Development of Neurons and Glial Cells in Cerebral Cortex, Cultured in the Presence or Absence of Bioelectric Activity: Morphological Observations

Accelerating bioelectric functional development of neural stem cells by graphene coupling: Implications for neural interfacing with conductive materials

In order to govern cell-specific behaviors in tissue engineering for neural repair and regeneration, a better understanding of material-cell interactions, especially the bioelectric functions, is extremely important. Graphene has been reported to be a potential candidate for use as a scaffold and neural interfacing material. However, the bioelectric evolvement of cell membranes on these conductive graphene substrates remains largely uninvestigated. In this study, we used a neural stem cell (NSC) model to explore the possible changes in membrane bioelectric properties - including resting membrane potentials and action potentials - and cell behaviors on graphene films under both proliferation and differentiation conditions. We used a combination of single-cell electrophysiological recordings and traditional cell biology techniques. Graphene did not affect the basic membrane electrical parameters (capacitance and input resistance), but resting membrane potentials of cells on graphene substrates were more strongly negative under both proliferation and differentiation conditions. Also, NSCs and their progeny on graphene substrates exhibited increased firing of action potentials during development compared to controls. However, graphene only slightly affected the electric characterizations of mature NSC progeny. The modulation of passive and active bioelectric properties on the graphene substrate was accompanied by enhanced NSC differentiation. Furthermore, spine density, synapse proteins expressions and synaptic activity were all increased in graphene group. Modeling of the electric field on conductive graphene substrates suggests that the electric field produced by the electronegative cell membrane is much higher on graphene substrates than that on control, and this might explain the observed changes of bioelectric development by graphene coupling. Our results indicate that graphene is able to accelerate NSC maturation during development, especially with regard to bioelectric evolvement. Our findings provide a fundamental understanding of the role of conductive materials in tuning the membrane bioelectric properties in a graphene model and pave the way for future studies on the development of methods and materials for manipulating membrane properties in a controllable way for NSC-based therapies.

Localization of heat shock proteins in cerebral cortical cultures following induction by celastrol

Hsp70, Hsp32, and Hsp27 were induced by celastrol in rat cerebral cortical cultures at dosages that did not affect cell viability. Pronounced differences were observed in the cellular localization of these heat shock proteins in cell types of cerebral cortical cultures. Celastrol-induced Hsp70 localized to the cell body and cellular processes of neurons that were identified by neuron-specific βIII-tubulin. Hsp70 was not detected in adjacent GFAP-positive glial cells that demonstrated a strong signal for Hsp27 and Hsp32 in both glial cell bodies and cellular processes. Cells in the cerebral cortex region of the brain are selectively impacted during the progression of Alzheimer's disease which is a "protein misfolding disorder." Heat shock proteins provide a line of defense against misfolded, aggregation-prone proteins. Celastrol is a potential agent to counter this neurodegenerative disorder as recent evidence indicates that in vivo administration of celastrol in a transgenic model of Alzheimer's reduces an important neuropathological hallmark of this disease, namely, amyloid beta pathology that involves protein aggregation.

Homeostatic structural plasticity increases the efficiency of small-world networks

In networks with small-world topology, which are characterized by a high clustering coefficient and a short characteristic path length, information can be transmitted efficiently and at relatively low costs. The brain is composed of small-world networks, and evolution may have optimized brain connectivity for efficient information processing. Despite many studies on the impact of topology on information processing in neuronal networks, little is known about the development of network topology and the emergence of efficient small-world networks. We investigated how a simple growth process that favors short-range connections over long-range connections in combination with a synapse formation rule that generates homeostasis in post-synaptic firing rates shapes neuronal network topology. Interestingly, we found that small-world networks benefited from homeostasis by an increase in efficiency, defined as the averaged inverse of the shortest paths through the network. Efficiency particularly increased as small-world networks approached the desired level of electrical activity. Ultimately, homeostatic small-world networks became almost as efficient as random networks. The increase in efficiency was caused by the emergent property of the homeostatic growth process that neurons started forming more long-range connections, albeit at a low rate, when their electrical activity was close to the homeostatic set-point. Although global network topology continued to change when neuronal activities were around the homeostatic equilibrium, the small-world property of the network was maintained over the entire course of development. Our results may help understand how complex systems such as the brain could set up an efficient network topology in a self-organizing manner. Insights from our work may also lead to novel techniques for constructing large-scale neuronal networks by self-organization.

