Effects of ∆9-tetrahydrocannabinol on aversive memories and anxiety: a review from human studies.

BACKGROUND: Posttraumatic stress disorder (PTSD) may stem from the formation of aberrant and enduring aversive memories. Some PTSD patients have recreationally used Cannabis, probably aiming at relieving their symptomatology. However, it is still largely unknown whether and how Cannabis or its psychotomimetic compound Δ9-tetrahydrocannabinol (THC) attenuates the aversive/traumatic memory outcomes. Here, we seek to review and discuss the effects of THC on aversive memory extinction and anxiety in healthy humans and PTSD patients. METHODS: Medline, PubMed, Cochrane Library, and Central Register for Controlled Trials databases were searched to identify peer-reviewed published studies and randomized controlled trials in humans published in English between 1974 and July 2020, including those using only THC and THC combined with cannabidiol (CBD). The effect size of the experimental intervention under investigation was calculated. RESULTS: At low doses, THC can enhance the extinction rate and reduce anxiety responses. Both effects involve the activation of cannabinoid type-1 receptors in discrete components of the corticolimbic circuitry, which could couterbalance the low "endocannabinoid tonus" reported in PTSD patients. The advantage of associating CBD with THC to attenuate anxiety while minimizing the potential psychotic or anxiogenic effect produced by high doses of THC has been reported. The effects of THC either alone or combined with CBD on aversive memory reconsolidation, however, are still unknown. CONCLUSIONS: Current evidence from healthy humans and PTSD patients supports the THC value to suppress anxiety and aversive memory expression without producing significant adverse effects if used in low doses or when associated with CBD. Future studies are guaranteed to address open questions related to their dose ratios, administration routes, pharmacokinetic interactions, sex-dependent differences, and prolonged efficacy.

Newly acquired and reactivated contextual fear memories are more intense and prone to generalize after activation of prelimbic cortex NMDA receptors.

Activity in the rodent prelimbic (PL) cortex contributes to consolidation, retrieval and reconsolidation of learned fear. The PL cortex is considered homologous to the primate dorsal anterior cingulate cortex (dACC). In patients with post-traumatic stress disorder (PTSD), the dACC is often reported to be hyperactive after acquisition and/or around the retrieval of the traumatic memory. It is still unknown, however, whether there is a relationship between altered dACC functioning at these time points and PTSD-associated behavioral outcomes, such as fear overgeneralization. The present study sought to investigate this matter by associating contextual fear conditioning with bilateral and selective activation of PL cortex N-methyl-D-aspartate (NMDA) glutamate receptors with NMDA (0.03-0.3nmol) while the learned fear was being consolidated, retrieved or reconsolidated. We report that this pharmacological intervention induced generalized fear expression and/or extinction deficits in animals subjected to a strong contextual fear conditioning protocol when conducted post-acquisition, pre-retrieval or post-retrieval. These results suggest that newly acquired and reactivated fear memories undergo abnormal consolidation or reconsolidation after PL cortex NMDA receptor activation. The consolidation or reconsolidation of a contextual fear memory trace induced by a weak fear training protocol was also potentiated by PL cortex NMDA receptor activation. Altogether, the present findings connect altered PL cortex activity with changes in specificity and/or intensity of a contextual fear memory, which might shed light on the PTSD neurobiology and related behavioral outcomes.

PTSD-like memory generated through enhanced noradrenergic activity is mitigated by a dual step pharmacological intervention targeting its reconsolidation.

