Convulsive effects of 3-aminopyridine on cortical neurones

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.

Electrophoretic drug delivery for seizure control

The persistence of intractable neurological disorders necessitates novel therapeutic solutions. We demonstrate the utility of direct in situ electrophoretic drug delivery to treat neurological disorders. We present a neural probe incorporating a microfluidic ion pump (μFIP) for on-demand drug delivery and electrodes for recording local neural activity. The μFIP works by electrophoretically pumping ions across an ion exchange membrane and thereby delivers only the drug of interest and not the solvent. This "dry" delivery enables precise drug release into the brain region with negligible local pressure increase. The therapeutic potential of the μFIP probe is tested in a rodent model of epilepsy. The μFIP probe can detect pathological activity and then intervene to stop seizures by delivering inhibitory neurotransmitters directly to the seizure source. We anticipate that further tailored engineering of the μFIP platform will enable additional applications in neural interfacing and the treatment of neurological disorders.

Reduced ictogenic potential of 4-aminopyridine in the hippocampal region in the pilocarpine model of epilepsy

It was previously shown that the ictogenic potential of 4-aminopyridine (4-AP) was reduced in the parahippocampal region of kainate treated chronic epileptic rats. In the actual study we investigated the potential of 4-aminopyridine (50 and 100μM) to induce seizure like events (SLEs) in combined entorhinal cortex hippocampal slices from Wistar rats following pilocarpine induced status epilepticus. The potential of 4-AP to induce SLEs in the entorhinal cortex was reduced in the latent period and in slices of chronic epileptic animals with a high seizure incidence in vivo (>2seizures/24h). 4-AP induced SLEs in slices from animals with a low incidence of seizures in vivo (<2seizures/24h) in a similar manner as compared to controls. The hippocampal formation displayed no SLEs, instead short recurrent epileptiform discharges (REDs) were evoked by application of 4-AP in areas CA3 and CA1. The incidence of REDs was largest in slices from control animals. This study shows that the reduced ictogenic potential of 4-AP is not restricted to kainate treated chronic epileptic animals as it can be found in the pilocarpine model as well. The underlying mechanisms may relate to altered expression and editing of voltage gated potassium channels.

Potassium currents in acutely isolated human hippocampal dentate granule cells

1. Properties of voltage- and Ca(2+)-dependent K+ currents were investigated in thirty-four dentate granule cells acutely isolated from the resected hippocampus of eleven patients with therapy-refractory temporal lobe epilepsy (TLE). 2. When intracellular Ca2+ was strongly buffered with 11.5 mM EGTA-1 mM Ca2+ in the recording pipette, K+ currents (IK) with a slow activation and biexponential time-dependent decay could be elicited, which showed a threshold for activation around -30 mV. 3. A contribution of Ca(2+)-dependent K+ currents became apparent with intracellular solution containing 1 mM BAPTA-0.1 mM Ca2+. Superfusion of low-Ca2+ extracellular solution blocked 43% of outward currents in this recording configuration. Outward current components could also be blocked by substituting 5 mM Ba2+ for extracellular Ca2+ (78%), or by application of 100 microM Cd2+ (25%). 4. The Ca(2+)-dependent K+ currents could be pharmacologically subdivided into two components. One component was sensitive to 500 microM tetraethylammmonium (TEA; 41%) and 10 nM charybdotoxin (CTX; 47.2%). The blocking effects of 10 nM CTX and 500 microM TEA were not additive, suggesting that both agents block the same conductance. A second, smaller outward current component was blocked by 50 nM apamin (13%). 5. A transient A-type K+ current could be observed in six neurones and showed a fast monoexponential time-dependent inactivation with a steady-state voltage dependence that was distinct from that of IK. The A-type current was blocked by 4-aminopyridine (4-AP) but not by TEA or low-Ca2+ solution. 6. We conclude that outward currents in human hippocampal dentate granule cells can be separated into at least four types by their kinetic and pharmacological properties. These include at least one voltage-dependent current similar to those observed in mammalian hippocampal neurones, and two Ca(2+)-dependent K+ currents that most probably correspond to SK- and BK-type currents. A classical A-type current could be detected in some patients with Ammon's horn sclerosis (AHS) but not in patients with lesion-associated TLE.

