Hypothalamic contribution to sleep–wake cycle development

Integrative Functions of the Hypothalamus: Linking Cognition, Emotion and Physiology for Well-being and Adaptability

Background

The hypothalamus, a small yet crucial neuroanatomical structure, integrates external (e.g., environmental) and internal (e.g., physiological/hormonal) stimuli. This integration governs various physiological processes and influences cognitive, emotional, and behavioral outcomes. It serves as a functional bridge between the nervous and endocrine systems, maintaining homeostasis and coordinating bodily functions.

Summary

Recent advancements in the neurobiology of the hypothalamus have elucidated its functional map, establishing a causal relationship between its responses-such as respiration, sleep, and stress-and various physiological processes. The hypothalamus facilitates and coordinates these complex processes by processing diverse stimuli, enabling the body to maintain internal balance and respond effectively to external demands. This review delves into the hypothalamus's intricate connections with cognition, emotion, and physiology, exploring how these interactions promote overall well-being and adaptability.

Key Message

Targeted external stimuli can modulate hypothalamic neuronal activities, impacting the physiological, cognitive, and emotional landscape. The review highlights non-invasive techniques, such as controlled breathing exercises, optimized sleep architecture, and stress management, as potential methods to enhance hypothalamic function. Ultimately, this comprehensive review underscores the multifaceted role of the hypothalamus in integrating signals, maintaining homeostasis, and influencing cognition, emotion, and physiology.

An integrate-and-fire mathematical model of sleep-wake neuronal networks in the developing mammal

Sleep behavior is present in nearly all animals, and is a vital part of growth, development, and overall health. Infant mammals cycle randomly between short bouts of sleep and wake, and the lengths of these bouts both follow an exponential distribution. As mammals mature into adulthood, the mean sleep and wake bout lengths increase, and we also observe a change in the distribution of wake bout lengths from exponential to power law. Focusing on three regions of the brainstem that are involved in sleep-wake regulation, we develop a novel integrate-and-fire neuronal network model to expand upon previous mathematical models of sleep-wake regulation in mammals, focusing on rats. This model allows fine control over neuronal connectivity while simultaneously increasing the size and complexity of the modeled system to make it more representative of reality. We establish a relationship between neuronal network structure and function that could explain the different sleep-wake behaviors observed in rats as they progress through development. We explore the relationship between three different neuronal populations as well as the overall network behavior of the system. We find that increasing synaptic connectivity strength between the wake-promoting region and the wake-active region accounts for the observed changes in mammalian sleep-wake patterns. This dynamic neuronal connectivity is a possible mechanism that accurately accounts for sleep-wake pattern changes observed during mammalian development.

Coincident development and synchronization of sleep-dependent delta in the cortex and medulla

In early development, active sleep is the predominant sleep state before it is supplanted by quiet sleep. In rats, the developmental increase in quiet sleep is accompanied by the sudden emergence of the cortical delta rhythm (0.5-4 Hz) around postnatal day 12 (P12). We sought to explain the emergence of the cortical delta by assessing developmental changes in the activity of the parafacial zone (PZ), a medullary structure thought to regulate quiet sleep in adults. We recorded from the PZ in P10 and P12 rats and predicted an age-related increase in neural activity during increasing periods of delta-rich cortical activity. Instead, during quiet sleep, we discovered sleep-dependent rhythmic spiking activity-with intervening periods of total silence-phase locked to a local delta rhythm. Moreover, PZ and cortical delta were coherent at P12 but not at P10. PZ delta was also phase locked to respiration, suggesting sleep-dependent modulation of PZ activity by respiratory pacemakers in the ventral medulla. Disconnecting the main olfactory bulbs from the cortex did not diminish cortical delta, indicating that the influence of respiration on delta at this age is not mediated indirectly through nasal breathing. Finally, we observed an increase in parvalbumin-expressing terminals in the PZ across these ages, supporting a role for local GABAergic inhibition in the PZ's rhythmicity. The unexpected discovery of delta-rhythmic neural activity in the medulla-when cortical delta is also emerging-provides a new perspective on the brainstem's role in regulating sleep and promoting long-range functional connectivity in early development.

DELTA-RHYTHMIC ACTIVITY IN THE MEDULLA DEVELOPS COINCIDENT WITH CORTICAL DELTA IN SLEEPING INFANT RATS

In early development, active sleep is the predominant sleep state before it is supplanted by quiet sleep. In rats, the developmental increase in quiet sleep is accompanied by the sudden emergence of the cortical delta rhythm (0.5-4 Hz) around postnatal day 12 (P12). We sought to explain the emergence of cortical delta by assessing developmental changes in the activity of the parafacial zone (PZ), a medullary structure thought to regulate quiet sleep in adults. We recorded from PZ in P10 and P12 rats and predicted an age-related increase in neural activity during increasing periods of delta-rich cortical activity. Instead, during quiet sleep we discovered sleep-dependent rhythmic spiking activity-with intervening periods of total silence-phase-locked to a local delta rhythm. Moreover, PZ and cortical delta were coherent at P12, but not at P10. PZ delta was also phase-locked to respiration, suggesting sleep-dependent modulation of PZ activity by respiratory pacemakers in the ventral medulla. Disconnecting the main olfactory bulbs from the cortex did not diminish cortical delta, indicating that the influence of respiration on delta at this age is not mediated indirectly through nasal breathing. Finally, we observed an increase in parvalbumin-expressing terminals in PZ across these ages, supporting a role for GABAergic inhibition in PZ's rhythmicity. The discovery of delta-rhythmic neural activity in the medulla-when cortical delta is also emerging-opens a new path to understanding the brainstem's role in regulating sleep and synchronizing rhythmic activity throughout the brain.

Development of the sleep-wake switch in rats during the P2-P21 early infancy period

In early infancy, rats randomly alternate between the sleeping and waking states-from postnatal day 2-10 (P2-P10), sleep and wake bouts are both exponentially distributed with increasing means, while from P10-P21 sleep and wake bout means continue to increase, though there is a striking qualitative shift in the distribution of wake bouts from exponential to power law. The behavioral states of sleep and wakefulness correspond to the activity of sleep-active and wake-active neuronal brainstem populations, with reciprocal inhibition between the two ensuring that only one population is active at a time. The locus coeruleus (LC) forms a third component of this circuit that rises in prominence during the P10-P21 period, as experimental evidence shows that an as-of-yet undeciphered interaction of the LC with sleep-active and wake-active populations is responsible for the transformation of the wake bout distribution from exponential to power law. Interestingly, the LC undergoes remarkable physiological changes during the P10-P21 period-gap junctions within the LC are pruned and network-wide oscillatory synchrony declines and vanishes. In this work, we discuss a series of models of sleep-active, wake-active, and the LC populations, and we use these models to postulate the nature of the interaction between these three populations and how these interactions explain empirical observations of sleep and wake bout dynamics. We hypothesize a circuit in which there is reciprocal excitation between the LC and wake-active population with inhibition from the sleep-active population to the LC that suppresses the LC during sleep bouts. During the P2-P10 period, we argue that a noise-based switching mechanism between the sleep-active and wake-active populations provides a simple and natural way to account for exponential bout distributions, and that the locked oscillatory state of the LC prevents it from impacting bout distributions. From P10-P21, we use our models to postulate that, as the LC gradually shifts from a state of synchronized oscillations to a state of continuous firing, reciprocal excitation between the LC and the wake-active population is able to gradually transform the wake bout distribution from exponential to power law.

