Melanopsin (Opn4) requirement for normal light-induced circadian phase shifting

Toward optimizing lighting as a countermeasure to sleep and circadian disruption in space flight

Light is being used as a pre-launch countermeasure to circadian and sleep disruption in astronauts. The effect of light on the circadian system is readily monitored by measurement of plasma melatonin. Our group has established an action spectrum for human melatonin regulation and determined the region of 446-477 nm to be the most potent for suppressing plasma melatonin. The aim of this study was to compare the efficacy of 460 and 555 nm for suppressing melatonin using a within-subjects design. Subjects (N=12) were exposed to equal photon densities (7.18 x 10(12) photons/cm2/s) at 460 and 555 nm. Melatonin suppression was significantly stronger at 460 nm (p<0.02). An extension to the action spectrum showed that 420 nm light at 16 and 32 microW/cm2 significantly suppressed melatonin (p<0.04 and p<0.002). These studies will help optimize lighting countermeasures to circadian and sleep disruption during spaceflight.

The cryptochromes

Cryptochromes are photoreceptors that regulate entrainment by light of the circadian clock in plants and animals. They also act as integral parts of the central circadian oscillator in animal brains and as receptors controlling photomorphogenesis in response to blue or ultraviolet (UV-A) light in plants. Cryptochromes are probably the evolutionary descendents of DNA photolyases, which are light-activated DNA-repair enzymes, and are classified into three groups -- plant cryptochromes, animal cryptochromes, and CRY-DASH proteins. Cryptochromes and photolyases have similar three-dimensional structures, characterized by an alpha/beta domain and a helical domain. The structure also includes a chromophore, flavin adenine dinucleotide (FAD). The FAD-access cavity of the helical domain is the catalytic site of photolyases, and it is predicted also to be important in the mechanism of cryptochromes.

The cryptochromes

Cryptochromes are photoreceptors that regulate entrainment of the circadian clock by light in plants and animals. They are related to DNA photolyases and have similar three-dimensional structures, characterized by a α/β domain and a helical domain and including a chromophore, flavin adenine dinucleotide.

Cryptochromes are photoreceptors that regulate entrainment by light of the circadian clock in plants and animals. They also act as integral parts of the central circadian oscillator in animal brains and as receptors controlling photomorphogenesis in response to blue or ultraviolet (UV-A) light in plants. Cryptochromes are probably the evolutionary descendents of DNA photolyases, which are light-activated DNA-repair enzymes, and are classified into three groups - plant cryptochromes, animal cryptochromes, and CRY-DASH proteins. Cryptochromes and photolyases have similar three-dimensional structures, characterized by an α/β domain and a helical domain. The structure also includes a chromophore, flavin adenine dinucleotide (FAD). The FAD-access cavity of the helical domain is the catalytic site of photolyases, and it is predicted also to be important in the mechanism of cryptochromes.

Regulation of prokineticin 2 expression by light and the circadian clock

Background

The suprachiasmatic nucleus (SCN) contains the master circadian clock that regulates daily rhythms of many physiological and behavioural processes in mammals. Previously we have shown that prokineticin 2 (PK2) is a clock-controlled gene that may function as a critical SCN output molecule responsible for circadian locomotor rhythms. As light is the principal zeitgeber that entrains the circadian oscillator, and PK2 expression is responsive to nocturnal light pulses, we further investigated the effects of light on the molecular rhythm of PK2 in the SCN. In particular, we examined how PK2 responds to shifts of light/dark cycles and changes in photoperiod. We also investigated which photoreceptors are responsible for the light-induced PK2 expression in the SCN. To determine whether light requires an intact functional circadian pacemaker to regulate PK2, we examined PK2 expression in cryptochrome1,2-deficient (Cry1-/-Cry2-/-) mice that lack functional circadian clock under normal light/dark cycles and constant darkness.

Results

Upon abrupt shifts of the light/dark cycle, PK2 expression exhibits transients in response to phase advances but rapidly entrains to phase delays. Photoperiod studies indicate that PK2 responds differentially to changes in light period. Although the phase of PK2 expression expands as the light period increases, decreasing light period does not further condense the phase of PK2 expression. Genetic knockout studies revealed that functional melanopsin and rod-cone photoreceptive systems are required for the light-inducibility of PK2. In Cry1-/-Cry2-/- mice that lack a functional circadian clock, a low amplitude PK2 rhythm is detected under light/dark conditions, but not in constant darkness. This suggests that light can directly regulate PK2 expression in the SCN.

Conclusion

These data demonstrate that the molecular rhythm of PK2 in the SCN is regulated by both the circadian clock and light. PK2 is predominantly controlled by the endogenous circadian clock, while light plays a modulatory role. The Cry1-/-Cry2-/- mice studies reveal a light-driven PK2 rhythm, indicating that light can induce PK2 expression independent of the circadian oscillator. The light inducibility of PK2 suggests that in addition to its role in clock-driven rhythms of locomotor behaviour, PK2 may also participate in the photic entrainment of circadian locomotor rhythms.

Pharmacological modulation of circadian rhythms: a new drug target in psychotherapeutics

Daily variation in an organism's physiology and behaviour is regulated by the synchrony that is achieved between the internal timing mechanisms - the circadian rhythms of the biological clock - and the prevailing environmental cues. Proper synchrony constitutes an adaptive response; improper or lost synchrony may well yield maladaptation and, in the case of humans, a psychiatric disorder. On a basic level, the circadian system is comprised of three parts: a central oscillator, its various neuronal inputs and its outputs. For all three of these parts, the dissemination of new information is moving at an unprecedented pace, and the number of molecular targets for the opportunistic pharmacologist is growing in step. Monoamines, neuropeptides, kinases - sorting through all these, much less developing one into a drug discovery programme, may be the biggest challenge. However, the potential benefits in targeting a basic flaw in a fundamental biological system may be enormous.

