Entrainment of the circadian rhythms of blinded infant rats by nursing mothers

Peripheral clock gene oscillations are perturbed in neonatal and adult rat offspring raised under adverse limited bedding conditions

Circadian (24-h) rhythms in the suprachiasmatic nucleus (SCN) are established in utero in rodents, but rhythmicity of peripheral circadian clocks appears later in postnatal development. Since peripheral oscillators can be influenced by maternal feeding and behavior, we investigated whether exposure to the adverse environmental conditions of limited bedding (LB) during postnatal life would alter rhythmicity in the SCN, adrenal gland and liver in neonatal (postnatal day PND10), juvenile (PND28) and adult rats. We also examined locomotor activity in adults. Limited bedding increased nursing time and slightly increased fragmentation of maternal behavior. Exposure to LB reduced the amplitude of Per2 in the SCN on PND10. Adrenal clock gene expression (Bmal1, Per2, Cry1, Rev-erbα, Dbp) and corticosterone secretion were rhythmic at all ages in NB offspring, whereas rhythmicity of Bmal1, Cry1 and corticosterone was abolished in neonatal LB pups. Circadian gene expression in the adrenal and liver was well established by PND28. In adults, liver expression of several circadian genes was increased at specific daytimes by LB and the microstructure of locomotor behavior was altered. Thus, changes in maternal care and behavior might provide important signals to the maturing peripheral oscillators and modify, in particular their output functions in the long-term.

Circadian Regulation of the Neuroimmune Environment Across the Lifespan: From Brain Development to Aging

Circadian clocks confer 24-h periodicity to biological systems, to ultimately maximize energy efficiency and promote survival in a world with regular environmental light cycles. In mammals, circadian rhythms regulate myriad physiological functions, including the immune, endocrine, and central nervous systems. Within the central nervous system, specialized glial cells such as astrocytes and microglia survey and maintain the neuroimmune environment. The contributions of these neuroimmune cells to both homeostatic and pathogenic demands vary greatly across the day. Moreover, the function of these cells changes across the lifespan. In this review, we discuss circadian regulation of the neuroimmune environment across the lifespan, with a focus on microglia and astrocytes. Circadian rhythms emerge in early life concurrent with neuroimmune sculpting of brain circuits and wane late in life alongside increasing immunosenescence and neurodegeneration. Importantly, circadian dysregulation can alter immune function, which may contribute to susceptibility to neurodevelopmental and neurodegenerative diseases. In this review, we highlight circadian neuroimmune interactions across the lifespan and share evidence that circadian dysregulation within the neuroimmune system may be a critical component in human neurodevelopmental and neurodegenerative diseases.

Circadian Regulation of Hormonal Timing and the Pathophysiology of Circadian Dysregulation

Circadian rhythms are endogenously generated, daily patterns of behavior and physiology that are essential for optimal health and disease prevention. Disruptions to circadian timing are associated with a host of maladies, including metabolic disease and obesity, diabetes, heart disease, cancer, and mental health disturbances. The circadian timing system is hierarchically organized, with a master circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and subordinate clocks throughout the CNS and periphery. The SCN receives light information via a direct retinal pathway, synchronizing the master clock to environmental time. At the cellular level, circadian rhythms are ubiquitous, with rhythms generated by interlocking, autoregulatory transcription-translation feedback loops. At the level of the SCN, tight cellular coupling maintains rhythms even in the absence of environmental input. The SCN, in turn, communicates timing information via the autonomic nervous system and hormonal signaling. This signaling couples individual cellular oscillators at the tissue level in extra-SCN brain loci and the periphery and synchronizes subordinate clocks to external time. In the modern world, circadian disruption is widespread due to limited exposure to sunlight during the day, exposure to artificial light at night, and widespread use of light-emitting electronic devices, likely contributing to an increase in the prevalence, and the progression, of a host of disease states. The present overview focuses on the circadian control of endocrine secretions, the significance of rhythms within key endocrine axes for typical, homeostatic functioning, and implications for health and disease when dysregulated. © 2022 American Physiological Society. Compr Physiol 12: 1-30, 2022.

Comparative Neurology of Circadian Photoreception: The Retinohypothalamic Tract (RHT) in Sighted and Naturally Blind Mammals

The mammalian eye contains two systems for light perception: an image detecting system constituted primarily of the classical photoreceptors, rods and cones, and a non-image forming system (NIF) constituted of a small group of intrinsically photosensitive retinal ganglion cells driven by melanopsin (mRGCs). The mRGCs receive input from the outer retina and NIF mediates light entrainment of circadian rhythms, masking behavior, light induced inhibition of nocturnal melatonin secretion, pupillary reflex (PLR), and affect the sleep/wake cycle. This review focuses on the mammalian NIF and its anatomy in the eye as well as its neuronal projection to the brain. This pathway is known as the retinohypothalamic tract (RHT). The development and functions of the NIF as well as the knowledge gained from studying gene modified mice is highlighted. Furthermore, the similarities of the NIF between sighted (nocturnal and diurnal rodent species, monkeys, humans) and naturally blind mammals (blind mole rats Spalax ehrenbergi and the Iberian mole, Talpa occidentalis) are discussed in relation to a changing world where increasing exposure to artificial light at night (ALAN) is becoming a challenge for humans and animals in the modern society.

