Thermoregulation in rats during early postnatal maturation: importance of nitric oxide

Recovery of ventilatory and metabolic responses to hypoxia in neonatal rats after chronic hypoxia

Chronic hypoxia (CH) during postnatal development attenuates the hypoxic ventilatory response (HVR) in mammals, but there are conflicting reports on whether this plasticity is permanent or reversible. This study tested the hypothesis that CH-induced respiratory plasticity is reversible in neonatal rats and investigated whether the initial plasticity or recovery differs between sexes. Rat pups were exposed to 3 d of normobaric CH (12 % O2) beginning shortly after birth. Ventilation and metabolic CO2 production were then measured in normoxia and during an acute hypoxic challenge (12 % O2) immediately following CH and after 1, 4-5, and 7 d in room air. CH pups hyperventilated when returned to normoxia immediately following CH, but normoxic ventilation was similar to age-matched control rats within 7 d after return to room air. The early phase of the HVR (minute 1) was only blunted immediately following the CH exposure, while the late phase of the HVR (minute 15) remained blunted after 1 and 4-5 d in room air; recovery appeared complete by 7 d. However, when normalized to CO2 production, the late phase of the hypoxic response recovered within only 1 d. The initial blunting of the HVR and subsequent recovery were similar in female and male rats. Carotid body responses to hypoxia (in vitro) were also normal in CH pups after approximately one week in room air. Collectively, these data indicate that ventilatory and metabolic responses to hypoxia recover rapidly in both female and male neonatal rats once normoxia is restored following CH.

Strategies for Hypothermia Compensation in Altricial and Precocial Newborn Mammals and Their Monitoring by Infrared Thermography

Thermoregulation in newborn mammals is an essential species-specific mechanism of the nervous system that contributes to their survival during the first hours and days of their life. When exposed to cold weather, which is a risk factor associated with mortality in neonates, pathways such as the hypothalamic–pituitary–adrenal axis (HPA) are activated to achieve temperature control, increasing the circulating levels of catecholamine and cortisol. Consequently, alterations in blood circulation and mechanisms to produce or to retain heat (e.g., vasoconstriction, piloerection, shivering, brown adipocyte tissue activation, and huddling) begin to prevent hypothermia. This study aimed to discuss the mechanisms of thermoregulation in newborn domestic mammals, highlighting the differences between altricial and precocial species. The processes that employ brown adipocyte tissue, shivering, thermoregulatory behaviors, and dermal vasomotor control will be analyzed to understand the physiology and the importance of implementing techniques to promote thermoregulation and survival in the critical post-birth period of mammals. Also, infrared thermography as a helpful method to perform thermal measurements without animal interactions does not affect these parameters.

Influence of chronic hyperoxia on the developmental time course of the hypoxic ventilatory response relative to other traits in rats

Newborn mammals exhibit a biphasic hypoxic ventilatory response (HVR) in which an initial increase in ventilation is followed by a decline back toward baseline levels. The magnitude of the secondary decline diminishes with postnatal age, but this transition occurs earlier in rat pups reared in moderate hyperoxia. This pattern is consistent with heterokairy, a form of developmental plasticity in which environmental factors alter the timing of developmental events. The present study investigated whether this plasticity is specific to the HVR or if hyperoxia instead accelerates overall development. Rat pups reared in 60 % O₂ (Hyperoxia) exhibited a less biphasic ventilatory response to 12 % O₂ than pups reared in 21 % O₂ (Control) at 4 days of age (P4) and transitioned to a sustained HVR by P10-11; Control rats exhibited a biphasic HVR at both ages. However, the average ages at which pups attained other key developmental milestones (i.e., fur development at P5, incisor eruption at P9, and eye opening at P15) were similar between treatment groups. Moreover, growth rates and maturation of the metabolic response to cooling were not accelerated, and may have been delayed slightly, relative to Control rats. For example, the capacity for pups to increase their metabolic rate at low ambient temperatures increased with age, but this thermogenic capacity tended to be reduced in Hyperoxia pups at both P4 and P10-11 (i.e., lower CO₂ production rates below the lower critical temperature). Collectively, these data support the conclusion that hyperoxia specifically advances the age at which rat pups exhibit a sustained HVR, altering the relative timing of developmental events rather than compressing the entire period of development.

A compendium of G-protein-coupled receptors and cyclic nucleotide regulation of adipose tissue metabolism and energy expenditure

With the ever-increasing burden of obesity and Type 2 diabetes, it is generally acknowledged that there remains a need for developing new therapeutics. One potential mechanism to combat obesity is to raise energy expenditure via increasing the amount of uncoupled respiration from the mitochondria-rich brown and beige adipocytes. With the recent appreciation of thermogenic adipocytes in humans, much effort is being made to elucidate the signaling pathways that regulate the browning of adipose tissue. In this review, we focus on the ligand-receptor signaling pathways that influence the cyclic nucleotides, cAMP and cGMP, in adipocytes. We chose to focus on G-protein-coupled receptor (GPCR), guanylyl cyclase and phosphodiesterase regulation of adipocytes because they are the targets of a large proportion of all currently available therapeutics. Furthermore, there is a large overlap in their signaling pathways, as signaling events that raise cAMP or cGMP generally increase adipocyte lipolysis and cause changes that are commonly referred to as browning: increasing mitochondrial biogenesis, uncoupling protein 1 (UCP1) expression and respiration.

