Pigment-dispersing hormone-immunoreactive neurons in the cockroach Leucophaea maderae share properties with circadian pacemaker neurons

Neurons immunoreactive with antisera against the crustacean peptide -pigment dispersing hormone fullfill several anatomical criteria proposed for circadian pacemakers in the brain of the cockroach Leucophaea maderae. These include position of somata, projections to the lamina and midbrain and possible coupling pathways between the two pacemakers through commissural fibers. In behavioral experiments combined with lesion studies and immunocytochemical investigations we examined whether the presence of pigment-dispersing hormone-immunoreactive arborizations in the midbrain of the cockroach correlates with the presence of circadian locomotor activity. No rhythm was detected after severing both optic stalks in any animal for at least 12 days. Within the same time pigment-dispersing hormone-immunoreactive fibers in the midbrain disappeared. Two to seven weeks after the operation some of the cockroaches regained circadian locomotor activity, while others remained arrhythmic. In all cockroaches which regained rhythmic behavior pigment-dispersing hormone-immunoreactive fibers had regenerated and had largely found their original targets within the brain. In all arrhythmic cockroaches either none or very little regeneration had occurred. The period of the regained circadian activity inversely correlated with the number of regenerated immunoreactive commissural fibers. These data provide further evidence for the involvement of pigment-dispersing hormone-immunoreactive neurons in circadian clocks of orthopteroid insects.Abbreviations DD constant darkness - PDH pigment-dispersing hormone - PDHLI pigmentdispersing hormone-like immunoreactivity - PDFL a pigment-dispersing factor containing cells in the lamina - PDFMe pigment-dispersing factor containing cells in the medulla - QV quantification value     

Complete suprachiasmatic lesions eliminate circadian rhythmicity of body temperature and locomotor activity in golden hamsters

The effects of suprachiasmatic and control lesions on the circadian rhythms of locomotor activity and body temperature were studied in golden hamsters (Mesocricetus auratus) maintained in constant light as well as constant darkness. Large suprachiasmatic lesions, but not control lesions, eliminated circadian rhythmicity in locomotor activity as well as in body temperature. Analysis of the robustness of the rhythms of locomotor activity and body temperature in unlesioned and lesioned animals suggests that, because body temperature rhythmicity is more robust than locomotor rhythmicity, lesions that spare a small number of suprachiasmatic cells might abolish the latter but not the former. Our results do not support the hypothesis that the body temperature rhythm is controlled by a circadian pacemaker distinct from the main pacemaker located in the suprachiasmatic nuclei.     

Aging alters properties of the circadian pacemaker controlling the locomotor activity rhythm in males of Drosophila nasuta

Effects of aging on the circadian rhythm of locomotor activity in males of Drosophila nasuta were investigated. The adult life of males was divided in 1-3 stages according to spontaneous changes in free-running period x in constant darkness (DD): stage 1, days 1-19; stage 2, days 20-36; stage 3, days 37-43. Stage 1 was characterized by a bimodal activity pattern with a short light-induced morning peak and a prolonged evening peak when the flies were entrained to light-dark cycles of 12 hours of light, 12 hours of darkness (LD 12:12). The morning peak had a phase angle difference Ψ_m (Ψ, the time from lights on in LD 12:12 cycles to the onset of morning peak) of about 0.1h, while Ψ_e (Ψ of evening peak) was about 9h at stage 1. The transient morning peak was curtailed at the end of stage 1. At stage 2, the Ψ_e was about 10h, and the activity end was delayed by an addition of about 3h of activity in the scotophase. The changes in W during DD free runs were determined in two groups of flies: flies reared in LD 12:12 and flies reared in DD. In both groups, W increased from about 23h at stage 1 to about 25h at stage 2. Stage 3 was characterized by arrhythmicity associated with highest mean activity level (total number of passes/fly/day) in the entrained and both free-running groups. The mean activity level increased significantly from stage 1 to stage 3 in all three groups of flies.     

