Developmental expression of N-methyl-D-aspartate (NMDA) receptor subunits in human white and gray matter: potential mechanism of increased vulnerability in the immature brain

Maturation of glutamatergic transmission onto dorsal raphe serotonergic neurons

Serotonergic neurons in the dorsal raphe nucleus (DRN) play important roles early in postnatal development in the maturation and modulation of higher-order emotional, sensory, and cognitive circuitry. The pivotal functions of these cells in brain development make them a critical substrate by which early experience can be wired into the brain. In this study, we investigated the maturation of synapses onto dorsal raphe serotonergic neurons in typically developing male and female mice using whole cell patch-clamp recordings in ex vivo brain slices. We show that while inhibition of these neurons is relatively stable across development, glutamatergic synapses greatly increase in strength between postnatal day 6 (P6) and P21-23. In contrast to forebrain regions, where the components making up glutamatergic synapses are dynamic across early life, we find that DRN excitatory synapses maintain a very high ratio of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) to N-methyl-d-aspartate (NMDA) receptors and a rectifying component of the AMPA response until adulthood. Overall, these findings reveal that the development of serotonergic neurons is marked by a significant refinement of glutamatergic synapses during the first three postnatal weeks. This suggests this time is a sensitive period of heightened plasticity for the integration of information from upstream brain areas. Genetic and environmental insults during this period could lead to alterations in serotonergic output, impacting both the development of forebrain circuits and lifelong neuromodulatory actions.NEW & NOTEWORTHY Serotonergic neurons are regulators of both the development of and ongoing activity in neuronal circuits controlling affective, cognitive, and sensory processing. Here, we characterize the maturation of extrinsic synaptic inputs onto these cells, showing that the first three postnatal weeks are a period of synaptic refinement and a potential window for experience-dependent plasticity in response to both enrichment and adversity.

Therapeutic Hypothermia for Hypoxic-Ischemic Brain Injury Is More Effective in Newborn Infants than in Older Patients: Review and Hypotheses

Posthypoxic therapeutic hypothermia has been tested in newborn infants, with seven randomized trials showing consistent evidence of reduction in death, cerebral palsy, and cognitive impairment at school age. In contrast, randomized trials of hypothermia after cardiac arrest in adults have not shown consistent evidence of lasting neurological protection. The apparently greater effectiveness of therapeutic hypothermia in newborns may be due to important biological and clinical differences. One such difference is that adults are heavily colonized with microbes, and many have active inflammatory processes at the time of arrest, but few newborns are heavily colonized or infected at the time of birth. Inflammation can interfere with hypothermia's neuroprotection. A second difference is that apoptosis is more commonly the pathway of neuronal death in newborns than in adults. Hypothermia inhibits apoptosis but not necrosis. Newborns have a larger endogenous supply of stem cells (which reduce apoptosis) than adults and this may favor regeneration and protection from hypothermia and regeneration. A third difference is that immature oligodendroglia are more sensitive to free radical attack then mature oligodendroglia. Hypothermia reduces free radical release. In addition, immature brain has increased N-methyl-D-aspartate receptor subunits compared with adults and hypothermia reduces excitotoxic amino acids. Adults suffering cardiac arrest often have comorbidities such as diabetes, hypertension, and atherosclerosis, which complicate recovery, but newborn infants rarely have comorbidities before asphyxia. Adult hypothermia treatment may have been too short as no trial has cooled for longer than 48 hours, some only 24 or 12 hours, but neonatal therapeutic hypothermia has routinely lasted 72 hours. We hypothesize that this combination of differences favors the effectiveness of therapeutic hypothermia in newborn infants compared with adults.

Differential responses of disease-related GRIN variants located in pore-forming M2 domain of N-methyl-D-aspartate receptor to FDA-approved inhibitors

N-methyl-D-aspartate receptors (NMDAR), ionotropic glutamate receptors, mediate a slow component of excitatory synaptic transmission in the central nervous system and play a key role in normal brain function and development. Genetic variations in GRIN genes encoding NMDAR subunits that alter the receptor's functional characteristics are associated with a wide range of neurological and neuropsychiatric conditions. Pathological GRIN variants located in the M2 re-entrant loop lining the channel pore cause significant functional changes, the most consequential alteration being a reduction in voltage-dependent Mg2+ inhibition. Voltage-dependent Mg2+ block is a unique feature of NMDAR biology whereby channel activation requires both ligand binding and postsynaptic membrane depolarization. Thus, loss of NMDAR Mg2+ block will have a profound impact on synaptic function and plasticity. Here, we choose 11 missense variants within the GRIN1, GRIN2A, and GRIN2B genes that alter residues located in the M2 loop and significantly reduce Mg2+ inhibition. Each variant was evaluated for tolerance to genetic variation using the 3-dimensional structure and assessed for functional rescue pharmacology via electrophysiological recordings. Three FDA-approved NMDAR drugs-memantine, dextromethorphan, and ketamine-were chosen based on their ability to bind near the M2 re-entrant loop, potentially rectifying dysregulated NMDAR function by supplementing the reduced voltage-dependent Mg2+ block. These results provide insight of structural determinants of FDA-approved NMDAR drugs at their binding sites in the channel pore and may further define conditions necessary for the use of such agents as potential rescue pharmacology.

Maturation of glutamatergic transmission onto dorsal raphe serotonergic neurons

Serotonergic neurons in the dorsal raphe nucleus (DRN) play important roles early in postnatal development in the maturation and modulation of higher order emotional, sensory, and cognitive circuitry. This unique position makes these cells a substrate by which early experience can be wired into brain. In this study, we have investigated the maturation of synapses onto dorsal raphe serotonergic neurons in typically developing male and female mice using whole-cell patch-clamp recordings in ex vivo brain slices. We show that while inhibition of these neurons is relatively stable across development, glutamatergic synapses greatly increase in strength between P6 and P21-23. In contrast to forebrain regions, where the components making up glutamatergic synapses are dynamic across early life, we find that the makeup of these synapses onto DRN serotonergic neurons is largely stable after P15. DRN excitatory synapses maintain a very high ratio of AMPA to NMDA receptors and a rectifying component of the AMPA response throughout the lifespan. Overall, these findings reveal that the development of serotonergic neurons is marked by a significant refinement of glutamatergic synapses during the first 3 postnatal weeks. This suggests this time as a sensitive period of heightened plasticity for integration of information from upstream brain areas and that genetic and environmental insults during this period could lead to alterations in serotonergic output, impacting both the development of forebrain circuits and lifelong neuromodulatory actions.

Brain Damage in the Preterm Infant: Clinical Aspects and Recent Progress in the Prevention and Treatment

Although the prevalence of brain injury and related neurodevelopmental disabilities resulting from preterm birth are major public health concerns, there are no definite neuroprotective strategies to prevent or reduce brain injury. The pattern of brain injury seen in preterm infants has evolved into more subtle lesions that are still essential to diagnose regarding neurodevelopmental outcomes. There is no specific effective method for the treatment of premature infant brain injury, and the focus of clinical treatment is still on prevention. Prevention of this injury requires insight into the pathogenesis, but many gaps exist in our understanding of how neonatal treatment procedures and medications impact cerebral hemodynamics and preterm brain injury. Many studies provide evidence about the prevention of premature infant brain injury, which is related to some drugs (such as erythropoietin, melatonin, mesenchymal stem cells, etc.). However, there are still some controversies about the quality of research and the effectiveness of therapy. This review aims to recapitulate the results of preclinical studies and provide an update on the latest developments around etiological pathways, prevention, and treatment.

Prenatal and postnatal insults differentially contribute to executive function and cognition: Utilizing touchscreen technology for perinatal brain injury research

The use of touchscreen technology to evaluate cognitive deficits in animal models has grown tremendously over the past 20 years. The touchscreen apparatus encompasses many advantages, namely a high level of standardization and translational capability. Improvements in technology in recent years have expanded the versatility of the touchscreen platform, as it is able to test distinct cognitive modalities including working memory, attention, discrimination, and association. Importantly, touchscreen technology has allowed researchers to explore deficits in multiple pillars of cognition in a wide variety of perinatal disorders with neurological sequelae across critical developmental windows. The touchscreen platform has been used to dissect deficits in antenatal CNS injury including fetal alcohol syndrome, prenatal opioid exposure, and chorioamnionitis, to peripartum insults such as term hypoxic-ischemic encephalopathy, to early postnatal insults including infantile traumatic brain injury. Most importantly, touchscreen technology offers the sensitivity necessary to detect subtle injury and treatment-induced changes in cognition and executive function beyond those offered by more rudimentary tests of rodent cognition. Understanding the pathophysiology of these disorders in rodents is paramount to addressing these deficits in human infants and dissecting the neural circuitry essential to perinatal brain injury pathophysiology and responsiveness to novel therapeutics. Touchscreen testing provides an effective, facile, sophisticated technique to accelerate the goal of improving cognitive and behavioral outcomes of children who suffer perinatal brain injury.

