Animal models of perinatal asphyxia: contributions, contradictions, clinical relevance

Developmental anatomy of the thalamus, perinatal lesions, and neurological development

The thalamic nuclei develop before a viable preterm age. GABAergic neuronal migration is especially active in the third trimester. Thalamic axons meet cortical axons during subplate activation and create the definitive cortical plate in the second and third trimesters. Default higher-order cortical driver connections to the thalamus are then replaced by the maturing sensory networks, in a process that is driven by first-order thalamic neurons. Surface electroencephalographic activity, generated first in the subplate and later in the cortical plate, gradually show oscillations based on the interaction of the cortex with thalamus, which is controlled by the thalamic reticular nucleus. In viable newborn infants, in addition to sensorimotor networks, the thalamus already contributes to visual, auditory, and pain processing, and to arousal and sleep. Isolated thalamic lesions may present as clinical seizures. In addition to asphyxia and stroke, infection and network injury are also common. Cranial ultrasound can be used to classify neonatal thalamic injuries based on functional parcelling of the mature thalamus. We provide ample illustration and a detailed description of the impact of neonatal focal thalamic injury on neurological development, and discuss the potential for neuroprotection based on thalamocortical plasticity.

Development of a neonatal Göttingen Minipig model for dose precision in perinatal asphyxia: technical opportunities, challenges, and potential further steps

Animal models provide useful information on mechanisms in human disease conditions, but also on exploring (patho)physiological factors affecting pharmacokinetics, safety, and efficacy of drugs in development. Also, in pediatric patients, nonclinical data can be critical for better understanding the disease conditions and developing new drug therapies in this age category. For perinatal asphyxia (PA), a condition defined by oxygen deprivation in the perinatal period and possibly resulting in hypoxic ischemic encephalopathy (HIE) or even death, therapeutic hypothermia (TH) together with symptomatic drug therapy, is the standard approach to reduce death and permanent brain damage in these patients. The impact of the systemic hypoxia during PA and/or TH on drug disposition is largely unknown and an animal model can provide useful information on these covariates that cannot be assessed separately in patients. The conventional pig is proven to be a good translational model for PA, but pharmaceutical companies do not use it to develop new drug therapies. As the Göttingen Minipig is the commonly used pig strain in nonclinical drug development, the aim of this project was to develop this animal model for dose precision in PA. This experiment consisted of the instrumentation of 24 healthy male Göttingen Minipigs, within 24 h of partus, weighing approximately 600 g, to allow the mechanical ventilation and the multiple vascular catheters inserted for maintenance infusion, drug administration and blood sampling. After premedication and induction of anesthesia, an experimental protocol of hypoxia was performed, by decreasing the inspiratory oxygen fraction (FiO₂) at 15%, using nitrogen gas. Blood gas analysis was used as an essential tool to evaluate oxygenation and to determine the duration of the systemic hypoxic insult to approximately 1 h. The human clinical situation was mimicked for the first 24 h after birth in case of PA, by administering four compounds (midazolam, phenobarbital, topiramate and fentanyl), frequently used in a neonatal intensive care unit (NICU). This project aimed to develop the first neonatal Göttingen Minipig model for dose precision in PA, allowing to separately study the effect of systemic hypoxia versus TH on drug disposition. Furthermore, this study showed that several techniques that were thought to be challenging or even impossible in these very small animals, such as endotracheal intubation and catheterization of several veins, are feasible by trained personnel. This is relevant information for laboratories using the neonatal Göttingen Minipig for other disease conditions or drug safety testing.

