Persistent LTP without triggered protein synthesis.

Protein synthesis is believed to be involved in stabilizing synaptic plasticity. Effects lasting longer than about 2-3h are considered to require synthesis of new proteins, implying a functional separation between early (E) and late (L) components. However, the issue of constitutive vs. new protein synthesis is still unclear, especially in young animals. Here, we examined the effects of two protein synthesis inhibitors, anisomycin and emetine, on long-term-potentiation (LTP) in CA1 area of hippocampal slices from 12- to 20-day-old rats. Either drug was applied from -30 min to +30 min with respect to LTP induction, a time window previously reported to be critical. However, the LTP remained stable under the entire recording period of 4h (anisomycin), or 8h (emetine). Proper preparation of emetine solution was evidenced by the fact that, in separate experiments, prolonged treatment with emetine gradually blocked baseline responses. Although no corresponding effect was observed with anisomycin, the drug was judged to be potent by its ability to inhibit yeast growth. The ability of anisomycin to inhibit protein synthesis was further confirmed by radiolabeling experiments assessing the degree of leucine incorporation. Our data suggest that LTP up to at least 8h is not dependent on triggered protein synthesis but can be attained by utilizing proteins already available at induction time.

Contribution of AMPA and NMDA receptors to early and late phases of LTP in hippocampal slices.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor mediated responses were investigated in rat hippocampal slices under 4h of long-term potentiation (LTP) expression. A modified medium containing the NMDA receptor antagonist AP5 and low concentration of Mg(2+) was used to monitor isolated AMPA responses. NMDA components were determined from composite excitatory postsynaptic potentials (EPSPs) under brief (15-20 min) wash-out of AP5. LTP was induced in a medium with low concentration of AP5, resulting in an about two-fold larger increase of the AMPA component than of the NMDA component at both 1h and 4h after induction. Similar results were obtained if LTP was induced in "normal Mg(2+)" and the NMDA components were assessed at the end of experiment, from either composite or isolated NMDA EPSPs, with or without blockade of GABAergic inhibition. It is generally believed that LTP undergoes biochemical and/or structural conversions during the first few hours. Our study, however, shows constant expression of LTP, at least in terms of AMPA versus NMDA components, during this time. The data support the notion that LTP initiates as a predominant amplification of AMPA receptors and remains so for at least 4h.

Characterization of NMDA induced depression in rat hippocampus: involvement of AMPA and NMDA receptors.

The involvement of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) vs. N-methyl-d-aspartate (NMDA) receptor mediated changes in NMDA-induced long-term depression (LTD) was assessed by monitoring isolated AMPA, isolated NMDA and composite field excitatory postsynaptic potentials (EPSP) in the CA1 area of acute rat hippocampal slices. Application of NMDA (20-50 microM) for 3-5 min led to LTD of both AMPA and NMDA receptor mediated EPSPs with near equal changes of the responses. However, AMPA EPSPs displayed a faster initial recovery than NMDA EPSPs. In addition, during the first 15-25 min after NMDA application, there was a superimposed potentiation of the later, but not early, part of AMPA EPSPs, implying a prolongation of waveform. In contrast, the NMDA EPSP waveform remained unaltered throughout the experiments. While it has been maintained that NMDA-induced depression is equivalent to stimulus-induced LTD, our results reveal additional complexity, suggesting a multitude of changes, most likely at the postsynaptic receptor level.

Electron energy-dependent product state distributions in the dissociative recombination of O2+.

We present product state distributions and quantum yields from the dissociative recombination reaction of O2+ in its electronic and vibrational ground states as a function of electron collision energy between 0 and 300 meV. The experiments have been performed in the heavy-ion storage ring, CRYRING, and use a cold hollow-cathode discharge source for the production of cold molecular oxygen ions. The branching fractions over the different dissociation limits show distinct oscillations while the resulting product quantum yields are largely independent of electron collision energy above 40 meV. The branching results are well reproduced assuming an isotropic dissociation process, in contrast with recent theoretical predictions.

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+.

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.

Dissociative recombination of the weakly bound NO-dimer cation: cross sections and three-body dynamics.

Dissociative recombination (DR) of the dimer ion (NO)(2) (+) has been studied at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm. The experiments were aimed at determining details on the strongly enhanced thermal rate coefficient for the dimer, interpreting the dissociation dynamics of the dimer ion, and studying the degree of similarity to the behavior in the monomer. The DR rate reveals that the very large efficiency of the dimer rate with respect to the monomer is limited to electron energies below 0.2 eV. The fragmentation products reveal that the breakup into the three-body channel NO+O+N dominates with a probability of 0.69+/-0.02. The second most important channel yields NO+NO fragments with a probability of 0.23+/-0.03. Furthermore, the dominant three-body breakup yields electronic and vibrational ground-state products, NO(upsilon=0)+N((4)S)+O((3)P), in about 45% of the cases. The internal product-state distribution of the NO fragment shows a similarity with the product-state distribution as predicted by the Franck-Condon overlap between a NO moiety of the dimer ion and a free NO. The dissociation dynamics seem to be independent of the NO internal energy. Finally, the dissociation dynamics reveal a correlation between the kinetic energy of the NO fragment and the degree of conservation of linear momentum between the O and N product atoms. The observations support a mechanism in which the recoil takes place along one of the NO bonds in the dimer.

Investigating the breakup dynamics of dihydrogen sulfide ions recombining with electrons.

This paper presents results concerning measurements of the dissociative recombination (DR) of dihydrogen sulfide ions. In combination with the ion storage ring CRYRING an imaging technique was used to investigate the breakup dynamics of the three-body channel in the DR of 32SD2(+). The two energetically available product channels S(3P) + 2D(2S) and S(1D) + 2D(2S) were both populated, with a branching fraction of the ground-state channel of 0.6(0.1). Information about the angle between the two deuterium atoms upon dissociation was obtained together with information about how the available kinetic energy was distributed between the two light fragments. The recombination cross sections as functions of energy in the interval of 1 meV to 0.3 eV in the center-of-mass frame are presented for 34SH2(+). The thermal rate coefficient for the DR of 34SH2(+) was determined to be (4.8+/-1.0) x 10(-7)(T/300)(-0.72+/-0.1) cm3 s(-1) over this interval.

Dissociative recombination study of Na+ (D2O) in a storage ring.

The dissociative recombination of Na(+)(D(2)O) ion has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The cross section has been measured as a function of center-of-mass energy ranging from 1 meV to 0.1 eV and found to have an E(-1.37) dependence. The rate coefficient has been deduced to be (2.3+/-0.32)x10(-7)(T(e)/300)(-0.95+/-0.01) cm(3) s(-1) for T(e)=50-1000 K. The branching ratios have been measured at 0 eV. Of the four energetically accessible dissociation channels, three channels are found to occur although the channel that breaks the weak Na(+)-D(2)O bond is by far dominant.