Neurobiology of hypoxic-ischemic injury in the developing brain

Pathophysiology of traumatic injury in the developing brain: an introduction and short update

Current understanding about the main peculiarities in pathophysiology of immature brain traumatic injury involves marked developmental discrepancy of biomechanical properties, aspects of altered features in water and electrolyte homeostasis as well as maturation dependent differences in structural and functional responses of major transmitter systems. Based on the fact that traumatic brain injury (TBI) is one of the major causes of morbidity and mortality in infants and children, the currently available epidemiological data are reviewed in order to gain insights about scope and dimension of health care engagement and derive the requirements for reinforced pathogenetic research. To this end, the main aspects of peculiarities in primary and secondary TBI mechanisms in the immature/developing brain are discussed, including structural and functional conditions resulting in a markedly diminished shear resistance of the immature brain tissue. As such, the immature brain tissue appears to be more susceptible to mechanical alterations, because similar mechanical load induces a more intense brain tissue displacement. Furthermore, available indications for increased incidence of brain swelling in the immature brain after TBI are reviewed, focusing on the interrelationship between the age-dependent differences in extracellular space and aquaporin-4 expression during brain maturation. The developmental differences of TBI induced cerebrovascular response as well as some relevant aspects of altered neurotransmission following TBI of the immature brain in regard to the glutamatergic and dopaminergic transmitter system are assessed. Thus, this mini-review highlights some progress but also an increased necessity for expanded pathogenetic research on a clinical scale in order to develop a solid foundation for adequate therapeutic strategies for the different life-threatening consequences of TBI in infancy and childhood, which mainly have failed up to now.

Doxycycline reduces cleaved caspase-3 and microglial activation in an animal model of neonatal hypoxia-ischemia

Neonatal hypoxia-ischemia (HI) is a major contributor to many perinatal neurologic disorders and, thus, the search for therapies and effective treatments for the associated brain damage has become increasingly important. The tetracycline derivative, doxycycline (DOXY), has been reported to be neuroprotective in adult animal models of cerebral ischemia. To investigate the putative neuroprotective effects of DOXY in an animal model of neonatal HI, a time-course study was run such that pups received either DOXY (10 mg/kg) or VEH immediately before hypoxia, 1, 2, or 3 hours after HI (n=6). At 7 days after injury, the pups were euthanized, and the brains were removed and processed for immunohistochemical and Western blot analyses using antibodies against specific markers for neurons, apoptotic markers, microglia, oligodendrocytes, and astrocytes. Results showed that in vulnerable brain regions including the hippocampal formation, thalamus, striatum, cerebral cortex and white matter tracts, DOXY significantly decreased caspase-3 immunoreactivity (a marker of apoptosis), promoted neuronal survival, inhibited microglial activation and reduced reactive astrocytosis compared with VEH-treated HI pups. These effects were found to occur in a time-dependent manner. Taken together, these results strongly suggest that doxycycline has potential as a pharmacological treatment for mild HI in neonates.

Effect of moderate hypercapnic hypoxia on cerebral dopaminergic activity and brain O2 uptake in intrauterine growth-restricted newborn piglets

There is evidence that intrauterine growth restriction (IUGR) is associated with altered dopaminergic function in the immature brain. Compelling evidence exists that in the newborn brain, specific structures are especially vulnerable to O2 deprivation. The dopaminergic system is shown to be sensitive to O2 deprivation in the immature brain. However, the respective enzyme activities have not been measured in the living neonatal brain after IUGR under hypercapnic hypoxia (H/H). Therefore, 18F-labeled 6-fluoro-L-3,4-dihydroxyphenylalanine (FDOPA) together with positron emission tomography was used to estimate the aromatic amino acid decarboxylase activity of the brain of seven normal weight (body weight 2078 +/- 434 g) and seven IUGR newborn piglets (body weight 893 +/- 109 g). Two positron emission tomography scans were performed in each piglet. All animals underwent a period of normoxia and moderate H/H. Simultaneously, cerebral blood flow was measured with colored microspheres and cerebral metabolic rate of O2 was determined. In newborn normal-weight piglets, the rate constant for FDOPA decarboxylation was markedly increased in mesostriatal regions during H/H, whereas brain oxidative metabolism remained unaltered. In contrast, moderate H/H induced in IUGR piglets a marked reduction of clearance rates for FDOPA metabolites (p <0.05), which was accompanied by a tendency of lowering the rate constant for FDOPA conversion. Furthermore, IUGR piglets maintained cerebral O2 uptake in the early period of H/H, but during the late period of H/H, a significantly reduced cerebral metabolic rate of O2 occurred (p <0.05). Thus, IUGR is accompanied by a missing activation of dopaminergic activity and attenuated brain oxidative metabolism during moderate H/H. This may indicate endogenous brain protection against O2 deprivation.

Auditory processing deficits in rats with neonatal hypoxic-ischemic injury

Hypoxia-ischemia (HI) refers to reduced blood oxygenation and/or a diminished amount of blood perfusing the brain, and is associated with premature birth/very low birth weight (VLBW). HI represents a common cause of injury to the perinatal brain. Indeed, a significant number of premature/VLBW infants go on to demonstrate cognitive/behavioral deficits, with particularly high incidence of disruptions in language development. Auditory processing deficits, in turn, have been suggested to play a causal role in the development of language impairments. Specifically, the inability to identify fast elements in speech is purported to exert cascading detrimental effects on phonological discrimination, processing, and identification. Based on this convergent evidence, the current studies address auditory processing evaluation in a rodent model of HI injury induced on postnatal days 1, 7, or 10 (which in turn is well accepted as modeling HI-related injury to the perinatal human). Induced injuries were followed by a battery of auditory testing, and a spatial maze assessment, performed both during juvenile and adult periods. Results indicate that rats suffering from these early HI insults performed significantly worse than shams on tasks requiring rapid auditory processing, and on a test of spatial learning (Morris water maze (MWM)), although these effects were not seen on simpler versions of auditory tasks or on a water escape assessment (thus ruling out hearing/motor impairments). Correlations were found between performance on rapid auditory and spatial behavioral tasks and neuroanatomical measures for HI animals such as: the volume of the hippocampus, cerebral cortex, ventricles, and/or the area of the corpus callosum. Cumulative findings suggest that perinatal HI injury in the rat may lead to neurodevelopmental damage associated, in turn, with auditory processing and/or learning and memory impairments. As such, the current model may have critical implications for the study of neurophysiological underpinnings of cognitive deficits in premature/VLBW infants.

cAMP response element-binding protein activation in ligation preconditioning in neonatal brain

Perinatal hypoxic-ischemic (HI) brain injury is a major cause of permanent neurological dysfunction in children. An approach to study the treatment of neonatal HI encephalopathy that allows for neuroprotection is to investigate the states of tolerance to HI. Twenty-four-hour carotid-artery ligation preconditioning established by delaying the onset of hypoxia for 24 hours after permanent unilateral carotid ligation rats markedly diminished the cerebral injury, however, the signaling mechanisms of this carotid-artery ligation preconditioning in neonatal rats remain unknown. Ligation of the carotid artery 24 hours before hypoxia provided complete neuroprotection and produced improved performance on the Morris water maze compared with ligation performed 1 hour before hypoxia. Carotid artery ligation 6 hours before hypoxia produced intermediate benefit. The 24-hour carotid-artery ligation preconditioning was associated with a robust and sustained activation of a transcription factor, the cAMP response element-binding protein (CREB), on its phosphorylation site on Ser133. Intracerebroventricular infusions of antisense CREB oligodeoxynucleotides significantly reduced the 24-hour carotid-artery ligation-induced neuroprotection effects by decreasing CREB expressions. Pharmacological activation of the cAMP-CREB signaling with rolipram 24 hours before hypoxia protected rat pups at behavioral and pathological levels by sustained increased CREB phosphorylation. This study suggests that 24-hour carotid-artery ligation preconditioning provides important mechanisms for potential pharmacological preconditioning against neonatal HI brain injury.

