Post-sigh sleep apneas in mice: Systematic review and data-driven definition

Sleep apneas can be categorized as post-sigh (prevailing in non-rapid eye movement sleep) or spontaneous (prevailing in rapid eye movement sleep) according to whether or not they are preceded by an augmented breath (sigh). Notably, the occurrence of these apnea subtypes changes differently in hypoxic/hypercapnic environments and in some genetic diseases, highlighting the importance of an objective discrimination. We aim to: (a) systematically review the literature comparing the criteria used in categorizing mouse sleep apneas; and (b) provide data-driven criteria for this categorization, with the final goal of reducing experimental variability in future studies. Twenty-two wild-type mice, instrumented with electroencephalographic/electromyographic electrodes, were placed inside a whole-body plethysmographic chamber to quantify sleep apneas and sighs. Wake\u2013sleep states were scored on 4-s epochs based on electroencephalographic/electromyographic signals. Literature revision showed that highly different criteria were used for post-sigh apnea definition, the intervals for apnea occurrence after sigh ranging from 1 breath up to 20 s. In our data, the apnea occurrence rate during non-rapid eye movement sleep was significantly higher than that calculated before the sigh only in the 1st and 2nd 4-s epochs following a sigh. These data suggest that, in mice, apneas should be categorized as post-sigh only if they start within 8 s from a sigh; the choice of shorter or longer time windows might underestimate or slightly overestimate their occurrence rate, respectively

Early-life nicotine or cotinine exposure produces long-lasting sleep alterations and downregulation of hippocampal corticosteroid receptors in adult mice

Early-life exposure to environmental toxins like tobacco can permanently re-program body structure and function. Here, we investigated the long-term effects on mouse adult sleep phenotype exerted by early-life exposure to nicotine or to its principal metabolite, cotinine. Moreover, we investigated whether these effects occurred together with a reprogramming of the activity of the hippocampus, a key structure to coordinate the hormonal stress response. Adult male mice born from dams subjected to nicotine (NIC), cotinine (COT) or vehicle (CTRL) treatment in drinking water were implanted with electrodes for sleep recordings. NIC and COT mice spent significantly more time awake than CTRL mice at the transition between the rest (light) and the activity (dark) period. NIC and COT mice showed hippocampal glucocorticoid receptor (GR) downregulation compared to CTRL mice, and NIC mice also showed hippocampal mineralocorticoid receptor downregulation. Hippocampal GR expression significantly and inversely correlated with the amount of wakefulness at the light-to-dark transition, while no changes in DNA methylation were found. We demonstrated that early-life exposure to nicotine (and cotinine) concomitantly entails long-lasting reprogramming of hippocampal activity and sleep phenotype suggesting that the adult sleep phenotype may be modulated by events that occurred during that critical period of life

Neural control of fasting-induced torpor in mice

Torpor is a peculiar mammalian behaviour, characterized by the active reduction of metabolic rate, followed by a drop in body temperature. To enter torpor, the activation of all thermogenic organs that could potentially defend body temperature must be prevented. Most of these organs, such as the brown adipose tissue, are controlled by the key thermoregulatory region of the Raphe Pallidus (RPa). Currently, it is not known which brain areas mediate the entrance into torpor. To identify these areas, the expression of the early gene c-Fos at torpor onset was assessed in different brain regions in mice injected with a retrograde tracer (Cholera Toxin subunit b, CTb) into the RPa region. The results show a network of hypothalamic neurons that are specifically activated at torpor onset and a direct torpor-specific projection from the Dorsomedial Hypothalamus to the RPa that could putatively mediate the suppression of thermogenesis during torpor

Obstructive sleep apneas naturally occur in mice during REM sleep and are highly prevalent in a mouse model of Down syndrome

Study objectives: The use of mouse models in sleep apnea study is limited by the belief that central (CSA) but not obstructive sleep apneas (OSA) occur in rodents. We aimed to develop a protocol to investigate the presence of OSAs in wild-type mice and, then, to apply it to a validated model of Down syndrome (Ts65Dn), a human pathology characterized by a high incidence of OSAs. Methods: In a pilot study, nine C57BL/6J wild-type mice were implanted with electrodes for electroencephalography (EEG), neck electromyography (nEMG), and diaphragmatic activity (DIA), and then placed in a whole-body-plethysmographic (WBP) chamber for 8 h during the rest (light) phase to simultaneously record sleep and breathing activity. CSA and OSA were discriminated on the basis of WBP and DIA signals recorded simultaneously. The same protocol was then applied to 12 Ts65Dn mice and 14 euploid controls. Results: OSAs represented about half of the apneic events recorded during rapid-eye-movement-sleep (REMS) in each experimental group, while the majority of CSAs were found during non-rapid eye movement sleep. Compared with euploid controls, Ts65Dn mice had a similar total occurrence rate of apneic events during sleep, but a significantly higher occurrence rate of OSAs during REMS, and a significantly lower occurrence rate of CSAs during NREMS. Conclusions: Mice physiologically exhibit both CSAs and OSAs. The latter appear almost exclusively during REMS, and are highly prevalent in Ts65Dn. Mice may, thus, represent a useful model to accelerate the understanding of the pathophysiology and genetics of sleep-disordered breathing and to help the development of new therapies

