Exercise evaluation with metabolic and ventilatory responses and blood lactate concentration in mice.

This study aimed to clarify the differential exercise capacity between 2-month-old and 10-month-old mice using an incremental running test. Metabolic and ventilatory responses and blood lactate concentration were measured to evaluate exercise capacity. We examined whether incremental running test results reflected metabolic and ventilatory responses and blood lactate concentration observed during the steady-state running test. Metabolic response significantly declined with age, whereas ventilatory response was similar between the groups. A low-intensity/moderate exercise load of 10/min in an incremental running test was performed on both mice for 30 min. They showed a characteristic pattern in ventilatory response in 10-month mice. The results of incremental running tests didn't necessarily reflect the steady-state metabolic and ventilatory responses because some parameters showed an approximation and others did not in incremental and steady-state tests, which changed with age. Our study suggests metabolic and ventilatory responses depending on age and provides basic knowledge regarding the objective and quantitative assessment of treadmill running in an animal model.

Optogenetic activation of DRN 5-HT neurons induced active wakefulness, not quiet wakefulness.

There has been a long-standing controversy regarding the physiological role of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) in sleep/wake architecture. Some studies have reported that 5-HT acts as a sleep-promoting agent, but several studies have suggested that DRN 5-HT neurons function predominantly to promote wakefulness and inhibit rapid eye movement (REM) sleep. Furthermore, recent studies have reported that there is a clear neurobiological difference between a waking state that includes alertness and active exploration (i.e., active wakefulness) and a waking state that is devoid of locomotion (i.e., quiet wakefulness). These states have also been shown to differ clinically in terms of memory consolidation. However, the effects of 5-HT neurons on the regulation of these two different waking states have not been fully elucidated. In the present study, we attempted to examine the physiological role of DRN 5-HT neurons in various sleep/wake states using optogenetic methods that allowed manipulation of cell-type specific neuronal activation with high temporal and anatomical precision. We crossed TPH2-tTA and TetO-ChR2(C128S) mice to obtain mice with channelrhodopsin-2 (ChR2) [C128S]-expressing central 5-HT neurons, and we activated DRN-5HT neurons or medullary 5-HT neurons. Optogenetic activation of DRN 5-HT neurons caused rapid transition from non-REM sleep to active wakefulness, not quiet wakefulness, whereas activation of medullary 5-HT neurons did not appear to affect sleep/wake states or locomotor activity. Our results may shed light on the physiological role of DRN 5-HT neurons in sleep/wake architecture and encourage further investigations of the cortical functional connectivity involved in sleep/wake state regulation.

Breathing is affected by dopamine D2-like receptors in the basolateral amygdala.

The precise mechanisms underlying how emotions change breathing patterns remain unclear, but dopamine is a candidate neurotransmitter in the process of emotion-associated breathing. We investigated whether basal dopamine release occurs in the basolateral amygdala (BLA), where sensory-related inputs are received and lead to fear or anxiety responses, and whether D1- and D2-like receptor antagonists affect breathing patterns and dopamine release in the BLA. Adult male mice (C57BL/6N) were perfused with artificial cerebrospinal fluid, a D1-like receptor antagonist (SCH 23390), or a D2-like receptor antagonist ((S)-(-)-sulpiride) through a microdialysis probe in the BLA. Respiratory variables were measured using a double-chamber plethysmograph. Dopamine release was measured by an HPLC. Perfusion of (S)-(-)-sulpiride in the BLA, not SCH 23390, specifically decreased respiratory rate without changes in local release of dopamine. These results suggest that basal dopamine release in the BLA, at least partially, increases respiratory rates only through post-synaptic D2-like receptors, not autoreceptors, which might be associated with emotional responses.

Effects of dorsomedial medullary 5-HT2 receptor antagonism on initial ventilatory airway responses to hypercapnic hypoxia in mice.

