Circadian and seasonal responses in Indian weaver bird: subjective interpretation of day and night depends upon both light intensity and contrast between illuminations

This study investigated whether changes in illumination modify perception of day and night conditions in a diurnal species, the Indian weaver bird. Birds were initially subjected to a 12-h light:12-h dark regime (12L:12D; L=20 lux, D =0.5 lux). After every 2 wks, the combinations of light illumination in L and D phases were changed as follows: 20:2 lux, 20:5 lux, 20:10 lux, 20:20 lux, 20:100 lux, and 20:200 lux. Finally, birds were released into dim constant light (0.5 lux) for 2 wks to determine the phase and period of the circadian activity rhythm. They were also laparotomized at periodic intervals to examine the effects of the light regimes on the seasonal testicular cycle. All individuals showed a consistently similar response. As evident by the activity pattern under these light regimes, both in total activity during contrasting light phases and during the 2?h in the beginning and end of first light phase, birds interpreted the period of higher light intensity as day, and the period of lower intensity as the night. During the period of similar light intensity, i.e., under LL, birds free-ran with a circadian period ( ~ 24 h). In bright LL (20 lux), the activity rhythm was less distinct, but periodogram analysis revealed the circadian period for the group as 24.46 (+/-) 0.41 h (mean???SE). However, in dim LL at the end of the experiment, all birds exhibited a circadian pattern with average period of 25.52 (+/-) 0.70 h. All birds also showed testicular growth and regression during the 16-wks study. It is suggested that weaver birds interpret day and night subjectively based on both the light intensity and contrast between illuminations during two phases over the 24 h.     

Circadian and Seasonal Responses in Indian Weaver Bird: Subjective Interpretation of Day and Night Depends Upon Both Light Intensity and Contrast Between Illuminations

This study investigated whether changes in illumination modify perception of day and night conditions in a diurnal species, the Indian weaver bird. Birds were initially subjected to a 12-h light:12-h dark regime (12L:12D; L?=?20 lux, D =?0.5 lux). After every 2 wks, the combinations of light illumination in L and D phases were changed as follows: 20:2 lux, 20:5 lux, 20:10 lux, 20:20 lux, 20:100 lux, and 20:200 lux. Finally, birds were released into dim constant light (0.5 lux) for 2 wks to determine the phase and period of the circadian activity rhythm. They were also laparotomized at periodic intervals to examine the effects of the light regimes on the seasonal testicular cycle. All individuals showed a consistently similar response. As evident by the activity pattern under these light regimes, both in total activity during contrasting light phases and during the 2?h in the beginning and end of first light phase, birds interpreted the period of higher light intensity as day, and the period of lower intensity as the night. During the period of similar light intensity, i.e., under LL, birds free-ran with a circadian period (～24?h). In bright LL (20 lux), the activity rhythm was less distinct, but periodogram analysis revealed the circadian period for the group as 24.46?±?0.41?h (mean?±?SE). However, in dim LL at the end of the experiment, all birds exhibited a circadian pattern with average period of 25.52?±?0.70?h. All birds also showed testicular growth and regression during the 16-wks study. It is suggested that weaver birds interpret day and night subjectively based on both the light intensity and contrast between illuminations during two phases over the 24?h. (Author correspondence: drskumar7@yahoo.com)     

Photic entrainment of circadian activity patterns in the tropical labrid fish Halichoeres chrysus