Unique effects of nicotine across the lifespan

Smoking remains the leading cause of preventable death in the United States. Although combustible cigarettes are largely being replaced by tobacco-free products, nicotine use continues to increase in vulnerable populations, including youth, adolescents, and pregnant women. Nicotine exerts unique effects on specific brain regions during distinct developmental periods due to the dynamic expression of nicotinic acetylcholine receptors (nAChRs) throughout the lifespan. Nicotine exposure is a health concern not only for adults but also has neurotoxic effects on the fetus, newborn, child, and adolescent. In this review, we aim to highlight the dynamic roles of nAChRs throughout gestation, adolescence, and adulthood. We also provide clinical and preclinical evidence of the neurodevelopmental, cognitive, and behavioral consequences of nicotine exposure at different developmental periods. This comprehensive review highlights unique effects of nicotine throughout the lifespan to help elucidate interventions and public health measures to protect sensitive populations from nicotine exposure.

Intranasal melatonin nanoniosomes: pharmacokinetic, pharmacodynamics and toxicity studies

AIM

Intranasal melatonin encapsulated in nanosized niosomes was preclinically evaluated.

METHODOLOGY

A formula of melatonin niosomes (MN) was selected through physicochemical and cytotoxic data for pharmacokinetic, pharmacodynamics and toxicity studies in male Wistar rats.

RESULTS

Intranasal MN was bioequivalent to intravenous injection of melatonin, providing therapeutic level doses. Acute and subchronic toxicity screening showed no abnormal signs, symptoms or hematological effects in any animals. Transient nasal irritations with no inflammation were observed with intranasal MN, leading it to be categorized as relatively harmless.

CONCLUSION

The intranasal MN could deliver melatonin to the brain to induce sleep and provide delayed systemic circulation, relative to intravenous injection and also distribute to peripheral tissue.

Deterministic stability regimes and noise-induced quasistable behavior in a pair of reciprocally inhibitory neurons

Reciprocal inhibition is a common motif exploited by neuronal networks; an intuitive and tractable way to examine the behaviors produced by reciprocal inhibition is to consider a pair of neurons that synaptically inhibit each other and receive constant or noisy excitatory driving currents. In this work, we examine reciprocal inhibition using two models (a voltage-based and a current-based integrate-and-fire model with instantaneous or temporally structured input), and we use analytic and computational tools to examine the bifurcations that occur and study the various possible monostable, bistable, and tristable regimes that can exist; we find that, depending on system parameters (and on choice of neuron model), there can exist up to 3 distinct monostable regimes (denoted M0, M1, M2), 3 distinct bistable regimes (denoted B, B1, B2), and a single tristable regime (denoted T). We also find that synaptic inhibition exerts independent control over the two neurons - inhibition from neuron 1 to neuron 2 governs the spiking behavior of neuron 2 but has no impact on the spiking behavior of neuron 1 (and vice versa). The excitatory driving current, however, does not exhibit this property - the excitatory current to neuron 1 affects the spiking behavior of both neurons 1 and 2 (as does the excitatory current to neuron 2). Furthermore, we develop a methodology to examine the behavior of the system when the excitatory driving currents are allowed to be noisy, and we investigate the relationship between the behavior of the noisy system with the stability regime of the corresponding deterministic system.

Role of the locus coeruleus in the emergence of power law wake bouts in a model of the brainstem sleep-wake system through early infancy

Infant rats randomly cycle between the sleeping and waking states, which are tightly correlated with the activity of mutually inhibitory brainstem sleep and wake populations. Bouts of sleep and wakefulness are random; from P2-P10, sleep and wake bout lengths are exponentially distributed with increasing means, while during P10-P21, the sleep bout distribution remains exponential while the distribution of wake bouts gradually transforms to power law. The locus coeruleus (LC), via an undeciphered interaction with sleep and wake populations, has been shown experimentally to be responsible for the exponential to power law transition. Concurrently during P10-P21, the LC undergoes striking physiological changes - the LC exhibits strong global 0.3 Hz oscillations up to P10, but the oscillation frequency gradually rises and synchrony diminishes from P10-P21, with oscillations and synchrony vanishing at P21 and beyond. In this work, we construct a biologically plausible Wilson Cowan-style model consisting of the LC along with sleep and wake populations. We show that external noise and strong reciprocal inhibition can lead to switching between sleep and wake populations and exponentially distributed sleep and wake bout durations as during P2-P10, with the parameters of inhibition between the sleep and wake populations controlling mean bout lengths. Furthermore, we show that the changing physiology of the LC from P10-P21, coupled with reciprocal excitation between the LC and wake population, can explain the shift from exponential to power law of the wake bout distribution. To our knowledge, this is the first study that proposes a plausible biological mechanism, which incorporates the known changing physiology of the LC, for tying the developing sleep-wake circuit and its interaction with the LC to the transformation of sleep and wake bout dynamics from P2-P21.

Large Scale Cortical Functional Networks Associated with Slow-Wave and Spindle-Burst-Related Spontaneous Activity

Cortical sensory systems are active with rich patterns of activity during sleep and under light anesthesia. Remarkably, this activity shares many characteristics with those present when the awake brain responds to sensory stimuli. We review two specific forms of such activity: slow-wave activity (SWA) in the adult brain and spindle bursts in developing brain. SWA is composed of 0.5-4 Hz resting potential fluctuations. Although these fluctuations synchronize wide regions of cortex, recent large-scale imaging has shown spatial details of their distribution that reflect underlying cortical structural projections and networks. These networks are regulated, as prior awake experiences alter both the spatial and temporal features of SWA in subsequent sleep. Activity patterns of the immature brain, however, are very different from those of the adult. SWA is absent, and the dominant pattern is spindle bursts, intermittent high frequency oscillations superimposed on slower depolarizations within sensory cortices. These bursts are driven by intrinsic brain activity, which act to generate peripheral inputs, for example via limb twitches. They are present within developing sensory cortex before they are mature enough to exhibit directed movements and respond to external stimuli. Like in the adult, these patterns resemble those evoked by sensory stimulation when awake. It is suggested that spindle-burst activity is generated purposefully by the developing nervous system as a proxy for true external stimuli. While the sleep-related functions of both slow-wave and spindle-burst activity may not be entirely clear, they reflect robust regulated phenomena which can engage select wide-spread cortical circuits. These circuits are similar to those activated during sensory processing and volitional events. We highlight these two patterns of brain activity because both are prominent and well-studied forms of spontaneous activity that will yield valuable insights into brain function in the coming years.

