Caffeine prevents prostaglandin E1-induced disturbances in respiratory control in neonatal rats: implications for infants with critical congenital heart disease.

Continuous infusion of prostaglandin E1 (PGE1) is used to maintain ductus arteriosus patency in infants with critical congenital heart disease, but it can also cause central apnea suggesting an effect on respiratory neural control. In this study, we investigated whether 1) PGE1 inhibits the various phases of the acute hypoxic ventilatory response (HVR; an index of respiratory control dysfunction) and increases apnea incidence in neonatal rats; and 2) whether these changes would be reversible with caffeine pretreatment. Whole body plethysmography was used to assess the HVR and apnea incidence in neonatal rats 2 h following a single bolus intraperitoneal injection of PGE1 with and without prior caffeine treatment. Untreated rats exhibited a biphasic HVR characterized by an initial increase in minute ventilation followed by a ventilatory decline of the late phase (~5th minute) of the HVR. PGE1 had a dose-dependent effect on the HVR. Contrary to our hypothesis, the lowest dose (1 µg/kg) of PGE1 prevented the ventilatory decline of the late phase of the HVR. However, PGE1 tended to increase postsigh apnea incidence and the coefficient of variability (CV) of breathing frequency, suggesting increased respiratory instability. PGE1 also decreased brainstem microglia mRNA and increased neuronal nitric oxide synthase (nNOS) and platelet-derived growth factor-ß (PDGF-ß) gene expression. Caffeine pretreatment prevented these effects of PGE1, and the adenosine A2A receptor inhibitor MSX-3 had similar preventative effects. Prostaglandin appears to have deleterious effects on brainstem respiratory control regions, possibly involving a microglial-dependent mechanism. The compensatory effects of caffeine or MSX-3 treatment raises the question of whether prostaglandin may also operate on an adenosine-dependent pathway.

Enhancement of phrenic long-term facilitation following repetitive acute intermittent hypoxia is blocked by the glycolytic inhibitor 2-deoxyglucose.

Moderate acute intermittent hypoxia (mAIH) elicits a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF). Preconditioning with modest protocols of chronic intermittent hypoxia enhances pLTF, demonstrating pLTF metaplasticity. Since "low-dose" protocols of repetitive acute intermittent hypoxia (rAIH) show promise as a therapeutic modality to restore respiratory (and nonrespiratory) motor function in clinical disorders with compromised breathing, we tested 1) whether preconditioning with a mild rAIH protocol enhances pLTF and hypoglossal (XII) LTF and 2) whether the enhancement is regulated by glycolytic flux. In anesthetized, paralyzed, and ventilated adult male Lewis rats, mAIH (three 5-min episodes of 10% O2) elicited pLTF (pLTF at 60 min post-mAIH: 49 ± 5% baseline). rAIH preconditioning (ten 5-min episodes of 11% O2/day with 5-min normoxic intervals, 3 times per week, for 4 wk) significantly enhanced pLTF (100 ± 16% baseline). XII LTF was unaffected by rAIH. When glycolytic flux was inhibited by 2-deoxy-d-glucose (2-DG) administered via drinking water (~80 mg·kg-1·day-1), pLTF returned to normal levels (58 ± 8% baseline); 2-DG had no effect on pLTF in normoxia-pretreated rats (59 ± 7% baseline). In ventral cervical (C4/5) spinal homogenates, rAIH increased inducible nitric oxide synthase mRNA vs. normoxic controls, an effect blocked by 2-DG. However, there were no detectable effects of rAIH or 2-DG on several molecules associated with phrenic motor plasticity, including serotonin 2A, serotonin 7, brain-derived neurotrophic factor, tropomyosin receptor kinase B, or VEGF mRNA. We conclude that modest, but prolonged, rAIH elicits pLTF metaplasticity and that a drug known to inhibit glycolytic flux (2-DG) blocks pLTF enhancement.

