Brown Norway and Zucker Lean rats demonstrate circadian variation in ventilation and sleep apnea

In light of breathing: environmental light is an important modulator of breathing with clinical implications

In vertebrate animals, the automatic, rhythmic pattern of breathing is a highly regulated process that can be modulated by various behavioral and physiological factors such as metabolism, sleep-wake state, activity level, and endocrine signaling. Environmental light influences many of these modulating factors both indirectly by organizing daily and seasonal rhythms of behavior and directly through acute changes in neural signaling. While several observations from rodent and human studies suggest that environmental light affects breathing, few have systematically evaluated the underlying mechanisms and clinical relevance of environmental light on the regulation of respiratory behavior. Here, we provide new evidence and discuss the potential neurobiological mechanisms by which light modulates breathing. We conclude that environmental light should be considered, from bench to bedside, as a clinically relevant modulator of respiratory health and disease.

Sleep Neurobiology and the Critical Care Environment

TOPIC

Sleep in the intensive care unit can be poorly consolidated and highly fragmented. This review examines the neurobiology of normal and abnormal sleep, with a focus on the changes that occur in the intensive care unit environment.

CLINICAL RELEVANCE

Patients in the intensive care unit demonstrate a lack of rapid-eye-movement sleep and an inability to effectively transition from light to deep stages of sleep. These abnormalities can adversely affect hemodynamic parameters and physiological and psychological outcomes.

PURPOSE

To describe the brain mechanisms and electroencephalographic characteristics of wakefulness and the different stages of sleep. This review also describes how sleep can be altered by hospitalization in the intensive care unit and how nurses can design interventions that improve sleep and outcomes.

CONTENT COVERED

The review examines sleep mechanisms, including brain electrical activity, regulatory centers in the brain, and circadian and diurnal patterns of sleep and hemodynamic function. Nursing interventions for specific patient risk factors in the intensive care unit are proposed.

Photoperiod Manipulation Reveals a Light-Driven Component to Daily Patterns of Ventilation in Male C57Bl/6J Mice

Obstructive sleep apnea is a common sleep disorder that increases risk for cardiovascular disease and mortality. The severity of sleep-disordered breathing in obstructive sleep apnea patients fluctuates with the seasons, opening the possibility that seasonal changes in light duration, or photoperiod, can influence mechanisms of breathing. Photoperiod can have profound effects on internal timekeeping and can reshape metabolic rhythms in mammals. While the daily rhythm in ventilation is largely shaped by the metabolic rate, less is known about whether ventilatory rhythms are altered in accordance with metabolism under different photoperiods. Here, we investigate the relationship between ventilation and metabolism under different photoperiods using whole-body plethysmography and indirect calorimetry. We find that the daily timing of ventilation is chiefly synchronized to dark onset and that light cues are important for maintaining daily ventilatory rhythms. Moreover, changes in ventilatory patterns are not paralleled by changes in oxygen consumption, energy expenditure, or respiratory exchange rate under different photoperiods. We conclude that ventilatory patterns are not only shaped by the metabolic rate and circadian timing but are also influenced by other light-driven factors. Collectively, these findings have clinical implications for the seasonal variations in sleep-disordered breathing found in individuals with obstructive sleep apnea.

Genetic depletion of 5-HT increases central apnea frequency and duration and dampens arousal but does not impact the circadian modulation of these variables

