. This conclusion can be reached by using the null hypothesis and parsimony arguments. First, we develop some theoretical and empirical considerations supporting the absence of specific effects after sleep deprivation. Then, we question the adaptive value of sleep traits by using non-coding DNA as a metaphor that shows that the complexity in the design is not a definitive proof of adaptation. We then propose that few, if any, phenotypic selectable traits do exist in sleep. Instead, the selection of efficient waking has been the major determinant of the most significant aspects in sleep structure. In addition, we suggest that the regulation of sleep is only a mechanism to enforce rest, a state that was challenged after the development of homeothermy. As a general conclusion, there is no direct answer to the problem of why we sleep; only an explanation of why such a complex set of mechanisms is used to perform what seems to be a simple function. This explanation should be reached by following the evolution of wakefulness rather than that of sleep. Sleep could have additional functions secondarily added to the trivial one, although, in this case, the necessity and sufficiency of these sleep functions should be demonstrated.

Role of melatonin in the eye and ocular dysfunctions

Melatonin is a ubiquitous molecule and widely distributed in nature, with functional activity occurring in unicellular organisms, plants, fungi, and animals. Several studies have indicated that melatonin synthesis occurs in the retina of most vertebrates, including mammals. The retinal biosynthesis of melatonin and the mechanisms involved in the regulation of this process have been extensively studied. Circadian clocks located in the photoreceptors and retinal neurons regulate melatonin synthesis in the eye. Photoreceptors, dopaminergic amacrine neurons, and horizontal cells of the retina, corneal epithelium, stroma endothelium, and the sclera all have melatonin receptors, indicating a widespread ocular function for melatonin. In addition, melatonin is an effective antioxidant which scavenges free radicals and up-regulates several antioxidant enzymes. It also has a strong antiapoptotic signaling function, an effect that it exerts even during ischemia. Melatonin cytoprotective properties may have practical implications in the treatment of ocular diseases, like glaucoma and age-related macular degeneration.

Light, photoreceptors, and circadian clocks

Research over the past decade has focused increasingly on the photoreceptor mechanisms that regulate the circadian system in all forms of life. Some of the results to emerge are surprising. For example, the rods and cones within the mammalian eye are not required for the alignment (entrainment) of circadian rhythms to the dawn-dusk cycle. There exists a population of directly light-sensitive ganglion cells within the eye that act as brightness detectors; these regulate both circadian rhythms and melatonin synthesis. An understanding of these "circadian photoreceptor" pathways, and the features of the light environment used for entrainment, have been and will continue to be heavily dependent on the appropriate use and measurement of light stimuli. Furthermore, if results from different laboratories, or species, are to be compared in any meaningful sense, standardized methods for light measurement and manipulation need to be adopted by circadian biologists. To this end, we describe light measurement in terms of both radiometric and photometric units and consider the appropriate use of light as a stimulus in circadian experiments. In addition, the construction of action spectra has been very helpful in associating photopigments with particular responses in a broad range of photobiological systems. Because the identity of the photopigments mediating circadian responses to light are often not known, we have also taken this opportunity to provide a step-by-step approach to conducting action spectra, including the construction of irradiance response curves, the calculation of relative spectral sensitivities, photopigment template fitting, and the underlying assumptions behind this approach. The aims of this chapter are to provide an accessible introduction to photobiological methods and explain why these approaches need to be applied to the study of circadian systems.

Melatonin in the eye: implications for glaucoma

Melatonin synthesis occurs in the retina of most animals as well as in humans. Circadian oscillators that control retinal melatonin synthesis have been identified in the eyes of different animal species. The presence of melatonin receptors is demonstrable by immunocytochemical studies of ocular tissues. These receptors may have different functional roles in different parts of the eye. In view that melatonin is a potent antioxidant molecule, it can be effective in scavenging free radicals that are generated in ocular tissues. By this mechanism melatonin could protect the ocular tissues against disorders like glaucoma, age-related macular degeneration, retinopathy of prematurity, photo-keratitis and cataracts. Although an increased intraocular pressure is an important risk factor in glaucoma, other concomitant phenomena like increased glutamate levels, altered nitric oxide metabolism and increased free radical generation seem to play a significant role in its pathogenesis. Data are discussed indicating that melatonin, being an efficient antioxidant with antinitridergic properties, has a promising role in the treatment and management of glaucoma.