Combined effects of intermittent hyperoxia and intermittent hypercapnic hypoxia on respiratory control in neonatal rats

Chronic exposure to intermittent hyperoxia causes abnormal carotid body development and attenuates the hypoxic ventilatory response (HVR) in neonatal rats. We hypothesized that concurrent exposure to intermittent hypercapnic hypoxia would influence this plasticity. Newborn rats were exposed to alternating bouts of hypercapnic hypoxia (10% O₂/6% CO₂) and hyperoxia (30-40% O₂) (5 cycles h^-1, 24 h d^-1) through 13-14 days of age; the experiment was run twice, once in a background of 21% O₂ and once in a background of 30% O₂ (i.e., "relative hyperoxia"). Hyperoxia had only small effects on carotid body development when combined with intermittent hypercapnic hypoxia: the carotid chemoafferent response to hypoxia was reduced, but this did not affect the HVR. In contrast, sustained exposure to 30% O₂ reduced carotid chemoafferent activity and carotid body size which resulted in a blunted HVR. When given alone, chronic intermittent hypercapnic hypoxia increased carotid body size and reduced the hypercapnic ventilatory response but did not affect the HVR. Overall, it appears that intermittent hypercapnic hypoxia counteracted the effects of hyperoxia on the carotid body and prevented developmental plasticity of the HVR.

Consequences of maternal omega-3 polyunsaturated fatty acid supplementation on respiratory function in rat pups

KEY POINTS

Incomplete development of the neural circuits that control breathing contributes to respiratory disorders in pre-term infants. Manifestations include respiratory instability, prolonged apnoeas and poor ventilatory responses to stimuli. Based on evidence suggesting that omega-3 polyunsaturated fatty acids (n-3 PUFA) improves brain development, we determined whether n-3 PUFA supplementation (via the maternal diet) improves respiratory function in 10-11-day-old rat pups. n-3 PUFA treatment prolonged apnoea duration but augmented the relative pulmonary surface area and the ventilatory response to hypoxia. During hypoxia, the drop in body temperature measured in treated pups was 1 °C less than in controls. n-3 PUFA treatment also reduced microglia cell density in the brainstem. Although heterogeneous, the results obtained in rat pups constitute a proof of concept that n-3 PUFA supplementation can have positive effects on neonatal respiration. This includes a more sustained hypoxic ventilatory response and a decreased respiratory inhibition during laryngeal chemoreflex.

ABSTRACT

Most pre-term infants present respiratory instabilities and apnoeas as a result of incomplete development of the neural circuits that control breathing. Because omega-3 polyunsaturated fatty acids (n-3 PUFA) benefit brain development, we hypothesized that n-3 PUFA supplementation (via the maternal diet) improves respiratory function in rat pups. Pups received n-3 PUFA supplementation from an enriched diet (13 g kg-1 of n-3 PUFA) administered to the mother from birth until the experiments were performed (postnatal days 10-11). Controls received a standard diet (0.3 g kg-1 of n-3 PUFA). Breathing was measured in intact pups at rest and during hypoxia (FiO2  = 0.12; 20 min) using whole body plethysmography. The duration of apnoeas induced by stimulating the laryngeal chemoreflex (LCR) was measured under anaesthesia. Lung morphology was compared between groups. Maternal n-3 PUFA supplementation effectively raised n-3 PUFA levels above control levels both in the blood and brainstem of pups. In intact, resting pups, n-3 PUFA increased the frequency and duration of apnoeas, especially in females. During hypoxia, n-3 PUFA supplemented pups hyperventilated 23% more than controls; their anapyrexic response was 1 °C less than controls. In anaesthetized pups, n-3 PUFA shortened the duration of LCR-induced apnoeas by 32%. The relative pulmonary surface area of n-3 PUFA supplemented pups was 12% higher than controls. Although n-3 PUFA supplementation augments apnoeas, there is no clear evidence of deleterious consequences on these pups. Based on the improved lung architecture and responses to respiratory challenges, this neonatal treatment appears to be beneficial to the offspring. However, further experiments are necessary to establish its overall safety.