BACKGROUND: Traumatic memories have been resilient to therapeutic approaches targeting their permanent attenuation. One of the potentially promising pharmacological strategies under investigation is the search for safe reconsolidation blockers. However, preclinical studies focusing on this matter have scarcely addressed abnormal aversive memories and related outcomes. METHODS: By mimicking the enhanced noradrenergic activity reported after traumatic events in humans, here we sought to generate a suitable condition to establish whether some clinically approved drugs able to disrupt the reconsolidation of conditioned fear memories in rodents would still be effective. RESULTS: We report that the α2-adrenoceptor antagonist yohimbine was able to induce an inability to restrict behavioral (fear) and cardiovascular (increased systolic blood pressure) responses to the paired context when administered immediately after acquisition, but not 6h later, indicating the formation of a generalized fear memory, which endured for over 29 days and was less susceptible to suppression by extinction. It was also resistant to reconsolidation disruption by the α2-adrenoceptor agonist clonidine or cannabidiol, the major non-psychotomimetic component of Cannabis sativa. Since signaling at N-methyl-D-aspartate (NMDA) receptors is important for memory labilization and because a dysfunctional memory may be less labile than is necessary to trigger reconsolidation on its brief retrieval and reactivation, we then investigated and demonstrated that pre-retrieval administration of the partial NMDA agonist D-cycloserine allowed the disrupting effects of clonidine and cannabidiol on reconsolidation. CONCLUSIONS: These findings highlight the effectiveness of a dual-step pharmacological intervention to mitigate an aberrant and enduring aversive memory similar to that underlying the post-traumatic stress disorder.

Enhanced noradrenergic activity potentiates fear memory consolidation and reconsolidation by differentially recruiting α1- and ß-adrenergic receptors.

Consolidation and reconsolidation are phases of memory stabilization that diverge slightly. Noradrenaline is known to influence both processes, but the relative contribution of α1- and ß-adrenoceptors is unclear. The present study sought to investigate this matter by comparing their recruitment to consolidate and/or reconsolidate a contextual fear memory trace under enhanced noradrenergic activity induced by yohimbine. We report that this α2-adrenoceptor antagonist was able to potentiate fear memory trace consolidation or reconsolidation when administered immediately after acquisition or retrieval, respectively, resulting in increased freezing expression. In either case, generalization of this response to an unpaired context was also seen when it achieved a ceiling level in the paired context. These effects endured for over 7 d and relied on action at central rather than peripheral sites, but were prevented when a memory trace was not acquired, when memory reactivation was omitted, or when administration of yohimbine was delayed until 6 h after acquiring or retrieving the memory trace. The ß-adrenoceptor antagonist propranolol was able to prevent the above-mentioned effects of yohimbine, while pretreatment with the α1-adrenoceptor antagonist prazosin blocked only its facilitating effects on memory reconsolidation. These results highlight a differential participation of α1- and ß-adrenoceptors in fear memory processing. Moreover, it was shown that the α2-adrenoceptor agonist clonidine, as opposed to yohimbine, mitigates fear expression by weakening memory consolidation or reconsolidation.

Activity in prelimbic cortex subserves fear memory reconsolidation over time.

The prelimbic cortex has been implicated in the consolidation of previously learned fear. Herein, we report that temporarily inactivating this medial prefrontal cortex subregion with the GABAA agonist muscimol (4.0 nmol in 0.2 µL per hemisphere) was able to equally disrupt 1-, 7-, and 21-d-old contextual fear memories after their brief retrieval in rats. In all cases, this effect was prevented when memory reactivation was omitted. These results indicate that recent and remote fear memories are susceptible to reconsolidation blockade induced by prelimbic cortex inactivation. It was also demonstrated that the disrupting effect of prelimbic cortex inactivation on fear memory persisted over 11 d, and did not show extinction-related features, such as reinstatement. Infusing the same dose and volume of muscimol bilaterally into the infralimbic cortex after brief retrieval/reactivation of the fear memory did not disrupt it, as seen in prelimbic cortex-inactivated animals. The expression of Zif268/Egr1, the product of an immediate early gene related to memory reconsolidation, was also less pronounced in the infralimbic cortex than in prelimbic cortex following memory retrieval/reactivation. Altogether, the present findings highlight that activity in the prelimbic cortex may reestablish reactivated aversive memories and, therefore, contribute to their maintenance over time.

On making (and turning adaptive to) maladaptive aversive memories in laboratory rodents.