An invertebrate‐like phototransduction cascade mediates light detection in the chicken retinal ganglion cells

Prebilaterian animals perceived ambient light through nonvisual rhabdomeric photoreceptors (RPs), which evolved as support of the chordate visual system. In vertebrates, the identity of nonvisual photoreceptors and the phototransduction cascade involved in nonimage forming tasks remain uncertain. We investigated whether chicken retinal ganglion cells (RGCs) could be nonvisual photoreceptors and the nature of the photocascade involved. We found that primary cultures of chicken embryonic RGCs express such RP markers as transcription factors Pax6 and Brn3, photopigment melanopsin, and G-protein q but not markers for ciliary photoreceptors (alpha-transducin and Crx). To investigate the photoreceptive capability of RGCs, we assessed the direct effect of light on 3H-melatonin synthesis in RGC cultures synchronized to 12:12 h light-dark cycles. In constant dark, RGCs displayed a daily variation in 3H-melatonin levels peaking at subjective day, which was significantly inhibited by light. This light effect was further increased by the chromophore all-trans-retinal and suppressed by specific inhibitors of the invertebrate photocascade involving phosphoinositide hydrolysis (100 microM neomycin; 5 microM U73122) and Ca2+ mobilization (10 mM BAPTA; 1 mM lanthanum). The results demonstrate that chicken RGCs are intrinsically photosensitive RPs operating via an invertebrate-like phototransduction cascade, which may be responsible for early detection of light before vision occurs.

Circadian rhythms of locomotor activity in the Lesotho mole-rat, Cryptomys hottentotus subspecies from Sani Pass, South Africa

The Lesotho mole-rat is a social subterranean rodent that occurs at altitude in the Drakensberg mountain range. As a consequence of living permanently underground these animals rarely if ever are exposed to light. The visual system of African mole-rats is particularly regressed whereas the circadian system is proportionately conserved. This study investigated the locomotor activity patterns of 12 Lesotho mole-rats maintained under a range of different lighting regimes. The majority (91.7%) of mole-rats entrained their activity patterns to a LD photoperiod of 12L/12D. The mole-rats displayed a monophasic nocturnal activity preference. Under constant dark (DD) most of the mole-rats (83.3%) showed a free running circadian activity pattern with a tau of 23.8 h to 24.4 h (mean+/-S.E.M.: 24.07 h+/-0.07 h; n=10). The phase of the activity rhythms each mole-rat exerted during the previous LD-cycle did not change when the animals started free-running after being placed in constant conditions. The duration of re-entrainment to a second bout of LD 12:12 amounted to 9.4+/-2.03 days (mean+/-S.E.M., n=10). Eleven mole-rats (91.7%) adjusted their locomotor activity rhythms to an inversed light regime DL 12:12 and displayed significant nocturnal activity preference. The animals required 9.73+/-2.01 days (mean+/-S.E.M., n=11) to adjust to the DL-photoperiod. The Lesotho mole-rat thus possesses a functional circadian clock that responds to a photic zeitgeber.

Nonvisual light responses in the Rpe65 knockout mouse: rod loss restores sensitivity to the melanopsin system

Intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment melanopsin (OPN4), together with rods and cones, provide light information driving nonvisual light responses. We examined nonvisual photoreception in mice lacking RPE65, a protein that is required for regeneration of visual chromophore in rods and cones. Although Rpe65 knockouts retain a small degree of rod function, we show here that circadian phase shifting responses in Rpe65(-/-) mice are attenuated far beyond what has been reported for rodless/coneless mice. Furthermore, the number of melanopsin-immunoreactive perikarya and the extent of dendritic arborizations were decreased in Rpe65 knockout mice compared with controls. To assess the nature of the photoreceptive defect in Rpe65 null mice, we eliminated either rods or melanopsin from Rpe65(-/-) retinas by generating (i) Rpe65(-/-) mice carrying a transgene (rdta) that results in selective elimination of rods and (ii) double knockout Rpe65(-/-);Opn4(-/-) mice. Surprisingly, rod loss in Rpe65 knockout mice resulted in restoration of circadian photosensitivity. Normal photoentrainment was lost in Rpe65(-/-);Opn4(-/-) mice, and, instead, a diurnal phenotype was observed. Our findings demonstrate that RPE65 is not required for ipRGC function but reveal the existence of a mechanism whereby rods may influence the function of ipRGCs.