Pineal-dependent locomotor activity of lamprey,Lampetra japonica,measured in relation to LD cycle and circadian rhythmicity

Summary Locomotor activity of the river lamprey, Lampetra japonica, was investigated under a light-dark (LD 1212) cycle and under continuous dark conditions. Intact lampreys were entrained to the light:dark cycle. They were active mainly in the early half of the dark period and inactive in light period. The light:dark entrainment continued in 72.7% of lampreys after the removal of bilateral eyes, but additional pinealectomy made the entrainment disappear in all lampreys. When lampreys were pinealectomized with their eyes intact, light: dark entrainment was abolished in most cases. The results indicate that the pineal organ of the lamprey is a photoreceptive organ responsible for synchronizing locomotor activity to LD cycle. Under continuous dark conditions, the locomotor activity began to free-run with a period of 21.3 ± 0.9 h (mean ± SD, n = 53). This circadian rhythmicity was not affected by the removal of lateral eyes but was abolished by pinealectomy. The pineal organ appears to function as an oscillator, or as one of the oscillators, for the circadian locomotor rhythm of lampreys.Abbreviations DD continuous dark - LD light:dark     

Juvenile hormone and circadian locomotor activity in the honey bee Apis mellifera

Age-related division of labor in honeybees is associated with plasticity in circadian rhythms. Young nest bees care for brood around the clock with no circadian rhythms while older foragers have strong circadian rhythms that are used for sun compass navigation and for timing visits to flowers. Since juvenile hormone (JH) is involved in the coordination of physiological and behavioral processes underlying age-related division of labor in honey bees, we tested the hypothesis that JH influences the ontogeny of circadian rhythms and other clock parameters in young worker bees. Treatments with the JH analog methoprene or allatectomy did not influence the onset of rhythmicity, overall locomotor activity, or the free-running period of rhythmic locomotor behavior. There were, however, significant differences in the onset of rhythmicity, overall locomotor activity, and longevity between bees from different source colonies, suggesting that there is significant genetic variation for these traits. Our results suggest that JH does not coordinate all aspects of division of labor in bees and that coordination of task performance with circadian rhythms is probably mediated by other regulatory systems.     

The role of the pineal and the retinae in the expression of circadian locomotor rhythmicity in the ruin lizard,Podarcis sicula

Summary A marked interspecific variability in the role played by the pineal and the retinae characterizes the circadian system of lizards. I examined the role played by these structures in a new model species, the ruin lizard, Podarcis sicula. In constant temperature and darkness pinealectomy as well as bilateral removal of the retinae produced significant changes (both lengthening and shortening) in the freerunning period of locomotor rhythms. Circadian activity time was also affected by pinealectomy. Circadian locomotor rhythmicity persisted in all cases even when both operations were combined in the same individuals. This demonstrates in Podarcis sicula the existence of an oscillatory system outside the pineal and the retinae which can drive locomotor rhythms. The period changes recorded after pinealectomy as well as after bilateral removal of the retinae specifically suggest that both the pineal and the retinae play a modulating role on circadian oscillators located elsewhere in the system, with the final effect of stabilizing the overt rhythms.Abbreviations DD constant darkness - LL constant light - PIN-X pinealectomy - RET-X bilateral removal of the retinae - SHAM sham pinealectomy - circadian activity time - freerunning circadian period     

Neural organization of the circadian system of the cockroach Leucophaea maderae

The cockroach Leucophaea maderae was the first animal in which lesion experiments localized an endogenous circadian clock to a particular brain area, the optic lobe. The neural organization of the circadian system, however, including entrainment pathways, coupling elements of the bilaterally distributed internal clock, and output pathways controlling circadian locomotor rhythms are only recently beginning to be elucidated. As in flies and other insect species, pigment-dispersing hormone (PDH)-immunoreac- tive neurons of the accessory medulla of the cockroach are crucial elements of the circadian system. Lesions and transplantation experiments showed that the endogeneous circadian clock of the brain resides in neurons associated with the accessory medulla. The accessory medulla is organized into a nodular core receiving photic input, and into internodular and peripheral neuropil involved in efferent output and coupling input. Photic entrainment of the clock through compound eye photoreceptors appears to occur via parallel, indirect pathways through the medulla. Light-like phase shifts in circadian locomotor activity after injections of γ-aminobutyric acid (GABA)- or Mas-allatotropin into the vicinity of the accessory medulla suggest that both substances are involved in photic entrainment. Extraocular, cryptochrome-based photoreceptors appear to be present in the optic lobe, but their role in photic entrainment has not been examined. Pigment-dispersing hormone-immunoreactive neurons provide efferent output from the accessory medulla to several brain areas and to the peripheral visual system. Pigment-dispersing hormone-immunoreactive neurons, and additional heterolateral neurons are, furthermore, involved in bilateral coupling of the two pacemakers. The neuronal organization, as well as the prominent involvement of GABA and neuropeptides, shows striking similarities to the organization of the suprachiasmatic nucleus, the circadian clock of the mammalian brain.     

