Parametric and nonparametric entrainment of circadian locomotor rhythm in the Japanese honeybee Apis cerana japonica

The circadian rhythm of locomotor activity in the Japanese honeybee Apis cerana japonica was studied to determine the involvement of parametric and/or nonparametric entrainment. The rhythm was entrained to a skeleton photoperiod in which a 1-h first light pulse was imposed in the morning along with a second light pulse in the evening, as well as to a complete photoperiodic regime (LD 12:12). However, the timing of peak activity relative to the lights-off in the evening in the skeleton photoperiod was earlier than that in the complete photoperiod. A single daily light pulse in the evening entrained the rhythm, whereas a daily light pulse in the morning allowed free-running as in constant darkness. The free-running period (τ) of locomotor activity in constant light became longer as the light intensity increased. A Winfree's type I phase response curve of the locomotor activity rhythm was obtained using a single 1-h light pulse. The results suggest that both parametric and nonparametric entrainment are involved in the circadian rhythm of individual locomotor activity in this honeybee.     

Period-Dependent Oscillatory Free-Run in the Locomotor Activity Rhythm of the Field Mouse Mus booduga Under Skeleton Photoperiodic Regimes

The entrainment behaviour of the circadian rhythm of locomotor activity in the field mouse Mus booduga was studied in order to evaluate the role of the animals' free-running period (τ) and the duration of skeleton photoperiods in determining entrainment of animals with τ values beyond and close to the “limits of entrainment”. We predicted that animals with τ lesser than the lower “limit of entrainment” would entrain only to short skeleton photoperiods (≤ 6 h) and not to longer skeleton photoperiods. Experimental animals (n = 25) were entrained to light/dark (LD) 12:12 h schedule, and then subjected to various skeleton photoperiods in which the duration of one of the two intervals of darkness was successively reduced while holding the zeitgeber period (T) constant. Some animals (n = 9) entrained to long as well as short photoperiods, whereas others (n = 5) entrained only to extremely short skeleton photoperiods of 6 h or less. The mean τ of the animals entraining to all photoperiods (23.78 ± 0.22 h) was significantly greater than that of the animals that entrained only to very short skeleton photoperiods (22.43 ± 0.41 h) (t _{df 12} = 5.3, p < 0.001). We also selected a few animals (n = 11) with average τ value of 23.13 ± 0.38 h and studied them under several skeleton photoperiods. To our surprise the animals which were subjected to restricted dark intervals invariably underwent “phase-jump” assuming the longer dark interval as “subjective night”. We suggest that the observed variation in entrainment behaviour might be due to the variation seen among individual animals in τ and the shape of their PRC. These results support the view that the duration of the skeleton photoperiod and the τ of an individual animal interact to determine its entrainment, and underscore the relevance of inter-individual variation in circadian organisation to studies of circadian rhythms.     

The Role of Wheel Running in the Coupling of Two Simultaneous Circadian Rhythms of Motor Activity in the Rat

Motor activity is among the non-photic stimuli that act on the internal clock. We have tested the role of motor activity in the circadian pattern of rats under conditions near the lower limits of entrainment, that induce circadian rhythm dissociation. Three groups of 8 rats each were used: a) rats kept individually in 25×25×15 cm cages, b) rats in 50×25×15 cm cages, and c) rats in 50×25×15 cm cages with access to a running wheel. All the rats were kept under light-dark cycles of 22 hours (T22, 11L:11D) for 50 days, after which they were transferred to constant darkness. All the rats without a running wheel showed a motor activity pattern with two statistically significant circadian rhythms in the periodogram of Sokolove and Bushell: one circadian component entrained by the LD cycle, and another free-running. The rats with access to a running wheel showed several patterns: 5 rats showed only one rhythm entrained to the LD cycle, 2 rats showed circadian rhythm dissociation, and 1 showed only a free running rhythm. We believe that the simultaneous manifestation of two circadian components reflects the functional dissociation of the oscillators population that constitutes the circadian pacemaker, of the rat. Physical exercise acts on the pacemaker reinforcing the strongest group of oscillators, which, depending on the structure of the circadian system of the rat, is usually the one entrained to the LD cycle. This study supports the hypothesis that motor activity couples the oscillators that form the circadian system of the rat.     

