, 1998). Consistent with

our findings, amygdala activation of the cortex has been shown to induce risk assessment (Gozzi et al., 2010), sustained attention (Holland and Gallagher, 1999), and moderate fear or vigilance (Davis and Whalen, 2001). Our findings suggest that vHPC can reduce or even prevent fear signaling in PL. This is consistent with behavioral and electrophysiological evidence that the hippocampus gates fear responses via the PFC (Hobin et al., 2003; Sotres-Bayon http://www.selleckchem.com/products/Methazolastone.html et al., 2004). For example, stimulation-induced depression of the HPC-PFC pathway impairs extinction (Hugues and Garcia, 2007). Hippocampal inhibition of PL also agrees with data from anaesthetized rats showing that stimulation of vHPC consistently activates interneurons prior to pyramidal cells in PL (Tierney et al., 2004). The vHPC also projects to the BLA (Orsini et al., 2011) and could conceivably inhibit PL tone responses via feed-forward Selleckchem Decitabine inhibition of BLA efferent to PL. Arguing against this, however, is our observation that interneurons local to PL are modulated by vHPC, and that vHPC and BLA manipulations were

able to differentially modulate tone responses of single neurons. Moreover, existing evidence suggests that the hippocampus excites rather than inhibits the BLA. The HPC-BLA pathway shows long-term potentiation (Maren and Fanselow, 1995) and inactivating the hippocampus decreases conditioned tone responses of BLA neurons (Maren and Hobin, 2007). Direct projections from vHPC to BLA may promote responding to unambiguous danger cues. Indeed, “fear cells” in BLA receive input from vHPC (Herry et al., 2008). Consistent with this, inactivation of vHPC prior to extinction increased pressing (decreased fear) (present study), and was previously shown to reduce conditioned freezing (Maren and Holt, 2004; Sierra-Mercado et al., 2011). We suggest that the hippocampal inhibition of spontaneous activity of PL becomes behaviorally apparent only after extinction, when amygdala output is reduced (Amano et al., 2010; Herry et al., 2008). The reduced excitatory drive of PL emanating from BLA is augmented

by increased inhibition of PL by vHPC (see circuit diagrams of Figure 4). The increase in PL activity with vHPC inactivation click here appears only after extinction, because PL activity before extinction is at a ceiling level. Thus, hippocampal projections to PL could effectively modulate behavioral responses to cues made ambiguous by prior extinction training. Additionally, fear-promoting and fear-inhibiting functions of vHPC may be mediated by either distinct subsets of hippocampal neurons (Tronson et al., 2009) or local circuits in PL-BLA, differentially engaged by conditioning or extinction. Our behavioral task was optimized to induce and detect moderate levels of fear suggestive of vigilance (readiness for danger). Suppression of bar pressing is more sensitive than freezing (Mast et al.