Imaging the aetiology of sleep disorders in dementia and Parkinson's disease

Sleep disorders are commonly observed in patients with neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease and often represent an early manifestation of the underlying degenerative process. The pathophysiology of sleep dysfunction in these conditions is complex and incompletely understood. However, in recent years, functional imaging in vivo with SPECT and PET has significantly improved our understanding of the possible molecular mechanisms. These include dysfunction of both dopaminergic and non-dopaminergic pathways involved in sleep/wakefulness control. This paper summarizes the main findings of the imaging studies performed to elucidate the aetiology of sleep disorders in Alzheimer's disease and Parkinson's disease.

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.

Switching mechanisms and bout times in a pair of reciprocally inhibitory neurons

Within the appropriate parameter regime, a deterministic model of a pair of mutually inhibitory neurons receiving excitatory driving currents exhibits bistability-each of the two stable states corresponds to one neuron being active and the other being quiescent. The presence of noise in the driving currents results in a system that randomly switches back and forth between these two states, causing alternating bouts of spiking activity. In this work, we examine the random bout durations of the two neurons and dependence on system parameters. We find that bout durations of each neuron are exponentially distributed, with changes in system parameters altering only the mean of the distribution. Synaptic inhibition independently controls the bout durations of the two neurons-the mean bout time of a neuron is a function of efferent (or outgoing) inhibition, and is independent of afferent (or incoming) inhibition. Furthermore, we find that the mean bout time of a neuron exhibits a critical dependence on the time course (rather than amplitude) of efferent inhibition-mean bout time of a neuron grows exponentially with the time course of efferent inhibition, and the growth rate of this exponential function depends only on the excitatory driving current to that neuron (and not on any other system parameters). We discuss the relevance of our results to the regulation of sleep-wake cycling by medullary and pontine structures within the brain.

The ontogeny of sleep-wake cycles in zebrafish: a comparison to humans

Zebrafish (Danio rerio) are used extensively in sleep research; both to further understanding of sleep in general and also as a model of human sleep. To date, sleep studies have been performed in larval and adult zebrafish but no efforts have been made to document the ontogeny of zebrafish sleep-wake cycles. Because sleep differs across phylogeny and ontogeny it is important to validate the use of zebrafish in elucidating the neural substrates of sleep. Here we describe the development of sleep and wake across the zebrafish lifespan and how it compares to humans. We find power-law distributions to best fit wake bout data but demonstrate that exponential distributions, previously used to describe sleep bout distributions, fail to adequately account for the data in either species. Regardless, the data reveal remarkable similarities in the ontogeny of sleep cycles in zebrafish and humans. Moreover, as seen in other organisms, zebrafish sleep levels are highest early in ontogeny and sleep and wake bouts gradually consolidate to form the adult sleep pattern. Finally, sleep percentage, bout duration, bout number, and sleep fragmentation are shown to allow for meaningful comparisons between zebrafish and human sleep.

Emergent dynamics in a model of visual cortex

This paper proposes that the network dynamics of the mammalian visual cortex are highly structured and strongly shaped by temporally localized barrages of excitatory and inhibitory firing we call ‘multiple-firing events’ (MFEs). Our proposal is based on careful study of a network of spiking neurons built to reflect the coarse physiology of a small patch of layer 2/3 of V1. When appropriately benchmarked this network is capable of reproducing the qualitative features of a range of phenomena observed in the real visual cortex, including spontaneous background patterns, orientation-specific responses, surround suppression and gamma-band oscillations. Detailed investigation into the relevant regimes reveals causal relationships among dynamical events driven by a strong competition between the excitatory and inhibitory populations. It suggests that along with firing rates, MFE characteristics can be a powerful signature of a regime. Testable predictions based on model observations and dynamical analysis are proposed.

Development of SCN connectivity and the circadian control of arousal: a diminishing role for humoral factors?

The suprachiasmatic nucleus (SCN) is part of a wake-promoting circuit comprising the dorsomedial hypothalamus (DMH) and locus coeruleus (LC). Although widely considered a "master clock," the SCN of adult rats is also sensitive to feedback regarding an animal's behavioral state. Interestingly, in rats at postnatal day (P)2, repeated arousing stimulation does not increase neural activation in the SCN, despite doing so in the LC and DMH. Here we show that, by P8, the SCN is activated by arousing stimulation and that selective destruction of LC terminals with DSP-4 blocks this activational effect. We next show that bidirectional projections among the SCN, DMH, and LC are nearly absent at P2 but present at P8. Despite the relative lack of SCN connectivity with downstream structures at P2, day-night differences in sleep-wake activity are observed, suggesting that the SCN modulates behavior at this age via humoral factors. To test this hypothesis, we lesioned the SCN at P1 and recorded sleep-wake behavior at P2: Day-night differences in sleep and wake were eliminated. We next performed precollicular transections at P2 and P8 that isolate the SCN and DMH from the brainstem and found that day-night differences in sleep-wake behavior were retained at P2 but eliminated at P8. Finally, the SCN or DMH was lesioned at P8: When recorded at P21, rats with either lesion exhibited similarly fragmented wake bouts and no evidence of circadian modulation of wakefulness. These results suggest an age-related decline in the SCN's humoral influence on sleep-wake behavior that coincides with the emergence of bidirectional connectivity among the SCN, DMH, and LC.

Carotid chemoreceptor development in mice

Mice are the most suitable species for understanding genetic aspects of postnatal developments of the carotid body due to the availability of many inbred strains and knockout mice. Our study has shown that the carotid body grows differentially in different mouse strains, indicating the involvement of genes. However, the small size hampers investigating functional development of the carotid body. Hypoxic and/or hyperoxic ventilatory responses have been investigated in newborn mice, but these responses are indirect assessment of the carotid body function. Therefore, we need to develop techniques of measuring carotid chemoreceptor neural activity from young mice. Many studies have taken advantage of the knockout mice to understand chemoreceptor function of the carotid body, but they are not always suitable for addressing postnatal development of the carotid body due to lethality during perinatal periods. Various inbred strains with well-designed experiments will provide useful information regarding genetic mechanisms of the postnatal carotid chemoreceptor development. Also, targeted gene deletion is a critical approach.

