Responses of foetal and newborn monkeys to asphyxia

A reliable and cost-effective protocol for creating bilirubin cerebral palsy model in rhesus macaque

BACKGROUND

Cerebral palsy is a severe motor disability in childhood that poses challenges for children, families, and society. Rhesus macaques are the preferred animals for cerebral palsy model, but surgical excision of motor cortex has low success rate and high cost. In this work, we created cerebral palsy rhesus macaque models by intrathecal injection of bilirubin.

METHODS

The puncture point for injection was identified as the intervertebral disc space two, located below the intersection of the iliac crest line and the posterior median line.

RESULTS

The models showed abnormal posture and increased muscle tension. Diffuse deposits of bilirubin were found in the basal ganglia from the magnetic resonance imaging. Pathological slides also revealed the presence of brain lesions, such as vacuole formation, contraction of neuronal nuclei, and deep staining of nuclei in the histopathological sections of the hippocampus and basal ganglia.

CONCLUSION

The model's symptoms closely resemble those observed in humans with spastic cerebral palsy.

Evaluation of Rhesus Macaque Models for Cerebral Palsy

Animal models play a central role in all areas of biomedical research. The similarities in anatomical structure and physiological characteristics shared by non-human primates (NHPs) and humans make NHPs ideal models with which to study human disorders, such as cerebral palsy (CP). However, the methodologies for systematically evaluating NHP models of CP have rarely been assessed, despite the long history of using NHP models to understand CP. Such models should be evaluated using multidisciplinary approaches prior to being used to research the diagnosis and treatment of CP. In this study, we evaluated rhesus macaque CP models established by partial resection of the motor cortex and intrathecal injection of bilirubin. Abnormal posture, motor dysfunction, gross and fine motor behavior, and muscular tension were evaluated, and changes in the cerebral cortex and basal ganglia were observed using 9.4 T magnetic resonance imaging. The results clearly demonstrated the utility of the established evaluation methodology for assessing CP models. This model evaluation methodology may guide researchers through the model building process.

Sex, drugs and rock and roll: tales from preterm fetal life

Premature fetuses and babies are at greater risk of mortality and morbidity than their term counterparts. The underlying causes are multifactorial, but include exposure to hypoxia. Immaturity of organs and their functional control may impair the physiological defence responses to hypoxia and the preterm fetal responses, or lack thereof, to moderate hypoxia appear to support this concept. However, as this review demonstrates, despite immaturity, the preterm fetus responds to asphyxia in a qualitatively similar manner to that seen at term. This highlights the importance in understanding metabolism versus homeostatic threat when assessing fetal responses to adverse challenges such as hypoxia. Data are presented to show that the preterm fetal adaptation to asphyxia is triphasic in nature. Phase one represents the rapid institution of maximal defences, designed to maintain blood pressure and central perfusion at the expense of peripheral organs. Phase two is one of adaptive compensation. Controlled reperfusion partially offsets peripheral tissue oxygen debt, while maintaining sufficient vasoconstriction to limit the fall in perfusion. Phase three is about decompensation. Strikingly, the preterm fetus generally performs better during phases two and three, and can survive for longer without injury. Paradoxically, however, the ability to survive can lead to longer exposure to hypotension and hypoperfusion and thus potentially greater injury. The effects of fetal sex, inflammation and drugs on the triphasic adaptations are reviewed. Finally, the review highlights the need for more comprehensive studies to understand the complexity of perinatal physiology if we are to develop effective strategies to improve preterm outcomes.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Animal models for the study of perinatal hypoxic-ischemic encephalopathy: a critical analysis

We critically evaluated various design features from 292 animal studies related to perinatal hypoxic-ischemic encephalopathy (HIE). Rodents were the most frequently used animals in HIE research (26%), followed by piglets (23%) and sheep (22%). Asphyxia with or without ischemia was the most predominant method of producing experimental brain damage, but there were significant variations in specific details, particularly regarding the method and duration of brain insult. In 71% (207/292) of studies the CNS outcomes were tested within 24 h of experimental insult and in 29% (85/292) they were tested 24 h or more after the insult. Acute CNS metabolic end-points were assessed in 82-100% of all studies. In 90% of studies the chronological age of the animal was equivalent to that of human term newborn infant. However, in only 23% (67/292) were clinical neurological, developmental or behavioral outcomes evaluated, and in only 26% (76/292) was neuropathology assessed. While no single animal model was found to be ideal for all HIE research, some models were distinctly superior to others, depending upon the specific research question. The fetal sheep, newborn lamb and piglet models are well suited for the study of acute and subacute metabolic and physiologic endpoints, whereas the rodent and primate models could be used for long-term neurological and behavioral outcome experiments as well. We also feel that standardizing the study design features, including an HI insult method that produces consistent and predictable brain damage is urgently needed. Studies in neuro-ethology should explore how well brains of various animals compare with that of the newborn human infant. There is also a need for developing animal models that mimic clinical entities in which long-term neuro-developmental and behavioral outcomes can be assessed.

