Calcium-sensing receptor and CPAP-induced neonatal airway hyperreactivity in mice.

BACKGROUND: Continuous positive airway pressure (CPAP) in preterm infants is initially beneficial, but animal models suggest longer term detrimental airway effects towards asthma. We used a neonatal CPAP mouse model and human fetal airway smooth muscle (ASM) to investigate the role of extracellular calcium-sensing receptor (CaSR) in these effects. METHODS: Newborn wild type and smooth muscle-specific CaSR-/- mice were given CPAP for 7 days via a custom device (mimicking CPAP in premature infants), and recovered in normoxia for another 14 days (representing infants at 3-4 years). Airway reactivity was tested using lung slices, and airway CaSR quantified. Role of CaSR was tested using NPS2143 (inhibitor) or siRNA in WT mice. Fetal ASM cells stretched cyclically with/without static stretch mimicking breathing and CPAP were analyzed for intracellular Ca2+ ([Ca2+]i) responses, role of CaSR, and signaling cascades. RESULTS: CPAP increased airway reactivity in WT but not CaSR-/- mice, increasing ASM CaSR. NPS2143 or CaSR siRNA reversed CPAP effects in WT mice. CPAP increased fetal ASM [Ca2+]I, blocked by NPS2143, and increased ERK1/2 and RhoA suggesting two mechanisms by which stretch increases CaSR. CONCLUSIONS: These data implicate CaSR in CPAP effects on airway function with implications for wheezing in former preterm infants. IMPACT: Neonatal CPAP increases airway reactivity to bronchoconstrictor agonist. CPAP increases smooth muscle expression of the extracellular calcium-sensing receptor (CaSR). Inhibition or absence of CaSR blunts CPAP effects on contractility. These data suggest a causal/contributory role for CaSR in stretch effects on the developing airway. These data may impact clinical recognition of the ways that CPAP may contribute to wheezing disorders of former preterm infants.

Cardiorespiratory anomalies and increased brainstem microglia in a rat model of neonatal opioid withdrawal syndrome.

Infants born with neonatal opioid withdrawal syndrome (NOWS) can display abnormal cardiorespiratory patterns including tachypnea, tachycardia, and impaired ventilatory responses to hypoxia (HVR) and hypercapnia (HCVR). Chronic morphine exposure is associated with increased midbrain microglial expression. Using a rat model of pre- and post-natal morphine exposure, we assessed cardiorespiratory features of NOWS (resting tachycardia and tachypnea) including the attenuated HVR and HCVR and whether they are associated with increased brainstem microglia expression. Pregnant rats (dams) received twice-daily subcutaneous injections of morphine (5 mg/kg) during the third (last) week of pregnancy to simulate 3rd trimester in utero opioid exposure. Offspring then received once-daily subcutaneous injections of morphine (0.5 mg/kg) until postnatal (P) day P10 days of age to simulate postnatal morphine therapy. Cardiorespiratory responses were assessed 24 h later (P11 days) following spontaneous withdrawal. Compared to saline-treated pups, morphine-exposed offspring exhibited tachycardia and tachypnea as well as an attenuated HVR and HCVR. Microglial cell counts were increased in the nucleus tractus solitarius (nTS), dorsal motor nucleus of the vagus (DMNV) and nucleus ambiguous (NAamb), but not the retrapezoid nucleus (RTN) or the non-cardiorespriatory region, the cuneate nucleus (CN). These data suggest that the cardiorespiratory features and autonomic dysregulation in NOWS infants may be associated with altered microglial function in specific brainstem cardiorespiratory control regions.

CPAP-induced airway hyper-reactivity in mice is modulated by hyaluronan synthase-3.

