Keeping an eye on retinal clocks

Salt concentration and solar orientation in two supralittoral sandhoppers: Talitrus saltator (Montagu) and Talorchestia ugolinii Bellan Santini and Ruffo

The influence of salt concentration in the seawater on solar orientation in Talitrus saltator and Talorchestia ugolinii was studied in a confined environment (transparent plexiglass bowls). Sodium and calcium concentrations strongly affect both sea-land orientation and the sun compass mechanism in T.saltator, whereas the behaviour of T. ugolinii is less influenced. The absence of Na(+) does not influence the sun compass mechanism, but causes an inversion in the mean direction of orientation in T. saltator. In T. ugolinii, there was no influence on the compass mechanism for solar orientation and no inversion in the directional choice. In the absence of Ca(2+), a photonegative tendency was observed for T saltator together with marked reduction in the capacity to go in any direction. However, the effect of Ca(2+) absence on the orientation capacity of T. saltator is reversible and the orientation capacity can be reduced in a few minutes. The different behaviour of the two species of sandhoppers is discussed.

Electroencephalographic signals synchronize with behaviors and are sexually dimorphic during the light-dark cycle in reproductive frogs

Male frogs behave differently from females during the breeding season, particularly with respect to courtship displays and in response to mating signals. In search of physiological correlates of these differences, the present study measured changes in baseline electroencephalogram (EEG) power output within four frequency bands in the telencephalon and mesencephalon, together with changes in locomotor activity as a function of the light-dark cycle in male and female Emei music frogs (Babina daunchina) at the reproductive stage. Previous studies have shown that male vocal activity varies both seasonally and daily in this species and that females use male advertisement calls to locate and select mates. The present results show that both EEG and locomotor activity exhibit highly correlated circadian patterns with peaks around light onset and offset. Importantly, during the reproductive stage, statistically significant sex differences in EEG output across brain regions during the light and dark phases were found indicating that sexual dimorphism exists for EEG activity which may underlie sexually specific information processing and behavioral activities.

Ocular diurnal rhythms and eye growth regulation: where we are 50 years after Lauber

Many ocular processes show diurnal oscillations that optimize retinal function under the different conditions of ambient illumination encountered over the course of the 24 h light/dark cycle. Abolishing the diurnal cues by the use of constant darkness or constant light results in excessive ocular elongation, corneal flattening, and attendant refractive errors. A prevailing hypothesis is that the absence of the Zeitgeber of light and dark alters ocular circadian rhythms in some manner, and results in an inability of the eye to regulate its growth in order to achieve emmetropia, the matching of the front optics to eye length. Another visual manipulation that results in the eye growth system going into a "default" mode of excessive growth is form deprivation, in which a translucent diffuser deprives the eye of visual transients (spatial or temporal) while not significantly reducing light levels; these eyes rapidly elongate and become myopic. It has been hypothesized that form deprivation might constitute a type of "constant condition" whereby the absence of visual transients drives the eye into a similar default mode as that in response to constant light or dark. Interest in the potential influence of light cycles and ambient lighting in human myopia development has been spurred by a recent study showing a positive association between the amount of time that children spent outdoors and a reduced prevalence of myopia. The growing eyes of chickens and monkeys show a diurnal rhythm in axial length: Eyes elongate more during the day than during the night. There is also a rhythm in choroidal thickness that is in approximate anti-phase to the rhythm in eye length. The phases are altered in eyes growing too fast, in response to form deprivation or negative lenses, or too slowly, in response to myopic defocus, suggesting an influence of phase on the emmetropization system. Other potential rhythmic influences include dopamine and melatonin, which form a reciprocal feedback loop, and signal "day" and "night" respectively. Retinal dopamine is reduced during the day in form deprived myopic eyes, and dopamine D2 agonists inhibit ocular growth in animal models. Rhythms in intraocular pressure as well, may influence eye growth, perhaps as a mechanical stimulus triggering changes in scleral extracellular matrix synthesis. Finally, evidence shows varying influences of environmental lighting parameters on the emmetropization system, such as high intensity light being protective against myopia in chickens. This review will cover the evidence for the possible influence of these various factors on ocular growth. The recognition that ocular rhythms may play a role in emmetropization is a first step toward understanding how they may be manipulated in treatment therapies to prevent myopia in humans.

