Organization principles of thalamocortical input onto deep-layer inhibitory neurons of the barrel cortex

Thalamus functions as the main route by which incoming sensory information reaches the cerebral cortex. Thalamic nuclei will receive input from the sensory receptors in the periphery and relay it to different areas in the cortex. (Boivie & Perl, 1975; Mountcastle, 1980; Brodal, 1981; Heimer, 1983). For this thesis, I use the vibrissal system as a model of thalamocortical input onto the cortex. In this system, the cortex will receive most of the thalamocortical (TC) projections from two thalamic nuclei, the ventral posteromedial nucleus (VPm) and the posteromedial complex (POm) (Landisman et al, 2007). A canonical pathway of information flow has been described for this system, in which thalamocortical projections will target neurons in layer 4, which in turn will synapse with neurons in layer 2/3, to finally end in pyramidal neurons in layer 5 (Gilbert et al., 1979). Although the canonical pathway is able to explain the activation of layer 4 neurons after the thalamic activation, it has become increasingly clear that neuronal responses from layer 5 do not fit with the signal flow of the canonical pathway (Egger et al., 2020). Considering the new evidence of thalamic activation of excitatory neurons in the deep layers, an investigation of the role of thalamocortical input onto the inhibitory population in the deep layers is required, as also is the evaluation of the role of the deep layer inhibitory neurons in modulating the responses of L5PTs. To this end, I systematically investigated how sensory input from primary thalamus is relayed to inhibitory neurons in the deep layers of the barrel cortex. I first recorded and labeled inhibitory neurons in the deep layers of the barrel cortex. Before the recording session, animals were injected with a modified rAAV conjugated with channelrhodopsin for optogenetic manipulation of thalamocortical synapses, and with m-Cherry as a fluorescent reporter. I recorded the spontaneous activity of these neurons, and their response to first, a multi-whisker deflection provided by ...