Protein quality control of N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated cation channels that mediate excitatory neurotransmission and are critical for synaptic development and plasticity in the mammalian central nervous system (CNS). Functional NMDARs typically form via the heterotetrameric assembly of GluN1 and GluN2 subunits. Variants within GRIN genes are implicated in various neurodevelopmental and neuropsychiatric disorders. Due to the significance of NMDAR subunit composition for regional and developmental signaling at synapses, properly folded receptors must reach the plasma membrane for their function. This review focuses on the protein quality control of NMDARs. Specifically, we review the quality control mechanisms that ensure receptors are correctly folded and assembled within the endoplasmic reticulum (ER) and trafficked to the plasma membrane. Further, we discuss disease-associated variants that have shown disrupted NMDAR surface expression and function. Finally, we discuss potential targeted pharmacological and therapeutic approaches to ameliorate disease phenotypes by enhancing the expression and surface trafficking of subunits harboring disease-associated variants, thereby increasing their incorporation into functional receptors.

Recent Investigations on Neurotransmitters' Role in Acute White Matter Injury of Perinatal Glia and Pharmacotherapies-Glia Dynamics in Stem Cell Therapy

Periventricular leukomalacia (PVL) and cerebral palsy are two neurological disease conditions developed from the premyelinated white matter ischemic injury (WMI). The significant pathophysiology of these diseases is accompanied by the cognitive deficits due to the loss of function of glial cells and axons. White matter makes up 50% of the brain volume consisting of myelinated and non-myelinated axons, glia, blood vessels, optic nerves, and corpus callosum. Studies over the years have delineated the susceptibility of white matter towards ischemic injury especially during pregnancy (prenatal, perinatal) or immediately after child birth (postnatal). Impairment in membrane depolarization of neurons and glial cells by ischemia-invoked excitotoxicity is mediated through the overactivation of NMDA receptors or non-NMDA receptors by excessive glutamate influx, calcium, or ROS overload and has been some of the well-studied molecular mechanisms conducive to the injury of white matter. Expression of glutamate receptors (GluR) and transporters (GLT1, EACC1, and GST) has significant influence in glial and axonal-mediated injury of premyelinated white matter during PVL and cerebral palsy. Predominantly, the central premyelinated axons express extensive levels of functional NMDA GluR receptors to confer ischemic injury to premyelinated white matter which in turn invoke defects in neural plasticity. Several underlying molecular mechanisms are yet to be unraveled to delineate the complete pathophysiology of these prenatal neurological diseases for developing the novel therapeutic modalities to mitigate pathophysiology and premature mortality of newborn babies. In this review, we have substantially discussed the above multiple pathophysiological aspects of white matter injury along with glial dynamics, and the pharmacotherapies including recent insights into the application of MSCs as therapeutic modality in treating white matter injury.

The roles of GluN3-containing N-methyl-D-aspartate receptor in central nerve system

The N-methyl-D-aspartate receptor (NMDAR) in central nerve system is mostly composed of GluN1 and GluN2 subunits. The classical NMDAR has been intensively studied. However, GluN3‑containing NMDAR is much less expressed and have atypical channel properties. Recently, accumulating evidences have revealed two types of GluN3‑containing NMDAR: glutamate-gated GluN1/GluN2/GluN3 NMDAR and glycine-gated GluN1/GluN3 NMDAR. The former may play important roles in regulating synapse maturation and pruning non-used synapses, and its elevated expression at the adult stage may alter synaptic reorganization in some neuropsychiatric disorders. The latter is expressed in the medial habenula and involves in control of aversion. This article reviews the recent progresses on the expression, functional properties of GluN3‑containing atypical NMDARs and the physiological and pathological relevance.

A new insight into mechanisms of interferon alpha neurotoxicity: Expression of GRIN3A subunit of NMDA receptors and NMDA-evoked exocytosis

Neurological and psychiatric side effects accompany the high-dose interferon-alpha (IFNA) therapy. The primary genes responsible for these complications are mostly unknown. Our genome-wide search in mouse and rat genomes for the conservative genes containing IFN-stimulated response elements (ISRE) in their promoters revealed a new potential target gene of IFNA, Grin3α, which encodes the 3A subunit of NMDA receptor. This study aimed to explore the impact of IFNA on the expression of Grin3α and Ifnα genes and neurotransmitters endo/exocytosis in the mouse brain. We administered recombinant human IFN-alpha 2b (rhIFN-α2b) intracranially, and 24 h later, we isolated six brain regions and used the samples for RT-qPCR and western blot analysis. Synaptosomes were isolated from the cortex to analyze endo/exocytosis with acridine orange and L-[14C]glutamate. IFNA induced an increase in Grin3α mRNA and GRIN3A protein, but a decrease in Ifnα mRNA and protein. IFNA did not affect the accumulation and distribution of L-[14C]glutamate and acridine orange between synaptosomes and the extra-synaptosomal space. It caused the more significant acridine orange release activated by NMDA or glutamate than from control mice's synaptosomes. In response to IFNA, the newly discovered association between elevated Grin3α expression and NMDA- and glutamate-evoked neurotransmitters release from synaptosomes implies a new molecular mechanism of IFNA neurotoxicity.

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

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

GRIN2A and GRIN2B and Their Related Phenotypes

Glutamate is the most relevant excitatory neurotransmitter of the central nervous system; it binds with several receptors, including N-methyl-D-aspartate receptors (NMDARs), a subtype of ionotropic glutamate receptor that displays voltage-dependent block by Mg2+ and a high permeability to Ca2+. GRIN2A and GRIN2B genes encode the GluN2A and GluN2B subunits of the NMDARs, which play important roles in synaptogenesis, synaptic transmission, and synaptic plasticity, as well as contributing to neuronal loss and dysfunction in several neurological disorders. Recently, individuals with a range of childhood-onset drug-resistant epilepsies, such as Landau–Kleffner or Lennox–Gastaut syndrome, intellectual disability (ID), and other neurodevelopmental abnormalities have been found to carry mutations in GRIN2A and GRIN2B, with high variable expressivity in phenotype. The first one is found mainly in epilepsy-aphasia syndromes, while the second one mainly in autism, schizophrenia, and ID, such as autism spectrum disorders. Brain magnetic resonance imaging alterations are found in some patients, even if without a clear clinical correlation. At the same time, increasing data on genotype–phenotype correlation have been found, but this is still not fully demonstrated. There are no specific therapies for the treatment of correlated NMDARs epilepsy, although some evidence with memantine, an antagonist of glutamate receptor, is reported in the literature in selected cases with mutation determining a gain of function.

GluN3A NMDA receptor subunits: more enigmatic than ever?

Non-conventional N-methyl-d-aspartate receptors (NMDARs) containing GluN3A subunits have unique biophysical, signalling and localization properties within the NMDAR family, and are typically thought to counterbalance functions of classical NMDARs made up of GluN1/2 subunits. Beyond their recognized roles in synapse refinement during postnatal development, recent evidence is building a wider perspective for GluN3A functions. Here we draw particular attention to the latest developments for this multifaceted and unusual subunit: from finely timed expression patterns that correlate with plasticity windows in developing brains or functional hierarchies in the mature brain to new insight onto presynaptic GluN3A-NMDARs, excitatory glycine receptors and behavioural impacts, alongside further connections to a range of brain disorders.

Effect of Neuroprotective Magnesium Sulfate Treatment on Brain Transcription Response to Hypoxia Ischemia in Neonate Mice

MgSO₄ is widely used in the prevention of preterm neurological disabilities but its modes of action remain poorly established. We used a co-hybridization approach using the transcriptome in 5-day old mice treated with a single dose of MgSO₄ (600 mg/kg), and/or exposed to hypoxia-ischemia (HI). The transcription of hundreds of genes was altered in all the groups. MgSO₄ mainly produced repressions culminating 6 h after injection. Bio-statistical analysis revealed the repression of synaptogenesis and axonal development. The putative targets of MgSO₄ were Mnk1 and Frm1. A behavioral study of adults did not detect lasting effects of neonatal MgSO₄ and precluded NMDA-receptor-mediated side effects. The effects of MgSO₄ plus HI exceeded the sum of the effects of separate treatments. MgSO₄ prior to HI reduced inflammation and the innate immune response probably as a result of cytokine inhibition (Ccl2, Ifng, interleukins). Conversely, MgSO₄ had little effect on HI-induced transcription by RNA-polymerase II. De novo MgSO₄-HI affected mitochondrial function through the repression of genes of oxidative phosphorylation and many NAD-dehydrogenases. It also likely reduced protein translation by the repression of many ribosomal proteins, essentially located in synapses. All these effects appeared under the putative regulatory MgSO₄ induction of the mTORC2 Rictor coding gene. Lasting effects through Sirt1 and Frm1 could account for this epigenetic footprint.

