Interaction of fMet-tRNA(fMet) with the C-terminal domain of translational initiation factor IF2 from Bacillus stearothermophilus

Analytical ultracentrifugation studies indicated that the C-terminal domains of IF2 comprising amino acid residues 520-741 (IF2 C) and 632-741 (IF2 C-2) bind fMet-tRNA with similar affinities (K(d) at 25 degrees C equal to 0.27 and 0.23 microM, respectively). Complex formation between fMet-tRNA(fMet) and IF2 C or IF2 C-2 is accompanied by barely detectable spectral changes as demonstrated by a comparison of the Raman spectra of the complexes with the calculated sum of the spectra of the individual components. These results and the temperature dependence of the K(d) of the protein-RNA complexes indicate that complex formation is not accompanied by obvious conformational changes of the components, and possibly depends on a rather small binding site comprising only a few interacting residues of both components.

Dopamine depresses polysynaptic inhibition in rat subicular neurons

Schizophrenia is considered to be associated with a hyperfunction of the dopaminergic system and with abnormalities in hippocampal information processing. To clarify whether an enhanced dopaminergic activity alters the hippocampal output, the effect of dopamine (DA) on inhibitory postsynaptic responses (IPSPs) in subicular neurons was examined. DA (200 microM) induced a small and inconsistent hyperpolarization that was accompanied by a reduction of membrane resistance. DA decreased polysynaptic IPSPs which was paralleled by a depression of isolated AMPA/kainate and NMDA receptor-mediated excitatory postsynaptic responses (EPSPs). In contrast, DA had no effect on isolated monosynaptic GABA(A) and GABA(B) receptor-mediated IPSP/Cs. We conclude that in addition to membrane effects, DA decreases polysynaptic IPSPs by attenuating the glutamatergic drive onto subicular interneurons.

Thermal stability and atomic-resolution crystal structure of the Bacillus caldolyticus cold shock protein

The bacterial cold shock proteins are small compact beta-barrel proteins without disulfide bonds, cis-proline residues or tightly bound cofactors. Bc-Csp, the cold shock protein from the thermophile Bacillus caldolyticus shows a twofold increase in the free energy of stabilization relative to its homolog Bs-CspB from the mesophile Bacillus subtilis, although the two proteins differ by only 12 out of 67 amino acid residues. This pair of cold shock proteins thus represents a good system to study the atomic determinants of protein thermostability. Bs-CspB and Bc-Csp both unfold reversibly in cooperative transitions with T(M) values of 49.0 degrees C and 77.3 degrees C, respectively, at pH 7.0. Addition of 0.5 M salt stabilizes Bs-CspB but destabilizes Bc-Csp. To understand these differences at the structural level, the crystal structure of Bc-Csp was determined at 1.17 A resolution and refined to R=12.5% (R(free)=17.9%). The molecular structures of Bc-Csp and Bs-CspB are virtually identical in the central beta-sheet and in the binding region for nucleic acids. Significant differences are found in the distribution of surface charges including a sodium ion binding site present in Bc-Csp, which was not observed in the crystal structure of the Bs-CspB. Electrostatic interactions are overall favorable for Bc-Csp, but unfavorable for Bs-CspB. They provide the major source for the increased thermostability of Bc-Csp. This can be explained based on the atomic-resolution crystal structure of Bc-Csp. It identifies a number of potentially stabilizing ionic interactions including a cation-binding site and reveals significant changes in the electrostatic surface potential.

Effects of standard anticonvulsant drugs on different patterns of epileptiform discharges induced by 4-aminopyridine in combined entorhinal cortex-hippocampal slices

Application of 4-aminopyridine (4-AP) has previously been reported to produce different patterns of epileptiform discharges in entorhinal cortex (EC)-hippocampal slices: recurrent short discharges (RSDs) in hippocampal area CA1, seizure-like events (SLEs) and negative-going potentials (NGPs) in the medial entorhinal cortex (mEC). Using recordings of field potentials, we investigated the pharmacological effects of the clinically employed standard anticonvulsant drugs phenytoin (PHT), carbamazepine (CBZ), valproic acid (VPA) and phenobarbital (PHB) and those of pentobarbital (PB) on 4-AP-induced epileptiform activity. The anticonvulsant drugs showed different effects: SLEs were completely blocked by all tested drugs. Valproic acid, which suppressed all epileptiform activities, seemed to have the most fundamental effect of all drugs on 4-AP induced activity, because under phenytoin and carbamazepine, some epileptiform activity was still observable. The RSDs in hippocampal area CA1 of the hippocampus did not respond to the different anticonvulsants. In contrast, PB decreased the frequency of the RSDs in CA1 and enhanced the frequency of the NGPs in the EC. We propose that the activities induced by 4-AP in the combined entorhinal cortex-hippocampal slices may provide an in vitro model for the development of new drugs against difficult-to-treat focal epilepsy.

