Splitting of the Circadian Rhythm of Activity in Hamsters. In: Aschoff J, Daan S, Groos GA, editors. Vertebrate Circadian Systems. Berlin: Springer Berlin Heidelberg; 1982. pp. 203-14. [DOI: 10.1007/978-3-642-68651-1_22]

Melatonin administration: effects on rodent circadian rhythms

Previous research has demonstrated a lack of effect of pinealectomy upon the generation of rodent circadian activity rhythms and only a subtle effect upon their reentrainment after phase-shifts of the light-dark cycle. In contrast, our pharmacological studies on melatonin indicate that the pineal gland may be more important than hitherto believed. The main outcome of a preliminary pilot experiment on the effect of daily injections of melatonin, arginine vasotocin, melanocyte-stimulating hormone and a beta-blocker on rat free-running activity rhythms in constant darkness was that melatonin appeared to have entrainment properties. This was clearly demonstrated in a second experiment although entrainment did not occur until the onset of the activity rhythm coincided with the daily injection. In contrast, when melatonin was administered ad lib, in the drinking water to six rats housed in constant dim light, there was apparently a lengthening of the free-running period. The effects of 17 days of melatonin injections given at four different times of day to different groups of rats on re-entrainment of activity rhythms after a 5 h phase-advance of darkness were assessed. Results were confounded by the response of some control rats. However, after an 8 h advance of darkness and daily injection at the time of day of the previous dark onset, melatonin-injected rats phase-advanced, whereas vehicle-injected and uninjected control rats phase-delayed. Thus melatonin can alter the direction, but not necessarily the rate, of re-entrainment. The relevance of some of these findings with pharmacological doses of melatonin to the function of endogenous melatonin is discussed.

Plasticity of hamster circadian entrainment patterns depends on light intensity

The multiple oscillatory basis of the mammalian circadian pacemaker is adduced by, among other phenomena, the occurrence of split locomotor activity rhythms in rodents after prolonged exposure to constant light. More recently, split rhythms entrained to a 24h light:dark:light:dark cycle have been documented following scheduled access of hamsters to a novel running wheel or by photoperiod manipulations alone. Because the incidence of constant light-induced splitting depends on light intensity, the role of this variable was assessed in this new splitting paradigm. Male Syrian hamsters, entrained to a 14h light:10h dark cycle, were transferred to individual running wheel cages 7h after light onset. Transfer coincided with the beginning of the scotophase of a new photocycle alternating between 5h of relative dark and 7h of light. For four weeks bright photophases (approximately 350 lux) were alternated with either dim (< 0.1 lux) or completely dark (0 lux) scotophases. An additional group received moderate intensity photophases (approximately 45 lux) paired with dim scotophase illumination. For an additional four weeks, all hamsters were exposed to the same bright:dim light:dark cycle. Dim light in the scotophase significantly increased the incidence of split activity rhythms relative to that observed with completely dark scotophases. Overall wheel-running rates and activity induced by a cage change were also increased in dim light-exposed animals. Group differences largely disappeared four weeks later when hamsters previously maintained in completely dark scotophases were exposed to dim scotophases. Photophase light intensity did not affect the overall incidence of splitting, but influenced the timing of activity in the afternoon scotophase. The effects of dim illumination may be mediated in part via enhanced locomotor responses to transfer to a new cage or by changes in coupling interactions between component oscillators.

Entrainment of circadian locomotor activity rhythm of the nocturnal field mouse Mus booduga using daily injections of melatonin

In this paper, we report the effects of daily injections of melatonin on the locomotor activity rhythm of the nocturnal field mouse Mus booduga. The locomotor activity rhythm of 45 animals was first monitored in constant darkness (DD) of the laboratory for about 15 days. The animals were then divided into three groups (experimental, vehicle-treated control, and the nontreated control groups) and subjected to three different treatments. The animals from the experimental group (n=19) were administered daily a single subcutaneous (s.c.) injection of melatonin (1 mg/kg) for about 45 days. The vehicle treated controls (n=13) were administered daily injections of 50% dimethyl sulfoxide (DMSO) for about 45 days, and the nontreated controls (n=13) were handled similar to the other two groups without being administered injections. Following the treatments, the animals were maintained in DD for about 20 days, after which the experiments were terminated. A significantly larger percentage of animals from the experimental group either entrained or showed phase control to daily treatments, compared to the animals from the two control groups. These results suggest that externally administered melatonin can influence the phase of the circadian locomotor activity rhythm of M. booduga. The fact that none of the nontreated controls showed any sign of phase control to daily handling, clearly demonstrates that the entrainment or phase control in the melatonin treated group of animals is caused by melatonin alone and not due to handling.

