Epileptogenic effect of hypoxia in the immature rodent brain

Mechanisms underlying the anti-epileptic efficacy of the ketogenic diet

The clinical efficacy of the ketogenic diet (KD) has now been well-documented. However, the underlying bases of KD antiepileptic efficacy are still a matter of speculation. A number of suggestions regarding underlying mechanisms have been offered, but all require rigorous testing. Development of appropriate animal model systems, and clear statement of experimentally testable hypotheses, are needed. Among the general hypotheses of interest are the following: (1) the KD alters the nature, and/or degree, of energy metabolism in the brain -- therefore altering brain excitability; (2) the KD leads to changes in cell (neuronal and perhaps glial) properties, which decrease excitability and dampen epileptiform discharge; (3) the KD induces changes in neurotransmitter function and synaptic transmission -- thus altering inhibitory-excitatory balance and discouraging hyper-synchronization; (4) the KD is associated with changes in a variety of circulating factors which act as neuromodulators that can regulate CNS excitability; and (5) the KD gives rise to alterations in brain extracellular milieu, which serve to depress excitability and synchrony. An understanding of the mechanism underlying KD antiepileptic efficacy will help us not only to optimize the clinical use of the ketogenic diet, but also to develop novel antiepileptic treatments.

Differential bicuculline-induced epileptogenesis in rat neonatal, juvenile and adult CA3 pyramidal neurons in vitro

The GABA(A) receptor antagonist bicuculline methiodide (BMI, 10 microM) transformed the evoked synaptic responses, recorded intracellularly from the CA3 area of neonatal (postnatal days 3-7, P3-P7), juvenile (P8-P20) and adult hippocampal slices, into long-lasting paroxysmal depolarizations (PDs), with repetitive action potentials (APs). In the same preparation, GABA(A)-mediated fast-IPSPs were depolarizing at resting membrane potential (RMP), with a reversal potential shifting to a hyperpolarizing direction with age (n=15, P6-P17). BMI provoked also spontaneous PDs in juvenile (20/30) and adult (7/10) but not in neonatal (0/12) neurons. PDs were depressed by either the NMDA receptor antagonist CPP (10 microM) or the non-NMDA antagonist CNQX (10 microM), but were blocked only by the combination of the two (n=6), indicating that activation of either NMDA or non-NMDA receptors can independently sustain PDs in immature hippocampus. In conclusion, these findings show that endogenous GABA tonically inhibits CA3 synaptic responses in neonatal life despite the depolarizing nature of GABA(A)-mediated potentials. Moreover, they suggest that during the 1st postnatal week, disinhibition alone is not sufficient to provoke spontaneous epileptiform discharges in CA3 hippocampal area.

Pathophysiology of perinatal brain damage

Perinatal brain damage in the mature fetus is usually brought about by severe intrauterine asphyxia following an acute reduction of the uterine or umbilical circulation. The areas most heavily affected are the parasagittal region of the cerebral cortex and the basal ganglia. The fetus reacts to a severe lack of oxygen with activation of the sympathetic-adrenergic nervous system and a redistribution of cardiac output in favour of the central organs (brain, heart and adrenals). If the asphyxic insult persists, the fetus is unable to maintain circulatory centralisation, and the cardiac output and extent of cerebral perfusion fall. Owing to the acute reduction in oxygen supply, oxidative phosphorylation in the brain comes to a standstill. The Na(+)/K(+) pump at the cell membrane has no more energy to maintain the ionic gradients. In the absence of a membrane potential, large amounts of calcium ions flow through the voltage-dependent ion channel, down an extreme extra-/intracellular concentration gradient, into the cell. Current research suggests that the excessive increase in levels of intracellular calcium, so-called calcium overload, leads to cell damage through the activation of proteases, lipases and endonucleases. During ischemia, besides the influx of calcium ions into the cells via voltage-dependent calcium channels, more calcium enters the cells through glutamate-regulated ion channels. Glutamate, an excitatory neurotransmitter, is released from presynaptic vesicles during ischemia following anoxic cell depolarisation. The acute lack of cellular energy arising during ischemia induces almost complete inhibition of cerebral protein biosynthesis. Once the ischemic period is over, protein biosynthesis returns to pre-ischemic levels in non-vulnerable regions of the brain, while in more vulnerable areas it remains inhibited. The inhibition of protein synthesis, therefore, appears to be an early indicator of subsequent neuronal cell death. A second wave of neuronal cell damage occurs during the reperfusion phase. This cell damage is thought to be caused by the post-ischemic release of oxygen radicals, synthesis of nitric oxide (NO), inflammatory reactions and an imbalance between the excitatory and inhibitory neurotransmitter systems. Part of the secondary neuronal cell damage may be caused by induction of a kind of cellular suicide programme known as apoptosis. Knowledge of these pathophysiological mechanisms has enabled scientists to develop new therapeutic strategies with successful results in animal experiments. The potential of such therapies is discussed here, particularly the promising effects of i.v. administration of magnesium or post-ischemic induction of cerebral hypothermia.

Acute and chronic effects of seizures in the developing brain: experimental models

Clinical experience suggests two major components to the relationship between brain development and epilepsy. First, the maturational state of the immature brain appears to generally decrease seizure threshold and contribute to a different seizure phenotype from the adult. Second, certain forms of seizures, when present during development, may modify brain maturation to result in chronic epilepsy and/or other neurocognitive deficits. Maturational studies in animals suggest there are numerous factors developmentally regulated in such a way as to increase excitability in immature neuronal networks in the forebrain. The developing brain appears to exhibit a transient overexpression of glutamate receptors, glutamate receptor subunit composition permissive of enhanced excitatory neurotransmission, a relative lack of GABAergic inhibitory transmission, and ion channel expression and homeostasis which enhance neuronal excitability. The increased excitatory "drive" that is likely to be critical for normal brain development may share common mechanisms with those responsible for rendering the immature brain more susceptible to seizures, seizure induced plasticity (epileptogenesis), and neuronal injury. Furthermore, the coincidence of seizures during early postnatal brain development may modify many of these parameters, which in turn may promote long term epilepsy.

Effect of seizures on cerebral hypoxic-ischemic lesions in immature rats

The present investigation was designed to study the effect of chemically induced seizures on cerebral hypoxic-ischemic (HI) damage in immature animals. Accordingly, cerebral HI was produced in 7-day postnatal (p7) rats and p13 rats by combined unilateral common carotid artery ligation and hypoxia with 8% oxygen. Seizures were induced chemically by the subcutaneous injection of kainic acid (KA) or inhalation of flurothyl vapor. Three types of experiments were conducted in each age group and for each convulsant. In some animals (group 1), seizures were produced at 24 h and again at 6 h prior to HI. In groups 2 and 3, seizures were induced 2 h or 24 h post HI, respectively. The results indicate that in group 1 animals, the first seizure significantly reduced duration of the second seizure challenge 18 h later at both p7 and p13 (p=0.001). Histologic examination of brains of animals in group 1 subjected to seizures prior to HI and their HI-only controls showed that seizures prior to HI conferred protection against cerebral damage. This effect was significant for flurothyl seizures in p13 rats for all cerebral regions, especially hippocampal CA1 (p=0.0004), and in p7 rats for hippocampus (p=0.04) and particularly cerebral cortex (p=0.007). For KA seizures, the protective effect was only significant in p13 rats and was limited to hippocampal CA regions and subiculum (p=0.0009). Histologic assessment of cerebral lesions of p7 and p13 rats in the other two groups showed no significant difference between the animals subjected to seizures 2 h or 24 h post HI and their HI-only controls (p>0.05). In conclusion, the results of the present study provide no evidence that seizures in early postnatal development aggravate pre-existing cerebral HI damage. They do suggest that seizures prior to HI or prior to a second seizure confer tolerance to both conditions.