The role of antioxidants in models of inflammation: Emphasis on l-arginine and arachidonic acid metabolism

Inflammatory processes are made up of a multitude of complex cascades. Under physiological conditions these processes aid in tissue repair. However, under pathophysiological environments, such as wound healing and hypoxia-ischaemia (HI), inflammatory mediators become imbalanced, resulting in tissue destruction. This review addresses the changes in reactive oxygen species (ROS), L-arginine and arachidonic acid metabolism in wound healing and HI and subsequent treatments with promising anti-oxidants. Even though these models may appear divergent, anti-oxidant treatments are nevertheless still having favourable effects. On the basis of recent findings, it is apparent that protection with anti-oxidants is not solely attributed to scavenging of ROS. In addition, the actions of anti-oxidants must be considered in light of the inflammatory process being assessed. To this end, there does not appear to be any universally applicable single mechanism to explain the actions of anti-oxidants.

Acute fetal hypoxia: the modulating effect of infection

The fetal brain is protected from the effects of acute hypoxia by a range of haemodynamic and metabolic compensations. Hypoxia alone is therefore an unusual cause of perinatal brain injury in either preterm or term infants. More recently, materno-fetal infection has been implicated as a causative factor in cases of cerebral palsy associated with preterm and term birth. This paper explores the concept that exposure to infection, and in particular pro-inflammatory cytokines, may reduce the threshold at which hypoxia becomes neurotoxic, so making the brain much more vulnerable to even mild hypoxic insults. The hypothesis is supported by an increasing body of evidence from animal studies that also demonstrate the importance of duration between exposure to infection and subsequent hypoxia. There are a number of clinical and research implications that centre around the role of antibiotics, mode and timing of delivery, maternal cooling during labour and the role of immune-modulating drugs.

Calcitonin gene-related peptide contributes to the umbilical haemodynamic defence response to acute hypoxaemia

Despite clinical advances in obstetric practice, undiagnosed fetal hypoxaemia still contributes to a high incidence of perinatal morbidity. The fetal defence to hypoxaemia involves a redistribution of blood flow away from peripheral circulations towards essential vascular beds, such as the umbilical, cerebral, myocardial and adrenal circulations. In marked contrast to other essential vascular beds, the mechanisms mediating maintained perfusion of the umbilical circulation during hypoxaemia remain unknown. This study determined the role of calcitonin gene-related peptide (CGRP) in the maintenance of umbilical blood flow during basal and hypoxaemic conditions. Under anaesthesia, five sheep fetuses were instrumented with catheters and a Transonic probe around an umbilical artery, inside the fetal abdomen, at 0.8 of gestation. Five days later, fetuses were subjected to 0.5 h hypoxaemia during either i.v. saline or a selective CGRP antagonist in randomised order. Treatment started 30 min before hypoxaemia and ran continuously until the end of the challenge. The CGRP antagonist did not alter basal blood gas or cardiovascular status in the fetus. A similar fall in Pa,O2 occurred in fetuses during either saline (21 +/- 0.8 to 9 +/- 0.9 mmHg) or antagonist treatment (20 +/- 0.9 to 9 +/- 1.2 mmHg). Hypoxaemia during saline led to significant increases in arterial blood pressure, umbilical blood flow and umbilical vascular conductance. In marked contrast, hypoxaemia during CGRP antagonist treatment led to pronounced falls in both umbilical blood flow and umbilical vascular conductance without affecting the magnitude of the hypertensive response. In conclusion, CGRP plays an important role in the umbilical haemodynamic defence response to hypoxaemia in the late gestation fetus.

Characterization of cellular and neurological damage following unilateral hypoxia/ischemia

Rodent models of stroke are often used to investigate the mechanisms that lead to ischemic neuronal damage. In this study, we used a model of cerebral hypoxia with ischemia to produce unilateral damage in C57Bl/6 mice. Lesion volume, ascertained by TTC staining, increased with longer durations of hypoxia. Additionally, cresyl violet, TUNEL, and FluoroJade staining showed a statistically significant increase in cellular damage in the ipsilateral cortex, CA1 pyramidal layer, and dentate gyrus of the hippocampus of ipsilateral hypoxic/ischemic tissue versus sham tissue. Astrocyte reactivity, determined by GFAP staining, was significantly higher in the ipsilateral H/I cortex and contralateral hippocampus compared to sham cortex and hippocampus, respectively. Increased microglia activation was evident in the H/I-treated cortex and hippocampus versus sham cortex and hippocampus, particularly within areas undergoing degeneration. To examine whether this model produces motor deficits, a battery of tests were administered before and after hypoxia. Following 45 min H/I, locomotor activity, rotarod performance and performance on an inverted wire hang test were all significantly decreased. These data indicate that the histological evidence of neuronal damage is consistent with functional deficits and suggest that this model may be useful for investigating strategies designed to protect neurons from hypoxia/ischemia-induced damage.

Differential vulnerability of immature murine neurons to oxygen-glucose deprivation

In vivo studies support selective neuronal vulnerability to hypoxia-ischemia (HI) in the developing brain. Since differences in intrinsic properties of neurons might be responsible, pure cultures containing immature neurons (6-8 days in vitro) isolated from mouse cortex and hippocampus, regions chosen for their marked vulnerability to oxidative stress, were studied under in vitro ischemic conditions-oxygen-glucose deprivation (OGD). Twenty-four hours of reoxygenation after 2.5 h of OGD induced significantly greater cell death in hippocampal than in cortical neurons (67.8% vs. 33.4%, P = 0.0068). The expression of neuronal nitric oxide synthase (nNOS) protein, production of nitric oxide (NO), and reactive oxygen species (ROS), as well as glutathione peroxidase (GPx) activity and intracellular levels of reduced glutathione (GSH), were measured as indicators of oxidative stress. Hippocampal neurons had markedly higher nNOS expression than cortical neurons by 24 h of reoxygenation, which coincided with an increase in NO production, and significantly greater ROS accumulation. GPx activity declined significantly in hippocampal but not in cortical neurons at 4 and 24 h after OGD. The decrease in GSH level in hippocampal neurons correlated with the decline of GPx activity. Our data suggest that developing hippocampal neurons are more sensitive to OGD than cortical neurons. This finding supports our in vivo studies showing that mouse hippocampus is more vulnerable than cortex after neonatal HI. An imbalance between excess prooxidant production (increased nNOS expression, and NO and ROS production) and insufficient antioxidant defenses created by reduced GPx activity and GSH levels may, in part, explain the higher susceptibility to OGD of immature hippocampal neurons.

