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Massimo Avoli

Publications and source records attributed to Massimo Avoli.

At least 19 recordsLinked to original sources
PubMed2006

The H current blocker ZD7288 decreases epileptiform hyperexcitability in the rat neocortex by depressing synaptic transmission.

Neurons respond to intracellular injection of hyperpolarizing current pulses by generating depolarizing sags contributed by a cation current termed Ih. Ih modulates neuron excitability and rhythmicity. It is, however, unclear whether the net effect of changing Ih leads to facilitation or depression of cortical epileptiform activity. Here, we addressed this issue by using field and intracellular recordings to study the effects of ZD7288 (10-100 microM), a bradycardic agent known to abolish Ih, on the epileptiform discharges (duration = 2.5 +/- 0.3 s, mean +/- SEM; interval of occurrence = 34.2 +/- 3.3 s, n = 30 slices) induced in rat neocortical slices by 4-aminopyridine and GABA receptor antagonists. ZD7288 abolished the depolarizing sags seen during injection of intracellular hyperpolarizing current pulses while increasing resting membrane potential and apparent input resistance. These effects, which were fully established with 10 microM ZD7288, were associated with a dose-dependent decrease in the occurrence of spontaneous epileptiform events and a reduction in their duration (the latter change occurring at doses > 20 microM). ZD7288 also caused a dose-dependent decrease of background postsynaptic potentials. Finally, ZD7288 could depress epileptiform activity during Cs+ pre-treatment, a procedure known to block Ih. These data indicate that ZD7288 hampers neocortical epileptiform synchronization, but also suggest that most of this action reflects the ability of ZD7288 to decrease synaptic transmission.

4-Aminopyridine↗
PubMed2006

Endogenous neurosteroids modulate epileptogenesis in a model of temporal lobe epilepsy.

Neurosteroids modulate seizure susceptibility, but their role in the regulation of epileptogenesis is unknown. Status epilepticus (SE) induces temporal lobe epileptogenesis following a latent period in which glial cells are activated. Here, we found that P450scc, the rate-limiting enzyme in steroid synthesis, is upregulated in hippocampal glia during the latent period after pilocarpine-induced SE in rats. More prolonged SE was associated with greater P450scc expression and longer latencies to the development of seizures, suggesting that enhanced steroid synthesis retards epileptogenesis. The 5alpha-reductase inhibitor finasteride, which blocks neurosteroid synthesis, reduced the latent period, indicating that glia-derived neurosteroids may be antiepileptogenic.

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗
PubMed2006

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.

Biopsy↗
PubMed2006

Subiculum network excitability is increased in a rodent model of temporal lobe epilepsy.

In this study, we used in vitro electrophysiology along with immunohistochemistry and molecular techniques to study the subiculum--a limbic structure that gates the information flow from and to the hippocampus--in pilocarpine-treated epileptic rats. Comparative data were obtained from age-matched nonepileptic controls (NEC). Subicular neurons in hippocampal-entorhinal cortex (EC) slices of epileptic rats were: (i) hyperexcitable when activated by CA1 or EC inputs; and (ii) generated spontaneous postsynaptic potentials at higher frequencies than NEC cells. Analysis of pharmacologically isolated, GABA(A) receptor-mediated inhibitory postsynaptic potentials revealed more positive reversal potentials in epileptic tissue (-67.8 +/- 6.3 mV, n = 16 vs. -74.8 +/- 3.6 mV in NEC, n = 13; P < 0.001) combined with a reduction in peak conductance (17.6 +/- 11.3 nS vs. 41.1 +/- 26.7 nS in NEC; P < 0.003). These electrophysiological data correlated in the epileptic subiculum with (i) reduced levels of mRNA expression and immunoreactivity of the neuron-specific potassium-chloride cotransporter 2; (ii) decreased number of parvalbumin-positive cells; and (iii) increased synaptophysin (a putative marker of sprouting) immunoreactivity. These findings identify an increase in network excitability within the subiculum of pilocarpine-treated, epileptic rats and point at a reduction in inhibition as an underlying mechanism.

Action Potentials↗
PubMed2006

NMDA receptor-mediated transmission contributes to network 'hyperexcitability' in the rat insular cortex.

