Department of Neurobiology, The University of Chicago, Chicago, IL 60637.
Argonne National Laboratory, Lemont, IL, 60439.
Proc Natl Acad Sci U S A. 2022 Sep 20;119(38):e2205209119. doi: 10.1073/pnas.2205209119. Epub 2022 Sep 12.
Neurons in the thalamic reticular nucleus (TRN) are a primary source of inhibition to the dorsal thalamus and, as they are innervated in part by the cortex, are a means of corticothalamic regulation. Previously, cortical inputs to the TRN were thought to originate solely from layer 6 (L6), but we recently reported the presence of putative synaptic terminals from layer 5 (L5) neurons in multiple cortical areas in the TRN [J. A. Prasad, B. J. Carroll, S. M. Sherman, 40, 5785-5796 (2020)]. Here, we demonstrate with electron microscopy that L5 terminals from multiple cortical regions make bona fide synapses in the TRN. We further use light microscopy to localize these synapses relative to recently described TRN subdivisions and show that L5 terminals target the edges of the somatosensory TRN, where neurons reciprocally connect to higher-order thalamus, and that L5 terminals are scarce in the core of the TRN, where neurons reciprocally connect to first-order thalamus. In contrast, L6 terminals densely innervate both edge and core subregions and are smaller than those from L5. These data suggest that a sparse but potent input from L5 neurons of multiple cortical regions to the TRN may yield transreticular inhibition targeted to higher-order thalamus.
丘脑网状核（TRN）中的神经元是对背侧丘脑的主要抑制源，并且由于它们部分受皮质的支配，因此是皮质丘脑调节的一种方式。以前，认为皮质对 TRN 的输入仅源自第 6 层（L6），但我们最近报道了多个皮质区域中的第 5 层（L5）神经元的假定突触末梢在 TRN 中存在[J. A. Prasad，B. J. Carroll，S. M. Sherman，40，5785-5796（2020）]。在这里，我们使用电子显微镜证明了来自多个皮质区域的 L5 末梢在 TRN 中形成真正的突触。我们进一步使用荧光显微镜将这些突触相对于最近描述的 TRN 细分定位，并表明 L5 末梢靶向体感 TRN 的边缘，其中神经元与高级丘脑反向连接，并且 L5 末梢在 TRN 的核心中稀少，其中神经元与一级丘脑反向连接。相比之下，L6 末梢密集地支配边缘和核心亚区，并且比 L5 的末梢小。这些数据表明，来自多个皮质区域的 L5 神经元的稀疏但有效的输入到 TRN 可能产生针对高级丘脑的跨网状抑制。