Fear conditioning and avoidance tasks usually elicit adaptive aversive memories. Traumatic memories are more intense, generalized, inflexible, and resistant to attenuation via extinction- and reconsolidation-based strategies. Inducing and assessing these dysfunctional, maladaptive features in the laboratory are crucial to interrogating posttraumatic stress disorder's neurobiology and exploring innovative treatments. Here we analyze over 350 studies addressing this question in adult rats and mice. There is a growing interest in modeling several qualitative and quantitative memory changes by exposing already stressed animals to freezing- and avoidance-related tests or using a relatively high aversive training magnitude. Other options combine aversive/fearful tasks with post-acquisition or post-retrieval administration of one or more drugs provoking neurochemical or epigenetic alterations reported in the trauma aftermath. It is potentially instructive to integrate these procedures and incorporate the measurement of autonomic and endocrine parameters. Factors to consider when defining the organismic and procedural variables, partially neglected aspects (sex-dependent differences and recent vs. remote data comparison) and suggestions for future research (identifying reliable individual risk and treatment-response predictors) are discussed.

Interactions of Noradrenergic, Glucocorticoid and Endocannabinoid Systems Intensify and Generalize Fear Memory Traces.

Systemic administration of drugs that activate the noradrenergic or glucocorticoid system potentiates aversive memory consolidation and reconsolidation. The opposite happens with the stimulation of endocannabinoid signaling under certain conditions. An unbalance of these interacting neurotransmitters can lead to the formation and maintenance of traumatic memories, whose strength and specificity attributes are often maladaptive. Here we aimed to investigate whether originally low-intensity and precise contextual fear memories would turn similar to traumatic ones in rats systemically administered with adrenaline, corticosterone, and/or the cannabinoid type-1 receptor antagonist/inverse agonist AM251 during consolidation or reconsolidation. The high dose of each pharmacological agent evaluated significantly increased freezing times at test in the conditioning context one and nine days later when given alone post-acquisition or post-retrieval. Their respective low dose produced no relative changes when given separately, but co-treatment of adrenaline with corticosterone or AM251 and the three drugs combined, but not corticosterone with AM251, produced results equivalent to those mentioned initially. Neither the high nor the low dose of adrenaline, corticosterone, or AM251 altered freezing times at test in a novel, neutral context two and ten days later. In contrast, animals receiving the association of their low dose exhibited significantly higher freezing times than controls. Together, the results indicate that newly acquired and destabilized threat memory traces become more intense and generalized after a combined interference acting synergistically and mimicking that reported in patients presenting stress-related psychiatric conditions.

Medial prefrontal cortex mechanisms of cannabidiol-induced aversive memory reconsolidation impairments.

Growing evidence indicates that cannabidiol (CBD), a substance present in the Cannabis sativa plant, has potential therapeutic value to regulate abnormal emotional memories associated with post-traumatic stress and drug use disorders. CBD can attenuate their valence after retrieval (i.e., during reconsolidation) or potentiate their suppression by extinction. Pharmacological research has now focused on elucidating how it acts. Systemic antagonism of cannabinoid type-1 (CB1) receptors has often prevented the abovementioned effects of CBD. However, it is unknown in which brain regions CBD stimulates CB1 receptors and how it interferes with local activity-related plasticity to produce these effects. The present study addressed these questions considering the reconsolidation of contextual fear memories in rats. We focused on the medial prefrontal cortex (mPFC), which comprises the anterior cingulate (AC), prelimbic (PL), and infralimbic (IL) subregions, as local activity or plasticity has been associated with the process to-be-investigated. Animals that received post-retrieval systemic CBD treatment presented relatively fewer cells expressing Zif268/Egr1 protein, a proxy for synaptic plasticity related to reconsolidation, in the AC and PL. At the same time, there were no significant differences in the IL. Pretreatment with the CB1 receptor antagonist/inverse agonist AM251 into the AC, PL, or IL prevented the impairing effects of systemic CBD treatment on reconsolidation. CBD also caused reconsolidation impairments when injected directly into the AC or PL but not the IL. Together, these findings show complementary mechanisms through which CBD may hinder the reconsolidation of destabilized aversive memories along the dorsoventral axis of the mPFC.