Light pulses do not induce c-fos or per1 in the SCN of hamsters that fail to reentrain to the photocycle

Circadian activity rhythms of most Siberian hamsters (Phodopus sungorus sungorus) fail to reentrain to a 5-h phase shift of the light-dark (LD) cycle. Instead, their rhythms free-run at periods close to 25 h despite the continued presence of the LD cycle. This lack of behavioral reentrainment necessarily means that molecular oscillators in the master circadian pacemaker, the SCN, were unable to reentrain as well. The authors tested the hypothesis that a phase shift of the LD cycle rendered the SCN incapable of responding to photic input. Animals were exposed to a 5-h phase delay of the photocycle, and activity rhythms were monitored until a lack of reentrainment was confirmed. Hamsters were then housed in constant darkness for 24 h and administered a 30-min light pulse 2 circadian hours after activity onset. Brains were then removed, and tissue sections containing the SCN were processed for in situ hybridization. Sections were probed with Siberian hamster c-fos and per1 mRNA probes because light rapidly induces these 2 genes in the SCN during subjective night but not at other circadian phases. Light pulses induced robust expression of both genes in all animals that reentrained to the LD cycle, but no expression was observed in any animal that failed to reentrain. None of the animals exhibited an intermediate response. This finding is the first report of acute shift in a photocycle eliminating photosensitivity in the SCN and suggests that a specific pattern of light exposure may desensitize the SCN to subsequent photic input.

Illumination of the melanopsin signaling pathway

In mammals, a small population of intrinsically photosensitive retinal ganglion cells (ipRGCs) plays a key role in the regulation of nonvisual photic responses, such as behavioral responses to light, pineal melatonin synthesis, pupillary light reflex, and sleep latency. These ipRGCs also express melanopsin (Opn4), a putative opsin-family photopigment that has been shown to play a role in mediating these nonvisual photic responses. Melanopsin is required for the function of this inner retinal pathway, but its precise role in generating photic responses has not yet been determined. We found that expression of melanopsin in Xenopus oocytes results in light-dependent activation of membrane currents through the Galpha(q)/Galpha(11) G protein pathway, with an action spectrum closely matching that of melanopsin-expressing ipRGCs and of behavioral responses to light in mice lacking rods and cones. When coexpressed with arrestins, melanopsin could use all-trans-retinaldehyde as a chromophore, which suggests that it may function as a bireactive opsin. We also found that melanopsin could activate the cation channel TRPC3, a mammalian homolog of the Drosophila phototransduction channels TRP and TRPL. Melanopsin therefore signals more like an invertebrate opsin than like a classical vertebrate rod-and-cone opsin.

Circadian gene expression patterns of melanopsin and pinopsin in the chick pineal gland

The directly light-sensitive chick pineal gland contains at least two photopigments. Pinopsin seems to mediate the acute inhibitory effect of light on melatonin synthesis, whereas melanopsin may act by phase-shifting the intrapineal circadian clock. In the present study we have investigated, by means of quantitative RT-PCR, the daily rhythm of photopigment gene expression as monitored by mRNA levels. Under a 12-h light/12-h dark cycle, the mRNA levels of both pigments were 5-fold higher in the transitional phase from light to dark than at night, both in vivo and in vitro. Under constant darkness in vivo and in vitro, the peak of pinopsin mRNA levels was attenuated, whereas that of melanopsin was not. Thus, whereas the daily rhythm of pinopsin gene expression is dually regulated by light plus the intrapineal circadian oscillator, that of melanopsin appears to depend solely on the oscillator.

Melanopsin-expressing ganglion cells in primate retina signal colour and irradiance and project to the LGN

Human vision starts with the activation of rod photoreceptors in dim light and short (S)-, medium (M)-, and long (L)- wavelength-sensitive cone photoreceptors in daylight. Recently a parallel, non-rod, non-cone photoreceptive pathway, arising from a population of retinal ganglion cells, was discovered in nocturnal rodents. These ganglion cells express the putative photopigment melanopsin and by signalling gross changes in light intensity serve the subconscious, 'non-image-forming' functions of circadian photoentrainment and pupil constriction. Here we show an anatomically distinct population of 'giant', melanopsin-expressing ganglion cells in the primate retina that, in addition to being intrinsically photosensitive, are strongly activated by rods and cones, and display a rare, S-Off, (L + M)-On type of colour-opponent receptive field. The intrinsic, rod and (L + M) cone-derived light responses combine in these giant cells to signal irradiance over the full dynamic range of human vision. In accordance with cone-based colour opponency, the giant cells project to the lateral geniculate nucleus, the thalamic relay to primary visual cortex. Thus, in the diurnal trichromatic primate, 'non-image-forming' and conventional 'image-forming' retinal pathways are merged, and the melanopsin-based signal might contribute to conscious visual perception.

Induction of photosensitivity by heterologous expression of melanopsin

Melanopsin has been proposed to be the photopigment of the intrinsically photosensitive retinal ganglion cells (ipRGCs); these photoreceptors of the mammalian eye drive circadian and pupillary adjustments through direct projections to the brain. Their action spectrum (lambda(max) approximately 480 nm) implicates an opsin and melanopsin is the only opsin known to exist in these cells. Melanopsin is required for ipRGC photosensitivity and for behavioural photoresponses that survive disrupted rod and cone function. Heterologously expressed melanopsin apparently binds retinaldehyde and mediates photic activation of G proteins. However, its amino-acid sequence differs from vertebrate photosensory opsins and some have suggested that melanopsin may be a photoisomerase, providing retinoid chromophore to an unidentified opsin. To determine whether melanopsin is a functional sensory photopigment, here we transiently expressed it in HEK293 cells that stably expressed TRPC3 channels. Light triggered a membrane depolarization in these cells and increased intracellular calcium. The light response resembled that of ipRGCs, with almost identical spectral sensitivity (lambda(max) approximately 479 nm). The phototransduction pathway included Gq or a related G protein, phospholipase C and TRPC3 channels. We conclude that mammalian melanopsin is a functional sensory photopigment, that it is the photopigment of ganglion-cell photoreceptors, and that these photoreceptors may use an invertebrate-like phototransduction cascade.

