Effects of interferons and hydrogen peroxide on CA3 pyramidal cells in rat hippocampal slice cultures

The Chemokine CCL2 Promotes Excitatory Synaptic Transmission in Hippocampal Neurons via GluA1 Subunit Trafficking

The CC chemokine ligand 2 (CCL2, also known as MCP-1) and its cognate receptor CCR2 have well-characterized roles in chemotaxis. CCL2 has been previously shown to promote excitatory synaptic transmission and neuronal excitability. However, the detailed molecular mechanism underlying this process remains largely unclear. In cultured hippocampal neurons, CCL2 application rapidly upregulated surface expression of GluA1, in a CCR2-dependent manner, assayed using SEP-GluA1 live imaging, surface GluA1 antibody staining, and electrophysiology. Using pharmacology and reporter assays, we further showed that CCL2 upregulated surface GluA1 expression primarily via Gαq- and CaMKII-dependent signaling. Consistently, using i.p. injection of lipopolysaccharide to induce neuroinflammation, we found upregulated phosphorylation of S831 and S845 sites on AMPA receptor subunit GluA1 in the hippocampus, an effect blocked in Ccr2-/- mice. Together, these results provide a mechanism through which CCL2, and other secreted molecules that signal through G-protein coupled receptors, can directly regulate synaptic transmission.

Unique aspects of IFN-γ/STAT1 signaling in neurons

The IFN-γ/STAT1 immune signaling pathway impacts many homeostatic and pathological aspects of neurons, beyond its canonical role in controlling intracellular pathogens. Well known for its potent pro-inflammatory and anti-viral functions in the periphery, the IFN-γ/STAT1 pathway is rapidly activated then deactivated to prevent excessive inflammation; however, neurons utilize unique IFN-γ/STAT1 activation patterns, which may contribute to the non-canonical neuron-specific downstream effects. Though it is now well-established that the immune system interacts and supports the CNS in health and disease, many aspects regarding IFN-γ production in the CNS and how neurons respond to IFN-γ are unclear. Additionally, it is not well understood how the diversity of the IFN-γ/STAT1 pathway is regulated in neurons to control homeostatic functions, support immune surveillance, and prevent pathologies. In this review, we discuss the neuron-specific mechanisms and kinetics of IFN-γ/STAT1 activation, the potential sources and entry sites of IFN-γ in the CNS, and the diverse set of homeostatic and pathological effects IFN-γ/STAT1 signaling in neurons has on CNS health and disease. We will also highlight the different contexts and conditions under which IFN-γ-induced STAT1 activation has been studied in neurons, and how various factors might contribute to the vast array of downstream effects observed.

Interferon-γ augments GABA release in the developing neocortex via nitric oxide synthase/soluble guanylate cyclase and constrains network activity

Interferon-γ (IFN-γ), a cytokine with neuromodulatory properties, has been shown to enhance inhibitory transmission. Because early inhibitory neurotransmission sculpts functional neuronal circuits, its developmental alteration may have grave consequences. Here, we investigated the acute effects of IFN-γ on γ-amino-butyric acid (GABA)ergic currents in layer 5 pyramidal neurons of the somatosensory cortex of rats at the end of the first postnatal week, a period of GABA-dependent cortical maturation. IFN-γ acutely increased the frequency and amplitude of spontaneous/miniature inhibitory postsynaptic currents (s/mIPSC), and this could not be reversed within 30 min. Neither the increase in amplitude nor frequency of IPSCs was due to upregulated interneuron excitability as revealed by current clamp recordings of layer 5 interneurons labeled with VGAT-Venus in transgenic rats. As we previously reported in more mature animals, IPSC amplitude increase upon IFN-γ activity was dependent on postsynaptic protein kinase C (PKC), indicating a similar activating mechanism. Unlike augmented IPSC amplitude, however, we did not consistently observe an increased IPSC frequency in our previous studies on more mature animals. Focusing on increased IPSC frequency, we have now identified a different activating mechanism-one that is independent of postsynaptic PKC but is dependent on inducible nitric oxide synthase (iNOS) and soluble guanylate cyclase (sGC). In addition, IFN-γ shifted short-term synaptic plasticity toward facilitation as revealed by a paired-pulse paradigm. The latter change in presynaptic function was not reproduced by the application of a nitric oxide donor. Functionally, IFN-γ-mediated alterations in GABAergic transmission overall constrained early neocortical activity in a partly nitric oxide-dependent manner as revealed by microelectrode array field recordings in brain slices analyzed with a spike-sorting algorithm. In summary, with IFN-γ-induced, NO-dependent augmentation of spontaneous GABA release, we have here identified a mechanism by which inflammation in the central nervous system (CNS) plausibly modulates neuronal development.

Interferon-γ enhances neocortical synaptic inhibition by promoting membrane association and phosphorylation of GABAA receptors in a protein kinase C-dependent manner

Interferon-γ (IFN-γ), an important mediator of the antiviral immune response, can also act as a neuromodulator. CNS IFN-γ levels rise acutely in response to infection and therapeutically applied IFN-γ provokes CNS related side effects. Moreover, IFN-γ plays a key role in neurophysiological processes and a variety of chronic neurological and neuropsychiatric conditions. To close the gap between basic research, behavioral implications and clinical applicability, knowledge of the mechanism behind IFN-γ related changes in brain function is crucial. Here, we studied the underlying mechanism of acutely augmented neocortical inhibition by IFN-γ (1.000 IU ml^-1) in layer 5 pyramidal neurons of male Wistar rats. We demonstrate postsynaptic mediation of IFN-γ augmented inhibition by pressure application of GABA and analysis of paired pulse ratios. IFN-γ increases membrane presence of GABA_AR γ₂, as quantified by cell surface biotinylation and functional synaptic GABA_AR number, as determined by peak-scaled non-stationary noise analysis. The increase in functional receptor number was comparable to the increase in underlying miniature inhibitory postsynaptic current (mIPSC) amplitudes. Blockage of putative intracellular mediators, namely phosphoinositide 3-kinase and protein kinase C (PKC) by Wortmannin and Calphostin C, respectively, revealed PKC-dependency of the pro-inhibitory IFN-γ effect. This was corroborated by increased serine phosphorylation of P-serine PKC motifs on GABA_AR γ₂ upon IFN-γ application. GABA_AR single channel conductance, intracellular chloride levels and GABA_AR driving force are unlikely to contribute to the effect, as shown by single channel recordings and chloride imaging. The effect of IFN-γ on mIPSC amplitudes was similar in female and male rats, suggesting a gender-independent mechanism of action. Collectively, these results indicate a novel mechanism for the regulation of inhibition by IFN-γ, which could impact on neocortical function and therewith behavior.

