Postsynaptic components of paroxysmal neuronal response in the strychninized neocortex

Neuronal response in the strychninized cortical suprasylvian gyrus was investigated in experiments on immobilized and unanesthetized cats using intracellular techniques. Paroxysmal depolarizing shifts (PDS) in neuronal membrane potential were recorded, consisting of a bursting discharge and slow depolarization wave. It was found when using intracortical stimulation that PDS can accumulate and change in shape and size. Bursting discharges in PDS were induced by large-scale EPSP which could be distinguished from paroxysmal response. Data from presumably intradendritic readings demonstrated the presence of large-scale EPSP during the generation of epileptiform discharges in the cortex. In a proportion of cells, PDS were accompanied by hyperpolarizing potentials — apparently IPSP, since they undergo reversal with intercellular administration of Cl^–. The contribution of excitatory and inhibitory synaptic influences to paroxysmal neuronal response is discussed.I. I. Mechnikov State University, Odessa. Translated from Neirofiologiya, Vol. 22, No. 5, pp. 642–649, September–October, 1990.     

Neuronal response in an epileptically excited isolated cortical slab to single electrical stimuli

Neuronal response to single stimuli was investigated in a cortical slab isolated from immobilized cats before, during, and after onset of induced epileptic states. Neurons of the isolated cortical slab were found to generate EPSP and paroxysmal depolarizing shifts (PDS) in membrane potential (MP) during the development of generalized epileptoid activity; these occurred together with refractory periods. Duration of the latter corresponds with the PDS plateau and repolarizing shifts in MP. Single electrical stimuli induced gradual alteration in PDS as these shifts developed. Neurons still maintain their ability to generate PDS arising in response to presentation of single stimuli once ictal activity has ceased. Postsynaptic response is not thought to play a decisive role in the genesis of epileptoid activity. Nonspecific factors and especially alterations in the concentration of electrogenic ions apparently contribute to this phenomenon.I. I. Mechnikov State University, Odessa. Translated from Neirofiziologiya, Vol. 21, No. 2, pp. 198–204, March–April, 1989.     

Neuronal responses of a chronically isolated slab of auditory cortex to intracortical stimulation during paroxysmal electrical activity in cats

Responses of 155 neurons 3 weeks after neuronal isolation of a slab of auditory cortex (area AI) to single intracortical stimulating pulses at the level of layer IV were studied in unanesthetized, curarized cats during paroxysmal electrical activity evoked by series of high-frequency (10–20 Hz) electrical stimulation by a current 2–5 times above threshold for the direct cortical response. In response to such stimulation a discharge of paroxysmal electrical activity, lasting from a few seconds to tens of seconds, appeared in the slab. As a rule it consisted of two phases — tonic and clonic. This indicates that cortical neurons can form both phases of paroxysmal cortical activity. Depending on behavior of the neurons during paroxysmal electrical activity and preservation of their ability to respond to intracortical stimulation at this time, all cells tested in the isolated slab were divided into four groups. Their distribution layer by layer and by duration of latent periods was studied. Two-thirds of the neurons tested were shown to generate spike activity during paroxysmal discharges whereas the rest exhibited no such activity. A special role of neurons in layer II in generation of paroxysmal activity in the isolated slab was noted. The view is expressed that at each moment functional neuronal circuits, independent of each other, exist in the slab and also, evidently in the intact cortex, which can interact with one another when conditions change.I. I. Mechnikov Odessa State University. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 3–11, January–February, 1984.     

Epileptiform activity in rat hippocampal slices isolated after local septal lesioning by ibotenic acid

In surviving slices of rat hippocampus, isolated from 1 to 4 weeks after septal lesioning by ibotenic acid, extracellular and intracellular responses were recorded in region CA₃. Spontaneous and evoked epileptiform focal discharges are described, synchronous with paroxysmal depolarization shifts (PDS) of the membrane potential and with burst activity of cells. It is shown that the development of synchronized population reactions and PDS have an "all or nothing" character. The values of the resting potential and input resistance of the neurons did not differ significantly from those of cells in the control group of slices. Histological analysis showed destruction of neurons in the dorsal part of the septum, with cells of the medial septum being unaffected. The role of intraseptal mechanisms in the generation of epileptiform activity in region CA₃ of hippocampal slices is discussed.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Department of Physiology and Biochemistry, University of Pisa, Italy. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 556–564, September–October, 1991.     

