Sex-dependent effects of endocannabinoid modulation of conditioned fear extinction in rats

Sex differences in fear expression and persistence in an animal model of Post-Traumatic Stress Disorder

Post-Traumatic Stress Disorder (PTSD) is a complex psychiatric condition arising from traumatic experiences, marked by abnormal fear memories. Despite women are twice as likely as men to develop PTSD, the biological mechanisms underlying this disparity remain inadequately explored, particularly in preclinical studies involving female subjects. Previous research shows that female rats exhibit active fear responses, while males display passive behaviors. Additionally, sex differences in ultrasonic vocalizations (USVs) during fear conditioning have been observed, indicating varying emotional responses. Here, we validated a traumatic stress model consisting of footshock exposure paired with social isolation - originally developed in male rats - on females for the first time, focusing on sex differences in fear memory expression, retention and extinction. Our findings reveal that only during trauma exposure, males predominantly exhibited passive responses, whereas females demonstrated more active responses, despite both sexes emitting similar numbers of alarm USVs. Females also showed lower levels of freezing and USV emissions throughout extinction sessions and displayed a higher extinction rate compared to males. Notably, only males displayed a conditioned fear response when triggered by a single mild stressor. These findings highlight sex differences in trauma responses and fear memory processes. The study emphasizes the importance of incorporating 22-kHz USV evaluations along with other behavioral metrics for a comprehensive understanding of fear memory. This research contributes to the existing literature on traumatic stress models as well as underscores the necessity of including female subjects in preclinical studies to better inform treatment and prevention strategies tailored to both sexes.

Interplay between endocannabinoid and endovanilloid mechanisms in fear conditioning

OBJECTIVE

The transient receptor potential cation channel, subfamily V (vanilloid), member 1 (TRPV1) mediates pain perception to thermal and chemical stimuli in peripheral neurons. The cannabinoid receptor type 1 (CB1), on the other hand, promotes analgesia in both the periphery and the brain. TRPV1 and CB1 have also been implicated in learned fear, which involves the association of a previously neutral stimulus with an aversive event. In this review, we elaborate on the interplay between CB1 receptors and TRPV1 channels in learned fear processing.

METHODS

We conducted a PubMed search for a narrative review on endocannabinoid and endovanilloid mechanisms on fear conditioning.

RESULTS

TRPV1 and CB1 receptors are activated by a common endogenous agonist, arachidonoyl ethanolamide (anandamide), Moreover, they are expressed in common neuroanatomical structures and recruit converging cellular pathways, acting in concert to modulate fear learning. However, evidence suggests that TRPV1 exerts a facilitatory role, whereas CB1 restrains fear responses.

CONCLUSION

TRPV1 and CB1 seem to mediate protective and aversive roles of anandamide, respectively. However, more research is needed to achieve a better understanding of how these receptors interact to modulate fear learning.

The effect of traumatic brain injury on learning and memory: A synaptic focus

Deficits in learning and memory are some of the most commonly reported symptoms following a traumatic brain injury (TBI). We will examine whether the neural basis of these deficits stems from alterations to bidirectional synaptic plasticity within the hippocampus. Although the CA1 subregion of the hippocampus has been a focus of TBI research, the dentate gyrus should also be given attention as it exhibits a unique ability for adult neurogenesis, a process highly susceptible to TBI-induced damage. This review examines our current understanding of how TBI results in deficits in synaptic plasticity, as well as how TBI-induced changes in endocannabinoid (eCB) systems may drive these changes. Through the synthesis and amalgamation of existing data, we propose a possible mechanism for eCB-mediated recovery in synaptic plasticity deficits. This hypothesis is based on the plausible roles of CB1 receptors in regulating inhibitory tone, influencing astrocytes and microglia, and modulating glutamate release. Dysregulation of the eCBs may be responsible for deficits in synaptic plasticity and learning following TBI. Taken together, the existing evidence indicates eCBs may contribute to TBI manifestation, pathogenesis, and recovery, but it also suggests there may be a therapeutic role for the eCB system in TBI.

Contributions of associative and non-associative learning to the dynamics of defensive ethograms

Defensive behavior changes based on threat intensity, proximity, and context of exposure, and learning about danger-predicting stimuli is critical for survival. However, most Pavlovian fear conditioning paradigms focus only on freezing behavior, obscuring the contributions of associative and non-associative mechanisms to dynamic defensive responses. To thoroughly investigate defensive ethograms, we subjected male and female adult C57BL/6J mice to a Pavlovian conditioning paradigm that paired footshock with a serial compound stimulus (SCS) consisting of distinct tone and white noise (WN) stimulus periods. To investigate how associative and non-associative mechanisms affect defensive responses, we compared this paired SCS-footshock group with four control groups that were conditioned with either pseudorandom unpaired presentations of SCS and footshock, shock only, or reversed SCS presentations with inverted tone--WN order, with paired or unpaired presentations. On day 2 of conditioning, the paired group exhibited robust freezing during the tone period with switching to explosive jumping and darting behaviors during the WN period. Comparatively, the unpaired and both reverse SCS groups expressed less tone-induced freezing and rarely showed jumping or darting during WN. Following the second day of conditioning, we observed how defensive behavior changed over two extinction sessions. During extinction, the tone-induced freezing decreased in the paired group and mice rapidly shifted from escape jumping during WN to a combination of freezing and darting. The unpaired, unpaired reverse, and shock-only groups displayed defensive tail rattling and darting during the SCS, with minimal freezing and jumping. Interestingly, the paired reverse group did not jump to WN, and tone-evoked freezing was resistant to extinction. These findings demonstrate that non-associative factors promote some defensive responsiveness, but associative factors are required for robust cue-induced freezing and high-intensity flight expression.

The Estrous Cycle Influences the Effects of Fatty Acid Amide Hydrolase and Monoacylglycerol Lipase Inhibition in the Anxiety-Like Behavior in Rats

Background: Sex differences in the response to the anxiety-related effects of cannabinoid drugs have been reported, with females being more sensitive than males. Evidence suggests that, according to sex and estrous cycle phase (ECP), the content of the endocannabinoids (eCBs) N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) varies in brain areas involved in the anxiety-like behavior. Methods: Considering the lack of studies evaluating sex and ECP differences in the eCB system in anxiety, using URB597, a fatty acid amide hydrolase inhibitor, or MJN110, a monoacylglycerol lipase inhibitor, we explored the effects of increasing AEA or 2-AG levels, respectively, in cycling and ovariectomized (OVX) female adult Wistar rats, as well as males, subjected to the elevated plus maze. Results: The administration of URB597 (0.1 or 0.3mg/kg; intraperitoneally) either increased or reduced the percentage of open arms time (%OAT) and open arms entries (%OAE), being anxiolytic in diestrus and anxiogenic in estrus. No effects were observed in proestrus or when all ECPs were analyzed together. Both doses produced anxiolytic-like effects in males. In OVX females, the anxiolytic-like effect of URB597 0.1 was associated with low levels of estradiol, whereas the anxiogenic-like effect of URB597 0.3 was spared by estradiol pretreatment. The systemic administration of MJN110 3.0 mg/kg reduced the risk assessment behavior (RAB), suggesting an anxiolytic-like effect independent of the ECP. When considering the ECP, MJN110 3.0 increased the %OAT and reduced the RAB, being anxiolytic in estrus and diestrus. No effects were observed in proestrus. Both doses of MJN110 were anxiogenic in males. In OVX females, the anxiolytic-like effect of MJN110 was dependent on low estradiol levels. Conclusion: Together, our findings support the evidence that females react differently to the effects of cannabinoids in the anxiety-like behavior; in addition, AEA and 2-AG modulation elicits anxiety-like responses that are closely influenced by hormone levels, mainly estradiol.

