Enhanced expression of the Flt‐1 and Flk‐1 receptor tyrosine kinases in a newborn piglet model of ischemic tolerance

Vascular endothelial growth factor (VEGF), an angiogenic factor induced by hypoxia, also exerts direct effects on neural tissues. VEGF up‐regulation after hypoxia coincides with expression of its two tyrosine kinase receptors Flt‐1(VEGFR‐1) and Flk‐1 (KDR/VEGFR‐2), which are the key mediators of physiological angiogenesis. We have recently shown that hypoxic‐preconditioning (PC) leading to tolerance to hypoxia–ischemia in neonatal piglet brain resulted in increased expression of VEGF. In this study, we used a hypoxic‐preconditioning model of ischemic tolerance to analyze the expression and cellular distribution of VEGF receptors and phosphorylation of cAMP‐response element‐binding protein (CREB) in newborn piglet brain. The response of Flt‐1 and Flk‐1 mRNA to PC alone was biphasic with peaks early (6 h) and late (1 week) after PC. The mRNA expression of Flt‐1 and Flk‐1 in piglets preconditioned 24 h prior to hypoxia–ischemia was significantly higher than non‐preconditioned piglets and remained up‐regulated up to 7 days. Furthermore, PC prior to hypoxia–ischemia significantly increased the protein levels of Flt‐1 and Flk‐1 compared with hypoxia–ischemia in a time‐dependent manner. Double‐immunolabeling indicated that both Flt‐1 and Flk‐1 are expressed in neurons and endothelial cells with a similar time course of expression following PC and that PC leads to the growth of new vessels. Finally, our data demonstrate that PC significantly phosphorylated and activated cAMP‐response element‐binding protein in nucleus. These results suggest that mechanism(s) initiated by PC can induce VEGF receptor up‐regulation in newborn brain and that VEGF–VEGF receptor‐coupled signal transduction pathways could contribute to the establishment of tolerance following hypoxia–ischemia.     

Effect of Neonatal Asphyxia on the Impairment of the Auditory Pathway by Recording Auditory Brainstem Responses in Newborn Piglets: A New Experimentation Model to Study the Perinatal Hypoxic-Ischemic Damage on the Auditory System

IntroductionHypoxia–ischemia (HI) is a major perinatal problem that results in severe damage to the brain impairing the normal development of the auditory system. The purpose of the present study is to study the effect of perinatal asphyxia on the auditory pathway by recording auditory brain responses in a novel animal experimentation model in newborn piglets.MethodHypoxia-ischemia was induced to 1.3 day-old piglets by clamping 30 minutes both carotid arteries by vascular occluders and lowering the fraction of inspired oxygen. We compared the Auditory Brain Responses (ABRs) of newborn piglets exposed to acute hypoxia/ischemia (n = 6) and a control group with no such exposure (n = 10). ABRs were recorded for both ears before the start of the experiment (baseline), after 30 minutes of HI injury, and every 30 minutes during 6 h after the HI injury.ResultsAuditory brain responses were altered during the hypoxic-ischemic insult but recovered 30-60 minutes later. Hypoxia/ischemia seemed to induce auditory functional damage by increasing I-V latencies and decreasing wave I, III and V amplitudes, although differences were not significant.ConclusionThe described experimental model of hypoxia-ischemia in newborn piglets may be useful for studying the effect of perinatal asphyxia on the impairment of the auditory pathway.     

Effect of 7-Nitroindazole Sodium on the Cellular Distribution of Neuronal Nitric Oxide Synthase in the Cerebral Cortex of Hypoxic Newborn Piglets