Respiratory dysfunction following neonatal sustained hypoxia exposure during a critical window of brain stem extracellular matrix formation.

The extracellular matrix (ECM) modulates brain maturation and plays a major role in regulating neuronal plasticity during critical periods of development. We examined 1) whether there is a critical postnatal period of ECM expression in brain stem cardiorespiratory control regions and 2) whether the attenuated hypoxic ventilatory response (HVR) following neonatal sustained (5 days) hypoxia [SH (11% O2, 24 h/day)] exposure is associated with altered ECM formation. The nucleus tractus solitarius (nTS), dorsal motor nucleus of the vagus, hypoglossal motor nucleus, cuneate nucleus, and area postrema were immunofluorescently processed for aggrecan and Wisteria floribunda agglutinin (WFA), a key proteoglycan of the ECM and the perineuronal net. From postnatal day ( P) 5 ( P5), aggrecan and WFA expression increased postnatally in all regions. We observed an abrupt increase in aggrecan expression in the nTS, a region that integrates and receives afferent inputs from the carotid body, between P10 and P15 followed by a distinct and transient plateau between P15 and P20. WFA expression in the nTS exhibited an analogous transient plateau, but it occurred earlier (between P10 and P15). SH between P11 and P15 attenuated the HVR (assessed at P16) and increased aggrecan (but not WFA) expression in the nTS, dorsal motor nucleus of the vagus, and area postrema. An intracisternal microinjection of chondroitinase ABC, an enzyme that digests chondroitin sulfate proteoglycans, rescued the HVR and the increased aggrecan expression. These data indicate that important stages of ECM formation take place in key brain stem respiratory neural control regions and appear to be associated with a heightened vulnerability to hypoxia.

Microglia modulate brainstem serotonergic expression following neonatal sustained hypoxia exposure: implications for sudden infant death syndrome.

KEY POINTS: Neonatal sustained hypoxia exposure modifies brainstem microglia and serotonin expression. The altered brainstem neurochemistry is associated with impaired ventilatory responses to acute hypoxia and mortality. The deleterious effects of sustained hypoxia exposure can be prevented by an inhibitor of activated microglia. These observations demonstrate a potential cause of the brainstem serotonin abnormalities thought to be involved in sudden infant death syndrome. ABSTRACT: We showed previously that the end of the second postnatal week (days P11-15) represents a period of development during which the respiratory neural control system exhibits a heightened vulnerability to sustained hypoxia (SH, 11% O2 , 5 days) exposure. In the current study, we investigated whether the vulnerability to SH during the same developmental time period is associated with changes in brainstem serotonin (5-HT) expression and whether it can be prevented by the microglia inhibitor minocycline. Using whole-body plethysmography, SH attenuated the acute (5 min) hypoxic ventilatory response (HVR) and caused a high incidence of mortality compared to normoxia rats. SH also increased microglia cell numbers and decreased 5-HT immunoreactivity in the nucleus of the solitary tract (nTS) and dorsal motor nucleus of the vagus (DMNV). The attenuated HVR, mortality, and changes in nTS and DMNV immunoreactivity was prevented by minocycline (25 mg kg(-1) /2 days during SH). These data demonstrate that the 5-HT abnormalities in distinct respiratory neural control regions can be initiated by prolonged hypoxia exposure and may be modulated by microglia activity. These observations share several commonalities with the risk factors thought to underlie the aetiology of sudden infant death syndrome, including: (1) a vulnerable neonate; (2) a critical period of development; (3) evidence of hypoxia; (4) brainstem gliosis (particularly the nTS and DMNV); and (5) 5-HT abnormalities.

Physiologic basis for intermittent hypoxic episodes in preterm infants.

