Raised Intracellular Calcium Contributes to Ischemia-Induced Depression of Evoked Synaptic Transmission

Oxygen-glucose deprivation (OGD) leads to depression of evoked synaptic transmission, for which the mechanisms remain unclear. We hypothesized that increased presynaptic [Ca2+]i during transient OGD contributes to the depression of evoked field excitatory postsynaptic potentials (fEPSPs). Additionally, we hypothesized that increased buffering of intracellular calcium would shorten electrophysiological recovery after transient ischemia. Mouse hippocampal slices were exposed to 2 to 8 min of OGD. fEPSPs evoked by Schaffer collateral stimulation were recorded in the stratum radiatum, and whole cell current or voltage clamp recordings were performed in CA1 neurons. Transient ischemia led to increased presynaptic [Ca2+]i, (shown by calcium imaging), increased spontaneous miniature EPSP/Cs, and depressed evoked fEPSPs, partially mediated by adenosine. Buffering of intracellular Ca2+ during OGD by membrane-permeant chelators (BAPTA-AM or EGTA-AM) partially prevented fEPSP depression and promoted faster electrophysiological recovery when the OGD challenge was stopped. The blocker of BK channels, charybdotoxin (ChTX), also prevented fEPSP depression, but did not accelerate post-ischemic recovery. These results suggest that OGD leads to elevated presynaptic [Ca2+]i, which reduces evoked transmitter release; this effect can be reversed by increased intracellular Ca2+ buffering which also speeds recovery

Elastic behavior of metal-assisted etched Si/SiGe superlattice nanowires containing dislocations

We systematically investigate structural parameters, such as shape, size, elastic strain, and relaxations, of metal-assisted etched vertically modulated Si/SiGe superlattice nanowires by using electron microscopy, synchrotron-based x-ray diffraction, and numerical linear elasticity theory. A vertical Si/Ge superlattice with atomically flat interfaces is grown by using molecular beam epitaxy on Si-buffered Si(001) substrates. The lattice constants for Si and Ge are 5.43 and 5.66 Å, respectively, which indicate a lattice mismatch of 4.2%. This results in a strained layer in the boundary between Si and Ge leading to dislocations. These substrates serve as the starting material for nanostructuring the surface by using metal-assisted etching. It is shown that the high quality crystalline structure is preserved in the fabrication process, while the lattice mismatch is partially relieved by dislocation formation. Despite this highly effective relaxation path, dislocations present in the parent superlattice do not vanish upon nanostructuring for wires with diameters of down to at least 80 nm. We relate these observations to the applicability of silicon-based nanowires for high-performance thermoelectric generators

Cubic Phase Sn-Rich GeSn Nanocrystals in a Ge Matrix

We report on the synthesis of a novel optoelectronic material, Sn-rich Ge_1–<i>x</i>Sn_<i>x</i> nanocrystals in a Ge matrix. The nanocrystals have been formed after annealing of a metastable Ge-rich Ge_1–<i>y</i>Sn_<i>y</i> film, which was embedded in the Ge matrix. Electron microscopy investigations have revealed that these nanocrystals possess two lattice types: (i) a diamondlike cubic structure with a high Sn fraction (<i>x</i> > 0.5) and (ii) an ordered zincblende structure (<i>x</i> = 0.5)

(A) Effect of calcium chelators and K channel antagonist, ChTX on fEPSP during OGD.

<p>1μM BAPTA-AM increases the amount of evoked neurotransmission remaining after 2 min (n = 7), 4 min (n = 6) and 6 min (n = 5) of ischemia relative to control (n = 7, 5, 5, respectively). Similarly, the amplitude of fEPSPs remaining after 2 min, 4 min, 6 min of OGD (n = 6) is increased after administration of 10 nM ChTX. Combining ChTX and BAPTA-AM led to almost no change in fEPSP amplitude up to 6 min of OGD (n = 6). <b>(</b>B) Effect of calcium chelators, and BK channel antagonist on recovery of fEPSP after OGD. BAPTA-AM (1 μM) decreases recovery time from 2 min (n = 7), 4 min (n = 6) and 6 min (n = 3) of <i>in vitro</i> OGD compared to control (n = 7, 5, 5, respectively). EGTA-AM (50μM) shows similar effects to BAPTA-AM (1μM) by decreasing time needed for electrophysiological recovery after 4 min of OGD (n = 6) but not after 6 min (n = 7). ChTX (10 nM) did not significantly decrease recovery time after 6 min of ischemia (n = 4) but a combination of BAPTA-AM (1 μM) and ChTX(10 nM) significantly decreased recovery time after 6 min of OGD (n = 6). <b>(</b>C) BAPTA-AM and BAPTA-AM + ChTX promote increased tissue resistance to a long ischemic episode. BAPTA-AM (1 μM) increases the amount of fEPSP remaining after 8 min of OGD and leads to full recovery after 40 min of reperfusion post ischemia (n = 3) when control tissue has surpassed the point of functional recovery (n = 4). A combination of BAPTA-AM (1 μM) and ChTX (10 nM) leads to almost no change in fEPSP amplitude after prolonged OGD and leads to almost full recovery after 40 min of reperfusion (n = 5). Data plotted as mean ± SE. *<i>p < 0</i>.<i>05</i>, ANOVA, all relative to control condition.</p

