Cromakalim (BRL 34915) counteracts the epileptiform activity elicited by diltiazem and verapamil in rats

The anticonvulsant retigabine potently suppresses epileptiform discharges in the low Ca ++ and low Mg++ model in the hippocampal slice preparation

Retigabine (N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester, D-23129) is a broad acting anticonvulsant currently undergoing phase II clinical trials. An opening effect on leakage conductance K+ channels, potentiation of GABA induced currents and a weak blocking effect on Na+ and Ca++ channels were previously reported. The goal of this study was to investigate whether retigabine is capable of blocking epileptiform discharges in the low Ca++ and low Mg++ model in the hippocampal slice preparations and whether the anti-burst activity can be related to the K+ channel opening effect. In the low Ca++ model, synaptic transmission is blocked and discharges evolve from ephaptically-coupled neurons. Compounds which directly interfere with the threshold for action potential induction via alteration of ion channel function (i.e. Na+ channel blocker) may alter the discharges, while compounds interfering with synaptic transmission are not active. Retigabine suppressed the discharges in a concentration-dependent manner. A significant reduction in frequency without effect on amplitude was observed after application of 1 microM, and a full block of all discharges after application of 25 microM. The opener of the ATP sensitive K+ channels cromakalim was also active. Application of 300 microM cromakalim yielded to a lower frequency with no effects on the amplitude of discharges. Treatment with phenytoin and carbamazepine resulted in a marked reduction in amplitude accompanied by a rise in frequency; only at higher concentrations was a full block observed. The effect of retigabine therefore differs from sodium channel blockers and can be related to the K+ channel opening effect. In the low Mg++ model, excitatory neurotransmission is augmented by reducing the Mg++ block of NMDA channels. This results in development of interictal-like epileptiform activity in area CA1 in isolated hippocampal slices. Treatment with retigabine 10 microM resulted in a significant reduction of the discharges, and discharges were fully blocked after application of 25 microM. Qualitatively similar effects were observed with cromakalim and valproate, albeit at higher concentrations. The data indicate that retigabine exerts potent broad spectrum activity making it an interesting candidate for treatment of drug resistant patients.

Characterization of the K+ channel opening effect of the anticonvulsant retigabine in PC12 cells

Retigabine (D-23129) is a new anticonvulsant compound which acts as a K+ channel opener in neuronal cells. The aim of the present study was to further characterize the retigabine induced K+ current. In nerve growth factor treated PC12 cells and in rat cortical neurones the application of retigabine activated a K+ current. In contrast, however, no K+ current activation was observed in untreated PC12 and in glial cells which were cultivated together with the neuronal cells. To characterise the retigabine activated K+ current, K+ channel blockers were used. The retigabine induced current was not affected by 1 and 10 mM 4-aminopyridine (4AP). Ba2+ 1 mM resulted in a reduction of 88.6+/-3.0% (n = 5); 10 mM abolished the current. Tetraetylamonium (TEA), 1 and 10 mM, reduced the current by 23.6+/-3.1 and 61.6+/-3.7%, respectively. To investigate the current/voltage (I/V) relation of the current initiated by retigabine (10 microM), cells were clamped to a holding potential of -80 mV and a ramp stimulation protocol (-120 to +60 mV in 5 s) was applied prior to and during application of retigabine. Subtraction of the two traces yielded the current induced by retigabine. A nearly linear relationship was determined between - 120 and -40 mV. At potentials positive to - 40 mV, the response was variable. This was due to the additionally observed weak blocking effect of retigabine on delayed rectifier (Kdr) currents. If the ramp was applied in the presence of 10 mM 4AP to block Kdr, a nearly linear I/V-relationship was present from -120 to +60 mV. The comparison of the I/V relation and pharmacology with published K+ channel subtypes gives evidence that an unknown neuronal K+ channel subtype may be involved.

The new anticonvulsant retigabine (D-23129) acts as an opener of K+ channels in neuronal cells

The patch-clamp technique was used to measure currents passing through K+ channels in neuronal cell preparations. Retigabine (D-23129, N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester) activated a K+ conductance in slightly depolarized NG108-15 neuronal cells in a dose-dependent manner (0.1-10 microM). At the K+ reversal potential, no current could be elicited and in hyperpolarized cells the current was reversed. A similar current was elicited in primary cultures of mouse cortical neurones and in differentiated hNT cells, a cell line derived from human neuronal cells. At higher concentrations, retigabine also partially blocked voltage activated K+ currents. None of the tested anticonvulsants, phenytoin, carbamazepine and valproate and none of the K+ channel openers cromakalim, diazoxide and pinacidil exerted a similar effect. The current was not affected by the K+ channel blocker glibenclamide (10 microM) but was fully blocked by application of Ba2+ (10.8 mM). Exchange of K+ with cesium in the intracellular space also fully abolished the current. It can be expected that the K+ channel opening effect contributes to the anticonvulsant activity of retigabine.

