Li Lu, Ebner Ford F
Allen Institute for Brain Science, Seattle, Washington; and
Department of Psychology, Vanderbilt University, Nashville, Tennessee.
J Neurophysiol. 2016 Nov 1;116(5):2368-2382. doi: 10.1152/jn.00424.2016. Epub 2016 Aug 31.
Cortex actively modulates the responses of thalamic relay neurons through corticothalamic (CT) projections. Here we investigated the temporal precision of CT modulation on sensory responses of relay neurons in rat ventral posterior medial thalamus (VPM) to direction-specific whisker stimuli. CT feedback levels were either augmented by cortical electrical microstimulation or depressed by cortical application of muscimol, a potent agonist of γ-aminobutyric acid A-type (GABA) receptors. To evaluate the temporal specificity of CT influence, we compared the early (3-10 ms after stimulus onset) and late (10-100 ms) response components of VPM single units to whisker deflections in preferred or nonpreferred directions before and after altering CT feedback levels under urethane anesthesia. The data showed that cortical feedback most strongly affected the late responses of single VPM units to whisker stimulation. That is, cortical stimulation consistently increased the late responses of VPM units in the corresponding (homologous) barreloids to the stimulus direction preferred by neurons in the cortical locus stimulated. However, cortical stimulation could either increase or decrease the early response, depending on whether or not cortical and thalamic loci were tuned to the same direction. Such bidirectional regulation of the early and late VPM responses is consistent with a mechanism of circuit-level disinhibition in vivo. The results support the theory that CT feedback on thalamic sensory responses is mediated by a time-dependent shift of the excitation-inhibition balance in the thalamo-cortico-thalamic loop, such as would occur during sensory feature integration, plasticity, and learning in the awake state.
皮层通过皮质丘脑（CT）投射积极调节丘脑中继神经元的反应。在此，我们研究了CT调制对大鼠腹后内侧丘脑（VPM）中继神经元对方向特异性触须刺激的感觉反应的时间精度。通过皮层电微刺激增强CT反馈水平，或通过皮层应用GABAA型（GABA）受体的强效激动剂蝇蕈醇来降低CT反馈水平。为了评估CT影响的时间特异性，我们比较了在乌拉坦麻醉下改变CT反馈水平之前和之后，VPM单个神经元对首选或非首选方向的触须偏转的早期（刺激开始后3-10毫秒）和晚期（10-100毫秒）反应成分。数据表明，皮层反馈对单个VPM神经元对触须刺激的晚期反应影响最强。也就是说，皮层刺激持续增加了相应（同源）桶状区中VPM神经元对皮层刺激位点神经元首选的刺激方向的晚期反应。然而，皮层刺激可以增加或减少早期反应，这取决于皮层和丘脑位点是否被调整到相同方向。VPM早期和晚期反应的这种双向调节与体内电路水平去抑制的机制一致。结果支持这样一种理论，即丘脑感觉反应的CT反馈是由丘脑-皮质-丘脑环路中兴奋-抑制平衡的时间依赖性转变介导的，就像在清醒状态下感觉特征整合、可塑性和学习过程中会发生的那样。
