小鼠的皮层、丘脑网状核可以被划分成多个功能区，背侧丘脑也分为几十个核团，他们形成的环路连接参与了感觉信息处理、记忆形成和注意力调控等多种认知功能。由于技术限制，多数在清醒动物中进行的皮层-丘脑环路的连接及功能研究，只涉及少数皮层功能区和特定的丘脑核团。这导致我们对各皮层功能区与丘脑网状核、背侧丘脑核团之间的功能性连接缺乏系统的理解。而系统地研究大范围脑区间的功能性投射需要高时空精度和高实验通量的研究方法。针对以上问题，本研究开发了小鼠背部皮层到皮下脑区的功能性投射的研究框架，综合了高时间精度、高通量的光遗传学控制，高通量的电生理记录和高精度记录位点重构等方法。该研究框架实现了在小鼠背部皮层进行高精度、高通量的可逆光抑制的同时，记录丘脑网状核和背侧丘脑中的数十个位点，并将记录位点在标准脑中进行精确比对定位。基于这套研究框架，我们绘制了背部皮层到丘脑网状核与背侧丘脑多个核团的功能性投射图谱，并在不同尺度上探究了皮层到丘脑网状核和背侧丘脑的功能性投射。首先，我们依据核团接收的皮层功能性投射强度，将背侧丘脑核团分为三类核群，并发现丘脑核团的皮层输入中心与核团的空间位置显著相关。其次，我们依据皮层投射强度对核团内的记录位点进行了聚类分析，发现多个核团从内侧前腹部到外侧后背部接收的皮层输入是从前侧皮层向后侧皮层过渡。该结果阐明了皮层的投射在丘脑网状核和背侧丘脑核团内部分布的空间规律。然后，我们证实了丘脑网状核和部分丘脑核团中存在大量神经元能够接收数个皮层区域的投射，说明这些核团中广泛存在信息的整合。同时，小部分相邻的丘脑网状核神经元具有不同的皮层输入，说明丘脑网状核参与调控的环路更加复杂。最后，本研究还将该图谱构建框架与动物行为系统结合，分析了在决策行为中不同活动特征的丘脑网状核前区神经元所接收的皮层投射差异。本研究开发了皮层到皮下脑区功能投射的高效研究方法，并首次绘制了皮层到丘脑网状核及背侧丘脑的功能投射图谱，展示了丘脑众多核团中复杂的信息整合和来自皮层的空间特异性输入，为深入认识皮层-丘脑之间的相互作用及环路研究提供了新的方向。并且我们运用该图谱分析了行为中有选择性的持续性活动的丘脑网状核神经元的皮层输入特征，为研究其在行为中的功能提供了新的思路。
The cortex, reticular nucleus of the thalamus (RT) and dorsal thalamus of mice can be divided into several functional areas or dozens of nuclei, respectively. They form circuits involved in multiple brain functions: sensation, memory and attention, for example. Most studies on functions of the cortex, RT and dorsal thalamus concentrated on limited areas or nuclei, which lacks comprehension of the systemic cortico-RT-thalamus circuits. However, the need of high temporal precision, high recording throughput, and accurate annotation of recording sites prevents researchers from systemic functional connection map.Here, to solve above problems, we developed a research framework to systematically map functional projections from the dorsal cortical areas to the subcortical regions in awake mice. The framework conbined optogenetic perturbation with high throughput and high temporal resolusion, multi-channel extracellular recording and high-precision reconstruction of recoding sites. In this framework, we are able to cyclically deliver multi-location opto-inactivation at the cortex and record dozens of sites in subcortical regions at the same time. Recording sites of different animals are further reconstructed in a thalamus model, and registered to the template thalamus for systematic analysis. Based on this framework, we mapped the cortical functional projection to RT and dorsal thalamic nuclei. And we analyzed the corticothalamic functional projection at three levels. Firstly, we classified dorsal thalamic nuclei into three major groups depending on their cortical inputs. The coordinates of cortical input center of these nuclei are related to the coordinates of nuclei locations. Secondly, we clustered sites of RT and dorsal thalamic nuclei according to their cortical functional projections. We found a topographic distribution of cortical projection in RT and many dorsal thalamic nuclei. Moreover, we confirmed that most RT neurons and part of thalamic neurons received projections from several cortical locations, which indicated that information convergence was normal in the thalamus. Finally, we found parts of anterior RT neurons have selective activity or ramping mode during the tactile-based delay response task. And selective neurons and ramping neurons of anterior RT had different cortical inputs with normal neurons.All in all, this project provided a efficient tool for exploring the functional projection from the dorsal cortical areas to the subcortical regions. Besides, we systematically mapped the corticoreticular and corticothalamic functional projections in awake mice for the first time. This map provided a new insight into the interaction between the cortex and the thalamus. Additionally, with the mapping framework, we inspected the cortical inputs of the selective and ramping RT neurons during the behavior task, which presented a new mind for researching the RT’s funtion in behaviors.