Silencing the circadian clock gene Clock using RNAi reveals dissociation of the circatidal clock from the circadian clock in the mangrove cricket

Whether a clock that generates a circatidal rhythm shares the same elements as the circadian clock is not fully understood. The mangrove cricket, Apteronemobius asahinai, shows simultaneously two endogenous rhythms in its locomotor activity; the circatidal rhythm generates active and inactive phases, and the circadian rhythm modifies activity levels by suppressing the activity during subjective day. In the present study, we silenced Clock (Clk), a master gene of the circadian clock, in A. asahinai using RNAi to investigate the link between the circatidal and circadian clocks. The abundance of Clk mRNA in the crickets injected with double-stranded RNA of Clk (dsClk) was reduced to a half of that in control crickets. dsClk injection also reduced mRNA abundance of another circadian clock gene period (per) and weakened diel oscillation in per mRNA expression. Examination of the locomotor rhythms under constant conditions revealed that the circadian modification was disrupted after silencing Clk expression, but the circatidal rhythm remained unaffected. There were no significant changes in the free-running period of the circatidal rhythm between the controls and the crickets injected with dsClk. Our results reveal that Clk is essential for the circadian clock, but is not required for the circatidal clock. From these results we propose that the circatidal rhythm of A. asahinai is driven by a clock, the molecular components of which are distinct from that of the circadian clock.     

Unique self-sustaining circadian oscillators within the brain of Drosophila melanogaster

In Drosophila circadian rhythms persist in constant darkness (DD). The small ventral Lateral Neurons (s-LN_v) mainly control the behavioral circadian rhythm in consortium with the large ventral Lateral Neurons (l-LN_v) and dorsal Lateral Neurons (LN_d). It is believed that the molecular oscillations of clock genes are the source of this persistent behavior. Indeed the s-LN_v, LN_d, Dorsal Neurons (DN)-DN₂ and DN₃ displayed self-sustained molecular oscillations in DD both at RNA and protein levels, except the DN₂ oscillates in anti-phase. In contrast, the l-LN_v and DN₁ displayed self-sustained oscillations at the RNA level, but protein oscillations quickly dampened. Having self-sustained and dampened molecular oscillators together in the DN groups suggested that they play different roles. However, all DN groups seemed to contribute together to the light-dark (LD) behavioral rhythm. The LD entrainment of LN oscillators is achieved through Rhodopsin (RH) and Cryptochrome (CRY). CRY's expression in all DN groups implicates also its role in LD entrainment of DN, like in DN₁. However, mutations in cry and glass that did not inflict LD synchronization of the DN₂, DN₃ oscillator implicate the existence of a novel photoreceptor at least in DN₃.     

Internal desynchronization of the circadian activity and feeding rhythm in an owl monkey (Aotus lemurinus griseimembra): a case study

In the free-running circadian locomotor activity rhythm of a 7-year-old male owl monkey (Aotus lemurinus griseimembra) kept under constant light and climatic conditions (LL 0.2 lux, 25°C ± 1°C, 60 ± 5% relative humidity [RH]), a second rhythm component developed that showed strong relative coordination with the free-running activity rhythm of 24.4h and a 24h rhythm. The simultaneously recorded feeding activity rhythm strongly resembled this rhythm component. Therefore, it seems justified to infer that there was an internal desynchronization between the two behavioral rhythms or their circadian pacemakers, that is, between the light-entrainable oscillator located in the suprachiasmatic nuclei (SCN) and a food-entrainable oscillator located outside the SCN. This internal desynchronization may have been induced and/or maintained by a zeitgeber effect of the (irregular) 24h feeding schedule on the food-entrainable oscillator. The weak relative coordination shown by the activity rhythm indicates a much weaker coupling of the light-entrainable oscillator to the food-entrainable oscillator than vice versa. (Chronobiology International, 17(2), 147-153, 2000)     