Protracted depolarizing potentials of neurons in a strychninized cortical slab isolated from the cat

Neuronal response in a cortical slab isolated from the cat during surface application of strychnine was investigated in experiments on immobilized unanesthetized animals by means of intracellular recording techniques. Protracted depolarizing potentials (PDP) were found to occur spontaneously and in response to a single intracortical electrical stimulus in a proportion of the neurons. These potentials could be triggered by transformation of response along the lines of "paroxysmal depolarizing shift" (PDS) — hyperpolarization, with hyperpolarization replaced by depolarizing potentials. A further increase in depolarizing after-potentials resulted in the generation of PDP. These changes were normally accompanied by enhanced summated epileptiform activity in the isolated cortical slab. It is postulated that PDP were triggered by increased calcium conductance at the neuronal membrane during intensification of paroxysmal response in the isolated cortical slab.I. I. Mechnikov University, Odessa. Translated from Neirofiziologiya, Vol. 22, No. 1, pp. 19–23, January–February, 1990.     

Neuronal electrical activity in the epileptic focus produced by electrical stimulation in an isolated cortical slab

Characteristics of neuronal activity in an isolated cortical slab were investigated during the onset of seizure spikes induced by frequent and powerful stimulation of the slab during experiments on unanesthetized immobilized cats. A high degree of coordination between the activity of cellular elements was found in the focus of epileptiform activity studied: convulsive shifts in membrane potential exactly corresponding to electrocorticograms of convulsive activity waves were observed in all neurons studied using intracellular techniques. No action potentials occurred in the soma of any of these neurons, moreover. Bursting spike discharges were recorded from neurons of the isolated slab at the same time. Findings from extra- and intracellular recordings of activity in the same neurons showed that action potentials are generated during convulsive activity at certain trigger zones remote from the cell in question without involving the soma, from which convulsive shifts in membrane potentials were recorded simultaneously. Mechanisms possibly underlying the generation of spike activity in neurons of the isolated slab undergoing development of generalized convulsive state are discussed.I. I. Mechnikov State University, Odessa. Translated from Neirofiziologiya, Vol. 20, No. 3, pp. 357–365, May–June, 1988.     

Effect of local polarization of after-discharges of cortical neurones

After strong tetanization epileptiform after-discharges occur in the neurones of the sensorimotor cortex of unanaesthetized rabbits, they take the form of bursts of impulses occurring at intervals of 150–600 msec. The bursts are caused by paroxysmal depolarization shifts (PDS) of the membrane potential (MP). When the MP is reduced to 10–20 mV, on account of the considerable damage to the neurone the PDS give way to hyperpolarization oscillations. Unlike the prolonged action potentials (AP), which are quite frequently recorded in damaged cells, the intracellular PDS and the extracellular bursts of after-discharges show no change in frequency when a current is passed through the recording microelectrode. It was found impossible to suppress the generation of PDS by means of a hyperpolarizing current (1–3·10^–9 A), or to evoke PDS by a depolarizing current. Therefore we were unable to confirm the hypothesis that PDS occur as a result of reorganization of the generation of the electrical impulse. Support is given to the hypothesis that the PDS are altered and enormously potentiated excitatory postsynaptic potentials (EPSP)Brain Institute, Academy of Medical Sciences of the USSR, Moscow, Translated from Neirofiziologiya, Vol. 2, No. 5, pp. 460–468, September–October, 1970.     