Rhabdomeric phototransduction initiated by the vertebrate photopigment melanopsin

Melanopsin is the photopigment that confers light sensitivity on intrinsically photosensitive retinal ganglion cells. Mammalian intrinsically photosensitive retinal ganglion cells are involved in the photic synchronization of circadian rhythms to the day-night cycle. Here, we report molecular components of melanopsin signaling using the cultured Xenopus dermal melanophore system. Photo-activated melanopsin is shown to initiate a phosphoinositide signaling pathway similar to that found in invertebrate photo-transduction. In melanophores, light increases the intracellular level of inositol trisphosphate and causes the dispersion of melanosomes. Inhibition of phospholipase C and protein kinase C and chelation of intracellular calcium block the effect of light on melanophores. At least four proteins, 43, 74, 90, and 134 kDa, are phosphorylated by protein kinase C upon light stimulation. This provides evidence of an invertebrate-like light-activated signaling cascade within vertebrate cells.

Cryptochromes and Circadian Photoreception in Animals

Cryptochromes are flavin- and folate-containing blue-light photoreceptors with a high degree of similarity to DNA photolyase, which repairs ultraviolet-induced DNA damage using blue light to initiate the repair reaction. Cryptochromes play essential roles in the maintenance of circadian rhythms in mice and Drosophila, and genetic data indicate that cryptochromes function as circadian photoreceptors in these and other animals. However, the photochemical reactions carried out by cryptochromes are not known at present.

Molecular cloning, localization and circadian expression of chicken melanopsin (Opn4): differential regulation of expression in pineal and retinal cell types

The avian retina and pineal gland contain autonomous circadian oscillators and photo-entrainment pathways, but the photopigment(s) that mediate entrainment have not been definitively identified. Melanopsin (Opn4) is a novel opsin involved in entrainment of circadian rhythms in mammals. Here, we report the cDNA cloning of chicken melanopsin and show its expression in retina, brain and pineal gland. Like the melanopsins identified in amphibians and mammals, chicken melanopsin is more similar to the invertebrate retinaldehyde-based photopigments than the retinaldehyde-based photopigments typically found in vertebrates. In retina, melanopsin mRNA is expressed in cells of all retinal layers. In pineal gland, expression was strong throughout the parenchyma of the gland. In brain, expression was observed in a few discrete nuclei, including the lateral septal area and medial preoptic nucleus. The retina and pineal gland showed distinct diurnal expression patterns. In pineal gland, melanopsin mRNA levels were highest at night at Zeitgeber time (ZT) 16. In contrast, transcript levels in the whole retina reached their highest levels in the early morning (ZT 0-4). Further analysis of melanopsin mRNA expression in retinal layers isolated by laser capture microdissection revealed different patterns in different layers. There was diurnal expression in all retinal layers except the ganglion cell layer, where heavy expression was localized to a small number of cells. Expression of melanopsin mRNA peaked during the daytime in the retinal pigment epithelium and inner nuclear layer but, like in the pineal, at night in the photoreceptors. Localization and regulation of melanopsin mRNA in the retina and pineal gland is consistent with the hypothesis that this novel photopigment plays a role in photic regulation of circadian function in these tissues.

The circadian Clock mutant mouse: impaired masking response to light

Synchronization of an internal clock (entrainment) and a direct response to light (masking) are complementary ways of restricting activity of an animal to day or night. The protein CLOCK has an important role in the oscillatory mechanism of mammalian pacemakers. Our data show that it is also involved in masking responses. Mice with the Clock/Clock mutation reduced their wheel running less than wildtypes when given 1-h light pulses of light (2-1,600 lx) in the night. With dimmer lights (<2 lx), there were no significant differences between mutant and wildtype mice. Impaired masking responses to light in Clock/Clock mice were confirmed in tests with ultradian light-dark cycles (3.5:3.5 h and 1:1 h). Tests with pulses of light longer than 1 h revealed that, although the mutants responded more slowly to light, they sustained the suppression of activity over the course of the 3-h tests better than wildtypes.

Direct visual and circadian pathways target neuroendocrine cells in primates

The effect of light on neuroendocrine functions is thought to be mediated through retinal inputs to the circadian pacemaker, the hypothalamic suprachiasmatic nucleus (SCN). The present studies were conducted to provide experimental evidence for this signaling modality in non-human primates. In the St. Kitts vervet monkey, anterograde tracing of SCN efferents revealed a monosynaptic pathway between the circadian clock and hypothalamic neurons producing luteinizing hormone-releasing hormone (LHRH). Using a variety of tracing techniques, direct retinal input was found to be abundant in the SCN and in other hypothalamic sites. Strikingly, in hypothalamic areas other than the SCN, primary visual afferents established direct contacts with neuroendocrine cells including those producing LHRH and dopamine, neurons that are the hypothalamic regulators of pituitary gonadotrops and prolactin. Thus, our data reveal for the first time in primates that light stimuli can reach the hypothalamo-pituitary-gonadal axis, directly providing a pathway independent of but parallel to that of the circadian clock for the photic modulation of hormone release.

Selective deficits in the circadian light response in mice lacking PACAP

Previous studies indicate that light information reaches the suprachiasmatic nucleus through a subpopulation of retinal ganglion cells that contain both glutamate and pituitary adenylyl cyclase-activating peptide (PACAP). Although the role of glutamate in this pathway has been well studied, the involvement of PACAP and its receptors is only beginning to be understood. To investigate the functions of PACAP in vivo, we developed a mouse model in which the gene coding for PACAP was disrupted by targeted homologous recombination. RIA was used to confirm a lack of detectable PACAP protein in these mice. PACAP-deficient mice exhibited significant impairment in the magnitude of the response to brief light exposures with both light-induced phase delays and advances of the circadian system impacted. This mutation equally impacted phase shifts induced by bright and dim light exposure. Despite these effects on phase shifting, the loss of PACAP had only limited effects on the generation of circadian oscillations, as measured by rhythms in wheel-running activity. Unlike melanopsin-deficient mice, the mice lacking PACAP exhibited no loss of function in the direct light-induced inhibition of locomotor activity, i.e., masking. Finally, the PACAP-deficient mice exhibited normal phase shifts in response to exposure to discrete dark treatments. The results reported here show that the loss of PACAP produced selective deficits in the light response of the circadian system.

