The anterior cingulate cortex and its role in controlling contextual fear memory to predatory threats

Predator exposure is a life-threatening experience and elicits learned fear responses to the context in which the predator was encountered. The anterior cingulate area (ACA) occupies a pivotal position in a cortical network responsive to predatory threats, and it exerts a critical role in processing fear memory. The experiments were made in mice and revealed that the ACA is involved in both the acquisition and expression of contextual fear to predatory threat. Overall, the ACA can provide predictive relationships between the context and the predator threat and influences fear memory acquisition through projections to the basolateral amygdala and perirhinal region and the expression of contextual fear through projections to the dorsolateral periaqueductal gray. Our results expand previous studies based on classical fear conditioning and open interesting perspectives for understanding how the ACA is involved in processing contextual fear memory to ethologic threatening conditions that entrain specific medial hypothalamic fear circuits.

Amygdalar CB2 cannabinoid receptor mediates fear extinction deficits promoted by orexin-A/hypocretin-1

Background and purpose: Anxiety is often characterized by an inability to extinguish learned fear responses. Orexins/hypocretins are involved in the modulation of aversive memories, and dysregulation of this system may contribute to the aetiology of anxiety disorders characterized by pathological fear. The mechanisms by which orexins regulate fear remain unknown. Experimental approach: We investigated the role of the endogenous cannabinoid system in the impaired fear extinction induced by orexin-A (OXA) in male mice. Behavioural pharmacology, neurochemical, molecular and genetic approaches were used. Key results: The selective inhibitor of 2-arachidonoylglycerol (2-AG) biosynthesis O7460 abolished the fear extinction deficits induced by OXA. Accordingly, increased 2-AG levels were observed in the amygdala and hippocampus of mice treated with OXA that do not extinguish fear, suggesting that high levels of this endocannabinoid are related to poor extinction. Impairment of fear extinction induced by OXA was associated with increased expression of CB2 cannabinoid receptor (CB2R) in microglial cells of the basolateral amygdala. Consistently, the intra-amygdala infusion of the CB2R antagonist AM630 completely blocked the impaired extinction promoted by OXA. Microglial and CB2R expression depletion in the amygdala with PLX5622 chow also prevented these extinction deficits. Conclusions and implications: We reveal that overactivation of the orexin system leads to impaired fear extinction through 2-AG and amygdalar CB2R. This novel mechanism may pave the way towards novel potential approaches to treat diseases associated with inappropriate retention of fear, such as post-traumatic stress disorder, panic anxiety and phobias.

A striatal circuit balances learned fear in the presence and absence of sensory cues

During fear learning, defensive behaviors need to be finely balanced, to allow animals to return to normal behaviors after the termination of threat-indicating sensory cues. Nevertheless, the circuits underlying such balancing are largely unknown. Here, we investigate the role of direct (D1R+) - and indirect (Adora+) pathway neurons of the amygdala-striatal transition zone (AStria) in fear learning. In-vivo Ca2+ imaging revealed that fear learning increased the responses of D1R+ AStria neurons to an auditory CS, given that the animal moved. In Adora+ neurons, fear learning also induced a differential activity during freezing and movement, albeit with little influence of the CS. In-vivo optogenetic silencing during the training day showed that plasticity in D1R+ AStria neurons contributes to auditory-cued fear memories, whereas Adora+ neurons suppressed learned freezing when no CS was present. Circuit tracing experiments identified cortical input structures to the AStria, and recording of optogenetically-evoked EPSCs at the corresponding projection revealed different forms of long-term plasticity at synapses onto D1R+ and Adora+ AStria neurons. Taken together, direct- and indirect pathways neurons of the AStria show differential signs of in-vivo and ex-vivo plasticity after fear learning, and balance defensive behaviors in the presence and absence of aversively motivated sensory cues.

Enhanced synchronization between prelimbic and infralimbic cortices during fear extinction learning

The ability to extinguish aversive memories when they are no longer associated with danger is critical for balancing survival with competing adaptive demands. Previous studies demonstrated that the infralimbic cortex (IL) is essential for extinction of learned fear, while neural activity in the prelimbic cortex (PL) facilitates fear responding and is negatively correlated with the strength of extinction memories. Though these adjacent regions in the prefrontal cortex maintain mutual synaptic connectivity, it has been unclear whether PL and IL interact functionally with each other during fear extinction learning. Here we addressed this question by recording local field potentials (LFPs) simultaneously from PL and IL of awake behaving rats during extinction of auditory fear memories. We found that LFP power in the fast gamma frequency (100–200 Hz) in both PL and IL regions increased during extinction learning. In addition, coherency analysis showed that synchronization between PL and IL in the fast gamma frequency was enhanced over the course of extinction. These findings support the hypothesis that interregional interactions between PL and IL increase as animals extinguish aversive memories.