Dogs as a Natural Animal Model of Epilepsy

Epilepsy is a common neurological disease in both humans and domestic dogs, making dogs an ideal translational model of epilepsy. In both species, epilepsy is a complex brain disease characterized by an enduring predisposition to generate spontaneous recurrent epileptic seizures. Furthermore, as in humans, status epilepticus is one of the more common neurological emergencies in dogs with epilepsy. In both species, epilepsy is not a single disease but a group of disorders characterized by a broad array of clinical signs, age of onset, and underlying causes. Brain imaging suggests that the limbic system, including the hippocampus and cingulate gyrus, is often affected in canine epilepsy, which could explain the high incidence of comorbid behavioral problems such as anxiety and cognitive alterations. Resistance to antiseizure medications is a significant problem in both canine and human epilepsy, so dogs can be used to study mechanisms of drug resistance and develop novel therapeutic strategies to benefit both species. Importantly, dogs are large enough to accommodate intracranial EEG and responsive neurostimulation devices designed for humans. Studies in epileptic dogs with such devices have reported ictal and interictal events that are remarkably similar to those occurring in human epilepsy. Continuous (24/7) EEG recordings in a select group of epileptic dogs for >1 year have provided a rich dataset of unprecedented length for studying seizure periodicities and developing new methods for seizure forecasting. The data presented in this review substantiate that canine epilepsy is an excellent translational model for several facets of epilepsy research. Furthermore, several techniques of inducing seizures in laboratory dogs are discussed as related to therapeutic advances. Importantly, the development of vagus nerve stimulation as a novel therapy for drug-resistant epilepsy in people was based on a series of studies in dogs with induced seizures. Dogs with naturally occurring or induced seizures provide excellent large-animal models to bridge the translational gap between rodents and humans in the development of novel therapies. Furthermore, because the dog is not only a preclinical species for human medicine but also a potential patient and pet, research on this species serves both veterinary and human medicine.

Current Therapies for Neonatal Hypoxic-Ischaemic and Infection-Sensitised Hypoxic-Ischaemic Brain Damage

Neonatal hypoxic-ischaemic brain damage is a leading cause of child mortality and morbidity, including cerebral palsy, epilepsy, and cognitive disabilities. The majority of neonatal hypoxic-ischaemic cases arise as a result of impaired cerebral perfusion to the foetus attributed to uterine, placental, or umbilical cord compromise prior to or during delivery. Bacterial infection is a factor contributing to the damage and is recorded in more than half of preterm births. Exposure to infection exacerbates neuronal hypoxic-ischaemic damage thus leading to a phenomenon called infection-sensitised hypoxic-ischaemic brain injury. Models of neonatal hypoxia-ischaemia (HI) have been developed in different animals. Both human and animal studies show that the developmental stage and the severity of the HI insult affect the selective regional vulnerability of the brain to damage, as well as the subsequent clinical manifestations. Therapeutic hypothermia (TH) is the only clinically approved treatment for neonatal HI. However, the number of HI infants needed to treat with TH for one to be saved from death or disability at age of 18-22 months, is approximately 6-7, which highlights the need for additional or alternative treatments to replace TH or increase its efficiency. In this review we discuss the mechanisms of HI injury to the immature brain and the new experimental treatments studied for neonatal HI and infection-sensitised neonatal HI.

How to Improve the Antioxidant Defense in Asphyxiated Newborns-Lessons from Animal Models

Oxygen free radicals have been implicated in brain damage after neonatal asphyxia. In the early phase of asphyxia/reoxygenation, changes in antioxidant enzyme activity play a pivotal role in switching on and off the cascade of events that can kill the neurons. Hypoxia/ischemia (H/I) forces the brain to activate endogenous mechanisms (e.g., antioxidant enzymes) to compensate for the lost or broken neural circuits. It is important to evaluate therapies to enhance the self-protective capacity of the brain. In animal models, decreased body temperature during neonatal asphyxia has been shown to increase cerebral antioxidant capacity. However, in preterm or severely asphyxiated newborns this therapy, rather than beneficial seems to be harmful. Thus, seeking new therapeutic approaches to prevent anoxia-induced complications is crucial. Pharmacotherapy with deferoxamine (DFO) is commonly recognized as a beneficial regimen for H/I insult. DFO, via iron chelation, reduces oxidative stress. It also assures an optimal antioxidant protection minimizing depletion of the antioxidant enzymes as well as low molecular antioxidants. In the present review, some aspects of recently acquired insight into the therapeutic effects of hypothermia and DFO in promoting neuronal survival after H/I are discussed.

