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The successful performance of a swallow requires dynamic integration between a wide range of sensory inputs and muscle activities to produce the coordinated kinematics of oropharyngeal structures. Damage to the recurrent laryngeal nerve (RLN) produces dysphagia in infants, with food or liquid entering the airway despite this nerve having minimal direct sensory or motor connections to the act of swallowing, apart from vocal fold closure. Previous results have demonstrated that a complete RLN lesion disrupts both performance and kinematics before initiation of the pharyngeal swallow in infants. We tested the hypothesis that a RLN lesion produces changes in the normal activity of oral floor, tongue, and infrahyoid muscles during a swallow. We recorded swallowing in our validated infant pig model, with synchronous high-speed imaging and fine-wire, chronic electromyography. We found changes in the timing, duration, and amplitude of the motor pattern in an array of muscles that are supplied by several different cranial and cervical nerves. Some of these changes in muscle activity are associated with the preparatory aspects of bolus aggregation or movement and so occur before the pharyngeal swallow. Taken with previous biomechanical results, these patterns suggest an intricate brain stem sensorimotor integration that occurs as part of a swallow. In particular, the execution of oral motor function is changed as a result of this simple lesion. NEW & NOTEWORTHY Damage to the recurrent laryngeal nerve compromises swallowing despite an absent or minimal contribution to either the motor or sensory aspects of this function. This study documents EMG changes, following RLN lesion, to non-RLN innervated muscles that are active during swallowing in an infant model. Some of these muscles fire before the pharyngeal swallow and are associated with the preparatory aspects of bolus aggregation and movement, suggesting important sensorimotor integration at a brain stem level.

The involvement of gastroesophageal refluxes in cardiorespiratory events of preterm infants remains controversial. While a few studies in full-term newborn animals have shown that stimulation of esophageal receptors leads to cardiorespiratory reflexes, the latter remain largely unknown, especially after premature birth. The present study aimed to 1 ) characterize the cardiorespiratory reflexes originating from esophageal receptors in newborn lambs and 2 ) test the hypotheses that preterm birth enhances reflex cardiorespiratory inhibition and that C-fibers are involved in these reflexes. Eight full-term lambs and 10 lambs born 14 days prematurely were studied. Following surgical instrumentation, a 6-h polysomnography was performed without sedation to record electrocardiogram, respiratory movements, arterial pressure, laryngeal constrictor muscle activity, state of alertness, and hemoglobin oxygen saturation. Five esophageal stimulations of the upper and/or lower esophagus, including rapid balloon inflation and/or HCl injection, were performed in random order. A second recording was performed in full-term lambs 24 h later, after C-fiber blockade by capsaicin. Results confirmed that esophageal stimulations induced inhibitory cardiorespiratory reflexes combined with protective mechanisms, including laryngeal closure, swallowing, coughing, increased arterial pressure, and arousal. Preterm birth heightened cardiorespiratory inhibition. The strongest cardiorespiratory inhibition was observed following simultaneous stimulation of the lower and upper esophagus. Finally, cardiorespiratory inhibition was decreased after C-fiber blockade. In conclusion, esophageal stimulation induces inhibitory cardiorespiratory reflexes, which are partly mediated by C-fibers and more pronounced in preterm lambs. Clinical relevance of these findings requires further studies, especially in conditions associated with increased cardiorespiratory events, e.g., neonatal infection. NEW & NOTEWORTHY Preterm birth heightens the cardiorespiratory events triggered by esophageal stimulation. The most extensive cardiorespiratory events are induced by simultaneous stimulation of the proximal and distal esophagus., (Copyright © 2017 the American Physiological Society.)

Arterial reservoir ("windkessel") function, whereby a part of left ventricular (LV) output is stored in elastic arteries during systole and discharged in diastole, is a well-established physiological phenomenon. However, its role in rapid reversal (to left-to-right) and a systolic-to-diastolic shift of shunting across the ductus arteriosus after birth is unknown. To address this question, ductal and aortic isthmus flows were measured with high-fidelity transit-time probes in six anesthetized preterm fetal lambs before and after cord clamping and subsequent early mechanical ventilation and for 30 min postbirth. Descending aortic flow was calculated as the sum of isthmus and ductal flows. Left-to-right ductal flow profiles were related to those of the isthmus and descending aorta, with upper body arterial reservoir discharge indicated by forward diastolic isthmus flow, and retrograde lower body arterial reservoir discharge by negative diastolic descending aortic flow. Left-to-right ductal shunting appeared immediately after cord clamping (P < 0.001), due entirely to newly emergent retrograde lower body reservoir discharge, and rose with ventilation via increased lower body reservoir discharge (P < 0.005), supplemented by upper body reservoir discharge after 45 s (P < 0.025) and LV systolic flow after 3 min (P = 0.025). The contribution of lower body reservoir discharge to left-to-right ductal shunting fell to 55 ± 8% at ≥15 min (P < 0.001) but remained higher (P < 0.002) than LV systolic flow (33 ± 8%) or upper body reservoir discharge (12 ± 5%). These results suggest that retrograde lower body arterial reservoir discharge plays a key role in rapid reversal and a systolic-to-diastolic shift of ductal shunting after cord clamping and early ventilation at birth., (Copyright © 2016 the American Physiological Society.)

Experimentally, a typical ∼2-min cord clamp-to-ventilation interval in preterm lambs is accompanied by increased hemodynamic lability of the birth transition. However, whether this lability is related to development of asphyxia after cord clamping, or can be avoided with a shorter clamp-to-ventilation interval, is unknown. To address these questions, anesthetized preterm fetal lambs (gestation 127 ± 2 days) were instrumented with ductus arteriosus and left pulmonary artery flow probes to obtain right ventricular (RV) output, brachiocephalic trunk and aortic isthmus flow probes to measure left ventricular (LV) output, and aortic trunk catheters for pressure measurement and blood gas analysis. With hemodynamics recorded continuously, fetuses were delivered onto the ewe's abdomen and the cord clamped for 1.5 min before ventilation (n = 8), with aortic sampling at 15, 30, 45, and 60 s, or for 0.5 min, with sampling at 15 s (n = 4). With 1.5-min cord clamping, an asphyxial state (Po2 < 10 mmHg) was evident at ≥45 s, with bradycardia and marked falls in LV and RV outputs (by 60% and 50%, P < 0.001), followed after ventilation onset by tachycardia and LV and RV output surges (4- and 3-fold, P < 0.001). By contrast, heart rate and outputs remained stable after 0.5-min cord clamping, with no postventilation change in heart rate or RV output, and a lesser rise in LV output (22%, P < 0.005). In preterm lambs, rapid development of an asphyxial state within 45 s in the cord clamp-to-ventilation interval increased hemodynamic lability of the birth transition, which was reduced with a shorter (∼0.5 min) cord clamp-to-ventilation interval., (Copyright © 2015 the American Physiological Society.)

At birth, the transition to newborn life is triggered by lung aeration, which stimulates a large increase in pulmonary blood flow (PBF). Current theories predict that the increase in PBF is spatially related to ventilated lung regions as they aerate after birth. Using simultaneous phase-contrast X-ray imaging and angiography we investigated the spatial relationships between lung aeration and the increase in PBF after birth. Six near-term (30-day gestation) rabbits were delivered by caesarean section, intubated and an intravenous catheter inserted, before they were positioned for X-ray imaging. During imaging, iodine was injected before ventilation onset, after ventilation of the right lung only, and after ventilation of both lungs. Unilateral ventilation increased iodine levels entering both left and right pulmonary arteries (PAs) and significantly increased heart rate, iodine ejection per beat, diameters of both left and right PAs, and number of visible vessels in both lungs. Within the 6th intercostal space, the mean gray level (relative measure of iodine level) increased from 68.3 ± 11.6 and 70.3 ± 7.5%·s to 136.3 ± 22.6 and 136.3 ± 23.7%·s in the left and right PAs, respectively. No differences were observed between vessels in the left and right lungs, despite the left lung not initially being ventilated. The increase in PBF at birth is not spatially related to lung aeration allowing a large ventilation/perfusion mismatch, or pulmonary shunting, to occur in the partially aerated lung at birth., (Copyright © 2014 the American Physiological Society.)