Central and baroreflex control of heart period as a function of ambient temperature in narcoleptic mice with genetic ablation of hypocretin neurons

Hypothalamic hypocretin (HCRT) neurons control wake-sleep behavior, cardiovascular system and body temperature, and their loss entails narcolepsy. Cardiovascular regulation results from the interplay between central autonomic commands (CAC) and the baroreflex. The cross-correlation function (CCF) between spontaneous fluctuations of heart period (HP) and blood pressure (BP) yields information on this interplay. We investigated whether HCRT neurons are required for appropriate autonomic control of HP at different ambient temperatures (Ta) and wake-sleep behaviors. Narcoleptic mice with genetic ablation of HCRT neurons (n=11) and wild-type controls (n=12) were implanted with a telemetric BP transducer and electrodes for discrimination of wake-sleep behaviors. The CCF between HP and BP was computed in each mouse at Ta of 25\ub0C, 30\ub0C, and 20\ub0C in random order. Data were analyzed with 3-way analysis of variance and t-tests (significance at P < 0.05). CCF changes depended on an interaction effect between ambient temperature and the wake-sleep behaviors regardless of the presence of HCRT neurons. In both mouse strains during rapid-eye-movement sleep, CAC on the heart prevailed at 30\ub0C whereas baroreflex control of HP prevailed at 20\ub0C. These results demonstrate that ambient temperature modulates the integration between cardiovascular control mechanisms during the wake-sleep cycle; this modulation is independent of the activity of HCRT neurons

Circadian rhythms of blood pressure occur during each wake-sleep state in mice

Blood pressure (BP) is higher in the activity than in the rest period. This daily BP rhythm has prognostic significance and is mainly attributed to behavioral factors, BP being higher in wakefulness than in sleep. A circadian BP rhythm independent of sleep is debated. Wake-sleep (W-S) episodes occur throughout the 24 hour day in rodents, allowing BP measurements in each W-S state at each time of day. Aim of this study was to investigate whether circadian BP rhythms occur in each W-S state in mice, which are the species of choice for functional genomics. Mice with B6 genetic background (n=26) were implanted with a telemetric BP transducer and electrodes to discriminate W-S states and recorded with 12:12 hours light-dark period. Mean BP values in wakefulness were averaged over 3-hour time bins and analyzed as z-scores. The same procedure was performed in non-rapid eye movement sleep and rapid eye movement (REM) sleep. Analysis of variance evidenced a significant time effect on BP in each W-S state as well as a significant W-S state x time interaction effect. The standard deviation of BP values among time bins was the highest in REM sleep. In an additional group of mice (n=3) recorded for 7 days in constant darkness, Lomb-Scargle periodogram demonstrated significant circadian rhythms of BP in each W-S state in free-running conditions. These findings provide evidence that circadian rhythms of BP occur during each W-S state in mice

Effects of ambient temperature on cardiovascular control during sleep in narcoleptic mice with genetic ablation of hypocretin neurons

Hypothalamic neurons releasing hypocretin (HCRT) control wake-sleep behavior, cardiovascular system and body temperature and their loss entails narcolepsy. We investigated whether HCRT neurons mediate sleep-dependent cardiovascular adaptations to changes in ambient temperature (Ta). Narcoleptic mice with genetic ablation of HCRT neurons (n=11) and wild-type controls (n=12) were implanted with a telemetric blood pressure transducer and electrodes to discriminate wake-sleep behavior. Recordings were performed in each mouse at ambient temperatures of 25\ub0C (acclimation temperature), 30\ub0C, and 20\ub0C in random order. Mean blood pressure (MBP) and heart rate (HR) were computed in each wake-sleep behavior and analyzed with 3-way analysis of variance and t-tests (significance at P < 0.05). Results revealed a significant interaction between the wake-sleep behavior and Ta on MBP and HR in both mouse strains, with MBP and HR rising at 20 \ub0C, particularly during wakefulness and non-rapid-eye-movement sleep. No effect involving the mouse strain was statistically significant. These results demonstrate that hypothalamic HCRT neurons are not necessary for sleep-dependent cardiovascular adaptations to changes in Ta