BACKGROUND: Continuous positive airway pressure (CPAP) is a primary mode of respiratory support for preterm infants. Animal studies have shown long-term detrimental effects on lung/airway development, particularly airway (AW) hyper-reactivity, as an unfortunate consequence of neonatal CPAP. Since the hyaluronan (HA) synthesizing enzyme hyaluronan synthase-3 (HAS3) is involved in various adult pulmonary disorders, the present study used a neonatal mouse model to investigate the role of HAS3 in CPAP-induced AW hyper-reactivity. METHODS: Male and female neonatal mice were fitted with a custom-made mask for delivery of daily CPAP 3 h/day for 7 days. At postnatal day 21 (2 weeks after CPAP ended), airway (AW) hyper-reactivity and HAS3 expression were assessed with and without in vitro HAS3 siRNA treatment. RESULTS: MRIs of 3-day-old mice confirmed that CPAP increased lung volume with incrementing inflation pressures. CPAP increased AW reactivity in both male and female mice, which was associated with increased airway smooth muscle and epithelial HAS3 immunoreactivity. CPAP did not affect HA accumulation, but HAS3 siRNA reversed CPAP-induced AW hyper-reactivity and reduced HAS3 expression. CONCLUSIONS: These data in mice implicate a role for HAS3 in long-term effects of CPAP in the developing airway in the context of preterm birth and CPAP therapy. IMPACT: Neonatal CPAP increases airway smooth muscle and epithelial HAS3 expression in mice. CPAP-induced airway hyper-reactivity is modulated by HAS3. These data enhance our understanding of the role mechanical forces play on lung development. These data are a significance step toward understanding CPAP effects on developing airway. These data may impact clinical recognition of the ways that CPAP may contribute to wheezing disorders of former preterm infants.

Prenatal Maternal Lipopolysaccharide and Mild Newborn Hyperoxia Increase Intrapulmonary Airway but Not Vessel Reactivity in a Mouse Model.

Maternal infection is a risk for preterm delivery. Preterm newborns often require supplemental oxygen to treat neonatal respiratory distress. Newborn hyperoxia exposure is associated with airway and vascular hyperreactivity, while the complications of maternal infection are variable. In a mouse model of prenatal maternal intraperitoneal lipopolysaccharide (LPS, embryonic day 18) with subsequent newborn hyperoxia (40% oxygen × 7 days) precision-cut living lung slices were used to measure intrapulmonary airway and vascular reactivity at 21 days of age. Hyperoxia increased airway reactivity to methacholine compared to room air controls. Prenatal maternal LPS did not alter airway reactivity in room air. Combined maternal LPS and hyperoxia exposures increased airway reactivity vs. controls, although maximal responses were diminished compared to hyperoxia alone. Vessel reactivity to serotonin did not significantly differ in hyperoxia or room air; however, prenatal maternal LPS appeared to attenuate vessel reactivity in room air. Following room air recovery, LPS with hyperoxia lungs displayed upregulated inflammatory and fibrosis genes compared to room air saline controls (TNFαR1, iNOS, and TGFß). In this model, mild newborn hyperoxia increases airway but not vessel reactivity. Prenatal maternal LPS did not further increase hyperoxic airway reactivity. However, inflammatory genes remain upregulated weeks after recovery from maternal LPS and newborn hyperoxia exposures.

CPAP protects against hyperoxia-induced increase in airway reactivity in neonatal mice.

BACKGROUND: Oxygen and continuous positive airway pressure (CPAP) are primary modes of respiratory support for preterm infants. Animal models, however, have demonstrated adverse unintended effects of hyperoxia and CPAP on lung development. We investigate the effects of combined neonatal hyperoxia and CPAP exposure on airway function and morphology in mice. METHODS: Newborn mice were exposed to hyperoxia (40% O2) 24 h/day for 7 consecutive days with or without daily (3 h/day) concomitant CPAP. Two weeks after CPAP and/or hyperoxia treatment ended, lungs were assessed for airway (AW) hyperreactivity and morphology. RESULTS: CPAP and hyperoxia exposure alone increased airway reactivity compared to untreated control mice. CPAP-induced airway hyperreactivity was associated with epithelial and smooth muscle proliferation. In contrast, combined CPAP and hyperoxia treatment no longer resulted in increased airway reactivity, which was associated with normalization of smooth muscle and epithelial proliferation to values similar to untreated mice. CONCLUSIONS: Our data suggest that the combination of CPAP and hyperoxia decreases the adverse consequences on airway remodeling of either intervention alone. The complex interaction between mechanical stretch (via CPAP) and hyperoxia exposure on development of immature airways has implications for the pathophysiology of airway disease in former preterm infants receiving non-invasive respiratory support. IMPACT: CPAP and mild hyperoxia exposure alone increase airway reactivity in the neonatal mouse model. In contrast, combined CPAP and hyperoxia no longer induce airway hyperreactivity. Combined CPAP and hyperoxia normalize smooth muscle and epithelial proliferation to control values. Interaction between CPAP-induced stretch and mild hyperoxia exposure on immature airways has important implications for airway pathophysiology in former preterm infants.