Perinatal exposure to hyperoxia (30-60% O2) alters the respiratory control system via modulation of peripheral arterial chemoreceptor development and function. Furthermore, hyperoxic exposure during the first two postnatal weeks of life can alternatively modulate the different phases of the hypoxic ventilatory response. Given the effects of perinatal hyperoxia, the aims of our study were 1) to determine the effect on survival time in response to lethal anoxic stimuli in rat pups and 2) to characterize the output of the isolated central respiratory network in response to acute hypoxic stimuli. We hypothesized that perinatal hyperoxic exposure would modify the neonatal rat ventilatory response to anoxia by affecting a central component of the respiratory network in addition to the maturation of the carotid body chemoreceptors. We found that animals continuously exposed to 60% oxygen up to age 5 days after parturition (P5) have reduced breathing frequency at baseline and within the first 10 min of a fatal anoxic challenge. Hyperoxic rat pups also have a shortened time to last gasp in response to anoxia that is not associated with lung injury or inflammation. This study is the first to demonstrate that these in vivo findings correlate with reduced phrenic burst frequency from the isolated brainstem ex vivo. Thus hyperoxic exposure reduced the phrenic burst frequency at baseline and in response to ex vivo anoxia. Importantly, our data suggest that perinatal hyperoxia alters ventilation and the response to anoxia at P5 in part by altering the frequency of phrenic bursts generated by the central respiratory network.

In this study we used a rat model for prenatal nicotine exposure to test whether clinically relevant concentrations of brain nicotine and cotinine are passed from dams exposed to nicotine to her pups, whether this changes the trigeminocardiac reflex (TCR), and whether serotonergic function in the TCR brainstem circuitry is altered. Pregnant Sprague-Dawley dams were exposed to 6 mg·kg(-1)·day(-1) of nicotine via osmotic minipumps for the duration of pregnancy. Following birth dams and pups were killed, blood was collected, and brain nicotine and cotinine levels were measured. A separate group of prenatal nicotine-exposed pups was used for electrophysiological recordings. A horizontal brainstem slice was obtained by carefully preserving the trigeminal nerve with fluorescent identification of cardiac vagal neurons (CVNs) in the nucleus ambiguus. Stimulation of the trigeminal nerve evoked excitatory postsynaptic current in CVNs. Our data demonstrate that prenatal nicotine exposure significantly exaggerates both the TCR-evoked changes in heart rate in conscious unrestrained pups, and the excitatory neurotransmission to CVNs upon trigeminal afferent nerve stimulation within this brainstem reflex circuit. Application of the 5-HT1A receptor antagonist WAY 100635 (100 μM) and 5-HT2A/C receptor antagonist ketanserin (10 μM)significantly decreased neurotransmission, indicating an increased facilitation of 5-HT function in prenatal nicotine-exposed animals. Prenatal nicotine exposure enhances activation of 5-HT receptors and exaggerates the trigeminocardiac reflex.

Recently, we found that the translocation of inhaled nanoparticles from the air space to secondary organs is age dependent and substantially greater in neonates than in adults (J Respir Crit Care Med 177: A48, 2008). One reason for this difference might be age-dependent differences in alveolar barrier integrity. Because the neonate lung is undergoing morphogenetic and fluid balance changes, we hypothesize that the alveolar barrier of developing lungs is more easily compromised and susceptible to foreign material influx than that of adult lungs. On the basis of these hypotheses, we predict that the postnatally developing lung is also more likely to allow the translocation of some materials from the air space to the lymphatic lumens. To test this idea, we intratracheally instilled methyl methacrylate into immature and adult lungs and compared lymphatic filling between these two age groups. Scanning electron microscopy of the resultant corrosion casts revealed peribronchial saccular and conduit lymphatic architecture. Deep pulmonary lymphatic casts were present on the majority (58.5%) of airways in immature lungs, but lymphatic casting in adult lungs, as anticipated, was much more infrequent (21.6%). Thus the neonate lung appears to be more susceptible than the adult lung to the passage of instilled methyl methacrylate from the air space into the lymphatics. We speculate that this could imply greater probability of translocation of other materials, such as nanoparticles, from the immature lung as well.

Autoresuscitation is a critical survival-promoting mechanism in mammals that allows recovery from primary apnea via hypoxia-induced gasping. Here we show, using head-out plethysmography, that gasping behavior is altered during autoresuscitation, and the autoresuscitation response is delayed, in neonatal 5-hydroxytryptamine (5-HT) neuron-deficient Pet-1 homozygous (Pet-1(-/-)) mice. When exposed to 97% N(2)-3% CO(2) on postnatal day 4.5, unanesthetized Pet-1(-/-) mice required over four times longer than age-matched wild-type controls to initiate gasping following primary apnea. When oxygen was made available before the first gasp, allowing autoresuscitation to occur, gasping frequency was decreased and the duration of the gasping period was extended in the Pet-1 mutants compared with wild type, resulting in a nearly threefold increase in the time needed for successful autoresuscitation. However, when the exposure to anoxia was unrelenting, gasping frequency, the form of the gasps, the total number of gasps produced, the duration of the gasping period, and time to last gasp were comparable to controls. Plethysmographic testing of the same mutants on postnatal day 9.5 revealed that their autoresuscitation responses, although improved compared with day 4.5, remained significantly longer than in wild-type controls. Our data indicate that despite a severe deficiency of central 5-HT neurons, Pet-1(-/-) neonatal mice are capable of gasping, but their gasping pattern is altered during autoresuscitation, leading to a prolongation of the time required to recover from hypoxia-induced apnea.

The therapeutic window following perinatal hypoxia-ischemia is brief, and early clinical signs of injury can be subtle. Electroencephalography (EEG) represents the most promising early diagnostic of hypoxia-ischemia; however, some studies have questioned the sensitivity and specificity of EEG. The present study investigated the use of both near-infrared spectroscopy (NIRS) measurements of the cerebral metabolic rate of oxygen (CMRO(2)) and amplitude-integrated EEG (aEEG) to detect the severity of hypoxia-ischemia after 1 h of reperfusion in newborn piglets (10 insult, 3 control). The CMRO(2) was measured before and after 1 h of reperfusion from hypoxia-ischemia, the duration of which was varied from piglet to piglet with a range of 3-24 min, under fentanyl/nitrous oxide anesthesia to mimic awake-like levels of cerebral metabolism. EEG data were collected throughout the study. On average, the CMRO(2) and mean aEEG background signals were significantly depressed following the insult (P < 0.05). Mean CMRO(2) and mean aEEG background were 2.61 +/- 0.11 ml O(2).min(-1).100 g(-1) and 20.4 +/- 2.7 microV before the insult and 1.58 +/- 0.09 ml O(2).min(-1).100 g(-1) and 11.8 +/- 2.9 microV after 1 h of reperfusion, respectively. Both CMRO(2) and aEEG displayed statistically significant correlations with duration of ischemia (P < 0.05; r = 0.71 and r = 0.89, respectively); however, only CMRO(2) was sensitive to milder injuries (<5 min). This study highlights the potential for combining NIRS measures of CMRO(2) with EEG in the neonatal intensive care unit to improve early detection of perinatal hypoxia-ischemia.