The Role of Wheel Running in the Coupling of Two Simultaneous Circadian Rhythms of Motor Activity in the Rat

Motor activity is among the non-photic stimuli that act on the internal clock. We have tested the role of motor activity in the circadian pattern of rats under conditions near the lower limits of entrainment, that induce circadian rhythm dissociation. Three groups of 8 rats each were used: a) rats kept individually in 25×25×15 cm cages, b) rats in 50×25×15 cm cages, and c) rats in 50×25×15 cm cages with access to a running wheel. All the rats were kept under light-dark cycles of 22 hours (T22, 11L:11D) for 50 days, after which they were transferred to constant darkness. All the rats without a running wheel showed a motor activity pattern with two statistically significant circadian rhythms in the periodogram of Sokolove and Bushell: one circadian component entrained by the LD cycle, and another free-running. The rats with access to a running wheel showed several patterns: 5 rats showed only one rhythm entrained to the LD cycle, 2 rats showed circadian rhythm dissociation, and 1 showed only a free running rhythm. We believe that the simultaneous manifestation of two circadian components reflects the functional dissociation of the oscillators population that constitutes the circadian pacemaker, of the rat. Physical exercise acts on the pacemaker reinforcing the strongest group of oscillators, which, depending on the structure of the circadian system of the rat, is usually the one entrained to the LD cycle. This study supports the hypothesis that motor activity couples the oscillators that form the circadian system of the rat.     

Significance of Nonparametric Light Effects in Entrainment of Circadian Rhythms in Owl Monkeys (Aotus Lemurinus Griseimembra) by Light-Dark Cycles

In a total of 12 adult Colombian owl monkeys, Aotus lemurinus griseimembra, the significance of nonparametric light effects for the entrainment of the circadian system by light-dark (LD) cycles was studied by carrying out (a) phase-response experiments testing the phase-shifting effect of 30-min light pulses (LPs) of 250 lx applied at various phases of the free-running circadian activity rhythm (LL 0.2 lx) and (b) synchronization experiments testing the entraining effect of 24-h single LP photoperiods consisting of 30-min L of 80 lx and 23.5-h D of 0.5 lx (sP 0.5) and skeleton photoperiods consisting of two 30-min LPs of 80 lx, given against a background illuminance of 0.5 lx either symmetrically at 12-h intervals (PP 12:12) or asymmetrically at 9- and 15-h intervals (PP 9:15). The phase-response characteristics in Aotus, as evidenced by the phase-response curve, generally correspond to those of nocturnal rodents, proving that this neotropical simian primate chronobiologically is a genuine nocturnal species. When free-running with a spontaneous period close to 24 h (24.3 ± 0.1 h), the PP 12:12 produced entrainment in only two of five owl monkeys, whereas the sP 0.5 entrained four of them. The PP 9:15, however, brought about stable entrainment of the circadian rhythms of locomotor activity, feeding activity, and core temperature in all animals tested (n = 8). Changes in phase position of the activity time with the endogenous rhythm entrained by a PP 12:12, by an sP 0.5, or by a PP 9:15 give evidence that both LPs of a skeleton photoperiod contribute to the phase setting of the circadian system. When free-running with a considerably lengthened spontaneous period (τ ≥ 25.5 h), even the sP 0.5 and the PP 9:15 failed to entrain the owl monkeys' circadian rhythms, whereas a 24-h photoperiod with a very long LP of 3 h caused entrainment. The results indicate that in Aotus lemurinus griseimembra, in addition to the nonparametric light effects, parametric light effects play a significant role in the entrainment of circadian rhythms by LD cycles.     

