The effects of pentylenetetrazol on molluscan neurons. II. Voltage clamp studies

Diversity of potassium channels contributing to differences in brain area-specific seizure susceptibility: sensitivity of different potassium channels to the epileptogenic agent pentylenetetrazol

The effect of the epileptogenic agent pentylenetetrazol on eight cloned voltage-operated mammalian potassium channels (expressed in oocytes of Xenopus laevis) was investigated in order to contribute to an explanation for the brain area-specific differences in seizure susceptibility. Pentylenetetrazol increased the potassium currents at more negative and decreased them at more positive potentials for the channels of the Kv1 gene family, whereas for the other channels the currents were decreased over the whole potential range. The sensitivities of the different potassium channels to the epileptogenic agent were different. At a potential of 0 mV, for example, there were strong reductions for the Kv1.1, Kv1.4 and Kv2.1 currents, whereas the decrease was smaller for the Kv1.3 and Kv1.6 currents and was negligible for the Kv1.2, Kv1.5 and Kv3.4 currents. Correlating these data with the distribution patterns of the potassium channels in the hippocampus, the neocortex and the cerebellum (representing examples of brain areas of distinct seizure susceptibility) revealed that in brain areas with higher seizure susceptibility the overall sensitivity of the potassium channels to the epileptogenic agent is augmented. As a whole, the findings give the first evidence that the differences in distributions and properties of potassium channels contribute to differences in the seizure susceptibility of brain areas.

Mechanism of action of the epileptogenic drug pentylenetetrazol on a cloned neuronal potassium channel

The action of the epileptogenic agent pentylenetetrazol (PTZ) on a cloned potassium channel of the rat brain was studied. The Kv1.1 channel was expressed in oocytes of Xenopus laevis and potassium currents were investigated in outside-out and inside-out membrane patches. The results show that PTZ increased the multi-channel potassium currents at strongly negative potentials and decreased them at potentials positive to -35 mV both in outside-out and inside-out membrane patches. The extent and manner of PTZ action, the concentration dependence as well as the onset and time course of the PTZ effect were the same both in outside-out and inside-out membrane patches. The single-channel potassium currents showed an increase in open probability and frequency of opening and a decrease in close time at -50 mV and vice versa at 0 mV with application of PTZ. The amplitude of single-channel current, the open time and the latency to the first channel opening remained almost unchanged under PTZ. The results indicate that PTZ acts via the cell membrane and influences the membrane-associated part of the potassium channel. Thereby, PTZ accelerates the transition from the inactivated to the open state of the channel at strongly negative potentials and reduces it at slightly negative and positive potentials. This mechanism may be the basis for a gate function which is in favour of the development of epileptic discharges.

Potassium currents in epilepsy: effects of the epileptogenic agent pentylenetetrazol on a cloned potassium channel

The effect of the epileptogenic agent pentylenetetrazol (PTZ) on the cloned rat brain potassium channel Kv1.1 (labelled also RCK1) was investigated in the Xenopus laevis oocyte expression system. The Kv1.1 channel was affected by PTZ in a voltage-dependent manner. PTZ increased the potassium currents at more negative potentials and decreased them at more positive potentials. At a potential of -50 mV the potassium currents were increased by 0.97 and at -20 mV decreased by 0.21 of control value with 100 mmol/l PTZ. The potential at which the inversion from increase to decrease occurred was -33 mV. The inactivation characteristic of the current was shifted to more negative potentials by PTZ. The PTZ effect was obtained at a threshold concentration of 1 mmol/l and increased with rising PTZ concentrations. After removal of the tissues covering the oocyte membrane, the PTZ effect was augmented; with a concentration of 10 mmol/l PTZ the potassium currents at 0 mV were decreased by 0.04 in oocytes with covering tissues and by 0.27 of control value in oocytes without covering tissues. Under current-clamp conditions, PTZ decreased small depolarizations and increased larger depolarizations. This effect of PTZ represents a 'discriminatory function' that may contribute to epileptogenesis in nervous tissues.

