4-Aminopyridine-induced spreading depression episodes in immature hippocampus: developmental and pharmacological characteristics

Region-specific Effects of Early-life Status Epilepticus on the Adult Hippocampal CA3 - Medial Entorhinal Cortex Circuitry In vitro: Focus on Interictal Spikes and Concurrent High-frequency Oscillations

Convulsive status epilepticus (SE) in immature life is often associated with lasting neurobiological changes. We provoked SE by pentylenetetrazole in postnatal day 20 rat pups and examined communication modalities between the temporal hippocampus and medial entorhinal cortex (mEC) in vitro. After a minimum of 40 days post-SE, we prepared combined temporal hippocampal - medial entorhinal cortex (mEC) slices from conditioned (SE) and naïve (N) adult rats and recorded 4-aminopyridine-induced spontaneous epileptiform interictal-like discharges (IED) simultaneously from CA3 and mEC layer V-VI. We analyzed IED frequency and high frequency oscillations (HFOs) in intact slices and after surgical separation of hippocampus from mEC, by two successive incisions (Schaffer collateral cut, Parasubiculum cut). In all slices, IED frequency was higher in CA3 vs mEC (5N, 4SE) and Raster plots indicated no temporal coincidence between them either in intact or in CA1-cut slices (4N, 4SE). IED frequency was significantly higher in SE mEC, but similar in SE and N CA3, independently of connectivity state. Ripples (R) and Fast Ripples (FR) coincided with IEDs and their power differed between SE and N intact slices (22N, 12SE), both in CA3 and mEC. CA3 FR/R ratios were higher in the absence of mEC (14N, 8SE). Moreover, SE (vs N) slices showed significantly higher FR/R ratios independently of the presence of mEC. Taken together, these findings suggest lasting effects of immature SE in network dynamics governing hippocampal-entorhinal communication which may impact adult cognitive, behavioral, and/or seizure threshold sequalae.

Association of cortical spreading depression and seizures in patients with medically intractable epilepsy

OBJECTIVE

Monitoring of the ultra-low frequency potentials, particularly cortical spreading depression (CSD), is excluded in epilepsy monitoring due to technical barriers imposed by the scalp ultra-low frequency electroencephalogram (EEG). As a result, clinical studies of CSD have been limited to invasive EEG. Therefore, the occurrence of CSD and its interaction with epileptiform field potentials (EFP) require investigation in epilepsy monitoring.

METHODS

Using a novel AC/DC-EEG approach, the occurrence of DC potentials in patients with intractable epilepsy presenting different symptoms of aura was investigated during long-term video-EEG monitoring.

RESULTS

Various forms of slow potentials, including simultaneous negative direct current (DC) potentials and prolonged EFP, propagated negative DC potentials, and non-propagated single negative DC potentials were recorded from the scalp of the epileptic patients. The propagated and single negative DC potentials preceded the prolonged EFP with a time lag and seizure appeared at the final shoulder of some instances of the propagated negative DC potentials. The slow potential deflections had a high amplitude and prolonged duration and propagated slowly through the brain. The high-frequency EEG was suppressed in the vicinity of the negative DC potential propagations.

CONCLUSIONS

The study is the first to report the recording of the propagated and single negative DC potentials with EFP at the scalp of patients with intractable epilepsy. The negative DC potentials preceded the prolonged EFP and may trigger seizures. The propagated and single negative DC potentials may be considered as CSD.

SIGNIFICANCE

Recordings of CSD may serve as diagnostic and prognostic monitoring tools in epilepsy.

Spreading Depolarization Facilitates the Transition of Neuronal Burst Firing from Interictal to Ictal State

The transition of neuronal burst firing from the interictal to ictal state contributes to seizure initiation in human temporal lobe epilepsy. The low-Mg²⁺ model of seizure is characterized by initial spontaneous interictal bursting events, which later developed into ictaform discharges. Both experimental and clinical studies point to a complex link between spreading depolarization (SD) and epileptiform field potentials (EFP), including SD-induced epileptic seizures. To investigate the mechanism of SD and EFP interactions, the effect of SD on the transition of interictal to ictal state in low-Mg²⁺ model of seizure was studied in the rat hippocampus in vitro. After the appearance of interictal activities, SD was elicited by local application of KCl. SD significantly increased the amplitude and duration of action potentials and after-hyperpolarization, and hyperpolarized the membrane potential. Furthermore, SD significantly increased the duration of interictal activities and the threshold potentials of interictal activities. In addition, SD significantly accelerated the transition from interictal to ictal state compared to the control tissues. Ictal activities after induction of SD exhibited a significantly longer duration. This study revealed that SD accelerates interictal-to-ictal transitions and facilitates development of ictaform discharges, possibly via the enhancement of neural synchronization, and points to the potential role of SD in seizure initiation.

Characterization of inhibitory GABA-A receptor activation during spreading depolarization in brain slice

Spreading depolarization (SD) is a slowly propagating wave of near complete depolarizations of neurons and glia. Previous studies have reported large GABA releases during SD, but there is limited understanding of how GABA release and receptor activation are regulated and influence the propagating SD wavefront, as well as an excitatory phase immediately following the passage of SD. The present study characterized GABA-A type receptor (GABA_AR) currents during SD generated by KCl microinjection in acute hippocampal slices from adult mice. Spontaneous GABA_AR-mediated currents (sIPSCs) were initially enhanced, and were followed by a large outward current at the wavefront. sIPSC were then transiently supressed during the late SD phase, resulting in a significant reduction of the sIPSC/sEPSC ratio. The large outward current generated during SD was eliminated by the GABA_AR antagonist gabazine, but the channel potentiator/agonist propofol failed to potentiate the current, likely because of a ceiling effect. Extracellular Cl⁻ decreases recorded during SD were reduced by the antagonist but were not increased by the potentiator. Together with effects of GABA_AR modulators on SD propagation rate, these results demonstrate a significant inhibitory role of the initial GABA_AR activation and suggest that intracellular Cl⁻ loading is insufficient to generate excitatory GABA_AR responses during SD propagation. These results provide a mechanistic explanation for facilitating effects of GABA_AR antagonists, and the lack of inhibitory effect of GABA_AR potentiators on SD propagation. In addition, selective suppression of GABA transmission in the late SD period and the lack of effect of GABA_A modulators on the duration of SD suggests that GABA modulation may not be effective approach to protect neurons during the vulnerable phase of SD.