Previously, we reported that the hypoxic ventilatory response (HVR) in rats was weakest at postnatal day (P) P13, concomitant with neurochemical changes in respiratory nuclei. A major determinant of minute ventilation (Ve) is reportedly the metabolic rate [O(2) consumption (Vo(2)) and CO(2) production (Vco(2))]. The present study aimed at testing our hypothesis that daily metabolic rates changed in parallel with ventilation during development and that a weak HVR at P13 was attributable mainly to an inadequate metabolic rate in hypoxia. Ventilation and metabolic rates were monitored daily in P0-P21 rats. We found that 1) ventilation and metabolic rates were not always correlated, and Ve/Vo(2) and Ve/Vco(2) ratios were not constant during development; 2) metabolic rate and Ve/Vo(2) and Ve/Vco(2) ratios at P0-P1 were significantly different from the remaining first postnatal week in normoxia and hypoxia; 3) at P13, metabolic rates and Ve/Vo(2) and Ve/Vco(2) ratios abruptly increased in normoxia and were compromised in acute hypoxia, unlike more stable trends during the remaining second and third postnatal weeks; and 4) the respiratory quotient (Vco(2)/Vo(2)) was quite stable in normoxia and fluctuated slightly in hypoxia from P0 to P21. Thus our data revealed heretofore unsuspected metabolic adjustments at P0-P1 and P13. At P0-P1, ventilation and metabolic rates were uncorrelated, whereas at P13, they were closely correlated under normoxia and hypoxia. The findings further strengthened the existence of a critical period of respiratory development around P13, when multiple physiological and neurochemical adjustments occur simultaneously.

Increases in positive end-expiratory pressure (PEEP) improve arterial oxygenation in preterm infants, but the effects on cardiopulmonary hemodynamics are understood poorly. We aimed to determine the effect of increased PEEP on cardiopulmonary hemodynamics and to compare measurements from indwelling flow probes with Doppler echocardiography. Preterm lambs (129 +/- 1 days) were ventilated initially with a tidal volume of 7 ml/kg and 4 cmH(2)O of PEEP. In ramp lambs (n = 7), PEEP was increased by 2-cmH(2)O increments to 10 cmH(2)O and then in decrements back to 4 cmH(2)O. PEEP was unchanged in controls (n = 6). Doppler echocardiographic flow measurements in the left pulmonary artery (LPA) and ductus arteriosus (DA) were correlated with flow probe measurements. Compared with controls, high PEEP reduced LPA flow from baseline (10-cmH(2)O PEEP: 43 +/- 8% vs. control: 83 +/- 21%; P = 0.029). High PEEP increased the proportion of right-to-left (R-L) shunting through the DA, with a trend to an increased oxygenation index compared with controls (oxygenation index: 44.5 +/- 13.5 at 10-cmH(2)O PEEP vs. 19.4 +/- 4.5 in controls; P = 0.07). Increasing PEEP decreased heart rate (17 beats/min; P = 0.03) and tended to lower systolic arterial pressure (5.0 mmHg; P = 0.052) compared with controls. Doppler echocardiography measurement of LPA flows correlated strongly with indwelling flow probe (r(2) = 0.73, P < 0.001), except during highly turbulent flows. Increases in PEEP have significant cardiopulmonary consequences in preterm lambs, including reduced LPA flow and increased R-L shunt through the DA. These changes are likely due to the concomitant increase in downstream pulmonary vascular resistance and increased cardiovascular constraint induced by PEEP.

During the neonatal period, high protein breakdown rate is a metabolic process inherent to elevated rates of protein accretion in skeletal muscle. To determine the relationship between hindlimb net movements of essential and nonessential amino acids in the regulation of hindlimb protein breakdown during an overnight fasting-feeding cycle, we infused overnight-food-deprived 10- and 28-day-old piglets with [1-(13)C]phenylalanine and [ring-(2)H(4)]tyrosine over 7 h (during 3 h of fasting and then during 4 h of feeding). Extraction rates for aspartate and glutamate after an overnight fast were 15% and 51% in the 10-day-old compared with 6% and 25% in the 28-day-old (P < 0.05) piglets, suggesting an altered requirement for precursors of amino acids to shuttle nitrogen to the liver as early life progresses. This occurred simultaneously with marginal positive hindlimb net balance of essential amino acids after an overnight fast, with negative net release of many nonessential amino acids, such as alanine, asparagine, glutamine, glycine, and proline. This suggests that newborn muscle does not undergo significant protein mobilization after a short period of fasting in support of an elevated rate of protein accretion. Furthermore, tyrosine efflux from hindlimb breakdown between overnight fasting and feeding periods was not different in the 10-day-old piglets, for which tyrosine was limiting, but when tyrosine supply balanced requirements in the 28-day-old piglet, hindlimb efflux was increased (P = 0.01). The results of the present study indicate that proteolysis and net movements of amino acids are coordinated mechanisms that sustain the elevated rate of net protein accretion during overnight feeding-fasting cycles in the neonate.

The effects of lung volume recruitment manouvres on pulmonary blood flow (PBF) during high-frequency oscillatory ventilation (HFOV) in preterm neonates are unknown. Since increased airway pressure adversely affects PBF, we compared the effects of two HFOV recruitment strategies on PBF and oxygenation index (OI). Preterm lambs (128+/-1 day gestation; term approximately 150 days) were anesthetized and ventilated using HFOV (10 Hz, 33% tI) with a mean airway pressure (Pao) of 15 cmH2O. Lung volume was recruited by either increasing Pao to 25 cmH2O for 1 min, repeated five times at 5-min intervals (Sigh group; n=5) or stepwise (5 cmH2O) changes in Pao at 5-min intervals incrementing up to 30 cmH2O then decrementing back to 15 cmH2O (Ramp group; n=6). Controls (n=5) received constant HFOV at 15 cmH2O. PBF progressively decreased (by 45+/-4%) and OI increased (by 15+/-6%, indicating reduced oxygenation) in controls during HFOV, which was similar to the changes observed in the Sigh group of lambs. In the Ramp group, PBF fell (by 54+/-10%) as airway pressure increased (r2=0.99), although the PBF did not increase again as the Pao was subsequently reduced. The OI decreased (by 47+/-9%), reflecting improved oxygenation at high Pao levels during HFOV in the Ramp group. However, high Pao restored retrograde PBF during diastole in four of six lambs, indicating the restoration of right-to-left shunting through the ductus arteriosus. Thus the choice of volume recruitment maneuvre influences the magnitude of change in OI and PBF that occurs during HFOV. Despite significantly improving OI, the ramp recruitment approach causes sustained changes in PBF.

Previous studies suggest that carotid body responses to long-term changes in environmental oxygen differ between neonates and adults. In the present study we tested the hypothesis that the effects of chronic intermittent hypoxia (CIH) on the carotid body differ between neonates and adult rats. Experiments were performed on neonatal (1-10 days) and adult (6-8 wk) males exposed either to CIH (9 episodes/h; 8 h/day) or to normoxia. Sensory activity was recorded from ex vivo carotid bodies. CIH augmented the hypoxic sensory response (HSR) in both groups. The magnitude of CIH-evoked hypoxic sensitization was significantly greater in neonates than in adults. Seventy-two episodes of CIH were sufficient to evoke hypoxic sensitization in neonates, whereas as many as 720 CIH episodes were required in adults, suggesting that neonatal carotid bodies are more sensitive to CIH than adult carotid bodies. CIH-induced hypoxic sensitization was reversed in adult rats after reexposure to 10 days of normoxia, whereas the effects of neonatal CIH persisted into adult life (2 mo). Acute intermittent hypoxia (IH) evoked sensory long-term facilitation of the carotid body activity (sensory LTF, i.e., increased baseline neural activity following acute IH) in CIH-exposed adults but not in neonates. The effects of CIH were associated with hyperplasia of glomus cells in neonatal but not in adult carotid bodies. These observations demonstrate that responses to CIH differ between neonates and adults with regard to the magnitude of sensitization of HSR, susceptibility to CIH, induction of sensory LTF, reversibility of the responses, and morphological remodeling of the chemoreceptor tissue.