Membrane currents elicited by the epileptogenic drug pentylenetetrazole in the native oocyte of Xenopus laevis

The effects of the epileptogenic agent pentylenetetrazole (PTZ) on membrane currents of native oocytes of Xenopus laevis were studied. PTZ elicits a response that consists of two inward currents. The first one is interpreted to be due to a decrease of potassium permeability since: (1) the input resistance is increased; (2) the equilibrium potential is near that of potassium; (3) the current is decreased during administration of potassium channel blocking agents; and (4) the PTZ response can be mimicked by blocking potassium channels without PTZ application. The second one is interpreted to be due to an increase of chloride permeability since: (1) the input resistance is decreased; (2) the equilibrium potential is near that of chloride; and (3) the response is decreased during administration of chloride blocking agents. These findings correspond to some extent with those made in neurons.

Effects of pentetrazol on neuronal activity and on extracellular calcium concentration in rat hippocampal slices

Effects of pentetrazol (PTZ) were studied on neuronal responses in dentate granule cells and area CA1 hippocampal pyramidal cells with intra- and extracellular recording techniques. PTZ induced spontaneous epileptiform field potential transients in areas CA3 and CA1, but not in the dentate gyrus. The concentration optimum for induction of spontaneous epileptiform activity was 2 mM. The epileptiform activity compared in many respects to that induced by GABA antagonists such as picrotoxin, bicuculline and penicillin. Paired pulse stimulus induced responses were affected by concentrations of 0.5 mM. In the concentration range 0.5-2 mM mostly disinhibitory effects were noted. Stimulus induced Ca2+ concentration changes were found to be maximally augmented at concentrations of 2-5 mM. In this range, intracellular studies revealed a block of frequency habituation and an increase in input resistance. The convulsant action of PTZ decreased at concentrations above 5 mM, probably due to a decrease of inward currents. We suggest that the action of PTZ in screening studies for anticonvulsants is mostly due to a decrease of GABAA-receptor mediated IPSPs.

Pentylenetetrazole-induced seizure activity produces an increased release of calcium from endoplasmic reticulum by mediating cyclic AMP-dependent protein phosphorylation in rat cerebral cortex

1. To determine the involvement of the convulsant agent pentylenetetrazole (PTZ) in intracellular calcium release in neurons, its effect on stored calcium in the endoplasmic reticulum of rat cortical neurons was tested. 2. Intraperitoneal injection of PTZ caused marked release of calcium from the endoplasmic reticulum which was similar to that observed when cortical slices were incubated with this convulsant. 3. Superfusion of dibutyryl cAMP and isobutylmethylxanthine to the cortical slices mimicked PTZ-induced calcium release from this reservoir. A similar effect was observed under depolarizing conditions brought about by either an elevation of extracellular K+ concentration or addition of veratridine. 4. Isoquinolinesulfonamide, a protein kinase inhibitor, reduced PTZ-stimulatory effect of calcium release and blocked the cAMP-induced calcium release. 5. Intracellular cAMP level was enhanced at about 3-fold by both intraperitoneal injection of PTZ and its superfusion. 6. These findings are taken to suggest that PTZ may release stored calcium in the endoplasmic reticulum by mediating a cAMP-dependent protein phosphorylation in cortical neurons.

Enhancement of cyclic AMP-dependent sodium current by the convulsant drug pentylenetetrazol

The convulsant drug pentylenetetrazol (PTZ) causes paroxysmal depolarizing shifts (PDS) and bursting in molluscan neurons. PDS has been found to be accompanied by increased levels of cyclic AMP (cAMP) and supported by persistent Na+ current. In neurons of the snail Lymnaea stagnalis the blocker of cAMP degradation isobutylmethylxanthine (IBMX) mimicks PTZ action. Na+ dependence of PTZ-induced inward shift in holding current in voltage-clamped cells supports the potential Na+ current origin of PDS. Intracellular cAMP injection elicits a transient Na+ current whose amplitude and duration are enhanced by both PTZ and IBMX. PTZ may cause PDS partly through slowing cAMP degradation, thus enhancing the cAMP-dependent Na+ current. PDS-generated bursts cause partial inactivation of the Na+ current, which may contribute towards burst termination.