Sulforaphane (SFA) protects neuronal cells from oxygen & glucose deprivation (OGD)

BACKGROUND

Perinatal brain injury results in neurodevelopmental disabilities (neuroDDs) that include cerebral palsy, autism, attention deficit disorder, epilepsy, learning disabilities and others. Commonly, injury occurs when placental circulation, that is responsible for transporting nutrients and oxygen to the fetus, is compromised. Placental insufficiency (PI) is a reduced supply of blood and oxygen to the fetus and results in a hypoxic-ischemic (HI) environment. A significant HI state in-utero leads to perinatal compromise, characterized by fetal growth restriction and brain injury. Given that over 80% of perinatal brain injuries that result in neuroDDs occur during gestation, prior to birth, preventive approaches are needed to reduce or eliminate the potential for injury and subsequent neuroDDs. Sulforaphane (SFA) derived from cruciferous vegetables such as broccoli sprouts (BrSps) is a phase-II enzyme inducer that acts via cytoplasmic Nrf2 to enhance the production of anti-oxidants in the brain through the glutathione pathway. We have previously shown a profound in vivo neuro-protective effect of BrSps/SFA as a dietary supplement in pregnant rat models of both PI and fetal inflammation. Strong evidence also points to a role for SFA as treatment for various cancers. Paradoxically, then SFA has the ability to enhance cell survival, and with conditions of cancer, enhance cell death. Given our findings of the benefit of SFA/Broccoli Sprouts as a dietary supplement during pregnancy, with improvement to the fetus, it is important to determine the beneficial and toxic dosing range of SFA. We therefore explored, in vitro, the dosing range of SFA for neuronal and glial protection and toxicity in normal and oxygen/glucose deprived (OGD) cell cultures.

METHODS

OGD simulates, in vitro, the condition experienced by the fetal brain due to PI. We developed a cell culture model of primary cortical neuronal, astrocyte and combined brain cell co-cultures from newborn rodent brains. The cultures were exposed to an OGD environment for various durations of time to determine the LD50 (duration of OGD required for 50% cell death). Using the LD50 as the time point, we evaluated the efficacy of varying doses of SFA for neuroprotective and neurotoxicity effects. Control cultures were exposed to normal media without OGD, and cytotoxicity of varying doses of SFA was also evaluated. Immunofluorescence (IF) and Western blot analysis of cell specific markers were used for culture characterization, and quantification of LD50. Efficacy and toxicity effect of SFA was assessed by IF/high content microscopy and by AlamarBlue viability assay, respectively.

RESULTS

We determined the LD50 to be 2 hours for neurons, 8 hours for astrocytes, and 10 hours for co-cultures. The protective effect of SFA was noticeable at 2.5 μM and 5 μM for neurons, although it was not significant. There was a significant protective effect of SFA at 2.5 μM (p<0.05) for astrocytes and co-cultures. Significant toxicity ranges were also confirmed in OGD cultures as ≥ 100 μM (p<0.05) for astrocytes, ≥ 50 μM (p<0.01) for co-cultures, but not toxic in neurons; and toxic in control cultures as ≥ 100 μM (p<0.01) for neurons, and ≥ 50 μM (p<0.01) for astrocytes and co-cultures. One Way ANOVA and Dunnett's Multiple Comparison Test were used for statistical analysis.

CONCLUSIONS

Our results indicate that cell death shows a trend to reduction in neuronal and astrocyte cultures, and is significantly reduced in co-cultures treated with low doses of SFA exposed to OGD. Doses of SFA that were 10 times higher were toxic, not only under conditions of OGD, but in normal control cultures as well. The findings suggest that: 1. SFA shows promise as a preventative agent for fetal ischemic brain injury, and 2. Because the fetus is a rapidly growing organism with profound cell multiplication, dosing parameters must be established to insure safety within efficacious ranges. This study will influence the development of innovative therapies for the prevention of childhood neuroDD.

Studying pre-treatment and ketamine-induced changes in white matter microstructure in the context of ketamine's antidepressant effects

Ketamine is increasingly being used as a therapeutic for treatment-resistant depression (TRD), yet the effects of ketamine on the human brain remain largely unknown. This pilot study employed diffusion magnetic resonance imaging (dMRI) to examine relationships between ketamine treatment and white matter (WM) microstructure, with the aim of increasing the current understanding of ketamine's neural mechanisms of action in humans. Longitudinal dMRI data were acquired from 13 individuals with TRD two hours prior to (pre-infusion), and four hours following (post-infusion), an intravenous ketamine infusion. Free-water imaging was employed to quantify cerebrospinal fluid-corrected mean fractional anisotropy (FA) in 15 WM bundles pre- and post-infusion. Analyses revealed that higher pre-infusion FA in the left cingulum bundle and the left superior longitudinal fasciculus was associated with greater depression symptom improvement 24 h post-ketamine. Moreover, four hours after intravenous administration of ketamine, FA rapidly increased in numerous WM bundles in the brain; this increase was significantly associated with 24 h symptom improvement in select bundles. Overall, the results of this preliminary study suggest that WM properties, as measured by dMRI, may have a potential impact on clinical improvement following ketamine. Ketamine administration additionally appears to be associated with rapid WM diffusivity changes, suggestive of rapid changes in WM microstructure. This study thus points to pre-treatment WM structure as a potential factor associated with ketamine's clinical efficacy, and to post-treatment microstructural changes as a candidate neuroimaging marker of ketamine's cellular mechanisms.

The Function of the NMDA Receptor in Hypoxic-Ischemic Encephalopathy

Hypoxic-ischemic encephalopathy (HIE) is one of the main forms of neonatal brain injury which could lead to neonatal disability or even cause neonatal death. Therefore, HIE strongly affects the health of newborns and brings heavy burden to the family and society. It has been well studied that N-methyl-D-aspartate (NMDA) receptors are involved in the excitotoxicity induced by hypoxia ischemia in adult brain. Recently, it has been shown that the NMDA receptor also plays important roles in HIE. In the present review, we made a summary of the molecular mechanism of NMDA receptor in the pathological process of HIE, focusing on the distinct role of GluN2A- and GluN2B-containing NMDA receptor subtypes and aiming to provide some insights into the clinical treatment and drug development of HIE.

The vulnerability of the immature brain

The concept of vulnerability of the immature brain is multifactorial by definition. Newer scientific work in this area has shifted and enlarged the concept from theoretical frameworks to the multiple levels (molecular, cellular, anatomic, network, behavioral) of the organization of the growing brain. The concept of vulnerability was first introduced by Donald O. Hebb in the 1950s and referred to the inability of the immature brain to completely recover normal development after a brain insult. The concept of vulnerability was further extended to the limitations of the brain in the development of specific skills in neuronal substrates originally used for other functions. We present an overview of some neurodevelopmental processes that characterize the immature brain and that can predict vulnerability in the case of disturbances: Hebb's principle, synaptic homeostasis, selective vulnerability of immature cells in mammals, and inherited constraint networks. A better understanding of the vulnerability mechanisms may help in early detection and prevention and further proposed individualized therapeutic approaches to enhance children's developmental outcomes.

Evaluation of Altered Glutamatergic Activity in a Piglet Model of Hypoxic-Ischemic Brain Damage Using 1H-MRS

Methods

Twenty-five newborn piglets were selected and then randomly assigned to the control group (n = 5) and the model group (n = 20) subjected to HI. HI was induced by blocking bilateral carotid blood flow under simultaneous inhalation of a 6% oxygen mixture. ¹H-MRS data were acquired from the basal ganglia at the following time points after HI: 6, 12, 24, and 72 h. Changes in protein levels of EAAT2 and GluR2 were determined by immunohistochemical analysis. Correlations among metabolite concentrations, metabolite ratios, and the protein levels of EAAT2 and GluR2 were investigated.

Results

The Glu level sharply increased after HI, reached a transient low level of depletion that approached the normal level in the control group, and subsequently increased again. Negative correlations were found between concentrations of Glu and EAAT2 protein levels (R _s = -0.662, P < 0.001) and between the Glu/creatine (Cr) ratio and EAAT2 protein level (R _s = -0.664, P < 0.001). Moreover, changes in GluR2 protein level were significantly and negatively correlated with those in Glu level (the absolute Glu concentration, R _s = -0.797, P < 0.001; Glu/Cr, R _s = -0.567, P = 0.003).

