Neonatal cerebral hypoxia-ischemia impairs plasticity in rat visual cortex

Neuroprotective efficacy of hypothermia and Inter-alpha Inhibitor Proteins after hypoxic ischemic brain injury in neonatal rats

Therapeutic hypothermia is the standard of care for hypoxic-ischemic (HI) encephalopathy. Inter-alpha Inhibitor Proteins (IAIPs) attenuate brain injury after HI in neonatal rats. Human (h) IAIPs (60 ​mg/kg) or placebo (PL) were given 15 ​min, 24 and 48 ​h to postnatal (P) day-7 rats after carotid ligation and 8% oxygen for 90 ​min with (30 ​°C) and without (36 ​°C) exposure to hypothermia 1.5 ​h after HI for 3 ​h. Hemispheric volume atrophy (P14) and neurobehavioral tests including righting reflex (P8-P10), small open field (P13-P14), and negative geotaxis (P14) were determined. Hemispheric volume atrophy in males was reduced (P ​< ​0.05) by 41.9% in the normothermic-IAIP and 28.1% in the hypothermic-IAIP compared with the normothermic-PL group, and in females reduced (P ​< ​0.05) by 30.3% in the normothermic-IAIP, 45.7% in hypothermic-PL, and 55.2% in hypothermic-IAIP compared with the normothermic-PL group after HI. Hypothermia improved (P ​< ​0.05) the neuroprotective effects of hIAIPs in females. The neuroprotective efficacy of hIAIPs was comparable to hypothermia in female rats (P ​= ​0.183). Treatment with hIAIPs, hypothermia, and hIAIPs with hypothermia decreased (P ​< ​0.05) the latency to enter the peripheral zone in the small open field test in males. We conclude that hIAIPs provide neuroprotection from HI brain injury that is comparable to the protection by hypothermia, hypothermia increases the effects of hIAIPs in females, and hIAIPs and hypothermia exhibit some sex-related differential effects.

Consequences of oxygen deprivation on myelination and sex-dependent alterations

Oxygen deprivation is one of the main causes of morbidity and mortality in newborns, occurring with a higher prevalence in preterm infants, reaching 20 % to 50 % mortality in newborns in the perinatal period. When they survive, 25 % exhibit neuropsychological pathologies, such as learning difficulties, epilepsy, and cerebral palsy. White matter injury is one of the main features found in oxygen deprivation injury, which can lead to long-term functional impairments, including cognitive delay and motor deficits. The myelin sheath accounts for much of the white matter in the brain by surrounding axons and enabling the efficient conduction of action potentials. Mature oligodendrocytes, which synthesize and maintain myelination, also comprise a significant proportion of the brain's white matter. In recent years, oligodendrocytes and the myelination process have become potential therapeutic targets to minimize the effects of oxygen deprivation on the central nervous system. Moreover, evidence indicate that neuroinflammation and apoptotic pathways activated during oxygen deprivation may be influenced by sexual dimorphism. To summarize the most recent research about the impact of sexual dimorphism on the neuroinflammatory state and white matter injury after oxygen deprivation, this review presents an overview of the oligodendrocyte lineage development and myelination, the impact of oxygen deprivation and neuroinflammation on oligodendrocytes in neurodevelopmental disorders, and recent reports about sexual dimorphism regarding the neuroinflammation and white matter injury after neonatal oxygen deprivation.

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.

Changing subplate circuits: Early activity dependent circuit plasticity

Early neural activity in the developing sensory system comprises spontaneous bursts of patterned activity, which is fundamental for sculpting and refinement of immature cortical connections. The crude early connections that are initially refined by spontaneous activity, are further elaborated by sensory-driven activity from the periphery such that orderly and mature connections are established for the proper functioning of the cortices. Subplate neurons (SPNs) are one of the first-born mature neurons that are transiently present during early development, the period of heightened activity-dependent plasticity. SPNs are well integrated within the developing sensory cortices. Their structural and functional properties such as relative mature intrinsic membrane properties, heightened connectivity via chemical and electrical synapses, robust activation by neuromodulatory inputs-place them in an ideal position to serve as crucial elements in monitoring and regulating spontaneous endogenous network activity. Moreover, SPNs are the earliest substrates to receive early sensory-driven activity from the periphery and are involved in its modulation, amplification, and transmission before the maturation of the direct adult-like thalamocortical connectivity. Consequently, SPNs are vulnerable to sensory manipulations in the periphery. A broad range of early sensory deprivations alters SPN circuit organization and functions that might be associated with long term neurodevelopmental and psychiatric disorders. Here we provide a comprehensive overview of SPN function in activity-dependent development during early life and integrate recent findings on the impact of early sensory deprivation on SPNs that could eventually lead to neurodevelopmental disorders.

Inter-alpha Inhibitor Proteins Ameliorate Brain Injury and Improve Behavioral Outcomes in a Sex-Dependent Manner After Exposure to Neonatal Hypoxia Ischemia in Newborn and Young Adult Rats

Hypoxic-ischemic (HI) brain injury is a major contributor to neurodevelopmental morbidities. Inter-alpha inhibitor proteins (IAIPs) have neuroprotective effects on HI-related brain injury in neonatal rats. However, the effects of treatment with IAIPs on sequential behavioral, MRI, and histopathological abnormalities in the young adult brain after treatment with IAIPs in neonates remain to be determined. The objective of this study was to examine the neuroprotective effects of IAIPs at different neurodevelopmental stages from newborn to young adults after exposure of neonates to HI injury. IAIPs were given as 11-sequential 30-mg/kg doses to postnatal (P) day 7-21 rats after right common carotid artery ligation and exposure to 90 min of 8% oxygen. The resulting brain edema and injury were examined by T2-weighted magnetic resonance imaging (MRI) and cresyl violet staining, respectively. The mean T2 values of the ipsilateral hemisphere from MRI slices 6 to 10 were reduced in IAIP-treated HI males + females on P8, P9, and P10 and females on P8, P9, P10, and P14. IAIP treatment reduced hemispheric volume atrophy by 44.5 ± 29.7% in adult male + female P42 rats and improved general locomotor abilities measured by the righting reflex over time at P7.5, P8, and P9 in males + females and males and muscle strength/endurance measured by wire hang on P16 in males + females and females. IAIPs provided beneficial effects during the learning phase of the Morris water maze with females exhibiting beneficial effects. IAIPs confer neuroprotection from HI-related brain injury in neonates and even in adult rats and beneficial MRI and behavioral benefits in a sex-dependent manner.

Neonatal Hypoxia-Ischemia Causes Persistent Intracortical Circuit Changes in Layer 4 of Rat Auditory Cortex

The connection between early brain injury and subsequent development of disorders is unknown. Neonatal hypoxia-ischemia (HI) alters circuits associated with subplate neurons (SPNs). SPNs are among the first maturing cortical neurons, project to thalamorecipient layer 4 (L4), and are required for the development of thalamocortical connections. Thus, early HI might influence L4 and such influence might persist. We investigated functional circuits to L4 neurons in neonatal rat HI models of different severities (mild and moderate) shortly after injury and at adolescence. We used laser-scanning photostimulation in slices of auditory cortex during P5-10 and P18-23. Mild injuries did not initially (P6/P7) alter the convergence of excitatory inputs from L2/3, but hyperconnectivity emerged by P8-10. Inputs from L4 showed initial hypoconnectivity which resolved by P8-10. Moderate injuries resulted in initial hypoconnectivity from both layers which resolved by P8-10 and led to persistent strengthening of connections. Inhibitory inputs to L4 cells showed similar changes. Functional changes were mirrored by reduced dendritic complexity. We also observed a persistent increase in similarity of L4 circuits, suggesting that HI interferes with developmental circuit refinement and diversification. Altogether, our results show that neonatal HI injuries lead to persistent changes in intracortical connections.

