Aminopyridine-induced seizure activity

Adam, amigo, brain, and K channel

Voltage-dependent K+ (Kv) channels are diverse, comprising the classical Shab - Kv2, Shaker - Kv1, Shal - Kv4, and Shaw - Kv3 families. The Shaker family alone consists of Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv1.6, and Kv1.7. Moreover, the Shab family comprises two functional (Kv2.1 and Kv2.2) and several "silent" alpha subunits (Kv2.3, Kv5, Kv6, Kv8, and Kv9), which do not generate K current. However, e.g., Kv8.1, via heteromerization, inhibits outward currents of the same family or even that of Shaw. This property of Kv8.1 is similar to those of designated beta subunits or non-selective auxiliary elements, including ADAM or AMIGO proteins. Kv channels and, in turn, ADAM may modulate the synaptic long-term potentiation (LTP). Prevailingly, Kv1.1 and Kv1.5 are attributed to respective brain and heart pathologies, some of which may occur simultaneously. The aforementioned channel proteins are apparently involved in several brain pathologies, including schizophrenia and seizures.

Electrophysiological and behavioral properties of 4-aminopyridine-induced epileptic activity in mice

4-aminopyridine (4-AP) is a widely used drug that induces seizure activity in rodents, especially in rats, although there is no consensus in the literature on the dose to be used in mice. The aim of the present study was to investigate the effect of the intraperitoneal administration of 4-AP in two doses (4 and 10 mg/kg) in vivo. EEG, movement, and video recordings were made simultaneously in male B6 mice to specify the details of the seizures and to determine whether there is a suitable non-lethal dose for seizure induction and for further molecular studies. Seizure behavior in mice differs from that seen in rats, with no characteristic stages of epileptic seizures, but with spiking and seizure activity. Seizure activity, although produced at both doses without being lethal, induced different changes of the EEG pattern. Smaller dose induced a lower amplitude seizure activity, decreased spiking activity and later onset of seizures, while higher dose induced a much more intense brain seizure activity and severe trembling. It is concluded that the intraperitoneal administration of 4-AP at a dose of 10 mg/kg induces explicit seizure activity in mice which is repeatable and can be suitable for further molecular research.

Acute Focal Seizures Start As Local Synchronizations of Neuronal Ensembles

Understanding seizure formation and spread remains a critical goal of epilepsy research. We used fast in vivo two-photon calcium imaging in male mouse neocortex to reconstruct, with single-cell resolution, the dynamics of acute (4-aminopyridine) focal cortical seizures as they originate within a spatially confined seizure initiation site (intrafocal region), and subsequently propagate into neighboring cortical areas (extrafocal region). We find that seizures originate as local neuronal ensembles within the initiation site. This abnormal hyperactivity engages increasingly larger areas in a saltatory fashion until it breaks into neighboring cortex, where it proceeds smoothly and is then detected electrophysiologically (LFP). Interestingly, PV inhibitory interneurons have spatially heterogeneous activity in intrafocal and extrafocal territories, ruling out a simple role of inhibition in seizure formation and spread. We propose a two-step model for the progression of focal seizures, where neuronal ensembles activate first, generating a microseizure, followed by widespread neural activation in a traveling wave through neighboring cortex during macroseizures.SIGNIFICANCE STATEMENT We have used calcium imaging in mouse sensory cortex in vivo to reconstruct the onset of focal seizures elicited by local injection of the chemoconvulsant 4-aminopyridine. We demonstrate at cellular resolution that acute focal seizures originate as increasingly synchronized local neuronal ensembles. Because of its spatial confinement, this process may at first be undetectable even by nearby LFP electrodes. Further, we establish spatial footprints of local neural subtype activity that correspond to consecutive steps of seizure microprogression. Such footprints could facilitate determining the recording location (e.g., inside/outside an epileptogenic focus) in high-resolution studies, even in the absence of a priori knowledge about where exactly a seizure started.

Role of inhibitory control in modulating focal seizure spread

Focal seizure propagation is classically thought to be spatially contiguous. However, distribution of seizures through a large-scale epileptic network has been theorized. Here, we used a multielectrode array, wide field calcium imaging, and two-photon calcium imaging to study focal seizure propagation pathways in an acute rodent neocortical 4-aminopyridine model. Although ictal neuronal bursts did not propagate beyond a 2-3-mm region, they were associated with hemisphere-wide field potential fluctuations and parvalbumin-positive interneuron activity outside the seizure focus. While bicuculline surface application enhanced contiguous seizure propagation, focal bicuculline microinjection at sites distant to the 4-aminopyridine focus resulted in epileptic network formation with maximal activity at the two foci. Our study suggests that both classical and epileptic network propagation can arise from localized inhibition defects, and that the network appearance can arise in the context of normal brain structure without requirement for pathological connectivity changes between sites.