Cannabidiol effects on fear processing and implications for PTSD: Evidence from rodent and human studies

Cannabidiol (CBD) modulates aversive memory and its extinction, with potential implications for treating anxiety- and stress-related disorders. Here, we summarize and discuss scientific evidence showing that CBD administered after the acquisition (consolidation) and retrieval (reconsolidation) of fear memory attenuates it persistently in rats and mice. CBD also reduces fear expression and enhances fear extinction. These effects involve the activation of cannabinoid type-1 (CB1) receptors in the dorsal hippocampus, bed nucleus of stria terminalis, and medial prefrontal cortex, comprising the anterior cingulate, prelimbic, and infralimbic subregions. Serotonin type-1A (5-HT1A) receptors also mediate some CBD effects on fear memory. CBD effects on fear memory acquisition vary, depending on the aversiveness of the conditioning procedure. While rodent findings are relatively consistent and encouraging, human studies investigating CBD's efficacy in modulating aversive/traumatic memories are still limited. More studies are needed to investigate CBD's effects on maladaptive, traumatic memories, particularly in post-traumatic stress disorder patients.

Endocannabinoid agonist 2-arachidonoylglycerol differentially alters diurnal activity and sleep during fentanyl withdrawal in male and female mice

Fentanyl has become the leading driver of opioid overdoses in the United States. Cessation of opioid use represents a challenge as the experience of withdrawal drives subsequent relapse. One of the most prominent withdrawal symptoms that can contribute to opioid craving and vulnerability to relapse is sleep disruption. The endocannabinoid agonist, 2-Arachidonoylglycerol (2-AG), may promote sleep and reduce withdrawal severity; however, the effects of 2-AG on sleep disruption during opioid withdrawal have yet to be assessed. Here, we investigated the effects of 2-AG administration on sleep-wake behavior and diurnal activity in mice during withdrawal from fentanyl. Sleep-wake activity measured via actigraphy was continuously recorded before and after chronic fentanyl administration in both male and female C57BL/6J mice. Immediately following cessation of fentanyl administration, 2-AG was administered intraperitoneally to investigate the impact of endocannabinoid agonism on opioid-induced sleep disruption. We found that female mice maintained higher activity levels in response to chronic fentanyl than male mice. Furthermore, fentanyl administration increased wake and decreased sleep during the light period and inversely increased sleep and decreased wake in the dark period in both sexes. 2-AG treatment increased arousal and decreased sleep in both sexes during first 24-h of withdrawal. On withdrawal day 2, only females showed increased wakefulness with no changes in males, but by withdrawal day 3 male mice displayed decreased rapid-eye movement sleep during the dark period with no changes in female mice. Overall, repeated administration of fentanyl altered sleep and diurnal activity and administration of the endocannabinoid agonist, 2-AG, had sex-specific effects on fentanyl-induced sleep and diurnal changes.

Predator odor stress reactivity, alcohol drinking and the endocannabinoid system

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are highly comorbid and individual differences in response to stress suggest resilient and susceptible populations. Using animal models to target neurobiological mechanisms associated with individual variability in stress coping responses and the relationship with subsequent increases in alcohol consumption has important implications for the field of traumatic stress and alcohol disorders. The current review discusses the unique advantages of utilizing predator odor stressor exposure models, specifically using 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) on better understanding PTSD pathophysiology and neurobiological mechanisms associated with stress reactivity and subsequent increases in alcohol drinking. Furthermore, there has been increasing interest regarding the role of the endocannabinoid system in modulating behavioral responses to stress with an emphasis on stress coping and individual differences in stress-susceptibility. Therefore, the current review focuses on the topic of endocannabinoid modulation of stress reactive behaviors during and after exposure to a predator odor stressor, with implications on modulating distinctly different behavioral coping strategies.

Pharmacological diacylglycerol lipase inhibition impairs contextual fear extinction in mice

Acquisition and extinction of associative fear memories are critical for guiding adaptive behavioral responses to environmental threats, and dysregulation of these processes is thought to represent important neurobehavioral substrates of trauma and stress-related disorders including posttraumatic stress disorder (PTSD). Endogenous cannabinoid (eCB) signaling has been heavily implicated in the extinction of aversive fear memories and we have recently shown that pharmacological inhibition of 2-arachidonoylglycerol (2-AG) synthesis, a major eCB regulating synaptic suppression, impairs fear extinction in an auditory cue conditioning paradigm. Despite these data, the role of 2-AG signaling in contextual fear conditioning is not well understood. Here, we show that systemic pharmacological blockade of diacylglycerol lipase, the rate-limiting enzyme catalyzing in the synthesis of 2-AG, enhances contextual fear learning and impairs within-session extinction. In sham-conditioned mice, 2-AG synthesis inhibition causes a small increase in unconditioned freezing behavior. No effects of 2-AG synthesis inhibition were noted in the Elevated Plus Maze in mice tested after fear extinction. These data provide support for 2-AG signaling in the suppression of contextual fear learning and the expression of within-session extinction of contextual fear memories.

The interaction of Synapsin 2a and Synaptogyrin-3 regulates fear extinction in mice

The mechanisms behind a lack of efficient fear extinction in some individuals are unclear. Here, by employing a principal components analysis-based approach, we differentiated the mice into extinction-resistant and susceptible groups. We determined that elevated synapsin 2a (Syn2a) in the infralimbic cortex (IL) to basolateral amygdala (BLA) circuit disrupted presynaptic orchestration, leading to an excitatory/inhibitory imbalance in the BLA region and causing extinction resistance. Overexpression or silencing of Syn2a levels in IL neurons replicated or alleviated behavioral, electrophysiological, and biochemical phenotypes in resistant mice. We further identified that the proline-rich domain H in the C-terminus of Syn2a was indispensable for the interaction with synaptogyrin-3 (Syngr3) and demonstrated that disrupting this interaction restored extinction impairments. Molecular docking revealed that ritonavir, an FDA-approved HIV drug, could disrupt Syn2a-Syngr3 binding and rescue fear extinction behavior in Syn2a-elevated mice. In summary, the aberrant elevation of Syn2a expression and its interaction with Syngr3 at the presynaptic site were crucial in fear extinction resistance, suggesting a potential therapeutic avenue for related disorders.

Pharmacological modulation of Kv3 voltage-gated potassium channels regulates fear discrimination and expression in a response-dependent manner

Various psychiatric diseases are characterized by aberrant cognition and emotional regulation. This includes inappropriately attributing affective salience to innocuous cues, which can be investigated using translationally relevant preclinical models of fear discrimination. Activity in the underpinning corticolimbic circuitry is governed by parvalbumin-expressing GABAergic interneurons, which also regulate fear discrimination. Kv3 voltage-gated potassium channels are highly expressed in these neurons and are important for controlling their activity, suggesting that pharmacological Kv3 modulation may regulate fear discrimination. We determined the effect of the positive Kv3 modulator AUT00206 given systemically to female rats undergoing limited or extended auditory fear discrimination training, which we have previously shown results in more discrimination or generalization, respectively, based on freezing at retrieval. We also characterized darting and other active fear-related responses. We found that limited training resulted in more discrimination based on freezing, which was unaffected by AUT00206. In contrast, extended training resulted in more generalization based on freezing and the emergence of discrimination based on darting during training and, to a lesser extent, at retrieval. Importantly, AUT00206 given before extended training had dissociable effects on fear discrimination and expression at retrieval depending on the response examined. While AUT00206 mitigated generalization without affecting expression based on freezing, it reduced expression without affecting discrimination based on darting, although darting levels were low overall. These results indicate that pharmacological Kv3 modulation regulates fear discrimination and expression in a response-dependent manner. They also raise the possibility that targeting Kv3 channels may ameliorate perturbed cognition and emotional regulation in psychiatric disease.