Neurons and networks in daily rhythms

Biological pacemakers dictate our daily schedules in physiology and behaviour. The molecules, cells and networks that underlie these circadian rhythms can now be monitored using long-term cellular imaging and electrophysiological tools, and initial studies have already suggested a theme--circadian clocks may be crucial for widespread changes in brain activity and plasticity. These daily changes can modify the amount or activity of available genes, transcripts, proteins, ions and other biologically active molecules, ultimately determining cellular properties such as excitability and connectivity. Recently discovered circadian molecules and cells provide preliminary insights into a network that adapts to predictable daily and seasonal changes while remaining robust in the face of other perturbations.

The rhythms of life: circadian output pathways in Neurospora

Research in Neurospora crassa pioneered the isolation of clock-controlled genes (ccgs), and more than 180 ccgs have been identified that function in various aspects of the fungal life cycle. Many clock-controlled genes are associated with damage repair, stress responses, intermediary metabolism, protein synthesis, and development. The expression of most of these genes peaks just before dawn and appears to prepare the cells for the desiccation, mutagenesis, and stress caused by sunlight. Progress on characterization of the output signaling pathways from the circadian oscillator mechanism to the ccgs is discussed. The authors also review evidence suggesting that, similar to other clock model organisms, a connection exists between the redox state of the cell and the Neurospora clock. The authors speculate that the clock system may sense not only light but also the redox potential of the cell through one of the PAS domains of the core clock components WC-1 or WC-2.

[Structural variations of the eye linked to daily vertical migrations of Daphnia longispina]

The role of the eyes, and more precisely that of the ommatidian pigments, in the control of daily rhythms of movement of Daphnia longispina is investigated. In the laboratory, under permanent light (LL), the pigmentary modifications observed are globally similar to those observed in situ, except in their timing, around dusk, LL cycles do not coincide with the ascent of daphnids but precede it. This is the expression of an endogenous free-running rhythm. Therefore, in D. longispina, an internal oscillator controls DVMs, which are circadian and not nycthemeral, and, in situ, illumination at dawn suffices to synchronize migrations on dark/light alternation.

Circadian rhythm of iguana electroretinogram: the role of dopamine and melatonin

The amplitude of the b-wave of the electroretinogram (ERG) varies with a circadian rhythm in the green iguana; the amplitude is high during the day(or subjective day) and low during the night (or subjective night). Dopamine and melatonin contents in the eye are robustly rhythmic under constant conditions; dopamine levels are high during the subjective day, and melatonin levels are high during the subjective night. Dopamine and melatonin affect the amplitude of the b-wave in an antagonistic and phase-dependent manner: dopamine D2-receptor agonists injected intraocularly during the subjective night produce high-amplitude b-waves characteristic of the subjective day, whereas melatonin injected intraocularly during the subjective day reduces b-wave amplitude. Sectioning the optic nerve abolishes the circadian rhythms of b-wave amplitude and of dopamine content. The results of this study suggest that in iguana, a negative feedback loop involving dopamine and melatonin regulates the circadian rhythm of the ERG b-wave amplitude that is at least in part generated in the brain.

Circadian phototransduction and the regulation of biological rhythms

The vertebrate circadian system that controls most biological rhythms is composed of multiple oscillators with varied hierarchies and complex levels of organization and interaction. The retina plays a key role in the regulation of daily rhythms and light is the main synchronizer of the circadian system. To date, the identity of photoreceptors/photopigments responsible for the entrainment of biological rhythms is still uncertain; however, it is known that phototransduction must occur in the eye because light entrainment is lost with eye removal. The retina is also rhythmic in physiological and metabolic activities as well as in gene expression. Retinal oscillators may act like clocks to induce changes in the visual system according to the phase of the day by predicting environmental changes. These oscillatory and photoreceptive capacities are likely to converge all together on selected retinal cells. The aim of this overview is to present the current knowledge of retinal physiology in relation to the circadian timing system.

