Moulder Krista L, Mennerick Steven
Department of Psychiatry, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Neurosci. 2005 Apr 13;25(15):3842-50. doi: 10.1523/JNEUROSCI.5231-04.2005.
The size of the readily releasable pool (RRP) of vesicles is critically important for determining the size of postsynaptic currents generated in response to action potentials. However, discrepancies in RRP estimates exist among methods designed to measure RRP size. In glutamatergic hippocampal neurons, we found that hypertonic sucrose application yielded RRP size estimates approximately fivefold larger than values obtained with high-frequency action potential trains commonly assumed to deplete the RRP. This discrepancy was specific for glutamatergic neurons, because no difference was found between sucrose and train estimates of RRP size in GABAergic neurons. A small component of the difference in excitatory neurons was accounted for by postsynaptic receptor saturation. Train estimates of vesicle pool size obtained using more stimuli revealed that action potential-elicited EPSCs did not truly reach a steady state during shorter trains, and RRP estimates were closer to sucrose estimates made in the same neurons. This suggested that reluctant vesicles may contribute to the total available pool. Two additional lines of evidence supported this hypothesis. First, RRP estimates from strongly depolarizing hyperkalemic solutions closely matched those obtained with sucrose. Second, when Ca2+ influx was enhanced during trains, train estimates of pool size matched those obtained with sucrose. These data suggest that glutamatergic hippocampal neurons maintain a heterogeneous population of vesicles that can be differentially released with varying Ca2+ influx, thereby increasing the range of potential synaptic responses.
囊泡的易释放池（RRP）大小对于确定动作电位引发的突触后电流大小至关重要。然而，在用于测量RRP大小的方法之间，RRP估计值存在差异。在谷氨酸能海马神经元中，我们发现应用高渗蔗糖得出的RRP大小估计值比通常认为会耗尽RRP的高频动作电位序列所获得的值大约大五倍。这种差异是谷氨酸能神经元特有的，因为在GABA能神经元中，蔗糖和序列对RRP大小的估计没有差异。兴奋性神经元差异的一小部分是由突触后受体饱和导致的。使用更多刺激获得的囊泡池大小的序列估计表明，在较短的序列期间，动作电位引发的兴奋性突触后电流（EPSC）并未真正达到稳态，并且RRP估计值更接近同一神经元中蔗糖的估计值。这表明反应迟缓的囊泡可能对总可用池有贡献。另外两条证据支持了这一假设。首先，来自强去极化高钾溶液的RRP估计值与蔗糖获得的估计值紧密匹配。其次，当在序列期间Ca2+内流增强时，池大小的序列估计值与蔗糖获得的估计值匹配。这些数据表明，谷氨酸能海马神经元维持着异质性的囊泡群体，这些囊泡可以随着不同的Ca2+内流而被差异释放，从而增加潜在突触反应的范围。
