It is well known that, under certain boundary conditions, the retrieval of a stable consolidated memory results into a labile one. During this unstable phase, memory can be vulnerable to interference by a number of pharmacological agents, including benzodiazepines. One of the goals of this study was to evaluate the vulnerability to midazolam (MDZ) after reactivation of recent and remote contextual fear memories in animals that experienced a stressful situation before learning. Animals were subjected to a restraint session and trained in a contextual fear paradigm the following day; consolidated memories were reactivated at different times after learning and different MDZ doses (1.5, 3.0 mg/kg) were administered to rats after reactivation. Our results show that MDZ did not affect memory reconsolidation in older-than-one-day memories of stressed animals, even after the administration of a higher MDZ dose and a longer reactivation session (5 min). In contrast, MDZ was effective in blocking reconsolidation at all memory ages in unstressed animals. In addition, the current research investigated whether activating NMDA sites before reactivation promotes the destabilization of resistant memories such as those of stressed animals. We tested the influence of pre-reactivation D-cycloserine (DCS), a partial NMDA agonist, on MDZ's effect on fear memory reconsolidation in stressed animals. Our findings indicate that DCS before reactivation promotes retrieval-induced lability in resistant memory traces, as MDZ-induced memory impairment in stressed rats became evident with pre-reactivation DCS but not after pre-reactivation sterile isotonic saline.

The present study investigates the fear memory resulting from the interaction of a stressful experience and the retrieval of an established fear memory trace. Such a combination enhanced both fear expression and fear retention in adult Wistar rats. Likewise, midazolam intra-basolateral amygdala (BLA) infusion prior to stress attenuated the enhancement of fear memory thus suggesting the involvement of a stress-induced reduction of the GABAergic transmission in BLA in the stress-induced enhancing effect. It has been suggested that, unlike the immediate-early gene Zif268 which is related to the reconsolidation process, the expression of hippocampal brain-derived neurotrophic factor (BDNF) is highly correlated with consolidation. We therefore evaluate the relative contribution of these two neurobiological processes to the fear memory resulting from the above-mentioned interaction. Intra-dorsal hippocampus (DH) infusions of either the antisense Zif268 or the inhibitor of the protein degradation (Clasto-Lactacystin b-Lactone), suggested to be involved in the retrieval-dependent destabilization process, did not affect the resulting contextual memory. In contrast, the knockdown of hippocampal BDNF mitigated the stress-induced facilitating influence on fear retention. In addition, the retrieval experience elevated BDNF level in DH at 60 min after recall exclusively in stressed animals. These findings suggest the involvement of a hippocampal BDNF sensitive mechanism in the stress-promoting influence on the fear memory following retrieval.

The present research investigated the resulting contextual fear memory and structural plasticity changes in the dorsal hippocampus (DH) following stress and fear conditioning. This combination enhanced fear retention and increased the number of total and mature dendritic spines in DH. Intra-basolateral amygdala (BLA) infusion of midazolam prior to stress prevented both the enhancement of fear retention and an increase in the density of total and mature dendritic spines in DH. These findings emphasize the role of the stress-induced attenuation of GABAergic neurotransmission in BLA in the promoting influence of stress on fear memory and on synaptic remodeling in DH. In conclusion, the structural remodeling in DH accompanied the facilitated fear memory following a combination of fear conditioning and stressful stimulation.It is well accepted that prior exposure to stressful events facilitates the emergence of fear memory (Shors et al. 1992;Beylin and Shors 1998;Shors 2001;Cordero et al. 2003;Rodriguez Manzanares et al. 2005). Consistent with this, recent data shows that the interaction of an unrelated aversive experience and an established fear memory trace results in a robust and persistent fear memory (Giachero et al. 2013). This stress-induced promoting influence was prevented by midazolam (MDZ) intra-basolateral amygdala (BLA) infusion prior to stress, indicating that such an influence is related to the modulation of the GABAergic transmission in BLA (Giachero et al. 2013).Dendritic spines represent the structural platform for excitatory synaptic contacts between neurons (Gray 1959;Chapleau and Pozzo-Miller 2007), with growing evidence suggesting that synaptic remodeling accompanies the formation of long-term memory (Restivo et al. 2009). In line with this view, the encoding of a novel contextual representation was shown to result in a rapid synaptic rearrangement through an increase in spine density, particularly in the hippocampus across multiple memory paradigms, including contextual fear memory (Leuner et al. 2003;Restivo et al. 2009;Vetere et al. 2011).The dorsal hippocampus (DH) is crucially involved in the contextual representation following fear conditioning (Kim and Fanselow 1992;Phillips and LeDoux 1995;Maren and Fanselow 1997;Fanselow and Dong 2010). Related to this, the present research investigates the structural plasticity in the DH that may underlie the contextual fear memory resulting from the interaction of fear conditioning and an unrelated stressful experience. The BLA is an essential component of the neural circuitry orchestrating emotional response to threatening stimuli (LeDoux 2000), and compelling evidence has shown that the activation of GABAa sites, specifically in BLA prior to stress, attenuates the enhancement of fear memory (Rodriguez Manzanares et al. 2005;Giachero et al. 2013). Therefore, we also explored the modulatory role of the GABAergic neurotransmission in the BLA in both fear retention and structural plasticity in the DH, which is potentially associated with the fear memory...