Sexually dimorphic effects of monoacylglycerol lipase inhibitor MJN110 on stress-related behaviour and drinking in Marchigian Sardinian alcohol-preferring rats

BACKGROUND AND PURPOSE

The endocannabinoid (eCB) system plays an important homeostatic role in the regulation of stress circuits and has emerged as a therapeutic target to treat stress disorders and alcohol use disorder (AUD). Extensive research has elucidated a role for the eCB anandamide (AEA), but less is known about 2-arachidonoylglycerol (2-AG) mediated signalling.

EXPERIMENTAL APPROACH

We pharmacologically enhanced eCB signalling by inhibiting the 2-AG metabolizing enzyme, monoacylglycerol lipase (MAGL), in male and female Marchigian Sardinian alcohol-preferring (msP) rats, a model of innate alcohol preference and stress hypersensitivity, and in control Wistar rats. We tested the acute effect of the selective MAGL inhibitor MJN110 in alleviating symptoms of alcohol drinking, anxiety, irritability and fear.

KEY RESULTS

A single systemic administration of MJN110 increased 2-AG levels in the central amygdala, prelimbic and infralimbic cortex but did not acutely alter alcohol drinking. MAGL inhibition reduced aggressive behaviours in female msPs, and increased defensive behaviours in male msPs, during the irritability test. Moreover, in the novelty-induced hypophagia test, MJN110 selectively enhanced palatable food consumption in females, mitigating stress-induced food suppression. Lastly, msP rats showed increased conditioned fear behaviour compared with Wistar rats, and MJN110 reduced context-associated conditioned fear responses, but not cue-probed fear expression, in male msPs.

CONCLUSIONS AND IMPLICATIONS

Acute inhibition of MAGL attenuated some stress-related responses in msP rats but not voluntary alcohol drinking. Our results provide new insights into the sex dimorphism documented in stress-induced responses. Sex-specific eCB-based approaches should be considered in the clinical development of therapeutics.

A Sexually Dimorphic Role for Intestinal Cannabinoid Receptor Subtype-1 in the Behavioral Expression of Anxiety

Background: Increasing evidence suggests that the endocannabinoid system (ECS) in the brain controls anxiety and may be a therapeutic target for the treatment of anxiety disorders. For example, both pharmacological and genetic disruption of cannabinoid receptor subtype-1 (CB1R) signaling in the central nervous system is associated with increased anxiety-like behaviors in rodents, while activating the system is anxiolytic. Sex is also a critical factor that controls the behavioral expression of anxiety; however, roles for the ECS in the gut in these processes and possible differences between sexes are largely unknown. Objective: In this study, we aimed to determine if CB1Rs in the intestinal epithelium exert control over anxiety-like behaviors in a sex-dependent manner. Methods: We subjected male and female mice with conditional deletion of CB1Rs in the intestinal epithelium (intCB1-/-) and controls (intCB1+/+) to the elevated plus maze (EPM), light/dark box, and open field test. Corticosterone (CORT) levels in plasma were measured at baseline and immediately after EPM exposure. Results: When compared with intCB1+/+ male mice, intCB1-/- male mice exhibited reduced levels of anxiety-like behaviors in the EPM and light/dark box. In contrast to male mice, no differences were found between female intCB1+/+ and intCB1-/- mice. Circulating CORT was higher in female versus male mice for both genotype groups at baseline and after EPM exposure; however, there was no effect of genotype on CORT levels. Conclusions: Collectively, these results indicate that genetic deletion of CB1Rs in the intestinal epithelium is associated with an anxiolytic phenotype in a sex-dependent manner.

The endocannabinoid system as a putative target for the development of novel drugs for the treatment of psychiatric illnesses

Cannabis is well established to impact affective states, emotion and perceptual processing, primarily through its interactions with the endocannabinoid system. While cannabis use is quite prevalent in many individuals afflicted with psychiatric illnesses, there is considerable controversy as to whether cannabis may worsen these conditions or provide some form of therapeutic benefit. The development of pharmacological agents which interact with components of the endocannabinoid system in more localized and discrete ways then via phytocannabinoids found in cannabis, has allowed the investigation if direct targeting of the endocannabinoid system itself may represent a novel approach to treat psychiatric illness without the potential untoward side effects associated with cannabis. Herein we review the current body of literature regarding the various pharmacological tools that have been developed to target the endocannabinoid system, their impact in preclinical models of psychiatric illness and the recent data emerging of their utilization in clinical trials for psychiatric illnesses, with a specific focus on substance use disorders, trauma-related disorders, and autism. We highlight several candidate drugs which target endocannabinoid function, particularly inhibitors of endocannabinoid metabolism or modulators of cannabinoid receptor signaling, which have emerged as potential candidates for the treatment of psychiatric conditions, particularly substance use disorder, anxiety and trauma-related disorders and autism spectrum disorders. Although there needs to be ongoing clinical work to establish the potential utility of endocannabinoid-based drugs for the treatment of psychiatric illnesses, the current data available is quite promising and shows indications of several potential candidate diseases which may benefit from this approach.

CB1R blockade unmasks TRPV1-mediated contextual fear generalization in female, but not male rats

Increasing evidence suggests that the neurobiological processes that govern learning and memory can be different in males and females, but many of the specific mechanisms underlying these sex differences have not been fully defined. Here we investigated potential sex differences in endocannabinoid (eCB) modulation of Pavlovian fear conditioning and extinction, examining multiple defensive behaviors, including shock responsivity, conditioned freezing, and conditioned darting. We found that while systemic administration of drugs acting on eCB receptors did not influence the occurrence of darting, females that were classified as Darters responded differently to the drug administration than those classified as Non-darters. Most notably, CB1R antagonist AM251 produced an increase in cue-elicited freezing and context generalization selectively in female Non-darters that persisted across extinction and extinction retrieval tests but was prevented by co-administration of TRPV1R antagonist Capsazepine. To identify a potential synaptic mechanism for these sex differences, we next employed biochemical and neuroanatomical tracing techniques to quantify anandamide (AEA), TRPV1R, and perisomatic CB1R expression, focusing on the ventral hippocampus (vHip) given its known role in mediating contextual fear generalization. These assays identified sex-specific effects of both fear conditioning-elicited AEA release and vHip-BLA circuit structure. Together, our data support a model in which sexual dimorphism in vHip-BLA circuitry promotes a female-specific dependence on CB1Rs for context processing that is sensitive to TRPV1-mediated disruption when CB1Rs are blocked.

