Simulated ischaemia induces Ca2+-independent glutamatergic vesicle release through actin filament depolymerization in area CA1 of the hippocampus

Transient, non-catastrophic brain ischaemia can induce either a protected state against subsequent episodes of ischaemia (ischaemic preconditioning) or delayed, selective neuronal death. Altered glutamatergic signalling and altered Ca(2+) homeostasis have been implicated in both processes. Here we use simultaneous patch-clamp recording and Ca(2+) imaging to monitor early changes in glutamate release and cytoplasmic [Ca(2+)] ([Ca(2+)](c)) in an in vitro slice model of hippocampal ischaemia. In slices loaded with the Ca(2+)-sensitive dye Fura-2, ischaemia leads to an early increase in [Ca(2+)](c) that precedes the severe ischaemic depolarization (ID) associated with pan necrosis. The early increase in [Ca(2+)](c) is mediated by influx through the plasma membrane and release from internal stores, and parallels an early increase in vesicular glutamate release that manifests as a fourfold increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs). However, the increase in mEPSC frequency is not prevented by blocking the increase in [Ca(2+)](c), and the early rise in [Ca(2+)](c) is not affected by blocking ionotropic and metabotropic glutamate receptors. Thus, the increase in [Ca(2+)](c) and the increase in glutamate release are independent of each other. Stabilizing actin filaments with jaspamide or phalloidin prevented vesicle release induced by ischaemia. Our results identify several early cellular cascades triggered by ischaemia: Ca(2+) influx, Ca(2+) release from intracellular stores, actin filament depolymerization, and vesicular release of glutamate that depends on actin dynamics but not [Ca(2+)](c). All of these processes precede the catastrophic ID by several minutes, and thus represent potential target mechanisms to influence the outcome of an ischaemic episode.

Dantrolene modulates the influence of steady magnetic fields on hippocampal evoked potentials in vitro

Direct current-generated magnetic fields (2-3 mT, 20-min exposure) exerted biphasic effects on the population spike recorded from hippocampal slices. The initial decrease in the potential, observed during exposure of the slices to magnetic fields was followed by a recovery/amplification phase, which began after terminating the magnetic field action. During that phase the population spike exceeded the amplitude observed before application of the magnetic fields. The pattern of magnetic fields influence was not affected either by (+)-5-methyl-10,11-dihydro-5H-dibenzo (a,d) cyclohepten-5, 10-imine maleate (MK801), or by D,L,-2amino-5phosphonovalerate (APV), a noncompetitive and competitive NMDA receptor antagonist, respectively. The rising phase of the potential, however, was eliminated by dantrolene, an inhibitor of intracellular Ca(2 +) channels. This suggests that intracellular calcium channels participate in the mechanism of the influence of the direct current magnetic fields on the function of the hippocampal tissue.

Early postischemic dantrolene-induced amelioration of poly(ADP-ribose) polymerase-related bioenergetic failure in neonatal rat brain slices

In the infant brain, ischemia-induced ionic and enzyme mechanisms may independently lead to cell death by energy depletion: resequestration of calcium mobilized from intracellular stores consumes ATP, and activated poly(ADP-ribose) polymerase (PARP) uses oxidized nicotinamide adenine dinucleotide to form polyADP-ribosyl nuclear proteins associated with DNA damage. Using 31P nuclear magnetic resonance spectroscopy, we have monitored intracellular pH and cellular energy metabolites in ex vivo neonatal rat cerebral cortex before, during, and after substrate and oxygen deprivation. In an insult that exhibited secondary energy failure and apoptosis we identified a relative 25% augmentation of high-energy phosphates at the end of recovery when the ryanodine-receptor antagonist, dantrolene, was introduced in the early (0- to 40-minute) but not late (40- to 120-minute) stage of recovery (P < 0.05). In contrast to the absence of a late dantrolene-sensitive effect, inhibition of PARP with 3-methoxybenzamide was as effective (P < 0.05) as early dantrolene, even when introduced after a 40-minute delay. The dantrolene and 3-methoxybenzamide effects on high-energy phosphates were not additive, rather the early dantrolene-sensitive effect nullified the potential 3-methoxybenzamide effect. Therefore, in this vascular-independent neonatal preparation, postischemic mobilization of calcium from intracellular stores is associated with PARP-related energy depletion. Inhibition of either of these processes confers improved postischemic bioenergetic recovery in the developing brain.