Partial microglial depletion is associated with impaired hippocampal synaptic and cognitive function in young and aged rats

The role of microglia in mediating age-related changes in cognition and hippocampal synaptic function was examined by microglial depletion and replenishment using PLX3397. We observed age-related differences in microglial number and morphology, as well as increased Iba-1 expression, indicating microglial activation. PLX3397 treatment decreased microglial number, with aged rats exhibiting the lowest density. Young rats exhibited increased expression of pro-inflammatory cytokines during depletion and repopulation and maintenance of Iba-1 levels despite reduced microglial number. For aged rats, several cytokines increased with depletion and recovered during repopulation; however, aged rats did not fully recover microglial cell number or Iba-1 expression during repopulation, with a recovery comparable to young control levels rather than aged controls. Hippocampal CA3-CA1 synaptic transmission was impaired with age, and microglial depletion was associated with decreased total synaptic transmission in young and aged rats. A robust decline in N-methyl-d-aspartate-receptor-mediated synaptic transmission arose in young depleted rats specifically. Microglial replenishment normalized depletion-induced synaptic function to control levels; however, recovery of aged animals did not mirror young. Microglial depletion was associated with decreased context-object discrimination memory in both age groups, which recovered with microglial repopulation. Aged rats displayed impaired contextual and cued fear memory, and microglial replenishment did not recover their memory to the level of young. The current study indicates that cognitive function and synaptic transmission benefit from the support of aged microglia and are hindered by removal of these cells. Replenishment of microglia in aging did not ameliorate age-related cognitive impairments or senescent synaptic function.

Interferon-γ acutely augments inhibition of neocortical layer 5 pyramidal neurons

BACKGROUND

Interferon-γ (IFN-γ, a type II IFN) is present in the central nervous system (CNS) under various conditions. Evidence is emerging that, in addition to its immunological role, IFN-γ modulates neuronal morphology, function, and development in several brain regions. Previously, we have shown that raising levels of IFN-β (a type I IFN) lead to increased neuronal excitability of neocortical layer 5 pyramidal neurons. Because of shared non-canonical signaling pathways of both cytokines, we hypothesized a similar neocortical role of acutely applied IFN-γ.

METHODS

We used semi-quantitative RT-PCR, immunoblotting, and immunohistochemistry to analyze neuronal expression of IFN-γ receptors and performed whole-cell patch-clamp recordings in layer 5 pyramidal neurons to investigate sub- and suprathreshold excitability, properties of hyperpolarization-activated cyclic nucleotide-gated current (I_h), and inhibitory neurotransmission under the influence of acutely applied IFN-γ.

RESULTS

We show that IFN-γ receptors are present in the membrane of rat's neocortical layer 5 pyramidal neurons. As expected from this and the putative overlap in IFN type I and II alternative signaling pathways, IFN-γ diminished I_h, mirroring the effect of type I IFNs, suggesting a likewise activation of protein kinase C (PKC). In contrast, IFN-γ did neither alter subthreshold nor suprathreshold neuronal excitability, pointing to augmented inhibitory transmission by IFN-γ. Indeed, IFN-γ increased electrically evoked inhibitory postsynaptic currents (IPSCs) on neocortical layer 5 pyramidal neurons. Furthermore, amplitudes of spontaneous IPSCs and miniature IPSCs were elevated by IFN-γ, whereas their frequency remained unchanged.

CONCLUSIONS

The expression of IFN-γ receptors on layer 5 neocortical pyramidal neurons together with the acute augmentation of inhibition in the neocortex by direct application of IFN-γ highlights an additional interaction between the CNS and immune system. Our results strengthen our understanding of the role of IFN-γ in neocortical neurotransmission and emphasize its impact beyond its immunological properties, particularly in the pathogenesis of neuropsychiatric disorders.

Interferon-γ potentiates GABAA receptor-mediated inhibitory currents in rat hippocampal CA1 pyramidal neurons

The neural transmission and plasticity can be differentially modulated by various elements of the immune system. Interferon-γ (IFN-γ) is a "pro-inflammatory" cytokine mainly produced by T lymphocytes, activates its corresponding receptor and plays important roles under both homeostatic and inflammatory conditions. However, the impact of IFN-γ on the γ-aminobutyric acid (GABA)-mediated currents in the hippocampus, a major brain region involved in the cognitive function, has not been investigated. Here we detected abundant expression of both IFN-γ receptor subunit gene transcripts (Ifngr1 and Ifngr2) in the rat hippocampus by quantitative PCR. In addition, we pre-incubated rat hippocampal slices with IFN-γ (100 ng/ml) and recorded GABA-activated spontaneous and miniature postsynaptic inhibitory currents (sIPSCs and mIPSCs) and tonic currents in hippocampal CA1 pyramidal neurons by the whole-cell patch-clamp method. The pre-incubation with IFN-γ increased the frequency but not the mean amplitude, rise time or decay time of both sIPSCs and mIPSCs in hippocampal CA1 pyramidal neurons, suggesting a presynaptic effect of IFN-γ. Moreover, the GABA-activated tonic currents were enhanced by IFN-γ. In conclusion, the potentiation of GABAergic currents in hippocampal neurons by IFN-γ may contribute to the disturbed neuronal excitability and cognitive dysfunction during neuroinflammation.

Restoring synaptic plasticity and memory in mouse models of Alzheimer's disease by PKR inhibition

Alzheimer’s disease (AD) is a neurodegenerative disorder associated with deficits in cognition and synaptic plasticity. While accumulation of amyloid β (Aβ) and hyper-phosphorylation of tau are parts of the etiology, AD can be caused by a large number of different genetic mutations and other unknown factors. Considering such a heterogeneous nature of AD, it would be desirable to develop treatment strategies that can improve memory irrespective of the individual causes. Reducing the phosphorylation of eukaryotic translation initiation factor 2α (eIF2α) was shown to enhance long-term memory and synaptic plasticity in naïve mice. Moreover, hyper-phosphorylation of eIF2α is observed in the brains of postmortem AD patients. Therefore, regulating eIF2α phosphorylation can be a plausible candidate for restoring memory in AD by targeting memory-enhancing mechanism. In this study, we examined whether PKR inhibition can rescue synaptic and learning deficits in two different AD mouse models; 5XFAD transgenic and Aβ_1–42-injected mice. We found that the acute treatment of PKR inhibitor (PKRi) can restore the deficits in long-term memory and long-term potentiation (LTP) in both mouse models without affecting the Aβ load in the hippocampus. Our results prove the principle that targeting memory enhancing mechanisms can be a valid candidate for developing AD treatment.

Brain Endothelial- and Epithelial-Specific Interferon Receptor Chain 1 Drives Virus-Induced Sickness Behavior and Cognitive Impairment

Sickness behavior and cognitive dysfunction occur frequently by unknown mechanisms in virus-infected individuals with malignancies treated with type I interferons (IFNs) and in patients with autoimmune disorders. We found that during sickness behavior, single-stranded RNA viruses, double-stranded RNA ligands, and IFNs shared pathways involving engagement of melanoma differentiation-associated protein 5 (MDA5), retinoic acid-inducible gene 1 (RIG-I), and mitochondrial antiviral signaling protein (MAVS), and subsequently induced IFN responses specifically in brain endothelia and epithelia of mice. Behavioral alterations were specifically dependent on brain endothelial and epithelial IFN receptor chain 1 (IFNAR). Using gene profiling, we identified that the endothelia-derived chemokine ligand CXCL10 mediated behavioral changes through impairment of synaptic plasticity. These results identified brain endothelial and epithelial cells as natural gatekeepers for virus-induced sickness behavior, demonstrated tissue specific IFNAR engagement, and established the CXCL10-CXCR3 axis as target for the treatment of behavioral changes during virus infection and type I IFN therapy.

