FAAH inhibitor, URB-597, promotes extinction and CB(1) antagonist, SR141716, inhibits extinction of conditioned aversion produced by naloxone-precipitated morphine withdrawal, but not extinction of conditioned preference produced by morphine in rats

Fatty Acid Amide Hydrolase (FAAH) Inhibition Plays a Key Role in Counteracting Acute Lung Injury

Acute lung injury (ALI) is a group of lung illnesses characterized by severe inflammation, with no treatment. The fatty acid amide hydrolase (FAAH) enzyme is an integral membrane protein responsible for the hydrolysis of the main endocannabinoids, such as anandamide (AEA). In pre-clinical pain and inflammation models, increasing the endogenous levels of AEA and other bioactive fatty acid amides (FAAs) via genetic deletion or the pharmacological inhibition of FAAH produces many analgesic benefits in several different experimental models. To date, nobody has investigated the role of FAAH inhibition on an ALI mouse model. Mice were subjected to a carrageenan injection and treated orally 1 h after with the FAAH inhibitor URB878 dissolved in a vehicle consisting of 10% PEG-400, 10% Tween-80 and 80% saline at different doses: The inhibition of FAAH activity was able to counteract not only the CAR-induced histological alteration, but also the cascade of related inflammatory events. URB878 clears the way for further studies based on FAAH inhibition in acute lung pathologies.

Molecular Targets of Cannabinoids Associated with Depression

Novel therapeutic strategies are needed to address depression, a major neurological disorder affecting hundreds of millions of people worldwide. Cannabinoids and their synthetic derivatives have demonstrated numerous neurological activities and may potentially be developed into new treatments for depression. This review highlights cannabinoid (CB) receptors, monoamine oxidase (MAO), N-methyl-D-aspartate (NMDA) receptor, gamma-aminobutyric acid (GABA) receptor, and cholecystokinin (CCK) receptor as key molecular targets of cannabinoids that are associated with depression. The anti-depressant activity of cannabinoids and their binding modes with cannabinoid receptors are discussed, providing insights into rational design and discovery of new cannabinoids or cannabimimetic agents with improved druggable properties.

Drug Addiction: Hyperkatifeia/Negative Reinforcement as a Framework for Medications Development

Compulsive drug seeking that is associated with addiction is hypothesized to follow a heuristic framework that involves three stages (binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation) and three domains of dysfunction (incentive salience/pathologic habits, negative emotional states, and executive function, respectively) via changes in the basal ganglia, extended amygdala/habenula, and frontal cortex, respectively. This review focuses on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the addiction cycle. Hyperkatifeia provides an additional source of motivation for compulsive drug seeking via negative reinforcement. Negative reinforcement reflects an increase in the probability of a response to remove an aversive stimulus or drug seeking to remove hyperkatifeia that is augmented by genetic/epigenetic vulnerability, environmental trauma, and psychiatric comorbidity. Neurobiological targets for hyperkatifeia in addiction involve neurocircuitry of the extended amygdala and its connections via within-system neuroadaptations in dopamine, enkephalin/endorphin opioid peptide, and γ-aminobutyric acid/glutamate systems and between-system neuroadaptations in prostress corticotropin-releasing factor, norepinephrine, glucocorticoid, dynorphin, hypocretin, and neuroimmune systems and antistress neuropeptide Y, nociceptin, endocannabinoid, and oxytocin systems. Such neurochemical/neurocircuitry dysregulations are hypothesized to mediate a negative hedonic set point that gradually gains allostatic load and shifts from a homeostatic hedonic state to an allostatic hedonic state. Based on preclinical studies and translational studies to date, medications and behavioral therapies that reset brain stress, antistress, and emotional pain systems and return them to homeostasis would be promising new targets for medication development. SIGNIFICANCE STATEMENT: The focus of this review is on neurochemical/neurocircuitry dysregulations that contribute to hyperkatifeia, defined as a greater intensity of negative emotional/motivational signs and symptoms during withdrawal from drugs of abuse in the withdrawal/negative affect stage of the drug addiction cycle and a driving force for negative reinforcement in addiction. Medications and behavioral therapies that reverse hyperkatifeia by resetting brain stress, antistress, and emotional pain systems and returning them to homeostasis would be promising new targets for medication development.

