Impaired Organization of GABAergic Neurons Following Prenatal Hypoxia

Maternal hyperhomocysteinemia increases seizures susceptibility of neonatal rats

AIMS

Neonatal seizures are severe pathologies which may result in long-term neurological consequences. High plasma concentrations of homocysteine - hyperhomocysteinemia (hHCy) - are associated with epilepsy. In the present study, we evaluated susceptibility to seizure of neonatal rats with prenatal hHCy.

MAIN METHODS

Prenatal hHCy was induced by feeding females with a high-methionine diet. Experiments were performed on pups during the first three postnatal weeks. Flurothyl-induced epileptic behavior was assessed according to Racine's scale. Epileptiform activity in the hippocampus was recorded using electrophysiological methods. The balance of excitation/inhibition, functional GABAergic inhibition and GABA reversal potential in hippocampal neurons were analyzed.

KEY FINDINGS

Rats with hHCy developed more severe stages of behavioral patterns during flurothyl-induced epilepsy with shorter latency. Electrophysiological recordings demonstrated higher background neuronal activity in rats with hHCy. Seizure-like events triggered by flurothyl (in vivo) or 4-aminopyridine (in vitro) showed shorter latency, higher power and amplitude. An increased glutamate/GABA synaptic ratio was shown in the pyramidal neurons of rats with hHCy and more slices demonstrated excitation by isoguvacine, a selective GABA(A) receptor agonist, during the first and second postnatal weeks. The GABA driving force and the reversal potential of GABA(A) currents were more positive during the second postnatal week for hHCy rats.

SIGNIFICANCE

The higher susceptibility to seizures in rats with prenatal hHCy due to a shift in the balance of excitation/inhibition toward excitation may underlie the clinical evidence about the association of hHCy with an increased risk of epilepsy.

Therapeutic Hypothermia Attenuates Cortical Interneuron Loss after Cerebral Ischemia in Near-Term Fetal Sheep

Therapeutic hypothermia significantly improves outcomes after neonatal hypoxic-ischemic (HI) encephalopathy but is only partially protective. There is evidence that cortical inhibitory interneuron circuits are particularly vulnerable to HI and that loss of interneurons may be an important contributor to long-term neurological dysfunction in these infants. In the present study, we examined the hypothesis that the duration of hypothermia has differential effects on interneuron survival after HI. Near-term fetal sheep received sham ischemia or cerebral ischemia for 30 min, followed by cerebral hypothermia from 3 h after ischemia end and continued up to 48 h, 72 h, or 120 h recovery. Sheep were euthanized after 7 days for histology. Hypothermia up to 48 h recovery resulted in moderate neuroprotection of glutamate decarboxylase (GAD)⁺ and parvalbumin⁺ interneurons but did not improve survival of calbindin⁺ cells. Hypothermia up to 72 h recovery was associated with significantly increased survival of all three interneuron phenotypes compared with sham controls. By contrast, while hypothermia up to 120 h recovery did not further improve (or impair) GAD⁺ or parvalbumin⁺ neuronal survival compared with hypothermia up to 72 h, it was associated with decreased survival of calbindin⁺ interneurons. Finally, protection of parvalbumin⁺ and GAD⁺ interneurons, but not calbindin⁺ interneurons, with hypothermia was associated with improved recovery of electroencephalographic (EEG) power and frequency by day 7 after HI. The present study demonstrates differential effects of increasing the duration of hypothermia on interneuron survival after HI in near-term fetal sheep. These findings may contribute to the apparent preclinical and clinical lack of benefit of very prolonged hypothermia.

Maternal Hypoxia Increases the Excitability of Neurons in the Entorhinal Cortex and Dorsal Hippocampus of Rat Offspring