A gene regulatory hierarchy for retinal ganglion cell specification and differentiation

Retinal ganglion cells (RGCs) are the first cell type to be specified during vertebrate retinogenesis. Specification and differentiation of the RGC lineage are a stepwise process involving a hierarchical gene regulatory network. During the past decade, a framework of the network has emerged and key transcriptional regulators have been identified. Pax6, Notch, Ath5, and the Brn3 (Pou4f) factors act at different levels of the regulatory hierarchy. In this review, we summarize the current understanding of the functions of these and other transcriptional factors in the specification and differentiation of the RGC lineage. We emphasize the regulatory relationships among transcription factors at different steps of RGC development. We discuss critical issues that need to be addressed before a complete understanding of the gene regulatory network for RGC development can be achieved. Future directions in RGC development will inevitably rely on combined genetic and genomics approaches.

Axotomized mouse retinal ganglion cells containing melanopsin show enhanced survival, but not enhanced axon regrowth into a peripheral nerve graft

Melanopsin is found in only approximately 2% of mouse retinal ganglion cells (RGCs), making these RGCs uniquely and directly photosensitive. Given that the majority of RGCs die after axotomy and that grafting of a peripheral nerve to the eye provides a permissive environment for axon regrowth, the present study examined the survival and axonal regrowth of melanopsin-containing RGCs in mice. One month after optic nerve transection and grafting, RGCs with regrown axons were labeled from the grafts and retinae were processed to visualize melanopsin and TUJ1. Melanopsin-positive and negative RGCs were counted and compared to axotomized RGCs from ungrafted eyes and uninjured RGCs. Melanopsin-positive RGCs showed a 3-fold increase in survival rate compared to non-melanopsin RGCs. Despite this enhanced survival, melanopsin-containing RGCs did not show increased axon regrowth into nerve grafts.

Altered endogenous activation of CREB in the suprachiasmatic nucleus of mice with retinal degeneration

The effect of cone- and rod-cell loss on the activation of transcription factor CREB (by phosphorylation at Ser133) was examined in the pacemaker of mammals, the suprachiasmatic nucleus (SCN). For this purpose, brain sections of rd/rd and wild-type C3H mice were immunolabeled with a polyclonal antibody that recognises p-CREB, i.e., the activated form of the protein. Both rd/rd and wild-type mice maintained in constant darkness showed a circadian variation of p-CREB nuclear staining: the number of immunopositive nuclear pixels at the subjective night was higher than the one observed at the subjective day. However, some differences were detected between both groups: (1) p-CREB immunolabelling in the SCN of rd/rd mice was significantly reduced throughout the 24-h cycle; (2) the time in which the activation of CREB begins to increase at the subjective night in these mice is delayed with regard to wild-type mice. When a light stimulus was given at the subjective night p-CREB immunostaining significantly increased in the SCN of both rd/rd and wild-type mice when compared to basal levels, while no significant effect was found when the stimulus was given at the subjective day. Taken together, our results suggest that despite lower levels of p-CREB, indicating that something is altered in the SCN of rd/rd mice, the main mechanisms of the clock (e.g., circadian oscillation, readjustment by light) are still fully functional in these mice. The present study supports the idea that the CREB/CRE pathway is a component of the circadian clock molecular mechanism.

Melanopsin--shedding light on the elusive circadian photopigment

Circadian photoentrainment is the process by which the brain's internal clock becomes synchronized with the daily external cycle of light and dark. In mammals, this process is mediated exclusively by a novel class of retinal ganglion cells that send axonal projections to the suprachiasmatic nuclei (SCN), the region of the brain that houses the circadian pacemaker. In contrast to their counterparts that mediate image-forming vision, SCN-projecting RGCs are intrinsically sensitive to light, independent of synaptic input from rod and cone photoreceptors. The recent discovery of these photosensitive RGCs has challenged the long-standing dogma of retinal physiology that rod and cone photoreceptors are the only retinal cells that respond directly to light and has explained the perplexing finding that mice lacking rod and cone photoreceptors can still reliably entrain their circadian rhythms to light. These SCN-projecting RGCs selectively express melanopsin, a novel opsin-like protein that has been proposed as a likely candidate for the photopigment in these cells. Research in the past three years has revealed that disruption of the melanopsin gene impairs circadian photo- entrainment, as well as other nonvisual responses to light such as the pupillary light reflex. Until recently, however, there was no direct demonstration that melanopsin formed a functional photopigment capable of catalyzing G-protein activation in a light-dependent manner. Our laboratory has recently succeeded in expressing melanopsin in a heterologous tissue culture system and reconstituting a pigment with the 11-cis-retinal chromophore. In a reconstituted biochemical system, the reconstituted melanopsin was capable of activating transducin, the G-protein of rod photoreceptors, in a light-dependent manner. The absorbance spectrum of this heterologously expressed melanopsin, however, does not match that predicted by previous behavioral and electophysiological studies. Although melanopsin is clearly the leading candidate for the elusive photopigment of the circadian system, further research is needed to resolve the mystery posed by its absorbance spectrum and to fully elucidate its role in circadian photoentrainment.

Retinal ganglion cells are autonomous circadian oscillators synthesizing N-acetylserotonin during the day

Retinal ganglion cells send visual and circadian information to the brain regarding the environmental light-dark cycles. We investigated the capability of retinal ganglion cells of synthesizing melatonin, a highly reliable circadian marker that regulates retinal physiology, as well as the capacity of these cells to function as autonomous circadian oscillators. Chick retinal ganglion cells presented higher levels of melatonin assessed by radioimmunoassay during both the subjective day in constant darkness and the light phase of a light-dark cycle. Similar changes were observed in mRNA levels and activity of arylalkylamine N-acetyltransferase, a key enzyme in melatonin biosynthesis, with the highest levels of both parameters during the subjective day. These daily variations were preceded by the elevation of cyclic-AMP content, the second messenger involved in the regulation of melatonin biosynthesis. Moreover, cultures of immunopurified retinal ganglion cells at embryonic day 8 synchronized by medium exchange synthesized a [3H]melatonin-like indole from [3H]tryptophan. This [3H]indole was rapidly released to the culture medium and exhibited a daily variation, with levels peaking 8 h after synchronization, which declined a few hours later. Cultures of embryonic retinal ganglion cells also showed self-sustained daily rhythms in arylalkylamine N-acetyltransferase mRNA expression during at least three cycles with a period near 24 h. These rhythms were also observed after the application of glutamate. The results demonstrate that chick retinal ganglion cells may function as autonomous circadian oscillators synthesizing a melatonin-like indole during the day.

