Extinction did not change the expression of PV in the SCR7 mouse soma of BA interneurons (Figures 4A and 4B). Next, we analyzed the presence of PV around the soma of BA fear neurons. We verified that our perisomatic PV immunolabeling represented perisomatic synapses (Figure S3A). Consistent with the extinction-induced increase in perisomatic GAD67, extinction also increased perisomatic PV around the silent fear neurons (Figure 4C). Again, there was no significant increase around the active fear neurons (Figure 4D). The effects of extinction on perisomatic PV seemed to reflect changes in synapse numbers (Figures S3B and S3C). Importantly, the increase in perisomatic PV that we detected with image analysis is similar to that reported

to increase perisomatic inhibition using electrophysiological analysis (Gittis et al., 2011 and Kohara et al., 2007). Thus, our data suggest an extinction-induced increase in perisomatic inhibition underlies the decreased number of active BA fear neurons and the resulting silencing of the fear memory circuit. This reveals a direct connection between extinction-induced structural and functional changes in the BA. We asked whether the extinction-induced increase in perisomatic PV might have reversed any fear conditioning-induced changes in those synapses, which would indicate that BA perisomatic inhibitory synapses were part

of the original fear circuit. To address this GSK1120212 question, we performed a separate experiment in which we compared a fear conditioned group (FC) with

a home cage group (HC) (Figures 5A and 5B). Consistent with our previous study (Reijmers et al., 2007), BA neurons activated during fear conditioning were tagged with long-lasting expression of GFP (Figure 5C). During retrieval on day 4, the FC group showed significant freezing (Figure 5D). The retrieval of contextual fear caused activation of both nontagged (GFP−Zif+; Figure 5E) and tagged (GFP+Zif+; Figure 5F) neurons in the BA, with a preferential reactivation of the tagged BA fear neurons (Figures 5E and 5F). 17-DMAG (Alvespimycin) HCl Importantly, we did not find fear conditioning-induced changes in perisomatic PV around silent or active fear neurons (Figures 5G and 5H). These data strongly suggest that the extinction-induced changes in PV+ perisomatic synapses constituted a new form of learning that occurred within the extinction circuit. In addition to PV+ perisomatic synapses, the BA also contains perisomatic inhibitory synapses that originate from cholecystokinin (CCK) interneurons (Yoshida et al., 2011). We therefore examined whether fear extinction also affected perisomatic CCK+ synapses. Extinction did not change the expression of CCK in the soma of BA interneurons (Figures 6A and 6B). In addition, perisomatic CCK around fear neurons, either silent or active, was not altered by fear extinction (Figures 6C, 6D, S4A, and S4B). Fear conditioning itself also did not change perisomatic CCK in the BA (Figures S4C and S4D).