Prenatal hypoxia is a widespread condition that causes various disturbances in later life, including aberrant central nervous system development, abnormalities in EEG rhythms, and susceptibility to seizures. Hypoxia in rats on the 14th day of embryogenesis (E14) disrupts cortical neuroblast radial migration, mainly affecting the progenitors of cortical glutamatergic neurons but not GABAergic interneurons or hippocampal neurons. Thus, hypoxia at this time point might affect the development of the neocortex to a greater extent than the hippocampus. In the present study, we investigated the long-term effects of hypoxia on the properties of the pyramidal neurons in the hippocampus and entorhinal cortex (EC) in 3-week-old rats subjected to hypoxia on E14. We observed a reduction in the total number of NeuN-positive neurons in EC but not in the CA1 field of the hippocampus, indicating an increased cell loss in EC. However, the principal neuron electrophysiological characteristics were altered in the EC and hippocampus of animals exposed to hypoxia. The whole-cell patch-clamp recordings revealed a similar increase in input resistance in neurons from the hippocampus and EC. However, the resting membrane potential was increased in the EC neurons only. The recordings of field postsynaptic potentials (fPSPs) in the CA1 hippocampal area showed that both the threshold currents inducing fPSPs and population spikes were lower in hypoxic animals compared to age-matched controls. Using the dosed electroshock paradigm, we found that seizure thresholds were lower in the hypoxic group. Thus, the obtained results suggest that maternal hypoxia during the generation of the pyramidal cortical neurons leads to the increased excitability of neuronal circuitries in the brain of young rats. The increased excitability can be attributed to the changes in intrinsic neuronal properties.

Early Postnatal Exposure to Intermittent Hypercapnic Hypoxia (IHH), but Not Nicotine, Decreases Reelin in the Young Piglet Hippocampus

This study evaluated the expression of reelin, an extracellular protein involved in lamination and migration of neurons, in the hippocampus of young piglets, and quantified to examine the following: (i) baseline levels within layers of the hippocampus and dentate gyrus (DG); (ii) differences between ventral and dorsal hippocampi; and (iii) changes attributable to postnatal exposure to continuous nicotine for 12 days, or intermittent hypercapnic hypoxia (IHH), with further analysis according to duration of IHH (1 vs 4 days). Additionally, we analysed whether any exposure altered DG morphology and whether it is related to altered reelin expression. Reelin was visualised via immunohistochemistry, and the number of positive reelin cells/mm² was measured in the CA4/Hilus, layers of the DG, and the CA1. The dorsal DG had significantly more reelin within the subgranular zone compared to the ventral DG (p < 0.01). There was no difference in reelin between nicotine (n = 5) and controls (n = 5). IHH exposed piglets (n = 10) had significantly lowered reelin in the CA1 (p = 0.05), specifically the stratum pyramidale (p = 0.04) and the hippocampal fissure (p = 0.02), compared to their controls (n = 7); the duration of IHH had no effect. No exposure was associated with an alteration in DG morphology. This study shows that postnatal IHH exposure decreased reelin expression in the developing piglet hippocampal CA1, suggesting that IHH may result in altered neuronal migration.

Influence of Cerebral Vasodilation on Blood Reelin Levels in Growth Restricted Fetuses

Fetal growth restriction (FGR) is one of the most important obstetric pathologies. It is frequently caused by placental insufficiency. Previous studies have shown a relationship between FGR and impaired new-born neurodevelopment, although the molecular mechanisms involved in this association have not yet been completely clarified. Reelin is an extracellular matrix glycoprotein involved in development of neocortex, hippocampus, cerebellum and spinal cord. Reelin has been demonstrated to play a key role in regulating perinatal neurodevelopment and to contribute to the emergence and development of various psychiatric pathologies, and its levels are highly influenced by pathological conditions of hypoxia. The purpose of this article is to study whether reelin levels in new-borns vary as a function of severity of fetal growth restriction by gestational age and sex. We sub-grouped fetuses in: normal weight group (Group 1, n = 17), FGR group with normal umbilical artery Doppler and cerebral redistribution at middle cerebral artery Doppler (Group 2, n = 9), and FGR with abnormal umbilical artery Doppler (Group 3, n = 8). Our results show a significant association of elevated Reelin levels in FGR fetuses with cerebral blood redistribution compared to the normal weight group and the FGR with abnormal umbilical artery group. Future research should focus on further expanding the knowledge of the relationship of reelin and its regulated products with neurodevelopment impairment in new-borns with FGR and should include larger and more homogeneous samples and the combined use of different in vivo techniques in neonates with impaired growth during their different adaptive phases.

How do established developmental risk-factors for schizophrenia change the way the brain develops?