Differential effects of pentylenetetrazole on ion currents of Aplysia neurones

The effect of the convulsant drug pentylenetetrazole (PTZ) on separated membrane current components has been studied in identified voltage-clamped Aplysia neurones. External PTZ blocks the voltage-dependent Na+, Ca2+ currents and the delayed rectifier current (INa, ICa and IK,V, respectively). The amplitude of the Ca2+-activated K+ current (IK,Ca) is increased. The amplitude of the fast inactivating K+ current (IA) is transiently increased at low concentrations of PTZ but is depressed at higher concentrations or after long-lasting application of the drug. The effect of PTZ on leakage current (IL) seems to depend on the cell type. In some cells (R-15, L-7, LP-1) IL is decreased while it is increased in other cells (L-11, BL-1, BR-1). PTZ accelerates the inactivation of IK,V and IA and shifts the current-voltage relation of ICa to negative voltages by 5-8 mV. Pressure injection of PTZ into the neurone did not affect IK,V or IK,Ca. Thus PTZ seems to act on the outside of the plasma membrane. The effect of external PTZ on INa, ICa, IK,V and IL is also observed if the internal Ca2+ activity is buffered with EGTA suggesting that an increase in the internal Ca2+ activity is not involved. At -40 mV PTZ induces a tetrodotoxin-insensitive inward current carried by Na+ ions. PTZ transforms the beating pacemaker cell L-11 into a bursting pacemaker and the bursting pacemaker cell R-15 exhibits 'square-wave'-like oscillations of the membrane potential.

Involvement of pentylenetetrazole in synapsin I phosphorylation associated with calcium influx in synaptosomes from rat cerebral cortex

To determine precisely how pentylenetetrazole (PTZ) is involved in the biochemical processes at the presynaptic nerve terminal, the effect of PTZ, under various conditions, on the phosphorylation of synapsin I (previously called protein I) was investigated, using 32Pi in synaptosomes from rat cerebral cortex. PTZ markedly stimulated the incorporation of 32P into this protein as determined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and autoradiography, but it failed to stimulate protein phosphorylation in Ca2+-free medium containing ethylene glycol bis-(beta-aminoethylether)-N',N'-tetraacetic acid (EGTA). Moreover, the PTZ-stimulated synapsin I phosphorylation was reversed by addition of EGTA sufficient to chelate all external free Ca2+. PTZ also stimulated synaptosomal accumulation of Ca2+. The PTZ-stimulatory effects of both synapsin I phosphorylation and synaptosomal accumulation of Ca2+ were inhibited markedly by tetrodotoxin as well as by cobalt chloride and lanthanum chloride. The calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7, strongly) and N-(6-aminohexyl)-1-naphthalenesulfonamide (W-5, weakly) reduced the PTZ-stimulatory effect on synapsin I phosphorylation by about 75 and 15%, respectively, whereas these antagonists had essentially no effect on PTZ-stimulated synaptosomal accumulation of Ca2+. These results suggest that PTZ causes the influx of Ca2+ into the presynaptic nerve terminal secondary to the elevated Na+ and is consequently involved in the synapsin I phosphorylation step, facilitating the Ca2+/calmodulin-mediated presynaptic event leading to seizure discharge.

Potassium, pentylenetetrazol, and anticonvulsants in mouse hippocampal slices

We studied the effect of varying potassium (K+) concentrations on spontaneous and pentylenetetrazol (PTZ)-induced population burst discharges in mouse hippocampal slices. Standard techniques were used to obtain extracellular recordings in the CA3 region of hippocampal slices from Swiss-Webster mice (21-28 days old). No spontaneous burst discharges occurred at 3.25 mM K+, but population bursts were observed in 20 and 90% of the slices at 6.25 and 9.25 mM K+, respectively. In the presence of 3.25 mM K+, PTZ produced bursts in 12% of the slices at a concentration of 200 micrograms/ml, in 36% at 300 micrograms/ml, and in 40% at 400 micrograms/ml. Slices exhibiting no burst discharges in the presence of 6.25 mM K+ could be induced to do so with the addition of PTZ; bursts were produced in 11% of these slices at a PTZ concentration of 100 micrograms/ml, in 65% at 150 micrograms/ml, and in 87% at 200 micrograms/ml. The PTZ-induced bursting activity was reversible. Clonazepam abolished the bursting elicited with 200 micrograms/ml PTZ at 6.25 mM K+, and phenytoin reduced, but did not stop, bursting activity. Ethosuximide (ETH) was ineffective in stopping or reducing the burst discharges at a concentration of 125 micrograms/ml ETH was there a consistent reduction in the frequency of population bursts. The induction of PTZ discharges in the hippocampal in vitro preparation offers the advantage of a simplified model for studying the pharmacology of antiepileptic drugs.