Maturation of sleep homeostasis in developing rats: a role for preoptic area neurons

The present study evaluated the hypothesis that developmental changes in hypothalamic sleep-regulatory neuronal circuits contribute to the maturation of sleep homeostasis in rats during the fourth postnatal week. In a longitudinal study, we quantified electrographic measures of sleep during baseline and in response to sleep deprivation (SD) on postnatal days 21/29 (P21/29) and P22/30 (experiment 1). During 24-h baseline recordings on P21, total sleep time (TST) during the light and dark phases did not differ significantly. On P29, TST during the light phase was significantly higher than during the dark phase. Mean duration of non-rapid-eye-movement (NREM) sleep bouts was significantly longer on P29 vs. P21, indicating improved sleep consolidation. On both P22 and P30, rats exhibited increased NREM sleep amounts and NREM electroencephalogram delta power during recovery sleep (RS) compared with baseline. Increased NREM sleep bout length during RS was observed only on P30. In experiment 2, we quantified activity of GABAergic neurons in median preoptic nucleus (MnPN) and ventrolateral preoptic area (VLPO) during SD and RS in separate groups of P22 and P30 rats using c-Fos and glutamic acid decarboxylase (GAD) immunohistochemistry. In P22 rats, numbers of Fos(+)GAD(+) neurons in VLPO did not differ among experimental conditions. In P30 rats, Fos(+)GAD(+) counts in VLPO were elevated during RS. MnPN neuronal activity was state-dependent in P22 rats, but Fos(+)GAD(+) cell counts were higher in P30 rats. These findings support the hypothesis that functional emergence of preoptic sleep-regulatory neurons contributes to the maturation of sleep homeostasis in the developing rat brain.

Dynamics of sleep-wake cyclicity across the fetal period in sheep (Ovis aries)

All adult mammals examined thus far exhibit sleep bout durations that follow an exponential distribution and wake bout durations that follow a power-law distribution. In altricial rodents such as rats and mice, exponential distributions of sleep bouts are found soon after birth, but the power-law distribution of wake bouts does not emerge until the third postnatal week. Also, both sleep and bouts consolidate across the early postnatal period. It is not known whether similar developmental processes occur in precocial species during the prenatal period. Here we characterize sleep-wake development in a precocial species, the domestic sheep (Ovis aries), from 114 to 148 days gestational age (DGA). Sleep and wake bout durations exhibited exponential distributions throughout the fetal period with some evidence of an emerging exponential-to-power-law transition for wake bouts toward the end of gestation. Both sleep and wake bouts consolidated in an orderly fashion across development and there was little evidence of circadian variation, even in the oldest subjects. These results indicate that similar patterns of sleep-wake organization are found prenatally in a precocial species as are found postnatally in altricial species. Data from more species are needed to fully realize the benefits of a developmental comparative approach for understanding the forces that have shaped the ontogeny and phylogeny of mammalian sleep.

Dynamics of sleep-wake cyclicity at night across the human lifespan

Studies in adult mammals (rats, cats, mice, and humans) have revealed a surprising regularity in the duration of sleep and wake bouts. In particular, wake bout durations exhibit a power-law distribution whereas sleep bout durations exhibit an exponential distribution. Moreover, in rodents, sleep bouts exhibit an exponential distribution at all ages examined, whereas wake bout durations exhibit exponential distributions early in ontogeny with a clear power-law emerging only at the older ages. Thus, the data examined thus far suggests a similar developmental trajectory for a wide range of mammals which in turn may offer a novel metric to directly compare human and animal sleep-wake data. Therefore, we tested the generalizability of these findings by examining the distributions of sleep and wake bouts during the night in a healthy human sample - from premature infants to 70-year-olds. We find that sleep bouts elongate over the first years. At the same time wake bouts shorten but elongate again with increasing age. Moreover, sleep bout durations exhibit exponential distributions at all ages tested, except for the youngest (premature infants). Wake bouts exhibit a power-law distribution - but only during a restricted time window during adulthood. We conclude that the developmental trajectory of human sleep-wake cycles does not map well onto those of rodents; however, the method of characterizing sleep-wake cycles, using bout distribution, holds great promise for classifying sleep, its disorders, and tracking its developmental milestones across the lifespan in humans.

An abrupt developmental shift in callosal modulation of sleep-related spindle bursts coincides with the emergence of excitatory-inhibitory balance and a reduction of somatosensory cortical plasticity

Transecting the corpus callosum of postnatal day (P)1-6 rats disinhibits the production of spindle bursts (SBs) within primary somatosensory cortex (S1), most notably during periods of sleep-related myoclonic twitching. Here we investigated developmental changes in this callosally mediated disinhibition and its association with cortical plasticity. Recordings in P2-15 subjects revealed that callosotomy-induced disinhibition is a transient feature of early development that disappears abruptly after P6. This abrupt switch was accompanied by sharp decreases in myoclonic twitching and equally sharp increases in spontaneous SBs and in the number of GABAergic and glutamatergic presynaptic terminals in S1. Expression of the K+Cl- cotransporter 2 (KCC2) also increased across these ages. To determine whether these developmental changes are associated with alterations in cortical plasticity, pups were callosotomized at P1, P6, or P8, and tested over the subsequent week. Regardless of age, callosotomy immediately disrupted SBs evoked by forepaw stimulation. Over the next week, the P1 and P6 callosotomy groups exhibited full recovery of function; in contrast, the P8 group did not exhibit recovery of function, thus indicating an abrupt decrease in cortical plasticity between P6 and P8. Together, our data demonstrate that callosotomy-induced disinhibition is a transient phenomenon whose disappearance coincides with the onset of increased intrinsic connectivity, establishment of excitatory-inhibitory balance, and diminished plasticity in S1. Accordingly, our findings indicate that callosotomy-induced disinhibition of twitch-related SBs is a bioassay of somatosensory cortical plasticity and, in addition, support the hypothesis that myoclonic twitches, like retinal waves, actively contribute to cortical development and plasticity.