Mossy fiber sprouting after recurrent seizures during early development in rats

In some children, epilepsy is a catastrophic condition, leading to significant intellectual and behavioral impairment, but little is known about the consequences of recurrent seizures during development. In the present study, we evaluated the effects of 15 daily pentylenetetrazol-induced convulsions in immature rats beginning at postnatal day (P) 1, 10, or 60. In addition, we subjected another group of P10 rats to twice daily seizures for 15 days. Both supragranular and terminal sprouting in the CA3 hippocampal subfield was assessed in Timm-stained sections by using a rating scale and density measurements. Prominent sprouting was seen in the CA3 stratum pyramidale layer in all rats having 15 daily seizures, regardless of the age when seizures began. Based on Timm staining in control P10, P20, and P30 rats, the terminal sprouting in CA3 appears to be new growth of axons and synapses as opposed to a failure of normal regression of synapses. In addition to CA3 terminal sprouting, rats having twice daily seizures had sprouting noted in the dentate supragranular layer, predominately in the inferior blade of the dentate, and had a decreased seizure threshold when compared with controls. Cell counting of dentate granule cells, CA3, CA1, and hilar neurons, with unbiased stereological methods demonstrated no differences from controls in rats with daily seizures beginning at P1 or P10, whereas adult rats with daily seizures had a significant decrease in CA1 neurons. Rats that received twice daily seizures on P10-P25 had an increase in dentate granule cells. This study demonstrates that, like the mature brain, immature animals have neuronal reorganization after recurrent seizures, with mossy fiber sprouting in both the CA3 subfield and supragranular region. In the immature brain, repetitive seizures also result in granule cell neurogenesis without loss of principal neurons. Although the relationship between these morphological changes after seizures during development and subsequent cognitive impairment is not yet clear, our findings indicate that during development recurrent seizures can result in significant alterations in cell number and axonal growth.

Electroencephalogram confirmatory rate in neonatal seizures

The electroencephalogram (EEG) is confirmatory in 70% of children and adults with seizures, although gestation- and etiology-specific EEG confirmatory rates in neonates have not been well defined. All neonates treated for seizures and who underwent EEG were identified from 4,575 neonates admitted between 1985 and 1996 to a neonatal intensive care unit. The relationship between EEG findings (epileptiform discharges and background abnormalities) and gestation, mortality rate, and seizure etiology was examined using the Student t test. One hundred eighty-three neonates treated for seizures underwent a total of 352 EEGs: 144 of these neonates (79%) had an abnormal EEG (epileptiform discharges in 113 (60%) and nonepileptiform background abnormalities in 31). The EEG confirmatory rate increased with gestation (63% at 28 weeks vs 77% at term, P < 0.02). Etiology for seizures also influenced the EEG confirmatory rate: central nervous system (CNS) infection 95% (P < 0.05), hypoxic-ischemic encephalopathy 80% (P < 0.05), germinal matrix-intraventricular hemorrhage 65%, and CNS malformations 65%. The EEG confirmatory rate was predictive of neonatal mortality (19% vs 6%, P < 0.03). The EEG was directly confirmatory (epileptiform discharges) in 60% and supportive (nonepileptiform background abnormalities) in a further 17% of neonates with seizures. Gestation and etiology influence the EEG confirmatory rate in neonatal seizures.

Neuropeptide-mediated excitability: a key triggering mechanism for seizure generation in the developing brain

Most human seizures occur early in life,consistent with established excitability-promoting features of the developing brain. Surprisingly, the majority of developmental seizures are not spontaneous but are provoked by injurious or stressful stimuli. What mechanisms mediate'triggering' of seizures and limit such reactive seizures to early postnatal life? Recent evidence implicates the excitatory neuropeptide, corticotropin-releasing hormone (CRH). Stress activates expression of the CRH gene in several limbic regions, and CRH-expressing neurons are strategically localized in the immature rat hippocampus, in which this neuropeptide increases the excitability of pyramidal cells in vitro. Indeed, in vivo, activation of CRH receptors--maximally expressed in hippocampus and amygdala during the developmental period which is characterized by peak susceptibility to 'provoked' convulsions--induces severe, age-dependent seizures. Thus, converging data indicate that activation of expression of CRH constitutes an important mechanism for generating developmentally regulated, triggered seizures, with considerable clinical relevance.

Changes in membrane and synaptic properties of thalamocortical circuitry caused by hydrogen peroxide

Free radical (FR) production was linked to the generation of epileptiform activity. We performed electrophysiological recordings in rat thalamocortical slices to investigate the effects of FRs on the intrinsic and synaptic properties of thalamic and cortical neurons. Whole cell recordings from identified cortical pyramidal neurons and thalamic neurons of the ventrobasal nucleus revealed that exposure to the FR-forming agent H2O2 (2.5 mM) decreased gamma-aminobutyric acid-A- and gamma-aminobutyric acid-B-mediated inhibition to 35.3 +/- 13.4% and 13.7 +/- 4.4% (means +/- SE) of control in cortical neurons and to 41.8 +/- 14.8% and 33.6 +/- 11.6% of control in thalamic neurons. H2O2 application increased excitatory transmission in thalamic neurons to 162.9 +/- 29.6% of control but caused no change in cortical neurons. H2O2 altered significantly the characteristic low-pass filter behavior of cortical and thalamic cells as determined by their input impedances. After 35 min of superfusion, the impedance of cortical neurons decreased by 67.0 +/- 14.5%, and thalamic decreased by 76.3 +/- 2.7% for the frequencies in the range 1-50 Hz while remaining constant for frequencies > 200 Hz. Neuronal hyperexcitability was manifested during H2O2 exposure by continuous firing and long depolarizing shifts in response to extracellular stimulation in both thalamocortical and cortical neurons only in slices preserving thalamocortical connections. In slices with severed thalamocortical connections, cortical neurons did not show signs of hyperexcitability. These observations indicate that FRs could promote hyperexcitability of thalamocortical circuits by altering the balance between excitation and inhibition and by transforming the characteristic low-pass filter behavior into a flat band-pass filter.

Seizure-induced neuronal injury: vulnerability to febrile seizures in an immature rat model

Febrile seizures are the most common seizure type in young children. Whether they induce death of hippocampal and amygdala neurons and consequent limbic (temporal lobe) epilepsy has remained controversial, with conflicting data from prospective and retrospective studies. Using an appropriate-age rat model of febrile seizures, we investigated the acute and chronic effects of hyperthermic seizures on neuronal integrity and survival in the hippocampus and amygdala via molecular and neuroanatomical methods. Hyperthermic seizures-but not hyperthermia alone-resulted in numerous argyrophilic neurons in discrete regions of the limbic system; within 24 hr of seizures, a significant proportion of neurons in the central nucleus of the amygdala and in the hippocampal CA3 and CA1 pyramidal cell layer were affected. These physicochemical alterations of hippocampal and amygdala neurons persisted for at least 2 weeks but were not accompanied by significant DNA fragmentation, as determined by in situ end labeling. By 4 weeks after the seizures, no significant neuronal dropout in these regions was evident. In conclusion, in the immature rat model, hyperthermic seizures lead to profound, yet primarily transient alterations in neuronal structure.

