Genetic labeling of axo-axonic cells in the basolateral amygdala

GABAergic neurons are classified into multiple subtypes based on morphology, physiological properties, and gene expression profiles. Although traditionally defined axo-axonic cells (AACs) are a unique type of interneuron that expresses parvalbumin and innervates the axon initial segment (AIS) of pyramidal neurons, a genetic marker for AACs in the basolateral amygdala (BLA) has not been identified. Here, we show that vasoactive intestinal peptide receptor 2 (Vipr2)-expressing interneurons exhibit anatomical and electrophysiological properties of AACs in the BLA. Using a reporter mouse expressing fluorescent proteins specifically in Vipr2⁺ cells, we analyzed the distribution, postsynaptic targeting and electrophysical properties of Vipr2⁺ cells in the BLA. More than half of the Vipr2⁺ cells showed parvalbumin immunoreactivity and innervated the AIS of pyramidal neurons in the BLA of Vipr2-tdTomato mice. Notably, most of the Vipr2⁺ cells showed fast-spiking properties. Furthermore, the use of a Cre-dependent adeno-associated virus led to more selective labeling of AACs in the BLA. These results suggest that AACs are genetically identifiable in the BLA without anatomical or physiological analysis.