Lighting the chandelier: new vistas for axo-axonic cells

GABAergic Excitation in the Basolateral Amygdala

“…The fact that interneurons in the amygdala that generate disynaptic excitation express parvalbumin, and their axons make cartridge synapses suggests that they are similar to the axoaxonic interneurons in the cortex. However, in contrast to the hippocampus and cortex, in which AACs selectively innervate the axon initial segment of pyramidal neurons with cartridge synapses (Howard et al, 2005), we have shown that these interneurons in the amygdala make cartridge as well as basket synapses. Furthermore, they clearly innervate both principal neurons as well as other parvalbuminpositive interneurons.…”

“…Furthermore, all four successfully reconstructed cells exhibiting feedback excitation had axons confined to the BL, and all exhibited closely spaced rows of boutons, or axonal cartridges. These cartridges are a hallmark of axoaxonic cells in cortex, hippocampus, and amygdala (Howard et al, 2005;Muller et al, 2005), the type of interneuron that has recently been reported to directly excite cortical layer 2/3 pyramidal neurons to threshold (Szabadics et al, 2006). Thus, given the similarity of this finding to that reported for cortical AACs, and the lack of a plausible alternative explanation, we suggest that AACs of the BL can also directly depolarize local principal neurons to threshold.…”

“…Chandelier synapses have also been described in the amygdala, and these terminals are known to express parvalbumin (Kemppainen and Pitkanen, 2000;McDonald and Betette, 2001;Muller et al, 2006). Because cortical AAC interneurons express parvalbumin (Howard et al, 2005), we tested whether interneurons that provide disynaptic excitation in the amygdala may also make chandelier synapses. We therefore reconstructed interneurons that showed feedback excitation (Fig.…”

“…Recently, cortical axoaxonic interneurons (AACs) that make "chandelier" synapses on the axon initial segment of pyramidal neurons (Howard et al, 2005) have been found to depolarize local pyramidal neurons because of a positively shifted GABA A reversal potential at the axon initial segment (Szabadics et al, 2006). Chandelier synapses have also been described in the amygdala, and these terminals are known to express parvalbumin (Kemppainen and Pitkanen, 2000;McDonald and Betette, 2001;Muller et al, 2006).…”

Highly Energized Inhibitory Interneurons are a Central Element for Information Processing in Cortical Networks

“…23 The characteristics of axo-axonic cells have been less explored but recent evidence suggests that these cells exert highly specialized functions during information processing in cortical networks. 52,87,90 For the hippocampus, it was demonstrated in vitro that axo-axonic cells generate action potentials at much higher rate (B100 Hz) compared with the frequency of gamma oscillations (B35 Hz) in the CA3 network, and that they can control the antidromic propagation of ectopically generated spikes from axons into the soma of pyramidal cells during gamma oscillations. 88 Moreover, axo-axonic cells in the CA3 network show significant phase coupling to hippocampal gamma oscillations in vivo, comparable in strength and phase to those of pyramidal cells and PV þ basket cells.…”