Impaired fear extinction in mice lacking protease nexin-1

The serine protease inhibitor protease‐nexin‐1 (PN‐1) has been shown to modulate N‐methyl‐d‐aspartate receptor (NMDAR)‐mediated synaptic currents and NMDAR‐dependent long‐term potentiation of synaptic transmission. Here, we analysed the role of PN‐1 in the acquisition and extinction of classical auditory fear conditioning, two distinct forms of learning that both depend on NMDAR activity in the amygdala. Immunostaining revealed that PN‐1 is expressed throughout the amygdala, primarily in γ‐aminobutyric acid containing neurons of the central amygdala and intercalated cell masses (ITCs) and in glia. Fear extinction was severely impaired in mice lacking PN‐1 (PN‐1 KO). Consistent with a role for the basal nucleus of the amygdala in fear extinction, we found that, compared with wild‐type (WT) littermate controls, PN‐1 KO mice exhibited decreased numbers of Fos‐positive neurons in the basal nucleus after extinction. Moreover, immunoblot analysis of laser‐microdissected amygdala sub‐nuclei revealed specific extinction‐induced increases in the level of phosphorylated alpha‐calcium/calmodulin protein kinase II in the medial ITCs and in the lateral subdivision of the central amygdala in WT mice. These responses were altered in PN‐1 KO mice. Together, these data indicate that lack of extinction in PN‐1 KO mice is associated with distinct changes in neuronal activity across the circuitry of the basal and central nuclei and the ITCs, supporting a differential impact on fear extinction of these amygdala substructures. They also suggest a new role for serine protease inhibitors such as PN‐1 in modulating fear conditioning and extinction.