Prognostic value of neonatal EEG following therapeutic hypothermia in survivors of hypoxic-ischemic encephalopathy

OBJECTIVE

Early prediction of neurological deficits following neonatal hypoxic-ischemic encephalopathy (HIE) may help to target support. Neonatal animal models suggest that recovery following hypoxia-ischemia depends upon cortical bursting. To test whether this holds in human neonates, we correlated the magnitude of cortical bursting during recovery (≥postnatal day 3) with neurodevelopmental outcomes.

METHODS

We identified 41 surviving infants who received therapeutic hypothermia for HIE (classification at hospital discharge: 19 mild, 18 moderate, 4 severe) and had 9-channel electroencephalography (EEG) recordings as part of their routine care. We correlated burst power with Bayley-III cognitive, motor and language scores at median 24 months. To examine whether EEG offered additional prognostic information, we controlled for structural MRI findings.

RESULTS

Higher power of central and occipital cortical bursts predicted worse cognitive and language outcomes, and higher power of central cortical bursts predicted worse motor outcome, all independently of structural MRI findings.

CONCLUSIONS

Clinical EEG after postnatal day 3 may provide additional prognostic information by indexing persistent active mechanisms that either support recovery or exacerbate brain damage, especially in infants with less severe encephalopathy.

SIGNIFICANCE

These findings could allow for the effect of clinical interventions in the neonatal period to be studied instantaneously in the future.

Inhibition of NOD1 Attenuates Neonatal Hypoxia-Ischemia Induced Long-Term Cognitive Impairments in Mice Through Modulation of Autophagy-Related Proteins

BACKGROUND

Autophagy is implicated in neonatal hypoxia-ischemia (HI) induced cognitive impairment. The nucleotide-oligomerizing domain-1 (NOD1), a protein involved in inflammatory responses, has been shown to activate autophagy to promote progression of other diseases. We aimed to investigate whether and how NOD1 is involved in HI-induced brain injury using an HI mouse model.

METHODS

We induced HI in neonatal mice and examined levels of NOD1 and genes associated with autophagy. We then inhibited NOD1 by intracerebroventricular injection of si-NOD1 following HI induction and tested the effects on autophagy, inflammatory responses and long-term behavioral outcomes through Morris water maze and open field tests.

RESULTS

We found that HI induction significantly elevated mRNA levels of NOD1 (3.54 folds change) and autophagy-related genes including Atg5 (3.89 folds change) and Beclin-1 (3.34 folds change). NOD1 inhibition following HI induction suppressed autophagy signaling as well as HI induced proinflammatory cytokine production. Importantly, NOD1 inhibition after HI improved long-term cognitive function, without impacting exploratory and locomotor activities.

CONCLUSION

We show here that NOD1 is involved in the pathogenesis of HI-induced brain injury through modulation of autophagy-related proteins and inflammatory responses. Our findings suggest that NOD1 may be a potent target for developing therapeutic strategies for treating HI-induced brain injury.

Sex specific correlation between GABAergic disruption in the dorsal hippocampus and flurothyl seizure susceptibility after neonatal hypoxic-ischemic brain injury

Since neonatal hypoxia-ischemia (HI) disrupts the hippocampal (Hp) GABAergic network in the mouse and Hp injury in this model correlates with flurothyl seizure susceptibility only in male mice, we hypothesized that GABAergic disruption correlates with flurothyl seizure susceptibility in a sex-specific manner. C57BL6 mice were exposed to HI (Vannucci model) versus sham procedures at P10, randomized to normothermia (NT) or therapeutic hypothermia (TH), and subsequently underwent flurothyl seizure testing at P18. Only in male mice, Hp atrophy correlated with seizure susceptibility. The number of Hp parvalbumin positive interneurons (PV⁺INs) decreased after HI in both sexes, but TH attenuated this deficit only in females. In males only, seizure susceptibility directly correlated with the number of PV⁺INs, but not somatostatin or calretinin expressing INs. Hp GABA_B receptor subunit levels were decreased after HI, but unrelated to later seizure susceptibility. In contrast, Hp GABA_A receptor α1 subunit (GABA_ARα1) levels were increased after HI. Adjusting the number of PV⁺ INs for their GABA_ARα1 expression strengthened the correlation with seizure susceptibility in male mice. Thus, we identified a novel Hp sex-specific GABA-mediated mechanism of compensation after HI that correlates with flurothyl seizure susceptibility warranting further study to better understand potential clinical translation.

Warped rhythms: Epileptic activity during critical periods disrupts the development of neural networks for human communication

It is well established that temporal lobe epilepsy-the most common and well-studied form of epilepsy-can impair communication by disrupting social-emotional and language functions. In pediatric epilepsy, where seizures co-occur with the development of critical brain networks, age of onset matters: The earlier in life seizures begin, the worse the disruption in network establishment, resulting in academic hardship and social isolation. Yet, little is known about the processes by which epileptic activity disrupts developing human brain networks. Here we take a synthetic perspective-reviewing a range of research spanning studies on molecular and oscillatory processes to those on the development of large-scale functional networks-in support of a novel model of how such networks can be disrupted by epilepsy. We seek to bridge the gap between research on molecular processes, on the development of human brain circuitry, and on clinical outcomes to propose a model of how epileptic activity disrupts brain development.

Effects of inter-alpha inhibitor proteins on brain injury after exposure of neonatal rats to severe hypoxia-ischemia

Hypoxic-ischemic (HI) brain injury is one of the most common neurological problems occurring in premature and full-term infants after perinatal complications. Hypothermia is the only treatment approved for HI encephalopathy in newborns. However, this treatment is only partially protective, cannot be used to treat premature infants, and has limited efficacy to treat severe HI encephalopathy. Inflammation contributes to the evolution of HI brain injury in neonates. Inter-alpha Inhibitor Proteins (IAIPs) are immunomodulatory proteins that have neuroprotective properties after exposure to moderate HI in neonatal rats. The objective of the current study was to determine the neuroprotective efficacy of treatment with IAIPs starting immediately after or with a delay of one hour after exposure to severe HI of 120 min duration. One hundred and forty-six 7-day-old rat pups were randomized to sham control, HI and immediate treatment with IAIPs (60 mg/kg) or placebo (PL), and sham, HI and delayed treatment with IAIPs or PL. IAIPs or PL were given at zero, 24, and 48 h after HI or 1, 24 and 48 h after HI. Total brain infarct volume was determined 72 h after exposure to HI. Treatment with IAIPs immediately after HI decreased (P < 0.05) infarct volumes by 58.0% and 44.5% in male and female neonatal rats, respectively. Delayed treatment with IAIPs after HI decreased (P < 0.05) infarct volumes by 23.7% in male, but not in female rats. We conclude that IAIPs exert neuroprotective effects even after exposure to severe HI in neonatal rats and appear to exhibit some sex-related differential effects.

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.

Caffeine Restores Background EEG Activity Independent of Infarct Reduction after Neonatal Hypoxic Ischemic Brain Injury

In human preterm newborns, caffeine increases brain activity and improves neurodevelopmental outcomes. In animal models of hypoxic ischemic brain injury, caffeine pretreatment reduces infarct volume. We studied the relationship between tissue neuroprotection and brain activity after injury to further understand caffeine neuroprotection. Rat dams received caffeine prior to birth or on postnatal day 3 (P3) through P16. Caffeine-treated and -untreated pups underwent the Vannucci procedure (unilateral carotid ligation, global hypoxia) on P2. A subset had EEG recordings. Brain hemispheric infarct volume was measured on P16. P2 hypoxic ischemia (HI) results in histologic brain injury (mean ± standard deviation infarct volume 10.3 ± 4.6%) and transient suppression of EEG activity. Caffeine pretreatment reduces brain injury (mean ± standard deviation infarct volume 1.6 ± 4.5%, p < 0.001) and improves amplitude-integrated EEG (aEEG) and EEG burst duration and amplitude. Caffeine treatment after HI does not reduce infarct volume (mean ± standard deviation 8.3 ± 4.1%, p = 1.0). However, caffeine posttreatment was equally effective at restoring aEEG amplitude and EEG burst duration and amplitude. Thus, caffeine supports brain background electrical activity independent of tissue neuroprotection.