Propagation of cortical spreading depression into the hippocampus: The role of the entorhinal cortex

Propagation of cortical spreading depression (CSD) to the subcortical structures could be the underlying mechanism of some neurological deficits in migraine with aura. The entorhinal cortex (EC) as a gray matter bridge between the neocortex and subcortical regions plays an important role in this propagation. In vitro combined neocortex-hippocampus brain slices were used to study the propagation pattern of CSD between the neocortex and the hippocampus. The effects of different compounds as well as tetanic electrical stimulations in the EC on propagation of CSD to the hippocampus were investigated. Repetitive induction of CSD by KCl injection in the somatosensory cortex enhanced the probability of CSD entrance to the hippocampus via EC. Local application of AMPA receptor blocker CNQX and cannabinoid receptor agonist WIN 55212-2 in EC facilitated the propagation of CSD to the hippocampus, whereas application of NMDA receptor blocker APV and GABA_A receptor blocker bicuculline in this region reduced the probability of CSD penetration to the hippocampus. Application of tetanic stimulation in EC also facilitated the propagation of CSD entrance to the hippocampus. Our data suggest the importance of synaptic plasticity of EC in filtering the propagation of CSD into subcortical structures and possibly the occurrence of concomitant neurological deficits. Synapse 68:574-584, 2014. © 2014 Wiley Periodicals, Inc.

Spreading convulsions, spreading depolarization and epileptogenesis in human cerebral cortex

Spreading depolarization of cells in cerebral grey matter is characterized by massive ion translocation, neuronal swelling and large changes in direct current-coupled voltage recording. The near-complete sustained depolarization above the inactivation threshold for action potential generating channels initiates spreading depression of brain activity. In contrast, epileptic seizures show modest ion translocation and sustained depolarization below the inactivation threshold for action potential generating channels. Such modest sustained depolarization allows synchronous, highly frequent neuronal firing; ictal epileptic field potentials being its electrocorticographic and epileptic seizure its clinical correlate. Nevertheless, Leão in 1944 and Van Harreveld and Stamm in 1953 described in animals that silencing of brain activity induced by spreading depolarization changed during minimal electrical stimulations. Eventually, epileptic field potentials were recorded during the period that had originally seen spreading depression of activity. Such spreading convulsions are characterized by epileptic field potentials on the final shoulder of the large slow potential change of spreading depolarization. We here report on such spreading convulsions in monopolar subdural recordings in 2 of 25 consecutive aneurismal subarachnoid haemorrhage patients in vivo and neocortical slices from 12 patients with intractable temporal lobe epilepsy in vitro. The in vitro results suggest that γ-aminobutyric acid-mediated inhibition protects from spreading convulsions. Moreover, we describe arterial pulse artefacts mimicking epileptic field potentials in three patients with subarachnoid haemorrhage that ride on the slow potential peak. Twenty-one of the 25 subarachnoid haemorrhage patients (84%) had 656 spreading depolarizations in contrast to only three patients (12%) with 55 ictal epileptic events isolated from spreading depolarizations. Spreading depolarization frequency and depression periods per 24 h recording episodes showed an early and a delayed peak on Day 7. Patients surviving subarachnoid haemorrhage with poor outcome at 6 months showed significantly higher total and peak numbers of spreading depolarizations and significantly longer total and peak depression periods during the electrocorticographic monitoring than patients with good outcome. In a semi-structured telephone interview 3 years after the initial haemorrhage, 44% of the subarachnoid haemorrhage survivors had developed late post-haemorrhagic seizures requiring anti-convulsant medication. In those patients, peak spreading depolarization number had been significantly higher [15.1 (11.4–30.8) versus 7.0 (0.8–11.2) events per day, P = 0.045]. In summary, monopolar recordings here provided unequivocal evidence of spreading convulsions in patients. Hence, practically all major pathological cortical network events in animals have now been observed in people. Early spreading depolarizations may indicate a risk for late post-haemorrhagic seizures.

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

GABA is the main inhibitory neurotransmitter in the adult forebrain, where it activates ionotropic type A and metabotropic type B receptors. Early studies have shown that GABA(A) receptor-mediated inhibition controls neuronal excitability and thus the occurrence of seizures. However, more complex, and at times unexpected, mechanisms of GABAergic signaling have been identified during epileptiform discharges over the last few years. Here, we will review experimental data that point at the paradoxical role played by GABA(A) receptor-mediated mechanisms in synchronizing neuronal networks, and in particular those of limbic structures such as the hippocampus, the entorhinal and perirhinal cortices, or the amygdala. After having summarized the fundamental characteristics of GABA(A) receptor-mediated mechanisms, we will analyze their role in the generation of network oscillations and their contribution to epileptiform synchronization. Whether and how GABA(A) receptors influence the interaction between limbic networks leading to ictogenesis will be also reviewed. Finally, we will consider the role of altered inhibition in the human epileptic brain along with the ability of GABA(A) receptor-mediated conductances to generate synchronous depolarizing events that may lead to ictogenesis in human epileptic disorders as well.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Infant Brain Stem Is Prone to the Generation of Spreading Depression During Severe Hypoxia

Spreading depression (SD) resembles a concerted, massive neuronal/glial depolarization propagating within the gray matter. Being associated with cerebropathology, such as cerebral ischemia or hemorrhage, epileptic seizures, and migraine, it is well studied in cortex and hippocampus. We have now analyzed the susceptibility of rat brain stem to hypoxia-induced spreading depression-like depolarization (HSD), which could critically interfere with cardiorespiratory control. In rat brain stem slices, severe hypoxia (oxygen withdrawal) triggered HSD within minutes. The sudden extracellular DC potential shift of approximately −20 mV showed the typical profile known from other brain regions and was accompanied by an intrinsic optical signal (IOS). Spatiotemporal IOS analysis revealed that in infant brain stem, HSD was preferably ignited within the spinal trigeminal nucleus and then mostly spread out medially, invading the hypoglossal nucleus, the nucleus of the solitary tract (NTS), and the ventral respiratory group (VRG). The neuronal hypoxic depolarizations underlying the generation of HSD were massive, but incomplete. The propagation velocity of HSD and the associated extracellular K+ rise were also less marked than in other brain regions. In adult brain stem, HSD was mostly confined to the NTS and its occurrence was facilitated by hypotonic solutions, but not by glial poisoning or block of GABAergic and glycinergic synapses. In conclusion, brain stem tissue reliably generates propagating HSD episodes, which may be of interest for basilar-type migraine and brain stem infarcts. The preferred occurrence of HSD in the infant brain stem and its propagation into the VRG may be of importance for neonatal brain stem pathology such as sudden infant death syndrome.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Repetitive Spreading Depression-Like Events Result in Cell Damage in Juvenile Hippocampal Slice Cultures Maintained in Normoxia