Little is known about lung function during early stages of postnatal maturation, although the complex structural changes associated with developing rat lung are well studied. We therefore analyzed corresponding functional (lung volume, respiratory mechanics, intrapulmonary gas mixing, and gas exchange) and structural (alveolar surface area, mean linear intercept length, and alveolar septal thickness) changes of the developing rat lung at 7-90 days. Total lung capacity (TLC) increased from 1.54 +/- 0.07 to 16.7 +/- 2.46 (SD) ml in proportion to body weight, but an increase in body weight exceeded an increase in lung volume by almost twofold. Series dead space volume increased from 0.21 +/- 0.03 to 1.38 +/- 0.08 ml but decreased relative to TLC from 14% to 8%, indicating that parenchymal growth exceeded growth of conducting airways. Diffusing capacity of CO (D(CO)) increased from 8.1 +/- 0.8 to 214.1 +/- 23.5 micromol min(-1) hPa(-1), corresponding to a substantial increase in surface area from 744 +/- 20 to 6,536 +/- 488 cm(2). D(CO) per unit of lung volume is considerably lower in the immature lung, inasmuch as D(CO)/TLC in 7-day-old rats was only 42% of that in adult (90 day-old) rats. In humans, however, infants and adults show comparable specific D(CO). Our functional and structural analysis shows that gas exchange is limited in the immature rat lung. The pivotal step for improvement of gas exchange occurs with the transition from bulk alveolarization to the phase of expansion of air spaces with septal reconstruction and microvascular maturation.

Developmental anomalies of central respiratory neural control contribute to newborn mortality and morbidity. Elucidation of the cellular, molecular, trophic, and genetic mechanisms involved in the formation and function of respiratory nuclei during prenatal development will provide a foundation for understanding pathologies. The pre-Bötzinger Complex (pre-BötC) is a specific group of neurons located in the ventrolateral medulla that is critical for respiratory rhythmogenesis. Thus it has become a major focus of research. Here, we provide an overview of current knowledge regarding the anatomical and functional emergence of the rodent pre-BötC during the prenatal period.

Modified natural surfactant preparations, used for treatment of respiratory distress syndrome in premature infants, contain phospholipids and the hydrophobic surfactant protein (SP)-B and SP-C. Herein, the individual and combined effects of SP-B and SP-C were evaluated in premature rabbit fetuses treated with airway instillation of surfactant and ventilated without positive end-expiratory pressure. Artificial surfactant preparations composed of synthetic phospholipids mixed with either 2% (wt/wt) of porcine SP-B, SP-C, or a synthetic poly-Leu analog of SP-C (SP-C33) did not stabilize the alveoli at the end of expiration, as measured by low lung gas volumes of approximately 5 ml/kg after 30 min of ventilation. However, treatment with phospholipids containing both SP-B and SP-C/SP-C33 approximately doubled lung gas volumes. Doubling the SP-C33 content did not affect lung gas volumes. The tidal volumes were similar in all groups receiving surfactant. This shows that SP-B and SP-C exert different physiological effects, since both proteins are needed to establish alveolar stability at end expiration in this animal model of respiratory distress syndrome, and that an optimal synthetic surfactant probably requires the presence of mimics of both SP-B and SP-C.

The respiratory control system exhibits considerable plasticity, similar to other regions of the nervous system. Plasticity is a persistent change in system behavior triggered by experiences such as changes in neural activity, hypoxia, and/or disease/injury. Although plasticity is observed in animals of all ages, some forms of plasticity appear to be unique to development (i.e., "developmental plasticity"). Developmental plasticity is an alteration in respiratory control induced by experiences during "critical" developmental periods; similar experiences outside the critical period will have little or no lasting effect. Thus complementary experiments on both mature and developing animals are generally needed to verify that the observed plasticity is unique to development. Frequently studied models of developmental plasticity in respiratory control include developmental manipulations of respiratory gas concentrations (O(2) and CO(2)). Environmental factors not specifically associated with breathing may also trigger developmental plasticity, however, including psychological stress or chemicals associated with maternal habits (e.g., nicotine, cocaine). Despite rapid advances in describing models of developmental plasticity in breathing, our understanding of fundamental mechanisms giving rise to such plasticity is poor; mechanistic studies of developmental plasticity are of considerable importance. Developmental plasticity may enable organisms to "fine tune" their phenotype to optimize the performance of this critical homeostatic regulatory system. On the other hand, developmental plasticity could also increase the risk of disease later in life. Future directions for studies concerning the mechanisms and functional implications of developmental plasticity in respiratory motor control are discussed.

The aim of the present study was to investigate the effect of hypercapnia and hypoxia on apnea and nonnutritive swallowing (NNS) frequency, as well as on the coordination between NNS and phases of the respiratory cycle in newborn lambs, while taking into account the potential effects of states of alertness. Six lambs were chronically instrumented for recording electroencephalogram, eye movements, diaphragm and thyroarytenoid muscle (a glottal adductor) activity, nasal airflow, and electrocardiogram. Polysomnographic recordings were performed in nonsedated lambs exposed to air (control), 10% O(2), and 5% CO(2) in a random order at 3, 4, and 5 days of age. Although hypercapnia decreased apnea frequency in wakefulness and active sleep (P = 0.002 vs. air and hypoxia), hypoxia had no significant effect on apnea. In addition, although hypercapnia increased NNS frequency during wakefulness and quiet sleep (P < 0.005 vs. air and hypoxia), hypoxia tended to decrease NNS frequency. Finally, only hypercapnia altered NNS-breathing coordination by increasing NNS at the transition from inspiration to expiration (ie-type NNS; P < 0.001 vs. air and hypoxia). In conclusion, whereas hypercapnia increases overall NNS frequency by specifically increasing ie-type NNS, hypoxia has the inverse tendency. Results were identical in all three states of alertness.

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.

Exposure of rat dams to hypergravity during pregnancy is associated with increased pup mortality, reduced food intake, and decreased rates of glucose oxidation and lipogenesis in mammary tissue. We hypothesized that increased pup mortality is due to changes in maternal metabolism and not to reduced food intake of dams. Effects of hypergravity on rate of glucose oxidation and lipogenesis in mammary, liver, and adipose tissue were measured in rat dams centrifuged at 2.0 G [hypergravity (HG)], kept at 1.0 G (control), or fed to match the intake of HG rats (pair fed) from gestation day 11 (G11) until G21 or postpartum day 3 (P3). Body weight, percent body fat, metabolizable energy, and nitrogen balance were significantly less in HG dams compared with controls (P<0.05); however, these factors were not different between HG and pair-fed dams. By P3, 100% of control and pair-fed pups survived, while only 49% of HG pups survived. At G21, rates of glucose oxidation and lipogenesis in mammary and adipose tissue were less in HG than in control and pair-fed dams (P<0.1 and P<0.05). In liver, at G21, the rate of lipogenesis was greater in HG than control and pair-fed dams (P<0.01); at P3, lipogenesis was greater in control than HG and pair-fed dams (P<0.05). Gene expression of ATP citrate lyase, acetyl-CoA carboxylase, and fatty acid synthase increased in liver from pregnancy to lactation in control and pair-fed dams but not HG dams. Thus reduced food intake and body mass due to hypergravity exposure cannot explain the dramatic decrease in HG pup survival.