Hyperpolarization-activated cation currents in stellate and pyramidal neurons of rat entorhinal cortex

Properties of hyperpolarization-activated cation currents (I(h)) were investigated in neurons of juvenile rat entorhinal cortex using the patch-clamp technique. A rat brain slice preparation containing the entorhinal cortex was used for whole-cell recordings of I(h) in pyramidal cells from layer IV and in stellate cells from layer II of the entorhinal cortex. In both stellate and pyramidal cells, I(h) activated at potentials more negative than -60 mV and did not show any time-dependent inactivation. Half-maximal activation of I(h) was achieved at -95.3 mV in pyramidal cells and at -95.0 mV in stellate cells. The channels were permeable for sodium and potassium ions. I(h) of pyramidal and stellate neurons was reduced by about 50% in the presence of 100 microM ZD7288. Extracellularly applied 1 mM Cs(+) decreased I(h) of pyramidal cells by 92%, whereas I(h) of stellate cells was only reduced by 70%. In both pyramidal and stellate neurons, I(h) was not significantly changed during the application of 1 mM Ba(2+). 8-Bromo-c-AMP increased amplitudes of I(h) in stellate cells, while I(h) of pyramidal cells remained unchanged. It is suggested that different types of hyperpolarization-activated cation channels are expressed in pyramidal and stellate cells of the entorhinal cortex.

Diminished glutathione levels cause spontaneous and mitochondria-mediated cell death in neurons from trisomy 16 mice: a model of Down's syndrome

It has been suggested that the increased neuronal death in cultures from trisomy 16 (Ts16) mice, a model of Down's syndrome, might result from a diminished concentration of reduced glutathione (GSH). In this study we used microfluorometric techniques to investigate the effect of GSH levels on neuronal survival in diploid and Ts16 cultures. Addition of the GSH precursors cysteine and cystine and the antioxidant tocopherol to the culture medium increased the GSH concentration up to 126.0% in diploid and up to 111.9% in Ts16 neurons. Moreover, we observed a reduced spontaneous neuronal death rate in diploid and Ts16 cultures. Following the application of 50-100 microM glutamate to culture medium, we found a GSH increase in the presence of cysteine, cystine, tocopherol, and cyclosporin A, an inhibitor of mitochondrial permeability transition (diploid, 105.8-110.8%; Ts16, 83.1-96.3%). However, only tocopherol and cyclosporin A had a protective effect on glutamate-induced neuronal death. The results suggest that reduced GSH levels affect the increase of a spontaneous and a mitochondria-mediated, cyclosporin A-sensitive type of neuronal cell death. Therefore, elevating intracellular GSH concentration may have neuroprotective effects in Down's syndrome and Alzheimer's disease.

Ca(2+)- and metabolism-related changes of mitochondrial potential in voltage-clamped CA1 pyramidal neurons in situ