Neurobiology of the fruit fly's circadian clock

Studying the fruit fly Drosophila melanogaster has revealed mechanisms underlying circadian clock function. Rhythmic behavior could be assessed to the function of several clock genes that generate circadian oscillations in certain brain neurons, which finally modulate behavior in a circadian manner. This review outlines how individual circadian pacemaker neurons in the fruit fly's brain control rhythm in locomotor activity and eclosion.

Residual photosensitivity in mice lacking both rod opsin and cone photoreceptor cyclic nucleotide gated channel 3 α subunit

The mammalian retina contains three classes of photoreceptor. In addition to the rods and cones, a subset of retinal ganglion cells that express the putative sensory photopigment melanopsin are intrinsically photosensitive. Functional and anatomical studies suggest that these inner retinal photoreceptors provide light information for a number of non-image-forming light responses including photoentrainment of the circadian clock and the pupil light reflex. Here, we employ a newly developed mouse model bearing lesions of both rod and cone phototransduction cascades (Rho−/−Cnga3−/−) to further examine the function of these non-rod non-cone photoreceptors. Calcium imaging confirms the presence of inner retinal photoreceptors inRho−/−Cnga3−/−mice. Moreover, these animals retain a pupil light reflex, photoentrainment, and light induction of the immediate early genec-fosin the suprachiasmatic nuclei, consistent with previous findings that pupillary and circadian responses can employ inner retinal photoreceptors.Rho−/−Cnga3−/−mice also show a light-dependent increase in the number of FOS-positive cells in both the ganglion cell and (particularly) inner nuclear layers of the retina. The average number of cells affected is several times greater than the number of melanopsin-positive cells in the mouse retina, suggesting functional intercellular connections from these inner retinal photoreceptors within the retina. Finally, however, while we show that wild types exhibit an increase in heart rate upon light exposure, this response is absent inRho−/−Cnga3−/−mice. Thus, it seems that non-rod non-cone photoreceptors can drive many, but not all, non-image-forming light responses.

Screening for novel ENU-induced rhythm, entrainment and activity mutants

Chemical mutagenesis has provided an opportunity to develop and expand the repertoire of behavioural mutants for gene function studies. With this in mind, we have established a screen in mice for mutations affecting circadian rhythms, entrainment to light and other wheel-running parameters. The screen consists of an assessment of mouse wheel-running activity in a 12:12 h light/dark cycle for 7-10 days followed by assessment in constant darkness for up to 20 days. Responses to light are assessed using two protocols; a 15 minute light pulse given at circadian time 16 on the tenth day in constant darkness and an additional 12 h of light upon transition from light/dark conditions to constant darkness. To date, approximately 1300 progeny of chemically mutagenised mice have been screened. Computer-aided assessment of wheel-running parameters has helped in identifying abnormal phenotypes in approximately 5% of all animals screened. Inheritance testing of mice with abnormal phenotypes has confirmed the number of robustly inherited mutant phenotypes to be 1% of the total screened. Confirmed mutants including those affecting free-running period, light-responsiveness and wheel-running endurance have been identified. Thus far, low-resolution map positions have been established for four mutants by completing genome scans in backcross progeny. Mutant loci do not correspond with those previously associated with wheel-running behaviour. This result confirms that phenotype-driven approaches such as this should continue to provide material for mammalian gene function studies.

N-ethyl-N-nitrosourea mouse mutants in the dissection of behavioural and psychiatric disorders

Twin and adoption studies have consistently implicated genetics in the aetiology of psychiatric and behavioural disorders. The identification of the genes and molecular pathways that are associated with these traits using linkage studies has been difficult because psychiatric disorders are almost always non-mendelian, heterogeneous, involve multiple genetic loci and are influenced significantly by environmental factors. Mouse models that are based on intermediate signatures of psychiatric disease and pharmacological responsiveness hold promise as a complementary approach to dissecting the molecular basis of neurobehavioural disorders. This has been made possible by the development and refinement of gene targeting technologies and the use of super-efficient chemical mutagens. N-ethyl-N-nitrosourea (ENU) mutagenesis in the mouse, when coupled to a battery of sensitive behavioural screens, is an effective way of creating and identifying novel mouse behavioural mutants. Here, the concept of screening for ENU mutants is introduced while progress with two behavioural screens, an "anxiety" screen and a circadian screen, are presented. It is hoped that the study of mouse mutants that have arisen from these screens will provide new insights into the genetic basis of abnormal behaviour and that they might lead to the development of novel therapeutic compounds for human psychiatric disease.

Preliminary evidence for spectral opponency in the suppression of melatonin by light in humans

Human adult males were exposed to light from blue light emitting diodes (18 lux; 29 microW/cm) and from clear mercury vapor lamps (450 lux; 170 microW/cm) during night-time experimental sessions. Both conditions suppressed nocturnal melatonin concentrations in blood plasma with the blue light more effective than mercury at melatonin suppression. No additive model incorporating opsin photopigments either alone or in combination could explain the results, but a model incorporating an opponent mechanism was consistent with the present data as well as data from previously published studies.

Resetting Mechanism of Central and Peripheral Circadian Clocks in Mammals

Almost all organisms on earth exhibit diurnal rhythms in physiology and behavior under the control of autonomous time-measuring system called circadian clock. The circadian clock is generally reset by environmental time cues, such as light, in order to synchronize with the external 24-h cycles. In mammals, the core oscillator of the circadian clock is composed of transcription/translation-based negative feedback loops regulating the cyclic expression of a limited number of clock genes (such as Per, Cry, Bmal1, etc.) and hundreds of output genes in a well-concerted manner. The central clock controlling the behavioral rhythm is localized in the hypothalamic suprachiasmatic nucleus (SCN), and peripheral clocks are present in other various tissues. The phase of the central clock is amenable to ambient light signal captured by the visual rod-cone photoreceptors and non-visual melanopsin in the retina. These light signals are transmitted to the SCN through the retinohypothalamic tract, and transduced therein by mitogen-activated protein kinase and other signaling molecules to induce Per gene expression, which eventually elicits phase-dependent phase shifts of the clock. The central clock controls peripheral clocks directly and indirectly by virtue of neural, humoral, and other signals in a coordinated manner. The change in feeding time resets the peripheral clocks in a SCN-independent manner, possibly by food metabolites and body temperature rhythms. In this article, we will provide an overview of recent molecular and genetic studies on the resetting mechanism of the central and peripheral circadian clocks in mammals.