Feeding entrainment of locomotor activity rhythms in the goldfish is mediated by a feeding-entrainable circadian oscillator

Periodic food availability can act as a potent zeitgeber capable of synchronizing many biological rhythms in fishes, including locomotor activity rhythms. In the present paper we investigated entrainment of locomotor rhythms to scheduled feeding under different light and feeding regimes. In experiment 1, fish were exposed to a 12:12?h light/dark cycle and fed one single daily meal in the middle of the light phase. In experiment 2, we tested the effect of random versus scheduled feeding on the daily distribution of activity. During random feeding, meals were randomly scheduled with intervals ranging from 12 to 36?h, while scheduled feeding consisted of one single daily meal set in the middle of the light or dark phase. Finally, in experiment 3, we studied the synchronization of activity rhythms to feeding under constant darkness (DD) and after shifting the feeding cycle by either advancing or delaying the feeding cycle by 9?h. The results revealed that goldfish synchronized to feeding, overcame light entrainment and significantly changed their daily distribution of activity according to their feeding schedule. In addition, the daily activity pattern modulated by feeding differed between layers: a peak of activity being noticeable directly after feeding at the bottom, while an anticipatory behaviour was obvious at the surface of the tank. Under DD and no food, free-running rhythms averaging 25.5?± 1.9?h (mean?±?SD) were detected. In conclusion, some properties of feeding entrainment (e.g. anticipation of the feeding time, free-running rhythms following termination of periodic feeding, and the stability of ø after shifting the feeding cycle) suggested that goldfish have (a) separate but tightly coupled light- and food-entrainable oscillators, or (b) a single oscillator that is entrainable by both light and food (one synchronizer being eventually stronger than the other).     

The Dual-Oscillator System of Drosophila melanogaster Under Natural-Like Temperature Cycles

Dual-oscillator systems that control morning and evening activities can be found in a wide range of animals. The two coupled oscillators track dawn and dusk and flexibly adapt their phase relationship to seasonal changes. This is also true for the fruit fly Drosophila melanogaster that serves as model organism to understand the molecular and anatomical bases of the dual-oscillator system. In the present study, the authors investigated which temperature parameters are crucial for timing morning and evening activity peaks by applying natural-like temperature cycles with different daylengths. The authors found that the morning peak synchronizes to the temperature increase in the morning and the evening peak to the temperature decrease in the afternoon. The two peaks did not occur at fixed absolute temperatures, but responded flexibly to daylength and overall temperature level. Especially, the phase of the evening peak clearly depended on the absolute temperature level: it was delayed at high temperatures, whereas the phase of the M peak was less influenced. This suggests that the two oscillators have different temperature sensitivities. The bimodal activity rhythm was absent in the circadian clock mutants Clk^Jrk and cyc⁰¹ and reduced in per⁰¹ and tim⁰¹ mutants. Whereas the activity of Clk^Jrk mutants just followed the temperature cycles, that of per⁰¹ and tim⁰¹ mutants did not, suggesting that these mutants are not completely clockless. This study revealed new characteristics of the dual-oscillator system in Drosophila that were not detected under different photoperiods. (Author correspondence: yoshii@cc.okayama-u.ac.jp)     

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

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

Dissociation between the circadian rhythm of locomotor activity and the pineal clock in the Japanese newt