Cerebral Response to Peripheral Challenge with a Viral Mimetic

It has been well established that peripheral inflammation resulting from microbial infections profoundly alters brain function. This review focuses on experimental systems that model cerebral effects of peripheral viral challenge. The most common models employ the induction of the acute phase response via intraperitoneal injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC). The ensuing transient surge of blood-borne inflammatory mediators induces a "mirror" inflammatory response in the brain characterized by the upregulated expression of a plethora of genes encoding cytokines, chemokines and other inflammatory/stress proteins. These inflammatory mediators modify the activity of neuronal networks leading to a constellation of behavioral traits collectively categorized as the sickness behavior. Sickness behavior is an important protective response of the host that has evolved to enhance survival and limit the spread of infections within a population. However, a growing body of clinical data indicates that the activation of inflammatory pathways in the brain may constitute a serious comorbidity factor for neuropathological conditions. Such comorbidity has been demonstrated using the PIC paradigm in experimental models of Alzheimer's disease, prion disease and seizures. Also, prenatal or perinatal PIC challenge has been shown to disrupt normal cerebral development of the offspring resulting in phenotypes consistent with neuropsychiatric disorders, such as schizophrenia and autism. Remarkably, recent studies indicate that mild peripheral PIC challenge may be neuroprotective in stroke. Altogether, the PIC challenge paradigm represents a unique heuristic model to elucidate the immune-to-brain communication pathways and to explore preventive strategies for neuropathological disorders.

Phasic Treatment with Interferon Gamma Stimulates Release of Exosomes that Protect Against Spreading Depression

The detrimental effects of T-cell-secreted interferon gamma (IFNγ) on oxidative stress (OS) and demyelination in multiple sclerosis (MS) are well recognized. Recently, we demonstrated that IFNγ-mediated damage to myelin also increases susceptibility to spreading depression (SD; the likely basis of migraine with aura). However, before onset of MS, induction of physiological levels of IFNγ, like that produced by environmental enrichment (EE), protects against demyelination and OS. Accordingly, we focused on the potential for physiological levels of IFNγ to protect against SD. EE, which occurs with a moderate and phasic increase in proinflammatory cytokines, reduces migraine frequency. Thus, we applied phasic or pulsed IFNγ to brain slice cultures to emulate EE. This treatment reduced OS, increased myelin basic protein, a marker for myelin, and reduced susceptibility to SD. Building on our research on exosomes in EE-based neuroprotection, we found that IFNγ stimulation of slice cultures induced release of exosomes, likely from the microglia that produce the same protective effects as IFNγ treatment when applied to naive cultures. Finally, nasal administration of IFNγ to rats recapitulated in vitro effects, reducing OS, increasing myelin, and reducing SD. These results support phasic IFNγ signaling as a therapeutic target for prevention of SD and, by extension, migraine.

Intrahippocampal injection of TsTX-I increases the levels of INF-γ in the cerebral tissue but not the levels of glutamate

TsTX-I, isolated from Tityus serrulatus scorpion venom, causes epileptic-like discharges when injected into the central nervous system. The involvement of excitatory amino acids and cytokines in this activity was investigated. Our results have demonstrated that TsTX-I increases the release of IFN-γ but does not alter the intracerebral concentration of the excitatory amino acids in rats. Thus, this cytokine seems to be more important in the convulsive process than glutamate.

Spreading depression transiently disrupts myelin via interferon-gamma signaling

Multiple sclerosis and migraine with aura are clinically correlated and both show imaging changes suggestive of myelin disruption. Furthermore, cortical myelin loss in the cuprizone animal model of multiple sclerosis enhances susceptibility to spreading depression, the likely underlying cause of migraine with aura. Since multiple sclerosis pathology involves inflammatory T cell lymphocyte production of interferon-gamma and a resulting increase in oxidative stress, we tested the hypothesis that spreading depression disrupts myelin through similar signaling pathways. Rat hippocampal slice cultures were initially used to explore myelin loss in spreading depression, since they contain T cells, and allow for controlled tissue microenvironment. These experiments were then translated to the in vivo condition in neocortex. Spreading depression in slice cultures induced significant loss of myelin integrity and myelin basic protein one day later, with gradual recovery by seven days. Myelin basic protein loss was abrogated by T cell depletion, neutralization of interferon-gamma, and pharmacological inhibition of neutral sphingomyelinase-2. Conversely, one day after exposure to interferon-gamma, significant reductions in spreading depression threshold, increases in oxidative stress, and reduced levels of glutathione, an endogenous neutral sphingomyelinase-2 inhibitor, emerged. Similarly, spreading depression triggered significant T cell accumulation, sphingomyelinase activation, increased oxidative stress, and reduction of gray and white matter myelin in vivo. Myelin disruption is involved in spreading depression, thereby providing pathophysiological links between multiple sclerosis and migraine with aura. Myelin disruption may promote spreading depression by enhancing aberrant excitability. Thus, preservation of myelin integrity may provide novel therapeutic targets for migraine with aura.

Genes and signaling pathways involved in memory enhancement in mutant mice

Mutant mice have been used successfully as a tool for investigating the mechanisms of memory at multiple levels, from genes to behavior. In most cases, manipulating a gene expressed in the brain impairs cognitive functions such as memory and their underlying cellular mechanisms, including synaptic plasticity. However, a remarkable number of mutations have been shown to enhance memory in mice. Understanding how to improve a system provides valuable insights into how the system works under normal conditions, because this involves understanding what the crucial components are. Therefore, more can be learned about the basic mechanisms of memory by studying mutant mice with enhanced memory. This review will summarize the genes and signaling pathways that are altered in the mutants with enhanced memory, as well as their roles in synaptic plasticity. Finally, I will discuss how knowledge of memory-enhancing mechanisms could be used to develop treatments for cognitive disorders associated with impaired plasticity.

Toll-like receptor 3 activation modulates hippocampal network excitability, via glial production of interferon-β

The family of toll-like receptors (TLR) plays a major role in innate immunity due to their pathogen-recognition abilities. TLR3 is a sensor for double-stranded RNA, and regulates host-defense responses to several viruses, via the production of type I interferons. Interferon-β (IFNβ) is a primary product of TLR3 activation, and its transcription is elevated in the CNS response to the synthetic TLR3 ligand, polyinosinic-polycytidylic acid (poly(I:C)). Peripheral infections, along with TLR-induced inflammatory mediators, are known to have detrimental effects on brain function, exerting a negative impact on cognition and enhancing seizure susceptibility. In this study, we assessed hippocampal function in vitro, in response to systemic delivery of a TLR3 agonist. Unlike agonists of other TLRs, intraperitoneal injection of poly(I:C) did not adversely affect evoked short- and long-term synaptic plasticity in mouse hippocampal slices. However, sustained and interictal-like spontaneous activity was observed in CA1 pyramidal cells in response to poly(I:C) and this was associated with alterations in the expression of phosphorylated NR2B subunit-containing NMDA receptors and an astrocyte-specific glutamate/aspartate transporter (GLAST) which impact on extracellular glutamate concentration and contribute to the genesis of epileptiform activity. We provide evidence for the production of IFNβ from microglia and astrocytes, and using mice deficient in the type I IFN receptor α 1 (IFNAR1), demonstrate that its subsequent activation is likely to underlie the TLR3-mediated modulation of hippocampal excitability.

