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

Anna Hoerder-Suabedissen,Shuichi Hayashi,Diana Casas-Torremocha,Patrick O Kanold,Zoltán Molnár,Francisco Clasca,Zachary Nolan,Eleanor Grant,Sarada Viswanathan,Yongsoo Kim,Louise Upton

doi:10.1093/cercor/bhy036

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.

Highlights

In rodents, layer 6 is comprised of 2 cytoarchitectonically distinct sublayers; the larger, upper layer 6a, and the lower compact layer 6b
Subplate cells are a diverse population of cells, many of which are lost during early postnatal development, but some survive as layer 6b cells forming a thin layer of cells between the white matter and layer 6a in the adult mouse brain (HoerderSuabedissen and Molnár 2015)
Since Drd1a-Cre labels a distinct subset of L6b, we do not know how representative these results are of all layer 6b projection neurons, a similar projection pattern was confirmed with biotinylated dextran amine (BDA) single-cell tracing that is not dependent on specific Cre expression

Summary

Introduction

Layer 6 is comprised of 2 cytoarchitectonically distinct sublayers; the larger, upper layer 6a, and the lower compact layer 6b. Transcriptomic analysis revealed clear distinctions between them (Hoerder-Suabedissen et al 2009; Belgard et al 2011; Oeschger et al 2012; Marx and Feldmeyer 2013; Tasic et al 2016; Chevée et al 2018). Layer 6b neurons have heterogeneous morphology (Marx and Feldmeyer 2013), developmental origin and birthdates (Hoerder-Suabedissen and Molnár 2013; Pedraza et al 2014). The adult layer 6b neurons are the only cortical neurons responsive to orexin, a peptide that is wake-promoting and produced in the lateral hypothalamus (Bayer et al, 2004; Wenger Combremont et al, 2016) and layer 6b cells selectively respond to the neuromodulators neurotensin (Case et al 2016) and cholecystokinin (CCK) (Chung et al 2009).

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Results

Conclusion