Circuits in the absence of cortical layers: increased callosal connectivity in reeler mice revealed by brain-wide input mapping of VIP neurons in barrel cortex

Abstract

1The neocortex is composed of layers. Whether layers constitute an essential 2 framework for the formation of functional circuits is not well understood. We 3 investigated if neurons require the layer organization to be embedded into brain-wide 4 circuits using the reeler mouse. This mutant is characterized by a migration deficit of 5 cortical neurons so that no layers are formed. Still, neurons retain their properties and 6 reeler mice show little cognitive impairment. We focused on VIP neurons because 7 they are known to receive strong long-range inputs and have a typical laminar bias 8 towards upper layers. In reeler these neurons are more distributed across the cortex. 9 We mapped the brain-wide inputs of VIP neurons in barrel cortex of wildtype and 10 reeler mice with rabies virus tracing. Innervation by subcortical inputs was not altered 11 in reeler, in contrast to the cortical circuitry. Numbers of long-range ipsilateral cortical 12 inputs were reduced in reeler, while contralateral inputs were strongly increased. 13Reeler mice had more callosal projection neurons. Hence, the corpus callosum was 14 larger in reeler as shown by structural imaging. We argue that in the absence of 15 cortical layers, circuits with subcortical structures are maintained but cortical neurons 16 establish a different network capable to preserve cognitive functions. 17 18 KEYWORDS 19 barrel cortex, corpus callosum, rabies tracing, reelin, VIP neurons 20 29input was increased compared to WT. Reeler mice had more callosal projecting 30 neurons (CPNs) and hence a larger corpus callosum. We argue that the absence of 31 layers has little effect on innervation by subcortical structures but induces a different 32 circuit arrangement within the cortex. 33 5 MATERIAL AND METHODS 1 2 Experimental animals 3 We crossed the reeler line (B6C3Fe a/a-Relnrl/J, The Jackson Laboratory, Bar 4 Harbor, USA) with the VIP-Cre line (VIPtm1(cre)Zjh, The Jackson Laboratory) to 5 breed VIP-Cre/reeler mice heterozygous for reelin mutation and homozygous for Cre. 6These animals were crossed to generate VIP-Cre/reeler mice homozygous for reelin 7 knockout. WT littermates or animals from the VIP-Cre line were used for comparison 8 in tracing experiments. For control experiments to check the quality of our Cre-9 dependent constructs, we used C57BL/6J wildtype mice (The Jackson Laboratory). 10For tracing experiments not requiring Cre expression and for imaging we used WT 11 and homozygous littermate pairs of the reeler line.

A method to estimate the cellular composition of the mouse brain from heterogeneous datasets

“…In total, we found 33 references providing values for GAD67 [5,6,31,43–45,54–56,58–69,69–80], 29 for PV [7,24–27,29,30,45,54–58,61–64,77,78,81–90], 14 for SST [7,8,27–29,45,56,58,63,64,81,82,85,86] and 9 for VIP [7,31,45,58,63,81,82,86,91], extracted from 54 different papers (see supplementary methods and Table 1). These estimates are in the form of density of neurons or cell counts within either a volume or a slice.…”