Differential expression of interferon-γ and chemokine genes distinguishes Rasmussen encephalitis from cortical dysplasia and provides evidence for an early Th1 immune response

Background

Rasmussen encephalitis (RE) is a rare complex inflammatory disease, primarily seen in young children, that is characterized by severe partial seizures and brain atrophy. Surgery is currently the only effective treatment option. To identify genes specifically associated with the immunopathology in RE, RNA transcripts of genes involved in inflammation and autoimmunity were measured in brain tissue from RE surgeries and compared with those in surgical specimens of cortical dysplasia (CD), a major cause of intractable pediatric epilepsy.

Methods

Quantitative polymerase chain reactions measured the relative expression of 84 genes related to inflammation and autoimmunity in 12 RE specimens and in the reference group of 12 CD surgical specimens. Data were analyzed by consensus clustering using the entire dataset, and by pairwise comparison of gene expression levels between the RE and CD cohorts using the Harrell-Davis distribution-free quantile estimator method.

Results

Consensus clustering identified six RE cases that were clearly distinguished from the CD cases and from other RE cases. Pairwise comparison showed that seven mRNAs encoding interferon-γ, CCL5, CCL22, CCL23, CXCL9, CXCL10, and Fas ligand were higher in the RE specimens compared with the CD specimens, whereas the mRNA encoding hypoxanthine-guanine phosphoribosyltransferase was reduced. Interferon-γ, CXCL5, CXCL9 and CXCL10 mRNA levels negatively correlated with time from seizure onset to surgery (P <0.05), whereas CCL23 and Fas ligand transcript levels positively correlated with the degree of tissue destruction and inflammation, respectively (P <0.05), as determined from magnetic resonance imaging (MRI) T2 and FLAIR images. Accumulation of CD4⁺ lymphocytes in leptomeninges and perivascular spaces was a prominent feature in RE specimens resected within a year of seizure onset.

Conclusions

Active disease is characterized by a Th1 immune response that appears to involve both CD8⁺ and CD4⁺ T cells. Our findings suggest therapeutic intervention targeting specific chemokine/chemokine receptors may be useful in early stage RE.

Insulin-like growth factor-1 lowers spreading depression susceptibility and reduces oxidative stress

Spreading depression (SD), the likely cause of migraine aura and perhaps migraine, is triggered by widespread and unfettered neuronal hyperexcitability. Migraine and the initiating hyperexcitability of seizure, which involve oxidative stress (OS), are likely interrelated. Environmental enrichment (EE) decreases seizure and can reduce migraine. EE's well-characterized neuroprotective effect involves insulin-like growth factor-1 (IGF-1). Accordingly, we asked if IGF-1 could mitigate the hyperexcitability that initiates SD using rat hippocampal slice cultures. We demonstrate that IGF-1 significantly decreased SD susceptibility and related OS. We mimicked OS of SD and observed that IGF-1 abolished hyperexcitability from OS. Application of an antioxidant significantly decreased SD susceptibility and co-administration of an antioxidant with IGF-1 produced no additive effect, whereas an oxidizer significantly increased SD, and this effect was abrogated by IGF-1. Moreover, IGF-1 significantly decreased baseline OS, despite seemingly paradoxically increasing CA3 bursting. These results suggest that IGF-1 increased endogenous antioxidants to levels sufficient to buffer against the OS of SD. Insulin similarly mitigated SD susceptibility, but required a far greater dose. Since brain IGF-1 increases with EE, and, like insulin, independently functions as an EE mimetic, we suggest that EE mimetics are a novel source of therapeutics for SD, and by extension, migraine.

Suppression of PKR promotes network excitability and enhanced cognition by interferon-γ-mediated disinhibition

The double-stranded RNA-activated protein kinase (PKR) was originally identified as a sensor of virus infection, but its function in the brain remains unknown. Here, we report that the lack of PKR enhances learning and memory in several behavioral tasks while increasing network excitability. In addition, loss of PKR increases the late phase of long-lasting synaptic potentiation (L-LTP) in hippocampal slices. These effects are caused by an interferon-γ (IFN-γ)-mediated selective reduction in GABAergic synaptic action. Together, our results reveal that PKR finely tunes the network activity that must be maintained while storing a given episode during learning. Because PKR activity is altered in several neurological disorders, this kinase presents a promising new target for the treatment of cognitive dysfunction. As a first step in this direction, we show that a selective PKR inhibitor replicates the Pkr(-/-) phenotype in WT mice, enhancing long-term memory storage and L-LTP.

General Anesthesia Causes Long-term Impairment of Mitochondrial Morphogenesis and Synaptic Transmission in Developing Rat Brain

BACKGROUND

Clinically used general anesthetics, alone or in combination, are damaging to the developing mammalian brain. In addition to causing widespread apoptotic neurodegeneration in vulnerable brain regions, exposure to general anesthesia at the peak of synaptogenesis causes learning and memory deficiencies later in life. In vivo rodent studies have suggested that activation of the intrinsic (mitochondria-dependent) apoptotic pathway is the earliest warning sign of neuronal damage, suggesting that a disturbance in mitochondrial integrity and function could be the earliest triggering events.

METHODS

Because proper and timely mitochondrial morphogenesis is critical for brain development, the authors examined the long-term effects of a commonly used anesthesia combination (isoflurane, nitrous oxide, and midazolam) on the regional distribution, ultrastructural properties, and electron transport chain function of mitochondria, as well as synaptic neurotransmission, in the subiculum of rat pups.

RESULTS

This anesthesia, administered at the peak of synaptogenesis, causes protracted injury to mitochondria, including significant enlargement of mitochondria (more than 30%, P < 0.05), impairment of their structural integrity, an approximately 28% increase in their complex IV activity (P < 0.05), and a twofold decrease in their regional distribution in presynaptic neuronal profiles (P < 0.05), where their presence is important for the normal development and functioning of synapses. Consequently, the authors showed that impaired mitochondrial morphogenesis is accompanied by heightened autophagic activity, decrease in mitochondrial density (approximately 27%, P < 0.05), and long-lasting disturbances in inhibitory synaptic neurotransmission. The interrelation of these phenomena remains to be established.

CONCLUSION

Developing mitochondria are exquisitely vulnerable to general anesthesia and may be important early target of anesthesia-induced developmental neurodegeneration.

Characterization of cortical neuronal and glial alterations during culture of organotypic whole brain slices from neonatal and mature mice

Background

Organotypic brain slice culturing techniques are extensively used in a wide range of experimental procedures and are particularly useful in providing mechanistic insights into neurological disorders or injury. The cellular and morphological alterations associated with hippocampal brain slice cultures has been well established, however, the neuronal response of mouse cortical neurons to culture is not well documented.

Methods

In the current study, we compared the cell viability, as well as phenotypic and protein expression changes in cortical neurons, in whole brain slice cultures from mouse neonates (P4–6), adolescent animals (P25–28) and mature adults (P50+). Cultures were prepared using the membrane interface method.

