Directly evoked paroxysmal' depolarizations of mouse hippocampal neurons in synaptically organized explants in long-term culture

Models of drug-induced epileptiform synchronization in vitro

Models of epileptiform activity in vitro have many advantages for recording and experimental manipulation. Neural tissues can be maintained in vitro for hours, and in neuronal or organotypic slice cultures for several weeks. A variety of drugs and other agents increase activity in these in vitro conditions, in many cases resulting in epileptiform activity, thus providing a direct model of symptomatic seizures. We review these preparations and the experimental manipulations used to induce epileptiform activity. The most common of drugs used are GABAA receptor antagonists and potassium channel blockers (notably 4-aminopyridine). Muscarinic agents also can induce epileptiform synchronization in vitro, and include potassium channel inhibition amongst their cellular actions. Manipulations of extracellular ions are reviewed in another paper in this special issue, as are ex vivo slices prepared from chronically epileptic animals and from people with epilepsy. More complex slices including extensive networks and/or several connected brain structures can provide insights into the dynamics of long range connections during epileptic activity. Visualization of slices also provides opportunities for identification of living neurons and for optical recording/stimulation and manipulation. Overall, the analysis of the epileptiform activity induced in brain tissue in vitro has played a major role in advancing our understanding of the cellular and network mechanisms of epileptiform synchronization, and it is expected to continue to do so in future.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Activity-dependent development of spontaneous bioelectric activity in organotypic cultures of rat occipital cortex

The development of spontaneous bioelectric activity (SBA) in organotypic tissue cultures (OTCs) from rat occipital cortex was studied by means of extracellular recording techniques in OTCs grown normally for 6-51 days in vitro (DIV), and in OTCs in which SBA had been silenced from DIV 4 on for 2 to 3 weeks by elevating the Mg(2+) levels in the growth medium. The proportions of spontaneously active neurones increased from about 25% at 6-14 DIV to more than 80% beyond the third week in vitro. Mature neurones discharged at shorter intervals and more vigorously than immature neurones; the developmental increase in firing rate was not significant, however. In OTCs 6-14 DIV the majority of spontaneously active neurones fired sluggishly in a regular manner. The remaining neurones fired action potentials in the form of discrete bursts resembling interictal activity in vivo. The proportions of these neurones increased from about 40% at 6-14 DIV to more than 80% beyond the third week in vitro. During development in vitro the mean burst duration increased from 3.5 s to about 8 s whereas the mean burst rate (between 0.7-1 bursts/min) remained constant. Activity-deprived neurons had low firing rates and fired action potentials in the form of discrete bursts with a mean burst rate of 0.4/min. The proportions of spontaneously active neurons, the variability of neuronal firing and the viability of the explants either were not altered by the activity blockade or had recovered to control values after 5-6 days in normal growth medium. We conclude that in OTCs of rat neocortex the absence of SBA during development in vitro delays the maturation of excitatory mechanisms responsible for the developmental increase in firing intensity. The development of burst firing modes is less affected by activity blockade.

Anatomical and Physiological Properties of GABAergic Neurotransmission in Organotypic Slice Cultures of Rat Hippocampus

The anatomical and physiological properties of GABAergic inhibitory neurotransmission were investigated in organotypic slice cultures of rat hippocampus. Interneurons and terminal-like elements containing GABA-like immunoreactivity were numerous in tissue kept for 13 - 26 days in culture and showed a similar morphology and distribution to those known from investigations on the hippocampal formation in situ. Furthermore, after 8 - 30 days in culture, spontaneous and evoked IPSPs were observed in all CA3 pyramidal cells tested, resulting from an increase in chloride conductance, and were shown to be mediated by activation of GABA receptors. No functional decrement in the efficacy of GABAergic inhibitory synaptic transmission following chronic isolation and long-term maintenance in vitro was noticed. In particular, neither the magnitude of the synaptic conductance underlying the inhibitory postsynaptic currents nor its reversal potential varied with time in culture. Taken together, the present physiological and immunohistochemical data show that GABAergic inhibition is well expressed in organotypic hippocampal slice cultures and is maintained over periods of at least 4 weeks in vitro.