Network Properties Revealed during Multi-Scale Calcium Imaging of Seizure Activity in Zebrafish

Seizures are characterized by hypersynchronization of neuronal networks. Understanding these networks could provide a critical window for therapeutic control of recurrent seizure activity, i.e., epilepsy. However, imaging seizure networks has largely been limited to microcircuits in vitro or small “windows” in vivo. Here, we combine fast confocal imaging of genetically encoded calcium indicator (GCaMP)-expressing larval zebrafish with local field potential (LFP) recordings to study epileptiform events at whole-brain and single-neuron levels in vivo. Using an acute seizure model (pentylenetetrazole, PTZ), we reliably observed recurrent electrographic ictal-like events associated with generalized activation of all major brain regions and uncovered a well-preserved anterior-to-posterior seizure propagation pattern. We also examined brain-wide network synchronization and spatiotemporal patterns of neuronal activity in the optic tectum microcircuit. Brain-wide and single-neuronal level analysis of PTZ-exposed and 4-aminopyridine (4-AP)-exposed zebrafish revealed distinct network dynamics associated with seizure and non-seizure hyperexcitable states, respectively. Neuronal ensembles, comprised of coactive neurons, were also uncovered during interictal-like periods. Taken together, these results demonstrate that macro- and micro-network calcium motifs in zebrafish may provide a greater understanding of epilepsy.

Reliable and Elastic Propagation of Cortical Seizures In Vivo

Mapping the fine-scale neural activity that underlies epilepsy is key to identifying potential control targets of this frequently intractable disease. Yet, the detailed in vivo dynamics of seizure progression in cortical microcircuits remain poorly understood. We combine fast (30-Hz) two-photon calcium imaging with local field potential (LFP) recordings to map, cell by cell, the spread of locally induced (4-AP or picrotoxin) seizures in anesthetized and awake mice. Using single-layer and microprism-assisted multilayer imaging in different cortical areas, we uncover reliable recruitment of local neural populations within and across cortical layers, and we find layer-specific temporal delays, suggesting an initial supra-granular invasion followed by deep-layer recruitment during lateral seizure spread. Intriguingly, despite consistent progression pathways, successive seizures show pronounced temporal variability that critically depends on GABAergic inhibition. We propose an epilepsy circuit model resembling an elastic meshwork, wherein ictal progression faithfully follows preexistent pathways but varies flexibly in time, depending on the local inhibitory restraint.

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.

Temporal lobe epileptiform activity following systemic administration of 4-aminopyridine in rats

PURPOSE

  The K(+) channel blocker 4-aminopyridine (4AP) induces epileptiform synchronization in brain slices maintained in vitro without interfering with γ-aminobutyric acid (GABA)A receptor-mediated inhibition and, actually, even enhancing it. The hypothesis that similar electrographic epileptiform patterns occur in vivo following systemic 4AP injection was tested here.

METHODS

  Sprague-Dawley rats (n = 13) were implanted with bipolar electrodes aimed at the hippocampal CA3 region, entorhinal cortex, subiculum, dentate gyrus, and amygdala. They were then injected with a single dose of 4AP (4-5 mg/kg, i.p.), and video-monitoring/electroencephalography (EEG) recordings were performed.

KEY FINDINGS

  4AP induced convulsive or nonconvulsive seizures in 12 of 13 rats, along with generalized fascicular twitching, wet-dog shakes, and myoclonic jerks. On EEG, we observed in 7 (58.3%) of 12 animals long-lasting interictal spikes from the subiculum before the occurrence of the first seizure. Once seizures had started, interictal spikes occurred in all areas with no fixed site of origin. Most seizures (41/60, 68.3%) were characterized by a low-voltage fast-activity onset pattern and were convulsive (48/60, 80%). 4AP also induced highly rhythmic theta (6-11 Hz) oscillations in CA3 and entorhinal cortex before seizure occurrence.

SIGNIFICANCE

  Our study shows that systemic 4AP administration in vivo can enhance theta oscillations and induce slow interictal spikes and low-voltage fast-onset seizures similar to those reported in brain slices. We propose that these effects may reflect, at least in part, enhanced GABAA receptor-mediated inhibition as reported in in vitro studies.

Is connexin36 critical for GABAergic hypersynchronization in the hippocampus?

