LD Cycle Research Articles

Non-Ocular Circadian Oscillators and Photoreceptors Modulate Long Term Memory Formation in Aplysia

In Aplysia californica, memory formation for long-term sensitization (LTS) and for a more complex type of associative learning, learning that food is inedible (LFI), is modulated by a circadian clock. For both types of learning, formation of long-term memory occurs during the day and significantly less during the night. Aplysia eyes contain a well-characterized circadian oscillator that is strongly coupled to the locomotor activity rhythm. Thus, the authors hypothesized that the ocular circadian oscillator was responsible for the circadian modulation of LFI and LTS. To test this hypothesis, they investigated whether the eyes were necessary for circadian modulation of LFI and LTS. Eyeless animals trained during the subjective day and tested 24 h later demonstrated robust long-term memory for both LFI and LTS, while eyeless animals trained and tested during the subjective night showed little or no memory for LFI or LTS. The amplitude of the rhythm of modulation in eyeless animals was similar to that of intact Aplysia, suggesting that extraocular circadian oscillators were mainly responsible for the circadian rhythms in long-term memory formation. Next, the authors investigated whether the eyes played a role in photic entrainment for circadian regulation of long-term memory formation. Eyeless animals were exposed to a reversed LD cycle for 7 days and then trained and tested for long-term memory using the LFI paradigm. Eyeless Aplysia formed significant long-term memory when trained during the projected shifted day but not during the projected shifted night. Thus, the extraocular circadian oscillator responsible for the rhythmic modulation of long-term memory formation can be entrained by extraocular photoreceptors.

Phenology of Chondrus ocellatus in Cheongsapo Near Busan, Korea

The reproductive phenology of Chondrus ocellatus and the effects of temperature and light on its growth were examined in Cheongsapo near Busan, Korea, from September 1994 to August 1995. The vegetative plants dominated over the year, with a peak occurrence in January. Gameto- and tetrasporophytes were most abundant in November and August. All vegetative and reproductive plants had a peak both in length and weight in October, when seawater temperature was highest (24°C). In laboratory culture, the maximum relative growth rate (RGR) of 2.94% day−1 was obtained at 20°C and 100 μmol photons m−2 s−1, whereas the lowest value was recorded at 25°C and 100 μmol photons m−2 s−1 in a 12: 12 h LD photoperiod regime. Among the three photoperiod regimes (8:16 h, 12:12 h, 16:8 h LD) tested, there was evidence of a higher RGR in the 12:12 h LD cycle. This result suggests that the growth and reproduction of C. ocellatus are correlated with the seawater temperature based on laboratory culture and field observations.

Scheduled wheel access during daytime: A method for studying conflicting zeitgebers

It is often stated that light is the primary environmental cue (zeitgeber) for entrainment of circadian clocks. Here, we use a new conflict test design in Syrian hamsters comparing the strength of a photic zeitgeber to that of a non-photic cue, i.e. wheel availability. Re-entrainment to an inverted LD cycle was significantly slowed down in the nocturnal hamster by restricting wheel access to the light phase of the inverted LD cycle. This effect is more pronounced if the illuminance level of the entraining lights is 0.1 lx compared to 6 lx. In this conflict design, the hamsters did not re-entrain to an inverted LD cycle for up to four weeks (when the experiment ended), but voluntarily ran during the light phase. This approximates the situation in people subjected to shift work or jet lag.

