The involvement of cannabinoids and mTOR in the reconsolidation of an emotional memory in the hippocampal-amygdala-insular circuit

The challenge of memory destabilisation: From prediction error to prior expectations and biomarkers

The re-ignition of memory reconsolidation research sparked by Karim Nader in the early 2000s led to great excitement that 'reconsolidation-based' interventions might be developed for mental health disorders such as post-traumatic stress disorder and substance use disorder. Two decades on, it is clear that reconsolidation-based interventions have been more challenging to translate to the clinic than initially thought. We argue that this challenge could be addressed with a better understanding of how prior expectations interact with information presented in a putative memory reactivation / cue reminder session, and through the identification of non-invasive biomarkers for memory destabilisation that would allow reminder sessions to be 'tuned' to enhance memory lability in an ad hoc manner.

Targeting the Endocannabinoid System in the Treatment of Posttraumatic Stress Disorder: A Promising Case of Preclinical-Clinical Translation?

The endocannabinoid (eCB) system is one the most ubiquitous signaling systems of the brain and offers a rich pharmacology including multiple druggable targets. Preclinical research shows that eCB activity influences functional connectivity between the prefrontal cortex and amygdala and thereby influences an organism's ability to cope with threats and stressful experiences. Animal studies show that CB1 receptor activation within the amygdala is essential for extinction of fear memories. Failure to extinguish traumatic memories is a core symptom of posttraumatic stress disorder, suggesting that potentiating eCB signaling may have a therapeutic potential in this condition. However, it has been unknown whether animal findings in this domain translate to humans. Data to inform this critical question are now emerging and are the focus of this review. We first briefly summarize the biology of the eCB system and the animal studies that support its role in fear extinction and stress responding. We then discuss the pharmacological eCB-targeting strategies that may be exploited for therapeutic purposes: direct CB1 receptor activation, using Δ9-tetrahydrocannabinol or its synthetic analogs; or indirect potentiation, through inhibition of eCB-degrading enzymes, the anandamide-degrading enzyme fatty acid amide hydrolase; or the 2-AG (2-arachidonoyl glycerol)-degrading enzyme monoacylglycerol lipase. We then review recent human data on direct CB1 receptor activation via Δ9-tetrahydrocannabinol and anandamide potentiation through fatty acid amide hydrolase blockade. The available human data consistently support a translation of animal findings on fear memories and stress reactivity and suggest a potential therapeutic utility in humans.

Translation of animal endocannabinoid models of PTSD mechanisms to humans: Where to next?

The endocannabinoid system is known to be involved in mechanisms relevant to PTSD aetiology and maintenance, though this understanding is mostly based on animal models of the disorder. Here we review how human paradigms can successfully translate animal findings to human subjects, with the view that substantially increased insight into the effect of endocannabinoid signalling on stress responding, emotional and intrusive memories, and fear extinction can be gained using modern paradigms and methods for assessing the state of the endocannabinoid system in PTSD.

Basolateral amygdala CB1 receptors gate HPA axis activation and context-cocaine memory strength during reconsolidation

Re-exposure to a cocaine-associated context triggers craving and relapse through the retrieval of salient context-drug memories. Upon retrieval, context-drug memories become labile and temporarily sensitive to modification before they are reconsolidated into long-term memory stores. The effects of systemic cannabinoid type 1 receptor (CB1R) antagonism indicate that CB1R signaling is necessary for cocaine-memory reconsolidation and associated glutamatergic plasticity in the basolateral amygdala (BLA); however, the contribution of BLA CB1R signaling to cocaine-memory reconsolidation is unknown. Here, we assessed whether intra-BLA CB1R manipulations immediately after cocaine-memory retrieval alter cocaine-memory strength indexed by subsequent drug context-induced cocaine-seeking behavior in an instrumental rodent model of drug relapse. Administration of the CB1R antagonist, AM251 (0.3 µg/hemisphere) into the BLA increased subsequent drug context-induced cocaine-seeking behavior in a memory retrieval-dependent and anatomically selective manner. Conversely, the CB1R agonist, WIN55,212-2 (0.5 or 5 µg/hemisphere) failed to alter this behavior. In follow-up experiments, cocaine-memory retrieval elicited robust hypothalamic-pituitary-adrenal axis activation, as indicated by a rise in serum corticosterone concentrations. Intra-BLA AM251 administration during memory reconsolidation selectively increased this cocaine-memory retrieval-induced corticosterone response. Intra-BLA corticosterone administration (3 or 10 ng/hemisphere) during memory reconsolidation did not augment subsequent cocaine-seeking behavior, suggesting that CB1R-dependent effects of corticosterone on memory strength, if any, are mediated outside of the BLA. Together, these findings suggest that CB1R signaling in the BLA gates cocaine-memory strength, possibly by diminishing the impact of cue-induced arousal on the integrity of the reconsolidating memory trace or on the efficacy of the memory reconsolidation process.

