Influence of Melatonin on the Rate of Rana pipiens Tadpole Metamorphosis In Vivo and Regression of Thyroxine-Treated Tail Tips In Vitro

Metamorphosis of Rana pipiens tadpoles may be retarded when the light phase of the light/dark (LD) cycle is shortened or when thyroxine (T₄) is given in the dark because melatonin peaks during the dark. Injection of premetamorphic tadpoles in spontaneous metamorphosis with melatonin (15 μg) retarded tail growth and hindlimb development on 18L:6D but had no significant effect on 6L:18D. During induced metamorphosis (30 μg/liter T₄), melatonin injections retarded tail resorption on 18L:6D and accelerated it on 6L:18D, but did not affect the hindlimb. When melatonin was injected during T₄ immersion at different times in the photophase on 18L:6D (L onset 0800 hr), tail regression was retarded by melatonin at 1430 or 2030 hr. At 0830 hr, shrinkage of tail length was accelerated whereas tail height was not affected. Tail tips in vitro induced to resorb by 0.2 μg/ml T₄ in Niu-Twitty solution regressed more slowly in the presence of melatonin (10 or 15 μg/ml) than with T₄ alone on both 6L:18D and 18L:6D. The findings implicate melatonin in LD cycle effects on tadpole metamorphic rate in vivo , show the importance of the time of melatonin injections, and indicate that melatonin antagonizes the metamorphic action of T₄ at the tissue level.     

Circadian regulation of pineal melatonin and reproduction in the Djungarian hamster

Gonadal state, pineal melatonin rhythms, and locomotor activity rhythms were examined in juvenile male Djungarian hamsters exposed to non-24-hr light cycles ("T-cycles") or to full photoperiods. At the end of 1 month, hamsters exposed to a 1-hr pulse of light every 24.33 hr (T 24.33) exhibited small testes, whereas those receiving the same amount of light every 24.78 hr (T 24.78) displayed stimulated gonads, ten-fold larger in size. Accompanying the nonstimulatory effect of the T 24.33 cycle were nocturnal peaks in both pineal melatonin content and serum melatonin concentration which were longer by approximately 4 hr than those observed on the photostimulatory T 24.78 cycle. Exposure to an intermediate-length T-cycle (T 24.53) resulted in a mixed gonadal response and in pineal and serum melatonin peaks of intermediate duration. Wheel-running activity was entrained to the T-cycles such that light was present only near the beginning of the subjective night, its phase (relative to activity onset) differing only slightly among T-cycle groups. Hence the durational differences observed in the melatonin peaks were apparently not due to the acute suppressive or phase-advancing effects of morning light on melatonin biosynthesis, but were rather the result of differences in the endogenous control of pineal activity by the circadian pacemaker system. While no strong correlation was detected between gonadal state and the phase of locomotor activity onset relative to the light pulse, a significant correlation was observed between gonadal state and the duration of daily locomotor activity (alpha). These data were compared to similar measures obtained from hamsters exposed to long-versus short-day full photoperiods (LD 16:8 vs. LD 10:14). In summary, the results of this study indicate involvement of the circadian pacemaker system of Djungarian hamsters in the control of pineal melatonin synthesis and secretion, and in photoperiodic time measurement. Furthermore, these data strengthen the hypothesis that it is the duration of nocturnal pineal melatonin secretion that is the critical feature of this neuroendocrine gland's photoperiodic signal.     

Daily light regulates seasonal responses in the migratory male redheaded bunting (Emberiza bruniceps)

