Faber D S, Lin J W, Korn H
Department of Physiology, State University of New York, Buffalo 14214.
Ann N Y Acad Sci. 1991;627:151-64. doi: 10.1111/j.1749-6632.1991.tb25920.x.
Comparison of the two afferent systems illustrates certain features common to synaptic transmission as well as differences that might be important for synaptic plasticity. Transmission at both the inhibitory and excitatory connections is satisfactorily described by a simple binomial model that considers the average probability of release to be the same at each active site, although it should be stressed that the best evidence derives from the first set of afferents. Another similarity between the two systems is that short-term changes in synaptic efficacy, namely, facilitation and depression, appear to be due to changes in p. We previously suggested that both phenomena occur during repetitive stimulation, with the dominant effect depending upon the initial probability of release. It remains to be seen if depression dominates at other inhibitory connections, although it is already clear that one cannot generalize about excitation, because some excitatory junctions have an initial high p and exhibit a marked depression rather than the facilitation described here. We have found no evidence for the notion that some synapses within a connection may be silent. That idea has been proposed, but not proven, for other synaptic connections in the vertebrate central nervous system. Indeed, it will be difficult to assess as long as quantal release cannot be reliably detected at these junctions, and morphological confirmation at the ultrastructural level will also be required. On the other hand, evidence from a few peripheral junctions where one presynaptic afferent establishes hundreds of contacts with its target cell, does suggest the possibility of silent synapses, or at least an extremely low probability of release in those cases. These situations may correspond to extremes of our finding that as the number of release sites increases, p decreases. Regardless, the inverse relation between n and p suggests caution should be exercised in interpreting data indicating that synaptic plasticity is associated with increased numbers of synapses between two cells. Although we have not detected silent synapses within a transmitting connection, we have observed chemically silent connections between neurons, and the evidence reviewed here suggests transmission may be blocked postsynaptically, as with the inhibitory connections, or presynaptically, as with the excitatory ones. Although the underlying mechanisms are only partially elucidated, it is also clear that such connections can be switched into a transmitting mode. Consequently, they may provide a significant reserve that might well become functional in different behavioral states or in response to certain patterns of activity.(ABSTRACT TRUNCATED AT 400 WORDS)
对两种传入系统的比较揭示了突触传递的某些共同特征以及对突触可塑性可能很重要的差异。抑制性和兴奋性连接的传递都可以用一个简单的二项式模型来令人满意地描述，该模型认为每个活动位点的平均释放概率相同，不过应该强调的是，最有力的证据来自第一组传入神经。这两个系统的另一个相似之处是，突触效能的短期变化，即易化和抑制，似乎是由于p的变化。我们之前提出，这两种现象在重复刺激期间都会发生，主导效应取决于初始释放概率。在其他抑制性连接中抑制是否占主导地位还有待观察，尽管很明显不能对兴奋进行一概而论，因为一些兴奋性突触的初始p值较高，表现出明显的抑制而非此处描述的易化。我们没有找到证据支持这样的观点，即一个连接内的一些突触可能是沉默的。这个想法已经针对脊椎动物中枢神经系统中的其他突触连接提出，但尚未得到证实。事实上，只要在这些突触处不能可靠地检测到量子释放，就很难评估，而且还需要超微结构水平的形态学证实。另一方面，来自少数外周突触的证据表明，一个突触前传入神经与其靶细胞建立了数百个接触，确实表明存在沉默突触的可能性，或者至少在这些情况下释放概率极低。这些情况可能对应于我们发现的极端情况，即随着释放位点数量的增加，p值会降低。无论如何，n和p之间的反比关系表明，在解释表明突触可塑性与两个细胞之间突触数量增加相关的数据时应谨慎。虽然我们在一个传导连接内没有检测到沉默突触，但我们观察到了神经元之间的化学沉默连接，这里回顾的证据表明，传递可能在突触后被阻断，就像抑制性连接一样，或者在突触前被阻断，就像兴奋性连接一样。虽然潜在机制只是部分得到阐明，但很明显，这样的连接可以切换到传导模式。因此，它们可能提供了一个重要的储备，很可能在不同的行为状态或对某些活动模式的反应中发挥作用。（摘要截于400字）