Minocycline worsens hypoxic-ischemic brain injury in a neonatal mouse model

Hypoxic-ischemic encephalopathy (HIE) is a leading cause of mortality and morbidity during the perinatal period, and currently no therapeutic drug is available. Minocycline, an antibiotic, has recently been shown to have neuroprotective effects distinct from its antimicrobial effect in several neurological disorders including ischemic brain injury. We examined the effect of minocycline on neonatal hypoxic-ischemic brain injury by using histologic scoring in both mouse and rat models. Mouse (C57Bl/6) and rat (SD) pups were exposed to a unilateral hypoxic-ischemic insult at 8 and 7 days of age, respectively. Minocycline hydrochloride was administered according to protocols that were reported to provide neuroprotection in adult or neonatal rats. Seven days after the insult, we examined brain injury in Nissl stained sections. Although minocycline ameliorated brain injury in the developing rat, it increased injury in the developing mouse. This detrimental effect in the mouse was consistent across different regions (cortex, striatum, and thalamus), with both single and multiple injection protocols and with both moderate and high-dose treatment (P < 0.05). The mechanism of the contrasting effects in mouse and rat is not clear and remains to be elucidated. Minocycline has been used as an antibiotic in the clinical setting for decades; therefore, it may be considered for use in infants with hypoxic-ischemic brain damage, based on prior reports of neuroprotection in the rat. However, it is important to examine this drug carefully before clinical use in human infants, taking our data in the mouse model into consideration.

Loss of glial glutamate transporters and induction of neuronal expression of GLT-1B in the hypoxic neonatal pig brain

The homeostasis of glutamate is critical to normal brain function; deficiencies in the regulation of extracellular glutamate are thought to be a major determinant of damage in hypoxic brains. Extracellular levels of glutamate are regulated mainly by plasmalemmal glutamate transporters. We have evaluated the distribution of the glutamate transporter GLAST and two splice variants of GLT-1 in the hypoxic neonatal pig brain using this as model of neonatal humans. In response to severe hypoxic insults, we observe a rapid loss of two glial glutamate transporters from specific brain regions, such as the CA1 region of the hippocampus, but not the dentate gyrus. The spatial distribution of loss accords with patterns of damage in these brains. Conversely, we demonstrate that hypoxia evokes the expression of a splice variant of GLT-1 in neurons. We suggest that this expression may be induced in response to elevated extracellular glutamate around these neurons, and that this splice variant may represent a useful marker for direct quantification of the extent of likely neuronal damage in hypoxic brains.

Maturational modulation of endothelium-dependent vasodilatation in ovine cerebral arteries

To address the hypothesis that maturation enhances endothelial vasodilator function in cerebral arteries, relaxant responses to ADP and A-23187 were determined in ovine carotid and cerebral arteries harvested from 25 newborn lambs (3-7 days) and 23 adult sheep. Maturation significantly increased pD(2) values for A-23187 (newborn range: 4.9 +/- 0.3 to 5.4 +/- 0.3; adult range: 6.0 +/- 0.2 to 7.1 +/- 0.2) and the maximal vasodilator response to A-23187 by 10-18%. In contrast, maturation decreased maximum responses to ADP by 5-25% with no change in pD(2). The magnitudes of endothelium-dependent relaxation were not affected by 10 microM indomethacin but were virtually abolished by 100 microM N(G)-nitro-L-arginine methyl ester/L-nitro arginine, indicating that nitric oxide (NO) is the primary endothelium-dependent vasodilator in these arteries. Maturation also modestly decreased endothelial NO synthase (eNOS) abundance in both carotid (32%) and cerebral (26%) arteries. Together, these findings reinforce the view that receptor coupling to endothelial activation is tightly regulated and may offset underlying changes in maximal endothelial vasodilator capacity. This capacity, in turn, appears to increase with postnatal age despite major growth and expansion of endothelial cell size and vascular wall volume. In ovine cerebral arteries, endothelial vasodilator capacity appears completely dependent on eNOS activity but not on cyclooxygenase activity. In turn, eNOS activity appears to be postnatally regulated by mechanisms independent of changes in eNOS abundance alone.

Neuronal pentraxin 1: a novel mediator of hypoxic-ischemic injury in neonatal brain

Neonatal hypoxic-ischemic brain injury is a major cause of neurological disability and mortality. Its therapy will likely require a greater understanding of the discrete neurotoxic molecular mechanism(s) triggered by hypoxia-ischemia (HI). Here, we investigated the role of neuronal pentraxin 1 (NP1), a member of a newly recognized subfamily of "long pentraxins," in the HI injury cascade. Neonatal brains developed marked infarcts in the ipsilateral cerebral hemisphere at 24 hr and showed significant loss of ipsilateral striatal, cortical, and hippocampal volumes at 7 d after HI compared with the contralateral hemisphere and sham controls. Immunofluorescence analyses revealed elevated neuronal expression of NP1 in the ipsilateral cerebral cortex from 6 hr to 7 d and in the hippocampal CA1 and CA3 regions from 24 hr to 7 d after HI. These same brain areas developed infarcts and terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling-positive cells within 24-48 hr of HI. In primary cortical neurons, NP1 protein was induced >2.5-fold (p < 0.001) after their exposure to hypoxia that caused approximately 30-40% neuronal death. Transfecting cortical neurons with antisense oligodeoxyribonucleotides directed against NP1 mRNA (NP1AS) significantly inhibited (p < 0.01) hypoxia-induced NP1 protein induction and neuronal death (p < 0.001), demonstrating a specific requirement of NP1 in hypoxic neuronal injury. NP1 protein colocalized and coimmunoprecipitated with the fast excitatory AMPA glutamate receptor subunit (GluR1) in primary cortical neurons, and hypoxia induced a time-dependent increase in NP1-GluR1 interactions. NPIAS also protected against AMPA-induced neuronal death (p < 0.05), implicating a role for NP1 in the excitotoxic cascade. Our results show that NP1 induction mediates hypoxic-ischemic injury probably by interacting with and modulating GluR1 and potentially other excitatory glutamate receptors.

Brain-stem auditory impairment during the neonatal period in term infants after asphyxia: dynamic changes in brain-stem auditory evoked response to clicks of different rates

OBJECTIVE

To explore dynamic changes in brain-stem auditory electrophysiology during the neonatal period in term infants after perinatal asphyxia.

METHODS

Sixty-eight term newborn infants who suffered asphyxia were studied on days 1, 3, 5, 7, 14 and 30 after birth. Brain-stem auditory evoked response (BAER) was recorded with clicks, delivered at 21, 51 and 91 s(-1) and > or =40 dB above BAER threshold of each subject.

RESULTS

During the neonatal period wave I latency in the infants after asphyxia increased slightly while later BAER components changed more significantly. On the first day after birth wave III and V latencies and I-V and III-V intervals increased significantly at all rates of clicks (ANOVA P<0.01-0.001). On day 3, the latencies and intervals increased further. III-V/I-III interval ratio increased at 51 and 91 s(-1), suggesting a relatively more significant increase in III-V interval than in I-III interval at higher rates. Thereafter, wave III and V latencies and all intervals decreased progressively, although these BAER variables were still significantly longer than in normal controls on days 5 and 7 (P<0.05-0.001) On day 30, all latencies and intervals approached near normal values, with a slight increase in wave V latency and I-V and III-V intervals at 51 and 91 s(-1).