Deletion of the K(V)1.1 potassium channel causes epilepsy in mice

Mice lacking the voltage-gated potassium channel alpha subunit, K(V)1.1, display frequent spontaneous seizures throughout adult life. In hippocampal slices from homozygous K(V)1.1 null animals, intrinsic passive properties of CA3 pyramidal cells are normal. However, antidromic action potentials are recruited at lower thresholds in K(V)1.1 null slices. Furthermore, in a subset of slices, mossy fiber stimulation triggers synaptically mediated long-latency epileptiform burst discharges. These data indicate that loss of K(V)1.1 from its normal localization in axons and terminals of the CA3 region results in increased excitability in the CA3 recurrent axon collateral system, perhaps contributing to the limbic and tonic-clonic components of the observed epileptic phenotype. Axonal action potential conduction was altered as well in the sciatic nerve--a deficit potentially related to the pathophysiology of episodic ataxia/myokymia, a disease associated with missense mutations of the human K(V)1.1 gene.

Developmental perspectives on epilepsy

This paper reviews current knowledge about epileptogenesis in the developing brain. Animal studies indicate that the maturational balance of excitatory and inhibitory neurotransmitter systems should result in an increased susceptibility to seizures. However, the reason for specific age-locked syndromes and the high rate of impairments secondary to early onset epilepsy remain mysterious. Present research activity is directed to prevention and amelioration of these severe cognitive and psychiatric impairments.

Acute and chronic increases in excitability in rat hippocampal slices after perinatal hypoxia In vivo

We have previously shown that hypoxia induces both acute and chronic epileptogenic effects that are age dependent. Global hypoxia (3-4% O2) induces seizure activity in the developing brain [postnatal day (P)10-12] but not at younger or older ages. Adult rats with prior seizures induced by hypoxia at P10 show increased seizure susceptibility to chemical convulsants compared with controls. In the present study, we tested the hypothesis that acute and chronic epileptogenic effects of hypoxia are demonstrable in hippocampus both in vivo and in vitro. Depth electrode recordings confirmed the presence of ictal activity within hippocampus in P10 rats during global hypoxia. Hippocampal slices prepared from P10 pups killed at 10 min after recovery from hypoxia showed evidence of increased excitability. Extracellular field recordings revealed that the amplitude and duration of long-term potentiation (LTP) was increased significantly in area CA1 of hippocampal slices removed from hypoxic pups. In addition, extracellular recordings within areas CA1 and CA3 showed significantly longer afterdischarge durations in response to kindling stimuli in slices from hypoxic pups compared with controls. To evaluate whether there were also long-term changes in hippocampal excitability, hippocampal slices were prepared from adult rats that had underwent hypoxia at P10 and compared with slices from adult litter-mate controls. A Mg2+-free medium was superfused to induce epileptiform activity within the slices. Extracellular recordings from stratum pyramidale of area CA1 showed that Mg2+-free media induced significantly more frequent ictal discharges in slices from previously hypoxic rats compared with controls. These results provide evidence that the naturally occurring stimulus of hypoxia can result in both acute and chronic changes in the excitability of the CA1 neuronal network. These results parallel our previous in vivo studies demonstrating that global hypoxia acutely increases excitability in the immature brain and that hypoxia during the age window approximately P10 results in long-lasting increases in seizure susceptibility within hippocampus. Our results suggest that the age-dependent epileptogenic effects of hypoxia are in part mediated by a direct and permanent effect on neuronal excitability within hippocampal neuronal networks.

Acute and long-lasting effects of neonatal hypoxia on (+)-3-[125I]MK-801 binding to NMDA brain receptors

The NMDA receptor subtype is the major excitatory mediator for glutamate neurotoxicity. To assess its participation in the noxious effects of postnatal hypoxia, we have characterized the binding of the ionophoric marker of NMDA receptor, dizocilpine (MK-801). Binding of (+)-3-[125I]MK-801 to NMDA brain receptors under nonequilibrium conditions was quantified by in vitro autoradiography in rats exposed to hypoxia induced by 93% N2/6.5% O2 exposure for 70 min on Postnatal Day 4. Acute and long-lasting effects were investigated at 4 h after injury and on Postnatal Day 40. At the acute stage, a transient decrease in binding was found in several specific brain areas, hypothalamus, amygdaloid nuclei, entorhinal cortex, perirhinal cortex, and hippocampus, and no differences were found in temporal cortex, thalamus, and geniculate nucleus, when compared to sham-treated animals. At this early age, there was no increase of binding when slices from both groups were incubated in the presence of glutamate and glycine (Glu/Gly), positive allosteric modulators of MK-801 binding. In the 40-day-old brains, the binding to the NMDA receptors of hypoxiatreated animals was not different with respect to controls in most of the areas studied, but the Glu/Gly stimulation of binding in hypoxic rats showed a reduced, or absent, response to the allosteric modulators. In contrast, control rats showed a remarkable increase of the specific binding induced by the presence of the modulators in the incubation buffer. Binding of (+)-3-[125I]MK-801 was also performed at a higher concentration to clarify whether the altered response to Glu/Gly may be due to differences in the number of channels; however, the density of NMDA receptors at this concentration was similar in both control and hypoxia-treated rats. We conclude that the effect of exposure of newborn rats to hypoxia can generate acute and long-lasting effects on the NMDA receptor. The deleterious action of this kind of noxa on the CNS could be exerted by interference with normal glutamatergic transmission and hence over normal growth and development.

Hypoxia-induced dysfunction in developing rat neocortex

Neocortical slices from young [postnatal day (P) 5-8], juvenile (P14-18), and adult (>P28) rats were exposed to long periods of hypoxia. Field potential (FP) responses to orthodromic synaptic stimulation, the extracellular DC potential, and the extracellular Ca2+ concentration ([Ca2+]o] were measured simultaneously in layers II/III of primary somatosensory cortex. Hypoxia caused a 42 and 55% decrease in the FP response in juvenile and adult cortex, respectively. FP responses recorded in slices from young animals were significantly more resistant to oxygen deprivation as compared with the juvenile (P < 0.01) and adult age group (P < 0.001) and declined by only 3% in amplitude. In adult cortex, hypoxia elicited, after 7 +/- 4.5 min (mean +/- SD), a sudden anoxic depolarization (AD) with an amplitude of 14 +/- 6 mV and a duration of 0.89 +/- 0.28 min at half-maximal amplitude. Although the AD onset latency was significantly longer in P5-8 (12.5 +/- 4.9 min, P < 0.001) and P14-18 (8.7 +/- 3.2 min, P < 0.002) cortex, the amplitude and duration of the AD was larger in young (45.7 +/- 7.6 mV, 2.19 +/- 0.71 min, both P < 0.001) and juvenile animals (29.9 +/- 9.1 mV, P < 0.001, 0.96 +/- 0.26 min, P > 0.05) when compared with the adults. The hypoxia-induced [Ca2+]o decrease was significantly (P < 0.002) larger in young cortex (1,115 +/- 50 microM) as compared with the adult (926 +/- 107 microM). Prolongation of hypoxia after AD onset for >5 min elicited in young and juvenile cortex a long-lasting AD with an amplitude of 40.5 mV associated with a decrease in [Ca2+]o by >1 mM. On reoxygenation, only slices from these age groups showed spontaneous repetitive spreading depression in 3 out of 26 cases. In adults, the same protocol caused a significantly (P < 0.05) smaller and shorter AD and never a spreading depression. However, recovery in synaptic transmission after this long-term hypoxia was better in young and juvenile cortex, indicating a prolonged or even irreversible deficiency in synaptic function in mature animals. Application of ketamine caused a 49% reduction in the initial amplitude of the AD in juvenile cortex but did not significantly affect the AD in slices from adult animals. These data indicate that the young and juvenile cortex tolerates much longer periods of oxygen deprivation as compared with the adult, but that a sufficiently long hypoxia causes severe pathophysiological activity in the immature cortex. This enhanced sensitivity of the immature cortex is at least partially mediated by activation of N-methyl-D-aspartate receptors.