Disrupting Reconsolidation by Systemic Inhibition of mTOR Kinase via Rapamycin Reduces Cocaine-Seeking Behavior

Drug addiction is considered maladaptive learning, and drug-related memories aroused by the presence of drug related stimuli (drug context or drug-associated cues) promote recurring craving and reinstatement of drug seeking. The mammalian target of rapamycin signaling pathway is involved in reconsolidation of drug memories in conditioned place preference and alcohol self-administration (SA) paradigms. Here, we explored the effect of mTOR inhibition on reconsolidation of addiction memory using cocaine self-administration paradigm. Rats received intravenous cocaine self-administration training for 10 consecutive days, during which a light/tone conditioned stimulus was paired with each cocaine infusion. After acquisition of the stable cocaine self-administration behaviors, rats were subjected to nosepoke extinction (11 days) to extinguish their behaviors, and then received a 15 min retrieval trial with or without the cocaine-paired tone/light cue delivery or without. Immediately or 6 h after the retrieval trial, rapamycin (10 mg/kg) was administered intraperitoneally. Finally, cue-induced reinstatement, cocaine-priming-induced reinstatement and spontaneous recovery of cocaine-seeking behaviors were assessed in rapamycin previously treated animals, respectively. We found that rapamycin treatment immediately after a retrieval trial decreased subsequent reinstatement of cocaine seeking induced by cues or cocaine itself, and these effects lasted at least for 28 days. In contrast, delayed intraperitoneal injection of rapamycin 6 h after retrieval or rapamycin injection without retrieval had no effects on cocaine-seeking behaviors. These findings indicated that mTOR inhibition within the reconsolidation time-window impairs the reconsolidation of cocaine associated memory, reduces cocaine-seeking behavior and prevents relapse, and these effects are retrieval-dependent and temporal-specific.

Dopamine, endocannabinoids and their interaction in fear extinction and negative affect in PTSD

There currently exist few frameworks for common neurobiology between reexperiencing and negative cognitions and mood symptoms of PTSD. Adopting a dopaminergic framework for PTSD unites many aspects of unique symptom clusters, and this approach also links PTSD symptomology to common comorbidities with a common neurobiological deficiency. Here we review the dopamine literature and incorporate it with a growing field of research that describes both the contribution of endocannabinoids to fear extinction and PTSD, as well as the interactions between dopaminergic and endocannabinoid systems underlying this disorder. Based on current evidence, we outline an early, preliminary model that links re-experiencing and negative cognitions and mood in PTSD by invoking the interaction between endocannabinoid and dopaminergic signalling in the brain. These interactions between PTSD, dopamine and endocannabinoids may have implications for future therapies for treatment-resistant and comorbid PTSD patients.

mTOR inhibition impairs extinction memory reconsolidation

Fear-motivated avoidance extinction memory is prone to hippocampal brain-derived neurotrophic factor (BDNF)-dependent reconsolidation upon recall. Here, we show that extinction memory recall activates mammalian target of rapamycin (mTOR) in dorsal CA1, and that post-recall inhibition of this kinase hinders avoidance extinction memory persistence and recovers the learned aversive response. Importantly, coadministration of recombinant BDNF impedes the behavioral effect of hippocampal mTOR inhibition. Our results demonstrate that mTOR signaling is necessary for fear-motivated avoidance extinction memory reconsolidation and suggests that BDNF acts downstream mTOR in a protein synthesis-independent manner to maintain the reactivated extinction memory trace.

Gestational poly(I:C) attenuates, not exacerbates, the behavioral, cytokine and mTOR changes caused by isolation rearing in a rat 'dual-hit' model for neurodevelopmental disorders