Restoration of Circadian Rhythmicity in Circadian Clock-Deficient Mice in Constant Light

In mammals, circadian rhythms in behavior and physiology are controlled by a central pacemaker, the SCN, and subordinated clocks throughout the body. On the molecular level, these clocks are based on transcriptional/translational feedback loops involving a set of clock genes that regulate their own transcription. Among the components driving the mammalian circadian clock are the Period 1 and 2 (Per1 and Per2) and Cryptochrome 1 and 2 (Cry1 and Cry2) genes. In the present study, the authors characterize the behavioral and molecular rhythms of Per2/Cry1 double mutant mice under 3 different lighting conditions. In an LD cycle, the activity of these animals is masked by light, while in DD, the mutants lose circadian rhythmicity but exhibit strong ultradian rhythms. In LL of higher intensity, circadian rhythms are restored on the behavioral level with a drastically shortened endogenous period. Furthermore, both in the SCN and in the periphery, clock gene rhythms are restored. Based on these observations and also on the fact that light-mediated induction of Per gene expression is preserved in these mutants, the authors propose a mechanism by which endogenous ultradian rhythms may relay timed light exposure to the SCN, leading to a reinitiation of self-sustained circadian rhythms in LL.

Development of a Two-Dimension Manifold to Represent High Dimension Mathematical Models of the Intracellular Mammalian Circadian Clock

A new focus for mathematical models of the circadian pacemaker involves the encapsulation within the models of detailed biological processes responsible for generating those circadian rhythms. Representing greater biological detail requires more mathematical equations, which pose a greater challenge for the analysis of such systems. Development of a method that retains the predominant dynamics while still providing biologically detailed information is advantageous. Two high-dimension mathematical models of intracellular mammalian circadian pacemakers, Leloup-Goldbeter and Forger-Peskin, with 19 and 73 differential equations, respectively, have been published. The authors projected each of these high-dimension models onto their respective manifold using proper orthogonal functions (POFs) obtained from the empirical decomposition of the model's phase space to obtain a 2-dimension model. The resulting 2-dimension model, represented by 2 differential equations, predicts most of the salient characteristics of a biological clock including approximately 24-h oscillations, entrainment to an LD cycle, phase response curves, and the amplitude recovery dynamics that emerge following amplitude suppression. The manifold representation simplifies the mathematical analysis, since only 2 variables need to be observed and analyzed to understand the behavior of the biological clock. This reduced model derived from a model based on biological variables can be used for the development and analysis of mathematical models of the coupled mammalian oscillators to understand the dynamics of the integrated circadian pacemaker.

Locomotor, feeding and melatonin daily rhythms in sharpsnout seabream ( Diplodus puntazzo)

Sharpsnout seabream is a marine teleost of increasing interest for Mediterranean aquaculture, but there is still a lack of information regarding its circadian organization. In this study, we have investigated sharpsnout seabream locomotor activity, feeding and plasma melatonin daily rhythms under a 12:12-h LD cycle, as well as the persistence of locomotor activity circadian rhythmicity under constant light (LL) conditions. When submitted to an LD cycle, most sharpsnout seabream displayed a diurnal locomotor pattern, with an average 74% of activity recorded during daytime. However, along the experiment 40% of fish spontaneously changed their locomotor rhythm phasing and became nocturnal. Feeding behaviour, nevertheless, remained strictly diurnal in all cases, with 97% of food demands being made during the light period. Free-running locomotor rhythms were recorded in one third of the fish kept under LL. Daily plasma melatonin levels displayed a rhythmic profile, with low daytime values (111 pg/ml) and high nighttime concentrations (791 pg/ml). Taken together, these results evidence a high degree of plasticity for sharpsnout seabream activity patterns, as well as phasing independence of locomotor and feeding rhythms. Finally, the existence of a well-defined daily rhythm of plasma melatonin was found.

The ant Camponotus atriceps shows electroretinogram circadian rhythms

We studied the circadian variation of the electroretinogram (ERG) response of individual isolated ants of two castes: major workers and virgin queens of the species Campanotus atriceps subjected to different experimental light protocols. The experiments demonstrated that both castes of this species show similar ERG endogenous circadian rhythms able to entrain to 24 h LD cycles changing phase and period. This is the first work reporting circadian changes in the eye's electrophysiological response to light in ants.