Dextromethorphan ameliorates effects of neonatal hypoxia on brain morphology and seizure threshold in rats

Hypoxic injury to the brain is mediated in part by NMDA receptors. Therefore, NMDA receptor blockade with dextromethorphan (DM), a non-competitive channel blocker, was hypothesized to ameliorate injury even when given after the hypoxic insult. Rats were exposed to 8% oxygen for 3 h on postnatal day 7. Within 20 min of exposure, animals received 30 mg/kg i.p. DM or normal saline. Littermates maintained in room air for 3 h also received DM or saline. At 14 days of age, 7 days after exposure, cortical thickness and hippocampal area were measured. At 70-90 days of age, approximately two months after exposure, in a separate group of rats, seizure threshold using pentylenetetrazol (PTZ) and passive avoidance learning and retention were determined. There were no gross changes in cellular morphology and no evidence for cellular necrosis in any of the exposure groups. However, cortical thickness was decreased in animals exposed to hypoxia. DM administration prevented this decrease. Hippocampal area was unaffected. Seizure susceptibility in adulthood was increased in animals exposed to hypoxia in the neonatal period. DM prevented the decrease in seizure threshold. There was no difference in passive avoidance learning or retention as a function of neonatal exposure condition. Mild to moderate hypoxia, previously thought not to produce any histologic changes, causes significant short-term loss of cortical thickness and long-term decrease in seizure threshold. DM appears to ameliorate these effects even when given after the hypoxic insult. These results implicate the glutamate receptor system in the pathophysiology of hypoxia damage and suggest that treatment with a glutamate receptor blocker when neonatal asphyxia is suspected would help ameliorate the consequences of such an insult.

Acute and chronic effects of hypoxia on the developing hippocampus

Perinatal hypoxia is associated with both seizures arising acutely and the subsequent development of temporal lobe epilepsy (as determined retrospectively). We therefore attempted to identify acute and chronic morphological and/or electrophysiological hippocampal pathologies associated with experimentally induced hypoxia in immature rats. Pups were exposed to 15 minutes of hypoxia on 3 successive days (starting on postnatal day 8; P8), or to 60 minutes of hypoxia on P10 with either one or multiple hypoxia-induced seizures. For animals experiencing multiple seizures, flurothyl seizure threshold was significantly lower than that of controls at 60 to 80 days, but not at 10 days, after hypoxia. Acutely, there was a treatment-related increase in the number and the density of pyknotic dentate and hilar neurons, in particular in animals experiencing multiple seizures. However, 60 to 80 days after the multiple-seizure protocol, the number of dentate and hilar neurons did not differ between control and experimental animals. Electrophysiological measures of pyramidal cell properties showed no striking difference between experimental and control animals at any time point. These results indicate that early postnatal hypoxia and hypoxia-induced seizure episodes decrease seizure threshold in the adult but produce minimal acute or chronic morphological or functional changes in the hippocampus.

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

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

Epilepsy in the developing brain: lessons from the laboratory and clinic

Children with epilepsy present unique challenges to the clinician. In addition to having differences in clinical and EEG phenomena, children differ from adults in regard to etiological factors, response to antiepileptic drugs (AEDs), and outcome. It is now recognized that the immature brain also differs from the mature brain in the basic mechanisms of epileptogenesis and propagation of seizures. The immature brain is more prone to seizures due to an imbalance between excitation and inhibition. gamma-Aminobutyric acid (GABA), the major CNS inhibitory neurotransmitter in the mature brain, can lead to depolarization in the hippocampal CA3 region in very young rats. There are also age-related differences in response to GABA agonists and antagonists in the substantia nigra, a structure important in the propagation of seizures. These age-related differences in response to GABAergic agents provide further evidence that the pathophysiology of seizures in the immature brain differs from that in the mature brain. Although prolonged seizures can cause brain damage at any age, the extent of brain damage after prolonged seizures is highly age dependent. Far less histological damage and fewer disturbances in cognition result from prolonged seizures in the immature brain than from seizures of similar duration and intensity in mature animals. However, detrimental effects of AEDs may be greater in the immature brain, than in the mature brain. These lessons from the animal laboratory raise questions about the appropriateness of current therapeutic approaches to childhood seizure disorders.

Hypoxia-induced hyperexcitability in vivo and in vitro in the immature hippocampus

Hypoxia is the most common cause of neonatal seizures and encephalopathy. We have previously developed an in vivo experimental model of perinatal hypoxia which exhibits age-dependent acute and chronic epileptogenic effects. Between postnatal day (P) 10-12, the rat exhibits acute seizure activity during global hypoxia, while no seizures are induced at earlier (P5) or older (P60) ages. Rats exposed to hypoxia between P10-12 have reduced seizure thresholds to chemical convulsants in adulthood. The nonNMDA antagonists NBQX appears to suppress both the acute and long term epileptogenic effects of hypoxia. The age-dependency of the hyperexcitable response to hypoxia in vivo can be reproduced in vitro using hippocampal slices. In Mg(2+)-free media, hypoxia induced ictal discharges within 60 s of onset in 79% of slices from normal P10 rat pups compared to 11% of adult slices (p < 0.001). Model systems such as that described here allow for correlation of in vitro and in vivo electrophysiology and should provide data regarding the pharmacological and physiological characteristics of hypoxia-induced seizure activity in the immature brain which could ultimately be applied to therapeutic strategies.

The effects of neonatal hypoxia on kindled seizure development and electroconvulsive shock profiles

PURPOSE

Our previous research indicated that the exposure of rat pups to an hypoxic environment during a discrete developmental period (postnatal days 10-15) produces short-term seizures and confers an enduring increase in susceptibility to pentylenetetrazol- and flurothyl-induced seizures. In this study, we evaluated the effects of hypoxic insult in this neonatal period of susceptibility to electrical kindling and corneal electroconvulsive shock.

METHODS

Ten-day-old rat pups were exposed to a 3% O2 environment, as previously described, and were either kindled or exposed to corneal electroshock at adulthood (70 days old).

RESULTS

Neither kindled seizure development from the septal nucleus or amygdala nor electroconvulsive shock profiles were significantly altered by hypoxic pretreatment.

CONCLUSIONS

Results indicate that hypoxia produces increases in seizure susceptibility that are observable in only some experimental seizure models but not in others. This outcome serves to target some anatomic systems more than others in the mechanisms involved in hypoxia-induced neural reorganization.

