New views on synapse-glia interactions

Enabling visualization of GFAP-positive retinal glial cells, neurons and microvasculature in three-dimensions

Glial cells are one of the most numerous cell types in the vertebrate retina and they serve to support neurovascular function. The principal glial cell in the retina is the Müller cell, accounting for approximately 90% of all retinal glial cells. Müller cells are phenotypically elongated in shape and were first described as 'radial fibers' by Heinrich Müller in 1851. Their structure spans the entire thickness of the retina, through all retinal layers from the internal to external limiting membrane. This unique three-dimensional spatial arrangement enables Müller cells' direct contact with almost all cell types in the retina to perform its function. Despite this, the current study of Müller cells has largely been limited to thin sections or in culture, which provide limited detail about its spatial arrangement and interconnection with other cell types. The novel technique described here enables the three-dimensional visualization of GFAP-positive Müller cell processes in rodent retina and is based on the isolated arterially perfused rat eye preparation. Our micro perfusion technique utilizes the microvasculature as the delivery channel to quickly and effectively preserve all retinal elements. Intravascular labelling enables visualization of the intact three-dimensional retinal microvasculature within its normal neuronal and glial confines. Additional immersion immunolabeling and subsequent clearing with RapiClear® enables the three-dimensional visualization of different retinal elements and their physical interaction. Volume rendering of confocal image stacks acquired from these specimens can facilitate the study of such interactions in normal and disease models to further our understanding. This technique may be replicated in human donor retinae for future investigations to provide insight into Müller cell form and spatial relationship with other cell types.

Differential roles of lysosomal cholesterol transporters in the development of C. elegans NMJs

Cholesterol homeostasis in neurons is critical for synapse formation and maintenance. Neurons with impaired cholesterol uptake undergo progressive synapse loss and eventual degeneration. To investigate the molecular mechanisms of neuronal cholesterol homeostasis and its role during synapse development, we studied motor neurons of Caenorhabditis elegans because these neurons rely on dietary cholesterol. Combining lipidomic analysis, we discovered that NCR-1, a lysosomal cholesterol transporter, promotes cholesterol absorption and synapse development. Loss of ncr-1 causes smaller synapses, and low cholesterol exacerbates the deficits. Moreover, NCR-1 deficiency hinders the increase in synapses under high cholesterol. Unexpectedly, NCR-2, the NCR-1 homolog, increases the use of cholesterol and sphingomyelins and impedes synapse formation. NCR-2 deficiency causes an increase in synapses regardless of cholesterol concentration. Inhibiting the degradation or synthesis of sphingomyelins can induce or suppress the synaptic phenotypes in ncr-2 mutants. Our findings indicate that neuronal cholesterol homeostasis is differentially controlled by two lysosomal cholesterol transporters and highlight the importance of neuronal cholesterol homeostasis in synapse development.

Glia-Driven Brain Circuit Refinement Is Altered by Early-Life Adversity: Behavioral Outcomes

Early-life adversity (ELA), often clinically referred to as "adverse childhood experiences (ACE)," is the exposure to stress-inducing events in childhood that can result in poor health outcomes. ELA negatively affects neurodevelopment in children and adolescents resulting in several behavioral deficits and increasing the risk of developing a myriad of neuropsychiatric disorders later in life. The neurobiological mechanisms by which ELA alters neurodevelopment in childhood have been the focus of numerous reviews. However, a comprehensive review of the mechanisms affecting adolescent neurodevelopment (i.e., synaptic pruning and myelination) is lacking. Synaptic pruning and myelination are glia-driven processes that are imperative for brain circuit refinement during the transition from adolescence to adulthood. Failure to optimize brain circuitry between key brain structures involved in learning and memory, such as the hippocampus and prefrontal cortex, leads to the emergence of maladaptive behaviors including increased anxiety or reduced executive function. As such, we review preclinical and clinical literature to explore the immediate and lasting effects of ELA on brain circuit development and refinement. Finally, we describe a number of therapeutic interventions best-suited to support adolescent neurodevelopment in children with a history of ELA.

Retinal glial remodeling by FGF21 preserves retinal function during photoreceptor degeneration

The group of retinal degenerations, retinitis pigmentosa (RP), comprises more than 150 genetic abnormalities affecting photoreceptors. Finding degenerative pathways common to all genetic abnormalities may allow general treatment such as neuroprotection. Neuroprotection may include enhancing the function of cells that directly support photoreceptors, retinal pigment epithelial cells, and Müller glia. Treatment with fibroblast growth factor 21 (FGF21), a neuroprotectant, from postnatal week 4-10, during rod and cone loss in P23H mice (an RP model) with retinal degeneration, preserved photoreceptor function and normalized Müller glial cell morphology. Single-cell transcriptomics of retinal cells showed that FGF21 receptor Fgfr1 was specifically expressed in Müller glia/astrocytes. Of all retinal cells, FGF21 predominantly affected genes in Müller glia/astrocytes with increased expression of axon development and synapse formation pathway genes. Therefore, enhancing retinal glial axon and synapse formation with neurons may preserve retinal function in RP and may suggest a general therapeutic approach for retinal degenerative diseases.

Fatty acid oxidation and photoreceptor metabolic needs

Photoreceptors have high energy demands and a high density of mitochondria that produce ATP through oxidative phosphorylation (OXPHOS) of fuel substrates. Although glucose is the major fuel for CNS brain neurons, in photoreceptors (also CNS), most glucose is not metabolized through OXPHOS but is instead metabolized into lactate by aerobic glycolysis. The major fuel sources for photoreceptor mitochondria remained unclear for almost six decades. Similar to other tissues (like heart and skeletal muscle) with high metabolic rates, photoreceptors were recently found to metabolize fatty acids (palmitate) through OXPHOS. Disruption of lipid entry into photoreceptors leads to extracellular lipid accumulation, suppressed glucose transporter expression, and a duel lipid/glucose fuel shortage. Modulation of lipid metabolism helps restore photoreceptor function. However, further elucidation of the types of lipids used as retinal energy sources, the metabolic interaction with other fuel pathways, as well as the cross-talk among retinal cells to provide energy to photoreceptors is not fully understood. In this review, we will focus on the current understanding of photoreceptor energy demand and sources, and potential future investigations of photoreceptor metabolism.

Calcium Signaling in Schwann cells

In addition to providing structural, metabolic and trophic support to neurons, glial cells of the central, peripheral and enteric nervous systems (CNS, PNS, ENS) respond to and regulate neural activity. One of the most well characterized features of this response is an increase of intracellular calcium. Astrocytes at synapses of the CNS, oligodendrocytes along axons of the CNS, enteric glia associated with the cell bodies and axonal varicosities of the ENS, and Schwann cells at the neuromuscular junction (NMJ) and along peripheral nerves of the PNS, all exhibit this response. Recent technical advances have facilitated the imaging of neural activity-dependent calcium responses in large populations of glial cells and thus provided a new tool to evaluate the physiological significance of these responses. This mini-review summarizes the mechanisms and functional role of activity-induced calcium signaling within Schwann cells, including terminal/perisynaptic Schwann cells (TPSCs) at the NMJ and axonal Schwann cells (ASCs) within peripheral nerves.

