Depolarizing effect of neocortical chandelier neurons

Alan Woodruff

doi:10.3389/neuro.04.015.2009

Abstract

Chandelier (or axo-axonic) cells are one of the most distinctive types of GABAergic interneurons in the cortex. Although they have traditionally been considered inhibitory neurons, data from rat and human neocortical preparations suggest that chandelier cells have a depolarizing effect on pyramidal neurons at resting membrane potential, and could even activate synaptic chains of neurons. At the same time, recent results from rat hippocampal chandeliers indicate a predominantly inhibitory effect on their postsynaptic targets. To better understand the function of chandelier neurons, we generated Nkx2.1Cre MADM mice, a strain of genetically engineered animals that, by expressing GFP in a subset of neocortical interneurons, enable the identification and targeting of chandelier cells in living brain slices. Using these mice, we characterized the basic electrophysiological properties of a homogeneous population of chandelier neurons from upper layers of somatosensory cortical slices. These chandelier cells have characteristic axon cartridges and stereotypical electrophysiological features, distinguishable from basket cells. To investigate the effect of chandelier cells on target neurons, we performed paired recordings from chandeliers and postsynaptic pyramidal cells. In both perforated patch and cell-attached configurations, chandelier PSPs have in every case a reversal potential that is depolarized from rest. Our results support the idea that chandelier cells depolarize pyramidal neurons and could potentially have an excitatory effect on the network at rest.

Highlights

Chandelier cells are one of the most distinctive types of GABAergic interneurons present in cortical circuits
IDENTIFICATION OF CHANDELIER CELLS IN Nkx2.1 MICE We used brain slices from the somatosensory cortex of p15–23 Nkx2.1 mice. These mice express GFP in interneurons derived from the Medial Ganglionic Eminence (MGE) (Figure 1B), a group which includes parvalbumin and somatostatin-positive neurons, among others (Xu et al, 2004, 2008)
We recorded from connected CC-pyramidal cell (PC) pairs, focusing on the cortical pyramidal neurons in layer 2/3, whose somata lie in the territories covered by the axons of the chandelier cells we studied, and that should receive GABAergic inputs at their axon initial segment

Summary

Introduction

Chandelier cells are one of the most distinctive types of GABAergic interneurons present in cortical circuits. Chandelier cells are rare and were only first described in the 1970s (Szentagothai and Arbib, 1974; Jones, 1975; Szentagothai, 1975) They form perhaps only a few percent of all GABAergic interneurons (Inda et al, 2007), and, distinctively, contact the axon initial segment of pyramidal cells (Somogyi, 1977; Fairen and Valverde, 1980; DeFelipe et al, 1985). The terms chandelier cells and axo-axonic cells are used interchangeably Due to their striking morphologies, chandelier neurons are often used as the best example to illustrate the purposeful design of cortical microcircuits. Chandelier cells appear ideally suited to shut off populations of pyramidal neurons, making chandelier cells potentially act as circuit switches

Objectives

Methods

Results

Conclusion