Yellow wrasses (Halichoeres chrysus) show clear daily activity patterns. The fish hide in the substrate at (subjective) night, during the distinct rest phase. Initial entrainment in a 12h:12h light-dark (12:12 LD) cycle (mean period 24.02h, SD 0.27h, n = 16 was followed by a free run (mean period 24.42h, SD 1.33h) after transition into constant dim light conditions. Light pulses of a comparable intensity as used in the light part of the LD cycles did not result in significant phase shifts of the free-running rhythm in constant darkness. Application of much brighter 3h light pulses resulted in a phase-response curve (PRC) for a fish species, with pronounced phase advances during late subjective night. The PRCs differed from those mainly obtained in other vertebrate taxa by the absence of significant phase delays in the early subjective night. At that circadian phase, significant tonic effects of the light pulses caused a shortening of the circadian period length. Entrainment to skeleton photoperiods of 1:11 LD was observed in five of six wrasses exposed, also after a 3h phase advance of this LD cycle. Subsequently, a 1:11.25 LD cycle resulted in entrainment in four of the six fish. It is suggested that the expression of the circadian system in fish can be interpreted as a functional response to a weak natural zeitgeber, as present in the marine environment. This response allows photic entrainment as described here in the yellow wrasse. (Chronobiology International, 17(5), 613-622, 2000)     

Circadian Rhythm in Pineal N-Acetyltransferase Activity: Rapid Phase Reversal and Response to Shorter than 24-Hour Cycles (IV)

N-Acetyltransferase (NAT) is an enzyme whose rhythmic activity in the pineal gland and retina is responsible for circadian rhythms in melatonin. The NAT activity rhythm has circadian properties such as persistence in constant conditions and precise control by light and dark. Experiments are reported in which chicks (Gallus domesticus), raised for 3 weeks in 12 h of light alternating with 12 h of dark (LD12:12), were exposed to 1-3 days of light-dark treatments during which NAT activity was measured in their pineal glands. (a) In LD12:12, NAT activity rose from less than 4.5 nmol/pineal gland/h during the light-time to 25-50 nmol/pineal gland/h in the dark-time. Constant light (LL) attenuated the amplitude of the NAT activity rhythm to 26-45% of the NAT activity cycle in LD12:12 during the first 24 h. (b) The timing of the increase in NAT activity was reset by the first full LD12:12 cycle following a 12-h phase shift of the LD12:12 cycle (a procedure that reversed the times of light and dark by imposition of either 24 h of light or dark). This result satisfies one of the criteria for NAT to be considered part of a circadian driving oscillator. (c) In less than 24-h cycles [2 h of light in alternation with 2 h of dark (LD2:2), 4 h of light in alternation with 4 h of dark (LD4:4), and 6 h of light in alternation with 6 h of dark (LD6:6)], NAT activity rose in the dark during the chicks' previously scheduled dark-time but not the previously scheduled light-time of LD12:12. In a cycle where 8 h of light alternated with 8 h of dark (LD8:8), NAT activity rose in both 8-h dark periods, even though the second one fell in the light-time of the prior LD12:12 schedule.(ABSTRACT TRUNCATED AT 250 WORDS)     

Direct modulation of activity and body temperature of owl monkeys (Aotus lemurinus griseimembra) by low light intensities

The activity pattern of Aotus lemurinus griseimembra can be predictably altered by varying the illuminance during the dark phase of a 12:12-hour light:dark rhythm. Intensities well below full-moon brightness (0.1-0.5 lx) severely inhibit activity. This modulation is not the result of a light-induced phase shift of the circadian rhythm, but it is primarily caused by masking due to direct effects of light on the motor system. Both proportional and differential effects of light are involved. Miniature transmitters were implanted intraperitoneally in two Aotus females so that the core temperature could be measured in parallel with locomotor activity. The responses to brief reductions of the dark-phase illuminance, from 10(-1) to 10(-3) lx, 10(-5) lx or physiological darkness, indicate that the direct effects of light that modulate the activity of the owl monkeys also affect their temperature time-course. The influence on the temperature rhythm, unlike that on the activity rhythm, varies greatly over the circadian period. The finding that the core temperature does not always change in parallel with locomotor activity and, to some extent, reacts differently to the light:dark alternation indicates that temperature does not simply follow activity passively, but rather is partially subject to a 'direct' masking influence of the light.     