Dual roles of astrocytes in plasticity and reconstruction after traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of fatality and disability worldwide. Despite its high prevalence, effective treatment strategies for TBI are limited. Traumatic brain injury induces structural and functional alterations of astrocytes, the most abundant cell type in the brain. As a way of coping with the trauma, astrocytes respond in diverse mechanisms that result in reactive astrogliosis. Astrocytes are involved in the physiopathologic mechanisms of TBI in an extensive and sophisticated manner. Notably, astrocytes have dual roles in TBI, and some astrocyte-derived factors have double and opposite properties. Thus, the suppression or promotion of reactive astrogliosis does not have a substantial curative effect. In contrast, selective stimulation of the beneficial astrocyte-derived molecules and simultaneous attenuation of the deleterious factors based on the spatiotemporal-environment can provide a promising astrocyte-targeting therapeutic strategy. In the current review, we describe for the first time the specific dual roles of astrocytes in neuronal plasticity and reconstruction, including neurogenesis, synaptogenesis, angiogenesis, repair of the blood-brain barrier, and glial scar formation after TBI. We have also classified astrocyte-derived factors depending on their neuroprotective and neurotoxic roles to design more appropriate targeted therapies.

Neuroglial Involvement in Abnormal Glutamate Transport in the Cochlear Nuclei of the Igf1 -/- Mouse

Insulin-like growth factor 1 (IGF-1) is a powerful regulator of synaptic activity and a deficit in this protein has a profound impact on neurotransmission, mostly on excitatory synapses in both the developing and mature auditory system. Adult Igf1^−/− mice are animal models for the study of human syndromic deafness; they show altered cochlear projection patterns into abnormally developed auditory neurons along with impaired glutamate uptake in the cochlear nuclei, phenomena that probably reflect disruptions in neuronal circuits. To determine the cellular mechanisms that might be involved in regulating excitatory synaptic plasticity in 4-month-old Igf1^−/− mice, modifications to neuroglia, astroglial glutamate transporters (GLTs) and metabotropic glutamate receptors (mGluRs) were assessed in the cochlear nuclei. The Igf1^−/− mice show significant decreases in IBA1 (an ionized calcium-binding adapter) and glial fibrillary acidic protein (GFAP) mRNA expression and protein accumulation, as well as dampened mGluR expression in conjunction with enhanced glutamate transporter 1 (GLT1) expression. By contrast, no differences were observed in the expression of glutamate aspartate transporter (GLAST) between these Igf1^−/− mice and their heterozygous or wildtype littermates. These observations suggest that congenital IGF-1 deficiency may lead to alterations in microglia and astrocytes, an upregulation of GLT1, and the downregulation of groups I, II and III mGluRs. Understanding the molecular, biochemical and morphological mechanisms underlying neuronal plasticity in a mouse model of hearing deficits will give us insight into new therapeutic strategies that could help to maintain or even improve residual hearing when human deafness is related to IGF-1 deficiency.

Mitochondrial DNA depletion by ethidium bromide decreases neuronal mitochondrial creatine kinase: Implications for striatal energy metabolism

Mitochondrial DNA (mtDNA), the discrete genome which encodes subunits of the mitochondrial respiratory chain, is present at highly variable copy numbers across cell types. Though severe mtDNA depletion dramatically reduces mitochondrial function, the impact of tissue-specific mtDNA reduction remains debated. Previously, our lab identified reduced mtDNA quantity in the putamen of Parkinson's Disease (PD) patients who had developed L-DOPA Induced Dyskinesia (LID), compared to PD patients who had not developed LID and healthy subjects. Here, we present the consequences of mtDNA depletion by ethidium bromide (EtBr) treatment on the bioenergetic function of primary cultured neurons, astrocytes and neuron-enriched cocultures from rat striatum. We report that EtBr inhibition of mtDNA replication and transcription consistently reduces mitochondrial oxygen consumption, and that neurons are significantly more sensitive to EtBr than astrocytes. EtBr also increases glycolytic activity in astrocytes, whereas in neurons it reduces the expression of mitochondrial creatine kinase mRNA and levels of phosphocreatine. Further, we show that mitochondrial creatine kinase mRNA is similarly downregulated in dyskinetic PD patients, compared to both non-dyskinetic PD patients and healthy subjects. Our data support a hypothesis that reduced striatal mtDNA contributes to energetic dysregulation in the dyskinetic striatum by destabilizing the energy buffering system of the phosphocreatine/creatine shuttle.

Roles of Glial Cells in Sculpting Inhibitory Synapses and Neural Circuits

Glial cells are essential for every aspect of normal neuronal development, synapse formation, and function in the central nervous system (CNS). Astrocytes secrete a variety of factors that regulate synaptic connectivity and circuit formation. Microglia also modulate synapse development through phagocytic activity. Most of the known actions of CNS glial cells are limited to roles at excitatory synapses. Nevertheless, studies have indicated that both astrocytes and microglia shape inhibitory synaptic connections through various mechanisms, including release of regulatory molecules, direct contact with synaptic terminals, and utilization of mediators in the extracellular matrix. This review summarizes recent investigations into the mechanisms underlying CNS glial cell-mediated inhibitory synapse development.

In Vitro Innervation as an Experimental Model to Study the Expression and Functions of Acetylcholinesterase and Agrin in Human Skeletal Muscle

Acetylcholinesterase (AChE) and agrin, a heparan-sulfate proteoglycan, reside in the basal lamina of the neuromuscular junction (NMJ) and play key roles in cholinergic transmission and synaptogenesis. Unlike most NMJ components, AChE and agrin are expressed in skeletal muscle and α-motor neurons. AChE and agrin are also expressed in various other types of cells, where they have important alternative functions that are not related to their classical roles in NMJ. In this review, we first focus on co-cultures of embryonic rat spinal cord explants with human skeletal muscle cells as an experimental model to study functional innervation in vitro. We describe how this heterologous rat-human model, which enables experimentation on highly developed contracting human myotubes, offers unique opportunities for AChE and agrin research. We then highlight innovative approaches that were used to address salient questions regarding expression and alternative functions of AChE and agrin in developing human skeletal muscle. Results obtained in co-cultures are compared with those obtained in other models in the context of general advances in the field of AChE and agrin neurobiology.

