Astrocytic contribution to functioning synapse formation estimated by spontaneous neuronal intracellular Ca2+ oscillations

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.

Napabucasin prevents brain injury in neuronal neonatal rat cells through suppression of apoptosis and inflammation

The present study investigates the protective effect of napabucasin on the expression of apoptosis markers and inflammatory factors in the neuronal rat cells with post-isolation damage. The level of ROS determined by the fluorescence measurement in the neuronal rat cells with post-isolation damage was 310.21 RFU compared to 21.45 RFU in sham cell cultures. Napabucasin treatment decreased ROS level in the neuronal rat cells with post-isolation damage in dose based manner. ROS level decreased to 278.67, 203.65, 163.32 and 26.87 RFU, respectively in 1, 2, 3 and 4 μM napabucasin treated cell cultures. Treatment with napabucasin increased GSH level significantly (P < 0.05) in the neuronal rat cells with post-isolation damage. Napabucasin treatment at with 1, 2, 3 and 4 μM concentrations increased SOD activity to 2.4, 3.6, 5.1 and 6.1 U/mg, respectively. Treatment with napabucasin increased the activity of catalase in dose based manner. Napabucasin treatment increased Gpx in injured brain cells of neonatal rats. A significant (P < 0.05) increase in the activity of AChE was observed in neuronal rat cells with post-isolation damage on treatment with napabucasin. Treatment with napabucasin reduced the level of TNF-α and IL-6 significantly (P < 0.05) compared to untreated group. Napabucasin treatment decreased the expression of Bax, caspase-3 and p53 proteins in the neuronal rat cells with post-isolation damage. Napabucasin treatment protects post-isolation damage in the neuronal cells of neonatal rats by suppression of apoptosis and oxidative stress. Therefore, napabucasin can be used for the treatment of brain injury.

N-terminal cleaved pancreatitis-associated protein-III (PAP-III) serves as a scaffold for neurites and promotes neurite outgrowth

Pancreatitis-associated protein (PAP)-III, also known as regenerating gene/regenerating islet-derived (Reg)-IIIγ, is a small secretory protein whose expression is substantially induced in injured nerves. Here, we found that PAP-III protein underwent proteolytic N-terminal processing by trypsin-like protease(s) in injured sciatic nerves after axotomy. In vitro studies demonstrated that the N terminus-truncated PAP-III (ΔN-PAP-III) polymerized into a filament with a relatively uniform diameter of 10-20 nm, and the filaments formed higher order structures in a Na(+) concentration-dependent manner. When the ΔN-PAP-III fibers were added to the culture media, the ΔN-PAP-III fibers were tightly attached to neurites and somata of primary cortical neurons in vitro. In contrast, little association with glial cells was observed. When dense matrices of ΔN-PAP-III fibers were sheeted on a culture dish, neurites preferentially adhered to the fibers, and neurite extension was enhanced. This neurite outgrowth activity was significantly suppressed by preincubation with antibodies against PAP-III. These results imply that the released PAP-III might be cleaved and forms ΔN-PAP-III fibers at the nerve injury sites. Consequently, these resulting fibers would provide regenerating axons with a platform for extension.

Intrinsic and extrinsic mechanisms control the termination of cortical interneuron migration

During development, neurons migrate from their site of origin to their final destinations. Upon reaching this destination, the termination of their migration is crucial for building functional architectures such as laminated structures and nuclei. How this termination is regulated, however, is not clear. Here, we investigated the contribution of cell-intrinsic mechanisms and extrinsic factors. Using GAD67-GFP knock-in mice and in utero electroporation cell labeling, we visualized GABAergic neurons and analyzed their motility in vitro. We find that the motility of GABAergic neurons in cortical slices gradually decreases as development proceeds and is almost abolished by the end of the first postnatal week. Consistent with this, a reduction of embryonic interneuron motility occurred in dissociated cultures. This is in part due to cell-intrinsic mechanisms, as a reduction in motility is observed during long-term culturing on glial feeder cells. Cell-intrinsic regulation is further supported by observations that interneurons labeled in early stages migrated more actively than those labeled in late stages in the same cortical explant. We found evidence suggesting that upregulation of the potassium-chloride cotransporter KCC2 underlies this intrinsic regulation. Reduced motility is also observed when embryonic interneurons are plated on postnatal cortical feeder cells, suggesting extrinsic factors derived from the postnatal cortex too contribute to termination. These factors should include secreted molecules, as cultured postnatal cortical cells could exercise this effect without directly contacting the interneuron. These findings suggest that intrinsic mechanisms and extrinsic factors coordinate to reduce the motility of migrating neurons, thereby leading to the termination of migration.