Ventilatory and chemoreceptor responses to hypercapnia in neonatal rats chronically exposed to moderate hyperoxia

Rats reared in hyperoxia hypoventilate in normoxia and exhibit progressive blunting of the hypoxic ventilatory response, changes which are at least partially attributed to abnormal carotid body development. Since the carotid body also responds to changes in arterial CO₂/pH, we tested the hypothesis that developmental hyperoxia would attenuate the hypercapnic ventilatory response (HCVR) of neonatal rats by blunting peripheral and/or central chemoreceptor responses to hypercapnic challenges. Rats were reared in 21% O₂ (Control) or 60% O₂ (Hyperoxia) until studied at 4, 6-7, or 13-14days of age. Hyperoxia rats had significantly reduced single-unit carotid chemoafferent responses to 15% CO₂ at all ages; CO₂ sensitivity recovered within 7days after return to room air. Hypercapnic responses of CO₂-sensitive neurons of the caudal nucleus tractus solitarius (cNTS) were unaffected by chronic hyperoxia, but there was evidence for a small decrease in neuronal excitability. There was also evidence for augmented excitatory synaptic input to cNTS neurons within brainstem slices. Steady-state ventilatory responses to 4% and 8% CO₂ were unaffected by developmental hyperoxia in all three age groups, but ventilation increased more slowly during the normocapnia-to-hypercapnia transition in 4-day-old Hyperoxia rats. We conclude that developmental hyperoxia impairs carotid body chemosensitivity to hypercapnia, and this may compromise protective ventilatory reflexes during dynamic respiratory challenges in newborn rats. Impaired carotid body function has less of an impact on the HCVR in older rats, potentially reflecting compensatory plasticity within the CNS.

Cardiorespiratory control and cytokine profile in response to heat stress, hypoxia, and lipopolysaccharide (LPS) exposure during early neonatal period

Sudden infant death syndrome (SIDS) is one of the most common causes of postneonatal infant mortality in the developed world. An insufficient cardiorespiratory response to multiple environmental stressors (such as prone sleeping positioning, overwrapping, and infection), during a critical period of development in a vulnerable infant, may result in SIDS. However, the effect of multiple risk factors on cardiorespiratory responses has rarely been tested experimentally. Therefore, this study aimed to quantify the independent and possible interactive effects of infection, hyperthermia, and hypoxia on cardiorespiratory control in rats during the neonatal period. We hypothesized that lipopolysaccharide (LPS) administration will negatively impact cardiorespiratory responses to increased ambient temperature and hypoxia in neonatal rats. Sprague-Dawley neonatal rat pups were studied at postnatal day 6-8. Rats were examined at an ambient temperature of 33°C or 38°C. Within each group, rats were allocated to control, saline, or LPS (200 μg/kg) treatments. Cardiorespiratory and thermal responses were recorded and analyzed before, during, and after a hypoxic exposure (10% O2). Serum samples were taken at the end of each experiment to measure cytokine concentrations. LPS significantly increased cytokine concentrations (such as TNFα, IL-1β, MCP-1, and IL-10) compared to control. Our results do not support a three-way interaction between experimental factors on cardiorespiratory control. However, independently, heat stress decreased minute ventilation during normoxia and increased the hypoxic ventilatory response. Furthermore, LPS decreased hypoxia-induced tachycardia. Herein, we provide an extensive serum cytokine profile under various experimental conditions and new evidence that neonatal cardiorespiratory responses are adversely affected by dual interactions of environmental stress factors.

Chronic intermittent hyperoxia alters the development of the hypoxic ventilatory response in neonatal rats

Chronic exposure to sustained hyperoxia alters the development of the respiratory control system, but the respiratory effects of chronic intermittent hyperoxia have rarely been investigated. We exposed newborn rats to short, repeated bouts of 30% O2 or 60% O2 (5 bouts h(-1)) for 4-15 days and then assessed their hypoxic ventilatory response (HVR; 10 min at 12% O2) by plethysmography. The HVR tended to be enhanced by intermittent hyperoxia at P4 (early phase of the HVR), but it was significantly reduced at P14-15 (primarily late phase of the HVR) compared to age-matched controls; the HVR recovered when individuals were returned to room air and re-studied as adults. To investigate the role of carotid body function in this plasticity, single-unit carotid chemoafferent activity was recorded in vitro. Intermittent hyperoxia tended to decrease spontaneous action potential frequency under normoxic conditions but, contrary to expectations, hypoxic responses were only reduced at P4 (not at P14) and only in rats exposed to higher O2 levels (i.e., intermittent 60% O2). Rats exposed to intermittent hyperoxia had smaller carotid bodies, and this morphological change may contribute to the blunted HVR. In contrast to rats exposed to intermittent hyperoxia beginning at birth, two weeks of intermittent 60% O2 had no effect on the HVR or carotid body size of rats exposed beginning at P28; therefore, intermittent hyperoxia-induced respiratory plasticity appears to be unique to development. Although both intermittent and sustained hyperoxia alter carotid body development and the HVR of rats, the specific effects and time course of this plasticity differs.

