Department of Neuroscience, Erasmus MC, University Medical Centre Rotterdam, Rotterdam, 3000 CA, The Netherlands.
Sorbonne Université, Inserm, CNRS, Institut de la Vision, 17 Rue Moreau, Paris, F-75012, France.
J Physiol. 2020 Oct;598(20):4603-4619. doi: 10.1113/JP279976. Epub 2020 Aug 6.
During development the giant, auditory calyx of Held forms a one-to-one connection with a principal neuron of the medial nucleus of the trapezoid body. While anatomical studies described that most of the target cells are temporarily contacted by multiple calyces, multi-calyceal innervation was only sporadically observed in in vivo recordings, suggesting a structure-function discrepancy. We correlated synaptic strength of inputs, identified in in vivo recordings, with post hoc labelling of the recorded neuron and synaptic terminals containing vesicular glutamate transporters (VGluT). During development only one input increased to the level of the calyx of Held synapse, and its strength correlated with the large VGluT cluster contacting the postsynaptic soma. As neither competing strong inputs nor multiple large VGluT clusters on a single cell were observed, our findings did not indicate a structure-function discrepancy.
In adult rodents, a principal neuron in the medial nucleus of the trapezoid (MNTB) is generally contacted by a single, giant axosomatic terminal called the calyx of Held. How this one-on-one relation is established is still unknown, but anatomical evidence suggests that during development principal neurons are innervated by multiple calyces, which may indicate calyceal competition. However, in vivo electrophysiological recordings from principal neurons indicated that only a single strong synaptic connection forms per cell. To test whether a mismatch exists between synaptic strength and terminal size, we compared the strength of synaptic inputs with the morphology of the synaptic terminals. In vivo whole-cell recordings of the MNTB neurons from newborn Wistar rats of either sex were made while stimulating their afferent axons, allowing us to identify multiple inputs. The strength of the strongest input increased to calyceal levels in a few days across cells, while the strength of the second strongest input was stable. The recorded cells were subsequently immunolabelled for vesicular glutamate transporters (VGluT) to reveal axosomatic terminals with structured-illumination microscopy. Synaptic strength of the strongest input was correlated with the contact area of the largest VGluT cluster at the soma (r = 0.8), and no indication of a mismatch between structure and strength was observed. Together, our data agree with a developmental scheme in which one input strengthens and becomes the calyx of Held, but not with multi-calyceal competition.
在发育过程中，Held 氏巨大听壶与梯形体内侧核的一个主神经元形成一一对应的连接。虽然解剖学研究描述了大多数靶细胞暂时被多个听壶接触，但在体内记录中仅偶尔观察到多听壶传入，表明存在结构与功能的不匹配。我们将体内记录中鉴定的输入突触强度与记录神经元的事后标记以及含有囊泡谷氨酸转运体（VGluT）的突触末端相关联。在发育过程中，只有一个输入增加到 Held 氏听壶突触的水平，其强度与接触突触后体的大 VGluT 簇相关。由于没有观察到竞争的强输入或单个细胞上的多个大 VGluT 簇，我们的发现并没有表明结构与功能的不匹配。
在成年啮齿动物中，梯形核（MNTB）中的一个主神经元通常被一个称为 Held 氏听壶的单一巨大轴突终末所接触。这种一一对应的关系是如何建立的仍然未知，但解剖学证据表明，在发育过程中，主神经元被多个听壶支配，这可能表明听壶竞争。然而，来自 MNTB 神经元的体内电生理记录表明，每个细胞仅形成一个强突触连接。为了测试突触强度和终末大小之间是否存在不匹配，我们比较了突触输入的强度与突触末端的形态。我们对新生 Wistar 大鼠的 MNTB 神经元进行了体内全细胞记录，同时刺激其传入轴突，从而能够识别多个输入。在几天内，所有细胞的最强输入的强度增加到听壶水平，而第二强输入的强度保持稳定。随后，对记录的细胞进行了囊泡谷氨酸转运体（VGluT）免疫标记，以使用结构照明显微镜揭示轴突体末端。最强输入的突触强度与最大 VGluT 簇在体部的接触面积相关（r=0.8），并且没有观察到结构与强度之间的不匹配。总之，我们的数据与一个发育方案一致，即一个输入增强并成为 Held 氏听壶，但不与多听壶竞争。