Brainstem and hypothalamic regulation of sleep pressure and rebound in newborn rats

Sleep pressure and rebound comprise the two compensatory or "homeostatic" responses to sleep deprivation. Although sleep pressure is expressed by infant rats as early as postnatal day (P)5, sleep rebound does not appear to emerge until after P11. We reexamined the developmental expression of these sleep-regulatory processes in P2 and P8 rats by depriving them of sleep for 30 min using a cold, arousing stimulus delivered to a cold-sensitive region of the snout. This method effectively increased sleep pressure over the 30-min period (i.e., increases in the number of arousing stimuli presented over time). Moreover, sleep rebound (i.e., increased sleep during the recovery period) is demonstrated for the first time at these ages. Next, we showed that precollicular transections in P2 rats prevent sleep rebound without affecting sleep pressure, suggesting that the brainstem is sufficient to support sleep pressure, but sleep rebound depends on neural mechanisms that lie rostral to the transection. Finally, again in P2 rats, we used c-fos immunohistochemistry to examine neural activation throughout the neuraxis during sleep deprivation and recovery. Sleep deprivation and rebound were accompanied by significant increases in neural activation in both brainstem and hypothalamic nuclei, including the ventrolateral preoptic area and median preoptic nucleus. This early developmental expression of sleep pressure and rebound and the apparent involvement of brainstem and hypothalamic structures in their expression further solidify the notion that sleep-wake processes in newborns-defined at these ages without reference to state-dependent EEG activity-provide the foundation on which the more familiar processes of adults are built.

Towards a dynamical network view of brain ischemia and reperfusion. Part II: a post-ischemic neuronal state space

The general failure of neuroprotectants in clinical trials of ischemic stroke points to the possibility of a fundamental blind spot in the current conception of ischemic brain injury, the "ischemic cascade". This is the second in a series of four papers whose purpose is to work towards a revision of the concept of brain ischemia by applying network concepts to develop a bistable model of brain ischemia. We here build the bistable network model of brain ischemia. The central concept is that of a post-ischemic state space. Ischemia, as a quantitative perturbation, is envisioned to push the brain through a series of four phenotypes as a function of the amount of ischemia: the homeostatic, preconditioned, delayed neuronal death and necrotic phenotypes. The phenotypes are meta-stable attractors in the landscape of the post-ischemic state space. The sequence of the phenotypes derives from the mutual antagonism between damage mechanisms and stress responses, each conceived as aggregate ensemble variables. The competition between damage mechanisms and stress responses is posited to have the form of a bistability. Application of bistability to brain ischemia is grounded in the incontrovertible fact that post-ischemic neurons face the mutually exclusive decision to either live or die.

Developmental emergence of power-law wake behavior depends upon the functional integrity of the locus coeruleus

STUDY OBJECTIVES

Daily amounts of sleep and wakefulness are accumulated in discrete bouts that exhibit distinct statistical properties. In adult mammals, sleep bout durations follow an exponential distribution whereas wake bout durations follow a power-law distribution. In infant Norway rats, however, wake bouts initially follow an exponential distribution and only transition to a power-law distribution beginning around postnatal day 15 (P15). Here we test the hypothesis that the locus coeruleus (LC), one of several wake-active nuclei in the brainstem, contributes to this developmental transition.

DESIGN

At P7, rats were injected subcutaneously with saline or DSP-4, a neurotoxin that targets noradrenergic (NA) LC terminals. Then, at P21, sleep and wakefulness during the day and night were monitored. The effectiveness of DSP-4 treatment was verified by measuring NA, dopamine (DA), and serotonin (5-HT) concentration in cortical and non-cortical tissue using high performance liquid chromatography.

RESULTS

In relation to controls, subjects treated with DSP-4 exhibited significant reductions only in cortical and non-cortical NA concentration. Consistent with our hypothesis, the wake bout durations of DSP-4 subjects more closely followed an exponential distribution, whereas those of control subjects followed the expected power-law distribution. Sleep bout distributions were unaffected by DSP-4.

CONCLUSIONS

These results suggest that the fundamental developmental transition in the statistical structure of wake bout durations is effected in part by changes in noradrenergic LC functioning. Considered within the domain of network theory, the hub-like connectivity of the LC may have important implications for the maintenance of network function in the face of random or targeted neural degeneration.

The dynamic effects of nicotine on the developing brain

Nicotinic acetylcholine receptors (nAChRs) regulate critical aspects of brain maturation during the prenatal, early postnatal, and adolescent periods. During these developmental windows, nAChRs are often transiently upregulated or change subunit composition in those neural structures that are undergoing major phases of differentiation and synaptogenesis, and are sensitive to environmental stimuli. Nicotine exposure, most often via tobacco smoke, but increasingly via nicotine replacement therapy, has been shown to have unique effects on the developing human brain. Consistent with a dynamic developmental role for acetylcholine, exogenous nicotine produces effects that are unique to the period of exposure and that impact the developing structures regulated by acetylcholine at that time. Here we present a review of the evidence, available from both the clinical literature and preclinical animal models, which suggests that the diverse effects of nicotine exposure are best evaluated in the context of regional and temporal expression patterns of nAChRs during sensitive maturational periods, and disruption of the normal developmental influences of acetylcholine. We present evidence that nicotine interferes with catecholamine and brainstem autonomic nuclei development during the prenatal period of the rodent (equivalent to first and second trimester of the human), alters the neocortex, hippocampus, and cerebellum during the early postnatal period (third trimester of the human), and influences limbic system and late monoamine maturation during adolescence.

PHOX2B mutations and ventilatory control

The transcription factor PHOX2B is essential for the development of the autonomic nervous system. In humans, polyalanine expansion mutations in PHOX2B cause Congenital Central Hypoventilation Syndrome (CCHS), a rare life-threatening disorder characterized by hypoventilation during sleep and impaired chemosensitivity. CCHS is combined with comparatively less severe impairments of autonomic functions including thermoregulation, cardiac rhythm, and digestive motility. Respiratory phenotype analyses of mice carrying an invalidated Phox2b allele (Phox2b+/- mutant mice) or the Phox2b mutation (+7 alanine expansion) found in patients with CCHS (Phox2b(27Ala/+) mice) have shed light on the role for PHOX2B in breathing control and on the pathophysiological mechanisms underlying CCHS. Newborn mice that lacked one Phox2b allele (Phox2b+/-) had sleep apneas and depressed sensitivity to hypercapnia. However, these impairments resolved rapidly, whereas the CCHS phenotype is irreversible. Heterozygous Phox2b(27Ala/+) pups exhibited a lack of responsiveness to hypercapnia and unstable breathing; they died within the first few postnatal hours. The generation of mouse models of CCHS provides tools for evaluating treatments aimed at alleviating both the respiratory symptoms and all other autonomic symptoms of CCHS.