Dual-compartment neurofluidic system for electrophysiological measurements in physically segregated and functionally connected neuronal cell culture

We developed a dual-compartment neurofluidic system with inter-connecting microchannels to connect neurons from their respective compartments, placed on a planar microelectrode arrays. The design and development of the compartmented microfluidic device for neuronal cell culture, protocol for sustaining long-term cultures, and neurite growth through microchannels in such a closed compartment device are presented. Using electrophysiological measurements of spontaneous network activity in the compartments and selective pharmacological manipulation of cells in one compartment, the biological origin of network activity and the fluidic isolation between the compartments are demonstrated. The connectivity between neuronal populations via the microchannels and the crossing-over of neurites are verified using transfection experiments and immunofluorescence staining. In addition to the neurite cross-over to the adjacent compartment, functional connectivity between cells in both the compartments is verified using cross-correlation (CC) based techniques. Bidirectional signal propagation between the compartments is demonstrated using functional connectivity maps. CC analysis and connectivity maps demonstrate that the two neuronal populations are not only functionally connected within each compartment but also with each other and a well connected functional network was formed between the compartments despite the physical barrier introduced by the microchannels.

Degeneration of cultured cortical neurons following prolonged inactivation: molecular mechanisms

Networks of neurons express persistent spontaneous network activity when maintained in dissociated cultures. Prolonged blockade of the spontaneous activity with tetrodotoxin (TTX) causes the eventual death of the neurons. In this study, we investigated some molecular mechanisms that may underlie the activity-suppressed slow degeneration of cortical neurons in culture. Already after 3-4 days of exposure to TTX, well before the neurons die, they began to express markers that lead to their eventual death, 7-10 days later. There was a reduction in glutamate receptor (GluR2) expression, a persistent increase in intracellular calcium concentration, activation of calpain, and an increase in spectrin breakdown products. At this point, blockade of GluR2-lacking GluR1 or calpain (either with a selective antagonist or through the natural regulator of calpain, calpastatin), protected cells from the toxic action of TTX. Subsequently, mitochondria lost their normal elongated shape as well as their membrane potential. Eventually, neurons activated caspase 3 and PUMA (p53 up-regulated modulator of apoptosis), hallmarks of neuronal apoptosis, and died. These experiments will lead to a better understanding of slow neuronal death, typical of neurodegenerative diseases.

Low-frequency stimulation induces stable transitions in stereotypical activity in cortical networks

Reverberating spontaneous synchronized brain activity is believed to play an important role in neural information processing. Whether and how external stimuli can influence this spontaneous activity is poorly understood. Because periodic synchronized network activity is also prominent in in vitro neuronal cultures, we used cortical cultures grown on multielectrode arrays to examine how spontaneous activity is affected by external stimuli. Spontaneous network activity before and after low-frequency electrical stimulation was quantified in several ways. Our results show that the initially stable pattern of stereotypical spontaneous activity was transformed into another activity pattern that remained stable for at least 1 h. The transformations consisted of changes in single site and culture-wide network activity as well as in the spatiotemporal dynamics of network bursting. We show for the first time that low-frequency electrical stimulation can induce long-lasting alterations in spontaneous activity of cortical neuronal networks. We discuss whether the observed transformations in network activity could represent a switch in attractor state.

Miniature synaptic currents become neurotoxic to chronically silenced neurons

When deprived of spontaneous ongoing network activity by chronic exposure to tetrodotoxin (TTX), cultured cortical neurons retract their dendrites, lose dendritic spines, and degenerate over a period of 1-2 weeks. Electrophysiological properties of these slowly degenerating neurons prior to their death are normal, but they express very large miniature excitatory postsynaptic currents (mEPSCs). Chronic blockade of these mEPSCs by the alpha-amino-5-hydroxy-3-methyl-4-isoxazole propionic acid (AMPA) receptor antagonist 6,7-Dinitroquinoxaline-2,3-dione (DNQX) had no effect of its own on cell survival, yet, paradoxically, it protected the TTX-silenced neurons from degenerating. TTX-treated neurons also exhibited deficient Ca(2+) clearance mechanisms. Thus, upscaled mEPSCs are sufficient to trigger apoptotic processes in otherwise chronically silenced neurons.

Dynamics and plasticity in developing neuronal networks in vitro

When dissociated cortical tissue is brought into culture, neurons readily grow out by forming axonal and dendritic arborizations and synaptic connections. These developing neuronal networks in vitro display spontaneous firing activity from about the end of the first week in vitro. When cultured on multielectrode arrays firing activity can be recorded from many neurons simultaneously over long periods of time. These experimental approaches provide valuable data for studying firing dynamics in neuronal networks in relation to an ongoing development of neurons and synaptic connectivity in the network. This chapter summarizes recent findings on the characteristics and developmental changes in the spontaneous firing dynamics. These changes include long-lasting transient periods of increased firing at individual sites on a time scale of days to weeks, and an age-specific repetitive pattern of synchronous network firing (network bursts) on a time scale of seconds. Especially the spatio-temporal organization of firing within network bursts showed great stability over many hours. In addition, a progressive day-to-day evolution was observed, with an initial broadening of the burst firing rate profile during the 3rd week in vitro (WIV) and a pattern of abrupt onset and precise spike timing from the 5th WIV onwards. These developmental changes are discussed in the light of structural changes in the network and activity-dependent plasticity mechanisms. Preliminary findings are presented on the pattern of spike sequences within network burst, as well as the effect of external stimulation on the spatio-temporal organization within network bursts.