Glia-neuron interactions in the mammalian retina

The mammalian retina provides an excellent opportunity to study glia-neuron interactions and the interactions of glia with blood vessels. Three main types of glial cells are found in the mammalian retina that serve to maintain retinal homeostasis: astrocytes, Müller cells and resident microglia. Müller cells, astrocytes and microglia not only provide structural support but they are also involved in metabolism, the phagocytosis of neuronal debris, the release of certain transmitters and trophic factors and K(+) uptake. Astrocytes are mostly located in the nerve fibre layer and they accompany the blood vessels in the inner nuclear layer. Indeed, like Müller cells, astrocytic processes cover the blood vessels forming the retinal blood barrier and they fulfil a significant role in ion homeostasis. Among other activities, microglia can be stimulated to fulfil a macrophage function, as well as to interact with other glial cells and neurons by secreting growth factors. This review summarizes the main functional relationships between retinal glial cells and neurons, presenting a general picture of the retina recently modified based on experimental observations. The preferential involvement of the distinct glia cells in terms of the activity in the retina is discussed, for example, while Müller cells may serve as progenitors of retinal neurons, astrocytes and microglia are responsible for synaptic pruning. Since different types of glia participate together in certain activities in the retina, it is imperative to explore the order of redundancy and to explore the heterogeneity among these cells. Recent studies revealed the association of glia cell heterogeneity with specific functions. Finally, the neuroprotective effects of glia on photoreceptors and ganglion cells under normal and adverse conditions will also be explored.

Thrombospondin 1 promotes synaptic formation in bone marrow-derived neuron-like cells

In this study, a combination of growth factors was used to induce bone marrow mesenchymal stem cells differentiation into neuron-like cells, in a broader attempt to observe the role of thrombospondin 1 in synapse formation. Results showed that there was no significant difference in the differentiation rate of neuron-like cells between bone marrow mesenchymal stem cells with thrombospondin induction and those without. However, the cell shape was more complex and the neurites were dendritic, with unipolar, bipolar or multipolar morphologies, after induction with thrombospondin 1. The induced cells were similar in morphology to normal neurites. Immunohistochemical staining showed that the number of positive cells for postsynaptic density protein 95 and synaptophysin 1 protein was significantly increased after induction with thrombospondin 1. These findings indicate that thrombospondin 1 promotes synapse formation in neuron-like cells that are differentiated from bone marrow mesenchymal stem cells.

Astrocyte glutamine synthetase: pivotal in health and disease

The multifunctional properties of astrocytes signify their importance in brain physiology and neurological function. In addition to defining the brain architecture, astrocytes are primary elements of brain ion, pH and neurotransmitter homoeostasis. GS (glutamine synthetase), which catalyses the ATP-dependent condensation of ammonia and glutamate to form glutamine, is an enzyme particularly found in astrocytes. GS plays a pivotal role in glutamate and glutamine homoeostasis, orchestrating astrocyte glutamate uptake/release and the glutamate-glutamine cycle. Furthermore, astrocytes bear the brunt of clearing ammonia in the brain, preventing neurotoxicity. The present review depicts the central function of astrocytes, concentrating on the importance of GS in glutamate/glutamine metabolism and ammonia detoxification in health and disease.

Abnormal cerebellar development and Purkinje cell defects in Lgl1-Pax2 conditional knockout mice

Lgl1 was initially identified as a tumour suppressor in flies and is characterised as a key regulator of epithelial polarity and asymmetric cell division. A previous study indicated that More-Cre-mediated Lgl1 knockout mice exhibited significant brain dysplasia and died within 24h after birth. To overcome early neonatal lethality, we generated Lgl1 conditional knockout mice mediated by Pax2-Cre, which is expressed in almost all cells in the cerebellum, and we examined the functions of Lgl1 in the cerebellum. Impaired motor coordination was detected in the mutant mice. Consistent with this abnormal behaviour, homozygous mice possessed a smaller cerebellum with fewer lobes, reduced granule precursor cell (GPC) proliferation, decreased Purkinje cell (PC) quantity and dendritic dysplasia. Loss of Lgl1 in the cerebellum led to hyperproliferation and impaired differentiation of neural progenitors in ventricular zone. Based on the TUNEL assay, we observed increased apoptosis in the cerebellum of mutant mice. We proposed that impaired differentiation and increased apoptosis may contribute to decreased PC quantity. To clarify the effect of Lgl1 on cerebellar granule cells, we used Math1-Cre to specifically delete Lgl1 in granule cells. Interestingly, the Lgl1-Math1 conditional knockout mice exhibited normal proliferation of GPCs and cerebellar development. Thus, we speculated that the reduction in the proliferation of GPCs in Lgl1-Pax2 conditional knockout mice may be secondary to the decreased number of PCs, which secrete the mitogenic factor Sonic hedgehog to regulate GPC proliferation. Taken together, these findings suggest that Lgl1 plays a key role in cerebellar development and folia formation by regulating the development of PCs.

Correlation of 3-mercaptopropionic acid induced seizures and changes in striatal neurotransmitters monitored by microdialysis

OBJECTIVES

The goal of this study was to use a status epilepticus steady-state chemical model in rats using the convulsant, 3-mercaptopropionic acid (3-MPA), and to compare the changes in striatal neurotransmission on a slow (5min) and fast (60s) timescale. In vivo microdialysis was combined with electrophysiological methods in order to provide a complete evaluation of the dynamics of the results obtained.

OBJECTIVE

To compare the effects of a steady-state chemical model pof status epilepticus on striatal amino-acid and amine neurotransmitters contents, as measured via in vivo microdialysis combined with electrophysiological methods. Measurements were performed on samples collected every 60s and every 5min. "Fast" (60s) and "slow" (5min) sampling timescales were selected, to gain more insight into the dynamics of GABA synthesis inhibition and of its effects on other neurotransmitters and on cortical electrical activity.

METHODS

3-MPA was administered in the form of an intra-venous load (60mg/kg) followed by a constant infusion (50mg/kg/min) for min. Microdialysis samples were collected from the striatum at intervals of 5min and 60s and analyzed for biogenic amine and amino acid neurotransmitters. ECoG activity was monitored via screws placed over the cortex.

RESULTS

In the 5min samples, glutamate (Glu) increased and γ-aminobutyric acid (GABA) decreased monotonically while changes in dopamine (DA) concentration were bimodal. In the sixty second samples, Glu changes were bimodal, a feature that was not apparent with the 5min samples. ECoG activity was indicative of status epilepticus.

CONCLUSIONS

This study describes the combination of in vivo microdialysis with electrophysiology to monitor the effect of 3-MPA on neurotransmission in the brain. This led to a better understanding of the chemical changes in the striatum due to the applied 3-MPA chemical model of status epilepticus.

Protective role of astrocytic leptin signaling against excitotoxicity

Both proconvulsive and anticonvulsive roles of leptin have been reported, suggesting cell-specific actions of leptin in different models of seizure and epilepsy. The goal of our study was to determine the regulation and function of astrocytic leptin receptors in a mouse model of epilepsy and glutamate-induced cytotoxicity. We show that in pilocarpine-challenged mice developing epilepsy with recurrent seizures after a latent period of 2 weeks, hippocampal leptin receptor (ObR) immunofluorescence was increased at 6 weeks. This was more pronounced in astrocytes than in neurons. In cultured astrocytes, glutamate increased ObRa and ObRb expression, whereas leptin pretreatment attenuated glial cytotoxicity by excess glutamate, reflected by better preserved adenosine triphosphate production. The protective role of astrocytic leptin signaling is further supported by the higher lethality of the astrocyte-specific leptin receptor knockout mice in the initial phase of seizure production. Thus, leptin signaling in astrocytes plays a protective role against seizure, and the effects are at least partially mediated by attenuation of glutamate toxicity. Astrocytic leptin signaling, therefore, may be a novel therapeutic target.

