Sleep-waking states develop independently in the isolated forebrain and brain stem following early postnatal midbrain transection in cats

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.

Neural Control of REM Sleep and Motor Atonia: Current Perspectives

PURPOSE OF REVIEW

Since the formal discovery of rapid eye movement (REM) sleep in 1953, we have gained a vast amount of knowledge regarding the specific populations of neurons, their connections, and synaptic mechanisms regulating this stage of sleep and its accompanying features. This article discusses REM sleep circuits and their dysfunction, specifically emphasizing recent studies using conditional genetic tools.

RECENT FINDINGS

Sublaterodorsal nucleus (SLD) in the dorsolateral pons, especially the glutamatergic subpopulation in this region (SLDGlut), are shown to be indispensable for REM sleep. These neurons appear to be single REM generators in the rodent brain and may initiate and orchestrate all REM sleep events, including cortical and hippocampal activation and muscle atonia through distinct pathways. However, several cell groups in the brainstem and hypothalamus may influence SLDGlut neuron activity, thereby modulating REM sleep timing, amounts, and architecture. Damage to SLDGlut neurons or their projections involved in muscle atonia leads to REM behavior disorder, whereas the abnormal activation of this pathway during wakefulness may underlie cataplexy in narcolepsy. Despite some opposing views, it has become evident that SLDGlut neurons are the sole generators of REM sleep and its associated characteristics. Further research should prioritize a deeper understanding of their cellular, synaptic, and molecular properties, as well as the mechanisms that trigger their activation during cataplexy and make them susceptible in RBD.

Rapid Eye Movement Sleep during Early Life: A Comprehensive Narrative Review

The ontogenetic sleep hypothesis suggested that rapid eye movement (REM) sleep is ontogenetically primitive. Namely, REM sleep plays an imperative role in the maturation of the central nervous system. In coincidence with a rapidly developing brain during the early period of life, a remarkably large amount of REM sleep has been identified in numerous behavioral and polysomnographic studies across species. The abundant REM sleep appears to serve to optimize a cerebral state suitable for homeostasis and inherent neuronal activities favorable to brain maturation, ranging from neuronal differentiation, migration, and myelination to synaptic formation and elimination. Progressively more studies in Mammalia have provided the underlying mechanisms involved in some REM sleep-related disorders (e.g., narcolepsy, autism, attention deficit hyperactivity disorder (ADHD)). We summarize the remarkable alterations of polysomnographic, behavioral, and physiological characteristics in humans and Mammalia. Through a comprehensive review, we offer a hybrid of animal and human findings, demonstrating that early-life REM sleep disturbances constitute a common feature of many neurodevelopmental disorders. Our review may assist and promote investigations of the underlying mechanisms, functions, and neurodevelopmental diseases involved in REM sleep during early life.

The Brainstem-Informed Autism Framework: Early Life Neurobehavioral Markers

Autism spectrum disorders (ASD) have long-term implications on functioning at multiple levels. In this perspective, we offer a brainstem-informed autism framework (BIAF) that traces the protracted neurobehavioral manifestations of ASD to early life brainstem dysfunctions. Early life brainstem-mediated markers involving functions of autonomic/arousal regulation, sleep-wake homeostasis, and sensorimotor integration are delineated. Their possible contributions to the early identification of susceptible infants are discussed. We suggest that the BIAF expands our multidimensional understanding of ASD by focusing on the early involvement of brainstem systems. Importantly, we propose an integrated BIAF screener that brings about the prospect of a sensitive and reliable early life diagnostic scheme for weighing the risk for ASD. The BIAF screener could provide clinicians substantial gains in the future and may carve customized interventions long before the current DSM ASD phenotype is manifested using dyadic co-regulation of brainstem-informed autism markers.

Differentiating Oneiric Stupor in Agrypnia Excitata From Dreaming Disorders

Oneiric Stupor (OS) in Agrypnia Excitata represents a peculiar condition characterized by the recurrence of stereotyped gestures such as mimicking daily-life activities associated with the reporting of a dream mentation consisting in a single oneiric scene. It arises in the context of a completely disorganized sleep structure lacking any physiological cyclic organization, thus, going beyond the concept of abnormal dream. However, a proper differential diagnosis of OS, in the complex world of the “disorders of dreaming” can become quite challenging. The aim of this review is to provide useful clinical and videopolygraphic data on OS to differentiate it from other dreaming disorders. Each entity will be clinically evaluated among the areas of dream mentation and abnormal sleep behaviors and its polygraphic features will be analyzed and distinguished from OS.