Melanopsin and non-melanopsin expressing retinal ganglion cells innervate the hypothalamic suprachiasmatic nucleus

Retinal input to the hypothalamic suprachiasmatic nucleus (SCN) synchronizes the SCN circadian oscillator to the external day/night cycle. Retinal ganglion cells that innervate the SCNviathe retinohypothalamic tract are intrinsically light sensitive and express melanopsin. In this study, we provide data indicating that not all SCN-projecting retinal ganglion cells express melanopsin. To determine the proportion of ganglion cells afferent to the SCN that express melanopsin, ganglion cells were labeled following transsynaptic retrograde transport of a recombinant of the Bartha strain of pseudorabies virus (PRV152) constructed to express the enhanced green fluorescent protein (EGFP). PRV152 injected into the anterior chamber of the eye retrogradely infects four retinorecipient nuclei in the brainviaautonomic circuits to the eye, resulting in transneuronally labeled ganglion cells in the contralateral retina 96 h after intraocular infection. In animals with large bilateral lesions of the lateral geniculate body/optic tract, ganglion cells labeled with PRV152 are retrogradely infected from only the SCN. In these animals, most PRV152-infected ganglion cells were immunoreactive for melanopsin. However, a significant percentage (10–20%) of EGFP-labeled ganglion cells did not express melanopsin. These data suggest that in addition to the intrinsically light-sensitive melanopsin-expressing ganglion cells, conventional ganglion cells also innervate the SCN. Thus, it appears that the rod/cone system of photoreceptors may provide signals to the SCN circadian system independent of intrinsically light-sensitive melanopsin ganglion cells.

Further evidence for the role of cryptochromes in retinohypothalamic photoreception/phototransduction

Cryptochrome is a blue-light absorbing photopigment that has been proposed to act as a photoreceptor for a variety of nonvisual light-responsive tasks. While mouse models have suggested an important role for cryptochrome in nonvisual photoreception, there are no biochemical data demonstrating the functional photoreceptive capability of cryptochrome in mice. There are two models that describe the effect of cryptochrome on light responsive events: (1) cryptochrome is a photoreceptor or (2) cryptochrome is required for either normal phototransduction from the retina to the brain or for normal transcriptional regulation in the brain, irrespective of light. To differentiate between these two models, we have examined the integrity of the regulatory mechanism of c-fos in cryptochromeless cell lines and in the suprachiasmatic nucleus (SCN) of cryptochromeless mice. Photoinduction of c-fos mRNA in the SCN can be used as a marker for circadian photoreception/phototransduction and it is drastically reduced in mice lacking cryptochromes. Our results indicate that light-independent transcription regulatory system of c-fos is normal in cryptochromeless mice and that the reduced c-fos light responsiveness in the absence of cryptochromes is due to a loss of photoreceptor function.

Light-induced c-fos in melanopsin retinal ganglion cells of young and aged rodless/coneless (rd/rd cl) mice

Non-rod, non-cone ocular photoreceptors have been shown to mediate a range of irradiance detection tasks. The strongest candidates for these receptors are melanopsin-positive retinal ganglion cells (RGCs). To provide a more complete understanding of these receptors in vivo, we have utilized a mouse that lacks rod and cone photoreceptors (rd/rd cl) and compared these animals to congenic wild-types. Using real-time polymerase chain reaction and immunohistochemistry, we address the following. (1) Is Fos expression within these RGCs driven by an input from the rods/cones or is it the product of the intrinsic photosensitivity of these neurons? We demonstrate that most Fos expression across the entire retina is due to the rods/cones, but in the absence of these photoreceptors, light will induce Fos within melanopsin RGCs. (2) Could the reported age-related decline in circadian photosensitivity of rodents be linked to changes in the population of melanopsin RGCs? We show that old mice experience an approximately 40% reduction in melanopsin RGCs. (3) Does the loss of inner retinal neurons affect the responses of melanopsin RGCs? Aged (approximately 700 days) rd/rd cl mice lose most of their inner retina but retain the retinal ganglion cell layer. In these mice, the proportion of melanopsin RGCs that express Fos in response to light is significantly reduced. Collectively, our data suggest that melanopsin RGCs form a heterogeneous population of neurons, and that most of the light-induced c-fos expression within these cells is associated with the endogenous photosensitivity of these neurons.

The chronosense – what light tells man about biological time

In the past 10 years, experimental studies have provided further evidence for the suggestion that the eye serves man as a dual sense organ, viz as a sense organ for sight but also for time and the regulation of biological rhythms. A small group of scientists interested in the adjustment of biological rhythms to the key Zeitgeber light wanted to answer the question whether rods and/or cones and/or other uncharacterized retinal photoreceptors contribute to this function in mammals. Intriguingly, in the course of elegant research, a number of laboratories around the world have been zeroing in on a novel non-rod, non-cone ocular photopigment which serves a number of responses to non-image-forming (NIF) photoreception in mammals. This paper intends to draw attention to possible implications of photoreception and phototransduction research for other scientific disciplines which study health and diesase effects in man. We therefore review the pivotal role of the photoreceptors -- old and new -- for the light-related timing and coordination of the interplay of otherwise less efficient biological rhythms. To distinguish our focus on time- and timing-related effects from classic image-forming (IF) and other NIF responses to ambient light, we refer informatively to chronoreceptors which mediate the sense of time, or chronosense. We conclude that syndisciplinary research into the physiology and pathophysiological implications of the chronosense is warranted and summarize a series of research questions.