Sustained hyperexcitability elicited by repetitive electric stimulation of organotypic hippocampal explants

Sustained or complex evoked extracellular slow-wave field potentials were recorded in the CA3/2 areas of organotypic hippocampal explants following stimulation of the dentate area. After repetitive electric stimulation, these discharges became more complex and/or self-sustaining. Self-sustaining discharges continued to occur for the duration of the experiment (15 min-10 h). These slow-wave discharges were evoked (or occurred spontaneously) over a wide range of extracellular K+ concentrations (3-9 mM) without addition of pharmacologic inhibitory antagonists, whereas in some explants raising extracellular K+ from 5.9 to 8-9 mM resulted in spontaneous discharges. The observation that epileptiform discharges in hippocampal explants often occurred spontaneously, were elicited by repetitive electric stimulation, and were recorded at K+ levels which are generally ineffective in acute adult hippocampal slices, indicates that excitability of these CNS explants may be significantly increased following altered neuronal and synaptic development (and/or reorganization) under isolated conditions in culture.

Intrinsic control of electroresponsive properties of transplanted mammalian brain neurons

The present study presents the first analysis of neurons in mammalian brain transplants based on intracellular recording. The results, obtained in brain slices including both donor and host tissue, showed that neuronal precursor cells in embryonic transplants retained their ability to complete their normal differentiation of cell-type-specific electroresponsive properties. Distortions in cell aggregation and synaptic connectivity did not affect this aspect of neuronal differentiation.

Hyperexcitability in organotypic mouse hippocampal explants

We have developed an organotypic hippocampal tissue culture model for analyses of sustained hyperexcitability in which repetitive electric stimulation of the dentate area enhances the amplitude and complexity of evoked normal and epileptiform field potentials recorded extracellularly from CA3/2 areas of neonatal mouse hippocampal explants. In explants where spontaneous field potentials are not detectable at the onset of the experiment, brief repetitive electric stimulation elicits self-sustained epileptiform discharges that continue for the duration of the recorded period (2-10 hours). Lowering the extracellular Ca++ level to 0.1-0.2 mM markedly attenuates these discharges and repetitive stimulation during a 2-4 hr period fails to elicit hyperexcitability. When tested after return to normal media repetitive stimulation can elicit hyperexcitability. Raising the extracellular K+ levels to 8-9 mM enhances the complexity of evoked as well as spontaneous field potentials and, in some cases, elicits self-sustained epileptiform discharges in the absence of repetitive electric stimulation.

Identification of cell types in rat hippocampal slices maintained in organotypic cultures

We have cultured transverse slices of the hippocampal formation from neonatal rats and have identified the cell types which appear in the outgrowth with cell type specific markers. Tetanus toxin and anti-tetanus toxoid, as well as antisera to neurofilaments and 14-3-2 protein, were used to identify neurons. Astrocytes were identified with antisera to glial fibrillary acidic protein and were the predominant non-neural cell type. Fibroblastic cells were labeled with antisera to fibronectin and to myosin and oligodendroglia were identified with antisera to galactocerebroside. The hippocampal neurons could be classified as 1 or the 3 types present in vivo (pyramidal cells, granule cells, or GABAergic interneurons) on the basis of their size, shape, location, or reaction with antisera to glutamic acid decarboxylase. Outgrowth of glial cells and neurites occurred within hours of explantation. Within a few days granule cell neurons migrated onto the glial cell layer from the explant. Their movement is probably related to their migration during in vivo development of the granule cell layer. Synapse formation was observed by electron microscopic analysis beginning about 3-5 days in vitro and areas of neuropil containing many synapses were observed after 3-4 weeks. This culture system should be useful for further studies on the cellular processes which occur during hippocampal development and plasticity.