This study analyzed the role of day length in regulation of seasonal body fattening and testicular growth in a latitudinal Palaearctic-Indian migrant, the redheaded bunting (Emberiza bruniceps). When exposed to increasing photoperiods (hours of light: hours of darkness; 11.5L:12.5D, 12L:12D, 12.5L:11.5D, 13L:11D, 14L:10D, and 18L:6D) for 9-12 weeks, buntings responded in a photoperiod-dependent manner and underwent growth and regression cycle under photoperiods of > or =12 hr per day. Also, the response to a long photoperiod of birds that were held under natural photoperiods at 27 degrees N for 2 years was similar to those who arrived the same year from their breeding grounds ( approximately 40 degrees N), suggesting that the experience of higher amplitude day-night (light-dark, LD) cycles during migratory and breeding seasons were not critical for the subsequent response (initiation-termination-reinitiation) cycle. Another experiment examined entrainment of the circadian photoperiodic rhythm in buntings by subjecting them to T=24+/-2 hr LD-cycles with 8 hr photophase and to T=22 and 24 hr with 11 hr photophase. The results showed a reduction in critical day length under T=22 hr LD-cycle. In the last experiment, we constructed an action spectrum for photoperiodic induction by exposing birds for 4.5 weeks to 13L:11D of white (control), blue (450 nm), or red (640 nm) light at irradiances ranging from 0.028 to 1.4 W m(-2). The threshold light irradiance for photoinduction was about 10-fold higher for blue light, than for red and white lights. These results conclude that the daily light of the environment regulates the endogenous program that times seasonal responses in body fattening and testicular cycles of the redheaded bunting.     

Photoperiodism of diapause induction in Thyrassia penangae (Lepidoptera: Zygaenidae)

Thyrassia penangae enters winter diapause as a prepupa in a cocoon. Photoperiodism of diapause induction was systematically investigated in this moth. The photoperiodic response curves under 24-h light-dark cycles showed that this insect is a typical long-day species. The critical daylength was 13 h 30 min at 25 °C, 13 h at 30 °C and 12 h 20 min at 28 °C. Transferring experiments from a short day (LD 12:12) to a long day (LD 15:9) or vice versa indicated that photoperiodic sensitivity mainly occurs during the larval period. In experiments using non-24-h light-dark cycles, when the length of photophase exceeded the critical daylength (13.5 h), was diapause inhibited effectively, even when the length of scotophase exceeded the critical nightlength (10.5 h). Only when a long scotophase was combined with a short photophase, diapause was induced effectively. This result suggests that daylength measurement is more important than nightlength measurement in T. penangae. Night interruption experiments under 24-h light-dark cycles exhibited two points of apparent light sensitivity, but the photosensitive position was highly influenced by temperature and the length of scotophase. Nanda-Hamner experiments failed to reveal the involvement of a circadian system in this photoperiodic time measurement. All light-dark cycles from LD 12:12 to LD 12:72 resulted in a short day response, and all cycles from LD 14:4 to LD 14:72 resulted in a long day response, suggesting that photoperiodic time measurement in this moth is performed by a day-interval timer or an hourglass-like clock.     

Circadian participation in the photoregulation of testis activity and prolactin secretion in the mink, a short-day breeder

It has been demonstrated that an endogenous mechanism is involved in photoperiodic time measurement in the mink, a short-day-breeding mannal. A study of testicular activity (testicular volume, plasma testosterone concentration) and plasma prolactin level was carried out in sexually resting minks (the experiment began in November). Groups of minks were kept in the natural photoperiod or subjected to different resonance light-dark (LD) cycles (LD 4:8, LD 4:20, LD 4:32, LD 4:44); an additional group of animals was reared in an ahemeral photoperiod (LD 4:16). A rapid increase of testicular activity was observed in control animals or those kept in LD 4:20 (T 24) and LD 4:44 (T 48). In the other groups of animals, those kept in LD 4:8 (T 12), LD 4:32 (T 36), and LD 4:16 (T 20), testicular function remained at rest. Prolactin secretion was, in contrast, stimulated in the groups kept in LD 4:8 (T 12). LD 4:32 (T 36), and LD 4:16 (T 20), and remained low in the groups kept in LD 4:20 (T 24) and LD 4:44 (T 48). These results show that the effects of the different photoperiodic regimens do not depend on the duration of the photophase, but rather on the period of the LD cycles. The LD cycles that allow an increase of testicular function are those that are inhibitory to reproduction in birds and long-day-breeding mammals. To explain these results, it is suggested that in the mink exposure to light during the circadian photosensitive phase induces inhibition of testicular activity and stimulation of prolactin secretion. To explain the opposite effects of a single photoperiod on testicular function and secretion of prolactin, the hypothesis has been advanced that, in the mink, long days might simultaneously inhibit hypothalamic luteinizing-hormone-releasing hormone (LH-RH) activity and prolactin-inhibiting factor (PIF) activity.     