The anatomical factors involved in upper airway closure of obstructive sleep apnea (OSA) have been established. However, the mechanisms of repetitive OSA are not well understood. We found that dorsomedial medullary 5-HT2 receptor activity is compensated for by hypercapnia and elicits the immediate onset of poikilocapnic hypoxic ventilatory airway responses. Therefore, the aim of this study was to test the hypothesis that hypercapnia compensates for the immediate onset of poikilocapnic hypoxic ventilatory airway responses induced by dorsomedial medullary 5-HT2 receptors. Adult male mice (C57BL/6N) were intraperitoneally anesthetized with pentobarbital sodium. Microdialysis probes were inserted into the dorsomedial medulla oblongata of the mice. The mice were placed in a double-chamber plethysmograph and were allowed to acclimatize and recover from anesthesia. Mice were then exposed to hypercapnic hypoxic gases (7 % O2/5 % CO2 in N2) with or without 5-HT2-antagonist (10(-5) M LY-53857) perfusion. Respiratory curves through the head and body chambers were recorded to measure ventilatory airway variables. Extracellular fluid was collected every 5 min for HPLC analysis of 5-HT concentration. Hypercapnic hypoxia elicited neither delayed onset of ventilatory augmentation nor immediate airway narrowing with dorsomedial medullary 5-HT2 antagonism. Hypoxic polypnea was shifted downward. The increases in dorsomedial medullary 5-HT release and ventilatory volume were not affected with or without 5-HT2 activity. In conclusion, the onset of poikilocapnic hypoxic ventilatory airway responses mediated via dorsomedial medullary 5-HT2 activity is compensated for by hypercapnia. Maintenance of PCO2 level and CO2 responsiveness, especially with lowered 5-HT2 activity, may be important for preventing repetitive OSA.

Chemical control of airway and ventilatory responses mediated via dorsomedial medullary 5-HT2 receptors.

The airway and ventilatory responses in mice elicited by the inhalation of hypercapnic or hypoxic gas mixtures were investigated by a microdialysis technique, 5-HT release analysis and double-chamber plethysmography with or without 5-HT2 receptor activity in the dorsomedial medulla oblongata (DMM). Hypercapnia elicited compensatory airway dilation and additive ventilatory augmentation to 5-HT2 receptor activity in the DMM. Hypoxia mediated immediate onset of hypoxic hyperventilation and airway dilation, and subsequent hypoxic ventilatory decline. Chemical control of the airway and ventilatory responses mediated via 5-HT2 receptors in the DMM are reviewed.

Dorsomedial medullary 5-HT2 receptors mediate immediate onset of initial hyperventilation, airway dilation, and ventilatory decline during hypoxia in mice.

The dorsomedial medulla oblongata (DMM) includes the solitary tract nucleus and the hypoglossal nucleus, to which 5-HT neurons project. Effects of 5-HT in the DMM on ventilatory augmentation and airway dilation are mediated via 5-HT2 receptors, which interact with the CO(2) drive. The interaction may elicit cycles between hyperventilation with airway dilation and hypoventilation with airway narrowing. In the present study, effects of 5-HT2 receptors in the DMM on hypoxic ventilatory and airway responses were investigated, while 5-HT release in the DMM was monitored. Adult male mice were anesthetized, and then a microdialysis probe was inserted into the DMM. The mice were placed in a double-chamber plethysmograph. After recovery from anesthesia, the mice were exposed to hypoxic gas (7% O(2) in N(2)) for 5 min with or without a 5-HT2 receptor antagonist (LY-53857) perfused in the DMM. 5-HT release in the DMM was increased by hypoxia regardless of the presence of LY-53857. Immediate onset and the peak of initial hypoxic hyperventilatory responses were delayed. Subsequent ventilatory decline and airway dilation during initial hypoxic hyperventilation were suppressed with LY-53857. These results suggest that 5-HT release increased by hypoxia acts on 5-HT2 receptors in the DMM, which contributes to the immediate onset of initial hypoxic hyperventilation, airway dilation, and subsequent ventilatory decline. Hypoxic ventilatory and airway responses mediated via 5-HT2 receptors in the DMM may play roles in immediate rescue and defensive adaptation for hypoxia and may be included in periodic breathing and the pathogenesis of obstructive sleep apnea.

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.