Morphological differentiation of nerve cells in thin organotypic cultures derived from rat hippocampus and cerebellum

Explants of cerebellum and hippocampus were cultured by means of the roller-tube technique. Large nerve cells such as Purkinje cells and pyramidal cells were injected with the fluorescent dye lucifer yellow. The results demonstrate that cultured neurons grow dendritic arborizations in a pattern that is reminiscent of their in situ counterparts. This finding supports the view that under the described culturing conditions nerve cells show a high degree of differentiation despite an altered biochemical and topographical environment.

Excitatory action of opioid peptides and opiates on cultured hippocampal pyramidal cells

Bath application of low concentrations of opioid peptides and higher concentrations of opiates increased the amplitude and duration of excitatory postsynaptic potentials of pyramidal cells and induced long-lasting depolarization shifts. These actions were reversible and blocked by the opiate antagonist naloxone. Synaptic isolation of the cells by exposure of the cultures to 8 mM Mg2+ not only abolished all spiking and synaptic activity, but also obliterated the peptide effects on pyramidal cells, although these cells were still excited by bath-applied glutamate. The opioid peptides had no detectable effect on resting membrane potential and on the input resistance of the penetrated cells. Experiments in which pyramidal cells were synaptically activated by field stimulation provided direct evidence for a disinhibitory action of the peptides.

Electrophysiology of dissociated hippocampal cultures from fetal mice

Action potentials, postsynaptic potentials (PSPs) and burst potentials have been recorded intracellularly from over 200 neurons in hippocampal cell cultures prepared from fetal mice of 13-18 days gestational age. Repetitively firing action potentials are elicited by intracellular electrical stimulation and often are preceded by stereotyped prepotentials which probably are generated on processes remote from the cell body. In some cells, action potentials are succeeded by long duration depolarizing afterpotentials (0.3--2 sec) with additional superimposed action potentials. Postburst afterhyperpolarization can last up to 2.5 sec. Action potentials are short (0.6--1.2 msec) with peak rates of rise from 64 to 267 V/sec (mean 139 +/- 13 V/sec, 24 cells) and corresponding rates of fall from 21 to 133 V/sec (mean 70 +/- 7 V/sec, 24 cells). Following single action potentials, the afterhyperpolarization is usually less than 10 msec. Inhibitory PSPs occur frequently (up to 70% incidence), have reversal potentials of--30 to--40 mV, and can be evoked in synaptically coupled cell pairs. Excitatory PSPs can initiate prepotentials and action potentials, suggesting dendritic and somatic loci respectively. Neural networks exhibit a broad range of electrophysiologic phenomenology including reciprocal innervation, multiple innervation and synchronous bursting among a widespread population of neurons.

Morphology of dissociated hippocampal cultures from fetal mice

Dissociated hippocampal cultures from fetal mice (13--18 days gestational age) can be maintained for up to two months in culture. Cells grow as either isolated neurons or in small neuronal aggregates. Neurons remain small with a soma diameter of 15--20 micrometer even in mature cultures and develop extensively branched processes during the first two weeks in culture. After this time, processes become more difficult to visualize with phase-contrast optics because of a tendency to grow within the underlying non-neuronal cells. However, the presence of processes has been proved by silver-staining which demonstrates an organizational complexity ranging from a loosely reticulated neuropil to fascicles containing many fibers. More detailed study of individual neuronal morphology was carried out in cells filled with the fluorescent dye, Lucifer Yellow CH, in conjunction with the intracellular recording of synaptic and action potentials from dye-containing micropipettes. Dye-filled cells show a well-developed branching morphology. Process specializations include spines, beading, and basket-like endings. Processes tend to emanate from one side of the soma, either originating at the cell body or from a single trunk. Commonly there are 2--4 orders of branching, but up to 6 orders can occur (counted centrifugally from the soma). Electron microscopy revealed synapses distributed predominantly on dendrites with a smaller number on somata. Dendritic spines are present and are contacted principally by asymmetric synaptic junctions. Symmetric synapses are relatively more common on somata and proximal dendrites.

