Functioning of the rat circadian system is modified by light applied in critical postnatal days

Lighting conditions influence biological clocks. The present experiment was designed to test the presence of a critical window of days during the lactation stage of the rat in which light has a decisive role on the development of the circadian system. Rats were exposed to 4, 8, or 12 days of constant light (LL) during the first days of life. Their circadian rhythm was later studied under LL and constant darkness. The response to a light pulse was also examined. Results show that the greater the number of LL days during lactation, the stronger the rhythm under LL and the smaller the phase shift due to the light pulse. These responses are enhanced when rats are exposed to LL days around postnatal day 12. A mathematical model was built to explain the responses of the circadian system with respect to the timing of LL during lactation, and we deduced that between postnatal days 10 to 20 there is a critical period of sensitivity to light; consequently, exposure to LL during this time modifies the circadian organization of the motor activity.     

Effect of light on the development of the circadian rhythm of motor activity in the mouse

In previous experiments, we found that rats raised in constant light (LL) manifested a more robust circadian rhythm of motor activity in LL and showed longer phase shifts after a light pulse in constant darkness (DD) than those raised under constant darkness. In addition, we observed that the effects produced by constant light differed depending on the time of postnatal development in which it was given. These results suggest that both sensitivity to light and the functioning of the circadian pacemaker of the rat could be affected by the environmental conditions experienced during postembryonic development. Thus, the present experiment aimed to study whether postnatal exposure to light could also affect the circadian system of the mouse. Three groups of mice were formed: One group was raised under constant darkness during lactation (DD group), the second under constant light (LL group), and the third under light-dark cycles (LD group). After lactation, the three groups were submitted first to constant light of high intensity, then to LD cycles, and finally to constant darkness. In the DD stage, a light pulse was given. Finally, mice were submitted to constant light of low intensity. We observed that the circadian rhythm of the DD group was more disturbed under constant light than the rhythm of the LL group, and that, when light intensity increased, the period of the rhythm of the DD group lengthened more than that of the LL group. No significant differences among the groups were found in the phase shift induced by the light pulse. Therefore, it appears that DD mice are more sensitive to light than their LL counterparts. However, at present there is no evidence to affirm that the light environment experienced by the mouse during postnatal development affects the circadian pacemaker. (Chronobiology International, 18(4), 683-696, 2001)     

EFFECT OF LIGHT ON THE DEVELOPMENT OF THE CIRCADIAN RHYTHM OF MOTOR ACTIVITY IN THE MOUSE

In previous experiments, we found that rats raised in constant light (LL) manifested a more robust circadian rhythm of motor activity in LL and showed longer phase shifts after a light pulse in constant darkness (DD) than those raised under constant darkness. In addition, we observed that the effects produced by constant light differed depending on the time of postnatal development in which it was given. These results suggest that both sensitivity to light and the functioning of the circadian pacemaker of the rat could be affected by the environmental conditions experienced during postembryonic development. Thus, the present experiment aimed to study whether postnatal exposure to light could also affect the circadian system of the mouse. Three groups of mice were formed: One group was raised under constant darkness during lactation (DD group), the second under constant light (LL group), and the third under light-dark cycles (LD group). After lactation, the three groups were submitted first to constant light of high intensity, then to LD cycles, and finally to constant darkness. In the DD stage, a light pulse was given. Finally, mice were submitted to constant light of low intensity. We observed that the circadian rhythm of the DD group was more disturbed under constant light than the rhythm of the LL group, and that, when light intensity increased, the period of the rhythm of the DD group lengthened more than that of the LL group. No significant differences among the groups were found in the phase shift induced by the light pulse. Therefore, it appears that DD mice are more sensitive to light than their LL counterparts. However, at present there is no evidence to affirm that the light environment experienced by the mouse during postnatal development affects the circadian pacemaker. (Chronobiology International, 18(4), 683–696, 2001)     