CB1R blockade unmasks TRPV1-mediated contextual fear generalization in female, but not male rats

Increasing evidence suggests that the neurobiological processes that govern learning and memory can be different in males and females, and here we asked specifically whether the endocannabinoid (eCB) system could modulate Pavlovian fear conditioning in a sex-dependent manner. Systemic (i.p.) injection of CB1R antagonist AM251 in adult male and female Sprague Dawley rats prior to auditory cued fear conditioning produced a female-specific increase in freezing that persisted across extinction and extinction retrieval tests but was prevented by co-administration of TRPV1R antagonist Capsazepine. Notably, AM251 also produced robust freezing in a novel context prior to auditory cue presentation the day following drug administration, but not the day of, suggesting that CB1R blockade elicited contextual fear generalization in females. To identify a potential synaptic mechanism for these sex differences, we next used liquid chromatography/tandem mass spectrometry, Western Blot, and confocal-assisted immunofluorescence techniques to quantify anandamide (AEA), TRPV1R, and perisomatic CB1R expression, respectively, focusing on the ventral hippocampus (vHip). Fear conditioning elicited increased vHip AEA levels in females only, and in both sexes, CB1R expression around vHip efferents targeting the basolateral amygdala (BLA) was twice that at neighboring vHip neurons. Finally, quantification of the vHip-BLA projections themselves revealed that females have over twice the number of neurons in this pathway that males do. Together, our data support a model in which sexual dimorphism in vHip-BLA circuitry promotes a female-specific dependence on CB1Rs for context processing that is sensitive to TRPV1-mediated disruption when CB1Rs are blocked.

The effectiveness of extinction training in male rats: Temporal considerations and brain mechanisms

The extinction of conditioned fear is frequently used in laboratories as a model for human exposure therapy and is crucial for studies of posttraumatic stress disorder (PTSD). However, the efficacy of specific protocols can vary greatly, and the underlying brain mechanisms are not sufficiently clarified. To address this issue, variable starting time (one or twenty-eight days after fear conditioning) and extinction protocols were used, and the efficacy and durability of fear extinction were also studied. Changes in the behavior, stress hormone levels and neuronal activation patterns of stressed rats were analyzed. Conditioned fear was rapidly and efficiently extinguished by all the protocols investigated. However, when these extinction protocols were initiated one day after fear training, conditioned fear relapsed spontaneously four weeks later. In contrast, when extinction trials were started 28 days after conditioning, no relapse occurred. Hormone measurements taken by the end of extinction trials indicated that adrenocorticotropin, but not corticosterone responses reflected behavioral extinction without any sign of relapse. The last extinction training increased the activation of the medial prefrontal cortex and decreased the activation of the central and medial amygdala when extinction began one day after fear conditioning. By contrast, the activation of the basolateral amygdala and the entire hippocampus decreased by the last training session when extinction started 28 days after fear conditioning. Our findings show that extinction training can extinguish remote fear memories more effectively than recent ones, and that the brain mechanisms underlying remote and recent fear memory extinction differ. Laboratory models should also focus on a later time point to increase their translational value.

Exposure to anandamide on young rats causes deficits in learning, temporal perception and induces changes in NMDA receptor expression

Human use of marijuana at an early age has been reported to lead to cognitive impairment. However, researchers have not yet clearly determined whether this impairment is due to marijuana-induced alterations in the developing nervous system and whether this deficit persists into adulthood after marijuana use has ceased. We administered anandamide to developing rats to assess the effect of cannabinoids on development. We subsequently evaluated learning and performance on a temporal bisection task in adulthood and assessed the expression of genes encoding principal subunits of NMDA receptors (Grin1, Grin2A, and Grin2B) in the hippocampus and prefrontal cortex. Rats in two age groups, namely, 21-day-old and 150-day-old rats, received intraperitoneal injections of anandamide or the vehicle for 14 days. Both groups performed a temporal bisection test, which included listening to tones of different durations and classifying them as short or long. The expression of the Grin1, Grin2A and Grin2B mRNAs was evaluated using quantitative PCR in both age groups after extracting mRNA from the hippocampus and prefrontal cortex. We observed a learning impairment in the temporal bisection task (p < 0.05) and changes in the response latency (p < 0.05) in rats that received anandamide. Furthermore, these rats exhibited decreased expression of Grin2b (p = 0.001) compared to those that received the vehicle. In human subjects, the use of cannabinoids during development induces a long-term deficit, but this deficit is not observed in subjects who use cannabinoids in adulthood. Rats treated with anandamide earlier in development took longer to learn the task, suggesting that anandamide exerts a harmful effect on cognition in developing rats. Administration of anandamide during early stages of development induced deficits in learning and other cognitive processes that depend on an adequate estimation of time. The cognitive demands of the environment must be considered when evaluating the cognitive effects of cannabinoids on developing or mature brains. High cognitive demands might induce differential expression of NMDA receptors that improves cognitive capacity, overcoming altered glutamatergic function.

TRPV1 modulation of contextual fear memory depends on stimulus intensity and endocannabinoid signalling in the dorsal hippocampus

The transient receptor potential vanilloid type-1 (TRPV1) channels have been implicated in the modulation of aversive responses. The endocannabinoid anandamide acts as an endogenous TRPV1 agonist, exerting opposite functions at TRPV1 and type-1 cannabinoid receptors (CB₁R). Here we tested the hypothesis that hippocampal TRPV1 modulates contextual fear memory retrieval and investigated the influence of the aversive stimulus intensity as well as the role of endocannabinoid signaling. Male C57BL/6J mice were tested for contextual fear memory after low-, moderate-, or high-intensity shock protocols. The selective TRPV1 blockers SB366791 (1-10 nmol) and 6-I-NC (2 nmol) were infused via intra-dorsal hippocampus before the retrieval test session. The local levels of endocannabinoids and Arc and Zif268 mRNAs, involved in synaptic plasticity and memory, were quantified. First, both TRPV1 blockers reduced memory retrieval in animals exposed to moderate or high (but not low) intensity training protocols. In the second series of results, the magnitude of the freezing responses positively correlated with the hippocampal anandamide levels; TRPV1 and CB₁R were found co-localized in this brain region; and the CB₁R antagonist, AM251, prevented the effects of SB366791. Thus, endocannabinoid signaling possibly mediates the effects of TRPV1 blockers. Finally, inhibition of memory retrieval by TRPV1 blockers increased Arc and Zif268 mRNAs and impaired fear memory reinstatement. In conclusion, the modulation of fear memories by dorsal hippocampal TRPV1 channels may depend on the aversive stimulus intensity and occur via anandamide/CB₁ signaling. Moreover, TRPV1 blockers promote Arc and Zif268 transcription, with subsequent attenuation of aversive memory reinstatement.

ChAT::Cre transgenic rats show sex-dependent altered fear behaviors, ultrasonic vocalizations and cholinergic marker expression

The cholinergic system is a critical regulator of Pavlovian fear learning and extinction. As such, we have begun investigating the cholinergic system's involvement in individual differences in cued fear extinction using a transgenic ChAT::Cre rat model. The current study extends behavioral phenotyping of a transgenic ChAT::Cre rat line by examining both freezing behavior and ultrasonic vocalizations (USVs) during a Pavlovian cued fear learning and extinction paradigm. Freezing, 22 kHz USVs, and 50 kHz USVs were compared between male and female transgenic ChAT::Cre+ rats and their wildtype (Cre-) littermates during fear learning, contextual and cue-conditioned fear recall, cued fear extinction, and generalization to a novel tone. During contextual and cued fear recall ChAT::Cre+ rats froze slightly more than their Cre- littermates, and displayed significant sex differences in contextual and cue-conditioned freezing, 22 kHz USVs, and 50 kHz USVs. Females showed more freezing than males in fear recall trials, but fewer 22 kHz distress calls during fear learning and recall. Females also produced more 50 kHz USVs during exposure to the testing chambers prior to tone (or shock) presentation compared with males, but this effect was blunted in ChAT::Cre+ females. Corroborating previous studies, ChAT::Cre+ transgenic rats overexpressed vesicular acetylcholine transporter immunolabeling in basal forebrain, striatum, basolateral amygdala, and hippocampus, but had similar levels of acetylcholinesterase and numbers of ChAT+ neurons as Cre- rats. This study suggests that variance in behavior between ChAT::Cre+ and wildtype rats is sex dependent and advances theories that distinct neural circuits and processes regulate sexually divergent fear responses.