Individual variability and photic entrainment of circadian rhythms in golden spiny mice

Golden spiny mice are diurnally active in most of their natural habitat. Their diurnal activity is ascribed to non-photic cues: competitive exclusion from the nocturnal niche, or thermoregulatory considerations. Here we studied the entrainment of golden spiny mice to light. In the laboratory, golden spiny mice were primarily nocturnal and displayed an unusual variety of rhythm patterns, with activity bursts occurring during both activity and rest periods. Spontaneous shifts of activity rhythms between light phases were sometimes recorded. In all cases but one, body temperature shifted in parallel with activity. Under DD conditions, the free running period (tau) of all individuals but one was shorter than 24 h, and in all individuals but the same one it was shorter than tau under LL conditions. In response to a 6 h phase delay, all individuals entrained to the new LD cycle in a relatively uniform way. During phase advance four out of the twelve individuals further delayed their activity and body temperature rhythms, and eight individuals advanced their activity rhythm, but the re-entrainment took them over twice as long as to re-entrain to the phase delay. We suggest that the golden spiny mouse is a nocturnal rodent whose circadian system developed the flexibility to be nocturnal or diurnal according to environmental conditions, or a nocturnal rodent in the process of turning diurnal, and that it has low sensitivity to the immediate masking effect of light on activity.

Effects of bilateral and unilateral ophthalmectomy on plasma melatonin in Rana tadpoles and froglets under various experimental conditions

The effect of ophthalmectomy (enucleation) on plasma melatonin in Rana tadpoles and froglets was studied under various experimental conditions to determine if ocular melatonin is released into the circulation from the eyes and to study the factors which might affect this process. Where operations occurred in early or mid-photophase on a 12 light:12 dark (12L:12D) cycle (light onset at 08:00 h), sampling in mid-light and mid-dark revealed that scotophase plasma melatonin was reduced at all developmental stages, with the more significant effects occurring before metamorphic climax. Experiments sampling prometamorphic tadpoles six times in a 24 h period on 18L:6D, 12L:12D, or 6L:18D five days after enucleation also showed a significant lowering of plasma melatonin in the dark, so that the scotophase peak was virtually eliminated on all the LD cycles. These findings indicated that the reduction in plasma melatonin after bilateral eye removal was independent of the LD cycle and the metamorphic stage, and that it abolished the diel melatonin rhythm at the expense of the scotophase peak. Experiments carried out for 5 weeks suggested that compensatory secretion of melatonin by other organs after eye removal might partially restore the plasma melatonin level over time. Unilateral ophthalmectomy tended to reduce, but not eliminate, the night peak of plasma melatonin, and did not result in a compensatory increase in ocular melatonin in the remaining eye. Ophthalmectomized tadpoles exhibited darkening of the skin after the operation, which was not associated with a significant change in pituitary α-melanotropin. The findings overall indicate that the eyes in Rana tadpoles and froglets contribute up to somewhat over one-half of the circulating melatonin, particularly during the scotophase, and provide experimental evidence for ocular secretion into the blood for the first time in the Amphibia.

History-Dependent Changes in Entrainment of the Activity Rhythm in the Syrian Hamster (Mesocricetus auratus)

The authors have studied the activity rhythm of Syrian hamsters exposed to square LD cycles with a 22-h period (T22) with the aim of testing the effects of the previous history on the rhythmic pattern. To do so, sequential changes of different lighting environments were established, followed by the same LD condition. Also, the protocol included T22 cycles with varying lighting contrasts to test the extent to which a computational model predicts experimental outcomes. At the beginning of the experiment, exposure to T22 with 300 lux and dim red light occurring respectively at photophase and scotophase (LD300/dim red) mainly generated relative coordination. Subsequent transfer to cycles with approximately 0.1-lux dim light during the scotophase (LD300/0.1) promoted entrainment to T22. However, a further reduction in light intensity to 10 lux during the photophase (LD10/0.1) generated weak and unstable T22 rhythms. When, after that, animals were transferred again to the initial LD300/dim red cycles, the amplitude of the rhythm still remained very low, and the phases were very unstable. Exposure to constant darkness partially restored the activity rhythm, and when, afterwards, the animals were submitted again to LD300/dim red cycles, a robust T22 rhythm appeared. The results demonstrate history-dependent changes in the hamster circadian system because the locomotor activity pattern under the same T22 cycle can show relative coordination or unstable or robust entrainment depending on the prior lighting condition. This suggests that the circadian system responds to environmental stimuli depending on its previous history. Moreover, computer simulations allow the authors to predict entrainment under LD300/0.1 cycles and indicate that most of the patterns observed in the animals due to the light in the scotophase can be explained by different degrees of coupling among the oscillators of the circadian system.