Properties of a delayed rectifier potassium current in dentate granule cells isolated from the hippocampus of patients with chronic temporal lobe epilepsy

PURPOSE

Properties of potassium outward currents were investigated in human hippocampal dentate gyrus granule cells from 11 hippocampal specimens obtained from patients with temporal lobe epilepsy (TLE) during resective surgery.

METHODS

Dentate granule cells were isolated enzymatically and outward currents analyzed by using the whole-cell configuration of the patch-clamp method. Hippocampal specimens were classified neuropathologically with respect to severe segmental cell loss, gliosis, and axonal sprouting (Ammon's horn sclerosis, AHS), or the presence of a focal lesion in the adjacent temporal lobe.

RESULTS

A delayed rectifier outward current (IK), but not an A-type potassium current (IA) or inwardly rectifying potassium currents, was observed in all cells. The average current density of IK, the time-dependent decay of IK, and the resting membrane characteristics were not significantly different between patients with and without AHS. The voltage of half-maximal activation V1/2(act) was 5.4 +/- 1.8 mV in AHS compared with -2.9 +/- 1.8 mV in lesion-associated epilepsy (NS). In contrast, V1/2(inact) was shifted in a hyperpolarizing direction in AHS (-67.7 +/- 0.6 mV) compared with that in hippocampi not showing AHS (-47.7 +/- 2.6 mV; p = 0.0017).

CONCLUSIONS

The altered steady-state voltage-dependence of IK may result in abnormal excitability of dentate granule cells in AHS and exert a marked influence on input-output properties of the dentate gyrus.

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.

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.

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.

Effects of the tetronic acid derivatives AO33 (losigamone) and AO78 on epileptiform activity and on stimulus-induced calcium concentration changes in rat hippocampal slices

The effects of members of a new class of anticonvulsants, the tetronic acid derivatives, were studied in 3 in vitro models of epileptogenesis in rat hippocampal slices; the picrotoxin, the low magnesium and the low calcium model. The effects of AO33 (losigamone) and AO78 on stimulus-induced decreases in extracellular calcium concentration were also investigated. In all 3 models of epileptogenesis, both drugs blocked spontaneous and reduced stimulus-induced epileptiform discharges dose dependently and reversibly. Stimulus-induced changes in [Ca2+]0 were markedly diminished by these agents. The fact that the tetronic acid derivatives block the low Ca seizure-like events which develop independently from chemical synaptic transmission suggests that these agents have non-synaptic or direct membrane actions with subsequently reduced cellular excitability.

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.

4-aminopyridine activates a cholinergic chloride conductance in isolated Helix neurons

Isolated Helix neurons were internally perfused under voltage-clamp, with drugs applied by the 'concentration clamp' which allowed an extremely rapid switching of the external solution. 4-Aminopyridine (4-AP) at more than 3 X 10(-4) M induced a chloride current (ICl) which could be blocked by D-tubocurarine. Acetylcholine (ACh) also induced ICl which was similarly blocked. Simultaneous application of 4-AP and ACh resulted in potentiation of ICl over the sum of respective currents. When preceded by either one, the other ICl was reduced in size, showing cross-desensitization. These data indicate that 4-AP activates ICl by binding to the cholinergic receptors, and reduces ACh-induced ICl by desensitizing a common receptor-channel complex.

4-Aminopyridine affects synaptosomal protein phosphorylation in rat hippocampal slices

Rat brain hippocampal slices were incubated with or without the convulsant 4-aminopyridine (4-AP). From these slices a crude mitochondrial/synaptosomal membrane fraction was prepared and analyzed for endogenous protein phosphorylation. 4-AP (10(-5) M) stimulated the phosphorylation of a 50 kDa protein by 86%. The phosphorylation of this 50 kDa protein is Ca2+/calmodulin-dependent and we suggest that this protein is the lower molecular weight subunit of Ca2+/calmodulin-dependent protein kinase II (CaMK II).

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.