Isolation and characterization of melanopsin and pinopsin expression within photoreceptive sites of reptiles

Non-mammalian vertebrates have multiple extraocular photoreceptors, mainly localised in the pineal complex and the brain, to mediate irradiance detection. In this study, we report the full-length cDNA cloning of ruin lizard melanopsin and pinopsin. The high level of identity with opsins in both the transmembrane regions, where the chromophore binding site is located, and the intracellular loops, where the G-proteins interact, suggests that both melanopsin and pinopsin should be able to generate a stable photopigment, capable of triggering a transduction cascade mediated by G-proteins. Phylogenetic analysis showed that both opsins are located on the expected branches of the corresponding sequences of ortholog proteins. Subsequently, using RT-PCR and RPA analysis, we verified the expression of ruin lizard melanopsin and pinopsin in directly photosensitive organs, such as the lateral eye, brain, pineal gland and parietal eye. Melanopsin expression was detected in the lateral eye and all major regions of the brain. However, different from the situation in Xenopus and chicken, melanopsin is not expressed in the ruin lizard pineal. Pinopsin mRNA expression was only detected in the pineal complex. As a result of their phylogenetic position and ecology, reptiles provide the circadian field with some of the most interesting models for understanding the evolution of the vertebrate circadian timing system and its response to light. This characterization of melanopsin and pinopsin expression in the ruin lizard will be important for future studies aimed at understanding the molecular basis of circadian light detection in reptiles.

The mPer1 clock gene expression in the rd mouse suprachiasmatic nucleus is affected by the retinal degeneration

Endogenous rhythms of mammals are controlled by the clock located in the suprachiasmatic nucleus (SCN). The molecular mechanism of a clock involves transcription/translation-based feedback loops in which the expression of the so called "clock genes" is suppressed periodically by their protein products. Previous studies reported influence of the eye itself on the circadian oscillation of the SCN, apart from the well-known photic readjustment of the central clock. With this in mind, we decided to analyze the mPer1 clock gene expression in the retinally degenerate (rd) mouse SCN by means of immunohistochemical techniques. Our objective was to detect possible alterations of the daily endogenous oscillation of PER1 protein in the SCN of these rd mice, as well as to make clear whether or not this protein was involved in the resetting of the central clock in a manner similar to wild-type animals. We found that the endogenous levels of PER1 protein were reduced in the SCN of rd mice throughout the 24-h cycle, which suggests that loss of classic photoreceptors influences somehow the main mechanism of the SCN clock. Light stimulation induced a parallel increase of Per1 expression at the subjective night, but not at the subjective day, in both rd and wild-type mice. Therefore, SCN readjustment by light in the rd mice occurs with a pattern similar to wild-type controls, despite the reduced PER1 protein levels detected. The effect of retinal degeneration on the circadian system and the possible interactions between the retinal and the SCN clocks are discussed.

Acute photoreceptor degeneration down-regulates melanopsin expression in adult rat retina

Melanopsin in retinal ganglion cells plays an important role in mammalian circadian systems. Previous studies indicate melanopsin is responsible for circadian photoentrainment independent of classical rods and cones. However, expression of melanopsin in ganglion cells may be regulated by photoreceptors. In this study, we investigated the effects of N-methyl-N-nitrosourea (MNU)-induced acute photoreceptor degeneration on melanopsin mRNA expression and protein distribution in adult rats. Expression of melanopsin was analyzed 0.5, 1, 5, 7, 13 and 28 days after MNU administration by real-time RT-PCR and immunohistochemistry. MNU-induced gradual degeneration of photoreceptors, and by day 7 most of the photoreceptors were lost. The number of ganglion cells did not change significantly at all time points after MNU injection. In contrast, melanopsin mRNA decreased gradually with the loss of photoreceptors, at the same time pituitary adenylate cyclase-activating polypeptide (PACAP) mRNA levels, which co-express with melanopsin in ganglion cells, were not affected by MNU treatment, indicating decrease of melanopsin mRNA levels is not due to ganglion cell damage. Distribution of melanopsin protein in the dendrites of ganglion cells dramatically decreased with the degeneration of photoreceptors, but its expression in the soma persisted for a long time. Our results suggest that intact photoreceptors maintain the expression of melanopsin and its distribution in ganglion cell dendrites.