Circadian Photoentrainment in Mice and Humans

Light around twilight provides the primary entrainment signal for circadian rhythms. Here we review the mechanisms and responses of the mouse and human circadian systems to light. Both utilize a network of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). In both species action spectra and functional expression of OPN4 in vitro show that melanopsin has a λ_max close to 480 nm. Anatomical findings demonstrate that there are multiple pRGC sub-types, with some evidence in mice, but little in humans, regarding their roles in regulating physiology and behavior. Studies in mice, non-human primates and humans, show that rods and cones project to and can modulate the light responses of pRGCs. Such an integration of signals enables the rods to detect dim light, the cones to detect higher light intensities and the integration of intermittent light exposure, whilst melanopsin measures bright light over extended periods of time. Although photoreceptor mechanisms are similar, sensitivity thresholds differ markedly between mice and humans. Mice can entrain to light at approximately 1 lux for a few minutes, whilst humans require light at high irradiance (>100’s lux) and of a long duration (>30 min). The basis for this difference remains unclear. As our retinal light exposure is highly dynamic, and because photoreceptor interactions are complex and difficult to model, attempts to develop evidence-based lighting to enhance human circadian entrainment are very challenging. A way forward will be to define human circadian responses to artificial and natural light in the “real world” where light intensity, duration, spectral quality, time of day, light history and age can each be assessed.

Development of the mammalian circadian clock

The mammalian circadian system is composed of a central clock situated in the hypothalamic suprachiasmatic nucleus (SCN) and peripheral clocks of each tissue and organ in the body. While much has been learned about the pre- and postnatal development of the circadian system, there are still many unanswered questions about how and when cellular clocks start to tick and form the circadian system. Most SCN neurons contain a cell-autonomous circadian clock with individual specific periodicity. Therefore, the network of cellular oscillators is critical for the coherent rhythm expression and orchestration of the peripheral clocks by the SCN. The SCN is the only circadian clock entrained by an environmental light-dark cycle. Photic entrainment starts postnatally, and the SCN starts to function gradually as a central clock that controls physiological and behavioral rhythms during postnatal development. The SCN exhibits circadian rhythms in clock gene expression from the embryonic stage throughout postnatal life and the rhythm phenotypes remain basically unchanged. However, the disappearance of coherent circadian rhythms in cryptochrome-deficient SCN revealed changes in the SCN networks that occur in postnatal weeks 2-3. The SCN network consists of multiple clusters of cellular circadian rhythms that are differentially integrated by the vasoactive intestinal polypeptide and arginine vasopressin signaling depending on the period of postnatal development.

Provoking predetermined aperiodic patterns in human brainwaves

In the present work, electroencephalographic recordings of healthy human participants were performed to study the entrainment of brainwaves using a variety of stimuli. First, the periodic entrainment of the brainwaves was studied using two different stimuli in the form of periodic auditory and visual signals. The entrainment with the periodic visual stimulation was consistently observed, whereas the auditory entrainment was inconclusive. Hence, a photic (visual) stimulus, where two frequencies were presented to the subject simultaneously, was used to further explore the bifrequency entrainment of human brainwaves. Subsequently, the evolution of brainwaves as a result of an aperiodic stimulation was explored, wherein an entrainment to the predetermined aperiodic pattern was observed. These results suggest that the aperiodic entrainment could be used as a tool for guided modification of brainwaves. This could find possible applications in processes such as epilepsy suppression and biofeedback.

Two coupled circadian oscillations regulate Bmal1-ELuc and Per2-SLR2 expression in the mouse suprachiasmatic nucleus

Circadian rhythms in clock genes, Bmal1 and Per2 expression were monitored simultaneously in the cultured slice of mouse suprachiasmatic nucleus (SCN) by dual bioluminescent reporters. In the neonatal SCN, the phase-relation between the Bmal1 and Per2 rhythms were significantly changed during culture. Medium exchange produced phase-dependent phase shifts (PRCm) in the Bmal1 rhythms, but not in the Per2 rhythms. As a result, the two circadian rhythms were temporally dissociated after medium exchange. In the adult SCN, the phase-relation between the two rhythms was kept constant during culture at least up to 20 cycles. The amplitude of PRCm in the adult SCN was significantly attenuated in the Bmal1 rhythm, whereas a PRCm was developed in the Per2 rhythm. The circadian period was not systematically affected by medium exchange in either of rhythms, regardless of whether it was in the neonatal or the adult SCN. Tetrodotoxin, a sodium channel blocker, enhanced the phase-response in both rhythms but abolished the phase-dependency. In addition, tetrodotoxin lengthened the circadian period independent of the phase of administration. Thus, the Bmal1 and Per2 rhythms in the SCN are dissociable and likely regulated by distinct circadian oscillators. Bmal1 is the component of a Bmal1/REV-ERBa/ROR loop and Per2 a Per/Cry/BMAL1/CLOCK loop. Both loops could be molecular mechanisms of the two circadian oscillators that are coupled through the protein product of Bmal1. The coupling strength between the two oscillations depends on developmental stages.