Intermittent hypoxic episodes are typically a consequence of immature respiratory control and remain a troublesome challenge for the neonatologist. Furthermore, their frequency and magnitude are commonly underestimated by clinically employed pulse oximeter settings. In extremely low birth weight infants the incidence of intermittent hypoxia [IH] progressively increases over the first 4 weeks of postnatal life, with a subsequent plateau followed by a slow decline beginning at weeks six to eight. Over this period of unstable respiratory control, increased oxygen-sensitive peripheral chemoreceptor activity has been associated with a higher incidence of apnea of prematurity. In contrast, infants with bronchopulmonary dysplasia [chronic neonatal lung disease] exhibit decreased peripheral chemosensitivity, although the effect on respiratory stability in this population is unclear. Such episodic hypoxia/reoxygenation in early life has the potential to sustain a proinflammatory cascade with resultant multisystem, including respiratory, morbidity. Therapeutic approaches for intermittent hypoxic episodes comprise careful titration of baseline or supplemental inspired oxygen as well as xanthine therapy to prevent apnea of prematurity. Characterization of the pathophysiologic basis for such intermittent hypoxic episodes and their consequences during early life is necessary to provide an evidence-based approach to their management.

Human bone marrow-derived mesenchymal stem cells rescue neonatal CPAP-induced airway hyperreactivity.

Continuous positive airway pressure (CPAP) is a primary non-invasive mode of respiratory support for preterm infants. However, emerging evidence suggests CPAP could be an underlying contributor to the unintended pathophysiology of wheezing and associated airway hyperreactivity (AHR) in former preterm infants. The therapeutic benefits of mesenchymal stem cells (MSCs) have been demonstrated in a variety of animal models and several clinical trials are currently underway to assess their safety profiles in the setting of prematurity and bronchopulmonary dysplasia (BPD). In the present study, using a mouse model of neonatal CPAP, we investigated whether conditioned medium harvested from cultures of human bone-marrow derived mesenchymal stem cells (hMSC) could rescue the CPAP-induced AHR, based upon previous observations of their anti-AHR properties. Newborn mice (male and female) were fitted with a custom-made mask for delivery of daily CPAP 3 h/day for the first 7 postnatal days. At postnatal day 21 (two weeks after CPAP ended), lungs were removed, precision-cut lung slices were sectioned and incubated for 48 h in vitro in conditioned medium collected from cultures of three different hMSC donors. As expected, CPAP resulted in AHR to methacholine compared to untreated control mice. hMSC conditioned medium from the cultures of all three donors completely reversed AHR. These data reveal potential therapeutic benefits of hMSC therapy, which may be capable of rescuing the long-term adverse effects of neonatal CPAP on human airway function.

Respiratory characteristics of the tammar wallaby pouch young and functional limitations in a newborn with skin gas exchange.

A short gestation, low birth weight and presence of cutaneous exchange of O2 and CO2 comprise altricial features of newborn marsupials and that collectively implies a highly immature respiratory system. In the present study, we investigated various respiratory characteristics of the neonatal/postnatal tammar wallaby, a species of marsupial in which > 30% of the newborn's total O2 demands are supported by cutaneous rather than pulmonary gas exchange. The ventilatory response (HVR) to acute hypoxia (10% inspired O2) was absent in the newborn (1 day old) pouch young; a hypoxic hypometabolism contributed entirely to the hyperventilation (increased pulmonary convection requirement). A high (compared to older animals) resting metabolic cost to breathe and an inefficient respiratory system suggest the lack of a HVR might be due to an energetic constraint that impinges on their ability to sustain an increase in ventilation. The latter was supported by the inability of the newborn to tolerate metabolic-ventilatory stimulation following administration of the metabolic uncoupler, 2,4-dinitrophenol (2,4-DNP). At 1 week of age, the cost of breathing was reduced, which coincided with the expression of a significant ventilatory response to hypoxia, a more energetically efficient respiratory system, and tolerance to 2,4-DNP. These data suggest this species of marsupial is born with major respiratory insufficiency, and that their pronounced dependence on the skin for metabolic gas exchange is of critical importance for survival.

Blood and urine biomarkers associated with long-term respiratory dysfunction following neonatal hyperoxia exposure: Implications for prematurity and risk of SIDS.