Long-term characterization of firing dynamics of spontaneous bursts in cultured neural networks

Extracellular action potentials were recorded from developing dissociated rat neocortical networks continuously for up to 49 days in vitro using planar multielectrode arrays. Spontaneous neuronal activity emerged toward the end of the first week in vitro and from then on exhibited periods of elevated firing rates, lasting for a few days up to weeks, which were largely uncorrelated among different recording sites. On a time scale of seconds to minutes, network activity typically displayed an ongoing repetition of distinctive firing patterns, including short episodes of synchronous firing at many sites (network bursts). Network bursts were highly variable in their individual spatio-temporal firing patterns but showed a remarkably stable underlying probabilistic structure (obtained by summing consecutive bursts) on a time scale of hours. On still longer time scales, network bursts evolved gradually, with a significant broadening (to about 2 s) in the third week in vitro, followed by a drastic shortening after about one month in vitro. Bursts at this age were characterized by highly synchronized onsets reaching peak firing levels within less than ca. 60 ms. This pattern persisted for the rest of the culture period. Throughout the recording period, active sites showed highly persistent temporal relationships within network bursts. These longitudinal recordings of network firing have, thus, brought to light a reproducible pattern of complex changes in spontaneous firing dynamics of bursts during the development of isolated cortical neurons into synaptically interconnected networks.

Chronic Suppression of Bioelectric Activity and Cell Survival in Primary Cultures of Rat Cerebral Cortex: Biochemical Observations

Chronic suppression of spontaneously occurring bioelectric activity (BEA) has been shown to increase neuronal cell death in tissue culture, but may also affect astrocytes. We investigated this process in primary cultures of rat cerebral cortex by measuring the levels of NSE (neuron-specific enolase) and GFAP (glial fibrillary acidic protein) in relation to general tissue markers, including measurements for cell death and proliferation. In electrically active (control) cultures, the content of DNA, protein, and NSE became maximal between 21 and 28 days in vitro (DIV) and thereafter decreased, whereas the content of GFAP rose continuously up to 43 DIV. Chronic suppression of BEA by tetrodotoxin (TTX; from 6 DIV) decreased the content of DNA, total protein, and especially NSE. The content of GFAP was decreased in all culture series investigated, but with great temporal variations among culture series. Chronic TTX treatment (started at 6 DIV) increased the efflux of lactate dehydrogenase, a marker for cell lysis, between 12 and 21 DIV, but this efflux was mainly derived from the supporting glial cells with which the cerebral cortex cultures were cocultured. Chronic, but not acute (7 h) TTX treatment decreased total [3H]thymidine incorporation into DNA from 14 DIV; this appeared to be due to a reduced number of astrocytes. Chronic suppression of BEA with xylocaine from 6 DIV had similar effects on DNA-, protein-, and NSE-content as TTX, but led to an increased content of GFAP at 21 DIV. Chronic suppression of synaptic transmission with 10 mM Mg2+ and 0.2 mM Ca2+, starting at 6 DIV, increased the content of DNA, protein, and GFAP at 21 DIV, but NSE was still decreased. We conclude that chronic suppression of BEA in cerebral cortex cultures enhances neuronal cell death, whereas astrocytes are differentially affected, depending on the suppressing agent. As astrocytes may have a modulating effect on neuronal survival, their involvement should be regarded when studying the effects of chronic suppression of BEA on neuronal development.

The role of calcium signaling in early axonal and dendritic morphogenesis of rat cerebral cortex neurons under non-stimulated growth conditions

The effects of depolarizing stimuli on neurite outgrowth have been shown to depend on an influx of extracellular calcium. However, the role of calcium under non-stimulated growth conditions is less well established. Here we investigated the contribution of calcium signaling to early neuronal morphogenesis of rat cerebral cortex neurons at three levels by blocking L-type voltage sensitive calcium channels, by depleting intracellular calcium or by blocking myosin light chain kinase. Detailed quantitative morphological analysis of neurons treated for 1 day revealed that depletion of intracellular calcium strongly decreased the density of filopodia, arrested axonal outgrowth and strongly decreased dendritic branching. Preventing calcium influx through L-type voltage sensitive calcium channels and blocking of myosin light chain kinase activity selectively decreased dendritic branching. Our observations support an essential role for basal intracellular calcium levels in axonal elongation. Furthermore, under non-stimulated conditions calcium entry through L-type voltage sensitive calcium channels and myosin light chain kinase play an important role in dendritic branching.