Negative surface potential shift and responses of neurons and glial cells during tetanic stimulation of the cortical surface

The negative potential shift in response to tetanic stimulation of the surface of the cortex or thalamic nucleus was recorded from the cortical surface in cats lightly anesthetized with pentobarbital. Parallel intracellular recordings were obtained of activity of neurons and glial cells. Glial cells responded to this stimulation by slow depolarization, which, under certain conditions of stimulation, was followed by slow hyperpolarization; hyperpolarization shifts were observed in neurons. Depolarization and hyperpolarization of glial cells, like hyperpolarization of neurons, did not correlate in time with the development of a negative shift of the surface potential. It is postulated that this shift is a response of complex origin involving the participation not only of glial cells, but also of cortical neurons.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 14, No. 3, pp. 248–253, May–June, 1982.     

Mechanisms of the effect of diazepam on paroxysmal electrical activity of an isolated cortical slab in cats

The effect of diazepam on paroxysmal global electrical activity of a neuronally isolated slab of auditory cortex and on inhibitory responses of its neurons due to intracortical electrical stimulation was investigated in cats. Diazepam (2 mg/kg, intravenously) caused inhibition of paroxysmal electrical activity and increased the number of inhibited neurons in both the acutely isolated slab and three weeks after isolation, compared with the intact cortex. However, the number of disynaptic responses was reduced under these circumstances, especially in the long-isolated slab. It is postulated that diazepam exerts its action through activation of GABA-ergic inhibitory neurons, by synchronizing inhibition and increasing the duration of the IPSPs. The action of diazepam is manifested first, probably, in the initial links of cortical neuron chains.I. I. Mechnikov Odessa State University. Translated from Neirofiziologiya, Vol. 17, No. 1, pp. 3–10, January–February, 1985.     

Self-sustained rhythmic spike and wave activity and response of glial and nerve cells in the cat sensorimotor cortex

During acute experiments on unanesthetized cats, immobilized with myorelaxants, it was found that during rhythmic stimulation (8–14 Hz, duration: 10 sec) of the ventroposterolateral thalamic nucleus brief hyperpolarization is succeeded by depolarization in the pyramidal neurons of the sensorimotor cortex. Following this depolarization, rhythmic (approximately 3 Hz) paroxysmal depolarizing shifts in membrane potential are produced by ending stimulation, succeeded by protracted hyperpolarization and termination of rhythmic wave activity. Depolarization only is observed in glial cells, however, while hyperpolarization sets in after hyperpolarization is completed in the neurons. It is suggested that long-term changes in the membrane potential of cortical cells could make some contribution to the setting up and termination of rhythmic spike and wave activity.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 18, No. 3, pp. 319–325, May–June, 1986.     

Postsynaptic neuronal response of a cat sensorimotor cortex during evoked and self-sustained rhythmic "spike and wave" activity

Intracellular correlates of complex sets of rhythmic cortical "spike and wave" potentials evoked in sensorimotor cortex and of self-sustained rhythmic "spike and wave" activity were examined during acute experiments on cats immobilized by myorelaxants. Rhythmic spike-wave activity was produced by stimulating the thalamic relay (ventroposterolateral) nucleus (VPLN) at the rate of 3 Hz; self-sustained afterdischarges were recorded following 8–14 Hz stimulation of the same nucleus. Components of the spike and wave afterdischarge mainly correspond to the paroxysmal depolarizing shifts of the membrane potential of cortical neurons in length. After cessation of self-sustained spike and wave activity, prolonged hyperpolarization accompanied by inhibition of spike discharges and subsequent reinstatement of background activity was observed in cortical neurons. It is postulated that the negative slow wave of induced spike and wave activity as well as slow negative potentials of direct cortical and primary response reflect IPSP in more deep-lying areas of the cell bodies, while the wave of self-sustained rhythmic activity is due to paroxysmal depolarizing shifts in the membrane potential of cortical neurons.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 18, No. 3, pp. 298–306, May–June, 1986.     