Prolonged seizures, e.g., induced by fever, experienced early in life are considered a precipitating injury for the subsequent development of temporal lobe epilepsy. During in vitro epileptiform activity, spreading depressions (SDs) have often been observed. However, their contribution to changes in the properties of juvenile neuronal tissue is unknown. We therefore used the juvenile hippocampal slice culture preparation (JHSC) maintained in normoxia (20% O(2)-5% CO(2)-75% N(2)) to assess the effect of repetitive SD-like events (SDLEs) on fast field potentials and cell damage. Repetitive SDLEs in the CA1 region could be induced in about two-thirds of the investigated JHSCs (n = 61) by repetitive electrical stimulation with 2-200 pulses. SDLEs were characterized by a transient large negative field potential shift accompanied by intracellular depolarization, ionic redistribution, slow propagation (assessed by intrinsic optical signals) and glutamate receptor antagonist sensitivity. The term "SDLE" was used because evoked fast field potentials were only incompletely suppressed and superimposed discharges occurred. With 20 +/- 1 repetitive SDLEs (interval of 10-15 min, n = 7 JHSCs), the events got longer, their amplitude of the first peak declined, while threshold for induction became reduced. Evoked fast field potentials deteriorated and cell damage (assessed by propidium iodide fluorescence) occurred, predominantly in regions CA1 and CA3. As revealed by measurements of tissue partial oxygen pressure during SDLEs repetitive transient anoxia accompanying SDLE might be critical for the observed cell damage. These results, limited so far to the slice culture preparation, suggest SDs to be harmful events in juvenile neuronal tissue in contrast to what is known about their effect on adult neuronal tissue.

Cortical spreading depression confounds concentration-dependent pial arteriolar dilation during N-methyl-D-aspartate superfusion

Pial arterioles do not express N-methyl-D-aspartate (NMDA) receptors but dilate in response to topical NMDA application. We explored the mechanism underlying NMDA-mediated responses in murine pial arterioles (11-31 microm), using a closed cranial window preparation, and found that arteriolar dilation was not concentration dependent. Pial arteriolar diameter abruptly increased within 3 min of superfusing 50 or 100 microM NMDA. Dilation reached a peak within 1 min (46 +/- 14%) and then declined to a plateau (28 +/- 13%) for the duration of superfusion. Whereas a higher concentration (200 microM) did not produce further dilation, lower concentrations (1-10 microM) did not dilate the arterioles at all. MK-801 (10 microM) abrogated the dilation response, whereas Nomega-nitro-L-arginine (1 mM) attenuated the peak and abolished the sustained dilation during NMDA superfusion. We determined that NMDA-induced pial arteriolar responses were evoked by cortical spreading depression, because abrupt vasodilation during 50 or 100 microM NMDA superfusion was associated with a large negative slow potential shift and electrocorticogram suppression that spread from the superfusion window to distant cortical areas. Our data suggest that the responses of pial arterioles to NMDA are caused in part by neurovascular coupling due to cortical spreading depression.

P/Q Ca2+ channel blockade stops spreading depression and related pyramidal neuronal Ca2+ rise in hippocampal organ culture

Ca2+ channels and pyramidal cell Ca2+ are involved in hippocampal spreading depression (SD), but their roles remain elusive. Accordingly, we characterized Ca2+ changes during SD in CA3 pyramidal neurons and determined whether Ca2+ channel antagonists could prevent SD. SD was induced in hippocampal organotypic cultures (HOTCs), in which experimental conditions can be rigorously controlled. SD was triggered by transient exposure to sodium acetate (NaAc)-based Ringer's coupled to an electrical pulse in the dentate gyrus and its occurrence confirmed with interstitial DC recordings. Pyramidal cell Ca2+ was measured with fura-2 filled cells and was quantified at the soma, proximal and more distal apical dendrites. Regional Ca2+ changes began simultaneously with the triggering pulse of SD and reached three distinct peaks before returning to baseline concomitant with the interstitial DC potential of SD. The first peak occurred within 5 s of the triggering pulse, was smallest, and heralded the onset of SD. The second Ca2+ change was the greatest and reached a peak 6 s later, during the early phase of SD. The third was intermediate in size and occurred 18 s later, as SD reached its maximum interstitial DC change. SD was prevented by nonselective Ca2+ blockade (Ni2+ and Cd2+) but not by either L-Ca2+ channel (nifedipine) or N-Ca2+ channel inhibition (omega-conotoxin GVIA). Importantly, SD was blocked by P/Q Ca2+ channel antagonism (omega-agatoxin-IVA), which also prompted a significant reduction in pyramidal cell Ca2+ change and hyperexcitability. These results show that the spatiotemporal pattern of pyramidal cell Ca2+ change with SD is multiphasic; they provide further evidence that these changes begin before electrophysiologic evidence of SD. Furthermore, they show that P/Q Ca2+ channel antagonism can prevent SD in HOTCs and it appears to do so by preventing the NaAc-induced increased pyramidal cell excitability from NaAc exposure, which may involve altered GABAergic transmission.

Adenosine-mediated synaptic depression and EPSP/spike dissociation following high potassium-induced depolarization in rat hippocampal slices

Simultaneous recordings of orthodromic PS, fEPSP and antidromic PS revealed EPSP/spike (E-S) dissociation, indicating a conversion of input/output relations from early and brief excitability to a late and prolonged depression during the recovery from depolarization induced by high levels of potassium. E-S potentiation was partially attenuated by pre-treating the slices with BAPTA-AM and lidocaine and totally eliminated by a submaximal concentration of muscimol. The time lag for recovery was decreased by the GABA(A) antagonist and completely eliminated by the A(1) antagonist. From these observations, we conclude that Ca(2+) dependent inhibitory suppression is the main cause of a brief period of E-S potentiation, and accumulation of adenosine is the mechanism responsible for prolonged depression of synaptic transmission

Network and pharmacological mechanisms leading to epileptiform synchronization in the limbic system in vitro

Seizures in patients presenting with mesial temporal lobe epilepsy result from the interaction among neuronal networks in limbic structures such as the hippocampus, amygdala and entorhinal cortex. Mesial temporal lobe epilepsy, one of the most common forms of partial epilepsy in adulthood, is generally accompanied by a pattern of brain damage known as mesial temporal sclerosis. Limbic seizures can be mimicked in vitro using preparations of combined hippocampus-entorhinal cortex slices perfused with artificial cerebrospinal fluid containing convulsants or nominally zero Mg(2+), in order to produce epileptiform synchronization. Here, we summarize experimental evidence obtained in such slices from rodents. These data indicate that in control animals: (i) prolonged, NMDA receptor-dependent epileptiform discharges, resembling electrographic limbic seizures, originate in the entorhinal cortex from where they propagate to the hippocampus via the perforant path-dentate gyrus route; (ii) the initiation and maintenance of these ictal discharges is paradoxically contributed by GABA (mainly type A) receptor-mediated mechanisms; and (iii) CA3 outputs, which relay a continuous pattern of interictal discharge at approximately 1Hz, control rather than sustain ictal discharge generation in entorhinal cortex. Recent work indicates that such a control is weakened in the pilocarpine model of epilepsy (presumably as a result of CA3 cell damage). In addition, in these experiments electrographic seizure activity spreads directly to the CA1-subiculum regions through the temporoammonic pathway. Studies reviewed here indicate that these changes in network interactions, along with other mechanisms of synaptic plasticity (e.g. axonal sprouting, decreased activation of interneurons, upregulation of bursting neurons) can confer to the epileptic, damaged limbic system, the ability to produce recurrent limbic seizures as seen in patients with mesial temporal lobe epilepsy.

