Ontogeny of sleep/wake and cardiorespiratory behavior in unanesthetized piglets

Improving pregnancy outcomes in humans through studies in sheep

Experimental studies that are relevant to human pregnancy rely on the selection of appropriate animal models as an important element in experimental design. Consideration of the strengths and weaknesses of any animal model of human disease is fundamental to effective and meaningful translation of preclinical research. Studies in sheep have made significant contributions to our understanding of the normal and abnormal development of the fetus. As a model of human pregnancy, studies in sheep have enabled scientists and clinicians to answer questions about the etiology and treatment of poor maternal, placental, and fetal health and to provide an evidence base for translation of interventions to the clinic. The aim of this review is to highlight the advances in perinatal human medicine that have been achieved following translation of research using the pregnant sheep and fetus.

Intermittent hypercapnic hypoxia effects on the nicotinic acetylcholine receptors in the developing piglet hippocampus and brainstem

This study investigated the effects of acute (1 day) vs repeated (4 days) exposure to intermittent hypercapnic hypoxia (IHH) on the immunohistochemical expression of α2, α3, α5, α7, α9 and β2 nicotinic acetylcholine receptor (nAChR) subunits in the developing piglet hippocampus and brainstem medulla, and how prior nicotine exposure alters the response to acute IHH. Five piglet groups included: 1day IHH (1D IHH, n=9), 4days IHH (4D IHH, n=8), controls exposed only to air cycles for 1day (1D Air, n=6) or 4days (4D Air, n=5), and pre-exposed to nicotine for 13days prior to 1day IHH (Nic+1D IHH, n=7). The exposure period alternated 6min of HH (8%O₂, 7%CO₂, balance N₂) and 6min of air over 48min, while controls were switched from air-to-air. Results showed that: 1. repeated IHH induces more changes in nAChR subunit expression than acute IHH in both the hippocampus and brainstem medulla, 2. In the hippocampus, α2 and β2 changed the most (increased) following IHH and the CA3, CA2 and DG were mostly affected. In the brainstem medulla, α2, α5, α9 and β2 were changed (decreased) in most nuclei with the hypoglossal and nucleus of the solitary tract being mostly affected. 3. Pre-exposure to nicotine enhanced the changes in the hippocampus but dampened those in the brainstem medulla. These findings indicate that the nAChRs (predominantly with the α2/β2 complex) are affected by IHH in critical hippocampal and brainstem nuclei during early brain development, and that pre-exposure to nicotine alters the pattern of susceptibility to IHH.

Cumulative effects of repetitive intermittent hypercapnic hypoxia on orexin in the developing piglet hypothalamus

Orexin neuropeptides (OxA and OxB) and their receptors (OX1R and OX2R) are involved in maintenance of sleep and wakefulness, and are regulated by various environmental stimuli. We studied piglets, in the early neonatal period, exposed to 48-min of intermittent hypercapnic hypoxia (IHH; 7% O2/8% CO2) alternating with air. Three groups of 13-14 day-old piglets with IHH exposure of 1-day (1D-IHH) (n=7), 2-days (2D-IHH) (n=7) and 4-days (4D-IHH) (n=8) were compared to controls (exposed only to air, n=8). Immunoreactivity of OxA and OxB was studied in the piglet hypothalamic regions of the dorsomedial hypothalamus (DMH), perifornical area (PeF) and lateral hypothalamic area (LH). Results showed that after 1D- and 2D-IHH, total OxA and OxB expression decreased by 20% (p ≤ 0.005) and 40% (p<0.001), respectively. After 4D-IHH, the decrease in OxA and OxB was 50% (p<0.001). These findings indicate that a chronic IHH exposure induces greater changes in orexin neuropeptide expression than an acute 1-day exposure in the hypothalamus. This may be causally related to the dysregulation of sleep.

Ventilatory response to hypercapnia and hypoxia after extensive lesion of medullary serotonergic neurons in newborn conscious piglets

Acute inhibition of serotonergic (5-HT) neurons in the medullary raphé (MR) using a 5-HT(1A) receptor agonist had an age-dependent impact on the "CO(2) response" of piglets (33). Our present study explored the effect of chronic 5-HT neuron lesions in the MR and extra-raphé on the ventilatory response to hypercapnia and hypoxia in piglets, with possible implications on the role of 5-HT in the sudden infant death syndrome. We established four experimental groups. Group 1 (n = 11) did not undergo any treatment. Groups 2, 3, and 4 were injected with either vehicle or the neurotoxin 5,7-dihydroxytryptamine in the cisterna magna during the first week of life (group 2, n = 9; group 4, n = 11) or second week of life (group 3, n = 10). Ventilation was recorded in response to 5% CO(2) (all groups) and 12% O(2) (group 2) during wakefulness and sleep up to postnatal day 25. Surprisingly, the piglets did not reveal changes in their CO(2) sensitivity during early postnatal development. Overall, considerable lesions of 5-HT neurons (up to 65% decrease) in the MR and extra-raphé had no impact on the CO(2) response, regardless of injection time. Postlesion raphé plasticity could explain why we observed no effect. 5,7-Dihydroxytryptamine-treated males, however, did present a lower CO(2) response during sleep. Hypoxia significantly altered the frequency during sleep in lesioned piglets. Further studies are necessary to elucidate the role of plasticity, sex, and 5-HT abnormalities in sudden infant death syndrome.

Comparative anatomical assessment of the piglet as a model for the developing human medullary serotonergic system

Because the piglet is frequently used as a model for developmental disorders of the medullary serotonergic (5-HT) system in the human infant, this review compares the topography and developmental profile of selected 5-HT markers between humans in the first year of life and piglets in the first 60 days of life. The distribution of tryptophan hydroxylase-immunoreactive 5-HT neurons in the human infant medulla is very similar, but not identical, to that in the piglet. One notable difference is the presence of compact clusters of 5-HT neurons at the ventral surface of the piglet medulla. While it lacks these distinctive clusters, the human infant medulla contains potentially homologous 5-HT neurons scattered along the ventral surface embedded in the arcuate nucleus. Each species shows evidence of age-related changes in the 5-HT system, but the changes are different in nature; in the human infant, statistically significant age-related changes are observed in the proportional distribution of medullary 5-HT cells, while in the piglet, statistically significant age-related changes are observed in the levels of 5-HT receptor binding in certain medullary nuclei. Analyses of 5-HT receptor binding profiles in selected nuclei in the two species suggest that the equivalent postnatal ages for 5-HT development in piglets and human infants are, respectively, 4 days and 1 month, 12 days and 4 months, 30 days and 6 months, and 60 days and 12 months. Collectively, when certain species differences are considered, these data support the use of the piglet as a model for the human infant medullary 5-HT system.

Effects of Prior Hypoxia Exposure, Endotoxin and Sleep State on Arousal Ability to Airway Obstruction in Piglets: Implications for Sudden Infant Death Syndrome

BACKGROUND

Respiratory tract infections may be an important component in many deaths attributed to sudden infant death syndrome (SIDS), although the mechanism of involvement remains unclear.

OBJECTIVES

The hypothesis was tested that prolonged hypoxia and a thermogenic state (simulating a fever due to respiratory tract infection) would impair respiratory responsiveness to airway obstruction during sleep.

METHODS

Thirty nine piglets aged 5-7 days were exposed to 24 h of moderate hypoxia and/or a low dose of endotoxin derived from Salmonella abortus equi. Responsiveness to complete and subtotal upper airway obstruction was tested during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The end-point for airway obstruction tests was taken as the first protective response, either arousal or initiation of mouth breathing. Responsiveness was assessed as response time and response threshold (measured as respiratory effort, i.e. esophageal pressure swing).