Apnea and respiratory failure often occur in infants with pulmonary disease. Bronchopulmonary C-fiber (PCF)-mediated apnea is an important component of respiratory dysfunction. This study was undertaken to define the postnatal development of PCF-mediated apnea. The experiments were conducted in five groups of anesthetized, tracheotomized, and spontaneously breathing rats with ages at postnatal days P1-3, P7-9, P14-16, P21-23, and P56-58. Right atrial bolus injection of three doses of capsaicin (Cap), equivalent to 2, 4, and 8 microg/kg used previously in 450-g rats, was applied to stimulate PCFs. We found that 1) Cap-induced apneic response [percent change from the baseline expiratory duration (Te) values (deltaTe%)] and the sensitivity of this response (deltaTe%.microg(-1)) were significantly greater in the rats P10; 2) the Cap-induced apneas were vagally dependent in all rats tested; and 3) bivagotomy-induced prolongation of Te was much greater in the rats P10. From these findings we concluded that, compared with the older rats (>P10), the newborn rats have a stronger PCF-mediated respiratory inhibition that may contribute to infants' vulnerability to respiratory failure.

Recent studies suggest that a bronchial-derived relaxing factor (BrDRF) decreases the contractility of newborn, but not fetal, rat pulmonary arteries (PAs) by a nitric oxide (NO)-mediated mechanism. We studied the effect of an adjacent bronchus on PA contractility to norepinephrine (NE) in late-gestation fetal (n = 7), neonatal (1 day old, n = 9), ventilated neonatal (24-h ventilation from birth with 100% oxygen, n = 9), and adult sheep (n = 6) in the presence and absence of the NO synthase inhibitor N(omega)-nitro-l-arginine (l-NNA). The sheep were anesthetized and killed, and fifth-generation PA rings with and without an attached adjacent bronchus (PA+Br) were contracted in standard tissue baths with NE (10(-8)-10(-6) M). NE contractions were expressed as fraction of KCl (118 mM) contraction and as grams of contraction force. NE contractions were significantly diminished by the presence of an attached bronchus in the neonatal and ventilated neonatal and adult, but not fetal, lambs. Hyperoxic ventilation markedly increased NE contractions in PA and PA+Br. l-NNA significantly enhanced NE contractions in PA+Br in postnatal but not in fetal lambs. Pretreatment with l-NNA abolished the difference between NE contractions in PA and PA+Br in neonatal but not in hyperoxic ventilated neonatal lambs. We conclude that there is a BrDRF that is developmentally regulated and has vascular activity postnatally but not during fetal life. The effect of BrDRF is predominantly mediated by NO in air-breathing neonatal lambs but may involve a second non-NO mediator following hyperoxic ventilation. We speculate that BrDRF may have an important role in postnatal changes in pulmonary arterial reactivity.

Previous work by our group has demonstrated substantial differences in lung volume and morphometric parameters between inbred mice. Specifically, adult C3H/HeJ (C3) have a 50% larger lung volume and 30% greater mean linear intercept than C57BL/6J (B6) mice. Although much of lung development occurs postnatally in rodents, it is uncertain at what age the differences between these strains become manifest. In this study, we performed quasi-static pressure-volume curves and morphometric analysis on neonatal mice. Lungs from anesthetized mice were degassed in vivo using absorption of 100% O2. Pressure-volume curves were then recorded in situ. The lungs were then fixed by instillation of Zenker's solution at a constant transpulmonary pressure. The left lung from each animal was used for morphometric determination of mean air space chord length (Lma). We found that the lung volume of C3 mice was substantially greater than that of B6 mice at all ages. In contrast, there was no difference in Lma (62.7 microm in C3 and 58.5 microm in B6) of 3-day-old mice. With increasing age (8 days), there was a progressive decrease in the Lma of both strains, with the magnitude of the decrease in B6 Lma mice exceeding that of C3. C3 lung volume remained 50% larger. The combination of parenchymal architectural similarity with lung air volume differences and different rates of alveolar septation support the hypothesis that lung volume and alveolar dimensions are independently regulated.

Impaired oxidative metabolism following hypoxia-ischemia (HI) is believed to be an early indicator of delayed brain injury. The cerebral metabolic rate of oxygen (CMRO2) can be measured by combining near-infrared spectroscopy (NIRS) measurements of cerebral blood flow (CBF) and cerebral deoxy-hemoglobin concentration. The ability of NIRS to measure changes in CMRO2 following HI was investigated in newborn piglets. Nine piglets were subjected to 30 min of HI by occluding both carotid arteries and reducing the fraction of inspired oxygen to 8%. An additional nine piglets served as sham-operated controls. Measurements of CBF, oxygen extraction fraction (OEF), and CMRO2 were obtained at baseline and at 6 h after the HI insult. Of the three parameters, only CMRO2 showed a persistent and significant change after HI. Five minutes after reoxygenation, there was a 28+/-12% (mean+/-SE) decrease in CMRO2, a 72+/-50% increase in CBF, and a 56+/-19% decrease in OEF compared with baseline (P<0.05). By 30 min postinsult and for the remainder of the study, there were no significant differences in CBF and OEF between control and insult groups, whereas CMRO2 remained depressed throughout the 6-h postinsult period. This study demonstrates that NIRS can measure decreases in CMRO2 caused by HI. The results highlight the potential for NIRS to be used in the neonatal intensive care unit to detect delayed brain damage.

In mature lungs, elevated positive end-expiratory pressure (PEEP) reduces pulmonary blood flow (PBF) and increases pulmonary vascular resistance (PVR). However, the effect of PEEP on PBF in preterm infants with immature lungs and a patent ductus arteriosus is unknown. Fetal sheep were catheterized at 124 days of gestation (term approximately 147 days), and a flow probe was placed around the left pulmonary artery to measure PBF. At 127 days, lambs were delivered and ventilated from birth with a tidal volume of 5 ml/kg and 4-cmH(2)O PEEP; PEEP was changed to 0, 8, and 12 cmH(2)O in random order, returning to 4 cmH(2)O between each change. Increasing PEEP from 4 to 8 cmH(2)O and from 4 to 12 cmH(2)O decreased PBF by 20.5 and 41.0%, respectively, and caused corresponding changes in PVR; reducing PEEP from 4 to 0 cmH(2)O did not affect PBF. Despite decreasing PBF, increasing PEEP from 4 to 8 cmH(2)O and 12 cmH(2)O improved oxygenation of lambs. Increasing and decreasing PEEP from 4 cmH(2)O significantly changed the contour of the PBF waveform; at a PEEP of 12 cmH(2)O, end-diastolic flow was reduced by 82.8% and retrograde flow was reestablished. Although increasing PEEP improves oxygenation, it adversely affects PBF and PVR shortly after birth, alters the PBF waveform, and reestablishes retrograde flow during diastole.

Medullary raphé serotonergic neurons are chemosensitive in culture and are situated adjacent to blood vessels in the brain stem. Selective lesioning of serotonergic raphé neurons decreases the ventilatory response to systemic CO2 in awake and sleeping adult rats. Abnormalities in the medullary serotonergic system, including the raphé, have been implicated in the sudden infant death syndrome (48). In this study, we ask whether serotonergic neurons in the medullary raphé and extra-raphé regions are involved in the CO2 response in unanesthetized newborn piglets, 3-16 days old. Whole body plethysmography was used to examine the ventilatory response to 5% CO2 before and during focal inhibition of serotonergic neurons by 8-hydroxy-2-di-n-propylaminotetralin (8-OH-DPAT), a 5-HT1A receptor agonist. 8-OH-DPAT (10 or 30 mM in artificial cerebrospinal fluid) decreased the CO2 response in wakefulness in an age-dependent manner, as revealed by a linear regression analysis that showed a significant negative correlation (P < 0.001) between the percent change in the CO2 response and piglet age. Younger piglets showed an exaggerated CO2 response. Control dialysis with artificial cerebrospinal fluid had no significant effect on the CO2 response. Additionally, 8-OH-DPAT increased blood pressure and decreased heart rate independent of age (P < 0.05). Finally, sleep cycling was disrupted by 8-OH-DPAT, such that piglets were awake more and asleep less (P < 0.05). Because of the fragmentary sleep data, it was not possible to examine the CO2 response in sleep. Inhibition of serotonergic medullary raphé and extra-raphé neurons decreases ventilatory CO2 sensitivity and alters cardiovascular variables and sleep cycling, which may contribute to the sudden infant death syndrome.