Developmental features of 4-aminopyridine induced epileptogenesis

4-Aminopyridine (4-AP, 50 microM), perfused in rat hippocampal slices from postnatal days 2-30 (P2-P30), induced in the CA3 area the appearance of spontaneous epileptiform discharges, short (interictal-like) and sustained (ictal-like), as well as slow potential. The duration of epileptiform discharges decreased and their rate of occurrence (frequency) increased with maturation: their duration during the 1st postnatal week was 4-6 times longer and their frequency 5 times lower, compared to those of the 4th postnatal week. Ictal discharges gradually disappeared at the end of the 4th postnatal week. Spontaneous synchronous activity-as a whole-often appeared in clusters separated by equal or longer length inactive periods, during the first two postnatal weeks. At the same period, ictal discharges were often followed by repetitive afterdischarges, forming sequences which lasted 0.7-1.5 min. Sectioning experiments showed that epileptiform discharges were generated in CA3, and their presence in CA1 depended on the integrity of CA1-CA3 synaptic connections. In conclusion, these findings demonstrate that (i) immature CA3 can generate synchronous epileptiform discharges as early as P2, (ii) such discharges are longer lasting and more complex during the early developmental stages and (iii) there are two time points (end of 2nd, end of 4th postnatal weeks), when maturational changes alter the epileptogenic properties of immature hippocampus.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

Age dependence of NMDA receptor involvement in epileptiform activity in rat hippocampal slices

The pattern of epileptiform activity recorded from a number of in vitro seizure models is age dependent: ictal discharges are observed in immature brain slices while interictal bursts are seen in adult brain slices. This study evaluated the involvement of the N-methyl-D-aspartate (NMDA) receptor in the age-dependency of epileptiform activity recorded in area CA1 of hippocampal slices in Mg(2+)-free medium. Incubation in Mg(2+)-free medium induced ictal activity in 84% of hippocampal slices from immature rats (postnatal 10-15 days). In contrast, adult slices responded with interictal bursting, while ictal activity was rare (9%). Bath application of the NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (DL-APV, 20 microM) converted ictal activity to interictal activity in the hippocampal slices from immature rats. In adult slices, bath application of NMDA (10-20 microM) in Mg(2+)-free medium induced ictal-like discharges. Perfusion with NMDA (20 microM) in a medium containing 1.5 mM Mg2+ induced ictal activity in immature slices while it evoked only interictal bursts in adult slices. These results suggest that differences in NMDA receptor function may be involved in the age-dependency of epileptiform activity induced by Mg(2+)-free medium. Enhanced NMDA receptor-mediated activity may partially underlie increased seizure susceptibility in the immature brain.

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.

Neonatal seizures: a clinician's overview

Seizures are the most frequent neurological event in newborns (NBs), provoked often by noxae not apt to cause them in later life. This is because receptor families of excitatory amino acids (EAA) are overexpressed at this stage of brain ontogenesis, which is also why most neonatal seizures rapidly abate, even when neurological deficits persist. The brain's immaturities dictate distinct seizure phenotypes. A classification proposed in the late 1960s has been criticized, and a new one has been advocated, based on correlations between EEGs and behaviors, leading to a classification of seizures into 'epileptic' and 'non-epileptic'. The taxonomic pitfalls of these classifications are discussed, and the notion advanced that many seizures fail to fulfil the criteria to label them as epileptic. While etiological factors have changed in time, the striking dichotomy in outcome has persisted. Many etiologies, often multifactorial, are unique in NBs, and they are discussed with reference to diagnosis and therapies. Four syndromes of NB seizures, accepted into the International Classification of the Epilepsies, are critically analyzed, some appearing to rest on fragile grounds. Controversies persist whether seizures per se are injurious to the immature brain. Clinical studies suggest that neither duration in days or length of seizure phenotypes correlates with outcomes, the most valid prognostic indices being offered by etiologies and by patterns of EEG polygraphy. However, because most seizures are symptomatic, it may be difficult to distinguish morbidity due to underlying pathology from that possibly added by seizures. Animal experiments suggested that they are injurious. The theory of energy failure, postulated to cause a cascade of events leading to inhibitions of DNA, proteins, lipids and disrupted neuronal proliferation, synaptogenesis, myelination, has largely been disproved. Brains of immature animals have been shown to have the oxidative machinery needed to fulfill energy demands, even during status convulsivus. They are also capable of using anaerobic metabolism and require less ATP when aerobic energy production ceases. Recent explanations for the injurious consequences of hypoxic ischemia and of prolonged convulsions postulate that neuronal damage occurs from excessive release of EAA which, by binding to their ligand-gated ionic receptors, cause a large influx of Ca2+, resulting in cell death. Because of the overabundance of EAA receptors in early ontogenesis, the excitotoxic hypothesis would appear attractive, but some observations militate against it. Among these is the dissociation found between the focal neurotoxicities induced by EAA injected into the brain and their absence following the concomitant convulsions. The latter are not blocked by pretreatment with EAA antagonists, while these prevent injuries caused by the injected EAA. There is no convincing evidence that excessive release of EAA occurs during NBs' seizures. Even if it does occur, it has been shown that immature neurons have a better capacity to self-protect from increased Ca2+ influx, and also that direct application of glutamate to immature neurons leads to significantly lower Ca2+ influx. These data raise doubts about the postulated excitotoxicity caused by NBs' seizures, being consistent with the fact that no one, so far, has observed neuronal damage from drug-induced convulsive states in NBs. Lack of overt neuronal injuries does not preclude that long-term subtle changes might be induced by noxae apt to provoke transient ictal events. Thus models developed in our laboratories demonstrate that long-term epileptogenicity results following postnatal O2 deprivation without evidence of neuronal injuries or of long-term behavioral or electrophysiological alteration. However, both age at which hypoxia occurs and specific proconvulsant methods used strictly determine whether increased epileptogenicity will occur.

NBQX blocks acute and late epileptogenic effects of perinatal hypoxia

Clinically, and in experimental models, perinatal hypoxic encephalopathy is commonly associated with seizures. We previously described a rat model in which hypoxia induces seizures and permanently increases in seizure susceptibility in immature rats [postnatal day (P) 10-12] but not in older rats. In the present study, we compared the effect of pretreatment with the excitatory amino acid antagonists MK-801 and NBQX versus lorazepam in our rat model of perinatal hypoxia. Animals exposed to hypoxia at P10 without treatment have frequent seizures during hypoxia and subsequently exhibit increased seizure susceptibility to flurothyl. Treatment with 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX 20 mg/kg) effectively suppressed hypoxia-induced seizures in immature rats and also protected against permanent changes in flurothyl threshold in adulthood, whereas treatment with MK-801 (1 mg/kg) or lorazepam (LZP 1 mg/kg) did not prevent these hypoxia-related epileptogenic effects. These results suggest that activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid (AMPA) receptors may partly mediate the age-dependent epileptogenic effect of hypoxia in the perinatal period.

Phosphocreatine and ATP regulation in the hypoxic developing rat brain

Decreased brain ATP and phosphocreatine (PCr) concentrations and intracellular pH were compared in hypoxic 4-, 10-11, and 24-25-day-old rats. Surface coil 31P-nuclear magnetic resonance (NMR) spectra were acquired in vivo every minute before, during, and after 7 min of breathing 4% O2. At all ages PCr decreased rapidly. At the two younger ages, the nucleoside triphosphate signal was still 80-85% of pre-hypoxic values, indicating 20-30% decrease in ATP, when PCr was almost fully depleted. At 24-25 days, PCr initially decreased 40-50% with an ATP loss of about 30%. Then, PCr and ATP decreased simultaneously. The decrease in brain pH was greatest at 24-25 days. More electrocortical seizure activity during hypoxia was seen at 10-11 days than at other ages. Seizure activity was seen only when ATP was less than 20% depleted and was not associated with more rapid decreases in ATP or PCr. At all ages, loss of electrocortical activity occurred when ATP was about 30% depleted. Brain creatine kinase catalyzed flux, measured by the NMR saturation transfer experiment before the hypoxic period, was 4-fold higher at 24-25 days than at 4- or 10-11 days. In conclusion, the temporally coupled depletion of PCr and ATP during hypoxia, which is characteristic of the mature brain, is seen only after the maturational increase in brain CK activity.