Potential of Cannabinoid Receptor Ligands as Treatment for Substance Use Disorders

Substance use disorder (SUD) is a major public health crisis worldwide, and effective treatment options are limited. During the past 2 decades, researchers have investigated the impact of a variety of pharmacological approaches to treat SUD, one of which is the use of medical cannabis or cannabinoids. Significant progress was made with the discovery of rimonabant, a selective CB1 receptor (CB1R) antagonist (also an inverse agonist), as a promising therapeutic for SUDs and obesity. However, serious adverse effects such as depression and suicidality led to the withdrawal of rimonabant (and almost all other CB1R antagonists/inverse agonists) from clinical trials worldwide in 2008. Since then, much research interest has shifted to other cannabinoid-based strategies, such as peripheral CB1R antagonists/inverse agonists, neutral CB1R antagonists, allosteric CB1R modulators, CB2R agonists, fatty acid amide hydrolase (FAAH) inhibitors, monoacylglycerol lipase (MAGL) inhibitors, fatty acid binding protein (FABP) inhibitors, or nonaddictive phytocannabinoids with CB1R or CB2R-binding profiles, as new therapeutics for SUDs. In this article, we first review recent progress in research regarding the endocannabinoid systems, cannabis reward versus aversion, and the underlying receptor mechanisms. We then review recent progress in cannabinoid-based medication development for the treatment of SUDs. As evidence continues to accumulate, neutral CB1R antagonists (such as AM4113), CB2R agonists (JWH133, Xie2-64), and nonselective phytocannabinoids (cannabidiol, β-caryophyllene, ∆9-tetrahydrocannabivarin) have shown great therapeutic potential for SUDs, as shown in experimental animals. Several cannabinoid-based medications (e.g., dronabinol, nabilone, PF-04457845) that entered clinical trials have shown promising results in reducing withdrawal symptoms in cannabis and opioid users.

Oleoyl glycine: interference with the aversive effects of acute naloxone-precipitated MWD, but not morphine reward, in male Sprague-Dawley rats

RATIONALE

Oleoyl glycine (OlGly), a recently discovered fatty acid amide that is structurally similar to N- acylethanolamines, which include the endocannabinoid, anandamide (AEA), as well as endogenous peroxisome proliferator-activated receptor alpha (PPARα) agonists oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), has been shown to interfere with nicotine reward and dependence in mice.

OBJECTIVES AND METHODS

Behavioral and molecular techniques were used to investigate the ability of OlGly to interfere with the affective properties of morphine and morphine withdrawal (MWD) in male Sprague-Dawley rats.

RESULTS

Synthetic OlGly (1-30 mg/kg, intraperitoneal [ip]) produced neither a place preference nor aversion on its own; however, at doses of 1 and 5 mg/kg, ip, it blocked the aversive effects of MWD in a place aversion paradigm. This effect was reversed by the cannabinoid 1 (CB1) receptor antagonist, AM251 (1 mg/kg, ip), but not the PPARα antagonist, MK886 (1 mg/kg, ip). OlGly (5 or 30 mg/kg, ip) did not interfere with a morphine-induced place preference or reinstatement of a previously extinguished morphine-induced place preference. Ex vivo analysis of tissue (nucleus accumbens, amygdala, prefrontal cortex, and interoceptive insular cortex) collected from rats experiencing naloxone-precipitated MWD revealed that OlGly was selectively elevated in the nucleus accumbens. MWD did not modify levels of the endocannabinoids 2-AG and AEA, nor those of the PPARα ligands, OEA and PEA, in any region evaluated.

CONCLUSION

Here, we show that OlGly interferes with the aversive properties of acute naloxone-precipitated morphine withdrawal in rats. These results suggest that OlGly may reduce the impact of MWD and may possess efficacy in treating opiate withdrawal.

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

RATIONALE

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

OBJECTIVE AND METHODS

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

RESULTS

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

CONCLUSION

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

Blockade of alcohol escalation and "relapse" drinking by pharmacological FAAH inhibition in male and female C57BL/6J mice

BACKGROUND

Anandamide (AEA)-dependent signaling is regulated by the catabolic enzyme fatty acid amide hydrolase (FAAH). Several lines of evidence have demonstrated that FAAH and AEA are involved in the behavioral effects of alcohol. Therefore, we investigated whether a selective FAAH inhibitor, URB597 (cyclohexylcarbamic acid 3'-[aminocarbonyl]-[1,1'-biphenyl]-3-yl ester), altered alcohol intake in mice in a voluntary alcohol drinking model.