Cerebral hypoxia results in generation of nitric oxide (NO) free radicals by Ca⁺⁺-dependent activation of neuronal nitric oxide synthase (nNOS). The present study tests the hypothesis that the hypoxia-induced increased expression of nNOS in cortical neurons is mediated by NO. To test this hypothesis the cellular distribution of nNOS was determined immunohistochemically in the cerebral cortex of hypoxic newborn piglets with and without prior exposure to the selective nNOS inhibitor 7-nitroindazole sodium (7-NINA). Studies were conducted in newborn piglets, divided into normoxic (n = 6), normoxic treated with 7-NINA (n = 6), hypoxic (n = 6) and hypoxic pretreated with 7-NINA (n = 6). Hypoxia was induced by lowering the FiO₂ to 0.05–0.07 for 1 h. Cerebral tissue hypoxia was documented by decrease of ATP and phosphocreatine levels in both the hypoxic and 7-NINA pretreated hypoxic groups (P < 0.01). An increase in the number of nNOS immunoreactive neurons was observed in the frontal and parietal cortex of the hypoxic as compared to the normoxic groups (P < 0.05) which was attenuated by pretreatment with 7-NINA (P < 0.05 versus hypoxic). 7-NINA affected neither the cerebral energy metabolism nor the cellular distribution of nNOS in the cerebral cortex of normoxic animals. We conclude that nNOS expression in cortical neurons of hypoxic newborn piglets is NO-mediated. We speculate that nNOS inhibition by 7-NINA will protect against hypoxia-induced NO-mediated neuronal death.     

Regional cerebral blood flow response during and after acute asphyxia in newborn piglets

The hemodynamic response during and after acute asphyxia was studied in 14 newborn piglets. An apnea-like asphyxial insult was produced in paralyzed mechanically ventilated piglets by discontinuing ventilation until the piglets became bradycardic (heart rate less than 80 beats/min). Seven piglets had organ blood flow measured by microspheres at control, during asphyxia (PO2 = 16 +/- 11 Torr, pH = 7.31 +/- 0.07, PCO2 = 47 +/- 9 Torr), and during recovery from asphyxia. During acute asphyxia, rapid organ blood flow redistribution occurred, producing decreased renal and skeletal muscle blood flow, while coronary blood flow increased. Although total brain blood flow changed little during asphyxia, regional cerebral blood flow (rCBF) analysis revealed significant nonhomogeneous blood flow distribution within the brain during asphyxia, with decreases to the cerebral gray and white matter and the choroid plexus, whereas brain stem structures had increased flow. During recovery with reventilation, total brain blood flow increased 24% above control, with a more uniform distribution and increased flow to all brain regions. The time course of rCBF changes during acute asphyxia was then determined in seven additional piglets with CBF measurements made sequentially at 30-60 s, 60-120 s, and 120-180 s of asphyxia. The vasoconstriction seen in cortical structures, concurrent with the reduction in skeletal and kidney blood flow, known to be sympathetically mediated, suggest a selective reflex effect in this brain region. The more gradual and progressive vasodilation in brain stem regions during asphyxia is consistent with chemical control. These findings demonstrate significant regional heterogeneity in CBF regulation in newborn piglets.     

Effect of alpha adrenergic blockade on brain blood flow and ventilation during hypoxia in newborn piglets.

The influence of cardiovascular changes on ventilation has been demonstrated in adult animals and humans (Jones, French, Weissman & Wasserman, 1981; Wasserman, Whipp & Castagna 1974). It has been suggested that neonatal hypoxic ventilatory depression may be related to some of the hemodynamic changes that occur during hypoxia (Brown & Lawson, 1988; Darnall, 1985; Suguihara, Bancalari, Bancalari, Hehre & Gerhardt, 1986). To test the possible relationship between the cardiovascular and ventilatory response to hypoxia in the newborn, eleven sedated spontaneously breathing piglets (age: 5.9 +/- 1.6 days; weight: 1795 +/- 317 g; SD) were studied before and after alpha adrenergic blockade with phenoxybenzamine. Minute ventilation (VE) was measured with a pneumotachograph, cardiac output (CO) by thermodilution and total and regional brain blood flow (BBF) with radiolabeled microspheres. Measurements were performed while the animals were breathing room air and after 10 min of hypoxia induced by breathing 10% O2. Hypoxia was again induced one hour after infusion of phenoxybenzamine (6 mg/kg over 30 min). After 10 min of hypoxia, in the absence of phenoxybenzamine, the animals responded with marked increases in VE (P less than 0.001), CO (P less than 0.001), BBF, and brain stem blood flow (BSBF) (P less than 0.02). However, the normal hemodynamic response to hypoxia was eliminated after alpha adrenergic blockade. There were significant decreases in systemic arterial blood pressure, CO, and BBF during hypoxia after phenoxybenzamine infusion; nevertheless, VE increased significantly (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)     