Characteristics of the Light-Induced Phase Response of Circadian Activity Rhythms in Common Marmosets,Callithrix j. jacchus [Primates-Cebidae]

Phase-response experiments using 1-h light pulses (LPs) of 1,100 lux applied under constant dim light of 0.3 lux were conducted with common marmosets, Callithrix j. jacchus, in order to obtain a complete phase-response curve established according to the common experimental procedure in a diurnal primate. Maximal phase delays of the free-running circadian activity rhythm (- 90 min) were induced by LPs delivered at circadian time (CT) 12; e.g., during the beginning of the marmosets' rest time, maximal advances (+ 25 min) were elicited by pulses administered during the late subjective night at CT 21. In contrast to rodents, neither regular transient cycles nor regular period responses resulted from LP applications at different phases. To check whether the underlying period length affects the phase response in primates as well, the marmosets' circadian timing system was entrained to 25 h by a lightrdark (LD) cycle of 12.5:12.5 h. The 1-h LPs were delivered during the first circadian cycle produced under constant dim light after the entraining LD periods. Here, LPs applied at CT 21 led to phase advances exceeding those measured during the steady-state free run. At CT 12, minor or no phase delays could be elicited. These findings show that the phase-shifting effect of LPs on the circadian system of marmosets is similar to that observed in other diurnal mammals. Some of the results indicate that in this diurnal primate, LP-induced phase shifts may be mediated in part by a light-induced increase in locomotor activity (arousal).     

Characteristics of the Light-Induced Phase Response of Circadian Activity Rhythms in Common Marmosets, Callithrix j. jacchus [Primates-Cebidae]

Phase-response experiments using 1-h light pulses (LPs) of 1,100 lux applied under constant dim light of 0.3 lux were conducted with common marmosets, Callithrix j. jacchus, in order to obtain a complete phase-response curve established according to the common experimental procedure in a diurnal primate. Maximal phase delays of the free-running circadian activity rhythm (- 90 min) were induced by LPs delivered at circadian time (CT) 12; e.g., during the beginning of the marmosets' rest time, maximal advances (+ 25 min) were elicited by pulses administered during the late subjective night at CT 21. In contrast to rodents, neither regular transient cycles nor regular period responses resulted from LP applications at different phases. To check whether the underlying period length affects the phase response in primates as well, the marmosets' circadian timing system was entrained to 25 h by a lightrdark (LD) cycle of 12.5:12.5 h. The 1-h LPs were delivered during the first circadian cycle produced under constant dim light after the entraining LD periods. Here, LPs applied at CT 21 led to phase advances exceeding those measured during the steady-state free run. At CT 12, minor or no phase delays could be elicited. These findings show that the phase-shifting effect of LPs on the circadian system of marmosets is similar to that observed in other diurnal mammals. Some of the results indicate that in this diurnal primate, LP-induced phase shifts may be mediated in part by a light-induced increase in locomotor activity (arousal).     

Entrainment to Light of Circadian Activity Rhythms in Tench (Tinca tinca)

The present article analyzes locomotor activity rhythms in Tinca tinca. To that end, three different experiments were conducted on 24 animals (20 g body weight) kept in pairs in 60‐liter aquaria fitted with infrared sensors connected to a computer to continuously record fish movements. The first experiment was designed to study the endogenous circadian clock under free‐running conditions [ultradian 40:40 min LD pulses and constant dark (DD)] and after shifting the LD cycle. Our results demonstrate that tench has a strictly nocturnal activity pattern, an endogenous rhythm being evident in 45.8% of the fish analyzed. The second experiment was conducted to test the influence of different photoperiods (LD 6:18, 12:12, 18:6, and 22:2) on locomotor activity, the results showing that even under an extremely long photoperiod, tench activity is restricted to dark hours. The third experiment examined the effect of light intensity on locomotor activity rhythms. When fish were exposed to decreasing light intensities (from 300:0 lux to 30:0, 3:0, and 0.3:0 lux) while maintaining a constant photoperiod (LD 12:12), the highest percentage of locomotor activity was in all cases associated with the hours of complete darkness (0 lux). In short, our results clearly show that (a) tench is a species with a strictly nocturnal behavior, and (b) daily activity rhythms gradually entrain after shifting the LD cycle and persist under free‐running conditions, pointing to their circadian nature. However, light strongly influences activity rhythms, since (c) the length of the active phase is directly controlled by the photophase, and (d) strictly nocturnal behavior persists even under very dim light conditions (0.3 lux). The above findings deepen our knowledge of tench behavior, which may help to optimize the aquacultural management of this species, for example, by adjusting feeding strategies to their nocturnal behavior.     