Effects of maternal separation on the dietary preference and behavioral satiety sequence in rats

This study investigated the effects of maternal separation on the feeding behavior of rats. A maternal separation model was used on postnatal day 1 (PND1), forming the following groups: in the maternal separation (MS) group, pups were separated from their mothers each day from PND1 to PND14, whereas in the control (C) group pups were kept with their mothers. Subgroups were formed to study the effects of light and darkness: control with dark and light exposure, female and male (CF and CM), and maternal separation with dark and light exposure, female and male (SDF, SDM, SLF and SLM). Female rats had higher caloric intake relative to body weight compared with male controls in the dark period only (CF=23.3±0.5 v. CM=18.2±0.7, P<0.001). Macronutrient feeding preferences were observed, with male rats exhibiting higher caloric intake from a protein diet as compared with female rats (CF=4.1±0.7, n=8 v. CM=7.0±0.5, n=8, P<0.05) and satiety development was not interrupted. Female rats had a higher adrenal weight as compared with male rats independently of experimental groups and exhibited a higher concentration of serum triglycerides (n=8, P<0.001). The study indicates possible phenotypic adjustments in the structure of feeding behavior promoted by maternal separation, especially in the dark cycle. The dissociation between the mother's presence and milk intake probably induces adjustments in feeding behavior during adulthood.

Embryonic development of circadian clocks in the mammalian suprachiasmatic nuclei

In most species, self-sustained molecular clocks regulate 24-h rhythms of behavior and physiology. In mammals, a circadian pacemaker residing in the hypothalamic suprachiasmatic nucleus (SCN) receives photic signals from the retina and synchronizes subordinate clocks in non-SCN tissues. The emergence of circadian rhythmicity during development has been extensively studied for many years. In mice, neuronal development in the presumptive SCN region of the embryonic hypothalamus occurs on days 12–15 of gestation. Intra-SCN circuits differentiate during the following days and retinal projections reach the SCN, and thus mediate photic entrainment, only after birth. In contrast the genetic components of the clock gene machinery are expressed much earlier and during midgestation SCN explants and isolated neurons are capable of generating molecular oscillations in culture. In vivo metabolic rhythms in the SCN, however, are observed not earlier than the 19th day of rat gestation, and rhythmic expression of clock genes is hardly detectable until after birth. Together these data indicate that cellular coupling and, thus, tissue-wide synchronization of single-cell rhythms, may only develop very late during embryogenesis. In this mini-review we describe the developmental origin of the SCN structure and summarize our current knowledge about the functional initiation and entrainment of the circadian pacemaker during embryonic development.

The development of sleep-wake rhythms and the search for elemental circuits in the infant brain

Despite the predominance of sleep in early infancy, developmental science has yet to play a major role in shaping concepts and theories about sleep and its associated ultradian and circadian rhythms. Here we argue that developmental analyses help us to elucidate the relative contributions of the brainstem and forebrain to sleep-wake control and to dissect the neural components of sleep-wake rhythms. Developmental analysis also makes it clear that sleep-wake processes in infants are the foundation for those of adults. For example, the infant brainstem alone contains a fundamental sleep-wake circuit that is sufficient to produce transitions among wakefulness, quiet sleep, and active sleep. In addition, consistent with the requirements of a "flip-flop" model of sleep-wake processes, this brainstem circuit supports rapid transitions between states. Later in development, strengthening bidirectional interactions between the brainstem and forebrain contribute to the consolidation of sleep and wake bouts, the elaboration of sleep homeostatic processes, and the emergence of diurnal or nocturnal circadian rhythms. The developmental perspective promoted here critically constrains theories of sleep-wake control and provides a needed framework for the creation of fully realized computational models. Finally, with a better understanding of how this system is constructed developmentally, we will gain insight into the processes that govern its disintegration due to aging and disease.

The effect of maternal stress activation on the offspring during lactation in light of vasopressin

Although it is obvious that preconceptional effects as well as stressors during pregnancy profoundly influence the progeny, the lactation period seems to be at least as important. Here we summarize how maternal stressors during the lactation period affect the offspring. As vasopressin is one of the crucial components both for stress adaptation and social behavior, special emphasis was given to this neuropeptide. We can conclude that stressing the mother does not have the same acute effect on the hypothalamo-pituitary-adrenocortical axis (as the main target of stress adaptation) of the pups as stressing the pups, but later endocrine and behavioral consequences can be similar. Vasopressin plays a role in acute and later consequences of perinatal stressor applied either to the mother or to the offspring, thereby contributing to transmitting the mothers' stress to the progeny. This mother-infant interaction does not necessarily mean a direct transmission of molecules, but rather is the result of programming the brain development through changes in maternal behavior. Thus, there is a time lag between maternal stress and stress-related changes in the offspring. The interactions are bidirectional as not only stress in the dam but also stress in the progeny has an effect on nursing.

The development of day-night differences in sleep and wakefulness in norway rats and the effect of bilateral enucleation

The suprachiasmatic nucleus exhibits circadian rhythmicity in fetal and infant rats, but little is known about the consequences of this rhythmicity for infant behavior. Here, in experiment 1, the authors measured sleep and wakefulness in rats during the day and night in postnatal day (P)2, P8, P15, and P21 subjects. As early as P2, day-night differences in sleep-wake activity were detected. Nocturnal wakefulness began to emerge around P15 and was reliably expressed by P21. The authors hypothesized that the process of photic entrainment over the 1st postnatal week, which depends on the development of connectivity between the retinohypothalamic tract (RHT) and the SCN, influences the later emergence of nocturnal wakefulness. To test this hypothesis, in experiment 2 infant rats were enucleated bilaterally at P3 and P11, that is, before and after photic entrainment. Whereas pups enucleated at P11 and tested at P21 exhibited increased wakefulness at night, identical to sham controls, pups enucleated at P3 and tested at P21 exhibited the opposite pattern of increased wakefulness during the day. Pups tested at P28 and P35 exhibited this same pattern of increased daytime wakefulness. All together, these results suggest that prenatal and postnatal experience modulates the development of species-typical circadian sleep-wake patterns. Moreover, the authors suggest that visual system stimulation, via the RHT's connections with the SCN, exerts an organizational influence on the developing circadian system and, consequently, contributes to the emergence of nocturnality in this species.