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

Ventilation oscillates throughout a day in parallel with oscillations in metabolic rate. Histamine affects ventilation and the balance of the energy metabolism via H1 receptors in the brain. We tested the hypothesis that the ventilatory response to hypoxia varies between light and dark periods and that histamine H1 receptors are required for the circadian variation, using wild-type (WT) and histamine H1 receptor knockout (H1RKO) mice. Mice were exposed to hypoxic gas (7% O(2) + 3% CO(2) in N(2)) during light and dark periods. Ventilation initially increased and then declined. In WT mice, minute ventilation (.Ve) during hypoxia was higher in the dark period than in the light period, which was an upward shift along with the baseline ventilation. Hypoxia decreased the metabolic rate, whereas O2 consumption (.VO(2)) and CO(2) excretion were higher in the dark period than in the light period. However, in H1RKO mice, changes in Ve during hypoxia between light and dark periods were minimal, because .Ve was increased relative to .VO(2), particularly in the light period. In H1RKO mice, the HCO(3)(-) concentration and base excess values were increased in arterial blood, and the level of ketone bodies was increased in the serum, indicating that metabolic acidosis occurred. Respiratory compensation takes part in the .Ve increase relative to .VO(2) during hypoxia. These results suggested that changes in .Ve during hypoxia vary between light and dark periods and that H1 receptors play a role in circadian variation in .Ve through control of the acid-base status and metabolism in mice.

Compensatory airway dilation and additive ventilatory augmentation mediated by dorsomedial medullary 5-hydroxytryptamine 2 receptor activity and hypercapnia.

5-HT2 receptor activity in the hypoglossal nucleus and hypercapnia is associated with airway dilation. 5-HT neurons in the medullary raphe and hypercapnia are responsible for tidal volume change. In this study, the effects of 5-HT2 receptors in the dorsomedial medulla oblongata (DMM), which receives projections from the medullary raphe, and hypercapnia on airway resistance and respiratory variables were studied in mice while monitoring 5-HT release in the DMM. A microdialysis probe was inserted into the DMM of anesthetized adult mice. Each mouse was placed in a double-chamber plethysmograph. After recovery from anesthesia, the mice were exposed to stepwise increases in CO(2) inhalation (5%, 7%, and 9% CO(2) in O(2)) at 8-min intervals with a selective serotonin reuptake inhibitor, fluoxetine, or fluoxetine plus a 5-HT2 receptor antagonist, LY-53857 in the DMM. In response to fluoxetine plus LY-53857 coperfusion, specific airway resistance was increased, and tidal volume and minute ventilation were decreased. CO(2) inhalation with fluoxetine plus LY-53857 coperfusion in the DMM largely decreased airway resistance and additively increased minute ventilation. Thus, 5-HT2 receptor activity in the DMM increases basal levels of airway dilation and ventilatory volume, dependent on central inspiratory activity and the volume threshold of the inspiratory off-switch mechanism. Hypercapnia with low 5-HT2 receptor activity in the DMM largely recovers airway dilation and additively increases ventilatory volume. Interaction between 5-HT2 receptor activity in the DMM and CO(2) drive may elicit a cycle of hyperventilation with airway dilation and hypoventilation with airway narrowing.

Lack of histamine type-1 receptors impairs the thermal response of respiration during hypoxia in mice (Mus musculus).

Thermoregulation and the hypoxic ventilatory response are modulated by histamine type-1 (H1) receptors in the brain. In this study, we tested the hypothesis that activation of H1 receptors is required for the thermal control of ventilation during normoxia and hypoxia, using conscious male wild-type and H1 receptor-knockout (H1RKO) mice (Mus musculus). Under normoxic conditions, hyperthermia (39 degrees C) decreased minute ventilation (V (E)) and oxygen consumption [Formula: see text] in both genotypes, suggesting that H1 receptors are not involved in thermal ventilatory control during normoxia. Pa(CO2) was unchanged in both hyperthermia and normothermia, suggesting that the thermal decrease in V (E) is optimized by metabolic demand. Acute hypoxic gas exposure (7% O(2)+3% CO(2) in N(2)) increased, and then decreased, V (E) in wild-type mice; this increase was augmented and sustained by hyperthermia. Hypoxic gas exposure reduced [Formula: see text] and [Formula: see text] in wild-type mice at both body temperatures; the reduced [Formula: see text] during combined hyperthermia and hypoxia was higher than during normothermia and hypoxia. In H1RKO mice, hyperthermia did not augment the V (E) response to hypoxia, and did not affect [Formula: see text] and [Formula: see text] during hypoxia. In conclusion, histamine participates in the thermal increase of ventilation during hypoxia by activating H1 receptors.

Contribution of histamine type-1 receptor to metabolic and behavioral control of ventilation.

