Entrainment of the circadian activity rhythm to the light-dark cycle can be altered by a short-acting benzodiazepine, triazolam

The circadian rhythm of locomotor activity in hamsters maintained in either constant darkness or constant light can be phase-shifted by a single injection of the short-acting benzodiazepine, triazolam. These results suggest that treatment with triazolam may also alter the entrainment pattern of circadian rhythms in animals that are synchronized to a light-dark (LD) cycle. To test this hypothesis, hamsters maintained on an LD 6:18 light cycle received daily injections of triazolam (or vehicle) for 10-12 days, and any subsequent effects on the phase relationship between the onset of activity and the LD cycle were determined. Daily injections of triazolam (but not vehicle) induced pronounced advances or delays in the phase relationship between the entrained activity rhythm and the LD cycle; the direction of the shift was dependent on the time of the injection. Taken together with data from previous studies, these results suggest that triazolam, and perhaps other short-acting benzodiazepines, can be used to manipulate the mammalian circadian clock under a variety of experimental conditions.     

The role of Clock in the plasticity of circadian entrainment

The mammalian circadian clock lying in suprachiasmatic nucleus (SCN) is synchronized to about 24 h by the environmental light-dark cycle (LD). The circadian clock exhibits limits of entrainment above and below 24 h, beyond which it will not entrain. Little is known about the mechanisms regulating the limits of entrainment. In this study, we show that wild-type mice entrain to only an LD 24 h cycle, whereas Clock mutant mice can entrain to an LD 24, 28, and 32 h except for LD 20 h and LD 36 h cycle. Under an LD 28 h cycle, Clock mutant mice showed a clear rhythm in Per2 mRNA expression in the SCN and behavior. Light response was also increased. This is the first report to show that the Clock mutation makes it possible to adapt the circadian oscillator to a long period cycle and indicates that the clock gene may have an important role for the limits of entrainment of the SCN to LD cycle.     

Daily and circadian variations of the pineal and ocular melatonin contents and their contributions to the circulating melatonin in the Japanese newt, Cynops pyrrhogaster

Daily and circadian variations of melatonin contents in the diencephalic region containing the pineal organ, the lateral eyes, and plasma were studied in a urodele amphibian, the Japanese newt (Cynops pyrrhogaster), to investigate the possible roles of melatonin in the circadian system. Melatonin levels in the pineal region and the lateral eyes exhibited daily variations with higher levels during the dark phase than during the light phase under a light-dark cycle of 12 h light and 12 h darkness (LD12:12). These rhythms persisted even under constant darkness but the phase of the rhythm was different from each other. Melatonin levels in the plasma also exhibited significant day-night changes with higher values at mid-dark than at mid-light under LD 12:12. The day-night changes in plasma melatonin levels were abolished in the pinealectomized (Px), ophthalmectomized (Ex), and Px+Ex newts but not in the sham-operated newts. These results indicate that in the Japanese newts, melatonin production in the pineal organ and the lateral eyes were regulated by both environmental light-dark cycles and endogenous circadian clocks, probably located in the pineal organ and the retina, respectively, and that both the pineal organ and the lateral eyes are required to maintain the daily variations of circulating melatonin levels.     

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.     

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).     

Expression of the melatonin receptor Mel(1c) in neural tissues of the reef fish Siganus guttatus

The golden rabbitfish, Siganus guttatus, is a reef fish exhibiting a restricted lunar-related rhythm in behavior and reproduction. Here, to understand the circadian rhythm of this lunar-synchronized spawner, a melatonin receptor subtype-Mel(1c)-was cloned. The full-length Mel(1c) melatonin receptor cDNA comprised 1747 bp with a single open reading frame (1062 bp) that encodes a 353-amino acid protein, which included 7 presumed transmembrane domains. Real-time PCR revealed high Mel(1c) mRNA expression in the retina and brain but not in the peripheral tissues. When the fish were reared under light/dark (LD 12:12) conditions, Mel(1c) mRNA in the retina and brain was expressed with daily variations and increased during nighttime. Similar variations were noted under constant conditions, suggesting that Mel(1c) mRNA expression is regulated by the circadian clock system. Daily variations of Mel(1c) mRNA expression with a peak at zeitgeber time (ZT) 12 were observed in the cultured pineal gland under LD 12:12. Exposure of the cultured pineal gland to light at ZT17 resulted in a decrease in Mel(1c) mRNA expression. When light was obstructed at ZT5, the opposite effect was obtained. These results suggest that light exerts certain effects on Mel(1c) mRNA expression directly or indirectly through melatonin actions.     