Conclusions

Changes in Glu level measured by ¹H-MRS were inversely correlated with those in EAAT2 and GluR2 protein levels following HI, and the results demonstrated that ¹H-MRS can reflect the early changes of glutamatergic activity in vivo.

Use of a physiologically based pharmacokinetic-pharmacodynamic model for initial dose prediction and escalation during a paediatric clinical trial

AIMS

To build and verify a physiologically based pharmacokinetic (PBPK) model for radiprodil in adults and link this to a pharmacodynamic (PD) receptor occupancy (RO) model derived from in vitro data. Adapt this model to the paediatric population and predict starting and escalating doses in infants based on RO. Use the model to guide individualized dosing in a clinical trial in 2- to 14-month-old children with infantile spasms.

METHODS

A PBPK model for radiprodil was developed to investigate the systemic exposure of the drug after oral administration in fasted and fed adults; this was then linked to RO via a PD model. The model was then expanded to include developmental physiology and ontogeny to predict escalating doses in infants that would result in a specific RO of 20, 40 and 60% based on average unbound concentration following a twice daily (b.i.d.) dosing regimen. Dose progression in the clinical trial was based on observed concentration-time data against PBPK predictions.

RESULTS

For paediatric predictions, the elimination of radiprodil, based on experimental evidence, had no ontogeny. Predicted b.i.d. doses ranged from 0.04 mg/kg for 20% RO, 0.1 mg/kg for 40% RO to 0.21 mg/kg for 60% RO. For all infants recruited in the study, observed concentration-time data following the 0.04 mg/kg and subsequent doses were within the PBPK model predicted 5^th and 95^th percentiles.

CONCLUSION

To our knowledge, this is the first time a PBPK model linked to RO has been used to guide dose selection and escalation in the live phase of a paediatric clinical trial.

Glutamate in schizophrenia: Neurodevelopmental perspectives and drug development

Research into the neurobiological processes that may lead to the onset of schizophrenia places growing emphasis on the glutamatergic system and brain development. Preclinical studies have shown that neurodevelopmental, genetic, and environmental factors contribute to glutamatergic dysfunction and schizophrenia-related phenotypes. Clinical research has suggested that altered brain glutamate levels may be present before the onset of psychosis and relate to outcome in those at clinical high risk. After psychosis onset, glutamate dysfunction may also relate to the degree of antipsychotic response and clinical outcome. These findings support ongoing efforts to develop pharmacological interventions that target the glutamate system and could suggest that glutamatergic compounds may be more effective in specific patient subgroups or illness stages. In this review, we consider the updated glutamate hypothesis of schizophrenia, from a neurodevelopmental perspective, by reviewing recent preclinical and clinical evidence, and discuss the potential implications for novel therapeutics.

Exposure to elevated embryonic kynurenine in rats: Sex-dependent learning and memory impairments in adult offspring

Distinct abnormalities in kynurenine pathway (KP) metabolism have been reported in various psychiatric disorders, including schizophrenia (SZ). Kynurenic acid (KYNA), a neuroactive metabolite of the KP, is elevated in individuals diagnosed with SZ and has been linked to cognitive impairments seen in the disorder. To further understand the role of KYNA in SZ etiology, we developed a prenatal insult model where kynurenine (100 mg/day) is fed to pregnant Wistar rats from embryonic day (ED) 15 to ED 22. As sex differences in the prevalence and severity of SZ have been observed, we presently investigated the impact of prenatal kynurenine exposure on KP metabolism and spatial learning and memory in male and female offspring. Specifically, brain tissue and plasma from offspring (control: ECon; kynurenine-treated: EKyn) in prepuberty (postnatal day (PD) 21), adolescence (PD 32-35), and adulthood (PD 56-85) were collected. Separate cohorts of adult offspring were tested in the Barnes maze to assess hippocampus- and prefrontal cortex-mediated learning and memory. Plasma tryptophan, kynurenine, and KYNA were unchanged between ECon and EKyn offspring across all three ages. Hippocampal and frontal cortex KYNA were elevated in male EKyn offspring only in adulthood, compared to ECon, while brain KYNA levels were unchanged in adult females. Male EKyn offspring were significantly impaired during acquisition of the Barnes maze and during reversal learning in the task. In female EKyn offspring, learning and memory remained relatively intact. Taken together, our data demonstrate that exposure to elevated kynurenine during the last week of gestation results in intriguing sex differences and further support the EKyn model as an attractive tool to study the pathophysiology of schizophrenia.

Hypoxic-ischemic-related cerebrovascular changes and potential therapeutic strategies in the neonatal brain

Perinatal hypoxic-ischemic (HI)-related brain injury is an important cause of morbidity and long-standing disability in newborns. The only currently approved therapeutic strategy available to reduce brain injury in the newborn is hypothermia. Therapeutic hypothermia can only be used to treat HI encephalopathy in full-term infants and survivors remain at high risk for a wide spectrum of neurodevelopmental abnormalities as a result of residual brain injury. Therefore, there is an urgent need for adjunctive therapeutic strategies. Inflammation and neurovascular damage are important factors that contribute to the pathophysiology of HI-related brain injury and represent exciting potential targets for therapeutic intervention. In this review, we address the role of each component of the neurovascular unit (NVU) in the pathophysiology of HI-related injury in the neonatal brain. Disruption of the blood-brain barrier (BBB) observed in the early hours after an HI-related event is associated with a response at the basal lamina level, which comprises astrocytes, pericytes, and immune cells, all of which could affect BBB function to further exacerbate parenchymal injury. Future research is required to determine potential drugs that could prevent or attenuate neurovascular damage and/or augment repair. However, some studies have reported beneficial effects of hypothermia, erythropoietin, stem cell therapy, anti-cytokine therapy and metformin in ameliorating several different facets of damage to the NVU after HI-related brain injury in the perinatal period.

Mean Diffusivity in Striatum Correlates With Acute Neuronal Death but Not Lesser Neuronal Injury in a Pilot Study of Neonatal Piglets With Encephalopathy

BACKGROUND

Diffusion MRI is routinely used to evaluate brain injury in neonatal encephalopathy. Although abnormal mean diffusivity (MD) is often attributed to cytotoxic edema, the specific contribution from neuronal pathology is unclear.

PURPOSE

To determine whether MD from high-resolution diffusion tensor imaging (DTI) can detect variable degrees of neuronal degeneration and pathology in piglets with brain injury induced by excitotoxicity or global hypoxia-ischemia (HI) with or without overt infarction.

STUDY TYPE

Prospective.

ANIMAL MODEL

Excitotoxic brain injury was induced in six neonatal piglets by intrastriatal stereotaxic injection of the glutamate receptor agonist quinolinic acid (QA). Three piglets underwent global HI or a sham procedure. Piglets recovered for 20-96 hours before undergoing MRI (n = 9).

FIELD STRENGTH/SEQUENCE

3.0T MRI with DTI, T₁ - and T₂ -weighted imaging.

ASSESSMENT

MD, fractional anisotropy (FA), and qualitative T₂ injury were assessed in the putamen and caudate. The cell bodies of normal neurons, degenerating neurons (excitotoxic necrosis, ischemic necrosis, or necrosis-apoptosis cell death continuum), and injured neurons with equivocal degeneration were counted by histopathology.

STATISTICAL TESTS

Spearman correlations were used to compare MD and FA to normal, degenerating, and injured neurons. T₂ injury and neuron counts were evaluated by descriptive analysis.

RESULTS

The QA insult generated titratable levels of neuronal pathology. In QA, HI, and sham piglets, lower MD correlated with higher ratios of degenerating-to-total neurons (P < 0.05), lower ratios of normal-to-total neurons (P < 0.05), and greater numbers of degenerating neurons (P < 0.05). MD did not correlate with abnormal neurons exhibiting nascent injury (P > 0.99). Neuron counts were not related to FA (P > 0.30) or to qualitative injury from T₂ -weighted MRI.

DATA CONCLUSION

MD is more accurate than FA for detecting neuronal degeneration and loss during acute recovery from neonatal excitotoxic and HI brain injury. MD does not reliably detect nonfulminant, nascent, and potentially reversible neuronal injury.