Early Regional Response of Apoptotic Activity in Newborn Piglet Brain Following Hypoxia and Ischemia

Responses of selected neuroregulatory proteins that promote (Caspase 3 and Bax) or inhibit (Bcl-2, high Bcl-2/Bax ratio) apoptotic cell death were measured in the brain of piglets subjected to precisely controlled hypoxic and ischemic insults: 1 h hypoxia (decreasing FiO₂ from 21 to 6%) or ischemia (ligation of carotid arteries and hemorrhage), followed by 0, 2 and 4 h recovery with 21% FiO₂. Protein expression was measured in cortex, hippocampus and striatum by Western blot. There were no significant differences in expression of Caspase-3 between sham operated, hypoxic and ischemic groups. There were significant regional differences in expression of Bcl-2 and Bax in response to hypoxia and ischemia. The changes in Bcl-2/Bax ratio were similar for hypoxia and ischemia except for striatum at zero time recovery, with ischemia giving lower ratios than hypoxia. The Bcl-2/Bax ratio was also lower for the striatum than for the other regions of the brain, suggesting this region is the more susceptible to apoptotic injury.     

Chronic hypoxia alters nitric oxide-dependent pulmonary vascular responses in lungs of newborn pigs

Fike, Candice D., and Mark R. Kaplowitz. Chronichypoxia alters nitric oxide-dependent pulmonary vascular responses inlungs of newborn pigs. J. Appl.Physiol. 81(5): 2078-2087, 1996.Almost all ofthe studies evaluating the effect of chronic hypoxia on lung nitricoxide production have been performed in adult animals. Because resultsof studies in adult lungs should not be extrapolated to represent thenewborn lung, we performed studies to determine whether decreasednitric oxide production might be involved in the pathogenesis ofchronic hypoxia-induced pulmonary hypertension in newborns. We keptnewborn pigs in chambers filled with room air (control) or 11-12%O₂ for either 3-5 (short) or10-12 (long) days. Using isolated lungs, we measured pulmonary vascular responses to agents that either stimulate or inhibit thesynthesis of nitric oxide. To define the vascular sites of alteredproduction of nitric oxide, we applied the micropuncture technique andmeasured small venular pressures before and after treatment with anitric oxide synthesis inhibitor. Pulmonary vascular responses toacetylcholine were blunted in chronically hypoxic piglets of both theshort and long groups. The nitric oxide synthesis inhibitor had adifferent effect in the lungs of control piglets than in those ofchronically hypoxic piglets of the long but not of the short group. Forthe long group, the nitric oxide synthesis inhibitors causedconstriction of both arteries and veins in lungs of control but not ofchronically hypoxic piglets. These findings support the idea thatdecreased pulmonary vascular nitric oxide production occurs withchronic hypoxia in newborn pigs and might therefore contribute to thepathogenesis of pulmonary hypertension in newborns.

     

Mechanism of DNA Fragmentation During Hypoxia in the Cerebral Cortex of Newborn Piglets

We have previously shown that hypoxia results in increased activity of caspase-9, caspase-3 and fragmentation of nuclear DNA in the cerebral cortex of newborn piglets. The present study tested the hypothesis that mechanism of DNA fragmentation during hypoxia in the cerebral cortex of newborn piglets is mediated by caspase-9-dependent caspase-3 activation. Newborn piglets were randomly assigned to normoxic, hypoxic, and hypoxic pretreated with a highly selective caspase-9 inhibitor, Z-LEHD-FMK groups. The data showed that cerebral tissue hypoxia results in increased expression of caspase-activated DNase (CAD) protein in the nucleus and fragmentation of nuclear DNA. A pretreatment with Z-LEHD-FMK attenuated the expression of CAD protein in the nucleus and the fragmentation of nuclear DNA. Based on these results, we conclude that the mechanism by which the nuclear DNA was fragmented is mediated by caspase-9-dependent caspase-3 activation and the consequence of caspase-activated DNase activation in the cerebral cortex of newborn piglets.     