Role of prelimbic cortex PKC and PKMζ in fear memory reconsolidation and persistence following reactivation.

The persistence of newly acquired memories is supported by the activity of PKMζ, an atypical isoform of protein kinase C (PKC). Whether the activity of conventional and atypical PKC isoforms contributes to reactivated memories to persist is still unknown. Similarly, whether memory reactivation is a prerequisite for interventions to be able to change memory persistence is scarcely investigated. Based on the above, we examined the role of conventional and atypical PKC isoforms in the prelimbic cortex in reconsolidation and persistence of a reactivated contextual fear memory in male Wistar rats. It is shown that (i) inhibiting the PKC activity with chelerythrine or the PKMζ activity with ZIP impaired the persistence of a reactivated memory for at least 21 days; (ii) ZIP given immediately after memory reactivation affected neither the reconsolidation nor the persistence process. In contrast, when given 1 h later, it impaired the memory persistence; (iii) chelerythrine given immediately after memory reactivation impaired the reconsolidation; (iv) omitting memory reactivation prevented the chelerythrine- and ZIP-induced effects: (v) the ZIP action is independent of the time elapsed between its administration and the initial memory test. The results indicate that prelimbic cortex PKC and PKMζ are involved in memory reconsolidation and persistence.

A time-dependent contribution of hippocampal CB1 , CB2 and PPARγ receptors to cannabidiol-induced disruption of fear memory consolidation.

BACKGROUND AND PURPOSE: In preclinical studies, cannabidiol (CBD) mitigates fear memories by facilitating their extinction or interfering with their generalization and reconsolidation. The brain regions and mechanisms underlying these effects, and their temporal window, are still poorly understood. Here, we have investigated related questions in the dorsal hippocampus (DH) during contextual fear consolidation. EXPERIMENTAL APPROACH: Adult male Wistar rats received CBD (10-30 pmol) intra-DH immediately, 1 or 3 hr after fear conditioning. Effects of CBD on consolidation were inferred behaviourally and by analysing expression of the activity-regulated, cytoskeleton-associated (Arc) protein. The contribution of anandamide, CB1 , CB2 , 5-HT1A , A2A , and PPARγ receptors was also assessed. KEY RESULTS: CBD impaired memory consolidation when given immediately or 1 hr after fear conditioning, but not after 3 hr. Expression of Arc protein in DH was reduced by systemic CBD treatment in both cases. Immediately after fear conditioning, CBD effects were abolished by CB1 or CB2 receptor blockade, partly reduced by 5-HT1A or A2A antagonism, and remained unchanged after antagonism of PPARγ receptors. One hour after fear conditioning, CBD effects were prevented only by PPARγ receptor antagonism. Also, inhibition of fatty acid amide hydrolase by URB597, impaired memory consolidation when infused immediately, but not 1 hr after fear conditioning. CONCLUSIONS AND IMPLICATIONS: CBD disrupts memory consolidation up to 1 hr after fear conditioning, allowing an extended window of opportunity to mitigate aversive memories after their acquisition. Our results suggest time-dependent participation of anandamide, CB1 , CB2 and PPARγ receptors in the DH, during this process.

Effects of Cannabinoid Drugs on Aversive or Rewarding Drug-Associated Memory Extinction and Reconsolidation.