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.

Changes in [Ca2+]i and membrane currents during impaired mitochondrial metabolism in dissociated rat hippocampal neurons

1. This study was designed to establish the basis for altered membrane excitability during the inhibition of mitochondrial metabolism in central mammalian neurons. Perforated whole-cell patch clamp and fluorimetric techniques were combined to examine changes in membrane currents, intracellular calcium ([Ca2+]i) and mitochondrial potential (DeltaPsim) in neurons dissociated from the CA1 subfield of the hippocampus of young rats. 2. On application of the mitochondrial inhibitor NaCN, or the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), the membrane potential hyperpolarized and membrane conductance increased. Under voltage clamp, an outward current was seen. The reversal potential of the current at -83 mV and its dependence on extracellular [K+] confirmed that this was a K+ conductance. 3. Simultaneous recordings of [Ca2+]i and current showed a striking correlation between a rise in [Ca2+]i and the developed outward current. Flash photolysis of the caged Ca2+ chelator, diazo-2, reversed both the rise in [Ca2+]i and the outward current. The current was reduced by 80 % by charybdotoxin, was attenuated by 10 mM TEA+ but was unaffected by apamin or by the KATP channel blocker tolbutamide (400 microM-1 mM). These data suggest strongly that the current is carried by Ca2+-dependent K+ channels. 4. Simultaneous recordings of membrane current, DeltaPsim and [Ca2+]i revealed the sequence of events in response to impaired mitochondrial function (CN, FCCP or anoxia): DeltaPsim depolarized, followed rapidly by an increase in [Ca2+]i followed in turn by the outward current. [Ca2+]i and membrane current recovered only after mitochondrial repolarization. 5. The rise in [Ca2+]i appeared to result from an increased Ca2+ influx through voltage-gated Ca2+ channels. It was dependent on extracellular Ca2+ and was much reduced by methoxyverapamil (D600). The rate of Mn2+ quench of fura-2 fluorescence was increased by the inhibitors, and the inhibitors induced a small inward current when K+ channels were blocked that preceded the rise in [Ca2+]i. However, the increase in [Ca2+]i showed no obvious dependence on membrane potential in cells clamped at a range of holding potentials from -90 to -45 mV. 6. Thus, removal of oxygen, uncoupling mitochondrial oxidative phosphorylation or inhibition of respiration, all lead to mitochondrial depolarization, an increased Ca2+ influx through (voltage-gated) channels, even at hyperpolarized membrane potentials, raising [Ca2+]i which in turn drives an increased K+ conductance that modulates membrane excitability.

Tolbutamide blocks Ca(2+)- and voltage-dependent K+ currents of hippocampal Ca1 neurons

In current-clamp recordings with KMeSO4 electrodes (either whole-cell or intracellular), though tolbutamide (0.5-1 mM) did not change the resting potential, it increased both input resistance (by 12 +/- 3.8%) and spontaneous firing, and spikes were evoked by smaller depolarizing pulses. Tolbutamide reduced in a dose-dependent manner both components of post-burst afterhyperpolarizations: IC50 was 0.15 mM for medium afterhyperpolarizations and 0.33 mM for slow afterhyperpolarizations. In whole-cell recordings under voltage-clamp, 0.5-1 mM tolbutamide depressed slow outward currents by 65 +/- 5.3%. The tolbutamide-sensitive current was Ca(2+)-dependent-tolbutamide being ineffective in Mn2+, low Ca(2+)-containing medium-though tolbutamide did not significantly depress high voltage-activated Ca2+ currents. Tolbutamide reduced C-type outward currents by 45 +/- 5.9% and M-type current inward relaxations by 41 +/- 12.9%, as well as Q-type current inward relaxations by 22 +/- 5.7%. Glyburide (10 microM) did not depress afterhyperpolarizations or outward currents, even in recordings with electrodes containing 1 mM guanosine diphosphate. We conclude that the most prominent effects of 0.5-1 mM tolbutamide on CA1 neurons are caused by suppression of Ca(2+)-and voltage-dependent outward currents, including IAHP, IC and IM.