Is extracellular calcium buffering involved in regulation of transmitter release at the neuromuscular junction?

During synaptic activity at the neuromuscular junction, sodium, potassium and calcium ions flow through both the postsynaptic and presynaptic membrane. These ionic fluxes can cause changes in the local extracellular concentration in the synaptic gap: a decrease in the concentration of the inwardly flowing ions (sodium and calcium) and an increase in the outwardly flowing potassium ions. To check whether depletion of calcium ions in the synaptic gap is involved in transmitter release, we have used calcium buffers to keep the extracellular calcium concentration almost constant. The expectation was that if depletion does occur, transmitter release will increase; if no depletion occurs, there will be no change in quantal release when the calcium concentration is the same in buffered and unbuffered bathing solutions. We report here that, surprisingly, perfusing the frog neuromuscular preparation with a calcium-buffered solution caused a decrease in transmitter release compared with that in an unbuffered solution with the same calcium concentration. This presumably indicates that the calcium level in the synaptic cleft is higher than that in the bulk extracellular medium. If such a mechanism operates physiologically, it may provide an energetically economical way to determine the level of evoked transmitter release and thus synaptic efficiency.     

Regulation of acetylcholinesterase appearance at neuromuscular junctions in vitro

The appearance of acetylcholinesterase (AChE) at newly formed nerve-muscle synapses depends on synaptic transmission. Synapses form when cultures are grown in the presence of acetylcholine receptor antagonists, but AChE does not accumulate at these synapses. The important component of transmission seems to be muscle activity. Treatment with dibutyryl cyclic GMP mimics muscle activity, directly inducing synaptic AChE appearance.     

Intracellular calcium dependence of transmitter release rates at a fast central synapse

Calcium-triggered fusion of synaptic vesicles and neurotransmitter release are fundamental signalling steps in the central nervous system. It is generally assumed that fast transmitter release is triggered by elevations in intracellular calcium concentration ([Ca2+]i) to at least 100 microM near the sites of vesicle fusion. For synapses in the central nervous system, however, there are no experimental estimates of this local [Ca2+]i signal. Here we show, by using calcium ion uncaging in the large synaptic terminals of the calyx of Held, that step-like elevations to only 10 microM [Ca2+]i induce fast transmitter release, which depletes around 80% of a pool of available vesicles in less than 3 ms. Kinetic analysis of transmitter release rates after [Ca2+]i steps revealed the rate constants for calcium binding and vesicle fusion. These show that transient (around 0.5 ms) local elevations of [Ca2+]i to peak values as low as 25 microM can account for transmitter release during single presynaptic action potentials. The calcium sensors for vesicle fusion are far from saturation at normal release probability. This non-saturation, and the high intracellular calcium cooperativity in triggering vesicle fusion, make fast synaptic transmission very sensitive to modulation by changes in local [Ca2+]i.     

Postsynaptic translation affects the efficacy and morphology of neuromuscular junctions

Long-term synaptic plasticity may be associated with structural rearrangements within the neuronal circuitry. Although the molecular mechanisms governing such activity-controlled morphological alterations are mostly elusive, polysomal accumulations at the base of developing dendritic spines and the activity-induced synthesis of synaptic components suggest that localized translation is involved during synaptic plasticity. Here we show that large aggregates of translational components as well as messenger RNA of the postsynaptic glutamate receptor subunit DGluR-IIA are localized within subsynaptic compartments of larval neuromuscular junctions of Drosophila melanogaster. Genetic models of junctional plasticity and genetic manipulations using the translation initiation factors eIF4E and poly(A)-binding protein showed an increased occurrence of subsynaptic translation aggregates. This was associated with a significant increase in the postsynaptic DGluR-IIA protein levels and a reduction in the junctional expression of the cell-adhesion molecule Fasciclin II. In addition, the efficacy of junctional neurotransmission and the size of larval neuromuscular junctions were significantly increased. Our results therefore provide evidence for a postsynaptic translational control of long-term junctional plasticity.     

麦草碱法黑液热解的产物特性及释放规律

对麦草碱法制浆黑液固形物(BLS) 400 ～ 800℃热解产物组成特性进行了研究.采用气相色谱(GC)和气相色谱-质谱(GC-MS)对热解气相产物和液相产物组成进行了全面分析;利用扫描电镜能谱仪(SEM-EDS)对固相产物的表面形貌及元素分布进行了表征.借助热重红外联用技术(TG-FTIR)探讨了热解过程中主要产物的释放规律.结果表明,BLS热解产物中,气相产物主要成分为H2、CO、CO2以及CxHy(CH4、C2H2、C2H4、C2H6)等,液相产物中主要是酚类、酮类、醇类、醚类、酸类、呋喃以及芳香化合物.TG-FTIR结果显示,BLS热解产物释放时间主要集中在l 000 ～2 000 s(300 ～600℃),此时BLS释放的产物主要为H2O、CO2、CH4、CO,以及烷烃类、芳烃类、酚醇类和酮类化合物.