Astrocyte-induced synaptogenesis is mediated by transforming growth factor β signaling through modulation of D-serine levels in cerebral cortex neurons

Assembly of synapses requires proper coordination between pre- and postsynaptic elements. Identification of cellular and molecular events in synapse formation and maintenance is a key step to understand human perception, learning, memory, and cognition. A key role for astrocytes in synapse formation and function has been proposed. Here, we show that transforming growth factor β (TGF-β) signaling is a novel synaptogenic pathway for cortical neurons induced by murine and human astrocytes. By combining gain and loss of function approaches, we show that TGF-β1 induces the formation of functional synapses in mice. Further, TGF-β1-induced synaptogenesis involves neuronal activity and secretion of the co-agonist of the NMDA receptor, d-serine. Manipulation of d-serine signaling, by either genetic or pharmacological inhibition, prevented the TGF-β1 synaptogenic effect. Our data show a novel molecular mechanism that might impact synaptic function and emphasize the evolutionary aspect of the synaptogenic property of astrocytes, thus shedding light on new potential therapeutic targets for synaptic deficit diseases.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

Targeting S100B in Cerebral Ischemia and in Alzheimer's Disease

S100B is an EF-hand calcium-binding protein that exerts both intracellular and extracellular effects on a variety of cellular processes. The protein is predominantly expressed in the central nervous system by astrocytes, both physiologically and during the course of neurological disease. In the healthy adult brain and during development, constitutive S100B expression acts as a trophic factor to drive neurite extension and to referee neuroplasticity. Yet, when induced during central nervous system disease, the protein can take on maladaptive roles and thereby exacerbate brain pathology. Based on genetic and pharmacological lines of evidence, we consider such deleterious roles of S100B in two common brain pathologies: ischemic stroke and Alzheimer's disease (AD). In rodent models of ischemic brain damage, S100B is induced early on during the subacute phase, where it exacerbates gliosis and delayed infarct expansion and thereby worsens functional recovery. In mouse models of AD, S100B drives brain inflammation and gliosis that accelerate cerebral amyloidosis. Pharmacological inhibition of S100B synthesis mitigates hallmark pathologies of both brain diseases, opening the door for translational approaches to treat these devastating neurological disorders.

Where the thoughts dwell: the physiology of neuronal-glial "diffuse neural net"

The mechanisms underlying the production of thoughts by exceedingly complex cellular networks that construct the human brain constitute the most challenging problem of natural sciences. Our understanding of the brain function is very much shaped by the neuronal doctrine that assumes that neuronal networks represent the only substrate for cognition. These neuronal networks however are embedded into much larger and probably more complex network formed by neuroglia. The latter, although being electrically silent, employ many different mechanisms for intercellular signalling. It appears that astrocytes can control synaptic networks and in such a capacity they may represent an integral component of the computational power of the brain rather than being just brain "connective tissue". The fundamental question of whether neuroglia is involved in cognition and information processing remains, however, open. Indeed, a remarkable increase in the number of glial cells that distinguishes the human brain can be simply a result of exceedingly high specialisation of the neuronal networks, which delegated all matters of survival and maintenance to the neuroglia. At the same time potential power of analogue processing offered by internally connected glial networks may represent the alternative mechanism involved in cognition.

Neuronal damage is much delayed and microgliosis is more severe in the aged hippocampus induced by transient cerebral ischemia compared to the adult hippocampus

Activation of astrocytes and microglia in the post-ischemic hippocampus has been investigated using ischemia models. The aim of this study was to investigate differences of delayed neuronal death and gliosis in the hippocampal CA1 region (CA1) between adult and aged gerbils. Delayed neuronal death in the CA1 was later in the aged gerbil than in the adult gerbil after ischemia/reperfusion (I/R). GFAP-immunoreactive ((+)) astrocytes and Iba-1(+) microglia were activated following neuronal damage in both adult and aged gerbils after I/R. Changes in GFAP immunoreactivity and protein levels were similar in both groups: they were distinctly increased from 3 days after I/R. Iba-1 immunoreactivity and protein levels in the aged sham gerbil were much higher than those in the adult sham gerbil. Activation of microglia in the CA1 of the aged group was slower, lower 4 days and much higher 7 days than that in the adult gerbil after I/R. These observations indicate that delayed neuronal death in the CA1 of the aged group is slower than that in the adult group after I/R. In addition, microglial activation, not astrocytes, in the aged ischemia group is slower and more intense than that in the adult ischemia group.

Neuroglia in neurodegeneration

Neuroglial cells are fundamental for control of brain homeostasis and they represent the intrinsic brain defence system. All forms in neuropathology therefore inevitably involve glia. The neurodegenerative diseases disrupt connectivity within brain circuits affecting neuronal-neuronal, neuronal-glial and glial-glial contacts. In addition neurodegenerative processes trigger universal and conserved glial reactions represented by astrogliosis and microglial activation. The complex of recently acquired knowledge allows us to regard the neurodegenerative diseases as primarily gliodegenerative processes, in which glial cells determine the progression and outcome of neuropathological process.

Role of glial cells in the formation and maintenance of synapses

Synaptogenesis is a decisive process for the development of the brain, its plasticity during adulthood and its regeneration after injury and disease. Despite tremendous progress during the last decades, it remains unclear, whether neurons can form synapses autonomously. In this review, I will summarize recent evidence that this is probably not the case and that distinct phases of synapse development depend on help from glial cells. The results supporting this view come from studies on the central and peripheral nervous system and on different experimental models including cultured cells as well as living flies, worms and mice. Our understanding of synapse-glia interactions in the developing, adult and diseased brain is likely to advance more rapidly as new experimental approaches to identify, visualize and manipulate glial cells in vivo become available.

Neurons and Brain Macrophages Regulate Connexin Expression in Cultured Astrocytes

Neurons and brain macrophages (BM), respectively, increase and inhibit gap junctional communication (GJC) and connexin expression in cultured astrocytes. Thus, in brain diseases and injuries, neuronal death associated with the BM activation may decrease GJC in astrocytes and therefore have a physiopathological relevance.

Roles of glial cells in synapse development

Brain function relies on communication among neurons via highly specialized contacts, the synapses, and synaptic dysfunction lies at the heart of age-, disease-, and injury-induced defects of the nervous system. For these reasons, the formation-and repair-of synaptic connections is a major focus of neuroscience research. In this review, I summarize recent evidence that synapse development is not a cell-autonomous process and that its distinct phases depend on assistance from the so-called glial cells. The results supporting this view concern synapses in the central nervous system as well as neuromuscular junctions and originate from experimental models ranging from cell cultures to living flies, worms, and mice. Peeking at the future, I will highlight recent technical advances that are likely to revolutionize our views on synapse-glia interactions in the developing, adult and diseased brain.

