Department of Neurobiology, University of Chicago, Chicago, IL, USA.
Eur J Neurosci. 2021 Feb;53(3):750-762. doi: 10.1111/ejn.15092. Epub 2021 Jan 10.
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.
许多研究都集中在初级感觉皮层的丘脑皮层突触上，但对高级丘脑皮质投射到高级皮质区域的研究却知之甚少。我们使用遗传标记结合大容量序列电子显微镜（即“连接组学”）来研究丘脑后内侧核到小鼠次级体感皮层的投射，从而开始解决这一差距。我们将一种表达 Cre 的病毒和一种表达 Cre 依赖性抗坏血酸过氧化物酶的病毒混合注入到这个丘脑核中，这种酶提供了电子致密的细胞质标记。这种“交叉”病毒方法特异性地标记了丘脑皮质轴突和突触，没有逆行标记，在皮质的所有层中都是如此。标记的丘脑皮质突触代表皮质体积中所有突触的 14%，与先前对一级丘脑皮质输入的估计一致。我们发现，与未标记的突触相比，标记的丘脑皮质突触：更大，更有可能含有一个线粒体，更频繁地靶向棘突树突，避免非棘突树突，并且经常支配带有棘突装置的较大棘突，以及其他差异。此外，标记的末梢在 2/3 层中更为常见，而标记和未标记的末梢之间的突触差异在 2/3 层中最大。这里报告的分层差异与初级感觉皮层中一级丘脑皮质连接的报告形成对比，例如，标记的末梢在 4 层中比在 2/3 层中更大（Viaene 等人，2011a）。这些数据首次提供了高级丘脑皮质突触超微结构的 glimpse，并指出需要进行更多的分析，因为这种连接可能代表了大多数丘脑皮质电路。
