He T, Singh V, Rumpal N, Lnenicka G A
Department of Biological Sciences, University at Albany, State University of New York, Albany, NY 12222, USA.
Neuroscience. 2009 Apr 10;159(4):1283-91. doi: 10.1016/j.neuroscience.2009.01.074. Epub 2009 Feb 7.
We determined whether two classes of Drosophila larval motor terminals with known differences in structure and transmitter release also showed differences in Ca(2+) regulation. Larval motor neurons can be separated into those producing large synaptic boutons (Ib) and those with small boutons (Is). Ib terminals release less transmitter during single action potentials (APs) than Is terminals, but show greater facilitation during high-frequency stimulation. We measured Ca(2+) transients produced by single APs and AP trains after loading the terminals with the dextran-conjugated Ca(2+) indicator Oregon Green 488 BAPTA-1 (OGB-1). The two pairs of Is and Ib terminals innervating muscle fiber 4 and fibers 6 and 7 were examined. The OGB-1 concentrations were measured in order to compare measurements from terminals with similar OGB-1 loading. For single APs, the change in OGB-1 fluorescence (DeltaF/F) in Is boutons was significantly larger than in Ib boutons due to greater Ca(2+) influx per bouton volume. The Is boutons had greater surface area and active zone number per bouton volume than Ib boutons; this could account for the differences in Ca(2+) influx and argues for similar Ca(2+) influx at Is and Ib active zones. As previously reported for the Ib boutons, the distal Is boutons had larger single-AP Ca(2+) transients than proximal ones on muscle fibers 6 and 7, but not on fiber 4. This difference was not due to proximal-distal differences in surface area or active zones per bouton volume and may be due to greater Ca(2+) influx at distal active zones. During AP trains, the Is Ca(2+) transients were larger in amplitude and had longer decay time constants than Ib ones. This can be explained by a slower rate of Ca(2+) extrusion from the Is boutons apparently due to lower plasma membrane Ca(2+) ATPase activity at Is boutons compared to Ib boutons.
我们确定了两类在结构和递质释放方面存在已知差异的果蝇幼虫运动终末在钙（Ca²⁺）调节上是否也存在差异。幼虫运动神经元可分为产生大突触小体的（Ib）和产生小突触小体的（Is）。Ib终末在单个动作电位（AP）期间释放的递质比Is终末少，但在高频刺激期间表现出更大的易化作用。在用葡聚糖偶联的钙指示剂俄勒冈绿488 BAPTA - 1（OGB - 1）加载终末后，我们测量了单个AP和AP串产生的钙瞬变。研究了支配肌肉纤维4以及纤维6和7的两对Is和Ib终末。测量了OGB - 1浓度，以便比较具有相似OGB - 1加载量的终末的测量结果。对于单个AP，由于每个突触小体体积内钙内流更多，Is突触小体中OGB - 1荧光的变化（ΔF/F）显著大于Ib突触小体。Is突触小体每单位突触小体体积的表面积和活性区数量比Ib突触小体更大；这可以解释钙内流的差异，并表明Is和Ib活性区的钙内流相似。如先前关于Ib突触小体的报道，在肌肉纤维6和7上，远端Is突触小体的单个AP钙瞬变比近端的大，但在纤维4上并非如此。这种差异不是由于每单位突触小体体积的表面积或活性区的近端 - 远端差异，可能是由于远端活性区的钙内流更多。在AP串期间，Is的钙瞬变在幅度上更大，衰减时间常数比Ib的更长。这可以通过Is突触小体中钙外流速率较慢来解释，这显然是由于与Ib突触小体相比，Is突触小体的质膜钙ATP酶活性较低。
