Simultaneous manifestation of free-running and entrained rhythms in the rat motor activity explained by a multioscillatory system

Reversible suppression of circadian-driven locomotor rhythms in mice using a gradual fragmentation of the day-night cycle

Circadian rhythms are regulated by molecular clockwork and drive 24-h behaviors such as locomotor activity, which can be rendered non-functional through genetic knockouts of clock genes. Circadian rhythms are robust in constant darkness (DD) but are modulated to become exactly 24 h by the external day-night cycle. Whether ill-timed light and dark exposure can render circadian behaviors non-functional to the extent of genetic knockouts is less clear. In this study, we discovered an environmental approach that led to a reduction or lack in rhythmic 24-h-circadian wheel-running locomotor behavior in mice (referred to as arrhythmicity). We first observed behavioral circadian arrhythmicity when mice were gradually exposed to a previously published disruptive environment called the fragmented day-night cycle (FDN-G), while maintaining activity alignment with the four dispersed fragments of darkness. Remarkably, upon exposure to constant darkness (DD) or constant light (LL), FDN-G mice lost any resemblance to the FDN-G-only phenotype and instead, exhibited sporadic activity bursts. Circadian rhythms are maintained in control mice with sudden FDN exposure (FDN-S) and fully restored in FDN-G mice either spontaneously in DD or after 12 h:12 h light-dark exposure. This is the first study to generate a light-dark environment that induces reversible suppression of circadian locomotor rhythms in mice.

Effects of Photoperiod on Rat Motor Activity Rhythm at the Lower Limit of Entrainment

The experiment described here studied the rat motor activity pattern as a function of the photoperiod of circadian light-dark cycles in the limits of entrainment (22-and 23-h periods). In most cases, the overt rhythm showed 2 circadian components: 1 that followed the external LD cycle and a 2nd rhythm that was free run. The expression of these components was directly dependent on the photoperiod, and there was a gradual transition in the manifestation of 1 or the other. The component with a period equal to that of the external cycle was more manifested under long photoperiods, while the other 1 was more expressed during short photoperiods. Also, the period of the free-running component was longer under T22 than T23. For each period, the free-running component was longer under a longer photoperiod. At first sight, the presence of these 2 components in most of the rats might appear to be due to the fact that in the limits of entrainment, some rats do not entrain and thus show a free-running rhythm plus masking. However, the gradation observed in the different patterns of the overt motor activity rhythm, especially those patterns related to the different balance between the 2 components and the length of the period of the free-running component under LD as a function of the photoperiod, suggests that the circadian system can be functionally dissociated.

In synch but not in step: Circadian clock circuits regulating plasticity in daily rhythms

The suprachiasmatic nucleus (SCN) is a network of neural oscillators that program daily rhythms in mammalian behavior and physiology. Over the last decade much has been learned about how SCN clock neurons coordinate together in time and space to form a cohesive population. Despite this insight, much remains unknown about how SCN neurons communicate with one another to produce emergent properties of the network. Here we review the current understanding of communication among SCN clock cells and highlight a collection of formal assays where changes in SCN interactions provide for plasticity in the waveform of circadian rhythms in behavior. Future studies that pair analytical behavioral assays with modern neuroscience techniques have the potential to provide deeper insight into SCN circuit mechanisms.

Photic desynchronization of two subgroups of circadian oscillators in a network model of the suprachiasmatic nucleus with dispersed coupling strengths

The suprachiasmatic nucleus (SCN) is the master circadian clock in mammals and is composed of thousands of neuronal oscillators expressing different intrinsic periods. These oscillators form a coupled network with a free-running period around 24 h in constant darkness and entrainable to the external light-dark cycle (T cycle). Coupling plays an important role in setting the period of the network and its range of entrainment. Experiments in rats have shown that two subgroups of oscillators within the SCN, a ventrolateral (VL) subgroup that receives photic input and a dorsomedial (DM) subgroup that is coupled to VL, can be desynchronized under a short (22-h) T cycle, with VL entrained to the cycle and DM free-running. We use a modified Goodwin model to understand how entrainment of the subgroups to short (22-h) and long (26-h) T cycles is influenced by light intensity, the proportion of neurons that receives photic input, and coupling heterogeneity. We find that the model's critical value for the proportion of photically-sensitive neurons is in accord with actual experimental estimates, while the model's inclusion of dispersed coupling can account for the experimental observation that VL and DM desynchronize more readily under the 22-h than under the 26-h T cycle. Heterogeneous intercellular coupling within the SCN is likely central to the generation of complex behavioral patterns.