The insular cortex (IC) plays distinct roles under physiological and pathological conditions. However, the mechanisms regulating excitability in this area remain unknown. By employing field potential and sharp-electrode intracellular recordings in horizontal rat brain slices comprising the IC and the perirhinal cortex, we studied here the intrinsic and network characteristics of neurons in the agranular IC. These cells generated regular action potential firing with weak adaptation during intracellular injection of depolarizing current pulses, and were pyramidal in shape when neurobiotin filled. Spontaneous, field events (duration = 2.3 +/- 0.25 s; intervals of occurrence = 44.9 +/- 6.3 s) were identified in 22/52 slices and corresponded in IC neurons to intracellular depolarizations with action potential firing. Similar field and intracellular discharges were elicited in all slices by electrical stimuli. Antagonizing N-methyl-d-aspartate (NMDA) receptors blocked the spontaneous activity and reduced or abolished the stimulus-induced discharges. In the latter cases, stimuli elicited depolarizing events that became hyperpolarizing at about -64 mV, suggesting the contribution of gamma-aminobutyric acid (GABA)(A) receptor-mediated conductances. Our findings identify for the first time some functional properties of agranular IC neurons and point at a powerful NMDA receptor-mediated mechanism implementing network hyperexcitability. This feature may contribute to the role of IC in neurological disorders.

Animals↗
PubMed2006

Neocortical hyperexcitability in a genetic model of absence seizures and its reduction by levetiracetam.

PURPOSE: To study the effect of the antiepileptic drug levetiracetam (LEV) on the patterns of intrinsic optical signals (IOSs) generated by slices of the somatosensory cortex obtained from 3- and 6-month-old WAG/Rij and age-matched, nonepileptic control (NEC) rats. METHODS: WAG/Rij and NEC animals were anesthetized with enfluorane and decapitated. Brains were quickly removed, and neocortical slices were cut coronally with a vibratome, transferred to a submerged tissue chamber, and superfused with oxygenated artificial cerebrospinal fluid (aCSF). Slices were illuminated with a dark-field condensor and examined with a x2.5 objective; images were processed with a real time digital video image-enhancement system. Images were acquired before (background) and during electrical stimulation with a temporal resolution of 10 images/s and were displayed in pseudocolors. Extracellular stimuli (200 micros; <4 V) were delivered through bipolar stainless steel electrodes placed in the white matter. RESULTS: IOSs recorded in NEC slices bathed in control aCSF became less intense and of reduced size with age (p < 0.05); this trend was not seen in WAG/Rij slices. Age-dependent decreases in IOS intensity and area size were also seen in NEC slices superfused with aCSF containing the convulsant 4-aminopyridine (4-AP, 5 microM); in contrast, significant increases in both parameters occurred with age in 4-AP-treated WAG/Rij slices (p < 0.05). Under any of these conditions, the IOS intensity and area size slices were larger in WAG/Rij than in NEC slices. LEV (50-500 microM) application to WAG/Rij slices caused dose-dependent IOS reductions that were evident both in control and in 4-AP-containing aCSF and were more pronounced in 6-month-old tissue. CONCLUSIONS: These data demonstrate age-dependent IOS modifications in NEC and WAG/Rij rat slices and identify a clear pattern of hyperexcitability that occurs in 6-month-old WAG/Rij neocortical tissue, an age when absence seizures occur in all animals. The ability of LEV to reduce these patterns of network hyperexcitability supports the potential use of this new antiepileptic drug in primary generalized epileptic disorders.

4-Aminopyridine↗
PubMed2006

High-frequency intracerebral EEG activity (100-500 Hz) following interictal spikes.

PURPOSE: High-frequency activity has been recorded with intracerebral microelectrodes in epileptic patients and related to seizure genesis. Our goal was to analyze high-frequency activity recorded with electroencephalograph (EEG) macroelectrodes during the slow wave immediately following interictal spikes, given the potential importance of this presumed hyperpolarization in transforming spikes into seizures. METHODS: Depth electrode EEG recordings from 10 patients with intractable focal epilepsy were low-pass filtered at 500 Hz and sampled at 2,000 Hz. Spikes were categorized according to localization and morphology. Segments of 256 ms were selected immediately following (postspike), and 2 s before each spike (baseline). Power was estimated in subgamma (0-40 Hz), gamma (40-100 Hz), high frequency (100-200 Hz), and very high frequency (250-500 Hz) bands. RESULTS: Changes in power above 100 Hz were seen in 22 of 29 spike categories, consisting primarily of a widespread decrease in frequencies above 100 Hz. This decrease became spatially more restricted as frequencies increased, and coincided with the localization of largest spikes for the highest frequencies. High-frequency power decreases were prominent in the hippocampus but less common in amygdala and neocortex. High-frequency power increases were observed in the amygdala. CONCLUSIONS: Thus high-frequency EEG activity can be recorded with macroelectrodes in humans and may provide insights on neuronal mechanisms related to human epilepsy. This activity undergoes consistent modifications after EEG spikes. We propose that the reduction in high frequencies reflects a postspike depression in neuronal activity that is more pronounced in the region of spike generation. This depression is almost always seen in hippocampus but less in amygdala.

