The behavior of presumed glial cells during seizure discharge in cat cerebral cortex

Increased Glial Fibrillary Acidic Protein Immunoreactivity in Astrocytes within the Lateral Vestibular Nucleus of the Cat following Labyrinthectomy and Vestibular Neurectomy

Unilateral vestibular injury evokes a characteristic pattern of acute disorganization of posture, locomotion, and eye movements. Following this acute stage, functional recovery occurs. In the present study, unilateral labyrinthectomy and vestibular neurectomy were performed in cats. The lateral vestibular nucleus (LVN) and vestibular nerve root entry zone on both sides of the brain stem were examined 24 hours 3 days and 8 weeks after operation by use of an immunochemical astrocyte marker, glial fibrillary acidic protein (GFAP). The results demonstrate extensive GFAP immunoreactivity within the ipsilateral nerve root following neurectomy, but not after labyrinthectomy Prominent GFAP-immunoreactive astrocytic processes were detected in the LVN both ipsilateral and contralateral to neurectomy and labyrinthectomy Within the ipsilateral LVN, the intensity of GFAP immunoreactivity was greater following neurectomy than after labyrinthectomy but the pattern of GFAP reactivity remained similar. In the contralateral LVN, GFAP reactivity was noted exclusively in the dorsal-rostral region corresponding to the zone of cerebellar afferents to the LVN. The results of the present study suggest that reactive astroglia may play an important role in the mechanism that leads to vestibular compensation.

Long-term potentiation of glial synaptic currents in cerebellar culture

Glial cells in the brain express neurotransmitter receptors and can respond appropriately to application of exogenous neurotransmitters such as glutamate. However, activation of receptors by endogenous, synaptically released transmitter has been difficult to demonstrate directly. Using cell-pair recording in cerebellar cultures from embryonic mouse, it is shown that activation of a cerebellar granule neuron can give rise to a rapid inward current in an adjacent glial cell. This current is mediated by activation of Ca2+-permeable AMPA/kainate receptors and is largely independent of glutamate reuptake or gap junctional coupling. Furthermore, prolonged stimulation of the granule neuron at 4 Hz can give rise to long-term potentiation (LTP) of the glial synaptic current that has similar properties to LTP of granule neuron-Purkinje neuron synaptic transmission--its induction is independent of postsynaptic depolarization, postsynaptic Ca2+ influx, or glutamate receptor activation but requires presynaptic Ca2+ influx. These findings suggest a model in which cerebellar LTP is both induced and expressed presynaptically and therefore may be detected by either neuronal or glial postsynaptic cells.

Changes in connexin43, the gap junction protein of astrocytes, during development of the rat pineal gland

The abundance of gap junctions between rat pineal astrocytes formed by connexin43 (Cx43) was studied during development. Levels and distribution of Cx43 were measured by immunoblotting and indirect immunofluorescence, respectively. The amount of Cx43 in cells located within the gland was low until about the 7th postnatal day and increased to adult values between the 14th and 21st days postpartum. Although astrocytes, recognized by their vimentin immunoreactivity, were scarce before birth, they were abundant by the 7th postnatal day suggesting that the low levels of Cx43 found at this age corresponded to a low expression of this protein. Localization of the immunoreactivity to Cx43 and vimentin showed a close correlation, indicating that mature or immature pineal astrocytes form gap junctions made of Cx43. Since Cx43 levels attained their adult values at about the time the innervation and the functional state of the gland reached maturity (2-3 weeks after birth), it is proposed that astrocyte gap junctions are involved in the function of the adult rat pineal gland.