Glial-derived neurotrophic factor (GDNF) prevents ethanol (EtOH) induced B92 glial cell death by both PI3K/AKT and MEK/ERK signaling pathways

We investigated the neuroprotective effect of glial-derived neurotrophic factor (GDNF) upon alcohol-exposed B92 cultures, as well as the role of the cytoskeleton and mitogen-activated protein kinase (MAPK) pathways in this effect. Ethanol (EtOH) was added to cultures, either alone or in combination with 30 ng/ml GDNF. Exposure to EtOH (86 and 172 mM; 60 and 120 min) increased the frequency of apoptotic cells identified by nuclear DNA staining with 4,6-diamidino-2-phenylindole (DAPI). Cultures treated with GDNF showed a decrease in ethanol-induced apoptosis. A jun N-terminal kinase (JNK) pathway is activated by EtOH and their pharmacological inhibition (by SP600125) neutralized ethanol-induced apoptosis, suggesting a role for JNK in EtOH neurotoxicity. Immunocytochemically detected phospho-JNK (p-JNK) showed an unusual filamental expression, and localized together with actin stress fibers. Examination of the cytoskeleton showed that EtOH depolymerized actin filaments, inducing p-JNK dissociation and translocation to the nucleus, which suggests that released p-JNK may contribute to glial cell death after EtOH exposure. Treatment with GDNF, in turn, may neutralize the ethanol-induced cell death pathway. Either a phosphatidylinositol 3-kinase (PI3K)/AKT pathway inhibitor (LY294002) or an inhibitor of the extracellular signal-regulated kinase (ERK) 1, 2 pathways (UO126) failed to neutralize GDNF protective effects. However, the simultaneous use of both inhibitors blocked the protective effect of GDNF, suggesting a role for both signaling cascades in the GDNF protection. These findings provide further insight into the mechanism involved in ethanol-induced apoptosis and the neurotrophic protection of glial cells.

Functional significance of the preconditioning-induced down-regulation of glutamate transporter GLT-1 in neuron/astrocyte co-cultures

In the brain, prior sublethal ischemia (preconditioning, PC) is known to produce tolerance of neurons to subsequent lethal ischemia. This study aims at elucidating what alterations were induced in neurons and/or astrocytes by PC treatment. The rise in the extracellular concentration of glutamate during ischemia was markedly suppressed by the prior PC treatment. Immunocytochemical and Western blot analyses demonstrated that the expression of the astrocytic glutamate transporter GLT-1 was transiently down-regulated after the PC insult. The PC insult possibly suppressed the neuron-derived factors up-regulating GLT-1. Here we show that PC-induced down-regulation of GLT-1 is crucial for the increased neuronal resistance to subsequent severe ischemic insult.

Astrocyte differentiation states and glioma formation

Gliomas are the most common primary malignancy in human central nervous system. Many similarities in cell morphology and expression of markers exist between cancerous cells and normal undifferentiated progenitor cells. At the molecular level, many important gene products are causally implicated in both the glial differentiation process and glial neoplasm formation. These observations raise the question of to what degree cell differentiation state influences glioma formation. In this review, we discuss new insights into the parallels between glial differentiation and glioma formation as well as the potential application of differentiation-inducing therapy.