Entrainment to light of circadian activity rhythms in tench (Tinca tinca)

The present article analyzes locomotor activity rhythms in Tinca tinca. To that end, three different experiments were conducted on 24 animals (20 g body weight) kept in pairs in 60-liter aquaria fitted with infrared sensors connected to a computer to continuously record fish movements. The first experiment was designed to study the endogenous circadian clock under free-running conditions [ultradian 40:40 min LD pulses and constant dark (DD)] and after shifting the LD cycle. Our results demonstrate that tench has a strictly nocturnal activity pattern, an endogenous rhythm being evident in 45.8% of the fish analyzed. The second experiment was conducted to test the influence of different photoperiods (LD 6:18, 12:12, 18:6, and 22:2) on locomotor activity, the results showing that even under an extremely long photoperiod, tench activity is restricted to dark hours. The third experiment examined the effect of light intensity on locomotor activity rhythms. When fish were exposed to decreasing light intensities (from 300:0 lux to 30:0, 3:0, and 0.3:0 lux) while maintaining a constant photoperiod (LD 12:12), the highest percentage of locomotor activity was in all cases associated with the hours of complete darkness (0 lux). In short, our results clearly show that (a) tench is a species with a strictly nocturnal behavior, and (b) daily activity rhythms gradually entrain after shifting the LD cycle and persist under free-running conditions, pointing to their circadian nature. However, light strongly influences activity rhythms, since (c) the length of the active phase is directly controlled by the photophase, and (d) strictly nocturnal behavior persists even under very dim light conditions (0.3 lux). The above findings deepen our knowledge of tench behavior, which may help to optimize the aquacultural management of this species, for example, by adjusting feeding strategies to their nocturnal behavior.     

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.     

Dim light at night increases body mass of female mice

During the past century, the prevalence of light at night has increased in parallel with obesity rates. Dim light at night (dLAN) increases body mass in male mice. However, the effects of light at night on female body mass remain unspecified. Thus, female mice were exposed to a standard light/dark (LD; 16?h light at ～150?lux/8?h dark at ～0?lux) cycle or to light/dim light at night (dLAN; 16?h light at ～150?lux/8?h dim light at ～5?lux) cycles for six weeks. Females exposed to dLAN increased the rate of change in body mass compared to LD mice despite reduced total food intake during weeks five and six, suggesting that dLAN disrupted circadian rhythms resulting in deranged metabolism.     

Societal synchronization in groups of rats or quail submitted to various lighting regimens

Groups of rats or of quail that had been previously synchronized in a light (L = 100 lux) dark (D) phase opposition (PO = LD and DL) were placed together in a L12:D12 or D12:L12 alternation or in continuous light (LL) or continuous darkness (DD). Emission of carbon dioxide (VCO2) which was continuously recorded in groups of individuals placed in respiratory chambers under controlled environmental conditions allows an index of their overall respiratory and metabolic exchanges to be found. In PO animals placed in LD or DL, the VCO2 circadian light dark synchronization comes back less quickly in rats than in quail, and the VCO2 variations at the light dark transitions (L-D and D-L) remain unchanged in rats, but are modified in quail. When PO animals are placed for 18 days in LL or DD, respiratory circadian rhythms disappear except in the grouped rats where they reappear after 4-5 days in DD.     