Many psychiatric illnesses have a multifactorial etiology involving genetic and environmental risk factors that trigger persistent neurodevelopmental impairments. Several risk factors have been individually replicated in rodents, to understand disease mechanisms and evaluate novel treatments, particularly for poorly-managed negative and cognitive symptoms. However, the complex interplay between various factors remains unclear. Rodent dual-hit neurodevelopmental models offer vital opportunities to examine this and explore new strategies for early therapeutic intervention. This study combined gestational administration of polyinosinic:polycytidylic acid (poly(I:C); PIC, to mimic viral infection during pregnancy) with post-weaning isolation of resulting offspring (to mirror adolescent social adversity). After in vitro and in vivo studies required for laboratory-specific PIC characterization and optimization, we administered 10 mg/kg i.p. PIC potassium salt to time-mated Lister hooded dams on gestational day 15. This induced transient hypothermia, sickness behavior and weight loss in the dams, and led to locomotor hyperactivity, elevated striatal cytokine levels, and increased frontal cortical JNK phosphorylation in the offspring at adulthood. Remarkably, instead of exacerbating the well-characterized isolation syndrome, gestational PIC exposure actually protected against a spectrum of isolation-induced behavioral and brain regional changes. Thus isolation reared rats exhibited locomotor hyperactivity, impaired associative memory and reversal learning, elevated hippocampal and frontal cortical cytokine levels, and increased mammalian target of rapamycin (mTOR) activation in the frontal cortex - which were not evident in isolates previously exposed to gestational PIC. Brains from adolescent littermates suggest little contribution of cytokines, mTOR or JNK to early development of the isolation syndrome, or resilience conferred by PIC. But notably hippocampal oxytocin, which can protect against stress, was higher in adolescent PIC-exposed isolates so might contribute to a more favorable outcome. These findings have implications for identifying individuals at risk for disorders like schizophrenia who may benefit from early therapeutic intervention, and justify preclinical assessment of whether adolescent oxytocin manipulations can modulate disease onset or progression.

Cannabinoids as therapeutics for PTSD

Post-traumatic stress disorder (PTSD) is a complex disorder that involves dysregulation of multiple neurobiological systems. The traumatic stressor plays a causal role in producing psychological dysfunction and the pattern of findings suggests that the hypothalamic-pituitary-adrenal (HPA) axis, which is instrumental for stress adaptation, is critically dysfunctional in PTSD. Given the lack of understanding of the basic mechanisms and underlying pathways that cause the disorder and its heterogeneity, PTSD poses challenges for treatment. Targeting the endocannabinoid (ECB) system to treat mental disorders, and PTSD in particular, has been the focus of research and interest in recent years. The ECB system modulates multiple functions, and drugs enhancing ECB signaling have shown promise as potential therapeutic agents in stress effects and other psychiatric and medical conditions. In this review, we focus on the interaction between the ECB-HPA systems in animal models for PTSD and in patients with PTSD. We summarize evidence supporting the use of cannabinoids in preventing and treating PTSD in preclinical and clinical studies. As the HPA system plays a key role in the mediation of the stress response and the pathophysiology of PTSD, we describe preclinical studies suggesting that enhancing ECB signaling is consistent with decreasing PTSD symptoms and dysfunction of the HPA axis. Overall, we suggest that a pharmacological treatment targeted at one system (e.g., HPA) may not be very effective because of the heterogeneity of the disorder. There are abnormalities across different neurotransmitter systems in the pathophysiology of PTSD and none of these systems function uniformly among all patients with PTSD. Hence, conceptually, enhancing ECB signaling may be a more effective avenue for pharmacological treatment.

Cannabinoid interventions for PTSD: Where to next?

Cannabinoids are a promising method for pharmacological treatment of post-traumatic stress disorder (PTSD). Despite considerable research devoted to the effect of cannabinoid modulation on PTSD symptomology, there is not a currently agreed way by which the cannabinoid system should be targeted in humans. In this review, we present an overview of recent research identifying neurological pathways by which different cannabinoid-based treatments may exert their effects on PTSD symptomology. We evaluate the strengths and weaknesses of each of these different approaches, including recent challenges presented to favourable options such as fatty acid amide hydrolase (FAAH) inhibitors. This article makes the strengths and challenges of different potential cannabinoid treatments accessible to psychological researchers interested in cannabinoid therapeutics and aims to aid selection of appropriate tools for future clinical trials.

Cannabinoid Regulation of Fear and Anxiety: an Update

PURPOSE OF REVIEW

Anxiety- and trauma-related disorders are prevalent and debilitating mental illnesses associated with a significant socioeconomic burden. Current treatment approaches often have inadequate therapeutic responses, leading to symptom relapse. Here we review recent preclinical and clinical findings on the potential of cannabinoids as novel therapeutics for regulating fear and anxiety.