Effect of light during lactation on the phasic and tonic responses of the rat pacemaker

The circadian system in mammals generates endogenous circadian rhythms and entrains them to external cycles. Here, we examine whether the lighting conditions under which rats are reared affect the properties of the circadian pacemaker. We maintained three groups of rats under constant darkness (DD-rats), constant bright light (LL-rats) or light-dark cycles of 24 hours (LD-rats) during lactation. We then studied motor activity rhythm under constant light of four intensities, and under seven light-dark cycles with periods ranging between 22 and 27 hours. Results show that neither the tau nor the phase angle to the external cycle differed between groups. Differences were found in the amplitude of the circadian rhythm and in the number of rats that became arrhythmic under LL. We conclude that the light received during lactation affects the strength of the circadian pacemaker and its sensitivity to light.     

Developmental manipulation of the circadian pacemaker in the cockroach: relationship between pacemaker period and response to light

Abstract Previous research has shown that fundamental properties of the circadian pacemaker that drives the rhythm of locomotor activity in the cockroach Leucophaea maderae L. are permanently altered by exposure of animals to 22 or 26 h light cycles during post-embryonic development (Barrett & Page, 1989; Page & Barrett, 1989). The present results document differences between animals exposed to either constant darkness (DD) or constant light (LL) during postembryonic development in the free-running period, the phase shifting response to light pulses, and the response to an LL to DD transition of the adult pacemaker. In addition, the changes in pacemaker period and in the phase shifting response that result from raising animals in several different lighting conditions are shown to be strongly correlated. The data suggest there is a developmentally labile interdependence between the period of the pacemaker and its sensitivity to light.     

Melatonin administration modifies circadian motor activity under constant light depending on the lighting conditions during suckling

Early lighting conditions have been described to produce long-term effects on circadian behavior, which may also influence the response to agents acting on the circadian system. It has been suggested that melatonin (MEL) may act on the circadian pacemaker and as a scavenger of reactive oxygen and nitrogen species. Here, we studied the oxidative and behavioral changes caused by prolonged exposure to constant light (LL) in groups of rats that differed in MEL administration and in lighting conditions during suckling. The rats were exposed to either a light–dark cycle (LD) or LL. At 40 days old, rats were treated for 2 weeks with a daily subcutaneous injection of MEL (10?mg/kg body weight) or a vehicle at activity onset. Blood samples were taken before and after treatment, to determine catalase (CAT) activity and nitrite level in plasma. As expected, LL-reared rats showed a more stable motor activity circadian rhythm than LD rats. MEL treatment produced more reactivity in LD- than in LL rats, and was also able to alter the phase of the rhythm in LD rats. There were no significant differences in nitrite levels or CAT activity between the groups, although both variables increased with time. Finally, we also tested depressive signs by means of sucrose consumption, and anhedonia was found in LD males treated with MEL. The results suggest that the lighting conditions in early infancy are important for the long-term functionality of the circadian system, including rhythm manifestation, responses to MEL and mood alterations.     

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.     

The Manifestation of the Motor Activity Circadian Rhythm of Blinded Rats Depends on the Lighting Conditions During Lactation

The purpose of this experiment was to examine the effect of different lighting conditions during lactation on the functioning of the circadian pacemaker in the adult rat in absence of the retinal input. We reared one group of rats under constant light (LL-rats) and the other under constant darkness (DD-rats). After weaning they were placed under light-dark cycles of 24h period for 29 days to eliminate the aftereffects of the previous lighting. All the animals were then binocularly enucleated and motor activity was recorded. Results reveal that, before and after the enucleation, the expression of the circadian rhythm was stronger in DD- than in the LL-rats. Our results indicate that lighting conditions during lactation modify the functioning of the circadian pacemaker.     

