INTERNAL TEMPORAL ORDER IN THE CIRCADIAN SYSTEM OF A DUAL-PHASING RODENT,THE OCTODON DEGUS

Daily rhythms in different biochemical and hematological variables have been widely described in either diurnal or nocturnal species, but so far no studies in the rhythms of these variables have been conducted in a dual-phasing species such as the degus. The Octodon degus is a rodent that has the ability to switch from diurnal to nocturnal activity under laboratory conditions in response to wheel-running availability. This species may help us discover whether a complete temporal order inversion occurs parallel to the inversion that has been observed in this rodent's activity pattern. The aim of the present study is to determine the phase relationships among 26 variables, including behavioral, physiological, biochemical, and hematological variables, during the day and at night, in diurnal and nocturnal degus chronotypes induced under controlled laboratory conditions through the availability of wheel running. A total of 39 male degus were individually housed under a 12:12 light-dark (LD) cycle, with free wheel-running access. Wheel-running activity (WRA) and body temperature (Tb) rhythms were recorded throughout the experiment. Melatonin, hematological, and biochemical variables were determined by means of blood samples obtained every 6?h (ZT1, ZT7, ZT13, and ZT19). In spite of great differences in WRA and Tb rhythms between nocturnal and diurnal degus, no such differences were observed in the temporal patterns of most of the biological variables analyzed for the two chronotypes. Variation was only found in plasma urea level and lymphocyte number. A slight delay in the phase of the melatonin rhythm was also observed. This study shows the internal temporal order of a dual-phasing mammal does not show a complete inversion in accordance with its activity and body temperature pattern; it would appear that the switching mechanism involved in the degu's nocturnalism is located downstream from the pacemaker. (Author correspondence: angerol@um.es).     

PACEMAKER PHASE CONTROL VERSUS MASKING BY LIGHT: SETTING THE CIRCADIAN CHRONOTYPE IN DUAL OCTODON DEGUS

There are two main processes involved in the expression of circadian rhythmicity: entrainment and masking. Whereas the first operates via the central pacemaker to anticipate predictable environmental conditions, masking (mainly induced by light) functions as a direct modulator of the circadian output signal induced by nonpredictable events. The Chilean rodent Octodon degus presents both diurnal and nocturnal chronotypes when given free access to an exercise wheel. Two steady-entrainment phases and graded masking by light seem to generate the wide variability of chronotypes in this species. The aim of this study was to characterize the differential masking by light according to the individual chronotypes, their stability over time, and the influence of wheel running availability and ambient temperature upon the degus' nocturnality. To this end, diurnal and nocturnal degus were subjected to ultradian cycles (1:1-h light-dark [LD]), with and without wheel running availability, and under both normal and high diurnal ambient temperature cycles. The present results show that diurnal and nocturnal degus present a stable masking by light, each according to its respective chronotype. Thus, whereas diurnal animals increased their activity with light, in nocturnal degus light induced a sharp drop in wheel running activity. These two types of masking responses appeared not only when the animals were synchronized to the 12:12-h LD cycle, but also under ultradian cycles. Different masking effects persisted when wheel running was made unavailable and when the animals shifted their circadian activity patterns in response to ultradian cycles or to diurnal exposure to high temperatures. In conclusion, our results show that the positive and negative masking effects of light on diurnal and nocturnal degus, respectively, seem to occur independently of relative phase control by the central pacemaker or the negative masking induced by high environmental temperatures. (Author correspondence: angerol@um.es)     

Scheduled voluntary wheel running activity modulates free-running circadian body temperature rhythms in Octodon degus