The effect of pentylenetetrazol on inward currents of non-bursting neurons and its role in plateau formation

The epileptogenic drug, pentylenetetrazol (PTZ) produces paroxysmal depolarization shifts in molluscan neurons that are similar to PDSs seen at a mammalian epileptic focus. Most research on molluscan neurons indicates that PTZ acts by altering ionic somatic conductances. This study was carried out to investigate the effect of PTZ on inward currents in isolated neurons of the pond snail, Lymnaea stagnalis, and to investigate how these altered currents might lead to the production of PDSs. In concentrations from 10 to 60 mM, PTZ decreased maximum inward current conductance and shifted the inactivation and activation curves to the left with the former shift being consistently greater. There was no change in reversal potential or time constants for activation and inactivation of inward currents. The effects of the PTZ-induced alterations in the inward currents were studied by incorporating them along with alterations of outward currents seen in this and other studies in a computer model for molluscan neuronal firing. The composite model reproduced in large part the intermediate changes in electrical activity seen before the development of the PDS as well as the PDS.

The effect of pentylenetetrazol on spike broadening and potassium inactivation

The effect of the convulsant drug, pentylenetetrazol (PTZ) on spike broadening and potassium current inactivation was studied. PTZ was found to decrease the time taken for a cell to reach maximal broadening as well as causing a decrease in the total amount of broadening. Voltage clamp studies showed that in the presence of PTZ potassium current inactivated less but exhibited a faster time constant of inactivation. By exerting an effect on potassium inactivation and thereby spike broadening, PTZ may alter synaptic efficacy. Such an effect on synaptic efficacy may partially underlie the drug's convulsive activity.

Cyclic AMP enhances calcium-dependent potassium current in Aplysia neurons

The effect on the Ca-dependent potassium current, IK(Ca), of procedures that increase intracellular cAMP levels was studied in Aplysia neurons using three different pharmacological approaches. Exposure to cAMP analogues which were either resistant to or protected from phosphodiesterase hydrolysis caused an increase in IK(Ca) from 30 to 50% in 10 min. The degree of reversibility of this effect varied from complete with db cAMP to very little with pcpt cAMP. Exposure to cholera toxin, which stimulates the synthesis of endogenous cAMP, increased IK(Ca) 25% in 10 min and the effect was not reversible. Both approaches were effective in all seven neuron types studied. Application of serotonin plus phosphodiesterase inhibitor caused an increase in IK(Ca) in neuron R15 but not in the other neuron types. Application of pentylene tetrazole (PTZ) led to a decrease in IK(Ca). It is proposed that elevation of cyclic AMP mediates an increased sensitivity of the IK(Ca) channel to Ca ions.

Conversion of beating to bursting pacemaker activity: action of quinidine

External quinidine converts the pacemaker neurone L-11, found in the Aplysia abdominal ganglion, from spontaneously "beating" to "bursting" discharge activity. Quinidine-induced bursting ceased when entry of Ca2+ ions into the cells was blocked in a Ca2+-free, Co2+-containing solution or if internal Ca2+ accumulation was prevented by the injection of EGTA. The analysis of membrane currents from voltage clamp experiments showed that quinidine blocks the Ca2+ inward current in a dose- and time-dependent manner. In addition, the currents were displaced to the left on the voltage axis, causing an increase of the inward current at negative membrane potentials. External quinidine suppresses the Ca2+-activated K+ current induced by intracellular Ca2+ injections and acts to prolong its decay phase. The slowing of the decay phase of the Ca2+-activated K+ current by quinidine was prevented after intracellular injection of EGTA, indicating that Ca2+ removal is impaired by the drug. It is suggested that the increase of Ca2+ inward current at negative potentials and the prolonged activation of the Ca2+-activated K+ current play a major role in causing the bursting discharge behavior in normally beating cells.

