Emotional arousal enhances consolidation of memory traces, a homeostatic response of our organism that is modulated by stress hormones (McGaugh and Roozendaal, 2002). There is extensive evidence derived from observations in both experimental animals and humans that the amygdala is indispensable to enable the acquisition and retention of lasting memories of emotional experiences. The compelling evidence led McGaugh and his collaborators (McGaugh et al., 2004) to propose that the activation of the BLA is fundamental in the establishment of an arousal state triggered by fear (as unconsciously occurs in humans, LeDoux, 2014) and that arousal is a major component in the establishment of posttraining memory consolidation. The elegant work of Campolongo and coauthors (2009) demonstrated that there are unsuspected players in addition to stress hormones (i.e., adrenaline and corticosteroids) that affect memory consolidation of fearful stimuli, namely the satiety factor OEA, normally secreted by the intestine after a fatty meal. Exogenous OEA at the concentration used here does not readily pass the blood brain barrier (Campolongo et al., 2009). By activating peripheral PPAR-α it engages the vagus nerve and activates the NTS that in turn provides the amygdala, together with the locus coeruleus (LC), with a dense supply of norepinephrine. Stimulation of NTS (Clayton et al., 2000) or LC (Sara, 2009; Rosa et al., 2013) contributes to memory processing by influencing noradrenergic neurotransmission in the amygdala. OEA enhances memory consolidation of the inhibitory avoidance task by inducing noradrenergic activation of BLA neurons (Campolongo et al., 2009). The cognitive effects of OEA are lost in PPAR-α mice and in rats that received intra-NTS infusion of lidocaine, further substantiating the hypothesis that vagal afferents contribute to the cognitive effects of OEA (Campolongo et al., 2009). Our present results show that OEA increases memory expression of another aversively motivated task, contextual fear conditioning, by eliciting histaminergic neurotransmission in the BLA. Accordingly, both depletion of releasable histamine in the brain with α-FMHis that blocks HDC, and intra-BLA infusion of histaminergic antagonists prevent the freezing-enhancing effects of OEA. The concept of several neurotransmitter systems contributing to emotional memory consolidation within the same brain region is indisputable (Izquierdo et al., 1992; McGaugh 2004; Izquierdo et al., 2016). We think that the emotional arousal that is generally considered indispensable for good memory consolidation of fear tasks is provided by both the noradrenergic and histaminergic transmission in the BLA. We recently observed that OEA elicits NTS activation in HDC-KO mice (hence unable to synthesize histamine) as well as in wild-type littermates (Umehara et al., 2016). Therefore, the regulatory action of histamine on the cerebral pathways triggered by OEA is most likely downstream of the NTS. A prudent interpretation of our results is that histamine released in the amygdala or in the LC (Korotkova et al., 2005; Lee et al., 2005) gates the effects of noradrenaline on BLA neurons, hence blocking either neurotransmitter within the BLA would lead to similar behavioral outcomes.

We previously documented that histamine neurotransmission in diverse brain areas is necessary for the consolidation of contextual fear memory, as infusions of H₃ receptors antagonists in the BLA decrease the freezing time of contextual fear-conditioned rats and decrease acetylcholine release in the same region (Passani et al., 2001). Also, in the nucleus basalis magnocellularis memory improvement for contextual fear conditioning requires the activation of H₂ receptors (Benetti et al., 2013), presumably by controlling the activity of the amygdalopetal cholinergic pathway (Power and McGaugh 2002). It is relatively surprising that both H₁ and H₂ receptor antagonists produce the same results. The arrangement of these receptors on BLA neurons is not known, and explanations may only be speculative. Jiang et al. (2005) recorded intracellular excitatory postsynaptic potentials from BLA neurons ex vivo in the presence of histamine and described the opposite responses of 2 neuronal populations with decreased or increased excitatory postsynaptic potential amplitude. The authors fell short of demonstrating what postsynaptic histamine receptors may partake in these effects; hence, it is not known if H₁ and H₂ receptors are on different cell populations. In the vascular system, H₁ and H₂ receptors may work synergistically via activation of common intracellular pathways (Panula et al., 2015), which is a mechanism that may also occur in neurons. It is plausible that when strong aversive stimuli are used, as in our paradigm, all histaminergic inputs to the BLA need to be blocked to prevent freezing-enhancing compounds such as OEA to exert their effect.

