Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, USA.
J Neurosci. 2012 Apr 4;32(14):4992-5001. doi: 10.1523/JNEUROSCI.4759-11.2012.
The generation of purposive movement by mammals involves coordinated activity in the corticospinal and corticostriatal systems, which are involved in different aspects of motor control. In the motor cortex, corticospinal and corticostriatal neurons are closely intermingled, raising the question of whether and how information flows intracortically within and across these two channels. To explore this, we developed an optogenetic technique based on retrograde transfection of neurons with deletion-mutant rabies virus encoding channelrhodopsin-2, and used this in conjunction with retrograde anatomical labeling to stimulate and record from identified projection neurons in mouse motor cortex. We also used paired recordings to measure unitary connections. Both corticospinal and callosally projecting corticostriatal neurons in layer 5B formed within-class (recurrent) connections, with higher connection probability among corticostriatal than among corticospinal neurons. In contrast, across-class connectivity was extraordinarily asymmetric, essentially unidirectional from corticostriatal to corticospinal. Corticostriatal neurons in layer 5A and corticocortical neurons (callosal projection neurons similar to corticostriatal neurons) similarly received a paucity of corticospinal input. Connections involving presynaptic corticostriatal neurons had greater synaptic depression, and those involving postsynaptic corticospinal neurons had faster decaying EPSPs. Consequently, the three connections displayed a diversity of dynamic properties reflecting the different combinations of presynaptic and postsynaptic projection neurons. Collectively, these findings delineate a four-way specialized excitatory microcircuit formed by corticospinal and corticostriatal neurons. The "rectifying" corticostriatal-to-corticospinal connectivity implies a hierarchical organization and functional compartmentalization of corticospinal activity via unidirectional signaling from higher-order (corticostriatal) to lower-order (corticospinal) output neurons.
哺乳动物有目的运动的产生涉及皮质脊髓和皮质纹状体系统的协调活动，这些系统涉及运动控制的不同方面。在运动皮层中，皮质脊髓和皮质纹状体神经元紧密交织在一起，这就提出了一个问题，即信息是否以及如何在这两个通道内和跨通道进行皮质内传递。为了探索这一点，我们开发了一种基于逆行转染神经元的光遗传学技术，使用缺失突变的狂犬病毒来编码通道视紫红质-2，然后结合逆行解剖学标记，刺激和记录小鼠运动皮层中已识别的投射神经元。我们还使用成对记录来测量单元连接。5B 层中的皮质脊髓和胼胝体投射的皮质纹状体神经元形成了同类型（递归）连接，皮质纹状体神经元之间的连接概率高于皮质脊髓神经元之间的连接概率。相比之下，跨类型连接具有极高的不对称性，基本上是从皮质纹状体到皮质脊髓的单向连接。5A 层中的皮质纹状体神经元和皮质皮质神经元（类似于皮质纹状体神经元的胼胝体投射神经元）同样接收很少的皮质脊髓输入。涉及到突触前皮质纹状体神经元的连接具有更大的突触抑制作用，而涉及到突触后皮质脊髓神经元的连接具有更快衰减的 EPSP。因此，这三种连接具有不同的动态特性，反映了不同的突触前和突触后投射神经元的组合。总的来说，这些发现描绘了由皮质脊髓和皮质纹状体神经元形成的四向专门兴奋性微电路。“整流”的皮质纹状体到皮质脊髓的连接暗示了皮质脊髓活动的分层组织和功能分区，通过从高级（皮质纹状体）到低级（皮质脊髓）输出神经元的单向信号传递。
