It may, however, have a developmental component that we cannot exclude. This impairment also cannot be considered as a general learning deficit of the PN-1 KO mice as their fear

conditioning learning is comparable to their WT littermates. In addition, while we found no evidence that they are more susceptible to learning fear, we cannot exclude that the threshold for fear acquisition is lower for PN-1 KO mice. Our study is the first demonstration as far as we know that a serpin can influence emotional learning such as fear extinction. Earlier reports have shown that serine proteases can influence fear conditioning. Acutely stressed mice lacking the protease tissue plasminogen activator exhibit reduced contextual fear learning compared with WT animals (Norris & Strickland, 2007). On the other hand, mice lacking another activity-dependent serine protease, neuropsin, display increased fear after cued fear conditioning compared with

WT littermates, even in Selleckchem CH5424802 the absence of stress (Horii et al., 2008). Mice with a targeted deletion of the cancer metabolism inhibitor serine protease-activated receptor-1 (PAR-1), also known as the thrombin receptor, show reduced fear retrieval after cued fear conditioning (Almonte et al., 2007). PN-1 inhibits many of the above involved proteases and reduces PAR-1 activation (Scott et al., 1985; Stone et al., 1987; Kvajo et al., 2004; Feutz et al., 2008). In addition to a reduced proteolytic inhibition, a further impact of the absence of PN-1 could be an altered cellular signaling triggered by high molecular weight complexes between PN-1 and its target proteins (Vaillant et al., 2007; Fayard et al., 2009). Consequently, our

results suggest a possible involvement of serine proteases in fear extinction ADP ribosylation factor as well. We evaluated short- and long-term patterns of neuronal activation in the amygdala by comparing Fos immunoreactivity and pαCamKII protein levels in the amygdala of WT and PN-1 KO mice to find cellular correlates of this behavioral deficit. We concentrated on the amygdala because of the striking pattern of PN-1 expression in GABAergic neurons as well as its central role in integrating fear inputs. It is possible that other affected brain areas contribute to the overall extinction deficit in the PN-1 KO mouse, e.g. the prefrontal cortex (Quirk & Mueller, 2008) or the hippocampus (Corcoran et al., 2005). In WT mice, Fos immunoreactivity increased in the no extinction and extinction groups as expected in the LA and BA after fear retrieval and extinction acquisition, compared with the naive control group (Herry & Mons, 2004). The Fos-immunopositive cells possibly represent subsets of the two populations of cells recently shown to be activated differentially by fear and extinction protocols (Herry et al., 2008). This response was shifted in PN-1 KO mice, namely the increase was higher than the WT response after fear retrieval in the no extinction group and lower than the WT in the extinction group.