Biomedical subjects

E J Speckmann

Publications and source records attributed to E J Speckmann.

At least 19 recordsLinked to original sources

PubMed1998

Characterization of ion currents elicited by a stream of fluid during spontaneous and ligand-induced chloride current oscillation in Xenopus laevis oocytes.

During Ca2+-activated C- current oscillations a mechanical deformation of the Xenopus laevis oocyte by a fluid stream evokes transient inward currents of high amplitude (stream evoked inward current, Ii,st). This current can be observed either in native or RNA-injected oocytes expressing ligand-controlled ion channels from rat brain. Ii,st reversed at the equilibrium potential of chloride and was blocked by 9-anthracene carboxylic acid (2 mM). Power spectral analysis of the oscillations did not reveal a correlation between the features of the oscillations and the amplitude of Ii,st. Antagonists of stretch-activated cation channels [gadolinium (100 microM) and lanthanum (1mM)] did not block Ii,st. Calcium channel blockers [cobalt and manganese (10 mM)] did not inhibited Ii,st and Ii,st could also be elicited in calcium-free medium. Preloading oocytes with pertussis toxin (PTX) for 17 h prevented current oscillations and Ii,st caffeine (10 mM), an antagonist of the liberation of calcium from intracellular stores, inhibited Ii,st. Our results proride evidence for modulation of the mechanosensitivity of chloride currents by activation of intracellular second messenger cascades.

Animals↗

PubMed1998

Gabapentin potentiation of the antiepileptic efficacy of vigabatrin in an in vitro model of epilepsy.

1. An enhancement of promoted release of gamma-aminobutyric acid (GABA) and a change in GABA-metabolism have been suggested as mechanisms of action of gabapentin. Vigabatrin is supposed to act mainly via inhibition of GABA-transaminase but it also interferes with GABA-release and GABA-uptake. On the basis of these mechanisms of action, a pharmacodynamic interaction of the two antiepileptic drugs could be supposed which might be of relevance in the sense of a rational polypharmacy. 2. To address the aforementioned hypothesis, experiments were carried out on hippocampal slices (n=107) of guinea-pigs (n=70). Epileptiform field potentials (e.f.p.) were induced by omission of magnesium from the bath solution and recorded in the stratum pyramidale of the CA3 region. Gabapentin (30-600 microM; 5.1-102.72 microg ml(-1)), vigabatrin (50-200 microM, 6.45-25.8 microg ml(-1)) and the GABA(A)-receptor antagonist bicuculline (100 microM) were added to the bath solution for 3 h. 3. Gabapentin, in concentrations up to 600 microM, failed to decrease the repetition rate or duration of e.f.p. (n=19). However, vigabatrin, evoked a dose-dependent reduction of the repetition rate of e.f.p. For a concentration of 100 microM (12.9 microg ml(-1)) there was a reduction down to 48+/-5% (mean+/-s.e.mean) of the initial value within 3 h (n=11). With simultaneous administration of vigabatrin (100 microM) and gabapentin (60 microM) for 3 h (n=15), the repetition rate of e.f.p. decreased down to 8+/-3%, which is significantly different from the values obtained after administration of 100 microM vigabatrin alone (P<0.0001). Both, the antiepileptic effect of vigabatrin alone and the enhancement by gabapentin were blocked by the GABA(A)-receptor antagonist bicuculline (100 microM, n=16). 4. These results demonstrate that gabapentin is able to augment the antiepileptic effects of vigabatrin significantly. It is possible that a change in the GABA-release machinery is induced by vigabatrin which then can be augmented by gabapentin.

4-Aminobutyrate Transaminase↗

PubMed1998

Hypothermia as cerebroprotective measure. Experimental hypoxic exposure of brain slices and clinical application in critically reduced cerebral perfusion pressure.