RESULTS

All animals demonstrated a thermogenic state following endotoxin delivery (drop in ear temperature of 5.8 +/- 0.2 degrees C and a small but significant increase in rectal temperature). Response time to subtotal airway obstruction was reduced during the heat conserving phase of the fever (thermogenesis; 2.8 +/- 0.5 s compared to 4.3 +/- 0.7 s during pre-endotoxin tests), but markedly increased during the recovery period (20.3 +/- 5.1 compared to 14.0 +/- 2.5 s pre-endotoxin) in NREM sleep. Response threshold was not significantly affected by either endotoxin or hypoxia in NREM sleep. Respiratory responsiveness to subtotal obstruction was markedly reduced during REM sleep (response time 40.3 +/- 10.9 s compared to 14.7 +/- 2.2 s in NREM; response threshold -14.0 +/- 1.3 mm Hg compared to -11.7 +/- 1.0 mm Hg in NREM).

CONCLUSIONS

This study has demonstrated in a neonatal animal model that respiratory responsiveness to airways obstruction is delayed during recovery from fever. The findings may have implications for the human infant recovering from a respiratory illness.

Effects of postnatal nicotine exposure on apoptotic markers in the developing piglet brain

Exposure to cigarette smoke is a risk factor for the sudden infant death syndrome (SIDS), but the ability to distinguish between the neuropathological effects of pre- versus postnatal exposure is limited in the clinical setting. To test whether postnatal nicotine exposure could contribute to the increased neuronal expression of apoptotic markers that we have previously observed in SIDS infants, as well as including study of gender influences, we developed a piglet model to mimic passive smoking in the early postnatal period. Piglets were exposed to nicotine (2 mg/kg/day infused via an implanted osmotic minipump) within 48 h of birth until the age of 13-14 days, when the brain was collected for study. Four piglet groups included: control females (n=7), control males (n=7), nicotine females (n=7), and nicotine males (n=7). Apoptotic markers included immunohistochemistry for activated caspase-3, and for DNA fragmentation or terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling (TUNEL) in seven nuclei of the brainstem caudal medulla and two subregions of the hippocampus (CA4 and dentate gyrus). Among control females compared with males, there was less active caspase-3 and less TUNEL in the dorsal motor nucleus of vagus (DMNV), and there was less TUNEL in the nucleus of the spinal trigeminal tract (NSTT). Compared with controls, nicotine-exposed male piglets had increased TUNEL staining in the cuneate nucleus (P=0.05), and increased active caspase-3 in the hypoglossal, gracile and dentate gyrus (P<0.05 for each). Nicotine-exposed females showed no change in TUNEL staining in any of the nuclei studied, but increased active caspase-3 in the hypoglossal, DMNV and NSTT (P<0.05 for each). These results show for the first time that postnatal nicotine exposure can lead to an increase in apoptotic markers in the brain. In piglets, these effects showed regional and gender-specific differences, suggesting that passive, postnatal nicotine exposure may be responsible for some neuropathological changes observed in infants dying from SIDS.

Habituation of arousal responses after intermittent hypercapnic hypoxia in piglets

RATIONALE

Clinical studies have demonstrated arousal deficits in infants suffering obstructive sleep apnea, and some infant deaths have been attributed to such an arousal deficit.

OBJECTIVES

To evaluate whether arousal deficits can be induced by intermittent asphyxia during normal development.

METHODS AND MEASUREMENTS

Young piglets were exposed to intermittent hypercapnic hypoxia for 4 days from age 9.55 +/- 0.5 days. Arousal responses were compared between control animals and animals exposed to intermittent hypercapnic hypoxia. Outcome measures included time to arouse after onset of the respiratory stimulus and frequency of arousals during recovery.

MAIN RESULTS

Arousal deficits emerged after successive exposures to hypercapnic hypoxia on Day 1, and were exacerbated on Day 4, although after overnight recovery, the deficit only became evident during the second and subsequent episode of hypercapnic hypoxia. On Day 1, time to arouse increased from 16.9 +/- 7.1 seconds in the first epoch to 41.7 +/- 28.6 seconds in the fourth epoch (p = 0.004 between cycles, one-way analysis of variance). In the recovery periods after hypercapnic hypoxia, there were 64% fewer arousals than baseline on Day 1 and 90% fewer arousals on Day 4. Respiratory effort, measured by VT across 10 breaths before the arousal, increased from 25.7 +/- 7.6 on Day 1 to 29.1 +/- 6.8 ml/kg on Day 4 (p < 0.001, two-way analysis of variance, Day 4 vs. Day 1, respectively).

CONCLUSIONS

These studies demonstrate that acute and chronic arousal deficits can be induced by intermittent asphyxia, on a background of otherwise normal postnatal development.

Brain-derived neurotrophic factor mRNA and protein in the piglet brainstem and effects of Intermittent Hypercapnic Hypoxia

Brain-derived neurotrophic factor (BDNF) is a neurotrophin essential for the development of normal respiratory rhythm and ventilatory control. Chronic exposure to Intermittent Hypercapnic Hypoxia (IHH) has been shown to alter ventilatory responses of piglets. This study investigated changes in BDNF distribution and expression in seven nuclei of the caudal medulla, from piglets exposed to IHH for 1, 2, 3, or 4 days before death, using non-radioactive in situ hybridisation (for mRNA) and immunohistochemistry (for protein). Compared to controls, BDNF mRNA was markedly increased across the entire medulla of the brainstem, after all durations of IHH (1-4 days). In contrast, BDNF protein expression increased after 1 day of exposure to IHH (p=0.003), but, thereafter, was not different to controls. Amongst individual nuclei, neurons of the dorsal motor nucleus of the vagus (DMNV) showed increased BDNF mRNA (p<0.01), but decreased protein expression (p=0.05) after all durations of IHH. In the ION, both mRNA and protein for BDNF were significantly increased after 1 day IHH (p<0.01 and p=0.001, respectively), but these increases were not sustained. This study is the first to investigate changes in BDNF expression in response to environmental challenges during postnatal development in the brainstem. Implications of the wide distribution of BDNF in the piglet caudal medulla and increased expression after IHH exposure are discussed, with particular reference to roles for BDNF-dependent neurons at this stage of development.

Respiratory responses to intermittent hypoxia in unsedated piglets: relation to substance P binding in brainstem

Respiratory responses to single intermittent hypoxia (5 min 21% O(2), 5 min 8% O(2) X6) in 5-6, 10-11, 21-22 and 26-27 day-old piglets, and to recurrent six daily intermittent hypoxia in 10-11 and 26-27 day-old piglets were assessed. Substance P binding in the piglets' brainstem immediately after the last hypoxic episode was measured. All piglets hyperventilated during hypoxia. Weight adjusted inspired ventilation, tidal volume and instantaneous flow decreased with age. The oldest piglets uniquely displayed attenuated ventilation and tidal volume during the sixth versus first hypoxic episode with single intermittent hypoxia, and reduced inspired ventilation and tidal volume during the first hypoxic episode on the sixth daily hypoxia compared to single hypoxia. By contrast, substance P binding was greatly reduced in the solitary, hypoglossal, paraambigual and lateral reticular brainstem nuclei of both younger and older piglets following either single or recurrent intermittent hypoxia. Thus, the reduction in membrane-bound neurokinin receptors by intermittent hypoxia, presumably consequent to endogenously released substance P, does not exclusively determine whether the ventilatory response to that hypoxia will be attenuated or not.