The recognition that schizophrenia is a disorder of neurodevelopment is widely accepted. The original hypothesis was coined more than 30 years ago and the wealth of supportive epidemiologically data continues to grow. A number of proposals have been put forward to suggest how adverse early exposures in utero alter the way the adult brain functions, eventually producing the symptoms of schizophrenia. This of course is extremely difficult to study in developing human brains, so the bulk of what we know comes from animal models of such exposures. In this review, I will summarise the more salient features of how the major epidemiologically validated exposures change the way the brain is formed leading to abnormal function in ways that are informative for schizophrenia symptomology. Surprisingly few studies have examined brain ontogeny from embryo to adult in such models. However, where there is longitudinal data, various convergent mechanisms are beginning to emerge involving stress and immune pathways. There is also a surprisingly consistent alteration in how very early dopamine neurons develop in these models. Understanding how disparate epidemiologically-validated exposures may produce similar developmental brain abnormalities may unlock convergent early disease-related pathways/processes.

Chronic Oxytocin treatment has long lasting therapeutic potential in a rat model of neonatal hypercapnic-hypoxia injury, through enhanced GABAergic signaling and by reducing hippocampal gliosis with its anti-inflammatory feature

Previous studies have shown that, oxytocin has anticonvulsant and neuroprotective effects. One of the most important complications of Hypercapnic-hypoxia is drug resistance epilepsy. Effects of chronic intraperitoneal oxytocin treatment on gliosis, neuroinflammation and seizure activity was investigated in a model in which rats were exposed to hypoxia on postnatal day 1 and later challenged to the seizure-inducing pentylenetetrazol Forty pups were included in the study on their first day of birth. 16 pups were exposed to 100% CO₂ for 5 minutes and other 16 pups for 10 minutes. The remaining 8 pups comprised the control group. Groups were classified according to oxytocin administration within the first 4 weeks. Pentylenetetrazol was administered 6 months after the oxytocin treatment. The Racine's Convulsion Scale and onset times of first myoclonic jerk (FMJ) were evaluated. To determine the mechanisms by which oxytocin exerted its effects on hypercapnic-anoxia exposed rats, we performed CA1 total neuron count & CA1 GFAP immunostaining, and measured brain levels of TNF-α and GAD-67. The Racine scale and TNF-α values were significantly lower in both groups that received oxytocin, while time-to-FMJ and GAD-67 level were significantly higher. The histopathological evaluations showed that oxytocin had significant ameliorative effects (especially regarding gliosis) on the hippocampus of hypoxic rats. Regarding the results of present study, it can be speculated that after acute hypercapnic-anoxia exposure, chronic Oxytocin treatment has long lasting therapeutic potential on rats, possibly by reducing the gliosis with its anti-inflammatory feature and by activating the GABA pathway.

Neuroprotective Effects of Growth Hormone (GH) and Insulin-Like Growth Factor Type 1 (IGF-1) after Hypoxic-Ischemic Injury in Chicken Cerebellar Cell Cultures

It has been reported that growth hormone (GH) and insulin-like growth factor 1 (IGF-1) exert protective and regenerative actions in response to neural damage. It is also known that these peptides are expressed locally in nervous tissues. When the central nervous system (CNS) is exposed to hypoxia-ischemia (HI), both GH and IGF-1 are upregulated in several brain areas. In this study, we explored the neuroprotective effects of GH and IGF-1 administration as well as the involvement of these endogenously expressed hormones in embryonic chicken cerebellar cell cultures exposed to an acute HI injury. To induce neural damage, primary cultures were first incubated under hypoxic-ischemic (<5% O₂, 1g/L glucose) conditions for 12 h (HI), and then incubated under normal oxygenation and glucose conditions (HI + Ox) for another 24 h. GH and IGF-1 were added either during or after HI, and their effect upon cell viability, apoptosis, or necrosis was evaluated. In comparison with normal controls (Nx, 100%), a significant decrease of cell viability (54.1 ± 2.1%) and substantial increases in caspase-3 activity (178.6 ± 8.7%) and LDH release (538.7 ± 87.8%) were observed in the HI + Ox group. On the other hand, both GH and IGF-1 treatments after injury (HI + Ox) significantly increased cell viability (77.2 ± 4.3% and 72.3 ± 3.9%, respectively) and decreased both caspase-3 activity (118.2 ± 3.8% and 127.5 ± 6.6%, respectively) and LDH release (180.3 ± 21.8% and 261.6 ± 33.9%, respectively). Incubation under HI + Ox conditions provoked an important increase in the local expression of GH (3.2-fold) and IGF-1 (2.5-fold) mRNAs. However, GH gene silencing with a specific small-interfering RNAs (siRNAs) decreased both GH and IGF-1 mRNA expression (1.7-fold and 0.9-fold, respectively) in the HI + Ox group, indicating that GH regulates IGF-1 expression under these incubation conditions. In addition, GH knockdown significantly reduced cell viability (35.9 ± 2.1%) and substantially increased necrosis, as determined by LDH release (1011 ± 276.6%). In contrast, treatments with GH and IGF-1 stimulated a partial recovery of cell viability (45.2 ± 3.7% and 53.7 ± 3.2%) and significantly diminished the release of LDH (320.1 ± 25.4% and 421.7 ± 62.2%), respectively. Our results show that GH, either exogenously administered and/or locally expressed, can act as a neuroprotective factor in response to hypoxic-ischemic injury, and that this effect may be mediated, at least partially, through IGF-1 expression.