Astrocytes selectively enhance N-type calcium current in hippocampal neurons

Astrocytes influence neuronal development, synapse formation, and synaptic transmission, partly through affecting neuronal calcium signals. In order to elucidate the extent to which astrocytes modulate neuronal voltage-gated calcium currents, we performed a whole-cell patch clamp analysis of neurons in astrocyte-deplete and astrocyte-enriched conditions. We demonstrate that hippocampal neurons in an astrocyte-enriched environment show augmentation of voltage-gated calcium current at 1-3 days in vitro. Further study in pairs of adjacent neurons showed that the augmentation in calcium current was dependent on direct contact with the astrocyte. Pharmacological analysis demonstrated the augmentation is selective for the N-type calcium current, although immunochemical labeling of the alpha1(B) subunit of the N-type calcium channel was unchanged. These findings show that astrocytes regulate neuronal voltage-gated calcium currents in a contact-dependent manner. The specificity of the effect for the N-type calcium current at early days in culture has special significance regarding the role of astrocytes in hippocampal synaptogenesis.

ACTH(1-24) down-regulates expression of ciliary neurotrophic factor mRNA in cultured rat astrocyte

We examined the effects on astrocytes of ACTH, which is used to treat West syndrome. We stimulated cultured rat astrocytes with ACTH(1-24), corticotropin-releasing factor, and dexamethasone, and examined changes in neurotrophic factor mRNAs by reverse transcription-PCR. Down-regulation of ciliary neurotrophic factor mRNA expression was observed by stimulation with ACTH(1-24), but the expression of nerve growth factor, brain-derived neurotrophic factor, and nerotrophin-3 mRNAs was unaffected. Northern blot analysis revealed that the decrease in ciliary neurotrophic factor mRNA occurred 4 h after stimulation with more than 10 nM of ACTH(1-24). Up-regulation of nerotrophin-3 mRNA expression was found after stimulation with 1 mM dexamethasone. These results suggest that ACTH(1-24) administrated in West syndrome may influence the expression of neurotrophic factors in astrocytes in vivo.

Interleukin-1beta induces the expression of lipocortin 1 mRNA in cultured rat cortical astrocytes

Lipocortin 1 (LC1) has been shown to increase in neuronal damage and act as a neuroprotectant and a neurotrophic factor. IL-1beta acts as a mediator of inflammation and has been reported as a potent inducer of various neurotrophic factors including nerve growth factor and fibroblast growth factor. In this study, we investigated the relationship between LC1 and IL-1beta in cultured rat astrocytes. Time-and dose-dependent experiments of IL-1beta on rat cortical astrocytes in culture revealed that the expression of LC1 mRNA was significantly augmented by IL-1beta at 8 h, 10 ng/ml. In addition, IL-1beta evoked an extracellular secretion of LC1 without its cytotoxic effects. The effect of IL-1beta was completely abolished when we treated cells with inhibitor of mitogen-activated protein kinases (MAPKs) (PD98059) (25 microM), phospholipase A(2) inhibitor mepacrine (30 microM) and protein synthesis inhibitor cycloheximide (CHX) (10 microg/ml). This suggests that induction of LC1 by IL-1beta is through a MAPKs and phospholipaseA(2) pathway and requires protein synthesis. These results indicate that IL-1beta released in the central nervous system (CNS) injury can stimulate the transcription of the LC1 gene. Subsequent synthesis and release of LC1 may provide trophic support to neurons and modulate the action of IL-1beta in brain damage.

Alterations in the expression of the AQP family in cultured rat astrocytes during hypoxia and reoxygenation

Aquaporins (AQPs) are a family of water-selective transporting proteins with homology to the major intrinsic protein (MIP) of lens [Cell 39 (1984) 49], that increase plasma membrane water permeability in secretory and absorptive cells. In the central nervous system (CNS), we detected the transcripts of AQP3, 5 and 8 in addition to the previously reported transcripts of AQP4 and 9 in astrocytes, of AQP3, 5 and 8 in neurons, of AQP8 in oligodendrocytes, and none of them in microglia using RNase protection assay and the reverse transcription-polymerase chain reaction (RT-PCR). Hypoxia evoked a marked decrease in the expression levels of AQP4, 5 and 9, but not of AQP3 and 8 mRNAs, and in astrocytes in vitro subsequent reoxygenation elicited the restoration of the expression of AQP4 and 9 to their basal levels. Interestingly, AQP5 showed a transient up-regulation (about 3-fold) and subsequent down-regulation of its expression within 20 h of reoxygenation after hypoxia. The changes in the profiles of AQP expression during hypoxia and reoxygenation were also observed by Western blot analysis. These results suggest that AQP5 may be one of the candidates for inducing the intracranial edema in the CNS after ischemia injury.