Endotoxin effects on markers of autonomic nervous system function in the piglet: implications for SIDS

The hypothesis was tested in 30 newborn piglets that the effects of a low dose of endotoxin (1 microg i.v. bolus; Salmonella abortus equi) would impair autonomic nervous system function. Two tests of autonomic function were performed following external warming (pre-endotoxin) and during endotoxin-generated thermogenesis: (1) analysis of heart rate variability in the time and frequency domains and (2) baroreflex sensitivity measured following intravenous injection of the vasoactive drugs nitroprusside and phenylephrine. Beat-to-beat heart rate variability (SDDeltaRR) fell by 2.2 ms from 7.0 ms before fever (p < 0.05). Low-frequency spectral power fell by 2.4 ms(2) from 4.1 ms(2) before fever (p < 0.05). The sensitivity of the baroreflex to changes in blood pressure induced by the vasoactive drugs decreased during fever by 0.72 ms/mm Hg for the nitroprusside test (p < 0.0005) and by 0.31 ms/mm Hg for the phenylephrine test (p < 0.005). These results indicate that in the piglet the balance of autonomic tone is altered and autonomic responsiveness reduced during the thermogenic phase of a fever. These findings are consistent with known risk factors for sudden infant death syndrome.

Increased neuronal cell death after intermittent hypercapnic hypoxia in the developing piglet brainstem

We examined the immunohistochemical expression of caspase-3 (CASP3), active caspase-3 and TUNEL in the normal piglet brainstem at 13-14 days of age and evaluated the effects of exposure to 2 vs. 4 days of intermittent hypercapnic hypoxia (IHH) on their expression. Eight nuclei from the level of the caudal medulla were studied. In control piglets, CASP3 was present in approximately 45% of neurons while active caspase-3 and TUNEL were present in approximately 5%, indicating that approximately half the neuronal population of the piglet medulla express caspase-3 in a latent state and that only approximately 5% undergo 'normal' programmed cell death. After 2 days of IHH, CASP3 increased in the nucleus of the solitary tract (NTS), gracile and cuneate nuclei (P<0.05 for all). Active caspase-3 increased in the dorsal motor nucleus of the vagus (DMNV) (P<0.05) but decreased in the lateral reticular nucleus (LRt) (P<0.05), while TUNEL increased in both the DMNV and LRt (P<0.05 for both). After 4 days of IHH, CASP3 remained elevated in the cuneate nucleus (P<0.01) but decreased in the hypoglossal and DMNV (P<0.05) when compared to controls. Active caspase-3 levels were not changed, whereas TUNEL was increased in the DMNV, LRt, and inferior olivary nucleus (P<0.05 for all). These results show that IHH induces neuronal cell death within certain nuclei in the piglet caudal medulla that are functionally important in cardiorespiratory, sleep and arousal control. This could have important implications for clinical conditions including obstructive apnea and prone sleeping as a risk for SIDS.

Responses to hypoxia during early development

Sustained hypoxia evokes a predictable cascade of ventilatory, neurochemical, and metabolic responses. Responses in immature animals are characterized by earlier and more marked depression of ventilation than fully mature animals. Ventilation during hypoxia reflects a collective system output, incorporating a number of compensatory mechanisms (stimulation or depression) from multiple systems. The time course of these responses is clearly developmentally regulated. When hypercapnia interacts with hypoxia, the ventilatory responses are enhanced but other responses are apparently unchanged. We propose a model in which responses to intermittent stimuli vary according to the point within the sequence of a single response where the stimulus interruption occurs. An intermittent stimulus may be seen as 'continuous' if the recurrence frequency exceeds a certain threshold, whereas application of slower cycles below such threshold may elicit discordant recruitment of the compensatory responses. Indeed, experimental observations on intermittent (hypercapnic or poikylocapnic) hypoxia show excitatory or depressant effects that are dictated by the cycle duration. Subject to further testing, this model may help explain how detrimental effects of hypoxic events in infancy only affect selected groups.

Effect of stimulus cycle time on acute respiratory responses to intermittent hypercapnic hypoxia in unsedated piglets

To determine whether stimulus frequency affects physiological compensation to an intermittent respiratory stimulus, we studied piglets (n = 43) aged 14.8 +/- 2.4 days. A 24-min total hypercapnic hypoxia (HH) (10% O(2)-6% CO(2)-balance N(2) = HH) was delivered in 24-, 8-, 4-, or 2-min cycles alternating with air. Controls (n = 10) breathed air continuously. Minute ventilation and temperature were not different between the 2-min and 24-min groups, with neither different from controls during recovery. Piglets exposed to 8-min cycles had ventilatory stimulation, whereas those exposed to 4-min cycles had significant depression of ventilation. Despite this, piglets in these intermediate intermittent HH (IHH) groups (8- and 4-min cycles) showed more severe acidosis and attenuated temperature changes (P < 0.001 and P < 0.01 for pH and temperature vs. 24 min, respectively). Cycle time affected the ability of young piglets to tolerate IHH. More severe respiratory acidosis developed when IHH was delivered in intermediate (4 min or 8 min) cycles compared with the same total dose as a single episode or in short (2 min) cycles.

Maturation of respiratory control in the behaving mammal

Whereas in vitro techniques have contributed greatly to our understanding of detailed neuronal mechanisms of respiratory control, the integrated function of respiratory behavior requires studying conscious, unsedated subjects. Noninvasive approaches, meticulous chronic instrumentation for the recording of multiple respiratory indices, and correlations with brain studies performed after physiological manipulations in vivo can all be employed to get to some understanding of the maturation of respiratory control in the mammal. This article is a selective and critical overview of recent literature on methodologies that can be used in behaving subjects, the relationship of respiration to sleep-wake states, respiratory patterns during normoxia, and on respiratory responsiveness to hypercarbia and hypoxia, all emphasizing processes during development. It is hoped that this review will encourage new investigators interested in the regulation of breathing to resort to experimental approaches that will reveal the mysteries of respiratory behavior in the integrated organism.

Central neuropeptide systems and respiratory control during development

The substance P/neurotachykinin-1 (NK-1) and the mu-opioid G protein-coupled receptor systems endow brainstem respiratory regions and display discrete developmental patterns. Hypoxia-induced neuropeptide release may increase receptor endocytosis, reducing receptor accessibility to ligands. We wondered whether the attenuated respiratory response to hypoxia of developing piglets after single (Respir. Physiol. 92 (1993a) 115) or repeated daily hypoxic exposure (J. Appl. Physiol. 83 (1997) 522) is influenced by differential endocytosis of NK-1 vs mu-opioid receptors. Whereas the long-term (24 h) response of both receptors to recurrent hypoxia in piglet brainstem is similar, i.e. upregulation, the short-term (5 min) response to single or recurrent hypoxia, albeit in rats, is different: radiolabelled NK-1 receptors are greatly reduced, suggesting enhanced endocytosis, but mu-opioid receptors remain unchanged, implying unaltered endocytosis. If confirmed in piglet brainstem, this difference would produce relatively more available mu-opioid receptors to opioid peptides in hypoxia that might contribute to the attenuated respiratory responses to single and repeated hypoxia during development.