Entrainment of the circadian pacemaker to nonphotic stimuli, such as scheduled wheel-running activity, is well characterized in nocturnal rodents, but little is known about activity-dependent entrainment in diurnal or crepuscular species. In the present study, effects of scheduled voluntary wheel-running activity on circadian timekeeping were investigated in Octodon degus, a hystricomorph rodent that exhibits robust crepuscular patterns of wakefulness. When housed in constant darkness, O. degus exhibited circadian rhythms in wheel-running activity and body temperature (Tb) with an average period length (tau) of 23.39 +/- 0.11 h. When wheel running was restricted to a fixed 2-h schedule every 24 h, tau increased on average 0.39 +/- 0.09 h but did not result in steady-state entrainment. Instead, relative coordination between the fixed running schedule and circadian timing was observed. Tau was greatest when scheduled wheel running occurred at CT 20.5 (0.4 h greater than DD baseline tau). Scheduled running activity also influenced Tb waveform symmetry, reflecting concomitant changes in the circadian activity-rest ratio (alpha:rho). Aftereffects of the scheduled wheel-running paradigm were also observed. In 2 animals, tau lengthened from 23.20 and 23.80 h to 24.14 and 24.15 h, respectively, and remained relatively stable for approximately 1 month during the wheel schedule. Although behavioral activity appears to be a weak zeitgeber in this species, these data suggest that nonphotic stimuli can phase delay the circadian pacemaker in O. degus at similar times of the day as in nocturnal hamsters and mice, and in humans.     

THE RESPONSE OF PER1 TO LIGHT IN THE SUPRACHIASMATIC NUCLEUS OF THE DIURNAL DEGU (OCTODON DEGUS)

Several studies suggest that the circadian systems of diurnal mammals respond differently to daytime light than those of nocturnal mammals. We hypothesized that the photosensitive “clock” gene Per1 would respond to light exposure during subjective day in the suprachiasmatic nucleus of the diurnal rodent, Octodon degus. Tissue was collected 1.5–2?h after a 30?min light pulse presented at five timepoints across the 24?h day and compared to controls maintained under conditions of constant darkness. Per1 mRNA was quantified using in situ hybridization. Results showed that the rhythmicity and photic responsiveness of Per1 in the degu resembles that of nocturnal animals. (Author correspondence: hagenaue@umich.edu)     

Regulation of Locomotor activity in fed,fasted, and food-restricted mice lacking tissue-type plasminogen activator

Background

Circadian rhythms of physiology and behavior are driven by a circadian clock located in the suprachiasmatic nucleus of the hypothalamus. This clock is synchronized to environmental day/night cycles by photic input, which is dependent on the presence of mature brain-derived neurotrophic factor (BDNF) in the SCN. Mature BDNF is produced by the enzyme plasmin, which is converted from plasminogen by the enzyme tissue-type plasminogen activator (tPA). In this study, we evaluate circadian function in mice lacking functional tPA.

Results

tPA^?/? mice have normal circadian periods, but show decreased nocturnal wheel-running activity. This difference is eliminated or reversed on the second day of a 48-h fast. Similarly, when placed on daily cycles of restricted food availability the genotypic difference in total wheel-running activity disappears, and tPA^?/? mice show equivalent amounts of food anticipatory activity to wild type mice.

Conclusions

These data suggest that tPA regulates nocturnal wheel-running activity, and that tPA differentially affects SCN-driven nocturnal activity rhythms and activity driven by fasting or temporal food restriction.

     

SYNCHRONIZATION TO LIGHT AND RESTRICTED-FEEDING SCHEDULES OF BEHAVIORAL AND HUMORAL DAILY RHYTHMS IN GILTHEAD SEA BREAM (SPARUS AURATA)

Food is not continuously available in the wild, and so most animals show a wide variety of circadian rhythms that can be entrained to feeding time. The aim of this research was to evaluate the effect of time-restricted feeding on the daily rhythms of gilthead sea bream, with food being provided during the day or night under a 12:12?h light-dark (LD) cycle or constant light (LL) conditions. Self-feeding and locomotor activity, as well as daily rhythms of cortisol, glucose, and melatonin, were evaluated. Fish synchronized their feeding behavior to the feeding phase, so that in LD they displayed 78% nocturnal feeding activity under night-feeding and 81% diurnal feeding activity under day-feeding, while under LL-feeding they displayed 72% of their daily activity during the 12?h feeding phase. In contrast, locomotor activity was mostly diurnal (66–71%), regardless of the feeding schedule, and it became arrhythmic under LL. Cortisol showed daily rhythms that peaked at different times, depending on the light and feeding schedule: one peak several hours before feeding under day-feeding and night-feeding, and two peaks under LL-feeding. Glucose displayed low-amplitude variations, with no daily rhythms being detected. Melatonin, however, showed a nocturnal rhythm, regardless of the feeding schedule, while the rhythm became attenuated under LL. Taken together, these results highlight the role of feeding on endocrine and metabolic rhythms, suggesting that feeding behavior should be considered when studying these variables. (Author correspondence: jflopez@um.es)     