Crossed and uncrossed retinal projections to the hamster circadian system

The hamster suprachiasmatic nucleus (SCN), site of the circadian clock, has been thought to be equally and completely innervated by each retina. This issue was studied in animals that had received an injection of the tracer cholera toxin subunit B (CTb) conjugated to Alexa 488 into the vitreous of one eye, with CTb-Alexa 594 injected into the other. Retinal projections to the SCN and other nuclei of the circadian system were simultaneously evaluated by using confocal laser microscopy. Each retina provides completely overlapping terminal fields throughout each SCN. Although SCN innervation by the contralateral retina is slightly denser than that from the ipsilateral retina, there are distinct SCN regions where input from one side is predominant, but not exclusive. A dense terminal field from the contralateral retina encompasses, and extends dorsally beyond, the central SCN subnucleus identified by calbindin-immunoreactive neurons. Surrounding the dense terminal field, innervation is largely derived from the ipsilateral retina. The densest terminal field in the intergeniculate leaflet is from the contralateral retina, which completely overlaps the ipsilateral projection. Most nuclei of the pretectum receive innervation largely, but not solely, from the contralateral retina, although the olivary pretectal nucleus has very dense patches of innervation derived exclusively from one retina or the other. Retina-dependent variation in terminal field density within the three closely examined nuclei may indicate areas of specialized function not previously appreciated. This issue is discussed in the context of the melanopsin-containing retinal ganglion cell projections to several nuclei in the circadian visual system.

Light-induced phase shift in the Syrian hamster (Mesocricetus auratus) is attenuated by the PACAP receptor antagonist PACAP6-38 or PACAP immunoneutralization

Circadian rhythms generated by the suprachiasmatic nucleus (SCN) are daily adjusted (entrained) by light via the retinohypothalamic tract (RHT). The RHT contains two neurotransmitters, glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP), which are believed to mediate the phase-shifting effects of light on the clock. In the present study we have elucidated the role of PACAP in light-induced phase shifting at early night in hamsters and shown that (i) light-induced phase delay of running-wheel activity was significantly attenuated by a specific PAC1 receptor antagonist (PACAP6-38) or by immunoblockade with a specific anti-PACAP antibody injected intracerebroventricularly before light stimulation; (ii) PACAP administered close to the SCN was able to phase-delay the circadian rhythm of running-wheel activity in a similar way to light; (iii) PACAP was present in the hamster RHT, colocalized with melanopsin, a recently identified opsin which has been suggested to be a circadian photopigment. The findings indicate that PACAP is a neurotransmitter of the RHT mediating photic information to the clock, possibly via melanopsin located exclusively on the PACAP-expressing cells of the RHT.

Retinal ganglion cell projections to the hamster suprachiasmatic nucleus, intergeniculate leaflet, and visual midbrain: bifurcation and melanopsin immunoreactivity

The circadian clock in the suprachiasmatic nucleus (SCN) receives direct retinal input via the retinohypothalamic tract (RHT), and the retinal ganglion cells contributing to this projection may be specialized with respect to direct regulation of the circadian clock. However, some ganglion cells forming the RHT bifurcate, sending axon collaterals to the intergeniculate leaflet (IGL) through which light has secondary access to the circadian clock. The present studies provide a more extensive examination of ganglion cell bifurcation and evaluate whether ganglion cells projecting to several subcortical visual nuclei contain melanopsin, a putative ganglion cell photopigment. The results showed that retinal ganglion cells projecting to the SCN send collaterals to the IGL, olivary pretectal nucleus, and superior colliculus, among other places. Melanopsin-immunoreactive (IR) ganglion cells are present in the hamster retina, and some of these cells project to the SCN, IGL, olivary pretectal nucleus, or superior colliculus. Triple-label analysis showed that melanopsin-IR cells bifurcate and project bilaterally to each SCN, but not to the other visual nuclei evaluated. The melanopsin-IR cells have photoreceptive characteristics optimal for circadian rhythm regulation. However, the presence of moderately widespread bifurcation among ganglion cells projecting to the SCN, and projection by melanopsin-IR cells to locations distinct from the SCN and without known rhythm function, suggest that this ganglion cell type is generalized, rather than specialized, with respect to the conveyance of photic information to the brain.

Seeing More Clearly: Recent Advances in Understanding Retinal Circuitry

Among 10 breakthroughs that Science announced at the end of 2002 was the discovery of a photosensing (melanopsin-containing) retinal ganglion cell (RGC) and its role in entraining the circadian clock. This breakthrough exemplifies the ultimate goal of neuroscience: to understand the nervous system from molecules to behavior. Light-sensing RGCs constitute one of a dozen discrete RGC populations coding various aspects of visual scenes by virtue of their unique morphology, physiology, and coverage of the retina. Interestingly, the function of the melanopsin-containing RGCs in entraining the circadian clock need not involve much retinal processing, making it the simplest form of processing in the retina. This review focuses on recent advances in our understanding of retinal circuitry, visual processing, and retinal development demonstrated by innovative experimental techniques. It also discusses the advantages of using the retina as a model system to address some of the key questions in neuroscience.

Light, timing of biological rhythms, and chronodisruption in man

This paper reviews abundant evidence suggesting that causes and course of aging and cancers can be considered as being both light- and rhythm-related. We define chronodisruption as a relevant disturbance of orderly biological rhythms over days and seasons and years in man. Light is the primary external mediator and melatonin a primary internal intermediary of such disturbances, which can result in earlier deaths via premature aging and cancers. We conclude that experimental and epidemiological research can provide further insights into common denominators of these chronic processes and may offer novel and uniform targets for prevention.

A broad role for melanopsin in nonvisual photoreception

The rod and cone photoreceptors that mediate visual phototransduction in mammals are not required for light-induced circadian entrainment, negative masking of locomotor activity, suppression of pineal melatonin, or the pupillary light reflex. The photopigment melanopsin has recently been identified in intrinsically photosensitive retinal ganglion cells (RGCs) that project to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus, suggesting that melanopsin might influence a variety of irradiance-driven responses. We have found novel projections from RGCs that express melanopsin mRNA to the ventral subparaventricular zone (vSPZ), a region involved in circadian regulation and negative masking, and the sleep-active ventrolateral preoptic nucleus (VLPO) and determined the subsets of melanopsin-expressing RGCs that project to the SCN, the pretectal area (PTA), and the IGL division of the lateral geniculate nucleus (LGN). Melanopsin was expressed in the majority of RGCs that project to the SCN, vSPZ, and VLPO and in a subpopulation of RGCs that innervate the PTA and the IGL but not in RGCs projecting to the dorsal LGN or superior colliculus. Two-thirds of RGCs containing melanopsin transcript projected to each of the SCN and contralateral PTA, and one-fifth projected to the ipsilateral IGL. Double-retrograde tracing from the SCN and PTA demonstrated a subpopulation of RGCs projecting to both sites, most of which contained melanopsin mRNA. Our results suggest that melanopsin expression defines a subset of RGCs that play a broad role in the regulation of nonvisual photoreception, providing collateralized projections that contribute to circadian entrainment, negative masking, the regulation of sleep-wake states, and the pupillary light reflex.

