Light and dark rations and the photic entrainment of circadian locomotor activity patterns in the South American Silver Catfish (Rhamdia quelen,Quoy & Gaimard, 1824)

The aim of this study was to evaluate the daily rhythm of locomotor activity in Rhamdia quelen (R. quelen). A total of 30 fish were enrolled in the study and were equally divided in 10 groups and maintained in 100 liters tanks. The locomotor activity was measured in fish maintained under the LD 12:12 photoperiod regime; thereafter, the LD cycle was reversed to DL in order to study the resynchronization and to explore the endogenous pacemaker. Subsequently, the fish were subjected to constant conditions of light to test whether or not locomotor rhythms are regulated by the endogenous circadian clock. The effect of increasing light length and intensity was studied on daily rhythm of locomotor activity of fish. Our results showed that the R. quelen is a strictly diurnal species, the rhythm of locomotory activity resynchronized quickly after inverting the LD cycle and persist under free course LL, suggesting a circadian origin. The light showed a significant masking effect often blocking the expression of the biological rhythm. The strictly diurnal behavior is controlled directly by the photoperiod and maintained even under very dim light (30 lux).     

Entrainment of eclosion and preliminary ontogeny of circadian clock gene expression in the flesh fly,Sarcophaga crassipalpis

Timing of circadian activities is controlled by rhythmic expression of clock genes in pacemaker neurons in the insect brain. Circadian behavior and clock gene expression can entrain to both thermoperiod and photoperiod but the availability of such cues, the organization of the brain, and the need for circadian behavior change dramatically during the course of insect metamorphosis. We asked whether photoperiod or thermoperiod entrains the clock during pupal and pharate adult stages by exposing flies to different combinations of thermoperiod and photoperiod and observing the effect on the timing of adult eclosion. This study used qRT-PCR to examine how entrainment and expression of circadian clock genes change during the course of development in the flesh fly, Sarcophaga crassipalpis. Thermoperiod entrains expression of period and controls the timing of adult eclosion, suggesting that the clock gene period may be upstream of the eclosion pathway. Rhythmic clock gene expression is evident in larvae, appears to cease during the early pharate adult stage, and resumes again by the time of adult eclosion. Our results indicate that both patterns of clock gene expression and the cues to which the clock entrains are dynamic and respond to different environmental signals at different developmental stages in S. crassipalpis.     

Roles of the circadian clock and endocrine regulator in the photoperiodic response of the brown‐winged green bug Plautia stali

The physiological mechanisms underlying photoperiodism in insects have been studied extensively, although the associated molecular machinery remains largely unknown. In the present study, we investigate the roles of the circadian clock gene cycle (cyc) and the endocrine regulator gene myoinhibitory peptide (Mip) in the photoperiodic response of the brown‐winged green bug Plautia stali Scott (Hemiptera, Pentatomidae). Typically, adult females of this species develop their ovaries under long‐day conditions, whereas they suppress its development under short‐day conditions. We find that RNA interference (RNAi) directed against cyc causes malfunction of the circadian clock governing the locomotor activity rhythm and yields abnormal activity profiles not only under constant darkness, but also under light/dark conditions. RNAi directed against cyc and Mip disrupts the photoperiodic response in ovarian development. cyc RNAi suppresses the ovarian development even under long‐day conditions, whereas Mip RNAi induces it even under short‐day conditions. We propose that the core circadian clock gene cyc regulates the photoperiodic response and that Mip is the causal regulator of juvenile hormone biosynthesis in the corpus allatum. Neither photoperiod, nor cyc RNAi affect Mip mRNA levels, and therefore it remains unknown how the photoperiodic information is processed and mediated by Mip.     

Effect of temperature on circadian rhythm controlling the crepuscular activity of the burying beetle Nicrophorus quadripunctatus Kraatz (Coleoptera: Silphidae)

Locomotor activity rhythm was examined at various temperatures under a 16 h light : 8 h dark photoperiod (LD 16:8) or LD 12:12 using adults of the burying beetle Nicrophorus quadripunctatus. At 20°C, the locomotor activity of the beetles showed a bimodal daily pattern with two peaks around lights on and lights off under both photoperiods. This bimodal activity rhythm persisted under constant darkness; therefore, the activity of adult N. quadripunctatus is controlled by a circadian clock. Adults showed a bimodal activity pattern for temperatures ranging from 15 to 25°C. The evening peak of the daily activity was earlier at lower temperatures. These findings suggest that in the field, N. quadripunctatus shows crepuscular activity, and is active earlier in the afternoon in cooler seasons. In this species, therefore, temperature appears to play an important role in the determination of daily activity patterns.     