Circadian Rhythms of Locomotor Activity in Lycosa tarentula (Araneae, Lycosidae) and the Pathways of Ocular Entrainment

Lycosa tarentula is a ground-living spider that inhabits a burrow where it awaits the appearance of prey or conspecifics. In this study, circadian rhythms of locomotor activity were examined as well as the ocular pathway of entrainment. Thirty-three adult virgin females were examined under constant darkness (DD); all of them exhibited robust circadian rhythms of locomotor activity with a period averaging 24.1h. Fourteen of these spiders were studied afterwards under an LD 12:12 cycle; they usually entrained to in the first or second day, even when the light intensity was as low as 1 lx. During the LD cycle, locomotor activity was generally restrained to the darkness phase, although several animals showed a small amount of diurnal activity. Ten males were also examined under LD; they were also nocturnal, but were much more active than the females. Seven females were examined under constant light (LL); under this they became arrhythmic. Except for the anterior median eyes (OMAs), all the eyes were capable of entraining the locomotor activity to an LD cycle. These results demonstrate that under laboratory conditions and low light intensities locomotor activity of Lycosa tarentula is circadian and in accordance with Aschoff's 'rule'. Only OMAs are unable to entrain the rhythm; the possible localization of circadian clock is therefore discussed.     

Comparison of the circadian eclosion rhythm between non-diapause and diapause pupae in the onion fly, Delia antiqua: the change of rhythmicity

When pupae of Delia antiqua were transferred to constant darkness (DD) from light-dark (LD) cycles or constant light (LL), the sensitivity to light of the circadian clock controlling eclosion increased with age. The daily rhythm of eclosion appeared in both non-diapause and diapause pupae only when this transfer was made during late pharate adult development. When transferred from LL to DD in the early pupal stage, the adult eclosion was weakly rhythmic in non-diapause pupae but arrhythmic in diapause pupae. However, the sensitivity of the circadian clock to temperature cycles or steps was higher in diapause pupae than in non-diapause pupae; in the transfer to a constant 20 degrees C from a thermoperiod of 25 degrees C (12 h)/20 degrees C (12 h) on day 10 after pupation or from chilling (7.5 degrees C) in DD, the adult eclosion from diapause pupae was rhythmic but that from non-diapause pupae arrhythmic. In a transfer to 20 degrees C from the thermoperiod after the initiation of eclosion, rhythmicity was observed in both types of pupae. The larval stage was insensitive to the effect of LD cycle initiating the eclosion rhythm. In D. antiqua pupae in the soil under natural conditions, therefore, the thermoperiod in the late pupal stage would be the most important 'Zeitgeber' for the determination of eclosion timing.     

Circadian locomotory rhythm and the influence of moulting in Australian field cricket nymphs

ABSTRACT. Evidence is presented for a circadian control of locomotory activity in the larval stadia of the cricket, Teleogryllus commodus Walker. Under light—dark cycles (LD), maximal activity occurs around the L/D transition and/or in the hours preceding it. Free-running rhythm patterns longer than 24 h are observed in constant light. Re-entrainment to phase advances in the LD cycle is also accompanied by several transient cycles. However, free-running rhythms under constant darkness or transients when exposed to LD cycle delays were not found. LD cycles during the eighth stadium set the phase of a free-running rhythm in the adult, even if the nymph does not show a rhythm. Nymphal activity is often erratic and is disrupted periodically by the moulting cycle, but moulting does not interrupt the operation of the circadian system. The daily timing of the moult itself is not under circadian control.     

About the Existence of Circadian Activity in Cave Crayfish

Evidence of a circadian clock mechanism was found in the cave crayfish Procambarus cavernicola. Analysis of motor activity recorded in this species during 12 consecutive days in either free running (constant darkness, DD or constant light, LL) or entrainment conditions (12 h of light alternated with 12 h of darkness, 12 : 12 LD) showed a well recognized circadian rhythm. In this rhythm however, the absence of synchronization by periodical external signals was notorious. The comparison between the motor circadian rhythm in cave crayfish and epigeous crayfish Procambarus clarkii (these last studied during juvenile and adult stages), evidenced strong similitude between the motor circadian rhythm of cave crayfish and juvenile epigeous crayfish.     

Phase of the circadian clock is accurately transferred from mother to daughter cells in the dinoflagellate Gonyaulax polyedra.

In the first cycle following transfer from a 12 h light-12 h dark cycle (LD12:12) to constant darkness (DD), the standard deviation in circadian phase among individual clocks in populations of Gonyaulax polyedra is approximately 60 min. When a culture is transferred to constant light conditions (LL) from an LD 12:12 cycle, the standard deviation increases in the first 2-3 d, but then remains unchanged, suggesting a lack of observable desynchronization in LL after the transient period. The synchrony in a cell population is preserved even after several cell divisions. The results indicate that variations in period among cells are small, that the period of an individual clock does not fluctuate randomly from day to day, and that the circadian phase of a mother cell is faithfully passed to the clocks of the daughter cells.     