Synchronous bursting of cortical GABAergic interneurons is important in epilepsies associated with excitatory GABAergic signalling. If electrical coupling was critical for the generation of this pathological activity, then the development of selective blockers of connexin36-based interneuronal gap junctions could be of therapeutic value. We have addressed this issue in the 4-aminopyridine model of epilepsy in vitro by comparing GABAergic epileptiform currents and their sensitivity to gap junction blockers in wild-type vs. connexin36 knockout mice. Although electrical coupling was abolished in stratum lacunosum-moleculare interneurons from knockout animals, epileptiform currents were not eliminated. Furthermore, epileptiform currents propagated similarly across hippocampal layers in the two genotypic groups. Blockade of electrical coupling with carbenoxolone suppressed amplitude, frequency and half-width of the epileptiform currents both in wild-type and in knockout animals, whereas mefloquine had no effects. Carbenoxolone also depressed responses to exogenous and synaptic GABA application onto interneurons. We conclude that, in the 4-aminopyridine model of epilepsy in vitro, connexin36 is not critical for the generation of epileptiform discharges in GABAergic networks and that the observed antiepileptic effects of carbenoxolone are likely to be due to blockade of GABAA receptors and not of connexin36-based gap junctions. Lastly, because of its chemical structure and its effects on amplitude and kinetics of GABAergic currents, we tested the hypothesis that carbenoxolone acted via specific sites on GABAA receptors, such as the one mediating the effects of the neurosteroid pregnenolone sulfate, or the allosteric regulatory site of benzodiazepines/β-carbolines. Our results suggest that neither of these is involved.

Quinine, a blocker of neuronal cx36 channels, suppresses seizure activity in rat neocortex in vivo

PURPOSE

The selective contribution of neuronal gap junction (GJ) communication via connexin 36 (Cx36) channels to epileptogenesis and to the maintenance and propagation of seizures was investigated in both the primary focus and the mirror focus by using pharmacologic approaches with the 4-aminopyridine in vivo epilepsy model.

METHODS

ECoG recording was performed on anesthetized adult rats, in which either quinine, a selective blocker of Cx36, or the broad-spectrum GJ blockers carbenoxolone and octanol were applied locally, before the induction or at already active epileptic foci.

RESULTS

The blockade of Cx36 channels by quinine before the induction of epileptiform activity slightly reduced the epileptogenesis. When quinine was applied after 25-30 repetitions of seizures, a new discharge pattern appeared with frequencies >15 Hz at the initiation of seizures. In spite of the increased number of seizures, the summated ictal activity decreased, because of the significant reduction in the duration of the seizures. The amplitudes of the seizure discharges of all the patterns decreased, with the exception of those with frequencies of 11-12 Hz. The blockade of Cx36 channels and the global blockade of the GJ channels resulted in qualitatively different modifications in ictogenesis.

CONCLUSIONS

The blockade of Cx36 channels at the already active epileptic focus has an anticonvulsive effect and modifies the manifestation of the 1- to 18-Hz seizure discharges. Our findings indicate that the GJ communication via Cx36 channels is differently involved in the synchronization of the activities of the networks generating seizure discharges with different frequencies. Additionally, we conclude that both neuronal and glial GJ communication contribute to the manifestation and propagation of seizures in the adult rat neocortex.

Uridine release during aminopyridine-induced epilepsy

Uridine, like adenosine, is released under sustained depolarization and it can inhibit hippocampal neuronal activity, suggesting that uridine may be released during seizures and can be involved in epileptic mechanisms. In an in vivo microdialysis study, we measured the extracellular changes of nucleoside and amino acid levels and recorded cortical EEG during 3-aminopyridine-induced epilepsy. Applying silver impregnation and immunohistochemistry, we examined the degree of hippocampal cell loss. We found that extracellular concentration of uridine, adenosine, inosine, and glutamate increased significantly, while glutamine level decreased during seizures. The release of uridine correlated with seizure activity. Systemic and local uridine application was ineffective. The number of parvalbumin- and calretinin-containing interneurons of dorsal hippocampi decreased. We conclude that uridine is released during epileptic activity, and suggest that as a neuromodulator, uridine may contribute to epilepsy-related neuronal activity changes, but uridine analogues having slower turnover would be needed for further investigation of physiological role of uridine.