Variability of diurnality in laboratory rodents

The locomotor activity rhythms of domestic mice, laboratory rats, Syrian hamsters, Siberian hamsters, Mongolian gerbils, degus, and Nile grass rats were compared. Running-wheel activity was monitored under a light–dark cycle with 12 h of light and 12 h of darkness per day. Nile grass rats were found to be reliably diurnal, whereas laboratory rats, Siberian hamsters, domestic mice, and Syrian hamsters were reliably nocturnal. Both diurnal and nocturnal subgroups were observed in Mongolian gerbils and degus. A downward gradient of diurnality was observed from Mongolian gerbils classified as diurnal, degus classified as diurnal, gerbils classified as nocturnal, and degus classified as nocturnal. Nocturnal degus remained nocturnal when tested with an infrared motion detector without running wheels. Thus, although the diurnal–nocturnal dichotomy could be applied to some of the species, it was not appropriate for others. The dichotomy may reflect researchers’ needs for systematization more than a natural distinction between species. Through mechanisms as yet poorly understood, the balance between entraining and masking processes seems to generate a gradient of temporal niches that runs from predominantly diurnal species to predominantly nocturnal species with many chronotypes in between, including species that exhibit wide intra-species gradients of temporal niche.     

CHRONOTYPES AND SUBJECTIVE SLEEP PARAMETERS IN EPILEPSY PATIENTS: A LARGE QUESTIONNAIRE STUDY

Accumulating evidence suggests epilepsy and seizures may influence circadian rhythms and that circadian rhythms may influence epilepsy. It is also conceivable that seizure timing influences the timing of daily activities, sleeping, and wakefulness (i.e., chronotype). Only one group has studied the distribution of chronotypes of epileptics, showing significant differences between the diurnal activity patterns in two groups of patients with different epilepsy syndromes. The authors performed a questionnaire-based study of 200 epilepsy patients to compare the distribution of chronotypes and subjective sleep parameters of sleep duration and time of mid-sleep on free days to the distribution in the general population (n?=?4042). Within this large group of epilepsy patients, we also compared the chronotypes of subsamples with well-defined epilepsy syndromes, i.e., temporal lobe epilepsy [TLE; n?=?46], frontal lobe epilepsy [FLE; n?=?30], and juvenile myoclonic epilepsy [JME; n?=?38]. In addition, 27 patients who had had surgery for TLE were compared with those with TLE who had not had surgery. Both the Morningness-Eveningness Questionnaire and Munich Chronotype Questionnaire were used to determine chronotypes and subjective sleep parameters. Significant differences in morningness/eveningness distribution, timing of mid-sleep (corrected for sleep duration), and total sleep time on free days were found between epileptics and healthy controls. Those with epilepsy were more morning oriented, had earlier mid-sleep on free days, and longer sleep duration on free days (p?<?.001). However, distributions of chronotypes and sleep parameters between the groups of people with TLE, FLE, and JME did not differ. Persons who had surgery for TLE had similar morningness-eveningness parameters and similar sleep durations compared to those without surgery, but mid-sleep on free days was earlier in operated patients (p?=?.039). In conclusion, this is the first large study focusing on chronotypes in people with epilepsy. We show that the distribution of chronotypes and subjective sleep parameters of epileptics, in general, is different from that of healthy controls. Nevertheless, no differences are observed between patients with specified epilepsy syndromes, although they exhibit seizures with different diurnal patterns. Our results suggest that epilepsy, itself, rather than seizure timing, has a significant influence on chronotype behavior and subjective sleep parameters. (Author correspondence: w.hofstra@mst.nl)     

Photic phase response curve in Octodon degus: assessment as a function of activity phase preference