Regulation of aquaporin-4 expression in astrocytes

Aquaporin-4 (AQP4), a mercury-insensitive water channel protein, is abundant in the central nervous system and is localized in astrocytes and ependymal cells. AQP4 is speculated to maintain the homeostasis of intracellular and extracellular water in the brain, but little is known about the mechanism of induction of its expression. To investigate the expressional regulation of AQP4, we analyzed changes in its expression during chemically induced differentiation of embryonal carcinoma cells (P19) to neuronal and astrocytic cells, and during the cell cycle of glioma cells. After exposure to retinoic acid for 4 days AQP4 mRNA expression started at the initiation of astrocytic differentiation of P19 cells at 6 days, and increased markedly by 21 days. AQP4 expression was parallel to that of GFAP, a marker intermediate filament of astrocytes. In glioma cell lines, AQP4 mRNA was not detected in the growing phase, but was induced when the cell cycle was arrested at G0/G1 by transient expression of p21. Although quiescent astrocytes in the G0/G1-phase cultured under the serum-free condition exhibited a high expression of AQP4, serum supplement moved them to the S-phase and markedly decreased the AQP expression. These results suggest that AQP4 expression may be induced not only at the initiation of astrocytic differentiation of neural stem cells, but also at the G0/G1-phase during the cell cycle of astrocytes.

Isolation and characterization of a new immortal rat astrocyte with a high expression of NGF mRNA

We have established a new line of immortalized rat astrocytes through transfection of plasmid pSV3-neo encoding the large T antigen of simian virus 40 into normal astrocytes. One of these immortalized astrocytes (ACT-57) with a flat and polygonal cell shape, exhibited stable growth in a chemically defined medium (modified N-2 medium) as well as in medium containing ordinary serum. ACT-57, retained a detectable level of expression of glial fibrillary acidic protein (GFAP) and its mRNA, and exhibited a stronger expression of nerve growth factor (NGF) mRNA than that of normal rat astrocytes or C6 glioma cells. NGF mRNA was significantly up-regulated by phorbol ester (12-O-tetradecanoylphorbol 13-acetate, TPA) and gamma-amino-n-butyric acid (GABA) but not by hydrocortisone. None of stimulants (TPA, dibutyryl cyclic AMP (db-cAMP), hydrocortisone, L-glutamate, carbacol, GABA, dopamine, or isoproterenol) changed the expression level of either brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). There was a discrete difference between ACT-57 and normal astrocytes in the response to GABA and isoproterenol. These findings imply that normal cortical astrocytes possess a functional heterogeneity whereas the clonal astrocyte, ACT-57, does not, indicating that ACT-57 cells may be useful for in vitro studies of neuron-astrocyte interactions involving the induction of neurotrophic factors such as NGF.

Effect of ethanol on muscarinic receptor-induced calcium responses in astroglia

The effects of ethanol on muscarinic receptor-mediated calcium responses were investigated in individual primary rat astrocytes and human 132 1N1 astrocytoma cells using indo-1/AM and image cytometry. After a 30-min incubation, carbachol-induced calcium responses were inhibited only at 100 or 250 mM ethanol. The effects of ethanol were more pronounced and occurred at lower concentrations with longer exposures, with significant inhibition seen at 10 mM following a 24-hr incubation. Thapsigargin- and glutamate-induced responses were unaffected by ethanol, indicating some selectivity in this inhibition. Upon removal of ethanol, inhibition of calcium responses persisted for up to 6-12 hr, with carbachol responses returning to control levels by 24 hr after washout. Ethanol exposure did not affect muscarinic-receptor binding in astrocytoma cells, but inhibited carbachol-induced IP(3) formation. Inhibition of (3)H-thymidine incorporation by ethanol also persisted upon removal of the alcohol, with a time-dependency similar to that of the calcium responses. These results indicate that ethanol inhibits muscarinic receptor-induced calcium responses in astroglia in a concentration- and duration-dependent manner. They also show that co-incubation with ethanol is not necessary for this effect, suggesting that long-term exposure to ethanol may modify, in a reversible manner, the coupling of muscarinic receptors with its effector. This effect of ethanol may play a role in ethanol's inhibition of carbachol-induced thymidine incorporation.