Unit activity in an epileptic focus formed in the cat motor cortex with tetanus toxin

Single unit activity was recorded intracellularly in the zone of an epileptic focus produced by injection of tetanus toxin into the cerebral cortex of cats. Epileptic activity of all neurons tested correlated with cortical discharges between fits. A group of neurons with continuous spontaneous activity, in which a steady fall of membrane potential and cyclic changes in the frequency of the spike discharges were observed was distinguished. In these neurons paroxysmal depolarization changes of membrane potential were found in the discharges between fits, without subsequent hyperpolarization of the membrane. Hyperpolarization potentials after paroxysmal depolarization shifts could be observed in neurons of other groups. The role of neurons of the different groups in the formation of an "epileptic aggregate," the main generator of pathologically enhanced excitation, is discussed.Institute of Normal and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Institute of Clinical and Experimental Neurology, Ministry of Health of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 10, No. 6, pp. 582–589, November–December, 1978.     

Changes in membrane potential and postsynaptic potentials evoked by strychnine in neocortical neurons

Intracellular responses of neurons of the suprasylvian fissure to intracortical stimulation before and during topical cortical strychnine application was studied in experiments on immobilized, unanesthetized cats (a local anesthetic was used). Untreated cortical neurons responded to intracortical stimulation with a monosynaptic excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP). Application of strychnine evoked epileptiform population activity and paroxysmal depolarizations of neuronal membrane potentials (MPs), followed by hyperpolarization. Increased hyperpolarizations, and the prolonged duration of their summation were responsible for an increased MP and reduced or abolished tonic spike activity. Intracellular application (as a result of diffusion from the microelectrode) of ethyleneglycoltetraacetate (EGTA) that blocked the calcium-dependent potassium membrane conductance (gK(Ca)) abolished the hyperpolarization. The development of epileptiform activity was accompanied by reduction of the IPSP, and an increase in the monosynaptic EPSP. The role of gK(Ca) and postsynaptic inhibition in epileptogenesis is discussed.I. I. Mechnikov State University, Odessa. Translated from Neirofiziologiya, Vol. 24, No. 6, pp. 684–691, November–December, 1992.     

An astrocytic basis of epilepsy

Hypersynchronous neuronal firing is a hallmark of epilepsy, but the mechanisms underlying simultaneous activation of multiple neurons remains unknown. Epileptic discharges are in part initiated by a local depolarization shift that drives groups of neurons into synchronous bursting. In an attempt to define the cellular basis for hypersynchronous bursting activity, we studied the occurrence of paroxysmal depolarization shifts after suppressing synaptic activity using tetrodotoxin (TTX) and voltage-gated Ca(2+) channel blockers. Here we report that paroxysmal depolarization shifts can be initiated by release of glutamate from extrasynaptic sources or by photolysis of caged Ca(2+) in astrocytes. Two-photon imaging of live exposed cortex showed that several antiepileptic agents, including valproate, gabapentin and phenytoin, reduced the ability of astrocytes to transmit Ca(2+) signaling. Our results show an unanticipated key role for astrocytes in seizure activity. As such, these findings identify astrocytes as a proximal target for the treatment of epileptic disorders.     

Spread of excitation in upper layers of the cat auditory cortex with participation of intracortical interneuronal connections

In chronically isolated slabs of the cat auditory cortex with additional transection of lower layers and preservation of the structural integrity of one, two, or three upper layers of cortex just under the pial membrane, impulse responses of slab neurons to stimulation applied at the additionally undercut section were studied. High effectiveness of axodendritic and axospinal excitatory contacts formed by nerve elements of intracortical origin in upper cortical layers was demonstrated. The participation of geniculocortical fibers in spread of excitation in the cortex through synaptic contacts in layer I with dendrites of underlying-layer pyramidal neurons is discussed. The capacity for generation of polysynaptic excitation responses by the neurons indicates preservation of complex interneuronal interactions in the isolated cortex slab preparations with their undercut lower layers.I. I. Mechnikov State University of Odessa, Odessa. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 80–87, January–February, 1991.     