Posttraumatic stress and drug use disorders may stem from aberrant memory formation. As the endocannabinoid (eCB) system has a pivotal role in emotional memory processing and related synaptic plasticity, here we seek to review and discuss accumulating evidence on how and where in the brain interventions targeting the eCB system would attenuate outcomes associated with traumatic events and/or drug addiction through memory extinction facilitation or reconsolidation disruption. Currently available data from mouse, rat, monkey and healthy human studies investigating the effects of cannabinoid drugs on extinction and reconsolidation of aversive memories are more consistent than those related to rewarding drug-associated memories. Interventions able to attenuate aversive memories by extinction facilitation or reconsolidation disruption have boosted the anandamide-induced activation of cannabinoid type-1 (CB1) receptors. A still limited number of studies report that CB1 receptor activation could also be effective in facilitating the extinction or disrupting the reconsolidation of rewarding drug-associated memories. The reinstatement of extinguished drug memories (relapse) is reduced by CB1 receptor antagonism. The cannabidiol has shown to be effective in any of the aforementioned cases, albeit its mechanism of action is not fully understood. Brain areas in which cannabinoid drugs induce these effects include the prefrontal cortex, amygdala, hippocampus, and/or nucleus accumbens. The potential role of 2-arachidonoylglycerol (2-AG) and cannabinoid type-2 (CB2) receptors in emotional memory extinction and reconsolidation is currently under investigation. Overall, preclinical data support a closer look into certain cannabinoid drugs owing to their safety and potential therapeutic value against stress-related and drug use disorders.

Cannabidiol disrupts the consolidation of specific and generalized fear memories via dorsal hippocampus CB1 and CB2 receptors.

Pharmacological interventions able to modulate a fear memory while it is consolidated could have therapeutic value in tempering those maladaptively overconsolidated. Animal and human studies have shown the intensity of unconditioned stimulus delivered during fear conditioning influences qualitative and quantitative aspects of the memory to be established. By varying the shock intensity used for contextual pairing in rats, here we induced specific and more generalized long-term fear memories to investigate whether, how and where in the brain the cannabidiol (CBD; 3.0-30 mg/kg i.p.) could impair their consolidation and related outcomes. When given immediately after their acquisition, it reduced respectively the conditioned fear expression, and fear generalization, ultrasonic vocalizations at 22-kHz and the relative resistance to extinction. CBD had no effects on short-term fear memory, and its delayed treatment no longer affected the consolidation process. As the dorsal hippocampus (DH) modulates fear memory specificity and generalization, and cannabinoid type-1 (CB1) and type-2 (CB2) receptors contribute to consolidation, we investigated their involvement in CBD effects. Both systemic and intra-DH treatment with the CB1 receptor antagonist/inverse agonist AM251 or the CB2 receptor antagonist/inverse agonist AM630 prevented the disrupting CBD effects on consolidation. Since the CBD effects on the endocannabinoid transmission are probably indirect, we investigated and demonstrated the FAAH inhibitor URB597 induced effects similar to those of CBD when given systemically or intra-DH. Altogether, the present results suggest the CBD disrupts the consolidation of different fear memories via anandamide-mediated activation of DH CB1 and CB2 receptors.

Evidence for an expanded time-window to mitigate a reactivated fear memory by tamoxifen.

The mechanisms underpinning the persistence of emotional memories are inaccurately understood. Advancing the current level of understanding with regards to this aspect is of potential translational value for the treatment of post-traumatic stress disorder (PTSD), which stems from an abnormal aversive memory formation. Tamoxifen (TMX) is a drug used in chemotherapy for breast cancer and associated with poor cognitive performances. The present study investigated whether the systemic administration of TMX (1.0-50mg/kg) during and/or beyond the reconsolidation time-window could attenuate a reactivated contextual fear memory in laboratory animals. When administered 0, 6 or 9h (but not 12h) post-memory retrieval and reactivation, TMX (50mg/kg) reduced the freezing behavior in male rats re-exposed to the paired context on day 7, but not on day 1, suggesting a specific impairing effect on memory persistence. Importantly, this effect lasts up to 21 days, but it is prevented by omitting the memory retrieval or memory reactivation. When female rats in the diestrous or proestrous phase were used, the administration of TMX 6h after retrieving and reactivating the fear memory also impaired its persistence. Altogether, regardless of the gender, the present results indicate that the TMX is able to disrupt the persistence of reactivated fear memories in an expanded time-window, which could shed light on a new promising therapeutic strategy for PTSD.