Photic influences on the developing mammal

In adult mammals, the daily light-dark cycle acts via the retinohypothalamic pathway to entrain the circadian clock in the suprachiasmatic nuclei (SCN) and to communicate information about daylength to photoperiodic species. Studies in rats show that during late fetal and early neonatal life, before the retinohypothalamic pathway has innervated the SCN, the maternal circadian system entrains the timing of the developing clock to prevailing lighting conditions. Although the nature of the maternal output signal(s) used to entrain the developing clock has not been elucidated, the maternal SCN are a necessary component of maternal entrainment during both prenatal and postnatal life. Maternal entrainment of the fetal and neonatal clock thus ensures that the developing circadian system is synchronized to the outside world until maturation of the retinohypothalamic pathway permits direct photic entrainment. The maternal circadian system is not only necessary for entrainment of the developing circadian system, but recent studies suggest it may also provide the immature mammal with important photoperiodic information. In the montane vole (Microtus montanus) and the Djungarian hamster (Phodopus sungorus), the prenatal photoperiod affects postnatal photoperiodic responses, and cross-fostering experiments show that this information about daylength is perceived by the fetus. This prenatal information, in conjunction with postnatal perception of photoperiod, allows the developing animal to determine which way the season is changing and to modify the rate of reproductive maturation accordingly.

Constant light disrupts the developing mouse biological clock

The central biological clock of the brain, contained within the suprachiasmatic nuclei (SCN) of mammals, orchestrates an orderly "internal day" of physiology and behavior. The developing biological clock begins to respond to light at an early stage and a particular concern in humans is whether light exposure has disruptive effects on the developing biological clock of infants exposed to constant lighting conditions in neonatal intensive care units (NICUs). Worldwide, eighteen million, or 14%, of newborns estimated to be of low birth weight, are exposed to artificial lighting environments in hospital nurseries annually. Here, we have tested whether constant light (LL) exposure disrupts the developing biological clock of mice, using a circadian reporter transgenic mouse model in which the organization of the central biological clock can be assayed by real-time gene expression imaging. We now find that LL has both acute and long-term disruptive effects on developing biological clocks and that cyclic lighting conditions are critical for developing circadian clocks to coordinate their molecular circadian mechanisms. This suggests that, from the perspective of developing circadian organization in humans, cyclic light conditions in NICUs are likely to be most appropriate for infants.

Ontogenetic development of the mammalian circadian system

This review summarizes the current knowledge about the ontogenetic development of the circadian system in mammals. The developmental changes of overt rhythms are discussed, although the main focus of the review is the underlying neuronal and molecular mechanisms. In addition, the review describes ontogenetic development, not only as a process of morpho-functional maturation. The need of repeated adaptations and readaptations due to changing developmental stage and environmental conditions is also considered. The review analyzes mainly rodent data, obtained from the literature and from the author's own studies. Results from other species, including humans, are presented to demonstrate common features and species-dependent differences. The review first describes the development of the suprachiasmatic nuclei as the central pacemaker system and shows that intrinsic circadian rhythms are already generated in the mammalian fetus. As in adult organisms, the period length is different from 24 h and needs continuous correction by environmental periodicities, or zeitgebers. The investigation of the ontogenetic development of the mechanisms of entrainment reveals that, at prenatal and early postnatal stages, non-photic cues deriving from the mother are effective. Light-dark entrainment develops later. At a certain age, both photic and non-photic zeitgebers may act in parallel, even though the respective time information is 12 h out of phase. That leads to a temporary internal desynchronization. Because rhythmic information needs to be transferred to effector organs, the corresponding neural and humoral signalling pathways are also briefly described. Finally, to be able to transform a rhythmic signal into an overt rhythm, the corresponding effector organs must be functionally mature. As many of these organs are able to generate their own intrinsic rhythms, another aspect of the review is dedicated to the development of peripheral oscillators and mechanisms of their entrainment. The latter includes control by the central pacemaker as well as by distinct environmental signals. Ecological aspects of the described developmental changes in the circadian system and some practical consequences are also briefly discussed.

Melanopsin containing retinal ganglion cells are light responsive from birth

Photoentrainment of the biological clock located in the suprachiasmatic nucleus (SCN) begins shortly after birth. Here we show using c-FOS immunoreactivity as a marker for neuronal activity that the melanopsin/PACAP containing retinal ganglion cells (RGCs) which project to the SCN as the retinohypothalamic tract (RHT) are responsive to light from birth. After postnatal day 12 where the classical photoreceptors become functional other RGCs and cells of the inner nuclear cell layer also respond to light. Light also induces c-FOS immunoreactivity in the retinorecipient SCN from the first postnatal day and accordingly PACAP immunoreactive fibres are visible in the SCN. The results indicate that the retina is light responsive before functional rods and cones and that the RHT is functional from birth supporting that photoentrainment of the biological clock begins shortly after birth.