The factors accounting for the maintenance of a low pulmonary vascular resistance postnatally are not completely understood. The aim of this study was to test the hypothesis that bronchial epithelium produces a factor capable of relaxing adjacent pulmonary arterial smooth muscle. We studied fourth-generation intralobar pulmonary arteries and bronchi of 4- to 8-day-old rats. Arteries were mounted on a wire myograph, alone or with the adjacent bronchus. The presence of the attached bronchus significantly reduced pulmonary artery force generation induced by the thromboxane analog (U-46619) or KCl whether the endothelium was present or absent (P < 0.01). The converse was not true in that bronchial force generation was not affected when studied with the adjacent pulmonary artery. Mechanical removal of the bronchial epithelium or addition of the nitric oxide (NO) synthase (NOS) nonspecific (N(G)-monomethyl-l-arginine) or the specific neuronal NOS (7-nitroindazole) inhibitors increased arterial force generation to levels comparable to the isolated artery preparation. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, significantly decreased (P < 0.01) NO release of pulmonary arteries only when the adjacent bronchus was present. We conclude that bronchial epithelium in the newborn rat produces a factor capable of lowering pulmonary vascular muscle tone. This relaxant effect can be suppressed by NOS and phosphatidylinositol 3-kinase kinase inhibition, suggesting an action via NOS phosphorylation and NO release. We speculate that such a mechanism may be operative in vivo and plays an important role in control of pulmonary vascular resistance in the early postnatal period.

Previously, we reported that cytochrome oxidase (CO) activity in the rat pre-Bötzinger complex (PBC) exhibited a plateau on postnatal days (P) 3-4 and a prominent decrease on P12 (Liu and Wong-Riley, J Appl Physiol 92: 923-934, 2002). These changes were correlated with a concomitant reduction in the expression of glutamate and N-methyl-d-aspartate receptor subunit 1 and an increase in GABA, GABAB, glycine receptor, and glutamate receptor 2. To determine whether changes were limited to the PBC, the present study aimed at examining the expression of CO in a number of brain stem nuclei, with or without known respiratory functions from P0 to P21 in rats: the ventrolateral subnucleus of the solitary tract nucleus, nucleus ambiguus, hypoglossal nucleus, nucleus raphe obscurus, dorsal motor nucleus of the vagus nerve, medial accessory olivary nucleus, spinal nucleus of the trigeminal nerve, and medial vestibular nucleus (MVe). Results indicated that, in all of the brain stem nuclei examined, CO activity exhibited a general increase with age from P0 to P21, with MVe having the slowest rise. Notably, in all of the nuclei examined except for MVe, there was a plateau or decrease at P3-P4 and a prominent rise-fall-rise pattern at P11-P13, similar to that observed in the PBC. In addition, there was a fall-rise-fall pattern at P15-P17 in these nuclei, instead of a plateau pattern in the PBC. Our data suggest that the two postnatal periods with reduced CO activity, P3-P4 and especially P12, may represent common sensitive periods for most of the brain stem nuclei with known or suspected respiratory control functions.

Exposing newborn rats to postnatal hyperoxia (60% O2) for 1-4 wk attenuates the ventilatory and phrenic nerve responses to acute hypoxia in adult rats. The goal of this research was to increase our understanding of the carotid chemoreceptor afferent neural input in this depressed response with different durations of postnatal hyperoxic exposure. Rats were exposed from a few days before birth to 1, 2, or 4 wk of 60% O2 and studied after 3-5 mo in normoxia. The rats were anesthetized with urethane. Whole carotid sinus nerve (CSN) responses to NaCN (40 microg/kg iv), 10 s of asphyxia and acute isocapnic hypoxia (arterial Po2 45 Torr) were determined. Mean CSN responses to stimuli after postnatal hyperoxia were reduced compared with controls. Responses in rats exposed to 1 wk of postnatal hyperoxia were less affected than those exposed to 2 and 4 wk of hyperoxia, which were equivalent to each other. These studies illustrate the importance of normoxia during the first 2 wk of life in development of carotid chemoreceptor afferent function.

Neonatal rats exposed to 60% O(2) for 14 days develop lung changes compatible with human bronchopulmonary dysplasia and pulmonary hypertension. Our aim was to evaluate and compare the newborn and adult rat pulmonary vascular and airway smooth muscle force generation and relaxation potential after exposure to 60% O(2) for 14 days. Vascular and airway intrapulmonary rings 100 microm in diameter were mounted on a myograph and bathed in Krebs-Henseleit solution bubbled with air- 6% CO(2) at 37 degrees C. Significant age-dependent changes in intrapulmonary arteries and their neighboring airway muscle properties were observed. Whereas hyperoxia enhanced force in neonatal vascular and airway muscle, the opposite was seen in adult samples. No changes in endothelium-dependent vascular relaxation were observed at either age, but the dose response to an endothelium-independent NO donor was altered. In the newborn experimental animals, the relaxation was reduced, whereas, in their adult counterparts, it was enhanced. After O(2) exposure, the bronchial muscle relaxation response to epithelium-dependent and -independent stimulation was not altered in either age group, whereas the epithelium-dependent response was decreased only in the adult. The antioxidant Trolox, or an endothelin-A and -B receptor antagonist, reversed the vascular and airway muscle's hyperoxia-induced changes. We conclude that chronic O(2) exposure in the newborn rat results in enhanced lung vascular and airway muscle contraction potential via a mechanism involving reactive oxygen species and the endothelin pathway. The present findings also suggest that the newborn is more susceptible to airway hyperresponsiveness after chronic O(2) exposure.

Our laboratory previously reported that active glottal closure was present in 90% of spontaneous central apneas in premature lambs while maintaining a high-apneic lung volume (Renolleau S, Letourneau P, Niyonsenga T, and Praud JP. Am J Respir Crit Care Med 159: 1396-1404, 1999.) The present study aimed at testing whether this mechanism limits postapnea oxygen desaturation. Four premature lambs were instrumented for recording states of alertness, thyroarytenoid muscle and diaphragm electromyographic (EMG) activity, nasal airflow, lung volume changes, and pulse oximetry. One thousand four hundred fifty-two spontaneous central apneas (isolated or during periodic breathing) were analyzed in nonsedated lambs. Apneas, with high lung volume maintained by active glottal closure, were compared with apneas, with a tracheostomy opened at apnea onset. Oxygen desaturation slopes were lower when high-apneic lung volume was actively maintained during both wakefulness and quiet sleep. Furthermore, oxygen desaturation slopes were lower after isolated apneas with continuous thyroarytenoid EMG during wakefulness, compared with apneas with noncontinuous thyroarytenoid EMG (= glottis opened shortly after apnea onset). These results highlight the importance of maintaining high-alveolar oxygen stores during central apneas by active glottal closure to limit desaturation in newborns.