Results

Propidium iodide labeling of nuclei (due to compromised cell membrane) and AlamarBlue™ (cell respiration) analysis demonstrated that neonatal tissue was significantly less vulnerable to long-term culture in comparison to the more mature brain tissues. Cultures from P6 animals showed a significant increase in the expression of synaptic markers and a decrease in growth-associated proteins over the entire culture period. However, morphological analysis of organotypic brain slices cultured from neonatal tissue demonstrated that there were substantial changes to neuronal and glial organization within the neocortex, with a distinct loss of cytoarchitectural stratification and increased GFAP expression (p<0.05). Additionally, cultures from neonatal tissue had no glial limitans and, after 14 DIV, displayed substantial cellular protrusions from slice edges, including cells that expressed both glial and neuronal markers.

Conclusion

In summary, we present a substantial evaluation of the viability and morphological changes that occur in the neocortex of whole brain tissue cultures, from different ages, over an extended period of culture.

Systemic interferon-alpha regulates interferon-stimulated genes in the central nervous system

The prime anti-viral cytokine interferon-alpha (IFN-alpha) has been implicated in several central nervous system (CNS) disorders in addition to its beneficial effects. Systemic IFN-alpha treatment causes severe neuropsychiatric complications in humans, including depression, anxiety and cognitive impairments. While numerous neuromodulatory effects by IFN-alpha have been described, it remains unresolved whether or not systemic IFN-alpha acts directly on the brain to execute its CNS actions. In the present study, we have analyzed the genes directly regulated in post-IFN-alpha receptor signaling and found that intraperitoneal administration of mouse IFN-alpha, but not human IFN-alpha, activated expression of several prototypic IFN-stimulated genes (ISGs), in particular signal transducers and activators of transcription (STAT1), IFN-induced 15 kDa protein (ISG15), ubiquitin-specific proteinase 18 (USP18) and guanylate-binding protein 3 (GBP3) in the brain. A similar temporal profile for the regulated expression of these IFN-alpha-activated ISG genes was observed in the brain compared with the peripheral organs. Dual labeling in situ hybridization combined with immunocytochemical staining demonstrated a wide distribution of the key IFN-regulated gene STAT1 transcripts in the different parenchyma cells of the brain, particularly neurons. The overall response to IFN-alpha challenge was abolished in STAT1 knockout mice. Together, our results indicate a direct, STAT1-dependent action of systemic IFN-alpha in the CNS, which may provide the basis for a mechanism in humans for neurological/neuropsychiatric illnesses associated with IFN-alpha therapy.

Pentylenetetrazol-induced kindling seizure attenuated by Ginkgo biloba extract (EGb 761) in mice

Ginkgo biloba extract (EGb 761) has been used therapeutically for centuries. It has attracted great attention as agents for improving circulation, particularly cerebral circulation, which may lead to improved mental function. Many researches hypothesized on the role of the extract in the treatment of diseases involving free radicals and oxidative damage. In the present study, anticonvulsant and antioxidant effects of EGb 761 were investigated in pentylenetetrazol (PTZ)-kindled mice. Valproic acid (VA), a major antiepileptic drug, was also tested for comparison. EGb 761-treated mice displayed a significant attenuated response to PTZ on the test day (day 26) compared with saline-treated and VA-treated animals. Valproic acid significantly increased seizure latency. Pretreatments with EGb 761 significantly protected against PTZ-induced convulsive behaviors (seizure latency, seizure score). EGb 761 and VA significantly decreased PTZ-induced oxidative injury in brain tissue. EGb 761 was found to be the most effective in preventing PTZ-induced oxidative damage among both substances studied. The data obtained support our speculation that neuroprotective action of EGb 761 may correlate with its ability to inhibit not only excessive reactive oxygen species (ROS) formation but also seizure generation. Taken together, the results of the present study show that the effect of EGb 761 on ROS production contributes to their neuroprotective action. It might be concluded that the suppression of seizure-induced ROS generation may be involved in the mechanism of action of antiepileptic drugs.

Neuromodulation by a cytokine: interferon-beta differentially augments neocortical neuronal activity and excitability

The immunomodulatory cytokine interferon-beta (IFN-beta) is used in the treatment of autoimmune diseases such as multiple sclerosis. However, the effect of IFN-beta on neuronal functions is currently unknown. Intracellular recordings were conducted on somatosensory neurons of neocortical layers 2/3 and 5 exposed to IFN-beta. The excitability of neurons was increased by IFN-beta (10-10,000 U/ml) in two kinetically distinct, putatively independent manners. First IFN-beta reversibly influenced the subthreshold membrane response by raising the membrane resistance R(M) 2.5-fold and the membrane time constant tau 1.7-fold dose-dependently. The effect required permanent exposure to IFN-beta and was reduced in magnitude if the extracellular K+ was lowered. However, the membrane response to IFN-beta in the subthreshold range was prevented by ZD7288 (a specific blocker of I(h)) but not by Ni2+, carbachol, or bicuculline, pointing to a dependence on an intact I(h). Second, IFN-beta enhanced the rate of action potential firing. This effect was observed to develop for >1 h when the cell was exposed to IFN-beta for 5 min or >5 min and showed no reversibility (< or =210 min). Current-discharge (F-I) curves revealed a shift (prevented by bicuculline) as well as an increase in slope (prevented by carbachol and Ni2+). Layer specificity was not observed with any of the described effects. In conclusion, IFN-beta influences the neuronal excitability in neocortical pyramidal neurons in vitro, especially under conditions of slightly increased extracellular K+. Our blocker experiments indicate that changes in various ionic conductances with different voltage dependencies cause different IFN-beta influences on sub- and suprathreshold behavior, suggesting a more general intracellular process induced by IFN-beta.

Uncoupling protein 2 prevents neuronal death including that occurring during seizures: a mechanism for preconditioning

The mitochondrial uncoupling protein (UCP2) is expressed in selected regions of the brain. Here we demonstrate that up-regulation of UCP2 is part of a neuroprotective set of responses to various cellular stresses in vitro and in vivo. PC12 cells, when transfected with UCP2, were protected against free radical-induced cell death. Seizure activity was associated with elevated UCP2 levels and mitochondrial uncoupling activity. In transgenic mice that expressed UCP2 constitutively in the hippocampus before seizure induction, a robust reduction in cell death was seen. Because UCP2 increased mitochondrial number and ATP levels with a parallel decrease in free radical-induced damage, it is reasonable to suggest that mitochondrial UCPs precondition neurons by dissociating cellular energy production from that of free radicals to withstand the harmful effects of cellular stress occurring in a variety of neurodegenerative disorders, including epilepsy.

Serum cytokine levels are altered in patients with West syndrome

The objective of this study is to explore the neuroimmunomodulator effect of interleukin (IL)-2, tumor necrosis factor (TNF)-alpha and interferon (IFN)-alpha in West syndrome (WS). Twenty-three cases of WS (13 males and 10 females, aged 4-14 months old) who first visited and consisted from 10 cryptogenic and 13 symptomatic, were enrolled in this study. Double-antibody sandwich enzyme-linked immunosorbent assay was used to measure serum IL-2, TNF-alpha and IFN-alpha levels in 23 patients with WS and the data were compared to those of 15 healthy infants who were matched with regard to age and sex. Levels of all three cytokines were significantly higher in both cryptogenic and symptomatic WS groups than the control group. Serum IL-2 levels in symptomatic WS were significantly higher than that in cryptogenic WS. There was a positive correlation between IL-2 and TNF-alpha in both cryptogenic and symptomatic WS groups. The immune systems of patients with WS are in an activated state. An imbalance in cytokine levels may be involved in the immunopathology of WS.