Δ9-Tetrahydrocannabinol alone and combined with cannabidiol mitigate fear memory through reconsolidation disruption.

Δ(9)-tetrahydrocannabinol (THC) and cannabidiol (CBD) are the major constituents of the Cannabis sativa plant, which is frequently consumed by subjects exposed to life-threatening situations to relief their symptomatology. It is still unknown, however, whether THC could also affect the maintenance of an aversive memory formed at that time when taken separately and/or in conjunction with CBD. The present study sought to investigate this matter at a preclinical level. We report that THC (0.3-10mg/kg, i.p.) was able to disrupt the reconsolidation of a contextual fear memory, resulting in reduced conditioned freezing expression for over 22 days. This effect was dependent on activation of cannabinoid type-1 receptors located in prelimbic subregion of the medial prefrontal cortex and on memory retrieval/reactivation. Since CBD may counteract the negative psychotropic effects induced by THC and has been shown to be a reconsolidation blocker, we then investigated and demonstrated that associating sub-effective doses of these two compounds was equally effective in attenuating fear memory maintenance in an additive fashion and in a dose ratio of 10 to 1, which contrasts with that commonly found in C. sativa recreational samples. Of note, neither THC alone nor CBD plus THC interfered with anxiety-related behaviors and locomotor activity, as assessed in the elevated plus-maze test, at a time point coinciding with that used to evaluate their effects on memory reconsolidation. Altogether, present findings suggest a potential therapeutic value of using THC and/or CBD to mitigate a dysfunctional aversive memory through reconsolidation disruption in post-traumatic stress disorder patients.

On disruption of fear memory by reconsolidation blockade: evidence from cannabidiol treatment.

The search for reconsolidation blockers may uncover clinically relevant drugs for disrupting memories of significant stressful life experiences, such as those underlying the posttraumatic stress disorder. Considering the safety of systemically administered cannabidiol (CBD), the major non-psychotomimetic component of Cannabis sativa, to animals and humans, the present study sought to investigate whether and how this phytocannabinoid (3-30 mg/kg intraperitoneally; i.p.) could mitigate an established memory, by blockade of its reconsolidation, evaluated in a contextual fear-conditioning paradigm in rats. We report that CBD is able to disrupt 1- and 7-days-old memories when administered immediately, but not 6 h, after their retrieval for 3 min, with the dose of 10 mg/kg being the most effective. This effect persists in either case for at least 1 week, but is prevented when memory reactivation was omitted, or when the cannabinoid type-1 receptors were antagonized selectively with AM251 (1.0 mg/kg). Pretreatment with the serotonin type-1A receptor antagonist WAY100635, however, failed to block CBD effects. These results highlight that recent and older fear memories are equally vulnerable to disruption induced by CBD through reconsolidation blockade, with a consequent long-lasting relief in contextual fear-induced freezing. Importantly, this CBD effect is dependent on memory reactivation, restricted to time window of <6 h, and is possibly dependent on cannabinoid type-1 receptor-mediated signaling mechanisms. We also observed that the fear memories disrupted by CBD treatment do not show reinstatement or spontaneous recovery over 22 days. These findings support the view that reconsolidation blockade, rather than facilitated extinction, accounts for the aforementioned CBD results in our experimental conditions.

Attenuation of anxiety-related behaviour after the antagonism of transient receptor potential vanilloid type 1 channels in the rat ventral hippocampus.

The neurochemical mechanisms supporting the role of the ventral hippocampus in anxiety are now under investigation. To address this issue, we examined whether the pharmacological blockade of transient receptor potential vanilloid type 1 (TRPV1) channels in the ventral hippocampus would attenuate anxiety-related behaviour. Rats infused with the TRPV1 channel antagonist capsazepine (2.0 nmol) showed less inhibitory avoidance than controls in the elevated plus-maze test. This result indicates an anxiolytic-like effect, and suggests a role for TRPV1 channels in regulating anxiety.