Ulrich Mußhoff

Intrinsic Excitability, Synaptic Potentials, and Short-Term Plasticity in Human Epileptic Neocortex

Although studies of epileptic human hippocampus suggest changes of synaptic and intrinsic excitability, few changes, save the appearance of spontaneous field/synaptic potentials, are known in epileptic neocortical tissue. However, invasive EEG and histological studies suggest that neocortical tissue, even in mesial temporal lobe epilepsy, can play an important role as an irritative zone or extrahippocampal focus. We hypothesized that intrinsic neuronal and synaptic excitability, as well as short‐term plasticity, are altered in neocortical areas, particularly with elevated K+ levels as occur during seizures. We analyzed neuronal firing properties, synaptic responses, and paired‐pulse plasticity in human neocortical slices from tissue resected during epilepsy surgery, both under normal and under pathological conditions, i.e., after elevating K+ (4/8 mM), with rat neocortical slices as controls. Neuronal firing properties were not different. We did find, however, alterations of synaptic responsiveness in epileptic tissue, i.e., an elevated network excitability with K+ elevations, and reduction of paired‐pulse depression.

Sensitivity of native and cloned hippocampal delayed-rectifier potassium channels to verapamil

The effects of the phenylalkylamine verapamil on native and cloned hippocampal voltage-operated potassium channels were investigated. Native channels were studied in acutely isolated CA1 neurons from the guinea pig with the whole-cell patch-clamp technique. Cloned channels were expressed in oocytes of Xenopus laevis and studied with the two-electrode voltage-clamp technique. Native potassium channels: Verapamil suppressed the potassium currents in micro- and submicromolar concentrations. The current suppression increased during the voltage step. The IC50 value of verapamil was 3 micromol/l and the Hill coefficient was 0.5 indicating a mixed population of potassium channels with distinct verapamil sensitivity. Cloned potassium channels: The hippocampal potassium channels Kv1.1, Kv1.2, Kv1.3, Kv2.1, Kv3.1 and Kv3.2 were affected by verapamil in micromolar concentrations. The effect increased with depolarization time, was voltage-dependent, reached 90% of the maximum within around 40 s after start of verapamil application, recovered slowly after wash-out and did not reach control values even after wash-out times of six minutes. The IC50 values differed markedly and were 35 micromol/l for the Kv1.1 channel, 98 micromol/l for the Kv1.2 channel, 12 micromol/l for the Kv1.3 channel, 226 micromol/l for the Kv2.1 channel, 6 micromol/l for the Kv3.1 channel and 11 micromol/l for the Kv3.2 channel.

Tunicamycin-induced inhibition of functional expression of glutamate receptors in Xenopus oocytes

The effects of tunicamycin, a specific inhibitor of N-linked glycosylation, on functional expression of glutamate receptor subtypes were investigated in RNA-injected oocytes. In the presence of tunicamycin the expression of ligand-operated receptors sensitive to kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and quisqualate were completely blocked. The inhibitory effect was reversible after removal of tunicamycin from the culture medium.

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.

Effects of lead on cloned voltage-operated neuronal potassium channels

The action of lead (Pb 2+) on cloned voltage-operated potassium channels of the rat brain was investigated in oocytes of Xenopus laevis. Pb2+ was found to decrease the potassium currents. This effect was due to a shift of the current-voltage relation in a positive direction (up to 30 mV). The Pb2+ effect appeared at a threshold concentration of about 0.1 μmol/l and was maximal at a concentration of about 30 μmol/l. At a potential of − 30 mV, the concentration needed for a 50% reduction of the potassium current was 1.0 μmol/l. The depressant effect of Pb2+ was obtained with all potassium channels tested (Kv1.1, Kv1.2, Kv1.4, Kv2.1, Kv3.4). It was minimal for the Kv2.1 channel and maximal for the Kv1.1 channel at potentials negative to 0 mV. An effect comparable with that of Pb2+ could not be induced by the application of magnesium or calcium. The external application of Pb2+ led to a decrease of potassium currents in outside-out but not in inside-out membrane patches. Overall, Pb2+ had a significant effect on the potassium channels which may contribute to the mechanisms of Pb2+ neurotoxicity.

Lead-induced blockade of kainate-sensitive receptor channels

The effects of bivalent lead on ion channels activated by kainate and α-amino-3-hydroxy-5-methyl-4-isoxazolpropionate (AMPA) were studied using Xenopus oocytes microinjected with mRNA from rat brain. Lead reduced kainate-induced membrane currents in a reversible and dose-dependent manner, without affecting membrane currents induced by AMPA. Lead decreased the kainate currents with a concentration of 0.1 μmol/l to 0.93 ± 0.01 and with a concentration of 100 μmol/l to 0.41 ± 0.04 of the control values. The blocking effect of lead on kainate responses was voltage dependent. The inhibition was strongest at - 90 mV to - 70 mV and became weaker at more positive membrane potentials. The effect of lead on the kainate-induced membrane currents remained unchanged when the concentration of kainate was increased. Hence lead probably represents a noncompetitive channel-blocking agent for non-N-methyl-d-aspartate (NMDA) receptor channels activated by kainate.

Activation of ATP-sensitive potassium channels in follicle-enclosedXenopus oocytes by the epileptogenic agent pentylenetetrazol

For further investigation of the epileptogenic properties of pentylenetetrazol (PTZ), membrane currents elicited by PTZ were analysed in nativeXenopus oocytes. PTZ elicited a sequence of membrane currents. Two inward currents have been described to be due to a decrease in potassium permeability and an increase in chloride permeability. Experiments performed up to 3 days after preparation of the oocytes showed that PTZ is also able to activate an outward current. This current is: (1) reversed near the potassium equilibrium potential, (2) associated with a decrease in membrane resistance, (3) reduced by tetraethylammonium and caesium, (4) abolished by defolliculation, and (5) blocked by glibenclamide. Thus, the current can be interpreted to be due to an activation of ATP-sensitive potassium (KATP) channels located in the follicle cells. An activation of KATP channels by PTZ may contribute to termination and re-initiation of seizure activity.

Decrease and increase of responses to glutamate receptor agonists in RNA-injected Xenopus oocytes by the epileptogenic agent pentylenetetrazol: Dependence on the agonist concentration

The effects of the epileptogenic agent pentylenetetrazol (PTZ) on current responses to glutamate (Glu) and to the Glu receptor agonist N-methyl-D-aspartate (NMDA), kainate (KA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and quisqualate (QA) were studied on RNA-injected Xenopus oocytes. PTZ antagonized the reactions to Glu as well as to NMDA, KA, AMPA and QA. With increasing concentration of AMPA, however, the depressive effect of PTZ turned to an augmentation. The complete change from a decreasing to an increasing effect of PTZ with elevated agonist concentration was unique for current responses by AMPA.

Suppression of a ligand operated membrane current by the epileptogenic agent pentylenetetrazol in oocytes of Xenopus laevis after injection of rat brain RNA

The effects of the epileptogenic substance pentylenetetrazol (PTZ) on ligand operated membrane channels were studied. For this purpose serotonin (5-HT) sensitive channels were expressed in RNA injected oocytes of Xenopus laevis. With simultaneous application of both substances, the response to 5-HT was reduced and eventually abolished by PTZ with increasing concentrations (5-100 mM). A reduction of the 5-HT response also appeared when PTZ was applied in various intervals (15 and 240 s) before 5-HT. It may be assumed that PTZ produces a component of its epileptogenic effect by acting on ligand operated membrane channels.