Interferon-gamma-induced changes in synaptic activity and AMPA receptor clustering in hippocampal cultures

Extended release of interferon-gamma (IFN-gamma) in the nervous system during immunological and infectious conditions may trigger demyelinating disorders and cause disturbances in brain function. The aim of this study was to examine the effects of IFN-gamma on neuronal function in rat hippocampal cell cultures by using whole cell patch clamp analysis together with quantitative immunocytochemistry. Acute application of IFN-gamma to differentiated neurons in culture caused no immediate neurophysiological responses, but recordings after 48 h of incubation displayed an increase in frequency of AMPA receptor (AMPAR)-mediated spontaneous excitatory postsynaptic currents (EPSCs). Quantitative immunocytochemistry for the AMPAR subunit GluR1 showed no alteration in receptor clustering at this time point. However, prolonged treatment with IFN-gamma for 2 weeks resulted in a significant reduction in AMPAR clustering on dendrites but no marked differences in EPSC frequency between treated neurons and controls could be observed. On the other hand, treatment of hippocampal neurons for 4 weeks, instituted at an immature stage (1 day in culture), caused a significant reduction in spontaneous EPSC frequency. These neurons developed with no overt alterations in dendritic arborization or in the appearance of dendritic spines as visualized by alpha-actinin immunocytochemistry. Nonetheless, there was a marked reduction in AMPAR clustering on dendrites. These observations show that a key immunomodulatory molecule, IFN-gamma, can cause long-term modifications of synaptic activity and perturb glutamate receptor clustering.

The role of cytokines and growth factors in seizures and their sequelae

Although the neuropathological changes caused by severe or repeated seizures have been well characterized, many questions about the molecular mechanisms involved remain unanswered. Neuronal cell death, reactive gliosis, enhanced neurogenesis, and axonal sprouting are four of the best-studied sequelae of seizures. In vitro, each of these pathological processes can be substantially influenced by soluble protein factors, including neurotrophins, cytokines, and growth factors. Furthermore, many of these proteins and their receptors are expressed in the adult brain and are up-regulated in response to neuronal activity and injury. We review the evidence that these intercellular signaling proteins regulate seizure activity as well as subsequent pathology in vivo. As nerve growth factor and brain derived neurotrophic factor are the best-studied proteins of this class, we begin by discussing the evidence linking these neurotrophins to epilepsy and seizure. More than a dozen additional cytokines, growth factors, and neurotrophins that have been examined in the context of epilepsy models are then considered. We discuss the effect of seizure on expression of cytokines and growth factors, and explore the regulation of seizure development and aftermath by exogenous application or antagonist perturbation of these proteins. The experimental evidence supports a role for these factors in each aspect of seizure and pathology, and suggests potential targets for future therapeutics.

Mechanisms underlying H(2)O(2)-mediated inhibition of synaptic transmission in rat hippocampal slices

Hydrogen peroxide (H(2)O(2)) inhibits the population spike (PS) evoked by Schaffer collateral stimulation in hippocampal slices. Proposed mechanisms underlying this effect include generation of hydroxyl radicals (.OH) and inhibition of presynaptic Ca(2+) entry. We have examined these possible mechanisms in rat hippocampal slices. Inhibition of the evoked PS by H(2)O(2) was sharply concentration-dependent: 1.2 mM H(2)O(2) had no effect, whereas 1.5 and 2.0 mM H(2)O(2) reversibly depressed PS amplitude by roughly 80%. The iron chelator, deferoxamine (1 mM), and the endogenous.OH scavenger, ascorbate (400 microM), prevented PS inhibition, confirming.OH involvement. Isoascorbate (400 microM), which unlike ascorbate is not taken up by brain cells, also prevented PS inhibition, indicating an extracellular site of.OH generation or action. We then investigated whether H(2)O(2)-induced PS depression could be overcome by prolonged stimulation, which enhances Ca(2+) entry. During 5-s, 10-Hz trains under control conditions, PS amplitude increased to over 200% during the first three-four pulses, then stabilized. In the presence of H(2)O(2), PS amplitude was initially depressed, but began to recover after 2.5 s of stimulation, finally reaching 80% of the control maximum. In companion experiments, we assessed the effect of H(2)O(2) on presynaptic Ca(2+) entry by monitoring extracellular Ca(2+) concentration ([Ca(2+)](o)) during train stimulation in the presence of postsynaptic receptor blockers. Evoked [Ca(2+)](o) shifts were apparently unaltered by H(2)O(2), suggesting a lack of effect on Ca(2+) entry. Taken together, these findings suggest new ways in which reactive oxygen species (ROS) might act as signaling agents, specifically as modulators of synaptic transmission.

Oxidative stress is involved in seizure-induced neurodegeneration in the kindling model of epilepsy

Reactive oxygen species have been implicated in the development of seizures under pathological conditions and linked to seizure-induced neurodegeneration. There has been little direct evidence, however, of free radical production resulting from seizures. Using amygdala-kindled rats, we have examined the generation of reactive oxygen species following seizures, and their possible contribution to seizure development and seizure-induced neuronal loss. The concentrations of two products of free radical-induced lipid peroxidation, malonaldehyde and 4-hydroxy-2(E)-nonenal, were measured using colorimetric assays. Lipid peroxidation was increased in both hemispheres of kindled rats as compared to sham-operated controls. Cell death was also significantly increased in all hippocampal areas. Antioxidants (vitamin E and glutathione) prevented the rise in lipid peroxides and hippocampal neuronal death during kindling, but did not arrest the development of seizures.Thus, epileptiform activity can result in free radical production which may be one of the factors leading to cell death.

Cytotoxicity of tributyltin in rat hippocampal slice cultures

The neurotoxic effects of tributyltin (TBT), an endocrine-disrupting chemical, were evaluated in organotypic slice cultures of immature rat hippocampus. Confocal microscopy study with propidium iodide showed that TBT induced severe neuronal death in a concentration- and time-dependent manner with CA3 > CA1 > dentate gyrus ranking of vulnerability of the hippocampal subfields. Dead or damaged neurons exhibited chromatin condensation, which is one of the morphological characteristics of apoptosis, as revealed by acridine orange staining. TBT neurotoxicity was alleviated by application of free radical scavengers or antioxidants, such as catalase, superoxide dismutase, Trolox and alpha-tocopherol but not by ascorbic acid or N-acetyl-L-cysteine, which suggests an involvement of free radicals, particularly reactive oxygen species. Neurons displayed a long-lasting increase in intracellular Ca2+ concentrations after TBT treatment. Although neither N-methyl-D-aspartate (NMDA) receptor inhibitors nor voltage-sensitive Ca2+ channel blockers protected hippocampal neurons against TBT neurotoxicity, non-NMDA receptor antagonist completely prevented TBT-induced neurodegeneration. These data suggest that TBT provokes apoptosis-like neuronal cell death, which might be mediated by intracellular Ca2+ elevation and free radical generation via non-NMDA receptor activation.