A Physiology-Based Pharmacokinetic Framework to Support Drug Development and Dose Precision During Therapeutic Hypothermia in Neonates

Therapeutic hypothermia (TH) is standard treatment for neonates (≥36 weeks) with perinatal asphyxia (PA) and hypoxic-ischemic encephalopathy. TH reduces mortality and neurodevelopmental disability due to reduced metabolic rate and decreased neuronal apoptosis. Since both hypothermia and PA influence physiology, they are expected to alter pharmacokinetics (PK). Tools for personalized dosing in this setting are lacking. A neonatal hypothermia physiology-based PK (PBPK) framework would enable precision dosing in the clinic. In this literature review, the stepwise approach, benefits and challenges to develop such a PBPK framework are covered. It hereby contributes to explore the impact of non-maturational PK covariates. First, the current evidence as well as knowledge gaps on the impact of PA and TH on drug absorption, distribution, metabolism and excretion in neonates is summarized. While reduced renal drug elimination is well-documented in neonates with PA undergoing hypothermia, knowledge of the impact on drug metabolism is limited. Second, a multidisciplinary approach to develop a neonatal hypothermia PBPK framework is presented. Insights on the effect of hypothermia on hepatic drug elimination can partly be generated from in vitro (human/animal) profiling of hepatic drug metabolizing enzymes and transporters. Also, endogenous biomarkers may be evaluated as surrogate for metabolic activity. To distinguish the impact of PA versus hypothermia on drug metabolism, in vivo neonatal animal data are needed. The conventional pig is a well-established model for PA and the neonatal Göttingen minipig should be further explored for PA under hypothermia conditions, as it is the most commonly used pig strain in nonclinical drug development. Finally, a strategy is proposed for establishing and fine-tuning compound-specific PBPK models for this application. Besides improvement of clinical exposure predictions of drugs used during hypothermia, the developed PBPK models can be applied in drug development. Add-on pharmacotherapies to further improve outcome in neonates undergoing hypothermia are under investigation, all in need for dosing guidance. Furthermore, the hypothermia PBPK framework can be used to develop temperature-driven PBPK models for other populations or indications. The applicability of the proposed workflow and the challenges in the development of the PBPK framework are illustrated for midazolam as model drug.

Electroencephalogram studies of hypoxic ischemia in fetal and neonatal animal models

Alongside clinical achievements, experiments conducted on animal models (including primate or non-primate) have been effective in the understanding of various pathophysiological aspects of perinatal hypoxic/ischemic encephalopathy (HIE). Due to the reasonably fair degree of flexibility with experiments, most of the research around HIE in the literature has been largely concerned with the neurodevelopmental outcome or how the frequency and duration of HI seizures could relate to the severity of perinatal brain injury, following HI insult. This survey concentrates on how EEG experimental studies using asphyxiated animal models (in rodents, piglets, sheep and non-human primate monkeys) provide a unique opportunity to examine from the exact time of HI event to help gain insights into HIE where human studies become difficult.