Acute lung injury in neonatal rats causes postsynaptic depression in nucleus tractus solitarii second-order neurons.

Acute Lung Injury (ALI) alters pulmonary reflex responses, in part due to changes in modulation within the lung and airway neuronal control networks. We hypothesized that synaptic efficacy of nucleus tractus solitarii (nTS) neurons, receiving input from lung, airway, and other viscerosensory afferent fibers, would decrease following ALI. Sprague Dawley neonatal rats (postnatal days 9-11) were given intratracheal installations of saline or bleomycin (a well-characterized model that reproduces the pattern of ALI) and then, one week later, in vitro slices were prepared for whole-cell and perforated whole-cell patch-clamp experiments (postnatal days 16-21). In preparations from ALI rats, 2nd-order nTS neurons had significantly decreased amplitudes of both spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs), compared to saline controls. Rise and decay times of sEPSCs were slower in whole-cell recordings from ALI animals. Similarly, the amplitude of tractus solitarii evoked EPSCs (TS-eEPSCs) were significantly lower in 2nd-order nTS neurons from ALI rats. Overall these results suggest the presence of postsynaptic depression at TS-nTS synapses receiving lung, airway, and other viscerosensory afferent tractus solitarii input after bleomycin-induced ALI.

Long-term effects of recurrent intermittent hypoxia and hyperoxia on respiratory system mechanics in neonatal mice.

BACKGROUND: Premature infants are at increased risk for wheezing disorders. Clinically, these neonates experience recurrent episodes of apnea and desaturation often treated by increasing the fraction of inspired oxygen (FIO2). We developed a novel paradigm of neonatal intermittent hypoxia with subsequent hyperoxia overshoots (CIHO/E) and hypothesized that CIHO/E elicits long-term changes on pulmonary mechanics in mice. METHODS: Neonatal C57BL/6 mice received CIHO/E, which consisted of 10% O2 (1 min) followed by a transient exposure to 50% FIO2, on 10-min repeating cycles 24 h/d from birth to P7. Baseline respiratory mechanics, methacholine challenge, RT-PCR for pro and antioxidants, radial alveolar counts, and airway smooth muscle actin were assessed at P21 after 2-wk room air recovery. Control groups were mice exposed to normoxia, chronic intermittent hyperoxia (CIHE), and chronic intermittent hypoxia (CIHO). RESULTS: CIHO/E and CIHE increased airway resistance at higher doses of methacholine and decreased baseline compliance compared with normoxia mice. Lung mRNA for NOX2 was increased by CIHO/E. Radial alveolar counts and airway smooth muscle actin was not different between groups. CONCLUSION: Neonatal intermittent hypoxia/hyperoxia exposure results in long-term changes in respiratory mechanics. We speculate that recurrent desaturation with hyperoxia overshoot may increase oxidative stress and contribute to wheezing in former preterm infants.

A critical postnatal period of heightened vulnerability to lipopolysaccharide.

