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

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.

Potassium conductance causing hyperpolarization of CA1 hippocampal neurons during hypoxia

In experiments on slices (from 100- to 150-g Sprague-Dawley rats) kept at 33 degreesC, we studied the effects of brief hypoxia (2-3 min) on CA1 neurons. In whole cell recordings from submerged slices, with electrodes containing only KMeSO4 and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, and in the presence of kynurenate and bicuculline (to minimize transmitter actions), hypoxia produced the following changes: under current clamp, 36 cells were hyperpolarized by 2.7 +/- 0.5 (SE) mV and their input resistance (Rin) fell by 23 +/- 2.7%; in 30 cells under voltage clamp, membrane current increased by 114 +/- 22.3 pA and input conductance (Gin) by 4.9 +/- 0.9 nS. These effects are much greater than those seen previously with K gluconate whole cell electrodes, but only half those seen with "sharp" electrodes. The hypoxic hyperpolarizations (or outward currents) were not reduced by intracellular ATP (1-5 mM) or bath-applied glyburide (10 microM): therefore they are unlikely to be mediated by conventional ATP-sensitive K channels. On the other hand, their depression by internally applied ethylene glycol-bis-(beta-aminoethyl ether)-N,N, N',N'-tetraacetic acid (1.1 and 11 mM) and especially 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (11-33 mM) indicated a significant involvement of Ca-dependent K (KCa) channels. The beta-adrenergic agonist isoprenaline (10 microM) reduced hypoxic hyperpolarizations and decreases in Rin (n = 4) (and in another 11 cells corresponding changes in Gin); and comparable but more variable effects were produced by internally applied 3':5'-adenosine cyclic monophosphate (cAMP, 1 mM, n = 6) and bath-applied 8-bromo-cAMP (n = 8). Thus afterhyperpolarization-type KCa channels probably take part in the hypoxic response. A major involvement of G proteins is indicated by the near total suppression of the hypoxic response by guanosine 5'-O-(3-thiotriphosphate) (0. 1-0.3 mM, n = 23) and especially guanosine 5'-O-(2-thiodiphosphate) (0.3 mM, n = 26), both applied internally. The adenosine antagonist 8-(p-sulfophenyl)theophylline (10-50 microM) significantly reduced hypoxic hyperpolarizations and outward currents in whole cell recordings (with KMeSO4 electrodes) from submerged slices but not in intracellular recordings (with KCl electrodes) from slices kept at gas/saline interface. In further intracellular recordings, antagonists of gamma-aminobutyric acid-B or serotonin receptors also had no clear effect. In conclusion, these G-protein-dependent hyperpolarizing changes produced in CA1 neurons by hypoxia are probably initiated by Ca2+ release from internal stores stimulated by enhanced glycolysis and a variable synergistic action of adenosine.

Effect of potassium channel openers on the firing rate of hippocampal pyramidal cells and A10 dopaminergic neurons in vitro

The effect of four KATP channel openers (KCOs) on the firing rate of CA1 pyramidal cells and A10 dopaminergic neurons was investigated using extracellular recording techniques in rat brain slices. Pinacidil, lemakalim, diazoxide and a new compound, BPDZ44, had an inhibitory effect on the electrical activity of CA1 pyramidal cells. They all had a similar potency. Only BPDZ44 and diazoxide inhibited the firing rate of A10 dopamine neurons. The sulfonylurea glipizide (1 microM) antagonized the effect of BPDZ44 and diazoxide on A10 neurons but failed to antagonize the effect of KCOs on CA1 pyramidal cells. These results show that differences exist among KCOs in their ability to decrease the electrical activity of various populations of central neurons.

Attenuation of the locomotor activating effects of D-amphetamine, cocaine, and scopolamine by potassium channel modulators

1. Locomotor activating effects of D-amphetamine, cocaine, and scopolamine were determined alone and after pretreatment with K-channel modulators in mice. 2. When administered alone, D-amphetamine (1.0- 30 mg/kg) and cocaine (3.0- 56 mg/kg) produced inverted U-shaped dose-effect curves characteristic of psychomotor stimulant drugs. 3. When administered alone, scopolamine (3.0-56 mg/kg) also produced dose-dependent increases in locomotor activity but these effects plateaued with similar increases in locomotor activity induced by 10-56 mg/kg of scopolamine. 4. Pretreatment with the K-channel blockers 4-aminopyridine (0.3-1.7 mg/kg), quinine (30-100 mg/kg) or apamin (0.3-1.0 mg/kg) attenuated the locomotor increases induced by d-amphetamine, cocaine, and scopolamine. 5. Like the K-channel blockers, pretreatment with the K-channel openers cromakalim (1.0-3.0 mg/kg) and pinacidil (3.0-10 mg/kg) also attenuated the locomotor increases induced by D-amphetamine and scopolamine but did not modify the locomotor activating effects of cocaine. 6. These results demonstrate that K-channel modulation modifies the effects of D-amphetamine, cocaine, and scopolamine. 7. The results also demonstrate that K-channel openers can differentially alter the behavioral effects of cocaine and D-amphetamine.

Actions of diazoxide on CA1 neurons in hippocampal slices from rats

Membrane effects of diazoxide (DZX) were examined in CA1 pyramidal neurons, mainly by whole-cell recording in slices kept at 33 degrees C (from Sprague-Dawley rats). Bath applications of DZX (0.65 mM) did not significantly change the resting input conductance; but instantaneous inward rectification was reduced by 47 +/- 14% (near -110 mV). There was a similar depression of a large, sustained voltage-dependent outward current (by 44 +/- 11% near 0 mV). A nearly identical reduction of the outward current recorded in a Ca current suppressing medium (but not in 30 mM tetraethylammonium) indicated that the DZX-sensitive current includes the delayed rectifier. In Mn, low-Ca medium containing tetraethylammonium and carbachol, DZX potentiated (by 43 +/- 12%) the D-type slowly decaying outward current seen after hyperpolarizing pulses at a holding potential of approximately -50 mV. DZX abolished or depressed slow inward currents, such as the tetrodotoxin-sensitive persistent Na current, high voltage activated Ca currents (IC50 = 0.47 mM), and the Q current. In 6 of 13 cells recorded with electrodes containing either guanosine or adenosine diphosphate, DZX potentiated the voltage-dependent outward current, but input conductances were reduced. In conclusion, although there was little indication that it activates classical KATP channels in CA1 neurons, DZX strongly depresses several voltage-dependent, slowly inactivating outward and inward currents, which are important modulators of cell excitability.