Surge of Peripheral Arginine Vasopressin in a Rat Model of Birth Asphyxia

Mammalian birth is accompanied by a period of obligatory asphyxia, which consists of hypoxia (drop in blood O₂ levels) and hypercapnia (elevation of blood CO₂ levels). Prolonged, complicated birth can extend the asphyxic period, leading to a pathophysiological situation, and in humans, to the diagnosis of clinical birth asphyxia, the main cause of hypoxic-ischemic encephalopathy (HIE). The neuroendocrine component of birth asphyxia, in particular the increase in circulating levels of arginine vasopressin (AVP), has been extensively studied in humans. Here we show for the first time that normal rat birth is also accompanied by an AVP surge, and that the fetal AVP surge is further enhanced in a model of birth asphyxia, based on exposing 6-day old rat pups to a gas mixture containing 4% O₂ and 20% CO₂ for 45 min. Instead of AVP, which is highly unstable with a short plasma half-life, we measured the levels of copeptin, the C-terminal part of prepro-AVP that is biochemically much more stable. In our animal model, the bulk of AVP/copeptin release occurred at the beginning of asphyxia (mean 7.8 nM after 15 min of asphyxia), but some release was still ongoing even 90 min after the end of the 45 min experimental asphyxia (mean 1.2 nM). Notably, the highest copeptin levels were measured after hypoxia alone (mean 14.1 nM at 45 min), whereas copeptin levels were low during hypercapnia alone (mean 2.7 nM at 45 min), indicating that the hypoxia component of asphyxia is responsible for the increase in AVP/copeptin release. Alternating the O₂ level between 5 and 9% (CO₂ at 20%) with 5 min intervals to mimic intermittent asphyxia during prolonged labor resulted in a slower but quantitatively similar rise in copeptin (peak of 8.3 nM at 30 min). Finally, we demonstrate that our rat model satisfies the standard acid-base criteria for birth asphyxia diagnosis, namely a drop in blood pH below 7.0 and the formation of a negative base excess exceeding -11.2 mmol/l. The mechanistic insights from our work validate the use of the present rodent model in preclinical work on birth asphyxia.

Animal models of hypoxic-ischemic encephalopathy: optimal choices for the best outcomes

Hypoxic-ischemic encephalopathy (HIE), a serious disease leading to neonatal death, is becoming a key area of pediatric neurological research. Despite remarkable advances in the understanding of HIE, the explicit pathogenesis of HIE is unclear, and well-established treatments are absent. Animal models are usually considered as the first step in the exploration of the underlying disease and in evaluating promising therapeutic interventions. Various animal models of HIE have been developed with distinct characteristics, and it is important to choose an appropriate animal model according to the experimental objectives. Generally, small animal models may be more suitable for exploring the mechanisms of HIE, whereas large animal models are better for translational studies. This review focuses on the features of commonly used HIE animal models with respect to their modeling strategies, merits, and shortcomings, and associated neuropathological changes, providing a comprehensive reference for improving existing animal models and developing new animal models.

Perinatal hypoxic-ischemic encephalopathy

Perinatal hypoxic-ischemic encephalopathy (HIE) is an important cause of brain injury in the newborn and can result in long-term devastating consequences. Perinatal hypoxia is a vital cause of long-term neurologic complications varying from mild behavioural deficits to severe seizure, mental retardation, and/or cerebral palsy in the newborn. In the mammalian developing brain, ongoing research into pathophysiological mechanism of neuronal injury and therapeutic strategy after perinatal hypoxia is still limited. With the advent of promising therapy of hypothermia in HIE, this paper reviews the pathophysiology of HIE and the future potential neuroprotective strategies for clinical potential for hypoxia sufferers.

Periventricular leucomalacia (PVL)-like lesions in two neonatal cynomolgus monkeys (Macaca fascicularis)

Periventricular leucomalacia (PVL) is a lesion of immature cerebral white matter that occurs in the perinatal period. In man, PVL is the predominant form of brain injury and a cause of cerebral palsy and cognitive deficits in premature infants. PVL affects fetuses and newborns, particularly those who have undergone oxygen deprivation as may occur in premature birth. Many clinical and pathological studies of PVL have been performed in man, but there is no clear definition of PVL in animals. A few spontaneous PVL-like cases in puppies or experimental cases in other animal species have been reported. The present study reports the histopathological and immunohistochemical features of PVL-like lesions in two neonatal cynomolgus monkeys. In both cases, there was cerebral white matter necrosis with marked infiltration of lipid-laden phagocytes and a reduction of neurons in the cerebral cortex. In case 1 there was extensive cavitation of the cerebral white matter. In case 2 there was reactive astrocytosis associated with a decrease in oligodendroglial cells and a decrease in cerebral white matter myelin. To our knowledge, this is the first report of PVL-like leucoencephalomalacia in non-human primates.

Traumatic brain injury in infants: the phenomenon of subdural hemorrhage with hemispheric hypodensity (“Big Black Brain”)

Clinical and experimental studies of traumatic brain injury during immaturity have been far less numerous than those involving adults, and many questions remain about differences in injury responses among patients of different ages. This chapter reviews a distinctive injury pattern common in infants, the so-called "big black brain" response to acute subdural hematoma. The pathophysiology of this injury remains incompletely understood. Insights from both clinical observation and experimental studies have helped to clarify the probable causes of this injury pattern, which appears to require a combination of stressors during a particular period of maturation.