Concentration- and time-dependent effect of aminooxyacetic acid on cortical epileptogenicity

In the present electrophysiological study the effect of aminooxyacetic acid (AOAA) on the cortical epileptogenicity, and on the basic electro-cortical activity was investigated in anesthetized rats. AOAA did not induce spontaneous epileptiform discharges but modified the somato-sensory evoked responses and the cortical epileptogenicity (induced by 4-aminopyridine) in the same manner depending on its concentration. AOAA at low concentrations increased the amplitude of evoked responses and the ipsilateral manifestation of epileptiform activity, however, at high concentrations significantly suppressed both the evoked responses and the induction and expression of seizures discharges. The anticonvulsive effect of AOAA was time-dependent (reached its maximum after 2h AOAA pre-treatment) and reversible. AOAA at low concentrations probably increases the efficacy of the NMDA excitatory system and decreases GABA-synthesis, resulting neuronal hyperexcitation. However, AOAA at high concentrations can lead to an effective cortical inhibition through intra- and extracellular accumulation of GABA. The gradual GABA accumulation - up to a certain level - at the synapses could also explain the time-dependency of the anticonvulsive effect of AOAA.

Effects of continuous low-dose exposure to organic and inorganic mercury during development on epileptogenicity in rats

The effects of chronic, low-dose fetal and lactational organic (MeHgCl) and inorganic (HgCl2) mercury intoxication on epileptogenicity were investigated and compared in rats. The main observations after both mercury treatments were a facilitated seizure expression and propagation evoked by 4-aminopyridine (4-AP). The seizure susceptibility of the offspring seemed to be in a parallel relation to the mercury concentration in the cortical tissue, which was significantly higher in treated animals as compared to the controls. While MeHgCl enhanced the number of ictal periods, HgCl2 facilitated the duration of seizure discharges in younger animals. HgCl2 intoxication seemed to be more permanent than MeHgCl. Changes in the summated ictal activity--which is an indication of epileptogenicity--were observed in the opposite directions in the two treated groups with increasing age. The amplitudes of seizure discharges were smaller in both mercury-treated groups than in the controls, which could be a consequence of a depressed proliferation of neurons or enhanced cell death in the neocortex. In summary, we observed that adult rats exposed to organic or inorganic mercury chemicals during their embryonic life, are more prone to epilepsy than control rats given only 4-AP.

Involvement of electrical coupling in the in vivo ictal epileptiform activity induced by 4-aminopyridine in the neocortex

In the present study we have investigated the possible role of gap junctions in the induction and manifestation of 4-aminopyridine-induced acute seizure activity both at the primary focus and at the mirror focus in anaesthetized rats by combining electrophysiological, pharmacological and molecular biological techniques. In the course of the intracellular recordings, unusual firing patterns that are assumed to be mediated by electrical coupling and appearing either randomly or in close time-locked manner with the ictal discharges were observed. In another series of experiments, a significant decrease in the intensity of seizure activity of the already active epileptic foci was detected when electrical synaptic transmission was blocked by carbenoxolone either at the primary focus or at the mirror focus. When electrical synaptic transmission was depressed relative to the initial baseline prior to the induction of epileptic focus, only a mild influence on the induction of seizure discharges occurred. The role of the gap junctional communication in the epileptiform activity was further investigated by following the expression pattern of two connexin genes. Both, connexin-32 and connexin-43 mRNA levels were significantly elevated at the primary focus as well as at the mirror focus, after 60 min of repeated ictal discharges. We conclude that gap junction communication probably became a part of the neuronal synchronization both in the primary and in the secondarily-induced acute epileptiform activity in the neocortex in vivo. These results, together with earlier observations, indicate a direction for the development of new drugs targeting gap junctions for therapeutic intervention.

In vivo optical mapping of epileptic foci and surround inhibition in ferret cerebral cortex

The population of neurons participating in an epileptiform event varies from moment to moment. Most techniques currently used to localize epileptiform events in vivo have spatial and/or temporal sampling limitations. Here we show in an animal model that optical imaging based on intrinsic signals is an excellent method for in vivo mapping of clinically relevant epileptiform events, such as interictal spikes, ictal onsets, ictal spread and secondary homotopic foci. In addition, a decrease in the optical signal correlates spatially with a decrease in neuronal activity recorded from cortex surrounding an epileptic focus. Optical mapping of epilepsy might be a useful adjunct in the surgical treatment of neocortical epilepsy, which critically depends on the precise localization of intrinsically epileptogenic neurons.