Adult↗
PubMed2005

Altered inhibition in lateral amygdala networks in a rat model of temporal lobe epilepsy.

Clinical and experimental evidence indicates that the amygdala is involved in limbic seizures observed in patients with temporal lobe epilepsy. Here, we used simultaneous field and intracellular recordings from horizontal brain slices obtained from pilocarpine-treated rats and age-matched nonepileptic controls (NECs) to shed light on the electrophysiological changes that occur within the lateral nucleus (LA) of the amygdala. No significant differences in LA neuronal intrinsic properties were observed between pilocarpine-treated and NEC tissue. However, spontaneous field activity could be recorded in the LA of 21% of pilocarpine-treated slices but never from NECs. At the intracellular level, this network activity was characterized by robust neuronal firing and was abolished by glutamatergic antagonists. In addition, we could identify in all pilocarpine-treated LA neurons: 1) large amplitude depolarizing postsynaptic potentials (PSPs) and 2) a lower incidence of spontaneous hyperpolarizing PSPs as compared with NECs. Single-shock stimulation of LA networks in the presence of glutamatergic antagonists revealed a biphasic inhibitory PSP (IPSP) in both NECs and pilocarpine-treated tissue. The reversal potential of the early GABA(A) receptor-mediated component, but not of the late GABA(B) receptor-mediated component, was significantly more depolarized in pilocarpine-treated slices. Furthermore, the peak conductance of both fast and late IPSP components had significantly lower values in pilocarpine-treated LA cells. Finally, paired-pulse stimulation protocols in the presence of glutamatergic antagonists revealed a less pronounced depression of the second IPSP in pilocarpine-treated slices compared with NECs. Altogether, these findings suggest that alterations in both pre- and postsynaptic inhibitory mechanisms contribute to synaptic hyperexcitability of LA networks in epileptic rats.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed2005

Volume-conducted epileptiform events between adjacent necortical slices in an interface tissue chamber.

"Far-field" artifacts are presumed to contribute negligibly to the field potential activity recorded from brain slices maintained in vitro. While performing paired intracellular and field potential recordings from rat neocortical slices superfused with medium containing 4-aminopyridine+GABA receptor antagonists, we identified: (i) epileptiform discharges characterized by concomitant field oscillations (amplitude = 2.6-6.4 mV) and intracellular depolarizations as well as (ii) smaller amplitude (0.3-1.3 mV) field epileptiform events that were not associated with any intracellular activity. By placing an additional extracellular recording electrode into adjacent slices, we discovered that large amplitude, epileptiform discharges were generated concomitant to those seen in the first slice at the field potential level only. In addition, we found in these slices small amplitude, field discharges that were synchronous with those recorded intracellularly in the original slice. Analysis of the changes in field potential amplitude over space demonstrated that this parameter was reduced by approximately 60% when the recording electrode was moved from the slice generating the epileptiform activity to the bathing medium and further decreased in a quasi-linear mode when recordings were obtained from an adjacent slice. In conclusion, these observations indicate that brain slices can, under appropriate conditions, produce field potentials that are of amplitude sufficient for being recorded from other slices in the tissue chamber. These findings suggest that caution should be taken in assuming that field potential activity seen in an in vitro brain slice is generated within the recorded tissue.

Action Potentials↗
PubMed2005

Rat subicular networks gate hippocampal output activity in an in vitro model of limbic seizures.

Evidence obtained from human epileptic tissue maintained in vitro indicates that the subiculum may play a crucial role in initiating epileptiform discharges in patients with mesial temporal lobe epilepsy. Hence, we used rat hippocampus-entorhinal cortex (EC) slices to identify the role of subiculum in epileptiform synchronization during bath application of 4-aminopyridine (4AP, 50 microM). In these slices, fast CA3-driven interictal-like events were restricted to the hippocampal CA3/CA1 areas and failed to propagate to the EC where slow interictal-like and ictal-like epileptiform discharges were recorded. However, antagonizing GABA(A) receptors with picrotoxin (50 microM) made CA3-driven interictal activity spread to EC. Sequential field potential analysis along the CA3-CA1-subiculum axis revealed that the amplitude of CA3-driven interictal discharges recorded in the presence of 4AP only diminished within the subiculum. Furthermore, CA1 electrical stimulation under control conditions elicited little or no subicular activation and never any response in EC; in contrast, robust subicular discharges that spread to EC could be evoked after picrotoxin. Intracellular recordings indicated that potentiation by picrotoxin was associated with blockade of hyperpolarizing IPSPs in subicular cells. Finally, when surgically isolated from adjacent structures, the subiculum generated low-amplitude synchronous discharges that corresponded to an intracellular hyperpolarization-depolarization sequence, were resistant to glutamatergic antagonists, and represented the activity of synchronized interneuronal networks. Bath application of picrotoxin abolished these 4AP-induced events and in their place robust network bursting occurred. In conclusion, our study demonstrates that the subiculum plays a powerful gating role on hippocampal output activity. This function depends on GABA(A) receptor-mediated inhibition and controls hippocampal-parahippocampal interactions that are known to modulate limbic seizures.