Circadian Rhythms and time Measurement in Locomotor Activity of the Scorpion Leiurus Quinquestriatus (Buthidae)

The circadian rhythms of locomotor activity of the scorpion Leiurus quinqueslriatus were examined under different light-dark cycles and in free-running conditions. The circadian rhythm is bimodal in LD 12:12 with alternating cycles of temperature (35°-25°C) with high intensity (1300 lux) or in LD 12: 12 with constant temperature 35° C with 300 lux. In LD 12:12 (1300 lux), in long or in short light spans with constant temperature, the bimodal pattern is slightly changed with the appearance of a third minor peak of activity. In free-running conditions, the bimodal rhythm of locomotor activity persists in DD with T about 24 hr, but in LL the rhythm becomes unimodal with T about 24 hr. Cosinor and power spectrum analysis showed the presence of more than one periodic component. It seems that there is a correlation between the range of light regimens, temperature, light intensity and the coincidence of these components. These components are independently entrained by the environmental light cycle. The mechanism of entrainment of components is discussed.     

Photoperiod modifies daily maps of light and dark sensitivity for N-acetyltransferase activity in pineal glands of 3-week oldGallus domesticus

Summary N-acetyltransferase (NAT) activity in pineal glands exhibits a circadian rhythm with peak activity occurring in the dark-time. We previously showed that inGallus domesticus chicks pretreated with LD12:12, NAT activity was increased by dark exposure (peak dark sensitivity occurred during the expected dark-time) or decreased by light at night (peak light sensitivity occurred early in the night during the time of dark sensitivity). In this study we mapped dark sensitivity vs time (for NAT activity increase in response to 2 h dark pulses), and light sensitivity vs time (for NAT activity decrease in response to 10 min or 30 min light pulses) over a cycle for 3-week old chicks,Gallus domesticus, pretreated with long (LD16:8) or short photoperiod (LD8:16). Sensitivity to light was increased in the second 8 h after L/D by LD8:16. Sensitivity to dark was increased in the first 8 h after L/D by LD16:8.Abbreviations LD16:8 a light-dark cycle consisting of 16 h of light alternating with 8 h of dark - LD8:16 a light-dark cycle consisting of 8 h of light alternating with 16 h of dark - DD constant dark - LL constant light - L/D lights-off - D/L lights-on - NAT pineal serotonin N-acetyltransferase - NAT activity is given in nmoles/pineal gland/h - chick used here to denote a young bird of either sex of the speciesGallus domesticus from hatching to three weeks of age     

PHOTIC ENTRAINMENT OF CIRCADIAN ACTIVITY PATTERNS IN THE TROPICAL LABRID FISH HALICHOERES CHRYSUS

Yellow wrasses (Halichoeres chrysus) show clear daily activity patterns. The fish hide in the substrate at (subjective) night, during the distinct rest phase. Initial entrainment in a 12h:12h light-dark (12:12 LD) cycle (mean period 24.02h, SD 0.27h, n = 16 was followed by a free run (mean period 24.42h, SD 1.33h) after transition into constant dim light conditions. Light pulses of a comparable intensity as used in the light part of the LD cycles did not result in significant phase shifts of the free-running rhythm in constant darkness. Application of much brighter 3h light pulses resulted in a phase-response curve (PRC) for a fish species, with pronounced phase advances during late subjective night. The PRCs differed from those mainly obtained in other vertebrate taxa by the absence of significant phase delays in the early subjective night. At that circadian phase, significant tonic effects of the light pulses caused a shortening of the circadian period length. Entrainment to skeleton photoperiods of 1:11 LD was observed in five of six wrasses exposed, also after a 3h phase advance of this LD cycle. Subsequently, a 1:11.25 LD cycle resulted in entrainment in four of the six fish. It is suggested that the expression of the circadian system in fish can be interpreted as a functional response to a weak natural zeitgeber, as present in the marine environment. This response allows photic entrainment as described here in the yellow wrasse. (Chronobiology International, 17(5), 613–622, 2000)     

Behavioral characterization and modulation of circadian rhythms by light and melatonin in C3H/HeN mice homozygous for the RORbeta knockout

