An ultrastructural connectomic analysis of a higher‐order thalamocortical circuit in the mouse,European Journal of Neuroscience

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An ultrastructural connectomic analysis of a higher‐order thalamocortical circuit in the mouse
European Journal of Neuroscience ( IF 2.7 ) Pub Date : 2020-12-23 , DOI: 10.1111/ejn.15092
Vandana Sampathkumar ₁ , Andrew Miller‐Hansen ₁ , S. Murray Sherman ₁ , Narayanan Kasthuri ₁

Affiliation

Many studies exist of thalamocortical synapses in primary sensory cortex, but much less in known about higher‐order thalamocortical projections to higher‐order cortical areas. We begin to address this gap using genetic labeling combined with large volume serial electron microscopy (i.e., “connectomics”) to study the projection from the thalamic posterior medial nucleus to the secondary somatosensory cortex in a mouse. We injected into this thalamic nucleus a cocktail combining a cre‐expressing virus and one expressing cre‐dependent ascorbate peroxidase that provides an electron dense cytoplasmic label. This “intersectional” viral approach specifically labeled thalamocortical axons and synapses, free of retrograde labeling, in all layers of cortex. Labeled thalamocortical synapses represented 14% of all synapses in the cortical volume, consistent with previous estimates of first‐order thalamocortical inputs. We found that labeled thalamocortical terminals, relative to unlabeled ones: were larger, were more likely to contain a mitochondrion, more frequently targeted spiny dendrites and avoided aspiny dendrites, and often innervated larger spines with spine apparatuses, among other differences. Furthermore, labeled terminals were more prevalent in layers 2/3 and synaptic differences between labeled and unlabeled terminals were greatest in layers 2/3. The laminar differences reported here contrast with reports of first‐order thalamocortical connections in primary sensory cortices where, for example, labeled terminals were larger in layer 4 than layers 2/3 (Viaene et al., 2011a). These data offer the first glimpse of higher‐order thalamocortical synaptic ultrastructure and point to the need for more analyses, as such connectivity likely represents a majority of thalamocortical circuitry.

中文翻译：

小鼠高级别丘脑皮质回路的超微结构组学分析

初级感觉皮层中存在丘脑突触的许多研究，但对高阶丘脑皮层投射到高阶皮质区域的了解却很少。我们开始使用基因标记结合大体积串联电子显微镜（即“连接体组学”）来研究小鼠从丘脑后内侧核到次级体感皮层的投射，从而解决这一差距。我们注入到这个丘脑核鸡尾酒组合CRE -表达病毒和一个表达CRE依赖的抗坏血酸过氧化物酶，可提供电子密集的胞质标记。这种“交叉”病毒方法在皮质的所有层中都专门标记了丘脑皮质轴突和突触，没有逆行标记。标记的丘脑皮质突触占皮质体积中所有突触的14％，与先前对一阶丘脑皮质输入的估计一致。我们发现带标签的丘脑皮质末端相对于未标记的丘脑皮质末端更大，更可能包含线粒体，更经常靶向刺状树突，避免了刺状树突，并经常用脊柱器械支配较大的刺，以及其他差异。此外，标记的末端在第2/3层中更为普遍，标记的和未标记的末端之间的突触差异在第2/3层中最大。此处报道的层流差异与初级感觉皮质中一级丘脑皮质连接的报道形成对比，例如，第4层中标记的末端大于第2/3层（Viaene等，2011a）。这些数据提供了更高级别的丘脑突触超微结构的第一印象，并指出需要进行更多分析，因为这种连通性可能代表了丘脑皮层电路的大部分。

更新日期：2021-02-15

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