Histaminergic neurons in the hypothalamus are well documented as being involved in the control of autonomic functions, such as the balance of energy metabolism and circadian rhythm. We tested the hypothesis that an activation of the histamine type-1 (H1) receptor is required for the control of ventilation during the course of a day in free-moving mice. Ventilation, aerobic metabolism, and electroencephalogram were measured by a whole-body-plethysmograph, a magnetic-type mass spectrometry system, and a telemetry system, respectively, in H1 receptor-knockout (H1RKO) and wild-type mice. Both genotypes showed daily oscillations in minute ventilation (V(E)) and oxygen consumption (VO(2)), with greater values during the dark period compared to the light period. In the latter, H1RKO mice showed increased V(E) and CO(2) excretion (VCO(2)) relative to wild-type mice, and V(E) was comparable to the VCO(2) increase. However, there was no change in VO(2) in H1RKO mice, suggesting that differences in VCO(2) between genotypes are responsible for differences in V(E) during the light period. During the dark period, VCO(2) was elevated in H1RKO mice compared with WT mice. Because there was no difference in V(E), the ratio of V(E) to VCO(2) was reduced in H1RKO mice. Electroencephalogram results suggested that this might be due to a depressed arousal state in H1RKO mice because the ratio of delta to theta band power spectrum densities was greater in H1RKO mice than in wild-type mice. We concluded that histamine modulates ventilation by affecting metabolism and arousal state via H1 receptors.

Impaired ventilation and metabolism response to hypoxia in histamine H1 receptor-knockout mice.

The role of central histamine in the hypoxic ventilatory response was examined in conscious wild-type (WT) and histamine type1 receptor-knockout (H1RKO) mice. Hypoxic gas (7% O(2) and 3% CO(2) in N(2)) exposure initially increased and then decreased ventilation, referred to as hypoxic ventilatory decline (HVD). The initial increase in ventilation did not differ between genotypes. However, H1RKO mice showed a blunted HVD, in which mean inspiratory flow was greater than that in WT mice. O(2) consumption (V(O2)) and CO(2) excretion were reduced 10min after hypoxic gas exposure in both genotypes, but (V(O2)) was greater in H1RKO mice than in WT mice. The ratio of minute ventilation to (V(O2)) during HVD did not differ between genotypes, indicating that ventilation is adequately controlled according to metabolic demand in both mice. Peripheral chemoreceptor sensitivity did not differ between genotypes. We conclude that central histamine contributes via the H1 receptor to changes in metabolic rate during hypoxia to increase HVD in conscious mice.

Effects of hyperthermia on ventilation and metabolism during hypoxia in conscious mice.

Hyperthermia and hypoxia influence ventilation and metabolism; however, their synergistic effects remain unanswered. We hypothesized that an enhancement of ventilation induced by hyperthermia is competitive with hypoxic hypometabolism. We then examined the relationship of body temperature, hypoxia, and respiration in conscious mice, measuring minute ventilation (VE), aerobic metabolism, and arterial blood gases. All parameters were measured at two different body temperatures (BTs), approximately 37 degrees C (normothermia) and 39 degrees C (hyperthermia), under both normoxia (room air inhalation) and hypoxia (7% O2 inhalation). Under normoxia, VE and O2 consumption (VO2) were lower at hyperthermia than at normothermia, and the VE-VO2 ratio remained constant. PaCO2 values were normal at both BTs under normoxia. Hypoxic gas inhalation increased VE, which reached a peak in 2 min, then decreased at both BTs. VE remained at a higher level during hyperthermia than during normothermia throughout the 10 min experiment. VO2 decreased during hypoxia at both BTs. Hypoxia increased the VE-VO2 ratio because of relatively high VE with respect to the decreased VO2, which means hyperventilation. At hypoxia under hyperthermia, serious hyperventilation occurred with a further increase in VE. The augmented ventilation may be due to the thermal stimulus and a lowered thermoregulatory set point for hypoxia. Thus hyperthermia reduces ventilation and metabolism to maintain normocapnia; as a result, thermogenesis is reduced under normoxia. Hyperthermia augments hyperventilation induced by hypoxia, leading to severe hypoxic hypocapnia. Thermal stimuli may impair the adjustment of ventilation and metabolism when O2 is limited.