Action Potentials↗
PubMed2005

Ripple activity in the dentate gyrus of dishinibited hippocampus-entorhinal cortex slices.

Fast oscillations at approximately 200 Hz, termed ripples, occur in the hippocampus and cortex of several species, including humans, and are thought to play a role in physiological (e.g., sensory information processing or memory consolidation) and pathological (e.g., seizures) processes. Blocking gamma-aminobutyric acid type A (GABA(A)) receptor-mediated inhibition represents one of the most often used models of epileptiform discharge. Here we found that bath application of the GABA(A) receptor antagonist picrotoxin (50 microM) to mouse hippocampus-entorhinal cortex slices induced spontaneous epileptiform activity (duration 536.6 +/- 146.1 msec, mean +/- SD; interval of occurrence 14.8 +/- 3.3 sec, n = 12) with two distinct phases of discharge; the first was characterized, in the dentate gyrus only, by high-frequency, field oscillations (ripples) at 206.3 +/- 23.4 Hz (n = 12), whereas the second component corresponded to afterdischarges in the theta range frequency. Ripples, which were also recorded in "minislices" only of the dentate gyrus, were unaffected by application of the mu-opioid receptor agonist (D-Ala2-N-Me-Phe,Gly-ol)enkephalin (10 microM; n = 6) or the N-methyl-D-aspartate (NMDA) receptor antagonist 3-(2-carboxy-piperazine-4-yl)-propyl-l-phosphonate (10 microM; n = 5). In contrast, the non-NMDA glutamatergic receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM; n = 5) completely blocked all picrotoxin-induced activities. In addition, application of the GABA(B) receptor agonist baclofen (0.01-0.5 microM; n = 6) dose dependently and reversibly abolished all picrotoxin-induced activities. We also found that application of the gap-junction decouplers carbenoxolone (0.2-0.5 mM; n = 6) or octanol (0.2-0.5 mM; n = 3) blocked the second phase while leaving ripples unchanged. These findings demonstrate that the disinhibited dentate gyrus can generate ripple activity at approximately 200 Hz that is contributed by ionotropic glutamatergic mechanisms and is not dependent on either GABA(A) receptor-mediated or gap-junction mechanisms.

Analgesics, Opioid↗
PubMed2005

Cellular and molecular mechanisms of epilepsy in the human brain.

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

Animals↗
PubMed2005

Impaired activation of CA3 pyramidal neurons in the epileptic hippocampus.

We employed in vitro and ex vivo imaging tools to characterize the function of limbic neuron networks in pilocarpine-treated and age-matched, nonepileptic control (NEC) rats. Pilocarpine-treated animals represent an established model of mesial temporal lobe epilepsy. Intrinsic optical signal (IOS) analysis of hippocampal-entorhinal cortex (EC) slices obtained from epileptic rats 3 wk after pilocarpine-induced status epilepticus (SE) revealed hyperexcitability in many limbic areas, but not in CA3 and medial EC layer III. By visualizing immunopositivity for FosB/DeltaFosB-related proteins which accumulate in the nuclei of neurons activated by seizures we found that: (1) 24 h after SE, FosB/DeltaFosB immunoreactivity was absent in medial EC layer III, but abundant in dentate gyrus, hippocampus proper (including CA3) and subiculum; (2) FosB/DeltaFosB levels progressively diminished 3 and 7 d after SE, whereas remaining elevated (p < 0.01) in subiculum; (3) FosB/DeltaFosB levels sharply increased 2 wk after SE (and remained elevated up to 3 wk) in dentate gyrus and in most of the other areas but not in CA3. A conspicuous neuronal damage was noticed in medial EC layer III, whereas hippocampus was more preserved. IOS analysis of the stimulus-induced responses in slices 3 wk after SE demonstrated that IOSs in CA3 were lower (p < 0.05) than in NEC slices following dentate gyrus stimulation, but not when stimuli were delivered in CA3. These findings indicate that CA3 networks are hypoactive in comparison with other epileptic limbic areas. We propose that this feature may affect the ability of hippocampal outputs to control epileptiform synchronization in EC.