The manifestation of the motor activity circadian rhythm of blinded rats depends on the lighting conditions during lactation

The purpose of this experiment was to examine the effect of different lighting conditions during lactation on the functioning of the circadian pacemaker in the adult rat in absence of the retinal input. We reared one group of rats under constant light (LL-rats) and the other under constant darkness (DD-rats). After weaning they were placed under light-dark cycles of 24h period for 29 days to eliminate the aftereffects of the previous lighting. All the animals were then binocularly enucleated and motor activity was recorded. Results reveal that, before and after the enucleation, the expression of the circadian rhythm was stronger in DD- than in the LL-rats. Our results indicate that lighting conditions during lactation modify the functioning of the circadian pacemaker.     

Splitting of the circadian rhythm of activity in hamsters: Effects of exposure to constant darkness and subsequent re-exposure to constant light

Summary The circadian rhythm of wheel running behavior was observed to dissociate into two distinct components (i.e. split) within 30 to 110 days in 56% of male hamsters exposed to constant light (Figs. 1–2). Splitting was abolished in all 16 animals that were transferred from constant light (LL) to constant darkness (DD) within 1–4 days of DD, and the components of the re-fused activity rhythm assumed a phase relationship that is characteristic of hamsters maintained in DD (Figs. 3–5). Re-fusion of the split activity rhythm was accompanied by a change in period (); in 14 animals increased while in the other 2 animals decreased after transfer to DD.After 10–30 days in DD, the hamsters were transferred back into LL at various time points throughout the circadian cycle. A few of these animals went through two or three LL to DD to LL transitions. The effect of re-exposure to LL was dependent on the phase relationship between the transition into LL and the activity rhythm. A rapid (i.e. 1–4 days) induction of splitting was observed in 7 of 9 cases when hamsters were transferred into LL 4–5 h after the onset of activity (Fig. 5). In the other 2 animals, the activity pattern was ultradian or aperiodic for 20 to 50 days before eventually coalescing into a split activity pattern. In contrast, transfer of animals (n = 13) from DD to LL at other circadian times did not result in the rapid induction of splitting and the activity rhythm continued to free-run with a single bout of activity (Fig. 5). Importantly, a transfer from DD to LL 4–5 h after the onset of activity did not induce splitting if the hamsters had not shown a split activity rhythm during a previous exposure to LL (n=10; Fig. 6).These studies indicate that transfer of split hamsters from LL to DD results in the rapid re-establishment of the normal phase relationship between the two circadian oscillators which underlie the two components of activity during splitting. In addition, there appears to be a history-dependent effect of splitting which renders the circadian system susceptible to becoming split again. The rapid re-initiation of the split condition upon transfer from DD to LL at only a specific circadian time is discussed in terms of the phase response curve for this species.Abbreviation PRC phase response curve This investigation was supported by NIH grants HD-09885 and HD-12622 from the National Institute of Child Health and Human Development and by a grant from the Whitehall FoundationRecipient of Research Career Development Award K04 HD-00249 from the National Institute of Child Health and Human Development     

Optic lobe circadian pacemaker sends its information to the contralateral optic lobe in the cricket Gryllus bimaculatus

The bilaterally paired optic lobe pacemakers of the cricket Gryllus bimaculatus are mutually coupled. In the present study we recorded the neural activity conveyed from the brain toward the optic lobe with a suction electrode to examine the coupling signals. The results demonstrated that the brain efferents to the optic lobe encode the circadian information: Both in constant light (LL) and constant darkness (DD), the neural activity of brain efferents showed a clear circadian rhythm with a nocturnal peak. Since the rhythm survived the severance of the contralateral optic nerve but disappeared when the contralateral optic lobe was removed, it is apparent that the rhythm originates from the contralateral optic lobe. The amplitude of the rhythm was greater in LL than in DD, suggesting that light affects the amplitude of the rhythm. This was confirmed by the fact that the light-induced response was under circadian control, being greater during the subjective night. These data suggest that the bilaterally paired optic lobe pacemakers exchange circadian information as well as light information. The data are also consistent with the results of previous behavioral experiment.Abbreviations DD constant darkness - LD light dark cycle - LL constant light     

Unequal coupling between locomotor pacemakers of the German cockroach,Blattella germanica (L.)