Light exposure during the early and late subjective night generally phase delays and advances circadian rhythms, respectively. However, this generality was recently questioned in a photic entrainment study in Octodon degus. Because degus can invert their activity phase preference from diurnal to nocturnal as a function of activity level, assessment of phase preference is critical for computations of phase reference [circadian time (CT) 0] toward the development of a photic phase response curve. After determining activity phase preference in a 24-h light-dark cycle (LD 12:12), degus were released in constant darkness. In this study, diurnal (n = 5) and nocturnal (n = 7) degus were randomly subjected to 1-h light pulses (30-35 lx) at many circadian phases (CT 1-6: n = 7; CT 7-12: n = 8; CT 13-18: n = 8; and CT 19-24: n = 7). The circadian phase of body temperature (Tb) onset was defined as CT 12 in nocturnal animals. In diurnal animals, CT 0 was determined as Tb onset + 1 h. Light phase delayed and advanced circadian rhythms when delivered during the early (CT 13-16) and late (CT 20-23) subjective night, respectively. No significant phase shifts were observed during the middle of the subjective day (CT 3-10). Thus, regardless of activity phase preference, photic entrainment of the circadian pacemaker in Octodon degus is similar to most other diurnal and nocturnal species, suggesting that entrainment mechanisms do not determine overt diurnal and nocturnal behavior.     

Two Steady‐Entrainment Phases and Graded Masking Effects by Light Generate Different Circadian Chronotypes in Octodon degus

Processes involved in the operation of the circadian pacemaker are well characterized; however; little is known about what mechanisms drive the overt diurnal, nocturnal, or crepuscular behavior in a species. In this context, dual‐phasing rodents, such as Octodon degus, emerge as a useful model to decipher these keys. Two main chronotypes, nocturnal and diurnal, have been traditionally described in laboratory‐housed degus based on the percentage of activity displayed by the animals during the scotophase or photophase. However, if one considers also the entrainment phase angle during the first days following a change from LD to DD conditions, a third chronotype (intermediate)—or more properly, a continuous grading of circadian expressions between diurnal and nocturnal chronotype—can be observed. Our experiments suggest the pacemaker of the diurnal animal is entrained to the photophase, and light does not exert a negative masking effect. The pacemaker of the nocturnal degus, on the other hand, is entrained to the scotophase, and light exerts a strong negative masking effect. Finally, the intermediate chronotype is characterized by variable negative masking effect of light overlapping a pacemaker entrained to the photophase. The phase shift inversion from diurnal to nocturnal chronotype is related to the availability of a wheel in the cage, and the effect may be located downstream from the clock. However, body temperature rhythm recordings, less affected by masking effects, point to an involvement of the circadian pacemaker in chronotype differentiation, as transient entrainment cycles, and not an abrupt phase shift, were detected after providing access to the wheel. The diurnality of degus seems to be the result of a variety of mechanisms, which may explain how different processes can lead to similar chronotypes.     

Evidence for a Plastic Dual Circadian Rhythm in the Oyster Crassostrea gigas

Although a significant body of literature has been devoted to the chronobiology of aquatic animals, how biological rhythms function in molluscan bivalves has been poorly studied. The first objective of this study was to determine whether an endogenous circadian rhythm does exist in the oyster, Crassostrea gigas. The second objective was to characterize it in terms of robustness. To answer these questions, the valve activity of 15 oysters was continuously recorded for 2 mo in the laboratory under different entrainment and free-running regimes using a high-frequency noninvasive valvometer. The present work demonstrates the presence of a circadian rhythm in the oyster Crassostrea gigas. First, oysters were entrained by 12?L:12 D conditions. Then, free-running conditions (D:D and L:L) indicated that the most frequently observed period ranged from 20 to 28?h, the circadian range. That endogenous circadian rhythm was characterized as weak. Indeed, the period (τ) of the individual animals exhibited high plasticity in D:D and L:L, and the animals immediately followed a 4-h phase advance or delay. Additionally, C. gigas appeared as a dual organism: all oysters were nocturnal at the beginning of the laboratory experiment (January), whereas they were diurnal at the end (March). That shift was progressive. Comparison with a full-year in situ record showed the same behavioral duality as observed in the laboratory: the animals were nocturnal in autumn–winter and diurnal in spring–summer. The significant advantage of a plastic and dual circadian rhythm in terms of adaptability in a highly changing environment is discussed. (Author correspondence: d.tran@epoc.u-bordeaux1.fr)     