Carbon acquisition of marine phytoplankton: Effect of photoperiod length

We investigated the carbon acquisition of three marine microalgae, Skeletonema costatum, Phaeocystis globosa, and Emiliania huxleyi in response to different light regimes. Rates of photosynthetic O2 evolution and CO2 and HCO3− uptake were measured by membrane inlet mass spectrometry in cells acclimated to cycles of 16 : 8 light : dark (LD; h : h) and 12 : 12 LD and were compared with those obtained under continuous light. In addition, cellular leakage was estimated for different photoperiods and ambient CO2 concentrations during growth. Maximum rates of photosynthesis more or less doubled under LD cycles compared with continuous light. In S. costatum and E. huxleyi, a remarkably higher contribution of HCO3− to the overall carbon uptake was observed under LD cycles. In contrast, P. globosa did not change its CO2 :HCO3− uptake ratio in response to daylength. Half saturation concentrations (K1/2) for O2 evolution and inorganic carbon (Ci) uptake were also influenced by the photoperiod. Under LD cycles K1/2 values for photosynthesis in S. costatum and P. globosa were similar or higher compared with continuous light, whereas they were much lower in E. huxleyi. With the exception of CO2 uptake in E. huxleyi and P. globosa, affinities for Ci decreased under the LD cycles. Cellular leakage was highest for E. huxleyi and lowest for S. costatum and generally decreased with increasing CO2 concentration. Although this study confirms species specific differences in the CO2-concentrating mechanisms (CCMs), the effect of daylength on CO2 and HCO3− uptake has hitherto not been described. We put forward the idea that variations in light condition influence the cellular carbon demand, thereby imposing a stronger control on CCM regulation than the naturally occurring changes in CO2 supply.

A Network of (Autonomic) Clock Outputs

The circadian clock in the suprachiasmatic nuclei (SCN) is composed of thousands of oscillator neurons, each dependent on the cell‐autonomous action of a defined set of circadian clock genes. A major question is still how these individual oscillators are organized into a biological clock that produces a coherent output capable of timing all the different daily changes in behavior and physiology. We investigated which anatomical connections and neurotransmitters are used by the biological clock to control the daily release pattern of a number of hormones. The picture that emerged shows projections contacting target neurons in the medial hypothalamus surrounding the SCN. The activity of these pre‐autonomic and neuro‐endocrine target neurons is controlled by differentially timed waves of vasopressin, GABA, and glutamate release from SCN terminals, among other factors. Together our data indicate that, with regard to the timing of their main release period within the LD cycle, at least four subpopulations of SCN neurons should be discernible. The different subgroups do not necessarily follow the phenotypic differences among SCN neurons. Thus, different subgroups can be found within neuron populations containing the same neurotransmitter. Remarkably, a similar distinction of four differentially timed subpopulations of SCN neurons was recently also discovered in experiments determining the temporal patterns of rhythmicity in individual SCN neurons by way of the electrophysiology or clock gene expression. Moreover, the specialization of the SCN may go as far as a single body structure, i.e., the SCN seems to contain neurons that specifically target the liver, pineal gland, and adrenal gland.