A piglet survival model of posthypoxic encephalopathy

The aim of this study was to produce a neonatal piglet model which, avoiding vessel ligation, exposed the whole animal to hypoxia and produced dose-dependent clinical encephalopathy and neuropathologic damage similar to that seen after birth asphyxia. Twenty-three piglets were halothane-anesthetized. Hypoxia was induced in 19 piglets by reducing the fractional concentration of inspired oxygen (FiO2) to the maximum concentration at which the EEG amplitude was below 7 microV (low amplitude) for 17-55 min. There were transient increases in Fio2 to correct bradycardia and hypotension. Posthypoxia, the piglets were extubated when breathing was stable. Four were sham-treated controls. We aimed at 72-h survival; seven died prematurely due to posthypoxic complications. EEG and a videotaped itemized neurologic assessment were recorded regularly. We found that 95% of the animals showed neuropathologic damage. The duration of low amplitude EEG during the insult and the arterial pH at the end of the insult correlated with cortical/white matter damage; r = 0.75 and 0.81, respectively. Early postinsult EEG background amplitude (r = 0.86 at 3 h) and neurologic score (r = 0.79 at 8 h) correlated with neuropathology. Epileptic seizures in seven animals were always associated with severe neuropathologic damage. We conclude that EEG-controlled hypoxia and subsequent intensive care enabled the animals to survive with an encephalopathy which correlated with the cerebral hypoxic insult. The encephalopathy was clinically, electrophysiologically, and neuropathologically similar to that in the asphyxiated term infant. This model is suitable for examining mechanisms of damage and evaluation of potential protective therapies after birth asphyxia.

The effect of age on susceptibility to brain damage in a model of global hemispheric hypoxia-ischemia

Stroke occurs in all age groups, ranging from the newborn to the elderly. The immature brain is generally believed to be more resistant to the damaging effects of cerebrovascular compromise compared to the more mature brain. However, recent experiments suggest that the correlation between brain damage and age is not linear. To determine the effects of age and development on hypoxic-ischemic brain damage, we developed a model whereby rats of increasing age received identical cerebrovascular insults, and assessed neuropathologic outcome. Male Wistar rats of 1, 3, 6, and 9 weeks and 6 months underwent unilateral common carotid artery ligation and exposure to 12% oxygen for 35 min. Animals were all spontaneously breathing under light halothane anesthesia (0.5%). Core temperatures were maintained at 37 degrees C. Blood pressures were monitored via indwelling carotid artery catheters on the side ipsilateral to the carotid artery ligation. Cerebral blood flow was assessed in separate groups utilizing Laser Doppler flowmetry. Physiologic monitoring revealed that under these experimental conditions, mean arterial blood pressure and cerebral blood flow decreased to the same extent in each of the age groups, verifying that all animals experienced an identical insult. Neuropathologic assessment at 7 days of recovery showed that brain damage was most severe in the 1 and 3 week old animals followed by those that were 6 months. The 6 and 9 week old groups had significantly less injury than the other 3 age groups. Hippocampal damage was most severe in the 3 week and 6 month old rats compared to all other age groups. Our findings contrast previously held beliefs regarding the enhanced tolerance of the immature brain to hypoxic-ischemic damage and demonstrates that, in a physiologically controlled in vivo model of hemispheric global ischemia, (1) the immature brain is, in fact, less resistant to hypoxic-ischemic brain damage than its adult counterpart, (2) the brain damaging effects of hypoxic-ischemia are age dependent, but do not increase linearly with advancing age and development, and (3) the intermediate age groups are more tolerant to hypoxic-ischemic brain injury than either very young or more mature ages.

Bradycardia preceding apneic attacks in low-birthweight infants. The relationship and management

Two premature infants had frequent episodes of prolonged apnea. The apneic spells were not due to the more commonly known causes of apnea in infancy, but were consistently preceded by severe bradycardia. Atropine or ephedrine produced favorable therapeutic results. Since severe bradycardia may be a cause of sudden death in infants, its recognition and treatment is important in the management of apneic infants.