A Self-Organising Model of Thermoregulatory Huddling

Endotherms such as rats and mice huddle together to keep warm. The huddle is considered to be an example of a self-organising system, because complex properties of the collective group behaviour are thought to emerge spontaneously through simple interactions between individuals. Groups of rodent pups display two such emergent properties. First, huddling undergoes a 'phase transition', such that pups start to aggregate rapidly as the temperature of the environment falls below a critical temperature. Second, the huddle maintains a constant 'pup flow', where cooler pups at the periphery continually displace warmer pups at the centre. We set out to test whether these complex group behaviours can emerge spontaneously from local interactions between individuals. We designed a model using a minimal set of assumptions about how individual pups interact, by simply turning towards heat sources, and show in computer simulations that the model reproduces the first emergent property--the phase transition. However, this minimal model tends to produce an unnatural behaviour where several smaller aggregates emerge rather than one large huddle. We found that an extension of the minimal model to include heat exchange between pups allows the group to maintain one large huddle but eradicates the phase transition, whereas inclusion of an additional homeostatic term recovers the phase transition for large huddles. As an unanticipated consequence, the extended model also naturally gave rise to the second observed emergent property--a continuous pup flow. The model therefore serves as a minimal description of huddling as a self-organising system, and as an existence proof that group-level huddling dynamics emerge spontaneously through simple interactions between individuals. We derive a specific testable prediction: Increasing the capacity of the individual to generate or conserve heat will increase the range of ambient temperatures over which adaptive thermoregulatory huddling will emerge.

Adenosine A1-receptor blockade impairs the ability of rat pups to autoresuscitate from primary apnea during repeated exposure to hypoxia

Failure of gasping to bring about autoresuscitation from hypoxia-induced apnea has been suggested to play a role in sudden unexpected infant death. Little is known, however, about factors that influence the ability of gasping to restore life during severe hypoxia in newborns. Given that adenosine modulates cardiac function during hypoxia-induced apnea and that cardiac dysfunction plays a role in mediating autoresuscitation failure, the present experiments were carried out on 34, 5- to 6-, and 10- to 11-day-old rat pups to investigate their ability to autoresuscitate from hypoxia-induced apnea during repeated exposure to hypoxia after adenosine A₁-receptor blockade. Each pup was placed into a temperature-controlled chamber regulated to 37 ± 1°C and repeatedly exposed to an anoxic gas mixture (97% N₂ and 3% CO₂) until the occurrence of autoresuscitation failure. One group was studied following administration of the selective adenosine A₁-receptor antagonist 8-Cyclopentyl-1,3,-dipropylxanthine (DPCPX) and one group was studied following vehicle. DPCPX significantly attenuated bradycardia during hypoxia-induced apnea and impaired the ability of both age groups of pups to autoresuscitate during repeated exposure to hypoxia (5–6 days tolerated – vehicle 17 ± 4 vs. DPCPX 10 ± 2 hypoxia exposures [P < 0.05]; 10–11 days tolerated – vehicle 10 ± 2 vs. DPCPX 7 ± 2 hypoxia exposures [P < 0.05]). Death in all pups resulted from the inability of gasping to restore cardiovascular function during hypoxia-induced apnea although the mechanism of cardiovascular dysfunction/failure was influenced and the occurrence hastened by DPCPX. Thus, our data provide evidence that adenosine acting via adenosine A₁-receptors enhances the ability of rat pups to tolerate repeated exposure to severe hypoxia during early postnatal maturation.

Role of TrkB during the postnatal development of the rat carotid body

Brain-derived neurotrophic factor (BDNF) supports innervation of the carotid body by neurons projecting from the petrosal ganglion. Although carotid body glomus cells also express TrkB, BDNF's high affinity receptor, the role of BDNF in carotid body growth and O2 sensitivity has not been studied. Neonatal rats were treated with the TrkB antagonist K252a (100 μg kg(-1), i.p., b.i.d.) or vehicle on postnatal days P0-P6 and studied on P7. Carotid body volume was decreased by 35% after chronic K252a (P<0.001); a reduction in carotid body size was also elicited using the more selective TrkB antagonist ANA-12 (500 μg kg(-1), i.p., b.i.d.). In contrast, single-unit chemoafferent responses to 5% O2, measured in vitro, were unaffected by chronic K252a administration. Normoxic and hypoxic ventilation, measured by head-body plethysmography, were also normal after chronic K252a administration, but acute K252a administration produced a slower, deeper breathing pattern during the transition into hypoxia. These data suggest that BDNF regulates postnatal carotid body growth but does not influence the development of glomus cell O2 sensitivity.