CONCLUSIONS

Perinatal asphyxia has a major effect on central auditory function, resulting in acute impairment. The impairment progresses during the first 3 days and then tends towards recovery. By 1 month the impaired auditory function has largely returned to normal. Significant increase in click rates can moderately improve the detection of auditory impairment.

SIGNIFICANCE

After perinatal asphyxia early detection of hypoxic-ischaemic damage to the central auditory system and initialisation of neuroprotective and therapeutic measures during the first hours after birth are critical to prevent or reduce deterioration of central impairment.

Neuroprotection of edaravone on hypoxic-ischemic brain injury in neonatal rats

Edaravone has an inhibitory effect on lipid peroxidation by scavenging free radicals and prevents vascular endothelial cell injury. We examined whether edaravone was effective on hypoxic-ischemic (HI) brain injury in immature brain or not using the Rice-Vannucci model. The initial dose, 3 mg/kg (0.05 ml) of edaravone, was injected intraperitoneally just before hypoxic exposure. Subsequently, the same dose was injected every 12 h until the animals were killed. Controls received saline injection as the same protocol. Macroscopic evaluation of brain injury revealed that the neuroprotective effect of edaravone on HI brain after 48 h post HI. TUNEL showed that edaravone injection decreased neurodegeneration. Quantitative analysis of cell death using H&E-stained 2.5 microm sections showed that there was a trend for both necrotic and apoptotic cells to decrease in edaravone injection group. Edaravone injection inhibited the release of cytochrome c from mitochondria to cytosol and caspase-3 activation in cortex and hippocampus between 24 and 168 h post HI. Our results suggest that edaravone is protective after HI insult in the immature brain by decreasing both apoptosis and necrosis and also by inhibiting mitochondrial injury.

Effect of hyperbaric oxygen on apoptosis in neonatal hypoxia-ischemia rat model

We have previously demonstrated that a transient exposure to hyperbaric oxygen (HBO) attenuated the neuronal injury after neonatal hypoxia-ischemia. This study was undertaken to determine whether HBO offers this neuroprotection by reducing apoptosis in injured brain tissue. Seven-day-old rat pups were subjected to unilateral carotid artery ligation followed by 2 h of hypoxia (8% oxygen). Apoptotic cell death was examined in the injured cortex and hippocampus tissue. Caspase-3 expression and activity increased at 18 and 24 h after the hypoxia-ischemia insult. At 18-48 h, poly(ADP-ribose) polymerase (PARP) cleavage occurred, which reduced the band at 116 kDa and enhanced the band at 85 kDa. There was a time-dependent increase in the number of terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL)-positive cells. A single HBO treatment (100% oxygen, 3 ATA for 1 h) 1 h after hypoxia reduced the enhanced caspase-3 expression and activity, attenuated the PARP cleavage, and decreased the number of TUNEL-positive cells observed in the cortex and hippocampus. These results suggest that the neuroprotective effect of HBO is at least partially mediated by the reduction of apoptosis.

Clinical profiles of children with cerebral palsy having lesions of the thalamus, putamen and/or peri-Rolandic area

Neurological findings, motor symptoms, mental abnormality and dysarthria were examined in 28 children with lesions in the thalamus, putamen, and/or peri-Rolandic area. The thalamus and putamen were involved in eight, and only the thalamus in ten of the children. Most of these 18 children had mild disabilities; they did not have severe mental retardation and could walk alone, speak words, and grasp an object. Dominant flexion of the hips was observed in many of the children who could walk. Two-thirds of these children had athetotic involuntary movement and the remaining had gross or fine motor abnormalities although they had no involuntary movement. In most of these children, reaching patterns were abnormal and were affected by shoulder retraction. Their abnormal movements were thought to be inappropriate muscle activity brought about by voluntary movements. In the remaining ten children, the thalamus, putamen, and peri-Rolandic area were all involved. Many had severe disabilities such as severe mental retardation and the inability to sit, speak words, or grasp an object. All had athetotic involuntary movements. Three children had spasticity of the lower extremities. Five children with severe disabilities and no spasticity were thought to have apparent weakness with athetosis.

Bcl-2 and Bax proteins are increased in neocortical but not in thalamic apoptosis following devascularizing lesion of the cerebral cortex in the rat: an immunohistochemical study

The hypothesis that devascularization of somatosensory and motor cortex causes apoptosis in infarcted regions and in the linked thalamic nuclei was evaluated. To unravel whether Bcl-related proteins, known to regulate apoptosis, participate in neuronal and glial responses to devascularization, we analyzed immunohistochemically the distribution and intensity of staining of Bcl-2 and Bax proteins at different time points after lesion. Both early (up to 6 h) and late (1-7 days) responses were studied. Devascularization led to rapid (within hours) apoptosis in the cortex and to a delayed (within 3-7 days) apoptosis in thalamic nuclei. In control groups, Bcl-2 and Bax immunoreactivity (IR) was detected in neurons and oligodendrocytes but not in astrocytes or microglia. Following devascularization, Bcl-2 IR and Bax IR increased in neurons before the onset of the apoptosis. In the ischemic focus, the increase reached maximal values 3 h after the lesion. The increase was of slower onset in the penumbra zone (24 h and after), a region in which both proteins were induced in astrocytes also. The change of Bax IR intensity exceeded four times that of Bcl-2 at all time points investigated, indicating a diminution of Bcl-2/Bax ratio that may direct neurons to apoptotic pathway. In numerous neurons, an increase of IR in the cytoplasm was accompanied by induction of nuclear staining. No changes of Bcl-2 and Bax IR were found in thalamic nuclei. Our results point to different mechanisms underlying apoptosis of cortical and thalamic neurons. Nuclear appearance of Bcl-2 and Bax suggests they possess regulatory role of gene expression changes triggered by cortical infarct.

The PDGF B-chain is involved in the ontogenic susceptibility of the developing rat brain to NMDA toxicity

Hypoxic-ischemic (H-I) injury to neonatal brains can cause a life-long neuronal deficit because of increased susceptibility in the neonatal period. Excitotoxicity due to overstimulation of the N-methyl-d-aspartate receptor (NMDAR) is assumed to be the basis of the injury. However, the ontogenic profile of the susceptibility does not directly correlate with the levels of NMDAR expression. Platelet-derived growth factor B-chain (PDGF-B) has been reported to protect neurons by suppressing the NMDA-evoked current and translocating the glutamate transporter to the cell membrane. Thus, we assessed the relationship between the susceptibility to H-I injury and the expression of PDGF-B in neonatal rat brain. PDGF-B infusion before and after an intrastriatal NMDA injection significantly reduced the size of the lesions in 7-day-old rats, when they are most susceptible and the neuronal expression of PDGF-B is low. Fourteen-day-old neonatal rats were found to be resistant to NMDA injury, even though NMDARs are expressed at high levels in the brain at this age. Inhibition of PDGF-B protein synthesis by antisense oligodeoxynucleotides increased the size of the NMDA-induced lesions up to 6-fold at postnatal day 14, when PDGF-B is expressed at high levels in neurons. These data suggest that PDGF-B is an important physiological modulator of NMDAR excitability in the developing brain, and that the balance between the expression of NMDAR and PDGF-B partly determines the ontogenic susceptibility to brain injury. Enhancement of the PDGF-B/receptor signal pathway might rescue neonatal brains at risk of H-I injury.