Neuroprotection by Argon Ventilation after Perinatal Asphyxia: A Safety Study in Newborn Piglets

MethodsEight newborn piglets (weight 1.4–3.0 kg) were used. Heart rate, blood pressure, regional cerebral saturation, and electrocortical brain activity were measured continuously. All experiments had a 30 min. baseline period, followed by three 60 min. periods of argon ventilation alternated with 30 min argon washout periods. Two animals were ventilated with increasing concentrations of argon (1h 30%, 1 h 50%, and 1 h 80%), two were subjected to 60 min. hypoxia (FiO₂ 0.08) before commencing 50% argon ventilation, and two animals received hypothermia following hypoxia as well as 50% argon ventilation. Two animals served as home cage controls and were terminated immediately.ResultsArgon ventilation did not result in a significant change of heart rate (mean ± s.d. −3.5±3.6 bpm), blood pressure (−0.60±1.11 mmHg), cerebral oxygen saturation (0.3±0.9%), electrocortical brain activity (−0.4±0.7 µV), or blood gas values. Argon ventilation resulted in elevated argon concentrations compared to the home cage controls (34.5, 25.4, and 22.4 vs. 7.3 µl/ml).ConclusionVentilation with up to 80% argon during normoxia, and 50% argon after hypoxia did not affect heart rate, blood pressure, cerebral saturation and electrocortical brain activity. Clinical safety studies of argon ventilation in humans seem justified.     

Effects of post-resuscitation treatment with N-acetylcysteine on cardiac recovery in hypoxic newborn piglets

Aims

Although N-acetylcysteine (NAC) can decrease reactive oxygen species and improve myocardial recovery after ischemia/hypoxia in various acute animal models, little is known regarding its long-term effect in neonatal subjects. We investigated whether NAC provides prolonged protective effect on hemodynamics and oxidative stress using a surviving swine model of neonatal asphyxia.

Methods and Results

Newborn piglets were anesthetized and acutely instrumented for measurement of systemic hemodynamics and oxygen transport. Animals were block-randomized into a sham-operated group (without hypoxia-reoxygenation [H–R, n = 6]) and two H-R groups (2 h normocapnic alveolar hypoxia followed by 48 h reoxygenation, n = 8/group). All piglets were acidotic and in cardiogenic shock after hypoxia. At 5 min after reoxygenation, piglets were given either saline or NAC (intravenous 150 mg/kg bolus + 20 mg/kg/h infusion) via for 24 h in a blinded, randomized fashion. Both cardiac index and stroke volume of H-R controls remained lower than the pre-hypoxic values throughout recovery. Treating the piglets with NAC significantly improved cardiac index, stroke volume and systemic oxygen delivery to levels not different from those of sham-operated piglets. Accompanied with the hemodynamic improvement, NAC-treated piglets had significantly lower plasma cardiac troponin-I, myocardial lipid hydroperoxides, activated caspase-3 and lactate levels (vs. H-R controls). The change in cardiac index after H-R correlated with myocardial lipid hydroperoxides, caspase-3 and lactate levels (all p<0.05).

Conclusions

Post-resuscitation administration of NAC reduces myocardial oxidative stress and caused a prolonged improvement in cardiac function and in newborn piglets with H-R insults.     

The effect of repeated intermittent hypoxia on pulmonary vasoconstriction in the newborn

The effect of repeated intermittent hypoxia upon the basal pulmonary vascular tone in the newborn period is unknown. We therefore studied the central hemodynamic response to seven repeated intermittent hypoxic challenges in acutely prepared piglets under 2 weeks of age. Catheters were placed in the aorta, pulmonary artery, and atria, and an electromagnetic flow probe was positioned around the main pulmonary artery. Each hypoxic challenge (Fio2 = 0.14) lasted 5 min, and was separated by an equal duration of ventilation with air. Nine control animals were ventilated with air for 90 min, a period of time equivalent to the seven challenges in the experimental group, and subjected to one hypoxic challenge at the end. Hypoxia uniformly induced pulmonary vasoconstriction. Repeated intermittent hypoxic challenges produced a progressive increase in pulmonary artery pressure and vascular resistance, both during air ventilation and hypoxia. For each challenge, the vascular resistance value achieved during hypoxia was directly related to the immediately preceding air ventilation one, and the magnitude of hypoxic pulmonary vasoconstriction, defined as the incremental change in resistance from air to hypoxia, was not different from the first to the last challenge in the experimental group. In the control group the pulmonary vascular tone did not change during the 90 min of air ventilation, and the single hypoxic challenge induced an increase in pulmonary vascular pressure and resistance similar in magnitude to the first challenge in the experimental group. Indomethacin administration to five experimental animals, after the last challenge, reversed the increase in air ventilation pulmonary artery pressure and vascular resistance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of hypoxic and hypercapnic stimuli on breathing pattern in the newborn rat