Heterogeneity in gap junction expression in astrocytes cultured from different brain regions

Heterogeneity among astrocytes suggests that their role in the central nervous system is more complex than is commonly recognized. This paper describes just such a functional difference, comparing gap junctions in astrocytes derived from two brain regions. Astrocytes, both in situ and in culture, employ gap junctions as a means of intercellular communication. Recent evidence utilizing cultured rat cortical and striatal astrocytes has shown that these channels consist of subunits of connexin 43, the same protein as that composing cardiac gap junctions. Here we report that astrocytes cultured from neonatal rat hypothalamus contain a greater number of functional channels than astrocytes from the striatum, a difference reflected in both connexin 43 protein and mRNA. Specifically, in hypothalamic astrocytes the level of connexin 43 protein was approximately four times that found in comparable cultures from the striatum, as determined by immunoblotting. Complementary results from immunocytochemical experiments using an antibody specific for connexin 43 reveal significantly greater fluorescence in astrocytes cultured from the hypothalamus as compared to those from the striatum. Northern blot analysis showed that connexin 43 mRNA levels were also approximately 4-fold greater in the hypothalamic cultures, consistent with the difference seen by immunoblotting. Finally, dye coupling studies using confluent cultures consistently showed that within 1 min Lucifer Yellow injected into striatal astrocytes spread to immediately surrounding cells while in hypothalamic astrocytes dye often spread to apparent third or fourth order neighbors within the same time period. Thus, the higher level of connexin 43 expression seen in hypothalamic astrocytes results in cells with greater numbers of functional channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical coupling between astrocytes and between oligodendrocytes studied in mammalian cell cultures

The characteristics of electrical coupling between astrocytes and between oligodendrocytes were analyzed in cell cultures derived from rodent central nervous system. Experiments were carried out by impaling one member of a glial pair with separate voltage recording and current passing electrodes (cell 1) and the other cell, a measured distance from the first, with a voltage-recording electrode (cell 2). Astrocyte pairs within 300 microns of one another were always coupled. The coupling ratio was determined for 23 astrocytic pairs various distances apart, and decreased with distance in a roughly exponential manner. The average coupling ratio of astrocytes within 100 microns of each other was 0.44 +/- 0.32. Oligodendrocytes were less strongly coupled to each other than astrocytes. Even cells immediately adjacent to one another were often uncoupled. Among coupled oligodendrocytes within 100 microns of each other, the average coupling ratio was 0.11 +/- 0.1. Current passage between pairs of astrocytes and pairs of oligodendrocytes was nonrectifying. Application of 0.5 mM BaCl2 or 44.6 mM CsCl (substituted for NaCl) depolarized and increased the input resistance of astrocytes and oligodendrocytes. These ions also increased the coupling ratio in astrocyte pairs and oligodendrocyte pairs; this effect was rapid in onset and completely reversible. Ba++ and Cs+ appear to block resting K+ conductance in glia and probably increase the coupling ratio by increasing the effective length constant of the glial membrane without any direct effect on junctional resistance. In three cases, oligodendrocyte pairs that appear uncoupled in normal solution exhibited coupling in the presence of BaCl2 or CsCl. This suggests that oligodendrocytes may be widely coupled by junctions that provide only weak electrical interaction; such junctions might be important for the exchange of small metabolically active molecules. The strong electrical coupling among astrocytes, in concert with their K+-selective membrane conductance, would provide for an electrical syncytium well designed to transport K+ away from areas of focal extracellular accumulation (i.e., the spatial buffer mechanism), and these cells, more than oligodendrocytes, may provide this function.

Cellular mechanisms of epilepsy: a status report

The cellular phenomena underlying focal epilepsy are currently understood in the context of contemporary concepts of cellular and synaptic function. Interictal discharges appear to be due to a combination of synaptic events and intrinsic currents, the exact proportion of which in any given neuron may vary according to the anatomic and functional substrate involved in the epileptic discharge and the epileptogenic agent used in a given model. The transition to seizure appears to be due to simultaneous increments in excitatory influences and decrements in inhibitory processes--both related to frequency-dependent neuronal events. A variety of specific hypotheses have been proposed to account for the increased excitability that occurs during epileptiform activity. Although each of the proposed mechanisms is likely to contribute significantly to the epileptic process, no single hypothesis provides an exclusive unifying framework within which all kinds of focal epilepsy can be understood. The spread of epileptic activity throughout the brain, the development of primary generalized epilepsy, the existence of "gating" mechanisms in specific anatomic locations, and the extrapolation of hypotheses derived from simple models of focal epilepsy to explain more complex forms of human epilepsy, all are not yet fully understood.