Male adult German cockroaches, Blattella germanica (L.), expressed robust locomotor circadian rhythmicity under 28 degrees C and constant darkness (DD) conditions. By surgically severing the connections between the optic lobes and midbrain and their subsequent regeneration, we demonstrated that the locomotor circadian pacemaker was located in the optic lobes and that it controlled the locomotor circadian rhythm through neural pathways. From the results that unilaterally optic tract severed males still showed locomotor circadian rhythmicity (93.1%, n=29) without significantly changing the circadian period (tau) or level of motor activity, we concluded that the right and left optic lobes each contain a circadian pacemaker competent to drive the locomotor circadian rhythm. These two pacemakers were strongly coupled since only one rhythm was expressed when the pacemakers were independently exposed to opposite lighting conditions (DD or LL) at the same time. However, an unequal contribution of each pacemaker in determining the overt circadian period was found under constant dim light (10 lux) conditions, revealing a major-minor coupling relationship between these two pacemakers, so that the unilaterally blinded male expressed either a LL-rhythm with a circadian period of 24.27+/-0.21 h (41.7%, n=24) or a DD-rhythm with a circadian period of 23.43+/-0.19 h (58.3%, n=24). However, higher intensity of photic information (200-300 lux) could overpower this relationship and cause the minor pacemaker to lead the rhythm.     

Developmental plasticity of the locomotor activity rhythm of Drosophila melanogaster

We used four replicate outbred populations of Drosophila melanogaster to investigate whether the light regimes experienced during the pre-adult (larval and pupal) and early adult stages influence the free-running period (τ_DD) of the circadian locomotor activity rhythm of adult flies. In a series of two experiments four different populations of flies were raised from egg to eclosion in constant light (LL), in light/dark (LD) 12:12 h cycle, and in constant darkness (DD). In the first experiment the adult male and female flies were directly transferred into DD and their locomotor activity was monitored, while in the second experiment the locomotor activity of the emerging adult flies was first assayed in LD 12:12 h for 15 days and then in DD for another 15 days. The τ_DD of the locomotor activity rhythm of flies that were raised in all the three light regimes, LL, LD 12:12 h and in DD was significantly different from each other. The τ_DD of the locomotor activity rhythm of the flies, which were raised in DD during their pre-adult stages, was significantly shorter than that of flies that were raised as pre-adults in LL regime, which in turn was significantly shorter than that of flies raised in LD 12:12 h regime. This pattern was consistent across both the experiments. The results of our experiments serve to emphasise the fact that in order to draw meaningful inferences about circadian rhythm parameters in insects, adequate attention should be paid to control and specify the environment in which pre-adult rearing takes place. The pattern of pre-adult and early adult light regime effects that we see differs from that previously observed in studies of mutant strains of D. melanogaster, and therefore, also points to the potential importance of inter-strain differences in the response of circadian organisation to external influences.     

The two-oscillator circadian system of tree shrews (Tupaia belangeri) and its response to light and dark pulses

The wheel-running activity rhythm of tree shrews (tupaias; Tupaia belangeri) housed in constant darkness (DD) phase-advanced following a 3-hr light pulse at circadian time (CT) 21. Dark pulses of 3 hr presented to tupaias in bright constant light (LL) did not induce significant phase shifts of the free-running activity rhythm, irrespective of the CT. In dim LL, tupaias showed simultaneous splitting of their circadian rhythm of wheel-running activity, nest-box activity, and feeding behavior. Light pulses of 6 hr and 2300 lux were presented to 13 tupaias with split wheel-running activity rhythms. These light pulses induced immediate phase shifts in the two components of the split rhythm in opposite directions. No differences were observed between the light-pulse phase response curves of the two components. Equally large immediate phase advances were induced in both components by light pulses of 230 lux, but not by 23 lux. The final phase shifts were small at all CTs. In two tupaias, activity rhythms transiently split and re-fused. Analysis of the relative position of the components in one of these indicates asymmetry in the coupling between the components.     