Of interest, BLA infusion of the H₁ or H₂ receptor antagonists did not affect the expression of baseline fear memory tested 72 hours after training; rather these treatments prevented the freezing enhancement induced by OEA. These results are in good agreement with our previous observations using the same behavioral protocol that neither pyrilamine nor zolantidine changed freezing responses when administered in the NBM (Benetti et al., 2013) but prevented the increased expression of fear memory induced by an H₃ receptor antagonist.

Rats depleted of histamine with i.c.v. injections of α-FMHis had a behavior comparable with controls treated with saline, but α-FMHis prevented the effect of OEA on freezing time. We explain this considering that the effect of α-FMHis lasts approximately 3 days (Benetti et al., 2015); therefore at the time of the retention test (4 days after α-FMHis infusion) histamine neurotransmission should be restored to normal levels, which presumably allows the expression of the memory trace. These results are in agreement with what we observed using another behavioral paradigm, inhibitory avoidance, as the memory impairment induced by pretraining injections of α-FMHis lasted 48 hours; the step-down latency returned to control levels after 1 week (Benetti et al., 2015). Despite the well-documented differences between inhibitory avoidance, a procedure that requires a discriminative response, and fear conditioning, a Pavlovian procedure that does not require decision making (Maren, 2003; Tinsley et al., 2004), OEA appears to enhance the expression of BLA-dependent emotional memory regardless of the type of behavioral response and adverse situation. The microdialysis experiments offered one further piece of data in favor of the hypothesis that OEA requires the histamine neurotransmission in the BLA to modulate emotional responses. OEA administered at the same dose that increased freezing time augmented significantly histamine release from the BLA, but not from hypothalamic nuclei where histaminergic and OEA signalling converge to regulate appetite (Provensi et al., 2014). This could be a quite puzzling observation, but we believe that the histaminergic system responds to various stimuli, in this case OEA administered systemically, by engaging different pathways according to the required behavioral response. As an example, in food-restricted mice OEA increases c-fos expression in the PVN and supraoptic nucleus but not in other hypothalamic nuclei that receive histaminergic afferents and control eating behavior such as the VMH (Provensi et al., 2014) or dorsomedial hypothalamus (Umehara et al., 2016). In the present study, during dialysate collection satiated rats are quiet or sleeping, and it is plausible that in this circumstance OEA does not engage histaminergic afferents to the PVN to regulate their activity; rather it recruits the histaminergic afferents to the BLA. Indeed, we believe that the TMN serves as a neuronal gateway that integrates hormonal, interoceptive, and environmental signals to coordinate the adequate behavioral response. This hypothesis implies that HA neurons in the TMN are organized in distinct subpopulations differently regulated and innervating specific brain areas. There is much experimental evidence supporting this view (Blandina et al., 2012; Munari et al., 2013).

What is the physiological significance of exaggerated emotional response induced by OEA? The few human studies published so far showed that blood concentrations of N-acyl-ethanolamides like OEA and other endocannabinoids increase in response to acute stress in healthy human volunteers (Dlugos et al., 2012), and subjects affected by PTSD have significantly higher plasma concentrations of OEA (Hauer et al. 2013; Schaefer et al., 2014). Despite the fact that these results are based on small and dis-homogeneous study groups, they may have pathophysiological and/or diagnostic relevance. In this respect, our results suggest that activation of the histaminergic system in the BLA has a “permissive” role on the memory enhancing effects of OEA. In particular, targeting the H₁ or H₂ receptor with classical clinically approved antihistamine compounds may modify the expression of emotional memory and reduce dysfunctional aversive memories as found in phobias and PTSD.