In hippocampal slices from rats, dialysis with rhodamine-123 (Rh-123) and/or fura-2 via the patch electrode allowed monitoring of mitochondrial potential (DeltaPsi) changes and intracellular Ca(2+) ([Ca(2+)](i)) of CA1 pyramidal neurons. Plasmalemmal depolarization to 0 mV caused a mean [Ca(2+)](i) rise of 300 nM and increased Rh-123 fluorescence signal (RFS) by </=50% of control. The evoked RFS, indicating depolarization of DeltaPsi, and the [Ca(2+)](i) transient were abolished by Ca(2+)-free superfusate or exposure of Ni(2+)/Cd(2+). Simultaneous measurements of RFS and [Ca(2+)](i) showed that the kinetics of both the Ca(2+) rise and recovery were considerably faster than those of the DeltaPsi depolarization. The plasmalemmal Ca(2+)/H(+) pump blocker eosin-B potentiated the peak of the depolarization-induced RFS and delayed recovery of both the RFS and [Ca(2+)](i) transient. Thus the DeltaPsi depolarization due to plasmalemmal depolarization is related to mitochondrial Ca(2+) sequestration secondary to Ca(2+) influx through voltage-gated Ca(2+) channels. CN(-) elevated [Ca(2+)](i) by <50 nM but increased RFS by 221% as a result of extensive depolarization of DeltaPsi. Oligomycin decreased RFS by 52% without affecting [Ca(2+)](i). In the presence of oligomycin, CN(-) and p-trifluoromethoxy-phenylhydrazone (FCCP) elevated [Ca(2+)](i) by <50 nM and increased RFS by 285 and 290%, respectively. Accordingly, the metabolism-related DeltaPsi changes are independent of [Ca(2+)](i). Imaging techniques revealed that evoked [Ca(2+)](i) rises are distributed uniformly over the soma and primary dendrites, whereas corresponding changes in RFS occur more localized in subregions within the soma. The results show that microfluorometric measurement of the relation between mitochondrial function and intracellular Ca(2+) is feasible in whole cell recorded mammalian neurons in situ.

Increased mitochondrial superoxide generation in neurons from trisomy 16 mice: a model of Down's syndrome

Increased neuronal cell death in neurodegenerative diseases has been suggested to result from an increased mitochondrial generation of radical oxygen species (ROS). To test this hypothesis, we investigated superoxide formation in cultured hippocampal neurons from diploid and trisomy 16 mice (Ts16), a model of Down's syndrome. Microflurometric techniques were used to measure superoxide-induced oxidation rate of hydroethidine (HEt) to ethidium and reduced nicotinamide adenine dinucleotide (NADH) and reduced nicotinamide adenine dinucleotide phosphate (NADPH) autofluorescence signal to monitor changes in neuronal energy metabolism. We found an increase in superoxide formation by more than 50% in Ts16 neurons in comparison with diploid control neurons. In the presence of the mitochondrial respiratory chain complex I inhibitor rotenone superoxide production was blocked in diploid neurons, but the increased superoxide generation in Ts16 neurons remained. Uncoupling of mitochondrial oxidative phosphorylation using carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) caused irreversible deficiency in the energy metabolism, monitored by NAD(P)H autofluorescence in Ts16 neurons, but not in diploid control neurons. These results suggest an increased basal generation of superoxide in Ts16 neurons, probably caused by a deficient complex I of mitochondrial electron transport chain, which leads to an impaired mitochondrial energy metabolism and finally neuronal cell death.

Structural Studies of Model RNA Helices with Relevance to Aminoacyl-tRNA Synthetase Specificity and HIV Reverse Transcription

Abstract We describe high-resolution crystal structures of synthetic nucleic-acid fragments determined as part of an effort to understand determinants of sequence-specific protein binding on the level of double-helix structure. In a first set of experiments, 7-base-pair RNA duplexes representing the acceptor-stem helix of Escherichia coli tRNA(Ala) and variants thereof were characterized at atomic resolution. The structures revealed a standard A-form double helix locally perturbed by a G·U wobble base pair at sequence position 3/70 of the tRNA. The G·U pair shows a characteristic hydration pattern which must be considered an integral part of the double-helix structure. It does not seem to exert a global effect on the duplex structure. A second experiment concerned the chimeric DNA-RNA hybrid structure formed transiently during initiation of minus-strand synthesis by the reverse transcriptase of HIV-1. The crystal structure of an 8-base-pair duplex with an RNA template strand derived from HIV-1 and a complementary strand representing the junction between the tRNA(Lys,3) RNA primer and the newly synthesized DNA strand was solved at a resolution of 1.9 Å. As before, the double helix was found to adopt standard A-type conformation with only local variations of backbone conformation. Based on the global helix structure as present in the crystal, it remains difficult to explain the preference of the reverse-transcriptase-associated RNAse H activity for certain sites of the template strand. Structural plasticity near the main cleavage site in suggested to govern cutting preferences. In both systems investigated, structural studies by NMR spectroscopy were carried out by others in parallel. In both cases, the solution structures are in partial disagreement with the crystallographic results by describing a significantly higher level of deviation from the canonical A-conformation.