Anti- and proconvulsive actions of levcromakalim, an opener of ATP-sensitive K+ channel, in the model of hippocampus-generating partial seizures in rats

We assessed the effect of an opener of ATP-sensitive K+ channel, levcromakalim (BRL 38227, (-)6-cyano-3,4-dihydro-2, 2-dimethyl-trans-4-(2-oxo-1-pyrrolidyl)-2H-1-benzopyran-3-ol) on seizure threshold and severity of the hippocampus-generating partial seizures in rats. For comparison, an ATP-sensitive K+ channel blocker, glibenclamide; K+ channel blocker, tetraethylammonium; Ca2+ channel antagonist, nimodipine and Ca2+ channel agonist, (+/-)-BAY K 8644 (1,4-dihydro-2, 6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridinecarboxyli c acid methyl ester) were also examined. Seizure threshold was determined using pulse number threshold and seizure severity was ascertained using afterdischarge duration. Levcromakalim decreased afterdischarge duration at 10 nmol i.c.v. and decreased pulse number threshold at 100 nmol i.c.v. Tetraethylammonium at 10 nmol i.c.v. increased afterdischarge duration selectively and at 100 nmol i.c.v. induced spontaneous seizures. Glibenclamide (1-100 nmol i.c.v.) failed to change pulse number threshold and afterdischarge duration. Nimodipine (40 mg/kg i.p.) decreased afterdischarge duration and pulse number threshold. BAY K 8644 (1 mg/kg i.p.) decreased pulse number threshold and increased afterdischarge duration. In addition, voltage-clamp recording from neuroblastoma x glioma hybrid cells indicates that levcromakalim inhibited the fast component of Ca(2+)-dependent K+ currents, in addition to the inhibition of T- and L-types of voltage-dependent Ca2+ currents reported (Ito et al., FEBS Lett. 262, 313, 1990). These results suggest that levcromakalim shows anti- and proconvulsive actions in the hippocampus-generating partial seizures in rats and these effects might be, at least partly, caused by inhibiting Ca2+ channel and Ca(2+)-dependent K+ channel, respectively.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Actions of cromakalim on outward currents of CA1 neurones in hippocampal slices

1. Membrane effects of cromakalim (Crom; 50-300 microM) were examined in CA1 neurones recorded mainly by intracellular, single-electrode voltage-clamping in slices (from Sprague-Dawley rats) kept in an interface chamber at 33 degrees C. 2. In 14 cells held at -63 +/- 3.5 mV, in the presence of tetrodotoxin, kynurenic acid and (in most cases) bicuculline, bath applied Crom produced no consistent change in holding current (-59 +/- 66 pA) or input conductance (GN) (-3.9 +/- 5.2%). 3. Overall there were no significant changes in instantaneous inward rectification or in Q-current inward relaxations. 4. In 18 out of 22 cells, outward currents, evoked by 0.5 s pulses to voltages > -50 and < -20 mV, were depressed by Crom (by 42 +/- 11%, for n = 22). Because this effect was consistently seen in Ca current-blocking media, containing either Mn and low Ca, or Cd (and also carbachol), the K channels depressed by Crom were probably of the delayed rectifier (IDR) type. 5. The Crom-control difference current (ICrom), obtained with slow depolarizing ramps, had a biphasic character, inward in the voltage (V) range > -50 < -20 mV (where outward currents are depressed by Crom) and tending outward for V > or = -20 mV. 6. In 10 out of 11 cells, Crom potentiated a D-like, slowly-inactivating outward current (by 88 +/- 31%, for n = 11). 7 The effects of Crom and of 2 min periods of anoxia were compared in 12 cells: unlike anoxia, Cromproduced no consistent increases in GN; the currents evoked in the same cells by anoxia differed significantly from those evoked by Crom (by 150 +/- 60 pA); the directions of current changes induced byCrom and anoxia respectively were not significantly correlated. Crom strongly depressed anoxic outward currents (by 80 +/- 12%, n = 4).8 Some Crom-induced effects (increases in D-like current and the outward current elicited at V>- 20 mV) were always reversed by tolbutamide (1 mM), but much less consistently by glibenclamide(10-30 microM).9 In conclusion, the effects of Crom, recorded with intracellular electrodes in CA1 neurones in slices,show little resemblance to the effects of activation of ATP-sensitive K channels.

Effects of adenosinergic drugs on hypoxia-induced electrophysiological changes in rat hippocampal slices

The effects of adenosinergic antagonists caffeine and DPCPX, and of the adenosinergic agonists L-PIA, CPA and CGS 21680 were investigated on fully and partially reversible hypoxia-induced electrophysiological changes in rat hippocampal slices. The influence of a high potassium solution and of the N-methyl-D-aspartate antagonist dizocilpine (MK 801) was also tested. The latency to obtain a 50% decrease in the amplitude of the CA1 population spike (CA1 PS) during a short- (5-10 min) lasting hypoxic period was significantly increased (P less than 0.01) by slice perfusion with caffeine (50 microM), DPCPX (0.2 microM), and by increasing (from 3 to 4 mM) the potassium concentration in the medium bathing the hippocampal slices. The latency was significantly decreased (P less than 0.01) by slice perfusion with L-PIA (0.2 microM) and CPA (0.05 microM). It was not significantly modified by CGS 21680 (5 microM). The incidence of reappearance of the CA1 PS during reoxygenation after long- (45 min) lasting hypoxia was significantly increased (P less than 0.05) by slice perfusion with MK 801 (50 microM), while it was not significantly affected by slice perfusion with caffeine (50 microM) or DPCPX (0.2 microM) or L-PIA (0.2 microM) or CPA (0.05 microM) or CGS 21680 (5 microM). The results indicate a prevalent involvement of the A1 adenosine receptors in the early mechanisms underlying hypoxia-induced reversible changes. Adenosine seems to have a limited role in the late mechanisms occurring after a long-lasting hypoxic period.