Hypoxic ventilatory response during light and dark periods and the involvement of histamine H1 receptor in mice

The Molecular Circadian Clock of Phox2b-expressing Cells Drives Daily Variation of the Hypoxic but Not Hypercapnic Ventilatory Response in Mice

While the suprachiasmatic nucleus (SCN) controls 24-h rhythms in breathing, including minute ventilation (V_E), the mechanisms by which the SCN drives these daily changes are not well understood. Moreover, the extent to which the circadian clock regulates hypercapnic and hypoxic ventilatory chemoreflexes is unknown. We hypothesized that the SCN regulates daily breathing and chemoreflex rhythms by synchronizing the molecular circadian clock of cells. We used whole-body plethysmography to assess ventilatory function in transgenic BMAL1 knockout (KO) mice to determine the role of the molecular clock in regulating daily rhythms in ventilation and chemoreflex. Unlike their wild-type littermates, BMAL1 KO mice exhibited a blunted daily rhythm in V_E and failed to demonstrate daily variation in the hypoxic ventilatory response (HVR) or hypercapnic ventilatory response (HCVR). To determine if the observed phenotype was mediated by the molecular clock of key respiratory cells, we then assessed ventilatory rhythms in BMAL1^fl/fl; Phox2b^Cre/+ mice, which lack BMAL1 in all Phox2b-expressing chemoreceptor cells (hereafter called BKOP). BKOP mice lacked daily variation in HVR, similar to BMAL1 KO mice. However, unlike BMAL1 KO mice, BKOP mice exhibited circadian variations in V_E and HCVR comparable to controls. These data indicate that the SCN regulates daily rhythms in V_E, HVR, and HCVR, in part, through the synchronization of the molecular clock. Moreover, the molecular clock of Phox2b-expressing cells is specifically necessary for daily variation in the hypoxic chemoreflex. These findings suggest that disruption of circadian biology may undermine respiratory homeostasis, which, in turn, may have clinical implications for respiratory disease.

Young and middle-aged mouse breathing behavior during the light and dark cycles

Unrestrained barometric plethysmography is a common method used for characterizing breathing patterns in small animals. One source of variation between unrestrained barometric plethysmography studies is the segment of baseline. Baseline may be analyzed as a predetermined time‐point, or using tailored segments when each animal is visually calm. We compared a quiet, minimally active (no sniffing/grooming) breathing segment to a predetermined time‐point at 1 h for baseline measurements in young and middle‐aged mice during the dark and light cycles. Additionally, we evaluated the magnitude of change for gas challenges based on these two baseline segments. C57BL/6JEiJ x C3Sn.BliA‐Pde6b⁺/DnJ male mice underwent unrestrained barometric plethysmography with the following baselines used to determine breathing frequency, tidal volume (VT) and minute ventilation (VE): (1) 30‐sec of quiet breathing and (2) a 10‐min period from 50 to 60 min. Animals were also exposed to 10 min of hypoxic (10% O₂, balanced N₂), hypercapnic (5% CO₂, balanced air) and hypoxic hypercapnic (10% O₂, 5% CO₂, balanced N₂) gas. Both frequency and VE were higher during the predetermined 10‐min baseline versus the 30‐sec baseline, while VT was lower (P < 0.05). However, VE/V_O2 was similar between the baseline time segments (P > 0.05) in an analysis of one cohort. During baseline, dark cycle testing had increased VT values versus those in the light (P < 0.05). For gas challenges, both frequency and VE showed higher percent change from the 30‐sec baseline compared to the predetermined 10‐min baseline (P < 0.05), while VT showed a greater change from the 10‐min baseline (P < 0.05). Dark cycle hypoxic exposure resulted in larger percent change in breathing frequency versus the light cycle (P < 0.05). Overall, light and dark cycle pattern of breathing differences emerged along with differences between the 30‐sec behavior observational method versus a predetermined time segment for baseline.