Pathophysiology of septic encephalopathy: a review

OBJECTIVES

Encephalopathy is a common complication of sepsis. This review describes the different pathologic mechanisms that may be involved in its etiology.

DATA SOURCES

The studies described here were derived from the database PubMed (http:¿¿www.nlm.nih.gov) and from references identified in the bibliographies of pertinent articles and books. The citations are largely confined to English language articles between 1966 and 1998. Older publications were used if they were of historical significance.

STUDY SELECTION

All investigations in which any aspect of septic encephalopathy was reported were included. This selection encompasses clinical, animal, and in vitro cell culture work.

DATA EXTRACTION

The literature cited was published in peer-reviewed clinical or basic science journals or in books.

DATA SYNTHESIS

Contradictions between the results of published studies are discussed.

CONCLUSIONS

The most immediate and serious complication of septic encephalopathy is impaired consciousness, for which the patient may require ventilation. The etiology of septic encephalopathy involves reduced cerebral blood flow and oxygen extraction by the brain, cerebral edema, and disruption of the blood-brain barrier that may arise from the action of inflammatory mediators on the cerebrovascular endothelium, abnormal neurotransmitter composition of the reticular activating system, impaired astrocyte function, and neuronal degeneration. Currently, there is no treatment.

Optical imaging reveals elevated intracellular chloride in hippocampal pyramidal neurons after oxidative stress

The accumulation of reactive oxygen species (ROS) in the brain is associated with several neurodegenerative conditions. ROS can affect ionic homeostasis leading to impaired neurotransmission. Here, we determined the ability of H(2)O(2), a membrane permeant ROS, to alter intraneuronal Cl(-), an important regulator of neuronal excitability. Real-time alterations in intracellular chloride, [Cl(-)]i, were measured with UV laser scanning confocal microscopy in hippocampal slices loaded with the cell-permeant form of 6-methoxy-N-ethylquinolium iodide (MEQ), a Cl(-)-sensitive fluorescent probe. In slices superfused with H(2)O(2) for 10 min, there was a significant decrease in MEQ fluorescence (elevation in [Cl(-)]i) in area CA1 pyramidal cell soma but not in interneurons located in stratum radiatum. Alterations in [Cl(-)]i induced by H(2)O(2) were prevented by the iron chelator deferoxamine and the vitamin E analog Trolox, suggesting the involvement of free radicals. The influx of Cl(-) probably occurred through the GABA-gated Cl(-) channel because the effects of H(2)O(2) were blocked by picrotoxin. In addition, HPLC analysis of the superfusates indicated that GABA and glutamate accumulated extracellularly after H(2)O(2) exposure. Excitatory amino acid receptor antagonists 2-amino-5-phoshopentanoic acid and 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo[f]quinoxaline-7-sulfonamide also attenuated the effect of H(2)O(2) on MEQ fluorescence. The changes in [Cl(-)]i induced by H(2)O(2) were Ca(2+)-dependent and Na(+)-independent. After exposure of slices to H(2)O(2), the ability of the GABA agonist muscimol to increase [Cl(-)]i was attenuated. Thus, ROS, like H(2)O(2), may impair transmembrane Cl(-) gradients and reduce inhibitory neurotransmission, further promoting neuronal damage in oxidative stress-related disease and in aging.

Cytokine and growth factor involvement in long-term potentiation

Hippocampal long-term potentiation (LTP) is one of the best-studied models of learning and memory at the molecular level. While it has long been known that tetanic stimulation causes changes at the synapse within seconds to minutes, recent research has begun to focus on factors that may affect synaptic plasticity on a longer time scale. One group of factors with many of the characteristics predicted for both short- and long-term actions at the synapse is the cytokines and growth factors. In vitro, these proteins can alter neuronal morphology, gene expression, and proliferation, and many cytokines and their receptors are present in the adult CNS. Because brainderived neurotrophic factor (BDNF) is the best-studied synaptic modulator of this class, we begin by discussing the experimental evidence linking BDNF to LTP. Ten cytokines and growth factors that have been examined in the context of hippocampal LTP are then considered. We discuss the effects of LTP on the expression of the cytokines and explore the regulation of synaptic plasticity by exogenous application or antagonist perturbation of these proteins. The available evidence strongly supports a role for these factors in synaptic modulation and should prompt further exploration of their functions at the synapse.

Structural and functional neuropathology in transgenic mice with CNS expression of IFN-alpha

Cytokines belonging to the type I interferon (e.g. interferon-alpha) family are important in the host response to infection and may have complex and broad ranging actions in the central nervous system (CNS) that may be beneficial or harmful. To better understand the impact of the CNS expression of the type I interferons (IFN), transgenic mice were developed that produce IFN-alpha(1) chronically from astrocytes. In two independent transgenic lines with moderate and low levels of astrocyte IFN-alpha mRNA expression respectively, a spectrum of transgene dose- and age-dependent structural and functional neurological alterations are induced. Structural changes include neurodegeneration with loss of cholinergic neurons, gliosis, angiopathy with mononuclear cell cuffing, progressive calcification affecting basal ganglia and cerebellum and the up-regulation of a number of IFN-alpha-regulated genes. At a functional level, in vivo and in vitro electrophysiological studies revealed impaired neuronal function and disturbed synaptic plasticity with pronounced hippocampal hyperexcitability. Severe behavioral alterations were also evident in higher expressor GFAP-IFNalpha mice which developed fatal seizures around 13 weeks of age precluding their further behavioral assessment. Modest impairments in discrimination learning were measured in lower expressor GFAP-IFNalpha mice at various ages (7-42 weeks). The behavioral and electrophysiological findings suggest regional changes in hippocampal excitability which may be linked to abnormal calcium metabolism and loss of cholinergic neurons in the GIFN mice. Thus, these transgenic mice provide a novel animal model in which to further evaluate the mechanisms that underlie the diverse actions of type I interferons in the intact CNS and to link specific structural changes with functional impairments.

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

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

Neuroimmunomodulatory actions of hypothalamic interferon-alpha

Recent studies have revealed that the brain produces interferon-alpha (IFN-alpha) in response to noninflammatory as well as inflammatory stress and that it might have a role in normal physiology. When administered intracerebrally, IFN-alpha causes diverse effects including fever, anorexia, analgesia and changes in the central neuronal activities. These responses are inhibited by the opioid receptor antagonist naloxone. This is consistent with the reports suggesting that recombinant human (rh) IFN-alpha binds to opioid receptors in rodent brain membrane. We revealed that rhIFN-alpha altered the activity of thermosensitive neurons in the medial preoptic area (MPO) and glucose-responsive neurons in the ventromedial hypothalamus in an opioid-receptor-dependent way. As a stress which produces opioid-dependent analgesia is known to suppress the cytotoxicity of splenic natural killer cells, we investigated whether the administration of beta-endorphin and rhIFN-alpha may induce a similar immunosuppression. We found that central, but not peripheral, injection of both compounds inhibited natural killer (NK) cytotoxicity. Further studies revealed that rhIFN-alpha decreased the activity of MPO neurons via opioid receptors and the altered activity of MPO neurons in turn resulted in the activation of corticotropin-releasing factor neurons, thereby suppressing NK cytotoxicity predominantly through activation of the splenic sympathetic nerve and beta-receptor mechanisms in splenocytes. Thus, IFN-alpha may alter the brain activity to exert a feedback effect on the immune system. Further detailed whole-cell clamping analyses on neuronal mechanisms in rat brain tissue slices showed that the inhibitory effect of rhIFN-alpha on N-methyl-D-aspartate-induced membrane current responses of MPO neurons was mediated not only by opioid receptors but also by the local production of reactive oxygen intermediates, nitric oxide and prostanoids, possibly due to neuron-glial cell interaction.