Differential immunoreactivity of microglial and astrocytic marker protein in the hippocampus of the seizure resistant and sensitive gerbils

In the present study, we compared differences in ionized calcium-binding adapter molecule 1 (Iba-1) and glial fibrillary acidic protein (GFAP) immunoreactivities for microglia and astrocytes, respectively, in the hippocampus of the seizure-resistant (SR) and seizure-sensitive (SS) gerbils. The density of Iba-1 immunoreactive microglia in the hippocampal CA1 region (CA1) and dentate gyrus (DG) of the SS gerbil was higher than that in the SR gerbil, and many Iba-1 immunoreactive microglia in the SS gerbil were hypertrophied in morphology. In contrast, we could not find significant difference in the density of GFAP immunoreactive astrocytes between the SR and SS gerbils. This result indicates that Iba-1 immunoreactive microglia in CA1 and DG of the SS gerbil are activated compared to those in the SR gerbil.

Analysis of age dependent effects of heat stress on EEG frequency components in rats

OBJECTIVE

To demonstrate changes in different frequencies of cerebral electrical activity or electroencephalogram (EEG) following exposure to high environmental heat in three different age groups of freely moving' rats.

METHODS

Rats were divided into three groups (i) acute heat stress--subjected to a single exposure for four hours at 38 degrees C; (ii) chronic heat stress--exposed for 21 days daily for one hour at 38 degrees C, and (iii) handling control groups. The digital polygraphic sleep-EEG recordings were performed just after the heat exposure from acute stressed rats and on 22nd day from chronic stressed rats by simultaneous recording of cortical EEG, EOG (electrooculogram), and EMG (electromyogram). Further, power spectrum analyses were performed to analyze the effects of heat stress.

RESULTS

The frequency analysis of EEG signals following exposure to high environmental heat revealed that in all three age groups of rats, changes in higher frequency components (beta 2) were significant in all sleep-wake states following both acute and chronic heat stress conditions. After exposure to acute heat, significant changes in EEG frequencies with respect to their control groups were observed, which were reversed partly or fully in four hours of EEG recording. On the other hand, due to repetitive chronic exposure to hot environment, adaptive and long-term changes in EEG frequency patterns were observed.

CONCLUSION

The present study has exhibited that the cortical EEG is sensitive to environmental heat and alterations in EEG frequencies in different sleep-wake states due to heat stress can be differentiated efficiently by EEG power spectrum analysis.

Effects of glucose and glutamine concentration in the formulation of the artificial cerebrospinal fluid (ACSF)

The composition of the ACSF is fundamental in controlling the extracellular environment of the brain slice preparation. 'Typical' formulations lack amino acids and contain a higher concentration of glucose (10 mM) than in the cerebrospinal fluid (0.47-4.4 mM). We examined the effects of different concentrations of glutamine, the most abundant amino acid in the CSF, and glucose on rat hippocampal slice physiology. Bipolar paired-pulse stimulation was applied to the Schaffer collaterals and population spikes were monitored in the CA1 pyramidal layer for approximately 1 hour. Addition of glutamine (0.5 mM) to slices superfused with 10 mM of glucose did not enhance population spike amplitude. Higher concentration of glutamine (2-5 mM) resulted in spreading-depression. Decreasing glucose concentration from 10 mM to 5 mM, in the absence of glutamine, attenuated population spikes. Decreasing glucose to 2 mM, in the absence of glutamine, suppressed evoked population spikes. Superfusing brain slices with ACSF containing 'physiological' concentrations of both glucose (2 mM) and glutamine (0.5 mM) similarly suppressed population spikes. In separate experiments, during high-K+ induced epileptiform activity, glutamine (0.5 mM) did not affect the burst duration, frequency or waveform. These results suggest that the concentration of glucose in ACSF should conservatively be 10 mM in order to maximize paired-pulse population responses while the presence of physiological concentration of glutamine (0.5 mM) has minimal effects on paired-pulse responses and high-K+ induced epileptiform activity. These results are discussed in the context of fundamental differences between in vitro brain slice superfusion and in vivo brain perfusion.

Local changes in the redox potential in the rabbit cerebral cortex accompanying the acquisition of a conditioned defensive reflex

The oxidative-reductive (redox) potential (E) of brain tissue depends on the ratio of the speeds of processes occurring in the glycolysis (the evolutionarily ancient energy compartment operating without oxygen) and oxidative metabolism (evolutionarily younger and energetically more efficient) compartments. E in the cortex was recorded using implanted platinum electrodes. A conditioned defensive reflex (CDR) was developed by combination of a light and electrocutaneous stimulation (ECS) of the ear. The results showed that after a series of combinations of the light and the ECS, the light started to elicit a change in E. By 200 combinations, the brain developed both increases and decreases in E during combinations. As the number of combinations increased, increases in E were gradually replaced by decreases. We believe that this dynamic of the balance of the major sources of brain energy supply suggests that formation of the CDR may involve a significant role for subcellular structures receiving energy from oxidative metabolism formed at the relatively young evolutionary level, while the major source of energy for brain function during performance of the acquired CDR is the older evolutionary compartment - glycolysis.

Clinical neurophysiology of aging brain: from normal aging to neurodegeneration

Physiological brain aging is characterized by a loss of synaptic contacts and neuronal apoptosis that provokes age-dependent decline of sensory processing, motor performance, and cognitive function. Neural redundancy and plastic remodelling of brain networking, also secondary to mental and physical training, promotes maintenance of brain activity in healthy elderly for everyday life and fully productive affective and intellectual capabilities. However, age is the main risk factor for neurodegenerative disorders such as Alzheimer's disease (AD) that impact on cognition. Oscillatory electromagnetic brain activity is a hallmark of neuronal network function in various brain regions. Modern neurophysiological techniques including electroencephalography (EEG), event-related potential (ERP), magnetoencephalography (MEG), and transcranial magnetic stimulation (TMS) can accurately index normal and abnormal brain aging to facilitate non-invasive analysis of cortico-cortical connectivity and neuronal synchronization of firing and coherence of rhythmic oscillations at various frequencies. The present review provides a perspective of these issues by assaying different neurophysiological methods and integrating the results with functional brain imaging findings. It is concluded that discrimination between physiological and pathological brain aging clearly emerges at the group level, with applications at the individual level also suggested. Integrated approaches utilizing neurophysiological techniques together with biological markers and structural and functional imaging are promising for large-scale, low-cost and non-invasive evaluation of at-risk populations. Practical implications of the methods are emphasized.

