Glutamatergic Nonpyramidal Neurons From Neocortical Layer VI and Their Comparison With Pyramidal and Spiny Stellate Neurons

Abstract

. The deeper part of neocortical layer VI is dominated by nonpyramidal neurons, which lack a prominent vertically ascending dendrite and predominantly establish corticocortical connections. These neurons were studied in rat neocortical slices using patch-clamp, single-cell reverse transcription-polymerase chain reaction, and biocytin labeling. The majority of these neurons expressed the vesicular glutamate transporter but not glutamic acid decarboxylase, suggesting that a high proportion of layer VI nonpyramidal neurons are glutamatergic. Indeed, they exhibited numerous dendritic spines and established asymmetrical synapses. Our sample of glutamatergic nonpyramidal neurons displayed a wide variety of somatodendritic morphologies and a subset of these cells expressed the Nurr1 mRNA, a marker for ipsilateral, but not commissural corticocortical projection neurons in layer VI. Comparison with spiny stellate and pyramidal neurons from other layers showed that glutamatergic neurons consistently exhibited a low occurrence of GABAergic interneuron markers and regular spiking firing patterns. Analysis of electrophysiological diversity using unsupervised clustering disclosed three groups of cells. Layer V pyramidal neurons were segregated into a first group, whereas a second group consisted of a subpopulation of layer VI neurons exhibiting tonic firing. A third heterogeneous cluster comprised spiny stellate, layer II/III pyramidal, and layer VI neurons exhibiting adaptive firing. The segregation of layer VI neurons in two different clusters did not correlate either with their somatodendritic morphologies or with Nurr1 expression. Our results suggest that electrophysiological similarities between neocortical glutamatergic neurons extend beyond layer positioning, somatodendritic morphology, and projection specificity.

Calcium imaging in single neurons from brain slices using bioluminescent reporters

“…Only two statistically significant differences were observed in sensor-expressing groups of cells relative to control. Furthermore, values obtained in each group were in the range of those earlier reported for layer V pyramidal neurons (Kawaguchi, 1993;Zhang, 2004;Christophe et al, 2005;Hattox and Nelson, 2007;Andjelic et al, 2009). These results suggest that overall morphological and electrophysiological properties of layer V pyramidal neurons were essentially preserved after overnight incubation and GFP-photoprotein expression.…”

“…Although GO-and GA N26D -expressing neurons reached higher mean AP frequency than GA-expressing and control neurons from acute slices, values of electrophysiological parameters of sensors-expressing neurons were in the range of those earlier reported for layer V pyramidal neurons, which display significant functional heterogeneity (Kawaguchi, 1993;Christophe et al, 2005;Hattox and Nelson, 2007;Andjelic et al, 2009). A similar absence of conspicuous short-term toxicity has been reported following expression of other proteins in hippocampal pyramidal neurons using recombinant Sindbis viruses (Okada et al, 2001(Okada et al, , 2003Poncer et al, 2002).…”

Orexin-dependent activation of layer VIb enhances cortical network activity and integration of non-specific thalamocortical inputs

“…The innermost layer VI is further partitioned into an upper part (layer VIa) containing a high density of glutamatergic pyramidal neurons and a deeper part (layer VIb) dominated by polymorphic non-pyramidal neurons (Töm-böl 1984;van Brederode and Snyder 1992;Zhang and Deschenes 1997;Mercer et al 2005;Andjelic et al 2009;Chen et al 2009;Marx and Feldmeyer 2013).…”

Decomposing spatially dependent and cell type specific contributions to cellular heterogeneity

“…It is also well-known that the behavior of different glutamatergic neurons can be very different (Andjelic et al, 2008;Tasic et al, 2016). By combining cell-type mapping .…”