Spreading depression in human neocortical slices

Cortical spreading depression (CSD) occurrence has been suggested to be associated with seizures, migraine aura, head injury and brain ischemia-infarction. Only few studies identified CSD in human neocortical slices and no comprehensive study so far evaluated this phenomenon in human. Using the neocortical tissue excised for treatment of intractable epilepsy, we aimed to investigate CSD in human. CSD was induced by KCl injection and by modulating T-type Ca(2+) currents in incubated human neocortical tissues in an interphase mode. The DC-fluctuations were recorded by inserting microelectrodes into different cortical layers. Local injection of KCl triggered single CSD that propagated at 3.1+/-0.1 mm/min. Repetitive CSD also occurred spontaneously during long lasting application (5 h) of the T-type Ca(2+) channel blockers amiloride (50 microM) or NiCl(2) (10 microM) which was concomitant with a reversible extracellular potassium increase up to 50 mM. CSD could be blocked by the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid in all cases. The results demonstrate that modulation of the Ca(2+) dynamics conditioned human neocortical slices and increased their susceptibility to generate CSD. Furthermore, these data indicate that glutamatergic pathway plays a role in CSD phenomenon in human.

Developmental characteristics of epileptiform activity in immature rat neocortex: a comparison of four in vitro seizure models

New-onset seizures and epilepsy have a relatively high incidence in infants and children. A leading hypothesis to explain an increased seizure susceptibility of the immature nervous system involves ontogenetic changes in different neurotransmitter systems, such as specific glutamate and GABA receptors. However, few studies have directly tested this hypothesis in a systematic fashion, especially in neocortical structures, where seizures in pediatric patients frequently arise. The present study investigated developmental changes in epileptiform activity in rat neocortical slices from four age groups (postnatal days P4--7, P13--16, P23--26, P41--47) due to four pharmacological conditions (4-aminopyridine, low magnesium, picrotoxin, CGP-35348) that differentially modulate glutamate and GABA systems. A characteristic age-dependence of the incidence of epileptiform activity was observed. In all pharmacological conditions, no epileptiform activity occurred in neocortical slices from P4--7 rats. Interictal discharges, ictal events, and spreading depression had a maximal incidence at P13--16 and decreased progressively in later age groups. 4-Aminopyridine, low magnesium, and picrotoxin induced all types of epileptiform activity with a similar age-dependent pattern, despite minor differences in quantitative characteristics of epileptiform activity between these three conditions. The GABA(B) antagonist, CGP-35348, did not elicit epileptiform activity in any age group, but could potentiate synaptic potentials. These findings establish that isolated neocortical tissue intrinsically displays ontogenetic changes in seizure susceptibility independent of systemic factors. The similar age-dependent patterns of epileptiform activity with multiple drugs support a concept of global developmental changes in excitability not specifically linked to any particular neurotransmitter system.

Endogenous acetylcholine facilitates epileptogenesis in immature rat neocortex

In the presence of the gamma-amino butyric acid-A (GABAA) antagonist bicuculline methiodide (50 microM), synchronous spontaneous and evoked potentials were recorded extracellularly from the deep layers of immature neocortex (postnatal days 10-31, P10-P31) in vitro. Addition of the anticholinesterase eserine (10 microM) depressed the amplitude (by 29.5+/-6.6%, n=13) and duration (by 26.3+/-4.7%, n=11) of the evoked field potentials in 13/19 slices (68%), and increased significantly the rates of occurrence of spontaneous epileptiform discharges or induced them in 9/19 slices (47%). All these effects were blocked by the muscarinic antagonist atropine (2.5 microM, n=3), suggesting that they were mediated by the activation of muscarinic receptors by endogenous acetylcholine. The cholinergic inhibitory effect is unlikely to terminate seizures, while the excitatory effect, could conceivably promote or aggravate their manifestation. In conclusion, these findings demonstrate that endogenous acetylcholine may contribute to epileptogenesis in immature neocortex.

Osmotic forces and gap junctions in spreading depression: a computational model

In a computational model of spreading depression (SD), ionic movement through a neuronal syncytium of cells connected by gap junctions is described electrodiffusively. Simulations predict that SD will not occur unless cells are allowed to expand in response to osmotic pressure gradients and K+ is allowed to move through gap junctions. SD waves of [K+]out approximately 25 to approximately 60 mM moving at approximately 2 to approximately 18 mm/min are predicted over the range of parametric values reported in gray matter, with extracellular space decreasing up to approximately 50%. Predicted waveform shape is qualitatively similar to laboratory reports. The delayed-rectifier, NMDA, BK, and Na+ currents are predicted to facilitate SD, while SK and A-type K+ currents and glial activity impede SD. These predictions are consonant with recent findings that gap junction poisons block SD and support the theories that cytosolic diffusion via gap junctions and osmotic forces are important mechanisms underlying SD.

Hyperthermic spreading depressions in the immature rat hippocampal slice

Febrile seizures are the most common seizure type in children (6 mo to 5 yr). The pathophysiology of febrile seizures is unknown. Current genetic studies show that some febrile seizures result from channelopathies. We have performed electrophysiological experiments in in vitro hippocampal slices to test a novel hypothesis that a disordered regulation of ionic homeostasis underlies the genesis of febrile seizures. In transverse hippocampal CA1 slices from 104 rats, temperature increase from 34 degrees to 40 degrees C produced a series of spreading depressions (SDs), called hyperthermic SDs. The hyperthermic SDs were age-dependent, occurring in only 1/17 8-16 day-old animals, 44/49 17-60 day-old animals, and 11/20 rats older than than 60 days. The hyperthermic SDs usually occurred on the rising phase of the temperature. The mean temperature to trigger a first hyperthermic SD was 38.8 +/- 1.3 degrees C (mean +/- SD, n = 44). The hyperthermic SDs induced a reversible loss of evoked synaptic potentials and a dramatic decrease of input resistance. Neuronal and field epileptiform bursting occurred in the early phases of the hyperthermic SD. During hyperthermic SDs, pyramidal cell membrane potential depolarized by 38.3 +/- 4.9 mV (n = 20), extracellular field shifted negative 18.5 +/- 3.9 mV (n = 44), and extracellular K(+) rose reversibly to 43.8 +/- 10.9 mM (n = 6). Similar SDs could be evoked by ouabain or transient hypoxia with normal temperature. Tetrodotoxin could block initial epileptiform bursting, without blocking SDs. Hyperthermia-induced SDs should be investigated as possible contributing factors to febrile seizures.