EVIDENCE LEVEL

1 TECHNICAL EFFICACY: Stage 2 J. Magn. Reson. Imaging 2020;52:1216-1226.

Age-Dependent Changes in Synaptic NMDA Receptor Composition in Adult Human Cortical Neurons

The molecular processes underlying the aging-related decline in cognitive performance and memory observed in humans are poorly understood. Studies in rodents have shown a decrease in N-methyl-D-aspartate receptors (NMDARs) that contain the GluN2B subunit in aging synapses, and this decrease is correlated with impaired memory functions. However, the age-dependent contribution of GluN2B-containing receptors to synaptic transmission in human cortical synapses has not been previously studied. We investigated the synaptic contribution of GluN2A and GluN2B-containing NMDARs in adult human neurons using fresh nonpathological temporal cortical tissue resected during neurosurgical procedures. The tissue we obtained fulfilled quality criteria by the absence of inflammation markers and proteomic degradation. We show an age-dependent decline in the NMDA/AMPA receptor ratio in adult human temporal cortical synapses. We demonstrate that GluN2B-containing NMDA receptors contribute to synaptic responses in the adult human brain with a reduced contribution in older individuals. With previous evidence demonstrating the critical role of synaptic GluN2B in regulating synaptic strength and memory storage in mice, this progressive reduction of GluN2B in the human brain during aging may underlie a molecular mechanism in the age-related decline in cognitive abilities and memory observed in humans.

Current Evidence on Cell Death in Preterm Brain Injury in Human and Preclinical Models

Despite tremendous advances in neonatal intensive care over the past 20 years, prematurity carries a high burden of neurological morbidity lasting lifelong. The term encephalopathy of prematurity (EoP) coined by Volpe in 2009 encompasses all aspects of the now known effects of prematurity on the immature brain, including altered and disturbed development as well as specific lesional hallmarks. Understanding the way cells are damaged is crucial to design brain protective strategies, and in this purpose, preclinical models largely contribute to improve the comprehension of the cell death mechanisms. While neuronal cell death has been deeply investigated and characterized in (hypoxic–ischemic) encephalopathy of the newborn at term, little is known about the types of cell death occurring in preterm brain injury. Three main different morphological cell death types are observed in the immature brain, specifically in models of hypoxic–ischemic encephalopathy, namely, necrotic, apoptotic, and autophagic cell death. Features of all three types may be present in the same dying neuron. In preterm brain injury, description of cell death types is sparse, and cell loss primarily concerns immature oligodendrocytes and, infrequently, neurons. In the present review, we first shortly discuss the different main severe preterm brain injury conditions that have been reported to involve cell death, including periventricular leucomalacia (PVL), diffuse white matter injury (dWMI), and intraventricular hemorrhages, as well as potentially harmful iatrogenic conditions linked to premature birth (anesthesia and caffeine therapy). Then, we present an overview of current evidence concerning cell death in both clinical human tissue data and preclinical models by focusing on studies investigating the presence of cell death allowing discriminating between the types of cell death involved. We conclude that, to improve brain protective strategies, not only apoptosis but also other cell death (such as regulated necrotic and autophagic) pathways now need to be investigated together in order to consider all cell death mechanisms involved in the pathogenesis of preterm brain damage.

Prenatal opioid exposure: The next neonatal neuroinflammatory disease

The rates of opioid use disorder during pregnancy have more than quadrupled in the last decade, resulting in numerous infants suffering exposure to opioids during the perinatal period, a critical period of central nervous system (CNS) development. Despite increasing use, the characterization and definition of the molecular and cellular mechanisms of the long-term neurodevelopmental impacts of opioid exposure commencing in utero remains incomplete. Thus, in consideration of the looming public health crisis stemming from the multitude of infants with prenatal opioid exposure entering school age, we undertook an investigation of the effects of perinatal methadone exposure in a novel preclinical model. Specifically, we examined the effects of opioids on the developing brain to elucidate mechanisms of putative neural cell injury, to identify diagnostic biomarkers and to guide clinical studies of outcome and follow-up. We hypothesized that methadone would induce a pronounced inflammatory profile in both dams and their pups, and be associated with immune system dysfunction, sustained CNS injury, and altered cognition and executive function into adulthood. This investigation was conducted using a combination of cellular, molecular, biochemical, and clinically translatable biomarker, imaging and cognitive assessment platforms. Data reveal that perinatal methadone exposure increases inflammatory cytokines in the neonatal peripheral circulation, and reprograms and primes the immune system through sustained peripheral immune hyperreactivity. In the brain, perinatal methadone exposure not only increases chemokines and cytokines throughout a crucial developmental period, but also alters microglia morphology consistent with activation, and upregulates TLR4 and MyD88 mRNA. This increase in neuroinflammation coincides with reduced myelin basic protein and altered neurofilament expression, as well as reduced structural coherence and significantly decreased fractional anisotropy on diffusion tensor imaging. In addition to this microstructural brain injury, adult rats exposed to methadone in the perinatal period have significant impairment in associative learning and executive control as assessed using touchscreen technology. Collectively, these data reveal a distinct systemic and neuroinflammatory signature associated with prenatal methadone exposure, suggestive of an altered CNS microenvironment, dysregulated developmental homeostasis, complex concurrent neural injury, and imaging and cognitive findings consistent with clinical literature. Further investigation is required to define appropriate therapies targeted at the neural injury and improve the long-term outcomes for this exceedingly vulnerable patient population.

Ion Channel Functions in Early Brain Development

During prenatal brain development, ion channels are ubiquitous across several cell types, including progenitor cells and migrating neurons but their function has not been clear. In the past, ion channel dysfunction has been primarily studied in the context of postnatal, differentiated neurons that fire action potentials - notably ion channels mutated in the epilepsies - yet data now support a surprising role in prenatal human brain disorders as well. Modern gene discovery approaches have identified defective ion channels in individuals with cerebral cortex malformations, which reflect abnormalities in early-to-middle stages of embryonic development (prior to ubiquitous action potentials). These human genetics studies and recent in utero animal modeling work suggest that precise control of ionic flux (calcium, sodium, and potassium) contributes to in utero developmental processes such as neural proliferation, migration, and differentiation.

Cannabidiol Administration Prevents Hypoxia-Ischemia-Induced Hypomyelination in Newborn Rats

Neonatal hypoxia-ischemia (HI) is a risk factor for myelination disturbances, a key factor for cerebral palsy. Cannabidiol (CBD) protects neurons and glial cells after HI insult in newborn animals. We hereby aimed to study CBD’s effects on long-lasting HI-induced myelination deficits in newborn rats. Thus, P7 Wistar rats received s.c. vehicle (HV) or cannabidiol (HC) after HI brain damage (left carotid artery electrocoagulation plus 10% O₂ for 112 min). Controls were non-HI pups. At P37, neurobehavioral tests were performed and immunohistochemistry [quantifying mature oligodendrocyte (mOL) populations and myelin basic protein (MBP) density] and electron microscopy (determining axon number, size, and myelin thickness) studies were conducted in cortex (CX) and white matter (WM). Expression of brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF) were analyzed by western blot at P14. HI reduced mOL or MBP in CX but not in WM. In both CX and WM, axon density and myelin thickness were reduced. MBP impairment correlated with functional deficits. CBD administration resulted in normal function associated with normal mOL and MBP, as well as normal axon density and myelin thickness in all areas. CBD’s effects were not associated with increased BDNF or GDNF expression. In conclusion, HI injury in newborn rats resulted in long-lasting myelination disturbance, associated with functional impairment. CBD treatment preserved function and myelination, likely as a part of a general neuroprotective effect.

De novo GRIN variants in NMDA receptor M2 channel pore-forming loop are associated with neurological diseases

N-methyl-D-aspartate receptors (NMDARs) mediate slow excitatory postsynaptic transmission in the central nervous system, thereby exerting a critical role in neuronal development and brain function. Rare genetic variants in the GRIN genes encoding NMDAR subunits segregated with neurological disorders. Here, we summarize the clinical presentations for 18 patients harboring 12 de novo missense variants in GRIN1, GRIN2A, and GRIN2B that alter residues in the M2 re-entrant loop, a region that lines the pore and is intolerant to missense variation. These de novo variants were identified in children with a set of neurological and neuropsychiatric conditions. Evaluation of the receptor cell surface expression, pharmacological properties, and biophysical characteristics show that these variants can have modest changes in agonist potency, proton inhibition, and surface expression. However, voltage-dependent magnesium inhibition is significantly reduced in all variants. The NMDARs hosting a single copy of a mutant subunit showed a dominant reduction in magnesium inhibition for some variants. These variant NMDARs also show reduced calcium permeability and single-channel conductance, as well as altered open probability. The data suggest that M2 missense variants increase NMDAR charge transfer in addition to varied and complex influences on NMDAR functional properties, which may underlie the patients' phenotypes.