Effect of hypoxia/ischemia on bicuculline-induced seizures in immature rats: behavioral and electrocortical phenomena

The relation between hypoxia/ischemia and subsequent alterations in seizure susceptibility in developing brain remains unclear. We assessed the behavioral and electrocorticographic (ECoG) effects of hypoxic/ischemic brain damage on bicuculline (BIC)-induced seizures in 7-day postnatal rats, and determined maturational changes in seizure susceptibility, behavior and ECoG activity. Rat pups were subjected to unilateral common carotid artery ligation, followed by exposure to 8% O2 at 37 degrees C for 2 h, an insult that produces brain damage in the cerebral hemisphere ipsilateral to carotid artery occlusion. The experimental group consisted of rat pups previously subjected to hypoxia/ischemia; control littermates received neither arterial ligation nor systemic hypoxia. Experimental animals received 4, 5, or 6 mg/kg BIC subcutaneously (s.c.) at 2 and 24 h, and at 3, 7, and 21 days of recovery from hypoxia/ischemia. Two animals at each interval of recovery, 1 each from the experimental and control groups, were used for ECoG monitoring. After BIC injection, animal behavior was observed for 2 h. Behaviors and seizures were classified in five categories based on severity, duration, and character: 1, mild irritability; 2, few clonic seizures and agitation; 3, few tonic-clonic seizures with swimming movements; 4, frequent tonic-clonic seizures with apneic episodes; 5, continuous tonic-clonic seizures and death. Rat pups previously subjected to hypoxia/ischemia had lesser seizure susceptibility than controls at 2-h recovery (p < 0.05) and greater susceptibility than controls at 24 h (p < 0.05). Tonic seizures were prominent at 2 and 24 h in both the experimental and control groups, whereas lesion-sided circling was prominent only in the hypoxic/ischemic rat pups.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental and neurologic status of children after heart surgery with hypothermic circulatory arrest or low-flow cardiopulmonary bypass

BACKGROUND

Deep hypothermia with either total circulatory arrest or low-flow cardiopulmonary bypass is used to support vital organs during heart surgery in infants. We compared the developmental and neurologic sequelae of these two strategies one year after surgery.

METHODS

Infants with D-transposition of the great arteries who underwent an arterial-switch operation were randomly assigned to a method of support consisting predominantly of circulatory arrest or a method consisting predominantly of low-flow bypass. Developmental and neurologic evaluations and magnetic resonance imaging (MRI) were performed at one year of age.

RESULTS

Of the 171 patients enrolled in the study, 155 were evaluated. After adjustment for the presence or absence of a ventricular septal defect, the infants assigned to circulatory arrest, as compared with those assigned to low-flow bypass, had a lower mean score on the Psychomotor Development Index of the Bayley Scales of Infant Development (a 6.5-point deficit, P = 0.01) and a higher proportion had scores < or = 80 (i.e., 2 SD or more below the population mean) (27 percent vs. 12 percent, P = 0.02). The score on the Psychomotor Development Index was inversely related to the duration of circulatory arrest (P = 0.02). The risk of neurologic abnormalities increased with the duration of circulatory arrest (P = 0.04). The method of support was not associated with the prevalence of abnormalities on MRI scans of the brain, scores on the Mental Development Index of the Bayley Scale, or scores on a test of visual-recognition memory. Perioperative electroencephalographic seizure activity was associated with lower scores on the Psychomotor Development Index (P = 0.002) and an increased likelihood of abnormalities on MRI scans of the brain (P < 0.001).

CONCLUSIONS

Heart surgery performed with circulatory arrest as the predominant support strategy is associated with a higher risk of delayed motor development and neurologic abnormalities at the age of one year than is surgery with low-flow bypass as the predominant support strategy.

Prenatal cocaine alters open-field behavior in young swine

Yucatan minisows received 2 mg/kg cocaine i.v. 4 times daily during the last third of gestation. Their piglets were fostered at birth to paired, unexposed sows with their litters, and studied at age 2 to 9 (young group) and 22 to 29 days (older group). Three to 5 exposed and unexposed piglets of each age group were videotaped together for 30 min on 5 consecutive days in an open-field environment. For each piglet, 41 behaviors were scored, timed, summed and clustered into 9 behavioral categories. With age, and independently of drug exposure, piglets spent more time in ingestion, immobility while alone and play/aggression, and less time in group locomotion. For the first 4 test days, the young exposed piglets spent more time in group immobility and less time in individual locomotion and rooting than their age-matched controls. In contrast, the older exposed and unexposed piglet groups did not differ in any of these behavioral clusters. These results suggest that prenatal cocaine exposure in neonatal swine may transiently affect responses to spatial novelty.

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

Animal models have contributed immensely to our understanding of hypoxic ischemic encephalopathy in the newborn. A number of animal models have been used, including both primate and subprimate species. Although the Rhesus monkey model has a dramatically similar pathological distribution of brain injury when compared with the human, other pathologic processes secondary to asphyxia may be more appropriately assessed in other species. The maxim that because primates are closer on the phylogenetic tree to humans than are subprimates all observations in the primate are applicable to the human is simply not true. Understanding of the neurochemical consequences of asphyxia in the past decade have arisen from experiments primarily in the neonatal rat. We have come to understand that not only is the hypoxic event of major significance, but that, once reperfused, reoxygenation causes further injury. Free-radical generation following reperfusion may be massive and may further contribute to cell membrane injury. These observations have lead to rational theoretic approaches to the treatment of hypoxic ischemic brain injury. On the other hand, previously used treatments such as osmotic agents and glucocorticoids would appear to be not only inefficacious but hazardous in the treatment of hypoxic ischemic brain injury. The role of nitric oxide (NO) in the pathogenesis of brain injury is yet uncertain, but there is little doubt that it plays a significant role. Although survival of the immature animal subjected to hypoxic environment is longer than in the mature animal, the central nervous system of the immature animal is more sensitive to glutamate and N-Methyl-D-aspartate (NMDA) receptor-mediated injury.

An animal model of hypoxia-induced perinatal seizures

Clinically, neonatal hypoxic encephalopathy is commonly associated with seizure activity. Here we describe a rodent model of cerebral hypoxia in which there is are age dependent effects of hypoxia, with hypoxia inducing seizure activity in the immature rat, but not in the adult. Global hypoxia (3-4% O2) induced acute seizure activity during a window of development between postnatal day (P5-17), peaking at P10-12. Animals which had been rendered hypoxic between P10-12 had long term decreases in seizure threshold, while animals exposed at younger (P5) or older (P60) ages did not. Antagonists of excitatory amino acid (EAA) transmission appear to be superior to benzodiazepines in suppressing the acute and long term effects of perinatal hypoxia, suggesting involvement of the EAA system in these phenomena. No significant histologic damage occurs in this model, suggesting that functional alterations take place in neurons when exposed to an hypoxic insult at a critical developmental stage. Future work is directed at evaluating molecular and cellular events underlying the permanent increase in seizure susceptibility produced by this model.

Hypoxic-ischemic injury in the neonatal rat brain: effects of pre- and post-treatment with the glutamate release inhibitor BW1003C87

In a model of perinatal hypoxia-ischemia (HI) we examined the neuroprotective efficacy of pre- and post-treatment with the glutamate release inhibitor BW1003C87 [5-(2,3,5-trichlorophenyl)-2,4-diamino-pyrimidine). Ipsilateral brain damage developed in 99% of rat pups subjected to HI (unilateral common carotid artery ligation and 100 min of 7.7% oxygen exposure) with a 26 +/- 16% (mean +/- S.D.) weight deficit of the damaged hemisphere 2 weeks after the insult. Pre-treatment with BW1003C87 (10 mg/kg intraperitoneally) reduced the brain damage by 46% (P < 0.05). A higher dose (20 mg/kg) of pre-treatment was not tolerated. Administration of BW1003C87 did not affect the rectal temperature of the rats. Post-treatment with BW1003C87 (10-30 mg/kg) offered no neuroprotection in this model. In conclusion, there was a neuroprotective effect from pre- but not post-treatment with BW1003C87 in this model, supporting the concept that intra-ischemic excitatory amino acid release is important for development of brain damage. The lack of post-treatment effect indicates that BW1003C87 did not attenuate deleterious EAA cycling during reflow in the neonatal brain.