Daily Changes in Ultraviolet Light Levels Can Synchronize the Circadian Clock of Bumblebees (Bombus terrestris)

Endogenous circadian clocks are synchronized to the 24-h day by external zeitgebers such as daily light and temperature cycles. Bumblebee foragers show diurnal rhythms under daily light:dark cycles and short-period free-running circadian rhythms in constant light conditions in the laboratory. In contrast, during the continuous light conditions of the arctic summer, they show robust 24-h rhythms in their foraging patterns, meaning that some external zeitgeber must entrain their circadian clocks in the presence of constant light. Although the sun stays above the horizon for weeks during the arctic summer, the light quality, especially in the ultraviolet (UV) range, exhibits pronounced daily changes. Since the photoreceptors and photopigments that synchronize the circadian system of bees are not known, we tested if the circadian clocks of bumblebees (Bombus terrestris) can be entrained by daily cycles in UV light levels. Bumblebee colonies were set up in the laboratory and exposed to 12?h:12?h UV?+?:UV? cycles in otherwise continuous lighting conditions by placing UV filters on their foraging arenas for 12?h each day. The activity patterns of individual bees were recorded using fully automatic radiofrequency identification (RFID). We found that colonies manipulated in such a way showed synchronized 24-h rhythms, whereas simultaneously tested control colonies with no variation in UV light levels showed free-running rhythms instead. The results of our study show that bumblebee circadian rhythms can indeed be synchronized by daily cycles in ambient light spectral composition. (Author correspondence: r.stanewsky@qmul.ac.uk)     

Circadian Rhythms of Self-feeding and Locomotor Activity in Zebrafish (Danio Rerio)

To investigate daily feeding rhythms in zebrafish, the authors have developed a new self-feeding system with an infrared photocell acting as a food-demand sensor, which lets small-size fish such as zebrafish trigger a self-feeder. In this paper, the authors used eight groups of 20 fish. Locomotor activity rhythms were also investigated by means of infrared sensors. Under a 12?h:12?h light (L)-dark (D) cycle, zebrafish showed a clear nocturnal feeding pattern (88.0% of the total daily food-demands occurring in the dark phase), concentrated during the last 4?h of the dark phase. In contrast, locomotor activity was mostly diurnal (88.2% of total daily activity occurring in the light phase). Moreover, both feeding and locomotor rhythms were endogenously driven, as they persisted under free-running conditions. The average period length (τ) of the locomotor and feeding rhythms was shorter (τ?=?22.9?h) and longer (τ?=?24.6?h) than 24?h, respectively. During the time that food availability was restricted, fish could only feed during ZT0–ZT12 or ZT12–ZT16. This resulted in feeding activity being significantly modified according to feeding time, whereas the locomotor activity pattern remained synchronized to the LD cycle and did not change during this trial. These findings revealed an independent phasing between locomotor and feeding activities (which were mostly nocturnal or diurnal, respectively), thus supporting the concept of multioscillatory control of circadian rhythmicity in zebrafish. (Author correspondence: javisan@um.es)     

Nocturnal to Diurnal Switches with Spontaneous Suppression of Wheel-Running Behavior in a Subterranean Rodent

Several rodent species that are diurnal in the field become nocturnal in the lab. It has been suggested that the use of running-wheels in the lab might contribute to this timing switch. This proposition is based on studies that indicate feed-back of vigorous wheel-running on the period and phase of circadian clocks that time daily activity rhythms. Tuco-tucos (Ctenomys aff. knighti) are subterranean rodents that are diurnal in the field but are robustly nocturnal in laboratory, with or without access to running wheels. We assessed their energy metabolism by continuously and simultaneously monitoring rates of oxygen consumption, body temperature, general motor and wheel running activity for several days in the presence and absence of wheels. Surprisingly, some individuals spontaneously suppressed running-wheel activity and switched to diurnality in the respirometry chamber, whereas the remaining animals continued to be nocturnal even after wheel removal. This is the first report of timing switches that occur with spontaneous wheel-running suppression and which are not replicated by removal of the wheel.     