Neonatal Ethanol Disturbs the Normal Maturation of Parvalbumin Interneurons Surrounded by Subsets of Perineuronal Nets in the Cerebral Cortex: Partial Reversal by Lithium

Reduction in parvalbumin-positive (PV+) interneurons is observed in adult mice exposed to ethanol at postnatal day 7 (P7), a late gestation fetal alcohol spectrum disorder model. To evaluate whether PV+ cells are lost, or PV expression is reduced, we quantified PV+ and associated perineuronal net (PNN)+ cell densities in barrel cortex. While PNN+ cell density was not reduced by P7 ethanol, PV cell density decreased by 25% at P90 with no decrease at P14. PNN+ cells in controls were virtually all PV+, whereas more than 20% lacked PV in ethanol-treated adult animals. P7 ethanol caused immediate apoptosis in 10% of GFP+ cells in G42 mice, which express GFP in a subset of PV+ cells, and GFP+ cell density decreased by 60% at P90 without reduction at P14. The ethanol effect on PV+ cell density was attenuated by lithium treatment at P7 or at P14-28. Thus, reduced PV+ cell density may be caused by disrupted cell maturation, in addition to acute apoptosis. This effect may be regionally specific: in the dentate gyrus, P7 ethanol reduced PV+ cell density by 70% at P14 and both PV+ and PNN+ cell densities by 50% at P90, and delayed lithium did not alleviate ethanol's effect.

Neonatal Hypoxia-Ischemia Causes Functional Circuit Changes in Subplate Neurons

Neonatal hypoxia-ischemia (HI) in the preterm human results in damage to subcortical developing white matter and cognitive impairments. Subplate neurons (SPNs) are among the first-born cortical neurons and are necessary for normal cerebral development. While moderate or severe HI at P1 in rats leads to SPN loss, it is unclear if HI, esp. forms not associated with overt cell loss lead to altered SPN circuits. Thus, we used two HI models with different severities in P1 rats. Cauterization of the common carotid artery (CCA) causes a largely transient and thus milder ischemia (HI-Caut) while CCA ligation causes more severe ischemia (HI-Lig). While HI-Lig caused subplate damage, HI-Caut did not cause overt histological damage on the light microscopic level. We used laser-scanning photostimulation (LSPS) in acute thalamocortical slices of auditory cortex during P5-10 to study the functional connectivity of SPNs. Both HI categories resulted in hyperconnectivity of excitatory and inhibitory circuits to SPNs. Thus, alterations on the circuit level are present in the absence of cell loss. Our results show that SPN circuits are uniquely susceptible to HI. Given the key developmental role of SPNs, our results suggest that altered SPN circuits might underlie the abnormal development of cortical function after HI.

The Integrative Function of Silent Synapses on Subplate Neurons in Cortical Development and Dysfunction

The thalamocortical circuit is of central importance in relaying information to the cortex. In development, subplate neurons (SPNs) form an integral part of the thalamocortical pathway. These early born cortical neurons are the first neurons to receive thalamic inputs and excite neurons in the cortical plate. This feed-forward circuit topology of SPNs supports the role of SPNs in shaping the formation and plasticity of thalamocortical connections. Recently it has been shown that SPNs also receive inputs from the developing cortical plate and project to the thalamus. The cortical inputs to SPNs in early ages are mediated by N-methyl-D-aspartate (NMDA)-receptor only containing synapses while at later ages α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptors are present. Thus, SPNs perform a changing integrative function over development. NMDA-receptor only synapses are crucially influenced by the resting potential and thus insults to the developing brain that causes depolarizations, e.g., hypoxia, can influence the integrative function of SPNs. Since such insults in humans cause symptoms of neurodevelopmental disorders, NMDA-receptor only synapses on SPNs might provide a crucial link between early injuries and later circuit dysfunction. We thus here review subplate associated circuits, their changing functions, and discuss possible roles in development and disease.

Thalamic inhibitory circuits and network activity development

Inhibitory circuits in thalamus and cortex shape the major activity patterns observed by electroencephalogram (EEG) in the adult brain. Their delayed maturation and circuit integration, relative to excitatory neurons, suggest inhibitory neuronal development could be responsible for the onset of mature thalamocortical activity. Indeed, the immature brain lacks many inhibition-dependent activity patterns, such as slow-waves, delta oscillations and sleep-spindles, and instead expresses other unique oscillatory activities in multiple species including humans. Thalamus contributes significantly to the generation of these early oscillations. Compared to the abundance of studies on the development of inhibition in cortex, however, the maturation of thalamic inhibition is poorly understood. Here we review developmental changes in the neuronal and circuit properties of the thalamic relay and its interconnected inhibitory thalamic reticular nucleus (TRN) both in vitro and in vivo, and discuss their potential contribution to early network activity and its maturation. While much is unknown, we argue that weak inhibitory function in the developing thalamus allows for amplification of thalamocortical activity that supports the generation of early oscillations. The available evidence suggests that the developmental acquisition of critical thalamic oscillations such as slow-waves and sleep-spindles is driven by maturation of the TRN. Further studies to elucidate thalamic GABAergic circuit formation in relation to thalamocortical network function would help us better understand normal as well as pathological brain development.

Seizure Susceptibility Correlates with Brain Injury in Male Mice Treated with Hypothermia after Neonatal Hypoxia-Ischemia

Hypoxic-ischemic encephalopathy is a common neonatal brain injury associated with significant morbidity and mortality despite the administration of therapeutic hypothermia (TH). Neonatal seizures and subsequent chronic epilepsy are frequent in this patient population and current treatments are partially effective. We used a neonatal murine hypoxia-ischemia (HI) model to test whether the severity of hippocampal and cortical injury predicts seizure susceptibility 8 days after HI and whether TH mitigates this susceptibility. HI at postnatal day 10 (P10) caused hippocampal injury not mitigated by TH in male or female pups. TH did not confer protection against flurothyl seizure susceptibility at P18 in this model. Hippocampal (R2 = 0.33, p = 0.001) and cortical (R2 = 0.33, p = 0.003) injury directly correlated with seizure susceptibility in male but not female pups. Thus, there are sex-specific consequences of neonatal HI on flurothyl seizure susceptibility in a murine neonatal HI model. Further studies are necessary to elucidate the underlying mechanisms of sex dimorphism in seizure susceptibility after neonatal HI.

The Superior Function of the Subplate in Early Neocortical Development

During early development the structure and function of the cerebral cortex is critically organized by subplate neurons (SPNs), a mostly transient population of glutamatergic and GABAergic neurons located below the cortical plate. At the molecular and morphological level SPNs represent a rather diverse population of cells expressing a variety of genetic markers and revealing different axonal-dendritic morphologies. Electrophysiologically SPNs are characterized by their rather mature intrinsic membrane properties and firing patterns. They are connected via electrical and chemical synapses to local and remote neurons, e.g., thalamic relay neurons forming the first thalamocortical input to the cerebral cortex. Therefore SPNs are robustly activated at pre- and perinatal stages by the sensory periphery. Although SPNs play pivotal roles in early neocortical activity, development and plasticity, they mostly disappear by programmed cell death during further maturation. On the one hand, SPNs may be selectively vulnerable to hypoxia-ischemia contributing to brain damage, on the other hand there is some evidence that enhanced survival rates or alterations in SPN distribution may contribute to the etiology of neurological or psychiatric disorders. This review aims to give a comprehensive and up-to-date overview on the many functions of SPNs during early physiological and pathophysiological development of the cerebral cortex.