Pubertal development of sex differences in circadian function: an animal model

The development of adult circadian function, particularly sexual dimorphism of function, has been well studied only in rapidly developed rodents. In such species development is complete by weaning. Data from adolescent humans suggest that significant development occurs during the pubertal period. We hypothesized that a more slowly developing rodent might better mimic the changes in circadian function around puberty in humans and allow us to determine the underlying neural changes. Entrained and free-running circadian rhythms were analyzed and correlated with pubertal development in male and female Octodon degus (degu) that remained gonadally intact or were gonadectomized at weaning. Brains were collected during development to measure androgen and estrogen receptors in the suprachiasmatic nuclei (SCN) Adult circadian period does not develop until 10-12 months of age in degus, long after the onset of gonadal maturation (3-5 months). The timing of circadian period maturation correlates with the appearance of steroid receptors in the SCN. Changes in free-running rhythms only occurred in gonadally intact degus. Adult phase angles of activity onset develop between 2 and 3 months of age (comparing results of two experiments), soon after the onset of pubertal changes.

CONCLUSION

The development of sexually dimorphic adult circadian period occurs after gonadal puberty is complete and requires the presence of gonadal steroids. The delay in development until after gonadal puberty is likely due to the delayed appearance of steroid receptors in the SCN. Phase is not sexually dimorphic and changes in the absence of steroid hormones.

The biological clock nucleus: a multiphasic oscillator network regulated by light

The circadian clock nucleus of the mammalian brain is composed of thousands of oscillator neurons, each driven by the cell-autonomous action of a defined set of circadian clock genes. A critical question is how these individual oscillators are organized into an internal clock that times behavior and physiology. We examined the neural organization of the suprachiasmatic nucleus (SCN) through time-lapse imaging of a short-half-life green fluorescent protein (GFP) reporter of the circadian clock gene Period 1 (Per1). Using brain slice preparations, Per1 promoter rhythms were resolved at the level of the SCN, and in individual neurons within the SCN, to determine the temporal patterns of rhythmicity resulting from exposure of mice to light/dark cycle (LD) and constant darkness (DD) conditions. Quantitative imaging and patch-clamp electrophysiology were used to define the relationship of Per1 gene expression to neurophysiological output on an individual neuron basis. We found that in both LD and DD, the overall rhythm of the clock nucleus is composed of individual cellular rhythms that peak in distinct phase groups at 3-4 hr intervals. However, the phase relationships of Per1 oscillations to locomotor activity and the phase relationships among individual neuronal oscillators within the SCN are different in LD and DD. There was a positive, linear correlation of Per1 transcription with neuronal spike frequency output, thus Per1::GFP rhythms are representative of physiological rhythmicity. Our results reveal multiple phase groupings of SCN oscillators and suggest that light regulation of oscillator interactions within the SCN underlies entrainment to the photoperiod.

Periodic absence of nursing mothers phase-shifts circadian rhythms of clock genes in the suprachiasmatic nucleus of rat pups

Effects of absence of nursing mothers on the circadian pacemaker of their offspring were examined by measuring clock genes, the rat Per1 (rPer1) and rPer2 expression rhythms in the pup suprachiasmatic nuclei (SCN). Neonate rats born to mothers kept under a 12-h light : 12-h dark cycle (LD) were blinded immediately after birth and exposed to periodic maternal deprivation where pups were deprived of their mothers during the light phase of 12-h for the first week of life. At postnatal day 6, the periodic maternal deprivation completely phase-reversed the circadian rhythms in expression of the clock genes in the pup SCN and in spontaneous locomotor activity after the pups were weaned at postnatal day 21. The periodic maternal absence also altered the patterns of stress-related gene expressions such as corticotropine-releasing hormone, arginine vasopressin, and glucocorticoid receptor in particular brain areas of the mother-deprived pups at P6. These findings indicate that periodic absence of the nursing mother in the first week of life produces a resetting effect on the neonatal circadian clock and induces stress responses in the hypothalamus-pituitary-adrenal axis.

Effects of nursing mothers on rPer1 and rPer2 circadian expressions in the neonatal rat suprachiasmatic nuclei vary with developmental stage

The ability of nursing mothers to entrain the circadian pacemaker of rat pups was examined by measuring the rat Per1 (rPer1) and rPer2 expression levels in the suprachiasmatic nuclei (SCN). Newborn rats from mothers under a light-dark cycle (LD) were blinded immediately after birth and reared by foster mothers under either LD (LD blind pups) or reversed light-dark cycle (DL; DL blind pups). At postnatal day (P)6, small but significant phase differences were observed in the circadian gene expression rhythms of the SCN not only between the blind and sighted pups, but also between the two groups of blind pups, indicating the involvement of both free-running and maternal influence in phase-resetting the circadian rhythms of blind pups. However, from P6 to P13 the circadian rhythms of both LD and DL blind pups showed phase delays of similar extent, which suggests that the influence of nursing mothers was lost. From P13 to P20 (the day of weaning), the rPer1 and rPer2 rhythms phase-shifted in a different manner, the rPer2 rhythm being related more closely to the behavioural rhythm than was the rPer1. This finding suggests a differential influence of mothers on the rPer1 and rPer2 rhythms in the third week of life. It is concluded that the ability of nursing mothers to entrain pup circadian oscillation depends on the developmental stage.