RHYTHMIC cFOS EXPRESSION IN THE VENTRAL SUBPARAVENTRICULAR ZONE INFLUENCES GENERAL ACTIVITY RHYTHMS IN THE NILE GRASS RAT,ARVICANTHIS NILOTICUS

Circadian rhythms in behavior and physiology are very different in diurnal and nocturnal rodents. A pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus is responsible for generating and maintaining circadian rhythms in mammals, and cellular and molecular rhythms within the SCN of diurnal and nocturnal rodents are very similar. The neural substrates determining whether an animal has a diurnal or nocturnal phase preference are thus likely to reside downstream of the SCN. The ventral subparaventricular zone (vSPVZ), a major target of the SCN that is important for the expression of circadian rhythmicity in nocturnal lab rats (Rattus norvegicus), exhibits different rhythms in cFos expression in diurnal Nile grass rats compared to lab rats. We examined the effects of chemotoxic lesions of the cFos-expressing cells of the vSPVZ on activity rhythms of grass rats to evaluate the hypothesis that these cells support diurnality in this species. Male grass rats housed in a 12:12 light:dark (LD) cycle were given bilateral injections of the neurotoxin n-methyl-D-L-aspartic acid (NMA) or vehicle aimed at the vSPVZ; cells in the SCN are resistant to NMA, which kills neurons in other brain regions, but leaves fibers of passage intact. vSPVZ-damaged grass rats exhibited highly unstable patterns of activity in constant darkness (DD) and in the LD cycle that followed. However, crepuscular bouts of activity could be seen in all animals with vSPVZ lesions. Damage to the vSPVZ reduced cFos expression in this area but not in the SCN. Using correlational analyses, we found that the number of cFos-ir cells in the vSPVZ was unrelated to several parameters of the activity rhythms during the initial post-surgical period, when animals were in LD. However, the number of cells expressing cFos in the vSPVZ was positively correlated with general activity during the subjective day relative to the subjective night when the animals were switched to DD, and this pattern persisted when a LD cycle was reinstated. Also, the number of cFos-ir cells in the vSPVZ was negatively correlated with the strength of rhythmicity in DD and the number of days required to re-entrain to a LD cycle following several weeks in DD. These data suggest that the vSPVZ emits signals important for the expression of stable diurnal activity patterns in grass rats, and that species differences in these signals may contribute to differences in behavioral and physiological rhythms of diurnal and nocturnal mammals. (Author correspondence: mschw009@umaryland.edu)     

PLASMA LEVELS OF INTERFERON-γ CORRELATE WITH AGE-RELATED DISTURBANCES OF CIRCADIAN RHYTHMS AND SURVIVAL IN A NON-HUMAN PRIMATE

Aging can be associated with changes in circadian rhythms and reduction in adaptive immune responses accompanied by expansion of memory T cells and elevated levels of pro-inflammatory cytokines. Recent findings suggest the cytokine interferon-γ (IFN-γ) can affect the function of the hypothalamic suprachiasmatic nucleus (SCN), the master mammalian circadian pacemaker, both in vitro and in vivo. We studied the correlation of plasma levels of IFN-γ and changes in circadian rhythms in a non-human primate species, the nocturnal mouse lemur (Microcebus murinus). Plasma IFN-γ and dehydroepiandrosterone sulfate (DHEA-S), a known biomarker of aging, were determined in middle- to old-age animals by immunoenzymoassay. Daily rhythms of locomotor activity and body temperature as well as survival time of the lemurs were recorded. With aging, mean levels of DHEA-S decreased whereas IFN-γ increased. Aged animals showed biological rhythm alterations characterized by a high percentage of diurnal activity, anticipation of the activity onset relative to lights-off, short free-running period, and delayed occurrence of minimal body temperature. The magnitude of these disturbances was correlated with the plasma level of IFN-γ but not DHEA-S. Most remarkably, in contrast to DHEA-S, increased levels of IFN-γ correlated with duration of the lifetime of the lemurs. These results show the degree of circadian rhythm alterations in an individual is correlated with plasma IFN-γ level during aging, and that plasma IFN-γ level may predict survival, at least in this non-human primate. (Author correspondence: fabienne.aujard@wanadoo.fr)     