Effects of age and clustered hypoxia on [(125)I] substance P binding to neurotachykinin-1 receptors in brainstem of developing swine

This work focused on the postnatal development of substance P-bound neurotachykinin-1 (NK-1) receptors in the porcine brainstem using 2-3-, 6-11-, 16-18-, and 21-28-day-old piglets versus adult, and on alterations in these receptors after single and six-daily repeated clustered hypoxia using 6-11- and 21-28-day-old piglets. NK-1 receptor localization and densities were determined by quantitative autoradiography using mono-iodinated Bolton-Hunter substance P ([(125)I]BHSP). Slide-mounted brainstem sections, incubated in [(125)I]BHSP and then exposed to film, have shown [(125)I]BHSP binding throughout many brainstem nuclei and tracts, including the ambigual/periambigual (nAmb), dorsal motor vagal (dmnv), gigantocellular (nGC), hypoglossal (nHyp), medial parabrachial (nPBM), lateral reticular (nRL), raphe magnus (nRMg), raphe obscurus (nROb) and solitary tract (nTS) nuclei. NK-1 receptor densities decreased with age. As compared to normoxia, NK-1 receptor densities increased significantly after the six-daily hypoxia protocol in nAmb, dmnv, nHyp, nRL, nRMg, nROb, and nTS of both the young and older age groups. This increase may represent receptor upregulation as an adaptation to repeated hypoxia.

Depression of ventilatory responses after daily, cyclic hypercapnic hypoxia in piglets

Ventilatory responses (VRs) were measured via a sealed face mask and pneumotachograph in 30 unsedated, mixed-breed miniature piglets at 12.6 +/- 2.3 days of age (day 1) and then repeated after seven daily 24-min exposures to 10% O(2)-6% CO(2) [hypercapnic hypoxia (HH)]. Arterial blood was sampled at baseline, after 10 min of exposure, and after 10 min of recovery. VRs included hypoxia (10% O(2) in N(2)), hypercapnia (6% CO(2) in air), and HH (10% O(2)-6% CO(2)-balance N(2)). Treatment groups (n = 10 each) were exposed to 24 min of HH from day 2 to 8 as sustained HH (24 min of HH and then 24 min of air) or cyclic HH (4 min of HH alternating with 4 min of air). Day 1 and 9 data were compared in treatment and control groups. After cyclic HH, respiratory responses to CO(2) were reduced during hypercapnia and during HH (P < 0.001 vs. control for minute ventilation in both). In both treatment groups, time to peak minute ventilation was delayed in hypoxia (P = 0.02, ANOVA), and response amplitude was increased (P < 0.001 and P = 0.003, sustained and cyclic HH, respectively, vs. control). Respiratory pattern was also altered during the VRs and among treatment groups. Stimulus presentation characteristics exert effects on VRs that are independent of those elicited by daily HH.

Respiratory responses to single and episodic hypoxia during development: mechanisms of adaptation

The respiratory responses of the developmental subject to single and repeated episodes of hypoxia are distinct. During a single exposure, the fetus responds with an arrest of breathing activity, and the neonate, with excitation followed by depression (the biphasic response). Mechanisms under active consideration include chemosensory resetting, hypometabolism, prevalence of inhibitory neurotransmitter/modulator influence, and supramedullary regulation of control functions. When exposed to recurrent episodic hypoxia, neonates respond with relative hypoventilation, i.e. tolerance to a subsequent hypoxic stimulus. Whereas the investigation of processes responsible for this tolerance is at its infancy, studies using chronic hypoxia appear to be a useful guide. So far, altered interstitial neuromodulator levels and central markers of programmed neuronal death are harbingers of future research in this field. The clarification of the mechanisms involved in response to recurrent episodic hypoxia during development will be of fundamental value and may be useful for the eventual treatment and/or prevention of harmful central respiratory-related processes.

Repeated prenatal cocaine increases met-enkephalin immunoreactivity in respiratory-related medulla of developing swine

Repeated prenatal exposure to cocaine is associated with attenuated respiratory and arousal responses in infants and piglets. As the normal development of these functions is influenced by medullary opioid systems, the present study explored the possible contribution of the opioid systems to the attenuation induced by cocaine. Methionine-enkephalin (met-enkephalin), an endogenous opioid peptide, was delineated by immunocytochemistry in respiratory- and arousal-related medullary regions of relatively immature (6-7-day-old) and more mature piglets (20-21-day-old). The animals were either unexposed, or exposed prenatally to 2 mg/kg cocaine four times daily administered to the pregnant sows intravenously throughout the last third of gestation. At control, met-enkephalin was found in the neurons, fibers and terminals of the respiratory- and arousal-related medullary regions throughout the age range studied. Prenatal cocaine exposure increased met-enkephalin immunoreactivity in the respiratory-related hypoglossal and solitary tract nuclei of both age groups. These findings support a modulatory role of met-enkephalin in the normal development of respiratory control, and an involvement of this peptide in the attenuation of respiration by repeated prenatal exposure to cocaine.

Age-dependent metabolic effects of repeated hypoxemia in piglets

The aim of this study was to determine whether repeated exposure to hypoxemia would modify the response to hypoxemia during maturation. We exposed piglets to three 1-h cycles of hypoxemia (PaO2 = 30 to 35 mmHg; 1 mmHg = 133.3 Pa) at 1 week (n = 9), 2-3 weeks (n = 10), and 4-5 weeks of age (n = 10). O2 consumption (VO2) and CO2 production (VCO2) were measured, and alveolar ventilation (VA) was derived from VCO2 and PaCO2. Levels of lactic acid (lactate) and serum catecholamines were also measured. With hypoxemia, time had a significant effect on VO2 and body temperature in an age-dependent fashion: that is, whereas the 1 week group and the 4-5 week group showed both variables decreasing over time, the 2-3 week group showed no drop in VO2 and a small increase in body temperature over time. Lactate levels increased with hypoxemia in all animals during the first exposure. However, with repeated exposures to hypoxemia, only the 2-3 week group continued to increase its lactate levels. Furthermore, the changes in lactate levels paralleled the changes in epinephrine levels with hypoxemia. We found, too, that although VA increased significantly with hypoxemia in all animals, this change was not modified by age or repeated exposures. No significant effects of age or repeated exposures were found in the cardiovascular response to hypoxemia. We concluded that, from a metabolic viewpoint, after repeated exposures to hypoxemia the 2-3 week animals responded differently.Key words: metabolic rate, lactic acid, maturation, catecholamines.

The effect of cold stimulation to the face on the metabolic rate of the febrile piglet

Sudden infant death syndrome has been associated with winter climates, infection, and overwrapping of babies. The hypothesis has been tested in this laboratory that two different causes of increased metabolic rate, high core temperature (via the van't Hoff or 'Q10' effect) and face-cooling, might synergistically induce hyperthermia. This proved not to be the case. We now report on a 'febrile' state adding Salmonella abortus equi pyrogens. The combination of face-cooling and pyrogen administration to 14 already hot piglets produced an increase in oxygen consumption of 47% in 6 of the animals (19% overall). Face-cooling alone caused a 6.5% fall in oxygen consumption, and injection of pyrogens alone had no effect on oxygen consumption. We conclude that there may be a danger of life-threatening hyperthermia in the combination of a cold face and febrile state.

Intracellular recording from posterior cricoarytenoid motoneurons in the rat

The ability to maintain coordinated vocal cord abduction and upper airway patency is dependent on the integrity of the posterior cricoarytenoid (PCA) motoneurons and their multiple neural connections. Study of the PCA motoneurons represents the initial step in understanding the complex mechanisms responsible for coordinated vocal cord abduction and may provide an insight into the possible pathological processes underlying the various clinical presentations of vocal cord dysfunction. Intracellular recordings were made from 11 PCA motoneurons in Sprague-Dawley rats, which all showed an inspiratory augmenting discharge pattern that is also characteristic of phrenic nerve activity. The resting membrane potential was -56+/-11 mV. Two PCA motoneurons were injected with Neurobiotin to demonstrate neuronal morphology, which was found to be similar to that obtained by retrograde labeling with cholera toxin B subunit. The technique described for intracellular recording of PCA motoneurons should allow more detailed morphological, electrophysiological, and immunohistochemical information to be obtained, to thereby identify some of the factors responsible for maintaining normal function of the PCA muscle.