Animal models of germinal matrix hemorrhage

Germinal matrix hemorrhage refers to bleeding that arises from the subependymal (or periventricular) germinal region of the immature brain. Clinical studies have shown that infants who experience germinal matrix hemorrhage can develop hydrocephalus or suffer from long-term neurologic dysfunction, including cerebral palsy, seizures, and learning disabilities. Understanding the causative factors and the pathogenesis of subsequent brain damage is important if germinal matrix hemorrhage is to be prevented or treated. Appropriate animal models are necessary to achieve this understanding. A number of animal species, including mice, rats, rabbits, sheep, pigs, dogs, cats, and primates, have been used to model germinal matrix hemorrhage. This literature review critically evaluates the animal models of germinal matrix hemorrhage. Each model has its own advantages and disadvantages; no single model is suitable for the study of all aspects of brain damage.

Brain Injury from Cardiac Arrest in Children

This article reviews the important differences between children and adults suffering brain injury following cardiac arrest. The differences in etiology, pathophysiology, neuronal vulnerability, and repair in the context of the developing brain are reviewed. The available clinical data are reviewed, and selected treatment priori-ties are declared. The article includes a discussion of knowledge gaps and future directions.

Hypoxic-ischemic insult decreases glutamate uptake by hippocampal slices from neonatal rats: prevention by guanosine

Brain injury secondary to hypoxic-ischemic disease is the predominant form of damage encountered in the perinatal period. The impact of neonatal hypoxia-ischemia (HI) in 7-day-old pups on the high-affinity [3H] glutamate uptake into hippocampal slices at different times after insult was examined. Immediately following, and 1 day after the insult there was no effect. But at 3 to 5 days after the HI insult, glutamate uptake into the hippocampus was markedly reduced; however, after 30 or 60 days the glutamate uptake into hippocampal slices returned to control levels. Also, this study demonstrated the effect of the nucleoside guanosine (Guo) on the [3H] glutamate uptake in neonatal HI injury, maintaining the [3H] glutamate uptake at control levels when injected before and after insult HI. We conclude that neonatal HI influences glutamate uptake a few days following insult, and that guanosine prevents this action.

Effects of sensorimotor restriction and anoxia on gait and motor cortex organization: implications for a rodent model of cerebral palsy

Chronic or acute perinatal asphyxia (PA) has been correlated with the subsequent development of cerebral palsy (CP), a developmental neurological disorder characterized by spasticity and motor abnormalities often associated with cognitive deficits. Despite the prevalence of CP, an animal model that mimics the lifetime hypertonic motor deficits is still not available. In the present study, the consequences of PA on motor behavior, gait and organization of the primary motor cortex were examined in rats, and compared with the behavioral and neurological consequences of early postnatal movement-restriction with or without oxygen deprivation. Rats subjected to PA had mild increases in muscular tone accompanied by subtle differences in walking patterns, paralleled by significantly altered but relatively modest disorganization of their primary motor cortices. Movement-restricted rats, suffering PA or not, had reduced body growth rate, markedly increased muscular tone at rest and with active flexion and extension around movement-restricted joints that resulted in abnormal walking patterns and in a profoundly distorted representation of the hind limbs in the primary motor cortex. Within the sensorimotor-restricted groups, non-anoxic rats presented the most abnormal pattern and the greatest cortical representational degradation. This outcome further supports the argument that PA per se may represent a substrate for subtle altered motor behaviors, and that PA alone is sufficient to alter the organization of the primary motor cortex. At the same time, they also show that early experience-dependent movements play a crucial role in shaping normal behavioral motor abilities, and can make a powerful contribution to the genesis of aberrant movement abilities.