Innervation of hippocampal explants by central catecholaminergic neurons in co-cultured fetal mouse brain stem explants

The ability of central catecholaminergic neurons to grow into and establish functional connections with the hippocampus in vitro was studied using organotypic tissue culture. Brain stem explanted from the region of the locus coeruleus and hippocampal explants, from 18-day fetal mice, were maintained as co-cultures and were also grown separately. After 1-4 weeks these tissues were analyzed by glyoxylic acid-induced histofluorescence, by light and electron microscopic radioautography after incubation with [3H]norepinephrine, and by electrophysiology. Brain stem explants exhibited specifically fluorescent catecholaminergic cell bodies and varicose fibers after 2-4 weeks in culture. In contrast, no fluorescent cells or neurites could be seen in isolated hippocampal cultures grown for 2-3 weeks in vitro. When hippocampal explants were grown near brain stem explants, catecholaminergic fibers grew out of the brain stem and entered the hippocampus. In additional experiments, co-cultures of brain stem and hippocampus were incubated with [3H]norepinephrine (0.5 micron) and the monoamine oxidase inhibitor nialamide (100 micron). Radioautographic analyses revealed that brain stem neurites which entered the hippocampus took up norepinephrine, whereas neurites in the isolated hippocampal explants did not. Electron microscopic studies of the hippocampus showed varicose axon terminals within the hippocampus to be preferentially labeled. Although close relationships could be seen between labeled axons and dendrites, junctions exhibiting the membranous modifications associated with synapses were never seen. Electrophysiological studies suggested that the catecholaminergic neurites within the hippocampus were functional. Complex synaptically mediated slow wave discharges could be evoked by electrical stimuli in isolated hippocampal explants. Introduction of the beta adrenergic antagonist propranolol (0.4-4.3 micron) did not alter, or slightly depressed, these hippocampal discharges. On the other hand, in hippocampus-brain stem co-cultures, these concentrations of propranolol enhanced the complex hippocampal responses to brain stem or hippocampal stimuli. Similar enhancement of hippocampal responses by propranolol also occurred in these cocultures after acute surgical extirpation of the brain stem explant. The data suggest, therefore, that the action of propranol was probably to block adrenergic inhibitory connections with hippocampal synaptic networks. These experiments provide morphological and electrophysiological evidence that catecholaminergic neurons from fetal mouse brain stem maintained in organotypic tissue culture can grow into and functionally innervate the hippocampus.

Development of kitten hippocampal neurons

Cells in the CA1 region of the hippocampus of kittens were studied using an in vitro slice preparation. Good quality intracellular records were obtained from over 100 cells from kittens 2 days to 4 weeks of age. Cell resistance was high in 2-day-old animals and decreased over the following 4 weeks. Both excitatory and inhibitory synaptic potentials were seen in all animals. EPSPs were only weakly effective in triggering spikes in the youngest kittens, but were greatly potentiated by repetitive stimulation at 3-10/sec; IPSPs caused a potent blockade of cell discharge in even the youngest preparations. Stimulation of the orthodromic input pathway led to a complex series of excitatory and inhibitory synaptic events which was not seen in the adult. In the 2- and 4-week-old kittens, a cell type with physiological properties different from the predominant pyramidal cell began to appear in recordings from the CA1 region. Technical difficulties inherent in in vivo recordings from neonatal animals were considerably less with the in vitro technique. Careful developmental studies may now be pursued in the slice at the single cell synaptic level.