Photic and Pineal Modulation of Food Anticipatory Circadian Activity Rhythms in Rodents

Restricted daily feeding schedules entrain circadian oscillators that generate food anticipatory activity (FAA) rhythms in nocturnal rodents. The location of food-entrainable oscillators (FEOs) necessary for FAA remains uncertain. The most common procedure for inducing circadian FAA is to limit food access to a few hours in the middle of the light period, when activity levels are normally low. Although light at night suppresses activity (negative masking) in nocturnal rodents, it does not prevent the expression of daytime FAA. Nonetheless, light could reduce the duration or magnitude of FAA. If so, then neural or genetic ablations designed to identify components of the food-entrainable circadian system could alter the expression of FAA by affecting behavioral responses to light. To assess the plausibility of light as a potential mediating variable in studies of FAA mechanisms, we quantified FAA in rats and mice alternately maintained in a standard full photoperiod (12h of light/day) and in a skeleton photoperiod (two 60 min light pulses simulating dawn and dusk). In both species, FAA was significantly and reversibly enhanced in the skeleton photoperiod compared to the full photoperiod. In a third experiment, FAA was found to be significantly attenuated in rats by pinealectomy, a procedure that has been reported to enhance some effects of light on behavioral circadian rhythms. These results indicate that procedures affecting behavioral responses to light can significantly alter the magnitude of food anticipatory rhythms in rodents.     

Entrainment and phase shifting of the circadian rhythm of cell division by light in cultures of the achlorophyllous ZC mutant ofEuglena gracilis

The photosynthesis-deficient ZC mutant ofEuglena gracilis Klebs (strain Z) was cultured at 16°C on an aerated, magnetically stirred, mineral medium containing 0.1% ethanol (pH 7.0). Cell division could be entrained by a 12: 12 light: dark cycle (LD: 12, 12) or even by a one-pulse skeleton photoperiod (LD: 1,23) The rhythm free-ran in DD for at least 8 days with a circadian period (=25.5 h) in populations that had been previously entrained by LD. The freerunning rhythm could be phase-shifted by a single 1-h light pulse (3000 lx). The strong (Type 0) phase-response curve derived from the resetting effects of such signals given at different circadian times was similar to that for the photosynthetic wild-type strain. These results demonstrate that the presence of a functional chloroplast compartment is not necessary for the circadian clock to function inEuglena and suggest that phase resetting of the circadian clock by light occurs via a similar pathway in both photosynthetic and non-photosynthetic cell types.     

Synergy between the light-induced acute response and the circadian cycle: a new mechanism for the synchronization of the Phaseolus vulgaris clock to light

PvLHY and Lhcb expression has been studied in primary bean leaves after exposure of etiolated leaves to two or three white light-pulses and under different photoperiods. Under the tested photoperiods, the steady-state mRNA levels exhibit diurnal oscillations with zenith in the morning between ZT21 and 4 for PvLHY and between ZT4 and 6 for Lhcb. Nadir is in the evening between ZT12 and 18 for PvLHY and ZT18 and 24 for Lhcb. Light-pulses to etiolated seedlings induce a differentiated acute response that is reciprocally correlated with the amplitude of the following circadian cycle. In addition, the clock modulates the duration of the acute response (descending part of the curve included), which according to the phase of the rhythm at light application extends from 7 to 18 h. This constitutes the response dynamics of the Phaseolus clock to light. Similarly, the waveform of PvLHY and Lhcb expression during the day of different photoperiods resembles in induction capability (accomplishment of peak after lights-on) and duration (from lights-on phase to trough) the phase-dependent progression of acute response in etiolated seedlings. Consequently, the peak of Lhcb (all tested photoperiods) and PvLHY (in LD 18:6) attained in the photophase corresponds to the acute response peak, while the peak of PvLHY during the scotophase (in LD 12:12 and 6:18) corresponds to the circadian peak. Thus, the effect of the response dynamics in the photoperiod determines the coincidence of the peak with the photo- or scotophase, respectively. This represents a new model mechanism for the adaptation of the Phaseolus clock to light.     