Long-Term Deficits in Cortical Circuit Function after Asphyxial Cardiac Arrest and Resuscitation in Developing Rats

Cardiac arrest is a common cause of global hypoxic-ischemic brain injury. Poor neurologic outcome among cardiac arrest survivors results not only from direct cellular injury but also from subsequent long-term dysfunction of neuronal circuits. Here, we investigated the long-term impact of cardiac arrest during development on the function of cortical layer IV (L4) barrel circuits in the rat primary somatosensory cortex. We used multielectrode single-neuron recordings to examine responses of presumed excitatory L4 barrel neurons to controlled whisker stimuli in adult (8 ± 2-mo-old) rats that had undergone 9 min of asphyxial cardiac arrest and resuscitation during the third postnatal week. Results indicate that responses to deflections of the topographically appropriate principal whisker (PW) are smaller in magnitude in cardiac arrest survivors than in control rats. Responses to adjacent whisker (AW) deflections are similar in magnitude between the two groups. Because of a disproportionate decrease in PW-evoked responses, receptive fields of L4 barrel neurons are less spatially focused in cardiac arrest survivors than in control rats. In addition, spiking activity among L4 barrel neurons is more correlated in cardiac arrest survivors than in controls. Computational modeling demonstrates that experimentally observed disruptions in barrel circuit function after cardiac arrest can emerge from a balanced increase in background excitatory and inhibitory conductances in L4 neurons. Experimental and modeling data together suggest that after a hypoxic-ischemic insult, cortical sensory circuits are less responsive and less spatially tuned. Modulation of these deficits may represent a therapeutic approach to improving neurologic outcome after cardiac arrest.

Stem Cell-Induced Biobridges as Possible Tools to Aid Neuroreconstruction after CNS Injury

Notch-induced mesenchymal stromal cells (MSCs) mediate a distinct mechanism of repair after brain injury by forming a biobridge that facilitates biodistribution of host cells from a neurogenic niche to the area of injury. We have observed the biobridge in an area between the subventricular zone and the injured cortex using immunohistochemistry and laser capture. Cells in the biobridge express high levels of extracellular matrix metalloproteinases (MMPs), specifically MMP-9, which co-localized with a trail of MSCs graft. The transplanted stem cells then become almost undetectable, being replaced by newly recruited host cells. This stem cell-paved biobridge provides support for distal migration of host cells from the subventricular zone to the site of injury. Biobridge formation by transplanted stem cells seems to have a fundamental role in initiating endogenous repair processes. Two major stem cell-mediated repair mechanisms have been proposed thus far: direct cell replacement by transplanted grafts and bystander effects through the secretion of trophic factors including fibroblast growth factor 2 (FGF-2), epidermal growth factor (EGF), stem cell factor (SCF), erythropoietin, and brain-derived neurotrophic factor (BDNF) among others. This groundbreaking observation of biobridge formation by transplanted stem cells represents a novel mechanism for stem cell mediated brain repair. Future studies on graft-host interaction will likely establish biobridge formation as a fundamental mechanism underlying therapeutic effects of stem cells and contribute to the scientific pursuit of developing safe and efficient therapies not only for traumatic brain injury but also for other neurological disorders. The aim of this review is to hypothetically extend concepts related to the formation of biobridges in other central nervous system disorders.

Neonatal Hypoxia Ischaemia: Mechanisms, Models, and Therapeutic Challenges

Neonatal hypoxia-ischaemia (HI) is the most common cause of death and disability in human neonates, and is often associated with persistent motor, sensory, and cognitive impairment. Improved intensive care technology has increased survival without preventing neurological disorder, increasing morbidity throughout the adult population. Early preventative or neuroprotective interventions have the potential to rescue brain development in neonates, yet only one therapeutic intervention is currently licensed for use in developed countries. Recent investigations of the transient cortical layer known as subplate, especially regarding subplate's secretory role, opens up a novel set of potential molecular modulators of neonatal HI injury. This review examines the biological mechanisms of human neonatal HI, discusses evidence for the relevance of subplate-secreted molecules to this condition, and evaluates available animal models. Neuroserpin, a neuronally released neuroprotective factor, is discussed as a case study for developing new potential pharmacological interventions for use post-ischaemic injury.

Genetic Otx2 mis-localization delays critical period plasticity across brain regions

Accumulation of non-cell autonomous Otx2 homeoprotein in postnatal mouse visual cortex (V1) has been implicated in both the onset and closure of critical period (CP) plasticity. Here, we show that a genetic point mutation in the glycosaminoglycan recognition motif of Otx2 broadly delays the maturation of pivotal parvalbumin-positive (PV+) interneurons not only in V1 but also in the primary auditory (A1) and medial prefrontal cortex (mPFC). Consequently, not only visual, but also auditory plasticity is delayed, including the experience-dependent expansion of tonotopic maps in A1 and the acquisition of acoustic preferences in mPFC, which mitigates anxious behavior. In addition, Otx2 mis-localization leads to dynamic turnover of selected perineuronal net (PNN) components well beyond the normal CP in V1 and mPFC. These findings reveal widespread actions of Otx2 signaling in the postnatal cortex controlling the maturational trajectory across modalities. Disrupted PV+ network function and deficits in PNN integrity are implicated in a variety of psychiatric illnesses, suggesting a potential global role for Otx2 function in establishing mental health.

Unbiased Quantification of Subplate Neuron Loss following Neonatal Hypoxia-Ischemia in a Rat Model

BACKGROUND

Cellular targets of neonatal hypoxia-ischemia (HI) include both oligodendrocyte and neuronal lineages with differences in the patterns of vulnerable cells depending upon the developmental stage at which the injury occurs. Injury to the developing white matter is a characteristic feature of human preterm brain injury. Data are accumulating, however, for neuronal injury in the developing cerebral cortex. In the most widely used rodent model of preterm HI brain injury, conflicting data have been reported regarding the sensitivity of subplate neurons to early neonatal HI, with some reports of selective vulnerability and others that find no increased loss of subplate neurons in comparison with other cortical layers. Methods used to identify subplate neurons and quantify their numbers vary across studies.

OBJECTIVE

To use recently developed cortical layer-specific markers quantified with definitive stereologic methods to determine the magnitude and specificity of subplate neuron cell loss following neonatal HI in a rodent model.

METHODS

Postnatal day 2 (P2) rats underwent right common carotid artery coagulation followed by 2-3 h of hypoxia (5.6% oxygen). Categorically moderately injured brains were stained with subplate and cortical layer III-V markers (Complexin3 and Foxp1, respectively) at P8 and P21 (Foxp1 only). An Optical Fractionator was used to quantify subplate and middle/lower cortical neuronal numbers and these were compared across groups (naive control, hypoxia hemisphere, and HI hemisphere).

RESULTS

Following HI at P2 in rats, the total Complexin3-expressing subplate neuron number decreases significantly in the HI hemisphere compared with naive controls or hypoxia alone (HI vs. control 26,747 ± 7,952 vs. 35,468 ± 8,029, p = 0.04; HI vs. hypoxia, 26,747 ± 7,952 vs. 40,439 ± 7,363, p = 0.003). In contrast, the total Foxp1-expressing layer III-V cell number did not differ across the 3 conditions at P8 (HI vs. control 1,195,085 ± 436,609 vs. 1,234,640 ± 178,540, p = 0.19; HI vs. hypoxia, 1,195,085 ± 436,609 vs. 1,289,195 ± 468,941, p = 0.35) and at P21 (HI vs. control 1,265,190 ± 48,089 vs. 1,195,632 ± 26,912, p = 0.19; HI vs. hypoxia, 1,265,190 ± 48,089 vs. 1,309,563 ± 41,669, p = 0.49).

CONCLUSIONS

There is significant biological variability inherent in both the subplate neuron cell number and the pattern and severity of cortical injury following HI at P2 in rats. Despite this variability, the subplate neuron cell number is lower following P2 HI in animals with mild or moderate cortical injury, whereas the middle-to-lower-layer cortical neuronal number is unchanged. In more severe cases, neurons are lost from the lower cortical layers, suggesting a relative vulnerability of subplate neurons.

