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

Prematurity, the diagnosis of bronchopulmonary dysplasia, and maturation of ventilatory control

Infants born before 32 weeks gestational age and receiving respiratory support at 36 weeks postmenstrual age (PMA) are diagnosed with bronchopulmonary dysplasia (BPD). This label suggests that their need for supplemental oxygen (O₂ ) is primarily due to acquired dysplasia of airways and airspaces, and that the supplemental O₂ is treating residual parenchymal lung disease. However, emerging evidence suggests that immature ventilatory control may also contribute to the need for supplemental O₂ at 36 weeks PMA. In all newborns, maturation of ventilatory control continues ex utero and is a plastic process. Among premature infants, supplemental O₂ mitigates the hypoxemic effects of delayed maturation of ventilatory control, as well as reduces the duration and frequency of periodic breathing events. Nevertheless, prematurity is associated with altered and occasionally aberrant maturation of ventilatory control. Infants born prematurely, with or without a diagnosis of BPD, are more prone to long-lasting effects of dysfunctional ventilatory control. This review addresses normal and abnormal maturation of ventilatory control and suggests how aberrant maturation complicates assigning the diagnosis of BPD. Greater awareness of the interaction between parenchymal lung disease and delayed maturation of ventilatory control is essential to understanding why a given premature infant requires and is benefitting from supplemental O₂ at 36 weeks PMA.

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

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

The physiological and behavioral response of pigs castrated with and without anesthesia or analgesia

Surgical castration is a common management practice performed on male pigs to prevent the occurrence of boar taint. Surgical castration is known to cause physiological and behavioral changes in pigs indicative of pain-induced distress; however, it is commonly performed without pain relief. The objective of this study was to evaluate the effectiveness of carbon dioxide gas (CO(2)) anesthesia and a non-steroidal anti-inflammatory drug (NSAID) to alleviate the pain caused by castration. At 3 d of age, male pigs were either control handled (CON), castrated without pain relief (CAS), given an NSAID and then immediately castrated (CAS+NSAID), anesthetized with CO(2) and then castrated (CAS+CO2), or anesthetized with CO(2) and given an NSAID at the time of castration (CAS+BOTH). Blood samples were collected before castration, and at 30, 60, 120, and 180 min, 24 h, and 3 d after castration or handling for analysis of cortisol, C-Reactive protein (CRP), and substance-P (SP) concentrations. This study was then repeated using the same treatment groups, and the behavioral response to castration and handling were measured using a 1-min scan sampling procedure. The percentage of stress vocalizations was recorded during the administration of all treatments. Anesthesia and analgesia did not effectively reduce (P > 0.05) the cortisol response to surgical castration. Overall, CRP concentrations were greater (P < 0.05) in CAS+CO2 pigs as compared with CON pigs. Sixty minutes after castration or handling, SP concentrations were greater (P < 0.08) in pigs given CO(2) anesthesia (CO2, CAS+CO2, and CAS+BOTH) than CON, CAS, and CAS+NSAID pigs. Pigs castrated without pain relief spent more (P < 0.001) time lying without contact than all other treatments during the first 30 min after castration, but thereafter CAS+CO2 pigs spent more (P < 0.001) time lying without contact than other treatments. During the first 30 min after the treatments were applied, CAS+CO2 pigs spent more (P < 0.01) time displaying pain-like behaviors than CON, CAS, CAS+NSAID, and CAS+BOTH pigs. The percentage of stress vocalizations was greater (P < 0.05) in CAS and CAS+NSAID pigs than all other treatments. Neither CO(2) anesthesia nor a NSAID, given separately or combined, markedly reduced the pain-induced distress caused by castration in pigs. More research is needed to evaluate practical methods of on-farm pain relief for pigs.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Long-term effects of the perinatal environment on respiratory control

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 (O2and CO2). 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.