EFFECTS OF LIGHT AND MELATONIN TREATMENT ON BODY TEMPERATURE AND MELATONIN SECRETION DAILY RHYTHMS IN A DIURNAL RODENT,THE FAT SAND RAT

Many mammals display predictable daily rhythmicity in both neuroendocrine function and behavior. The basic rest-activity cycles are usually consistent for a given species and vary from night-active (nocturnal), those mostly active at dawn and dusk (i.e., crepuscular), and to day-active (diurnal) species. A number of daily rhythms are oppositely phased with respect to the light/dark (LD) cycle in diurnal compared with nocturnal mammals, whereas others are equally phased with respect to the LD cycle, regardless of diurnality/nocturnality. Pineal produced melatonin (MLT) perfectly matches this phase-locked feature in that its production and secretion always occurs during the night in both diurnal and nocturnal mammals. As most rodents studied to date in the field of chronobiology are nocturnal, the aim in this study was to evaluate the effect of light manipulations and different photoperiods on a diurnal rodent, the fat sand rat, Psammomys obesus. The authors studied its daily rhythms of body temperature (T_b) and 6-sulphatoxymelatonin (6-SMT) under various photoperiodic regimes and light manipulations (acute and chronic exposures) while maintaining a constant ambient temperature of 30°C?±?1°C. The following protocols were used: (A) Control (CON) conditions 12L:12D; (A1) exposure to one light interference (LI) of CON-acclimated individuals for 30?min, 5?h after lights-off; (A2) short photoperiod (SP) acclimation (8L:16D) for 3 wks; (A3) 3 wks of SP acclimation with chronic LI of 15?min, three times a night at 4-h intervals; (A4) chronic exposure to constant dim blue light (470nm, 30 lux) for 24?h for 3 wks (LL). (B) The response to exogenous MLT administration, provided in drinking water, was measured under the following protocols: (B1) After chronic exposure to SP with LI, MLT was provided once, starting 1?h before the end of photophase; (B2) after a continuous exposure to dim blue light, MLT was provided at 15:00?h for 2?h for 2 wks; (B3) to CON animals, MLT was given intraperitoneally (i.p.) at 14:00?h. The results demonstrate that under CON acclimation, Psammomys obesus has robust T_b and 6-SMT daily rhythms in which the acrophase (peak time) of T_b is during the photophase, whereas that of 6-SMT is during scotophase. LI resulted in an elevation of T_b and a reduction of 6-SMT levels. A significant difference in the response was noted between acute and chronic exposure to LI, particularly in 6-SMT levels, which were lower than CON after LI and higher after chronic LI, implying an acclimation process. Constant exposure to blue light abolished T_b and 6-SMT rhythms in all the animals. MLT administration resumed the T_b daily rhythm in these animals, and had a recovery effect on the chronic LI-exposed animals, resulting in a T_b decrease. Altogether, the authors show in this study the different modifications of T_b rhythms and MLT levels in response to environmental light manipulations. These series of experiments may serve as a basis for establishing P. obesus as an animal model for further studies in chronobiology. (Author correspondence: hagitsc@research.haifa.ac.il)     

From daily behavior to hormonal and neurotransmitters rhythms: Comparison between diurnal and nocturnal rat species