Circadian biology: the physiology of inner retinal photoreceptors

Non-traditional photoreceptors detect overall irradiance in the vertebrate retina. Such cells in the mouse inner retina show increased intracellular Ca(2+) levels following illumination. Neurons in the outer retina of fish also display characteristics appropriate for an irradiance detector.

Making (a) sense of non-visual ocular photoreception

A subset of intrinsically photosensitive retinal ganglion cells transduce information about ambient lighting conditions to areas of the brain involved in tasks including entrainment of the circadian clock, pupillary light reflexes and melatonin synthesis. The phototransduction system(s) utilized by these cells are unknown. Melanopsin and cryptochromes have been proposed as candidate photopigments for this system. Recent analyses of retinal degenerate mice lacking melanopsin or cryptochromes indicates that outer and inner photoreceptors can both contribute to non-visual photoresponses, and that both melanopsin and cryptochromes play important roles in this process.

Calcium imaging reveals a network of intrinsically light-sensitive inner-retinal neurons

BACKGROUND

Mice lacking rod and cone photoreceptors (rd/rd cl) are still able to regulate a range of responses to light, including circadian photoentrainment, the pupillary light reflex, and suppression of pineal melatonin by light. These data are consistent with the presence of a novel inner-retinal photoreceptor mediating non-image-forming irradiance detection.

RESULTS

We have examined the nature and extent of intrinsic light sensitivity in rd/rd cl retinae by monitoring the effect of light stimulation (470 nm) on intracellular Ca(2+) via FURA-2 imaging. Using this approach, which does not rely on pharmacological or surgical isolation of ganglion cells from the rod and cone photoreceptors, we identified a population of light-sensitive neurons in the ganglion cell layer (GCL). Retinal illumination induced an increase of intracellular Ca(2+) in approximately 2.7% of the neurons. The light-evoked Ca(2+) fluxes were dependent on the intensity and duration of the light stimulus. The light-responsive units formed an extensive network that could be uncoupled by application of the gap junction blocker carbenoxolone. Three types of light-evoked Ca(2+) influx were observed: sustained, transient, and repetitive, which are suggestive of distinct functional classes of GCL photoreceptors.

CONCLUSIONS

Collectively, our data reveal a heterogeneous syncytium of intrinsically photosensitive neurons in the GCL coupled to a secondary population of light-driven cells, in the absence of rod and cone inputs.

VA opsin, melanopsin, and an inherent light response within retinal interneurons

BACKGROUND

Although photoreception is best understood in rods and cones, it is increasingly clear that these are not the only photoreceptive cells of the vertebrate retina. While considerable attention has been paid to the role of melanopsin in the generation of intrinsic light sensitivity in the retinal ganglion cells of mammals, nothing is known about the photoreceptive capacity of the horizontal cells of the fish retina in which both VA opsin and melanopsin are expressed. As yet, there has been little more than speculation as to the physiological function of these opsins within local retinal circuit neurons.

RESULTS

VA opsin and melanopsin have been isolated and localized within the well-characterized cyprinid retina of the roach (Rutilus rutilus). Parallel electrophysiological studies identified a novel subtype of horizontal cell (HC-RSD) characterized by a depolarizing response that fits an opsin photopigment with a lambda(max) of 477 nm. The HC-RSD cells mediate responses to light that are characterized by long integration times, well beyond those observed for rods and cones. Significantly, HC-RSD responses persist when the conventional photoreceptor inputs are saturated by background light.

CONCLUSIONS

The syncytium of coupled horizontal cells has long been considered to provide a signal of overall retinal irradiance. Our data suggest that this light information is, at least in part, derived from a population of intrinsically photosensitive VA opsin and/or melanopsin horizontal cells.

Melanopsin is required for non-image-forming photic responses in blind mice

Although mice lacking rod and cone photoreceptors are blind, they retain many eye-mediated responses to light, possibly through photosensitive retinal ganglion cells. These cells express melanopsin, a photopigment that confers this photosensitivity. Mice lacking melanopsin still retain nonvisual photoreception, suggesting that rods and cones could operate in this capacity. We observed that mice with both outer-retinal degeneration and a deficiency in melanopsin exhibited complete loss of photoentrainment of the circadian oscillator, pupillary light responses, photic suppression of arylalkylamine-N-acetyltransferase transcript, and acute suppression of locomotor activity by light. This indicates the importance of both nonvisual and classical visual photoreceptor systems for nonvisual photic responses in mammals.

Melanopsin and rod-cone photoreceptive systems account for all major accessory visual functions in mice

In the mammalian retina, besides the conventional rod-cone system, a melanopsin-associated photoreceptive system exists that conveys photic information for accessory visual functions such as pupillary light reflex and circadian photo-entrainment. On ablation of the melanopsin gene, retinal ganglion cells that normally express melanopsin are no longer intrinsically photosensitive. Furthermore, pupil reflex, light-induced phase delays of the circadian clock and period lengthening of the circadian rhythm in constant light are all partially impaired. Here, we investigated whether additional photoreceptive systems participate in these responses. Using mice lacking rods and cones, we measured the action spectrum for phase-shifting the circadian rhythm of locomotor behaviour. This spectrum matches that for the pupillary light reflex in mice of the same genotype, and that for the intrinsic photosensitivity of the melanopsin-expressing retinal ganglion cells. We have also generated mice lacking melanopsin coupled with disabled rod and cone phototransduction mechanisms. These animals have an intact retina but fail to show any significant pupil reflex, to entrain to light/dark cycles, and to show any masking response to light. Thus, the rod-cone and melanopsin systems together seem to provide all of the photic input for these accessory visual functions.