Nocturnal illumination maintains reproductive function and simulates the period-lengthening effect of constant light in the mature male Djungarian hamster (Phodopus sungorus)

Mature male Djungarian hamsters (Phodopus sungorus) were placed in individual light-tight, sound attenuated chambers and exposed to one of four lighting conditions for a duration of approximately seven weeks. The four lighting conditions were: constant light (LL); constant dark (DD); feedback lighting (LDFB; a condition that illuminates the cage in response to locomotor activity); or a feedback lighting neighbor control (LDFB NC; the animal receives the same light pattern as a paired animal in feedback lighting, but has no control over it). Exposure of hamsters to LL or LDFB produced significantly and similarly longer free-running periods of the locomotor activity rhythm than exposure of animals to DD. Hamsters exposed to LDFB NC did not free-run or entrain, but rather displayed "relative coordination". The paired testes and sex accessory glands weights suggest that in the Djungarian hamster, LL and LDFB exposed animals maintained reproductive function, whereas DD exposed animals did not. Animals exposed to LDFB NC had intermediate paired testes weights. Since several previous studies have demonstrated that short pulses of light, which are coincident with the subjective night, are photostimulatory, it is not surprising that LDFB maintained reproductive function in the mature Djungarian hamster. Feedback lighting, however, has been shown to be an insufficient stimulus to maintain reproductive function of mature male and female Syrian hamsters, and to the reproductive maturation of immature Djungarian hamsters. The results suggest that there may be slight, but significant differences in the way these two species interpret photoperiod, as well as a developmental change in the photoperiodic response of Djungarian hamsters.

The clocks controlling the tide-associated rhythms of intertidal animals

The living clock that governs tide-associated organismic rhythms has previously been assumed to have a fundamental period of approximately 12.4 h, an interval that reflects the average period of the ebb and flow of the tide. But, in 1986, marine chronobiologists began to accumulate laboratory results that could not be explained by the action of such a clock. Prime among these findings was the discovery that, occasionally, one of the two daily peaks in an organism's rhythm assumed a different period from its partner. Similar results have since been observed in a host of different organisms. These data led to the circalunidian-clock hypothesis that envisions two basic 24.8 h clocks, coupled together in antiphase, as the driving force for these rhythms. There is, however, only a slight difference (50 minutes) in running times between a solar-day clock with a period of approximately 24 h and a lunar-day clock with a period of approximately 24.8 h, both of which display "circa" periods that overlap. Here, I postulate that the two clocks are fundamentally one and the same. BioEssays 22:32-37, 2000.

Melatonin's role in vertebrate circadian rhythms

The circadian secretion of melatonin by the pineal gland and retinae is a direct output of circadian oscillators and of the circadian system in many species of vertebrates. This signal affects a broad array of physiological and behavioral processes, making a generalized hypothesis for melatonin function an elusive objective. Still, there are some common features of melatonin function. First, melatonin biosynthesis is always associated with photoreceptors and/or cells that are embryonically derived from photoreceptors. Second, melatonin frequently affects the perception of the photic environment and has as its site of action structures involved in vision. Finally, melatonin affects overt circadian function at least partially via regulation of the hypothalamic suprachiasmatic nucleus (SCN) or its homologues. The mechanisms by which melatonin affects circadian rhythms and other downstream processes are unknown, but they include interaction with a class of membrane-bound receptors that affect intracellular processes through guanosine triphosphate (GTP)-binding protein second messenger systems. Investigation of mechanisms by which melatonin affects its target tissues may unveil basic concepts of neuromodulation, visual system function, and the circadian clock.

Circadian rhythms in mouse suprachiasmatic nucleus explants on multimicroelectrode plates

The suprachiasmatic nucleus (SCN) of the mammalian hypothalamus functions as a circadian pacemaker. This study used multimicroelectrode plates to measure extracellular action potential activity simultaneously from multiple sites within the cultured mouse SCN. Neurons within the isolated mouse SCN expressed a circadian rhythm in spontaneous firing rate for weeks in culture.