Suppression by phenytoin of convulsant-induced afterdischarges at presynaptic nerve terminals

The mechanisms underlying the induction of afterdischarges at presynaptic nerve terminals by convulsant aminopyridines and their suppression by the anticonvulsant drug phenytoin were studied at the frog neuromuscular preparation. Addition of aminopyridine to the perfusing solution induced the appearance of afterdischarges in motor nerve fibres following their primary response to a single nerve stimulus. The afterdischarges seemed to originate at or near the nerve terminals and to propagate both antidromically and orthodromically. The latter resulted in repetitive activation of the neuromuscular synapse. Focal recordings of nerve terminal potentials suggested that aminopyridines may induce afterdischarges by slowing spike repolarization and thereby producing a prolonged depolarization of nerve terminals. Phenytoin suppressed the aminopyridine-induced afterdischarges and the resultant repetitive excitation of the postsynaptic muscle fibres. This effect of phenytoin was associated with a depression of the action potential at the motor nerve terminals but not at their parent axons. These results single the presynaptic nerve terminals as preferential sites for convulsant and anticonvulsant actions.

Neocortical cytopathology in focal aminopyridine seizures as related to the intracortical diffusion of [3H] 4-aminopyridine. Electrophysiologic and light-microscopic studies

Focal seizure was induced in rat and cat neocortex by the topical application of aminopyridines. The epileptic character of the focal events was followed by surface and intracellular recordings of seizure activity. The pathologic alterations in the neurons, the glial cells, and the protein permeability of the neocortical blood vessels were investigated by means of light microscopy, using standard histological stainings and immunohistochemical detection of endogenous serum albumin. Diffusion of [3H] 4-aminopyridine in the neocortex was studied by light-microscopic autoradiography. The spreading of the neuropathologic changes strictly followed the diffusion of the tritiated compound, suggesting the gradual involvement of neocortical layers in the seizure process.

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.

Effects of 4-aminopyridine on acetylcholine output from the cerebral cortex of the rat in vivo

1 The effects of 4-aminopyridine (4AP) on the output of acetylcholine (ACh) from the cerebral cortex were investigated in unanaesthetized freely moving rats and in anaesthetized rats by means of the ;cup technique'. ACh was determined by bioassay on the dorsal muscle of the leech.2 In unanaesthetized rats intraperitoneal injection of 4AP (3 mg/kg) had no effect on the cortical output of ACh.3 After injection of morphine (10 mg/kg s.c.), which depressed the spontaneous output of ACh, 4AP increased the cortical output to a level significantly higher than that determined before morphine injection.4 In rats anaesthetized with either urethane or pentobarbitone, drugs known to decrease cortical output of ACh, 4AP (i.v. or i.p.) elicited a significant increase in the output of ACh. The time-courses of the 4AP-induced effects were different depending on the anaesthetic drug used: an immediate increase slowly fading in urethane anaesthesia and a gradual increase after delayed onset in pentobarbitone-anaesthetized rats.5 In some urethane-anaesthetized rats, respiratory frequency was kept constant (tracheotomy, connection to respirator, bilateral vagotomy) and prazosin (1 mg/kg i.v.) was administered to reduce the 4AP-induced increase of blood pressure. Cortical output of ACh was not related to changes in blood pressure. Moreover, the 4AP-induced increase in cortical ACh output was not related to changes in respiratory frequency.6 In summary systemic administration of 4AP in subconvulsive doses (1 and 3 mg/kg) increased cortical output of ACh in rats anaesthetized with urethane or pentobarbitone or after injection of morphine, but not in untreated freely moving rats. It is suggested that the anaesthetic agents and morphine may cause an imbalance between excitatory and inhibitory central pathways, and that this imbalance may play a role in their depressant effect on cortical output of ACh and/or in the 4AP-induced facilitation described in this paper.

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.

Effect of epileptogenic agents on the incorporation of 3H-glycine into proteins in the cat's cerebral cortex

Filter paper strips soaked in 3H-glycine solution were applied to acoustic cortex of cats, anaesthetized with Nembutal and pretreated with epileptogenic agents (Metrazol, G-penicillin, and 3-amino-pyridine) and cycloheximide. The untreated contralateral hemisphere served as control. After 1 h incubation, both cortical samples were excised simultaneously and fixed in Bouin solution for autoradiography. Incorporation was blocked by cycloheximide. There was no glycine incorporation on the penicillin-treated side, while pyramidal cells were intensively labelled in layers II-V of the mirror focus. 3-Aminopyridine produced the same result. Metrazol as convulsant proved to be far weaker than the previous two. The intensity of incorporation was significantly more intensive in the mirror focus than in the primary one. Penicillin and 3-aminopyridine, while provoking cortical seizures, seem to inhibit glycine incorporation into a neuron-specific, function-dependent protein contained by the labelled cells in the autoradiogram.

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.