Clinical disorders of brain plasticity

Clinical disorders of brain plasticity are common in the practice of child neurology. Children have an enhanced capacity for brain plasticity compared to adults as demonstrated by their superior ability to learn a second language or their capacity to recover from brain injuries or radical surgery such as hemispherectomy for epilepsy. Basic mechanisms that support plasticity during development include persistence of neurogenesis in some parts of the brain, elimination of neurons through apoptosis or programmed cell death, postnatal proliferation and pruning of synapses, and activity-dependent refinement of neuronal connections. Brain plasticity in children can be divided into four types: adaptive plasticity that enhances skill development or recovery from brain injury; impaired plasticity associated with cognitive impairment; excessive plasticity leading to maladaptive brain circuits; and plasticity that becomes the brain's 'Achilles' Heel' because makes it vulnerable to injury. A broad group of pediatric neurologic disorders can be understood in terms of their impact on fundamental mechanisms for brain plasticity. These include neurofibromatosis, tuberous sclerosis, Fragile X syndrome, other inherited forms of mental retardation, cretinism, Coffin-Lowry syndrome, lead poisoning, Rett syndrome, epilepsy, hypoxic-ischemic encephalopathy and cerebral palsy.

Relationship between cerebral interstitial levels of amino acids and phosphorylation potential during secondary energy failure in hypoxic-ischemic newborn piglets

The aim of this study was to determine the validity of the hypothesis that excitatory amino acids are related to phosphorylation potential during primary and secondary cerebral energy failure observed in asphyxiated infants. We report here the results of experiments using newborn piglets subjected to severe transient cerebral hypoxia-ischemia followed by resuscitation. We examined cerebral energy metabolism by phosphorus nuclear magnetic resonance spectroscopy and changes in levels of amino acid neurotransmitters in the cortex by microdialysis before, during, and up to 24 h after the hypoxic-ischemic insult. The concentrations of aspartate, glutamate, taurine, and gamma-aminobutyric acid were significantly elevated during the hypoxic-ischemic insult compared with prebaseline values. Shortly after resuscitation, glutamate, taurine, and gamma-aminobutyric acid concentrations decreased but then began to increase again. These secondary elevations were greater than the primary elevations. A negative linear correlation was found between primary interstitial levels of glutamate and taurine and minimum values of phosphocreatine/inorganic phosphate during the secondary energy failure. The cerebral energy state depended on the time course of changes in excitatory amino acids, suggesting that amino acids play distinct roles during the early and delayed phases of injury.

Postnatal hypoxic-ischemic brain injury alters mechanisms mediating neuronal glucose transport

We examined the effect of hypoxic ischemia and hypoxia vs. normoxia on postnatal murine brain substrate transporter concentrations and function. We detected a transient increase in the neuronal brain glucose transporter isoform (GLUT-3) in response to hypoxic ischemia after 4 h of reoxygenation. This increase was associated with no change in GLUT-1 (blood-brain barrier/glial isoform), monocarboxylate transporter isoforms 1 and 2, synapsin I (neuronal marker), or Bax (proapoptotic protein) but with a modest increase in Bcl-2 (antiapoptotic mitochondrial protein) protein concentrations. At 24 h of reoxygenation, the increase in GLUT-3 disappeared but was associated with a decline in Bcl-2 protein concentrations and the Bcl2:Bax ratio, an increase in caspase-3 enzyme activity (apoptotic effector enzyme), and extensive DNA fragmentation, which persisted later in time (48 h) only in the hippocampus. Hypoxia alone in the absence of ischemia was associated with a transient but modest increase in GLUT-3 and synapsin I protein concentrations, which did not cause significant apoptosis and/or necrosis. Assessment of glucose transporter function by 2-deoxyglucose (2-DG) uptake using two distinct techniques, namely positron emission tomography (PET) and the modified Sokoloff method, revealed a discrepancy due to glucose uptake by extracranial Harderian glands that masked the accurate detection of intracranial brain glucose uptake by PET scanning. The modified Sokoloff method assessing 2-DG uptake revealed that the transient increase in GLUT-3 was critical in protecting against a decline in brain glucose uptake. We conclude that hypoxic-ischemic brain injury is associated with transient compensatory changes targeted at protecting glucose delivery to fuel cellular energy metabolism, which then may delay the processes of apoptosis and cell necrosis.

Increased susceptibility of S100B transgenic mice to perinatal hypoxia-ischemia

S100B is a glial-derived protein that is a well-established biomarker for severity of neurological injury and prognosis for recovery. Cell-based and clinical studies have implicated S100B in the initiation and maintenance of a pathological, glial-mediated proinflammatory state in the central nervous system. However, the relationship between S100B levels and susceptibility to neurological injury in vivo has not been determined. We used S100B transgenic (Tg) and knockout (KO) mice to test the hypothesis that overexpression of S100B increases vulnerability to cerebral hypoxic-ischemic injury and that this response correlates with an increase in neuroinflammation from activated glia. Postnatal day 8 Tg mice subjected to hypoxia-ischemia showed a significant increase in mortality compared with KO and wild-type mice. Tg mice also exhibited greater cerebral injury and volume loss in the ischemic hemisphere after an 8-day recovery, as assessed by histopathology and magnetic resonance imaging. Measurement of glial fibrillary acidic protein and S100B levels showed a significant increase in the Tg mice, consistent with heightened glial activation and neuroinflammation in response to injury. This is the first demonstration to our knowledge that overexpression of S100B in vivo enhances pathological response to injury.

p21ras activation following hypoxia-ischemia in the newborn rat brain is dependent on nitric oxide synthase activity but p21ras does not contribute to neurologic injury

Hypoxia-ischemia (HI) in the perinatal period is associated with significant infant mortality and neurologic morbidity. Increase in the activity of nitric oxide synthase (NOS) and increased release of nitric oxide (NO) are cardinal events in the pathophysiology of stroke and perinatal asphyxia. Cell culture studies suggest that the GTP-binding protein p21ras (Ras) is activated by NO in an NMDA-receptor-dependent pathway. These findings imply that Ras may be activated in vivo by NO released in response to glutamate stimulation during HI. The contribution of downstream Ras activation to neurologic injury after perinatal HI is unknown. We used a postnatal day 7 rat model of perinatal hypoxia-ischemia to determine the response of Ras to HI, the role of NO in Ras activation and the effect of Ras inhibition on neurologic injury in vivo. Ras is activated in both hippocampus and cortex within 2 h after HI. This increase is prevented by treatment with the NOS inhibitor, aminoguanidine (AG) and by a farnesyl/protein transferase inhibitor, manumycin (MAN). Inhibition of NOS, but not Ras, significantly reduces neurologic injury after a 7-day recovery period. This data suggests that Ras is activated during the initiation of the cellular response to HI in both hippocampus and cortex and that this activation is NO-dependent. Ras does not, however, contribute to the pathophysiologic NO-dependent mechanisms of neurologic injury in this model.