Behavioral and neurochemical sources of variability of circadian period and phase: studies of circadian rhythms of npy-/- mice

The cycle length or period of the free-running rhythm is a key characteristic of circadian rhythms. In this study we verify prior reports that locomotor activity patterns and running wheel access can alter the circadian period, and we report that these treatments also increase variability of the circadian period between animals. We demonstrate that the loss of a neurochemical, neuropeptide Y (NPY), abolishes these influences and reduces the interindividual variability in clock period. These behavioral and environmental influences, from daily distribution of peak locomotor activity and from access to a running wheel, both act to push the mean circadian period to a value < 24 h. Magnitude of light-induced resetting is altered as well. When photoperiod was abruptly changed from a 18:6-h light-dark cycle (LD18:6) to LD6:18, mice deficient in NPY were slower to respond to the change in photoperiod by redistribution of their activity within the prolonged dark and eventually adopted a delayed phase angle of entrainment compared with controls. These results support the hypothesis that nonphotic influences on circadian period serve a useful function when animals must respond to abruptly changing photoperiods and point to the NPYergic pathway from the intergeniculate leaflet innervating the suprachiasmatic nucleus as a circuit mediating these effects.     

Circadian rhythm of locomotor activity in the Japanese honeybee, Apis cerana japonica

Abstract.  To reveal circadian characteristics and entrainment mechanisms in the Japanese honeybee Apis cerana japonica , the locomotor-activity rhythm of foragers is investigated under programmed light and temperature conditions. After entrainment to an LD 12 : 12 h photoperiodic regime, free-running rhythms are released in constant dark (DD) or light (LL) conditions with different free-running periods. Under the LD 12 : 12 h regime, activity offset occurs approximately 0.4 h after lights-off transition, assigned to circadian time (Ct) 12.4 h. The phase of activity onset, peak and offset, and activity duration depends on the photoperiodic regimes. The circadian rhythm can be entrained to a 24-h period by exposure to submultiple cycles of LD 6 : 6 h, as if the locomotive rhythm is entrained to LD 18 : 6 h. Phase shifts of delay and advance are observed when perturbing single light pulses are presented during free-running under DD conditions. Temperature compensation of the free-running period is demonstrated under DD and LL conditions. Steady-state entrainment of the locomotor rhythm is achieved with square-wave temperature cycles of 10 °C amplitude, but a 5 °C amplitude fails to entrain.     

Forced desynchronization model for a diurnal primate

The circadian system is organized in a hierarchy of multiple oscillators, with the suprachiasmatic nucleus (SCN) as the master oscillator in mammals. The SCN is formed by a group of coupled cell oscillators. Knowledge of this coupling mechanism is essential to understanding entrainment and the expression of circadian rhythms. Some authors suggest that light-dark (LD) cycles with periods near the limit of entrainment may be good models for promoting internal desynchronization, providing knowledge about the coupling mechanism. As such, we evaluated the circadian activity rhythm (CAR) pattern of marmosets in LD cycles at lower limits of entrainment in order to study induced internal dissociation. To that end, two experiments were conducted: (1) 6 adult females were under symmetrical LD cycles T21, T22 and T21.5 for 60, 35 and 48 days, respectively; and (2) 4 male and 4 female adults were under T21 for 24 days followed by 18 days of LL, back to T21 for 24 days, followed by 14 days of LL. The CAR of each animal was continuously recorded. In experiment 1, vocalizations were also recorded. Under Ts shorter than 24 days, a dissociation pattern was observed for CAR and vocalizations. Two simultaneous circadian components emerged, one with the same period as the LD cycle, called the light-entrained component, and the other in free-running, denominated the non-light-entrained component. Both components were displayed in the CAR for all the animals in T21, five animals (83.3%) in T21.5 and two animals (33.3%) in T22. Our results are in accordance with the multioscillatory nature of the circadian system. Dissociation is partial synchronization to the LD cycle, with at least one group of oscillators synchronized by relative coordination and masking, while another group of oscillators free runs, but is also masked by the LD cycle. Since only T21 promoted the emergence of both circadian components in the circadian rhythms of all marmosets, it was considered the promoter period of circadian rhythm dissociation in this species, and is proposed as a good animal model for forced desynchronization in non-human diurnal primates.     