We examined the impact of serotonin (5-HT) on the frequency and duration of central apneic events and the frequency of accompanying arousals during nonrapid and rapid eye movement (NREM and REM, respectively) sleep across the light/dark cycle. Electroencephalography, electromyography, core body temperature, and activity were recorded for 24 h following implantation of telemeters in wild-type (Tph2+/+) and tryptophan hydroxylase 2 knockout (Tph2−/−) male mice. The frequency and duration of central apneic events were increased, the number of apneic events coupled to an arousal was decreased, and the ventilatory sensitivity to hypoxia and hypercapnia was decreased in the Tph2−/− compared with the Tph2+/+ mice during NREM sleep. Apnea frequency and duration were similar in the Tph2−/− and Tph2+/+ mice during REM sleep. The duration of apneic events during REM compared with NREM sleep was similar in the Tph2−/− mice. In contrast, the duration was greater during REM sleep in the Tph2+/+ mice. Our results also revealed that apnea frequency was greater during the light compared with the dark cycle. Circadian modulation of this variable was evident in both the Tph2−/− and Tph2+/+ mice during NREM and REM sleep. We conclude that depletion of 5-HT increases the frequency and duration of central apneic events, dampens arousal, and blunts the ventilatory response to hypoxia and hypercapnia during NREM sleep but is not essential for the circadian modulation of these variables.

NEW & NOTEWORTHY The presence of serotonin (5-HT) in the central nervous system diminishes the frequency of central apneic events. This neuromodulator also moderates the duration of central apneic events and promotes arousal from central events if they occur during nonrapid eye movement (NREM) sleep. However, 5-HT is not responsible for the circadian modulation of apnea frequency, which we found was greater during NREM sleep in the light compared with the dark cycle.

Mathematical Models of Sleep and Circadian Rhythms: A Case for Using the 2-Process Model in Neuroscience Nursing

Acute and chronic neurological disorders impair sleep. Despite the availability of theoretical/mathematical frameworks about sleep, the nursing profession rarely incorporates these models. The purpose of this article was to analyze the 2-process model of sleep regulation using Fawcett and DeSanto-Madeya's method, a systematic approach for determining whether a theory is relevant to nursing. The 2-process model has 3 concepts: process S (sleep-dependent process), process C (circadian-timing-dependent process), and total sleep propensity (summation of processes S and C). Nonnursing theories do not explicitly incorporate nursing metaparadigm concepts-person, health, environment, and nursing-but the 2-process model is congruent with nursing's philosophy. The model guided studies quantifying sleep and circadian patterns in other fields, and nurses could use this framework to measure the impact of nursing interventions. Strengths of the 2-process model include parsimony (conciseness without oversimplification) and the ability to empirically test propositions related to processes S and C. The 2-process model is relevant to neuroscience nursing-by measuring sleep/circadian-related variables (electroencephalogram, core body temperature, salivary melatonin). Nurses have opportunities to design, test, and use interventions that improve sleep in patients with neurological conditions.

Modulation of Motoneuronal Activity With Sleep-Wake States and Motoneuronal Gene Expression Vary With Circadian Rest-Activity Cycle

In both nocturnal and diurnal mammals, sleep and wake states differentially aggregate during the rest and active phases of circadian cycle. Closely associated with this rhythm are prominent changes in motor activity. Here, we quantified the magnitudes of electromyographic activity (EMG) measured separately during different sleep-wake states across the rest-activity cycle, thereby separating amplitude measurements from the known dependance of the timing of wake and sleep on the phase of circadian rest-activity cycle. In seven rats chronically instrumented for electroencephalogram and EMG monitoring, nuchal and lingual muscle EMGs were measured as a commonly used postural output in behavioral sleep studies and as a cranial motor output with potential clinical relevance in obstructive sleep apnea (OSA) syndrome, respectively. We found that, for both motor outputs, EMG measured during wake episodes was significantly higher during the active phase, than during the rest phase, of circadian cycle. The corresponding patterns observed during slow-wave sleep (SWS) and rapid eye movement sleep (REMS) were different. During SWS, lingual EMG was very low and did not differ between the rest and active phase, whereas nuchal EMG had pattern similar to that during wakefulness. During REMS, lingual EMG was, paradoxically, higher during the rest phase due to increased twitching activity, whereas nuchal EMG was very low throughout the rest and active periods (postural atonia). In the follow-up comparison of differences in transcript levels in tissue samples obtained from the medullary hypoglossal motor nucleus and inferior olive (IO) at rest onset and active period onset conducted using microarrays, we identified significant differences for multiple transcripts representing the core members of the molecular circadian clock and other genes important for the regulation of cell metabolism and activity (up to n = 130 at p < 0.001). Collectively, our data indicate that activity of motoneurons is regulated to optimally align it with the rest-activity cycle, with the process possibly involving transcriptional mechanisms at the motoneuronal level. Our data also suggest that OSA patients may be relatively better protected against sleep-related upper airway obstructions during REMS episodes generated during the rest phase, than during active phase, of the circadian cycle.