Evidence of respiratory abnormalities and vulnerability to infection during a critical period of development have been implicated in Sudden Infant Death Syndrome (SIDS). Here we investigated whether the acute hypoxic ventilatory response (HVR) exhibits a heightened vulnerability to the endotoxin lipopolysaccharide (LPS) during a critical period of development. The acute HVR was measured 2h after an i.p. injection of saline or LPS (0.1mg/kg) at various postnatal (P) ages (P5, P10, or P20days). LPS attenuated the early (1-2min) and late (4-6min) phase of the acute HVR in P10 but not P5 or P20 rats. The P10 age group exhibited the largest increase in brainstem TNFα and iNOS mRNA expression following LPS. LPS also caused a higher mortality rate in P10 rats (48%) compared to P5 (12%) and P20 (0%) age groups. After stratifying LPS treated P10 rats into survivors vs non-survivors, only the latter exhibited an attenuated HVR (specifically the early phase). Thus, the heightened vulnerability to endotoxin exposure during this critical period of development is characterized by a depression of the ventilatory response to acute hypoxia in association with an increased incidence of mortality. These data share similarities with some of the circumstances surrounding a SIDS scenario, including evidence of infection, increased brainstem cytokine expression, a disturbance in respiratory control, and a peak incidence of mortality during a critical period of development.

Increased airway reactivity in a neonatal mouse model of continuous positive airway pressure.

BACKGROUND: Continuous positive airway pressure (CPAP) is a primary form of respiratory support used in the intensive care of preterm infants, but its long-term effects on airway (AW) function are unknown. METHODS: We developed a neonatal mouse model of CPAP treatment to determine whether it modifies later AW reactivity. Unanesthetized spontaneously breathing mice were fitted with a mask to deliver CPAP (6 cmH2O, 3 h/day) for 7 consecutive days starting at postnatal day 1. AW reactivity to methacholine was assessed using the in vitro living lung slice preparation. RESULTS: One week of CPAP increased AW responsiveness to methacholine in male, but not female mice, compared to untreated control animals. The AW hyper-reactivity of male mice persisted for 2 wk (at P21) after CPAP treatment ended. Four days of CPAP, however, did not significantly increase AW reactivity. Females also exhibited AW hyper-reactivity at P21, suggesting a delayed response to early (7 d) CPAP treatment. The effects of 7 d of CPAP on hyper-reactivity to methacholine were unique to smaller AWs whereas larger ones were relatively unaffected. CONCLUSION: These data may be important to our understanding of the potential long-term consequences of neonatal CPAP therapy used in the intensive care of preterm infants.

Airway Hyperreactivity Is Delayed after Mild Neonatal Hyperoxic Exposure.

BACKGROUND: Wheezing disorders are prominent in former preterm infants beyond the neonatal period. OBJECTIVES: We used a neonatal mouse model to investigate the time course of airway hyperreactivity in response to mild (40% oxygen) or severe (70% oxygen) neonatal hyperoxia. METHODS: After hyperoxic exposure during the first week of postnatal life, we measured changes in airway reactivity using the in vitro living lung slice preparation at the end of exposure [postnatal day 8 (P8)] and 2 weeks later (P21). This was accompanied by measures of smooth muscle actin, myosin light chain (MLC) and alveolar morphology. RESULTS: Neither mild nor severe hyperoxia exposure affected airway reactivity to methacholine at P8 compared to normoxic controls. In contrast, airway reactivity was enhanced at P21 in mice exposed to mild (but not severe) hyperoxia, 2 weeks after exposure ended. This was associated with increased airway α-smooth muscle actin expression at P21 after 40% oxygen exposure without a significant increase in MLC. Alveolar morphology via radial alveolar counts was comparably diminished by both 40 and 70% oxygen at both P8 and P21. CONCLUSIONS: These data demonstrate that early mild hyperoxia exposure causes a delayed augmentation of airway reactivity, suggesting a long-term alteration in the trajectory of airway smooth muscle development and consistent with resultant symptomatology.

Intrapulmonary lipopolysaccharide exposure upregulates cytokine expression in the neonatal brainstem.