An in vitro human neocortical and rodent hippocampus brain slice technique was used under repeated hypoxia to investigate the cerebroprotective effect of hypothermia. As a hallmark of the neuronal hypoxic reaction anoxic terminal negativity (ATN) was registered to test whether hypothermia delays the onset of ATN. The experiments clearly confirm an assumed protective effect of hypothermia in vitro and in vivo and give for the first time evidence of the lack of the protective effect of hypothermia once hypoxia has occurred under normothermic conditions, probably by a critical depletion of cellular ATP-stores. In patients with severe traumatic brain injury and critically low cerebral perfusion pressure mild hypothermia is able to improve clinical outcome.

Adult↗

PubMed1998

Spontaneous sharp waves in human neocortical slices excised from epileptic patients.

Human neocortical temporal lobe tissue resected for treatment of pharmacoresistant epilepsy was investigated. In slices prepared from this tissue, field potentials sometimes superimposed by population spikes were found to appear spontaneously. In individual slices, they were generalized or highly localized to a field of approximately 200 microns in diameter. Synchronous with these potentials, hyperpolarizing and depolarizing postsynaptic potentials were recorded from neurons in the vicinity of the field potential electrode. Hyperpolarizing postsynaptic potentials appeared to be mainly chloride mediated. All potentials, i.e. sharp field potentials as well as postsynaptic potentials, were reversibly suppressed by blockade of the non-NMDA (non-N-methyl-D-aspartate) glutamate-subreceptor and of the GABAA (gamma-aminobutyric acid) receptor, and by application of the organic calcium channel blocker verapamil. By contrast, all potentials remained unaffected by blockade of the NMDA glutamate-subreceptor and the GABAB receptor. The antiepileptic drugs carbamazepine and phenytoin failed to suppress the spontaneous potentials at therapeutic concentrations. Washout of Mg2+ from the superfusate left the spontaneous potentials unchanged or converted them to ictal-type discharges. This epileptiform activity was not suppressed, but augmented by blockade of the GABAA receptor. As a whole, the spontaneously appearing field potentials may be assumed to reflect a state of increased neuronal synchronization.

Action Potentials↗

PubMed1997

Epileptic neurons induce augmenting synaptic depolarizations in non-epileptic neurons (buccal ganglia, Helix pomatia).

Spread of epileptic activity was studied by inducing epileptiform activity (pentylenetetrazol, PTZ) in one part of a nervous system and by analyzing responses of neurons in a non-PTZ-treated part (identified neurons, paired buccal ganglia, Helix pomatia). Paroxysmal depolarization shifts (PDS) induced time-locked depolarizations in non-epileptic neurons (latency ca. 5 s, duration ca. 1 min, amplitude < or =20 mV). Amplitudes were augmenting during several hours of epileptic activity. Depolarizations were accompanied by an increase in membrane resistance and they were blocked in 'high Mg-low Ca' solutions. It is assumed that the potentials represent a typical widespread response of non-epileptic neurons to PDS of other neurons. This response may be induced via non-specific releases of substances of the epileptically active neurons thereby activating neighboring neurons which in turn activate neurons in control ganglion.

Animals↗

PubMed1997

Strychnine-induced epileptiform activity in hippocampal and neocortical slice preparations: suppression by the organic calcium antagonists verapamil and flunarizine.

Alongside GABA, glycine is the major inhibitory transmitter in the central nervous system. Application of the glycine receptor blocker strychnine is known to evoke epileptiform phenomena. The present paper addresses the question whether postsynaptic calcium currents through L-type channels contribute to strychnine-induced epileptiform field potentials (EFP). To test for this, the antiepileptic effect of the specific L-type calcium channel blocker, verapamil, in hippocampal and neocortical slices was investigated. In parallel with this, the antiepileptic efficacy of the unspecific calcium channel modulator, flunarizine, was tested with respect to pharmacotherapy of epilepsies. In both preparations, the L-type calcium channel blocker, verapamil, was able to suppress EFP. In neocortical slices, EFP were blocked in all experiments, whereas in hippocampal slices, in 3 out of 11 experiments, no complete suppression occurred. Flunarizine acted in a similar way. It is concluded that L-type calcium channels are involved in strychnine-induced epilepsy, but to a greater extent in the neocortex than in the hippocampus.

