Department of Neurobiology and Behavior, and Program in Neuroscience, State University of New York (SUNY)-Stony Brook, Stony Brook, New York 11794, USA.
J Neurosci. 2013 Feb 27;33(9):4181-91. doi: 10.1523/JNEUROSCI.4423-12.2013.
Thalamocortical circuits are central to sensory and cognitive processing. Recent work suggests that the thalamocortical inputs onto L4 and L6, the main input layers of neocortex, are activated differently by visual stimulation. Whether these differences depend on layer-specific organization of thalamocortical circuits; or on specific properties of synapses onto receiving neurons is unknown. Here we combined optogenetic stimulation of afferents from the visual thalamus and paired recording electrophysiology in L4 and L6 of rat primary visual cortex to determine the organization and plasticity of thalamocortical synapses. We show that thalamocortical inputs onto L4 and L6 differ in synaptic dynamics and sensitivity to visual drive. We also demonstrate that the two layers differ in the organization of thalamocortical and recurrent intracortical connectivity. In L4, a significantly larger proportion of excitatory neurons responded to light activation of thalamocortical terminal fields than in L6. The local microcircuit in L4 showed a higher degree of recurrent connectivity between excitatory neurons than the microcircuit in L6. In addition, L4 recurrently connected neurons were driven by thalamocortical inputs of similar magnitude indicating the presence of local subnetworks that may be activated by the same axonal projection. Finally, brief manipulation of visual drive reduced the amplitude of light-evoked thalamocortical synaptic currents selectively onto L4. These data are the first direct indication that thalamocortical circuits onto L4 and L6 support different aspects of cortical function through layer-specific synaptic organization and plasticity.
丘脑皮层回路是感觉和认知处理的核心。最近的工作表明，视觉刺激以不同的方式激活新皮层的主要输入层 L4 和 L6 的丘脑皮层输入。这些差异是取决于丘脑皮层回路的特定层组织；还是取决于传入神经元上突触的特定特性尚不清楚。在这里，我们结合了来自视觉丘脑的传入纤维的光遗传学刺激和大鼠初级视觉皮层 L4 和 L6 的配对记录电生理学，以确定丘脑皮层突触的组织和可塑性。我们表明，L4 和 L6 的丘脑皮层输入在突触动力学和对视觉驱动的敏感性方面存在差异。我们还证明，这两层在丘脑皮层和皮层内回传连接的组织上存在差异。在 L4 中，与 L6 相比，对丘脑皮层终端场的光激活有反应的兴奋性神经元比例显著更高。L4 中的局部微电路在兴奋性神经元之间表现出更高程度的递归连接性，而 L6 中的微电路则较低。此外，L4 的递归连接神经元受到类似幅度的丘脑皮层输入的驱动，这表明存在局部子网，这些子网可能被相同的轴突投射激活。最后，短暂的视觉驱动操纵选择性地降低了 L4 上光诱发的丘脑皮层突触电流的幅度。这些数据首次直接表明，L4 和 L6 的丘脑皮层回路通过特定层的突触组织和可塑性支持皮层功能的不同方面。