Function and Mechanism of Myelin Regulation in Alcohol Abuse and Alcoholism

Excessive alcohol use has adverse effects on the central nervous system (CNS) and can lead to alcohol use disorders (AUDs). Recent studies have suggested that myelin reductions may directly contribute to CNS dysfunctions associated with AUDs. Myelin consists of compact lipid membranes wrapped around axons to provide electrical insulation and trophic support. Regulation of myelin is considered as a new form of neural plasticity due to its profound impacts on the computation of neural networks. In this review, the authors first discuss experimental evidence showing how alcohol exposure causes demyelination in different brain regions, often accompanied by deficits in cognition and emotion. Next, they discuss postulated molecular and cellular mechanisms underlying alcohol's impact on myelin. It is clear that more extensive investigations are needed in this important but underexplored research field in order to gain a better understanding of the myelin-behavior relationship and to develop new treatment strategies for AUDs.

Glutamate Transport and Preterm Brain Injury

Preterm birth complications are the leading cause of child death worldwide and a top global health priority. Among the survivors, the risk of life-long disabilities is high, including cerebral palsy and impairment of movement, cognition, and behavior. Understanding the molecular mechanisms of preterm brain injuries is at the core of future healthcare improvements. Glutamate excitotoxicity is a key mechanism in preterm brain injury, whereby the accumulation of extracellular glutamate damages the delicate immature oligodendrocytes and neurons, leading to the typical patterns of injury seen in the periventricular white matter. Glutamate excitotoxicity is thought to be induced by an interaction between environmental triggers of injury in the perinatal period, particularly cerebral hypoxia-ischemia and infection/inflammation, and developmental and genetic vulnerabilities. To avoid extracellular build-up of glutamate, the brain relies on rapid uptake by sodium-dependent glutamate transporters. Astrocytic excitatory amino acid transporter 2 (EAAT2) is responsible for up to 95% of glutamate clearance, and several lines of evidence suggest that it is essential for brain functioning. While in the adult EAAT2 is predominantly expressed by astrocytes, EAAT2 is transiently upregulated in the immature oligodendrocytes and selected neuronal populations during mid-late gestation, at the peak time for preterm brain injury. This developmental upregulation may interact with perinatal hypoxia-ischemia and infection/inflammation and contribute to the selective vulnerability of the immature oligodendrocytes and neurons in the preterm brain. Disruption of EAAT2 may involve not only altered expression but also impaired function with reversal of transport direction. Importantly, elevated EAAT2 levels have been found in the reactive astrocytes and macrophages of human infant post-mortem brains with severe white matter injury (cystic periventricular leukomalacia), potentially suggesting an adaptive mechanism against excitotoxicity. Interestingly, EAAT2 is suppressed in animal models of acute hypoxic-ischemic brain injury at term, pointing to an important and complex role in newborn brain injuries. Enhancement of EAAT2 expression and transport function is gathering attention as a potential therapeutic approach for a variety of adult disorders and awaits exploration in the context of the preterm brain injuries.

How Early Can a Seizure Happen? Pathophysiological Considerations of Extremely Premature Infant Brain Development

Seizures in neonates represent a neurologic emergency requiring prompt recognition, determination of etiology, and treatment. Yet, the definition and identification of neonatal seizures remain challenging and controversial, in part due to the unique physiology of brain development at this life stage. These issues are compounded when considering seizures in premature infants, in whom the complexities of brain development may engender different clinical and electrographic seizure features at different points in neuronal maturation. In extremely premature infants (< 28 weeks gestational age), seizure pathophysiology has not been explored in detail. This review discusses the physiological and structural development of the brain in this developmental window, focusing on factors that may lead to seizures and their consequences at this early time point. We hypothesize that the clinical and electrographic phenomenology of seizures in extremely preterm infants reflects the specific pathophysiology of brain development in that age window.

CXCR2 Blockade Mitigates Neural Cell Injury Following Preclinical Chorioamnionitis

Minimizing central nervous system (CNS) injury from preterm birth depends upon identification of the critical pathways that underlie essential neurodevelopmental and CNS pathophysiology. While chorioamnionitis (CHORIO), is a leading cause of preterm birth, the precise mechanism linking prenatal brain injury and long-term CNS injury is unknown. The chemokine (C-X-C motif) ligand 1 (CXCL1) and its cognate receptor, CXCR2, are implicated in a variety of uterine and neuropathologies, however, their role in CNS injury associated with preterm birth is poorly defined. To evaluate the putative efficacy of CXCR2 blockade in neural repair secondary to CHORIO, we tested the hypothesis that transient postnatal CXCR2 antagonism would reduce neutrophil activation and mitigate cerebral microstructural injury in rats. To this end, a laparotomy was performed on embryonic day 18 (E18) in Sprague Dawley rats, with uterine arteries transiently occluded for 60 min, and lipopolysaccharide (LPS, 4 μg/sac) injected into each amniotic sac. SB225002, a CXCR2 antagonist (3 mg/kg), was administered intraperitoneally from postnatal day 1 (P1)-P5. Brains were collected on P7 and P21 and analyzed with western blot, immunohistochemistry and ex vivo diffusion tensor imaging (DTI). Results demonstrate that transient CXCR2 blockade reduced cerebral neutrophil activation (myeloperoxidase expression/MPO) and mitigated connexin43 expression, indicative of reduced neuroinflammation at P7 (p < 0.05 for all). CXCR2 blockade also reduced alpha II-spectrin calpain-mediated cleavage, improved pNF/NF ratio, and minimized Iba1 and GFAP expression consistent with improved neuronal and axonal health and reduced gliosis at P21. Importantly, DTI revealed diffuse white matter injury and decreased microstructural integrity following CHORIO as indicated by lower fractional anisotropy (FA) and elevated radial diffusivity (RD) in major white matter tracts (p < 0.05). Early postnatal CXCR2 blockade also reduced microstructural abnormalities in white matter and hippocampus at P21 (p < 0.05). Together, these data indicate that transient postnatal blockade of CXCR2 ameliorates perinatal abnormalities in inflammatory signaling, and facilitates neural repair following CHORIO. Further characterization of neuroinflammatory signaling, specifically via CXCL1/CXCR2 through the placental-fetal-brain axis, may clarify stratification of brain injury following preterm birth, and improve use of targeted interventions in this highly vulnerable patient population.

The Potential Role of Ferroptosis in Neonatal Brain Injury

Ferroptosis is an iron-dependent form of cell death that is characterized by early lipid peroxidation and different from other forms of regulated cell death in terms of its genetic components, specific morphological features, and biochemical mechanisms. Different initiation pathways of ferroptosis have been reported, including inhibition of system X_c ^-, inactivation of glutathione-dependent peroxidase 4, and reduced glutathione levels, all of which ultimately promote the production of reactive oxygen species, particularly through enhanced lipid peroxidation. Although ferroptosis was first described in cancer cells, emerging evidence now links mechanisms of ferroptosis to many different diseases, including cerebral ischemia and brain hemorrhage. For example, neonatal brain injury is an important cause of developmental impairment and of permanent neurological deficits, and several types of cell death, including iron-dependent pathways, have been detected in the process of neonatal brain damage. Iron chelators and erythropoietin have both shown neuroprotective effects against neonatal brain injury. Here, we have summarized the potential relation between ferroptosis and neonatal brain injury, and according therapeutic intervention strategies.

Enhanced autophagy contributes to excitotoxic lesions in a rat model of preterm brain injury

Cystic periventricular leukomalacia is commonly diagnosed in premature infants, resulting from severe hypoxic-ischemic white matter injury, and also involving some grey matter damage. Very few is known concerning the cell death pathways involved in these types of premature cerebral lesions. Excitotoxicity is a predominant mechanism of hypoxic-ischemic injury in the developing brain. Concomitantly, it has been recently shown that autophagy could be enhanced in excitotoxic conditions switching this physiological intracellular degradation system to a deleterious process. We here investigated the role of autophagy in a validated rodent model of preterm excitotoxic brain damage mimicking in some aspects cystic periventricular leukomalacia. An excitotoxic lesion affecting periventricular white and grey matter was induced by injecting ibotenate, a glutamate analogue, in the subcortical white matter (subcingulum area) of five-day old rat pups. Ibotenate enhanced autophagy in rat brain dying neurons at 24 h as shown by increased presence of autophagosomes (increased LC3-II and LC3-positive dots) and enhanced autophagic degradation (SQSTM1 reduction and increased number and size of lysosomes (LAMP1- and CATHEPSIN B-positive vesicles)). Co-injection of the pharmacological autophagy inhibitor 3-methyladenine prevented not only autophagy induction but also CASPASE-3 activation and calpain-dependent cleavage of SPECTRIN 24 h after the insult, thus providing a strong reduction of the long term brain injury (16 days after ibotenate injection) including lateral ventricle dilatation, decreases in cerebral tissue volume and in subcortical white matter thickness. The autophagy-dependent neuroprotective effect of 3-methyladenine was confirmed in primary cortical neuronal cultures using not only pharmacological but also genetic autophagy inhibition of the ibotenate-induced autophagy. Strategies inhibiting autophagy could then represent a promising neuroprotective approach in the context of severe preterm brain injuries.

