Rollenhagen Astrid, Sätzler Kurt, Rodríguez E Patricia, Jonas Peter, Frotscher Michael, Lübke Joachim H R
Institute of Neuroscience and Biophysics INB-3, Research Centre Jülich, D-52425 Jülich, Germany.
J Neurosci. 2007 Sep 26;27(39):10434-44. doi: 10.1523/JNEUROSCI.1946-07.2007.
Synapses are the key elements for signal processing and plasticity in the brain. To determine the structural factors underlying the unique functional properties of the hippocampal mossy fiber synapse, the complete quantitative geometry was investigated, using electron microscopy of serial ultrathin sections followed by computer-assisted three-dimensional reconstruction. In particular, parameters relevant for transmitter release and synaptic plasticity were examined. Two membrane specializations were found: active zones (AZs), transmitter release sites, and puncta adherentia, putative adhesion complexes. Individual boutons had, on average, 25 AZs (range, 7-45) that varied in shape and size (mean, 0.1 microm2; range, 0.07-0.17 microm2). The mean distance between individual AZs was 0.45 microm. Mossy fiber boutons and their target structures were mostly ensheathed by astrocytes, but fine glial processes never reached the active zones. Two structural factors are likely to promote synaptic cross talk: the short distance between AZs and the absence of fine glial processes at AZs. Thus, synaptic cross talk may contribute to the efficacy of hippocampal mossy fiber synapses. On average, a bouton contained 20,400 synaptic vesicles; approximately 900 vesicles were located within 60 nm from the active zone, approximately 4400 between 60 and 200 nm, and the remaining beyond 200 nm, suggesting large readily releasable, recycling, and reserve pools. The organization of the different pools may be a key structural correlate of presynaptic plasticity at this synapse. Thus, the mossy fiber bouton differs fundamentally in structure and function from the calyx of Held and other central synapses.
突触是大脑中信号处理和可塑性的关键要素。为了确定海马苔藓纤维突触独特功能特性背后的结构因素，我们使用连续超薄切片电子显微镜和计算机辅助三维重建技术，对其完整的定量几何结构进行了研究。特别地，我们检查了与递质释放和突触可塑性相关的参数。我们发现了两种膜特化结构：活性区（AZs），即递质释放位点，以及点状黏附结构，即假定的黏附复合体。单个终扣平均有25个活性区（范围为7 - 45个），其形状和大小各不相同（平均为0.1平方微米；范围为0.07 - 0.17平方微米）。单个活性区之间的平均距离为0.45微米。苔藓纤维终扣及其靶结构大多被星形胶质细胞包裹，但细小的胶质突起从未到达活性区。有两个结构因素可能会促进突触间的串扰：活性区之间的短距离以及活性区缺乏细小的胶质突起。因此，突触间串扰可能有助于海马苔藓纤维突触的效能。平均而言，一个终扣含有20400个突触小泡；约900个小泡位于距活性区60纳米以内，约4400个在60至200纳米之间，其余的在200纳米以外，这表明存在大量易于释放、循环利用和储备的小泡池。不同小泡池的组织方式可能是该突触前突触可塑性的关键结构相关因素。因此，苔藓纤维终扣在结构和功能上与Held壶腹及其他中枢突触有根本区别。