Integrative Analysis of Disease Signatures Shows Inflammation Disrupts Juvenile Experience-Dependent Cortical Plasticity

Throughout childhood and adolescence, periods of heightened neuroplasticity are critical for the development of healthy brain function and behavior. Given the high prevalence of neurodevelopmental disorders, such as autism, identifying disruptors of developmental plasticity represents an essential step for developing strategies for prevention and intervention. Applying a novel computational approach that systematically assessed connections between 436 transcriptional signatures of disease and multiple signatures of neuroplasticity, we identified inflammation as a common pathological process central to a diverse set of diseases predicted to dysregulate plasticity signatures. We tested the hypothesis that inflammation disrupts developmental cortical plasticity in vivo using the mouse ocular dominance model of experience-dependent plasticity in primary visual cortex. We found that the administration of systemic lipopolysaccharide suppressed plasticity during juvenile critical period with accompanying transcriptional changes in a particular set of molecular regulators within primary visual cortex. These findings suggest that inflammation may have unrecognized adverse consequences on the postnatal developmental trajectory and indicate that treating inflammation may reduce the burden of neurodevelopmental disorders.

IGF1R+ Dental Pulp Stem Cells Enhanced Neuroplasticity in Hypoxia-Ischemia Model

Until now, the surface markers of multipotent mesenchymal stem cells (MSCs) had not been fully identified. Here, we found that the IGF1 receptor (IGF1R), regarded as a pluripotent marker of embryonic stem cells (ESCs), was also expressed in human dental pulp derived-mesenchymal stem cells (hDSCs), which displayed a potential for both self-renewal and multipotency. hDSC-secreted IGF1 interacted with IGF1R through an autocrine signaling pathway to maintain this self-renewal and proliferation potential. Stereotaxic implantation of immunosorted IGF1R⁺ hDSCs in rats with neonatal hypoxia-ischemia (NHI) promoted neuroplasticity, improving the neurological outcome by increasing expression of the anti-apoptotic protein Bcl-2, which enhanced both neurogenesis and angiogenesis. In addition, treatment with IGF1R⁺ hDSCs significantly modulated neurite regeneration and anti-inflammation in vivo in NHI rats and in vitro in primary cortical cultures under oxygen/glucose deprivation. Autocrine regulatory expression of IGF1R contributed to maintaining the self-renewal capacity of hDSCs. Furthermore, implantation of IGF1R⁺ hDSCs increased neuroplasticity with neurite regeneration and immunomodulation in and the NHI rat model.

Cardiac Arrest-Induced Global Brain Hypoxia-Ischemia during Development Affects Spontaneous Activity Organization in Rat Sensory and Motor Thalamocortical Circuits during Adulthood

Normal maturation of sensory information processing in the cortex requires patterned synaptic activity during developmentally regulated critical periods. During early development, spontaneous synaptic activity establishes required patterns of synaptic input, and during later development it influences patterns of sensory experience-dependent neuronal firing. Thalamocortical neurons occupy a critical position in regulating the flow of patterned sensory information from the periphery to the cortex. Abnormal thalamocortical inputs may permanently affect the organization and function of cortical neuronal circuits, especially if they occur during a critical developmental window. We examined the effect of cardiac arrest (CA)-associated global brain hypoxia-ischemia in developing rats on spontaneous and evoked firing of somatosensory thalamocortical neurons and on large-scale correlations in the motor thalamocortical circuit. The mean spontaneous and sensory-evoked firing rate activity and variability were higher in CA injured rats. Furthermore, spontaneous and sensory-evoked activity and variability were correlated in uninjured rats, but not correlated in neurons from CA rats. Abnormal activity patterns of ventroposterior medial nucleus (VPm) neurons persisted into adulthood. Additionally, we found that neurons in the entopeduncular nucleus (EPN) in the basal ganglia had lower firing rates yet had higher variability and higher levels of burst firing after injury. Correlated levels of power in local field potentials (LFPs) between the EPN and the motor cortex (MCx) were also disrupted by injury. Our findings indicate that hypoxic-ischemic injury during development leads to abnormal spontaneous and sensory stimulus-evoked input patterns from thalamus to cortex. Abnormal thalamic inputs likely permanently and detrimentally affect the organization of cortical circuitry and processing of sensory information. Hypoxic-ischemic injury also leads to abnormal single neuron and population level activity in the basal ganglia that may contribute to motor dysfunction after injury. Combination of deficits in sensory and motor thalamocortical circuit function may negatively impact sensorimotor integration in CA survivors. Modulation of abnormal activity patterns post-injury may represent a novel therapeutic target to improve neurologic function in survivors.

Vascular Dynamics Aid a Coupled Neurovascular Network Learn Sparse Independent Features: A Computational Model

Cerebral vascular dynamics are generally thought to be controlled by neural activity in a unidirectional fashion. However, both computational modeling and experimental evidence point to the feedback effects of vascular dynamics on neural activity. Vascular feedback in the form of glucose and oxygen controls neuronal ATP, either directly or via the agency of astrocytes, which in turn modulates neural firing. Recently, a detailed model of the neuron-astrocyte-vessel system has shown how vasomotion can modulate neural firing. Similarly, arguing from known cerebrovascular physiology, an approach known as “hemoneural hypothesis” postulates functional modulation of neural activity by vascular feedback. To instantiate this perspective, we present a computational model in which a network of “vascular units” supplies energy to a neural network. The complex dynamics of the vascular network, modeled by a network of oscillators, turns neurons ON and OFF randomly. The informational consequence of such dynamics is explored in the context of an auto-encoder network. In the proposed model, each vascular unit supplies energy to a subset of hidden neurons of an autoencoder network, which constitutes its “projective field.” Neurons that receive adequate energy in a given trial have reduced threshold, and thus are prone to fire. Dynamics of the vascular network are governed by changes in the reconstruction error of the auto-encoder network, interpreted as the neuronal demand. Vascular feedback causes random inactivation of a subset of hidden neurons in every trial. We observe that, under conditions of desynchronized vascular dynamics, the output reconstruction error is low and the feature vectors learnt are sparse and independent. Our earlier modeling study highlighted the link between desynchronized vascular dynamics and efficient energy delivery in skeletal muscle. We now show that desynchronized vascular dynamics leads to efficient training in an auto-encoder neural network.

Reduced Cortical Activity Impairs Development and Plasticity after Neonatal Hypoxia Ischemia

Survivors of preterm birth are at high risk of pervasive cognitive and learning impairments, suggesting disrupted early brain development. The limits of viability for preterm birth encompass the third trimester of pregnancy, a "precritical period" of activity-dependent development characterized by the onset of spontaneous and evoked patterned electrical activity that drives neuronal maturation and formation of cortical circuits. Reduced background activity on electroencephalogram (EEG) is a sensitive marker of brain injury in human preterm infants that predicts poor neurodevelopmental outcome. We studied a rodent model of very early hypoxic-ischemic brain injury to investigate effects of injury on both general background and specific patterns of cortical activity measured with EEG. EEG background activity is depressed transiently after moderate hypoxia-ischemia with associated loss of spindle bursts. Depressed activity, in turn, is associated with delayed expression of glutamate receptor subunits and transporters. Cortical pyramidal neurons show reduced dendrite development and spine formation. Complementing previous observations in this model of impaired visual cortical plasticity, we find reduced somatosensory whisker barrel plasticity. Finally, EEG recordings from human premature newborns with brain injury demonstrate similar depressed background activity and loss of bursts in the spindle frequency band. Together, these findings suggest that abnormal development after early brain injury may result in part from disruption of specific forms of brain activity necessary for activity-dependent circuit development.

SIGNIFICANCE STATEMENT

Preterm birth and term birth asphyxia result in brain injury from inadequate oxygen delivery and constitute a major and growing worldwide health problem. Poor outcomes are noted in a majority of very premature (<25 weeks gestation) newborns, resulting in death or life-long morbidity with motor, sensory, learning, behavioral, and language disabilities that limit academic achievement and well-being. Limited progress has been made to develop therapies that improve neurologic outcomes. The overall objective of this study is to understand the effect of early brain injury on activity-dependent brain development and cortical plasticity to develop new treatments that will optimize repair and recovery after brain injury.

