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Accueil > Agenda > Les séminaires Jean Roche > Presynaptic action potential amplification by voltage-gated Na+ channels in (...)

Presynaptic action potential amplification by (...)

Lundi 19 septembre 2005, 11h, salle Lissitzky.


1 : Neuron. 2005 Feb 3 ;45(3):405-17. Comment in : · · Neuron. 2005 sep 3 ;45(3):327-9.

Presynaptic action potential amplification by voltage-gated Na+ channels in hippocampal mossy fiber boutons.

Engel D, Jonas P.

Physiologisches Institut der Universitat Freiburg, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany.

Action potentials in central neurons are initiated near the axon initial segment, propagate into the axon, and finally invade the presynaptic terminals, where they trigger transmitter release. Voltage-gated Na(+) channels are key determinants of excitability, but Na(+) channel density and properties in axons and presynaptic terminals of cortical neurons have not been examined yet. In hippocampal mossy fiber boutons, which emerge from parent axons en passant, Na(+) channels are very abundant, with an estimated number of approximately 2000 channels per bouton. Presynaptic Na(+) channels show faster inactivation kinetics than somatic channels, suggesting differences between subcellular compartments of the same cell. Computational analysis of action potential propagation in axon-multibouton structures reveals that Na(+) channels in boutons preferentially amplify the presynaptic action potential and enhance Ca(2+) inflow, whereas Na(+) channels in axons control the reliability and speed of propagation. Thus, presynaptic and axonal Na(+) channels contribute differentially to mossy fiber synaptic transmission.

2 : J Physiol. 2004 Sep 15 ;559(Pt 3):721-8. Epub 2004 Jul 29.

Alpha 5 subunit-containing GABAA receptors affect the dynamic range of mouse hippocampal kainate-induced gamma frequency oscillations in vitro.

Towers SK, Gloveli T, Traub RD, Driver JE, Engel D, Fradley R, Rosahl TW, Maubach K, Buhl EH, Whittington MA.

School of Biomedical Sciences, University of Leeds, LS2 9NQ, UK.

Though all in vitro models of gamma frequency network oscillations are critically dependent on GABAA receptor-mediated synaptic transmission little is known about the specific role played by different subtypes of GABAA receptor. Strong expression of the alpha5 subunit of the GABAA receptor is restricted to few brain regions, amongst them the hippocampal dendritic layers. Receptors containing this subunit may be expressed on the extrasynaptic membrane of principal cells and can mediate a tonic GABAA conductance. Using hippocampal slices of wild-type (WT) and alpha5-/- mice we investigated the role of alpha5 subunits in the generation of kainate-induced gamma frequency oscillations (20-80 Hz). The change in power of the oscillations evoked in CA3 by increasing network drive (kainate, 50-400 nm) was significantly greater in alpha5-/- than in WT slices. However, the change in frequency of gamma oscillations with increasing network drive seen in WT slices was absent in alpha5-/- slices. Raising the concentration of extracellular GABA by bathing slices in the GABA transaminase inhibitor vigabatrin and blocking uptake with tiagabine reduced the power of gamma oscillations more in WT slices than alpha5-/- slices (43%versus 15%). The data suggest that loss of this GABAA receptor subunit alters the dynamic profile of gamma oscillations to changes in network drive, possibly via actions of GABA at extrasynaptic receptors.

3 : J Neurophysiol. 2004 Jan ;91(1):25-39. Epub 2003 Sep 17.

Transmitter metabolism as a mechanism of synaptic plasticity : a modeling study.

Axmacher N, Stemmler M, Engel D, Draguhn A, Ritz R.

Johannes-Muller-Institut fur Physiologie, Humboldt-Universitat Berlin, 10117 Berlin, Germany.

The nervous system adapts to experience by changes in synaptic strength. The mechanisms of synaptic plasticity include changes in the probability of transmitter release and in postsynaptic responsiveness. Experimental and neuropharmacological evidence points toward a third variable in synaptic efficacy : changes in presynaptic transmitter concentration. Several groups, including our own, have reported changes in the amplitude and frequency of postsynaptic (miniature) events indicating that alterations in transmitter content cause alterations in vesicular transmitter content and vesicle dynamics. It is, however, not a priori clear how transmitter metabolism will affect vesicular transmitter content and how this in turn will affect pre- and postsynaptic functions. We therefore have constructed a model of the presynaptic terminal incorporating vesicular transmitter loading and the presynaptic vesicle cycle. We hypothesize that the experimentally observed synaptic plasticity after changes in transmitter metabolism puts predictable restrictions on vesicle loading, cytoplasmic-vesicular transmitter concentration gradient, and on vesicular cycling or release. The results of our model depend on the specific mechanism linking presynaptic transmitter concentration to vesicular dynamics, that is, alteration of vesicle maturation or alteration of release. It also makes a difference whether differentially filled vesicles are detected and differentially processed within the terminal or whether vesicle filling acts back onto the terminal by presynaptic autoreceptors. Therefore, the model allows one to decide, at a given synapse, how transmitter metabolism is linked to presynaptic function and efficacy.