Mammalian species can be defined as diurnal or nocturnal, depending on the temporal niche during which they are active. Even if general activity occurs during nighttime in nocturnal rodents, there is a patchwork of general activity patterns in diurnal rodents, including frequent bimodality (so-called crepuscular pattern, i.e., dawn and dusk peaks of activity) and a switch to a nocturnal pattern under certain circumstances. This raises the question of whether crepuscular species have a bimodal or diurnal - as opposed to nocturnal - physiology. To this end, we investigated several daily behavioral, hormonal and neurochemical rhythms in the diurnal Sudanian grass rat (Arvicanthis ansorgei) and the nocturnal Long-Evans rat (Rattus norvegicus). Daily rhythms of general activity, wheel-running activity and body temperature, with or without blocked wheel, were diurnal and bimodal for A. ansorgei, and nocturnal and unimodal for Long-Evans rats. Moreover, A. ansorgei and Long-Evans rats exposed to light-dark cycles were respectively more and less active, compared to conditions of constant darkness. In contrast to other diurnal rodents, wheel availability in A. ansorgei did not switch their general activity pattern. Daily, unimodal rhythm of plasma leptin was in phase-opposition between the two rodent species. In the hippocampus, a daily, unimodal rhythm of serotonin in A. ansorgei occurred 7 h earlier than that in Long-Evans rats, whereas a daily, unimodal rhythm of dopamine was unexpectedly concomitant in both species. Multiparameter analysis demonstrates that in spite of bimodal rhythms linked with locomotor activity, A. ansorgei have a diurnally oriented physiology.     

Restraint stress delays reentrainment in male and female diurnal and nocturnal rodents

A temporary loss of normal circadian entrainment, such as that associated with shift work and transmeridian travel, can result in an array of detrimental symptoms, making rapid reentrainment of rhythmicity essential. While there is a wealth of literature examining the effects of stress on the entrained circadian system, less is known about the influence of stress on circadian function following a phase shift of the light: dark (LD) cycle. The authors find that recovery of locomotor activity synchronization is altered by restraint stress in the diurnal rodent Octodon degus (degu) and the nocturnal rat. In the first experiment, degus were subjected to a 6-h phase advance of the LD cycle. Sixty minutes after the new lights-on, animals underwent 60 min of restraint stress. The number of days it took each animal to reentrain its activity rhythms to the new LD cycle was recorded and compared to the number of days it took the animal to reentrain under control conditions. When subjected to restraint stress, degus took 30% longer to reentrain their activity rhythms (p < 0.01). In a second experiment, rats underwent a similar experimental paradigm. As with the degus, stress significantly delayed the reentrainment of rats' activity rhythms (p < 0.01). There was no interaction between sex and stress on the rate of reentrainment for either rats or degus. Furthermore, there was no effect of stress on the free-running activity rhythm of degus, suggesting that the effect of stress on reentrainment rate is not secondary to alterations of period length. Together, these data point to a detrimental effect of stress on recovery of entrainment of circadian rhythms, which is independent of activity niche and sex.     

Dim Light at Night Increases Immune Function in Nile Grass Rats,a Diurnal Rodent

With the widespread adoption of electrical lighting during the 20th century, human and nonhuman animals became exposed to high levels of light at night for the first time in evolutionary history. This divergence from the natural environment may have significant implications for certain ecological niches because of the important influence light exerts on the circadian system. For example, circadian disruption and nighttime light exposure are linked to changes in immune function. The majority of studies investigating the effects of light exposure and circadian disruption on the immune system use nocturnal rodents. In diurnal species, many hormones and immune parameters vary with secretion patterns 180° out of phase to those of nocturnal rodents. Thus, the authors investigated the effects of nighttime light exposure on immunocompetence in diurnal Nile grass rats (Arvicanthis niloticus). Rats were housed in either standard 14-h light (L):10-h dark (D) cycles with L ～150 lux and D 0 lux or dim light at night (dLAN) cycles of LD 14:10 with L ～150 lux and D 5 lux for 3 wks, then tested for plasma bactericidal capacity, as well as humoral and cell-mediated immune responses. Rats exposed to dLAN showed increased delayed-type hypersensitivity pinna swelling, which is consistent with enhanced cell-mediated immune function. dLAN rats similarly showed increased antibody production following inoculation with keyhole lymphocyte hemocyanin (KLH) and increased bactericidal capacity. Daytime corticosterone concentrations were elevated in grass rats exposed to nighttime dim light, which may have influenced immunological measures. Overall, these results indicate nighttime light affects immune parameters in a diurnal rodent. (Author correspondence: fonken.1@osu.edu)