The role of Clock in the plasticity of circadian entrainment

The mammalian circadian clock lying in suprachiasmatic nucleus (SCN) is synchronized to about 24 h by the environmental light-dark cycle (LD). The circadian clock exhibits limits of entrainment above and below 24 h, beyond which it will not entrain. Little is known about the mechanisms regulating the limits of entrainment. In this study, we show that wild-type mice entrain to only an LD 24 h cycle, whereas Clock mutant mice can entrain to an LD 24, 28, and 32 h except for LD 20 h and LD 36 h cycle. Under an LD 28 h cycle, Clock mutant mice showed a clear rhythm in Per2 mRNA expression in the SCN and behavior. Light response was also increased. This is the first report to show that the Clock mutation makes it possible to adapt the circadian oscillator to a long period cycle and indicates that the clock gene may have an important role for the limits of entrainment of the SCN to LD cycle.     

Locomotor activity rhythm in four wrasse species under varying light conditions

Under controlled laboratory conditions, the locomotor activity rhythms of four species of wrasses (Suezichthys gracilis, Thalassoma cupido, Labroides dimidiatus andCirrhilabrus temminckii) were individually examined using an actograph with infra-red photo-electric switches in a dark room at temperatures of 21.3–24.3°C, for 7 to 14 days. The locomotor activity ofS. gracilis occurred mostly during the light period under a light-dark cycle regimen (LD 12:12; 06:00-18:00 light, 18:00-06:00 dark). The locomotor activity commenced at the beginning of the light period and continued until a little before the beginning of dark period. The diel activity rhythm of this species synchronizes with LD. Under constant illumination (LL) this species shows distinct free-running activity rhythms varying in length from 23 hrs. 39 min. to 23 hrs. 47 min. Therefore,S. gracilis appears to have a circadian rhythm under LL. However, in constant darkness (DD), the activity of this species was greatly suppressed. All the fish showed no activity rhythms in DD conditions. After DD, the fish showed the diel activity rhythm with the resumption of LD, but this activity began shortly after the beginning of light period. The fish required several days to synchronize with the activity in the light period. Therefore,S. gracilis appeared to continue the circadian rhythm under DD. InT. cupido, the locomotor activity commenced somewhat earlier than the beginning of the light period and continued until the beginning of the dark period under LD. The diel activity rhythm of this species synchronizes with LD. Under LL, four of the five specimens of this species tested showed free-running activity rhythms for the first 5 days or longer varying in length from 22 hrs. 54 min. to 23 hrs. 39 min. Although the activity of this species was suppressed under DD, two of five fish showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 38 min. to 23 hrs. 50 min. under DD. Therefore, it was ascertained thatT. cupido has a circadian rhythm. InL. dimidiatus, the locomotor activity rhythm under LD resembled that observed inT. cupido. The diel activity rhythm of this species synchronizes with LD. Under LL, four of seven of this species showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 07 min. to 25 hrs. 48 min. Although the activity of this species was suppressed under DD, three of five fish showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 36 min. to 23 hrs. 41 min. under DD. Therefore, it was ascertained thatL. dimidiatus has a circadian rhythm. Almost all locomotor activity of C.temminckii occurred during the light period under LD. The diel activity rhythm of this species coincides with LD. Under LL, two of four of this species showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 32 min. to 23 hrs. 45 min. Although the activity of this species was suppressed under DD, one of the four fish showed free-running activity rhythms throughout the experimental period. The length of the free-running period was 23 hrs. 21 min. under DD. Therefore,C. temminckii appeared to have a circadian rhythm. According to field observations,S. gracilis burrows and lies in the sandy bottom whileT. cupido, L. dimidiatus, andC. temminckii hide and rest in spaces among piles of boulders or in crevices of rocks during the night. It seems that the differences in nocturnal behavior among the four species of wrasses mentioned above are closely related to the intensity of endogenous factors in their locomotor activity rhythms.     