Former preterm infants, many of whom required supplemental O2 support, exhibit sleep disordered breathing and attenuated ventilatory responses to acute hypoxia (HVR) beyond their NICU stay. There is an increasing awareness that early detection of biomarkers in biological fluids may be useful predictors/identifiers of short- and long-term morbidities. In the present study, we identified serotonin (5-HT), dopamine (DA) and hyaluronan (HA) as three potential biomarkers that may be increased by neonatal hyperoxia and tested whether they would be associated with an impaired HVR in a rat model of supplemental O2 exposure. Neonatal rats (postnatal age (P) 6 days, P6) exposed to hyperoxia (40% FIO2, 24 h/day between P1-P5 days of age) exhibited an attenuated early (1 min), but not the late (4-5 min) phase of the HVR compared to normoxia control rats; the attenuated early phase HVR was associated with increased levels of DA (urine and serum), 5-HT (platelet poor plasma only, PPP), and HA (serum only). At P21, both the early and late phases of the HVR were attenuated, but serum and urine levels of all 3 biomarkers were similar to age-matched control rats. These data indicate that changes in several serum and/or urine biomarkers (5-HT, DA, and HA) following short-term (days) neonatal hyperoxia can signify long-term (weeks) respiratory control dysfunction. Further studies are needed to determine whether early detection of similar biomarkers could be convenient predictors of increased risk of abnormalities in respiratory control including sleep disordered breathing in former preterm infants who had received prior supplemental O2 and who might also be at increased risk of SIDS.

Episodic spinal serotonin receptor activation elicits long-lasting phrenic motor facilitation by an NADPH oxidase-dependent mechanism.

Phrenic long-term facilitation (pLTF) is a serotonin (5-HT)-dependent augmentation of phrenic motor output induced by acute intermittent hypoxia (AIH). AIH-induced pLTF requires spinal NADPH oxidase activity and reactive oxygen species (ROS) formation. Since 5-HT receptor activation stimulates NADPH oxidase activity in some cell types, we tested the hypothesis that episodic spinal 5-HT receptor activation (without AIH) is sufficient to elicit an NADPH oxidase-dependent facilitation of phrenic motor output (pMF). In anaesthetised, artificially ventilated adult male rats, episodic intrathecal 5-HT injections (3 x 6 microl injections at 5 min intervals) into the cerebrospinal fluid (CSF) near cervical spinal segments containing the phrenic motor nucleus elicited a progressive increase in integrated phrenic nerve burst amplitude (i.e. pMF) lasting at least 60 min post-5-HT administration. Hypoglossal (XII) nerve activity was unaffected, suggesting that effective doses of 5-HT did not reach the brainstem. A single 5-HT injection was without effect. 5-HT-induced pMF was dose dependent, but exhibited a bell-shaped dose-response curve. Activation of different 5-HT receptor subtypes, specifically 5-HT(2) versus 5-HT(7) receptors, may underlie the bell-shaped dose-response curve via a mechanism of 'cross-talk' inhibition. Pre-treatment with NADPH oxidase inhibitors, apocynin or diphenylenodium (DPI), blocked 5-HT induced pMF. Thus, episodic spinal 5-HT receptor activation is sufficient to elicit pMF by an NADPH oxidase-dependent mechanism, suggesting common mechanisms of ROS formation with AIH-induced pLTF. An understanding of the mechanisms giving rise to AIH-induced pLTF and 5-HT induced pMF may inspire novel therapeutic strategies for respiratory insufficiency in diverse conditions, such as sleep apnoea, cervical spinal injury or amyotrophic lateral sclerosis.

NADPH oxidase activity is necessary for acute intermittent hypoxia-induced phrenic long-term facilitation.