Respiratory and metabolic responses to early postnatal chronic intermittent hypoxia and sustained hypoxia in the developing rat

Exposure to sustained hypoxia (SH) differentially modifies the hypoxic ventilatory response (HVR) in adults and developing rats. We examined the possibility that postnatal intermittent hypoxia (IH), a more prevalent clinical condition than SH, may lead to significant modifications of ventilatory patterning during development. Sprague-Dawley rat pups were exposed as of the d 1 of life to either SH (10% O2) or IH [alternating room air (RA) and 10% O2 every 90 s] for up to 30 d; controls were exposed to normoxia. HVR (10% O2 for 20 min) was assessed in unrestrained pups at 5, 10, 15, and 30 d of age using whole-body plethysmography. IH pups displayed higher normoxic ventilation (VE) at all ages (p < 0.001 versus control; n = 12 per group), which was not observed in SH animals until 10 d of exposure (p < 0.001 versus control; n = 12 per group). Furthermore, both SH and IH modified properties of peak HVR (pHVR), as well as those of the ensuing hypoxic ventilatory decline (HVD); however, the ventilatory strategies adopted after SH and IH greatly differed. We conclude that both postnatal IH and SH modify normal ventilatory patterning and induce altered HVR, but differ in the ventilatory strategies adopted to mount HVR responses.

Early postnatal chronic intermittent hypoxia modifies hypoxic respiratory responses and long-term phrenic facilitation in adult rats

Acute isocapnic intermittent hypoxia elicits time-dependent, serotonin-dependent enhancement of phrenic motor output in anesthetized rats (phrenic long-term facilitation, pLTF). In adult rats, pLTF is enhanced by chronic intermittent hypoxia (CIH). To test the hypothesis that early postnatal CIH induces persistent modifications of ventilation and pLTF, we exposed male Sprague-Dawley rat pups on their first day of life to a CIH profile consisting of alternating room air and 10% oxygen every 90 s for 30 days during daylight hours (RAIH) or to comparable exposures consisting of room air throughout (RARA). One month after cessation of CIH, respiratory responses were recorded using whole body plethysmography, and integrated phrenic nerve activity was recorded in urethane-anesthetized, vagotomized, paralyzed, and ventilated rats at baseline and after exposures to three 5-min hypoxic episodes [inspired O2 fraction (FiO2)=0.11] separated by 5 min of hyperoxia (FiO2=0.5). RAIH rats displayed greater normoxic ventilation and also increased burst frequency compared with RARA rats (P<0.01). Ventilatory responses to hypoxia and short-term phrenic responses during acute hypoxic challenges were reduced in RAIH rats (P<0.01). Although pLTF was present in both RAIH and RARA rats, it was diminished in RAIH rats (minute activity: 74+/-2% in RARA vs. 55+/-5% in RAIH at 60 min; P<0.01). Thus we conclude that early postnatal CIH modifies normoxic and hypoxic ventilatory and phrenic responses that persist at 1 mo after cessation of CIH (i.e., metaplasticity) and markedly differ from previously reported increased neural plasticity changes induced by CIH in adult rats.

Developmental plasticity of respiratory control following intermittent hypoxia

During development, windows of increased vulnerability to noxious stimulus occur. These critical periods of maturation represent times at which the maturing animal is uniquely susceptible to external perturbations that may alter the ultimate configuration of neural networks and their associated function(s), thereby inducing persistent (mal)adaptive changes. In contrast, when comparable perturbations are applied to adult animals the associated adaptive changes do not typically persist. This principle has been demonstrated in models of respiratory plasticity in developing mammals including exposure to sustained hypoxia, hyperoxia, and pharmacological agents. Recently, intermittent hypoxia (IH) during development has also been implicated as a potent inducer of respiratory plasticity. Altered ventilatory patterning induced by IH is distinct from other stimuli and elicits markedly different responses in the developing mammal as compared to the adult. Furthermore, adaptations to acute IH (AIH) exposure may involve mechanisms that differ from those invoked by chronic IH exposure (CIH). Thus, critical examination of IH exposure paradigms is also an important consideration. Greater understanding of IH-induced ventilatory plasticity, particularly in the developing animal, will undoubtedly increase our understanding of IH related diseases such as sleep disordered breathing, and perhaps provide future directions for intervention strategies.

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.