Measurement of sealing resistance of cell-electrode interfaces in neuronal cultures using impedance spectroscopy

Sealing resistance is highly significant with respect to the electrical neuron-electrode contact because it decreases the stimulation threshold of neurons cultured on a planar micro-electrode array. A method is proposed for measurement of the sealing resistance using impedance spectroscopy. The effect of the sealing resistance on the total impedance spectrum of a cell-electrode interface is modelled for complete coverage of the electrode by the cell. Sensitivity analysis demonstrates that the impedance spectrum is determined by four parameters: two electrode parameters, the sealing resistance and the shunt capacitance between the lead of the electrode and the culture medium. Experimental verification of the model is performed by simultaneous measurement of the impedance spectrum and electrode coverage. A good and unique fit between the simulated and measured impedance spectra was obtained by varying the two electrode parameters and the sealing resistance.

Depolarization stimulates lamellipodia formation and axonal but not dendritic branching in cultured rat cerebral cortex neurons

Electric activity is known to have profound effects on growth cone morphology and neurite outgrowth, but the nature of the response varies strongly between neurons derived from different species or brain areas. To establish the role of electric activity in neurite outgrowth and neuronal morphogenesis of rat cerebral cortex neurons, cultured neurons were depolarized for up to 72 h and quantitatively analyzed for changes in axonal and dendritic morphology. Depolarization with 25 mM potassium chloride induced a rapid increase in lamellipodia in almost all growth cones and along both axons and dendrites. Lamellipodia formation was dependent on an influx of extracellular calcium through L-type voltage-sensitive calcium channels. Prolonged depolarization for 24 h induced an increase in total axonal length, mainly due to an increase in branching. After three days of depolarization axonal outgrowth was largely the same as in control neurons, suggesting accommodation of the growth cones to chronic depolarization. Dendrites showed very little change during the first three days in culture, and dendritic length or branching were not affected by depolarization. Thus, in early cerebral cortex neurons depolarization specifically stimulates axonal outgrowth through increased branching. This increase in branching may be a consequence of the earlier increase in lamellipodia formation. In contrast, early dendrites seem to be unable to translate the increase in lamellipodia into changes in outgrowth or branching. This difference between axons and dendrites could be due to differences in the stabilization of the tubulin cytoskeleton.

An increase in intracellular levels of cyclic AMP produces trophic effects on striatal neurons developing in culture

Cyclic AMP-dependent kinases have been suggested to constitute signal transduction pathways involved in the regulation of neuronal development and survival. The present study examined whether elevated levels of cyclic AMP exhibit trophic activities on rat striatal neurons grown under serum-free culture conditions. Treatment with dibutyryl cyclic AMP, a permeable cyclic AMP, increased GABA uptake and immunocytochemically detectable levels of proteins such as c-Fos and calbindin-D28k. Neuronal survival was promoted by dibutyryl cyclic AMP only in lower density cultures. Chronic exposure of neurons to dibutyryl cyclic AMP enhanced the morphological development of calbindin-D28k-positive neurons. Furthermore, pretreatment with dibutyryl cyclic AMP afforded neuroprotection against N-methyl-D-aspartate-induced excitotoxicity. The dibutyryl cyclic AMP-induced trophic effects above were blocked by adenosine 3',5'-cyclic monophosphothioate, a specific inhibitor of cyclic AMP-dependent kinases. We also examined whether cyclic AMP is involved in trophic effects provided by membrane depolarization induced by high K+ and growth factors such as basic fibroblast growth factor and insulin-like growth factor-1. Depolarization, but not the growth factors, increased intracellular levels of cyclic AMP. Adenosine 3',5'-cyclic monophosphothioate diminished depolarization increases in GABA uptake, whereas it did not affect the trophic effect of the growth factors. Co-treatment with the growth factors and dibutyryl cyclic AMP produced additive effects on both increases in GABA uptake and neuroprotection against excitotoxicity. The present results indicate that cyclic AMP-dependent kinases play roles in mediating differentiation and survival of developing striatal neurons. Signalling pathways activated by either basic fibroblast growth factor or insulin-like growth factor-1 are independent of those involving cyclic AMP. In contrast, depolarization-induced trophic effects are mediated, at least in part, by cyclic AMP-dependent pathways. Protective actions of dibutyryl cyclic AMP against excitotoxic injury as well as the additive effects with the growth factors are of potential interest in the experimental therapy of acute or chronic neurodegenerative diseases.