Photoperiodic induction on the growth rate and gonads maturation in the crayfish Procambarus clarkii during ontogeny

Five groups of 20 juvenile instars of crayfish each, were submitted to five experimental photoperiodic cycles of different photophase and scotophase characteristics during 13 weeks. The animals' total length and weight were measured weekly. When the size and maturation of the first animals were adequate, all the animals were sacrificed, and the histological study of gonads was performed. The results indicate that animals submitted to a light-dark (LD) cycle 12:12 with light interruptions in the scotophase, showed the greatest gonadal induction. However the greatest growth induction appeared in animals submitted to control condition (LD 12:12). These results could indicate for both functions two different models of photoperiodic induction.     

Disturbance of plasma melatonin profile by high dose melatonin administration inhibits testicular maturation of precocious male masu salmon

We have previously shown that the testicular development of underyearling male masu salmon Oncorhynchus masou reared under a long photoperiod was accelerated by oral melatonin treatment (0.5 mg melatonin/kg body weight/day), suggesting that melatonin mediates photoperiodic signaling. In this study, we further examined the effects of a disturbance in the plasma melatonin profile on gonadal development in underyearling male masu salmon by administering a higher dose of melatonin. Fish randomly selected in June were divided into two groups. They were reared under a light:dark (LD) cycle of 16:8 (lights on 04:00-20:00 hr) and fed with pellets sprayed with melatonin or vehicle twice a day at 08:30 and at 15:30 hr (7.5 mg melatonin/kg body weight/day) until October. Fish were sampled on Day 0, 25, 60, 90 and 120. The plasma melatonin levels were high in the dark phase and low in the light phase in the control group, while they were constantly high with no significant change in the melatonin-treated group. Melatonin treatment had inhibitory effects on the gonadosomatic index and plasma testosterone levels. Pituitary salmon gonadotropin-releasing hormone content and luteinizing hormone content were significantly lower in the melatonin-treated group on Day 60 and 90, respectively. These results indicate that the plasma melatonin profile is important for mediating photoperiodic signals that regulate brain-pituitary-gonadal axis in underyearling precocious male masu salmon.     

Circadian rhythm of sex pheromone-releasing behaviour in females of the dermestid beetle,Trogoderma glabrum: Regulation by photoperiod

The effects on sex pheromone-releasing, or calling behaviour, of diel photoperiods of varying daylength, of light cycle phase shifts, and of continuous illumination were investigated in Trogoderma glabrum females. On light régimes with 8 to 20 hr daylengths, calling maxima tended to centre close to photophase midpoints. Although influencing the time of day at which calling occurred, daylength had little effect on the amount of activity or the length of the calling period. When 16 : 8 LD light cycles were advanced or delayed by 4 hr, the time of day at which calling peaks were observed shifted within 2 to 4 cycles so that a constant phase relationship with photoperiod was maintained. Daily calling peaks were evident in groups of females exposed to between 1 and 5 days of continuous illumination, but mean calling time occurred earlier in the day as light exposures were lengthened. It was concluded that a circadian rhythm of calling behaviour exists in T. glabrum females. and that the rhythm can be entrained to 24 hr periodicity by photoperiod.     

Responsiveness to photoperiodic changes in the circannual rhythm of the varied carpet beetle, <Emphasis Type="Italic">Anthrenus verbasci</Emphasis>

The circannual pupation rhythm of Anthrenus verbasci is entrained to an environmental cycle by changes in photoperiod. Exposure of larvae reared under short-day conditions to 4 weeks of long days can induce phase-dependent phase shifts. In the present study, we examined the range of photoperiodic changes effective for phase shifts at 20°C. For larvae under light/dark (LD) 12:12 conditions, 4-week exposure to LD 14:10 caused a clear phase delay, as great an extent as that brought about by exposure to LD 15:9 and LD 16:8. In contrast, the delay brought about by exposure to LD 13:9 was slight. For larvae under LD 10:14, exposure to LD 14:10 and LD 16:8 for 4 weeks induced a phase delay, but exposure to LD 12:12 did not. These results indicate that a clear phase delay is induced when the photoperiodic change exceeds a critical value in the photophase between 13 and 14 h, regardless of the amplitude of the change. Although phase advances were smaller than phase delays, they depended on the amplitude of photoperiodic changes rather than the absolute photophase duration in contrast to the case of the phase delay.     