Relationship of the ventilatory response to hypoxia with neonatal apnea in preterm infants

OBJECTIVE

Correlate the ventilatory response of preterm infants to hypoxic exposure with incidence of neonatal apnea. Study design Seventeen stable convalescing premature infants underwent bedside cardiorespiratory monitoring of respiration using respiratory inductance plethysmography, heart rate, and oxygen saturation (SaO(2)) for a 12-hour period. These studies were scored for number of apneas > or =15 and > or =20 seconds. Infants then underwent a 3-minute hypoxic exposure. Minute ventilation (V(E)) was calculated for 30-second epochs from the time inspired oxygen reached 15%. Linear regression analysis was used to correlate the change in V(E) normalized for decrease in SaO(2) (DeltaV(E)/DeltaSaO(2)) during the first and third minutes of hypoxic exposure with the number of apneic episodes during the 12-hour study.

RESULTS

The majority of infants exhibited an anticipated biphasic ventilatory response to hypoxia. There was a significant positive correlation between DeltaV(E)/DeltaSaO(2) during the first and third minutes of hypoxic exposure and number of apneic episodes > or =15 and > or =20 seconds during the preceding 12 hours.

CONCLUSIONS

Preterm infants with a greater number of apneic episodes exhibit an increased ventilatory response to hypoxic exposure, suggesting that apnea of prematurity may be associated with enhanced peripheral chemoreceptor activity.

nCPAP improves abnormal autonomic function in at-risk-for-SIDS infants with OSA

We evaluated cardiovascular autonomic control and arousability during sleep in infants with obstructive sleep apnea (OSA) before and after 10 +/- 4 (mean +/- SD) days of treatment with nasal continuous positive airway pressure (nCPAP). Six OSA infants and 12 age-matched control infants were studied with polygraphic sleep studies at the age of 13 +/- 4 wk. During the study, 45 degrees head-up tilt tests were performed in slow-wave and rapid eye movement sleep. Blood pressure (BP) and heart rate (HR) were continuously monitored. All OSA infants had decreased initial BP and HR responses, followed by hypotension in two and hypertension in two. OSA infants displayed higher arousal thresholds in response to the tilt in rapid eye movement sleep (P < 0.005) and higher baseline HR (P < 0.05) than controls. nCPAP treatment normalized BP and HR responses as well as arousal thresholds to tilting and stabilized HR levels. OSA in infants may be linked with cardiovascular autonomic control disturbances and decreased arousability during sleep. These defects are improved by control of OSA with nCPAP.

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.

Neurokinin-1 versus mu-opioid receptor binding in rat nucleus tractus solitarius after single and recurrent intermittent hypoxia

G protein-coupled excitatory neurokinin-1 and inhibitory mu-opioid receptors exist in respiratory brainstem with their peptides and influence breathing. To assess their putative role in respiratory responses to hypoxia, neurokinin-1, and mu-opioid receptor binding was determined in the respiratory nucleus tractus solitarius of the mature rat after single and recurrent intermittent hypoxia versus normoxia. Hypoxia comprised six 5-min bouts of 8% O(2)-92% N(2) interceded by 5-min bouts in 21% O(2)-79% N(2) (normoxia), either on 6 consecutive days (recurrent intermittent hypoxia) or on the 6th day only (single intermittent hypoxia). Controls comprised six daily sessions in normoxia. To examine the plasticity in receptor response, brains were collected 5min, 2h, or 24h after the last gaseous exposure. Sections from each brainstem underwent quantitative film autoradiography with iodinated substance P and DAMGO for neurokinin-1 and mu-opioid receptors, respectively. Neurokinin-1 receptor binding decreased 5min after single and recurrent hypoxia and 2h after recurrent hypoxia, whereas mu-opioid binding remained unchanged. The binding of both receptors increased 24h after recurrent intermittent hypoxia. Neurokinin versus mu-opioid binding differences immediately posthypoxia might affect physiological responses to episodic hypoxia.

Developmental plasticity in 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.

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.