Developmental expression and regulation of alpha 1 adrenergic receptors in cultured cortical neurons

The expression and distribution of alpha 1 receptors in cultured neurons derived from rat visual cortex were studied with confocal microscopy using the fluorescently labeled alpha 1 adrenergic receptor selective antagonist BODIPY FL prazosin. The receptors were found to be clustered on neuronal somata and on proximal dendrites. We found that expression of the alpha 1 receptor is regulated both by neuronal excitability and by the usage of the receptor itself. The specificity of receptor regulation to the blockade of a particular receptor class was also studied. We approached this by comparing the effects of treatment with the alpha 1 adrenoreceptor antagonist prazosin and M1 muscarinic receptor antagonist pirenzepine on the expression of alpha 1 and M1 receptors in cultured cortical neurons. The results showed that blockade of muscarinic receptors with pirenzepine up-regulated muscarinic receptor expression selectively without changing alpha 1 receptor expression. Conversely blockade of alpha 1 receptors up-regulated alpha 1 expression but not muscarinic receptor expression. This implies that the expression levels of M1 and alpha 1 receptors are both regulated through specific signal transduction pathways. The interactions between neuronal activity and receptor activation (or blockade) on receptor expression were studied as well. In these experiments we compared the effects of high K+, tetrodotoxin (TTX), prazosin, and noradrenaline on the expression of alpha 1 and M1 receptors in cultured neurons. The results show that high K+ exposure increased both alpha 1 and M1 receptor expression regardless of the presence of receptor agonists or antagonists. On the other hand, TTX exposure reduced both alpha 1 and M1 receptor expression regardless of the presence of the receptor agonists or antagonists. This implies that regulation of receptor number is predominantly regulated by neuronal activity rather than by receptor occupancy.

Neuronal regulation of astrocyte morphology in vitro is mediated by GABAergic signaling

The addition of isolated neurons to monolayers of cultured astrocytes induced a morphological change in the astrocytes that came into contact with the added neuronal cell bodies or neurites. The change, which included an increase in the complexity of cell shape, took at least 3 days to become detectable and was enhanced in proportion to the number of attached neurons. Astrocytes that did not make contact with any neurons had a less complex contour, comparable to those in control cultures with no neurons added. Treatment of neuron-astrocyte cocultures with a sodium channel blocker, tetrodotoxin, suppressed the neuron-induced morphological changes in astrocytes. A GABAA-receptor antagonist, bicuculline, mimicked the inhibitory effect of tetrodotoxin. In cultures without added neurons, morphological alteration of astrocytes was also observed when cultures were incubated for 1 or more days with exogenous GABA together with a GABA-uptake inhibitor, 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol. The effect of exogenous GABA was mimicked by treatment with a GABAA-receptor agonist, muscimol, and blocked by bicuculline treatment. These results suggest that GABA released from neurons with their activity serves as a signal from neurons to astrocytes that triggers the morphological change in astrocytes through the activation of GABAA receptors.

Development and activity-dependent expression of neuronal marker proteins in organotypic cultures of rat visual cortex

We are interested in activity-dependent mechanisms which govern the structural and functional maturation of neurons in the visual cortex. We have asked whether the expression of neuronal markers microtubule-associated proteins tau, MAP-2, synaptophysin (p38), and the growth-associated protein GAP-43 are dependent on cortical afferents or spontaneous activity. As a model system we have employed organotypic monocultures of rat visual cortex (OTCs, isolated from subcortical structures) in comparison with visual cortex in vivo (innervated by thalamic and other afferents) at different postnatal ages. We know from previous work that the OTCs, like the cortex in vivo, display a high rate of spontaneously generated action potentials. Therefore, as a third objective, we have analysed OTCs grown as monocultures under chronic blockade of spontaneous action potentials. Protein expression was detected by protein blots and/or immunohistochemistry. The proteins examined in this study are expressed in OTCs, even when grown under activity blockade. However, the pattern of expression differs from the cortex in vivo. Tau is expressed much weaker in OTCs than in cortex in vivo. The expression of the major band of about 50 kDa increases over time in vivo and in OTCs. Smaller isoforms of tau are dramatically downregulated, and larger (adult) isoforms do not appear within 35 days in vitro (DIV). Under activity blockade the expression of tau reaches a maximum by 21 DIV and decreases dramatically, so that the protein is hardly detectable by 47 DIV. MAP-2-immunoreactive proteins are localized in somata and dendrites, but also persist in axons. The expression in OTCs of p38 and GAP-43 correlates well with the expression observed in vivo. Synaptophysin (p38) occurs with a similar time course and amount in OTCs as in cortex in vivo. Synaptic boutons appear in all layers, and specialized terminal elements have been observed. Activity blockade slightly affects the p38 expression, although the late postnatal decline in p38 immunoreactivity observed on protein blots from cortex in vivo and in normal OTCs appears more accentuated in activity-blocked OTCs. The GAP-43 expression is prominent from birth onwards in vivo and in OTCs. However, in normal OTCs GAP-43 is not declining as it is in vivo, although it is downregulated in activity-blocked OTCs. As a major finding we report that neuronal markers which are normally expressed in immature neurons and axons during the period of differentiation and structural plasticity are continuously expressed in OTCs, suggesting that a monocultured cortex retains the ability for growth and structural changes longer than the cortex in vivo.