Effects of retigabine (D-23129) on different patterns of epileptiform activity induced by low magnesium in rat entorhinal cortex hippocampal slices

PURPOSE

The objective of this study was to evaluate the effect of a new antiseizure drug, retigabine (D-23129; N-(2-amino-4-[fluorobenzylamino]-phenyl) carbamic acid ethyl ester) on low-Mg2+-induced epileptiform discharges in rat in vitro.

METHODS

Three types of epileptiform discharges (recurrent short discharges in the hippocampus, seizure-like events, and late recurrent discharges in the entorhinal cortex) were elicited in rat combined entorhinal cortex-hippocampal slices by perfusion with low-Mg2+-artificial cerebrospinal fluid (ACSF). The antiepileptic properties of retigabine were evaluated as effect on the frequency and amplitude of the epileptiform activities as well as time of onset of the effect in the entorhinal cortex (EC) and in hippocampal area CA1 (CA1) by using extracellular recording techniques.

RESULTS

Retigabine (20 microM) reversibly suppressed the recurrent short discharges otherwise sensitive only to high doses of valproate (VPA) but insensitive to standard antiepileptic drugs (AEDs) in CA1, whereas 10 microM reduced the frequency of discharges by 34+/-18.8%, with no significant effect on the amplitude. In EC, retigabine (50 microM) reversibly suppressed the seizure-like events, whereas 20 microM blocked seizure-like events in 71.5% of the slices. The seizure-like events were also sensitive to standard AEDs. Late recurrent discharges in EC that are not blocked by standard AEDs were reversibly suppressed by retigabine (100 microM), whereas 50 microM reduced the frequency of the discharges by 94.4+/-7.7%, and 20 microM, by 74.2+/-18.0%, with no significant effect on the amplitude.

CONCLUSIONS

Retigabine is an effective AED with suppressive effects on recurrent short discharges and on late recurrent discharges normally insensitive to standard AEDs.

Sodium bromide: effects on different patterns of epileptiform activity, extracellular pH changes and GABAergic inhibition

Results regarding the anticonvulsant potency of bromide have been questioned, and the mechanisms of its action are unclear. Using combined rat hippocampus-entorhinal cortex slices we analyzed the effects of NaBr on four types of epileptiform discharges in two different models of epilepsy, the low-Ca2+ and the low-Mg2+ model. NaBr concentration-dependently reduced the frequency and finally blocked the low Ca2+-induced discharges. Low Mg2+-induced short recurrent discharges were also reduced in a concentration-dependent manner. In the entorhinal cortex the frequency of seizure-like events was reduced by 3 and 5 mM and the discharges were blocked by 7 mM NaBr. Also, the late recurrent discharges in the entorhinal cortex which do not respond to most clinically employed anticonvulsants were reduced by concentrations of 10 and 15 mM and completely blocked by 30 mM NaBr. Using pH-sensitive microelectrodes different effects of NaBr were seen than those of acetazolamide on extracellular pH under control conditions and after stimulation. Acetazolamide at 1 mM caused a reversible acidification of delta pH: 0.2+/-0.14 at rest whereas no change on extracellular pH was seen with 5 mM NaBr. Acetazolamide increased the transient alkalosis induced by repetitive stimulation of the stratum radiatum in area CA1 and reduced the subsequent acidosis. NaBr also increased the alkalosis but had no effect on the subsequent acidosis. A significant increase in paired-pulse inhibition was seen in a paired-pulse stimulation protocol used to monitor the efficacy of GABAergic inhibition at concentrations of 5 mM NaBr. This finding was confirmed in whole-cell patch clamp recordings from cultured hippocampal neurons showing an increase in inhibitory postsynaptic current amplitude. In summary, our results suggest a broad-spectrum anticonvulsant activity which is likely to be caused by its effects on membrane excitability, by an increase in GABAergic inhibition and is less likely caused by its effects on extracellular pH.

Alterations of glial cell function in temporal lobe epilepsy

PURPOSE

Comparison of extracellular K+ regulation in sclerotic and nonsclerotic epileptic hippocampus.

METHODS

Measurements of K+ signals with double-barreled K+-selective reference microelectrodes in area CAI of slices from human and rat hippocampus, induction of increases in extracellular potassium concentration by repetitive alvear stimulation or iontophoresis. and block of inward-rectifying and background K+ channels in astrocytes by barium.