Endocannabinoid metabolism inhibition has no effect on spontaneous fear recovery or extinction resistance in Lister hooded rats

Endocannabinoid transmission is emerging as a target for treating anxiety-related disorders, given its regulation of fear extinction. Boosting anandamide levels via inhibition of its metabolism by fatty acid amide hydrolase (FAAH) can enhance extinction, whereas inhibiting monoacylglycerol lipase (MAGL) to elevate 2-arachidonoylglycerol levels can impair extinction. However, whether endocannabinoids regulate fear relapse over time or extinction resistance remains unclear. In two experiments using auditory fear conditioned rats, we examined the effects of the FAAH inhibitor URB597 and the MAGL inhibitor JZL184 administered systemically on 1) spontaneous fear recovery after delayed extinction, and 2) extinction resistance resulting from immediate extinction [the immediate extinction deficit (IED)]. In Experiment 1, URB597 or JZL184 was given immediately after delayed extinction occurring 24 h after conditioning. Extinction recall and spontaneous fear recovery were tested drug-free 1 and 21 days later, respectively. We found no effects of either drug on extinction recall or spontaneous fear recovery. In Experiment 2, URB597 or JZL184 was given before immediate extinction occurring 30 min after conditioning and extinction recall was tested drug-free the next day. We also examined the effects of propranolol, a beta-adrenoceptor antagonist that can rescue the IED, as a positive control. JZL184 enhanced fear expression and impaired extinction learning but we found no lasting effects of URB597 or JZL184 on cued extinction recall. Propranolol reduced fear expression but, unexpectedly, had no enduring effect on extinction recall. The results are discussed in relation to various methodological differences between previous studies examining endocannabinoid and adrenergic regulation of fear extinction.

ThermoTRP channels in pain sexual dimorphism: new insights for drug intervention

Chronic pain is a major burden for the society and remains more prevalent and severe in females. The presence of chronic pain is linked to persistent alterations in the peripheral and the central nervous system. One of the main types of peripheral pain transducers are the transient receptor potential channels (TRP), also known as thermoTRP channels, which intervene in the perception of hot and cold external stimuli. These channels, and especially TRPV1, TRPA1 and TRPM8, have been subjected to profound investigation because of their role as thermosensors and also because of their implication in acute and chronic pain. Surprisingly, their sensitivity to endogenous signaling has been far less studied. Cumulative evidence suggests that the function of these channels may be differently modulated in males and females, in part through sexual hormones, and this could constitute a significant contributor to the sex differences in chronic pain. Here, we review the exciting advances in thermoTRP pharmacology for males and females in two paradigmatic types of chronic pain with a strong peripheral component: chronic migraine and chemotherapy-induced peripheral neuropathy (CIPN). The possibilities of peripheral druggability offered by these channels and the differential exploitation for men and women represent a development opportunity that will lead to a significant increment of the armamentarium of analgesic medicines for personalized chronic pain treatment.

Involvement of D2-like dopaminergic receptors in contextual fear conditioning in female rats: influence of estrous cycle

Introduction: Dopamine has been increasingly recognized as a key neurotransmitter regulating fear/anxiety states. Nevertheless, the influence of sex and estrous cycle differences on the role of dopamine in fear responses needs further investigation. We aimed to evaluate the effects of sulpiride (a dopaminergic D2-like receptor antagonist) on contextual fear conditioning in females while exploring the influence of the estrous cycle. Methods: First, using a contextual fear conditioning paradigm, we assessed potential differences in acquisition, expression, and extinction of the conditioned freezing response in male and female (split in proestrus/estrus and metestrus/diestrus) Wistar rats. In a second cohort, we evaluated the effects of sulpiride (20 and 40 mg/kg) on contextual conditioned fear in females during proestrus/estrus and metestrus/diestrus. Potential nonspecific effects were assessed in motor activity assays (catalepsy and open-field tests). Results: No sex differences nor estrous cycle effects on freezing behavior were observed during the fear conditioning phases. Sulpiride reduced freezing expression in female rats. Moreover, females during the proestrus/estrus phases of the estrous cycle were more sensitive to the effects of sulpiride than females in metestrus/diestrus. Sulpiride did not cause motor impairments. Discussion: Although no sex or estrous cycle differences were observed in basal conditioned fear expression and extinction, the estrous cycle seems to influence the effects of D2-like antagonists on contextual fear conditioning.

Darting across space and time: parametric modulators of sex-biased conditioned fear responses

Pavlovian fear conditioning is a widely used behavioral paradigm for studying associative learning in rodents. Despite early recognition that subjects may engage in a variety of both conditioned and unconditioned responses, the last several decades have seen the field narrow its focus to measure freezing as the sole indicator of conditioned fear. We previously reported that female rats were more likely than males to engage in darting, an escape-like conditioned response that is associated with heightened shock reactivity. To determine how experimental parameters contribute to the frequency of darting in both males and females, we manipulated factors such as chamber size, shock intensity, and number of trials. To better capture fear-related behavioral repertoires in our animals, we developed ScaredyRat, an open-source custom Python tool that analyzes Noldus Ethovision-generated raw data files to identify darters and quantify both conditioned and unconditioned responses. We found that, like freezing, conditioned darting occurrences scale with experimental alterations. While most darting occurs in females, we found that with an extended training protocol, darting can emerge in males as well. Collectively, our data suggest that darting reflects a behavioral switch in conditioned responding that is a product of an individual animal's sex, shock reactivity, and experimental parameters, underscoring the need for careful consideration of sex as a biological variable in classic learning paradigms.

Leitl K, Megan Gray J, N Petrie G, J.Tellez-Monnery K, N.Hill M. Sex and stressor modality influence acute stress-induced dynamic changes in corticolimbic endocannabinoid levels in adult Sprague Dawley rats

Research over the past few decades has established a role for the endocannabinoid system in contributing to the neural and endocrine responses to stress exposure. The two endocannabinoid ligands, anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), both play roles in regulating the stress response and both exhibit dynamic changes in response to stress exposure. Most of this previous research, however, was conducted in male rodents. Given that, especially in rodents, the stress response is influenced by sex, an understanding of how these dynamic responses of endocannabinoids in response to stress is influenced by sex could provide insight into sex differences of the acute stress response. We exposed adult, Sprague Dawley rats to different commonly utilized acute stress modalities, specifically restraint, swim and foot shock stress. Thirty minutes following stress onset, we excised the amygdala, hippocampus and medial prefrontal cortex, corticolimbic brain regions involved in the stress response, to measure endocannabinoid levels. When AEA levels were altered in response to restraint and swim stress, they were reduced, whereas exposure to foot shock stress led to an increase in the amygdala. 2-AG levels, when they were altered by stress exposure were only increased, specifically in males in the amygdala following swim stress, and in the hippocampus and medial prefrontal cortex overall following foot shock stress. This increase in 2-AG levels following stress only in males was the only sex difference found in stress-induced changes in endocannabinoid levels. There were no consistent sex differences observed. Collectively, these data contribute to our further understanding of the interactions between stress and endocannabinoid function.