We aimed to investigate whether newborn rats respond to acute hypoxia with a biphasic pattern as other newborn species, the characteristics of their ventilatory response to hypercapnia, and the ventilatory response to combined hypoxic and hypercapnic stimuli. First, we established that newborn unanesthetized rats (2-4 days old) exposed to 10% O2 respond as other species. Their ventilation (VE), measured by flow plethysmography, immediately increased by 30%, then dropped and remained around normoxic values within 5 min. The drop was due to a decrease in tidal volume, while frequency remained elevated. Hence, alveolar ventilation was about 10% below normoxic value. At the same time O2 consumption, measured manometrically, dropped (-23%), possibly indicating a mechanism to protect vital organs. Ten percent CO2 in O2 breathing determined a substantial increase in VE (+47%), indicating that the respiratory pump is capable of a marked sustained hyperventilation. When CO2 was added to the hypoxic mixture, VE increased by about 85%, significantly more than without the concurrent hypoxic stimulus. Thus, even during the drop in VE of the biphasic response to hypoxia, the respiratory control system can respond with excitation to a further increase in chemical drive. Analysis of the breathing patterns suggests that in the newborn rat in hypoxia the inspiratory drive is decreased but the inspiratory on-switch mechanism is stimulated, hypercapnia increases ventilation mainly through an increase in respiratory drive, and moderate asphyxia induces the most powerful ventilatory response by combining the stimulatory action of hypercapnia and hypoxia.     

Mitochondria are More Resistant to Hypoxic Depolarization in the Newborn than in the Adult Brain

Hypoxic-ischemic brain injury subsequent to asphyxia represents a major cause of morbidity and death in the newborn. The newborn brain has been considered more resistant to hypoxia than the adult brain because of lower energy demand. The mechanisms underlying hypoxic brain injury, in particular the age-related vulnerability, are still only partially understood. The mitochondrial function is pivotal for the function and survival of neurons. Acutely isolated CA1 neurons from neonatal (3-6 days) and adult rats (5-6 weeks) were loaded with Rh 123, and the effect of hypoxia on the inner mitochondrial membrane potential (Delta psi(m)) was compared. During prolonged hypoxia (15 min), Delta psi(m) was lost in a majority of the neonatal neurons (83%) and in all the adult neurons. During hypoxia (5 min) followed by reoxygenation the mitochondria in 23% of the neonatal neurons were completely depolarized, whereas 85% of the adult neurons demonstrated a complete loss of Delta psi(m). In conclusion hippocampal CA1 mitochondria in the newborn rat are more resistant to hypoxic depolarization than in the adult rat.     

Effect of Hypoxia on Caspase-3, -8, and -9 Activity and Expression in the Cerebral Cortex of Newborn Piglets

Caspases play an important role in programmed cell death. Caspase-3 is a key executioner of apoptosis, whose activation is mediated by the initiator caspases, caspase-8 and caspase-9. The present study tested the hypothesis that cerebral hypoxia results in increased activation and expression of caspases-3, -8, and -9 in the cytosolic fraction of the cerebral cortex of newborn piglets. To test this hypothesis the activity and expression of caspases-3, -8, and -9 were determined in newborn piglets divided into normoxic and hypoxic groups. Caspase activity was determined spectrofluorometrically using enzyme specific substrates. The expression of caspase protein was assessed by Western blot analysis using enzyme specific antibody. Caspases-3, -8, and -9 activity and expression was significantly higher in the hypoxic group than in the normoxic group. These results demonstrate that hypoxia induces activation and increased expression of both the initiator caspases and the executioner caspase in the cerebral cortex of newborn piglets. We conclude that hypoxia results in stimulation of both the pathways of caspase-3 activation.     