4 : J Physiol. 2001 Sep 1 ;535(Pt 2):473-82.

Plasticity of rat central inhibitory synapses through GABA metabolism.

Engel D, Pahner I, Schulze K, Frahm C, Jarry H, Ahnert-Hilger G, Draguhn A.

Institut fur Physiologie der Charite, Humboldt-Universitat, Tucholskystrasse 2, 10117 Berlin, Germany.

1. The production of the central inhibitory transmitter GABA (gamma-aminobutyric acid) varies in response to different patterns of activity. It therefore seems possible that GABA metabolism can determine inhibitory synaptic strength and that presynaptic GABA content is a regulated parameter for synaptic plasticity. 2. We altered presynaptic GABA metabolism in cultured rat hippocampal slices using pharmacological tools. Degradation of GABA by GABA-transaminase (GABA-T) was blocked by gamma-vinyl-GABA (GVG) and synthesis of GABA through glutamate decarboxylase (GAD) was suppressed with 3-mercaptopropionic acid (MPA). We measured miniature GABAergic postsynaptic currents (mIPSCs) in CA3 pyramidal cells using the whole-cell patch clamp technique. 3. Elevated intra-synaptic GABA levels after block of GABA-T resulted in increased mIPSC amplitude and frequency. In addition, tonic GABAergic background noise was enhanced by GVG. Electron micrographs from inhibitory synapses identified by immunogold staining for GABA confirmed the enhanced GABA content but revealed no further morphological alterations. 4. The suppression of GABA synthesis by MPA had opposite functional consequences : mIPSC amplitude and frequency decreased and current noise was reduced compared with control. However, we were unable to demonstrate the decreased GABA content in biochemical analyses of whole slices or in electron micrographs. 5. We conclude that the transmitter content of GABAergic vesicles is variable and that postsynaptic receptors are usually not saturated, leaving room for up-regulation of inhibitory synaptic strength. Our data reveal a new mechanism of plasticity at central inhibitory synapses and provide a rationale for the activity-dependent regulation of GABA synthesis in mammals.

5 : Neuroreport. 2001 Jun 13 ;12(8):1593-6.

Efficacy of background GABA uptake in rat hippocampal slices.

Frahm C, Engel D, Draguhn A.

Johannes-Muller-Institut fur Physiologie der Charite, Humboldt-Universitat, Berlin, Germany.

GABA uptake is crucial for the termination of inhibitory synaptic events. In addition, GABA transporters may also control the level of diffusely distributed GABA in the extracellular space. We analysed this function by superfusing rat hippocampal slices with different concentrations of GABA. Whole-cell patch clamp recordings of CA1 pyramidal cells revealed small increases in chloride conductance at 5-10 microM GABA which increased dramatically upon addition of the GABA uptake blocker tiagabine. Tiagabine alone induced a significant chloride conductance indicating that spontaneous release of GABA in hippocampal slices is neutralized by GAT-1, the main hippocampal GABA transporter. Thus, GAT-1 clears the extracellular space in the hippocampus from diffusely distributed GABA with high efficacy.

6 : Neuron. 2001 Jan ;29(1):185-96. Comment in : · · Neuron. 2001 Jan ;29(1):1-3.

Disruption of ClC-3, a chloride channel expressed on synaptic vesicles, leads to a loss of the hippocampus.

Stobrawa SM, Breiderhoff T, Takamori S, Engel D, Schweizer M, Zdebik AA, Bosl MR, Ruether K, Jahn H, Draguhn A, Jahn R, Jentsch TJ.

Zentrum fur Molekulare Neurobiologie Hamburg, Universitat Hamburg, Martinistrasse 85, D-20246, Hamburg, Germany.

Several plasma membrane chloride channels are well characterized, but much less is known about the molecular identity and function of intracellular Cl- channels. ClC-3 is thought to mediate swelling-activated plasma membrane currents, but we now show that this broadly expressed chloride channel is present in endosomal compartments and synaptic vesicles of neurons. While swelling-activated currents are unchanged in mice with disrupted ClC-3, acidification of synaptic vesicles is impaired and there is severe postnatal degeneration of the retina and the hippocampus. Electrophysiological analysis of juvenile hippocampal slices revealed no major functional abnormalities despite slightly increased amplitudes of miniature excitatory postsynaptic currents. Mice almost lacking the hippocampus survive and show several behavioral abnormalities but are still able to acquire motor skills.

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