Increase in glutamate as a sensitive indicator of extracellular matrix integrity in peritumoral edema: a 3.0-tesla proton magnetic resonance spectroscopy study

OBJECT

The authors of previous studies based on diffusion tensor imaging have indicated that there are two types of peritumoral edema-namely, edema with preserved structural integrity of the glial matrix and edema with compromised glial matrix. The authors of this study hypothesized that functionality of the glutamate (Glu)-glutamine shuttle, a vital neuron-glia interaction, may be differentially affected by peritumoral edema. They tested this hypothesis using proton magnetic resonance (MR) spectroscopy on a 3.0-tesla system that is capable of quantifying Glu without need of editing.

METHODS

Twenty-three patients, each with a single brain tumor mass and peritumoral edema (nine high-grade gliomas, eight metastatic brain tumors, and six meningiomas), and nine healthy individuals participated in this study. Single-voxel proton MR imaging targeting the region of peritumoral edema was performed using a 3.0-tesla system. Glutamate levels in the peritumoral edema of nonglial tumors was significantly elevated (p < 0.01) compared with edema associated with glial tumors or normal white matter. The finding confirmed that peritumoral edema in nonglial tumors is distinct from that of glial tumors, as previously indicated in diffusion tensor imaging studies. The authors hypothesized that the former condition represents a compensatory increase in activities of the Glu-glutamine shuttle brought about by simple expansion of the extracellular space due to edema.

CONCLUSIONS

The assessment of Glu concentrations in peritumoral edema using 3.0-tesla proton MR spectroscopy may be developed into an objective index of the structural integrity of the glial matrix.

Glutamate release from astrocytes in physiological conditions and in neurodegenerative disorders characterized by neuroinflammation

Although glial cells have been traditionally viewed as supportive partners of neurons, studies of the last 20 years demonstrate that astrocytes possess functional receptors for neurotransmitters and other signaling molecules and respond to their stimulation via release of chemical transmitters (called gliotransmitters) such as glutamate, ATP, and d-serine. Notably, astrocytes react to synaptically released neurotransmitters with intracellular calcium ([Ca(2+)](i)) elevations, which result in the release of glutamate via regulated exocytosis and possibly other mechanisms. These findings have led to a new concept of neuron-glia intercommunication where astrocytes play an unsuspected dynamic role by integrating neuronal inputs and modulating synaptic activity. The additional discovery that glutamate release from astrocytes is controlled by molecules linked to inflammatory reactions, such as the cytokine tumor necrosis factor-alpha (TNF-alpha) and prostaglandins, suggests that glia-to-neuron signaling may be sensitive to changes in production of these mediators in pathological conditions. Indeed, a local, parenchymal brain inflammatory reaction (neuroinflammation) characterized by astrocytic and microglial activation has been reported in several neurodegenerative disorders, including Alzheimer's disease and AIDS dementia complex. This transition to a reactive state may be accompanied by a disruption of the cross talk normally occurring between astrocytes and neurons and so contribute to disease development. The findings reported in this chapter suggest that a better comprehension of the glutamatergic interplay between neurons and glia may provide information about normal brain function and also highlight possible molecular targets for therapeutic interventions in pathology.

Erratum to "Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintenance of epilepsy." [Pharmacol. Ther. 105(3) (2005) 229-266]

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury [central nervous system (CNS) insult]. (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels ([Ca(2+)](i)) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but the share a common molecular mechanism for producing brain damage--an increase in extracellular glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Guanosine effect on cholesterol efflux and apolipoprotein E expression in astrocytes

The main source of cholesterol in the central nervous system (CNS) is represented by glial cells, mainly astrocytes, which also synthesise and secrete apolipoproteins, in particular apolipoprotein E (ApoE), the major apolipoprotein in the brain, thus generating cholesterol-rich high density lipoproteins (HDLs). This cholesterol trafficking, even though still poorly known, is considered to play a key role in different aspects of neuronal plasticity and in the stabilisation of synaptic transmission. Moreover, cell cholesterol depletion has recently been linked to a reduction in amyloid beta formation. Here we demonstrate that guanosine, which we previously reported to exert several neuroprotective effects, was able to increase cholesterol efflux from astrocytes and C6 rat glioma cells in the absence of exogenously added acceptors. In this effect the phosphoinositide 3 kinase/extracellular signal-regulated kinase 1/2 (PI3K/ERK1/2) pathway seems to play a pivotal role. Guanosine was also able to increase the expression of ApoE in astrocytes, whereas it did not modify the levels of ATP-binding cassette protein A1 (ABCA1), considered the main cholesterol transporter in the CNS. Given the emerging role of cholesterol balance in neuronal repair, these effects provide evidence for a role of guanosine as a potential pharmacological tool in the modulation of cholesterol homeostasis in the brain.

Progesterone-induced agrin expression in astrocytes modulates glia-neuron interactions leading to synapse formation

Experimental evidence recently obtained suggests that synaptogenesis is a tripartite event in which not only pre- and post-synaptic neurons but also glial cells play a key role. However, the molecular mechanisms by which glia modulate the formation of synapses in the CNS remain poorly understood. In the present study, we analyzed the role of astrocytes in synapse formation in cultured hippocampal rat neurons. For these experiments, hippocampal neurons were cultured in the presence or absence of a monolayer of astrocytes. Our results indicated that hippocampal neurons cultured in the presence of astrocytes formed more synapses than the ones cultured in their absence only when kept in N2 serum-free medium. To get insights into the potential molecular mechanisms underlying this effect, we analyzed the expression of proteins known to induce synapse formation in hippocampal neurons. A significant increase in agrin expression was detected in astrocytes cultured in N2 serum-free medium when compared with the ones cultured in serum containing medium. Experiments performed using different components of the N2 mixture indicated that progesterone induced the expression of agrin in astrocytes. Taken collectively, these results provide evidence supporting a role for astrocytes in synapse formation in central neurons. Furthermore, they identified agrin as a potential mediator of this effect, and astrocytes as a bridge between the endocrine and nervous systems during synaptogenesis.

Astrocytes regulate inhibitory synapse formation via Trk-mediated modulation of postsynaptic GABAA receptors

Astrocytes promote the formation and function of excitatory synapses in the CNS. However, whether and how astrocytes modulate inhibitory synaptogenesis are essentially unknown. We asked whether astrocytes regulate the formation of inhibitory synapses between hippocampal neurons during maturation in vitro. Neuronal coculture with astrocytes or treatment with astrocyte-conditioned medium (ACM) increased the number of inhibitory presynaptic terminals, the frequency of miniature IPSCs, and the number and synaptic localization of GABA(A) receptor (GABA(A)R) clusters during the first 10 d in vitro. We asked whether neurotrophins, which are potent modulators of inhibitory synaptic structure and function, mediate the effects of astrocytes on inhibitory synapses. ACM from BDNF- or tyrosine receptor kinase B (TrkB)-deficient astrocytes increased inhibitory presynaptic terminals and postsynaptic GABA(A)R clusters in wild-type neurons, suggesting that BDNF and TrkB expression in astrocytes is not required for these effects. In contrast, although the increase in the number of inhibitory presynaptic terminals persisted, no increase was observed in postsynaptic GABA(A)R clusters after ACM treatment of hippocampal neurons lacking BDNF or TrkB. These results suggest that neurons, not astrocytes, are the relevant source of BDNF and are the site of TrkB activation required for postsynaptic GABA(A)R modulation. These data also suggest that astrocytes may modulate postsynaptic development indirectly by stimulating Trk signaling between neurons. Together, these data show that astrocytes modulate inhibitory synapse formation via distinct presynaptic and postsynaptic mechanisms.