Dye-coupling between glial cells in the guinea pig neocortical slice

Physiologically identified glial cells in guinea pig neocortical slices were injected with the low molecular weight, fluorescent dye Lucifer yellow CH. The stained aggregates which resulted consisted of one brightly stained, central cell surrounded by numerous lightly stained cells. The central cell had well defined feathery processes and resembled a protoplasmic astrocyte. The surrounding cells appeared also to be glial cells but lacked sufficient detail to be further categorized. This first demonstration of dye-coupling between neocortical glial cells strongly suggests that these cells are connected together via low resistance junctions capable of passing ionic current as well as dye.

Changes in excitatory and inhibitory synaptic potentials leading to epileptogenic activity

The effects of the epileptogenic agent, penicillin, on excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) were studied in the hippocampal in vitro slice preparation. The actions of penicillin were compared to those of bicuculline, an antagonist of gamma-aminobutyric acid. Neither substance enhanced monosynaptic EPSP amplitude in CA1 pyramidal cells, but both penicillin and bicuculline depressed IPSPs. Large depolarizations that gave rise to cellular bursting activity did develop after addition of penicillin or bicuculline to the bathing medium. There was also an increase in the incidence of fast prepotentials of 'd-spikes.' These observations are interpreted according to the hypothesis that penicillin and bicuculline block cellular IPSPs, and consequently allow remote intrinsic excitatory events to invade the cell soma and trigger action potentials.

Mechanisms underlying epileptiform burst discharge

Studies of hyperexcitable cells in alumina-gel and penicillin epileptogenic foci suggest that epileptid or triggered by a number of extrinsic factors which disrupt the normal depolarizing-hyperpolarizing balance. This paper presents the hypothesis that the most important influence for burst discharge is influx of calcium ions. This excitatory effect is modulated by synaptic inputs, hyperpolarizing conductances, cell morphology and membrane characteristics, and the extracellular milieu. The hypothesis incorporates findings from both penicillin and alumina models of epilepsy and provides experimentally testable concepts for further investigation.

Spreading depression in isolated carp retina

(1) Spreading depression (SD) could be elicited in isolated carp retina by KCI application, the concomitants of which were similar to those described in other vertebrates. (2) The threshold for generating SD was greatly reduced by brief immersion of the retina in low ci-ringer's solutions. The properties of SD waves were almost the same with treated and untreated retinas, except for intervals. (3) Extracellular negative potential shifts during SD, averaging 4.6 mV in amplitude and 27 sec in duration, were recorded in whole retinal layers with the maximum amplitude about at the inner plexiform layer. (4) The PIII potential of the local electroretinogram was virtually unaffected by SD. (5) Both L-and C-type S-potentials could be evoked with increase of 20-40% in amplitude around the peak of slow membrane depolarizations (mean value of maximal amplitude 5.8 mV) during SD in horizontal cells. (6) Increase in spike number was observed in both on-and off-center ganglion cells before and after the spike cessation during SD in the untreated retina. However, the off-discharges, which were a unique response to light in the immersed retina, only decreased during SD.