Phrenic long-term facilitation (pLTF) following acute intermittent hypoxia (AIH) is a form of spinal, serotonin-dependent synaptic plasticity that requires reactive oxygen species (ROS) formation. We tested the hypothesis that spinal NADPH oxidase activity is a necessary source of ROS for pLTF. Sixty minutes post-AIH (three 5-min episodes of 11% O(2), 5 min intervals), integrated phrenic and hypoglossal (XII) nerve burst amplitudes were increased from baseline, indicative of phrenic and XII LTF. Intrathecal injections (approximately C(4)) of apocynin or diphenyleneiodonium chloride (DPI), two structurally and functionally distinct inhibitors of the NADPH oxidase complex, attenuated phrenic, but not XII, LTF. Immunoblots from soluble (cytosolic) and particulate (membrane) fractions of ventral C(4) spinal segments revealed predominantly membrane localization of the NADPH oxidase catalytic subunit, gp91(phox), whereas membrane and cytosolic expression were both observed for the regulatory subunits, p47(phox) and RAC1. Immunohistochemical analysis of fixed tissues revealed these same subunits in presumptive phrenic motoneurons of the C(4) ventral horn, but not in neighbouring astrocytes or microglia. Collectively, these data demonstrate that NADPH oxidase subunits localized within presumptive phrenic motoneurons are a major source of ROS necessary for AIH-induced pLTF. Thus, NADPH oxidase activity is a key regulator of spinal synaptic plasticity, and may be a useful pharmaceutical target in developing therapeutic strategies for respiratory insufficiency in patients with, for example, cervical spinal injury.

Respiratory long-term facilitation following intermittent hypoxia requires reactive oxygen species formation.

Acute intermittent hypoxia (AIH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). LTF is a progressive and sustained increase in respiratory motor output as expressed in phrenic and hypoglossal (XII) nerve activity. Since reactive oxygen species (ROS) play important roles in several forms of neuroplasticity, and ROS production is increased by intermittent hypoxia, we tested the hypothesis that ROS are necessary for phrenic and XII LTF following AIH. Urethane-anesthetized, paralyzed, vagotomized and pump-ventilated Sprague-Dawley rats were exposed to AIH (11% O2, 3, 5 min episodes, 5 min intervals), and both phrenic and XII nerve activity were monitored for 60 min post-AIH. Although phrenic and XII LTF were observed in control rats, i.v. manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP), a superoxide anion scavenger, attenuated both phrenic and XII LTF in a dose dependent manner. Localized application of MnTMPyP (5.5 mM; 10 microl) to the intrathecal space of the cervical spinal cord (C4) abolished phrenic, but not XII LTF. Thus, ROS are necessary for AIH-induced respiratory LTF, and the relevant ROS appear to be localized near respiratory motor nuclei since cervical MnTMPyP injections impaired phrenic (and not XII) LTF. Phrenic LTF is a novel form of ROS-dependent neuroplasticity since its ROS-dependence resides in the spinal cord.

Reactive oxygen species and respiratory plasticity following intermittent hypoxia.

The neural network controlling breathing exhibits plasticity in response to environmental or physiological challenges. For example, while hypoxia initiates rapid and robust increases in respiratory motor output to defend against hypoxemia, it also triggers persistent changes, or plasticity, in chemosensory neurons and integrative pathways that transmit brainstem respiratory activity to respiratory motor neurons. Frequently studied models of hypoxia-induced respiratory plasticity include: (1) carotid chemosensory plasticity and metaplasticity induced by chronic intermittent hypoxia (CIH), and (2) acute intermittent hypoxia (AIH) induced phrenic long-term facilitation (pLTF) in naïve and CIH preconditioned rats. These forms of plasticity share some mechanistic elements, although they differ in anatomical location and the requirement for CIH preconditioning. Both forms of plasticity require serotonin receptor activation and formation of reactive oxygen species (ROS). While the cellular sources and targets of ROS are not well known, recent evidence suggests that ROS modify the balance of protein phosphatase and kinase activities, shifting the balance towards net phosphorylation and favoring cellular reactions that induce and/or maintain plasticity. Here, we review possible sources of ROS, and the impact of ROS on phosphorylation events relevant to respiratory plasticity.

