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PubMed2006

Enhanced circadian photoresponsiveness after prolonged dark adaptation in seven species of diurnal and nocturnal rodents.

Previous studies in mice and Syrian hamsters have described an enhancement of circadian photoresponsiveness after exposure to darkness for several weeks. The present study investigated the generality of the phenomenon in 3 diurnal and 4 nocturnal rodent species. In four of the species tested, phase delays of the running-wheel activity rhythm evoked by 1-h light pulses were several-fold larger after 3 to 4 weeks of exposure to darkness than after a single day. This drastic change in photoresponsiveness has important implications for the understanding of the process of photic entrainment. Differences between species that showed a significant effect of dark adaptation and species that showed no effect were not accounted for by temporal niche (diurnal versus nocturnal) or photic sensitivity (albino versus pigmented). Further research is needed to elucidate the mechanisms responsible for inter-species differences in the occurrence of enhanced photoresponsiveness after dark adaptation and to identify the neural substrates of this phenomenon in species that exhibit it.

Analysis of Variance↗
PubMed2005

Analysis of PDE6 function using chimeric PDE5/6 catalytic domains.

cGMP-phosphodiesterases of the PDE6 family are expressed in retinal photoreceptor cells, where they mediate the phototransduction cascade. A system for expression of PDE6 in vitro is lacking, thus straining progress in understanding the structure-function relationships of the photoreceptor enzyme. Here, we report generation and characterization of bacterially expressed chimeric PDE5/6 catalytic domains which are highly soluble, catalytically active, and sensitive to inhibition by the PDE6 Pgamma subunit. Two flexible PDE6 loops, H and M, impart chimeric PDE5/6 catalytic domains with PDE6-like properties. The replacement of the PDE6 H-loop into the PDE5 catalytic domain increases the catalytic rate and the K(m) value for cGMP hydrolysis, whereas the substitution of the M-loop produces catalytic PDE domains responsive to Pgamma. Multiple PDE6 segments preventing functional expression of the catalytic domain are identified, supporting the requirement for specialized photoreceptor chaperones to assist PDE6 folding in vivo.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed2006

Rhodopsin-transducin coupling: role of the Galpha C-terminus in nucleotide exchange catalysis.

In the early steps of visual signal transduction, light-activated rhodopsin (R*) catalyzes GDP/GTP exchange in the heterotrimeric G protein (Galphabetagamma) transducin. We recently reported that the catalytic interaction involves two sequential steps. An initial docking between R* and Gbetagamma leads to conformational changes which make the C-terminus of Galpha (CTalpha) available for binding to R*. Binding of CTalpha by R* then triggers GDP/GTP exchange in the Galpha subunit. To further study this two-step mechanism, we investigated different single amino acid substitutions within CTalpha and discuss the effects of high affinity mutations on nucleotide exchange catalysis.

Amino Acid Substitution↗
PubMed1999

Proton circulation during the photocycle of sensory rhodopsin II.

Sensory rhodopsin II (SRII) in Halobacterium salinarum membranes is a phototaxis receptor that signals through its bound transducer HtrII for avoidance of blue-green light. In the present study we investigated the proton movements during the photocycle of SRII in the HtrII-free and HtrII-complexed form. We monitored sustained light-induced pH changes with a pH electrode, and laser flash-induced pH changes with the pH indicator pyranine using sealed membrane vesicles and open sheets containing the free or the complexed receptor. The results demonstrated that SRII takes up a proton in M-to-O conversion and releases it during O-decay. The uptake and release are from and to the extracellular side, and therefore SRII does not transport the proton across the membrane. The pH dependence of the SRII photocycle indicated the presence of a protonatable group (pK(a) approximately 7.5) in the extracellular proton-conducting path, which plays a role in proton uptake by the Schiff base in the M-to-O conversion. The extracellular proton circulation produced by SRII was not blocked by HtrII complexation, unlike the cytoplasmic proton conduction in SRI that was found in the same series of measurements to be blocked by its transducer, HtrI. The implications of this finding for current models of SRI and SRII signaling are discussed.

Absorption↗
PubMed2002

A mouse-like retinal cone phenotype in the Syrian hamster: S opsin coexpressed with M opsin in a common cone photoreceptor.

Previous immunocytochemical, physiological, and molecular studies have reported that the Syrian hamster lacks a shortwave-sensitive (S) cone photopigment but retains circadian responses to ultraviolet (UV) light. Using opsin antibodies and a sensitive detection protocol, we here show that S opsin immunoreactivity colocalizes with M opsin immunoreactivity in a common type of cone photoreceptor. S opsin signal within individual cone outer segments is low and continuously decreases from the ventral to dorsal retina. Only double-labeled cones were found. During development, S opsin expression precedes that of M opsin, but there is no indication of transdifferentiation. Our results imply that in the Syrian hamster low levels of S opsin colocalize with M opsin in a common cone phenotype. We suggest that, similar to other murid rodents, the S pigment absorbs maximally in the UV range, and thus may contribute to mediating the circadian response of the Syrian hamster to UV light.