Mechanisms of vertebrate synaptogenesis

The formation of synapses in the vertebrate central nervous system is a complex process that occurs over a protracted period of development. Recent work has begun to unravel the mysteries of synaptogenesis, demonstrating the existence of multiple molecules that influence not only when and where synapses form but also synaptic specificity and stability. Some of these molecules act at a distance, steering axons to their correct receptive fields and promoting neuronal differentiation and maturation, whereas others act at the time of contact, providing positional information about the appropriateness of targets and/or inductive signals that trigger the cascade of events leading to synapse formation. In addition, correlated synaptic activity provides critical information about the appropriateness of synaptic connections, thereby influencing synapse stability and elimination. Although synapse formation and elimination are hallmarks of early development, these processes are also fundamental to learning, memory, and cognition in the mature brain.

In vivo modulatory action of extracellular glutamate on the anticonvulsant effects of hippocampal dopamine and serotonin

PURPOSE

Our recent work (Clinckers et al., J Neurochem 2004;89:834-43) demonstrated that intrahippocampal perfusion of 2 nM dopamine or serotonin via a microdialysis probe offered complete protection against focal pilocarpine-induced limbic seizures and did not influence basal extracellular hippocampal glutamate levels. Ten nanomolar dopamine or serotonin perfusion, however, worsened seizures and was accompanied by significant extracellular glutamate increases to approximately 200%. The significance of these glutamate elevations in seizure generation remains unclear. The present microdialysis study investigated the modulatory role of extracellular hippocampal glutamate levels in these monoaminergic protective and proconvulsant effects.

METHODS

A first group of male Wistar albino rats was perfused intrahippocampally for 240 min with 6.25 microM glutamate alone to increase extracellular levels by 200%. Other animals were perfused with anticonvulsant concentrations of monoamines throughout the experiments while receiving continuous coperfusions of 6.25 microM glutamate either before, during, and after (240 min) or only after (100 min) pilocarpine perfusion (40 min). Rats were scored for epileptic behavior, and the mean scores were compared with those of the control group. Microdialysates were analyzed for monoamine and glutamate content with microbore liquid chromatography.

RESULTS

No convulsions occurred during glutamate perfusion alone. When monoamines and glutamate were coperfused before pilocarpine administration, the anticonvulsant effect of the monoamines was lost. Glutamate addition after pilocarpine administration did not affect monoaminergic seizure protection.

CONCLUSIONS

These results indicate that extracellular glutamate increases per se do not necessarily induce seizures but that they can modulate the anticonvulsant effects exerted by hippocampal monoamines.

Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintainance of epilepsy

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury (central nervous system [CNS] insult), (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels [Ca(2+)](i) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but they share a common molecular mechanism for producing brain damage-an increase in extracellular glutamate concentration that causes increased intracellular neuronal calcium, leading to neuronal injury and/or death. Neurons that survive the injury induced by glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Architectural organisation of neuronal activity-associated magnetic fields: a hypothesis for memory

Despite intensive investigation into the mechanisms underlying the memory process, the physical bases for this superior cognitive function remain elusive. Neuronal activity-associated magnetic fields (NAAMFs) hypothesis of memory suggests that items of information are stored as three-dimensional bundles of magnetic fields associated to the complex but extremely organised cerebral cortex. The present paper proposes a plausible architectural organisation of neuronal activity-associated magnetic fields that may explain how information could be stored in the human cerebral cortex. Magnetic fields generated as consequence of neuronal minicolumns activation could modify the basal "electromagnetic status" of the closest astrocytes allowing codification and storage of information.

Immature developmental pattern of the monosynaptic reflex in isolated spinal cord of glial mutant taiep rats

There is increasing evidence suggesting that glial cells play a crucial role in the formation and maturation of neural circuits. However, little is known about the effects of glial alterations on the establishment of functional circuitry in vivo during the development. The taiep rat, a long-lived neurological mutant characterized by early astrogliosis and demyelination affecting selectively the CNS, provides an interesting model to study the glia-neuron interaction in situ. In the present study, we evaluated the functional development of segmental neural circuits recording the monosynaptic reflex responses (MSR) in the isolated spinal cord of neonatal taiep rats. To evaluate the developmental changes during the first two postnatal weeks, we measured the latency of MSR, the magnitude of depression to paired pulses and the time course of post-tetanic recovery. During the early postnatal period, the MSR of control rats reduced their latency and decreased their sensitivity to depression, as a function of age. By contrast, the MSR of taiep rats failed to develop further from neonatal stage. Near the end of the second postnatal week, the MSR latencies were still prolonged, and the MSR showed a significantly stronger paired pulse depression, and higher post-tetanic recovery times than the age-matched controls. The lack of MSR maturation in taiep rats suggests an early alteration of functional mechanisms underlying the maturation of the spinal reflexes, probably due to the characteristic glial dysfunction(s) of this mutant.

Electro-encephalogram disturbances in different sleep-wake states following exposure to high environmental heat

In this study, cerebral electrical activity or electro-encephalogram (EEG) was studied following exposure to high environmental heat, in three different age groups of freely moving rats. Each age group was subdivided into three groups: the acute heat stress group, subjected to a single exposure of 4 h at 38 degrees C in the biological oxygen demand incubator; the chronic heat stress group, exposed for 21 days, for 1 h each day, at 38 degrees C in the incubator; and the handling control group. The polygraphic sleep-wake recordings involved simultaneous recordings of cortical EEG, electro-oculogram (EOG), and electromyogram (EMG), on paper and in digital form on computer hard disk, just after the heat exposure for the acute stressed rats and on the 22nd day for the chronic stressed rats. The power spectrum was calculated for 2s epochs of the EEG signals. Quantitative analyses of EEG (qEEG) showed that, in all three age groups, changes in higher-frequency components (beta2) were significant in all sleep-wake states following both acute and chronic heat stress conditions. The power of beta2 activity in all three age groups after acute heat exposure was significantly decreased during slow wave sleep (SWS) (p < 0.05) and rapid eye movement sleep (p < 0.05), whereas the reverse was observed in the awake state (p < 0.05). Following chronic heat exposure, beta2 activity was found to increase in all three sleep-wake stages in all groups of rats (p < 0.01 for SWS in the weaning group and p < 0.05 for other data). Thus the study demonstrated that the cortical EEG is sensitive to environmental heat, and alterations in EEG frequencies in different states of mental consciousness due to high heat can be differentiated efficiently by EEG power spectrum analysis.