Postnatal Erythropoietin Mitigates Impaired Cerebral Cortical Development Following Subplate Loss from Prenatal Hypoxia-Ischemia

Preterm birth impacts brain development and leads to chronic deficits including cognitive delay, behavioral problems, and epilepsy. Premature loss of the subplate, a transient subcortical layer that guides development of the cerebral cortex and axonal refinement, has been implicated in these neurological disorders. Subplate neurons influence postnatal upregulation of the potassium chloride co-transporter KCC2 and maturation of γ-amino-butyric acid A receptor (GABAAR) subunits. We hypothesized that prenatal transient systemic hypoxia-ischemia (TSHI) in Sprague-Dawley rats that mimic brain injury from extreme prematurity in humans would cause premature subplate loss and affect cortical layer IV development. Further, we predicted that the neuroprotective agent erythropoietin (EPO) could attenuate the injury. Prenatal TSHI induced subplate neuronal loss via apoptosis. TSHI impaired cortical layer IV postnatal upregulation of KCC2 and GABAAR subunits, and postnatal EPO treatment mitigated the loss (n ≥ 8). To specifically address how subplate loss affects cortical development, we used in vitro mechanical subplate ablation in slice cultures (n ≥ 3) and found EPO treatment attenuates KCC2 loss. Together, these results show that subplate loss contributes to impaired cerebral development, and EPO treatment diminishes the damage. Limitation of premature subplate loss and the resultant impaired cortical development may minimize cerebral deficits suffered by extremely preterm infants.

Non-invasive imaging and analysis of cerebral ischemia in living rats using positron emission tomography with 18F-FDG

Stroke is the third leading cause of death among Americans 65 years of age or older(1). The quality of life for patients who suffer from a stroke fails to return to normal in a large majority of patients(2), which is mainly due to current lack of clinical treatment for acute stroke. This necessitates understanding the physiological effects of cerebral ischemia on brain tissue over time and is a major area of active research. Towards this end, experimental progress has been made using rats as a preclinical model for stroke, particularly, using non-invasive methods such as (18)F-fluorodeoxyglucose (FDG) coupled with Positron Emission Tomography (PET) imaging(3,10,17). Here we present a strategy for inducing cerebral ischemia in rats by middle cerebral artery occlusion (MCAO) that mimics focal cerebral ischemia in humans, and imaging its effects over 24 hr using FDG-PET coupled with X-ray computed tomography (CT) with an Albira PET-CT instrument. A VOI template atlas was subsequently fused to the cerebral rat data to enable a unbiased analysis of the brain and its sub-regions(4). In addition, a method for 3D visualization of the FDG-PET-CT time course is presented. In summary, we present a detailed protocol for initiating, quantifying, and visualizing an induced ischemic stroke event in a living Sprague-Dawley rat in three dimensions using FDG-PET.

Process of cortical network formation and impact of early brain damage

PURPOSE OF REVIEW

The aim is to review mechanisms that are central to the formation of proper cortical circuitry and relevant to perinatal brain injury and premature birth.

RECENT FINDINGS

Clinical investigations using noninvasive imaging techniques suggest that impaired connectivity of cortical circuitry is associated with perinatal adverse conditions. Recent experimental and translational studies revealed developmental mechanisms that are critical for circuit formation and potentially at risk in the perinatal period. These include existence of last wave genesis, migration and integration of gamma-aminobutyric acid (GABA) interneurons in the perinatal period; maturation of GABA interneuron networks that are central to critical period plasticity; transient connections by subplate neurons that guide thalamocortical connectivity, and a perineuronal microglia network that maintains axonal growth and neuronal survival as well as executing synaptic pruning. In addition, recent work has demonstrated that birth plays a key role in triggering the maturation cascade of cortical circuits.

SUMMARY

Altered maturation of cortical circuits is an increasingly recognized aspect of perinatal injury and premature birth. Potential mechanisms are revealed but further translational studies are required to associate fine changes at the cellular and molecular level with imaging data in experimental models.

Subplate in a rat model of preterm hypoxia-ischemia

Objective

Hypoxia–ischemia (HI) in preterm infants primarily leads to injuries in the cerebral white matter. However, there is growing evidence that perinatal injury in preterms can also involve other zones including the cortical gray matter. In a neonatal rat model of HI, selective vulnerability of subplate has been suggested using BrdU birth-dating methods. In this study, we aimed to investigate the neuropathological changes of the subplate and deep layers of the cortex following cerebral HI in neonatal rats with specific cell markers.

Methods

P2 rats underwent permanent occlusion of the right common carotid artery followed by a period of hypoxia. P8 rats were analyzed using immunohistochemistry; subplate and deep layers cells were quantified and compared with sham-operated case.

Results

A large variability in the extent of the cerebral injury was apparent. For the three analyzed subplate populations (Nurr1+, Cplx3+, and Ctgf+ cells), no significant cell reduction was observed in mild and moderate cases. Only in severe cases, subplate cells were strongly affected, but these injuries were always accompanied by the cell reductions in layers VI and V.

Interpretation

We could therefore not confirm a specific vulnerability of subplate cells compared to other deep layers or the white matter in our model.

Interneurons from embryonic development to cell-based therapy

Many neurologic and psychiatric disorders are marked by imbalances between neural excitation and inhibition. In the cerebral cortex, inhibition is mediated largely by GABAergic (γ-aminobutyric acid-secreting) interneurons, a cell type that originates in the embryonic ventral telencephalon and populates the cortex through long-distance tangential migration. Remarkably, when transplanted from embryos or in vitro culture preparations, immature interneurons disperse and integrate into host brain circuits, both in the cerebral cortex and in other regions of the central nervous system. These features make interneuron transplantation a powerful tool for the study of neurodevelopmental processes such as cell specification, cell death, and cortical plasticity. Moreover, interneuron transplantation provides a novel strategy for modifying neural circuits in rodent models of epilepsy, Parkinson's disease, mood disorders, and chronic pain.

In vivo visuotopic brain mapping with manganese-enhanced MRI and resting-state functional connectivity MRI

The rodents are an increasingly important model for understanding the mechanisms of development, plasticity, functional specialization and disease in the visual system. However, limited tools have been available for assessing the structural and functional connectivity of the visual brain network globally, in vivo and longitudinally. There are also ongoing debates on whether functional brain connectivity directly reflects structural brain connectivity. In this study, we explored the feasibility of manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administration for visuotopic brain mapping and understanding of physiological transport in normal and visually deprived adult rats. In addition, resting-state functional connectivity MRI (RSfcMRI) was performed to evaluate the intrinsic functional network and structural-functional relationships in the corresponding anatomical visual brain connections traced by MEMRI. Upon intravitreal, subcortical, and intracortical Mn(2+) injection, different topographic and layer-specific Mn enhancement patterns could be revealed in the visual cortex and subcortical visual nuclei along retinal, callosal, cortico-subcortical, transsynaptic and intracortical horizontal connections. Loss of visual input upon monocular enucleation to adult rats appeared to reduce interhemispheric polysynaptic Mn(2+) transfer but not intra- or inter-hemispheric monosynaptic Mn(2+) transport after Mn(2+) injection into visual cortex. In normal adults, both structural and functional connectivity by MEMRI and RSfcMRI was stronger interhemispherically between bilateral primary/secondary visual cortex (V1/V2) transition zones (TZ) than between V1/V2 TZ and other cortical nuclei. Intrahemispherically, structural and functional connectivity was stronger between visual cortex and subcortical visual nuclei than between visual cortex and other subcortical nuclei. The current results demonstrated the sensitivity of MEMRI and RSfcMRI for assessing the neuroarchitecture, neurophysiology and structural-functional relationships of the visual brains in vivo. These may possess great potentials for effective monitoring and understanding of the basic anatomical and functional connections in the visual system during development, plasticity, disease, pharmacological interventions and genetic modifications in future studies.