Behavioral and respiratory characteristics during sleep in neonatal DBA/2J and A/J mice

The ventilatory response to hypoxia depends on the carotid body function and sleep-wake states. Therefore, the response must be measured in a consistent sleep-wake state. In mice, EMG with behavioral indices (coordinated movements, CMs; myoclonic twitches, MTs) has been used to assess sleep-wake states. However, in neonatal mice EMG instrumentation could induce stress, altering their behavior and ventilation. Accordingly, we examined: (1) if EMG can be eliminated for assessing sleep-wake states; and (2) behavioral characteristics and carotid body-mediated respiratory control during sleep with EMG (EMG+) or without EMG (EMG-). Seven-day-old DBA/2J and A/J mice were divided into EMG+ and EMG- groups. In both strains, CMs occurred when EMG was high; MTs were present during silent/low EMG activity. The durations of high EMG activity and of CMs were statistically indifferent. Thus, CMs can be used to indicate wake state without EMG. The stress caused by EMG instrumentation may be distinctively manifested based on genetic background. Prolonged agitation was observed in some EMG+ DBA/2J (5 of 13), but not in A/J mice. The sleep time and MT counts were indifferent between the groups in DBA/2J mice. The EMG+ A/J group showed longer sleep time and less MT counts than the EMG- A/J group. Mean respiratory variables (baseline, hyperoxic/hypoxic responses) were not severely influenced by EMG+ in either strain. Individual values were more variable in EMG+ mice. Carotid body-mediated respiratory responses (decreased ventilation upon hyperoxia and increased ventilation upon mild hypoxia) during sleep were clearly observed in these neonatal mice with or without EMG instrumentation.

Synchronous bursts of neuronal activity in the developing hippocampus: modulation by active sleep and association with emerging gamma and theta rhythms

The neonatal hippocampus exhibits regularly recurring waves of synchronized neuronal activity in vitro. Because active sleep (AS), characterized by bursts of phasic motor activity in the form of myoclonic twitching, may provide conditions that are conducive to activity-dependent development of hippocampal circuits, we hypothesized that the waves of synchronous neuronal activity that have been observed in vitro would be associated with AS-related twitching. Using unanesthetized 1- to 12-d-old rats, we report here that the majority of neurons in CA1 and the dentate gyrus (DG) are significantly more active during AS than during either quiet sleep or wakefulness. Neuronal activity typically occurs in phasic bursts, during which most neurons are significantly cross-correlated both within and across the CA1 and DG fields. All AS-active neurons increase their firing rates during periods of myoclonic twitching of the limbs, and a subset of these neurons exhibit a burst of activity immediately after limb twitches, suggesting that the twitches themselves provide sensory feedback to the infant hippocampus, as occurs in the infant spinal cord and neocortex. Finally, the synchronous bursts of neuronal activity are coupled to the emergence of the AS-related hippocampal gamma rhythm during the first postnatal week, as well as the emergence of the AS-related theta rhythm during the second postnatal week. We hypothesize that the phasic motor events of active sleep provide the developing hippocampus with discrete sensory stimulation that contributes to the development and refinement of hippocampal circuits as well as the development of synchronized interactions between hippocampus and neocortex.

Balancing paediatric anaesthesia: preclinical insights into analgesia, hypnosis, neuroprotection, and neurotoxicity

Logistical and ethical reasons make conducting clinical research in paediatric practice difficult, and therefore safe and efficacious advances are dependent on good preclinical research. For example, notable advances have been made in preclinical studies of pain processing that correlate well with patient data. Other areas of paediatric anaesthetic research remain in their infancy including mechanisms of anaesthesia and anaesthetic neuroprotection and neurotoxicity. Animal data have identified the potential 'double-edged' sword of administering anaesthetic agents in the young; although these agents can be neuroprotective in certain circumstances, they can be neurotoxic in others. The potential for this toxicity must be balanced against the importance of providing adequate anaesthesia for which there can be no compromise. We review the current state of preclinical research in paediatric anaesthesia and identify areas which require further exploration in order to provide the foundations for well-conducted clinical trials.

Perinatal alcohol exposure leads to prolonged upregulation of hypothalamic GABA A receptors and increases behavioral sensitivity to gaboxadol

Prenatal alcohol exposure (AE) is associated with lasting abnormalities of sleep and motor development, but the underlying mechanisms are unknown. We hypothesized that AE alters development of GABAergic signaling in the hypothalamic regions important for the control of sleep and motor activity. Alcohol (5.25 g/(kg day)) was administered intragastrically to male rats on postnatal days (PD) 4-9, a period of brain development equivalent to the human third trimester (AE group). Control pups were sham-intubated (S group). Motor activity was monitored on PD27 and 28. On PD29 and 30, GABA A receptor subunit mRNA levels and alpha4 and delta subunit proteins were quantified by RT-PCR and immunoblotting, respectively, in the wake- and motor activity-promoting perifornical (PF) region of the posterior hypothalamus and the sleep-promoting ventrolateral preoptic (VLPO) region of the anterior hypothalamus. Then, in 47-52-day-old rats, motor activity was quantified following administration of GABA A receptor agonist, gaboxadol (5 mg/kg s.c.). In the PF region, mRNA and protein levels for the alpha4 and delta subunits were significantly higher and beta3 and gamma2 subunit mRNAs were also increased in the AE group. In the VLPO region, only the delta subunit mRNA was increased. Spontaneous motor activity was lower and suppressed more by gaboxadol in the AE than S group, and the latency to a transient total loss of activity after gaboxadol was shorter in the AE group. Thus, perinatal AE leads to GABA A receptor overexpression in the vigilance- and motor activity-promoting hypothalamic PF region, with the neurochemical and functional outcomes lasting long beyond the period of the insult.

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.

A preliminary study of sleep ontogenesis in the ferret (Mustela putorius furo)

We investigated sleep ontogenesis in the ferret-a placental mammal that is highly altricial compared to other mammalian species. Because altriciality is linked with elevated rapid-eye-movement (REM) sleep amounts during infancy, it was expected that ferret kits would display very high levels of this state. Longitudinal polysomnographic measurements were made from 8 ferret kits from approximately eye-opening (postnatal day [P]30)-P50 using an experimental routine that minimized the effects of maternal separation. These data were compared to values from 8 adult ferrets (>3 months of age) and 6 neonatal cats (mean age: P31.7). We find that the polygraphic features of REM and non-REM (NREM) sleep are present by at least P30. Over the next 2 weeks, REM sleep amounts slightly declined while wakefulness and NREM sleep amounts increased. However, a comparison to published values from developing cats and rats showed that the ferret did not exhibit a disproportionate amount of REM sleep at similar postnatal ages or relative to a common developmental milestone (eye-opening).

Non-invasive in vivo imaging of calcium signaling in mice

Rapid and transient elevations of Ca(2+) within cellular microdomains play a critical role in the regulation of many signal transduction pathways. Described here is a genetic approach for non-invasive detection of localized Ca(2+) concentration ([Ca(2+)]) rises in live animals using bioluminescence imaging (BLI). Transgenic mice conditionally expressing the Ca(2+)-sensitive bioluminescent reporter GFP-aequorin targeted to the mitochondrial matrix were studied in several experimental paradigms. Rapid [Ca(2+)] rises inside the mitochondrial matrix could be readily detected during single-twitch muscle contractions. Whole body patterns of [Ca(2+)] were monitored in freely moving mice and during epileptic seizures. Furthermore, variations in mitochondrial [Ca(2+)] correlated to behavioral components of the sleep/wake cycle were observed during prolonged whole body recordings of newborn mice. This non-invasive imaging technique opens new avenues for the analysis of Ca(2+) signaling whenever whole body information in freely moving animals is desired, in particular during behavioral and developmental studies.