The functional role of metabotropic glutamate receptors in epileptiform activity induced by 4-aminopyridine in the rat amygdala slice

The metabotropic glutamate receptor (mGluR) antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG; 500 microM), was tested on intracellularly recorded epileptiform activity induced by 4-aminopyridine (4-AP) in amygdala neurons. Superfusing 4-AP (1 mM) produced interictal spiking followed by ictal bursting. MCPG prevented the progressive transition from interictal spiking to ictal bursting but affected neither induction of interictal spiking nor maintenance of ongoing ictal bursting. These data suggest that mGluRs may be involved in the induction of ictal seizure events.

Cellular mechanisms of neocortical secondary epileptogenesis

Firing activity, membrane parameters and postsynaptic responses were studied by recording intracellularly from different types of neurons during the development of a secondary neocortical epileptiform focus (mirror focus, Mf) contralateral to the site of an aminopyridine-induced focus (primary focus, Pf) in anesthetized rats. Three different stages in the development of secondary epileptogenesis were observed. (i) in the Pf stage epileptiform discharges appeared only in the ECoG recorded from the Pf, but neurons in the Mf showed reduced firing activity; (ii) in the Pf + Mf stage, synchronous ictal epileptiform activity occurred in the Pf and Mf. Changes in the balance between inhibition and excitation, appearance of novel electrophysiological phenomena (e.g. antidromic like action potentials, PDS (paroxysmal depolarization shift) potentials, rebound bursts), enhanced intrinsic bursting, and a transition from regular spiking to bursting were observed at the cellular level; (iii) in the Pf/Mf stage in 10% of the animals, the surface epileptic discharges were in synchrony with cellular activity in the Mf but were temporally independent of Pf activity, suggesting that during secondary epileptogenesis the Pf and the Mf can have underlying epileptogenic mechanisms which are different in origin.

Effects of carbamazepine and baclofen on 4-aminopyridine-induced epileptic activity in rat hippocampal slices

1. Rat transverse hippocampal slices exposed to 100 microM 4-aminopyridine (4-AP) generate spontaneous epileptic discharges ranging in duration from short 50 ms 'interictal' bursts to long 0.5-2 s 'polyspike' activity. 2. Here we compared the effects of the commonly used anticonvulsant, carbamazepine (40 microM) and the antispastic drug, baclofen (2 microM) on the various types of burst. 3. Carbamazepine completely abolished long bursts whilst leaving shorter bursts intact. This is consistent with its known anticonvulsant properties. 4. Baclofen greatly reduced the frequency of short bursts but did not block the long bursts. Rather, they became significantly more prolonged, indicating that baclofen does not have an anticonvulsant action, and may be proconvulsant. 5. These results conflict with conclusions based on studies using models that exhibited only interictal bursts, and emphasize the need to use experimental epilepsies which generate several types of epileptic discharge to evaluate the effects of putative anticonvulsant drugs. 6. The present findings suggest that GABAB receptors play a role in the transition of benign interictal bursts to longer polyspike activity which could develop into seizures in the whole animal.

Stimulation of substantia nigra pars reticulata suppresses neocortical seizures

The effects of unilateral electrical stimulation of the substantia nigra pars reticulata (SNpr) on the electrocorticographic (ECoG) manifestations of seizures were studied in anesthetized rats. Epileptiform activity was provoked in the primary focus (Pf) by unilateral, local application of 3-aminopyridine which induced secondary focus in the homologous area of the contralateral cortex (mirror focus, Mf). The position of the electrode for stimulation of SNpr was contralateral to the Pf. The results showed a strong suppressive nigral effect on cortical seizure propagation and on seizure susceptibility in both hemispheres. Stimulation of the SNpr prevented the manifestation of sustained epileptiform events, decreased the rate of seizure appearance in the Mf, delayed the onset of paroxysmal activity and markedly reduced the amplitude and duration of ictal episodes at both foci. Seizure potentials of lower frequencies disappeared, while the relative proportion of those of higher frequency increased in SNpr-stimulated animals. SNpr stimulation had no significant effect on fully developed seizures. Our observations support the idea that SNpr might be involved in the control of cortical seizure susceptibility, regulating other structures which are possibly involved in the generation and propagation of seizure.