The circadian locomotor activity rhythm of the Japanese newt has been thought to be driven by a putative brain oscillator(s) subordinate to the pineal clock. The existence of mutual coupling between the pineal clock and the brain oscillator(s) in vivo was examined. We covered the newt's skull with aluminum foil and simultaneously reversed the light-dark cycle, thereby allowing the pineal organ to be exposed to constant darkness while the rest of the animal was exposed to the reversed light-dark cycle. In control animals, whose heads were covered with transparent plastic, the rhythm of synaptic ribbon number in the pineal photoreceptor cells was entrained to the reversed light-dark cycle. Rhythms from newts whose heads were shielded, however, were similar to those observed in the unoperated newts kept under constant darkness. The locomotor activity rhythms of both head-covered animals and control animals were entrained to the reversed light-dark cycle. These data suggest that extrapineal photoreception can entrain the putative brain oscillator(s), but not the pineal clock. Thus, at least in an aspect of photic entrainment, there seems to be little or no mutual coupling between the pineal clock and the putative brain oscillator(s) in the circadian system of the Japanese newt.Abbreviations LD light-dark - DD constant darkness - SCN suprachiasmatic nucleus - SR synaptic ribbon     

Analysis of coupling between optic lobe circadian pacemakers in the cricket Gryllus bimaculatus

The coupling mechanism between weakly coupled two optic lobe circadian pacemakers in the cricket Gryllus bimaculatus was investigated by recording the locomotor activity, under light-dark cycles with various lengths, after the optic nerve was unilaterally severed. The activity rhythm split into two components under the light cycles different from 24 h: one was readily entrained to the light cycle and the other only loosely entrained or freeran. Additional removal of the optic lobe on the intact side resulted in a loss of the entrained component and that on the blinded side caused the reverse effect, indicating that the entrained component was driven by the pacemaker on the intact side and the other by the one on the blinded side. The synchronization between the two components was achieved only in light cycles with a limited length between 23 and 25 h. Without this range, the desynchronization of the components occurred. In the split rhythm, the phase-dependent modulation of the period of freerunning component and the mutual suppression of locomotor activity during the subjective day phase were clearly observed. The suppression was also evident in the lights-on peak that was the masking effect of light. The light cycle with dim light significantly reduced the ratio of animals with the pacemaker coupling as well as the magnitude of the period modulation. These results suggest (1) that the mutual coupling is achieved only when the difference in the periods between the two pacemakers is within an allowable range, (2) that the photic information is also involved in the mechanism of mutual coupling, and (3) that the suppression of activity occurs at the regulatory center for locomotion.Abbreviations CT circadian time - DD constant darkness - LL constant light - LD light to dark cycle - T length of light to dark cycle - freerunning period     

Social stimuli fail to act as entraining agents of circadian rhythms in the golden hamster

Summary The ability of social stimuli to act as entraining agents of circadian rhythms was investigated in golden hamsters (Mesocricetus auratus). In a first experiment, pairs of male hamsters (one of them enucleated and the other intact) were maintained under a light-dark (LD) cycle with a period of 23.3 h. Running-wheel activity was recorded to determine the effect of social interaction on the free-running circadian rhythm of activity. In several pairs, general activity and body temperature were also recorded. In all pairs the intact animals entrained to the LD cycle, whereas the activity rhythms of the enucleated animals free-ran with periods of approximately 24 h and showed no apparent sign of synchronization or relative coordination with the other member of the pair. In a second experiment, male hamsters maintained in constant darkness received pulses of social interaction, which have been reported to induce phase shifts of the activity rhythm. Consistent phase shifts in the running-wheel activity rhythm were not induced by the social pulses in our experiment. These results suggest strongly that social stimuli are not effective entraining agents of circadian rhythms in the golden hamster.Abbreviations CT circadian time - LD light-dark     

A short red light pulse during dark phase of LD-cycle perturbs the hamster's circadian clock

In this study we investigated the influence of red light, which naturally occurs during dawn and dusk, on locomotor activity and body temperature rhythms of Djungarian hamsters (Phodopus sungarus). A single weak red light pulse given 2 h before regular lights on had acute as well as long-term effects persisting for several days following exposure. The hamsters immediately stopped their locomotor activity, accompanied by a drop in body temperature. In the following undisturbed nights (LD 168) the nocturnal activity stopped earlier than usual. This lasting effect of the light pulse was more pronounced than the acute effect. The activity phase compressed gradually during 3 to 5 days after the light pulse was administered while time of activity onset was almost unaffected. It took 6 to 11 days for complete recovery of the original activity phase. The maximal activity compression and the recovery period depended on the duration of the single red light pulse and its intensity. Red light pulses of 15 min duration were about twice effective as 1 min pulses; and the effect of a red light pulse of 130 mW/m² was about 1.5 times stronger than a 30 mW/m² red light pulse. The maximal value of activity phase compression reached in this experiment was 2.5+0.2 h with a recovery period of 11.1±0.3 days following a given red light pulse of 90 mW/m² and 15 min. The morning oscillator seems to be persistently affected. This indicates a very high photosensitivity of the Djungarian hamster's circadian system to red light.Abbreviations T _b body temperature - DD constant darkness - LD light:dark cycle - LL constant light - duration of activity phase - CT circadian time - PRC phase response curve - SCN suprachiasmatic nuclei     