Influence of Photoperiod, Daylength, and Feeding Schedule on Tadpole Growth and Development

The metamorphic rate of Rana pipiens tadpoles was studied under different photoperiods, daylengths, and feeding schedules. Tail resorption and hindlimb growth and development induced by immersion in 30 μg/l thyroxine (T₄) were accelerated under longer photoperiods and continuous light when 6L: 18D, 12L: 12D, 18L: 6D, and 24L regimes were compared. Constant light exposure did not produce faster development than an 18 hr photoperiod, and initially was less effective. The rate of spontaneous and T₄-induced metamorphosis was greater the shorter the day on 9L:9D, 12L: 12D, or 15L: 15D cycles, although all groups received the same overall amount of light, but in different dosages. When feeding schedule but not the LD cycle was varied, groups of tadpoles fed on 18, 24, or 30 hr regimes showed no differences in growth and development rate on 19L: 5D, and only random variations under continuous light. Differences in metamorphic rate on 18, 24, or 30 hr days are not due to the feeding schedules, but to the LD cycles. From these experiments we conclude that illumination, particularly the length and frequency of the photoperiod, affects the utilization of T₄. Development rates independent of the total amount of illumination, but related to daylength and light schedule, suggest interaction of light with an endogenous timing mechanism.     

Effect of different light-dark schedules on survival from heart failure

Our earlier work showed that life in constant light prolonged life for hamsters with an inherited cardiomyopathy when compared to littermates spending their lives in 24 hour days (lights on 12 hr each day). This study was designed to begin evaluating the mechanism for this effect. 4-5.5 month old cardiomyopathic hamsters (CMHs) were placed on one of 5 different light-dark (LD) schedules for the remainder of their lives: (1) LD 12:12 (moderate total light exposure, short photoperiodic effect, 24 hr daylength); (2) LD 12:13 (same total light as LD 12:12, long photoperiodic effect, non-24 hr daylength); (3) LD 6:30 (less total light than LD 12:12, long photoperiodic effect, non-24 hr daylength); (4) LD 18:6 (more total light than LD 12:12, long photoperiodic effect, 24 hr daylength); (5) constant light (high total light exposure, long photoperiodic effect, non-24 hr daylength). CMHs living on the first two non-24 hr schedules lived longer than LD 12:12 controls. This study therefore suggests that manipulating the biological clock can have positive therapeutic consequences. However, in contrast to our earlier studies, hamsters living in constant light were not protected--perhaps because the hamsters began the treatment later in their lives or because their inherited disease was less severe than had been the case in hamsters used in previous studies. Defining the conditions that diminish or enhance the photobiological effect is an important challenge for future research.     

Effects of Photophase and Altitude on Oviposition Rhythm of the Himalayan Strains of Drosophila Ananassae

The effects of varying photophase and altitude of origin on the phase angle difference (Ψ) of the circadian rhythm of oviposition during entrainment to light‐dark (LD) cycles and the aftereffects of such photophases on the period of the free‐running rhythm (τ) in constant darkness (DD) were evaluated in two Himalayan strains of Drosophila ananassae, the high‐altitude (HA) strain from Badrinath (5,123 m above sea level=ASL) and the low‐altitude (LA) strain from Firozpur (179 m ASL). The Ψ (i.e., the hours from lights‐on of the LD cycle to oviposition median) of both strains was determined in LD cycles in which the photophase at 100 lux varied from 6 to 18 h/24 h. The HA strain was entrained by all LD cycles except the one with 6 h photophase in which it was weakly rhythmic, but the LA strain was entrained by only three LD cycles with photophases of 10, 12, and 14 h, but photophases of 6, 8, 16, and 18 h rendered it arrhythmic. Lights‐off transition of LD cycles was the phase‐determining signal for both strains as oviposition medians of the HA strain occurred～6 h prior to lights‐off, while those of the LA strain occurred～1 h after lights‐off. The Ψ of the HA strain increased from～2 h in 8 h photophase to～11 h in 18 h photophase, while that of the LA strain increased from～11 h in 10 h photophase to～15 h in 14 h photophase. The aftereffects of photophase of the prior entraining LD cycles on τ in DD were determined by transferring flies from LD cycles to DD. The τ of the HA strain increased from～19 to～25 h when transferred to DD from LD 8:16 and LD 18:6 cycles, respectively, whereas the τ of the LA strain increased from～26 to～28 h when transferred to DD from LD 10:14 and LD 14:10 cycles, respectively. Thus, these results demonstrate that the photophases of entraining LD cycles and the altitude of origin affected several parameters of entrainment and the period of the free‐running rhythm of these strains.     