The epileptiform activity induced by 4-aminopyridine in rat amygdala slices: antagonism by non-N-methyl-D-aspartate receptor antagonists

The effects of excitatory amino acid receptor antagonists kynuretic acid and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on epileptiform activity induced by 4-aminopyridine (4-AP) were studied in rat amygdala slices using intracellular recording techniques. Five to 10 min after switching to 4-AP-containing solution, spontaneous epileptiform bursts were observed in 35 out of 45 slices studied. The spontaneous epileptiform events consisted of an initial burst followed by a number of afterdischarges. Superfusion with kynuretic acid, a broad-spectrum excitatory amino acid receptor antagonist, reversibly reduced the duration of the spontaneous bursting discharges in a dose-dependent manner. The frequency of spontaneous bursting was also decreased. The IC50, estimated from the graph of the concentration-response relationship, was approximately 130 microM. In addition, CNQX which is a specific non-N-methyl-D-aspartate (NMDA) receptor antagonist blocked the spontaneous and evoked epileptiform bursting. In 11 out of 15 neurons tested, there was a residual depolarizing component remained. This depolarizing component was reversibly blocked by specific NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleate (D,L-APV). Relative to the CNQX-sensitive component, the D,L-APV-sensitive component is much smaller in amplitude and shorter in duration indicating that NMDA receptor plays only a minor role in this process. These data suggest that the generation or propagation of spontaneous epileptiform events induced by 4-AP in the neurons of basolateral amygdala is mediated by excitatory amino acids and that activation of non-NMDA receptors is of primary importance.

Orally administered 4-aminopyridine improves clinical signs in multiple sclerosis

4-Aminopyridine (4-AP), a potassium channel blocker, restores conduction in blocked, demyelinated animal nerve. Its administration to multiple sclerosis (MS) patients produces transient neurological improvements. Vision improves after either oral or intravenous administration, whereas motor function improvement has been reported only with the latter. To assess further its potential as a practical symptomatic treatment, we studied the efficacy of single, oral doses of 4-AP on both visual and motor signs in MS. Twenty temperature-sensitive male MS patients were given either 10 to 25 mg of 4-AP or identically appearing lactose placebo capsules. Static quantitative perimetry, critical flicker-fusion, visual acuity, visual evoked potentials, and videotaped neurological examinations were monitored. All of 15 MS patients given 4-AP mildly to markedly improved. Motor functions (power, coordination, gait) improved in 9 of 13 involved, vision in 11 of 13, and oculomotor functions in 1 of 2. Improvements developed gradually at doses as low as 10 mg, usually beginning within 60 minutes after drug administration, and reversed gradually over 4 to 7 hours. No serious adverse effects occurred. No significant changes were observed in 5 MS patients given placebo. We conclude that orally administered 4-AP produces clinically important improvements in multiple, chronic deficits in MS. Further studies are warranted to assess efficacy and safety of prolonged administration.

Properties of depolarizing plateau potentials in aminopyridine-induced ictal seizure foci of cat motor cortex

The mechanisms of generation of self-sustained depolarizing plateau potentials (DPs) were studied in intracellular recordings in aminopyridine-induced ictal seizure foci in the motor cortex of the cat. In some experiments single-electrode voltage clamp techniques were used and intracellular pressure injection of aminopyridine (Ap), phorbol esters (PhEs) and tetraethylammonium (TEA) was carried out. After several ictal episodes, DPs with bursts of action potentials or with spike inactivation developed gradually in the clonic and interictal phases, without synchronism with surface ictal seizure potentials. In many cases DPs were followed by hyperpolarizing afterpotentials and neuronal inhibition. In bursting neurons DPs originated from the augmented depolarizing envelope of bursts of action potentials. In non-bursting neurons DPs were initiated from summated depolarizing afterpotentials and slow spikes with high threshold, resembling Ca-spikes. In a few neurons DPs were triggered by enlarged excitatory postsynaptic potentials. It was possible to evoke DPs by injections of depolarizing current pulses into single neurons of the Ap-focus, or by intracellular injection of AP, PhEs or TEA. We conclude that DPs are not causal cellular bases of the ictal paroxysmal discharges, rather they occur as consequences of abnormal neuronal activity. It is suggested that DPs are intrinsic regenerative membrane events induced by a transient dominance of voltage-dependent inward currents (carried primarily by calcium ions although sodium ions may contribute) by simultaneous decreases in concurrent outward potassium currents.

The effects of 4-aminopyridine on the cat spinal cord: rhythmic antidromic discharges recorded from the dorsal roots

In a previous paper, we have reported that 4-aminopyridine (4-AP, i.v., 10 mg/kg) induces in decerebrate spinal and paralyzed cats, a sustained rhythmic activity (2.5-8.5 Hz) in various muscle nerves. We describe here that similar discharges are recorded from the proximal stump of cut cutaneous nerves. The latter rhythmic activity arises from intense antidromic discharges in the dorsal roots. The rhythmic discharges are recorded from dorsal roots of both spinal cord enlargements as well as from thoracic roots. The rhythmic activity is highly synchronous among adjacent dorsal roots. Bilateral activity is also highly cross-correlated, but may be dissociated by unilateral stimulation of one dorsal root. It is not yet possible to determine the precise site where the antidromic discharges recorded from the dorsal roots are generated. 4-AP could act directly at the terminal level of the primary afferents or could activate interneurons impinging upon the terminals.