Lateral hypothalamic neurotensin neurons promote arousal and hyperthermia

Sleep and wakefulness are greatly influenced by various physiological and psychological factors, but the neuronal elements responsible for organizing sleep-wake behavior in response to these factors are largely unknown. In this study, we report that a subset of neurons in the lateral hypothalamic area (LH) expressing the neuropeptide neurotensin (Nts) is critical for orchestrating sleep-wake responses to acute psychological and physiological challenges or stressors. We show that selective activation of Nts^LH neurons with chemogenetic or optogenetic methods elicits rapid transitions from non-rapid eye movement (NREM) sleep to wakefulness and produces sustained arousal, higher locomotor activity (LMA), and hyperthermia, which are commonly observed after acute stress exposure. On the other hand, selective chemogenetic inhibition of Nts^LH neurons attenuates the arousal, LMA, and body temperature (Tb) responses to a psychological stress (a novel environment) and augments the responses to a physiological stress (fasting).

A neurotensin-producing subset of neurons in the lateral hypothalamus promote arousal and thermogenesis; these neurons are necessary for appropriate sleep-wake and body temperature responses to various stressors.

Adjusting sleep-wake behavior in response to environmental and physiological challenges may not only be of protective value, but can also be vital for the survival of the organism. For example, while it is crucial to increase wake to explore a novel environment to search for potential threats and food sources, it is also necessary to decrease wake and reduce energy expenditure during prolonged absence of food. In this study, we report that a subset of neurons in the lateral hypothalamic area (LH) expressing the neuropeptide neurotensin (Nts) is critical for orchestrating sleep-wake responses to such challenges. We show that brief activation of Nts^LH neurons in mice evokes immediate arousals from sleep, while their sustained activation increases wake, locomotor activity, and body temperature for several hours. In contrast, when Nts^LH neurons are inhibited, mice are neither able to sustain wake in a novel environment nor able to reduce wake during food deprivation. These data suggest that Nts^LH neurons may be necessary for generating appropriate sleep-wake responses to a wide variety of environmental and physiological challenges.

Are there Sleep-promoting Neurons in the Mouse Parafacial Zone?

Although recent studies have reported that gamma-aminobutyric acid (GABA) neurons in the parafacial zone (PZ) of the rostral medulla are needed for the induction of slow-wave sleep (SWS) and that the PZ is a medullary SWS-promoting center, it remains unknown whether the PZ contains SWS-active or sleep-promoting neurons. In the present study, a total of 125 neurons were recorded, for the first time, in non-anesthetized, head-restrained mice during the complete wake-sleep cycle throughout the PZ of the rostral medulla. The vast majority (87.2%) of the neurons displayed increased activity during both wakefulness (W) and paradoxical (or rapid eye movement) sleep (PS) compared to during SWS (W/PS-active neurons) and a few (8.0%) discharged phasically and selectively during PS (PS-active neurons), but none discharged maximally during SWS (SWS-active neurons) or displayed a higher rate of spontaneous discharge during both SWS and PS than during W (SWS/PS-active neurons). These findings do not support the view that the GABAergic PZ is a medullary SWS-promoting center.

Brainstem regulation of slow-wave-sleep

Recent work has helped reconcile puzzling results from brainstem transection studies first performed over 60 years ago, which suggested the existence of a sleep-promoting system in the medullary brainstem. It was specifically shown that GABAergic neurons located in the medullary brainstem parafacial zone (PZ^GABA) are not only necessary for normal slow-wave-sleep (SWS) but that their selective activation is sufficient to induce SWS in behaving animals. In this review we discuss early experimental findings that inspired the hypothesis that the caudal brainstem contained SWS-promoting circuitry. We then describe the discovery of the SWS-promoting PZ^GABA and discuss future experimental priorities.

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.

Identification and characterization of a sleep-active cell group in the rostral medullary brainstem

Early transection and stimulation studies suggested the existence of sleep-promoting circuitry in the medullary brainstem, yet the location and identity of the neurons comprising this putative hypnogenic circuitry remains unresolved. In the present study, we sought to uncover the location and identity of medullary neurons that might contribute to the regulation of sleep. Here we show the following in rats: (1) a delimited node of medullary neurons located lateral and dorsal to the facial nerve-a region we termed the parafacial zone (PZ)-project to the wake-promoting medial parabrachial nucleus; (2) PZ neurons express c-Fos after sleep but not after wakefulness and hence are sleep active; and (3) cell-body-specific lesions of the PZ result in large and sustained increases (50%) in daily wakefulness at the expense of slow-wave sleep (SWS). Using transgenic reporter mice [vesicular GABA/glycine transporter (Vgat)-GFP], we then show that >50% of PZ sleep-active neurons are inhibitory (GABAergic/glycinergic, VGAT-positive) in nature. Finally, we used a Cre-expressing adeno-associated viral vector and conditional Vgat(lox/lox) mice to selectively and genetically disrupt GABA/glycinergic neurotransmission from PZ neurons. Disruption of PZ GABAergic/glycinergic neurotransmission resulted in sustained increases (40%) in daily wakefulness at the expense of both SWS and rapid eye movement sleep. These results together reveal the location and neurochemical identity of a delimited node of sleep-active neurons within the rostral medullary brainstem.