Synaptogenesis and distribution of presynaptic axonal varicosities in low density primary cultures of neocortex: an immunocytochemical study utilizing synaptic vesicle-specific antibodies, and an electrophysiological examination utilizing whole cell recording

Low-density primary cultures of neocortical neurons were utilized to examine: (i) early interactions of growing neurites with morphological characteristics of axons with other neuronal elements, and (ii) the distribution of presynaptic axonal varicosities closely apposed to MAP-2 immunoreactive, putatively postsynaptic, dendrites. At the light microscopical level axonal varicosities, presumably presynaptic terminals, were identified using immunocytochemistry incorporating antibodies specific for the synaptic vesicle antigens synaptophysin and synapsin. The presence of synaptophysin- and synapsin-immunoreactive swellings along axonal processes was first detected at 5 days post-plating and was also apparent in axons growing in isolation. At 5-7 days in vitro, immunolabelled axonal varicosities in close apposition to putative postsynaptic dendrites (MAP-2 immunoreactive) dendrites were detected. Electrophysiologically active synaptic contacts can also readily be detected at this stage. After 3 weeks in vitro presynaptic contacts do appear to be distributed heterogeneously along postsynaptic dendrites of many neurons in culture. As the culture matures a higher number of presynaptic profiles can be seen along dendrites, with a centrifugal distribution, e.g. a higher density of presynaptic axonal terminals in close apposition to more distal regions of larger dendrites, putatively considered to be apical dendrites of pyramidal-like neurons. In our cultures, the overall increase in the density and the pattern of distribution of presynaptic axon terminals immunoreactive for synaptic vesicle antigens closely apposed to putative post-synaptic structures mimics the general postnatal increase of synaptic density in the neocortex in vivo. Thus, low density primary cultures of neocortical neurons offer a valuable system to explore and manipulate (i) the molecular and cellular basis of neocortical synaptogenesis, and (ii) the pharmacology of neocortical synaptic transmission.

Activity-dependent plasticity of inhibitory and excitatory amino acid transmitter systems in cultured rat cerebral cortex

Chronic suppression of spontaneous bioelectric activity in cultures of dissociated fetal rat cerebral cortex increases neuronal cell death and results in electrophysiological changes which indicate an altered balance between excitatory and inhibitory neurotransmission in culture. To delineate whether alterations in neurotransmitter release could underlie this imbalance, we investigated the effects of chronic tetrodotoxin (TTX) treatment on the content and release of glutamate, aspartate and gamma-aminobutyric acid (GABA) in culture. Chronic TTX treatment decreased the content of all amino acids investigated. However, only GABA was decreased relative to the neuronal marker NSE (neuron-specific enolase), indicating a disproportionate loss of GABA production following chronic silencing. Depolarization-induced release of GABA, glutamate and aspartate increased about 10-fold between 7 and 21 days in control cultures. Chronic TTX treatment significantly increased the depolarization-induced release of glutamate and aspartate at 7 days in vitro relative to control levels. At all ages it caused a two-fold increase in the ratio of evoked excitatory amino acid release to that of GABA. These observations suggest that chronic silencing of developing neocortex cell cultures increases the ratio of excitatory to inhibitory synaptic activity either by differential cell death or by reduced synaptic efficiency, on which a decrease in GABA neurotransmission appears to play a major role. Since similar mechanisms may be involved in activity-dependent plasticity in vivo, these cultures provide a useful model to analyse this phenomenon at the cell biological and molecular level.