Penicillin- and barium-induced epileptiform bursting in hippocampal neurons: actions on Ca++ and K+ potentials

Both barium (Ba++) and penicillin produce spontaneous epileptiform burst generation in hippocampal neurons in vitro. Recent investigations suggest that Ba++ acts by both adding to a calcium (Ca++)-mediated depolarization and reducing potassium (K+) conductance. In contrast, it has been proposed that penicillin produces burst generation by attenuating inhibitory postsynaptic potentials. However, some evidence suggests that penicillin may also directly alter intrinsic membrane properties. We therefore compared the actions of penicillin and Ba++ on three intrinsic Ca++- or K+-mediated membrane events, namely, CA++ spikes, Ca++-dependent anomalous rectification, and K+-dependent afterhyperpolarization. Ba++ augmented the Ca++ potentials and attenuated the K+-dependent afterhyperpolarization; penicillin had no demonstrable effect on these events. Ba++ produced rhythmical burst firing and oscillations of the membrane potentials, while penicillin caused sporadic burst generation followed by a longlasting afterhyperpolarization. Synchronized, orthodromically evoked burst firing occurred after exposure to penicillin but not to Ba++. Ba++ and penicillin are prototypes of agents which induce epileptogenesis in mammalian cortical neurons by two different but probably interrelated mechanisms. Ba++ causes burst generation by disrupting a delicate balance between depolarizing Ca++ potentials and repolarizing, hyperpolarizing K+ potentials. Penicillin does not affect Ca++- or K+-mediated membrane events; other data suggest that it produces burst generation in hippocampal pyramidal neurons by attenuating gamma-aminobutyric acid-mediated synaptic inhibition, which in turn ordinarily limits intrinsic bursting.

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.

Extracellular free calcium and potassium during paroxsmal activity in the cerebral cortex of the cat

Extracellular calcium and potassium activities (aCa and aK) as well as neuronal activity were simultaneously recorded with ion-sensitive electrodes in the somatosensory cortex of cats. Baseline aCa was 1.2-1.5 mM/l, baseline aK 2.7-3.2 mM/l. Transient decreases in aCa and simultaneous increases in aK were evoked by repetitive stimulation of the contralateral forepaw, the nucleus ventroposterolateralis thalami and the cortical surface. Considerable decreases in aCa (by up to 0.7 mM/l) were found during seizure activity. A fall in aCa preceded the onset of paroxysmal discharges and the rise in aK after injection of pentylene tetrazol. The decrease in aCa led also the rise in aK during cyclical spike driving in a penicillin focus. It is concluded that alterations of Ca++ dependent mechanisms participate in the generation of epileptic activity.

The effects of pentylenetetrazol on molluscan neurons. I. Intracellular recording and stimulation

The effects of the convulsant drug pentylenetetrazol (PTZ) were studied in neurons from isolated ganglia of the nudibranch molluscs, Archidoris montereyensis and Anisodoris nobilis, using conventional techniques of intracellular recording and constant current stimulation. PTZ was selected because it causes changes in the intracellularly recorded responses similar to the depolarization shifts recorded in mammalian epileptic neurons. When perfusate containing 120-140 mM PTZ is introduced, the intracellular recording is characterized by an initial silent period followed by small oscillations in membrane potential and irregular firing of spikes. Within 5-15 min, bursts of 2-3 spikes occurred followed by the appearance of episodic prolonged depolarizations with superimposed high-frequency spikes. In the presence of PTZ the prolonged depolarizations were evoked by intracellular stimulation and at the termination of conditioning hyperpolarizations. The prolonged depolarizations were also recorded in neurons isolated from all synpatic input by axonal ligation. Prolonged depolarizations showed threshold behavior since they can be terminated early by an intracellularly applied hyperpolarizing current.