A Stochastic Burst Follows the Periodic Morning Peak in Individual Drosophila Locomotion

Coupling between cyclically varying external light and an endogenous biochemical oscillator known as the circadian clock, modulates a rhythmic pattern with two prominent peaks in the locomotion of Drosophila melanogaster. A morning peak appears around the time lights turn on and an evening peak appears just before lights turn off. The close association between the peaks and the external 12:12 hour light/dark photoperiod means that respective morning and evening peaks of individual flies are well-synchronized in time and, consequently, feature prominently in population-averaged data. Here, we report on a brief but strong stochastic burst in fly activity that, in contrast to morning and evening peaks, is detectable only in single fly recordings. This burst was observed across 3 wild-type strains of Drosophila melanogaster. In a single fly recording, the burst is likely to appear once randomly within 0.5–5 hours after lights turn on, last for only 2–3 minutes and yet show 5 times greater activity compared to the maximum of morning peak with data binned in 3 minutes. Owing to its variable timing and short duration, the burst is virtually undetectable in population-averaged data. We use a locally-built illumination system to study the burst and find that its incidence in a population correlates with light intensity, with ~85% of control flies showing the behavior at 8000 lux (1942 μW/cm²). Consistent with that finding, several mutant flies with impaired vision show substantially reduced frequency of the burst. Additionally, we find that genetic ablation of the clock has insignificant effect on burst frequency. Together, these data suggest that the pronounced burst is likely generated by a light-activated circuit that is independent of the circadian clock.     

The role of circadian clock genes in the photoperiodic timer of the linden bug Pyrrhocoris apterus during the nymphal stage

Many insects survive seasonal adversities during diapause, a form of programmed developmental and metabolic arrest. Photoperiodically regulated entry into diapause allows multivoltine insect species to optimize the number of generations. The molecular mechanism of the photoperiodic timer is unknown in insects. In the present study, we take advantage of the robust reproductive diapause response in the linden bug Pyrrhocoris apterus and explore the fifth‐instar nymphal stage, which is the most photoperiod‐sensitive stage. The nymphs display daily changes in locomotor activity during short days; this differs from the activity observed during long days. We find evidence of cyclical expression of the circadian clock genes, per and cyc, in nymphal heads; in addition, per expression is also photoperiod‐dependent. The RNA interference‐mediated knockdown of the two circadian clock genes, Clk and cyc, during the nymphal stage results in reproductive arrest in adult females. Furthermore, Clk and cyc knockdown induces the expression of the storage protein hexamerin in the fat body, whereas the expression of vitellogenin diminishes. Taken together, these data support the involvement of circadian clock genes in photoperiodic timer and/or diapause induction.     

Effects of altered photoperiod on circadian clock and lipid metabolism in rats

Disruption of circadian clock timekeeping due to changes in the photoperiod enhances the risk of lipid metabolism disorders and metabolic syndrome. However, the effects of altered photoperiods on the circadian clock and lipid metabolism are not well understood. To explore the effects of altered photoperiods, we developed a rat model where rats were exposed to either short-day or long-day conditions. Our findings demonstrated that altered photoperiods mediated circadian clocks by partly disrupting rhythmicity and shifting phase values of clock genes. We also showed that compared to long-day conditions, rats under short-day conditions exhibited more photoperiodic changes in a variety of physiological outputs related to lipid metabolism, such as significant increases in serum triglyceride (TG), high-density lipoprotein, and leptin levels, as well as increased body weight, fat:weight ratio, and hepatic TG levels. These increments were gained possibly through upregulated expression of forkhead box O1 (FoxO1), which partly mediates the expression of peroxisome proliferator-activated receptorα (PPARα) to increase the expression of phosphoenolpyruvate carboxykinase (PEPCK), peroxisome proliferator-activated receptor-g coactivator-1β (PGC1β), and fatty acid synthase (Fasn). In addition, the oscillation rhythms of FoxO1, PEPCK, PGC1β, and Fasn expression levels in the livers of rats exposed to a short-day photoperiod were more robust than those exposed to a long-day photoperiod. These findings suggest that a change in photoperiod can partly disrupt the circadian rhythmcity of clock genes, impair lipid metabolism, and promote obesity.