Circadian desynchronization

The suprachiasmatic nucleus (SCN) coordinates via multiple outputs physiological and behavioural circadian rhythms. The SCN is composed of a heterogeneous network of coupled oscillators that entrain to the daily light-dark cycles. Outside the physiological entrainment range, rich locomotor patterns of desynchronized rhythms are observed. Previous studies interpreted these results as the output of different SCN neural subpopulations. We find, however, that even a single periodically driven oscillator can induce such complex desynchronized locomotor patterns. Using signal analysis, we show how the observed patterns can be consistently clustered into two generic oscillatory interaction groups: modulation and superposition. In seven of 17 rats undergoing forced desynchronization, we find a theoretically predicted third spectral component. Combining signal analysis with the theory of coupled oscillators, we provide a framework for the study of circadian desynchronization.

Entrainment of 2 subjective nights by daily light:dark:light:dark cycles in 3 rodent species

Recent work with exotic 24-h light:dark:light:dark (LDLD) cycles indicates surprising flexibility in the entrainment patterns of Syrian hamsters. Following exposure to an LDLD cycle, hamsters may adopt a form of rhythm splitting in which markers of subjective night (e.g., activity, melatonin) are expressed in each of the twice daily scotophases. This pattern contrasts markedly with that of conventionally entrained hamsters in which markers of subjective night are expressed once daily in only 1 of the 2 dark periods. The "split" entrainment pattern was examined further here in Syrian and Siberian hamsters and in mice exposed to LDLD 7:5:7:5, a condition that reliably induces split activity rhythms in all 3 species. The phase angle of entrainment and activity duration were generally similar comparing the 2 daily activity bouts in each species. The stability of this split entrainment state was assessed by deletions of photophases on individual days, by exposure to skeleton photoperiods, and by transfer to constant darkness. As in Syrian hamsters, the one-time substitution of darkness for one 7-h photophase did not grossly alter activity patterns of Siberian hamsters but acutely disrupted the split rhythms of mice. Skeleton light pulses of progressively shorter duration did not significantly alter split entrainment patterns of either Syrian or Siberian hamsters. Both species continued to exhibit stable entrainment with activity expressed in alternate scotophases of an LD 1:5 cycle presented 4 times daily. In contrast, the split activity rhythms of mice were not maintained under skeleton pulses. In constant darkness, rhythms of Siberian hamsters remained distinctly split for a minimum of 2 cycles. Split entrainment to these novel LDLD and 4-pulse skeleton lighting regimes demonstrates a marked degree of plasticity common to the circadian systems of several rodent species and identifies novel entrainment patterns that may be reliably elicited with simple environmental manipulations. Inter- and intraspecific differences in the stability of split activity rhythms likely reflect differences in coupling interactions between the component circadian oscillators, which, adopting separate phase relations to these novel LD cycles, yield a split entrainment pattern.

Effect of short light-dark cycles on young and adult TGR(mREN2)27 rats

Animals placed under short light-dark (LD) cycles show a dissociation of their circadian rhythms. However, this effect has only been studied in Wistar rats and with the motor activity (MA) rhythm. Thus, in the present experiment, we studied in TGR(mREN2)27 (TGR) rats, a strain of hypertensive rats, the effect of a short LD cycle on the circadian rhythms of MA, heart rate (HR), and blood pressure (BP). Our aim was (1) to investigate whether the exposure of TGR rats to a short LD cycle induced a dissociation of their circadian rhythms, (2) to study the effect of short LD cycles on the development of the circadian rhythms of TGR rats, and (3) to compare the effect of short LD cycles on young and adult TGR rats. One group of TGR rats was maintained under LD cycles of 22h periods (group G22). The progress in time of their rhythms was compared to that of TGR rats of the same age that had been kept under LD cycles of 24h periods (group G24). For the third point, the rhythms of a group of 5-week-old TGR rats kept under LD 22h cycles (young rats) were compared to those of a group of 11-week-old TGR rats (adult rats). Results showed that there is a dissociation of the circadian rhythms of all the variables monitored in TGR rats maintained under LD 22h cycles, independent of age. We have also found that group G22 showed a higher increase in BP with age and a higher mortality due to malignant hypertension compared to group G24. Finally, it seems that it is harder for young rats to entrain to short LD cycles than for adult rats, and young rats have a higher mortality due to malignant hypertension than adult rats. In conclusion, we demonstrated that short LD cycles produce a dissociation in the HR, BP, and MA circadian rhythms. The results of this experiment, compared to those previously obtained in Wistar rats, suggest that the light perception, the responses of the circadian system to light, or both are altered in the TGR rats.