Intracellular [Cl(-)] modulates synchronous electrical activity in rat neocortical neurons in culture by way of GABAergic inputs

The influence of GABAergic neurons on spontaneous electrical activities of neocortical neurons in culture, which was estimated to be about 9.5% of the total neurons by immunohistochemistry, was examined using dual whole-cell recording. Synchronized depolarization or hyperpolarization was observed in recorded neurons with pipettes containing low [Cl(-)] solution, while synchronized bursting of action potentials (APs) was observed with pipettes containing high [Cl(-)] solution. Spontaneous currents (SCs) were synchronous in all pairs tested with either pipettes containing low or high [Cl(-)] solution and spontaneous outward currents (SOCs) observed at around -30 mV were sensitive to the GABA-A receptor antagonist, bicuculline. Their reversal potential (V(rev)) was linearly related to the logarithm of Cl(-) activity in the pipette (-56.9 mV/decade). The intracellular chloride concentration was estimated from the V(rev) of SCs with gramicidin perforated-patch recordings and was between 5.9 and 28.1 mM (mean: 13.0 mM). These results suggest that GABA depolarized some neurons and hyperpolarized others, depending on the E(Cl). Bicuculline decreased the frequency of periodic depolarized potentials and increased their amplitudes. However, perfusion with low [Cl(-)] bath solution did not decrease the frequency. Our data indicate that recurrent subthreshold electrical activities by GABAergic inputs along with glutamatergic inputs take part in deterring synchronized bursting and that intracellular [Cl(-)] can modulate this bursting.

Membrane-bound form of ADP-ribosyl cyclase in rat cortical astrocytes in culture

ADP-ribosyl cyclase activities in cultured rat astrocytes were examined by using TLC for separation of enzymatic products. A relatively high rate of [3H]cyclic ADP-ribose production converted from [3H]NAD+ by ADP-ribosyl cyclase (2.015+/-0.554 nmol/min/mg of protein) was detected in the crude membrane fraction of astrocytes, which contained approximately 50% of the total cyclase activity in astrocytes. The formation rate of [3H]ADP-ribose from cyclic ADP-ribose by cyclic ADP-ribose hydrolase and/or from NAD+ by NAD glycohydrolase was low and enriched in the cytosolic fraction. Although NAD+ in the extracellular medium was metabolized to cyclic ADP-ribose by incubating cultures of intact astrocytes, the presence of Triton X-100 in the medium for permeabilizing cells increased cyclic ADP-ribose production three times as much. Isoproterenol and GTP increased [3H]cyclic ADP-ribose formation in crude membrane-associated cyclase activity. This isoproterenol-induced stimulation of membrane-associated ADP-ribosyl cyclase activity was confirmed by cyclic GDP-ribose formation fluorometrically. This stimulatory action was blocked by prior treatment of cells with cholera toxin but not with pertussis toxin. These results suggest that ADP-ribosyl cyclase in astrocytes has both extracellular and intracellular actions and that signals of beta-adrenergic stimulation are transduced to membrane-bound ADP-ribosyl cyclase via G proteins within cell surface membranes of astrocytes.