Non-human primate models of neonatal brain injury

Nonhuman primate species have been selectively used in the scientific investigation of adult and newborn neurological diseases. The rhesus monkey has been utilized in models of term asphyxial insults, accurately reflecting the mechanisms and neuropathology demonstrated in the newborn human infant. More recently, a premature baboon model developed for evaluation of bronchopulmonary dysplasia has been applied to the investigation of cerebral development and injury, revealing high similarity in neuropathology to the premature human infant. Given the differences in the outcomes of neuroprotective therapies between lower order species, such as the rat, and human trials in disorders such as stroke, nonhuman primate models may provide an invaluable resource for safety and efficacy testing before trials in human newborns. This article summarizes both models of brain injury. The histologic findings from the models are compared with neuropathological studies in human infants.

Periventricular and Subcortical Leukoencephalopathy in two Dachshund Puppies

Two wirehaired dachshund puppies were presented with generalized tremor and gait abnormalities characterized by mild ataxia, tetraparesis and slightly abnormal proprioception. Neurological examination led to the suspicion of a diffuse generalized white matter lesion. Computerized tomography and pathological examination revealed a remarkable unilateral dilatation of the lateral ventricles in each dog. Histopathological examination showed a severe reduction of stainable myelin, widespread mild perineuronal oedema with vacuolations and multifocal reactive astrocytosis affecting the subcortical and deep periventricular white, and to a lesser degree, grey matter of the cerebral hemispheres, most prominently at the level of the optic chiasm. Axons showed a moderately reduced packing density; some axons were irregularly shaped and slightly thickened. There was no evidence of myelin breakdown products and neurones appeared to be well preserved. Brain stem, cerebellum and spinal cord were normal, as was the peripheral nervous system. This leukoencephalopathy in two dachshund puppies most closely resembles human periventricular leukomalacia caused by pre- or perinatal hypoxia-ischaemia.

Models of brain injury and alterations in synaptic plasticity

Animal models are crucial for understanding human pathophysiological processes and for understanding how connections are injured, lost, or even regenerated and/or repaired. When animal models are used in conjunction with theoretical computational models, an ideal combination is achieved that potentially yields insight and encourages the formation of new theories concerning connectionism, cognitive functioning, and synaptic mechanisms. Mechanisms regulating glutamate receptor activation and intracellular calcium levels are important for normal synaptic transmission. These mechanisms (and others) are also critical during and after brain injury when the potential exists for these mechanisms to function pathologically. Interestingly enough, the regulation of glutamate receptor activation and intracellular calcium levels is also involved in normal processes of neuronal and synaptic plasticity. In addition, studies have shown that neurotrophins and cytokines, which are released after brain injury, can be neuroprotective and may also be important in synaptic plasticity. Furthermore, synaptic plasticity is a phenomenon thought by many to be necessary for memory encoding. If this is the case, then research described in this review has significant scientific merit concerning plasticity and memory and clinical benefit for understanding pathophysiologic processes associated with brain injury and memory impairment. This paper reviews the application of experimental animal models of brain injury for simulating conditions of stroke, trauma, and epilepsy (and/or seizure generation) and the associated cellular mechanisms of brain injury. The paper also briefly addresses the advantage of using computational models in combination with experimental models for hypothesis building and for aiding in the interpretation of empirical data. Finally, it reviews studies concerning brain injury and synaptic plasticity.

Brain damage in preterm newborns: might enhancement of developmentally regulated endogenous protection open a door for prevention?

We present a two-component model of brain white matter damage in preterm neonates. The insult component comprises infection and hypoxia-ischemia, which are both associated with inflammation-related abnormalities in the white matter. The developmental component comprises at least three factors, ie, immaturity of the ependymal/endothelial, oligodendroglial, and endogenous protection systems. All three factors are likely contributors to an increased vulnerability of the preterm newborn's white matter. In this article, we focus on recent developments in oligodendrocyte biology that support the view of certain cytokines and growth factors as oligotrophins based on their capability to enhance oligodendrocyte development or survival. We suggest that research into networks of developmentally regulated endogenous protectors (such as oligotrophins) is necessary to broaden our perspectives in brain injury prevention in preterm newborns.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.