Development of a postnatal 3-day-old rat model of mild hypoxic-ischemic brain injury

Improvements in both obstetric and paediatric care have been responsible for a continuing reduction in mortality in extremely premature infants. However, higher survival rates have been at the expense of more long-term neurological damage. Various animal models have been developed to study the effect of hypoxic-ischemic insults on the brain. However, established models like the postnatal day 7 rat model represent damage found in term infants rather than in preterm infants of 24-28 weeks' gestation, and produce a severe form of injury resulting in high mortality rates. In this study we developed a reliable model of minor hypoxic-ischemic brain injury in postnatal day 3 rats. At this maturity, the pattern of damage represents that expected in a preterm infant suffering a non-lethal perinatal insult. We found that minor changes in duration of insult and both temperature and humidity produced wide fluctuations in the degree of injury observed. By maintaining strict control over experimental conditions including duration of insult, temperature and humidity, we produced a reliable model of minor injury primarily affecting all five areas of the cerebral cortex, and also the thalamus (area 7) and basal ganglia (area 8). Differences were significant compared to normal controls and sham-operated animals (p<0.05). These areas represent the primary motor, insular, visual and temporal cortices. The overall mortality rate in this study was 12.3%.

Time course of brainstem pathophysiology during first month in term infants after perinatal asphyxia, revealed by MLS BAER latencies and intervals

Dynamic changes in electrophysiology of brainstem auditory neurons during the first month after birth were studied in 51 term infants after perinatal asphyxia using maximum length sequence brainstem auditory evoked responses. The responses were recorded on d 1, 3, 5, 7, 10, 15, and 30 after birth. On d 1, wave III and V latencies and all interpeak intervals increased significantly at all repetition rates of clicks used (91-910/s), especially the higher rates (ANOVA, p < 0.05-0.0001). On d 3, all these latencies and intervals increased further and differed more significantly from the normal control subjects. Thereafter, the latencies and intervals decreased progressively. On d 7, wave V latency and all intervals still differed significantly from the control subjects. These dynamic changes were more significant at higher rates of clicks than at lower rates. On d 10 and 15, all intervals decreased significantly. On d 30, all wave latencies decreased to the values in the normal control subjects on the same day. The intervals also approached normal values, although the III-V and I-V intervals still increased slightly. These results indicate that hypoxic-ischemic brain damage persists during the first week, with a peak on d 3, and recovers progressively thereafter. By 1 mo, the damage has largely returned to normal. Maximum length sequence brainstem auditory evoked responses results correlated well with the stage of hypoxic-ischemic encephalopathy during the first week. The present study revealed a general time course of brainstem pathophysiology after asphyxia, although there were individual variations. Our findings can be used as a reference to monitor cerebral function and help judge the value of neuroprotective or therapeutic interventions. The first week, particularly the first 3 d, is a critical period of hypoxic-ischemic brain damage, and early intervention may prevent or reduce deterioration of the damage.

L-carnitine reduces brain injury after hypoxia-ischemia in newborn rats

Perinatal hypoxia-ischemia remains a significant cause of neonatal mortality and neurodevelopmental disability. Numerous lines of evidence indicate that cerebral ischemic insults disrupt normal respiratory activity in mitochondria. Carnitine (3-hydroxy-4-N-trimethylammonium-butyrate) has an essential role in fatty acid transport in the mitochondrion and in modulating potentially toxic acyl-CoA levels in the mitochondrial matrix. There are no naturally occurring esterases available to reduce the accumulation of acyl-CoA but this process can be overcome by exogenous carnitine. We used a newborn rat model of perinatal hypoxia-ischemia to test the hypothesis that treatment with l-carnitine would reduce the neuropathologic injury resulting from hypoxia-ischemia in the developing brain. We found that treatment with l-carnitine during hypoxia-ischemia reduces neurologic injury in the immature rat after both a 7- and 28-d recovery period. We saw no neuroprotective effect when l-carnitine was administered after hypoxia-ischemia. Treatment with d-carnitine resulted in an increase in mortality during hypoxia-ischemia. Carnitine is easy to administer, has low toxicity, and is routinely used in neonates as well as children with epilepsy, cardiomyopathy, and inborn errors of metabolism. l-Carnitine merits further investigation as a treatment modality for the asphyxiated newborn or as prophylaxis for the at-risk fetus or newborn.

Inner retinal neurons display differential responses to N-methyl-D-aspartate receptor activation

The N-methyl-D-aspartate (NMDA) responses of neurons from within the inner rabbit retina were mapped using a channel permeable cation, 1-amino-4-guanidobutane (agmatine, AGB). Serial sections were subsequently probed with immunoglobulins targeting AGB, glutamate, gamma-aminobutyric acid (GABA), and glycine to visualize the NMDA responses of neurochemical subpopulations of neurons. Most inner retinal subpopulations of neurons demonstrated an NMDA concentration-dependent increase in activation. This NMDA-induced activation displayed a distinct pattern, with the most sensitive class to least sensitive class ranking being GC > GABA cAC > GABA/Gly cAC > Gly cAC > GABA dAC (GC, ganglion cells; AC, amacrine cells; c, conventional; d, displaced; Gly, glycine). The variable NMDA response may reflect differences in NMDA receptor subunit disposition or differences in receptor density. In addition to the variable NMDA activation pattern, we found that virtually all ganglion cells (87%) showed NMDA-gated AGB entry, compared with only 58% of amacrine cells. We conclude that a large cohort of amacrine cells do not possess functional NMDA receptors. In addition to most ganglion cells being activated by NMDA, a large subpopulation displayed the highest sensitivity to NMDA application. The functional significance of this finding is that the ganglion cell population will be the first neuronal class to be susceptible to glutamate-induced neurotoxicity mediated through the NMDA receptor. The addition of betaxolol significantly reduced NMDA-mediated AGB entry into most neuronal groups (ganglion cells, GABA, and glycine amacrine cells), with the greatest effect being on ganglion cells. Betaxolol had no significant effect on NMDA-gated entry of AGB on the GABA/Gly amacrine cell population.

A1 adenosine receptors mediate hypoxia-induced ventriculomegaly

Periventricular leukomalacia is characterized by a reduction in brain matter and secondary ventriculomegaly and is a major cause of developmental delay and cerebral palsy in prematurely born infants. Currently, our understanding of the pathogenesis of this condition is limited. In animal models, features of periventricular leukomalacia can be induced by hypoxia and activation of A1 adenosine receptors (A1ARs). Using mice that are deficient in the A1AR gene (A1AR-/-), we show that A1ARs play a prominent role in the development of hypoxia-induced ventriculomegaly in neonates. Supporting a role for adenosine in the pathogenesis of developmental brain injury, ventriculomegaly was also observed in mice lacking the enzyme adenosine deaminase, which degrades adenosine. Thus, adenosine acting on A1ARs appears to mediate hypoxia-induced brain injury ventriculomegaly during early postnatal development.