Blockade of GABA uptake potentiates GABA-induced depolarizations in adult mouse cortical slices

The electrophysiological effects of GABA and the GABA uptake inhibitor, NO-711, were investigated on cortical slices prepared from adult audiogenic seizure-prone DBA/2 mice. GABA, perfused for 1 min, elicited depolarizing responses which had a mean duration of 80-100 s and were concentration-dependent (0.1-32 mM). NO-711 (25 microM), perfused for 15 min, produced depolarizations with a mean duration of 50-60 s and these persisted for 4-5 h. The responses to both compounds were blocked by the GABA(A) receptor antagonist, bicuculline. Pre-treatment of the slices with NO-711 potentiated the responses to GABA and moved the concentration-response curve to the left. The EC50 to GABA following pre-treatment with NO-711 was 111+/-24 microM from a control value of 1.17+/-0.19 mM. These results demonstrate that in cortical slices GABA has a depolarizing action rather than the conventional one of hyperpolarization and that this response is potentiated by inhibition of GABA reuptake.

Termination of epileptic afterdischarge in the hippocampus

The mechanism of afterdischarge termination in the various hippocampal regions was examined in the rat. Stimulation of the perforant path or the commissural system was used to elicit afterdischarges. Combination of multiple site recordings with silicon probes, current source density analysis, and unit recordings in the awake animal allowed for a high spatial resolution of the field events. Interpretation of the field observations was aided by intracellular recordings from anesthetized rats. Irrespective of the evoking conditions, afterdischarges always terminated first in the CA1 region. Termination of the afterdischarge was heralded by a large DC shift initiated in dendritic layers associated with a low amplitude "afterdischarge termination oscillation" (ATO) at 40 to 80 Hz in the cell body layer. ATOs were also observed in the CA3 region and the dentate gyrus. The DC shift spread at the same velocity (0. 1-0.2 mm/sec) in all directions and could cross the hippocampal fissure. All but 1 of the 25 putative interneurons in the CA1 and dentate regions ceased to fire before the onset of ATO. Intracellularly, ATO and the emerging DC potential were associated with fast depolarizing potentials and firing of pyramidal cells and depolarization block of spike initiation, respectively. Both field ATO and the intracellular depolarization shift were replicated by focal microinjection of potassium. We hypothesize that [K+]o lost by the intensely discharging neurons during the afterdischarge triggers propagating waves of depolarization in the astrocytic network. In turn, astrocytes release potassium, which induces a depolarization block of spike generation in neurons, resulting in "postictal depression" of the EEG.

Neuroprotective electrical stimulation of cerebellar fastigial nucleus attenuates expression of periinfarction depolarizing waves (PIDs) and inhibits cortical spreading depression

In rat, electrical stimulation of the cerebellar fastigial nucleus (FN) for 1 h reduces the volume of focal ischemic infarctions produced by occluding the middle cerebral artery (MCAO), even 10 days later. The mechanism by which this 'central neurogenic neuroprotection' salvages ischemic brain is not known but does not result from changes in cerebral perfusion. MCAO also triggers periodic periinfarction depolarizing waves (PIDs) in the ischemic penumbra, the territory of salvage. These may contribute to neuronal death and promote infarct expansion. Conceivably, FN stimulation, which can otherwise modify cortical excitability, may alter the development of PIDs. We investigated in anesthetized rats whether FN stimulation modifies PIDs expression and, if so, the threshold for evoking cortical spreading depression (CSD), a process sharing characteristics with PIDs and an index of cortical excitability. Stimulation of FN immediately or 72 h before MCAO decreased infarction volumes by approximately 45% (p<0.01), increased PID latency >10-fold, and decreased the number of PIDs by >50% (p<0.001). In normal rats, stimulation of FN increased the threshold current for eliciting CSD by 175% and slowed its propagation velocity by 35% (p<0.01 for each) immediately, but not 72 h, after FN stimulation. We conclude: FN stimulation elicits long-lasting suppression of PIDs in parallel with neuroprotection. However, PIDs suppression over time is unlikely to result from a major increase in cortical tolerance to depolarization and probably is not the principal mechanism of salvage.

Calcium waves precede electrophysiological changes of spreading depression in hippocampal organ cultures

Although intercellular Ca2+ waves resemble spreading depression (SD) and occur in hippocampal organ cultures (HOTCs), SD has not been reported in these cultures. Accordingly, electrophysiological and Ca2+ imaging techniques were used to examine potential interrelations between Ca2+ waves and electrophysiological changes of SD. Our results show, for the first time, that HOTCs can support SD. Furthermore, two distinct Ca2+ waves were found to precede SD. The first traveled >100 micron/sec along the pyramidal cell dendritic layer. The second subsequently traveled mostly perpendicular to the pyramidal cell layer from CA3 (or CA1) but also in all directions from its area of initiation. This second, slower wave spread with the interstitial DC change of SD at millimeters per minute but always ahead of it by 6-16 sec. Heptanol, which uncouples gap junctions, blocked both of these Ca2+ waves and SD. Thus, two types of Ca2+ waves occur with the initiation and propagation of SD. The first might reflect interneuronal changes linked by gap junctions, whereas the second might stem from interastrocyte changes linked via similar connections. Because individual cells can be followed in space and time for protracted periods in HOTCs, this preparation may be ideal for studies designed to explore not only the mechanisms of SD but also the long-term consequences of SD, such as ischemic tolerance.

Postnatal conditioning for spreading cortical depression in the rat brain

The cerebral cortex of anaesthetised 2- to 12-day-old rats was superfused with artificial cerebrospinal fluid containing 100 mM acetate substituted for chloride to condition the brain for spreading depression (SD). After such superfusion, the earliest SD-like events were found at day 9 and full blown SD at day 10, whereas in the unconditioned brain the first SD occurred between days 12 and 15. Acetate conditioning of the cerebral cortex may be used to unmask neuronal and glial properties that are hidden in early stages of development.