Circadian Rhythms in Heart Rate, Motility, and Body Temperature of Wild‐type C57 and eNOS Knock‐out Mice Under Light‐dark, Free‐run, and After Time Zone Transition

The nitric oxide (NO) system is involved in the regulation of the cardiovascular system in controlling central and peripheral vascular tone and cardiac functions. It was the aim of this study to investigate in wild‐type C57BL/6 and endothelial nitric oxide synthase (eNOS) knock‐out mice (eNOS‐/‐) the contribution of NO on the circadian rhythms in heart rate (HR), motility (motor activity [MA]), and body temperature (BT) under various environmental conditions. Experiments were performed in 12∶12 h of a light:dark cycle (LD), under free‐run in total darkness (DD), and after a phase delay shift of the LD cycle by −6 h (i.e., under simulation of a westward time zone transition). All parameters were monitored by radiotelemetry in freely moving mice. In LD, no significant differences in the rhythms of HR and MA were observed between the two strains of mice. BT, however, was significantly lower during the light phase in eNOS‐/‐ mice, resulting in a significantly greater amplitude. The period of the free‐running rhythm in DD was slightly shorter for all variables, though not significant. In general, rhythmicity was greater in eNOS‐/‐ than in C57 mice both in LD and DD. After a delay shift of the LD cycle, HR and BT were resynchronized to the new LD schedule within 5–6 days, and resynchronization of MA occurred within 2–3 days. The results in telemetrically instrumented mice show that complete knock‐out of the endothelial NO system—though expressed in the suprachiasmatic nuclei and in peripheral tissues—did not affect the circadian organization of heart rate and motility. The circadian regulation of the body temperature was slightly affected in eNOS‐/‐ mice.

Individual Differences in Rhythms of Behavioral Sleep and Its Neural Substrates in Nile Grass Rats

Laboratory populations of grass rats (Arvicanthis niloticus) housed with a running wheel show considerable variation in patterns of locomotor activity. At the extremes are "day-active" (DA) animals with a monophasic distribution of running throughout the light phase and "night-active" (NA) animals exhibiting a biphasic pattern with an extended peak at the beginning of the dark phase and a brief peak shortly before lights-on. Here, the authors use this intraspecific variation to explore interactions between circadian and homeostatic influences on sleep and the effects of these interactions on the activity of brain regions involved in sleep regulation. Male animals were singly housed with running wheels in a 12:12 LD cycle, videotaped for 24 h, and perfused at ZT 4 or 16. Behavioral sleep was scored from the videotapes, and brains were processed for cFos immunoreactivity (cFos-ir). Sleep duration within the light and dark phases was higher in NA and DA animals, respectively, but these groups did not differ with respect to total sleep. In both groups, sleep bouts were shortest in the light phase and longest between ZT 20 and ZT 23. In the ventrolateral preoptic area (VLPO), cFos-ir was higher at ZT 16 than at ZT 4 in DA but not NA grass rats, and it was correlated with behavioral sleep at ZT 16 but not ZT 4. In OXA neurons, cFos-ir was high at ZT 4 in DA grass rats and at ZT 16 in NA grass rats, and it was correlated with behavioral sleep at both times. In the lower subparaventricular zone (LSPV), cFos-ir was higher at ZT 16 in both DA and NA animals, and it was unrelated to behavioral sleep. Thus, patterns of cFos-ir in the LSPV and OXA neurons were most tightly linked to time and sleep, respectively, whereas cFos-ir in the VLPO was influenced by an interaction between these 2 variables.

Circadian Rhythm of a TCA Cycle Enzyme Is Apparently Regulated at the Translational Level in the Dinoflagellate Lingulodinium polyedrum

Previously, the authors have reported that intracellular amounts of several metabolic-related enzymes from the photosynthetic dinoflagellate Lingulodinium polyedrum(formerly Gonyaulax polyedra) showed a daily rhythm under a 12:12 h LD cycle. This led the authors to hypothesize that a circadian clock controls metabolism, including the tricarboxylic acid (TCA) cycle. In this study, the authors investigated daily changes in the levels of mRNA, protein, and enzyme activity of several metabolic enzymes during 12:12 h LD, 8:16 h LD, and constant light conditions. The NADP-dependent isocitrate dehydrogenase (NADPICDH) in the TCA cycle exhibited circadian changes of protein abundance and enzyme activity under all conditions, whereas its mRNA level remained constant throughout the cycle. These results indicate that the rhythm of NADPICDH is regulated by a circadian control of protein synthesis or modification rather than by message levels and suggest that the TCA cycle may be controlled by the circadian clock system.