RESULTS

In the CA1 pyramidal layer from normal rat hippocampus, barium augmented extracellular K+ accumulation induced by iontophoresis or antidromic stimulation in a dose-dependent manner. Similarly, barium augmented stimulus-induced K+ signals from nonsclerotic hippocampi (human mesial temporal lobe epilepsy). In contrast, barium failed to do so in sclerotic hippocampi (human mesial temporal lobe epilepsy, rat pilocarpine model).

CONCLUSIONS

Our findings suggest that in areas of reduced neuronal density (hippocampal sclerosis), glial cells adapt to permit rather large increases in extracellular potassium accumulation. Such increases might be involved in the transmission of activity through the sclerotic area.

Retigabine strongly reduces repetitive firing in rat entorhinal cortex

Retigabine (D-23129) [N-(2-amino-4-(4-fluorobenzylamino)phenyl) carbamic acid ethyl ester] is a novel antiepileptic drug. The compound was shown to possess anticonvulsant properties both in vivo and in vitro. We investigated the effects of retigabine on neurones in the rat medial entorhinal cortex using conventional intracellular recordings in combined hippocampal-entorhinal cortex slices. Retigabine strongly reduced the number of action potentials elicited by 1 s long depolarising current injections. Both the amplitudes of monosynaptic inhibitory postsynaptic potentials/currents (IPSP/Cs) and the amplitudes of excitatory postsynaptic potentials (EPSPs) remained unaffected. The drug increased outward rectification and induced a membrane-potential hyperpolarisation in most of the tested neurones. The findings suggest that retigabine exerts its anticonvulsant effects by activation of a K(+)conductance, however it cannot be excluded from our experiments that other mechanisms may be involved in the effect of retigabine on membrane properties.

Morphological, immunophenotypical and electrophysiological properties of resting microglia in vitro

Morphological, immunophenotypical and electrophysiological properties were investigated in isolated cultured murine microglia before and after exposure to astrocyte-conditioned medium (ACM). Following application of ACM, microglial cells underwent a dramatic shape transformation from an amoeboid appearance to a ramified morphology. In parallel to morphological changes, a downregulation of macrophage surface antigens was observed in microglia exposed to ACM. Staining intensities for major histocompatibility complex (MHC) class II molecules and for the adhesion molecules leukocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) were significantly decreased in ramified microglia 5 days after exposure to ACM. In microglial cells treated daily with ACM over a period of 5 days, the smallest staining intensities for all surface antigens as well as the smallest ramification index as a measure for the highest degree of ramification were determined. In addition, upregulation of delayed rectifier K + currents was observed in microglia exposed to ACM for 1 day or treated daily with ACM for 5 days. In contrast, untreated amoeboid microglia or ramified microglia analysed 5 days after exposure to ACM did not express delayed rectifier K + currents. Analyses of the resting membrane potential and expression levels and properties of inward rectifier K + currents did not reveal any differences between untreated and ACM-treated microglia. It is suggested that electrophysiological properties of microglia do not strongly correlate with the morphology or the immunophenotype of microglial cells.

Vertebrate-type and plant-type ferredoxins: crystal structure comparison and electron transfer pathway modelling

Crystallographic analysis of a fully functional, truncated bovine adrenodoxin, Adx(4-108), has revealed the structure of a vertebrate-type [2Fe-2S] ferredoxin at high resolution. Adrenodoxin is involved in steroid hormone biosythesis in adrenal gland mitochondria by transferring electrons from adrenodoxin reductase to different cytochromes P450. Plant-type [2Fe-2S] ferredoxins interact with photosystem I and a diverse set of reductases.A systematic structural comparison of Adx(4-108) with plant-type ferredoxins which share about 20 % sequence identity yields these results. (1) The ferredoxins of both types are partitioned into a large, strictly conserved core domain bearing the [2Fe-2S] cluster and a smaller interaction domain which is structurally different for both subfamilies. (2) In both types, residues involved in interactions with reductase are located at similar positions on the molecular surface and coupled to the [2Fe-2S] cluster via structurally equivalent hydrogen bonds. (3) The accessibility of the [2Fe-2S] cluster differs between Adx(4-108) and the plant-type ferredoxins where a solvent funnel leads from the surface to the cluster. (4) All ferredoxins are negative monopoles with a clear charge separation into two compartments, and all resulting dipoles but one point into a narrow cone located in between the interaction domain and the [2Fe-2S] cluster, possibly controlling predocking movements during interactions with redox partners. (5) Model calculations suggest that FE1 is the origin of electron transfer pathways to the surface in all analyzed [2Fe-2S] ferredoxins and that additional transfer probability for electrons tunneling from the more buried FE2 to the cysteine residue in position 92 of Adx is present in some.