The effects of pinealectomy and blinding on the circadian locomotor activity rhythm in the Japanese newt,Cynops pyrrhogaster

We examined the effects of pinealectomy and blinding (bilateral ocular enucleation) on the circadian locomotor activity rhythm in the Japanese newt, Cynops pyrrhogaster. The pinealectomized newts were entrained to a light-dark cycle of 12 h light and 12 h darkness. After transfer to constant darkness they showed residual rhythmicity for at least several days which was gradually disrupted in prolonged constant darkness. Blinded newts were also entrained to a 12 h light/12 h dark cycle. In subsequent constant darkness they showed free-running rhythms of locomotor activity. However, the freerunning periods noticeably increased compared with those observed in the previous period of constant darkness before blinding. In blinded newts entrained to the light/dark cycle the activity rhythms were gradually disrupted after pinealectomy even in the presence of the light/dark cycle. These results suggest that both the pineal and the eyes are involved in the newt's circadian system, and also suggest that the pineal of the newt acts as an extraretinal photoreceptor which mediates the entrainment of the locomotor activity rhythm.Abbreviations circadian period - DD constant darkness - LD cycle, light-dark cycle - LD 12:12 light-dark cycle of 12 h light and 12 h darkness     

Circadian Rhythm of Total Protein Synthesis in the Cytoplasm and Chloroplasts of Gonyaulaxpolyedra

Protein synthesis of Gonyaulax polyedra was analyzed by means of electron microscopic autoradiographs under constant conditions at different times of the 24-hr cycle. Circadian rhythmic changes in the synthesis rate of total protein were determined in the cytoplasm and chloroplasts of growing cells. Three independent series of experiments in constant light showed a maximum of grains per unit area during the 'subjective' dark phase (=phase that corresponds to the dark phase during a 12:12 hr LD cycle) in both compartments. Minimum and maximum grain number are different by a factor of 5-10. The maximum of total protein synthesis coincided with the maximum phase shift by cycloheximide pulses (1) suggesting protein species within the total pool involved in the mechanism of the circadian clock. A similar rhythm of lower amplitude was observed in the mitochondria, but this rhythm cannot with certainty be attributed to these organelles. In a slowly growing culture a rhythm of total protein synthesis was observed that showed a smaller amplitude and a different phasing.     

Circadian Rhythms of Locomotor Activity in Lycosa tarentula (Araneae,Lycosidae) and the Pathways of Ocular Entrainment

Lycosa tarentula is a ground-living spider that inhabits a burrow where it awaits the appearance of prey or conspecifics. In this study, circadian rhythms of locomotor activity were examined as well as the ocular pathway of entrainment. Thirty-three adult virgin females were examined under constant darkness (DD); all of them exhibited robust circadian rhythms of locomotor activity with a period averaging 24.1h. Fourteen of these spiders were studied afterwards under an LD 12:12 cycle; they usually entrained to in the first or second day, even when the light intensity was as low as 1 lx. During the LD cycle, locomotor activity was generally restrained to the darkness phase, although several animals showed a small amount of diurnal activity. Ten males were also examined under LD; they were also nocturnal, but were much more active than the females. Seven females were examined under constant light (LL); under this they became arrhythmic. Except for the anterior median eyes (OMAs), all the eyes were capable of entraining the locomotor activity to an LD cycle. These results demonstrate that under laboratory conditions and low light intensities locomotor activity of Lycosa tarentula is circadian and in accordance with Aschoff's 'rule'. Only OMAs are unable to entrain the rhythm; the possible localization of circadian clock is therefore discussed.     

A circadian rhythm of thermal preference in the ant Camponotus mus: masking and entrainment by temperature cycles

Abstract. Along a stable temperature gradient and under a LD 12:12 h cycle, nurse workers of the ant Camponotus mus Roger 1863 (Hymenoptera: Formicidae) select for the brood two different temperatures daily: 30.8°C at the middle of the light period (circadian phase = 90°), and 27.5°C 8 h later, during the dark period (circadian phase = 210°), this rhythm being of endogenous nature.When a 24 h temperature cycle was superimposed along the thermal gradient, so that the immobile brood experienced a temperature transition as they receive when translocated by nurses (8 h at 30.8°C and 16 h at 27.5°C), no brood translocations occurred.The thermal cycle masked the rhythm of brood translocation when temperature fitted the daily pattern expected by nurses.When the same temperature cycle was presented with a phase-advance, nurses did not tolerate the early thermal increase and removed the brood as temperature rose.However, when workers experienced this new phase relationship between light and temperature cycles for more than 10 days, brood translocations were suppressed.Records under constant conditions of light and temperature indicated that the overt rhythm was locked-on to the expected early increase in temperature, so that the temperature cycle dominated over the LD cycle in resetting brood-carrying activity.