Tissue culture studies of demyelinating disease: a critical review

Tissue culture studies of human and experimental demyelinating diseases have demonstrated that sera from patients with multiple sclerosis reversibly demyelinate central nervous system cultures. Similar changes are evoked by sera from animals with experimental allergic encephalomyelitis induced by inoculation with whole central nervous system tissue but not by encephalitogenic myelin basic protein. Sera and buffy coat or lymph node cells from humans with idiopathic polyneuritis and animals with experimental allergic neuritis demyelinate cultures of peripheral nervous system tissue. While these studies have contributed to speculations about pathogenetic mechanisms of demyelinating diseases, including the role of both circulating antibodies and delayed hypersensitivity factors, a number of important questions remain unanswered. Among these are the identity of the antigens that evoke antimyelin antibodies and the precise relationship of serum or cellular antimyelin factors to the pathogenesis or clinical course of the demyelinating diseases. Further studies with this technique may provide more complete information about the role of immunological events in induction of disease.

Electrophysiology and cytology of hippocampal formation transplants in the anterior chamber of the eye. I. Intrinsic organization

Pieces of the immature hippocampal formation were transplanted to the anterior chamber of adult rat eyes. The transplants survived, became vascularized from the host irides, and proliferated extensively in proportion to the age of the donor fetus. These transplants mature and develop an adult organization in oculo. This maturation proceeds unimpaired in the absence of gyrus dentatus or after superior cervical ganglionectomy.

A comparative statistical study of hippocampal neuronal spontaneous spike activity in situ and in vitro

The statistical characteristics of the spontaneous spike activity of rat hippocampal neurons in fields CA1-2 were compared in situ and in tissue culture. Statistical analyses have shown strong similarities in estimators of basic numerical characteristics of interspike interval (SIS) distributions. These similarities may serve as evidence of maintenance of normal functional properties and an "organotypic arrangement" of neurons in tissue culture, and they are also indicative of an intrahippocampal origin of the spontaneous impulse activity in the hippocampus. On the other hand, some differences are noted in the tests of firing patterns. Interpretation of these results leads to some assumptions about mechanisms of the phenomenon under study.

The effects of putative neurotransmitters on the resting membrane potential of dissociated brain neurones in culture

Cultures established from mechanically dissociated neonatal mouse brains were found to be suitable for electrophysiological investigation of drug action. During culture most cells were aggregated into either monolayer regions or thick cords joining monolayer regions. A few cells remained isolated. The neurones in the monolayer regions were distinguished from glial cells by differential staining, and were found to be the best subject for intracellular recording. Frequency of resting membrane potentials of these cells proved to be reproducible in cultures of the same age, and were a useful index of sensitivity to bath applied drugs. Acetylcholine, dopamine, histamine, serotonin and noradrenaline depolarized various neurones; GABA caused hyperpolarization, while glutamate and glycine had no significant effect. Antagonism of the responses to acetylcholine, dopamine, serotonin and GABA was seen using atropine, pimozide, methysergide and bicuculline respectively. It is concluded that dissociated brain neurones in culture show chemosensitivity and may be useful in further pharmacological studies.

Spontaneous bioelectric activity of cultured Purkinje cells during exposure to glutamate, glycine, and strychnine

The addition of glutamate to the bathing medium increased the average firing rate of cerebellar rat Purkinje cells in vitro. At concentrations lower than 10(-6) M, there was no deviation from controls in the firing pattern or rate that was detectable. At 10(-3) M glutamate, the amplitude of the action potentials was gradually decreased until all activity was abolished. The action of glutamate was rapid in onset and reversible. Glycine produced sustained depression of firing at concentrations higher than 10(-3) M. This inhibition was strychnine-insensitive and considered nonspecific. Strychnine, on the other hand, exerted an excitatory influence on Purkinje cells when applied at low concentrations (10(-8) TO 10(-6) M). The firing became more irregular and complex discharges appeared. Higher concentrations of strychnine (greater than 10(-5) M) inhibited the spontaneous activity. The effect of strychnine was partly reversible. The data suggest that low concentrations of strychnine lower the threshold for inputs at excitatory as well as inhibitory synapses.