METHODS

Mice, subjected to 3 weeks of chronic intermittent access (IA) in a two-bottle choice paradigm with 24-h access every other day, developed rapid escalation of alcohol intake and high preference. We evaluated the pharmacological effects of URB597 after both acute (1-day) withdrawal from chronic IA and 1-week withdrawal using the alcohol deprivation effect (ADE) model. AEA and N-acyl ethanolamide (NAE) abundances were determined after chronic IA, acute (1-day), or long-term (1 and 2 weeks) withdrawal in four brain regions.

RESULTS

Acute pretreatment with URB597 reduced alcohol intake and preference after acute withdrawal. This effect was blocked by pretreatment with a selective type 1 cannabinoid receptor (CB1) antagonist, suggesting a CB1-mediated mechanism. Both single- and multiple-dosing regimens with an effective dose of URB597 prevented the ADE, with no tolerance development after the multi-dosing regimen. AEA and NAE levels were transiently increased in all brain regions measured after acute withdrawal, indicating that the endocannabinoid system is involved in acute alcohol withdrawal stress response.

CONCLUSION

FAAH inhibitors reduce alcohol escalation and "relapse" drinking in mice.

The effects of enhancing endocannabinoid signaling and blocking corticotrophin releasing factor receptor in the amygdala and hippocampus on the consolidation of a stressful event

Current clinical and pre-clinical data suggest that both cannabinoid agents and blockage of CRF through corticotrophin releasing factor receptor type 1 (CRFr1) may offer therapeutic benefits for post-traumatic stress disorder (PTSD). Here we aim to determine whether they are more effective when combined when microinjected into the basolateral amygdala (BLA) or CA1 area of the hippocampus after exposure to a stressful event in the shock/reminders rat model for PTSD. Injection of the fatty acid amide hydrolase (FAAH) inhibitor URB597 after the shock into either the BLA or CA1 facilitated extinction, and attenuated startle response and anxiety-like behavior. These preventive effects of URB597 were found to be mediated by the CB1 receptor. Intra-BLA and intra-CA1 microinjection of the CRFr1 antagonist, CP-154,526 attenuated startle response. When microinjected into the BLA, CP-154,526 also attenuated freezing behavior during exposure to the first reminder and decreased anxiety-like behavior. The combined treatment of URB597 and CP-154,526 was not more effective than the separate treatments. Finally, mRNA levels of CRF, CRFr1 and CB1r were significantly higher in the BLA of rats exposed to shock and reminders compared to non-shocked rats almost one month after the shock. Taken together, the results show that enhancing endocannabinoid signaling in the amygdala and hippocampus produced a more favorable spectrum of effects than those caused by the CRFr1 antagonist. The findings suggest that FAAH inhibitors may be used as a novel treatment for stress-related anxiety disorders.

Effects of drugs of abuse on hippocampal plasticity and hippocampus-dependent learning and memory: contributions to development and maintenance of addiction

It has long been hypothesized that conditioning mechanisms play major roles in addiction. Specifically, the associations between rewarding properties of drugs of abuse and the drug context can contribute to future use and facilitate the transition from initial drug use into drug dependency. On the other hand, the self-medication hypothesis of drug abuse suggests that negative consequences of drug withdrawal result in relapse to drug use as an attempt to alleviate the negative symptoms. In this review, we explored these hypotheses and the involvement of the hippocampus in the development and maintenance of addiction to widely abused drugs such as cocaine, amphetamine, nicotine, alcohol, opiates, and cannabis. Studies suggest that initial exposure to stimulants (i.e., cocaine, nicotine, and amphetamine) and alcohol may enhance hippocampal function and, therefore, the formation of augmented drug-context associations that contribute to the development of addiction. In line with the self-medication hypothesis, withdrawal from stimulants, ethanol, and cannabis results in hippocampus-dependent learning and memory deficits, which suggest that an attempt to alleviate these deficits may contribute to relapse to drug use and maintenance of addiction. Interestingly, opiate withdrawal leads to enhancement of hippocampus-dependent learning and memory. Given that a conditioned aversion to drug context develops during opiate withdrawal, the cognitive enhancement in this case may result in the formation of an augmented association between withdrawal-induced aversion and withdrawal context. Therefore, individuals with opiate addiction may return to opiate use to avoid aversive symptoms triggered by the withdrawal context. Overall, the systematic examination of the role of the hippocampus in drug addiction may help to formulate a better understanding of addiction and underlying neural substrates.