Postnatal development of eupneic ventilation and metabolism in rats chronically exposed to moderate hyperoxia

Newborn rats chronically exposed to moderate hyperoxia (60% O2) exhibit abnormal respiratory control, including decreased eupneic ventilation. To further characterize this plasticity and explore its proximate mechanisms, rats were exposed to either 21% O2 (Control) or 60% O2 (Hyperoxia) from birth until studied at 3-14 days of age (P3-P14). Normoxic ventilation was reduced in Hyperoxia rats when studied at P3, P4, and P6-7 and this was reflected in diminished arterial O2 saturations; eupneic ventilation spontaneously recovered by P13-14 despite continuous hyperoxia, or within 24h when Hyperoxia rats were returned to room air. Normoxic metabolism was also reduced in Hyperoxia rats but could be increased by raising inspired O2 levels (to 60% O2) or by uncoupling oxidative phosphorylation within the mitochondrion (2,4-dinitrophenol). In contrast, moderate increases in inspired O2 had no effect on sustained ventilation which indicates that hypoventilation can be dissociated from hypometabolism. The ventilatory response to abrupt O2 inhalation was diminished in Hyperoxia rats at P4 and P6-7, consistent with smaller contributions of peripheral chemoreceptors to eupneic ventilation at these ages. Finally, the spontaneous respiratory rhythm generated in isolated brainstem-spinal cord preparations was significantly slower and more variable in P3-4 Hyperoxia rats than in age-matched Controls. We conclude that developmental hyperoxia impairs both peripheral and central components of eupneic ventilatory drive. Although developmental hyperoxia diminishes metabolism as well, this appears to be a regulated hypometabolism and contributes little to the observed changes in ventilation.

Hyperoxia-induced developmental plasticity of the hypoxic ventilatory response in neonatal rats: contributions of glutamate-dependent and PDGF-dependent mechanisms

Rats reared in hyperoxia exhibit a sustained (vs. biphasic) hypoxic ventilatory response (HVR) at an earlier age than untreated, Control rats. Given the similarity between the sustained HVR obtained after chronic exposure to developmental hyperoxia and the mature HVR, it was hypothesized that hyperoxia-induced plasticity and normal maturation share common mechanisms such as enhanced glutamate and nitric oxide signaling and diminished platelet-derived growth factor (PDGF) signaling. Rats reared in 21% O2 (Control) or 60% O2 (Hyperoxia) from birth until 4-5 days of age were studied after intraperitoneal injection of drugs targeting these pathways. Hyperoxia rats receiving saline showed a sustained HVR to 12% O2, but blockade of NMDA glutamate receptors (MK-801) restored the biphasic HVR typical of newborn rats. Blockade of PDGF-β receptors (imatinib) had no effect on the pattern of the HVR in Hyperoxia rats, although it attenuated ventilatory depression during the late phase of the HVR in Control rats. Neither nitric oxide synthase inhibitor used in this study (nNOS inhibitor I and l-NAME) altered the pattern of the HVR in Control or Hyperoxia rats. Drug-induced changes in the biphasic HVR were not correlated with changes in metabolic rate. Collectively, these results suggest that developmental hyperoxia hastens the transition from a biphasic to sustained HVR by upregulating glutamate-dependent mechanisms and downregulating PDGF-dependent mechanisms, similar to the changes underlying normal postnatal maturation of the biphasic HVR.

Developmental hyperoxia alters CNS mechanisms underlying hypoxic ventilatory depression in neonatal rats

Newborn mammals exhibit a biphasic hypoxic ventilatory response (HVR), but the relative contributions of carotid body-initiated CNS mechanisms versus central hypoxia on ventilatory depression during the late phase of the HVR are not well understood. Neonatal rats (P4-5 or P13-15) were treated with a nonselective P2 purinergic receptor antagonist (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, or PPADS; 125mgkg(-1), i.p.) to pharmacologically denervate the peripheral chemoreceptors. At P4-5, rats reared in normoxia showed a progressive decline in ventilation during a 10-min exposure to 12% O2 (21-28% decrease from baseline). No hypoxic ventilatory depression was observed in the older group of neonatal rats (i.e., P13-15), suggesting that the contribution of central hypoxia to hypoxic ventilatory depression diminishes with age. In contrast, rats reared in moderate hyperoxia (60% O2) from birth exhibited no hypoxic ventilatory depression at either age studied. Systemic PPADS had no effect on the ventilatory response to 7% CO2, suggesting that the drug did not cross the blood-brain barrier. These findings indicate that (1) CNS hypoxia depresses ventilation in young, neonatal rats independent of carotid body activation and (2) hyperoxia alters the development of CNS pathways that modulate the late phase of the hypoxic ventilatory response.