Ontogenesis of morphine-induced behavior in the cat

We analyzed the behavioral responses to a single dose of morphine in kittens at postnatal (P) ages 7, 15, 30, 60, 90, and 120 days. Each kitten received 0.5 or 3.0 mg/kg i.p. of morphine sulphate or saline vehicle. An average of 6.5 kittens were studied at each dose and age. An ethogram was constructed, based on morphine effects in adult cats, to score appropriate behaviors from direct observation and video sampling. After injection behaviors were sampled for periods of 2 min every 15-30 min for a total of 4 h. The frequency of each selected behavior was scored at 2 s intervals during each of the 2 min periods and it was expressed as a percent of all time samples scored for the 4 h period. Statistical comparisons were made with control (saline) littermates. At P7-15 the drug's main effect was behavioral depression; i.e., kittens, away from the litter, laid sprawled as if with no muscle tonus; Nursing was suppressed and Vocalization was distressed. Mainly with the higher dose, at P30, morphine-specific behaviors appeared for the first time. With the kitten in a Sitting position, these included stereotypical Head and Paw Movements and body Torsion. At P60 other drug-elicited behaviors emerged, including Spinning, Retching, and Vomiting. By P90-120 the frequency of Head (16.0%) and Paw (16.9%) Movements doubled relative to P30-60. Morphine significantly changed frequencies of newly matured behaviors (in control kittens) including Sniffing and Licking (increased), and Grooming (decreased/blocked). Retching and Vomiting increased to adult levels. Morphine-induced hyperthermia was first detected at P60 and peaked by P90-P120. The early behavioral depression shifted to a pattern of increasing activity starting at P30 and peaking at P90-120, at which time Sleep was absent and Laying was reduced, while Walking and Sitting were increased. We concluded that the maturation of the stereotypical behavioral responses to morphine in cats begins at about P30 and is completed between P90 and 120. Results are discussed in terms of developmental parameters and putative brain sites of morphine's actions.

Metabolic maturation of the brain: A study of local cerebral protein synthesis in the developing cat

We used quantitative L-[1-(14)C]leucine autoradiography to study the maturation of cerebral protein synthesis metabolism in kittens, starting at birth and through postnatal age (P) 180 days as well as in adult cats. We found that at birth most brain structures show protein synthesis (nmol/min/g; lCPS(leu)) rates already within the range of adult values (with some exceptions; e.g., the hippocampus and putamen). Likewise, most structures show a transient developmental peak during which the rates climb to levels higher than in adulthood. This peak often occurred at P60, but in some regions lasted from P30 to P90. Therefore, there is some regional heterogeneity in the maturation of brain protein synthesis. These results are compared with our previous findings on the maturation of cerebral glucose utilization and oxidative metabolism. We discuss the meaning of these maturational profiles in terms of time course of morphological development and of maturation of behavior in the cat. Correlations with findings in other mammalian species are also discussed.

Postnatal growth and morphological development of the brain: a species comparison

The objective of this report is to summarize the available literature regarding the postnatal growth and morphological development of the brain and compare the timelines for these events between humans and experimental species. While not the primary focus of this report, in acknowledgement of the evident role of maturation of neurotransmitter systems in development, a brief description of the comparative development of the NMDA receptor is included. To illustrate the challenges faced in estimating developmental toxicity potential in humans, the importance of postnatal experience in CNS development is also briefly reviewed. This review is part of the initial phase of a project undertaken by the Developmental and Reproductive Toxicology Technical Committee of the ILSI Health and Environmental Sciences Institute (HESI) to bring together information on a selected number of organ systems and compare their postnatal development across several species (Hurtt and Sandler: Birth Defects Res Part B 68:307-308, 2003).

The brain stem but not forebrain independently supports morphine tolerance and withdrawal effects in cats

We employed polygraphic recordings and behavioral measures to study the effects of chronic morphine use upon the isolated forebrain and the decerebrate animal in cats with a midbrain transection. Cats received morphine for 12 days, and 24 h recording sessions were conducted on days 1 and 11. For the decerebrate cat, the percent time of rapid eye movement (REM) sleep was reduced during the 24 h period on both days 1 and 11. However, the values on day 11 were consistently higher than the values on day 1. Other tolerance indicators were decreases in the number of early behavioral signs and in the onset delay for REM sleep, together with an increase in onset time for motor activation. After naloxone (day 12) all cats displayed "wet shakes," tachypnea and eye squinting, as well as either pyloerection, elevated tail, salivation, licking, micturition, and yawning. In the isolated forebrain, the percent time for waking increased through the first 18 h post-morphine on both days 1 and 11. Conversely, the duration of non-REM (NREM) sleep and of drowsiness decreased. But importantly, the duration of sleep-waking states did not vary between days 11 and 1, indicating absence of tolerance. Additionally, after naloxone, the isolated forebrain entered NREM sleep, contrasting with opposite findings in intact cats. Therefore, while we could not demonstrate chronic use effects in the isolated forebrain, the decerebrate cat still displayed typical tolerance/withdrawal manifestations. This suggests that the effects of chronic opiate use are deeply seated in the brain stem, which might help understanding the ingrained nature of physical dependence.