Evolving visual pigments: hints from the opsin-based proteins in a phylogenetically old "eyeless" invertebrate

Visual pigments are photosensitive receptor proteins that trigger the transduction process producing the visual excitation once they have absorbed photons. In spite of the molecular and morpho-functional complexity that has characterized the development of animal eyes and eyeless photoreceptive systems, opsin-based protein family appears ubiquous along metazoan visual systems. Moreover, in addition to classic rhodopsin photoreceptors, all Metazoa have supplementary non-visual photosensitive structures, mainly located in the central nervous system, that sense light without forming an image and that rather regulate the organism's temporal physiology. The investigation of novel non-visual photopigments exerting extraretinal photoreception is a challenging field in vision research. Here we propose the cnidarian Hydra as a useful tool of investigation for molecular and functional differences between these pigment families. Hydra is the first metazoan owning a nervous system and it is an eyeless invertebrate showing only an extraocular photoreception, as it has no recognized visual or photosensitive structures. In this paper we provide an overview of the molecular and functional features of the opsin-based protein subfamilies and preliminary evidences in a phylogenetically old species of both image-forming and non-visual opsins. Then we give new insights on the molecular biology of Hydra photoreception and on the evolutionary pathways of visual pigments.

Conserved cis-elements in the Xenopus red opsin promoter necessary for cone-specific expression

The long-wavelength sensitive (red) opsin genes encode proteins which play a central role in daytime and color vision in vertebrates. We used transgenic Xenopus to identify 5' cis-elements in the red cone opsin promoter necessary for cone-specific expression. We found a highly conserved extended region (-725 to -173) that was required for restricting GFP transgene expression to cones. We further identified a short element (5'-CCAATTAAGAGAT-3') highly conserved amongst tetrapods, including humans, necessary to restrict expression to cones in the retina. These results identify novel conserved elements that regulate spatial expression of tetrapod red cone opsin genes.

Roles of PACAP‐Containing Retinal Ganglion Cells in Circadian Timing

The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms in physiology and behavior. The clock-driven rhythms need daily adjustment (entrainment) to be synchronized with the astronomical day of 24 h. The most important stimulus for entrainment of the clock is the light-dark (LD) cycle. In this review functional elements of the light entrainment pathway will be considered with special focus on the neurotransmitter pituitary adenylate cyclase-activating polypeptide (PACAP), which is found exclusively in the monosynaptic neuronal pathway mediating light information to the SCN, the retinohypothalamic tract (RHT). The retinal ganglion cells of the RHT are intrinsically photosensitive due to the expression of melanopsin and seem to constitute a non-image forming photosensitive system in the mammalian eye regulating circadian timing, masking behavior, light-regulated melatonin secretion, and the pupillary light reflex. Evidence from in vitro and in vivo studies and studies of mice lacking PACAP and the specific PACAP receptor (PAC1) indicate that PACAP and glutamate are neurotransmitters in the RHT which in a clock and concentration-dependent manner interact during light entrainment of the clock.

Non-image-forming ocular photoreception in vertebrates

It has been accepted for a hundred years or more that rods and cones are the only photoreceptive cells in the retina. The light signals generated in rods and cones, after processing by downstream retinal neurons (bipolar, horizontal, amacrine and ganglion cells), are transmitted to the brain via the axons of the ganglion cells for further analysis. In the past few years, however, convincing evidence has rapidly emerged indicating that a small subset of retinal ganglion cells in mammals is also intrinsically photosensitive. Melanopsin is the signaling photopigment in these cells. The main function of the inner-retina photoreceptors is to generate and transmit non-image-forming visual information, although some role in conventional vision (image detection) is also possible.

Adrenergic response patterns in vas deferens isolated from rats under diurnal rhythms

The aim of this study was to verify, by means of functional methods, whether the circadian rhythm changes adrenergic response patterns in the epididymal half of the vas deferens isolated from control rats as well as from rats submitted to acute stress. The experiments were performed at 9:00 a.m., 3:00 p.m., 9:00 p.m., and 3:00 a.m. The results showed a light-dark dependent variation of the adrenergic response pattern on organs isolated from control as well as from stressed rats. In the control group, only the phenylephrine sensitivity was changed throughout the circadian rhythm. Under the stress condition, both norepinephrine and phenylephrine response patterns were changed, mainly during darkness. The maximal contractile response to both alpha- and beta-agonist and alpha1-agonist was increased in the dark phase, corresponding to high plasmatic concentrations of endogenous melatonin. The vas deferens isolated from stressed rats during the light phase simultaneously incubated with exogenous melatonin showed the same pattern of response obtained in the dark phase, thus indicating a peripheric action of melatonin on this organ. Therefore, the circadian rhythms are important to the adrenergic response pattern in rat vas deferens from both control and stressed rats. In conclusion, we suggest a melatonin modulation on alpha1-postsynaptic adrenergic response in the rat vas deferens.

