Cholecystokinin-expressing basket cells (CCKBCs) and parvalbumin-containing basket cells (PVBCs) form 2 parallel inhibitory circuits, but both innervate axo-axonic cells (AACs).

<p>(A) Horizontal section taken from the amygdala region of a transgenic mouse expressing red fluorescent protein under the control of the Cck promoter (CCK-DsRed). BLA, basolateral amygdala complex; EC, entorhinal cortex; Hip, hippocampus; DG, dentate gyrus; Me, medial amygdala; Ce, central amygdala; Pir, piriform cortex. Scale: 500 μm. (B) Red fluorescent protein (DsRed) expression and cholinergic fibers visualized by immunostaining against the vesicular acetylcholine transporter (VAChT) parcel out the basolateral amygdala complex into 3 parts: the anterior part of the basal amygdala (BAa) contains a pronounced DsRed signal and strong VAChT immunoreactivity, the posterior part of the BA (BAp) is characterized by low and high levels of DsRed and VAChT expression, respectively, and the lateral amygdala (LA) has low levels of both signals. Scale: 200 μm. (C) Maximum z intensity projection image of a PVBC and a CCKBC recorded in the BAa of a PV-eGFP x CCK-DsRed double transgenic mouse. The soma of the PVBC showed eGFP, but not DsRed signal (solid arrowhead), and its boutons were immunostained for calbindin (Calb, bottom left panels) that typifies PVBCs. The axon terminals of the CCKBC, which soma contained DsRed but not eGFP signal (open arrowhead), were immunoreactive for cannabinoid receptor type 1 (CB1) (bottom right panels). (D) Maximum z intensity projection image of an AAC and a CCKBC recorded simultaneously in the BAa. The axon collaterals of the AAC, which soma contained eGFP but not DsRed signal (solid arrowhead), formed close apposition with the axon initial segments visualized by immunostaining against ankyrin G (AnkG) and lacked Calb (bottom left panels), typical for AACs. The boutons of the CCKBC (open arrowhead), which soma showed DsRed but not eGFP signal, expressed CB1 (bottom right panels), characteristic for CCKBCs. Scale: 50 μm, insets 2 μm. (E) Representative traces from whole-cell paired recordings from post hoc identified different interneuron→interneuron pairs. Ten superimposed consecutive traces are in gray, averages are in bold. Scale: y: 10 mV/10 pA, x: 20 ms. (F) Connection probability matrix of interneuron→interneuron synaptic coupling was obtained by paired recordings. (G) Comparison of the amplitude of inhibitory postsynaptic currents (IPSCs) (18.8 ± 5.5 pA, <i>n</i> = 11 CCKBC→CCKBC pairs; 30.8 ± 18.3 pA, <i>n</i> = 9 CCKBC→AAC pairs; 32.2 ± 9.0 pA, <i>n</i> = 6 PVBC→PVBC pairs; 21.1 ± 4.5 pA, <i>n</i> = 6 PVBC→AAC pairs, Kruskal-Wallis ANOVA <i>p</i> = 0.2) (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001421#pbio.2001421.s011" target="_blank">S1 Data</a>). Each data point on the plot is an average obtained from a pair recording, and lines represent mean. (H) Comparison of the short-term plasticity of the different types of interneuron→interneuron connections (IPSC₁₀/IPSC₁: 5.72 ± 1.91, <i>n</i> = 7 CCKBC→CCKBC pairs; 2.87 ± 0.52, <i>n</i> = 7 CCKBC→AAC pairs; 0.48 ± 0.05, n = 6 PVBC→PVBC pairs; 0.53 ± 0.08, <i>n</i> = 4 PVBC→AAC pairs. Kruskal-Wallis ANOVA, <i>p</i> < 0.001) (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2001421#pbio.2001421.s011" target="_blank">S1 Data</a>). Data are presented as mean ± SEM.