Methyl‐isobutyl amiloride reduces brain Lac/NAA,cell death and microglial activation in a perinatal asphyxia model

Na⁺/H⁺ exchanger (NHE) blockade attenuates the detrimental consequences of ischaemia and reperfusion in myocardium and brain in adult and neonatal animal studies. Our aim was to use magnetic resonance spectroscopy (MRS) biomarkers and immunohistochemistry to investigate the cerebral effects of the NHE inhibitor, methyl isobutyl amiloride (MIA) given after severe perinatal asphyxia in the piglet. Eighteen male piglets (aged < 24 h) underwent transient global cerebral hypoxia‐ischaemia and were randomized to (i) saline placebo; or (ii) 3 mg/kg intravenous MIA administered 10 min post‐insult and 8 hourly thereafter. Serial phosphorus‐31 (³¹P) and proton (¹H) MRS data were acquired before, during and up to 48 h after hypoxia‐ischaemia and metabolite‐ratio time‐series Area under the Curve (AUC) calculated. At 48 h, histological and immunohistochemical assessments quantified regional tissue injury. MIA decreased thalamic lactate/N‐acetylaspartate and lactate/creatine AUCs (both p < 0.05) compared with placebo. Correlating with improved cerebral energy metabolism, transferase mediated biotinylated d‐UTP nick end‐labelling (TUNEL) positive cell density was reduced in the MIA group in cerebral cortex, thalamus and white matter (all p < 0.05) and caspase 3 immunoreactive cells were reduced in pyriform cortex and caudate nucleus (both p < 0.05). Microglial activation was reduced in pyriform and midtemporal cortex (both p < 0.05). Treatment with MIA starting 10 min after hypoxia‐ischaemia was neuroprotective in this perinatal asphyxia model.     

Region-specific effects on brain metabolites of hypoxia and hyperoxia overlaid on cerebral ischemia in young and old rats: a quantitative proton magnetic resonance spectroscopy study

Background  

Both hypoxia and hyperoxia, deregulating the oxidative balance, may play a role in the pathology of neurodegenerative disorders underlain by cerebral ischemia. In the present study, quantitative proton magnetic resonance spectroscopy was used to evaluate regional metabolic alterations, following a 24-hour hypoxic or hyperoxic exposure on the background of ischemic brain insult, in two contrasting age-groups of rats: young - 3 months old and aged - 24 months old.     

Early cerebral metabolic and electrophysiological recovery during controlled hypoxemic resuscitation in piglets

We tested the hypothesis that controlledhypoxemic resuscitation improves early cerebral metabolic andelectrophysiological recovery in hypoxic newborn piglets. Severelyhypoxic anesthetized piglets were randomly divided into threeresuscitation groups: hypoxemic, 21%O₂, and 100%O₂ groups (8 in each group). Thehypoxemic group was mechanically ventilated with 12-18%O₂ adjusted to achieve a cerebralvenous O₂ saturation of17-23% (baseline; 45 ± 1%). Base excess (BE) reached22 ± 1 mM at the end of hypoxia. During a 2-h resuscitationperiod, no significant differences in time to recovery ofelectroencephalography (EEG), quality of EEG at recovery, orextracellular hypoxanthine concentrations in the cerebral cortex andstriatum were found among the groups. BE and plasma hypoxanthine,however, normalized significantly more slowly during controlledhypoxemic resuscitation than during resuscitation with 21 or 100%O₂. We conclude that early brainrecovery during controlled hypoxemic resuscitation was as efficient as,but not superior to, recovery during resuscitation with 21 or 100%O₂. The systemic metabolicrecovery from hypoxia, however, was delayed during controlled hypoxemicresuscitation.