Maternal entrainment of the developing circadian system in the Siberian hamster (Phodopus sungorus)

The aim of these studies was to investigate maternal entrainment of developing circadian locomotor activity rhythms in the Siberian hamster. In Experiment 1, mothers were transferred from a 16:8 LD cycle into constant dim red light (DD) from the day of parturition, and wheel-running activity of the mother and pups was individually monitored from the time of weaning. The phases of the individual pups' rhythms were found to be synchronized both to the phase of the mother and to the phase of lights off (ZT 12) of the photo cycle that the mother was exposed to until the day of parturition. To investigate whether this synchrony might reflect direct effects of light acting upon the fetal circadian system in late gestation, the experiment was repeated but with mothers placed into DD early in pregnancy (< or = day 7 of gestation). The results were similar to the first study, suggesting that the mother rather than the photo cycle during the latter part of gestation entrains the developing circadian system. The third experiment investigated whether this entrainment occurred during the postnatal period. Breeding pairs were maintained on alternative light-dark cycles, LD and DL, that were 12 h out of phase. Litters born to mothers on one light-dark cycle were exchanged on the day of birth with foster mothers from the reversed light-dark cycle, then raised in DD. Control litters exchanged between mothers from the same light-dark cycle had similar litter synchrony as shown by nonfostered litters of Experiment 1. However, pups cross-fostered with mothers on reversed LD cycles showed a very different distribution of pup phases. Pups were not synchronized to their natural mother but to their foster mother. Moreover, pups were more scattered over the 24-h period and were found to be significantly synchronized to the phase of the reversed LD cycle. These results demonstrate the occurrence of postnatal entrainment in the Siberian hamster. The increased scatter produced by the cross-fostering paradigm results from some litters being completely entrained to the phase of the foster mother, some with an intermediate distribution between the phase of the natural and foster mothers, and a minority being associated with the phase of the natural mother. These results suggest that Siberian hamster pups are initially synchronized either prenatally or at birth but that the mother continues to provide entrainment signals during the postnatal period.

Effect of maternal deprivation on N-acetyltransferase activity rhythm in blinded rat pups

It has been reported that the rhythms of infant rats synchronize with the mother's rhythm until the light-dark cycle comes and has strong effects on their endogenous clocks. We found that periodic maternal deprivation (PMD) was able to cause a phase shift of serotonin N-acetyltransferase (NAT) in neonatal blinded rat pups. PMD in which contact with the mother was allowed for only 4 h caused a phase shift of NAT rhythm, irrespective of the timing of contact with the mother in a day. Acute single mother deprivation caused an excess of NAT activity for more hours than usual and contact with the mother prevented such an excessive response. Mother deprivation may act as a cold stress, since artificial warming of pups gave the same results as contact with the mother. When the pups were artificially warmed by a heater during a 1-week deprivation period, a flat 24-h pattern of NAT was observed. The mechanism causing a phase shift of NAT activity rhythm of rat pups may be complicated.

Adult timing after preweaning shifts of Zeitgeber in rats: crossed sensitization to time?

Preweaning albino rats were exposed from days 1 to 18 of life to successive 6-h shifts in light and temperature Zeitgebers, (1-18 rats), whereas controls (C) were raised under constant 12:12 L:D and temperature cycles. Cyclic Peak Interval performance at adulthood (100 days) showed that 1-18 rats were more accurate and sensitive to time than C subjects. These effects, which were akin to a crossed senzitization to time, were interpreted within the framework of scalar timing theory and the temporal information-processing model. They seemed not to depend upon changes in the pacemaker rate (lambda) or the memory constant K*, but to a change at the level of the decision process: 1-18 rats used a smaller response threshold than controls.

Ontogeny of pineal melatonin rhythm in rats under 12:12-hr and 14:14-hr light:dark conditions

The aim of the study was to determine whether a discrepancy between the genetically determined endogenous circadian period and an abnormally long Zeitgeber period disturbs the development of melatonin synthesis. Breeding pairs of rats were kept under 12:12- or 14:14-hr light:dark (LD) conditions. Pineal melatonin contents in the offspring were measured by radioimmunoassay. At 2 weeks of age high melatonin contents were found from lights-off to lights-on in both conditions suggesting dominance of the photic regulation. At 3 weeks of age the signs of the circadian regulation in the melatonin profiles were evident: a lag period after the light offset in control conditions and a significant decline before the light onset in both conditions. However, in 14:14-hr LD conditions the melatonin content did not decrease to daytime levels until the lights were on. This could suggest incomplete maturation of the circadian system. The phase relationships between the melatonin peak and LD cycle were different in the two conditions. A statistically significant LD difference was first found at the age of 8-10 days in male pups and at 14 days in female pups under both lighting. The results suggest that the abnormally long LD cycle did not cause any major disorders in the development of photic or circadian regulation of the melatonin synthesis.

Circadian rhythms in the release of vasoactive intestinal polypeptide and arginine-vasopressin in organotypic slice culture of rat suprachiasmatic nucleus

Temporal profiles of the amount of vasoactive intestinal polypeptide (VIP) were examined in the medium of organotypic suprachiasmatic nucleus (SCN) slice cultures over a 2-day period. Arginine-vasopressin (AVP) level was also measured in the same medium. The slices of the SCN were obtained from 7-8-day-old rats and cultured individually in tubes on a roller drum for 14 days. The VIP amount in the medium of SCN culture showed a circadian rhythm with a approximately 22-h period. Circadian rhythms with identical periods were also observed in AVP amount of the same culture. However, the peak time of the VIP rhythm was slightly ahead of that of the AVP rhythm. Furthermore, the total VIP amount in the medium over a 24-h period was six times as large as that of AVP. These results suggest that there is a circadian rhythm of VIP which is released from the ventrolateral SCN.