RECENT FINDINGS

Evidence from preclinical studies has shown that the non-psychotropic phytocannabinoid cannabidiol and the endocannabinoid anandamide have acute anxiolytic effects and also regulate learned fear by dampening its expression, enhancing its extinction and disrupting its reconsolidation. The findings from the relevant clinical literature are still very preliminary but are nonetheless encouraging. Based on this preclinical evidence, larger-scale placebo-controlled clinical studies are warranted to investigate the effects of cannabidiol in particular as an adjunct to psychological therapy or medication to determine its potential utility for treating anxiety-related disorders in the future.

Tempering aversive/traumatic memories with cannabinoids: a review of evidence from animal and human studies

RATIONALE

Aversive learning and memory are essential to cope with dangerous and stressful stimuli present in an ever-changing environment. When this process is dysfunctional, however, it is associated with posttraumatic stress disorder (PTSD). The endocannabinoid (eCB) system has been implicated in synaptic plasticity associated with physiological and pathological aversive learning and memory.

OBJECTIVE AND METHODS

The objective of this study was to review and discuss evidence on how and where in the brain genetic or pharmacological interventions targeting the eCB system would attenuate aversive/traumatic memories through extinction facilitation in laboratory animals and humans. The effect size of the experimental intervention under investigation was also calculated.

RESULTS

Currently available data indicate that direct or indirect activation of cannabinoid type-1 (CB1) receptor facilitates the extinction of aversive/traumatic memories. Activating CB1 receptors around the formation of aversive/traumatic memories or their reminders can potentiate their subsequent extinction. In most cases, the effect size has been large (Cohen's d ≥ 1.0). The brain areas responsible for the abovementioned effects include the medial prefrontal cortex, amygdala, and/or hippocampus. The potential role of cannabinoid type-2 (CB2) receptors in extinction learning is now under investigation.

CONCLUSION

Drugs augmenting the brain eCB activity can temper the impact of aversive/traumatic experiences by diverse mechanisms depending on the moment of their administration. Considering the pivotal role the extinction process plays in PTSD, the therapeutic potential of these drugs is evident. The sparse number of clinical trials testing these compounds in stress-related disorders is a gap in the literature that needs to be addressed.

Role of endocannabinoids in the hippocampus and amygdala in emotional memory and plasticity

Posttraumatic stress disorder (PTSD) is characterized by the reexperiencing of a traumatic event and is associated with slower extinction of fear responses. Impaired extinction of fearful associations to trauma-related cues may interfere with treatment response, and extinction deficits may be premorbid risk factors for the development of PTSD. We examined the effects of exposure to a severe footshock followed by situational reminders (SRs) on extinction, plasticity, and endocannabinoid (eCB) content and activity in the hippocampal CA1 area and basolateral amygdala (BLA). We also examined whether enhancing eCB signaling before extinction, using the fatty acid amide hydrolase (FAAH) inhibitor URB597, could prevent the shock/SRs-induced effects on fear response and plasticity. URB597 administered systemically (0.3 mg/kg) or locally into the CA1 or BLA (0.1 µg/side) prior to extinction decreased fear retrieval and this effect persisted throughout extinction training and did not recuperate during spontaneous recovery. A low dose of the CB1 receptor antagonist AM251 (0.3 mg/kg i.p. or 0.01 µg/0.5 µl intra-CA1 or intra-BLA) blocked these effects suggesting that the effects of URB597 were CB1 receptor-dependent. Exposure to shock and reminders induced behavioral metaplasticity with opposite effects on long-term potentiation (LTP) in the hippocampus (impairment) and the BLA (enhancement). URB597 was found to prevent the opposite shock/SR-induced metaplasticity in hippocampal and BLA-LTP. Exposure to shock and reminders might cause variation in endogenous cannabinoid levels that could affect fear-circuit function. Indeed, exposure to shock and SRs affected eCB content: increased 2-arachidonoyl-glycerol (2-AG) and N-arachidonylethanolamine (AEA) levels in the CA1, decreased serum and BLA AEA levels while shock exposure increased FAAH activity in the CA1 and BLA. FAAH inhibition before extinction abolished fear and modulated LTP in the hippocampus and amygdala, brain regions pertinent to emotional memory. The findings suggest that targeting the eCB system before extinction may be beneficial in fear memory attenuation and these effects may involve metaplasticity in the CA1 and BLA.