Prenatal Bisphenol A Exposure is Linked to Epigenetic Changes in Glutamate Receptor Subunit Gene Grin2b in Female Rats and Humans

Bisphenol A (BPA) exposure has been linked to neurodevelopmental disorders and to effects on epigenetic regulation, such as DNA methylation, at genes involved in brain function. High doses of BPA have been shown to change expression and regulation of one such gene, Grin2b, in mice. Yet, if such changes occur at relevant doses in animals and humans has not been addressed. We investigated if low-dose developmental BPA exposure affects DNA methylation and expression of Grin2b in brains of adult rats. Furthermore, we assessed associations between prenatal BPA exposure and Grin2b methylation in 7-year old children. We found that Grin2b mRNA expression was increased and DNA methylation decreased in female, but not in male rats. In humans, prenatal BPA exposure was associated with increased methylation levels in girls. Additionally, low APGAR scores, a predictor for increased risk for neurodevelopmental diseases, were associated with higher Grin2b methylation levels in girls. Thus, we could link developmental BPA exposure and low APGAR scores to changes in the epigenetic regulation of Grin2b, a gene important for neuronal function, in a sexual dimorphic fashion. Discrepancies in exact locations and directions of the DNA methylation change might reflect differences between species, analysed tissues, exposure level and/or timing.

Glial function (and dysfunction) in the normal & ischemic brain

Astrocytes are the most abundant cell type in the central nervous system (CNS). Once considered to be of fairly homogeneous phenotype throughout the brain and spinal cord, they are now understood to be heterogeneous in both structure and function. They are important in brain functions as diverse as ion and fluid balance in the interstitial space, contributing to integrity of the neurovascular unit (blood-brain barrier), neurotransmitter regulation, metabolism of energy substrates and possibly even axonal regeneration. After ischemic or hemorrhagic brain/spinal cord injury, formation of an astrocytic scar adjacent to the 'lesion' is a characteristic histopathologic feature, and this astrogliosis can be demonstrated by immunohistochemistry, usually using primary antibodies to glial fibrillary acidic protein (GFAP). Astrocytes interact with microglia and oligodendroglia in novel ways that will be discussed in this review. This article is part of the Special Issue entitled 'Cerebral Ischemia'.

Neuronalinjury and roles of apoptosis and autophagy in a neonatal rat model of hypoxia-ischemia-induced periventricular leukomalacia

As research into periventricular leukomalacia (PVL) gradually increases, concerns are emerging about long‑term neuron injury. The present study aimed to investigate neuronal injury and the relevant alterations in apoptosis and autophagy in a PVL model established previously. A rat model of hypoxia‑ischemia‑induced PVL was established. In the model group, Sprague‑Dawley (SD) rats [postnatal day 3 (P3)] were subjected to right common carotid artery ligation followed by suturing and exposed to 6‑8% oxygen for 2 h; in the control group, SD rats (P3) were subjected to right common carotid artery dissection followed by suturing, without ligation and hypoxic exposure. At 1, 3, 7 and 14 days following modeling, brain tissue samples were collected and stained with hematoxylin and eosin. Cellular apoptosis was detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and the protein and mRNA expression alterations of neuronal nuclei (NeuN), caspase‑3 and Beclin 1 in the model group were detected by western blot analysis and reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) analyses. Compared with the control group, the protein and mRNA expression levels of NeuN (a marker of mature neurons) were markedly reduced, the number of positive cells was increased as detected by TUNEL, and the protein and mRNA expression levels of caspase‑3 and Beclin 1 were elevated in the model group. In the rat model of hypoxia‑ischemia‑induced PVL, oligodendrocyte injury and myelinization disorders were observed, in addition to neuron injury, a decrease in mature neurons and the co‑presence of apoptosis and autophagy. However, apoptosis and autophagy exist in different phases: Apoptosis is involved in neuron injury, while autophagy is likely to have a protective role.

Targeting Sentinel Proteins and Extrasynaptic Glutamate Receptors: a Therapeutic Strategy for Preventing the Effects Elicited by Perinatal Asphyxia?

Perinatal asphyxia (PA) is a relevant cause of death at the time of labour, and when survival is stabilised, associated with short- and long-term developmental disabilities, requiring inordinate care by health systems and families. Its prevalence is high (1 to 10/1000 live births) worldwide. At present, there are few therapeutic options, apart from hypothermia, that regrettably provides only limited protection if applied shortly after the insult.PA implies a primary and a secondary insult. The primary insult relates to the lack of oxygen, and the secondary one to the oxidative stress triggered by re-oxygenation, formation of reactive oxygen (ROS) and reactive nitrogen (RNS) species, and overactivation of glutamate receptors and mitochondrial deficiencies. PA induces overactivation of a number of sentinel proteins, including hypoxia-induced factor-1α (HIF-1α) and the genome-protecting poly(ADP-ribose) polymerase-1 (PARP-1). Upon activation, PARP-1 consumes high amounts of ATP at a time when this metabolite is scarce, worsening in turn the energy crisis elicited by asphyxia. The energy crisis also impairs ATP-dependent transport, including glutamate re-uptake by astroglia. Nicotinamide, a PARP-1 inhibitor, protects against the metabolic cascade elicited by the primary stage, avoiding NAD+ exhaustion and the energetic crisis. Upon re-oxygenation, however, oxidative stress leads to nuclear translocation of the NF-κB subunit p65, overexpression of the pro-inflammatory cytokines IL-1β and TNF-α, and glutamate-excitotoxicity, due to impairment of glial-glutamate transport, extracellular glutamate overflow, and overactivation of NMDA receptors, mainly of the extrasynaptic type. This leads to calcium influx, mitochondrial impairment, and inactivation of antioxidant enzymes, increasing further the activity of pro-oxidant enzymes, thereby making the surviving neonate vulnerable to recurrent metabolic insults whenever oxidative stress is involved. Here, we discuss evidence showing that (i) inhibition of PARP-1 overactivation by nicotinamide and (ii) inhibition of extrasynaptic NMDA receptor overactivation by memantine can prevent the short- and long-term consequences of PA. These hypotheses have been evaluated in a rat preclinical model of PA, aiming to identify the metabolic cascades responsible for the long-term consequences induced by the insult, also assessing postnatal vulnerability to recurrent oxidative insults. Thus, we present and discuss evidence demonstrating that PA induces long-term changes in metabolic pathways related to energy and oxidative stress, priming vulnerability of cells with both the neuronal and the glial phenotype. The effects induced by PA are region dependent, the substantia nigra being particularly prone to cell death. The issue of short- and long-term consequences of PA provides a framework for addressing a fundamental issue referred to plasticity of the CNS, since the perinatal insult triggers a domino-like sequence of events making the developing individual vulnerable to recurrent adverse conditions, decreasing his/her coping repertoire because of a relevant insult occurring at birth.

Stem cells and cell-based therapies for cerebral palsy: a call for rigor

Cell-based therapies hold significant promise for infants at risk for cerebral palsy (CP) from perinatal brain injury (PBI). PBI leading to CP results from multifaceted damage to neural cells. Complex developing neural networks are injured by neural cell damage plus unique perturbations in cell signaling. Given that cell-based therapies can simultaneously repair multiple injured neural components during critical neurodevelopmental windows, these interventions potentially offer efficacy for patients with CP. Currently, the use of cell-based interventions in infants at risk for CP is limited by critical gaps in knowledge. In this review, we will highlight key questions facing the field, including: Who are optimal candidates for treatment? What are the goals of therapeutic interventions? What are the best strategies for agent delivery, including timing, dosage, location, and type? And, how are short- and long-term efficacy reliably tracked? Challenges unique to treating PBI with cell-based therapies, and lessons learned from cell-based therapies in closely related neurological disorders in the mature central nervous system, will be reviewed. Our goal is to update pediatric specialists who may be counseling families about the current state of the field. Finally, we will evaluate how rigor can be increased in the field to ensure the safety and best interests of this vulnerable patient population.