Intrinsic theta-frequency membrane potential oscillations in layer III/V perirhinal cortex neurons of the rat

The firing of a proportion of neurons in the in vivo perirhinal cortex, a brain region involved in object recognition memory, has recently been shown to be synchronized with hippocampal theta activity. The purpose of the present study was to determine whether neurons located in perirhinal cortex have intrinsic properties that might encourage their participation in theta activity. To these ends, current clamp recordings were made from 98 neurons located in layer III/V of the in vitro rat perirhinal cortex. The intrinsic properties of these neurons were investigated, and a subset of 61 neurons were tested for the presence of membrane potential oscillations at threshold levels of depolarization. Thirty-nine percent of these neurons displayed a theta-frequency membrane potential oscillation (MPO; mean frequency = 8.6 Hz). When depolarized past spike threshold, these neurons tended to fire in clusters, with a within-cluster interspike interval close to the peak to peak interval of the MPOs. Neurons that did not generate MPOs generated nonaccomodating action potential trains with a frequency that spanned the theta range. Biocytin staining indicated that MPOs could be generated in cells with both pyramidal and nonpyramidal morphology. These findings demonstrate that a large proportion of perirhinal neurons exhibit intrinsic properties that could assist in the entrainment and synchronization of theta-frequency oscillations. These properties may enhance the communication of information between the perirhinal cortex, entorhinal cortex, and hippocampus.

Age-dependence of the anticonvulsant effects of the GABA uptake inhibitor tiagabine in vitro

Epileptic syndromes frequently start at childhood and therefore it is crucial to test new anticonvulsants at immature stages of the nervous system. We compared the effects of the gamma-aminobutyric acid (GABA) uptake inhibitor tiagabine [(R)-N-(4, 4-bis(3-methyl-2-thienyl)but)3-en-1-yl nipecotic acid] on low-Mg(2+)-induced epileptic discharges in brain slices from rat pups (p 5-8) and juvenile animals (p 15-20). In tissue from rat pups, tiagabine slightly reduced epileptiform activity in hippocampal area CA1 but had no effect in the entorhinal cortex. In juvenile rats, epileptiform discharges were unaffected in CA1 but suppressed by 60% in the entorhinal cortex. While tiagabine increases its efficacy with age, in-situ hybridisation and PCR analysis show that mRNA coding for the neuronal GABA-transporter GAT-1 is already present at p 5. We therefore conclude that the increasing efficacy of tiagabine during ontogenesis is due to functional maturation of GABAergic synapses rather than to up-regulation of GAT-1 expression.

Carbachol-induced changes in excitability and [Ca2+]i signalling in projection cells of medial entorhinal cortex layers II and III

The entorhinal cortex (EC) is a major gateway for sensory information into the hippocampus and receives a cholinergic input from the forebrain. Therefore, we studied muscarinic effects on excitability and intracellular Ca2+ signalling in layer II stellate and layer III pyramidal projection neurons of the EC. In both classes of neurons, local pressure-pulse application of carbachol (1 mM) caused small, atropine-sensitive membrane depolarizations that were not accompanied by any detectable changes in [Ca2+]i. At a higher concentration (10 mM), carbachol induced a larger membrane depolarization associated with synaptic oscillations and epileptiform activity in both classes of neurons. In contrast to the intrinsic theta rhythm in stellate cells with one dominant peak frequency at approximately 7 Hz, the synaptically mediated oscillation induced by carbachol showed three characteristic peaks in the theta and gamma frequency range at approximately 11, 23 and 40 Hz. Although carbachol-induced epileptiform activity was associated with increases in intracellular free Ca2+ in both layer II and III cells, the observed [Ca2+]i accumulation was significantly larger in layer III than in layer II cells. Responses to intracellular current injections showed differences in Ca2+ accumulation in layer II and III cells at the same membrane potentials, suggesting a dominant expression of low- and high-voltage-activated Ca2+ channels in these layer II and III cells, respectively. In conclusion, we present evidence for significant differences in the [Ca2+]i regulation between layer II stellate and layer III pyramidal cells of the medial EC.