Daily acute intermittent hypoxia improves breathing function with acute and chronic spinal injury via distinct mechanisms.

Daily acute intermittent hypoxia (dAIH) elicits respiratory plasticity, enhancing respiratory motor output and restoring breathing capacity after incomplete cervical spinal injuries (cSCI). We hypothesized that dAIH-induced functional recovery of breathing capacity would occur after both acute (2 weeks) and chronic (8 weeks) cSCI, but through distinct cellular mechanisms. Specifically, we hypothesized that dAIH-induced breathing recovery would occur through serotonin-independent mechanisms 2wks post C2 cervical hemisection (C2Hs), versus serotonin-dependent mechanisms 8wks post C2Hs. In two independent studies, dAIH or sham (normoxia) was initiated 1 week (Study 1) or 7 weeks (Study 2) post-C2Hs to test our hypothesis. Rats were pre-treated with intra-peritoneal vehicle or methysergide, a broad-spectrum serotonin receptor antagonist, to determine the role of serotonin signaling in dAIH-induced functional recovery. Our data support the hypothesis that dAIH-induced recovery of breathing capacity transitions from a serotonin-independent mechanism with acute C2Hs to a serotonin-dependent mechanism with chronic C2Hs. An understanding of shifting mechanisms giving rise to dAIH-induced respiratory motor plasticity is vital for clinical translation of dAIH as a therapeutic modality.

Intratracheal LPS administration attenuates the acute hypoxic ventilatory response: Role of brainstem IL-1ß receptors.

Perinatal inflammation and infection are commonly associated with various respiratory morbidities in preterm infants including apnea of prematurity. In this study, we investigated whether pulmonary inflammation via intra-tracheal micro-injection of lipopolysaccharide (LPS) into neonatal rats modifies respiratory neural control via an IL-1ß receptor-dependent mechanism. Prior to an intra-tracheal micro-injection of LPS (1mg/kg), 10day old (Postnatal age, P10) rats received an intraperitoneal (i.p.) or intracisternal (i.c.) micro-injection of the IL-1ß receptor antagonist AF12198. Whole-body plethysmography was performed two hours later to assess the magnitude of the acute hypoxic (HVR) and hypercapnic (HCVR) ventilatory responses. Intra-tracheal LPS dose-dependently attenuated the acute HVR compared to saline (control) treated rats, whereas the HCVR was not affected. Pre-treatment with an i.c. (but not i.p.) micro-injection of AF12198 15min prior to LPS prevented the attenuated HVR. These data indicate that intrapulmonary inflammation affects brainstem respiratory neural pathways mediating the ventilatory response to acute hypoxia via an IL-1ß-dependent pathway. These findings are relevant to our understanding of the way that pulmonary inflammation may affect central neural mechanisms of respiratory insufficiency commonly seen in preterm infants.

Development of the respiratory system in marsupials.

Marsupials at birth are small and relatively undeveloped. At birth, the lung in some species is at the canalicular stage of development and though lung diffusion and metabolic rate are strongly correlated, the allometric exponent suggests that smaller newborns have relatively smaller diffusing capacity with respect to their demand for oxygen. Without improvement in functional or structural parameters newborn marsupials are reliant to varying degrees on skin gas exchange to compensate for the immaturity of the lung. Indeed, in some species there is complete reliance on the skin for gas exchange at birth. Nevertheless, with an early dependence on ventilation, the CNS would appear already to contain neurons with properties and connections that permit rhythmic motor output at birth and pulmonary reflexes mature soon after. Despite appropriate neural control and the presence of surfactant, the highly compliant nature of the newborn chest wall results in substantial chest wall distortion during inspiratory effort which reduce the efficacy of the lung for ventilation. This review explores the morpho-functional development of the respiratory system, including oxygen transport and cardiac shunts, and the establishment of convective requirement in marsupials, a group that places emphasis on extended postnatal development.