Animals↗
PubMed2004

GABA(B) receptor activation and limbic network ictogenesis.

Rat brain slices containing interconnected hippocampus and entorhinal cortex (EC) responded to 4-aminopyridine (50 microM) application by generating: (i) CA3-driven interictal discharges that propagated to the EC; and (ii) N-methyl-D-aspartic (NMDA) acid receptor-dependent ictal events originating in EC (cf. J. Neurosci. 17 (1997) 9308 for experiments made in brain slices). Ictal discharges disappeared within 1-2 h, but were re-established by cutting the Schaffer collaterals, which abolished CA3-driven interictal discharge propagation to EC. In intact slices, GABA(B) receptor activation by baclofen (5-40 microM): (i) depressed CA3-driven interictal activity; and (ii) disclosed non-NMDA glutamatergic receptor-dependent ictal discharges originating in CA3 and propagating to EC. These effects were reversed by the GABA(B) receptor antagonist CGP 35348 (0.5 mM). Application of increasing baclofen doses to slices in which hippocampus and EC networks were surgically isolated decreased epileptiform events with an IC50 that was lower in EC (0.6 microM; n = 12) than in CA3 (2.5 microM; n = 12). Hence, under control conditions, EC ictogenesis depends on NMDA receptor function and is controlled by CA3-driven output activity; in contrast, following GABA(B) receptor activation EC excitability is depressed to a greater extent than CA3, which leads to non-NMDA glutamatergic receptor-mediated ictogenesis in CA3. We propose that GABA(B) receptor modulation may represent an important mechanism for setting the site of initiation, the modalities of propagation and the glutamatergic receptor properties of ictogenesis in the limbic system and, perhaps, in mesial temporal lobe epilepsy patients.

4-Aminopyridine↗
PubMed2004

4-Aminopyridine-induced epileptogenesis depends on activation of mitogen-activated protein kinase ERK.

Extracellular signal-regulated kinases such as ERK1 [p44 mitogen-activated protein kinase (MAPK)] and ERK2 (p42 MAPK) are activated in the CNS under physiological and pathological conditions such as ischemia and epilepsy. Here, we studied the activation state of ERK1/2 in rat hippocampal slices during application of the K(+) channel blocker 4-aminopyridine (4AP, 50 micro m), a procedure that enhances synaptic transmission and leads to the appearance of epileptiform activity. Hippocampal slices superfused with 4AP-containing medium exhibited a marked activation of ERK1/2 phosphorylation that peaked within about 20 min. These effects were not accompanied by changes in the activation state of c-Jun N-terminal kinase (JNK), another member of the MAP kinase superfamily. 4AP-induced ERK1/2 activation was inhibited by the voltage-gated Na(+) channel blocker tetrodotoxin (1 micro m). We also found that application of the ERK pathway inhibitors U0126 (50 micro m) or PD98059 (100 micro m) markedly reduced 4AP-induced epileptiform synchronization, thus abolishing ictal discharges in the CA3 area. The effects induced by U0126 or PD98059 were not associated with changes in the amplitude and latency of the field potentials recorded in the CA3 area following electrical stimuli delivered in the dentate hylus. These data demonstrate that activation of ERK1/2 accompanies the appearance of epileptiform activity induced by 4AP and suggest a cause-effect relationship between the ERK pathway and epileptiform synchronization.

4-Aminopyridine↗
PubMed2003

Epileptiform synchronization in the human dysplastic cortex.

Taylor's focal cortical dysplasia corresponds to a localized disruption of the normal cortical lamination with an excess of large, aberrant cells. Sustained epileptic discharges originate from the dysplastic neocortex and this tissue retains sufficient connectivity for expressing seizure abnormalities. In this brief review, we summarize the findings obtained by analyzing surgically-resected human tissue with focal cortical dysplasia that was maintained in vitro in a brain slice preparation. These data have been compared with those obtained from human cortex with normal structural organization; such tissue was available from patients undergoing surgery for a variety of neurological disorders, most often for mesial temporal lobe epilepsy. These studies have shown that: (i). slices obtained from focal cortical dysplastic tissue have an intrinsic ability to generate ictal-like epileptiform events when challenged with the convulsant drug 4-aminopyridine; (ii). 4-aminopyridine-induced ictal discharges are not seen in neocortical slices obtained from neocortical samples with no or minimal structural lesion; (iii). these ictal discharges are caused by the activation of excitatory amino acid receptors, and in particular those of the N-methyl-D aspartate type; (iv). focal cortical dysplastic tissue also generates synchronous potentials that are mainly contributed by GABA(A) receptor-mediated conductances.

4-Aminopyridine↗