Is interferon-alpha a neuromodulator?

Interferons were initially characterized for their ability to 'interfere' with viral replication, slow cell proliferation, and profoundly alter immunity. They are a group of hormone-like molecules synthesized and secreted by macrophages, monocytes, T lymphocytes, glia, and neurons. These cytokines have been shown to have several regulatory roles and diverse biological activities, including control of cellular and humoral immune responses, inflammation, and tumor regression. In addition, there are many reports indicating that interferon-alpha (IFN-alpha) participates in the regulation of various cellular and humoral processes such as the endocrine system modulates behavior, brain activity, temperature, glucose sensitive neurons, feeding pattern and opiate activity. Therefore, IFN-alpha can be considered as a physiological modulator, with only one of its functions being the ability to hinder viral replication intracellularly.

Biphasic effect of hydrogen peroxide on field potentials in rat hippocampal slices

In the CA1 region of rat hippocampal slices, H2O2 (0.294-2.94 mM) caused initial augmentation, and subsequent long-lasting depression, of population spikes and excitatory postsynaptic potentials. The effect of H2O2 may not be mediated by its degradation product, hydroxyl radicals, because an iron chelator deferoxamine did not block the effect. A catalase inhibitor 3-amino-1,2,4-triazole only modestly attenuated the initial augmentation, suggesting that the effect of H2O2 is not attributable to catalase-dependent O2 generation, either. An N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonovaleric acid had no influence on the effect of H2O2, whereas a gamma-aminobutyric acid type A receptor channel blocker picrotoxin attenuated long-lasting depression, indicating that gamma-aminobutyric acid-mediated inhibition is altered during the depression phase. The initial augmentation but not subsequent depression was attenuated by a phospholipase A2/C inhibitor 4-bromophenacyl bromide, suggesting the involvement of lipid signaling molecule(s) in the enhancement of excitatory synaptic transmission. These results suggest that H2O2 regulates hippocampal synaptic transmission via multiple mechanisms.

Major histocompatibility complex (MHC) class I gene expression in single neurons of the central nervous system: differential regulation by interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha

This study examined the effect of the pro-inflammatory cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) on the induction of MHC class I-related genes in functionally mature brain neurons derived from cultures of dissociated rat hippocampal tissue. Patch clamp electrophysiology combined with single cell RT-PCR demonstrated that approximately 50% of the untreated neurons contained mRNA for MHC class I heavy chains, while, with few exceptions, the cells failed to transcribe beta2-microglobulin and TAP1/TAP2 gene transcripts. No constitutive expression of MHC class I protein was detectable by confocal laser microscopy on the surface of neurons. All neurons transcribed the alpha-chain of the interferon-type II receptor (binding IFN-gamma) along with the p55 receptor for TNF-alpha. Sustained exposure to IFN-gamma resulted in transcription of beta2-microglobulin and TAP1/TAP2 genes and MHC class I surface expression in a minor part of the neurons, but did not alter their electrophysiological activities as assessed by whole cell electrophysiology. Suppression of neuronal electric activity by the sodium channel blocker tetrodotoxin drastically increased to almost 100% IFN-gamma-mediated induction of MHC class I chains, of both TAP transporters, and of membrane expression of MHC class I protein. The effect of tetrodotoxin is at least partly reverted by the neurotransmitter glutamate. In contrast to IFN-gamma, treatment with TNF-alpha did neither upregulate TAP1/TAP2 nor beta2-microglobulin gene expression, but induced MHC class I heavy chain gene transcription in all neurons. Consequently, no MHC class I molecules were detectable on the membranes of TNF-alpha-treated neurons.

Long-term hippocampal slices: a model system for investigating synaptic mechanisms and pathologic processes

Organotypic cultures provide a unique strategy with which to examine many aspects of brain physiology and pathology. Long-term slice cultures from the hippocampus, a region involved in memory encoding and one that exhibits early degeneration in Alzheimer's disease and ischemia, are particularly valuable in this regard due to their expression of synaptic plasticity mechanisms (e.g., long-term potentiation) and responsiveness to pathological insults (e.g., excitotoxicity). Long-term slices can be prepared from hippocampi at the second or third postnatal week of development and thus incorporate a number of relatively mature features; further signs of maturation and the preservation of adult-like characteristics occur over succeeding weeks. The stability of the cultured slice renders it an appropriate model for studying 1) prolonged regulation/stabilization events linked to synaptogenesis and certain forms of plasticity, 2) temporal patterns of cellular atrophy associated with pathogenic conditions such as ischemia and epilepsia, and 3) slow processes associated with aging and age-related pathologies.

Stable maintenance of glutamate receptors and other synaptic components in long-term hippocampal slices

Cultured hippocampal slices retain many in vivo features with regard to circuitry, synaptic plasticity, and pathological responsiveness, while remaining accessible to a variety of experimental manipulations. The present study used ligand binding, immunostaining, and in situ hybridization assays to determine the stability of AMPA- and NMDA-type glutamate receptors and other synaptic proteins in slice cultures obtained from 11 day postnatal rats and maintained in culture for at least 4 weeks. Binding of the glutamate receptor ligands [3H]AMPA and [3H]MK-801 exhibited a small and transient decrease immediately after slice preparation, but the binding levels recovered by culture day (CD) 5-10 and remained stable for at least 30 days in culture. Autoradiographic analyses with both ligands revealed labeling of dendritic fields similar to adult tissue. In addition, slices at CD 10-20 expressed a low to high affinity [3H]AMPA binding ratio that was comparable with that in the adult hippocampus (10:1). AMPA receptor subunits GluR1 and GluR2/3 and an NMDA receptor subunit (NMDAR1) exhibited similar postcutting decreases as that exhibited by the ligand binding levels, followed by stable recovery. The GluR4 AMPA receptor subunit was not evident during the first 10 CDs but slowly reached detectable levels thereafter in some slices. Immunocytochemistry and in situ hybridization techniques revealed adult-like labeling of subunit proteins in dendritic processes and their mRNAs in neuronal cell body layers. Long-term maintenance was evident for other synapse-related proteins, including synaptophysin, neural cell adhesion molecule isoforms (NCAMs), and an AMPA receptor related antigen (GR53), as well as for certain structural and cytoskeletal components (e.g., myelin basic protein, spectrin, microtubule-associated proteins). In summary, following an initial and brief depression, many synaptic components were expressed at steady-state levels in long-term hippocampal slices, thus allowing the use of such a culture system for investigations into mechanisms of brain synapses.