Bennett M R
Department of Physiology F13, University of Sydney, N.S.W., Australia.
Prog Neurobiol. 1995 Jul;46(4):331-50. doi: 10.1016/0301-0082(94)00061-l.
Spontaneous synaptic potentials were identified at the motor endplate 40 years ago. These were shown to possess amplitudes that could be described by a Gaussian distribution as could the amplitudes of evoked synaptic potentials under conditions of very low probability for secretion. As these Gaussians were identical, the idea of a unit or quantum of transmission was conceived. The failure to obtain similar Gaussian distributions for both spontaneous and low-probability evoked potentials during development of endplates indicated that a unit of transmission was not operating. However both the spontaneous and very low-probability evoked potentials could each be described by mixtures of Gaussians indicating a subunit of transmission might be operative. There are no ganglionic or central synapses at which comparisons have been made between spontaneous and low-probability evoked potentials that show each can be described by a Gaussian distribution, let alone that these are the same indicating a unit of transmission as originally conceived. There is some evidence that mixtures of Gaussians can be used to describe both spontaneous and very low-probability evoked synaptic potential amplitudes, opening up the possibility for a subunit of transmission at these synapses. The vesicle hypothesis, that the quantum of transmission at the endplate is due to the exocytosis of the contents of a synaptic vesicle, was also enunciated nearly 40 years ago. The existence of subunits of transmission has required reconsideration of this hypothesis. Three alternatives are considered: in one, the calcium-transient hypothesis, the subunit of secretion is due to the release of calcium from one of several calcium stores in the nerve terminal, so that several subunits are released when a number of these calcium stores are engaged in a regenerative response to the terminal action potential; a second alternative, the mediatophore hypothesis, is that a subunit of secretion occurs when a single transmitter transport protein channels transmitter across the terminal membrane, several such mediatophore proteins acting in concert then give multiple subunit release; finally, there is the vesicle fusion-pore hypothesis, in which individual transient openings of a fusion-pore channel joining a synaptic vesicle to the terminal membrane are responsible for secretion of a transmitter subunit, with multiple transients giving several subunits. Perhaps we will have distinguished between these possibilities before the quantal hypothesis is 50 years old.
40年前在运动终板发现了自发突触电位。研究表明，这些电位的幅度可用高斯分布来描述，在分泌概率极低的情况下诱发的突触电位幅度也可用高斯分布来描述。由于这些高斯分布是相同的，因此提出了传递单位或量子的概念。在终板发育过程中，自发电位和低概率诱发电位未能获得类似的高斯分布，这表明传递单位不起作用。然而，自发电位和极低概率诱发电位都可以用高斯混合分布来描述，这表明传递亚单位可能起作用。目前还没有在神经节或中枢突触进行过自发电位和低概率诱发电位的比较，结果表明每种电位都可用高斯分布来描述，更不用说两者相同表明存在最初设想的传递单位了。有证据表明，高斯混合分布可用于描述自发电位和极低概率诱发电位的幅度，这为这些突触处存在传递亚单位提供了可能性。囊泡假说认为，终板处的传递量子是由于突触囊泡内容物的胞吐作用，这一假说也在近40年前提出。传递亚单位的存在需要重新考虑这一假说。文中考虑了三种替代假说：一种是钙瞬变假说，分泌亚单位是由于神经末梢中几个钙库之一释放钙所致，因此当多个钙库对终板动作电位产生再生反应时，会释放几个亚单位；第二种替代假说，即介质载体假说，认为当单个递质转运蛋白将递质转运穿过终板膜时会发生分泌亚单位，几个这样的介质载体蛋白协同作用会导致多个亚单位释放；最后是囊泡融合孔假说，其中连接突触囊泡和终板膜的融合孔通道的单个瞬时开放负责递质亚单位的分泌，多个瞬时开放会产生几个亚单位。也许在量子假说诞生50周年之前，我们就能区分这些可能性了。