UNLABELLED: Perinatal inflammation and neonatal sepsis trigger lung and brain injury. We hypothesized that endotoxin exposure in the immature lung upregulates proinflammatory cytokine expression in the brainstem and impairs respiratory control. Lipopolysaccharide (LPS) or saline was administered intratracheally to vagal intact or denervated rat pups. LPS increased brainstem IL-1ß and vagotomy blunted this response. There was an attenuated ventilatory response to hypoxia and increased brainstem IL-1ß expression after LPS. CONCLUSION: Intratracheal endotoxin exposure in rat pups is associated with upregulation of IL-1ß in the brainstem that is vagally mediated and associated with an impaired hypoxic ventilatory response.

Lung and brainstem cytokine levels are associated with breathing pattern changes in a rodent model of acute lung injury.

Acute lung injury evokes a pulmonary inflammatory response and changes in the breathing pattern. The inflammatory response has a centrally mediated component which depends on the vagi. We hypothesize that the central inflammatory response, complimentary to the pulmonary inflammatory response, is expressed in the nuclei tractus solitarii (nTS) and that the expression of cytokines in the nTS is associated with breathing pattern changes. Adult, male Sprague-Dawley rats (n=12) received intratracheal instillation of either bleomycin (3units in 120µl of saline) or saline (120µl). Respiratory pattern changed by 24h. At 48h, bronchoalveolar lavage fluid and lung tissue had increased IL-1ß and TNF-α levels, but not IL-6. No changes in these cytokines were noted in serum. Immunocytochemical analysis of the brainstem indicated increased expression of IL-1ß in the nTS commissural subnucleus that was localized to neurons. We conclude that breathing pattern changes in acute lung injury were associated with increased levels of IL-1ß in brainstem areas which integrate cardio-respiratory sensory input.

Vagal afferents modulate cytokine-mediated respiratory control at the neonatal medulla oblongata.

Perinatal sepsis and inflammation trigger lung and brain injury in preterm infants, and associated apnea of prematurity. We hypothesized that endotoxin exposure in the immature lung would upregulate proinflammatory cytokine mRNA expression in the medulla oblongata and be associated with impaired respiratory control. Lipopolysaccharide (LPS, 0.1mg/kg) or saline was administered intratracheally to rat pups and medulla oblongatas were harvested for quantifying expression of mRNA for proinflammatory cytokines. LPS-exposure significantly increased medullary mRNA for IL-1ß and IL-6, and vagotomy blunted this increase in IL-1ß, but not IL-6. Whole-body flow plethysmography revealed that LPS-exposed pups had an attenuated ventilatory response to hypoxia both before and after carotid sinus nerve transection. Immunochemical expression of IL-1ß within the nucleus of the solitary tract and area postrema was increased after LPS-exposure. In summary, intratracheal endotoxin-exposure in rat pups is associated with upregulation of proinflammatory cytokines in the medulla oblongata that is vagally mediated for IL-1ß and associated with an impaired hypoxic ventilatory response.

Effect of intermittent hypercapnia on respiratory control in rat pups.

Preterm infants are subject to fluctuations in blood gas status associated with immature respiratory control. Intermittent hypoxia during early postnatal life has been shown to increase chemoreceptor sensitivity and destabilize the breathing pattern; however, intermittent hypercapnia remains poorly studied. Therefore, to test the hypothesis that intermittent hypercapnia results in altered respiratory control, we examined the effects of daily exposure to intermittent hypercapnia on the ventilatory response to subsequent hypercapnic and hypoxic exposure in neonatal rat pups. Exposure cycles consisted of 5 min of intermittent hypercapnia (5% CO(2), 21% O(2), balance N(2)) followed by 10 min of normoxia. Rat pups were exposed to 18 exposure cycles each day for 1 week, from postnatal day 7 to 14. We analyzed diaphragm electromyograms (EMGs) from pups exposed to subsequent acute hypercapnic (5% CO(2)) and hypoxic (12% O(2)) challenges. In response to a subsequent hypercapnia challenge, there was no significant difference in the ventilatory response between control and intermittent hypercapnia-exposed groups. In contrast, intermittent hypercapnia-exposed rat pups showed an enhanced ventilatory response to hypoxic challenge with an increase in minute EMG to 118 +/- 14% of baseline versus 107 +/- 13% for control pups (p < 0.05). We speculate that prior hypercapnic exposure may increase peripheral chemoreceptor response to subsequent hypoxic exposures and result in perturbed neonatal respiratory control.