Animals↗

PubMed1997

Shifts of cortical d.c. potential induced by application of gamma-aminobutyric acid in rats in vivo.

Generally, increases in cortical activity go in parallel with negative shifts and decreases with positive shifts of cortical d.c. potentials. The aim of the present investigation was to test the effects of the inhibitory transmitter gamma-aminobutyric acid (GABA) and of GABA receptor agonists on cortical d.c. potentials. Concomitant changes of local pH were measured to get first insights as to the mechanisms of the evoked d.c. changes. The experiments were carried out on anesthetized and artificially ventilated rats. d.c. potentials were recorded at a cortical depth of about 1000 microm by glass microelectrodes. Extracellular pH was measured by ion-selective microelectrodes. GABA (0.1 mol/l), the GABA(A) receptor agonist muscimol (0.1 mmol/l) and the GABA(B) receptor agonist baclofen (0.1 mmol/l) were microejected by pressure pulses at a distance of 20-40 microm from the recording electrode. GABA evoked positive d.c. shifts with low pressure ejection and long application times. With increasing pressure the positive d.c. shifts were initially superimposed by negative ones. The GABA(A) receptor agonist muscimol elicited negative and the GABA(B) receptor agonist baclofen positive displacements of the d.c. potential independent of application time or pressure. The negative d.c. shifts induced by GABA and muscimol were associated with an extracellular alkalization of up to 0.1 pH units. The findings led one to assume (1) that the negative d.c. shift after GABA application was due to a neuronal depolarization and to an increase in excitation via local alkalization and (2) that the positive d.c. shift mirrored neuronal hyperpolarization.

Animals↗

PubMed1997

Effects of n-hexane and its metabolites on cloned voltage-operated neuronal potassium channels.

In order to study the mechanisms of acute n-hexane intoxication, the effects of n-hexane and its metabolites 2-hexanol, methyl-n-butyl ketone, 2,5-hexanediol and 2,5-hexanedione on the cloned voltage-operated potassium channels Kv1.1, Kv1.4, Kv2.1 and Kv3.4 were investigated with electrophysiological techniques in the expression system of Xenopus oocytes. n-Hexane had no effect at any channel, whereas some of its metabolites led to reductions of the potassium currents. The greatest effects obtained were caused by 2-hexanol at the Kv2.1 channel, resulting in reductions of 13% at 0 mV with a concentration of 500 mg/l and IC50 of ca. 3500 mg/l. The reduction appeared to be caused by a shift of the current-voltage relation to the right. Methyl-n-butyl ketone showed smaller effects, whereas 2,5-hexanedione and 2,5-hexandiol were nearly ineffective. Concerning the different potassium channels, the sensitivity to the metabolites differed. The metabolites showed greatest sensitivity towards the Kv2.1 channel and lowest sensitivity towards the Kv3.4 channel. Since the n-hexane metabolite concentrations in the brain during acute n-hexane intoxication are unknown, the relevance of the data is still unclear. The size of the effects and the currently available data on tissue concentration, however, make it more likely that the action of n-hexane and its metabolites on voltage-operated potassium channels is not a major mechanism for acute neurotoxicity.

Animals↗

PubMed1997

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.

Animals↗

PubMed1997

Follicular tissues reduce drug effects on ion channels in oocytes of Xenopus laevis.

The influence of follicular tissues on drug effects on ion channels in Xenopus oocytes was tested by investigating the pharmacological properties of a cloned potassium channel in oocytes with and without follicular tissues. The data show that the efficacy of blocking agents (ranging from metal ions to peptides) is drastically reduced by the follicular tissues (reductions by as much as 90% and increases of the IC50 values up to 30-fold). Furthermore, the time course of the blocking effect was slowed down by the tissues (increases of the t50 values up to 40-fold). The described impairment could be mitigated, but not abolished by partial removal of the follicular tissues (so-called defolliculation, leaving only the vitelline envelope and part of the follicle cells on the oocyte surface). The results indicate that the follicular tissues can induce significant errors in pharmacological measurements on membrane proteins in Xenopus oocytes.