This study reports for the first time the effects of retinoid-related orphan receptors [RORbeta; receptor gene deletion RORbeta(C3H)(-/-)] in C3H/HeN mice on behavioral and circadian phenotypes. Pineal melatonin levels showed a robust diurnal rhythm with high levels at night in wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The RORbeta(C3H)(-/-) mice displayed motor ("duck gait," hind paw clasping reflex) and olfactory deficits, and reduced anxiety and learned helplessness-related behaviors. Circadian rhythms of wheel-running activity in all genotypes showed entrainment to the light-dark (LD) cycle, and free running in constant dark, with RORbeta(C3H)(-/-) mice showing a significant increase in circadian period (tau). Melatonin administration (90 microg/mouse sc for 3 days) at circadian time (CT) 10 induced phase advances, while exposure to a light pulse (300 lux) at CT 14 induced phase delays of circadian activity rhythms of the same magnitude in all genotypes. In RORbeta(C3H)(-/-) mice a light pulse at CT 22 elicited a larger phase advance in activity rhythms and a slower rate of reentrainment after a 6-h advance in the LD cycle compared with (+/+) mice. Yet, the rate of reentrainment was significantly advanced by melatonin administration at the new dark onset in both (+/+) and (-/-) mice. We conclude that the RORbeta nuclear receptor is not involved in either the rhythmic production of pineal melatonin or in mediating phase shifts of circadian rhythms by melatonin, but it may regulate clock responses to photic stimuli at certain time domains.     

Dim nighttime light impairs cognition and provokes depressive-like responses in a diurnal rodent

Circadian disruption is a common by-product of modern life. Although jet lag and shift work are well-documented challenges to circadian organization, many more subtle environmental changes cause circadian disruption. For example, frequent fluctuations in the timing of the sleep/wake schedule, as well as exposure to nighttime lighting, likely affect the circadian system. Most studies of these effects have focused on nocturnal rodents, which are very different from diurnal species with respect to their patterns of light exposure and the effects that light can have on their activity. Thus, the authors investigated the effect of nighttime light on behavior and the brain of a diurnal rodent, the Nile grass rat. Following 3 weeks of exposure to standard light/dark (LD; 14:10 light [~150 lux] /dark [0 lux]) or dim light at night (dLAN; 14:10 light [~150 lux] /dim [5 lux]), rats underwent behavioral testing, and hippocampal neurons within CA1, CA3, and the dentate gyrus (DG) were examined. Three behavioral effects of dLAN were observed: (1) decreased preference for a sucrose solution, (2) increased latency to float in a forced swim test, and (3) impaired learning and memory in the Barnes maze. Light at night also reduced dendritic length in DG and basilar CA1 dendrites. Dendritic length in the DG positively correlated with sucrose consumption in the sucrose anhedonia task. Nighttime light exposure did not disrupt the pattern of circadian locomotor activity, and all grass rats maintained a diurnal activity pattern. Together, these data suggest that exposure to dLAN can alter affective responses and impair cognition in a diurnal animal.     