     

Metabolic Responses of the Dopaminergic System during Hypoxia in Newborn Brain

The purpose of this review is to describe the relationship between the dopamine and amino acid neurotransmitter systems and cortical oxygen pressure during different levels of cerebral hypoxia using newborn piglets as an animal model, adding new data from our laboratory. The extracellular dopamine increases as the oxygen pressure in the cortex decreases. The relationship between oxygen pressure and dopamine levels is the same whether the hypoxia is induced by reduced F_iO₂ (high-flow hypoxia) or by hypocapnia-induced cerebral vasoconstriction (low-flow hypoxia). Thus it appears that, particularly in mild hypoxia, the extracellular level of dopamine depends primarily on the oxygen concentration in the tissue with minimal influence of parameters such as blood flow and pH. There is no "oxygen reserve" in the brain of newborn piglets and the extracellular levels of dopamine in the striatum increase almost linearly with decrease in oxygen pressure, with even small decreases in oxygen pressure resulting in increased dopamine levels. In contrast, the changes in extracellular concentrations of the excitatory amino acids glutamate and aspartate are variable and transient. In a majority of 2- to 5 day-old piglets even very low oxygen pressures in the brain did not result in significant alterations in the extracellular levels of glutamate and aspartate. These changes in the dopaminergic system may contribute directly and indirectly to the neuronal damage that occurs during hypoxic/ischemic insult and reoxygenation in newborn brain, particularly in the striatum. A variety of mechanisms are discussed by which dopamine, in particular extracellular dopamine, can increase cellular toxicity.     

Effect of acute hypoxia on respiration and brain stem blood flow in the piglet

Changes in local brain stem perfusion that alter extracellular fluid Pco2 and/or [H+] near central chemoreceptors may contribute to the decrease in respiration observed during hypoxia after peripheral chemoreceptor denervation and to the delayed decrease observed during hypoxia in the newborn. In this study, we measured the changes in respiration and brain stem blood flow (BBF) during 2-4 min of hypoxic hypoxia in both intact and denervated piglets and calculated the changes in brain stem Pco2 and [H+] that would be expected to occur as a result of the changes in BBF. All animals were anesthetized, spontaneously breathing, and between 2 and 7 days of age. Respiratory and other variables were measured before and during hypoxia in all animals, and BBF (microspheres) was measured in a subgroup of intact and denervated animals at 0, 30, and 260 s and at 0 and 80 s, respectively. During hypoxia, minute ventilation increased and then decreased (biphasic response) in the intact animals but decreased only in the denervated animals. BBF increased in a near linear fashion, and calculated brain stem extracellular fluid Pco2 and [H+] decreased over the first 80 s both before and after denervation. We speculate that a rapid increase in BBF during acute hypoxia decreases brain stem extracellular fluid Pco2 and [H+], which, in turn, negatively modulate the increase in respiratory drive produced by peripheral chemoreceptor input to the central respiratory generator.     

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

Waters, Karen A., André Laferrière, JuliePaquette, Cynthia Goodyer, and Immanuela R. Moss. Curtailedrespiration by repeated vs. isolated hypoxia in maturing piglets isunrelated to NTS ME or SP levels. J. Appl.Physiol. 83(2): 522-529, 1997.In earlydevelopment, respiratory disorders can produce recurring hypoxicepisodes during sleep. To examine possible effects of daily repeatedvs. isolated hypoxic hypoxia, cardiorespiratory functions and central,respiratory-related neuromodulator levels in 21- to 32-day-old,chronically instrumented, unsedated piglets were compared between afifth sequential daily hypoxia and an isolated hypoxia (10%O₂-90%N₂ for 30 min). Diaphragmaticelectromyographic activity, heart rate and arterial pressure, and pHand gas tensions were measured. In vivo microdialysis, via chronicallyimplanted guides, served to sample interstitial substance P (SP) andmethionine-enkephalin (ME) at the level of the respiratory-relatednucleus tractus solitarii (NTS). Compared with an isolated hypoxia,repeated hypoxia resulted in 1)lower respiratory frequency (f), ventilation equivalent, and arterialpH, higher arterial PO₂during hypoxia, and lower f in recovery from hypoxia; and2) increased SP concentrations butno change in ME concentrations. We conclude that, in these maturingswine, repeated vs. isolated hypoxic exposure curtails respiratoryresponses to hypoxia by a mechanism(s) unrelated to SP or ME levels atthe NTS.