Epileptiform activity induced by 4-aminopyridine in entorhinal cortex hippocampal slices of rats with a genetically determined absence epilepsy (GAERS)

Patients with absence epilepsy frequently develop convulsions later in life. We were therefore interested whether tissue from rats with a genetic absence epilepsy is more prone to seizure generation than normal animals. We compared the epileptiform activities induced by 4-aminopyridine (4-AP) induced in hippocampal-entorhinal cortex slices from genetic absence epilepsy rats of Strasbourg (GAERS, age 6 months) in which absence seizures have been present for about 4 months and from control non epileptic rats (NE). 4-AP induced short recurrent discharges in area CA1 of rat hippocampus, seizure-like events and interictal discharges in the entorhinal cortex. The various epileptiform discharges did not differ between the two strains in amplitude, duration and frequency. However, the latency for induction of different epileptiform activities by 50 microM 4-AP was significantly shorter in GAERS (about 16 min) than in NE rats (about 25 min). We also analysed differences in evoked field potentials (fp) in hippocampal area CA1 before, during and after application of 4-AP. Before application of 4-AP, responses to stimulation of Schaffer collateral were smaller in GAERS than in NE rats. Paired pulse potentiation was significantly larger in GAERS than in NE rats. 4-AP in the bath augmented the size of the evoked field potentials and this increase was larger in GAERS than in NE rats. Our findings show a greater excitability of hippocampal area CA1 in GAERS rats and a greater ability to develop 4-AP-induced epileptiform activity in combined hippocampal-enthorhinal cortex slices in GAERS than in NE rats.

Potent depression of stimulus evoked field potential responses in the medial entorhinal cortex by serotonin

1. The entorhinal cortex (EC), main input structure to the hippocampus, gets innervated by serotonergic terminals from the raphe nuclei and expresses 5-HT-receptors at high density. Using extra- and intracellular recording techniques we here investigated the effects of serotonin on population and cellular responses within the EC. 2. Stimulation in the lateral entorhinal cortex resulted in complex field potential responses in the superficial EC. The potentials are composed of an early antidromic and a late orthodromic component reflecting the efferent and afferent circuitry. 3. Serotonin (5-HT) reduced synaptic potentials of the stimulus evoked extracellular field potential at all concentrations tested (0. 1 - 100 microM; 59%-depression by 10 microM serotonin), while the antidromic response was not significantly changed by up to 50 microM 5-HT. Depression of field potential responses by serotonin was associated with a significant increase in paired-pulse facilitation from 1.15 to 1.88. 4. The effects of serotonin on field potential responses were mimicked by 5-HT1A-receptor agonists (8-OH-DPAT, 5-CT) and partially prevented by the 5-HT1A-receptor antagonist (S-UH-301). Moreover, the 5-HT1A-receptor antagonist WAY100635 reduced the effect of 5-CT. 5. Fenfluramine, a serotonin releaser, mimics the effects of serotonin on stimulus-evoked field potential responses, indicating that synaptically released serotonin can produce the changes in reactivity to afferent stimulation. 6. Depression of isolated AMPA-receptor mediated EPSCs by serotonin as well as fenfluramine was associated with an increase in paired pulse facilitation, indicating a presynaptic locus of action. 7. We conclude that physiological concentrations of serotonin potently suppresses excitatory synaptic transmission in the superficial entorhinal cortex by a presynaptic mechanism.

Effects of ionotropic glutamate receptor blockade and 5-HT1A receptor activation on spreading depression in rat neocortical slices

The effect of the AMPA antagonist NBQX (10 microM), NMDA antagonist ketamine (100 microM) and 5-HT1A agonist 8-OH-DPAT (1, 10 and 100 microM) on the properties of a KCl-induced spreading depression (SD) was studied in parietal cortical slices of adult rats. Whereas NBQX did not significantly affect the SD, ketamine significantly (p < 0.01) reduced the amplitude of the first SD peak (12.8 +/- 4.6 mV) and blocked the second SD peak when compared with the controls (19.8 +/- 5.2 mV and 25 +/- 5 mV, respectively). Ketamine also decreased the SD duration at half maximal amplitude from 34.9 +/- 12.4 s to 22.2 +/- 12 s (p < 0.05). 8-OH-DPAT attenuated the duration of the SD from 42 +/- 15.6 s to 21.2 +/- 10.6 s (p < 0.05, 100 microM). These data indicate that not only NMDA receptor blockade, but also activation of the 5-HT1A receptor attenuates the SD and may be beneficial in the reduction of ischemic injury following focal cerebral ischemia.