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Stressor modality influences acute stress-induced changes in endocannabinoid levels in corticolimbic brain regions.

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There are minimal sex differences with regards to basal levels or stress-induced changes in endocannabinoid levels.

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AEA decreased from acute restraint and swim stresses but increased from acute foot shock stress.

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2-AG levels only showed increases when they were impacted by the different stress modalities.

Circadian regulation of memory under stress: Endocannabinoids matter

Organisms ranging from plants to higher mammals have developed 24-hour oscillation rhythms to optimize physiology to environmental changes and regulate a plethora of neuroendocrine and behavioral processes, including neurotransmitter and hormone regulation, stress response and learning and memory function. Compelling evidence indicates that a wide array of memory processes is strongly influenced by stress- and emotional arousal-activated neurobiological systems, including the endocannabinoid system which has been extensively shown to play an integral role in mediating stress effects on memory. Here, we review findings showing how circadian rhythms and time-of-day influence stress systems and memory performance. We report evidence of circadian regulation of memory under stress, focusing on the role of the endocannabinoid system and highlighting its circadian rhythmicity. Our discussion illustrates how the endocannabinoid system mediates stress effects on memory in a circadian-dependent fashion. We suggest that endocannabinoids might regulate molecular mechanisms that control memory function under circadian and stress influence, with potential important clinical implications for both neurodevelopmental disorders and psychiatric conditions involving memory impairments.

Post-trauma behavioral phenotype predicts the degree of vulnerability to fear relapse after extinction in male rats

Current treatments for trauma-related disorders remain ineffective for many patients.^1,2 Fear extinction deficiency is a prominent feature of these diseases,³ and many behavioral treatments rely on extinction training.^4,5 However, in many patients, therapy is followed by a relapse of symptoms, and the underpinnings of such interindividual variations in vulnerability to relapse remain unknown.^6-8 Here, we modeled interindividual differences in post-therapy fear relapse with an ethologically relevant trauma recovery paradigm. After fear conditioning, male rats underwent fear extinction while foraging in a large enriched arena, permitting the expression of a wide spectrum of behaviors. An automated multidimensional behavioral assessment revealed that post-conditioning fear response profiles clustered into two groups: some animals expressed fear by freezing more, whereas others darted more, as if fleeing from danger. Remarkably, the tendency of an animal to dart or to freeze after CS presentation during the first extinction session was, respectively, associated with stronger or weaker fear renewal. Moreover, genome-wide transcriptional profiling revealed that these groups differentially regulated specific sets of genes, some of which were previously implicated in anxiety and trauma-related disorders. Our results suggest that post-trauma behavioral phenotypes and the associated gene expression landscapes can serve as markers of fear relapse susceptibility and thus may be instrumental for future development of more effective treatments for psychiatric patients.

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

Anxiety and stress disorders are 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 are unknown. Here we investigated the role of the endogenous cannabinoid system in the impaired fear extinction induced by orexin-A (OXA) in male mice. 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. These results show that overactivation of the orexin system leads to impaired fear extinction through 2-AG and amygdalar CB2R. This novel mechanism could be of relevance for the development of novel potential approaches to treat diseases associated with inappropriate retention of fear, such as post-traumatic stress disorder, panic anxiety and phobias.

Cannabinoid and vanilloid pathways mediate opposing forms of synaptic plasticity in corticotropin-releasing hormone neurons

Activity-dependent release of retrograde signaling molecules form micro-feedback loops to regulate synaptic function in neural circuits. Single neurons can release multiple forms of these signaling molecules, including endocannabinoids and endovanilloids, which act via cannabinoid (CB) receptors and transient receptor potential vanilloid 1 (TRPV1) receptors. In hypothalamic corticotrophin-releasing hormone (CRH) neurons, endocannabinoids acting via CB1 receptors have been shown to play an important role in regulating excitability and hence stress hormone secretion. However, the importance of endovanilloid signaling in CRH neurons is currently unclear. Here, we show that, in response to postsynaptic depolarization, CRH neurons release endocannabinoid/endovanilloid molecules that can activate CB1 and TRPV1 receptors. Activation of CB1 receptors suppresses glutamate neurotransmission whereas activation of TRPV1 enhances spontaneous glutamate transmission. However, the excitatory effects of TRPV1 are normally masked by the inhibitory effects of CB1. When the degradation of the endocannabinoid 2-arachidonoylglycerol (2-AG) was inhibited, this revealed tonic activation of CB1 receptors, suggesting tonic endocannabinoid release. However, we found no evidence for tonic activation of TRPV1 receptors under similar conditions. These findings show that activation of CRH neurons can drive the release of signaling molecules that activate parallel endocannabinoid and endovanilloid receptor pathways to mediate opposing forms of synaptic plasticity.

The role of the cannabinoid system in fear memory and extinction in male and female mice

The prevalence of post-traumatic stress disorder (PTSD) is higher in women than in men. Among both humans and mice, females exhibit higher resistance to fear extinction than males, suggesting that differences between sexes in fear-extinction processes are involved in the pathophysiology of such fear-related diseases. Sex differences in molecular mechanisms underlying fear memory and extinction are unclear. The cannabinoid (CB) system is well known to be involved in fear memory and extinction, but this involvement is based mainly on experiments using male rodents. It is not known whether there are sex differences in the role of the CB system in fear memory and extinction. To explore this possibility, we investigated the effects of pharmacological manipulations of the CB system on the retrieval and extinction of contextual fear memory in male and female mice. WIN55,212-2, a CB receptor (CBR) agonist, augmented the retrieval of fear memory in both sexes, but SR141716 (a CB1R antagonist) did not affect it in either sex. An enhancement of 2-arachidonylglycerol (2-AG, one of the two major endocannabinoids) via JZL184 (an inhibitor of the 2-AG hydrolase monoacylglycerol lipase [MAGL]), augmented the retrieval of fear memory through the activation of CB1R but not CB2R in female mice. In contrast, the enhancement of N-arachidonylethanolamine (AEA, the other major endocannabinoid) via URB597, an inhibitor of an AEA hydrolase (fatty acid amide hydrolase-1) did not show any effects on the retrieval of fear memory in either sex. WIN55,212-2, SR141716, and JZL184 inhibited fear extinction irrespective of sex. URB enhanced fear extinction in females that were in diestrus phase at the first extinction session, but not in males. These results suggest that although the role of CB1R in the retrieval and extinction of contextual fear memory is common among males and females, the effects of an increase in endocannabinoid levels on the retrieval or extinction of contextual fear memory differ between the sexes.