Effects of Preparation Time on Phase of Cultured Tissues Reveal Complexity of Circadian Organization

The phases of central (SCN) and peripheral circadian oscillators are held in specific relationships under LD cycles but, in the absence of external rhythmic input, may damp or drift out of phase with each other. Rats exposed to prolonged constant light become behaviorally arrhythmic, perhaps as a consequence of dissociation of phases among SCN cells. The authors asked whether individual central and peripheral circadian oscillators were rhythmic in LL-treated arrhythmic rats and, if rhythmic, what were the phase relationships between them. The authors prepared SCN, pineal gland, pituitary, and cornea cultures from transgenic Period1-luciferaserats whose body temperature and locomotor activity were arrhythmic and from several groups of rhythmic rats held in LD, DD, and short-term LL. The authors measured mPer1gene expression by recording light output with sensitive photomultipliers. Most of the cultures from all groups displayed circadian rhythms. This could reflect persistent rhythmicity in vivo prior to culture or, alternatively, rhythmicity that may have been initiated by the culture procedure. To test this, the authors cultured tissues at 2 different times 12 h apart and asked whether phase of the rhythm was related to culture time. The pineal, pituitary, and SCN cultures showed partial or complete dependence of phase on culture time, while peak phases of the cornea cultures were independent of culture time in rhythmic rats and were randomly distributed regardless of culture time in arrhythmic animals. These results suggest that in behaviorally arrhythmic rats, oscillators in the pineal, pituitary, and SCN had been arrhythmic or severely damped in vivo, while the cornea oscillator was free running. The peak phases of the SCN cultures were particularly sensitive to some aspect of the culture procedure since rhythmicity of SCN cultures from robustly rhythmic LD-entrained rats was strongly influenced when the procedure was carried out at any time except the 2nd half of the day.

Circadian patterns in the activity of the Brazilian cave cricket Strinatia brevipennis (Ensifera: Phalangopsidae)

This paper presents data on the activity rhythms of the Brazilian cave cricket, Strinatia brevipennis. Recordings were made in the laboratory in a sound-proof constant environment. Recordings were made either under constant darkness or LD cycles, with food provided ad libitum or not, and with crickets isolated or with a conspecific near the cage. Some crickets were tested with pulses of sound. Raw data are presented in the form of single plot actograms and analyzed using the phase weighted stack (PWS) method. These cave crickets showed a somewhat erratic patterns of activity, although a circadian component could be detected. All the environmental variants tested (food availability, presence of conspecific and sound pulses) seemed to mask any activity rhythm indicating they are possibly important in determining the temporal organization of these crickets. Their activity patterns are discussed in terms of the plasticity of a cave animal's circadian system.

Reentrainment of the Circadian Pacemaker through Three Distinct Stages

Circadian rhythms are endogenously generated by a central pacemaker and are synchronized to the environmental LD cycle. The rhythms can be resynchronized, or reentrained, after a shift of the LD cycle, as in traveling across time zones. The authors have performed high-resolution mapping of the pacemaker to analyze the reentrainment process using rat pineal melatonin onset (MT(on)) and melatonin offset (MT(off)) rhythms as markers. Following LD (12:12) delays of 3, 6, and 12 h, MT(on) was phase locked immediately, whereas MT(off) shifted rapidly during the initial 1 through 3 cycles. In all animals, the MT(off) shifted beyond their expected phase positions in the new LD cycle, which resulted in a transient expansion of melatonin secretion duration for several cycles. It took MT(off) only 1, 2, or 3 cycles to complete most of the required phase shifts after 3, 6, or 12 h of the LD cycle delays, respectively. However, the final stabilization of phase relationships of both MT(on) and MT(off) required at least 6 cycles for rats experiencing a 3-h LD delay and much longer for the rest. These results reaffirmed the notion that both onset and offset phases of melatonin rhythms are important markers for the pacemaker and demonstrated that the reentrainment of the central pacemaker to a delay shift of the LD cycle is a 3-step process: an immediate phase lock of onset and a rapid delay shift of offset rhythms, overshoot of the offset, and, finally, a slow adjustment of both onset and offset phases. This study represents the 1st detailed analysis of the pacemaker behavior during reentrainment using melatonin and supports the notion that the eventual adaptation of the circadian pacemaker to a new time zone is a time-consuming process.