The circadian rhythm of thermoregulation in Japanese quail. II. Multioscillator control

Most biochemical, physiological, and behavioral processes in vertebrates show significant daily rhythms. Under constant conditions, these rhythms exhibit an endogenous periodicity around 24 h showing that they are driven by an internal circadian clock. In Japanese quail, the circadian clock driving activity and body temperature rhythms is functionally organized as a dual-oscillator system. Under certain conditions, such as switching birds from light:dark (LD) 12:12 to continuous darkness (DD), the body temperature rhythm splits into two circadian components that free-run independently before recoupling in a normal phase-relationship. The behavior of the activity rhythm parallels that of the body temperature rhythm, supporting the hypothesis that both rhythms are driven by the same set of oscillators. In some instances, recoupling fails to occur and birds continue to exhibit two circadian components that free-run independently. Dual-oscillator control of body temperature was observed in normal birds, pinealectomized birds, and optic nerve sectioned birds. However, birds were rendered arrhythmic by complete eye removal. It is proposed that the central circadian system (suprachiasmatic nuclei?) acts as a complex pacemaker that is functionally organized as two sets of oscillators and that circadian input from the eyes is necessary to preserve the integrity of this complex pacemaker.

Rubidium chloride fuses split circadian activity rhythms in hamsters housed in bright constant light

Chronotypic effects of rubidium (Rb) were examined in hamsters whose circadian activity rhythms had split into two components while they were housed in bright constant light. Seven of 12 hamsters receiving RbCl in drinking water for 10 weeks showed fusing of the components into an intact rhythm compared with none of 7 control hamsters (p = 0.016). Rb may modify coupling between circadian oscillators via reduced photic input to the suprachiasmatic nuclei. Alternative mechanisms include changes in potassium metabolism or endocrine function or behavioral changes that in turn alter circadian function. This normalization of a circadian anomaly by a putative antidepressant suggests that Rb may be valuable in strengthening coupling between oscillators in cases of human chronopathology, including those implicated in the etiology of some affective disorders.

Expression of circadian rhythmicity in Djungarian hamsters under constant light: effects of light intensity and the circadian system's state

Djungarian hamsters (Phodopus sungorus), were exposed to constant light with increasing intensities (20, 60, 350 lux), and wheel running activity was recorded. With increasing light intensity the percentage of hamsters showing a split in their daily activity pattern increased and the free running period was lengthened for both the unsplit and the split state. The fact that the free running period of both states depended on the light intensity together with the observation that the highest incidence of a circadian activity occurred under 350 lux, provoked the idea that the emergence of splitting or a circadian rhythmicity is a direct consequence of the light induced lengthening of the free running period. However, analysis of the data failed to support the idea that emergence of a split or a circadian activity is a threshold phenomenon with respect to the free running period. Due to differences in circadian function some Djungarian hamsters do not exhibit photoinduction following short day exposure. In these individuals splitting also occurred but required exposure to a higher light intensity than in photo-responsive hamsters. This observation is in accordance with the idea that the two phenotypes differ in the interaction of the two component oscillators underlying circadian rhythmicity.

Contributions made to chronobiology by studies of fiddler crab rhythms

One of the classic organisms used in chronobiological research is the fiddler crab (genus Uca), an animal unique in that it displays both circadian and tidal (i.e., circalunidian) rhythms. The pioneering work on this animal helped produce the early evidence for many of the standard properties now recognized for all circadian rhythms: near temperature independence of the period, phase lability and setability, the light and temperature sensitivity rhythms expressed by phase response curves, and the persistence of rhythms in organs isolated from a multicellular animal. Importantly, results arising from studies of this crab--and a few other organisms--resulted in the development of the exogenous timing hypothesis. While philosophically sound, the lack of supporting evidence for this hypothesis has resulted in it being discarded by most chronobiologists; but while still in its prime, it drew great interest, and therefore grant support, to the field in general, stimulated a great deal of research that otherwise might not have been performed, and resulted in the discovery of environmental stimuli previously unsuspected to influence organisms. As could be expected, continuing work with this crab, using modern approaches and statistical techniques, has modified earlier findings and interpretations, has revealed new properties, and has resulted in the creation of new hypotheses. The review and update is a synthesis of 45 years of this work.