Circulating endocannabinoids and genetic polymorphisms as predictors of posttraumatic stress disorder symptom severity: heterogeneity in a community-based cohort

The endocannabinoid signaling system (ECSS) regulates fear and anxiety. While ECSS hypoactivity can contribute to symptoms of established post-traumatic stress disorder (PTSD), the role of the ECSS in PTSD development following trauma is unknown. A prospective, longitudinal cohort study of 170 individuals (47% non-Hispanic Caucasian and 70% male) treated at a level 1 trauma center for traumatic injury was carried out. PTSD symptom assessments and blood were obtained during hospitalization and at follow-up (6-8 months post injury). Serum concentrations of the endocannabinoids N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG) were determined at both time points and selected genetic polymorphisms in endocannabinoid genes, including rs324420 in fatty acid amide hydrolase, were assessed. For the entire sample, serum concentrations of AEA at hospitalization were significantly higher in those diagnosed with PTSD at follow-up (p = 0.030). Serum concentrations of 2-AG were significantly, positively correlated with PTSD symptom severity at follow-up only in minorities (p = 0.014). Minority participants (mostly Black/African American) also demonstrated significant, negative correlations between serum AEA concentrations and PTSD symptom severity both measured at hospitalization (p = 0.015). The A/A genotype at rs324420 was associated with significantly higher PTSD symptom severity (p = 0.025) and occurred exclusively in the Black participants. Collectively, these results are contrary to our hypothesis and find positive associations between circulating endocannabinoids and risk for PTSD. Minority status is an important modulator of the association between endocannabinoids and risk for PTSD, suggesting that the ECSS contributes to risk most significantly in these individuals and the contextual factors related to these findings should be further explored.

Sex-dependent Effects of the Drugs of Abuse Amphetamine and the Smart Drug 3,4-Methylenedioxypyrovalerone on Fear Memory Generalization in Rats

In recent years there has been an increase in the development of new synthetic drugs, among which the "bath salt" 3,4-methylenedioxypyrovalerone (MDPV), a psychostimulant with a mechanism of action similar to those of cocaine and amphetamine, stands out. Drugs of abuse have been consistently shown to affect memory function in male rodents. We have recently shown that amphetamine and MDPV induce generalization of fear memory in an inhibitory avoidance discrimination task in male rats. Although abuse of illicit drugs is more prevalent in men than in women, several studies have demonstrated that females are more vulnerable to the effects of drugs of abuse than males and the effects caused by substance dependence on memory in females are still under-investigated. Thus, we examined the effects of subchronic amphetamine or MDPV administrations on memory in a contextual fear conditioning/generalization paradigm in adult male and female rats. Animals were given daily subchronic injections of the drugs, starting 6 days prior the beginning of the behavioral procedures until the end of the paradigm. On day 1 of the experimental protocol, all rats were exposed to a safe context and, the day after, to a slightly different chamber where they received an unsignaled footshock. Twenty-four and forty-eight hours later, freezing behavior and emission of 22 kHz-ultrasonic vocalizations (USVs) were measured in the two different contexts to assess fear memory retention and generalization. Our results indicate that MDPV treatment altered freezing in both sexes, USVs were affected by amphetamine in males while by MDPV in females.

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.

A salience hypothesis of stress in PTSD

Attention to key features of contexts and things is a necessary tool for all organisms. Detecting these salient features of cues, or simply, salience, can also be affected by exposure to traumatic stress, as has been widely reported in individuals suffering from post-traumatic stress disorder (PTSD). Interestingly, similar observations have been robustly replicated across many animal models of stress as well. By using evidence from such rodent stress paradigms, in the present review, we explore PTSD through the lens of salience processing. In this context, we propose that interaction between the neurotrophin brain-derived neurotrophic factor (BDNF) and glucocorticoids determines the long lasting cellular and behavioural consequences of stress salience. We also describe the dual effect of glucocorticoid therapy in the amelioration of PTSD symptoms. Finally, by integrating in vivo observations at multiple scales of plasticity, we propose a unifying hypothesis that pivots on a crucial role of glucocorticoid signalling in dynamically orchestrating stress salience.

Sexually Dimorphic Expression of Fear-conditioned Analgesia in Rats and Associated Alterations in the Endocannabinoid System in the Periaqueductal Grey

The endocannabinoid system within the periaqueductal grey (PAG) has been implicated in fear-conditioned analgesia (FCA), the profound suppression of pain upon re-exposure to a context previously paired with an aversive stimulus. Since the endocannabinoid and nociceptive systems exhibit sexual dimorphism, the aim of the present study was to assess possible sex differences in the expression of FCA, fear in the presence of nociceptive tone, and associated sex-dependent alterations in the endocannabinoid system within the PAG. Male and female Sprague-Dawley rats received footshock (10 × 1s; 0.4 mA; every 60 s) or no-footshock in a conditioning arena and 23.5 h later received intraplantar injection of formalin (2.5%) under brief isoflourane anaesthetic into the right hind paw. Nociceptive and fear-related behaviours were assessed 30 min later. Levels of endocannabinoids, N-acylethanolamines and neurotransmitters in the PAG were assessed by LC-MS/MS and expression of endocannabinoid system-related proteins by Western immunoblotting. Male, but not female, rats exhibited robust FCA and greater expression of fear-related behaviours than females. Fear-conditioned formalin-treated males, but not females, had higher levels of N-oleoylethanolamine (OEA) and γ-aminobutyric acid (GABA) in the PAG, compared with non-fear-conditioned controls. There was no effect of fear conditioning on the levels of FAAH or CB₁ receptor expression (CB1R) in the PAG of male or female formalin-treated rats. Non-fear-conditioned females had higher levels of CB1R and PPARγ expression than non-fear-conditioned male counterparts. In summary, our results provide evidence of sexual dimorphism in the expression of FCA and fear-related behaviours, and associated alterations in components of the endocannabinoid system and GABA within the PAG.

Sex differences in anxiety and depression: circuits and mechanisms

Epidemiological sex differences in anxiety disorders and major depression are well characterized. Yet the circuits and mechanisms that contribute to these differences are understudied, because preclinical studies have historically excluded female rodents. This oversight is beginning to be addressed, and recent studies that include male and female rodents are identifying sex differences in neurobiological processes that underlie features of these disorders, including conflict anxiety, fear processing, arousal, social avoidance, learned helplessness and anhedonia. These findings allow us to conceptualize various types of sex differences in the brain, which in turn have broader implications for considering sex as a biological variable. Importantly, comparing the sexes could aid in the discovery of novel therapeutics.

Amygdalar endocannabinoids are affected by predator odor stress in a sex-specific manner and modulate acoustic startle reactivity in female rats

Understanding sex differences in behavioral and molecular effects of stress has important implications for understanding the vulnerability to chronic psychiatric disorders associated with stress response circuitry. The amygdala is critical for emotional learning and generating behavioral responses to stressful stimuli, and preclinical studies indicate that amygdalar endocannabinoid (eCB) signaling regulates emotional states. This study measured eCB contents in the basolateral (BLA) and central (CeA) amygdala of male and female rats exposed to predator odor stress (bobcat urine) and tested for contextual avoidance 24 h later. Stressed females had lower levels of 2-arachidonoyl glycerol (2-AG) in the BLA and higher levels of anandamide (AEA) in the CeA, while exposure to bobcat urine did not affect amygdalar eCB contents in males. We previously reported that female rats exposed to bobcat urine exhibit blunted acoustic startle reactivity (ASR) 48 h after predator odor stress. Therefore, we tested the hypothesis that intra-BLA injection of a diacylglycerol lipase (DAGL) inhibitor (which would be expected to reduce 2-AG levels in BLA) and intra-CeA injection of a fatty acid amide hydrolase (FAAH) inhibitor (which would be expected to increase AEA levels in CeA) would mimic previously observed predator odor stress-induced reductions in ASR. Contrary to our hypothesis, microinjections of either the DAGL inhibitor DO34 into the BLA or the FAAH inhibitor URB597 into the CeA significantly increased ASR in females compared to vehicle-treated rats. These findings describe sex-specific effects of predator odor stress on amygdalar eCBs, and new roles for amygdalar eCBs in regulating behavior in females.