Astrocytes are required for the oscillatory activity in cultured hippocampal neurons

Synchronous oscillations of intracellular calcium concentration ([Ca2+]i) and of membrane potential occurred in a limited population of glutamatergic hippocampal neurons grown in primary cultures. The oscillatory activity occurred in synaptically connected cells only when they were in the presence of astrocytes. Microcultures containing only one or a few neurons also displayed oscillatory activity, provided that glial cells participated in the network. The glutamate-transporter inhibitors L-trans-pyrrolidine-2, 4-dicarboxylic acid (PDC) and dihydrokainate, which produce an accumulation of glutamate in the synaptic microenvironment, impaired the oscillatory activity. Moreover, in neurons not spontaneously oscillating, though in the presence of astrocytes, oscillations were induced by exogenous L-glutamate, but not by the stereoisomer D-glutamate, which is not taken up by glutamate transporters. These data demonstrate that astrocytes are essential for neuronal oscillatory activity and provide evidence that removal of glutamate from the synaptic environment is one of the major mechanisms by which glial cells allow the repetitive excitation of the postsynaptic cell.

Dual intracellular recording of neocortical neurons in a neuron-glia co-culture system

Measurement of synchronized activities in neocortical networks is critical for understanding integrated brain function. We describe here a procedure for intracellular recording of activity in two functionally connected neurons using patch electrodes in neuron-glia co-cultures. Rat neocortical neurons survive on monolayers of astrocytes for far longer periods (3-4 weeks) than in their absence. This prolonged survival facilitates the detection of functional synapses which are readily identified in vitro by means of dual intracellular recording using two patch electrodes. The neuron-glia co-culture system appears to be useful not only for studying the formation of neural networks in vitro but also for determining the effects of physiologically active substances and chemicals on synapse formation.

Functional synapses are formed between human NTera2 (NT2N, hNT) neurons grown on astrocytes

The formation of functional synapses is a late milestone of neuronal differentiation. The establishment of functional synapses can be used to assess neuronal characteristics of different cell lines. In the present study, we examined the in vitro conditions that influence the ability of human neurons derived from the NT2 cell line (NT2N neurons) to establish synapses. The morphologic, immunologic, and electrophysiologic characteristics of these synapses was examined. In the absence of astrocytes, NT2N neurons rarely formed synapses and their action potentials were weak and uncommon. In contrast, when plated on primary astrocytes, NT2N neurons were able to form both glutamatergic excitatory (71%) and GABAergic inhibitory (29%) functional synapses whose properties (kinetics, ion selectivity, pharmacology, and ultrastructure) were similar to those of synapses of neurons in primary cultures. In addition, coculture of NT2N neurons with astrocytes modified the morphology of the neurons and extended their in vitro viability to more than 1 year. Because astrocyte-conditioned medium did not produce these effects, we infer that direct contact between NT2N neurons and astrocytes is required. These results suggest that NT2N neurons are similar to primary neurons in their synaptogenesis and their requirement for glial support for optimal survival and maturation. This system provides a model for further investigations into the neurobiology of synapses formed by human neurons.

The role of glial cells in synaptic function

Glial cells represent the most abundant cell population in the central nervous system and for years they have been thought to provide just structural and trophic support to neurons. Recently, several studies were performed, leading to the identification of an active interaction between glia and neurons. This paper focuses on the role played by glial cells at the level of the synapse, reviewing recent data defining how glia is determinant in synaptogenesis, in the modulation of fully working synaptic contacts and in synaptic plasticity.

Effects of ethanol on calcium homeostasis in the nervous system: implications for astrocytes

Ethanol is a major health concern, with neurotoxicity occurring after both in utero exposure and adult alcohol abuse. Despite a large amount of research, the mechanism(s) underlying the neurotoxicity of ethanol remain unknown. One of the cellular aspects that has been investigated in relationship to the neuroteratogenicity and neurotoxicity of ethanol is the maintenance of calcium homeostasis. Studies in neuronal cells and other cells have shown that ethanol can alter intracellular calcium levels and affect voltage and receptor-operated calcium channels, as well as G protein-mediated calcium responses. Despite increasing evidence of the important roles of glial cells in the nervous systems, few studies exist on the potential effects of ethanol on calcium homeostasis in these cells. This brief review discusses a number of reported effects of alcohol on calcium responses that may be relevant to astrocytes' functions.