Polarized distribution of the trans-Golgi network marker TGN38 during the in vitro development of neocortical neurons: effects of nocodazole and brefeldin A

Neurons are polarized secretory cells whose cytoplasm and plasma membrane are polarized to form two compartments: dendrites and axons. In mature, fully polarized neurons, the microtubule-associated protein Map2 is targeted to dendrites, while tau is mainly restricted to axons. However, the intraneuronal distribution of secretory pathway organelles, such as the endoplasmic reticulum and the Golgi complex, which give rise to all constitutive, regulated and lysosome vesicles, is poorly understood. Thus, to investigate the distribution of the trans-Golgi network during the development and maturation of rat neocortical neurons in vitro, we have utilized an antibody recognizing a 38 kDa trans-Golgi network-specific protein, TGN38, and immunofluorescence microscopy. Before neurons have established polarity. TGN38 immunoreactivity outlines several vesicles dispersed throughout the cell body cytoplasm; these converge close to a major Map2-immunopositive process during the establishment of neuronal polarity, and later merge into a single structure located at the base of a thick Map2-immunopositive process, approximately 18 h after plating. At this stage TGN38 immunoreactivity is located within 45 degrees of the major Map2-immunoreactive process in 54% of neurons, while in only 6% of cells it is located at the opposite pole. After 3 days in vitro, during the segregation of microtubule-associated proteins to either dendrites or axons. TGN38 immunoreactivity clusters continue to be located close to a major dendrite, and in some neurons these clusters begin to enter a major Map2-immunoreactive process. At 10 days in vitro TGN38 immunoreactivity extends into a major dendrite for 5-30 microns in many neurons. Thus, the distribution of TGN38 immunoreactivity becomes polarized, being localized within a single, usually the major, neocortical dendrite. Our results also show that the morphological appearance of TGN38-immunoreactive structures is microtubule-dependent, since nocodazole treatment of polarized neurons induces scattering of TGN38-immunoreactive vesicles throughout the cell body's cytoplasm. Treatment with brefeldin A induces scattering of small TGN38-immunoreactive vesicles throughout the neuronal cytoplasm and processes, a different response to that observed in non-neuronal cells.

Use of recombinant vectors derived from herpes simplex virus 1 mutant tsK for short-term expression of transgenes encoding cytoplasmic and membrane anchored proteins in postmitotic polarized cortical neurons and glial cells in vitro

We constructed three recombinant vectors derived from the herpes simplex virus type 1 mutant tsK, each of which contained a different transgene under the control of the herpes simplex virus type 1 immediate early 3 promoter inserted into the thymidine kinase locus: the prokaryotic enzymes beta-galactosidase and chloramphenicol acetyl transferase, and a fusion gene consisting of human tissue inhibitor of metalloproteinases linked to the last exon of Thy-1, which encodes for a glycosyl-phosphatidyl-inositol membrane anchor. Infection of postmitotic neocortical and hippocampal neurons in low-density primary cultures with these vectors, achieved reliable expression of all three foreign gene products in various neocortical cell types, e.g. pyramidal neurons, non-pyramidal neurons, and glial cells. The percentage of neurons expressing transgenes ranged from 1 to 46% depending on the multiplicity of infection (highest assayed = 5); the percentage of glial cells expressing transgenes ranged from 0.5 to 98% (highest multiplicity assayed = 3.4). Expression of transgenes could be detected for up to three days in approximately 20% of neurons infected at a multiplicity of infection of 1. Infection of neurons with tk K-derived recombinant vectors inhibited their protein synthesis by 40-50% at a multiplicity of infection of 10, but no effect was observed at a multiplicity of infection of 1. Infection of glial cells with the same vectors at a multiplicity of infection of 1 inhibited protein synthesis by more than 90%. Analysis of neuronal viability at different times post-infection indicated that more than 98% of neurons expressing transgenes 48 h post-infection were viable. Thus, low-density neuronal cultures can be used to assess the efficiency of herpes simplex virus type 1-derived gene transfer vectors and transgene expression in developing cortical postmitotic cells, before and after they establish polarity. In addition, we show that two cytoplasmic enzymes, beta-galactosidase and chloramphenicol acetyl transferase, are able to diffuse freely in the cytoplasm reaching even growth cones in young neurons, while the chimeric protein tissue inhibitor of metalloproteinases/Thy-1 is correctly targeted to the plasma membrane via a glycosyl-phosphatidylinositol anchor. This model system should be useful for investigation of cellular and molecular aspects of the development and establishment of neuronal polarity, as well as for analysis of signals involved in protein targeting in postmitotic neurons.