Glial cells and extracellular potassium: their relationship in mammalian cortex

Simultaneous recordings were made of glial cell potentials and the extracellular potassium concentration ([K+]O) in cat cortex in an attempt to provide more quantitative information about the sensitivity of mammalian neuroglia to changes in [K+]O. A penicillin epileptogenic focus served to generate both transient and sustained elevations in [K+]O, thus allowing measurement of glial membrane potential (Vm) at both resting and increased [K+]O levels many times during the same experiment. Resting Vm averaged--92.6 +/- 10.9 mV for 33 glial cells. With each surface interictal spike, glial cells exhibited slow depolarizations averaging 18.4 +/- 6.5 mV which mirrored rises in [K+]O in many respects. Several discrepancies were found, however, between transient and focal rises in [K+]O and the associated glial cell depolarizations which made it difficult to determine accurately the effect of changes in [K+]O on glial Vm. For example, the amplitude of the glial depolarization caused by a single interictal discharge showed no constant relationship to depth below the cortical surface in contrast to the consistent laminar profile recorded by the K+ electrode. Thus, large glial membrane depolarizations could be recorded at times when there was little or no increase in measured [K+]O. Agreement between changes in [K+]O and glial cell depolarizations was closer to that predicted by the Nernst equation during sustained elevations in [K+]O such as occurred during ictal episodes ('seizures'). These findings may be related in part to methodology as a consequence of the different spatial relationships which exist between glial membrane, K+-electrode tip and released K+. In addition, though, they may indicate the presence of a functional glial syncytium.

Kinetics of potassium movement in norman cortex

Artificial CSF containing 12mEq/1 of potassium (4 times normal concentration) was perfused over the surface of normal neocortex in 10 locally or barbiturate anesthetized cats. Extracellular potassium concentrations ([K+]OS) were measured as a function of time to 8 h and of depth to 3 mm. Analysis of the [K+]O versus depth and [K+]O versus time profiles showed that the distribution of extracellular potassium could be modelled by a process of diffusion with diffusion coefficient = 1.03 +/- 0.16 sq.mm/h, and a surface barrier to diffusion with magnitude given by a barrier constant 0.8 +/- 0.2 mm-1, and with a small correction for active uptake into cells and blood vessels. The profiles obtained differ considerably from those which could be predicted from kinetic properties of potassium in aqueous solution.

The effect of convulsions induced by flurothyl on ribonucleic acid synthesis in rat cerebral cortex during the recovery phase

The effect of convulsions, induced by flurothyl, on RNA synthesis in purified unfractionated nuclei and the cytoplasm of rat cerebral cortex was studied by using a double-label technique involving injection of [3H]- and [14C]-orotate intracisternally. 2. Intact RNA was extracted in 80% yield by an enzymic method by using a proteinase in the presence of sodium dodecyl sulphate followed by deoxyribonuclease. Electrophoresis on 1.5% polyacrylamide-0.5% agarose gels revealed the presence of giant nuclear RNA of size up to approx. 300X 10(6) daltons and mRNA of maximal mol.wt. 9 X 10(6)-16 X 10(6). 3. Nuclear RNA synthesis was decreased to 27% in the first 15 min after convulsions but rapidly increased, so that at 1 1/2 h it was 124% of the control, and at 6 h 147%. 4. Labelling of cytoplasmic RNA was decreased to 15% at 15 min after convulsions but had not recovered to control values by 6 h. 5. Analysis of radioactive gel patterns and the 3H/14C ratio at six time-points (15 min-6h) showed that the major effect was inhibition of the processing of heterogeneous nuclear RNA resulting in a sharp decline in the export of newly synthesized RNA from the nucleus. 6. Cytoplasmic RNA patterns indicated that specific messengers were synthesized at different times during the recovery of the cell after convulsions.

Blockage of cortical spreading depression by picrotoxin foci of paroxysmal activity

1 per cent Picrotoxin placed on cortex of rat caused paroxysmal ECoG discharges with concomitant increase in [Ke"] from 3 to 6.7 mM with oscillations corresponding to ictal (maximum) and interictal (minimum) spiking. Invasion of the epileptogenic focus by spreading depression was blocked when the amplitude of oscillations of [Ke+] reached 2.6 mM. Epileptogenic activity induced by topical 10 per cent pentazol caused a less marked increase in [Ke+] (4.6 mM) and did not prevent depression from invading the focal area, but did diminish [Ke+] from the normal of 60 to 70 mM to 39 mM. It is concluded that seizure-induced depolarization of neural elements in deep cortical layers, though inadequate to trigger spreading depression, does prevent it from spreading, in part by activating the sodium pump.