Periodic mother deprivation during the light period reversed the phase of serotonin N-acetyltransferase activity rhythm of the pineal gland in rat pups

It has been reported that nursing mother rats can postnatally entrain the circadian rhythms of blinded rat pups, such as locomotor, drinking, and corticosterone rhythms. To gain more insight in the mechanism of the postnatal entrainment of such pups' circadian rhythms, we examined the serotonin N-acetyltransferase (NAT) activity rhythm in blinded rat pups subjected to periodic mother deprivation (PMD) in which mothers were periodically deprived of their pups during either half of a day. We found that only PMD during the light period shifted the phase of NAT activity rhythm in the pups. To cause a reversal of the NAT activity rhythm, it was necessary to repeat PMD for more than 6 days. PMD for 6 h each day also shifted the phase of the blinded rat pups, but it did not reverse the NAT rhythm, even when it was repeated for 10 days. In 9-h deprivation for 10 days, however, deprivation during the first 9 h of the light period reversed the phase, although the latter 9 h failed to cause reversal of the phase. On the other hand, restricted feeding of the mother took more than 11 days to reverse the phase and a foster mother in the cross-fostering experiment failed to affect the phase of pup's rhythm, when the rhythm was determined on the 11th postnatal day. These facts indicate that PMD during the light period is a potent entrainer of the pups' circadian NAT rhythms and provide a useful method of exploring the underlying mechanism of the entrainment of the pups' rhythm by the mother.

Perinatal development of human circadian rhythms: role of the foetal biological clock

The development of circadian rhythms and the neuronal mechanisms underlying their generation (particularly the suprachiasmatic nucleus of the hypothalamus) were reviewed. Based on perinatal animal studies and data from human foetuses and/or preterm infants it was concluded that human circadian rhythms are present as early as at 30 weeks of gestation. The significance of the mother and/or the environment regarding the entrainment of the "endogenous" foetal biological clock was emphasized.

Maternal behaviour in the Wistar rat under atypical Zeitgeber

In previous experiments, neonatal rats subjected to atypical Zeitgeber (light and temperature) produce adult animals that present a different reactivity in some test situations. On the other hand, a great body of evidence indicates that physiological and psychological processes, including the maturation of the circadian system, are regulated by maternal behaviour. In order to investigate in which manner the atypical Zeitgeber, mentioned above, influences maternal behaviour, mothers with their young were observed systematically. More precisely, the question was: is the changed reactivity due to the fact that the young experience a situation of classical early privation (e.g., a decrease in social or other sensory stimulation or in nutrition) or is it due to a situation in which only the temporal pattern of the maternal behaviour is modified and thus disturbs the proper development of the circadian system of the young. Results tend to show that the only difference between the experimental and the control group is a modification in the circadian rhythm of several behavioural items. Only one activity (licking of the young) shows a significant difference in the overall mean value but in favour of the experimental group. We would suggest that the modified temporal pattern of the maternal care could be a poor timegiver for the young, in such a way that the development of their circadian rhythms is disturbed. The unusual reactivity in adulthood could depend on this abnormal development of the circadian system and not on a care privation.

Diurnal rhythms of body temperature, drinking and activity over reproductive cycles

These studies describe diurnal rhythm changes in female rats during gestation and lactation. In Experiment I, we measured the diurnal temperature rhythm (DTR) of 20 females through gestation, lactation and the post-lactational period and found that rhythm amplitude decreased during gestation and increased during lactation. Phase changes were also common features of the DTR during these states. In Experiment II, we measured drinking rhythms in 12 females during lactation and post-lactation and found phase and amplitude changes that were similar to the DTR changes seen in Experiment I. In Experiment III, we correlated the behavior of lactating females with their DTR and found that there was a consistent internal organization between behavior and Tb regardless of the degree of change in the DTR during the postpartum period. Females showing large phase changes in their DTR patterns were distinguishable from those showing smaller phase changes, however, on the basis of the absolute temporal organization of their behavior during lactation. All phase and amplitude changes disappeared immediately after pups were removed from the cage.

Effects of feeding and light cycles on activity rhythms of maternally isolated rat pups

Motor activity of infant rat pups was measured continuously between days 3 and 18 of postnatal age. Mother-reared rats on a 12:12 LD cycle exhibited significant rhythmic activity in the circadian range as early as day 5 of postnatal age. Some of the pups reared in isolation from maternal and sibling contact and kept on 12:12 LD cycles, feeding cycles, or combinations of feeding, temperature, and LD cycles also showed rhythmic activity but it was less persistent and of a lower amplitude than the rhythms of the mother-reared group. In the isolated rat pups nocturnal partitioning of activity was strengthened in the presence of both a light-dark cycle and a feeding cycle but only when the feeding resembled more natural nursing rhythms. In animals kept on constant light and a feeding cycle, activity occurred slightly more often during the 12-hr interval of decreased food intake. The addition of a temperature cycle--cooler nocturnal temperature--decreased the proportions of nocturnal motor activity. These results indicate that feeding and light-dark cycles may contribute to the synchronization of activity rhythms during the early postnatal period.

Developmental appearance of light-dark entrainment in the rat

Previous studies show that the developing circadian system is entrained by a maternal signal during the prenatal and early postnatal period. The present study investigated the developmental onset of retina-mediated, light-dark entrainment. Juvenile rats were exposed to phase-shifts in the light-dark cycle at different ages and the pineal N-acetyltransferase rhythm was monitored on postnatal day 10. The results show that retina-mediated, light-dark entrainment begins by postnatal day 6 and overrides maternal entrainment by postnatal day 8.