On Melatonin Suppression from Polychromatic and Narrowband Light

This study investigates the relative strengths of food and light zeitgebers in synchronization of circadian rhythms of Indian weaver birds and the role of the pineal gland in food-induced synchronization of the circadian activity rhythms. Two experiments were performed. In the first experiment, six birds were concurrently exposed for 10 days to PA 12/12 (12 h food present: 12 h food absent) and LD 12/12 (12 h light: 12 h dark). Then, the PA 12/12 cycle was reversed: food was present during the dark period of the LD 12/12 cycle. After 15 days, birds were released into constant dim light (LL_dim). During exposure to overlapping light and food availability periods, birds were active only during the daytime. When light and food availability periods were presented in antiphase, two of six birds became night active. However, with the removal of the light zeitgeber (i.e., under LL_dim), all birds were synchronized with reversed PA 12/12; hence, they were active during the subjective night (i.e., the period corresponding to darkness [ZT12-0] of the preceding LD 12/12). The second experiment examined whether the pineal contributed to the food-induced synchronization. After two weeks of concurrent PA 12/12 and LD 12/12 exposure, six birds were released into LL_dim for 2.5 weeks. Under LL_dim, five of six birds were synchronized to PA 12/12 with the circadian period (tau, τ)?=?24 h. The LD 12/12 was restored, and after seven days, birds were pinealectomized (pinx). After 2.5 weeks, pinx birds were again released into LL_dim for 2.5 weeks. Under LL_dim, pinx birds did not become arrhythmic; instead, they appeared synchronized to PA 12/12 with τ?=?24 h (n?=?4) or ～24 h (n?=?2). We conclude that both food and light act as zeitgebers, although light appears to be the relatively stronger cue when the two are present together, as in the natural environment. We also found that the pineal is not necessary for food-induced synchronization. The findings suggest that food cycles could act as the synchronizer of circadian rhythmicity in biological functions in individuals held in an aperiodic environment.     

Food availability affects circadian clock-controlled activity and Zugunruhe in the night migratory male blackheaded bunting (Emberiza melanocephala)

This study investigated the functional linkage between food availability and activity behavior in the Palaearctic Indian night migratory blackheaded bunting (Emberiza melanocephala) subjected to artificial light-dark (LD) cycles. Two experiments were performed on photosensitive birds. In the first one, birds were exposed to short days (LD 10/14; Experiment 1A), long days (LD 13/11; Experiment 1B), or increasing daylengths (8 to 13?h light/d; Experiment 1C) and presented with food either for the whole or a restricted duration of the light period. In Experiments 1A and 1B, illumination of the light and dark periods or of the dark period, alone, was changed to assess the influence of the light environment on direct and circadian responses to food cycles. In the second experiment, birds were exposed to LD 12/12 or LD 8/16 with food availability overlapping with the light (light and food presence in phase) or dark period (light and food presence in antiphase). Also, birds were subjected to constant dim light (LL(dim)) to examine the phase of the activity rhythms under synchronizing influence of the food cycles. Similarly, the presentation of food ad libitum (free food; FF) during an experiment examined the effects of the food-restriction regimes on activity rhythms. A continuous measurement of the activity-rest pattern was done to examine both the circadian and direct effects of the food and LD cycles. Measurement of activity at night enabled assessment of the migratory phenotype, premigratory restlessness, or Zugunruhe. The results show that (i) light masked the food effects if they were present together; (ii) birds had a higher anticipatory activity and food intake during restricted feeding conditions; and (iii) food at night alone reduced both the duration and amount of Zugunruhe as compared to food during the day alone. This suggests that food affects both the daily activity and seasonal Zugunruhe, and food cycles act as a synchronizer of circadian rhythms in the absence of dominant natural environmental synchronizers, such as the light-dark cycle.     

Characteristics of the circadian activity rhythm in common marmosets (Callithrix j. jacchus)

The circadian activity rhythm of the common marmoset, Callithrix j. jacchus was investigated by long-term recording of the locomotor activity of 15 individuals (5 males, 10 females) from 1.5 to 8 years old, both under constant illumination and under LD 12:12. The mean period of the spontaneous circadian rhythm was 23.2 ± 0.3 h. Neither sex-specific differences nor a systematic influence of light intensity on the spontaneous period were observed, but the period was dependent on the duration of the trial and on the age of the individual. Due to the short spontaneous period, in LD 12:12 there was a distinct advance of the activity phase with respect to the light time and a masking of the true onset of activity by the inhibitory direct effect of low light intensity during the dark time. After an 8 h delay shift of the LD 12:12, re-entrainment of the circadian activity rhythm required an average of 6.8 ± 0.7 days; the average re-entrainment time after an 8 h phase advance of the LD cycle was 8.6 ± 1.3 day. This directional effect is ascribed to characteristics of the phase-response curve. No ultradian components were observed, either in the LD-entrained or the free-running circadian activity rhythm.