Brain injury in premature neonates: A primary cerebral dysmaturation disorder?

With advances in neonatal care, preterm neonates are surviving with an evolving constellation of motor and cognitive disabilities that appear to be related to widespread cellular maturational disturbances that target cerebral gray and white matter. Whereas preterm infants were previously at high risk for destructive brain lesions that resulted in cystic white matter injury and secondary cortical and subcortical gray matter degeneration, contemporary cohorts of preterm survivors commonly display less severe injury that does not appear to involve pronounced glial or neuronal loss. Nevertheless, these milder forms of injury are also associated with reduced cerebral growth. Recent human and experimental studies support that impaired cerebral growth is related to disparate responses in gray and white matter. Myelination disturbances in cerebral white matter are related to aberrant regeneration and repair responses to acute death of premyelinating late oligodendrocyte progenitors (preOLs). In response to preOL death, early oligodendrocyte progenitors rapidly proliferate and differentiate, but the regenerated preOLs fail to normally mature to myelinating cells required for white matter growth. Although immature neurons appear to be more resistant to cell death from hypoxia-ischemia than glia, they display widespread disturbances in maturation of their dendritic arbors, which further contribute to impaired cerebral growth. These complex and disparate responses of neurons and preOLs thus result in large numbers of cells that fail to fully mature during a critical window in development of neural circuitry. These recently recognized forms of cerebral gray and white matter dysmaturation raise new diagnostic challenges and suggest new therapeutic directions centered on reversal of the processes that promote dysmaturation.

Hypoxia-induced developmental delays of inhibitory interneurons are reversed by environmental enrichment in the postnatal mouse forebrain

Infants born premature experience hypoxic episodes due to immaturity of their respiratory and central nervous systems. This profoundly affects brain development and results in cognitive impairments. We used a mouse model to examine the impact of hypoxic rearing (9.5-10.5% O2) from postnatal day 3 to 11 (P3-P11) on GABAergic interneurons and the potential for environmental enrichment to ameliorate these developmental abnormalities. At P15 the numbers of cortical interneurons expressing immunohistochemically detectable levels of parvalbumin (PV), somatostatin (SST), and vasoactive intestinal peptide were decreased in hypoxic-reared mice by 59%, 32%, and 38%, respectively, compared with normoxic controls. Hypoxia also decreased total GABA content in frontal neocortex by 31%. However, GAD67-EGFP knock-in mice reared under hypoxic conditions showed no changes in total number of GAD67-EGFP(+) cells and no evidence of increased interneuron death, suggesting that the total number of interneurons was not decreased, but rather, that hypoxic-rearing decreased interneuron marker expression in these cells. In adulthood, PV and SST expression levels were decreased in hypoxic-reared mice. In contrast, intensity of reelin (RLN) expression was significantly increased in adult hypoxic-reared mice compared with normoxic controls. Housing mice in an enriched environment from P21 until adulthood normalized phenotypic interneuron marker expression without affecting total interneuron numbers or leading to increased neurogenesis. Our data show that (1) hypoxia decreases PV and SST and increases RLN expression in cortical interneurons during postnatal cortical development and (2) enriched environment has the capacity to normalize the interneuron abnormalities in cortex.

A theory of the transition to critical period plasticity: inhibition selectively suppresses spontaneous activity

What causes critical periods (CPs) to open? For the best-studied case, ocular dominance plasticity in primary visual cortex in response to monocular deprivation (MD), the maturation of inhibition is necessary and sufficient. How does inhibition open the CP? We present a theory: the transition from pre-CP to CP plasticity arises because inhibition preferentially suppresses responses to spontaneous relative to visually driven input activity, switching learning cues from internal to external sources. This differs from previous proposals in (1) arguing that the CP can open without changes in plasticity mechanisms when activity patterns become more sensitive to sensory experience through circuit development, and (2) explaining not simply a transition from no plasticity to plasticity, but a change in outcome of MD-induced plasticity from pre-CP to CP. More broadly, hierarchical organization of sensory-motor pathways may develop through a cascade of CPs induced as circuit maturation progresses from "lower" to "higher" cortical areas.

Non-injurious neonatal hypoxia confers resistance to brain senescence in aged male rats

Whereas brief acute or intermittent episodes of hypoxia have been shown to exert a protective role in the central nervous system and to stimulate neurogenesis, other studies suggest that early hypoxia may constitute a risk factor that influences the future development of mental disorders. We therefore investigated the effects of a neonatal “conditioning-like” hypoxia (100% N₂, 5 min) on the brain and the cognitive outcomes of rats until 720 days of age (physiologic senescence). We confirmed that such a short hypoxia led to brain neurogenesis within the ensuing weeks, along with reduced apoptosis in the hippocampus involving activation of Erk1/2 and repression of p38 and death-associated protein (DAP) kinase. At 21 days of age, increased thicknesses and cell densities were recorded in various subregions, with strong synapsin activation. During aging, previous exposure to neonatal hypoxia was associated with enhanced memory retrieval scores specifically in males, better preservation of their brain integrity than controls, reduced age-related apoptosis, larger hippocampal cell layers, and higher expression of glutamatergic and GABAergic markers. These changes were accompanied with a marked expression of synapsin proteins, mainly of their phosphorylated active forms which constitute major players of synapse function and plasticity, and with increases of their key regulators, i.e. Erk1/2, the transcription factor EGR-1/Zif-268 and Src kinase. Moreover, the significantly higher interactions between PSD-95 scaffolding protein and NMDA receptors measured in the hippocampus of 720-day-old male animals strengthen the conclusion of increased synaptic functional activity and plasticity associated with neonatal hypoxia. Thus, early non-injurious hypoxia may trigger beneficial long term effects conferring higher resistance to senescence in aged male rats, with a better preservation of cognitive functions.

Thalamocortical dysfunction and thalamic injury after asphyxial cardiac arrest in developing rats

Global hypoxia-ischemia interrupts oxygen delivery and blood flow to the entire brain. Previous studies of global brain hypoxia-ischemia have primarily focused on injury to the cerebral cortex and to the hippocampus. Susceptible neuronal populations also include inhibitory neurons in the thalamic reticular nucleus. We therefore investigated the impact of global brain hypoxia-ischemia on the thalamic circuit function in the somatosensory system of young rats. We used single neuron recordings and controlled whisker deflections to examine responses of thalamocortical neurons to sensory stimulation in rat survivors of 9 min of asphyxial cardiac arrest incurred on postnatal day 17. We found that 48-72 h after cardiac arrest, thalamocortical neurons demonstrate significantly elevated firing rates both during spontaneous activity and in response to whisker deflections. The elevated evoked firing rates persist for at least 6-8 weeks after injury. Despite the overall increase in firing, by 6 weeks, thalamocortical neurons display degraded receptive fields, with decreased responses to adjacent whiskers. Nine minutes of asphyxial cardiac arrest was associated with extensive degeneration of neurites in the somatosensory nucleus as well as activation of microglia in the reticular nucleus. Global brain hypoxia-ischemia during cardiac arrest has a long-term impact on processing and transfer of sensory information by thalamic circuitry. Thalamic circuitry and normalization of its function may represent a distinct therapeutic target after cardiac arrest.