PHOTIC ENTRAINMENT OF CIRCADIAN ACTIVITY PATTERNS IN THE TROPICAL LABRID FISH HALICHOERES CHRYSUS

Yellow wrasses (Halichoeres chrysus) show clear daily activity patterns. The fish hide in the substrate at (subjective) night, during the distinct rest phase. Initial entrainment in a 12h:12h light-dark (12:12 LD) cycle (mean period 24.02h, SD 0.27h, n = 16 was followed by a free run (mean period 24.42h, SD 1.33h) after transition into constant dim light conditions. Light pulses of a comparable intensity as used in the light part of the LD cycles did not result in significant phase shifts of the free-running rhythm in constant darkness. Application of much brighter 3h light pulses resulted in a phase-response curve (PRC) for a fish species, with pronounced phase advances during late subjective night. The PRCs differed from those mainly obtained in other vertebrate taxa by the absence of significant phase delays in the early subjective night. At that circadian phase, significant tonic effects of the light pulses caused a shortening of the circadian period length. Entrainment to skeleton photoperiods of 1:11 LD was observed in five of six wrasses exposed, also after a 3h phase advance of this LD cycle. Subsequently, a 1:11.25 LD cycle resulted in entrainment in four of the six fish. It is suggested that the expression of the circadian system in fish can be interpreted as a functional response to a weak natural zeitgeber, as present in the marine environment. This response allows photic entrainment as described here in the yellow wrasse. (Chronobiology International, 17(5), 613–622, 2000)     

Circadian Rhythms and time Measurement in Locomotor Activity of the Scorpion Leiurus Quinquestriatus (Buthidae)

The circadian rhythms of locomotor activity of the scorpion Leiurus quinqueslriatus were examined under different light-dark cycles and in free-running conditions. The circadian rhythm is bimodal in LD 12:12 with alternating cycles of temperature (35°-25°C) with high intensity (1300 lux) or in LD 12: 12 with constant temperature 35° C with 300 lux. In LD 12:12 (1300 lux), in long or in short light spans with constant temperature, the bimodal pattern is slightly changed with the appearance of a third minor peak of activity. In free-running conditions, the bimodal rhythm of locomotor activity persists in DD with T about 24 hr, but in LL the rhythm becomes unimodal with T about 24 hr. Cosinor and power spectrum analysis showed the presence of more than one periodic component. It seems that there is a correlation between the range of light regimens, temperature, light intensity and the coincidence of these components. These components are independently entrained by the environmental light cycle. The mechanism of entrainment of components is discussed.     

Adult locomotor rhythmicity as "hands" of the maternal photoperiodic clock regulating larval diapause in the blowfly, Calliphora vicina.

Some basic properties of the adult locomotor activity rhythm and of the maternal induction of larval diapause in Calliphora vicina are described. Diapause responses in Nanda-Hamner experiments indicate that circadian rhythmicity is involved in photoperiodic time measurement (PPTM). However, although the locomotor rhythm shows long-lasting changes in free-running period (aftereffects of photoperiod and constant light) and occasional "splitting," thereby indicating a structural complexity to the circadian system, the overt rhythm may be used as an indicator of phase relationships (or "hands") of the covert system involved in PPTM, within the framework of a simple external-coincidence model for the diapause clock. Thus, in light-dark (LD) cycles close to "resonance" with the circadian pacemaker(s) (T 24, LD 12:12; T 48, LD 12:36; and T 72, LD 12:60), light is restricted to the subjective day and diapause incidence is high. In T 36 (LD 12:24) and T 60 (LD 12:48), light falls into the subjective night and illuminates the postulated light-sensitive phase (phi i), and diapause incidence is low. Within the primary range of entrainment, light invades the late subjective night in T 20 (LD 12:8), illuminates phi i, and causes low incidence of diapause; however, it invades the early subjective night in T 30 (LD 12:18) and diapause remains high.     