Type I glutaric aciduria, part 2: a model of acute striatal necrosis

Type I glutaric aciduria (GA1) is an inborn error of organic acid metabolism that is associated with acute neurological crises, typically precipitated by an infectious illness. The neurological crisis coincides with swelling, metabolic depression, and necrosis of basal ganglia gray matter, especially the putamina and can be visualized as focal, stroke-like, signal hyperintensity on MRI. Here we focus on the stroke-like nature of striatal necrosis and its similarity to brain injury that occurs in infants after hypoxia-ischemia or systemic intoxication with 3-nitropropionic acid (NPA). These conditions share several features including abrupt onset, preferential effect in the striatum and age-specific susceptibility. The pathophysiology of the conditions is reviewed and a model proposed herein. We encourage investigators to test this model in an appropriate experimental system.

Neonatal hypoxia-ischemia reduces ganglioside, phospholipid and cholesterol contents in the rat hippocampus

Hypoxia-ischemia is a common cause of neonatal brain damage producing serious impact on cerebral maturation. This report demonstrates that rats submitted to hypoxia-ischemia present a marked decrease in hippocampal gangliosides, phospholipids and cholesterol contents as from 7 days after the injury. Although chromatographic profiles of the different ganglioside species (GM1, GD1a, GD1b, and GT1b) from the hippocampus of hypoxic-ischemic hippocampi groups (HI) were apparently unaffected, as compared with controls, there were quantitative absolute reductions in HI. The phospholipid patterns were altered in HI as from the 14th to the 30th day after the injury, where phosphatidylcholine (PC) quantities were higher than phosphatidylethanolamine (PE); additionally, the cardiolipin band was detected only in hippocampi of control adult rats. In general, the absolute quantities of phospholipids were lower in HI than in correspondent controls since 7th day after the injury. Considering that reported effects were maintained, we suggest they express a late biochemical response triggered by the neonatal hypoxic-ischemic episode; the consequences would be cell death and a delay on brain development, expressed by a reduction on synaptogenesis and myelinogenesis processes.

Selective vulnerability of subplate neurons after early neonatal hypoxia-ischemia

Neonatal hypoxia-ischemia in the preterm human leads to selective injury to the subcortical developing white matter, which results in periventricular leukomalacia (PVL), a condition associated with abnormal neurodevelopment. Maturation-dependent vulnerability of late oligodendrocyte progenitors is thought to account for the cellular basis of this condition. A high frequency of cognitive and sensory deficits with decreasing gestational age suggests pervasive abnormalities of cortical development. In a neonatal rat model of hypoxic-ischemic injury that produces the characteristic pattern of subcortical injury associated with human PVL, selective subplate neuron death is seen. The premature subplate neuron death occurs after thalamic axons have reached their targets in cortex. Thus, as expected, thalamocortical connections form normally, including patterned connections to somatosensory cortex. However, deficits in motor function still occur, as in babies with PVL. Subplate neuron cell death in PVL provides another mechanism for abnormal neurodevelopment after neonatal hypoxia-ischemia.

Brain injury in children with congenital heart disease

The incidence of neurodevelopmental impairment in children with congenital heart disease is high. Its aetiology is multiple and complex. Prevention and treatment must start during the preoperative period and continue through the intra- and postoperative periods. Research has resulted in a clearer understanding of the relationship between congenital heart disease and the brain, and of the effects of cardiopulmonary bypass, hypothermia and circulatory arrest. This has led to modifications in management which may improve neurological outcome in the future.

Recombinant peroxiredoxin 5 protects against excitotoxic brain lesions in newborn mice

The pathophysiology of brain lesions associated with cerebral palsy is multifactorial and likely involves excess release of glutamate and excess production of free radicals, among other factors. Theoretically, antioxidants could limit the severity of these brain lesions. Peroxiredoxins are a family of peroxidases widely distributed in eukaryotes and prokaryotes. Peroxiredoxin 5 (PRDX5) is a recently discovered mammalian member of this family of antioxidant enzymes that is able to reduce hydrogen peroxide and alkyl hydroperoxides. The present study was designed to examine the neuroprotective effects of recombinant PRDX5 against neonatal excitotoxic challenge in both in vivo and in vitro experiments. For in vivo experiments, mice (postnatal day 5) were injected intraneopallially with ibotenate acting on NMDA and metabotropic receptors, or S-bromowillardiine acting on AMPA-kainate receptors to produce excitotoxic stress and brain lesions. Systemically administered recombinant PRDX5 provided protection against ibotenate-induced excitotoxic stress. Brain lesions of animals given ibotenate and PRDX5 were up to 63% smaller than that given ibotenate alone. However, PRDX5 provided no prevention from lesions induced with S-bromowillardiine. A mutated recombinant PRDX5 that is devoid of peroxidase activity was also tested and showed no protection against lesions induced by either ibotenate or S-bromowillardiine. Two classical antioxidants, N-acetylcysteine and catalase-PEG, provided the same neuroprotective effect as PRDX5. For in vitro experiments, neocortical neurons were exposed to 300 microM NMDA alone, NMDA plus recombinant PRDX5, or NMDA, recombinant PRDX5 and dithiothreitol, a classical electron donor for peroxiredoxins. Recombinant PRDX5 plus dithiothreitol displayed a synergistic neuroprotective effect on NMDA-induced neuronal death. These findings indicate that reactive oxygen species production participates in the formation of NMDA receptor-mediated brain lesions in newborn mice and that antioxidant compounds, such as PRDX5, provide some neuroprotection in these models.

Fructose-1,6-biphosphate prevents excitotoxic neuronal cell death in the neonatal mouse brain

The excitotoxic cascade may represent an important pathway leading to brain damage and cerebral palsy. Brain lesions induced in newborn mice by ibotenate (acting on N-methyl-D-aspartate receptors) and by S-bromowillardiine (acting on alpha-3-amino-hydroxy-5-methyl-4-isoxazole propionic acid and kainate receptors) mimic some aspects of white matter cysts and transcortical necrosis observed in human perinatal brain damage. Fructose 1,6-biphosphate (FBP) is a high-energy glycolytic pathway intermediate which, in therapeutic doses, is non-toxic and neuroprotective in hypoxic-ischemic models of brain injury. Mechanisms of action include modulation of intracellular calcium through phospholipase C (PLC) activation. The goal of this study was to determine the neuroprotective effects of FBP in a mouse model of neonatal excitotoxic brain injury. Mice that received intraperitoneal FBP had a significant reduction in size of ibotenate-induced (80% reduction) or S-bromowillardiine-induced (40% reduction) cortical plate lesions when compared with control animals. Studies of fragmented DNA and cleaved caspase 3 confirmed the survival promoting effects of FBP. FBP had no detectable effect on excitotoxic white matter lesions. The effects of FBP were antagonized by co-administration of PLC, protein kinase C or mitogen-associated protein kinase inhibitors but not by protein kinase A inhibitor. A moderate, transient cooling of pups immediately after the insult extended the therapeutic window for FBP, as FBP administered 24 h after ibotenate was still significantly neuroprotective in these pups. This data extends the neuroprotective profile of FBP in neonatal brain injury and identifies gray matter lesions involving N-methyl-D-aspartate receptors as a major target for this promising drug.