Identification of posterior cricoarytenoid motoneurons in the rat

The posterior cricoarytenoid (PCA) muscle is the sole abductor of the larynx and is controlled by motoneurons located in the nucleus ambiguus. These motoneurons receive inputs from a variety of interneurons, including those that impart respiratory modulation, and are responsible for the phasic inspiratory activity of the PCA muscle. Identification of PCA motoneurons is therefore an essential initial step in understanding the mechanisms responsible for coordinated vocal cord abduction. We identified PCA motoneurons in the rat model by retrograde labeling, and following antidromic activation. A total of 194 neurons were identified by retrograde labeling with cholera toxin B subunit (CTB). Labeling was exclusively ipsilateral where the contralateral vagus and superior laryngeal nerves had been divided. The neurons were multipolar, with dimensions of 33.2 +/- 6.4 microm (mean +/- standard deviation) in length and 22.4 +/- 3.4 microm in width. The neurons were located within a range of 0.6 to 2.4 mm caudal to the caudal pole of the facial nerve, 1.2 to 1.7 mm lateral to the midline, and 1.5 to 2.3 mm deep to the dorsal surface of the medulla. The PCA motoneurons were antidromically activated by focal stimulation of the PCA muscle. The extracellular field was recorded in 5 rats, and the PCA motoneurons were found within a range of 0.8 to 1.7 mm caudal to the caudal pole of the facial nerve, 1.5 to 2.0 mm lateral to the midline, and 1.9 to 2.4 mm deep to the dorsal surface of the medulla. The mean conduction velocity ranged from 37.0 +/- 5.8 to 68.6 +/- 5.0 m/s. An extracellular antidromic field potential, which corresponds to the distribution of the PCA motoneuron pool demonstrated by retrograde labeling with CTB, can be reliably obtained in a rat model following focal PCA muscle stimulation.

The area postrema of newborn swine is activated by hypercapnia: relevance to sudden infant death syndrome?

This study was performed to investigate a role of the neonatal area postrema (AP) in the chemoreceptor response to hypercapnia which is defective in sudden infant death syndrome (SIDS). AP responses to CO2 inhalation were monitored in 1 to 5 week old piglets by mapping neurons that were induced to express the c-fos gene product, Fos--a marker of functional activation. Interpretive confounds were minimized by controlling for hypoxia, the effects of surgical procedures and ambient environmental stressors on neuronal activity (c-fos expression). The AP demonstrated a powerful and reproducible response in neonatal swine breathing 10% CO2 for 1 h. Intensely immunolabeled nuclei were detected throughout the longitudinal extent of the circumventricular organ, and were especially heavily concentrated at rostral levels proximal to obex. Quantitative analysis verified statistically significant increases in numbers of cells that were induced to express Fos-like immunoreactivity (FLI) in the AP of CO2- stimulated piglets as compared to control groups. No detectable age-related differences were observed in AP response patterns. Conclusions. The AP responds to hypercapnic stress in the newborn piglet. A mature circumventricular organ response in the neonate may be crucial in defending against common environmental stressors, such as nicotine exposure--an emetic agent acting via the AP and a major risk factor in SIDS. Hence, a defect of the AP or its network may underlie a loss of state-dependent controls over cardiopulmonary reflex function in SIDS.

Effect of sleep state on the laryngeal chemoreflex in neonatal piglets

The laryngeal chemoreflex (LCR) is a brain stem-mediated response that is a potential mechanism for sudden infant death syndrome. The vast majority of sudden infant death occurs during sleep, yet it remains to be established whether there is a particular sleep state that makes an infant animal more susceptible to apneic events via the LCR. The purpose of this study was to investigate the LCR during different sleep states in the neonatal piglet. In this study, continuous physiologic monitoring and electroencephalographic, electro-oculographic, and electromyographic techniques were utilized to study neonatal piglets during a hypnotic induced sleep model. Propofol drip anesthetic was utilized to provide an anesthetic state and was titrated for dose-dependent sedation. The LCR was initiated in 11 animals during quiet sleep, rapid eye movement sleep, and the anesthetic state. Baseline respiratory and cardiovascular responses were measured. Durations of apnea were recorded and compared. This study found that despite known physiologic differences in respiratory control during different sleep states as compared to the anesthetic state, there appears to be no increased risk of profound apnea in one state versus another in piglets 19 to 28 days old.

Mu- and delta-opioid receptor densities in respiratory-related brainstem regions of neonatal swine

The piglet displays similar postnatal development in respiration and sleep-wake behavior to the human. To shed light on the possible influence of opioid systems on these functions, this study assessed the density of mu- and delta-opioid receptors in brainstems of 2-3 and 5-7 (young), 14-17 (intermediate) and 20-21 (older) day-old piglets, using quantitative autoradiography. Serial 10 microns sections from fresh-frozen brains were incubated with either mu-(125I-DAGO) or delta-(125I-DPDPE) opioid ligands. The binding characteristics of each receptor remained unchanged over the age-range studied. delta-opioid receptor density was minimal in the young piglets, and increased over the age-range studied in all brainstem regions. mu-opioid receptor density exceeded delta-opioid density in all brainstem regions in young and older piglets, and remained unchanged with age. We conclude that, as in other species, the development of delta-opioid receptors in swine lags behind that of mu-opioid receptors, and that the distribution of each in the piglet's brainstem is distinct. The present findings help explain the changing influence of the mu- and delta-opioid systems on breating and state during postnatal development.

Correlation of laryngeal chemoreflex severity with laryngeal muscle response

OBJECTIVES

To examine the relationship between the severity of the laryngeal chemoreflex (LCR) and the pattern of laryngeal muscle activity during an LCR-induced apnea.

METHODS

The laryngeal mucosa of 20 piglets aged 17 to 20 days was stimulated under both hypoxic and normoxic conditions. Respiration, blood pressure, and activity of the thyroarytenoid (TA) and posterior cricoarytenoid (PCA) muscles were monitored during the LCR-induced apnea.

RESULTS

Hypoxemia resulted in a shorter average apnea duration but a greater degree of hypotension. All piglets recovered spontaneously following normoxic LCR stimulation. Hypoxic stimulation resulted in two divergent apneic responses: transient with a spontaneous recovery (17 piglets) or profound requiring resuscitation (three piglets). An increase in TA muscle activity and a decrease in PCA muscle activity was the most common response to LCR stimulation. The response of the TA and PCA muscles was maintained in piglets destined for spontaneous recovery. Decreasing TA activity and increasing PCA activity correlated with the development of a profound response.

CONCLUSIONS

The piglet demonstrates two distinct responses to hypoxic laryngeal chemostimulation that correlate with the activity of the intrinsic laryngeal muscles. Failure to maintain the activity of the TA and PCA muscles during a profound response is associated with the development of severe cardiovascular instability. This study suggests that the critical event involving the LCR is the development of complications secondary to hypoxia.