Paradoxical masking effects of bright photophase and high temperature in Drosophila malerkotliana

Synergic contribution of light and temperature is known to cause a paradoxical masking effect (inhibition of activity by bright light and high temperature) on various rhythms of animals. The present study reports the paradoxical masking effects of 1000-lux photophase at 25°C on the locomotor activity rhythm of Drosophila malerkotliana. Flies were subjected to light (L)-dark (D) 12:12 cycles wherein the photophase was varied from 10 to 1000 lux, whereas the scotophase was set to 0 lux in these and subsequent LD cycles. At 10, 100, and 500 lux, the flies were diurnal; however, at 1000 lux they were nocturnal. Transfer from LD 12:12 cycles to continuous darkness (DD) initiated free-running rhythmicity in all flies. Free-running rhythms of the flies switched from the 10-lux to the 500-lux groups started from the last activity-onset phase of the rhythm following 3-5 transient cycles, suggesting involvement of the circadian pacemaker. In contrast, the free-running rhythm of the flies of the 1000-lux group began abruptly from the last lights-on phase of the LD cycle, indicating noninvolvement of the pacemaker. Furthermore, all flies showed nocturnal activity in the two types of LD 12:12 cycles when the photophase was 1000 lux. The first type of LD cycles had three succeeding photophases of 100, 1000, and again 100 lux, whereas the second type of LD cycles had only one photophase of 1000 lux, but the LD 12:12 cycles were reversed to DL 12:12 cycles. Apparently, the combined effects of light and temperature caused such paradoxical masking effects. This hypothesis was tested by repeating the above experiments at 20°C. Flies in all experiments exhibited a diurnal activity pattern, even when the photophase was 1000 lux. Thus, the present study demonstrates that the paradoxical masking effect in D. malerkotliana was caused by the additive influence of light intensity and temperature. This strategy appears to have physiological significance, i.e., to shun and thus protect against the bright photophase at high temperature in the field.     

Circadian behavioral and melatonin rhythms in the European starling under light-dark cycles with steadily changing periods: evidence for close mutual coupling?

In European starlings exposed to constant conditions, circadian rhythms in locomotion and feeding can occasionally exhibit complete dissociation from each other. Whether such occasional dissociation between two behavioral rhythms reflects on the strength of the mutual coupling of their internal oscillators has not been investigated. To examine this, as well as to elucidate the role of melatonin in this system, we simultaneously measured the rhythms of locomotion, feeding and melatonin secretion in starlings exposed to light-dark (LD) cycles of low intensity with steadily changing periods (T). In birds initially entrained to T 24 LD cycles (12L:12D, 10:0.2 lx), beginning on day 15, T was either lengthened to 26.5 h (experiment 1) or shortened to T 21.5 h (experiment 2) by changing the daily dark period 4 min each day. After 18 and 19 cycles of T 26.5 and T 21.5, respectively, birds were released into constant dim light conditions (LL(dim); 0.2 lx) for about 2 weeks. Locomotor and feeding rhythms were continuously recorded. Plasma melatonin levels were measured at three times: in T 24, when T equaled 26 or 22 h and at the end of T 26.5 or T 21.5 exposure. The results show that, contrary to our expectations, the three rhythms were not dissociated. Rather they remained synchronized and changed their phase angle difference with the light zeitgeber concomitantly and at the same rate. The melatonin rhythm stayed in synchrony with the behavioral rhythms and as a consequence, peaked either during day or at night, depending on the phase relationship between the activity rhythm and the zeitgeber cycle.     