Impact of environmental thermal stimulation on activation of hypothalamic neuronal nitric oxide synthase during the prenatal ontogenesis in Muscovy ducks

The aim of the study is to investigate the influence of prenatal temperature stimulation on neuronal NO synthase (nNOS) expression in the anterior hypothalamus of Muscovy duck embryos. Experiments were performed on embryonic day (E) E20, E23, E28, and E33 using histochemistry for identification of the nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) as marker of NOS-containing neurons. Until the experiments, all duck embryos were incubated under standard temperature conditions (37.5°C). During 3 hours before the start of the experiments, one group was incubated at 37.5°C (control group), the second was warm-experienced at 39°C, and the third was cold-experienced at 34°C. In normal and warm-incubated duck embryos, nNOS activity could be first detected on E23. Particularly, after cold stimulation, a significant increase in nNOS activity was found in all embryos investigated even on day 20. Warm stimulation obviously induces the opposite effect, but at later embryonic age (E33). It can be concluded that probably in late-term bird embryos NO acts as a mediator of the neuronal cold pathway in the anterior hypothalamus, which might be improved by prenatal cold stimulation.

Impaired ventilatory and thermoregulatory responses to hypoxic stress in newborn phox2b heterozygous knock-out mice

The Phox2b genesis necessary for the development of the autonomic nervous system, and especially, of respiratory neuronal circuits. In the present study, we examined the role of Phox2b in ventilatory and thermoregulatory responses to hypoxic stress, which are closely related in the postnatal period. Hypoxic stress was generated by strong thermal stimulus, combined or not with reduced inspired O(2). To this end, we exposed 6-day-old Phox2b(+/-) pups and their wild-type littermates (Phox2b(+/+)) to hypoxia (10% O(2)) or hypercapnia (8% CO(2)) under thermoneutral (33°C) or cold (26°C) conditions. We found that Phox2b(+/-) pups showed less normoxic ventilation (V(E)) in the cold than Phox2b(+/+) pups. Phox2b(+/-) pups also showed lower oxygen consumption (VO(2)) in the cold, reflecting reduced thermogenesis and a lower body temperature. Furthermore, while the cold depressed ventilatory responses to hypoxia and hypercapnia in both genotype groups, this effect was less pronounced in Phox2b(+/-) pups. Finally, because serotonin (5-HT) neurons are pivotal to respiratory and thermoregulatory circuits and depend on Phox2b for their differentiation, we studied 5-HT metabolism using high pressure liquid chromatography, and found that it was altered in Phox2b(+/-) pups. We conclude that Phox2b haploinsufficiency alters the ability of newborns to cope with metabolic challenges, possibly due to 5-HT signaling impairments.

Potentiation of the hypoxic ventilatory response by 1 day of hyperoxia in neonatal rats

The O(2) sensitivity of the neonatal rat carotid body is increased after 1 day in moderate hyperoxia (60% O(2)) (Donnelly et al., 2009). We investigated whether this enhanced peripheral chemosensitivity increases the hypoxic ventilatory response (HVR) and tested the hypothesis that this plasticity is mediated by the superoxide anion. Neonatal rats (7 d old) were injected with saline or MnTMPyP, a superoxide scavenger, and placed into 60% O(2) for 23-28h. Baseline ventilation was reduced and the acute HVR (12% O(2)) was enhanced in hyperoxia-treated rats relative to age-matched controls; MnTMPyP did not block these effects. An additional group of rats was studied after only 30min in 60% O(2). This shorter exposure had no effect on normoxic ventilation or the HVR. We conclude that 1 d, but not 30min, of 60% O(2) augments the HVR of neonatal rats and that production of the superoxide anion does not contribute to this plasticity.

Neonatal stress and attenuation of the hypercapnic ventilatory response in adult male rats: the role of carotid chemoreceptors and baroreceptors

Neonatal maternal separation (NMS) is a form of stress that disrupts respiratory control development. Awake adult male rats previously subjected to NMS show a ventilatory response to hypercapnia (HCVR; Fi(CO(2)) = 0.05) 47% lower than controls; however, the underlying mechanisms are unknown. To address this issue, we first tested the hypothesis that carotid bodies contribute to NMS-related attenuation of the HCVR by using carotid sinus nerve section or Fi(O(2)) manipulation to maintain Pa(O(2)) constant (iso-oxic) during hypercapnic hyperpnea. We then determined whether NMS-related augmentation of baroreflex sensitivity contributes to the reduced HCVR in NMS rats. Nitroprusside and phenylephrine injections were used to manipulate arterial blood pressure in both groups of rats. Pups subjected to NMS were separated from their mother 3 h/day from postnatal days 3 to 12. Control rats were undisturbed. At adulthood, rats were anesthetized [urethane (1g/kg) + isoflurane (0.5%)], and diaphragmatic electromyogram (dEMG) was measured under baseline and hypercapnic conditions (Pa(CO(2)): 10 Torr above baseline). The relative minute activity response to hypercapnia of anesthetized NMS rats was 34% lower than controls. Maintaining Pa(O(2)) constant during hypercapnia reversed this phenotype; the HCVR of NMS rats was 45% greater than controls. Although the decrease in breathing frequency during baroreflex activation was greater in NMS rats, the change observed within the range of pressure change observed during hypercapnia was minimal. We conclude that NMS-related changes in carotid body sensitivity to chemical stimuli and/or its central integration is a key mechanism in the attenuation of HCVR by NMS.