Neurotrophic action of lipocortin 1 derived from astrocytes on cultured rat cortical neurons

The lipocortins are a family of structurally related proteins, namely an annexin family, that exerts a variety of cellular functions through Ca2+-dependent binding to phospholipase A2 [EC 3.1. 1.4], including a crucial role in the central nervous system (CNS) such as antipyrogenic, thermoregulatory and neuroprotective agents in vivo. To elucidate the paradigm of lipocortin 1 functions in the CNS, we have first demonstrated (1) the induction and subsequent extracellular secretion of LC1 by glucocorticoid in cultured rat astrocytes, and (2) neurotrophic activities (survival-promoting, neuritogenic and synaptogenic actions on rat cortical neurons) of recombinant LC1. Time-and dose-dependent experiments of a synthetic glucocorticoid, dexamethasone (DEX), on rat cortical astrocytes in culture revealed that the expression of the intracellular LC1 mRNA and protein were significantly augmented by DEX (1 microM). In addition, DEX evoked an extracellular secretion of LC1 without its cytotoxic effects. Furthermore, the recombinant LC1 appeared to promote not only the survival and neurite outgrowth but also the synaptogenesis of embryonal rat cortical neurons. These results suggest that LC1 induced and selectively released from astrocytes by either endogenously or exogenously introduced glucocorticoids may play a specific and essential role on development and regeneration of the central nervous system.

Astrocytic gap junction blockage and neuronal Ca2+ oscillation in neuron-astrocyte cocultures in vitro

We have investigated the effects of gap junction inhibitors, octanol, halothane, sodium propionate and lindane, on neuronal periodic Ca2+ transients in neuron-astrocyte coculture systems. Octanol reduced the amplitude and frequency of Ca2+ oscillations in dose-dependent manner. One mM octanol caused a complete disappearance of Ca2+ oscillations. Similar suppressions were obtained by halothane (1 mM) and sodium propionate (25 mM). In contrast, lindane (300 nM) uniquely raised the basal level of [Ca2+], in oscillating neurons as well as the height of apparent amplitude without changes in the frequency. The current results imply that octanol, halothane and sodium propionate might lower the frequency of spontaneous Ca2+ oscillations by blocking the gap junctional communication of neighboring astrocytes and that lindane, though also blocking the gap junctions, might not affect the frequency but reversely increase both the basal [Ca2+]i and the amplitude, probably due to an increase of neuronal [Ins (1.4.5)P3]i. These findings strongly suggest that astrocytes contribute to the generation of periodic neuronal Ca2+ oscillations through astrocytic gap junctional communications and/or other signaling components between astrocytes and neurons.

Functional synapses in synchronized bursting of neocortical neurons in culture

Spontaneous electrical activities in pairs of neocortical neurons in culture were simultaneously recorded using a whole cell current clamp technique. Synchronous bursting activities were observed in all 59 pairs tested. In 52 pairs of neurons electrically stimulated, EPSPs were recorded in 20 pairs (39%), among which 3 pairs (6%) showed bidirectional coupling. The response latency observed was 4. 05+/-0.61 ms (mean+/-S.E.M.). The synaptic delay was estimated at 1. 5-1.9 ms, suggesting the response latency is derived from a polysynaptic connection. The burst latency which was defined as the time difference of the onset of bursting in each neuron was 5.87+/-0. 47 ms (mean+/-S.E.M.), and was weakly correlated with the spatial distance between the neurons (37.5-600 micro(m) apart) (Rs=0.362, tied P value=0.0065). No synchronized bursting was observed in bathing solution with a low Ca2+ concentration (0.4 mM) or in bathing solution containing 50 microM D-AP5 and 15 microM CNQX. No dye-coupling between bursting neurons was observed on injection of the small molecule dye Lucifer yellow or the neurotracer neurobiotin. Disrupting neural connections completely by cutting the cell layer, caused disappearance of synchronized bursting with each neuron bursting independently. In conclusion, these results are consistent with the hypothesis that synchronized bursting in cultured neocortical neurons is attributed to connections by way of several synapses rather than by way of gap junctions and/or diffusible factors.