Hypoxia-induced dysfunction in developing rat neocortex

Neocortical slices from young [postnatal day (P) 5-8], juvenile (P14-18), and adult (>P28) rats were exposed to long periods of hypoxia. Field potential (FP) responses to orthodromic synaptic stimulation, the extracellular DC potential, and the extracellular Ca2+ concentration ([Ca2+]o] were measured simultaneously in layers II/III of primary somatosensory cortex. Hypoxia caused a 42 and 55% decrease in the FP response in juvenile and adult cortex, respectively. FP responses recorded in slices from young animals were significantly more resistant to oxygen deprivation as compared with the juvenile (P < 0.01) and adult age group (P < 0.001) and declined by only 3% in amplitude. In adult cortex, hypoxia elicited, after 7 +/- 4.5 min (mean +/- SD), a sudden anoxic depolarization (AD) with an amplitude of 14 +/- 6 mV and a duration of 0.89 +/- 0.28 min at half-maximal amplitude. Although the AD onset latency was significantly longer in P5-8 (12.5 +/- 4.9 min, P < 0.001) and P14-18 (8.7 +/- 3.2 min, P < 0.002) cortex, the amplitude and duration of the AD was larger in young (45.7 +/- 7.6 mV, 2.19 +/- 0.71 min, both P < 0.001) and juvenile animals (29.9 +/- 9.1 mV, P < 0.001, 0.96 +/- 0.26 min, P > 0.05) when compared with the adults. The hypoxia-induced [Ca2+]o decrease was significantly (P < 0.002) larger in young cortex (1,115 +/- 50 microM) as compared with the adult (926 +/- 107 microM). Prolongation of hypoxia after AD onset for >5 min elicited in young and juvenile cortex a long-lasting AD with an amplitude of 40.5 mV associated with a decrease in [Ca2+]o by >1 mM. On reoxygenation, only slices from these age groups showed spontaneous repetitive spreading depression in 3 out of 26 cases. In adults, the same protocol caused a significantly (P < 0.05) smaller and shorter AD and never a spreading depression. However, recovery in synaptic transmission after this long-term hypoxia was better in young and juvenile cortex, indicating a prolonged or even irreversible deficiency in synaptic function in mature animals. Application of ketamine caused a 49% reduction in the initial amplitude of the AD in juvenile cortex but did not significantly affect the AD in slices from adult animals. These data indicate that the young and juvenile cortex tolerates much longer periods of oxygen deprivation as compared with the adult, but that a sufficiently long hypoxia causes severe pathophysiological activity in the immature cortex. This enhanced sensitivity of the immature cortex is at least partially mediated by activation of N-methyl-D-aspartate receptors.

Tigabine hydrochloride, an inhibitor of gamma-aminobutyric acid (GABA) uptake, induces cortical depolarizations in vitro

The effect of the gamma-aminobutyric acid uptake inhibitor tiagabine hydrochloride was studied on electrical responses in cortical wedges prepared from 20-30 day-old, audiogenic seizure-prone DBA/2 mice. Perfusion of tiagabine (50 microM) for 15 min, evoked large, slow depolarizations with a frequency of 6-8/h which persisted for 4-5 h. The GABA(A) receptor antagonists, bicuculline (10 microM) and picrotoxin (100 microM), inhibited established depolarizations. These depolarizations were also calcium-dependent and blocked by tetrodotoxin. The non-opioid antitussive, dextromethorphan, which has been shown to inhibit glutamate release, irreversibly blocked the depolarizations. Conversely, 4-aminopyridine (50 microM), a potassium channel antagonist, markedly potentiated the responses. The NMDA receptor antagonist, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, had no effect on the depolarizations at concentrations up to 100 microM but the AMPA/kainate receptor antagonist, 6,7-dinitroquinoxaline-2.3-dione at high concentrations (100 and 200 microM), reversibly decreased the frequency without affecting the amplitude. It is concluded that the tiagabine-induced depolarizations in this in vitro preparation were initiated through GABA(A) receptors leading, possibly, to a release of excitatory amino acids.

Astrocytic neurotransmitter receptors in situ and in vivo

In the brain, astrocytes are associated intimately with neurons and surround synapses. Due to their close proximity to synaptic clefts, astrocytes are in a prime location for receiving synaptic information from released neurotransmitters. Cultured astrocytes express a wide range of neurotransmitter receptors, but do astrocytes in vivo also express neurotransmitter receptors and, if so, are the receptors activated by synaptically released neurotransmitters? In recent years, considerable efforts has gone into addressing these issues. The experimental results of this effort have been compiled and are presented in this review. Although there are many different receptors which have not been identified on astrocytes in situ, it is clear that astrocytes in situ express a number of different receptors. There is evidence of glutamatergic, GABAergic, adrenergic, purinergic, serotonergic, muscarinic, and peptidergic receptors on protoplasmic, fibrous, or specialized (Bergmann glia, pituicytes, Müller glia) astrocytes in situ and in vivo. These receptors are functionally coupled to changes in membrane potential or to intracellular signaling pathways such as activation of phospholipase C or adenylate cyclase. The expression of neurotransmitter receptors by astrocytes in situ exhibits regional and intraregional heterogeneity and changes during development and in response to injury. There is also evidence that receptors on astrocytes in situ can be activated by neurotransmitter(s) released from synaptic terminals. Given the evidence of extra-synaptic signaling and the expression of neurotransmitter receptors by astrocytes in situ, direct communication between neurons and astrocytes via neurotransmitters could be a widespread form of communication in the brain which may affect many different aspects of brain function, such as glutamate uptake and the modulation of extracellular space.

Hippocampal astrocytes in situ respond to glutamate released from synaptic terminals

A long-standing question in neurobiology is whether astrocytes respond to the neuronal release of neurotransmitters in vivo. To address this question, acutely isolated hippocampal slices were loaded with the calcium-sensitive dye Calcium Green-1 and the responses of the astrocytes to electrical stimulation of the Schaffer collaterals were monitored by confocal microscopy. To confirm that the responsive cells were astrocytes, the slices were immunostained for the astrocytic marker glial fibrillary acidic protein. Stimulation of the Schaffer collaterals (50 Hz, 2 sec) resulted in increases in the concentration of intracellular calcium ([Ca2+]i) in the astrocytes located in the stratum radiatum of CA1. The astrocytic responses were blocked by the sodium channel blocker tetrodotoxin, the voltage-dependent calcium channel blocker omega-conotoxin-MVIIC, and the selective metabotropic glutamate receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG). These results suggest that the astrocytic responses were induced by stimulation of metabotropic glutamate receptors on the astrocytes by neuronally released glutamate. The astrocytic responses to neuronal stimulation were enhanced in the presence of the K+ channel antagonist 4-aminopyridine (4-AP). Inhibition of the astrocytic responses in the presence of 4-AP required the presence of both MCPG and the ionotropic glutamate receptor antagonist kynurenic acid. These results suggest that higher levels of neuronal activity result in stimulation of both metabotropic and ionotropic glutamate receptors on the astrocytes. Overall, the results indicate that hippocampal astrocytes in situ are able to respond to the neuronal release of the neurotransmitter glutamate with increases in [Ca2+]i.