Neuronal models for sleep-wake regulation and synaptic reorganization in the sleeping hippocampus

In this article, we discuss mathematical models that address the control of sleep-wake behavior in the infant and adult rodent and a model that addresses changes in single-cell firing patterns in the hippocampus across wake and rapid eye movement (REM) sleep states. Each of the models describes the dynamics of experimentally identified neuronal components--either the firing activity of wake-and sleep-promoting neuronal populations or the spiking activity of hippocampal pyramidal neurons. Our discussion of each model illustrates how a mathematical model that describes the temporal dynamics of the modeled neuronal components can reveal specifics about proposed neuronal mechanisms that underlie sleep-wake regulation or sleep-specific firing patterns. For example, the dynamics of the models developed for sleep-wake regulation in the infant rodent lend insight into the involved brain-stem neuronal populations and the evolution of the network during maturation. The results of the model for sleep-wake regulation in the adult rodent suggest distinct properties of the involved neuronal populations and their interactions that account for long-lasting and brief waking bouts. The dynamics of the model for sleep-specific hippocampal neural activity proposes neural mechanisms to account for observed activity changes that can invoke synaptic reorganization associated with learning and memory consolidation.

Brainstem cholinergic modulation of muscle tone in infant rats

In week-old rats, lesions of the dorsolateral pontine tegmentum (DLPT) and nucleus pontis oralis (PnO) have opposing effects on nuchal muscle tone. Specifically, pups with DLPT lesions exhibit prolonged bouts of nuchal muscle atonia (indicative of sleep) and pups with PnO lesions exhibit prolonged bouts of high nuchal muscle tone (indicative of wakefulness). Here we test the hypothesis that nuchal muscle tone is modulated, at least in part, by cholinergically mediated interactions between these two regions. First, in unanesthetized pups, we found that chemical infusion of the cholinergic agonist carbachol (22 mm, 0.1 microL) within the DLPT produced high muscle tone. Next, chemical lesions of the PnO were used to produce a chronic state of high nuchal muscle tone, at which time the cholinergic antagonist scopolamine (10 mm, 0.1 microL) was infused into the DLPT. Scopolamine effectively decreased nuchal muscle tone, thus suggesting that lesions of the PnO increase muscle tone via cholinergic activation of the DLPT. Using 2-deoxyglucose autoradiography, metabolic activation throughout the DLPT was observed after PnO lesions. Finally, consistent with the hypothesis that PnO inactivation produces high muscle tone, infusion of the sodium channel blocker lidocaine (2%) into the PnO of unanesthetized pups produced rapid increases in muscle tone. We conclude that, even early in infancy, the DLPT is critically involved in the regulation of muscle tone and behavioral state, and that its activity is modulated by a cholinergic mechanism that is directly or indirectly controlled by the PnO.

Developmental divergence of sleep-wake patterns in orexin knockout and wild-type mice

Narcolepsy, a disorder characterized by fragmented bouts of sleep and wakefulness during the day and night as well as cataplexy, has been linked in humans and nonhuman animals to the functional integrity of the orexinergic system. Adult orexin knockout mice and dogs with a mutation of the orexin receptor exhibit symptoms that mirror those seen in narcoleptic humans. As with narcolepsy, infant sleep-wake cycles in humans and rats are highly fragmented, with consolidated bouts of sleep and wakefulness developing gradually. Based on these common features of narcoleptics and infants, we hypothesized that the development of sleep-wake fragmentation in orexin knockout mice would be expressed as a developmental divergence between knockouts and wild-types, with the knockouts lagging behind the wild-types. We tested this hypothesis by recording the sleep-wake patterns of infant orexin knockout and wild-type mice across the first three postnatal weeks. Both knockouts and wild-types exhibited age-dependent, and therefore orexin-independent, quantitative and qualitative changes in sleep-wake patterning. At 3 weeks of age, however, by which time the sleep and wake bouts of the wild-types had consolidated further, the knockouts lagged behind the wild-types and exhibited significantly more bout fragmentation. These findings suggest the possibility that the fragmentation of behavioural states that characterizes narcolepsy in adults reflects reversion back toward the more fragmented sleep-wake patterns that characterize infancy.

The preoptic hypothalamus and basal forebrain play opposing roles in the descending modulation of sleep and wakefulness in infant rats

Recent findings in infant rats suggest that the preoptic area (POA) and/or basal forebrain (BF) contribute to developmental changes in sleep and wake organization between postnatal day 2 (P2) and P9. To examine the contributions of these forebrain areas to sleep and wakefulness, separate lesions of the POA or BF, or combined lesions (POA + BF), were performed at P9, and precollicular transections were performed at P2. In addition, modafinil, a drug of unknown mechanism of action the effects of which on sleep and wakefulness have been hypothesized to result from inhibition of POA activity, was administered at P2 and P9. Finally, extracellular neuronal activity was recorded from the POA and BF. POA lesions decreased sleep bout durations and increased wake bout durations. BF lesions inhibited sleep bout durations to a lesser extent, while leaving wake bout durations unaffected. POA + BF lesions produced a combination of these effects, resulting in short bouts of sleep and wakefulness similar to those of transected P8 rats. Even at P2, transections decreased sleep bout durations. The finding, however, that the sleep-inhibiting and wake-promoting effects of modafinil were more potent at P9 than at P2 suggests increasing sleep-wake modulation by the POA between these two ages. Finally, neuronal recordings confirmed the presence of state-dependent neurons within the infant POA and BF. We propose that the POA, in addition to promoting sleep, inhibits wakefulness via direct and indirect inhibitory connections with wake-promoting neurons in the BF, and that this inhibitory influence increases across early development.

On the co-occurrence of startles and hippocampal sharp waves in newborn rats

Hippocampal sharp waves (SPWs) are among the earliest neural population patterns observed in infant mammals. Similarly, startles are among the earliest behavioral events observed. Here we provide evidence indicating that these two events are linked mechanistically soon after birth in freely moving and head-fixed 1 to 4-day-old rats. EMG electrodes and intrahippocampal silicon depth electrodes were used to detect the presence of startles and SPWs, respectively. In intact pups, the majority of sharp waves were preceded by startles (average latency: 161 ms). When the hippocampal formation was surgically separated from the brainstem, however, sharp waves and startles still occurred, but now independently. In addition, unrelated to startles or SPWs, gamma oscillations were detected in several subjects, as were neocortical "spindles" that propagated passively into the hippocampus. The co-occurrence of sharp waves and startles provides the opportunity for Hebbian changes in synaptic efficacy and, thus, is poised to contribute to the assembly of neural circuits early in development.