Influence of the photocycle and thermocycle on rhythms of plasma thyroxine and plasma and ocular melatonin in late metamorphic stages of the bullfrog tadpole, Rana catesbeiana

The diel fluctuations in plasma thyroxine (T(4)) and plasma and ocular melatonin entrain to the light/dark (LD) cycle in the bullfrog tadpole, although the phase of the rhythms changes during development. Previous studies on the rhythmicity of these hormones were conducted under various LD cycles, but with a constant temperature, raising the question of the role of the natural thermocycle in determining the phase of the rhythms, and the changes that occur in the hormone levels and rhythms during late metamorphosis. To study this question, tadpoles were acclimated to simulated natural conditions of 14.5L:9.5D with a corresponding thermocycle in which the thermophase was 28 degrees C and the cryophase was 18 degrees C, or to the same thermocycle under constant light (24L). On both photoregimens, the diel fluctuations changed between prometamorphosis and metamorphic climax. However, more statistically significant rhythms, as indicated by the cosinor, occurred on 14.5L:9.5D than on 24L. At climax on the LD cycle, all hormones peaked around the same time in the late scotocryophase, whereas on 24L, plasma T(4) peaked in the thermophase and plasma and ocular melatonin peaks occurred some distance from each other early in the cryophase. The earlier peaks of plasma and ocular melatonin on 24L were due to a transient rise in these hormones at the onset of the cryophase, which was not sustained in the absence of an LD cycle. On 14.5L:9.5D with a corresponding thermocycle, the hormone rhythms had nearly the same phases as was found in previous work on 12L:12D at a constant temperature of 22 degrees C, allowing for minor phase shifting due to the photocycle differences, indicating that in this species laboratory studies on constant temperature give valid results even in the absence of a thermocycle. The findings show that the phases of the hormone rhythms are determined by the LD cycle although the onset of the cryophase, in the absence of a photocycle, may exert some influence on the nighttime rise in melatonin. The developmental rise in plasma T(4), and drop in plasma melatonin, occurred on both 14.5L:9.5D and 24L, indicating, taken together with previous work, that these climactic changes were independent of temperature and light cycling.     

Photoperiodic control of diapause induction and termination in Ostrinia nubilalis: two different clocks?

Both diapause induction and diapause termination are under photoperiodic control in the lepidopteran, Ostrinia nubilalis. In the present study, induction of diapause was maximal in light-dark (LD) cycles that contained 12 hr of light alternating with 12 hr of darkness (LD 12:12). Termination of diapause was maximal in LD 16:8. Diapause termination also occurred rapidly in non-24-hr LD cycles that possessed an 8-hr dark phase. In each of these cases, the period of the LD cycle was not important. Diapause termination did not, however, occur rapidly in non-24-hr LD cycles that lacked an 8-hr dark phase. Thus, the clock mechanism underlying the termination response resembles an hourglass in its behavior. This is in contrast with what is known about induction of diapause. Here it has been demonstrated that the circadian system is somehow involved. It is thus possible that two different physiological clocks underlie these responses.     

Light pulses entrain the circadian activity rhythm of a diurnal rodent (Ammospermophilus leucurus)

Circadian rhythms of wheel-running activity of the antelope ground squirrel (Ammospermophilus leucurus) were entrained by light-dark cycles (LD: 100 1x vs total darkness) with periods (T) between ca 23.75 and 24.75 hr. Two 1-hr light pulses per cycle ('skeleton photoperiods') with T = 24.25 hr as well as one 1-hr light pulse per cycle with Ts of 23.75 and 24.25 hr were effective in entraining the circadian activity rhythms in at least 50% of the antelope ground squirrels. Phase and period responses to single 1-hr light pulses were measured which depend on the initial phase and period of the rhythm. It is concluded that discrete (phasic) light input contributes to the mechanism of entrainment to LD cycles in diurnal rodents.     