Sleep apnea in obese miniature pigs

We postulated that three extremely obese Yucatan miniature pigs would have more sleep apnea than three nonobese Yucatan miniature pigs. Pigs were studied with the use of electroencephalograms, inductance plethysmography, oximetry, expired nasal CO2, or thermistors. All of the obese pigs, but none of the nonobese pigs, had both sleep apnea (8.5, 10.3, and 97.0 in obese pigs vs. O apnea + hypopnea/h in all nonobese pigs; P < 0.05) and oxyhemoglobin desaturation episodes during sleep [9.4 +/- 3.0 vs. 0 + 0.53 (SD) mean desaturation episodes/h in obese pigs vs. nonobese pigs, respectively; P < 0.05]. Two of the extremely obese pigs had obstructive sleep apnea, whereas the third obese pig had central sleep apnea. We conclude that sleep apnea occurs in extremely obese Yucatan minipigs and suggest that this animal can be used as a model for sleep apnea in obesity.

Prenatal cocaine alters normoxic sleep-wake and diaphragmatic EMG patterns in piglets

This study assessed in piglets the effects of prenatal cocaine administration on sleep-wake states (SWS) and respiratory parameters, utilizing diaphragmatic electromyogram (EMGdi) recordings during normoxia before and after hypoxia (0.10 F(I,O2), 10 min). We asked whether the respiratory effects were linked to a specific SWS, and whether there was a difference in respiratory measures between the two normoxic conditions. Unsedated, chronically instrumented 3-9- or 21-31-day-old piglets, representing distinct stages in developmental respiratory control, were used. In pre-hypoxic normoxia, prenatal cocaine enhanced sleep at the expense of wakefulness and increased EMGdi amplitude, slope, and area in both age groups regardless of SWS; after the hypoxia, the respiratory findings persisted in the young group, but disappeared in the older group [corrected]. In the young group and regardless of SWS, interbreath interval (ttot) and expiratory duration (ttot - tEMGdi[duration of EMGdi]) were shorter in the cocaine-exposed than in the unexposed piglets, and ttot, tEMGdi, and (ttot - tEMGdi) decreased from pre- to post-hypoxic normoxia. In the older group, ttot and (ttot - tEMGdi) differed among SWS, but were unaffected by drug treatment; tEMGdi was higher with cocaine exposure in pre-, but not in post-hypoxic normoxia, and two-thirds of the EMGdi measurements during post-hypoxic normoxia exhibited a similar magnitude in the drug-treated and untreated groups regardless of SWS. We conclude that 1) prenatal cocaine alters both SWS and EMGdi, but the EMGdi effects are independent of SWS; and 2) the similar EMGdi patterns in the older group after hypoxia, regardless of drug treatment, suggest that hypoxia and chronic prenatal cocaine might alter EMGdi by similar mechanisms.

Respiratory activity of the rat posterior cricoarytenoid muscle

An anatomic and electrophysiological study of the rat posterior cricoarytenoid (PCA) muscle is described. The intramuscular nerve distribution of the PCA branch of the recurrent laryngeal nerve was demonstrated by a modified Sihler's stain. The nerve to the PCA was found to terminate in superior and inferior branches with a distribution that appeared to be confined to the PCA muscle. Electromyography (EMG) recordings of PCA muscle activity in anesthetized rats were obtained under stereotaxic control together with measurement of phrenic nerve discharge. A total of 151 recordings were made in 7 PCA muscles from 4 rats. Phasic inspiratory activity with a waveform similar to that of phrenic nerve discharge was found in 134 recordings, while a biphasic pattern with both inspiratory and post-inspiratory peaks was recorded from random sites within the PCA muscle on 17 occasions. The PCA EMG activity commenced 24.6 +/- 2.2 milliseconds (p < .0001) before phrenic nerve discharge. The results are in accord with findings of earlier studies that show that PCA muscle activity commences prior to inspiratory airflow and diaphragmatic muscle activity. The data suggest that PCA and diaphragm motoneurons share common or similar medullary pre-motoneurons. The earlier onset of PCA muscle activity may indicate a role for medullary pre-inspiratory neurons in initiating PCA activity.

Repeated microdialysis from the nucleus tractus solitarii of chronically instrumented, unsedated piglets

Normal development of respiratory rhythm and control, and perturbations thereof, likely relate to neuromodulators in brainstem regions. To assess the feasibility of repeated neurochemical sampling by in vivo microdialysis from the respiratory-related nucleus tractus solitarii (NTS) during development. 19-24 day-old piglets (n = 7) were implanted under anesthesia with chronic microdialysis guides near NTS around obex. Unsedated piglets then underwent in vivo microdialysis twice, 3 days apart, through probes inserted acutely via the guides to abut against the NTS. The probe tips, surrounded by normal neurons and only diffuse gliosis, either intersected, or were within < or = 300 microns from the NTS. Thirty-min microdialysates were collected for 120 min in normoxia, HPLC-fractionated, and assayed for substance-P (SP), a respiratory excitatory neuropeptide. SP levels stabilized within 90 min from probe placement, and did not differ between the 2 experimental days. Thus, repeated in vivo microdialysis from NTS of conscious piglets is feasible, and can illuminate respiratory-related normal and pathological neurochemical processes during development.

Curtailed respiration by repeated vs. isolated hypoxia in maturing piglets is unrelated to NTS ME or SP levels

In early development, respiratory disorders can produce recurring hypoxic episodes during sleep. To examine possible effects of daily repeated vs. isolated hypoxic hypoxia, cardiorespiratory functions and central, respiratory-related neuromodulator levels in 21- to 32-day-old, chronically instrumented, unsedated piglets were compared between a fifth sequential daily hypoxia and an isolated hypoxia (10% O2-90% N2 for 30 min). Diaphragmatic electromyographic activity, heart rate and arterial pressure, and pH and gas tensions were measured. In vivo microdialysis, via chronically implanted guides, served to sample interstitial substance P (SP) and methionine-enkephalin (ME) at the level of the respiratory-related nucleus tractus solitarii (NTS). Compared with an isolated hypoxia, repeated hypoxia resulted in 1) lower respiratory frequency (f), ventilation equivalent, and arterial pH, higher arterial PO2 during hypoxia, and lower f in recovery from hypoxia; and 2) increased SP concentrations but no change in ME concentrations. We conclude that, in these maturing swine, repeated vs. isolated hypoxic exposure curtails respiratory responses to hypoxia by a mechanism(s) unrelated to SP or ME levels at the NTS.

Developmental changes in the hypoxic response of the hypoglossus respiratory motor output in vitro

The transverse brain stem slice of mice containing the pre-Bötzinger complex (PBC), a region essential for respiratory rhythm generation in vitro, was used to study developmental changes of the response of the in vitro respiratory network to severe hypoxia (anoxia). This preparation generates, at different postnatal stages [postnatal day (P)0-22], spontaneous rhythmic activity in hypoglossal (XII) rootlets that are known to occur in synchrony with periodic bursts of neurons in the PBC. It is assumed that this rhythmic activity reflects respiratory rhythmic activity. At all examined stages anoxia led to a biphasic response: the frequency of rhythmic XII activity initially increased ("primary augmentation") and then decreased ("secondary depression"). In neonates (P0-7), anoxia did not significantly affect the amplitude of integrated XII bursts. Secondary depression never led to a cessation of rhythmic activity. In mice older than P7, augmentation was accompanied by a significant increase in the amplitude of XII bursts. A significant decrease of the amplitude of XII bursts occurred during secondary depression. This depression led always to cessation of rhythmic activity in XII rootlets. The anoxia-induced response of the respiratory rhythmic XII motor output is biphasic and changes during development in a similar way to the in vivo respiratory network. Whether this biphasic response is due to a biphasic response of the respiratory rhythm generator and/or to a biphasic modulation of the XII motor nucleus remains unresolved and needs further cellular analysis. We propose that the transverse slice is a useful model system for examination of the mechanisms underlying the hypoxic response.