Mutation in the myelin proteolipid protein gene alters BK and SK channel function in the caudal medulla.

Proteolipid protein (Plp) gene mutation in rodents causes severe CNS dysmyelination, early death, and lethal hypoxic ventilatory depression (Miller et al., 2004). To determine if Plp mutation alters neuronal function critical for control of breathing, the nucleus tractus solitarii (nTS) of four rodent strains were studied: myelin deficient rats (MD), myelin synthesis deficient (Plp(msd)), and Plp(null) mice, as well as shiverer (Mbp(shi)) mice, a myelin basic protein mutant. Current-voltage relationships were analyzed using whole-cell patch-clamp in 300 microm brainstem slices. Voltage steps were applied, and inward and outward currents quantified. MD, Plp(msd), and Plp(null), but not Mbp(shi) neurons exhibited reduced outward current in nTS at P21. Apamin blockade of SK calcium-dependent currents and iberiotoxin blockade of BK calcium-dependent currents in the P21 MD rat demonstrated reduced outward current due to dysfunction of these channels. These results provide evidence that Plp mutation specifically alters neuronal excitability through calcium-dependent potassium channels in nTS.

Developmental changes in brainstem neurons regulating lower airway caliber.

Premature infants are at risk for lower airway obstruction; however, maturation of reflex pathways regulating lower airway patency is inadequately studied. We hypothesized that postnatal maturation causes developmental change in brainstem efferent airway-related vagal preganglionic neurons (AVPNs) within the rostral nucleus ambiguus (rNA) that project to the airways and in pulmonary afferent fibers that terminate in the nucleus tractus solitarius (NTS). Ferrets aged 7, 14, 21, and 42 d received intrapulmonary injection of cholera toxin (CT)-beta subunit, a transganglionic retrograde tracer. Five days later, their brainstem was processed for dual immunolabeling of CT-beta and the cholinergic marker, choline acetyl transferase. CT-beta-labeled AVPNs and CT-beta-labeled afferent fiber optical density (OD) were analyzed. There was a significantly higher CT-beta-labeled cell number within the rNA at the youngest compared with older ages. All efferent CT-beta-labeled cells expressed choline acetyl transferase. OD of CT-beta-labeled afferent fibers was also higher at 7 d compared with 14 d. We conclude that the number of efferent AVPNs and afferent fiber OD both diminish over the second postnatal week. We speculate that exposure to injurious agents in early postnatal life may inhibit natural remodeling and thereby enhance later vulnerability to airway hyperreactivity.

Hyperbilirubinemia diminishes respiratory drive in a rat pup model.

Although apnea is common in premature babies, there is a paucity of information concerning the pathophysiologic basis of these episodes and their relationship to other perinatal conditions such as hyperbilirubinemia. Unconjugated hyperbilirubinemia in premature infants, even in moderately high levels, may cause encephalopathy affecting brainstem functions and has been linked to increased incidence of apnea in these infants. Thus, there is a need to clarify mechanisms by which bilirubin may alter respiratory control and induce apnea of prematurity. In this study, bilirubin or placebo was infused i.v. in 9-d-old rat pups (n = 36). Serum hyperbilirubinemia peaked in the first hours after bilirubin infusion. Twenty-four hours after bilirubin infusion, respiration was recorded by plethysmography at rest and under hypercapnic and hypoxic conditions. In treated pups, minute ventilation in room air was significantly reduced, hyperventilatory response to CO2 was blunted, and hypoxic ventilatory depression was increased, compared with placebo-injected rat pups. Brainstem bilirubin deposition and immunoreactivity to bilirubin was detected in the brainstem on histologic analysis. We speculate that high serum bilirubin levels may cause prolonged inhibition of brainstem autonomic function and that this could underlie the exacerbation of apnea noted in premature babies who have experienced jaundice.

