Subset of Cortical Layer 6b Neurons Selectively Innervates Higher Order Thalamic Nuclei in Mice

Abstract

The thalamus receives input from 3 distinct cortical layers, but input from only 2 of these has been well characterized. We therefore investigated whether the third input, derived from layer 6b, is more similar to the projections from layer 6a or layer 5. We studied the projections of a restricted population of deep layer 6 cells (“layer 6b cells”) taking advantage of the transgenic mouse Tg(Drd1a-cre)FK164Gsat/Mmucd (Drd1a-Cre), that selectively expresses Cre-recombinase in a subpopulation of layer 6b neurons across the entire cortical mantle. At P8, 18% of layer 6b neurons are labeled with Drd1a-Cre::tdTomato in somatosensory cortex (SS), and some co-express known layer 6b markers. Using Cre-dependent viral tracing, we identified topographical projections to higher order thalamic nuclei. VGluT1+ synapses formed by labeled layer 6b projections were found in posterior thalamic nucleus (Po) but not in the (pre)thalamic reticular nucleus (TRN). The lack of TRN collaterals was confirmed with single-cell tracing from SS. Transmission electron microscopy comparison of terminal varicosities from layer 5 and layer 6b axons in Po showed that L6b varicosities are markedly smaller and simpler than the majority from L5. Our results suggest that L6b projections to the thalamus are distinct from both L5 and L6a projections.

Long‐range projections from sparse populations of GABAergic neurons in murine subplate

“…Without additional birthdating (which is incompatible with carbocyanine tracing), it is not possible to determine whether contralaterally projecting GABAergic subplate cells preferentially survive during the first postnatal week, or whether long‐range axons to other targets are withdrawn during this time window. However, we would like to highlight, that other subplate and layer 6b cells also form long‐range projections including to the contralateral hemisphere (Hoerder‐Suabedissen & Molnár, ), some of which persist into adulthood (Hoerder‐Suabedissen et al, ). It would be interesting to follow this up in mature brains, using alternative methods to carbocyanine dye tracing, which is not a method suitable beyond the end of the second postnatal week.…”

Reconstruction of 1,000 projection neurons reveals new cell types and organization of long-range connectivity in the mouse brain

“…However, these projections were not always mutually exclusive. Whether L6-CT neurons in the motor cortex form discrete cell-types, as has been suggested in the sensory cortex (Hoerder-Suabedissen et al, 2018;Shima et al, 2016), or represent a continuum of projection patterns will require additional investigation.…”

The distribution, number, and certain neurochemical identities of infracortical white matter neurons in a lar gibbon (Hylobates lar) brain

“…The density of WMICs decreases during ontogeny from the prenatal to the adult brain (Duque, Krsnik, Kostovic, & Rakic, 2016;Kanold & Luhmann, 2010), but this change varies depending on the cortical region examined, brain size and the extent of cortical gyrencephaly (García-Marin et al, 2010;Kostovic & Rakic, 1980Suárez-Solá et al, 2009). Hodological studies of WMICs indicate that they are integrated into the circuitry of the overlying region of cortical gray matter (Clancy et al, 2001;Frazer et al, 2017;Shering & Lowenstein, 1994;Tomioka et al, 2005;Tomioka & Rockland, 2007;von Engelhardt et al, 2011), while functionally, subpopulations have been linked to vasodilation (Suárez-Solá et al, 2009), homeostatic sleep regulation (Kilduff, Cauli, & Gerashchenko, 2011), the regulation of information transfer (Colombo, 2018), and the regulation of arousal and transthalamic cortico-cortical communication (Hoerder-Suabedissen et al, 2018;Molnár, 2018). These characteristics of WMICs indicate that they are not just a remnant of the subplate, and form a distinct functional neural system (Colombo, 2018;Hoerder-Suabedissen et al, 2018;Hoerder-Suabedissen & Molnár, 2015;Molnár, 2018;Suárez-Solá et al, 2009).…”