Animals↗

PubMed1997

Effects of Pb2+ on delayed-rectifier potassium channels in acutely isolated hippocampal neurons.

Effects of Pb2+ on delayed-rectifier potassium channels in acutely isolated hippocampal neurons. J. Neurophysiol. 78: 2649-2654, 1997. The effects of Pb2+ on delayed-rectifier potassium currents were studied in acutely isolated hippocampal neurons (CA1 neurons, CA3 neurons, granule cells) from the guinea pig using the patch-clamp technique in the whole cell configuration. Pb2+ in micromolar concentrations decreased the potassium currents in a voltage-dependent manner, which appeared as a shift of the current-voltage relation to positive potentials. The effect was reversible after washing. The concentration-responsiveness measured in CA1 neurons revealed an IC50 value of 30 mu mol/l at a potential of -30 mV. The half-maximal shift of the current-voltage relation was reached at 33 mu mol/l and the maximal obtainable shift was 13.4 mV. For the different types of hippocampal neurons, the shift of the current-voltage relation was distinct and was 7.9 mV in CA1 neurons, 13.7 mV in CA3 neurons, and 14.2 mV in granule cells with 50 micro mol/l Pb2+. The effects described here of Pb2+ on the potassium currents in hippocampal neurons and the differences between the types of hippocampal neurons correspond with the known properties and distributions of cloned potassium channels found in the hippocampus. As a whole, our results demonstrate that Pb2+ in micromolar concentration is a voltage-dependent, reversible blocker of delayed-rectifier potassium currents of hippocampal neurons. This effect has to be taken into consideration as a possible contributing mechanism for the neurological symptoms of enhanced brain activity seen during Pb2+ intoxication.

4-Aminopyridine↗

PubMed1997

Generation of field potentials in the brain.

Neuronal function is closely correlated with bioelectrical signals. These electrical events can be detected as field potentials in the space surrounding cellular elements of the nervous system. The basic mechanisms underlying the generation of field potentials are described.

Animals↗

PubMed1997

Membrane currents elicited by the organic calcium channel blocker verapamil in native and rat brain RNA-injected oocytes of Xenopus laevis.

For further analysis of the action of the diphenylalkylamine verapamil (CAS 152-11-4), the ability of verapamil to elicit membrane currents by itself was investigated in native and rat brain. RNA-injected oocytes of Xenopus laevis. Administration of verapamil elicited inward currents which remained constant or increased slightly during ongoing application. In native and RNA-injected oocytes the current responses were similar in shape, but larger in size in RNA-injected oocytes. The currents increased up to the maximal tested concentration of 1 mmol/l verapamil; the threshold concentration was below 80 mumol/l. After removal of follicular tissues the verapamil response was nearly doubled. During verapamil administration the input resistance was increased up to 1.7 of the initial value. The current response to verapamil can be subdivided into an early and late component. The equilibrium potential of the early component ranged between -80 and -110 mV; the late component which increased slightly during verapamil application, had an equilibrium potential between 0 and -20 mV. Under the influence of potassium channel blockers (tetraethylammonium and cesium chloride) or chloride channel blockers (anthracene-9-carbonic acid and the indanyloxy-acetic acid derivative IAA-94) the verapamil induced currents were reduced. Thus, the results indicate that beside the calcium channel-blocking effect, verapamil can induce currents by itself, presumably by acting on the potassium and chloride leakage.

Animals↗

PubMed1996

Anoxic terminal negative DC-shift in human neocortical slices in vitro.