Animals↗
PubMedDate not supplied

The eye as metronome of the body.

Vision is much more than just resolving small objects. In fact, the eye sends visual information to the brain that is not consciously perceived. One such pathway entails visual information to the hypothalamus. The retinohypothalamic tract (RHT) mediates light entrainment of circadian rhythms. Retinofugal fibers project to several nuclei of the hypothalamus. These and further projections to the pineal via the sympathetic system provide the anatomical substrate for the neuro-endocrine control of diurnal and longer rhythms. Without the influence of light and dark, many rhythms desynchronize and exhibit free-running periods of approximately 24.2-24.9 hours in humans. This review will demonstrate the mechanism by which the RHT synchronizes circadian rhythms and the importance of preserving light perception in those persons with impending visual loss.

Animals↗
PubMed1999

Development of a circadian melatonin rhythm in embryonic zebrafish.

We investigated the time course of circadian system development in zebrafish and the role of environmental light cycles in this process, using a rhythm in melatonin content of embryos and larvae as a marker of circadian function. When zebrafish were raised in a cycle of 14 h light and 10 h dark at 28.5 degrees C, nocturnal increases in melatonin content were detectable beginning on the second night post-fertilization (PF). When embryos were transferred to constant darkness (DD) at the end of the second light period, a circadian rhythm of melatonin content persisted for at least three cycles. However, when embryos were transferred from light to DD at 14 h PF, no rhythm was detectable in the population. Phase-locked circadian melatonin rhythms were measurable after embryos were exposed to a transition from constant light (LL) to darkness at 26 or 32 h PF, but not at 20 h. These data indicate that a circadian oscillator that regulates melatonin synthesis becomes functional and responsive by light between 20 and 26 h PF. At this stage, pineal photoreceptors have begun to differentiate, but retinal photoreceptors have not, suggesting that the first circadian melatonin rhythms are of pineal origin. The absence of melatonin rhythms in populations of embryos exposed to DD beginning at earlier stages indicates that there is no timed developmental event that sets the circadian clock in the absence of environmental input. Exposure to DD starting at 14 or 20 h PF did not retard overall development as determined by gross morphological staging criteria, and did not prevent later synchronization of melatonin rhythms by light-dark (LD) cycles.

Animals↗
PubMed2003

Strange vision: ganglion cells as circadian photoreceptors.

A novel photoreceptor of the mammalian retina has recently been discovered and characterized. The novel cells differ radically from the classical rod and cone photoreceptors. They use a unique photopigment, most probably melanopsin. They have lower sensitivity and spatiotemporal resolution than rods or cones and they seem specialized to encode ambient light intensity. Most surprisingly, they are ganglion cells and, thus, communicate directly with the brain. These intrinsically photosensitive retinal ganglion cells (ipRGCs) help to synchronize circadian rhythms with the solar day. They also contribute to the pupillary light reflex and other behavioral and physiological responses to environmental illumination.

Animals↗
PubMed2002

Zebrafish melanopsin: isolation, tissue localisation and phylogenetic position.

Photoreception is best understood in retinal rods and cones, but it is not confined to these cells. In non-mammals, intrinsically photosensitive cells have been identified within several structures including the pineal, hypothalamus and skin. More recently novel light sensitive cells have been identified in the inner/basal retina of both teleosts and rodents. Melanopsin has been proposed as the photopigment mediating many of these non-rod, non-cone responses to light. However, much about the melanopsin gene family remains to be clarified including their potential role as photopigments, and taxonomic distribution. We have isolated the first orthologue of melanopsin from a teleost fish and show expression of this gene in a sub-set of retinal horizontal cells (type B). Zebrafish melanopsin, and orthologues of this gene, differ markedly from the vertebrate photopigment opsins. The putative counterion is not a glutamate but a tyrosine, the putative G-protein binding domain in the third cytoplasmic loop is not conserved, and they show low levels of amino acid identity (approximately 27%) to both the known photopigment opsins and to other members of the melanopsin family. Mouse melanopsin is only 58% identical to Xenopus, and 68% identical to zebrafish. By contrast, the photosensory opsin families show approximately 75% conservation. On the basis of their structure, genomic organisation, discrete evolutionary lineage, and their co-expression with other opsins, the melanopins are not obvious photosensory opsins. They might represent a separate branch of photopigment evolution in the vertebrates or they may have a non-direct photosensory function, perhaps as a photoisomerase, in non-rod, non-cone light detection.