Temperature effect on entrainment, phase shifting, and amplitude of circadian clocks and its molecular bases

Effects of temperature and temperature changes on circadian clocks in cyanobacteria, unicellular algae, and plants, as well as fungi, arthropods, and vertebrates are reviewed. Periodic temperature with periods around 24 h even in the low range of 1-2 degrees C (strong Zeitgeber effect) can entrain all ectothermic (poikilothermic) organisms. This is also reflected by the phase shifts-recorded by phase response curves (PRCs)-that are elicited by step- or pulsewise changes in the temperature. The amount of phase shift (weak or strong type of PRC) depends on the amplitude of the temperature change and on its duration when applied as a pulse. Form and position of the PRC to temperature pulses are similar to those of the PRC to light pulses. A combined high/low temperature and light/dark cycle leads to a stabile phase and maximal amplitude of the circadian rhythm-when applied in phase (i.e., warm/light and cold/dark). When the two Zeitgeber cycles are phase-shifted against each other the phase of the circadian rhythm is determined by either Zeitgeber or by both, depending on the relative strength (amplitude) of both Zeitgeber signals and the sensitivity of the species/individual toward them. A phase jump of the circadian rhythm has been observed in several organisms at a certain phase relationship of the two Zeitgeber cycles. Ectothermic organisms show inter- and intraspecies plus seasonal variations in the temperature limits for the expression of the clock, either of the basic molecular mechanism, and/or the dependent variables. A step-down from higher temperatures or a step-up from lower temperatures to moderate temperatures often results in initiation of oscillations from phase positions that are about 180 degrees different. This may be explained by holding the clock at different phase positions (maximum or minimum of a clock component) or by significantly different levels of clock components at the higher or lower temperatures. Different permissive temperatures result in different circadian amplitudes, that usually show a species-specific optimum. In endothermic (homeothermic) organisms periodic temperature changes of about 24 h often cause entrainment, although with considerable individual differences, only if they are of rather high amplitudes (weak Zeitgeber effects). The same applies to the phase-shifting effects of temperature pulses. Isolated bird pineals and rat suprachiasmatic nuclei tissues on the other hand, respond to medium high temperature pulses and reveal PRCs similar to that of light signals. Therefore, one may speculate that the self-selected circadian rhythm of body temperature in reptiles or the endogenously controlled body temperature in homeotherms (some of which show temperature differences of more than 2 degrees C) may, in itself, serve as an internal entraining system. The so-called heterothermic mammals (undergoing low body temperature states in a daily or seasonal pattern) may be more sensitive to temperature changes. Effects of temperature elevation on the molecular clock mechanisms have been shown in Neurospora (induction of the frequency (FRQ) protein) and in Drosophila (degradation of the period (PER) and timeless (TIM) protein) and can explain observed phase shifts of rhythms in conidiation and locomotor activity, respectively. Temperature changes probably act directly on all processes of the clock mechanism some being more sensitive than the others. Temperature changes affect membrane properties, ion homeostasis, calcium influx, and other signal cascades (cAMP, cGMP, and the protein kinases A and C) (indirect effects) and may thus influence, in particular, protein phosphorylation processes of the clock mechanism. The temperature effects resemble to some degree those induced by light or by light-transducing neurons and their transmitters. In ectothermic vertebrates temperature changes significantly affect the melatonin rhythm, which in turn exerts entraining (phase shifting) functions.