GABA-mediated synchronous potentials and seizure generation

This article summarizes findings related to a synchronous, GABA-mediated potential that may contribute to the initiation and spread of epileptiform discharges within the brain. This phenomenon is observed in cortical structures such as the hippocampus, the entorhinal cortex, and the neocortex during application of low concentrations of 4-aminopyridine and is characterized at the intracellular level by a long-lasting membrane depolarization. The synchronous, GABA-mediated potential continues to occur after blockade of excitatory synaptic transmission and relays on the synchronous firing of inhibitory interneurons and consequent activation of postsynaptic (mainly type A) GABA receptors leading to a transient elevation of [K+]O. Studies performed in young rat hippocampus indicate that the synchronous, GABA-mediated potential may play a role in initiating ictal discharges under normal conditions (i.e., when excitatory amino acid receptors are operant). Moreover, a similar phenomenon may also occur in adult rat entorhinal cortex. These findings therefore indicate a novel role that is played by GABAA receptors in limbic structures. The ability of this synchronous GABA-mediated potential to propagate in the absence of excitatory synaptic transmission may also be relevant for the propagation of synchronous activity outside conventional neuronal-synapse dependent pathways. This condition may occur in brain structures with neuronal loss and consequent disruption of normal excitatory synaptic connections such as mesial limbic structures of temporal lobe epilepsy patients with Ammon's horn sclerosis.

Synchronization of rat hippocampal neurons in the absence of excitatory amino acid-mediated transmission

Extracellular and intracellular recordings and measurements of extracellular K+ concentration ([K+]o) were performed in the adult rat hippocampus in an in vitro slice preparation. Excitatory amino acid receptor antagonists, as well as the K(+)-channel blockers 4-aminopyridine (4AP, 50 microM) and/or tetraethylammonium (TEA, 5 mM), were added to the bath. Synchronous, negative-going field potentials were recorded in the CA3 stratum radiatum during application of 4AP and excitatory amino acid receptor antagonists. Each of these events was associated with an intracellular long-lasting depolarization and a concomitant rise in [K+]o that attained peak values of 4.3 +/- 0.1 mM (mean +/- S.E.M., n = 6 slices) and lasted 29 +/- 3 s. These field potentials were still recorded in CA3 stratum radiatum after addition of TEA. Under these conditions, prolonged field potentials (40.2 +/- 4.5 s, n = 18) characterized by a prominent positive component; discharge of population spikes also occurred. [K+]o increases associated with these prolonged field-potential discharges had a considerable variability in magnitude (peak value = 3.8-14.1 mM, 6.1 +/- 0.7 mM, n = 5) and duration (14-210 s; 48 +/- 13 s, n = 5). In 8% of the cases spreading depression-like episodes were observed. [K+]o increases during spreading depression-like episodes attained peak values of 11-27 mM (22.8 +/- 0.2 mM, n = 2) and had a duration of 160-396 s (244 +/- 29 s, n = 2). All types of synchronous activity were abolished by the GABAA-receptor antagonist bicuculline methiodide (10 microM) (n = 11). A similar effect was obtained by applying Ca(2+)-free/high-Mg2+ medium (n = 5). Simultaneous field-potential recordings in CA3, CA1, dentate area and subiculum demonstrated that negative-going potentials and prolonged field-potential discharges occurred in all areas in a synchronous fashion. Spreading depression-like episodes were more frequently recorded in the CA1 than in the CA3 area and were not seen in the subiculum or dentate area. These experiments indicate that a glutamatergic-independent, synchronous GABA-mediated potential which is elicited by 4AP in the adult rat hippocampus continues to occur in the presence of TEA. In addition, concomitant application of these K(+)-channel blockers induces a novel type of prolonged field-potential discharge as well as spreading depression-like episodes. Since all synchronous potentials (including spreading depression-like episodes) were abolished by bicuculline methiodide, we conclude that their occurrence is presumably dependent upon the post-synaptic activation of GABAA receptors located on neuronal and glial elements. As excitatory synaptic transmission was nominally blocked under our experimental conditions, we also propose that rises in [K+]o and consequent redistribution processes are per se sufficient to make all types of synchronous activity propagate.

Extracellular free potassium and calcium during synchronous activity induced by 4-aminopyridine in the juvenile rat hippocampus

1. Field potential recordings and measurements of the extracellular concentration of free K+ ([K+]o) and Ca2+ ([Ca2+]o) were made during application of 4-aminopyridine (4-AP, 50 microM) in hippocampal slices that were obtained from 11- to 32-day-old rats. 2. Spontaneous field potentials recorded under this experimental condition in the CA3 stratum radiatum of slices from rats < 23 days old consisted of interictal (duration, 0.2-1.4 s; intervals of occurrence, 0.9-3.4 s) and ictal epileptiform discharges (duration, 5-46 s; intervals of occurrence, 22-259 s) and negative-going potentials that often preceded the onset of ictal discharge. Ictal activity became rare in slices from rats > 25 days old. 3. The negative-going potential (which also corresponded to the ictal discharge onset) was associated with [K+]o increases to 9.4 +/- 3.6 mM (mean +/- S.D.) from 3.25 mM baseline (n = 11 slices). [K+]o remained elevated at 5-6 mM throughout the ictal event. Decreases in [Ca2+]o (from 1.8 mM baseline to 1.3 +/- 0.1 mM, n = 7) were observed during the ictal discharge. 4. Interictal and ictal discharges were abolished by the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX, 10 microM). CNQX and the NMDA receptor antagonist 3-((+/-)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP) did not influence negative-going potentials or the associated [K+]o increases (peak values were 8.7 +/- 3.2 mM, n = 8), that were blocked, however, by bicuculline methiodide (BMI, 10 microM). 5. The mu-opioid receptor agonist (D-Ala2,N-Me-Phe4,Gly5-ol)-enkephalin (DAGO, 10 microM) which inhibits GABA release from interneurons, prevented the occurrence of both GABA-mediated synchronous potentials and subsequent ictal discharges (n = 6) as well as the [K+]o elevations. DAGO effects were antagonized by naloxone (10 microM; n = 4). 6. The GABA-mediated [K+]o elevations changed as a function of age. In hippocampal slices obtained from 11- to 17-day-old rats, peak values of 10.6 +/- 2.0 mM (n = 10) and half-width durations of 8.7 +/- 1.3 s (n = 7) were observed. In slices obtained from 25- to 32-day-old animals these parameters were 5.2 +/- 0.5 mM (n = 13) and 4.6 +/- 1.1 s (n = 4), respectively. 7. This study shows that, in the juvenile rat hippocampus, 4-AP induces a glutamatergic independent synchronous potential that is due to GABA released from inhibitory terminals and is associated with an increase in [K+]o. This [K+]o elevation undergoes age-dependent changes, and is instrumental in synchronizing neurons thus initiating prolonged epileptiform discharges.

