A Novel Layer 4 Corticofugal Cell Type/Projection Involved in Thalamo-Cortico-Striatal Sensory Processing.

In sensory cortices, the information flow has been thought to be processed vertically across cortical layers, with layer 4 being the major thalamo-recipient which relays thalamic signals to layer 2/3, which in turn transmits thalamic information to layer 5 and 6 to then leave the cortex to reach subcortical and cortical long-range structures. Although several exceptions to this model have been described, neurons in layer 4 are still considered to establish only local (i.e., interlaminar and short-range) connections. Here, taking advantage of anatomic, electrophysiological, and optogenetic techniques, we describe, for the first time, a long-range corticostriatal class of pyramidal neurons in layer 4 (CS-L4) of the mouse auditory cortex that receive direct thalamic inputs. The CS-L4 neurons are embedded in a feedforward inhibitory circuit involving local parvalbumin neurons and establish connections in the posterior striatum in yet another feedforward inhibitory thalamo→cortico(L4)→striatal circuit which potentially contributes in controlling control the output of striatal spiny projection neurons. SIGNIFICANCE STATEMENT The assumption has been that layer 4 neurons are the main thalamic recipient layer, projecting to the upper cortical layer 2/3. However, no study has revealed a detailed understanding of the circuit mechanisms by which layer 4 sends a projection to a subcortical structure, such as the striatum, and differentially innervate the spiny projection neurons (SPNs) and intrastriatal parvalbumin-expressing neurons. For the first time, our results demonstrate that the auditory cortex projects to the posterior part of the dorsal striatum via pyramidal neurons located in layer 4 (CS-L4 neurons). Here we propose a new wiring diagram that implemented the old one, in which layer 4 is not only involved in the transfer of thalamic input to the upper layer 2/3, but can also exert a direct top-down control, bypassing intracortical processing of subcortical structures, such as the posterior part of the dorsal striatum. This poses a new conceptual cell element (CS-L4 neurons) for experimental and theoretical work of the cortical function.
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