Respiratory responses to rapid-onset, repetitive vs. continuous hypoxia in piglets

Repeated, frequent hypoxic exposures may precede Sudden Infant Death. This study assessed whether such hypoxic modality, vs. continuous hypoxia, compromised compensatory cardiorespiratory responses. Following aseptic, chronic instrumentation, 10 to 20 day-old, unsedated piglets underwent measurements of arterial O2 saturation, pH and gas tensions, respiration, heart rate, physical activity, O2 consumption and rectal temperature on several experimental days. The piglets were exposed to 21 min of either 10% or 6% O2 in N2, each comprising either seven, 3-min exposures alternating with 3-min intervals in 21% O2 balance N2, or 7 consecutive hypoxic exposures. Responses to 6% hypoxia were greater than those to 10% hypoxia. In 10% hypoxia, responses to repetitive vs. continuous exposure differed only in PaCO2. In 6% hypoxia, repetitive vs. continuous exposure resulted in lower respiratory frequency (p < 0.05) and in lower ventilation equivalent (p = 0.07) despite higher activity levels. Thus, the mode of hypoxic exposure determines the extent of the respiratory response: Severe, repetitive hypoxia mitigates protective respiratory responses when compared to equivalent, but sustained hypoxia.

Physiological studies of gastro-oesophageal reflux and airway protective responses in the young animal and human infant

1. The mechanisms that underlie the Sudden Infant Death Syndrome (SIDS) must explain its two unique features; age at death and death during apparent sleep. 2. The occurrence of gastro-oesophageal reflux (GOR) during active sleep in infants presenting with apparent life threatening episodes (ALTE) and their similar age distribution to SIDS infants, suggested that reflux could be a cause of asphyxia. 3. Sleep related GOR was found to be a physiological and not a pathological event in normal, healthy term infants. 4. In healthy term infants, those infants that were formula-fed (who have a higher incidence of SIDS) had significantly longer oesophageal clearance times for acid reflux and significantly more active sleep compared with breast fed infants. 5. In very preterm infants (who are at increased risk for SIDS), both the frequency and duration of reflux during active sleep was significantly less at term equivalent age compared with healthy term infants, suggesting additional factors must operate to promote an ALTE. 6. One mechanism which may explain the pathogenesis of GOR could be that the reflux reaches the level of the pharynx and this, in turn, stimulates laryngeal receptors to produce apnoea. 7. Simulated reflux to the level of the pharynx in the sleeping piglet evoked airway protective responses, namely swallow, arousal and occasionally expectoration, but neither apnoea nor oxygen desaturation. 8. In the same piglets treated with pentobarbitone sodium, swallowing was impaired and arousal depressed. Simulated reflux to the pharynx produced significant apnoea and oxygen desaturation and death in two of five piglets.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiorespiratory and sleep-wake behavior in developing swine: kappa-opioid influence

Effects of specific kappa-opioid antagonism with norbinaltorphimine (NorBNI) on sleep-wake state, blood pressure and heart rate, and on diaphragmatic and posterior cricoarytenoid electromyographic activities were assessed in 3 to 13 and 23 to 33 day-old, chronically instrumented, unanesthetized piglets. Preliminary experiments established the pharmacodynamics and dose-response for NorBNI. In the main study, each piglet was studied twice daily, once before and once after 3.7 mg kg-1 NorBNI iv, for up to five consecutive days. During each study session, piglets underwent 10 min trials with 21% O2 in 79% N2 followed by 10% O2 in 90% N2 while lying in a sling within a plexiglass box. Sleep-wake distribution and cardiorespiratory functions matured with age. NorBNI produced a modest increase of arterial pressure and heart rate in the older group only, and altered neither state nor respiration at either age. These results suggest that, in the developing piglet model, the kappa-opioid system influences neither breathing nor state, but modulates cardiovascular regulation to a modest degree and later in ontogeny.

Age-related mu-, delta-and kappa-opioid ligands in respiratory-related brain regions of piglets: effect of prenatal cocaine

Neonates, as compared to older subjects, exhibit increased signs of relative respiratory suppression such as apnea, periodic breathing and only transient hyperventilatory response to hypoxia. Prenatal cocaine exposure exaggerates the respiratory pattern disturbances observed in infants. As endogenous opioids cause central suppression of breathing, we tested their possible involvement in these effects by assessing opioid content in respiratory-related brainstem regions of 2 to 5 (young) and 18 to 22 (older) day-old piglets, unexposed or preexposed to cocaine during 0.66 to 1.0 gestation. The selected ages represent distinct stages in the postnatal development of respiration. beta-Endorphin, methionine-enkephalin, dynorphin A and dynorphin B from the tractus solitarii, ambigualis, gigantoreticularis and parabrachialis medialis nuclei were separated by high performance liquid chromatography, then quantified by radioimmunoassays. Opioid content was higher in the brain regions of the young than of the older piglets, and increased after cocaine exposure in both age groups, but more in the young. These findings support the possible contribution of high opioid content to the relative suppression of respiratory function in early life, and to the exaggerated respiratory dysrhythmia observed in cocaine preexposed neonates.

Prenatal cocaine alters open-field behavior in young swine

Yucatan minisows received 2 mg/kg cocaine i.v. 4 times daily during the last third of gestation. Their piglets were fostered at birth to paired, unexposed sows with their litters, and studied at age 2 to 9 (young group) and 22 to 29 days (older group). Three to 5 exposed and unexposed piglets of each age group were videotaped together for 30 min on 5 consecutive days in an open-field environment. For each piglet, 41 behaviors were scored, timed, summed and clustered into 9 behavioral categories. With age, and independently of drug exposure, piglets spent more time in ingestion, immobility while alone and play/aggression, and less time in group locomotion. For the first 4 test days, the young exposed piglets spent more time in group immobility and less time in individual locomotion and rooting than their age-matched controls. In contrast, the older exposed and unexposed piglet groups did not differ in any of these behavioral clusters. These results suggest that prenatal cocaine exposure in neonatal swine may transiently affect responses to spatial novelty.

The micro-environment of the sleeping newborn piglet covered by bedclothes: gas exchange and temperature

This study followed the thermal and gaseous micro-environment of the newborn piglet sleeping under two levels of bedding insulation. After 1 h in the piglet's thermal comfort zone, the head of each piglet was covered for 2 h with bedding either 12 mm or 21 mm thick. Body temperature rose rapidly, but the arterial gases showed no change in the direction of asphyxia; with the thicker covering PaO2 was unchanged and PaCO2 fell and pH rose. These changes were despite a small rise in environmental PCO2 and fall in PO2 but were also minimized by the piglet's ability to avoid profound hypocapnia by panting at a very low tidal volume. No significant changes in blood gases occurred with the thinner bedding; the temperature rose markedly but more slowly. It is suggested that human babies submerged under bedclothes for any length of time would be more likely to succumb to the effects of hyperthermia than of asphyxia.

Respiratory electromyographic estimates of ventilatory functions in piglets

The best electromyographic (EMG) predictors of respiratory drive (P100), tidal volume (VT) and ventilation (VE) were determined from diaphragmatic (DI) and posterior cricoarytenoid (PCA) EMG measures in 8-48-day-old, anesthetized piglets. Progressive hypercapnia was employed to obtain a wide range of muscle activity. A custom-designed, microcomputer-based system was employed to measure the duration, peak amplitude, rate of rise (initial slope) as well as the summed total and initial (first 100 ms) EMG activity from the DI and the PCA. For each respiratory function, the following combinations of EMG measures were identified as significant predictors using regression analyses: (1) for P100, DI amplitude, PCA initial area and PCA rate of rise; (2) for VT, DI amplitude, PCA duration and DI duration; (3) for VE, DI amplitude, DI initial area, PCA initial area, PCA rate of rise, PCA duration, DI area and DI rate of rise. Thus, whereas the traditionally employed measure of DI amplitude is an important correlate of P100, VT or VE, a complete estimate of these respiratory functions requires the inclusion of initial EMG measures and duration.