The postnatal development of ventilatory reflexes originating from bronchopulmonary receptors was assessed in preterm vs. full-term lambs. Ventilation and arterial pressure were repeatedly measured in 10 preterm (gestational age, 132 days) and 7 full-term lambs without sedation from day 1 to day 42. The Hering-Breuer inhibitory reflex (slowly adapting stretch receptors) was assessed by the increase in expiratory time during end-inspiratory occlusion. The pulmonary chemoreflex (C-fiber endings) was assessed by the initial apnea + bradycardia + systemic hypotension and the secondary tachypnea after capsaicin intravenous injection. Results show the following. 1) Premature birth did not modify the maturation of the Hering-Breuer reflex. 2) Whereas a classic pulmonary chemoreflex was observed in the very first hours of life in preterm lambs, the tachypneic component of this reflex was weaker than in full-term lambs on day 1. 3) Premature birth led to a reversed postnatal maturation of this tachypneic response (tendency to increase with postnatal age). Our findings suggest that premature birth in lambs modifies postnatal maturation of the pulmonary chemoreflex.

Surfactant protein (SP)-C is characterized by alpha-helix structure and palmitoyl groups attached to two cysteine residues. We examined the function of palmitoylation and dimerization in promotion of tidal volume in immature newborn rabbits. Reconstituted surfactants were made from a mixture of synthetic phospholipids and porcine SP-B (basic mixture) by adding various forms of SP-Cs: normal SP-C isolated from porcine lungs and monomeric or dimeric forms of SP-C. These latter two were isolated from patients with pulmonary alveolar proteinosis and were less palmitoylated. Animals were ventilated at an inspiratory pressure of 25 cmH2O. Median tidal volumes were <2 ml/kg in nontreated controls, 7.7 ml/kg in animals receiving the basic mixture without SP-C, and >18 ml/kg in animals treated with reconstituted surfactants containing 3% normal or 2% dimeric SP-C (P < 0.05 vs. basic mixture). The physiological effect of basic mixture was not improved by monomeric SP-C. We conclude that palmitoyl groups are important for the physiological effects of SP-C and that the dimeric form also improves physiological effects.

Prolonged fetal tracheal occlusion (TO) accelerates lung growth but leads to loss of alveolar epithelial type II (AE2) cells. In contrast, temporary TO leads to recovery of AE2 cells and their ability to produce surfactant. The aim of this study was to determine the effects of temporary TO in fetal sheep with lung hypoplasia on postnatal lung function, structure, and surfactant protein mRNA expression. Diaphragmatic hernia (DH) was created in 22 fetal sheep at 65 days of gestation. TO was performed between 110 days of gestation and full term (DH/TO, n = 7) and between 110 and 130 days of gestation (DH/TO+R, n = 6). Sham-operated fetuses (n = 11) served as controls. Lambs were delivered at approximately 139 days of gestation, and blood gas tensions were monitored over a 2-h resuscitation period. Temporary TO increased growth of the hypoplastic lung and restored surfactant protein mRNA expression and AE2 cell density but did not improve respiratory function above that of animals that underwent prolonged TO; DH/TO and DH/TO+R lambs were hypoxic and hypercapnic compared with Sham animals. Lung compliance remained low in DH/TO+R lambs, most likely as a consequence of the persistent increase in alveolar wall thickness in these animals.

Previously, we found that the rat pre-Bötzinger complex (PBC) exhibited reduced cytochrome oxidase (CO) activity on postnatal days (P) 3-4 and especially on P12, with a concomitant decrease in glutamate and N-methyl-d-aspartate receptor subunit 1, and an increase in GABA, GABA(B), glycine receptor, and glutamate subunit 2. We hypothesized that the PBC would be more affected by carotid body denervation (CBD) during the two critical windows than at other times. Pairs of CBD and sham animals at each postnatal day from P2 to P14 and at P21 were operated on and survived for 3 days. Brain stems were processed for CO and neurokinin-1 receptor for the identification of PBC. Results indicate that CBD caused a significant loss in body weight in all animals and a reduction in PBC somal size when the surgery was between P2 and P7. CBD also induced a significant decrease in CO activity of the PBC in most animals and a distinct delay, as well as prolongation of the maturational process, especially when induced close to P3 and P11-P13.

Acute ozone exposure evokes adverse respiratory responses, particularly in children. With repeated ozone exposures, however, despite the persistent lung inflammation and increased sensory nerve excitability, the central nervous system reflex responses, i.e., rapid shallow breathing and decreased lung function, adapt, suggesting changes in central nervous system signaling. We determined whether repeated ozone exposures altered the behavior of nucleus tractus solitarius (NTS) neurons where reflex respiratory motor outputs are first coordinated. Whole cell recordings were performed on NTS neurons in brain stem slices from infant monkeys exposed to filtered air or ozone (0.5 ppm, 8 h/day for 5 days every 14 days for 11 episodes). Although episodic ozone exposure depolarized the membrane potential, increased the membrane resistance, and increased neuronal spiking responses to depolarizing current injections (P < 0.05), it decreased the excitability to vagal sensory fiber activation (P < 0.05), suggesting a diminished responsiveness to sensory transmission, despite overall increases in excitability. Substance P, implicated in lung and NTS signaling, contributed to the increased responsiveness to current injections but not to the diminished sensory transmission. The finding that NTS neurons undergo plasticity with repeated ozone exposures may help to explain the adaptation of the respiratory motor responses.

The aim of the present study was to test whether breathing pattern conditioning may occur just after birth. We hypothesized that sensory stimuli signaling the resumption of maternal care after separation may trigger an arousal and/or orienting response accompanied with concomitant respiratory changes. We performed a conditioning experiment in 2-day-old mice by using an odor (lemon) as the conditioned stimulus (CS) and maternal care after 1 h without the mother as the unconditioned stimulus (US). Each pup underwent two acquisition trials, in which the CS was presented immediately before (experimental paired group, n = 30) or 30 min before (control unpaired group, n = 30) contact with the mother. Conditioning was tested by using noninvasive whole body plethysmography to measure the respiratory response to the CS for 1 min. We found significantly stronger respiratory responses to the CS in the experimental group than in the control group. In contrast, somatomotor activity did not differ significantly between groups. Our results confirm the sensitivity of breathing to conditioning and indirectly support the hypothesis that learned feedforward processes may complement feedback pathways during postnatal maturation of respiratory control.

Development of the mammalian respiratory control system begins early in gestation and does not achieve mature form until weeks or months after birth. A relatively long gestation and period of postnatal maturation allows for prolonged pre- and postnatal interactions with the environment, including experiences such as episodic or chronic hypoxia, hyperoxia, and drug or toxin exposures. Developmental plasticity occurs when such experiences, during critical periods of maturation, result in long-term alterations in the structure or function of the respiratory control neural network. A critical period is a time window during development devoted to structural and/or functional shaping of the neural systems subserving respiratory control. Experience during the critical period can disrupt and alter developmental trajectory, whereas the same experience before or after has little or no effect. One of the clearest examples to date is blunting of the adult ventilatory response to acute hypoxia challenge by early postnatal hyperoxia exposure in the newborn. Developmental plasticity in neural respiratory control development can occur at multiple sites during formation of brain stem neuronal networks and chemoafferent pathways, at multiple times during development, by multiple mechanisms. Past concepts of respiratory control system maturation as rigidly predetermined by a genetic blueprint have now yielded to a different view in which extremely complex interactions between genes, transcriptional factors, growth factors, and other gene products shape the respiratory control system, and experience plays a key role in guiding normal respiratory control development. Early-life experiences may also lead to maladaptive changes in respiratory control. Pathological conditions as well as normal phenotypic diversity in mature respiratory control may have their roots, at least in part, in developmental plasticity.

We hypothesized that administration of estradiol benzoate to males and testosterone propionate to female neonatal rat pups alters sex-specific ventilatory responses to aspartic acid with correspondent changes in N-methyl-D-aspartate receptor subunit 1 (NR1) expression determined by Western blot in specific brain regions. One-day-old rat pups received estradiol benzoate, testosterone propionate, or vehicle and were studied at weanling and adulthood. Different groups had distinct patterns of changes in tidal volume and frequency of breathing after aspartic acid administration. NR1 expression in hypothalamus was altered by age, sex, and treatment. Medullary and pontine NR1 expression correlated with baseline ventilation and magnitude of the ventilatory response to aspartic acid in some groups. Thus 1) tidal volume and breathing frequency patterns in response to aspartic acid are gender, age, and treatment dependent; 2) sex, age, and exogenous steroid hormones affect NR1 expression primarily in the hypothalamus; and 3) there is correlation between NR1 expression in pons and medulla with ventilatory parameters.