Mechanism of aminopyridine-induced ictal seizure activity in the cat neocortex

Intracellular recordings were obtained from neurons in the motor cortex of anesthetized cats in order to examine membrane and synaptic processes involved in aminopyridine (AP)-induced ictal seizure activity. Depolarizing and hyperpolarizing membrane potential sequences which behaved as large, synchronized excitatory and inhibitory postsynaptic potentials, were found to accompany the ictal seizure potentials. After several repetitions of the seizure attack, partial responses, bursts and depolarizing plateaus with spike inactivation occurred. In layers IV and V we found non-pyramidal tract neurons showing endogenous bursting ability activated by AP. These neurons seemed to be the initiators of the rhythmic synchronous activity of the epileptic neuron population. Our results suggest that AP-induced epileptogenesis represents an adequate model of ictal events in the neocortex.

A GABAergic depolarizing potential in the hippocampus disclosed by the convulsant 4-aminopyridine

Intracellular recordings from hippocampal pyramidal cells in the CA1 subfield of the 'in vitro' slice in the presence of 4-aminopyridine (4-AP, 5-50 microM) revealed a long-lasting (up to 1.5 s) depolarizing potential which occurred either spontaneously or following orthodromic stimulation. This potential was: capable of blocking both direct and synaptic activation of the cell; sensitive to bath application of low concentrations of bicuculline methiodide; and associated to an extracellular current sink in the dendrites as suggested by the extracellular field potentials recorded at different levels along an axis perpendicular to the stratum pyramidale. It is concluded that the long-lasting depolarizing potential evoked by 4-AP is caused by the activation of GABA receptors localized in the dendritic region of the CA1 subfield.

Depression of spike adaptation and afterhyperpolarization by 4-aminopyridine in hippocampal neurons

In guinea pig hippocampal slices, 4-aminopyridine (4-AP) in concentrations of 100-500 microM reduced the adaptation of CA3 pyramidal neurons to depolarizing stimuli, resulting in a prolongation of repetitive firing during injection of long-lasting depolarizing currents. Concurrently, there was a decrease in the 'sag' of potential after spike bursts. Furthermore, 4-AP decreased or abolished the hyperpolarizing potential (the afterhyperpolarization) which normally followed repetitive firing of the neurons. The findings suggest that 4-AP could interfere with the Ca2+-activated K+ current in hippocampal CA3 pyramidal neurons.

The effects of 4-aminopyridine on the spinal cord: rhythmic discharges recorded from the peripheral nerves

The effects of an intravenous injection (20 mg/kg) of 4-aminopyridine (4-AP) were initially investigated in acute low spinal cats (Th 13), in which L-DOPA had induced fictive locomotion after paralysis. 4-AP first accelerated the locomotor rhythm and could also change markedly the pattern of activation of some muscle nerves. Shortly after, the locomotor activity was replaced by synchronous rhythmic discharges (2.5-8.5 Hz) in flexor and extensor muscle nerves of the same limb girdle. Similar rhythmic activity was recorded after 4-AP alone (5-20 mg/kg) in the acute decerebrate spinal cat. Whilst the mean rate of the rhythmic activity could differ in the two limb girdles, discharges generated in one girdle appeared to be strongly influenced by those generated in the other. After a complete section of the spinal cord (Th13), the activity persisted in both the rostral and caudal segments although the interactions between the two disappeared. The persistence of the rhythmic activity caudal to the section underscores its spinal origin. In the chronic spinal rat, such rhythmic activity could still be induced in the lumbo-sacral cord despite degeneration of descending pathways. It appears that large doses of 4-AP exert potent effects on the spinal cord which can override other patterns of activity and synchronize the electrical activity of many neuronal elements.

Effects of 4-aminopyridine (4-AP) on rat neostriatal neurons in an in vitro slice preparation

Effects of 4-aminopyridine (4-AP) on the rat neostriatal neuron were studied using the in vitro slice preparation. The intracellularly recorded neurons had resting membrane potentials of more than 50 mV and were capable of generating action potentials with the amplitude greater than 60 mV. Application of 4-AP in the superfusing media depolarizes the cell membrane and increases its input resistance. Local electrical stimulation induces excitatory postsynaptic potentials (EPSPs) overlapping with inhibitory postsynaptic potentials (IPSPs) in these neurons. 4-AP application enhances the amplitude and duration of the postsynaptic potentials. With application of higher concentration of 4-AP, local stimulation induces a second EPSP and a bicuculline sensitive long duration depolarization. These results indicate that 4-AP clearly has effects on local stimulation-induced postsynaptic responses of neostriatal neurons. Possible mechanisms underlying the 4-AP actions on neurotransmission in the neostriatal slice are discussed.