Probing inner retinal circuits in the rod pathway: a comparison of c-fos activation in mutant mice

We have used wild-type mice and mice possessing defects in specific retinal circuits in order to more clearly define functional circuits of the inner retina. The retina of the nob mouse lacks communication between photoreceptors and depolarizing bipolar cells (DBCs). Thus, all light driven activity in the nob mouse is mediated via remaining hyperpolarizing bipolar cell (HBC) circuits. Transducin null (Tr alpha-/-) mice lack rod photoreceptor activity and thus remaining retinal circuits are solely generated via cone photoreceptor activity. Activation in inner retinal circuits in each of these mice was identified by monitoring light-induced expression of an immediate early gene, c-fos. The number of cells expressing c-fos in the inner retina was dependent upon stimulus intensity and was altered in a systematic fashion in mice with known retinal mutations. To determine whether c-fos is activated via circuits other than photoreceptors in the outer retina, we examined c-fos expression in tulp1-/- mice that lack photoreceptors in the outer retina; these mice showed virtually no c-fos activity following light exposure. Double-labeling immunohistochemical studies were carried out to more clearly define the population of c-fos expressing amacrine cells. Our results indicate that c-fos may be used to map functional circuits in the retina.

Extraocular photoreception and circadian entrainment in nonmammalian vertebrates

In mammals both the regulation of circadian rhythms and photoperiodic responses depend exclusively upon photic information provided by the lateral eyes; however, nonmammalian vertebrates can also rely on multiple extraocular photoreceptors to perform the same tasks. Extraocular photoreceptors include deep brain photoreceptors located in several distinct brain sites and the pineal complex, involving intracranial (pineal and parapineal) and extracranial (frontal organ and parietal eye) components. This review updates the research field of the most recent acquisitions concerning the roles of extraocular photoreceptors on circadian physiology and behavior, particularly photic entrainment and sun compass orientation.

Mammalian photoentrainment: results, methods, and approaches

Research on circadian biology over the past decade has paid increasing attention to the photoreceptor mechanisms that align the molecular clock to the 24-h light/dark cycle, and some of the results to emerge are surprising. For example, the rods and cones within the mammalian eye are not required for entrainment. A population of directly light-sensitive ganglion cells exists within the retina and acts as brightness detectors. This article provides a brief history of the discovery of these novel ocular photoreceptors and then describes the methods that have been used to study the photopigments mediating these responses to light. Photopigment characterization has traditionally been based on a number of complementary approaches, but one of the most useful techniques has been action spectroscopy. A photopigment has a discrete absorbance spectrum, which describes the probability of photons being absorbed as a function of wavelength, and the magnitude of any light-dependent response depends on the number of photons absorbed by the photopigment. Thus, a description of the spectral sensitivity profile (action spectrum) of any light-dependent response must, by necessity, match absorbance spectra of the photopigment mediating the response. We provide a step-by-step approach to conducting action spectra, including the construction of irradiance response curves, the calculation of relative spectral sensitivities, and photopigment template fitting, and discuss the underlying assumptions behind this approach. We then illustrate action spectrum methodologies by an in-depth analysis of action spectra obtained from rodless/coneless (rd/rd cl) mice and discuss, for the first time, the full implications of these findings.

Could visible light contribute to the development of leukaemia and other cancers in children?