Latitudinal clines in the properties of a circadian pacemaker

The circadian rhythm of eclosion activity and its pacemaker were analyzed in a series of latitudinal races of Drosophila auraria ranging from 34.2 degrees to 42.9 degrees N in Japan. The phase of the rhythm (psi EL) to the daily photoperiod (PP) changes as daylength is increased, and the slope of psi EL (PP) changes with latitude. Is is sufficiently greater in the north to cause a phase reversal of northern and southern races on long versus short photoperiods. This reversal is found in assays of the pacemaker's phase (psi PL) as well as that of the rhythm (psi EL). Assay of the pacemaker shows that its period (tau) is longer in northern than in southern races, and that the amplitude of its phase response curve (PRC) is lower in the north. The period of the rhythm in all latitudinal races is longer than 24 hr in short photoperiods (LD 1:23), but is probably less than 24 hr (as an aftereffect of photoperiod) in longer days such as LD 14:10. The observed north-south differences in the phase relation of both pacemaker and rhythm to the light cycle are explained by the latitudinal clines in pacemaker properties and a postulated aftereffect of photoperiod on tau. It is suggested that the latitudinal cline in PRC amplitude has functional significance in conserving the amplitude of the pacemaker's signal to the rest of the system it times. Computer simulation shows that without such a reduction in the perceived light intensity, pacemaker amplitude will be lowered by the increase in duration of the daily light at higher latitudes.     

Possible Evidence for Shift Work Schedules in the Media Workers of the Ant Species Camponotus compressus

The locomotor activity rhythm of the media workers of the ant species Camponotus compressus was monitored under constant conditions of the laboratory to understand the role of circadian clocks in social organization. The locomotor activity rhythm of most ants entrained to a 24 h light/dark (12:12 h; LD) cycle and free-ran under constant darkness (DD) with circadian periodicities. Under entrained conditions about 75% of media workers displayed nocturnal activity patterns, and the rest showed diurnal activity patterns. In free-running conditions these ants displayed three types of activity patterns (turn-around). The free-running period (τ) of the locomotor activity rhythm of some ants (10 out of 21) showed period lengthening, and those of a few (6 out of 21) showed period shortening, whereas the locomotor activity rhythm of the rest of the ants (5 out of 21) underwent large phase shifts. Interestingly, the pre-turn-around τ of those ants that showed nocturnal activity patterns during earlier LD entrainment was shorter than 24 h, which became greater than 24 h after 6–9 days of free-run in DD. On the other hand, the pre-turn-around τ of those ants, which exhibited diurnal patterns during earlier LD entrainment, was greater than 24 h, which became shorter than 24 h after 6–9 days of free-run in DD. The patterns of activity under LD cycles and the turn-around of activity patterns in DD regime suggest that these ants are shift workers in their respective colonies, and they probably use their circadian clocks for this purpose. Circadian plasticity thus appears to be a general strategy of the media workers of the ant species C. compressus to cope with the challenges arising due to their roles in the colony constantly exposed to a fluctuating environment.     