Photoperiodic time measurement in the male deer mouse, Peromyscus maniculatus

Weanling male deer mice, Peromyscus maniculatus, were exposed for three weeks either to light-dark (LD) cycles with periods (T=L+D) ranging from T=23 (1L:22D) to T=25.16 (1L:24.16D) or to 24-h LD cycles with photoperiods ranging from 1 (1L:23D) to 19 (19L:5D) h. Both the circadian locomotor activity rhythms and the response of the reproductive system to these LD cycles were assessed. The results demonstrate that the photoperiodic effectiveness of light depends on the phase of the light relative to the animal's circadian system, as marked by the circadian activity rhythm. Light falling during the animal's subjective night, from activity onset to at least 11.8 h after activity onset, stimulates growth and maturation of the reproductive system, whereas light falling during the rest of the circadian cycle is nonstimulatory.     

History-dependent changes in entrainment of the activity rhythm in the Syrian hamster (Mesocricetus auratus)

The authors have studied the activity rhythm of Syrian hamsters exposed to square LD cycles with a 22-h period (T22) with the aim of testing the effects of the previous history on the rhythmic pattern. To do so, sequential changes of different lighting environments were established, followed by the same LD condition. Also, the protocol included T22 cycles with varying lighting contrasts to test the extent to which a computational model predicts experimental outcomes. At the beginning of the experiment, exposure to T22 with 300 lux and dim red light occurring respectively at photophase and scotophase (LD300/dim red) mainly generated relative coordination. Subsequent transfer to cycles with approximately 0.1-lux dim light during the scotophase (LD300/0.1) promoted entrainment to T22. However, a further reduction in light intensity to 10 lux during the photophase (LD10/0.1) generated weak and unstable T22 rhythms. When, after that, animals were transferred again to the initial LD300/dim red cycles, the amplitude of the rhythm still remained very low, and the phases were very unstable. Exposure to constant darkness partially restored the activity rhythm, and when, afterwards, the animals were submitted again to LD300/dim red cycles, a robust T22 rhythm appeared. The results demonstrate history-dependent changes in the hamster circadian system because the locomotor activity pattern under the same T22 cycle can show relative coordination or unstable or robust entrainment depending on the prior lighting condition. This suggests that the circadian system responds to environmental stimuli depending on its previous history. Moreover, computer simulations allow the authors to predict entrainment under LD300/0.1 cycles and indicate that most of the patterns observed in the animals due to the light in the scotophase can be explained by different degrees of coupling among the oscillators of the circadian system.     

Paradoxical Masking Effects of Bright Photophase and High Temperature in Drosophila malerkotliana

Synergic contribution of light and temperature is known to cause a paradoxical masking effect (inhibition of activity by bright light and high temperature) on various rhythms of animals. The present study reports the paradoxical masking effects of 1000-lux photophase at 25°C on the locomotor activity rhythm of Drosophila malerkotliana. Flies were subjected to light (L)-dark (D) 12:12 cycles wherein the photophase was varied from 10 to 1000 lux, whereas the scotophase was set to 0 lux in these and subsequent LD cycles. At 10, 100, and 500 lux, the flies were diurnal; however, at 1000 lux they were nocturnal. Transfer from LD 12:12 cycles to continuous darkness (DD) initiated free-running rhythmicity in all flies. Free-running rhythms of the flies switched from the 10-lux to the 500-lux groups started from the last activity-onset phase of the rhythm following 3–5 transient cycles, suggesting involvement of the circadian pacemaker. In contrast, the free-running rhythm of the flies of the 1000-lux group began abruptly from the last lights-on phase of the LD cycle, indicating noninvolvement of the pacemaker. Furthermore, all flies showed nocturnal activity in the two types of LD 12:12 cycles when the photophase was 1000 lux. The first type of LD cycles had three succeeding photophases of 100, 1000, and again 100 lux, whereas the second type of LD cycles had only one photophase of 1000 lux, but the LD 12:12 cycles were reversed to DL 12:12 cycles. Apparently, the combined effects of light and temperature caused such paradoxical masking effects. This hypothesis was tested by repeating the above experiments at 20°C. Flies in all experiments exhibited a diurnal activity pattern, even when the photophase was 1000 lux. Thus, the present study demonstrates that the paradoxical masking effect in D. malerkotliana was caused by the additive influence of light intensity and temperature. This strategy appears to have physiological significance, i.e., to shun and thus protect against the bright photophase at high temperature in the field. (Author correspondence: drdsjoshi08@gmail.com)