Adaptive and pathological inhibition of neuroplasticity associated with circadian rhythms and sleep

Developmental changes in obstructive sleep apnea and sleep architecture in Down syndrome

BACKGROUND

Down syndrome (DS, also known as Trisomy 21) is a condition associated with abnormal neurodevelopment and a higher risk for sleep apnea. Our study sought to better understand and characterize the age-related developmental differences in sleep architecture and obstructive sleep apnea (OSA) severity in children with DS compared to euploid individuals.

METHODS

Retrospective review of polysomnograms in over 4151 infants, children, and adolescents in the pediatric sleep center at Children's National Hospital in Washington D.C. (0-18 years) including 218 individuals with DS.

RESULTS

The primary findings of our study are that: (1) severe OSA (obstructive apnea-hypopnea index ≥ 10/h) was more prevalent in the DS group (euploid 18% vs. DS 34%, p < 0.001) with the highest OSA severity being present in young children (<3 years old) and adolescents (>10 years old), (2) abnormalities in sleep architecture in children with DS were characterized by a prolonged rapid-eye movement (REM) sleep onset latency (SOL) (euploid 119 min vs. DS 144 min, p < 0.001) and greater arousal indexes (euploid 10.7/h vs. DS 12.2/h, p < 0.001), (3) developmental changes in the amount of REM sleep or slow wave sleep were not different in DS individuals relative to euploid children, (4) multivariate analyses showed that OSA and REM sleep latency differences between DS and euploid individuals were still present after adjusting by age, biological sex, and body mass index.

CONCLUSION

Severe OSA is highly prevalent in children with DS and follows an age-dependent "U" distribution with peaks in newborns/infants and children >10 years of age. Children with DS also have disturbances in sleep architecture characterized by a longer REM SOL and elevated arousal indexes. As sleep cycle generation and continuity play crucial roles in neuroplasticity and cognitive development, these findings offer clinically relevant insights to guide anticipatory guidance for infants, children, and adolescents with DS.

Handling, task complexity, time-of-day, and sleep deprivation as dynamic modulators of recognition memory in mice

Sleep is essential for optimal cognitive functioning. Although we lack a complete understanding of the role of sleep in memory consolidation, we know that various factors that disturb sleep or sleep quality have consequences for cognitive performance. Such factors can be unintended components of behavioral experiments on rodents and other experimental animals that generate differing results from different labs. These experimental variables include habituation to handling, intended or unintended sleep deprivation, task complexity, time of testing, and environmental features. We have examined how these variables impact recognition memory in C57BL/6 mice. Handled mice outperformed their non-handled counterparts across different combinations of delay phase duration and lighting conditions. Results also suggest that simple task recall is more resistant to diurnal variation and the impairing effects of sleep deprivation than is complex task recall. This study underscores the role of protocol and environmental factors in recognition memory and in conflicting results from different laboratories.

Functional Interactions Between Sleep and Circadian Rhythms in Learning and Learning Disabilities

The propensity for sleep is timed by the circadian system. Many studies have shown that learning and memory performance is affected by circadian phase. And, of course it is well established that critical processes of memory consolidation occur during and depend on sleep. This chapter presents evidence that sleep and circadian rhythms do not just have separate influences on learning and memory that happen to coincide because of the circadian timing of sleep, but rather sleep and circadian systems have a critical functional interaction in the processes of memory consolidation. The evidence comes primarily from research on two models of learning disability: Down's syndrome model mice and Siberian hamsters. The Down's syndrome model mouse (Ts65Dn) has severe learning disability that has been shown to be due to GABAergic over-inhibition. Short-term, chronic therapies with GABA_A antagonists restore learning ability in these mice long-term, but only if the antagonist treatments are given during the dark or sleep phase of the daily rhythm. The Siberian hamster is a model circadian animal except for the fact that a light treatment that gives the animal a phase advance on one day and a phase delay on the next day can result in total circadian arrhythmia for life. Once arrhythmic, the hamsters cannot learn. Learning, but not rhythmicity, is restored by short-term chronic treatment with GABA antagonists. Like many other species, if these hamsters are made arrhythmic by SCN lesion, their learning is unaffected. However, if made arrhythmic and learning disabled by the light treatment, subsequent lesions of their SCNs restore learning. SCN lesions also appear to restore learning in the Ts65Dn mice. The collective work on these two animal models of learning disability suggests that the circadian system modulates neuroplasticity. Our hypothesis is that a previously unrecognized function of the circadian system is to dampen neuroplasticity during the sleep phase to stabilize memory transcripts during their transfer to long-term memory. Thus, sleep and circadian systems have integrated roles to play in memory consolidation and do not just have separate but coincident influences on that process.