Development of hamster circadian rhythms: prenatal entrainment of the pacemaker

Individual hamster pups were maintained in constant dim light from just prior to birth, and their circadian wheel-running activity rhythms were recorded beginning at 18 days of age. Records of the postweaning free-running activity rhythm were used to determine the phase of a pup's rhythm on the day of weaning. Two groups of pups (LD and DL) were born to mothers that had been entrained before birth to light-dark cycles 12 hr out of phase. Both groups of pups were raised in constant dim light by foster mothers that had been entrained to only one of the prenatal cycles (LD). Thus pups born to mothers from different cycles were exposed to identical rhythmic environments postnatally. Despite the similar postnatal treatment, the two groups of pups showed activity rhythms at weaning with very different phases. The LD pups, born to and raised by LD mothers, were approximately synchronous with one another and with their foster mothers. The DL pups, born to DL mothers, but raised by LD mothers, were not synchronous with one another or with their foster mothers. Half of the DL pups showed phases predicted by their prenatal treatment, but the other half showed scattered phases. The results demonstrate that phase at weaning is affected by prenatal rhythmicity, and suggest that the circadian pacemaker underlying the activity rhythm is already functional and entrained at, or before, birth.

Periodic exposure to mother is potent zeitgeber of rat pups' rhythm

To study the possibility and potency of periodic maternal deprivation (PMD) in entraining the circadian rhythm of rat pups, access to the natural mother of which pups were optically enucleated on the day of birth (day 1) was restricted to either light phase (L-group) or dark phase (D-group) for various durations beginning on various days during the nursing period. Drinking rhythm of both groups was determined once per one or two weeks between the 4th and 8th postnatal week. The corticosterone rhythm was determined to confirm the results obtained by the measurement of water intake. Both rhythms were clearly observed by 5 weeks of age, and a reversed phase relationship was observed even when PMD was performed for a short period during days 1-4 or days 15-21. On the other hand, no phase angle difference was observed between L- and D-groups, when pups were periodically exposed to their original mother either during days 1-2 or days 18-21. These facts indicate that periodic exposure to mother can set the phase of the blinded pups' rhythm and that the period required for entrainment of the rhythm is as short as 4 or 7 days in the early or late times during the nursing period respectively, showing the high potency of PMD in entraining the circadian rhythm of blinded pups.

Development of the circadian rhythm of body temperature in rats

Seven male and seven female rat pups, all from separate litters of 10 pups each, had biotelemetry thermistors implanted into their peritoneal cavities when they were 10-15 days old. Body temperatures (Tb) were recorded by an Apple computer every 10 minutes until the rats were 60-75 days old so that the development of the circadian temperature rhythm (CTR) could be studied. The data were compared with eight parameters taken from adult CTRs. All rats were maintained on a 12 hr light/12 hr dark photoperiod. The results indicate that the CTR begins to develop after thermoregulatory mechanisms mature to produce an adult like daily range of TbS around day 24. From days 25 to 45, there was a steady increase in the percentage of TbS greater than the daily mean occurring in the dark. This process organized Tb into the characteristic sinusoidal waveform of the adult. Proper phasing of the trough and peak relative to the light/dark cycle took place between days 30-36. By day 42, 90% of TbS higher than the daily mean were occurring in the dark, while those lower than the mean were in the light. This is the typical adult pattern. The waveform's appearance continued to develop until day 50.

Restricted access to natural mother shifted endogenous rhythm of rat pups

For further investigation of the role of rat dams in setting the phase of pups' endogenous circadian rhythm, periodic maternal deprivation experiments were performed. Access of intact original mothers to blinded pups was restricted to light phase (L-Suckling pups) or dark phase (D-suckling pups) under 12-12 h light-dark alternation throughout the nursing period. Free-running patterns of drinking and corticosterone rhythms were determined for 3 consecutive weeks after weaning. In each determination of the rhythm, water consumption and corticosterone levels were measured every 4 hr. Both rhythms were clearly observed in both L- and D-suckling pups. Acrophase of water intake rhythm at 5 weeks of age in L-suckling and D-suckling groups were 0758 h and 1954 h while those of corticosterone rhythm were 0143 h and 0948 h respectively. Reversed phase relationship of rhythms between L- and D-suckling groups were observed throughout the period examined. These results indicate that suckling can act as a Zeitgeber for the endogenous rhythm of pups.

Critical period for the entrainment of the circadian rhythm in blinded pups by dams

The critical period for the entrainment of circadian rhythm in blinded pups by a foster mother with a rhythm inverted to that of the natural mother was determined by a convenient method for the analysis of the circadian rhythm of drinking behavior. The pups, optically enucleated on day 1, were transferred to the foster mother at various ages, measured in days from birth, and raised until weaning. When the pups were transferred before 4 days of age, the drinking rhythm of the adopted pups was in phase with that of the blinded natural pups born of and raised by the foster mother. On the other hand, when the mother was exchanged after 10 days of age, the rhythm of the adopted pups was out of phase with that of the natural pups of the foster mother. Pups exchanged on 5 or 7 days of age showed inconsistent phase of rhythm, either being entrained by the foster or the natural mothers, or affected by both mothers. These results indicate that blinded pups must be adopted by the foster mothers earlier than day 4 for the foster mothers to fully entrain the circadian rhythm of blinded pups.