Connexin Hemichannel Mimetic Peptide Attenuates Cortical Interneuron Loss and Perineuronal Net Disruption Following Cerebral Ischemia in Near-Term Fetal Sheep

Perinatal hypoxia-ischemia is associated with disruption of cortical gamma-aminobutyric acid (GABA)ergic interneurons and their surrounding perineuronal nets, which may contribute to persisting neurological deficits. Blockade of connexin43 hemichannels using a mimetic peptide can alleviate seizures and injury after hypoxia-ischemia. In this study, we tested the hypothesis that connexin43 hemichannel blockade improves the integrity of cortical interneurons and perineuronal nets. Term-equivalent fetal sheep received 30 min of bilateral carotid artery occlusion, recovery for 90 min, followed by a 25-h intracerebroventricular infusion of vehicle or a mimetic peptide that blocks connexin hemichannels or by a sham ischemia + vehicle infusion. Brain tissues were stained for interneuronal markers or perineuronal nets. Cerebral ischemia was associated with loss of cortical interneurons and perineuronal nets. The mimetic peptide infusion reduced loss of glutamic acid decarboxylase-, calretinin-, and parvalbumin-expressing interneurons and perineuronal nets. The interneuron and perineuronal net densities were negatively correlated with total seizure burden after ischemia. These data suggest that the opening of connexin43 hemichannels after perinatal hypoxia-ischemia causes loss of cortical interneurons and perineuronal nets and that this exacerbates seizures. Connexin43 hemichannel blockade may be an effective strategy to attenuate seizures and may improve long-term neurological outcomes after perinatal hypoxia-ischemia.

Animal models for neonatal brain injury induced by hypoxic ischemic conditions in rodents

Neonatal hypoxia-ischemia and resulting encephalopathies are of significant concern. Intrapartum asphyxia is a leading cause of neonatal death globally. Among surviving infants, there remains a high incidence of hypoxic-ischemic encephalopathy due to neonatal hypoxic-ischemic brain injury, manifesting as mild conditions including attention deficit hyperactivity disorder, and debilitating disorders such as cerebral palsy. Various animal models of neonatal hypoxic brain injury have been implemented to explore cellular and molecular mechanisms, assess the potential of novel therapeutic strategies, and characterize the functional and behavioural correlates of injury. Each of the animal models has individual advantages and limitations. The present review looks at several widely-used and alternative rodent models of neonatal hypoxia and hypoxia-ischemia; it highlights their strengths and limitations, and their potential for continued and improved use.

Brain oxidative damage in murine models of neonatal hypoxia/ischemia and reoxygenation

The brain is one of the main organs affected by hypoxia and reoxygenation in the neonatal period and one of the most vulnerable to oxidative stress. Hypoxia/ischemia and reoxygenation leads to impairment of neurogenesis, disruption of cortical migration, mitochondrial damage and neuroinflammation. The extent of the injury depends on the clinical manifestation in the affected regions. Preterm newborns are highly vulnerable, and they exhibit severe clinical manifestations such as intraventricular hemorrhage (IVH), retinopathy of prematurity (ROP) and diffuse white matter injury (DWMI) among others. In the neonatal period, the accumulation of high levels of reactive oxygen species exacerbated by the immature antioxidant defense systems in represents cellular threats that, if they exceed or bypass physiological counteracting mechanisms, are responsible of significant neuronal damage. Several experimental models in mice mimic the consequences of perinatal asphyxia and the use of oxygen in the reanimation process that produce brain injury. The aim of this review is to highlight brain damage associated with oxidative stress in different murine models of hypoxia/ischemia and reoxygenation.