Division of Cerebral Circuitry, NIPS
Department of Physiological Sciences
The Graduate University for Advanced Studies (SOKENDAI)
It is well known that the cortex receives signals from the basal ganglia and cerebellum via thalamo-cortical afferents. About 4 - 16% of the thalamo-cortical excitatory afferents make synaptic contacts on spines each of which is also postsynaptic to an inhibitory terminal, dually innervated spine (DiS) (J Neurosci, 2007, 27, 1139- 1150). Our previous morphological and physiological analyses (eLife, 2015, elife 4) as well as optogenetic methods demonstrated that the inhibitory synapse to the DiS can effectively veto the local excitatory synaptic inputs. Furthermore, the inhibitory synapses on the DiS display more dynamic properties than the inhibitory synapses on the dendritic shaft (Neuron, 2016, 90: 662-664). One of the important functions of the thalamocortical system is to provide feedforward inhibition. We showed that FS basket cells receive thalamo-cortical innervation on their soma, and believe that this link is likely involved in feedforward inhibition (Cereb Cor, 2016, 26: 2689-2704).
To understand this intriguing cortical microcircuit architecture further, 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.