Thalamo-cortical synaptic network in motor cortex analyzed with large volume electron microscopy

It is well known that the motor cortex receives signals from the basal ganglia and cerebellum via thalamo-cortical afferents. We investigated how the thalamo-cortical axon terminals participate in the cortical microcircuit in rat frontal cortex. The axon arborization pattern across cortical layers varies significantly among thalamocortical afferents from three motor-related thalamic nuclei: the ventral medial nucleus (VM) the ventral anterior (VA) and the ventral lateral (VL) thalamic complex which relay motor information from the basal ganglia (VM/VA) and the cerebellum (VL) respectively (2015, Cereb Cor 25: 221-235). We hypothesized that the synaptic connections of VM/VA and VL afferents in the cortical microcircuits are different. A viral vector (pal-GFP AAV) was injected into each of three motor-related thalamic nuclei. Their target structures in the motor cortex was investigated using a correlated light and electron microscopy (CLEM) with a laser confocal microscopy and automated tape-collecting ultramicrotomy (ATUM) with scanning electron microscopy (SEM). To identify the GFP labeled axonal fibers subsequently at the electron microscopy we stained blood vessels with lectin and cellular nuclei with DAPI and used them as landmarks in cortical tissue sections. Firstly images were taken with a laser confocal microscopy. Then the tissue sections were embedded in plastic and sectioned with ATUM for SEM observation. GFP-labeled thalamo-cortical fibers and their target structures were identified in serial electron micrographs and reconstructed three-dimensionally. Our preliminary results indicated that the VA fibers mainly targeted dendritic spines of the layer 5 pyramidal cell.