Distinct signaling pathways from the circadian clock participate in regulation of rhythmic conidiospore development in Neurospora crassa

Several different environmental signals can induce asexual spore development (conidiation) and expression of developmentally regulated genes in Neurospora crassa. However, under constant conditions, where no environmental cues for conidiation are present, the endogenous circadian clock in N. crassa promotes daily rhythms in expression of known developmental genes and of conidiation. We anticipated that the same pathway of gene regulation would be followed during clock-controlled conidiation and environmental induction of conidiation and that the circadian clock would need only to control the initial developmental switch. Previous experiments showed that high-level developmental induction of the clock-controlled genes eas (ccg-2) and ccg-1 requires the developmental regulatory proteins FL and ACON-2, respectively, and normal developmental induction of fl mRNA expression requires ACON-2. We demonstrate that the circadian clock regulates rhythmic fl gene expression and that fl rhythmicity requires ACON-2. However, we find that clock regulation of eas (ccg-2) is normal in an fl mutant strain and ccg-1 expression is rhythmic in an acon-2 mutant strain. Together, these data point to the endogenous clock and the environment following separate pathways to regulate conidiation-specific gene expression.

Distribution of melatonin MT1 receptor immunoreactivity in human retina

Melatonin is synthesized in the pineal gland and retina during the night. Retinal melatonin is believed to be involved in local cellular modulation and in regulation of light-induced entrainment of circadian rhythms. The present study provides the first immunohistochemical evidence for the localization of melatonin 1a-receptor (MT1) in human retina of aged subjects. Ganglion, amacrine, and photoreceptor cells expressed MT1. In addition, MT1 immunoreactivity was localized to cell processes in the inner plexiform layer and to central vessels of the retina, as well as to retinal vessels but not to ciliary or choroidal vessels. These results support a variety of cellular and vascular effects of melatonin in the human retina. Preliminary evidence from patients with Alzheimer's disease (AD) revealed increased MT1 immunoreactivity in ganglion and amacrine cells, as well as in vessels. In AD cases photoreceptor cells were degenerated and showed low MT1 expression.

Circadian systems: different levels of complexity

After approximately 50 years of circadian research, especially in selected circadian model systems (Drosophila, Neurospora, Gonyaulax and, more recently, cyanobacteria and mammals), we appreciate the enormous complexity of the circadian programme in organisms and cells, as well as in physiological and molecular circuits. Many of our insights into this complexity stem from experimental reductionism that goes as far as testing the interaction of molecular clock components in heterologous systems or in vitro. The results of this enormous endeavour show circadian systems that involve several oscillators, multiple input pathways and feedback loops that contribute to specific circadian qualities but not necessarily to the generation of circadian rhythmicity. For a full appreciation of the circadian programme, the results from different levels of the system eventually have to be put into the context of the organism as a whole and its specific temporal environment. This review summarizes some of the complexities found at the level of organisms, cells and molecules, and highlights similar strategies that apparently solve similar problems at the different levels of the circadian system.

Sexual differences and effect of photoperiod on melatonin receptor in avian brain

Several data suggest that melatonin may influence avian reproduction by acting at the level of the hypothalamic-hypophisial-gonadal axis, and/or on neural circuits controlling reproductive behaviours. The action of melatonin is exerted through specific receptors whose distribution and pharmacological properties have been extensively investigated. This review will focus on the distribution, sexual dimorphism, and dependence upon the photoperiod of melatonin binding sites in avian species with a special emphasis on Japanese quail. Melatonin receptors are widely distributed in avian brain. They are mostly present in the visual pathways of all the investigated species and in the song controlling nuclei of oscine birds. Sexual dimorphism of melatonin binding sites (higher density in males than in females) was detected in some telencephalic nuclei of songbirds, in the visual pathways, and in the preoptic area of quail. The last region plays a key role in the activation of male quail copulatory behaviour and it hosts a large population of gonadotropin-releasing hormone-containing neurons. Sexual dimorphism of melatonin-binding sites in the above-mentioned regions suggests a differential role for this hormone in the modulation of visual perception, gonadotropin production, and seasonally activated behaviours in male and female quail. Further studies are necessary to understand interrelationships among photic cues, gonadal steroids, density, and sexually dimorphic distribution of melatonin receptors.