Effects of intolerance of uncertainty on subjective and psychophysiological measures during fear acquisition and delayed extinction

Individuals who score high in self-reported Intolerance of Uncertainty (IU) tend to find uncertainty unacceptable and aversive. In recent years, research has shed light on the role of IU in modulating subjective (i.e. expectancy ratings) and psychophysiological responses (i.e. skin conductance) across different classical fear conditioning procedures, particularly that of immediate extinction. However, there remain gaps in understanding how IU, in comparison to other negative emotionality traits (STAI-T), impact different types of subjective and psychophysiological measures during different classical fear conditioning procedures. Here, we analyzed IU, STAI-T, subjective (i.e. fear ratings) and psychophysiological (i.e. skin conductance, auditory startle blink) data recorded during fear acquisition training and 24h-delayed extinction training (n = 66). Higher IU, over STAI-T, was: (1) significantly associated with greater fear ratings to the learned fear cue during fear acquisition training, and (2) at trend associated with greater fear ratings to the learned fear versus safe cue during delayed extinction training. Both IU and STAI-T were not related to skin conductance or auditory startle blink during fear acquisition training and delayed extinction training. These results add to and extend our current understanding of the role of IU on subjective and physiological measures during different fear conditioning procedures, particularly that of delayed extinction training. Implications of these findings and future directions are discussed.

Active suppression prevents the return of threat memory in humans

Unbidden distressing memories inflict serious damage on mental health. Extant research highlights the importance of associative learning in modulating aversive memory. We report that conscious active suppression eliminates learned fear responses independent of memory triggers and is related to individual difference in thought control ability; in contrast, thought diversion only reduces cue-specific fear response. These results suggest potential avenues for treatment of persistent maladaptive memories by engaging declarative mnemonic control mechanisms.

Systematic Review and Methodological Considerations for the Use of Single Prolonged Stress and Fear Extinction Retention in Rodents

Posttraumatic stress disorder (PTSD) is a mental health condition triggered by experiencing or witnessing a terrifying event that can lead to lifelong burden that increases mortality and adverse health outcomes. Yet, no new treatments have reached the market in two decades. Thus, screening potential interventions for PTSD is of high priority. Animal models often serve as a critical translational tool to bring new therapeutics from bench to bedside. However, the lack of concordance of some human clinical trial outcomes with preclinical animal efficacy findings has led to a questioning of the methods of how animal studies are conducted and translational validity established. Thus, we conducted a systematic review to determine methodological variability in studies that applied a prominent animal model of trauma-like stress, single prolonged stress (SPS). The SPS model has been utilized to evaluate a myriad of PTSD-relevant outcomes including extinction retention. Rodents exposed to SPS express an extinction retention deficit, a phenotype identified in humans with PTSD, in which fear memory is aberrantly retained after fear memory extinction. The current systematic review examines methodological variation across all phases of the SPS paradigm, as well as strategies for behavioral coding, data processing, statistical approach, and the depiction of data. Solutions for key challenges and sources of variation within these domains are discussed. In response to methodological variation in SPS studies, an expert panel was convened to generate methodological considerations to guide researchers in the application of SPS and the evaluation of extinction retention as a test for a PTSD-like phenotype. Many of these guidelines are applicable to all rodent paradigms developed to model trauma effects or learned fear processes relevant to PTSD, and not limited to SPS. Efforts toward optimizing preclinical model application are essential for enhancing the reproducibility and translational validity of preclinical findings, and should be conducted for all preclinical psychiatric research models.

A multisensory circuit for gating intense aversive experiences

The ventral hippocampus (vHPC) is critical for both learned and innate fear, but how discrete projections control different types of fear is poorly understood. Here, we report a novel excitatory circuit from a subpopulation of the ventral hippocampus CA1 subfield (vCA1) to the inhibitory peri-paraventricular nucleus of the hypothalamus (pPVN) which then routes to the periaqueductal grey (PAG). We find that vCA1→pPVN projections modulate both learned and innate fear. Fiber photometric calcium recordings reveal that activity in vCA1→pPVN projections increases during the first moments of exposure to an unconditioned threat. Chemogenetic or optogenetic silencing of vCA1→pPVN cell bodies or vCA1→pPVN axon terminals in the pPVN enhances the initial magnitude of both active and passive unconditioned defensive responses, irrespective of the sensory modalities engaged by a particular innate threat. Moreover, silencing produces a dramatic impact on learned fear without affecting milder anxiety-like behaviors. We also show that vCA1→pPVN projections monosynaptically route to the PAG, a key brain region that orchestrates the fear response. Surprisingly, optogenetic silencing of vCA1 terminals in the pPVN titrates the level of c-Fos neural activity in the PAG differently for learned versus innate threats. Together, our results show how a novel vCA1→pPVN circuit modulates neuronal activity in the PAG to regulate both learned and innate fear. These findings have implications for how initial trauma processing may influence maladaptive defensive behaviors across fear and trauma-related disorders.