Developmental features of 4-aminopyridine induced epileptogenesis

4-Aminopyridine (4-AP, 50 microM), perfused in rat hippocampal slices from postnatal days 2-30 (P2-P30), induced in the CA3 area the appearance of spontaneous epileptiform discharges, short (interictal-like) and sustained (ictal-like), as well as slow potential. The duration of epileptiform discharges decreased and their rate of occurrence (frequency) increased with maturation: their duration during the 1st postnatal week was 4-6 times longer and their frequency 5 times lower, compared to those of the 4th postnatal week. Ictal discharges gradually disappeared at the end of the 4th postnatal week. Spontaneous synchronous activity-as a whole-often appeared in clusters separated by equal or longer length inactive periods, during the first two postnatal weeks. At the same period, ictal discharges were often followed by repetitive afterdischarges, forming sequences which lasted 0.7-1.5 min. Sectioning experiments showed that epileptiform discharges were generated in CA3, and their presence in CA1 depended on the integrity of CA1-CA3 synaptic connections. In conclusion, these findings demonstrate that (i) immature CA3 can generate synchronous epileptiform discharges as early as P2, (ii) such discharges are longer lasting and more complex during the early developmental stages and (iii) there are two time points (end of 2nd, end of 4th postnatal weeks), when maturational changes alter the epileptogenic properties of immature hippocampus.

Effect of 4-aminopyridine in acute spinal cord injury

BACKGROUND

The demyelination process has been proven to be an important factor contributing to long-term sensory and motor impairments after spinal cord injury (SCI). The loss of myelin promotes exposure of K+ channels in internodal region of the damaged myelinated axons leading to K+ efflux into the neurons with subsequent blockage of action potentials. The potassium channel blocker 4-aminopyridine (4-AP) has been effective in restoring some sensory and motor impairment in incomplete SCI patients. The effect of this compound given immediately after an acute injury is not known. The objective of this study was to determine if blockage of K+ ions efflux immediately after an acute SCI would improve neuronal conduction in this model of injury.

METHODS

Cortical somatosensory evoked potentials (SSEPs) were recorded before and after a weight-induced compression injury of 120 grams, and were monitored up to 5 hours postinjury. A randomized treatment was initiated with administration of either vehicle or 4-AP. All 4-AP treatments were given as intravenous bolus injections of 1.0, 0.5, and 0.3 mg/kg at 1, 2, and 3 hours after the trauma.

RESULTS

The SSEPs were abolished immediately after the injury in all control and treated animals. Both groups showed spontaneous recovery of the SSEPs at the rate of 44.5% for the 4-AP treated and nontreated groups at the second hour postinjury. This recovery rate remained the same for both groups at the end of the experiments.

CONCLUSIONS

Based on the recovery of the SSEPs, our data indicate that early administration of 4-AP lacks any beneficial effect on axonal function during acute stage of spinal cord injury.

Pharmacology and electrophysiology of a synchronous GABA-mediated potential in the human neocortex

Spontaneous synchronous field potentials of negative polarity (duration = 200-700 ms, inter-event interval = 9.1 +/- 2.9 s; n = 27 slices) were recorded, during application of 4-aminopyridine (50 microM), from the superficial/middle layers of slices of human neocortex obtained in the course of neurosurgery for the relief of intractable seizures. The negative-going field potential corresponded to an intracellular long-lasting (duration = 200-1600 ms) depolarization that could be preceded by preceded by an excitatory postsynaptic potential-hyperpolarizing inhibitory postsynaptic potential sequence and followed by a long-lasting hyperpolarization. This synchronous activity continued to occur following blockade of excitatory synaptic transmission by excitatory amino acid receptor antagonists, but was greatly reduced and eventually disappeared during application of the GABAA receptor antagonist bicuculline methiodide. Simultaneous extracellular recordings from three sites in the slice located along an axis parallel to the pia showed that successive synchronous field potentials could originate from any of the three areas. They invaded the other two sites in c. 35.5% of the cases, while propagation to another site only or no propagation at all was observed, respectively, in 44.4% and 20% of instances. The velocity of lateral propagation of the synchronous field potential was 7.9 +/- 2.5 mm/s (range = 4.5-11.8 mm/s, n = 6). The modalities of origin and propagation remained the same after blockade of excitatory amino acid receptors. Under these conditions, however, there was a higher incidence of non-propagation and the velocity was significantly lower than in control (5.6 +/- 1.9 mm/s; range = 2.8-7.7 mm/s, n = 6). These data indicate that, in the human neocortex, 4-aminopyridine can reveal a synchronous field potential that correlates with an intracellular long-lasting depolarization and is mainly due to the activation of postsynaptic GABAA receptors. The action of excitatory amino acid receptors is not necessary for the generation and propagation of these GABA-mediated potentials. We propose that this potential represents a novel mechanism for synchronization and spread of neuronal activity, including seizure-like discharges in the human neocortex.

Strategies for the development of drugs for pharmacoresistant epilepsies

Presently, most strategies for development of antiepileptic drugs (AEDs) center around seizure models that are known to respond to presently marketed AEDs. These strategies do not take into account that epilepsy can be a progressive disease. Moreover, region-specific aspects of epileptogenesis are rarely considered when new AEDs are developed. Seizures in the temporal lobe are often difficult to treat. Animal studies on various seizure models in the hippocampus and the entorhinal cortex (EC) suggest that these structures do not a priori produce seizures that are difficult to treat. However, seizure-like events in the EC tend to progress to a state of status epilepticus-like activity that cannot be suppressed by presently marketed AEDs. Loss of gamma-aminobutyric acid (GABA)ergic neurotransmission and increased excitatory synaptic coupling seem to cooperate for induction of this state. Epilepsy induced alterations in the interaction between the EC and the hippocampus may lead to alterations that facilitate precipitation of seizures. Because of the recurrent interaction between the hippocampus and the EC, these seizures may reach an intensity that is no longer controllable by presently available AEDs. Ontogenetic alterations of the circuitry between the EC and the hippocampus, seizure-induced stabilization of synaptic connections overexpressed during ontogenesis, seizure-induced lesions and subsequent rearrangements of internal cell properties, and synaptic arrangements and kindling-like alterations of nerve cell and glial behavior may all be involved in the generation of a neuronal aggregate whose balance between inhibitory and excitatory processes becomes readily disturbed. Strategies for the development of AEDs treating such seizures should suppress hyperactivity and prevent progression of epileptogenesis. AEDs directed against seizures may be effective if they can be given in sufficient concentrations to suppress very intense local seizures.