Anoxic decrease in potassium outward currents of hippocampal cultured neurons in absence and presence of dithionite

The effect of brief anoxia on voltage dependent K(+)-currents of hippocampal cultured neurons was studied. The oxygen scavenger dithionite (hydrosulphite) was previously used for creating zero oxygen pressure. However, dithionite consumes O(2) in parallel with generation of superoxide radicals and is a strongly reducing agent. In this study anoxia was produced by perfusion of the neurons with a solution bubbled with nitrogen for 1 h using a chamber with an argon layer isolating the anoxic bath flow from atmospheric oxygen in presence and absence of dithionite. Oxygen partial pressure of dithionite-free solution was determined by oxygen dependent quenching of the phosphorescence of Pd-coproporphyrin to be 0.15+/-0. 02 Torr (values are given as mean+/-S.D., n=6). Slow (I(K))- and fast (I(A))-inactivating K(+)-currents were measured with the patch clamp technique in the whole cell configuration. Exposure of the neurons to anoxia reversibly decreased the amplitude of I(K) at a test pulse of 0 mV to 77+/-12% (n=7) in absence and to 83+/-7% (n=6) in presence of 2 mM dithionite; the amplitude of I(A) decreased to 78+/-11% in absence and to 82+/-9% in presence of 2 mM dithionite. Voltage dependence of activation and inactivation shifted 5 min after exposure to anoxia reversibly by about 6 mV in depolarizing direction. The decay times of inactivation were insensitive to anoxia. Dithionite had no significant effects on K(+)-currents. In 15 of 21 neurons not employed for analysis on K(+)-currents, a reversible increase in holding current under dithionite was observed. In absence of dithionite in 4 of 19 neurons the holding current reversibly increased during anoxia. Although dithionite does not affect K(+)-currents, changes in holding current show that the dithionite may affect neurons independently of oxygen deprivation.

Ca(2+)-dependent inactivation of high-threshold Ca(2+) currents in hippocampal granule cells of patients with chronic temporal lobe epilepsy

Intracellular Ca(2+) represents an important trigger for various second-messenger mediated effects. Therefore a stringent control of the intracellular Ca(2+) concentration is necessary to avoid excessive activation of Ca(2+)-dependent processes. Ca(2+)-dependent inactivation of voltage-dependent calcium currents (VCCs) represents an important negative feedback mechanism to limit the influx of Ca(2+) that has been shown to be altered in the kindling model of epilepsy. We therefore investigated the Ca(2+)-dependent inactivation of high-threshold VCCs in dentate granule cells (DGCs) isolated from the hippocampus of patients with drug-refractory temporal lobe epilepsy (TLE) using the patch-clamp method. Ca(2+) currents showed pronounced time-dependent inactivation when no extrinsic Ca(2+) buffer was present in the patch pipette. In addition, in double-pulse experiments, Ca(2+) entry during conditioning prepulses caused a reduction of VCC amplitudes elicited during a subsequent test pulse. Recovery from Ca(2+)-dependent inactivation was slow and only complete after 1 s. Ca(2+)-dependent inactivation could be blocked either by using Ba(2+) as a charge carrier or by including bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) or EGTA in the intracellular solution. The influence of the cytoskeleton on Ca(2+)-dependent inactivation was investigated with agents that stabilize and destabilize microfilaments or microtubules, respectively. From these experiments, we conclude that Ca(2+)-dependent inactivation in human DGCs involves Ca(2+)-dependent destabilization of both microfilaments and microtubules. In addition, the microtubule-dependent pathway is modulated by the intracellular concentration of GTP, with lower concentrations of guanosine triphosphate (GTP) causing increased Ca(2+)-dependent inactivation. Under low-GTP conditions, the amount of Ca(2+)-dependent inactivation was similar to that observed in the kindling model. In summary, Ca(2+)-dependent inactivation was present in patients with TLE and Ammon's horn sclerosis (AHS) and is mediated by the cytoskeleton similar to rat pyramidal neurons. The similarity to the kindling model of epilepsy may suggest the possibility of altered Ca(2+)-dependent inactivation in patients with AHS.