Early alcohol exposure disrupts visual cortex plasticity in mice

There is growing evidence that deficits in neuronal plasticity underlie the cognitive problems seen in fetal alcohol spectrum disorders (FASD). However, the mechanisms behind these deficits are not clear. Here we test the effects of early alcohol exposure on ocular dominance plasticity (ODP) in mice and the reversibility of these effects by phosphodiesterase (PDE) inhibitors. Mouse pups were exposed to 5 g/kg of 25% ethanol i.p. on postnatal days (P) 5, 7 and 9. This type of alcohol exposure mimics binge drinking during the third trimester equivalent of human gestation. To assess ocular dominance plasticity animals were monocularly deprived at P21 for 10 days, and tested using optical imaging of intrinsic signals. During the period of monocular deprivation animals were treated with vinpocetine (20mg/kg; PDE1 inhibitor), rolipram (1.25mg/kg; PDE4 inhibitor), vardenafil (3mg/kg; PDE5 inhibitor) or vehicle solution. Monocular deprivation resulted in the expected shift in ocular dominance of the binocular zone in saline controls but not in the ethanol group. While vinpocetine successfully restored ODP in the ethanol group, rolipram and vardenafil did not. However, when rolipram and vardenafil were given simultaneously ODP was restored. PDE4 and PDE5 are specific to cAMP and cGMP respectively, while PDE1 acts on both of these nucleotides. Our findings suggest that the combined activation of the cAMP and cGMP cascades may be a good approach to improve neuronal plasticity in FASD models.

Changing microcircuits in the subplate of the developing cortex

Subplate neurons (SPNs) are a population of neurons in the mammalian cerebral cortex that exist predominantly in the prenatal and early postnatal period. Loss of SPNs prevents the functional maturation of the cerebral cortex. SPNs receive subcortical input from the thalamus and relay this information to the developing cortical plate and thereby can influence cortical activity in a feedforward manner. Little is known about potential feedback projections from the cortical plate to SPNs. Thus, we investigated the spatial distribution of intracortical synaptic inputs to SPNs in vitro in mouse auditory cortex by photostimulation. We find that SPNs fell into two broad classes based on their distinct spatial patterns of synaptic inputs. The first class of SPNs receives inputs from only deep cortical layers, while the second class of SPNs receives inputs from deep as well as superficial layers including layer 4. We find that superficial cortical inputs to SPNs emerge in the second postnatal week and that SPNs that receive superficial cortical input are located more superficially than those that do not. Our data thus suggest that distinct circuits are present in the subplate and that, while SPNs participate in an early feedforward circuit, they are also involved in a feedback circuit at older ages. Together, our results show that SPNs are tightly integrated into the developing thalamocortical and intracortical circuit. The feedback projections from the cortical plate might enable SPNs to amplify thalamic inputs to SPNs.

Neonatal fluoxetine exposure affects the action potential properties and dendritic development in cortical subplate neurons of rats

Selective serotonin reuptake inhibitor (SSRI)-type antidepressants might be given to depressive pregnant women and the developing fetuses are thus exposed to these drugs. Since serotonin plays important roles in the maturation of the nervous system, early SSRI exposure might influence the fetal brain development. To test this hypothesis, we treated the neonatal rat pups with fluoxetine (Flx) from the day of birth to postnatal day (P) 4, comparable to the third trimester of human gestation, and observed the physiological and morphological features of subplate neurons (SPns), a group of cells important for early cortical development and vulnerable to neonatal neural insults. Using whole-cell patch-clamp recording technique, we examined the passive membrane properties and characteristics of action potential (AP). In SPns of Flx-treated rats, the rheobase for generating an AP was increased and the width of APs was reduced, especially in the falling phase. In the morphological aspect, the dendritic remodeling of SPns including dendritic branching, elongation and pruning were affected by early Flx treatment. Together, our results demonstrate that the teratogenic effect of early SSRI exposure on the structure and function of developing SPns and these changes may lead to undesired brain activity and distorted behaviors later in life.

Global impairment and therapeutic restoration of visual plasticity mechanisms after a localized cortical stroke

We tested the influence of a photothrombotic lesion in somatosensory cortex on plasticity in the mouse visual system and the efficacy of anti-inflammatory treatment to rescue compromised learning. To challenge plasticity mechanisms, we induced monocular deprivation (MD) in 3-mo-old mice. In control animals, MD induced an increase of visual acuity of the open eye and an ocular dominance (OD) shift towards this eye. In contrast, after photothrombosis, there was neither an enhancement of visual acuity nor an OD-shift. However, OD-plasticity was present in the hemisphere contralateral to the lesion. Anti-inflammatory treatment restored sensory learning but not OD-plasticity, as did a 2-wk delay between photothrombosis and MD. We conclude that (i) both sensory learning and cortical plasticity are compromised in the surround of a cortical lesion; (ii) transient inflammation is responsible for impaired sensory learning, suggesting anti-inflammatory treatment as a useful adjuvant therapy to support rehabilitation following stroke; and (iii) OD-plasticity cannot be conceptualized solely as a local process because nonlocal influences are more important than previously assumed.

Harnessing neuroplasticity for clinical applications

Neuroplasticity can be defined as the ability of the nervous system to respond to intrinsic or extrinsic stimuli by reorganizing its structure, function and connections. Major advances in the understanding of neuroplasticity have to date yielded few established interventions. To advance the translation of neuroplasticity research towards clinical applications, the National Institutes of Health Blueprint for Neuroscience Research sponsored a workshop in 2009. Basic and clinical researchers in disciplines from central nervous system injury/stroke, mental/addictive disorders, paediatric/developmental disorders and neurodegeneration/ageing identified cardinal examples of neuroplasticity, underlying mechanisms, therapeutic implications and common denominators. Promising therapies that may enhance training-induced cognitive and motor learning, such as brain stimulation and neuropharmacological interventions, were identified, along with questions of how best to use this body of information to reduce human disability. Improved understanding of adaptive mechanisms at every level, from molecules to synapses, to networks, to behaviour, can be gained from iterative collaborations between basic and clinical researchers. Lessons can be gleaned from studying fields related to plasticity, such as development, critical periods, learning and response to disease. Improved means of assessing neuroplasticity in humans, including biomarkers for predicting and monitoring treatment response, are needed. Neuroplasticity occurs with many variations, in many forms, and in many contexts. However, common themes in plasticity that emerge across diverse central nervous system conditions include experience dependence, time sensitivity and the importance of motivation and attention. Integration of information across disciplines should enhance opportunities for the translation of neuroplasticity and circuit retraining research into effective clinical therapies.

Subplate in the developing cortex of mouse and human

The subplate is a largely transient zone containing precocious neurons involved in several key steps of cortical development. The majority of subplate neurons form a compact layer in mouse, but are dispersed throughout a much larger zone in the human. In rodent, subplate neurons are among the earliest born neocortical cells, whereas in primate, neurons are added to the subplate throughout cortical neurogenesis. Magnetic resonance imaging and histochemical studies show that the human subplate grows in size until the end of the second trimester. Previous microarray experiments in mice have shown several genes that are specifically expressed in the subplate layer of the rodent dorsal cortex. Here we examined the human subplate for some of these markers. In the human dorsal cortex, connective tissue growth factor-positive neurons can be seen in the ventricular zone at 15-22 postconceptional weeks (PCW) (most at 17 PCW) and are present in the subplate at 22 PCW. The nuclear receptor-related 1 protein is mostly expressed in the subplate in the dorsal cortex, but also in lower layer 6 in the lateral and perirhinal cortex, and can be detected from 12 PCW. Our results suggest that connective tissue growth factor- and nuclear receptor-related 1-positive cells are two distinct cell populations of the human subplate. Furthermore, our microarray analysis in rodent suggested that subplate neurons produce plasma proteins. Here we demonstrate that the human subplate also expresses α2zinc-binding globulin and Alpha-2-Heremans-Schmid glycoprotein/human fetuin. In addition, the established subplate neuron marker neuropeptide Y is expressed superficially, whereas potassium/chloride co-transporter (KCC2)-positive neurons are localized in the deep subplate at 16 PCW. These observations imply that the human subplate shares gene expression patterns with rodent, but is more compartmentalized into superficial and deep sublayers. This increased complexity of the human subplate may contribute to differential vulnerability in response to hypoxia/ischaemia across the depth of the cortex. Combining knowledge of cell-type specific subplate gene expression with modern imaging methods will enable a better understanding of neuropathologies involving the subplate.