Paroxysmal long-lasting depolarizations in cultured hippocampal neurons are generated by activation of NMDA and non-NMDA receptors

In primary cultures of hippocampal neurons from the embryonic rat, spontaneous depolarizations lasting up to 6 sec and resembling paroxysmal depolarization shifts (PDSs) appeared after 11 days in vitro. These depolarizations are presumably generated by synaptic events, because: (1) both their appearance and duration are independent of membrane potential, (2) the amplitudes of the underlying currents depend monotonically on membrane potential, and (3) they are reversed at the reversal potential of the excitatory postsynaptic potentials (EPSPs). In addition, PDSs disappeared reversibly when sodium-dependent action potentials were blocked by tetrodotoxin (10 microM) and when synaptic transmission was reduced by elevated Mg2+ (5 mM). Further, the fact that these depolarizations can appear simultaneously in two neurons in paired recordings also points to a synaptic origin. Inhibition of glutaminergic synaptic transmission by kynurenic acid (50 microM) and the NMDA-antagonist D-2-amino-5-phosphonovaleric acid (APV; 50 microM) led to a marked shortening of the depolarizations. This blocking effect of kynurenic acid and APV and comparison with the currents elicited by locally applied glutamate or NMDA provide evidence for an activation of both types of glutamate receptors to induce PDSs. The role of alteration of glutaminergic synaptic transmission in the induction and maintenance of these depolarizations is discussed in the context of results from the literature on the appearance of PDSs in cultures grown under chronic blockade of glutamate receptors.

A developmental decrease in NMDA-mediated spontaneous firing in cultured rat cerebral cortex

In primary cultures of fetal rat cerebral cortex chronic manipulation of the level and/or pattern of bioelectric activity leads to plastic changes in bioelectric activity, opposite to those seen during the manipulation. This suggests the presence of adaptive mechanisms which regulate functional development in the neuronal network. Since NMDA receptors play an important role in early postnatal bioelectric activity and have been implicated in activity-dependent plasticity in vivo, the involvement of NMDA and non-NMDA receptors in spontaneously occurring bioelectric activity was investigated in cultured rat cerebral cortex by assaying the effects of NMDA and non-NMDA antagonists on neuronal firing. In addition, the physiological consequences of chronic suppression of bioelectric activity were investigated following development in the presence of tetrodotoxin. NMDA receptors appeared at all ages to be more crucial for spontaneous bioelectric activity than non-NMDA receptors, although their relative importance decreased during the first 3 weeks. Whereas the NMDA antagonist APV strongly reduced burst firing, the non-NMDA antagonist DNQX tended to increase burst firing slightly. Following chronic suppression of bioelectric activity, non-variable burst firing was increased, thus replicating previous findings in cerebral cortex culture grown under different conditions. The prominence of NMDA receptor activation in spontaneous bioelectric activity in early cultures suggests a role for these receptors in activity-dependent functional plasticity, as found in vivo.

Effects of spontaneous bioelectric activity and gangliosides on cell survival in vitro

Chronic suppression of spontaneous bioelectric activity in spinal cord explants in the presence of tetrodotoxin (TTX) during network formation caused a large reduction in cell number (lowered DNA levels). The addition of gangliosides failed to protect against this cell loss. Conversely, the omission of galactose from the growth medium had no effect on DNA levels. It was concluded that the presence or absence of afferent selectivity is unlikely to require the survival of a regionally specific subpopulation of preferred dorsal root ganglion target cells. Neocortical explants also showed a large reduction in DNA levels following chronic TTX treatment, and morphometric analysis confirmed that neuronal survival was affected to the same degree. Chronic ganglioside supplementation failed to influence DNA and cell counts in either control or TTX-treated explants, but one of the added gangliosides (GD1a) stimulated extensive neuritic outgrowth in electrically silenced cultures. Particular ganglioside species, therefore, may exert a growth stimulating influence that can partially compensate for the absence of bioelectric self-stimulation during early development.

Spontaneous firing as an epigenetic factor in brain development--physiological consequences of chronic tetrodotoxin and picrotoxin exposure on cultured rat neocortex neurons

Functional consequences of either suppressing or intensifying spontaneous neuronal firing have been studied in developing rat cerebral cortex cultures using, respectively, tetrodotoxin (TTX) and picrotoxin (PTX) added chronically to the growth medium. Simple measures derived from the interspike interval histogram were able to powerfully discriminate between age and treatment groups. After return to control medium, most TTX-treated neurons spontaneously displayed stereotyped clustering of action potentials ('phasic' firing) which closely resembled the characteristic firing patterns seen acutely in the presence of PTX. The 'TTX-syndrome' thus suggests that GABAergic synaptic inhibition is ineffective in cortical networks grown under conditions which prevent the expression of bioelectric activity. In contrast, after return to control medium, neurons which had been partially disinhibited throughout development (by continuous exposure to PTX) had even less phasic firing than was measured in age-matched controls. Based upon these and previous findings, a two (main) factor model is put forth which can economically account for the major effects. The working hypothesis embodied in this model is that phasic neuronal discharges not only accelerate the maturation of excitatory connections within the neocortex but, even more important, are crucial for the development of adequate inhibitory synaptic transmission.