Juvenile Traumatic Brain Injury Results in Cognitive Deficits Associated with Impaired Endoplasmic Reticulum Stress and Early Tauopathy

The leading cause of death in the juvenile population is trauma, and in particular neurotrauma. The juvenile brain response to neurotrauma is not completely understood. Endoplasmic reticulum (ER) stress has been shown to contribute to injury expansion and behavioral deficits in adult rodents and furthermore has been seen in adult postmortem human brains diagnosed with chronic traumatic encephalopathy. Whether endoplasmic reticulum stress is increased in juveniles with traumatic brain injury (TBI) is poorly delineated. We investigated this important topic using a juvenile rat controlled cortical impact (CCI) model. We proposed that ER stress would be significantly increased in juvenile rats following TBI and that this would correlate with behavioral deficits using a juvenile rat model. A juvenile rat (postnatal day 28) CCI model was used. Binding immunoglobulin protein (BiP) and C/EBP homologous protein (CHOP) were measured at 4 h in the ipsilateral pericontusion cortex. Hypoxia-inducible factor (HIF)-1α was measured at 48 h and tau kinase measured at 1 week and 30 days. At 4 h following injury, BiP and CHOP (markers of ER stress) were significantly elevated in rats exposed to TBI. We also found that HIF-1α was significantly upregulated 48 h following TBI showing delayed hypoxia. The early ER stress activation was additionally asso-ciated with the activation of a known tau kinase, glycogen synthase kinase-3β (GSK-3β), by 1 week. Tau oligomers measured by R23 were significantly increased by 30 days following TBI. The biochemical changes following TBI were associated with increased impulsive-like or anti-anxiety behavior measured with the elevated plus maze, deficits in short-term memory measured with novel object recognition, and deficits in spatial memory measured with the Morris water maze in juvenile rats exposed to TBI. These results show that ER stress was increased early in juvenile rats exposed to TBI, that these rats developed tau oligomers over the course of 30 days, and that they had significant short-term and spatial memory deficits following injury.

Long-term impact of intrauterine neuroinflammation and treatment with magnesium sulphate and betamethasone: Sex-specific differences in a preterm labor murine model

Preterm infants are at significantly increased risk for lifelong neurodevelopmental disability with male offspring disproportionately affected. Corticosteroids (such as betamethasone) and magnesium sulphate (MgSO₄) are administered to women in preterm labor to reduce neurologic morbidity. Despite widespread use of MgSO₄ in clinical practice, its effects on adult offspring are not well known nor have sex-specific differences in therapeutic response been explored. The objective of our study was to examine the long-term effects of perinatal neuroinflammation and the effectiveness of prenatal MgSO₄/betamethasone treatments between males and females in a murine model via histologic and expression analyses. Our results demonstrate that male but not female offspring exposed to intrauterine inflammation demonstrated impaired performance in neurodevelopmental testing in early life assessed via negative geotaxis, while those exposed to injury plus treatment fared better. Histologic analysis of adult male brains identified a significant reduction in hippocampal neural density in the injured group compared to controls. Evaluation of key neural markers via qRT-PCR demonstrated more profound differences in gene expression in adult males exposed to injury and treatment compared to female offspring, which largely showed resistance to injury. Prenatal treatment with MgSO₄/betamethasone confers long-term benefits beyond cerebral palsy prevention with sex-specific differences in response.

N-Methyl d-Aspartate Receptor Expression Patterns in the Human Fetal Cerebral Cortex

N-methyl d-aspartate receptors (NMDARs), a subtype of glutamate receptor, have important functional roles in cellular activity and neuronal development. They are well-studied in rodent and adult human brains, but limited information is available about their distribution in the human fetal cerebral cortex. Here we show that 3 NMDAR subunits, NR1, NR2A, and NR2B, are expressed in the human cerebral cortex during the second trimester of gestation, a period of intense neurogenesis and synaptogenesis. With increasing fetal age, expression of the NMDAR-encoding genes Grin1 (NR1) and Grin2a (NR2A) increased while Grin2b (NR2B) expression decreased. The protein levels of all 3 subunits paralleled the changes in gene expression. On cryosections, all 3 subunits were expressed in proliferative ventricular and subventricular zones, in radial glia, and in intermediate progenitor cells, consistent with their role in the proliferation of cortical progenitor cells and in the determination of their respective fates. The detection of NR1, NR2A, and NR2B in both glutamatergic and GABAergic neurons of the cortical plate suggests the involvement of NMDARs in the maturation of human cortical neurons and in early synapse formation. Our results and previous studies in rodents suggest that NMDAR expression in the developing human brain is evolutionarily conserved.

Acute oligodendrocyte loss with persistent white matter injury in a third trimester equivalent mouse model of fetal alcohol spectrum disorder

Alcohol exposure during central nervous system (CNS) development can lead to fetal alcohol spectrum disorder (FASD). Human imaging studies have revealed significant white matter (WM) abnormalities linked to cognitive impairment in children with FASD; however, the underlying mechanisms remain unknown. Here, we evaluated both the acute and long-term impacts of alcohol exposure on oligodendrocyte number and WM integrity in a third trimester-equivalent mouse model of FASD, in which mouse pups were exposed to alcohol during the first 2 weeks of postnatal development. Our results demonstrate a 58% decrease in the number of mature oligodendrocytes (OLs) and a 75% decrease in the number of proliferating oligodendrocyte progenitor cells (OPCs) within the corpus callosum of alcohol-exposed mice at postnatal day 16 (P16). Interestingly, neither mature OLs nor OPCs derived from the postnatal subventricular zone (SVZ) were numerically affected by alcohol exposure, indicating heterogeneity in susceptibility based on OL ontogenetic origin. Although mature OL and proliferating OPC numbers recovered by postnatal day 50 (P50), abnormalities in myelin protein expression and microstructure within the corpus callosum of alcohol-exposed subjects persisted, as assessed by western immunoblotting of myelin basic protein (MBP; decreased expression) and MRI diffusion tensor imaging (DTI; decreased fractional anisotropy). These results indicate that third trimester-equivalent alcohol exposure leads to an acute, albeit recoverable, decrease in OL lineage cell numbers, accompanied by enduring WM injury. Additionally, our finding of heterogeneity in alcohol susceptibility based on the developmental origin of OLs may have therapeutic implications in FASD and other disorders of WM development.

D-cycloserine in Schizophrenia: New Strategies for Improving Clinical Outcomes by Enhancing Plasticity

Background

Dysregulation of N-methyl D-aspartate (NMDA) receptor signaling is strongly implicated in schizophrenia. Based on the ketamine model of NMDA receptor hypoactivity, therapeutic approaches designed to maintain a sustained increase in agonist activity at the glycine site of the NMDA receptor have produced promising, although inconsistent, efficacy for negative symptoms.

Methods

A review of the published literature on D-cycloserine (DCS) pharmacology in animal models and in clinical studies was performed. Findings relevant to DCS effects on memory and plasticity and their potential clinical application to schizophrenia were summarized.

Results

Studies in animals and clinical trials in patients with anxiety disorders have demonstrated that single or intermittent dosing with DCS enhances memory consolidation. Preliminary trials in patients with schizophrenia suggest that intermittent dosing with DCS may produce persistent improvement of negative symptoms and enhance learning when combined with cognitive behavioral therapy for delusions or with cognitive remediation. The pharmacology of DCS is complex, since it acts as a “super agonist” at NMDA receptors containing GluN2C subunits and, under certain conditions, it may act as an antagonist at NMDA receptors containing GluN2B subunits.

Conclusions

There are preliminary findings that support a role for D-cycloserine in schizophrenia as a strategy to enhance neuroplasticity and memory. However, additional studies with DCS are needed to confirm these findings. In addition, clinical trials with positive and negative allosteric modulators with greater specificity for NMDA receptor subtypes are needed to identify the optimal strategy for enhancing neuroplasticity in schizophrenia.

Embracing oligodendrocyte diversity in the context of perinatal injury

Emerging evidence is fueling a new appreciation of oligodendrocyte diversity that is overturning the traditional view that oligodendrocytes are a homogenous cell population. Oligodendrocytes of distinct origins, maturational stages, and regional locations may differ in their functional capacity or susceptibility to injury. One of the most unique qualities of the oligodendrocyte is its ability to produce myelin. Myelin abnormalities have been ascribed to a remarkable array of perinatal brain injuries, with concomitant oligodendrocyte dysregulation. Within this review, we discuss new insights into the diversity of the oligodendrocyte lineage and highlight their relevance in paradigms of perinatal brain injury. Future therapeutic development will be informed by comprehensive knowledge of oligodendrocyte pathophysiology that considers the particular facets of heterogeneity that this lineage exhibits.