Animals↗
PubMed1999

Circadian regulation of iodopsin and clock is altered in the retinal degeneration chicken retina.

We are interested in determining if the visual phototransduction cascade plays a role in light entrainment of photoreceptor circadian oscillators. In this study, we compared mRNA levels of iodopsin and the chicken homolog of Clock (cClock) in the retinas of normal and rd (retinal degeneration) chickens that lack functional rod and cone phototransduction cascades. Iodopsin is a circadian-regulated, photoreceptor-specific gene expressed in chicken retina, and Clock is a transcription factor that has been shown to play a role in the circadian clock mechanism in mouse and Drosophila. The results of our analyses show that cClock and iodopsin transcript levels undergo daily oscillations in retinas of normal animals housed under 12 h light:12 h dark (12L:12D) conditions, and that these oscillations are maintained in the absence of light. Levels of these transcripts in the retinas of rd/rd chickens housed under cyclic light conditions did not change significantly over the course of a 12L:12D cycle; however, there was evidence that the photoreceptor oscillators were entrained in these animals. Comparisons of our normal and rd/rd data suggest that there are at least two light entrainment pathways that impinge on the oscillators found in photoreceptor cells, one of which is effectively disabled by the GC1 null mutation carried by the rd chicken.

Amino Acid Sequence↗
PubMed1999

Cellular circadian clocks in the pineal.

Daily rhythms are a fundamental feature of all living organisms; most are synchronized by the 24 hr light/dark (LD) cycle. In most species, these rhythms are generated by a circadian system, and free run under constant conditions with a period close to 24 hr. To function properly the system needs a pacemaker or clock, an entrainment pathway to the clock, and one or more output signals. In vertebrates, the pineal hormone melatonin is one of these signals which functions as an internal time-keeping molecule. Its production is high at night and low during day. Evidence indicates that each melatonin producing cell of the pineal constitutes a circadian system per se in non-mammalian vertebrates. In addition to the melatonin generating system, they contain the clock as well as the photoreceptive unit. This is despite the fact that these cells have been profoundly modified from fish to birds. Modifications include a regression of the photoreceptive capacities, and of the ability to transmit a nervous message to the brain. The ultimate stage of this evolutionary process leads to the definitive loss of both the direct photosensitivity and the clock, as observed in the pineal of mammals. This review focuses on the functional properties of the cellular circadian clocks of non-mammalian vertebrates. How functions the clock? How is the photoreceptive unit linked to it and how is the clock linked to its output signal? These questions are addressed in light of past and recent data obtained in vertebrates, as well as invertebrates and unicellulars.

Animals↗
PubMed2001

Human melatonin suppression by light: a case for scotopic efficiency.

Human adult males were exposed to combinations of two illuminances and two spectral power distributions over the course of four nighttime sessions. A dose-dependent response of acute melatonin suppression to light was found, but photopic (cone-based) illuminance did not adequately predict suppression. When melatonin suppression was plotted against scotopic (rod-based) illuminance, the data formed a nearly monotonic function, implicating rods, or a rod-dominated mechanism, in the human melatonin regulation system. The results do not, however, rule out mechanisms other than rods, including novel photoreceptors, as candidates for melatonin regulation in humans.

Adult↗
PubMed2001

The circadian clock of fruit flies is blind after elimination of all known photoreceptors.

Circadian rhythms are entrained by light to follow the daily solar cycle. We show that Drosophila uses at least three light input pathways for this entrainment: (1) cryptochrome, acting in the pacemaker cells themselves, (2) the compound eyes, and (3) extraocular photoreception, possibly involving an internal structure known as the Hofbauer-Buchner eyelet, which is located underneath the compound eye and projects to the pacemaker center in the brain. Although influencing the circadian system in different ways, each input pathway appears capable of entraining circadian rhythms at the molecular and behavioral level. This entrainment is completely abolished in glass(60j) cry(b) double mutants, which lack all known external and internal eye structures in addition to being devoid of cryptochrome.

Animals↗
PubMed1999

Light receptor kinases in plants!

Plants must adapt to a capricious light environment, but the mechanism by which light signals are transmitted to cause changes in development has long eluded us. The search might be over, however, as two photoreceptors, phytochrome and NPH1, have been shown to autophosphorylate in a light-dependent fashion.

Arabidopsis Proteins↗
PubMedDate not supplied

Residual photosensitivity in mice lacking both rod opsin and cone photoreceptor cyclic nucleotide gated channel 3 alpha 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 in Rho(-/-) Cnga3(-/-) mice. Moreover, these animals retain a pupil light reflex, photoentrainment, and light induction of the immediate early gene c-fos in 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 in Rho(-/-) Cnga3(-/-) mice. Thus, it seems that non-rod non-cone photoreceptors can drive many, but not all, non-image-forming light responses.

Animals↗