Hypoglossal motoneurons are endogenously activated by serotonin during the active period of circadian cycle

In obstructive sleep apnea patients, contraction of lingual muscles protects the pharyngeal airway from collapse. Hypoglossal (XII) motoneurons innervate the muscles of the tongue and are themselves under wake-related excitatory drives, including that mediated by serotonin (5-HT). Estimates of endogenous 5-HT activation vary among different studies. We tested whether endogenous drive mediated by 5-HT is present in rat XII motoneurons when measured during the active period of the circadian cycle. We monitored sleep-wake states and lingual and nuchal electromyograms (EMGs) while perfusing the XII nucleus with a vehicle or a 5-HT₂ receptor antagonist (mianserin, 0.2mM) at the active period onset. EMG levels were measured during each behavioral state and normalized by the mean EMG activity during wakefulness at 4-7am. Wake-related lingual EMG was significantly lower during mianserin perfusion than with the vehicle (53.0±9.7% vs. 84.5±8.7%; p=0.002). Mianserin had no effect on nuchal EMG or sleep-wake behavior. Thus, rat XII motoneurons receive endogenous serotonergic activation during wakefulness when measured during the dark period. This indicates that XII motoneuronal activity is enhanced by 5-HT output during the active period of the circadian cycle.

The pedunculopontine tegmentum controls renal sympathetic nerve activity and cardiorespiratory activities in nembutal-anesthetized rats

Elevated renal sympathetic nerve activity (RSNA) accompanies a variety of complex disorders, including obstructive sleep apnea, heart failure, and chronic kidney disease. Understanding pathophysiologic renal mechanisms is important for determining why hypertension is both a common sequelae and a predisposing factor of these disorders. The role of the brainstem in regulating RSNA remains incompletely understood. The pedunculopontine tegmentum (PPT) is known for regulating behaviors including alertness, locomotion, and rapid eye movement sleep. Activation of PPT neurons in anesthetized rats was previously found to increase splanchnic sympathetic nerve activity and blood pressure, in addition to altering breathing. The present study is the first investigation of the PPT and its potential role in regulating RSNA. Microinjections of DL-homocysteic acid (DLH) were used to probe the PPT in 100-μm increments in Nembutal-anesthetized rats to identify effective sites, defined as locations where changes in RSNA could be evoked. A total of 239 DLH microinjections were made in 18 rats, which identified 20 effective sites (each confirmed by the ability to evoke a repeatable sympathoexcitatory response). Peak increases in RSNA occurred within 10–20 seconds of PPT activation, with RSNA increasing by 104.5 ± 68.4% (mean ± standard deviation) from baseline. Mean arterial pressure remained significantly elevated for 30 seconds, increasing from 101.6 ± 18.6 mmHg to 135.9 ± 36.4 mmHg. DLH microinjections also increased respiratory rate and minute ventilation. The effective sites were found throughout the rostal-caudal extent of the PPT with most located in the dorsal regions of the nucleus. The majority of PPT locations tested with DLH microinjections did not alter RSNA (179 sites), suggesting that the neurons that confer renal sympathoexcitatory functions comprise a small component of the PPT. The study also underscores the importance of further investigation to determine whether sympathoexcitatory PPT neurons contribute to adverse renal and cardiovascular consequences of diseases such as obstructive sleep apnea and heart failure.