Chronic treatment with URB597 ameliorates post-stress symptoms in a rat model of PTSD

Activating the endocannabinoid system has become a major focus in the search for novel therapeutics for anxiety and deficits in fear extinction, two defining features of PTSD. We examined whether chronic treatment with the fatty acid amide hydrolase (FAAH) inhibitor URB597 (0.2, 0.3, 0.4 mg/kg, i.p.) or the CB1/2 receptor agonist WIN55,212-2 (0.25, 0.5 mg/kg, i.p.) injected for 3 weeks to rats exposed to the shock and reminders model of PTSD would attenuate post-stress symptoms and affect basolateral amygdala (BLA) and CA1 CB1 receptors. Exposure to shock and reminders enhanced acoustic startle response and impaired extinction. Rats exposed to shock and reminders and chronically treated with URB597 demonstrated normalized startle response and intact extinction kinetics. WIN55,212-2 only affected the startle response. The therapeutic effects of URB597 and WIN55,212-2 were found to be CB1 receptor dependent, as these effects were blocked when a low dose of the CB1 receptor antagonist AM251 (0.3 mg/kg, i.p. for 3 weeks) was co-administered. Moreover, URB597, but not WIN55,212-2, normalized the shock/reminders-induced upregulation in CB1 receptor levels in the BLA and CA1. One hour after the shock, N-arachidonoylethanolamine (AEA) was increased in the BLA and decreased in the CA1. Circulating 2-arachidonoylglycerol (2-AG) concentrations were decreased in shocked rats, with no significant effect in the BLA or CA1. FAAH activity was increased in the CA1 of shocked rats. Chronic cannabinoid treatment with URB597 can ameliorate PTSD-like symptoms suggesting FAAH inhibitors as a potentially effective therapeutic strategy for the treatment of disorders associated with inefficient fear coping.

Differential roles of the infralimbic and prelimbic areas of the prefrontal cortex in reconsolidation of a traumatic memory

Studies about reconsolidation of conditioned fear memories have shown that pharmacological manipulation at memory reactivation can attenuate or enhance the subsequent expression of the conditioned fear response. Here we examined the effects of a single injection of the mTOR inhibitor rapamycin (Rap) into the infralimbic (IL) and prelimbic (PL) areas [which compose the ventromedial prefrontal cortex (PFC)] on reconsolidation and extinction of a traumatic fear memory. We found opposite effects of Rap infused into the PL and IL on reconsolidation and extinction: intra-PL Rap and systemic Rap impaired reconsolidation and facilitated extinction whereas intra-IL Rap enhanced reconsolidation and impaired extinction. These effects persisted at least 10 days after reactivation. Shock exposure induced anxiety-like behavior and impaired working memory and intra-IL and -PL Rap normalized these effects. Finally, when measured after fear retrieval, shocked rats exhibited reduced and increased phosphorylated p70s6K levels in the IL and basolateral amygdala, respectively. No effect on phosphorylated p70s6K levels was observed in the PL. The study points to the differential roles of the IL and PL in memory reconsolidation and extinction. Moreover, inhibiting mTOR via rapamycin following reactivation of a fear memory may be a novel approach in attenuating enhanced fear memories.

Role of the agranular insular cortex in contextual control over cocaine-seeking behavior in rats

RATIONALE

Environmental stimulus control over drug relapse requires the retrieval of context-response-cocaine associations, maintained in long-term memory through active reconsolidation processes. Identifying the neural substrates of these phenomena is important from a drug addiction treatment perspective.

OBJECTIVES

The present study evaluated whether the agranular insular cortex (AI) plays a role in drug context-induced cocaine-seeking behavior and cocaine memory reconsolidation.

METHODS

Rats were trained to lever press for cocaine infusions in a distinctive context, followed by extinction training in a different context. Rats in experiment 1 received bilateral microinfusions of vehicle or a GABA agonist cocktail (baclofen and muscimol (BM)) into the AI or the overlying somatosensory cortex (SSJ, anatomical control region) immediately before a test of drug-seeking behavior (i.e., non-reinforced lever presses) in the previously cocaine-paired context. The effects of these manipulations on locomotor activity were also assessed in a novel context. Rats in experiment 2 received vehicle or BM into the AI after a 15-min reexposure to the cocaine-paired context, intended to reactivate context-response-cocaine memories and initiate their reconsolidation. The effects of these manipulations on drug context-induced cocaine-seeking behavior were assessed 72 h later.

RESULTS

BM-induced pharmacological inactivation of the AI, but not the SSJ, attenuated drug context-induced reinstatement of cocaine-seeking behavior without altering locomotor activity. Conversely, AI inactivation after memory reactivation failed to impair subsequent drug-seeking behavior and thus cocaine memory reconsolidation.

CONCLUSIONS

These findings suggest that the AI is a critical element of the neural circuitry that mediates contextual control over cocaine-seeking behavior.