Circadian Intraocular Pressure Rhythms in Athletic Horses under Different Lighting Regime

The present study was undertaken to investigate the existence of intraocular pressure (IOP) rhythms in athletic thoroughbred horses maintained under a 24 h cycle of light and darkness (LD) or under constant light (LL) or constant dark (DD) conditions. We identified an IOP circadian rhythm that is entrained to the 24 h LD cycle. IOP was low during the dark phase and high during the light phase, with a peak at the end of the light phase (ZT10). The circadian rhythm of IOP persisted in DD (with a peak at CT9.5), demonstrating an endogenous component in IOP rhythm. As previously shown in other mammalian species, horse IOP circadian rhythmicity was abolished in LL. Because tonometry is performed in horses for the diagnosis of ophthalmologic diseases, such as glaucoma or anterior uveitis, the daily variation in IOP must be taken into account in clinical practice to properly time tests and to interpret clinical findings.     

Avian Melatonin Synthesis: Photic and Circadian Regulation of Serotonin N-Acetyltransferase mRNA in the Chicken Pineal Gland and Retina

Abstract: The circadian rhythms in melatonin production in the chicken pineal gland and retina reflect changes in the activity of serotonin N -acetyltransferase (arylalkylamine N -acetyltransferase; AA-NAT; EC 2.3.1.87). Here we determined that the chicken AA-NAT mRNA is detectable in follicular pineal cells and retinal photoreceptors and that it exhibits a circadian rhythm, with peak levels at night. AA-NAT mRNA was not detected in other tissues. The AA-NAT mRNA rhythm in the pineal gland and retina persists in constant darkness (DD) and constant lighting (LL). The amplitude of the pineal mRNA rhythm is not decreased in LL. Light appears to influence the phase of the clock driving the rhythm in pineal AA-NAT mRNA in two ways: The peak is delayed by ∼6 h in LL, and it is advanced by >4 h by a 6-h light pulse late in subjective night in DD. Nocturnal AA-NAT mRNA levels do not change during a 20-min exposure to light, whereas this treatment dramatically decreases AA-NAT activity. These observations suggest that the rhythmic changes in chicken pineal AA-NAT activity reflect, at least in part, clock-generated changes in mRNA levels. In contrast, changes in mRNA content are not involved in the rapid light-induced decrease in AA-NAT activity.     

Clock-controlled endogenous melatonin rhythms in Nile tilapia (Oreochromis niloticus niloticus) and African catfish (Clarias gariepinus)

The purpose of this work was to investigate the circadian melatonin system in two tropical teleost species characterized by different behavioral habits, Nile tilapia (diurnal) and African catfish (nocturnal). To do so, fish were subjected to either a control photoperiod (12L:12D), continuous light (LL) or darkness (DD), or a 6L:6D photoperiod. Under 12L:12D, plasma melatonin levels were typically low during the photophase and high during the scotophase in both species. Interestingly, in both species, melatonin levels significantly decreased prior to the onset of light, which in catfish reached similar basal levels to those during the day, demonstrating that melatonin production can anticipate photic changes probably through circadian clocks. Further evidence for the existence of such pacemaker activity was obtained when fish were exposed to DD, as a strong circadian melatonin rhythm was maintained. Such an endogenous rhythm was sustained for at least 18 days in Nile tilapia. A similar rhythm was shown in catfish, although DD was only tested for four days. Under LL, the results confirmed the inhibitory effect of light on melatonin synthesis already reported in other species. Finally, when acclimatized to a short photo-cycle (6L:6D), no endogenous melatonin rhythm was observed in tilapia under DD, with melatonin levels remaining high. This could suggest that the circadian clocks cannot entrain to such a short photocycle. Additional research is clearly needed to further characterize the circadian axis in teleost species, identify and localize the circadian clocks, and better understand the environmental entrainment of fish physiology.     