Increased flexibility and selectivity in spatial learning of transgenic mice ectopically expressing the neural cell adhesion molecule L1 in astrocytes

The expression of the neural cell adhesion molecule L1 is altered by neuronal activity and promotes neurite outgrowth in vitro. To study the effects of L1 on learning and synaptic plasticity, transgenic mice have been created which express L1 ectopically in glial fibrillary acidic protein (GFAP) expressing astrocytes. Ninety mice, including GFAP-L1-transgenic mice from two genetic backgrounds and their littermates, were tested for swimming navigation learning in the Morris water maze according to a standardized protocol. While learning the position of an invisible target platform and also relearning its position after relocation, GFAP-L1-transgenic mice spent a greater fraction of their swim time in the target quadrant. Moreover, they showed a more rapid improvement of escape performance during the first day of training. Factor analysis revealed that this difference in swimming pattern could not be explained by non-cognitive factors. Factor analysis also revealed that, during a probe trial, the GFAP-L1-transgenic mice spent comparatively less time in the old target quadrant than predicted by the increased searching they had shown during acquisition learning. Hence, ectopic expression of L1 by astrocytes in mice appears to be linked to a factor which increases behavioural flexibility and selectivity while learning and relearning, but concomitantly may lead to a relative reduction of spatial retention.

Involvement of amyloid precursor protein in functional synapse formation in cultured hippocampal neurons

Amyloid precursor protein (APP) is known to be widely expressed in neuronal cells, and enriched in the central and peripheral synaptic sites. Although it has been proposed that APP functions in synaptogenesis, no direct evidence has yet been reported. In this study we investigated the involvement of APP in functional synapse formation by monitoring spontaneous oscillations of intracellular Ca2+ concentration ([Ca2+]i) in cultured hippocampal neurons. As more and more neurons form synapses with each other during the culture period, increasing numbers of neuronal cells show synchronized spontaneous oscillations of [Ca2+]i. The number of neurons that showed synchronized spontaneous oscillations of [Ca2+]i was significantly lower when cultured in the presence of monoclonal antibody 22C11 against the N-terminal portion of APP. Moreover, incubation with excess amounts of the secretory form of APP or the N-terminal fragment of APP also inhibited the increase in number of neurons with synchronized spontaneous oscillations of [Ca2+]i. The addition of monoclonal antibody 22C11 or secretory form of APP did not, however, affect MAP-2-positive neurite outgrowth. These findings suggest that APP play a role in functional synapse formation during CNS development.

New views on synapse-glia interactions

Although glial cells ensheath synapses throughout the nervous system, the functional consequences of this relationship are uncertain. Recent studies suggest that glial cells may promote the formation of synapses and help to maintain their function by providing nerve terminals with energy substrates and glutamate precursors.

Appearance of neurofilament subunit epitopes correlates with electrophysiological maturation in cortical embryonic neurons cocultured with mature astrocytes

E14 rat cortical neurons which have almost no glial progenitors were cocultured with a homogeneous population of mature type 1 astrocytes at a 4/1 ratio in serum free medium. Maturation of neurons was evaluated using a set of well characterized antibodies and two new monoclonal antibodies (MN2E4 and MN3H6) raised against various neurofilament subunits and whole-cell patch clamp experiments. We observed that this coculture method leads to a well-timed and very homogeneous neuronal maturation and that sequential appearance of neurofilament subunits in developing neurons correlates with the electrophysiological maturation. This sequence, early expression of the 68 kDa neurofilament subunit and late appearance of the 200 kDa neurofilament subunit, occurs in normal brain development, which validates this culture model as a useful tool for studying neuronal maturation and differentiation. MN2E4 staining (non-phosphorylated 200 kDa cytoskeletal protein antibody) appeared just before the neurons became excitable. It could thus be used as a functional neuronal marker. MN3H6 staining (phosphorylated 160-200 kDa neurofilament subunit antibody) appeared just after the neurons made synaptic contacts and generated synaptically driven spike bursts. This finding indicated that some phosphorylated epitopes of 160-200 kDa neurofilament followed synaptogenesis. These processes may play a key role in stabilizing the synapses to achieve a functional neuronal network.