# Spatial distribution of neurons innervated by chandelier cells

## TL;DR

This study shows that chandelier cells in the mouse brain selectively and unevenly innervate nearby pyramidal neurons, influencing local brain circuits.

## Contribution

The paper provides new insights into the spatial connectivity and influence of chandelier cells on cortical circuits.

## Key findings

- A single chandelier cell innervates 18–22% of nearby pyramidal neurons.
- Innervated neurons are clustered, not randomly distributed, within the chandelier cell's axonal arbor.
- The innervation density peaks within 30–60 µm from the chandelier cell's body.

## Abstract

Chandelier (or axo-axonic) cells are a distinct group of GABAergic interneurons that innervate the axon initial segments of pyramidal cells and are thus thought to have an important role in controlling the activity of cortical circuits. To examine the circuit connectivity of chandelier cells (ChCs), we made use of a genetic targeting strategy to label neocortical ChCs in upper layers of juvenile mouse neocortex. We filled individual ChCs with biocytin in living brain slices and reconstructed their axonal arbors from serial semi-thin sections. We also reconstructed the cell somata of pyramidal neurons that were located inside the ChC axonal trees and determined the percentage of pyramidal neurons whose axon initial segments were innervated by ChC terminals. We found that the total percentage of pyramidal neurons that were innervated by a single labeled ChC was 18–22 %. Sholl analysis showed that this percentage peaked at 22–35 % for distances between 30 and 60 µm from the ChC soma, decreasing to lower percentages with increasing distances. We also studied the three-dimensional spatial distribution of the innervated neurons inside the ChC axonal arbor using spatial statistical analysis tools. We found that innervated pyramidal neurons are not distributed at random, but show a clustered distribution, with pockets where almost all cells are innervated and other regions within the ChC axonal tree that receive little or no innervation. Thus, individual ChCs may exert a strong, widespread influence on their local pyramidal neighbors in a spatially heterogeneous fashion.