The intergeniculate leaflet partially mediates effects of light on circadian rhythms

Photic signals affect circadian activity rhythms by both phasic and tonic mechanisms that modulate pacemaker phase and period. In mammals, the effects of light on circadian activity are mediated by the retina, which communicates with the suprahiasmatic nucleus (SCN) by two different anatomical routes: the retino-hypothalamic tract (RHT), originating in the retina, and the geniculo-hypothalamic tract (GHT), arising from a retino-recipient nucleus, the intergeniculate leaflet (IGL). We assessed the roles of these two afferent systems in mediating phasic and tonic effects of light on circadian activity in IGL-lesioned animals. Destruction of the IGL significantly affected phase shifts produced by brief light pulses (phasic effect) and modified the change in period (tau) of the free-running activity rhythm produced by changing the level of constant light (LL) (tonic effect). Phase advances produced by brief light pulses were decreased in amplitude while phase delays were increased in IGL-lesioned animals as compared to controls. The free-running period in constant dark (tau DD) of IGL-lesioned animals was greater than tau DD of controls, and the lengthening of tau normally produced by LL was not observed or was greatly reduced in IGL-lesioned animals. Entrainment to light-dark cycles was unaffected by the lesions, as were other aspects of the circadian activity rhythm that normally change in response to LL (e.g., activity-rest ratio, total activity, splitting). Our data support the interpretation that the IGL plays a significant role in relaying information regarding illumination intensity to the SCN.     

Exposure to T‐Cycles of 22 and 23 h during Lactation Modifies the Later Dissociation of Motor Activity and Temperature Circadian Rhythms in Rats

Early environmental conditions may affect the development and manifestation of circadian rhythms. This study sought to determine whether the maintenance of rats under different T‐cycles during lactation influences the subsequent degree of dissociation of the circadian rhythms of motor activity and core body temperature. Two groups of 22 day‐old Wistar rats were kept after weaning under T‐cycles of 22 h (T22) or 23 h (T23) for 70 days. Subsequently, they were kept in constant darkness (DD). Half of the animals in each group were born and reared under these experimental conditions, while the other half were reared until weaning under 24 h LD cycles (T24). Rats transferred from T24 to T22 or T23 showed two circadian components in motor activity and temperature, one entrained by light and the other free‐running. In T22, there was also desynchronization between temperature and motor activity. Rats submitted to T23 from birth showed higher stability of the 23 h component than rats transferred from T24 to T23 after weaning. However, in comparison to rats born under T24 and subsequently changed to T22, animals submitted to T22 from birth showed shorter values of the period of the non‐light‐dependent component during T22, more aftereffects when transferred to DD, and a lack of desynchronization between motor activity and temperature. The results suggest that T‐cycles in the early environment may modify overt rhythms by altering the internal coupling of the circadian pacemaker.     

Constant light disrupts the circadian but not the circatidal rhythm in mangrove crickets

Mangrove crickets have a circatidal activity rhythm (~12.6 h cycles) with a circadian modulation under constant darkness (DD), whereby activity levels are higher during subjective night low tides than subjective day low tides. This study explored the locomotor activity rhythm of mangrove crickets under constant light (LL). Under LL, the crickets also exhibited a clear circatidal activity rhythm with a free-running period of 12.6 ± 0.26 h (mean ± SD, n = 6), which was not significantly different from that observed under DD. In contrast, activity levels were almost the same between subjective day and night, unlike those under DD, which were greater during subjective night. The loss of circadian modulation under LL may be explained by the suspension of the circadian clock in these conditions. These results strongly suggest that the circatidal activity rhythm is driven by its own clock system, distinct from the circadian clock.