Pharyngeal dilator muscle contractile and endurance properties in neonatal piglets

Pharyngeal dilator muscles are critical for maintaining upper airway patency in the neonatal period. The present study examined in vitro the contractile properties of a pharyngeal dilator muscle, the sternohyoid, in 1-7-day-old piglets (n = 24). Isometric contraction and half-relaxation times were 36.7 +/- 1.1 and 30.9 +/- 1.2 msec, respectively. Twitch potentiation ('staircase phenomenon') and post-tetanic potentiation were noted following repetitive stimulation. During prolonged repetitive stimulation with a standard (40 Hz) fatigue test, muscle force declined gradually over time, with loss of half of the initial force occurring over 138 +/- 11 sec, and a 2-min fatigue index (ratio of force at 2 min to initial force) of 0.52 +/- 0.03. An additional 10 piglets were studied at ages of 14-20 days. Muscle from older piglets had comparable isometric twitch kinetics as that of younger animals. However, sternohyoid muscle from the older piglets had worse endurance than muscle from the younger animals, as indicated by a shorter time required for force to decrease by half (86 +/- 10 sec, P < 0.01) and a lower 2-min fatigue index (0.36 +/- 0.03, P < 0.01). These data indicate that for the sternohyoid muscle of the newborn piglet (a) physiological properties are consistent with moderate to fast contraction with good endurance, (b) force potentiates during repetitive twitch stimulation and following a brief period of tetanic stimulation, and (c) there is worsening of endurance but no change in isometric twitch kinetics with increasing age during the first weeks of life.

Hypoxia, sleep and respiration in relation to opioids in developing swine

To test the role of mu and delta opioid systems in neonates during hypoxia, a total of sixteen, 4-11 (n = 7) and 26-33-day-old piglets (n = 9) were instrumented aseptically for assessment of sleep/wake states (S/W), electromyographic activities of the diaphragm and posterior cricoarytenoid muscles (EMGdi, EMG-pca, respectively), heart rate, and arterial pressures, pH and gas tensions. During daily sessions for 5 consecutive days, the piglets inhaled 10% O2/90% N2 for 10 min twice per session, first before any drug, then after either naltrexone (2 mg.kg-1 i.v.), a predominantly mu opioid antagonist, or naltrindole (4 mg.kg-1 i.v.), a specific delta opioid antagonist. During hypoxia, young, in contrast to older piglets, spent more time asleep, and increased sleep during the second half of the hypoxic exposure before, but not after each antagonist. They also exhibited, overall, higher breathing frequency, and lower slope, amplitude, area and initial area of EMGdi and EMGpca activity than older piglets. Naltrindole stimulated EMGpca activity in both age groups, and naltrexone increased the breathing frequency and slope of EMGdi in the older group. We conclude that hypoxia enhances the activation of central mu and delta opioid systems which influence S/W and respiration.

Oxygen consumption in the newborn piglet during combined cold face/hot body exposure

There is increasing evidence that hyperthermia can occur in cool climates because of overwrapping and that this practice precedes many deaths labelled as SIDS. We have attempted to test the hypothesis that the interaction of a cold face and a hot body might lead to further hyperthermia in a piglet model. Twelve non-sedated newborn piglets were studied over the first 10 days of development. Oxygen consumption was measured continuously during sleep. Animals were exposed to cold face conditions initially while the animal's body was kept warm and then while the body was hyperthermic. The results show that stimulation of the face with cool ambient air during conditions of raised metabolic activity (hot body) causes a fall in oxygen consumption towards basal levels. These studies do not, therefore, support the hypothesis that a further increase in metabolic rate occurs during combined cold face and hot body exposure in the piglet model.

Effects of pentobarbital on proopiomelanocortin opioid products of neonatal piglets during normoxia and hypoxia

Abstract Endogenous opioids have been shown to suppress physiological functions in the neonate. It has been suggested that anesthesia with barbiturates might enhance this suppression by influencing opioid systems directly. To explore this possibility, naive piglets, 2.2+/-0.8 (X+/-SD) days old, underwent one of five protocols: 1) normoxia (control); 2) 10% 0(2)/90% N(2) (hypoxia); 3) saline injection ip during normoxia (sham anesthesia); 4) pentobarbital sodium, 25 mg/kg ip, during normoxia (barbiturate anesthesia); and 5) pentobarbital sodium, 25 mg/kg ip, during hypoxia (combined hypoxia and barbiturate anesthesia). Following the inhalation of either gas mixture for at least 30 min, and precisely 30 min after an injection, blood, cerebrospinal fluid and a dorsal medullary slice containing the nucleus tractus solitarii were collected and processed for measurement by radioimmunoassay of opioid proopiomelanocortin products. These comprised beta-lipotropin (the precursor), beta-endorphin-like immunoreactivity (containing the active peptide beta-endorphin) and N-acetyl beta-endorphin (a deactivated peptide). The most striking result was seen in the cerebrospinal fluid: As compared to barbiturate anesthesia, peptide levels with all other treatments, including combined hypoxia and barbiturate anesthesia, were consistently higher. In the plasma, peptide levels after either combined hypoxia and barbiturate anesthesia or hypoxia alone were generally higher than those of their respective controls (sham anesthesia, control). Plasma levels of beta-endorphin-like immunoreactivity and estimated beta-endorphin with combined hypoxia and barbiturate anesthesia were also higher than those with barbiturate anesthesia. The latter pattern was reversed in the nucleus tractus solitarii, in which beta-endorphin-like immunoreactivity and estimated beta-endorphin levels were lower with combined hypoxia and barbiturate anesthesia than with barbiturate anesthesia alone, although no significant differences were achieved. These results suggest that pentobarbital may decrease the central neuronal release of active endorphins, and thus decrease the quantity of these ligands available for interaction with opioid receptors. Hypoxia, on the other hand, appears to increase such release even in the presence of pentobarbital. Thus, during a hypoxic insult, the suppressive influence of opioids on physiological functions would be enhanced regardless of the presence of barbiturate anesthesia.

Modulation by mu opioid antagonism of sleep and respiration in neonatal swine

Young (3-13 days) and older (26-34 days) piglets were instrumented aseptically for chronic recording of sleep/wake states (biparietal electrocorticogram, horizontal and vertical electrooculogram, submental muscle electromyogram (EMG)), heart rate, arterial pressure, pH and gas tensions, posterior cricoarytenoid and diaphragmatic EMG (EMGpca, EMGdi). After recovery from surgery, piglets underwent 1 h daily recordings for 5 consecutive days. Experimental sessions comprised control periods followed by study periods with CTOP (10-40 micrograms/kg i.v.), a somatostatin analogue with mu opioid antagonistic activity. In the young group, CTOP decreased percent time spent in active sleep (AS), increased heart rate during wakefulness, increased breathing frequency during transitional and quiet sleep (TS, QS) and decreased the duration of EMGdi activity during TS, QS and AS. In the older group, CTOP decreased the duration of EMGdi activity during QS. Changes in cardiorespiratory functions with age simulated those reported previously (Scott et al. (1990) Respir. Physiol. 80: 83-102). We conclude that, in early neonatal life, the mu opioid system influences both sleep pattern and respiratory timing, and that this influence diminishes with postnatal age.