Development behaviors: delayed appearance in monkeys asphyxiated at birth

Developmental behaviors were studied in monkeys subjected to asphyxia at birth. Visual depth perception, visual pla ing, and locomotion appeared significantly later than in nonasphyxiated monkeys. After these behaviors had been established in asphyxiates, however, there was little difference from those observed in normal monkeys. These results were compared with reports of permanent learning deficits that occur in monkeys asphyxiated at birth for similar periods of time. Such comparison suggests that the neural structures responsible for the developmental behaviors studied are not damaged by asphyxia to the same extent as those for acquisition. Delay in development may be an early indication of brain damage with subsequent mental retardation.

Effects of neonatal hypoglycaemia on the nervous system: a pathological study

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Respiratory Distress: Relation to Prematurity and Other Factors in Newborn Monkeys

A respiratory distress syndrome resembling that seen in human infants was encountered in 4 out of 90 rhesus monkey infants after uncomplicated births. These were nonviable immature infants weighing less than 350 grams. A much higher incidence of respiratory distress was observed in those whose births were complicated by experimental procedures, mainly asphyxiation. Thirty-four out of 68 infants developed the syndrome, the incidence being greatest among the least mature.

Neuropathology of Certain Forms of Mental Retardation: Experiments on monkeys illustrate probable mechanisms of brain damage in human infants

In presenting the neuropathology of asphyxia neonatorum and some related conditions, as illustrated in experiments with monkeys (12), I have tried to make six principal points. (i) Newborn monkeys need not be asphyxiated to the point of terminal apnea to suffer structural brain damage. Mental retardation has not been proved, but neither has it been excluded, in these monkeys. (ii) Asphyxia neonatorum requiring resuscitation of the offspring, which otherwise would die, is associated with a remarkably constant syndrome of bilaterally symmetrical, nonhemorrhagic lesions in thalamic and brain-stem nuclei, mainly those of afferent systems. The individuals in this category clearly are retarded. (iii) Asphyxia neonatorum of this second degree may be associated with postpartum complications leading to neocortical atrophy, often of considerable magnitude. The individuals are markedly retarded, often palsied, epileptic, and in a few instances even comatose. (iv) Increased intrauterine pressure in the monkey during prolonged labor leads to fetal and postpartum depression, in connection with which cerebral cortical injury occurs in the absence of typical asphyxial lesions. (v) The relationship of cerebral hemorrhages to mental retardation is not clear, but their presence at autopsy probably signifies trauma during birth, or is an agonal artifact associated with death after postpartum depression. (vi) Finally, kernicterus, a condition in which bile pigment escapes into the brain tissues from the blood when the bilirubin level is high and when there is, in combination with it, some depressing factor such as asphyxia, has been produced in the newborn monkey. It, too, is associated with mental retardation.

Hemodynamic Changes in Fetal Lamb in Utero in Response to Asphyxia, Hypoxia, and Hypercapnia

The effects of hypoxia (administration of 10 per cent O 2 in N 2 and constriction of the uterine arteries), of hypercapnia (administration of 8 per cent CO 2 in air), and of asphyxia (obstruction of the umbilical circulation) were studied on the fetus in utero in near-term pregnant ewes under spinal anesthesia.

Hypoxia and hypercapnia produced increases in the maternal arterial pressure, maternal heart rate, and uterine blood flow. Fetal arterial pressure and carotid and umbilical blood flows increased, while the fetal heart rate decreased. Fetal femoral flow decreased during hypoxia and increased during hypercapnia. There was a delay of five to six minutes between the initiation of hypoxia or hypercapnia and the appearance of fetal circulatory changes.

Transient reduction of uterine blood flow evoked insignificant hemodynamic alterations in the mother and fetus. A reduction of flow greater than 50 per cent of control values produced a fetal bradycardia along with increases in fetal arterial pressure, carotid and umbilical flows.

Constriction of the umbilical veins with the umbilical arteries intact produced a prompt bradycardia together with a decrease in fetal arterial pressure and regional blood flows. Constriction of the umbilical arteries alone also produced a bradycardia with an initial rise in arterial pressure and carotid flow. Similar changes were observed after cutting the umbilical cord. Elimination of the low resistance of the placenta played a major role in these changes.

Placing the lamb in the head-down position caused a marked increase in carotid blood flow without greatly modifying arterial pressure.