FAAH inhibitor OL-135 disrupts contextual, but not auditory, fear conditioning in rats

Anxiety disorders are among the most prevalent psychological disorders, have significant negative impacts on quality of life and the healthcare system, and yet effective treatments remain elusive. Manipulating the endocannabinoid system has demonstrated potential for treating anxiety, although the side effects of direct manipulations of cannabinoid receptors keeps them from widespread clinical use. Disrupting the degradation enzyme fatty acid amide hydrolase (FAAH) enhances endogenous signaling and may produce similar efficacy without the side effects. The current experiments examine the effects of low (5.6mg/kg) or moderate (10.0mg/kg) doses of OL-135, a FAAH inhibitor, on the acquisition and consolidation of classical fear conditioning, a common model of trauma-induced anxiety. The acquisition of contextual, but not auditory, fear conditioning was disrupted by both doses of OL-135. Shock reactivity was not affected. Due to the additional neural circuitry required for contextual, but not auditory, fear conditioning, these data suggest that endocannabinoid signaling outside the amygdala may be critical for a subset of fearful memories.

2-AG promotes the expression of conditioned fear via cannabinoid receptor type 1 on GABAergic neurons

RATIONALE

The contribution of two major endocannabinoids, 2-arachidonoylglycerol (2-AG) and anandamide (AEA), in the regulation of fear expression is still unknown.

OBJECTIVES

We analyzed the role of different players of the endocannabinoid system on the expression of a strong auditory-cued fear memory in male mice by pharmacological means.

RESULTS

The cannabinoid receptor type 1 (CB1) antagonist SR141716 (3 mg/kg) caused an increase in conditioned freezing upon repeated tone presentation on three consecutive days. The cannabinoid receptor type 2 (CB2) antagonist AM630 (3 mg/kg), in contrast, had opposite effects during the first tone presentation, with no effects of the transient receptor potential vanilloid receptor type 1 (TRPV1) antagonist SB366791 (1 and 3 mg/kg). Administration of the CB2 agonist JWH133 (3 mg/kg) failed to affect the acute freezing response, whereas the CB1 agonist CP55,940 (50 μg/kg) augmented it. The endocannabinoid uptake inhibitor AM404 (3 mg/kg), but not VDM11 (3 mg/kg), reduced the acute freezing response. Its co-administration with SR141716 or SB366791 confirmed an involvement of CB1 and TRPV1. AEA degradation inhibition by URB597 (1 mg/kg) decreased, while 2-AG degradation inhibition by JZL184 (4 and 8 mg/kg) increased freezing response. As revealed in conditional CB1-deficient mutants, CB1 on cortical glutamatergic neurons alleviates whereas CB1 on GABAergic neurons slightly enhances fear expression. Moreover, 2-AG fear-promoting effects depended on CB1 signaling in GABAergic neurons, while an involvement of glutamatergic neurons remained inconclusive due to the high freezing shown by vehicle-treated Glu-CB1-KO.

CONCLUSIONS

Our findings suggest that increased AEA levels mediate acute fear relief, whereas increased 2-AG levels promote the expression of conditioned fear primarily via CB1 on GABAergic neurons.

The potential of inhibitors of endocannabinoid metabolism as anxiolytic and antidepressive drugs--A practical view

The endocannabinoid system, comprising cannabinoid CB1 and CB2 receptors, their endogenous ligands anandamide and 2-arachidonoylglyerol, and their synthetic and metabolic enzymes, are involved in many biological processes in the body, ranging from appetite to bone turnover. Compounds inhibiting the breakdown of anandamide and 2-arachidonoylglycerol increase brain levels of these lipids and thus modulate endocannabinoid signalling. In the present review, the preclinical evidence that these enzymes are good targets for development of novel therapies for anxiety and depression are discussed from a practical, rather than mechanistic, point of view. It is concluded that the preclinical data are promising, albeit tempered by problems of tolerance as well as effects upon learning and memory for irreversible monoacylglycerol lipase inhibitors, and limited by a focus upon male rodents alone. Clinical data so far has been restricted to safety studies with inhibitors of anandamide hydrolysis and a hitherto unpublished study on such a compound in elderly patients with major depressive disorders, but under the dose regimes used, they are well tolerated and show no signs of "cannabis-like" behaviours.