Effects of ketamine on neuronal epileptiform responses in the cat neocortex

The effects of ketamine, an antagonist of NMDA receptors, on the neuronal epileptiform responses evoked by applications of strychnine, penicillin, or bicuculline to the suprasylvian gyrus were studied in cats. Ketamine either exerted no effect, or slightly decreased interictal high-amplitude depolarizing shifts of the membrane potential and depolarizing afterpotentials, which appeared spontaneously or were evoked by intracortical stimulation. Repetitive electrical stimulation of the epileptogenic cortical regions resulted in the appearance of autogenerated ictal activity lasting up to several tens of seconds; this activity was produced against the background of a depolarization of neuronal membranes. After ketamine injections, such stimulations evoked no ictal activity in the neurons, or the discharges became much shorter. The results of our study show that the NMDA-dependent postsynaptic components play a more important role in the development of neocortical ictal activity compared with the interictal activity.Neirofiziologiya/Neurophysiology, Vol. 27, No. 1, pp. 32–35, January–February, 1995.     

Glial origin of the slow negative potential of the direct cortical response: Microelectrode study and mathematical analysis

The slow negative potential of the direct cortical response is similar in its shape, time course, and relationship to repetitive stimulation to depolarization of cortical glial cells but differs from the IPSP of cortical neurons. According to the results of digital spectral (frequency) analysis, the basis of the slow negative potential is the glial component formed by summation of components which coincide with glial depolarization processes with an accuracy determined by a constant factor. The much smaller component (as regards relative contribution) is the indirect result of the development of an IPSP in the neurons.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 14, No. 1, pp. 76–84, January–February, 1982.     

Glial origin of negative shifts of cortical surface potential during tetanic stimulation: Microelectrode study and mathematical analysis

Experiments on anesthetized cats showed that a negative shift of potential on the surface of the cerebral cortex caused by its tetanic stimulation is similar in shape and time course to the depolarization shift of membrane potential of the glial cells, but has a more rapid decline. The hyperpolarization shifts of membrane potential of neurons differed in shape and time course from the negative shift of cortical surface potential. It is concluded that the contribution of hyperpolarization of neurons to the surface-negative potential shift during tetanic stimulation may be manifested visibly only at the beginning (the first 200–300 msec) of such stimulation. The negative potential shift on the cortical surface is due mainly to depolarization of glial cells under the influence of K⁺ secreted from excited nerve cells.     

Continuous increase of epileptogenic effects following application of proteolytic enzymes (buccal ganglia of Helix pomatia)

Epileptic activity of neurons consists of paroxysmal depolarization shifts (PDS) which can be induced presumably in any nervous system by application of an epileptogenic drug. The spontaneous appearance of epileptic activity, however, is based on a largely unknown process which increases susceptibility to epileptic activity (seizure susceptibility in man). It is presently shown that the treatment of ganglia with proteolytic enzymes (Pronase) decreases the effective concentration of epileptogenic drugs, i.e. increases seizure susceptibility. Since proteolytic enzymes are known to primarily affect glial cells a contribution of glia to seizure susceptibility is discussed.     

Computational models of epileptiform activity in single neurons

A series of original computational models written in NEURON of increasing physiological and morphological complexity were developed to determine the dominant causes of epileptiform behavior. Current injections to a model hippocampal pyramidal neuron consisting of three compartments produced the sustained depolarizations (SD) and simple paroxysmal depolarizing shifts (PDS) characteristic of ictal and interictal behavior in a cell, respectively. Our results indicate that SDs are the result of the semi-saturation of Na+, Ca2+ and K+ active channels, particularly the CaN, with regular Na+/K+ spikes riding atop a saturated depolarization; PDS rides on a similar semi-saturated depolarization whose shape depends more heavily on interactions between low-threshold voltage-gated Ca2+ channels (CaT) and Ca(2+)-dependent K+ channels. Our results reflect and predict recent physiological data, and we report here a cellular basis of epilepsy whose mechanisms reside mainly in the membrane channels, and not in specific morphology or network interactions, advancing a possible resolution to the cellular/network debate over the etiology of epileptiform activity.