Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611.
Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611
J Neurosci. 2020 Apr 1;40(14):2849-2858. doi: 10.1523/JNEUROSCI.2277-19.2020. Epub 2020 Feb 19.
Cortical projections to the thalamus arise from corticothalamic (CT) neurons in layer 6 and pyramidal tract-type (PT) neurons in layer 5B. We dissected the excitatory synaptic connections in the somatosensory thalamus formed by CT and PT neurons of the primary somatosensory (S1) cortex, focusing on mouse forelimb S1. Mice of both sexes were studied. The CT neurons in S1 synaptically excited S1-projecting thalamocortical (TC) neurons in subregions of both the ventral posterior lateral and posterior (PO) nuclei, forming a pair of recurrent cortico-thalamo-cortical (C-T-C) loops. The PT neurons in S1 also formed a recurrent loop with S1-projecting TC neurons in the same subregion of the PO. The PT neurons in the adjacent primary motor (M1) cortex formed a separate recurrent loop with M1-projecting TC neurons in a nearby subregion of the PO. Collectively, our results reveal that C-T-C circuits of mouse forelimb S1 are primarily organized as multiple cortical cell-type-specific and thalamic subnucleus-specific recurrent loops, with both CT and PT neurons providing the strongest excitatory input to TC neurons that project back to S1. The findings, together with those of related studies of C-T-C circuits, thus suggest that recurrently projecting thalamocortical neurons are the principal targets of cortical excitatory input to the mouse somatosensory and motor thalamus. Bidirectional cortical communication with the thalamus is considered an important aspect of sensorimotor integration for active touch in the somatosensory system, but the cellular organization of the circuits mediating this process is not well understood. We used an approach combining cell-type-specific anterograde optogenetic excitation with single-cell recordings targeted to retrogradely labeled thalamocortical neurons to dissect these circuits. The findings reveal a consistent pattern: cortical projections to the somatosensory thalamus target thalamocortical neurons that project back to the same cortical area. Commonalities of these findings to previous descriptions of related circuits in other areas suggest that cortico-thalamo-cortical circuits may generally be organized primarily as recurrent loops.
皮质丘脑投射由 6 层的皮质丘脑（CT）神经元和 5B 层的皮质脊髓束型（PT）神经元产生。我们剖析了来自初级体感（S1）皮质的 CT 和 PT 神经元在体感丘脑中形成的兴奋性突触连接，重点研究了鼠标前肢 S1。研究了雌雄小鼠。S1 的 CT 神经元突触兴奋了腹后外侧核（VP）和后核（PO）的 S1 投射丘脑皮质（TC）神经元，形成了一对皮质丘脑皮质（C-T-C）的循环。S1 的 PT 神经元也与 PO 的同一亚区的 S1 投射 TC 神经元形成了一个循环。毗邻的初级运动（M1）皮质中的 PT 神经元与 PO 的附近亚区中的 M1 投射 TC 神经元形成了单独的循环。总的来说，我们的结果表明，鼠标前肢 S1 的 C-T-C 回路主要是由多个皮质细胞类型特异性和丘脑亚核特异性的循环回路组成的，CT 和 PT 神经元为投射回 S1 的 TC 神经元提供了最强的兴奋性输入。这些发现与 C-T-C 回路的相关研究结果一起表明，反向投射的丘脑皮质神经元是皮质兴奋性输入到鼠标体感和运动丘脑的主要靶标。与丘脑的双向皮质通讯被认为是体感系统主动触摸中的感觉运动整合的一个重要方面，但介导这一过程的回路的细胞组织尚不清楚。我们使用了一种结合细胞类型特异性顺行光遗传学兴奋与针对逆行标记的丘脑皮质神经元的单细胞记录的方法来剖析这些回路。这些发现揭示了一个一致的模式：体感丘脑的皮质投射靶向投射回同一皮质区域的丘脑皮质神经元。这些发现与以前在其他区域描述的相关回路的描述有共同之处，这表明皮质丘脑皮质回路可能通常主要组织为循环回路。