Allergic lung inflammation affects central noradrenergic control of cholinergic outflow to the airways in ferrets.

Brain stem noradrenergic cell groups mediating autonomic responses to stress project to airway-related vagal preganglionic neurons (AVPNs). In ferrets, their activation produces withdrawal of cholinergic outflow to the airways via release of norepinephrine and activation of alpha(2A)-adrenergic receptors (alpha(2A)-AR) expressed by AVPNs. In these studies, we examined the effects of allergen exposure of the airway (AE) with ovalbumin on noradrenergic transmission regulating the activity of AVPNs and, consequently, airway smooth muscle tone. Experiments were performed in vehicle control (Con) and AE ferrets. Microperfusion of an alpha(2A)-AR agonist (guanabenz) in close proximity to AVPNs elicited more pronounced effects in Con than AE ferrets, including a decrease in unit activity and reflexly evoked responses of putative AVPN neurons with a corresponding decrease in cholinergic outflow to the airways. Although no differences were found in the extent of noradrenergic innervation of the AVPNs, RT-PCR and Western blot studies demonstrated that AE and repeated exposure to antigen significantly reduced expression of alpha(2A)-ARs at message and protein levels. These findings indicate that, in an animal model of allergic asthma, sensitization and repeated challenges with a specific allergen diminish central inhibitory noradrenergic modulation of AVPNs, possibly via downregulation of alpha(2A)-AR expression by these neurons.

Effect of supplemental oxygen on reinitiation of breathing after neonatal resuscitation in rat pups.

To test our hypothesis that resuscitation with 21% and 40% oxygen (O2) would shorten time to onset of respiratory activity when compared with resuscitation with 100% O2, diaphragmatic electromyogram (EMG) electrodes were inserted in Sprague-Dawley rat pups, age 8-10 d before intubation and mechanical ventilation with 5% O2 to induce cessation of respiratory activity. Each animal was then resuscitated with 100% and 21% O2 (n = 10) or 100% and 40% O2 (n = 11) for 30 s before the ventilator was disconnected. Recovery of diaphragm activity was compared between resuscitation groups. Blood gas status and heart rate data were characterized in additional rat pups. Time to first respiratory effort was 36 +/- 21 s (mean +/- SD) for room air resuscitation and 72 +/- 22 s for 100% O2, (p = 0.002). In contrast, there was no difference in time to onset of diaphragm activity when resuscitation with 40% O2 was compared with 100% O2: 84 +/- 27 s versus 76 +/- 23 s, respectively (p > 0.05). Resuscitation with 100% and 40% O2 both resulted in hyperoxia and hypocapnia when compared with room air, without effect on heart rate. Our findings indicate that even modest hyperoxic resuscitation will result in a delayed onset of respiration compared with normoxic gas, via a mechanism that may involve both hyperoxemic and hypocapnic inhibition of chemoreceptors.

Adenosine A2A receptors mediate GABAergic inhibition of respiration in immature rats.

Adenosine is a known inhibitor of respiratory output during early life. In this study we investigated the developmental changes in adenosine A2A-receptor activation on respiratory timing, as well as the relationship between adenosine and GABA. The specific adenosine A2A-receptor agonist CGS-21680 (CGS) or vehicle control was injected into the fourth ventricle of 14-day (n = 9), 21-day (n = 9), and adult (n = 5) urethane-anesthetized rats while diaphragm electromyogram was monitored as an index of respiratory neural output. CGS injection resulted in a decrease in frequency and/or apnea in all 14-day-old rats and in 66% of 21-day-old rats. There was no effect of CGS injection on respiratory timing in adult rats. Prior injection of the GABA(A)-receptor blocker bicuculline at 14 and 21 days eliminated the CGS-induced decrease in frequency and apnea. We conclude from these studies that the inhibitory effect of A2A-receptor activation on respiratory drive is age dependent and is mediated via GABAergic inputs to the inspiratory timing neural circuitry. These findings demonstrate an important mechanism by which xanthine therapy alleviates apnea of prematurity.