Changes in carotid body and nTS neuronal excitability following neonatal sustained and chronic intermittent hypoxia exposure.

We investigated whether pre-treatment with neonatal sustained hypoxia (SH) prior to chronic intermittent hypoxia (SH+CIH) would modify in vitro carotid body (CB) chemoreceptor activity and the excitability of neurons in the caudal nucleus of the solitary tract (nTS). Sustained hypoxia followed by CIH exposure simulates an oxygen paradigm experienced by extremely premature infants who developed persistent apnea. Rat pups were treated with 5 days of SH (11% O2) from postnatal age 1 (P1) followed by 10 days of subsequent chronic intermittent hypoxia (CIH, 5% O2/5 min, 8 h/day, between P6 and P15) as described previously (Mayer et al., Respir. Physiol. Neurobiol. 187(2): 167-75, 2013). At the end of SH+CIH exposure (P16), basal firing frequency was enhanced, and the hypoxic sensory response of single unit CB chemoafferents was attenuated. Further, basal firing frequency and the amplitude of evoked excitatory post-synaptic currents (ESPC's) of nTS neurons was augmented compared to age-matched rats raised in normoxia. These effects were unique to SH+CIH exposure as neither SH or CIH alone elicited any comparable effect on chemoafferent activity or nTS function. These data indicated that pre-treatment with neonatal SH prior to CIH exposure uniquely modified mechanisms of peripheral (CB) and central (nTS) neural function in a way that would be expected to disturb the ventilatory response to acute hypoxia.

Ventilation and metabolism in a large semifossorial marsupial: the effect of graded hypoxia and hypercapnia.

Metabolic and ventilatory variables were measured in a large semifossorial marsupial, the hairy-nosed wombat (Lasiorhinus latifrons, 21.9 kg). In normoxia, the rate of oxygen consumption was 63% of that predicted for a similar-sized marsupial, and the level of ventilation (V(E)) was such that the convective requirement (V(E)/VO2) was similar to other mammals. Exposure to hypercapnia (5% CO(2)) evoked a hyperventilatory response (3.55 x normoxia) that was no different to that observed for epigeal (surface-dwelling) marsupials; the increase in V(E) was primarily achieved with an increase in tidal volume. Exposure to hypoxia (15% to 8% O(2)) resulted in a hyperventilation (principally through an increase in frequency), although the response was blunted (in 8% O(2), 1.85 x normoxia) and only at the severest levels did hypometabolism contribute. The attenuated response to hypoxia in the wombat is presumably a reflection of a semifossorial lifestyle and a tolerance to this respiratory stimulant.

Spinal nNOS regulates phrenic motor facilitation by a 5-HT2B receptor- and NADPH oxidase-dependent mechanism.

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF) by a mechanism that requires spinal serotonin (5-HT) receptor activation and NADPH oxidase (NOX) activity. Here, we investigated whether: (1) spinal nitric oxide synthase (NOS) activity is necessary for AIH-induced pLTF; (2) episodic exogenous nitric oxide (NO) is sufficient to elicit phrenic motor facilitation (pMF) without AIH (i.e. pharmacologically); and (3) NO-induced pMF requires spinal 5-HT2B receptor and NOX activation. In anesthetized, mechanically ventilated adult male rats, AIH (3 × 5-min episodes; 10% O2; 5 min) elicited a progressive increase in the amplitude of integrated phrenic nerve bursts (i.e. pLTF), which lasted 60 min post-AIH (45.1 ± 8.6% baseline). Pre-treatment with intrathecal (i.t.) injections of a neuronal NOS inhibitor (nNOS-inhibitor-1) near the phrenic motor nucleus attenuated pLTF (14.7 ± 2.5%), whereas an inducible NOS (iNOS) inhibitor (1400 W) had no effect (56.3 ± 8.0%). Episodic i.t. injections (3 × 5µl volume; 5 min) of a NO donor (sodium nitroprusside; SNP) elicited pMF similar in time-course and magnitude (40.4 ± 6.0%, 60 min post-injection) to AIH-induced pLTF. SNP-induced pMF was blocked by a 5-HT2B receptor antagonist (SB206553), a superoxide dismutase mimetic (MnTMPyP), and two NOX inhibitors (apocynin and DPI). Neither pLTF nor pMF was affected by pre-treatment with a protein kinase G (PKG) inhibitor (KT-5823). Thus, spinal nNOS activity is necessary for AIH-induced pLTF, and episodic spinal NO is sufficient to elicit pMF by a mechanism that requires 5-HT2B receptor activation and NOX-derived ROS formation, which indicates AIH (and NO) elicits spinal respiratory plasticity by a nitrergic-serotonergic mechanism.