The putative essential nutrient pyrroloquinoline quinone is neuroprotective in a rodent model of hypoxic/ischemic brain injury

Pyrroloquinoline quinone is a ubiquitous redox cofactor and putative essential nutrient in mammals. Pyrroloquinoline quinone has recently been demonstrated to depress N-methyl-D-asparate induced electrical responses and is neuroprotective in vitro. In addition, pyrroloquinoline quinone has been demonstrated to act as a free radical scavenger in mammalian tissues. In this study, we demonstrate a neuroprotective effect of pyrroloquinoline quinone in an in vivo cerebral hypoxia/ischemia model in the rodent. Significant reduction in infarct size resulted from pyrroloquinoline quinone pretreatment and also when pyrroloquinoline quinone was administered following induction of hypoxia/ischemia. The neuroprotective effect was not dependent on change in core or cranial temperatures, as there was no difference between temperature measurements in pyrroloquinoline quinone-treated and vehicle-treated controls. No changes in electroencephalographic activity were observed at neuroprotective doses. These findings suggest that pyrroloquinoline quinone may represent a novel class of quinoid reagents of potential use in the treatment of neurological disorders that involve excitotoxicity. This study demonstrates a protective effect of the novel essential nutrient pyrroloquinoline quinone on brain injury in a rodent model of cerebral hypoxia/ischemia. Pyrroloquinoline quinone was neuroprotective when administered before and even after the insult, and did not appear to have significant neurobehavioral side effects. Pyrroloquinoline quinone represents a new class of agents with potential use in the therapy of stroke.

Zonisamide reduces hypoxic-ischemic brain damage in neonatal rats irrespective of its anticonvulsive effect

The neuroprotective effect of a novel anticonvulsant, zonisamide, was investigated in neonatal rats with hypoxic-ischemic brain damage. Rats underwent left carotid ligation followed by hypoxic exposure (8% O2) for 2.5 h. When zonisamide (75 mg/kg) was administered i.p. 1 h before hypoxia, it reduced the cortical infarction volume to 6 +/- 5% (mean +/- S.E.M.) from 68 +/- 7% in vehicle-treated controls and the striatal volume to 8 +/- 4% from 78 +/- 7%. Zonisamide also reduced neuronal necrosis in 5 hippocampal regions (the dentate gyrus, CA4, CA3, CA1, and the subiculum). The plasma zonisamide concentration before and after hypoxia was 47.9 +/- 2.0 microgram/ml and 42.3 +/- 3.9 microgram/ml, respectively. Epidural electrodes were implanted in 6 pups one day before hypoxia-ischemia. Electroencephalograms were recorded during hypoxia-ischemia in rats given zonisamide or vehicle before the insult. The intensity of seizure activities was similar in the zonisamide-treated pups and the vehicle-treated controls. These findings demonstrate that zonisamide reduces neonatal hypoxic-ischemic brain damage and that this protective effect does not depend on its anticonvulsant action.

The effects of perinatal anoxia or hypoxia on hippocampal kindling development in rats

The effects of anoxia and hypoxia (3% oxygen) at 10-12 post days of age on the development of ventral hippocampal kindling and its transfer to the contralateral ventral hippocampus were studied in adult male Sprague-Dawley rats. During oxygen deprivation, the heart rate decreased to 15% of the prehypoxic value in the animals exposed to anoxia and 40% in those exposed to hypoxia. As is observed in asphyxia of human newborns, our study included both ischemia and hypoxia. The susceptibility to kindling, which was measured by kindling rate, afterdischarge threshold, generalized seizure threshold, and total afterdischarge duration to stage 5, had a tendency to be enhanced in rats exposed to hypoxia compared with controls. The facilitating effects on primary site kindling were enhanced in the animals exposed to hypoxia compared with those exposed to anoxia. Transfer, which was indicated by kindling rate and afterdischarge threshold, was also slightly facilitated in the rats exposed to anoxia or hypoxia in the perinatal period. These results reveal that perinatal oxygen deficiency may not be sufficient to lead to the development of temporal lobe epilepsy. However, it is possible that perinatal hypoxia results in some pathophysiological change in the brain which leads to greater seizure susceptibility in adulthood.

Age-related electrocorticographic and respiratory adaptation to repeated hypoxia

Severe hypoxia is known to produce depression in electrical brain activity and perturbation of respiratory pattern. In piglets undergoing chronic recording of brain and respiratory muscle activities, a depressed electrocorticogram (ECoG) was observed in response to rapidly induced (< 30 s), brief (10 min), and moderate hypoxia (10% O2 in 90% N2) in 16 out of 42 study sessions in young (3- to 11-day-old) animals only. Responses to hypoxia were monitored over 4 consecutive days. In five cases, the latency to the onset of the ECoG depression increased progressively over the 4 test days, and its duration decreased progressively. An associated respiratory gasping pattern also exhibited gradual remission over consecutive days. These changes in the responses to repeated hypoxia demonstrate adaptation of mechanisms underlying neuronal perturbation by oxygen deprivation.

Potassium-induced changes in excitability in the hippocampal CA1 region of immature and adult rats

Orthodromic and spontaneous population spike activity was measured in vitro in the CA1 region of rat hippocampal slices to determine maturational differences in excitability and susceptibility to K(+)-induced seizures. Several indices of excitability in the CA1 region changed in a non-monotonic fashion during maturation, in response to step-wise increases in bath [K+]. Slices from rats 18-22 days old, showed a greater probability of both spontaneous epileptiform activity and episodes of seizure-like activity followed by spreading depression, and more prolonged durations of evoked seizure-like events. Elevation of [K+] in the bathing medium increased these indices in a similar manner in older rats but not to the same degree as in 18- to 22-day-old rats. However, the threshold level of bath [K+] resulting in evoked bursts of population spikes was lower in adult and 28- to 32-day-old rats than in 18- to 22-day-old rats, suggesting that excitability is not uniformly greater at any given age. In 10- to 15-day-old rats, elevation of bath [K+] either produced persistent blockade of population responses, or increased the amplitude of the initial population spike, without producing bursts. Basal levels of [K+] in the interstitium of the slices corresponded to the various levels of [K+] placed in the bathing medium and there were no differences among age groups. Therefore, differences in basal [K+]o cannot account for the maturational changes in excitability and seizure activity. The period from 18-22 days of age in the rat is a useful focal point for investigating mechanisms underlying maturational changes in propensity to develop seizures.

Profound, reversible energy loss in the hypoxic immature rat brain

The goal of this study was to compare the effects of oxygen deprivation on cellular energy state and pH in the developing and adult rat brain. Relative quantities of phosphocreatine (PC), inorganic phosphorus (P(i)), and nucleoside triphosphates (NTP), and intracellular pH, were determined using in vivo 31P NMR spectroscopy at different postnatal ages (postnatal day (P) 2-6, P9-13, P16-20, P23-27) in the hypoxic rat brain (7 min, 4% O2). While a significant increase in P(i) was seen at all ages during hypoxia, a severe but reversible reduction in concentrations of PC (80-100% decrease) and NTP (40-50% decrease) was observed only at P9-13. This dramatic response was not seen in older (> P16) or younger (< P6) animals. These latter groups responded with moderate decreases in brain PC (50-60% decrease) and NTP (20-40% decrease). In addition, the youngest animals showed much less intracellular brain acidosis than the other age groups. The transient period of development during which the brain exhibits heightened susceptibility to hypoxic energy failure coincides with known changes in brain energy production pathways and susceptibility to hypoxia-induced excitability.

