Long-range Circuits Of Intratelencephalic Neurons In The Mice Sensorimotor Cortex

The sensorimotor cortex is an important region that involves locomotion,which is divided into the primary motor cortex,secondary motor cortex,and sensory cortex according to anatomical structure and functional differences.These subregions form complex circuit structures with other areas through different projection neurons.Intratelencephalic（IT）neurons project widely to the bilateral telencephalon,especially the adjacent cortical regions and striatum.Different subregions perform unique functions at different stages of locomotion.To understand the circuit mechanisms of the functional differences of these neurons,it is necessary to analyze the fine structure at the mesoscopic level.The researches on the circuits of IT neurons in these regions are still insufficient,and there is a lack of systematic analysis of the input and output connection.This thesis utilized Plxn D1-2A-Cre ER mice that specifically target IT neurons,combined with virus tracer,fluorescence micro-optical sectioning tomography（fMOST）,and optogenetic techniques to explore the long-range connection patterns and function of IT neurons in six subregions.（1）The whole-brain distribution pattern of input circuits was analyzed.Retrograde transsynaptic tracing and fMOST techniques were employed to obtain the input datasets of IT neurons in six subregions.The main input neurons of these subregions were mainly located in the cortex and thalamus,and showed topological mapping relationships.The cortical inputs were mainly derived from neurons of layers 2/3 and 5a in the adjacent area of the injection site and the homologous cortical area on the contralateral side.The distribution center of input neurons in the cortex gradually shifted backward or outward with the location of start cells.The anterior-medial and posterior-medial motor area of secondary motor cortex received more inputs from the dorsolateral and dorsomedial thalamus respectively,the primary motor cortex from the ventral anterolateral and posterolateral thalamus,and the sensory cortex from the ventral and posterolateral thalamus.（2）The projection patterns of IT neurons were analyzed.Based on anterograde tracing and fMOST techniques,the projection datasets of six subregions were acquired.IT neurons at the population level widely innervated to the bilateral cortex,striatum,pallidum,and hippocampus,and the axonal fibers were topologically distributed in downstream targets.The intermediate-medial(MOp^IMM)and anterior-lateral motor area(MOp^ALM)of primary motor cortex to the striatum presented parallel projections.Through sparse labeling and whole-brain imaging,the fine morphology of 483 IT neurons at the mesoscopic level was analyzed.The axonal terminals of IT neurons from the motor cortex to the contralateral cortex were more located in layers 1 and 2/3,while the sensory cortex was in layers 2/3 and6.IT neurons were divided into 12 subtypes according to the distribution of axonal terminals.Due to different proportions,neuronal populations in six subregions preferentially projected to distinct downstream targets.In addition to the bilateral sensorimotor cortex（IT 12,about20%）,the primary motor cortex also innervated the bilateral caudoputamen（IT 6,about25%）and ipsilateral pallidum（IT 5,about 15%）,while the primary sensory cortex also regulated the ipsilateral caudoputamen（IT 2,about 17%）.（3）The structure and function of two parallel circuits from the primary motor cortex to the striatum were analyzed.Through optogenetic manipulations,it was found that the test mice showed sustained locomotion impairment during and after the inhibition of the bilateral MOp^ALM to the ventral caudoputamen（CP.v）circuit.While after the inhibition of the bilateral MOp^IMM to the dorsal caudoputamen circuit impaired the locomotion.These two circuits played different roles in the locomotion of mice.CP.v neurons receiving inputs from MOp^ALM mainly projected to the globus pallidus and reticular part of substantia nigra（SNr）.Compared with GP,inhibition of CP.v neurons induced more SNr neurons to express the c-Fos protein.It was confirmed that the MOp^ALM-CP.v-SNr circuit played an important role in the locomotion of mice,and inhibition of this circuit weaken the locomotion.In conclusion,this thesis analyzed the circuit structure of IT neurons in six subregions,including the distribution pattern of upstream neurons and downstream fibers,and revealed the whole-brain projection rules in three-dimensional space at the single-neuron level.The circuits from the primary motor cortex to striatum were involved in the locomotion.These results will help to understand the mechanism of neurological diseases associated with movement and provide guidance for clinical prevention and treatment.