Neuropathological alterations in the neocortex of rats subjected to focal aminopyridine seizures

Acute focal seizures were produced in anaesthetized albino rats by topical application of 3- and 4-aminopyridine on the exposed fronto-parietal neocortical areas. After 15 min of seizure activity neuronal and glial changes were studied by light (toluidine blue and Golgi staining) and electron microscopy. Shrinkage and increased electron density of some pyramidal cells, astrocytic swelling and depletion of 40-60 nm synaptic vesicles from the nerve terminals in layers I, II and III were found. The possible significance of the alterations in the seizure is discussed.

Convulsant actions of 4-aminopyridine on the guinea-pig olfactory cortex slice

The effects of bath-applied 4-aminopyridine on neurones and extracellular potassium and calcium concentrations were recorded in slices of guinea-pig olfactory cortex. Neurones were orthodromically activated by stimulating the lateral olfactory tract. 4-Aminopyridine (3-10 microM) had the following effects: (1) an increase in the frequency and amplitude of spontaneous postsynaptic potentials; (2) a prolongation and oscillatory behaviour of orthodromically evoked postsynaptic potentials; (3) induction of spontaneous or stimulus-evoked seizure-type discharges which were accompanied by large rises in extracellular potassium and falls in calcium concentration; (4) a prolongation of the lateral olfactory tract population fibre spike. Prior to paroxysmal depolarization, membrane potential, input resistance and soma spike duration were unaffected. In the seconds before seizure discharges, a late hyperpolarizing potential (evoked by orthodromic stimulation) was reduced in amplitude or abolished. Diphenylhydantoin (50 microM) or magnesium ions (5 mM) prevented paroxysmal activity. Our results show that 4-aminopyridine can produce seizure-type discharges in a brain slice preparation. The role of increased spontaneous potentials and possible loss of synaptic inhibition as causal factors for such discharges is discussed.

Comparative study of aminopyridine-induced seizure activities in primary and mirror foci of cat's cortex

Epileptogenic activity was induced by topical application of 3-Ap on the somatosensory cortex of anaesthetized cats. Surface epileptiform events were recorded and single cell activity was studied with intracellular electrodes both in the primary and in the mirror focus. Surface records showed that soon after the onset of paroxysmal activity in the primary focus, an epileptic mirror focus developed with variable delay. After MnCl2 treatment of the primary focus, the mirror focus generated independent epileptic events. In the primary and in the mirror focus 122 neurones were recorded intracellularly during seizure activity. Different firing patterns were observed. In the primary focus 31% and in the mirror focus 28% of the recorded neurones generated PDSs. The firing of the neurones in both foci was often phase-locked with surface waves. PDSs usually occurred asynchronously with the surface activity. Ionic mechanisms which may underlie the effect of 3-Ap are discussed.

On modelling the variability of interspike intervals during epileptic unit activity

In spite of the fact that the participation of well defined ionic particles in generating convulsive unit discharges is established, there is a gap between the data on ionic movements and on first-order statistics of firing patterns. Our aim was to tight this gap by studying the effectiveness of functionally separated electrical conductances of membrane during the generation of consecutive interspike interval histograms (IIHs) of unitary discharges. On account of the non-stationarity of the process curve fitting analysis which based on the simple modifications of the integrate-and-fire model has been implemented in the sequential interspike interval histogram procedure (SIIH). The experimental data were recorded from cat cortex treated with 3-Aminopyridine (3-Ap) by glass microelectrodes during nembutal anesthesia. Assuming the normal distribution of input parameters it is concluded, that the efficiency of the fluctuations of the active spike-generating conductance gg and the passive diffusional conductance gl may increase during the generation of the unimodal IIHs and the first mode of the bimodal IIHs. The simple conductance coupling gl=gg + b may participate in gg activation, moreover, the reciprocally coupled mechanism gg=c/gl may be driven by gl activation (a, b, c are the coupling constants). A temporal separation of processes governed by gg or gl respectively was observed. The time-independent occurrences of the reciprocally coupled conductance processes may be involved in the unit activities represented by the prolonged IIHs and second modes of the bimodal IIHs.