The Function(s) of Sleep

Sleep is a highly conserved phenomenon in endotherms, and therefore it must serve at least one basic function across this wide range of species. What that function is remains one of the biggest mysteries in neurobiology. By using the word neurobiology, we do not mean to exclude possible non-neural functions of sleep, but it is difficult to imagine why the brain must be taken offline if the basic function of sleep did not involve the nervous system. In this chapter we discuss several current hypotheses about sleep function. We divide these hypotheses into two categories: ones that propose higher-order cognitive functions and ones that focus on housekeeping or restorative processes. We also pose four aspects of sleep that any successful functional hypothesis has to account for: why do the properties of sleep change across the life span? Why and how is sleep homeostatically regulated? Why must the brain be taken offline to accomplish the proposed function? And, why are there two radically different stages of sleep?The higher-order cognitive function hypotheses we discuss are essential mechanisms of learning and memory and synaptic plasticity. These are not mutually exclusive hypotheses. Each focuses on specific mechanistic aspects of sleep, and higher-order cognitive processes are likely to involve components of all of these mechanisms. The restorative hypotheses are maintenance of brain energy metabolism, macromolecular biosynthesis, and removal of metabolic waste. Although these three hypotheses seem more different than those related to higher cognitive function, they may each contribute important components to a basic sleep function. Any sleep function will involve specific gene expression and macromolecular biosynthesis, and as we explain there may be important connections between brain energy metabolism and the need to remove metabolic wastes.A deeper understanding of sleep functions in endotherms will enable us to answer whether or not rest behaviors in species other than endotherms are homologous with mammalian and avian sleep. Currently comparisons across the animal kingdom depend on superficial and phenomenological features of rest states and sleep, but investigations of sleep functions would provide more insight into the evolutionary relationships between EEG-defined sleep in endotherms and rest states in ectotherms.

The interdependence of excitation and inhibition for the control of dynamic breathing rhythms

The preBötzinger Complex (preBötC), a medullary network critical for breathing, relies on excitatory interneurons to generate the inspiratory rhythm. Yet, half of preBötC neurons are inhibitory, and the role of inhibition in rhythmogenesis remains controversial. Using optogenetics and electrophysiology in vitro and in vivo, we demonstrate that the intrinsic excitability of excitatory neurons is reduced following large depolarizing inspiratory bursts. This refractory period limits the preBötC to very slow breathing frequencies. Inhibition integrated within the network is required to prevent overexcitation of preBötC neurons, thereby regulating the refractory period and allowing rapid breathing. In vivo, sensory feedback inhibition also regulates the refractory period, and in slowly breathing mice with sensory feedback removed, activity of inhibitory, but not excitatory, neurons restores breathing to physiological frequencies. We conclude that excitation and inhibition are interdependent for the breathing rhythm, because inhibition permits physiological preBötC bursting by controlling refractory properties of excitatory neurons.

Sleep-Dependent Potentiation in the Visual System Is at Odds with the Synaptic Homeostasis Hypothesis

STUDY OBJECTIVES

Two commentaries recently published in SLEEP came to very different conclusions regarding how data from a mouse model of sleep-dependent neural plasticity (orientation-specific response potentiation; OSRP) fit with the synaptic homeostasis hypothesis (SHY). To assess whether SHY offers an explanatory mechanism for OSRP, we present new data on how cortical neuron firing rates are modulated as a function of novel sensory experience and subsequent sleep in this model system.

METHODS

We carried out longitudinal extracellular recordings of single-neuron activity in the primary visual cortex across a period of novel visual experience and subsequent sleep or sleep deprivation. Spontaneous neuronal firing rates and visual responses were recorded from the same population of visual cortex neurons before control (blank screen) or novel (oriented grating) stimulus presentation, immediately after stimulus presentation, and after a period of subsequent ad lib sleep or sleep deprivation.

RESULTS

Firing rate responses to visual stimuli were unchanged across waking experience, regardless of whether a blank screen or an oriented grating stimulus was presented. Firing rate responses to stimuli of the presented stimulus orientation were selectively enhanced across post-stimulus sleep, but these changes were blocked by sleep deprivation. Neuronal firing increased significantly across bouts of post-stimulus rapid eye movement (REM) sleep and slow wave sleep (SWS), but not across bouts of wake.

CONCLUSIONS

The current data suggest that following novel visual experience, potentiation of a subset of V1 synapses occurs across periods of sleep. This finding cannot be explained parsimoniously by SHY.

Sex-specific sleep patterns among university students in Lebanon: impact on depression and academic performance

Good sleep quality and quantity are fundamental to the maintenance of normal physiological processes. Changes in sleep patterns are commonly observed among young adults and are shown to impact neurocognitive, academic, and psychological well-being. Given the scarcity of sleep information about Lebanon and acknowledging the sex differences in various sleep dimensions, we conducted a study that aimed at assessing sex differences in sleep habits among university students in Lebanon in relation to psychoacademic status. A total of 540 students (50.6% females) completed a questionnaire that inquired about sociodemographics and evaluated sleep quality and depression using the Pittsburg Sleep Quality Index (PSQI) and Center for Epidemiological Studies Depression Scale (CES-D), respectively. The mean PSQI global score (6.57±3.49) indicated poor sleep, with no significant differences between men and women. The sleep/wake rhythm was delayed on weekends for both sexes. Females exhibited earlier bedtimes and rise times and longer sleep durations on both weekdays and weekends. However, unlike males females showed a greater phase delay in wake times than bedtimes on weekends (149 minutes vs 74 minutes, respectively). In all, 70.9% of females suffered from depressive symptoms, which was a significantly higher proportion compared with 58.5% of males (P<0.01). Based on the mean cumulative self-reported grade point average (GPA), the academic performance of females was significantly better than that of males (2.8±0.61 vs 2.65±0.61, P<0.05, respectively). Depression, as scored by CES-D, in females was significantly negatively correlated with the cumulative GPA (r=-0.278, P<0.01), earlier wake time (r=-0.168, P<0.05), and average sleep duration (r=-0.221, P<0.01) on weekdays. GPA of males was significantly correlated with bedtime on weekends (r=-0.159, P<0.05). We conclude that sex differences in sleep timing, such as bedtime/rise time and nocturnal sleep duration, rather than sleep quality exist among Lebanese university students. Sex-specific sleep patterns have differential impact on psychological and academic well-being.