Method to Obtain Pattern of Breathing in Senescent Mice through Unrestrained Barometric Plethysmography

Unrestrained barometric plethysmography (UBP) is a method for quantifying the pattern of breathing in mice, where breathing frequency, tidal volume, and minute ventilation are routinely reported. Moreover, information can be collected regarding the neural output of breathing, including the existence of central apneas and augmented breaths. An important consideration for UBP is obtaining a breathing segment with a minimal impact of anxious or active behaviors, to elucidate the response to breathing challenges. Here, we present a protocol that allows for short, quiet baselines to be obtained in aged mice, comparable to waiting for longer bouts of quiet breathing. The use of shorter time segments is valuable, as some strains of mice may be increasingly excitable or anxious, and longer periods of quiet breathing may not be achieved within a reasonable timeframe. We placed 22 month-old mice in a UBP chamber and compared four 15 s quiet breathing segments between minutes 60-120 to a longer 10 min quiet breathing period that took 2-3 h to acquire. We also obtained counts of central apneas and augmented breaths prior to the quiet breathing segments, following a 30 min familiarization period. We show that 10 min of quiet breathing is comparable to using a much shorter 15 s duration. Additionally, the time leading up to these 15 s quiet breathing segments can be used to gather data regarding apneas of central origin. This protocol allows investigators to collect pattern-of-breathing data in a set amount of time and makes quiet baseline measures feasible for mice that may exhibit increased amounts of excitable behavior. The UBP methodology itself provides a useful and noninvasive way to collect pattern-of-breathing data and allows for mice to be tested over several time points.

Arterial blood sampling in male CD-1 and C57BL/6J mice with 1% isoflurane is similar to awake mice

Isoflurane (ISO) is a commonly used anesthetic that offers rapid recovery for laboratory animal research. Initial studies indicated no difference in arterial Pco₂ (PaCO2) or pH between conscious (NO ISO) and 1% ISO-exposed CD-1 mice. Our laboratory investigated whether arterial blood sampling with 1% ISO is a suitable alternative to NO ISO sampling for monitoring ventilation in a commonly studied mouse strain. We hypothesized similar blood chemistry, breathing patterns, and cardiovascular responses with NO ISO and 1% ISO. C57BL/6J mice underwent unrestrained barometric plethysmography to quantify the pattern of breathing. Mice exposed to hypoxic and hypercapnic gas under 1% ISO displayed blunted responses; with air, there were no breathing differences. Blood pressure and heart rate were not different between NO ISO and 1% ISO-exposed mice breathing air. Oxygen saturation was not different between groups receiving 2% ISO, 1% ISO, or air. Breathing frequency stabilized at ~11 min of 1% ISO following 2% ISO exposure, suggesting that 11 min is the optimal time for a sample in C57BL/6J mice. Blood samples at 1% ISO and NO ISO revealed no differences in blood pH and PaCO2 in C57BL/6J mice. Overall, this method reveals similar arterial blood sampling values in awake and 1% ISO CD-1 and C57BL/6J mice exposed to air. Although this protocol may be appropriate in other mouse strains when a conscious sample is not feasible, caution is warranted first to identify breathing frequency responses at 1% ISO to tailor the protocol.

NEW & NOTEWORTHY Conscious arterial blood sampling is influenced by extraneous factors and is a challenging method due to the small size of mice. Through a series of experiments, we show that arterial blood sampling with 1% isoflurane (ISO) is an alternative to awake sampling in C57BL/6J and CD-1 male mice breathing air. Monitoring breathing frequency during 1% ISO is important to the protocol and should be closely followed to confirm adequate recovery after the catheter implantation.

Systemic Oxygen Transport with Rest, Exercise, and Hypoxia: A Comparison of Humans, Rats, and Mice

