The Stimulus Selectivity and Connectivity of Layer Six Principal Cells Reveals Cortical Microcircuits Underlying Visual Processing

Abstract

SummarySensory computations performed in the neocortex involve layer six (L6) cortico-cortical (CC) and cortico-thalamic (CT) signaling pathways. Developing an understanding of the physiological role of these circuits requires dissection of the functional specificity and connectivity of the underlying individual projection neurons. By combining whole-cell recording from identified L6 principal cells in the mouse primary visual cortex (V1) with modified rabies virus-based input mapping, we have determined the sensory response properties and upstream monosynaptic connectivity of cells mediating the CC or CT pathway. We show that CC-projecting cells encompass a broad spectrum of selectivity to stimulus orientation and are predominantly innervated by deep layer V1 neurons. In contrast, CT-projecting cells are ultrasparse firing, exquisitely tuned to orientation and direction information, and receive long-range input from higher cortical areas. This segregation in function and connectivity indicates that L6 microcircuits route specific contextual and stimulus-related information within and outside the cortical network.

Infrabarrels Are Layer 6 Circuit Modules in the Barrel Cortex that Link Long-Range Inputs and Outputs

“…Previous studies showed that L6 CT and CC neurons exhibit distinctive intrinsic properties, including excitability (Brumberg et al, 2003; Kinnischtzke et al, 2016; Kumar and Ohana, 2008; Velez-Fort et al, 2014; Yang et al, 2014), however the results of these studies differed substantially. Clarifying these differences is critical for understanding L6 circuit mechanisms.…”

“…Broadly speaking, three main types of L6 neurons can be defined by their axonal projections (Zhang and Deschenes, 1997): excitatory corticothalamic (CT) and corticocortical (CC) projection neurons, and local inhibitory interneurons. The L6 projection neurons differ in their intrinsic properties, dendrite and axon morphologies, long-range inputs and outputs, and in vivo activity patterns (Kumar and Ohana, 2008; Narayanan et al, 2015; Oberlaender et al, 2012; Velez-Fort et al, 2014; Yang et al, 2014); this has led to suggestions that L6 CT and CC cells participate in distinct subcircuits within the infragranular network (Kumar and Ohana, 2008; Velez-Fort et al, 2014). …”

Monosynaptic Circuit Tracing with Glycoprotein-Deleted Rabies Viruses

“…2). This has been done by either infecting neurons in the desired starter cell location with a Credependent helper virus (Figure 2; Wall et al, 2010; among many other publications cited previously), or by crossing the Cre driver line with a responder mouse line that has Cre-dependent expression of TVA and G (Li et al, 2013;Takatoh et al, 2013;Stanek et al, 2014;Bourane et al, 2015). Following adequate expression of the Cre-dependent gene products, EnvA ϩ RVdG is injected which, as described above, can result in selective infection of starter cells.…”

“…Published studies using monosynaptic RV tracing have described the connections to single neurons (Marshel et al, 2010;Rancz et al, 2011;Vélez-Fort et al, 2014), to cell types defined by their projection to particular locations (Stepien et al, 2010;Yonehara et al, 2011;Takatoh et al, 2013;Cruz-Martín et al, 2014;Levine et al, 2014;Ni et al, 2014;Sreenivasan et al, 2015), to cells targeted by their birth date (Arenkiel et al, 2011;Nakashiba et al, 2012;Vivar et al, 2012;Deshpande et al, 2013;Li et al, 2013;, to neurons with particular cell-surface receptors , and to genetically identified cell types in Cre-or tTA-expressing mouse lines (Haubensak et al, 2010;Weible et al, 2010;Watabe-Uchida et al, 2012;Fu et al, 2014;Kohara et al, 2014;Ogawa et al, 2014;Pollak Dorocic et al, 2014;Sun et al, 2014;. The results have led to novel insights into the relationships between the organization and function of neural circuits throughout the nervous system; from muscles or peripheral sensory receptors to the brain; and in nearly every neural system, from vision to motor function and neuromodulatory systems.…”

Challenges in Retinal Circuit Regeneration

“…Moreover, electroporation and targeted patching of single cells of interest in vivo to introduce TVA, followed by infection of the TVA-expressing cells with EnvA-pseudotyped rabies viruses, allows analysis of single neuronal networks with single-cell resolution. 147,157,158) The most striking feature of RABVΔG is that complementation of G in trans enables transsynaptic spreading of RABVΔG to directly-connected, presynaptic neurons 45,46,119,[147][148][149] (Figs. 4E, F).…”