Neonatal maternal deprivation response and developmental changes in gene expression revealed by hypothalamic gene expression profiling in mice

Neonatal feeding problems are observed in several genetic diseases including Prader-Willi syndrome (PWS). Later in life, individuals with PWS develop hyperphagia and obesity due to lack of appetite control. We hypothesized that failure to thrive in infancy and later-onset hyperphagia are related and could be due to a defect in the hypothalamus. In this study, we performed gene expression microarray analysis of the hypothalamic response to maternal deprivation in neonatal wild-type and Snord116del mice, a mouse model for PWS in which a cluster of imprinted C/D box snoRNAs is deleted. The neonatal starvation response in both strains was dramatically different from that reported in adult rodents. Genes that are affected by adult starvation showed no expression change in the hypothalamus of 5 day-old pups after 6 hours of maternal deprivation. Unlike in adult rodents, expression levels of Nanos2 and Pdk4 were increased, and those of Pgpep1, Ndp, Brms1l, Mett10d, and Snx1 were decreased after neonatal deprivation. In addition, we compared hypothalamic gene expression profiles at postnatal days 5 and 13 and observed significant developmental changes. Notably, the gene expression profiles of Snord116del deletion mice and wild-type littermates were very similar at all time points and conditions, arguing against a role of Snord116 in feeding regulation in the neonatal period.

Effects of temperature on ventilatory response to hypercapnia in newborn mice heterozygous for transcription factor Phox2b

Congenital central hypoventilation syndrome (CCHS) is a rare disease with variable severity, generally present from birth and chiefly characterized by impaired chemosensitivity to hypercapnia. The main cause of CCHS is a mutation in the PHOX2B gene, which encodes a transcription factor involved in the development of autonomic medullary reflex pathways. Temperature regulation is abnormal in many patients with CCHS. Here, we examined whether ambient temperature influenced CO2sensitivity in a mouse model of CCHS. A weak response to CO2at thermoneutrality (32°C) was noted previously in 2-day-old mice with an invalidated Phox2b allele ( Phox2b+/−), compared with wild-type littermates. We exposed Phox2b+/− pups to 8% CO2at three ambient temperatures (TAs): 29°C, 32°C, and 35°C. We measured breathing variables and heart rate (HR) noninvasively using a novel whole body flow plethysmograph equipped with contact electrodes. Body temperature and baseline breathing increased similarly with TA in mutant and wild-type pups. The hypercapnic ventilatory response increased linearly with TA in both groups, while remaining smaller in mutant than in wild-type pups at all TAs. The differences between the absolute increases in ventilation in mutant and wild-type pups become more pronounced as temperature increased above 29°C. The ventilatory abnormalities in mutant pups were not associated with significant impairments of heart rate control. In both mutant and wild-type pups, baseline HR increased with TA. In conclusion, TA strongly influenced the hypercapnic ventilatory response in Phox2b+/− mutant mice. These findings suggest that abnormal temperature regulation may contribute to the severity of respiratory impairments in CCHS patients.

Ventilatory response to hypoxia during endotoxemia in young rats: role of nitric oxide

Administration of Escherichia coli endotoxin attenuates the ventilatory response to hypoxia (VRH) in newborn piglets, but the mechanisms responsible for this depression are not clearly understood. Nitric oxide (NO) production increases during sepsis and elevated NO levels can inhibit carotid body function. The role of endothelial NO on the VRH during endotoxemia was evaluated in 26 young rats. Minute ventilation (VE) and oxygen consumption (VO2) were measured in room air (RA) and during 30 min of hypoxia (10% O2) before and after E. coli endotoxin administration. During endotoxemia, animals received placebo (PL, n = 8); a nonselective nitric oxide synthase (NOS) inhibitor (NG-nitro-L-arginine methyl ester, L-NAME, n = 9), or a neuronal NOS (nNOS) inhibitor (7-nitroindazole, 7-NI, n = 9). During endotoxemia, a larger increase in VE was observed only during the first min of hypoxia in the L-NAME group when compared with PL or 7-NI (p < 0.001). VRH was similar in the PL and 7-NI groups. A larger decrease in VO2 at 30 min of hypoxia was observed in L-NAME and 7-NI groups when compared with PL (p < 0.03). These data demonstrate that the attenuation of the early VRH during endotoxemia is in part mediated by an inhibitory effect of endothelial NO on the respiratory control mechanisms.