This paper suggests to rigorously test the hypothesis that there are causal links between visible light and the development of leukaemia and other cancers in children. Light can be considered as a candidate risk factor because it suppresses melatonin biosynthesis which may play a role in a series of anticancer defences. Indeed, melatonin may offer some protection against all "hallmarks of cancer" [i.e., self-sufficiency in growth signals; insensitivity to growth-inhibitory signals; evasion of programmed cell death (apoptosis); limitless replicative potential; sustained angiogenesis; tissue invasion and metastasis] recently suggested by Hanahan and Weinberg. Already ongoing investigations into the possible nexus of light, endocrine systems and the development of cancers will be further fueled by recent insights into photoreception and -- transduction, including the discovery of "novel" photoreceptors in the eye. Among a variety of different photosensory tasks, these receptors constitute crucial gates for light information from the environment which is employed for the temporal organization of our physiology and it has been proposed that chronodisruption, i.e., a significant disturbance of the coordination and thus order of biological rhythms, could contribute to the development of cancers. With regard to public health, the pervasive exposures to light -- at work and in homes -- imply that visible radiation could be a strong risk factor defined epidemiologically as a causal contributor to disease in a large proportion of cases. Importantly, if light were to be corroborated as a contributor to cancers in children, it would be amenable to manipulations with the perspective of reducing inherent risks significantly. In fact, it could be much easier -- and much more effective -- to reevaluate and modify lighting systems than to manipulate other possible determinants of the chronic processes of cancer such as genetic, nutritional or lifestyle factors.

Do magnetic fields cause increased risk of childhood leukemia via melatonin disruption?

Epidemiological studies have reported associations between exposure to power frequency magnetic fields and increased risk of certain cancer and noncancer illnesses. For childhood leukemia, a doubling of risk has been associated with exposures above 0.3/0.4 microT. Here, we propose that the melatonin hypothesis, in which power frequency magnetic fields suppress the nocturnal production of melatonin in the pineal gland, accounts for the observed increased risk of childhood leukemia. Such melatonin disruption has been shown in animals, especially with exposure to electric and/or rapid on/off magnetic fields. Equivocal evidence has been obtained from controlled laboratory magnetic field exposures of volunteers, although the exposure conditions are generally atypical of neighborhood exposures. In contrast, support for the hypothesis is found in the body of studies showing magnetic field disruption of melatonin in human populations chronically exposed to both electric and magnetic fields associated with electricity distribution. Further support comes from the observation that melatonin is highly protective of oxidative damage to the human haemopoietic system. Aspects of the hypothesis are amenable to further investigation.

Melanopsin expression in the chick retina and pineal gland

The chick retina and pineal gland exhibit circadian oscillations in biochemical and physiological processes in vivo and in vitro and entrain to 24 h LD cycles. However, the phototransductive mechanisms responsible for entrainment are largely unknown. Experiments utilizing mice lacking functional rods and cones reveal normal circadian responses to light. These studies also implicate a subset of photoresponsive cells in the ganglion cell layer (GCL) which express melanopsin (opn4). The current study investigates opn4 mRNA distribution and regulation in the chick. Opn4 mRNA is widespread in the central nervous system and peripheral tissues, with highest levels of transcript abundance in the pineal gland and retina. Opn4 mRNA is also regulated on a circadian basis in the pineal gland, reaching peak values of accumulation in the late subjective night. In the retina, opn4 is not robustly regulated at the transcriptional level, exhibiting a slight increase in mRNA abundance during subjective night. In situ hybridization (ISH) revealed opn4 mRNA in areas associated with phototransduction and the visual system, including the pineal gland and optic tectum of the brain, along with GCL, inner nuclear layer (INL), and photoreceptor layer (PL) of the retina. Opn4 is also present in cerebellum. Our results present an opsin-like gene located in two circadian oscillators associated with circadian phototransduction and melatonin biosynthesis that may play a role in entrainment of these tissues' clocks.

Extensive duplications of phototransduction genes in early vertebrate evolution correlate with block (chromosome) duplications

Many gene families in mammals have members that are expressed more or less uniquely in the retina or differentially in specific retinal cell types. We describe here analyses of nine such gene families with regard to phylogenetic relationships and chromosomal location. The families are opsins, G proteins (alpha, beta, and gamma subunits), phosphodiesterases type 6, cyclic nucleotide-gated channels, G-protein-coupled receptor kinases, arrestins, and recoverins. The results suggest that multiple new gene copies arose in all of these families very early in vertebrate evolution during a period with extensive gene duplications. Many of the new genes arose through duplications of large chromosome regions (blocks of genes) or even entire chromosomes, as shown by linkage with other gene families. Some of the phototransduction families belong to the same duplicated regions and were thus duplicated simultaneously. We conclude that gene duplications in early vertebrate evolution probably helped facilitate the specialization of the retina and the subspecialization of different retinal cell types.