Brain plasticity in paediatric neurology

Plasticity includes the brain's capacity to be shaped or moulded by experience, the capacity to learn and remember, and the ability to reorganize and recover after injury. Mechanisms for plasticity include activity-dependent refinement of neuronal connections and synaptic plasticity as a substrate for learning and memory. The molecular mechanisms for these processes utilize signalling cascades that relay messages from synaptic receptors to the nucleus and the cytoskeleton to control the structure of axons and dendrites. Several paediatric neurological disorders such as neurofibromatosis-1, Fragile X syndrome, Rett syndrome, and other syndromic and non-specific forms of mental retardation involve lesions in these signalling pathways. Acquired disorders such as hypoxic-ischaemic encephalopathy, lead poisoning and epilepsy also involve signalling pathways including excitatory glutamate receptors. Information about these 'plasticity pathways' is useful for understanding their pathophysiology and potential therapy.

Caspase inhibitors reduce the apoptotic but not necrotic component of kainate injury in primary murine cortical neuronal cultures

Excitotoxicity has been demonstrated to play a major role in ischemic neuronal injury. While the necrotic component of excitotoxicity has been well demonstrated, apoptosis has also been shown to play a role. We sought to quantitate and modulate the apoptotic component of kainate-induced injury. Experiments were performed in mouse primary cortical neuronal cultures after three or 10 days in vitro. Cell death was assessed by Hoechst/propidium iodide staining and cell counting. Apoptosis was further confirmed with inhibition by caspase inhibitors. Exposure of three-day old neurons to 100 microM kainate produced an injury in which 56% +/- 0.9% of cells showed apoptotic nuclei and 13.5% +/- 2.0% showed necrotic nuclei. After 10 days in vitro neurons were more easily injured by kainate, but the cell death had primarily necrotic characteristics. Inhibition of both caspases 1 and 3 significantly reduced the apoptotic injury of 3-day old neurons. Neither reduced the necrotic component. Inhibition of protein synthesis with cycloheximide was also effective in reducing the apoptotic injury without affecting the necrotic injury. Kainate injury causes both apoptosis and necrosis, with the injury depending on both the dose of kainate and the age of the culture. The apoptotic component can be selectively reduced by caspase inhibition or cycloheximide.

DAPK catalytic activity in the hippocampus increases during the recovery phase in an animal model of brain hypoxic-ischemic injury

Death-associated protein kinase (DAPK) is a pro-apoptotic, calmodulin (CaM)-regulated protein kinase whose mRNA levels increase following cerebral ischemia. However, the relationship between DAPK catalytic activity and cerebral ischemia is not known. This knowledge is critical as DAPK function is dependent on the catalytic activity of its kinase domain. Consequently, we examined DAPK catalytic activity in a rat model of neonatal cerebral hypoxia-ischemia (HI). An increase in DAPK specific activity was found in homogenates of the hippocampus from the injured right hemisphere, compared to the uninjured left hemisphere, 7 days after injury. The results raised the possibility that an upregulation of DAPK activity might be associated with the recovery phase of HI, during which neuronal repair and differentiation are initiated. Therefore, we examined the change of DAPK in an experimentally tractable cell culture model of neuronal differentiation. We found that DAPK catalytic activity and protein levels increase after nerve growth factor (NGF)-induced differentiation of rat PC12 cells. These results suggest that DAPK may have a previously unappreciated role in neuronal development or recovery from injury, and that potential future therapies targeting DAPK should consider a restricted time window.

Teratogen-induced limb defects

Limb abnormalities are one of the most common and visible phenotypic effects of several human teratogens. The specific effects are different for most teratogens and include effects on limb morphogenesis (thalidomide, warfarin, phenytoin, valproic acid) and the effect of vascular disruption on a limb that had formed normally (misoprostol, chorionic villus sampling, and phenytoin). Either duplication (preaxial polydactyly of hands and feet) or deficiency (absence of thumb) is a common effect of thalidomide; no other human teratogen identified to date has this effect on the developing limb. Procedures during pregnancy, including chorionic villus sampling and dilation and curettage, produce defects of vascular disruption. For common exposures, such as alcohol and cocaine, it has been difficult to confirm objectively the exposure during embryogenesis and to ascribe specific limb defects that are produced. The molecular basis for the limb defects produced by the recognized human teratogens remains unknown.

Periventricular leukomalacia, inflammation and white matter lesions within the developing nervous system

Periventricular leukomalacia (PVL) occurring in premature infants, represents a major precursor for neurological and intellectual impairment, and cerebral palsy in later life. The disorder is characterized by multifocal areas of necrosis found deep in the cortical white matter, which are often symmetrical and occur adjacent to the lateral ventricles. There is no known cure for PVL. Factors predisposing to PVL include birth trauma, asphyxia and respiratory failure, cardiopulmonary defects, premature birth/low birthweight, associated immature cerebrovascular development and lack of appropriate autoregulation of cerebral blood flow in response to hypoxic-ischemic insults. The intrinsic vulnerability of oligodendrocyte precursors is considered as central to the pathogenesis of PVL. These cells are susceptible to a variety of injurious stimuli including free radicals and excitotoxicity induced by hypoxic-ischemic injury (resulting from cerebral hypoperfusion), lack of trophic stimuli, as well as secondary associated events involving microglial and astrocytic activation and the release of pro-inflammatory cytokines TNF-alpha and IL-6. It is yet unclear whether activated astrocytes and microglia act as principal participants in the development of PVL lesions, or whether they are representatives of an incidental pathological response directed towards repair of tissue injury in PVL. Nevertheless, the accumulated evidence points to a pathological contribution of microglia towards damage. The topography of lesions in PVL most likely reflects a combination of the relatively immature cerebrovasculature together with a failure in perfusion and/or hypoxia during the greatest period of vulnerability occurring around mid-to-late gestation. Mechanisms underlying the pathogenesis of PVL have so far been related to prenatal ischemic injury to the brain initiated within the third trimester, which result in global cognitive and developmental delay and motor disturbances. Over the past few years, several epidemiological and experimental studies have implicated intrauterine infection and chorioamnionitis as causative in the pathogenesis of PVL. In particular, recent investigations have shown that inflammatory responses in the fetus and neonate can contribute towards neonatal brain injury and development-related disabilities including cerebral palsy. This review presents current concepts on the pathogenesis of PVL and emphasizes the increasing evidence for an inflammatory pathogenic component to this disorder, either resulting from hypoxic-ischemic injury or from infection. These findings provide the basis for clinical approaches targeted at protecting the premature brain from inflammatory damage, which may prove beneficial for treating PVL, if identified early in pathogenesis.

Mechanisms of hypoxic neurodegeneration in the developing brain

Asphyxia and other insults to the developing brain are responsible for several human neurodevelopmental disorders. The pattern of neonatal brain injury differs from that seen in the adult nervous system, and there are wide differences in regional vulnerability. Recent evidence suggests that two events that contribute to this pattern of selective vulnerability are developmental changes in excitatory glutamate-containing neurotransmitter circuits and the propensity for immature neurons to die by apoptosis rather than necrosis. Developmental up-regulation of NMDA receptors with enhanced function and increased expression of caspase-3 at critical periods in development are linked to these mechanisms. Although these molecular changes enhance the developing brain's capacity for plasticity by helping to prune redundant synapses and neurons, they can become "Achilles heels" in the face of a brain energy crisis.