Decay time in the presynaptic component action potential (fiber volley) was lengthened in the presence of OGD and chelator + ChTX.

<p>(A) Fiber volley recorded in the presence of BAPTA-AM + ChTX + OGD (solid line) and in presence of BAPTA-AM + ChTX only (dotted line). Fiber volleys were aligned by their negative peaks and superimposed to compare their amplitude and decay time. (B) Decay time of the fiber volley was increased in BAPTA-AM + ChTX + OGD condition versus BAPTA-AM + ChTX alone. It remained unchanged in control, OGD, BAPTA-AM and ChTX condition. (C) Amplitude of the fiber volley was unchanged in the presence of BAPTA-AM + ChTX and BAPTA-AM + ChTX + OGD, as well as control, OGD, BAPTA-AM and ChTX conditions. * p < 0.05, Student t-test</p

Intrinsic properties of the postsynaptic CA1 neurons did not change during transient OGD (n = 8).

<p>(A) Sample recording from a cell when constant current steps were applied via the patching electrode to quantify the spike and membrane properties from the hyperpolarizing and depolarizing voltage response. The current step protocol was used through all experiments when I-V curves were obtained. (B) 2–4 minutes of transient OGD did not cause significant changes in the transmembrane potential, threshold of firing action potential, and the size of AP. (C) 2–4 minutes of transient OGD did not cause significant changes in input resistance. * <i>p</i> < 0.05. mEPSC: miniature excitatory postsynaptic current</p

Effects of transient OGD on mEPSCs in CA1 neurons (n = 8).

<p>Experiments were performed in the presence of TTX (1.0 μM). (A) Voltage clamp recording from a CA1 pyramidal cell before, during transient ischemia, and 10 minutes after recovery. Note the transit inward currents, which represented mEPSCs, increase in frequency. (B) Frequency of mEPSCs increased during transient OGD. (C) Cumulative curve plot showing reduced intervals (increased frequencies) of the mEPSCs during OGD compared to control and recovery. (D) Amplitude of mEPSCs did not change in transient ischemia. (E) The decay time of mEPSC was not altered by the 2 min OGD.</p

Presynaptic intracellular calcium fluorescence measurement from the CA1-stratum radiatum region using Ca Green-1.

<p>Calcium increases in the presynaptic terminal following OGD and is reduced by administration of cell- permeant calcium chelators (BAPTA-AM). (X40 N/A 0.8). A. High power images of axons locally loaded with Ca Green-1 (B) The effect of OGD on intracellular calcium in control (n = 5) and in the presence of 1 μM BAPTA-AM in presynaptic terminals (n = 6).</p

Ischemia depresses the amplitude of evoked synaptic transmission without changes in the presynaptic volley amplitude.

<p>(A). fEPSPs were recorded from the stratum radiatum in the CA1 region while the Schaffer collaterals were stimulated every 15 s. fEPEP amplitude decreased reversibly in a time-dependent manner after 2 min (n = 7), 4 min (n = 5), and 6 min (n = 5) of OGD. (B) fEPSP amplitudes produced by 2 min, 4 min, 6 min of <i>in vitro</i> ischemia relative to controls. (C) Fiber volley amplitude after 8 min of OGD (n = 6, p>0.05) Average plotted as mean ± SE. * p < 0.001: paired student t-test. OGD: Oxygen-glucose deprivation, fEPSP: field excitatory postsynaptic potentials.</p