Circadian activity rhythms and sensitivity to noise in the Mongolian gerbil (Meriones unguiculatus)

Since consistent data on endogenous circadian rhythms of Mongolian gerbils are not available, the main aim of our study was to identify suitable conditions to receive stable and reproducible free-running rhythms of activity under different light intensities. Another objective was to determine the role of social cues as an exogenous zeitgeber in the absence of a light-dark (LD) cycle. We performed two long-term sets of experiments with adult male gerbils kept in climatic chambers under various photoperiods of at least 30 days each. In all cases, the time of lights on in the chambers differed from the daily starting hour of work in the animal house. Always, two animals per chamber were kept separately in cages with a running wheel while their activity was monitored continuously. During the first set, only three of eight animals developed intra- and interindividual variable free-running rhythms. The activity patterns seemed to be influenced by human activities outside, indicating high sensitivity to external factors. Subsequently, we damped the chambers and the room and restricted access to the room. In the following noise-reduced set, all gerbils developed comparable free-running rhythms of activity. We determined the mean of the free-running period tau, the activity-rest relationship alpha/theta and the amount of running wheel activity per day: tau = 23.7h +/- 0.08h under low light (5 lux) and 25.5h +/- 0.19h under high light intensities (450 lux); alpha/theta = 0.53 +/- 0.08 under 5 lux and 0.34 +/- 0.04 under 450 lux. The amount of daily activity was 12 times as high under 5 lux as under 450 lux. There was no indication that the two animals in one chamber socially synchronized each other. In conclusion, the pronounced rhythm changes in accordance with Aschoff's theory support the view that gerbils are mainly nocturnal animals.

Endocrine activity during sleep

Almost all functions of humans are subject to cyclic changes and are governed by the nervous system. Most rhythms are driven by an internal biological clock located in the hypothalamic suprachiasmatic nucleus (SCN) and can be synchronized by external signals such as light-dark cycles. Homeostatic activities such as body temperature, blood volume, water balance and sleep, are rhythmic. Likewise, most hormones are secreted in a rhythmic fashion. Both sleep and circadian effects interact to produce the overall rhythmic pattern of the pituitary and pituitary-dependent hormones. Some of the 24-h hormonal rhythms depend on the circadian clock (ACTH, cortisol and melatonin), or are sleep related (prolactin and TSH). GH secretion is influenced by the first slow wave sleep (SWS) episode at the beginning of the night. Pulses of prolactin and GH are positively linked to increases in delta wave activity, i.e. deepest phases of sleep, occurring primarily during the first third of the night. Pulses of TSH and cortisol are related to superficial phases of sleep. As a result of the consolidation of the sleep period in humans, the wake-sleep transition is associated with physiological changes with the endocrine system being part of the adaptive mechanism to reduce physical activity during sleep.

Feeding behavior and entrainment limits in the circadian system of the rat

The entrainment limits of the circadian rhythms of feeding activity were studied in Wistar rats exposed to gradually increasing and decreasing or to static light-dark cycles. In the former, the entrainment limits of feeding behavior were 22 h 10 min and 26 h 40 min. In the latter, the upper limit was higher, because rats under zeitgeber period (t) length = 27 h (t27) and t28 met the criteria of entrainment. The lower limit, on the other hand, was not modified because none of the t22 animals showed entrained rhythms and one-half of the t23 rats exhibited two components in their circadian feeding rhythms, one with a period of 23 h and the other free running. This 23-h component reflected not only the masking effect of light-dark cycles but also seemed a true light-entrained component. In well-synchronized animals, food intake seemed to depend more on the number of cycles that the animal experienced than on actual time lived; however, other feeding parameters, such as meal frequency and feeding duration, remained constant when expressed per 24 h, irrespective of the t cycle. These results concerning feeding duration, meal frequency, and food intake revealed that the homeostatic and circadian controls interacted to a degree that depended on the type of variable considered. In conclusion, the entrainment limits appeared much more imprecise than they were previously thought to be, because the circadian system can only be partially synchronized near its entrainment limits. The hypothesis that the rat's circadian system is composed of multiple oscillators with different intrinsic frequencies and varying capacities for light synchronization would explain the partial desynchronization observed near the entrainment limits.

Dissociation of the rat motor activity rhythm under T cycles shorter than 24 hours

Since the suprachiasmatic nuclei (SCN) were identified as the principal mammalian circadian clock, many studies describing their morphology and physiology have been carried out. Today, the multioscillatory nature of the SCN, which explains the dissociation of the circadian rhythms under some experimental conditions, is widely accepted. Here, we study the simultaneous presence of two circadian rhythms in the motor activity of the rat when exposed to symmetric light-dark (LD) cycles shorter than 24 h (T21, T21.5, T22, T22.5, T23, and T23.5). One rhythmic component was entrained by the external LD cycle whereas the other ran free with a period longer than 24 h. The results show that two circadian rhythms were present only when T was shorter than T23, whereas at T23.5 only one entrained component was manifested. The manifestation of the two circadian components depends quantitatively on the period of the external cycle--i.e., the strength of the entrained rhythm increases when the external T is closer to 24 h--whereas that of the nonentrained rhythm decreases. The dissociation of the motor activity rhythm and the gradual appearance of the two components are explained by considering the entrainment of a multioscillatory system as not taking place as a whole but rather in a partial manner, in such a way that some oscillators may entrain but not others. The effect of the entrained oscillators is added to the masking effect of the LD cycles.