Vulnerability of neonatal respiratory neural control to sustained hypoxia during a uniquely sensitive window of development.

The first postnatal weeks represent a period of development in the rat during which the respiratory neural control system may be vulnerable to aberrant environmental stressors. In the present study, we investigated whether sustained hypoxia (SH; 11% O2) exposure starting at different postnatal ages differentially modifies the acute hypoxic (HVR) and hypercapnic ventilatory response (HCVR). Three different groups of rat pups were exposed to 5 days of SH, starting at either postnatal age 1 (SH1-5), 11 (SH11-15), or 21 (SH21-25) days. Whole body plethysmography was used to assess the HVR and HCVR the day after SH exposure ended. The primary results indicated that 1) the HVR and HCVR of SH11-15 rats were absent or attenuated (respectively) compared with age-matched rats raised in normoxia; 2) there was a profoundly high (∼84% of pups) incidence of unexplained mortality in the SH11-15 rats; and 3) these phenomena were unique to the SH11-15 group with no comparable effect of the SH exposure on the HVR, HCVR, or mortality in the younger (SH1-5) or older (SH21-25) rats. These results share several commonalities with the risk factors thought to underlie the etiology of sudden infant death syndrome, including 1) a vulnerable neonate; 2) a critical period of development; and 3) an environmental stressor.

Adrenergic α1 receptor activation is sufficient, but not necessary for phrenic long-term facilitation.

Acute intermittent hypoxia (AIH; three 5-min hypoxic episodes) causes a form of phrenic motor facilitation (pMF) known as phrenic long-term facilitation (pLTF); pLTF is initiated by spinal activation of Gq protein-coupled 5-HT2 receptors. Because α1 adrenergic receptors are expressed in the phrenic motor nucleus and are also Gq protein-coupled, we hypothesized that α1 receptors are sufficient, but not necessary for AIH-induced pLTF. In anesthetized, paralyzed, and ventilated rats, episodic spinal application of the α1 receptor agonist phenylephrine (PE) elicited dose-dependent pMF (10 and 100 µM, P < 0.05; but not 1 µM). PE-induced pMF was blocked by the α1 receptor antagonist prazosin (1 mM; -20 ± 20% at 60 min, -5 ± 21% at 90 min; n = 6). Although α1 receptor activation is sufficient to induce pMF, it was not necessary for AIH-induced pLTF because intrathecal prazosin (1 mM) did not alter AIH-induced pLTF (56 ± 9% at 60 min, 78 ± 12% at 90 min; n = 9). Intravenous (iv) prazosin (150 µg/kg) appeared to reduce pLTF (21 ± 9% at 60 min, 26 ± 8% at 90 min), but this effect was not significant. Hypoglossal long-term facilitation was unaffected by intrathecal prazosin, but was blocked by iv prazosin (-4 ± 14% at 60 min, -13 ± 18% at 90 min), suggesting different LTF mechanisms in different motor neuron pools. In conclusion, Gq protein-coupled α1 adrenergic receptors evoke pMF, but they are not necessary for AIH-induced pLTF.