Relationship between infant and mother circadian rest-activity rhythm pre- and postpartum, in comparison to an infant with free-running rhythm

Some infants show a free-running rhythm in their rest-activity. We do not know why, nor do we know exactly what the entrainment factors are for the development of the normal 24-h rest-activity rhythm. Actigraphic recordings on 10 primiparae during late pregnancy and these mothers and their infants during the 2nd, 6th, and 12th wks after birth were made over 3-5 continuous days to investigate maternal and infant entrainment. One infant showed a free-running rest-activity circadian rhythm. In late pregnancy, the period in the autocorrelogram of the mother with the free-running infant was longer than the significant period of the mean autocorrelogram of the mothers with non-free-running infants. The finding of this study indicates the free-running rhythm of infant is not reset by maternal entrainment factors.     

Exposure to T-cycles of 22 and 23 h during lactation modifies the later dissociation of motor activity and temperature circadian rhythms in rats

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

Drosophila cryb mutation reveals two circadian clocks that drive locomotor rhythm and have different responsiveness to light

Cryptochrome (CRY) is a blue-light-absorbing protein involved in the photic entrainment of the circadian clock in Drosophila melanogaster. We have investigated the locomotor activity rhythms of flies carrying cryb mutant and revealed that they have two separate circadian oscillators with different responsiveness to light. When kept in constant light conditions, wild-type flies became arrhythmic, while cryb mutant flies exhibited free-running rhythms with two rhythmic components, one with a shorter and the other with a longer free-running period. The rhythm dissociation was dependent on the light intensities: the higher the light intensities, the greater the proportion of animals exhibiting the two oscillations. External photoreceptors including the compound eyes and the ocelli are the likely photoreceptors for the rhythm dissociation, since rhythm dissociation was prevented in so1;cryb and norpAP41;cryb double mutant flies. Immunohistochemical analysis demonstrated that the PERIOD expression rhythms in ventrally located lateral neurons (LNvs) occurred synchronously with the shorter period component, while those in the dorsally located per-expressing neurons showed PER expression most likely related to the longer period component, in addition to that synchronized to the LNvs. These results suggest that the Drosophila locomotor rhythms are driven by two separate per-dependent clocks, responding differentially to constant light.     

Tau differences between short-day responsive and short-day nonresponsive white-footed mice (Peromyscus leucopus) do not affect reproductive photoresponsiveness

In laboratory-bred rodent populations, intraspecific variation in circadian system organization is a known cause of individual variation in reproductive photoresponsiveness. The authors sought to determine whether circadian system variation accounted for individual variation in reproductive photoresponsiveness in a single, highly genetically variable population of Peromyscus leucopus recently derived from the wild. Running-wheel activity patterns of male and female mice, aged 70 to 90 days, from artificially selected lines of reproductively photoresponsive (R) and nonresponsive (NR) lines were monitored under short-day photoperiod (8 h light, 16 h dark), long-day photoperiod (16 h light, 8 h dark), and constant darkness (DD). NR mice displayed a significantly longer mean free-running period (24.08 h) in DD compared with R mice (23.75 h), due in large part to a difference between NR and R females (24.25 h vs. 23.74 h, respectively). All other entrainment characteristics (alpha, phase angle of activity) under short days, long days, and DD were similar between R and NR mice. Variation in free-running period and entrainment characteristics has been shown to affect photoresponsiveness in other rodent species by altering the manner in which the circadian system interprets short days. To determine whether variation in photoresponsiveness in P. leucopus is due to differences in free-running period instead of variation downstream from the central circadian clock in the pathway controlling photoresponsiveness, the authors exposed young R and NR mice to DD and measured the effect on reproductive organ development. If variation in free-running period affected how the circadian system of mice interpreted short days, then both R and NR mice exposed to DD should have exhibited a delay in gonadal development. Only R mice exhibited pubertal delay in DD. NR mice exhibited large paired testes, paired seminal vesicles, paired ovaries, and uterine weight typical of mice nonresponsive to short days, whereas R mice exhibited reproductive organ weight typical of mice responsive to short days. These data suggest that despite significant differences in free-running period between R and NR mice, individual variation in photoresponsiveness is not due to differences in how the circadian systems of R and NR mice interpret the LD cycle.