Principles of connectivity among morphologically defined cell types in adult neocortex

Abstract

Introduction The intricate microcircuitry of the cerebral cortex is thought to be a critical substrate from which arise the impressive capabilities of the mammalian brain. Until now, our knowledge of the stereotypical connectivity in neocortical microcircuits has been pieced together from individual studies of the connectivity between small numbers of neuronal cell types. Here, we provide unbiased, large-scale profiling of neuronal cell types and connections to reveal the essential building blocks of the cortex and the principles governing their assembly into cortical circuits. Using advanced techniques for tissue slicing, multiple simultaneous whole-cell recording, and morphological reconstruction, we are able to provide a comprehensive view of the connectivity between diverse types of neurons, particularly among types of γ-aminobutyric acid–releasing (GABAergic) interneurons, in the adult animal. Rationale We took advantage of a method for preparing high-quality slices of adult tissue and combined this technique with octuple simultaneous, whole-cell recordings followed by an improved staining method that allowed detailed recovery of axonal and dendritic arbor morphology. These data allowed us to perform a census of morphologically and electrophysiologically defined neuronal types (primarily GABAergic interneurons) in neocortical layers 1, 2/3, and 5 (L1, L23, and L5, respectively) and to observe their connectivity patterns in adult animals. Results Our large-scale, comprehensive profiling of neocortical neurons differentiated 15 major types of interneurons, in addition to two lamina-defined types of pyramidal neurons (L23 and L5). Cortical interneurons comprise two types in L1 (eNGC and SBC-like), seven in L23 (L23MC, L23NGC, BTC, BPC, DBC, L23BC, and ChC), and six in L5 (L5MC, L5NGC, L5BC, SC, HEC, and DC) (see the figure). Each type has stereotypical electrophysiological properties and morphological features and can be differentiated from all others by cell type-specific axonal geometry and axonal projection patterns. Importantly, each type of neuron has its own characteristic input-output connectivity profile, connecting with other constituent neuronal types with varying degrees of specificity in postsynaptic targets, laminar location, and synaptic characteristics. Despite specific connection patterns for each cell type, we found that a small number of simple connectivity motifs are repeated across layers and cell types defining a canonical cortical microcircuit. Conclusion Our comprehensive profiling of neuronal cell types and connections in adult neocortex provides the most complete wiring diagram of neocortical microcircuits to date. Compared with current genetic labels for cell class, which paint the cortex in broad strokes, our analysis of morphological and electrophysiological properties revealed new cell classes and allowed us to derive a small number of simple connectivity rules that were repeated across layers and cell types. This detailed blueprint of cortical wiring should aid efforts to i...

Response to Comment on “Principles of connectivity among morphologically defined cell types in adult neocortex”

“…(ii) Severed connections are a well-known limitation of slice recordings, which we discussed explicitly on p. 3 of the supplementary materials (SM) for (2). However, we do not agree that our study is substantially more susceptible to connection loss than previous patching studies, most of which also used 300-to 350-mm slices [we found fewer than half a dozen multipatching studies using 450-to 500-mm slices (3)(4)(5)].…”

“…Pearson correlation between connection probability and the correction factor is 0.02. (C) Original connection probability (p) matrix as shown in (2). (D) Corrected connection probability p/(1 -q) matrix.…”

“…We patched cells at a wide range of intersomatic distances [see fig. S13A in (2)]; this range reflects the sparsity of labeled interneurons and is not a limitation of octuple patching.…”

Neuroinformatics for Degenerate Brains

“…12 In general, intermediary cell types are observed that seem to defy the classification scheme, whether one uses schemes based on morphology, physiology and molecular markers 13 or on gene expression. 14 Conversely, a recent bigdata study 15 reports that it is sufficient to use 3 simple connectivity motives to come to a definite classification, suggesting that also in cortex it may be more useful to consider broader groupings of cell types instead of clearly separable types.…”

Whole cell patch clamp analysis of non-synaptic and synaptic signaling in rodent and human supragranular neocortex

“…This in turn triggers polysynaptic series of events downstream in the network (Komlósi et al, 2012;Molnár et al, 2008;Szegedi et al, 2016;Szegedi et al, 2017). Such powerful synaptic connections between pyramidal cells and interneurons have not been reported in rodent neocortex (Jiang et al, 2015;Miles and Wong, 1986). These specific features were speculated to represent an evolutionary adaptation to the human brain indicating that they might be important for information processing in the human neocortex (Woodruff and Yuste, 2008).…”