Influence of adenosine A(1)-receptor blockade and vagotomy on the gasping and heart rate response to hypoxia in rats during early postnatal maturation

Failure to autoresuscitate from apnea has been suggested to play a role in sudden infant death. Little is known, however, about factors that influence the gasping and heart rate response to severe hypoxia that are fundamental to successful autoresuscitation in the newborn. The present experiments were carried out on 184 rat pups to investigate the influence of the parasympathetic nervous system, as well as adenosine, in mediating the profound bradycardia that occurs with the onset of hypoxic-induced primary apnea and in modulating hypoxic gasping. On days 1 to 2, days 5 to 6, and days 10 to 11 postpartum and following bilateral cervical vagotomy (VAG) or administration of a selective adenosine A(1) receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine; DPCPX), each pup was exposed to a single period of severe hypoxia produced by breathing an anoxic gas mixture (97% N(2)-3% CO(2)). Exposure to severe hypoxia resulted in an age-dependent decrease in heart rate (P < 0.001), accentuated with increasing postnatal age, that was attenuated in all age groups by DPCPX but not by VAG. Furthermore, DPCPX but not VAG decreased the time to last gasp but increased the total number of gasps in the 1- to 2-day-old and 5- to 6-day-old pups but not in the 10- to 11-day-old pups during exposure to severe hypoxia. Thus our data provide evidence that adenosine acting via adenosine A(1) receptors plays a role in modulating hypoxic gasping and in mediating the profound bradycardia that occurs coincident with hypoxic-induced primary apnea in rats during early postnatal life.

Bicyclic monoterpene diols stimulate release of nitric oxide from skin cells, increase microcirculation, and elevate skin temperature

Bicyclic monoterpene diols (BMTd) stimulate nitric oxide synthesis in melanoma and neuronal cells, representing cell types arising from embryonic neural crest tissue. This study shows that an equimolar mixture of the BMTd's 2,3-cis/exo-pinanediol and 2,3-cis/exo-camphanediol stimulates nitric oxide synthesis in epithelial cells of the skin, specifically normal human epidermal keratinocytes (NHEK) and normal human microvascular endothelial cells (HMVEC). A 1 mM mixture increased nitric oxide 3-fold in HMVEC in the first 24 h after treatment, and a 2 mM mixture produced an equivalent increase in NHEK. We hypothesized that an increase in nitric oxide in skin would lead to an increase in microcirculation, thereby increasing skin temperature. We found that twice daily application of 1mM BMTd lotion significantly increased arm skin temperature by 0.5 degrees C in 14 days compared to placebo, while a 2 mM mixture significantly increased skin temperature by 0.3 degrees C in 7 days (P < or = 0.05; ANOVA). A single application of a 2 mM BMTd mixture applied 30 min before a 30 min cold challenge (6 degrees C), maintained facial skin temperature 1.4 degrees C above untreated control sites (P < or = 0.05; ANOVA). We also tested whether BMTd treatment would benefit people with dark circles under their eyes. Twenty-six panelists with dark undereye circles completed 2-week, twice daily application of a lotion containing the 1mM mixture to one eye while the other eye was untreated. Seven of 26 subjects showed a reduction of darkness of undereye circles (P < or = 0.05; paired t test). Application of 2 mM BMTd lotion to lips resulted in a significant increase in their redness, as measured by the erythema index (P < or = 0.05; ANOVA). These results show that a mixture of BMTd's increases nitric oxide, and application to skin increases microcirculation and skin temperature.

Prior exposure to hypoxic-induced apnea impairs protective responses of newborn rats in an exposure-dependent fashion: influence of normoxic recovery time

Experiments were carried out to determine whether prior exposure to hypoxic-induced apnea impairs protective responses of newborn rats. Ninety-five, 5- to 6-day-old rat pups were instrumented for respiratory measurements and placed prone in a metabolic chamber regulated to 37.0°C. The time to first and last gasp as well as the number of gasps were determined on exposure to unrelenting hypoxia after each pup had experienced 0, 1, 2, 3, 4, 9, or 14 hypoxic-induced apnea/autoresuscitation cycles (HIA/AR) at 5-min intervals. Prior exposure to HIA/AR did not significantly alter the time to first gasp, but it decreased the time to last gasp after two HIA/AR and the number of gasps after three HIA/AR on exposure to unrelenting hypoxia. When the normoxic recovery time after 9 HIA/AR was varied from 5 to 120 min, the time to last gasp as well as the total number of gasps increased on exposure to unrelenting hypoxia but only at 120 min (i.e., the number of gasps was similar but the time to last gasp was still decreased compared with that observed in naive animals exposed to unrelenting hypoxia). Thus prior exposure to hypoxic-induced apnea as may occur during obstructive sleep apnea or positional asphyxia decreases the number and duration of potential autoresuscitation producing gasps on exposure to unrelenting hypoxia for a period of up to and exceeding 120 min, respectively. The mechanism by which prior exposure to hypoxic-induced apnea influences the duration and number of hypoxic-induced gasps is unknown.