Neonatal Alcohol Exposure Permanently Disrupts the Circadian Properties and Photic Entrainment of the Activity Rhythm in Adult Rats

Alcohol exposure during the period of rapid brain development produces structural damage in different brain regions, including the suprachiasmatic nucleus (SCN), that may have permanent neurobehavioral consequences. Thus, this study examined the long-term effects of neonatal alcohol exposure on circadian behavioral activity in adult rats. Artificially reared Sprague-Dawley rat pups were exposed to alcohol (EtOH; 4.5 g/kg/day) or isocaloric milk formula (gastrostomy control; GC) on postnatal days 4-9. At 2 months of age, rats from the EtOH, GC, and suckle control (SC) groups were housed individually, and properties of the circadian rhythm in wheel-running behavior were continuously analyzed during exposure to a 12-hr light:12-hr dark photoperiod (LD 12:12) or constant darkness (DD). Neonatal alcohol exposure had distinctive effects on the rhythmic properties and quantitative parameters of adult wheel-running behavior. EtOH-treated animals were distinguished by unstable and altered entrainment to LD 12:12 such that their daily onsets of activity were highly variable and occurred at earlier times relative to control animals. In DD, circadian regulation of wheel-running behavior was altered by neonatal alcohol exposure such that the free-running period of the activity rhythm was shorter in EtOH-exposed rats than in control animals. Total amount of daily wheel-running activity in EtOH-treated rats was greater than that observed in the SC group. In addition, the circadian activity patterns of EtOH-exposed rats were fragmented such that the duration of the active phase and the number of activity bouts per day were increased. These data indicate that neonatal alcohol exposure produces permanent changes in the circadian regulation of the rat activity rhythm and its entrainment to LD cycles. These long-term alterations in circadian behavior, along with the developmental alcohol-induced changes in SCN endogenous rhythmicity, may have important implications in clinical sleep-wake disturbances observed in neonates, children, and adults exposed to alcohol in utero.

Light-dark cycle synchronization of circadian rhythm in blind primates.

BackgroundRecently, several papers have shown that a small subset of retinal ganglion cells (RGCs), which project to the suprachiasmatic nucleus (SCN) and contain a new photopigment called melanopsin, are the photoreceptors involved in light-dark entrainment in rodents. In our primate colony, we found a couple of common marmosets (Callithrix jacchus) that had developed progressive and spontaneous visual deficiency, most likely because of retinal degeneration of cones and/or rods. In this study, we evaluated the photoresponsiveness of the circadian system of these blind marmosets.MethodsTwo blind and two normal marmosets were kept in cages with a controlled light-dark cycle (LD) to study photoentrainment, masking, and phase response to a dark pulse.ResultsBlind marmosets were entrained with the new LD cycle when light onsets were delayed and advanced by 6 hours. In constant light conditions, blind marmosets free-ran with a period of 23.2 hours, while normal animals free-ran with a period of 23.6 hours. All marmosets responded to dark pulses in the early subjective day with phase delays and with phase advances in the late subjective day.ConclusionOur results demonstrate that light can synchronize circadian rhythms of blind marmosets and consequently, that this species could be a good primate model for circadian photoreception studies.