Induced recovery of hypoxic phrenic responses in adult rats exposed to hyperoxia for the first month of life

1. Adult rats exposed to hyperoxia for the first month of life have permanently attenuated ventilatory and phrenic nerve responses to hypoxia. We tested the hypothesis that the blunted hypoxic phrenic response in hyperoxia-treated rats (inspired O(2) fraction, F(I,O2) = 0.6 for 28 post-natal days) could be actively restored to normal by intermittent (alternating 12 % O(2)/air at 5 min intervals; 12 h per night for 1 week) or sustained (12 % O(2) for 1 week) hypoxia. 2. Phrenic responses to isocapnic hypoxia (P(a,O2) = 60, 50 and 40 +/- 2 mmHg) were assessed in the following groups of anaesthetized, vagotomized adult Sprague-Dawley rats (age 4 months), treated with a neuromuscular blocking agent and ventilated: control, hyperoxia-treated and hyperoxia-treated exposed to either intermittent or sustained hypoxia as adults. Experiments on intermittent and sustained hypoxia-treated rats were performed on the morning following hypoxic exposures. 3. Both intermittent and sustained hypoxia enhanced hypoxic phrenic responses in hyperoxia-treated rats when expressed as minute phrenic activity (P < 0.05). Increases in phrenic burst amplitude during hypoxia were greater in hyperoxia-treated rats after intermittent hypoxia (P < 0.05), and a similar but non-significant trend was observed after sustained hypoxia. Hypoxia-induced changes in phrenic burst frequency were not significantly different among groups. 4. The estimated carotid body volume in control rats (11.5 (+/- 0.7) x 10(6) microm(3)) was greater than in the other treatment groups (P < 0.05). However, carotid body volume was significantly greater in hyperoxia-treated rats exposed to sustained hypoxia (6.3 (+/- 0.3) x 10(6) microm(3); P < 0.05) compared to hyperoxia-treated rats (3.3 (+/- 0.2) x 10(6) microm(3)) or hyperoxia-treated rats exposed to intermittent hypoxia (3.8 (+/- 0.3) x 10(6) microm(3)). 5. Hypoxic phrenic responses in hyperoxia-treated rats 1 week after intermittent hypoxia were similar to responses measured immediately after intermittent hypoxia, indicating persistent functional recovery. 6. The results indicate that diminished hypoxic phrenic responses in adult rats due to hyperoxia exposure for the first 28 post-natal days can be reversed by intermittent or sustained activation of the hypoxic ventilatory control system. Although the detailed mechanisms of functional recovery are unknown, we suggest that sustained hypoxia restores carotid chemoreceptor sensitivity, whereas intermittent hypoxia primarily augments central integration of synaptic inputs from chemoafferent neurons.

Respiratory plasticity following intermittent hypoxia: developmental interactions

Intermittent hypoxia (IH) is the most frequent form of hypoxia occurring in the developing mammal. On one hand, the maturational process of neural, mechanical, pulmonary, and sleep state-dependent factors will favor the occurrence of IH during early postnatal life. On the other hand, it has also become clear that hypoxia, even when short lasting, can modify subsequent respiratory responses to hypoxia and induce a variety of genes whose consequences will persist for much longer periods than the duration of the hypoxic stimulus itself, i.e., functional and adaptive plasticities. The dynamic interactions between the overall duration and recurring frequency of IH, the severity of IH, and the level of neural maturity at the time of IH will modify the ventilatory, metabolic, and cardiovascular responses to hypoxia. We propose that the earlier IH will occur in the developmental course the more likely that the physiological responses to an ulterior hypoxic challenge will be altered even into adulthood. At this point in time, a critical examination of the field would suggest that the short-term alterations of the hypoxic ventilatory response (HVR) of the developing mammal to IH are qualitatively similar to those of the adult and display a biphasic pattern, namely, initial enhancement of the HVR followed by a reduction in HVR. However, the short- and long-term effects of IH on the modulation of neurotransmitter release, receptor binding and expression, intracellular signaling cascades, transcriptional regulation, and gene expression as a function of animal maturity are almost completely unknown. Further delineation of such complex responses to IH may permit the formulation of interventional strategies aiming at reducing the overall vulnerability of the young infant and child to apnea and sudden death.

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.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.