In animal models, the hallmark of a hypoxic condition is a strong negative shift of the DC potential (anoxic terminal negativity, ATN). This DC-shift is interpreted to be primarily due to a breakdown of the membrane potential of neurons. Such massive neuronal depolarizations have not been reported for all human neocortical neurons in vitro even during prolonged hypoxic periods. This poses the question whether ATN develop also in human neocortical slices made hypoxic. ATN could be observed when human brain slice preparations (n = 15, 13 patients) were subjected to periods of hypoxia (10 to 120 min). These ATN were usually monophasic and appeared with a latency of 16 +/- 4 min (mean +/- S.E.M.). Separating the ATN according to their slopes of rise, steep (> 10 mV/min) and flat (< 10 mV/min) ATN could be distinguished. Steep and flat ATN may be regarded as two different entities of reactions since steep ATN had also greater amplitudes and slopes of decay as compared a flat ATN. With repetitive hypoxias, the latency of both the steep and flat ATN was reduced for the following hypoxic episodes. During hypoxic DC-shifts, evoked potentials were suppressed. With the 1st through 4th hypoxia, they recovered fully within 30 min after reoxygenation when hypoxia was terminated at the plateau of ATN; with extension of hypoxia, recovery was only partial. From the 5th hypoxia onwards, recovery usually did not take place or was not complete.

Adolescent↗

PubMed1996

Simultaneous blockade of intracellular calcium increases and of neuronal epileptiform depolarizations by verapamil.

The specific L-type calcium channel blocker verapamil exerts an antiepileptic effect on neurons. This effect is assumed to depend on the blockade of transmembraneous calcium flux during epileptic discharges. In order to test this hypothesis, fura-dextran loaded snail neurons were rendered epileptic by pentylenetetrazole (40 mmol/l). The effect of verapamil (20 or 40 mumol/l) on free intracellular calcium ([Ca2+]i) transients was investigated by means of fluorescence ratio-imaging and simultaneous intracellular membrane potential recording. During epileptic depolarization [Ca2+]i increased especially in the outermost submembraneous areas of the neuron. [Ca2+]i reached peak values 6-22 s after the onset of epileptic depolarizations. Application of verapamil progressively shortened the epileptic depolarizations. This shortening of epileptic depolarizations developed along with a diminution of the submembraneous calcium signals down to noise level. The effect was found to be reversible. It is concluded that the antiepileptic effect of verapamil depends largely on its ability to block transmembraneous calcium flux.

Animals↗

PubMed1996

The effects of verapamil and flunarizine on epileptiform activity induced by bicuculline and low Mg2+ in neocortical tissue of epileptic and primary non-epileptic patients.

In human neocortical slices the specific L-type calcium channel blocker verapamil had been shown to be antiepileptic in the low Mg(2+)-model of epilepsy. The present investigation demonstrated: (1) verapamil exerted also an antiepileptic effect on epileptiform field potentials (EFP) induced by the GABAA-antagonist bicuculline. (2) The unspecific calcium channel modulator flunarizine, which in contrast to verapamil penetrates the blood-brain barrier, depressed EFP in the low Mg(2+)-model and in the bicuculline model. (3) There was no significant difference in the antiepileptic efficacy of verapamil and flunarizine in epileptic (epilepsy surgery) and primary non-epileptic (tumor surgery) neocortical slices.

Astrocytoma↗

PubMed1996

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.

Animals↗

PubMed1996

Expression and functional characterization of a melatonin-sensitive receptor in Xenopus oocytes.

Melatonin (MEL) plays a central role in the regulation of seasonal cycles and in the control of circadian rhythms in mammals. Functional MEL-sensitive receptors were expressed in Xenopus laevis oocytes following injection of poly (A)+ RNA from rat brain. Administration of 0.1-100 micromol/l MEL to voltage-clamped oocytes (holding potential: -70 mV) elicited oscillatory inward currents (reversal potential: -24 mV) which could be blocked by 9-anthracenecarboxylic acid and caffeine. After preincubation with pertussis toxin (PTX) the MEL response disappeared. The expressed MEL-sensitive receptor probably activates Ca(2+)-dependent chloride currents via a PTX-sensitive G protein and the phosphoinositol pathway.

Animals↗