Differences in c-fos immunoreactivity due to age and mode of seizure induction

The aim of this study was to determine whether the regional distribution and time course of immunoreactivity to the c-fos protein varies with maturation and method of seizure induction. The effect of the two chemical convulsants, pentylenetetrazol (PTZ) and flurothyl, on the spatial and temporal pattern of c-fos-like immunoreactivity in immature (postnatal day (P) 10) was compared to that in adult rats. Patterns of c-fos-like immunoreactivity following O2 deprivation were also evaluated at the 2 ages because hypoxia is acutely epileptogenic in immature animals but not adults. C-fos-like immunoreactivity was examined at 2, 4, and 6 h after onset of chemically induced seizures or O2 deprivation at both ages. After PTZ or flurothyl seizures, both ages exhibited similar patterns of IR in amygdala, pyriform cortex, and hypothalamus. Age-dependent regional differences were most prominent in cortex: superficial layers of retrosplenial, cingulate, and neocortex stained in adults; staining was confined to deep layers of neocortex in P10 rats. Intense staining of dentate gyrus and hippocampus occurred with more prolonged seizures, but not brief seizures. PTZ administration resulted in staining at 2 h after seizure onset and was reduced by 4 h in adults, but immunoreactivity was not seen until 4 and 6 h after seizure onset in immature rats, indicating an age effect on the time course of IR. In immature rats, immunoreactivity patterns after hypoxia were markedly different from PTZ or flurothyl: staining was confined to layer VI of neocortex in these animals, and rarely involved limbic structures. These differences in the pattern of c-fos immunoreactivity suggest that the neuronal populations involved in epileptogenesis are influenced by age as well as seizure phenotype and intensity.

Age-dependent changes in long-term seizure susceptibility and behavior after hypoxia in rats

We showed that hypoxia is acutely epileptogenic in immature but not in adult rats. In the present study, we evaluated whether hypoxia results in an increase in long-term seizure susceptibility to flurothyl and whether this is associated with impaired performance on behavioral tests. We also determined whether these long-term outcomes are dependent on age at time of O2 deprivation. Long Evans hooded rats were rendered hypoxic on either postnatal day (P)5, P10, or P60. Sixty to 75 days after hypoxia, rats were tested for performance in water maze, open field, and handling tests and for seizure susceptibility to flurothyl. Hypoxia at P10 significantly increased seizure susceptibility to flurothyl, whereas hypoxia at P5 and P60 induced no long-term changes in seizure threshold. At P10, greater seizure severity during hypoxia and more prolonged exposure to hypoxia significantly increased long-term seizure susceptibility. This long-term change in seizure susceptibility appeared to be dissociated from any long-term neurobehavioral consequences, because only animals rendered hypoxic as adults (P60) had impaired behavioral performance. The results suggest that hypoxia-induced seizures can alter long-term seizure susceptibility and that this long-term effect is dependent on age and on severity of seizure activity at the time of previous hypoxia.

Perinatal hypoxic ischemic encephalopathy affects the proportion of GABA-immunoreactive neurons in the cerebral cortex of the rat

The hypothesis was tested whether perinatal hypoxic ischemia leads to a preferential degeneration of the GABAergic inhibitory neurons in the cerebral cortex which, in turn, could account for the reported higher risk of developing epilepsy later in life. To that end rat pups, aged 12-13 days, were made hypoxic by employing a combination of unilateral ligation of one of the carotid arteries and a 90-min exposure to 8% O2. After recovery periods of 3, 7, 35 and 150 days, the animals were sacrificed by perfusion fixation and the brains embedded in Epon. Transverse semi-thin sections were alternately stained with an antibody against GABA and with Toluidine blue. By using an unbiased morphometric method (the disector) the number of GABA-immunoreactive (GABA-IR) neurons and the total number of nerve cells per unit volume of tissue were estimated in corresponding neocortical areas in the ipsilateral (damaged) and contralateral ('control') hemisphere. For all animals with post-ischemic survival times of 3 and 7 days GABA-IR cells constituted a lower proportion of the total number of nerve cells in the damaged than in the 'control' cortical areas. This finding was consistent with the outcome of an earlier in vitro study. By contrast, in all animals with a survival time of 35 and 150 days, the proportion of GABA-IR neurons was higher on the damaged than on the 'control' side. This switch in the direction of the left/right differences, apparently depending on the length of the post-ischemic survival time, was statistically significant. No lateralization in the proportion of GABA-IR cells was detected in the cerebral cortex of the control rats. These observations, therefore, do not support the hypothesis that perinatal hypoxic ischemia ultimately leads to a preferential loss of GABAergic neurons in the cerebral cortex.

Utility of serial EEGs in neonates during extracorporeal membrane oxygenation

We found electroencephalographic (EEG) studies to be useful for monitoring cerebral function, for confirming seizure activity, and for limited prediction of short-term outcome in 145 neonates who required extra-corporeal membrane oxygenation (ECMO) of reversible respiratory failure. The EEG tracings were classified as normal or as mildly, moderately, or markedly abnormal; abnormal recordings were further classified as focal, diffuse, or predominantly lateralized. A significant decrease in frequency and degree of EEG abnormalities was observed in recordings obtained after ECMO compared to those obtained prior to (P = .001) or during ECMO (P = .001). There was no significant increase in marked EEG abnormalities when recordings obtained before and during ECMO were compared (P = 0.41). Of 11 infants with electrographic seizures during ECMO, 7 (64%) either died during their nursery courses or were developmentally handicapped at age 1 year which is a significantly greater adverse outcome than that observed in infants without EEG seizure activity (P less than .003). No consistently lateralized EEG abnormalities were observed during or after ECMO when compared to tracings obtained before cannulation of the right common carotid artery. There was no acute change in EEG rhythm or amplitude over the right cerebral hemisphere during right common carotid artery cannulation. Our observations support the value of serial EEG in the assessment of cerebral function in critically ill infants undergoing ECMO. They further suggest that, in this patient population, cannulation of the right common carotid artery is a safe procedure that does not result in lateralized abnormalities of cerebral electrical activity.

Differential effects of perinatal hypoxia and anoxia on long term seizure susceptibility in the rat

We have previously demonstrated that hypoxia is acutely epileptogenic in the immature rat but not in the adult. The window during which hypoxia induces seizures in the rat ranges from postnatal day (P) 5-17, with the most severe seizures occurring at P10-12. Perinatal hypoxia resulted in significantly more acute seizure activity than perinatal anoxia. The present study evaluates the long term effects of perinatal hypoxia versus anoxia. Animals were exposed to hypoxia (3%O2) or anoxia (0%O2) at P10 and challenged later in adulthood (P55-60) with administration of pentylenetetrazol (PTZ) (45 mg/kg subcutaneously). Compared to normal littermate controls, the animals which had been exposed to perinatal hypoxia had a significantly higher frequency of generalized convulsions (GC) and a significantly shorter latency to the first myoclonic jerk (MJ) after PTZ. In contrast, perinatal anoxia did not alter long term seizure susceptibility. These results are discussed in context of previous studies which have shown variable long term effects using different models of perinatal hypoxia and/or ischemia.