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.

Melatonin and circadian control in mammals

Although pinealectomy has little influence on the circadian locomotor rhythms of laboratory rats, administration of the pineal hormone melatonin has profound effects. Evidence for this comes from studies in which pharmacological doses of melatonin are administered under conditions of external desynchronization, internal desynchronization, steady state light-dark conditions, and phase shifts of the zeitgeber. Taken together with recent findings on melatonin receptor concentration in the rat hypothalamus, particularly at the level of the suprachiasmatic nuclei, these results suggest that melatonin is a potent synchronizer of rat circadian rhythms and has a direct action on the circadian pacemaker. It is possible, therefore, that the natural role of endogenous melatonin is to act as an internal zeitgeber for the total circadian structure of mammals at the level of cell, tissue, organ, whole organism and interaction of that organism with environmental photoperiod changes.

Effects of constant darkness and constant light on circadian organization and reproductive responses in the ram

The relationship between circadian rhythms in the blood plasma concentrations of melatonin and rhythms in locomotor activity was studied in adult male sheep (Soay rams) exposed to 16-week periods of short days (8 hr of light and 16 hr of darkness; LD 8:16) or long days (LD 16:8) followed by 16-week periods of constant darkness (dim red light; DD) or constant light (LL). Under both LD 8:16 and LD 16:8, there was a clearly defined 24-hr rhythm in plasma concentrations of melatonin, with high levels throughout the dark phase. Periodogram analysis revealed a 24-hr rhythm in locomotor activity under LD 8:16 and LD 16:8. The main bouts of activity occurred during the light phase. A change from LD 8:16 to LD 16:8 resulted in a decrease in the duration of elevated melatonin secretion (melatonin peak) and an increase in the duration of activity corresponding to the changes in the ratio of light to darkness. In all rams, a significant circadian rhythm of activity persisted over the first 2 weeks following transfer from an entraining photoperiod to DD, with a mean period of 23.77 hr. However, the activity rhythms subsequently became disorganized, as did the 24-hr melatonin rhythms. The introduction of a 1-hr light pulse every 24 hr (LD 1:23) for 2 weeks after 8 weeks under DD reinduced a rhythm in both melatonin secretion and activity: the end of the 1-hr light period acted as the dusk signal, producing a normal temporal association of the two rhythms. Under LL, the 24-hr melatonin rhythms were disrupted, though several rams still showed periods of elevated melatonin secretion. Significant activity rhythms were either absent or a weak component occurred with a period of 24 hr. The introduction of a 1-hr dark period every 24 hr for 2 weeks after 8 weeks under LL (LD 23:1) failed to induce or entrain rhythms in either of the parameters. The occurrence of 24-hr activity rhythm in some rams under LL may indicate nonphotoperiodic entrainment signals in our experimental facility. Reproductive responses to the changes in photoperiod were also monitored. After pretreatment with LD 8:16, the rams were sexually active; exposure to LD 16:8, DD, or LL resulted in a decline in all measures of reproductive function. The decline was slower under DD than LD 16:8 or LL.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of circadian function in Djungarian hamsters insensitive to short day photoperiod

Djungarian hamsters (Phodopus sungorus sungorus) depend mainly on day length to cue seasonal adjustments. However, not all individuals respond to short day conditions. A previous study from this laboratory proposed that nonresponsiveness to short day conditions rests with a defect in the circadian organization of these hamsters. In this study we found pronounced differences between responsive and nonresponsive hamsters in the expression of circadian rhythmicity under constant darkness and under constant illumination. While responsive hamsters showed a free-running activity pattern with a period of 23.86 +/- 0.04 h and responded to brief light pulses with the expected phase delays and phase advances, nonresponsive hamsters exhibited a period of 24.04 +/- 0.05 h and responded to light pulses with phase advances. Furthermore, 9 out of 15 responsive hamsters showed a clear split in the activity pattern within 8 weeks under constant light (80-100 lux), while only 1 of the 7 nonresponsive hamsters exhibited a split activity pattern. As a result of these differences in circadian function, nonresponsive Djungarian hamsters are incapable of proper photoperiod time measurement and photoperiod-induced seasonality.