Gasping is a critically important mechanism for autoresuscitation and survival during extreme tissue hypoxia. Evidence of antecedent hypoxia in sudden infant death syndrome suggests that intermittently occurring hypoxic episodes may modify gasping and autoresuscitation. To examine this issue, an intermittent hypoxia (IH) profile consisting of alternating room air and 10% O(2)-balance N(2) every 90 s was applied to pregnant Sprague-Dawley rats (IHRA; n = 50) and to pups after a normal pregnancy (RAIH; n = 50) as well as to control pups (RARA; n = 50). At postnatal day 5, pups were exposed to 95% N(2)-5% CO(2), and gasping and the ability to autoresuscitate were assessed. Compared with RARA, IHRA- and RAIH-exposed pups had a reduced number of gasps, decreased overall gasp duration, and were less likely to autoresuscitate on introduction of room air to the breathing mixture during the last phase of gasping (P < 0.001 vs. RARA). We conclude that both prenatal and early postnatal IH adversely affect gasping and related survival mechanisms.

The objective of the present study was to test the hypothesis that in neonatal piglets there would be no hypoventilation after sham denervation or aortic denervation (AOD) alone, but there would be transient hypoventilation after carotid body denervation (CBD) and the hypoventilation would be greatest after combined carotid and aortic denervation (CBD+AOD). There was a significant (P < 0.05) hypoventilation in CBD and CBD+AOD piglets denervated at 5, 15, and 25 days of age. The hypoventilation in CBD+AOD piglets denervated at 5 days of age was greater (P < 0.05) than that of all other groups. Conversely, sham-denervated and AOD piglets did not hypoventilate after denervation. Injections of sodium cyanide showed that aortic chemoreceptors were a site of recovery of peripheral chemosensitivity after CBD. This aortic sodium cyanide response was abolished by prior injection of a serotonin 5a receptor blocker. Residual peripheral chemosensitivity after CBD+AOD was localized to the left ventricle. We conclude that 1) aortic chemoreceptors contribute to eupneic breathing in piglets that were carotid denervated at 5 days of age and 2) there are multiple sites of residual peripheral chemosensitivity after CBD.

Pulmonary hypertension and blunted pulmonary vascular responses to ACh develop when newborn pigs are exposed to chronic hypoxia for 3 days. To determine whether a cyclooxygenase (COX)-dependent contracting factor, such as thromboxane, is involved with altered pulmonary vascular responses to ACh, newborn piglets were raised in 11% O(2) (hypoxic) or room air (control) for 3 days. Small pulmonary arteries (100-400 microm diameter) were cannulated and pressurized, and their responses to ACh were measured before and after either the COX inhibitor indomethacin; a thromboxane synthesis inhibitor, dazoxiben or feregrelate; or the thromboxane-PGH(2)-receptor antagonist SQ-29548. In control arteries, indomethacin reversed ACh responses from dilation to constriction. In contrast, hypoxic arteries constricted to ACh before indomethacin and dilated to ACh after indomethacin. Furthermore, ACh constriction in hypoxic arteries was nearly abolished by either dazoxiben, feregrelate, or SQ-29548. These findings suggest that thromboxane is the COX-dependent contracting factor that underlies the constrictor response to ACh that develops in small pulmonary arteries of piglets exposed to 3 days of hypoxia. The early development of thromboxane-mediated constriction may contribute to the pathogenesis of chronic hypoxia-induced pulmonary hypertension in newborns.

Muscle fiber length adaptation to static stretch or shortening depends on age, with sarcomere addition in young muscle being dependent on mobility. Series sarcomere number can also increase in young animals in response to increased muscle excursion, but it is not clear whether adult muscles respond similarly. The ankle flexor retinaculum was transected in neonatal and adult rats to increase tibialis anterior muscle excursion. Sarcomere number in tibialis anterior was determined after 8 wk of adaptation. Muscle moment arm and excursion were increased 30% (P < 0.01) in both age groups. Muscle cross-sectional area was reduced by 12% (P < 0.01) in response to the increased mechanical advantage, and this reduction was unaffected by age. Fiber length change was also unaffected by age, with both groups showing a trend (P < 0.10) for slightly (6%) increased fiber length. Retinaculum transection results in shorter muscle length in all joint configurations, so this trend opposes the fiber length decrease predicted by an adaptation to muscle length and indicates that fiber length is influenced by dynamic mechanical signals in addition to static length.

Nicotine exposure modifies the expression of catecholamine and opioid neurotransmitter systems involved in attenuation of hypoxic chemosensitivity. We used in situ hybridization histochemistry to determine the effect of prenatal and early postnatal nicotine exposure on tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DbetaH), preproenkephalin (PPE), and D2-dopamine receptor mRNA levels in the rat carotid body and petrosal ganglion during postnatal development. In the carotid body, nicotine increased TH mRNA expression in animals at 0 and 3 postnatal days (both, P < 0.05 vs. control) without affecting TH mRNA levels at 6 and 15 days. At 15 postnatal days, DbetaH mRNA levels were increased in the carotid body of nicotine-exposed animals. Dopamine D2-receptor mRNA levels in the carotid body increased with postnatal age but were unaffected by nicotine exposure. PPE was not expressed in the carotid body at any of the ages studied in control or treated animals. In the petrosal ganglion, nicotine increased the number of ganglion cells expressing TH mRNA in animals at 3 days (P < 0.01 vs. control). DbetaH mRNA expression was not induced nor was PPE mRNA expression increased in the petrosal ganglion in treated animals. Prenatal nicotine exposure upregulates mRNAs involved in the synthesis of two inhibitory neuromodulators, dopamine and norepinephrine, in peripheral arterial chemoreceptors, which may contribute to abnormalities in cardiorespiratory control observed in nicotine exposed animals.

Recently, Wong et al. (Wong KA, Bano A, Rigaux A, Wang B, Bharadwaj B, Schurch S, Green F, Remmers JE, and Hasan SU, J Appl Physiol 85: 849-859, 1998) demonstrated that fetal lambs that have undergone vagal denervation prenatally do not establish adequate alveolar ventilation shortly after birth. In their study, however, vagal denervation was performed prenatally and the deleterious effects of vagal denervation on breathing patterns and gas exchange could have resulted from the prenatal actions of the neurotomy. To quantify the relative roles of pre- vs. postnatal vagal denervation on control of breathing, we studied 14 newborn lambs; 6 were sham operated, and 8 were vagally denervated below the origin of the recurrent laryngeal nerve. Postoperatively, all denervated animals became hypoxemic and seven of eight succumbed to respiratory failure. In vagally denervated lambs, expiratory time increased, whereas respiratory rate, minute ventilation, and lung compliance decreased compared with the sham-operated animals. In the early postoperative period, the frequency of augmented breaths was lower but gradually increased over time in the denervated vs. sham-operated group. The dynamic functional residual capacity was significantly higher than the passive functional residual capacity among the sham-operated group compared with the denervated group. No significant differences were observed in the prevalence of various sleep states and in the amount of total phospholipids or large- and small-aggregate surfactants between the two groups. We provide new evidence indicating that intrauterine actions of denervation are not required to explain the effects of vagal denervation on postnatal survival. Our data suggest that vagal input is critical in the maintenance of normal breathing patterns, end-expiratory lung volume, and gas exchange during the early neonatal period.