Effect of Estrous Cycle on Behavior of Females in Rodent Tests of Anxiety

Anxiety disorders are more prevalent in women than in men. In women the menstrual cycle introduces another variable; indeed, some conditions e.g., premenstrual syndrome, are menstrual cycle specific. Animal models of fear and anxiety, which form the basis for research into drug treatments, have been developed almost exclusively, using males. There remains a paucity of work using females and the available literature presents a confusing picture. One confound is the estrous cycle in females, which some authors consider, but many do not. Importantly, there are no accepted standardized criteria for defining cycle phase, which is important given the rapidly changing hormonal profile during the 4-day cycle of rodents. Moreover, since many behavioral tests that involve a learning component or that consider extinction of a previously acquired association require several days to complete; the outcome may depend on the phase of the cycle on the days of training as well as on test days. In this article we consider responsiveness of females compared to males in a number of commonly used behavioral tests of anxiety and fear that were developed in male rodents. We conclude that females perform in a qualitatively similar manner to males in most tests although there may be sex and strain differences in sensitivity. Tests based on unconditioned threatening stimuli are significantly influenced by estrous cycle phase with animals displaying increased responsiveness in the late diestrus phase of the cycle (similar to the premenstrual phase in women). Tests that utilize conditioned fear paradigms, which involve a learning component appear to be less impacted by the estrous cycle although sex and cycle-related differences in responding can still be detected. Ethologically-relevant tests appear to have more translational value in females. However, even when sex differences in behavior are not detected, the same outward behavioral response may be mediated by different brain mechanisms. In order to progress basic research in the field of female psychiatry and psychopharmacology, there is a pressing need to validate and standardize experimental protocols for using female animal models of anxiety-related states.

Increasing Endocannabinoid Tone Alters Anxiety-Like and Stress Coping Behaviour in Female Rats Prenatally Exposed to Valproic Acid

Given the sex differences evident in the prevalence of autism, there is an increased awareness of the importance of including females in autism research to determine sexual dimorphism and sex-specific treatments. Cannabinoids and endocannabinoid modulators have been proposed as potential novel treatments for autism-related symptoms; however, few studies to date have examined if these pharmacological agents elicit sex-specific effects. The aim of the present study was to use the valproic acid (VPA) model of autism to compare the behavioural responses of male and female rats and examine the effects of increasing endocannabinoid tone on the behavioural responses of VPA-exposed female rats. These data revealed that VPA-exposed male, but not female, rats exhibit reduced social responding in the three-chamber and olfactory habituation/dishabituation (OHD) test during adolescence. In comparison, VPA-exposed female, but not male, adolescent rats exhibited anxiety-like behaviour in the elevated plus maze (EPM) and open field test (OFT). In VPA-exposed female rats, increasing 2-AG levels augmented anxiety-like behaviour in the EPM and OFT, while increasing AEA levels reduced stress coping behaviour in the swim stress test. These data highlight sexual dimorphic behaviours in the VPA model and indicate that enhancing endocannabinoid levels may exacerbate negative affective behaviour in VPA-exposed females. Thus, considerations should be paid to the possible sex-specific effects of cannabinoids for the treatment of symptoms associated with autism.

The role of endocannabinoids in consolidation, retrieval, reconsolidation, and extinction of fear memory

Endocannabinoids are involved in various physiological functions, including synaptic plasticity and memory, and some psychiatric disorders, such as posttraumatic stress disorder (PTSD), through the activation of cannabinoid (CB) receptors. Patients with PTSD often show excessive fear memory and impairment of fear extinction (FE). It has been reported that the stability of acquired fear memory is altered through multiple memory stages, such as consolidation and reconsolidation. FE also affects the stability of fear memory. Each stage of fear memory formation and FE are regulated by different molecular mechanisms, including the CB system. However, to the best of our knowledge, no review summarizes the role of the CB system during each stage of fear memory formation and FE. In this review, we summarize the roles of endocannabinoids in fear memory formation and FE. Moreover, based on the summary, we propose a new hypothesis for the role of endocannabinoids in fear regulation, and discuss treatment for PTSD using CB system-related drugs.

Cocaine-induced Fos expression in the rat brain: Modulation by prior Δ9-tetrahydrocannabinol exposure during adolescence and sex-specific effects

It has been suggested that cannabis consumption during adolescence may be an initial step to cocaine use in adulthood. Indeed, previous preclinical data show that adolescent exposure to cannabinoids (both natural and synthetic) potentiates cocaine self-administration in rats. Here we aimed at gaining a deeper understanding of the cellular activation patterns induced by cocaine as revealed by Fos imaging and how these patterns may change due to adolescent exposure to THC. Male and female Wistar rats were administered every other day THC (3 mg/kg i.p.) or vehicle from postnatal day 28-44. At adulthood (PND90) they were given an injection of cocaine (20 mg/kg i.p.) or saline and sacrificed 90 min later. Cocaine-induced Fos activation was measured by immunohistochemistry as an index of cellular activation. We found that cocaine-induced activation in the motor cortex was stronger in THC-exposed rats. Moreover, there was significant sex-dependent interaction between cocaine and adolescent THC exposure in the dorsal hypothalamus, suggesting that cocaine induced a more robust cellular activation in THC-exposed females but not in THC-treated males. Other THC- and cocaine-induced effects were also evident. These results add to the previous literature suggesting that the behavioral, cellular, molecular, and brain-activating actions of cocaine are modulated by early experience with cannabinoids and provide additional knowledge that may explain the enhanced actions of cocaine in rats exposed to cannabinoids during their adolescence.

In it together? The case for endocannabinoid-noradrenergic interactions in fear extinction

Anxiety and trauma-related disorders, such as post-traumatic stress disorder (PTSD), are debilitating mental illnesses with great personal and socioeconomic costs. Examining memory formation and relevant behavioural responding associated with aversive stimuli may improve our understanding of the neurobiology underlying fear memory processing and PTSD treatment. The neurocircuitry underpinning learned fear and its inhibition through extinction is complex, involving synergistic interactions between different neurotransmitter systems in inter-connected brain areas. Endocannabinoid and noradrenergic transmission have both been implicated separately in fear memory processing and PTSD, but potential interactions between these systems in relation to fear extinction have received little attention to date. Their receptors are expressed together in brain areas crucial for fear extinction, which is enhanced by both cannabinoid and noradrenergic receptor activation in these areas. Moreover, cannabinoid signalling modulates the activity of locus coeruleus noradrenaline (NA) neurons and the release of NA in the medial prefrontal cortex, a brain area that is crucial for fear extinction. Interestingly, endocannabinoid-noradrenergic system interactions have been shown to regulate the encoding and retrieval of fear memory. Thus, noradrenergic regulation of fear extinction may also be driven indirectly in part via cannabinoid receptor signalling. In this perspective paper, we collate the available relevant literature and propose a synergistic role for the endocannabinoid and noradrenergic systems in regulating fear extinction, the study of which may further our understanding of the neurobiological substrates of PTSD and its treatment.