Maturation of astrocyte morphology and the establishment of astrocyte domains during postnatal hippocampal development

Mature protoplasmic astrocytes exhibit an extremely dense ramification of fine processes, yielding a 'spongiform' morphology. This complex morphology enables protoplasmic astrocytes to maintain intimate relationships with many elements of the brain parenchyma, most notably synapses. Recently, it has been demonstrated that astrocytes establish individual cellular-level domains within the neuropil, with limited overlap occurring between the extents of neighboring astrocytes. The highly ramified nature of protoplasmic astrocytes is closely associated with their ability to create such domains. This study was an attempt to characterize the development of spongiform processes and the establishment of astrocyte domains. A combination of immunolabeling for the astrocyte-specific markers glial fibrillary acidic protein and S100beta with intracellular dye labeling in fixed tissue slices allowed for the identification of immature astrocytes and the elucidation of their complete, well-preserved morphologies. We find that during the first two postnatal weeks astrocytes extend stringy, filopodial processes. Fine, spongiform processes appear during the third week. Protoplasmic astrocytes are quite heterogeneous in morphology at 1-week postnatum, but there is a remarkable consistency in morphology by 2 weeks of age. Finally, protoplasmic astrocytes initially extend long, overlapping processes during the first two postnatal weeks. The subsequent elaboration of spongiform processes results in the development of boundaries between neighboring astrocyte domains. Stray processes that encroach on neighboring domains are eventually pruned by 1 month of age. These observations suggest that domain formation is largely the consequence of competition between astrocyte processes, similar to the well-studied competitive interactions between certain neuronal dendritic fields.

Integrated technology for evaluation of brain function and neural plasticity

The study of neural plasticity has expanded rapidly in the past decades and has shown the remarkable ability of the developing, adult, and aging brain to be shaped by environmental inputs in health and after a lesion. Robust experimental evidence supports the hypothesis that neuronal aggregates adjacent to a lesion in the sensorimotor brain areas can take over progressively the function previously played by the damaged neurons. It definitely is accepted that such a reorganization modifies sensibly the interhemispheric differences in somatotopic organization of the sensorimotor cortices. This reorganization largely subtends clinical recovery of motor performances and sensorimotor integration after a stroke. Brain functional imaging studies show that recovery from hemiplegic strokes is associated with a marked reorganization of the activation patterns of specific brain structures. To regain hand motor control, the recovery process tends over time to bring the bilateral motor network activation toward a more normal intensity/extent, while overrecruiting simultaneously new areas, perhaps to sustain this process. Considerable intersubject variability exists in activation/hyperactivation pattern changes over time. Some patients display late-appearing dorsolateral prefrontal cortex activation, suggesting the development of "executive" strategies to compensate for the lost function. The AH in stroke often undergoes a significant "remodeling" of sensory and motor hand somatotopy outside the "normal" areas, or enlargement of the hand representation. The UH also undergoes reorganization, although to a lesser degree. Although absolute values of the investigated parameters fluctuate across subjects, secondary to individual anatomic variability, variation is minimal with regards to interhemispheric differences, due to the fact that individual morphometric characters are mirrored in the two hemispheres. Excessive interhemispheric asymmetry of the sensorimotor hand areas seems to be the parameter with highest sensitivity in describing brain reorganization after a monohemispheric lesion, and mapping motor and somatosensory cortical areas through focal TMS, fMRI, PET, EEG, and MEG is useful in studying hand representation and interhemispheric asymmetries in normal and pathologic conditions. TMS and MEG allow the detection of sensorimotor areas reshaping, as a result of either neuronal reorganization or recovery of the previously damaged neural network. These techniques have the advantage of high temporal resolution but also have limitations. TMS provides only bidimensional scalp maps, whereas MEG, even if giving three-dimensional mapping of generator sources, does so by means of inverse procedures that rely on the choice of a mathematical model of the head and the sources. These techniques do not test movement execution and sensorimotor integration as used in everyday life. fMRI and PET may provide the ideal means to integrate the findings obtained with the other two techniques. This multitechnology combined approach is at present the best way to test the presence and amount of plasticity phenomena underlying partial or total recovery of several functions, sensorimotor above all. Dynamic patterns of recovery are emerging progressively from the relevant literature. Enhanced recruitment of the affected cortex, be it spared perilesional tissue, as in the case of cortical stroke, or intact but deafferented cortex, as in subcortical strokes, seems to be the rule, a mechanism especially important in early postinsult stages. The transfer over time of preferential activation toward contralesional cortices, as observed in some cases, seems, however, to reflect a less efficient type of plastic reorganization, with some aspects of maladaptive plasticity. Reinforcing the use of the affected side can cause activation to increase again in the affected side with a corresponding enhancement of clinical function. Activation of the UH MI may represent recruitment of direct (uncrossed) corticospinal tracts and relate more to mirror movements, but it more likely reflects activity redistribution within preexisting bilateral, large-scale motor networks. Finally, activation of areas not normally engaged in the dysfunctional tasks, such as the dorsolateral prefrontal cortex or the superior parietal cortex in motor paralysis, might reflect the implication of compensatory cognitive strategies. An integrated approach with technologies able to investigate functional brain imaging is of considerable value in providing information on the excitability, extension, localization, and functional hierarchy of cortical brain areas. Deepening knowledge of the mechanisms regulating the long-term recovery (even if partial), observed for most neurologic sequelae after neural damage, might prompt newer and more efficacious therapeutic and rehabilitative strategies for neurologic diseases.

Immunocytochemical expression of monocarboxylate transporters in the human visual cortex at midgestation

Lactate and the other monocarboxylates are a major energy source for the developing brain. We investigated the immunocytochemical expression of two monocarboxylate transporters, MCT1 and MCT2, in the human visual cortex between 13 and 26 post-ovulatory weeks. We used immunoperoxidase and immunofluorescence techniques to determine whether these transporters co-localized with markers for blood vessels (CD34), neurons (microtubule-associated protein 2 [MAP2], SMI 311), radial glia (vimentin), or astrocytes (glial fibrillary acidic protein [GFAP], S100beta protein). MCT1 immunoreactivity was visible in blood vessel walls as early as the 13th week of gestation mainly in the cortical plate and subplate. At this stage, less than 10% of vessels in the ventricular layer expressed MCT1, whereas all blood vessels walls showed this immunoreactivity at the 26th gestational week. Starting at the 19th week of gestation, sparse MCT1 positive cell bodies were detected, some of them co-localized with MAP2 immunoreactivity. MCT2 immunoreactivity was noted in astrocytic cell bodies from week 19 and spread subsequently to the astrocyte end-feet in contact with blood vessels. MCTs immunoreactivities were most marked in the subplate and deep cortical plate, where the most differentiated neurons were located. Our findings suggest that monocarboxylate trafficking between vessels (MCT1), astrocytes (MCT2) and some postmitotic neurons (MCT1) could develop gradually toward 20 gestational weeks (g.w.). These data suggest that lactate or other monocarboxylates could represent a significant energy source for the human visual cortex at this early stage.