The objective of this article is to compare and contrast the known characteristics of the systemic O₂ transport of humans, rats, and mice at rest and during exercise in normoxia and hypoxia. This analysis should help understand when rodent O₂ transport findings can-and cannot-be applied to human responses to similar conditions. The O₂ -transport system was analyzed as composed of four linked conductances: ventilation, alveolo-capillary diffusion, circulatory convection, and tissue capillary-cell diffusion. While the mechanisms of O₂ transport are similar in the three species, the quantitative differences are naturally large. There are abundant data on total O₂ consumption and on ventilatory and pulmonary diffusive conductances under resting conditions in the three species; however, there is much less available information on pulmonary gas exchange, circulatory O₂ convection, and tissue O₂ diffusion in mice. The scarcity of data largely derives from the difficulty of obtaining blood samples in these small animals and highlights the need for additional research in this area. In spite of the large quantitative differences in absolute and mass-specific O₂ flux, available evidence indicates that resting alveolar and arterial and venous blood PO₂ values under normoxia are similar in the three species. Additionally, at least in rats, alveolar and arterial blood PO₂ under hypoxia and exercise remain closer to the resting values than those observed in humans. This is achieved by a greater ventilatory response, coupled with a closer value of arterial to alveolar PO₂ , suggesting a greater efficacy of gas exchange in the rats. © 2018 American Physiological Society. Compr Physiol 8:1537-1573, 2018.

Spontaneous locomotor activity correlates with the degranulation of mast cells in the meninges rather than in the thalamus: disruptive effect of cocaine

Mast cells are located in the central nervous system (CNS) of many mammals and stress induces their degranulation. We postulated that mast cells are associated with wakefulness and stimulatory tone in the CNS, as reflected by spontaneous motor activity. Because stress also precipitates drug-seeking behavior in cocaine addicts, we also postulated that cocaine manifests its effects through this relationship. We investigated the influence of single and repeated injections of cocaine on circulating corticosterone, motor activity and degranulation of mast cells in both the thalamus and meninges of mice. Mice were subjected to 5 consecutive days of cocaine or saline followed by a single injection of cocaine or saline 11 days later. Spontaneous locomotor activity was measure for 1h after the final injection before death. Neither a single injection nor prior treatment with cocaine increased motor activity compared to saline-injected controls, however, repeated administration of cocaine induced a significant sensitization to its behavioral effect when delivered 11 days later. In mice that received only saline, motor activity correlated positively with mast cell degranulation in the meninges but not in the thalamus. Cocaine, regardless of the treatment schedule, disrupted this correlation. The concentration of corticosterone did not differ amongst groups and did not correlate with either behavior or mast cell parameters in any group. The correlation between behavioral activity and the mast cell degranulation in the meninges suggests that these parameters are linked. The disruptive effect of cocaine on this relationship indicates a role downstream from mast cells in the regulation of motor activity.

Effects of fasting on hypoxic ventilatory responses and the contribution of histamine H1 receptors in mice

We tested the hypothesis that fasting affects hypoxic ventilatory responses through metabolic changes via histamine H1 receptors. Wild-type (WT) and histamine H1 receptor knockout (H1RKO) mice were studied in fed and fasted states. In the fed WT, hypoxic-gas exposure elicited an increase and a subsequent decline in ventilation (hypoxic ventilatory decline or HVD). HVD was influenced by fasting in breathing pattern with metabolic rate. Fasting elicited hypoglycemia, a drop in R, and increases in free fatty acid and ketone bodies in the serum. In H1RKO, HVD was blunted in the fed state, but it appeared in the fasted state. There was a minimal drop in R following fasting and a low triglyceride concentration. Thus, fasting affects HVD through a change in energy mobilization from glucose to lipid metabolism. Histamine H1 receptors are involved in HVD during fed and fasted states, resulting in adaptation to the environmental conditions.

The waking brain: an update

Wakefulness and consciousness depend on perturbation of the cortical soliloquy. Ascending activation of the cerebral cortex is characteristic for both waking and paradoxical (REM) sleep. These evolutionary conserved activating systems build a network in the brainstem, midbrain, and diencephalon that contains the neurotransmitters and neuromodulators glutamate, histamine, acetylcholine, the catecholamines, serotonin, and some neuropeptides orchestrating the different behavioral states. Inhibition of these waking systems by GABAergic neurons allows sleep. Over the past decades, a prominent role became evident for the histaminergic and the orexinergic neurons as a hypothalamic waking center.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Time-dependent ventilatory response to poikilocapnic hypoxia during light and dark periods and the role of histamine H1 receptors in mice

We tested the hypothesis that the biphasic ventilatory response to poikilocapnic hypoxia shows circadian variation and contribution of histamine H1 receptors in mice. Initial increases in ventilation were augmented during dark periods. H1 receptors had no major relationship with circadian variation, but affected the declined phase.