Effects of prolonged exposure to darkness on circadian photic responsiveness in the mouse

Circadian rhythms in mammals are generated by an endogenous pacemaker but are modulated by environmental cycles, principally the alternation of light and darkness. Although much is known about nonparametric effects of light on the circadian system, little is known about other effects of photic stimulation. In the present study, which consists of a series of five experiments in mice, various manipulations of photic stimulation were used to dissect the mechanisms responsible for a variation in the magnitude of light-induced phase-shifts that results from prolonged exposure to darkness. The results confirmed previous observations that prolonged exposure to darkness causes an increase in the magnitude of phase shifts (both phase advances and phase delays) evoked by discrete light pulses. The results also indicated that the increase in responsiveness results from the lack of exposure to light per se and not from collateral effects of exposure to constant darkness such as the lack of previous entrainment. The lack of exposure to light causes the circadian system to undergo a process of dark adaptation similar to dark adaptation in the visual system but with a much slower temporal course. The results suggest that circadian dark adaptation may take place at the retinal level, but it is not clear whether it involves a change in the sensitivity or maximal responsiveness of the system.     

Pineal and retinal melatonin is involved in the control of circadian locomotor activity and body temperature rhythms in the pigeon

Summary Although pinealectomy or blinding resulted in loss of the clarity of the free-running rhythm of locomotor activity and body temperature and reduced the peak level of circulating melatonin rhythms to approximately a half in intact pigeons, neither pinealectomy nor blinding abolished any of these rhythms. However, when pinealectomy and blinding were combined, the rhythms of locomotor activity and body temperature disappeared in prolonged constant dim light, and melatonin concentration was reduced to the minimum level of detection. In order to examine the role of melatonin in the pigeon's circadian system, it was administered either daily or continuously to PX + EX-pigeons in LLdim. Daily administration of melatonin restored circadian rhythms of locomotor activity which entrained to melatonin injections, but continuous administration did not induce any remarkable change of locomotor activity. These results suggest that melatonin synthesized in the pineal body and the eye contributes to circulating melatonin and its rhythmicity is important for the control of circadian rhythms of locomotor activity and body temperature in the pigeon.Abbreviations LD Light-dark - LLdim constant dim light - LLbright constant bright light - PX pinealectomy - EX blinding - SCN suprachiasmatic nucleus     

Circadian organization in the ruin lizard Podarcis sicula: the role of the suprachiasmatic nuclei of the hypothalamus

The effects of electrolytic lesions to the suprachiasmatic nuclei of the hypothalamus (SCN) on circadian rhythms of locomotor activity were examined in ruin lizards Podarcis sicula maintained in constant darkness and constant temperature (29°C). All lizards (N=15) in which the lesion damaged 80% or more of the SCN became behaviorally arrhythmic. On the contrary, locomotor rhythms persisted in all cases (N=11) when the SCN remained intact and lesions were confined to neighbouring regions of the preoptic area. Taken together with previous work which demonstrates that the pineal and the retinae are not essential for the persistence of circadian locomotor rhythmicity in Podarcis sicula and with recent evidence showing the homology between the SCN of lizards and those of mammals the present results strongly support the view that the SCN of Podarcis sicula contain the primary pacemaker(s) for locomotor rhythms.Abbreviations DD constant darkness - LL constant light - SCN suprachiasmatic nuclei of the hypothalamus - PH nucleus periventricularis hypothalami - OC optic chiasm - te length of circadian activity - freerunning circadian period