Dynamics of sleep-wake cyclicity in developing rats

Adult mammals cycle between periods of sleep and wakefulness. Recent assessments of these cycles in humans and other mammals indicate that sleep bout durations exhibit an exponential distribution, whereas wake bout durations exhibit a power-law distribution. Moreover, it was found that wake bout distributions, but not sleep bout distributions, exhibit scale invariance across mammals of different body sizes. Here we test the generalizability of these findings by examining the distributions of sleep and wake bout durations in infant rats between 2 and 21 days of age. In agreement with Lo et al., we find that sleep bout durations exhibit exponential distributions at all ages examined. In contrast, however, wake bout durations also exhibit exponential distributions at the younger ages, with a clear power-law distribution only emerging at the older ages. Further analyses failed to find substantial evidence either of short- or long-term correlations in the data, thus suggesting that the durations of current sleep and wake bouts evolve through time without memory of the durations of preceding bouts. These findings further support the notion that bouts of sleep and wakefulness are regulated independently. Moreover, in light of recent evidence that developmental changes in sleep and wake bouts can be attributed in part to increasing forebrain influences, these findings suggest the possibility of identifying specific neural circuits that modulate the changing complexity of sleep and wake dynamics during development.

Extraocular muscle activity, rapid eye movements and the development of active and quiet sleep

Rapid eye movements (REMs), traditionally measured using the electrooculogram (EOG), help to characterize active sleep in adults. In early infancy, however, they are not clearly expressed. Here we measured extraocular muscle activity in infant rats at 3 days of age (P3), P8 and P14-15 in order to assess the ontogeny of REMs and their relationship with other forms of sleep-related phasic activity. We found that the causal relationship between extraocular muscle twitches and REMs strengthened during the first two postnatal weeks, reflecting increased control of the extraocular muscles over eye movements. As early as P3, however, phasic bursts of extraocular muscle twitching occurred in synchrony with twitching in other muscle groups, producing waves of phasic activity interspersed with brief periods of quiescence. Surprisingly, the tone of the extraocular muscles, invisible to standard EOG measures, fluctuated in synchrony with the tone of other muscle groups; focal electrical stimulation within the dorsolateral pontine tegmentum, an area that has been shown to contain wake-on neurons in P8 rats, resulted in the simultaneous activation of high tone in both nuchal and extraocular muscles. Finally, when state-dependent neocortical electroencephalographic activity was observed at P14, it had already integrated fully with sleep and wakefulness as defined using electromyographic criteria alone; this finding is not consistent with the notion that active sleep in infants at this age is 'half-activated.' All together, these results indicate exquisite temporal organization of sleep soon after birth and highlight the possible functional implications of homologous activational states in striated muscle and neocortex.

Sleep-disordered Breathing in Newborn Mice Heterozygous for the Transcription Factor Phox2b

RATIONALE

Central congenital hypoventilation syndrome (CCHS) is a rare autosomal dominant syndrome present from birth, and characterized by depressed ventilation during sleep. Heterozygous mutations of the homeobox gene Phox2b were recently found in a very high proportion of patients.

OBJECTIVES

To determine whether newborn mice with heterozygous targeted deletion of the transcription factor Phox2b would display sleep-disordered breathing.

METHODS

We measured breathing pattern using whole-body plethysmography in wild-type and mutant 5-day-old mice, and we classified sleep-wake states using nuchal EMG and behavioral scores.

RESULTS

We found that sleep apnea total time was approximately six times longer (8.9 +/- 12 vs. 1.5 +/- 2.2 seconds, p < 0.0015), and ventilation during active sleep was 21% lower (18.4 +/- 5.1 vs. 23.3 +/- 5.5 ml/g/second, p < 0.006) in mutant than in wild-type pups. During wakefulness, apnea time and ventilation were not significantly different between mutant and wild-type pups. Mutant and wild-type pups showed highly similar sleep-wake states.

CONCLUSION

Although their respiratory phenotype was much less severe than CCHS, the Phox2b(+/-) mutant mice showed sleep-disordered breathing, which partially modeled the key feature of CCHS.

The neural substrates of infant sleep in rats

Sleep is a poorly understood behavior that predominates during infancy but is studied almost exclusively in adults. One perceived impediment to investigations of sleep early in ontogeny is the absence of state-dependent neocortical activity. Nonetheless, in infant rats, sleep is reliably characterized by the presence of tonic (i.e., muscle atonia) and phasic (i.e., myoclonic twitching) components; the neural circuitry underlying these components, however, is unknown. Recently, we described a medullary inhibitory area (MIA) in week-old rats that is necessary but not sufficient for the normal expression of atonia. Here we report that the infant MIA receives projections from areas containing neurons that exhibit state-dependent activity. Specifically, neurons within these areas, including the subcoeruleus (SubLC), pontis oralis (PO), and dorsolateral pontine tegmentum (DLPT), exhibit discharge profiles that suggest causal roles in the modulation of muscle tone and the production of myoclonic twitches. Indeed, lesions in the SubLC and PO decreased the expression of muscle atonia without affecting twitching (resulting in “REM sleep without atonia”), whereas lesions of the DLPT increased the expression of atonia while decreasing the amount of twitching. Thus, the neural substrates of infant sleep are strikingly similar to those of adults, a surprising finding in light of theories that discount the contribution of supraspinal neural elements to sleep before the onset of state-dependent neocortical activity.

Unexpectedly, the anatomy and neurophysiology of brainstem areas associated with sleep in the neonatal rat are strikingly similar to the adult

Thermal and nutritional modulation of sleep in infant rats

Infant rats cycle rapidly between periods of high muscle tone (indicative of wakefulness) and periods of atonia (indicative of sleep). Here, the influence of air temperature on sleep in 8-day-old rats was examined by testing pups at thermoneutrality (35 degrees C) and during moderate (28 degrees C) and extreme (20 degrees C) cold challenge; also, pups were tested 1, 4, and 8 hr after infusion of milk to assess the effects of food deprivation on sleep. Whereas moderate cooling slightly reduced sleep durations and altered the temporal patterning of myoclonic twitching, extreme cooling substantially decreased sleep durations and inhibited twitching. In contrast, food deprivation had little effect. Therefore, thermoregulatory mechanisms engaged during moderate cooling sustain sleep, whereas extreme cooling overwhelms these mechanisms, thereby promoting arousal.