Biological timing and the clock metaphor: oscillatory and hourglass mechanisms

Living organisms have developed a multitude of timing mechanisms--"biological clocks." Their mechanisms are based on either oscillations (oscillatory clocks) or unidirectional processes (hourglass clocks). Oscillatory clocks comprise circatidal, circalunidian, circadian, circalunar, and circannual oscillations--which keep time with environmental periodicities--as well as ultradian oscillations, ovarian cycles, and oscillations in development and in the brain, which keep time with biological timescales. These clocks mainly determine time points at specific phases of their oscillations. Hourglass clocks are predominantly found in development and aging and also in the brain. They determine time intervals (duration). More complex timing systems combine oscillatory and hourglass mechanisms, such as the case for cell cycle, sleep initiation, or brain clocks, whereas others combine external and internal periodicities (photoperiodism, seasonal reproduction). A definition of a biological clock may be derived from its control of functions external to its own processes and its use in determining temporal order (sequences of events) or durations. Biological and chemical oscillators are characterized by positive and negative feedback (or feedforward) mechanisms. During evolution, living organisms made use of the many existing oscillations for signal transmission, movement, and pump mechanisms, as well as for clocks. Some clocks, such as the circadian clock, that time with environmental periodicities are usually compensated (stabilized) against temperature, whereas other clocks, such as the cell cycle, that keep time with an organismic timescale are not compensated. This difference may be related to the predominance of negative feedback in the first class of clocks and a predominance of positive feedback (autocatalytic amplification) in the second class. The present knowledge of a compensated clock (the circadian oscillator) and an uncompensated clock (the cell cycle), as well as relevant models, are briefly re viewed. Hourglass clocks are based on linear or exponential unidirectional processes that trigger events mainly in the course of development and aging. An important hourglass mechanism within the aging process is the limitation of cell division capacity by the length of telomeres. The mechanism of this clock is briefly reviewed. In all clock mechanisms, thresholds at which "dependent variables" are triggered play an important role.

The rat arylalkylamine N-acetyltransferase E-box: differential use in a master vs. a slave oscillator

The rat arylalkylamine N-acetyltransferase (AA-NAT) gene encodes the key enzyme whose rhythmic expression drives the nocturnal production of melatonin. It is of interest that this enzyme is expressed virtually exclusively in two phylogenetically related tissues: retinal photoreceptors, which harbor an endogenous clock, and pinealocytes which, in higher vertebrates, function strictly in response to the master oscillator in the suprachiasmatic nucleus (SCN). While much is known about AA-NAT transcriptional regulation in the rat pineal gland (a slave oscillator), a full understanding of the mechanisms controlling AA-NAT gene expression in the retina by the clock is lacking. In the present study we have identified a functional E box in the first intron of the rat AA-NAT gene which is capable of mediating transcriptional upregulation via the action of a bMAL/CLOCK heterodimer. This is the first report to characterize the AA-NAT gene as a possible direct transcriptional target of the biological clock loop in a master oscillator.

Symphony of rhythms in the Xenopus laevis retina

The photoreceptor layer in the retina of Xenopus laevis harbors a circadian clock. Many molecular components known to drive the molecular clock in other organisms have been identified in Xenopus, such as XClock, Xper2, and Xcrys, demonstrating phylogenetic conservation. This model system displays a wide array of rhythms, including melatonin release, ERG rhythms, and retinomotor movements, suggesting that the ocular clock is important for proper retinal function. A flow-through culture system allows measurements of retinal rhythms such as melatonin release in vitro over time from a single eyecup. This system is suited for pharmacological perturbations of the clock, and has led to important observations regarding the circadian control of melatonin release, the roles of light and dopamine as entraining agents, and the circadian mechanisms regulating retinomotor movements. The development of a transgenic technique in Xenopus allows precise and reliable molecular perturbations. Since it is possible to follow rhythms in eyecups obtained from adults or tadpoles, the combination of the flow-through culture system and the transgenic technique leads to the fast generation of transgenic tadpoles to monitor the effects of molecular perturbations on the clock.