Comparative effects of pulmonary and parenteral Δ⁹-tetrahydrocannabinol exposure on extinction of opiate-induced conditioned aversion in rats

RATIONALE

Evidence suggesting that the endogenous cannabinoid (eCB) system can be manipulated to facilitate or impair extinction of learned behaviours has important consequences for opiate withdrawal and abstinence. We demonstrated that the fatty acid amide hydrolase (FAAH) inhibitor URB597, which increases eCB levels, facilitates extinction of a naloxone-precipitated morphine withdrawal-induced conditioned place aversion (CPA).

OBJECTIVES

The potential of the exogenous CB1 ligand, Δ(9)-tetrahydrocannabinol (Δ(9)-THC), to facilitate extinction of this CPA was tested. Effects of both pulmonary and parenteral Δ(9)-THC exposure were evaluated using comparable doses previously determined.

METHODS

Rats trained to associate a naloxone-precipitated morphine withdrawal with a floor cue were administered Δ(9)-THC-pulmonary (1, 5, 10 mg vapour inhalation) or parenteral (0.5, 1.0, 1.5 mg/kg intraperitoneal injection)-prior to each of 20 to 28 extinction/testing trials.

RESULTS

Vapourized Δ(9)-THC facilitated extinction of the CPA in a dose- and time-dependent manner: 5 and 10 mg facilitated extinction compared to vehicle and 1 mg Δ(9)-THC. Injected Δ(9)-THC significantly impaired extinction only for the 1.0-mg/kg dose: it prolonged the CPA fourfold longer than the vehicle and 0.5- and 1.5-mg/kg doses.

CONCLUSIONS

These data suggest that both dose and route of Δ(9)-THC administration have important consequences for its pharmacokinetic and behavioural effects; specifically, pulmonary exposure at higher doses facilitates, whereas pulmonary and parenteral exposure at lower doses impairs, rates of extinction learning for CPA. Pulmonary-administered Δ(9)-THC may prove beneficial for potentiation of extinction learning for aversive memories, such as those supporting drug-craving/seeking in opiate withdrawal syndrome, and other causes of conditioned aversions, such as illness and stress.

Interaction between the cholecystokinin and endogenous cannabinoid systems in cued fear expression and extinction retention

Post-traumatic stress disorder (PTSD) is thought to develop, in part, from improper inhibition of fear. Accordingly, one of the most effective treatment strategies for PTSD is exposure-based psychotherapy. Ideally, neuroscience would inform adjunct therapies that target the neurotransmitter systems involved in extinction processes. Separate studies have implicated the cholecystokinin (CCK) and endocannabinoid systems in fear; however, there is a high degree of anatomical colocalization between the cannabinoid 1 receptor (Cnr1) and CCK in the basolateral amygdala (BLA), a brain region critical for emotion regulation. Although most research has focused on GABA and GABAergic plasticity as the mechanism by which Cnr1 mediates fear inhibition, we hypothesize that a functional interaction between Cnr1 and CCKB receptor (CCKBR) is critical for fear extinction processes. In this study, systemic pharmacological manipulation of the cannabinoid system modulated cued fear expression in C57BL/6J mice after consolidation of auditory fear conditioning. Knockout of the CCKBR, however, had no effect on fear- or anxiety-like behaviors. Nonetheless, administration of a Cnr1 antagonist increased freezing behavior during a cued fear expression test in wild-type subjects, but had no effect on freezing behavior in CCKBR knockout littermates. In addition, we found that Cnr1-positive fibers form perisomatic clusters around CCKBR-positive cell bodies in the BLA. These CCKBR-positive cells comprise a molecularly heterogenous population of excitatory and inhibitory neurons. These findings provide novel evidence that Cnr1 contributes to cued fear expression via an interaction with the CCK system. Dysfunctional Cnr1-CCKBR interactions might contribute to the etiology of, or result from, fear-related psychiatric disease.

Glucocorticoid receptor antagonism disrupts the reconsolidation of social reward-related memories in rats

Reconsolidation is the process whereby consolidated memories are destabilized upon retrieval and restabilized to persist for later use. Although the neurobiology of the reconsolidation of both appetitive and aversive memories has been intensively investigated, reconsolidation of memories of physiologically relevant social rewards has received little attention. Social play, the most characteristic social behaviour displayed by young mammals, is highly rewarding, illustrated by the fact that it can induce conditioned place preference (CPP). Here, we investigated the role of signalling mechanisms implicated in memory processes, including reconsolidation, namely glucocorticoid, mineralocorticoid, NMDA glutamatergic and CB1 cannabinoid receptors, in the reconsolidation of social play-induced CPP in rats. Systemic treatment with the glucocorticoid receptor antagonist mifepristone before, but not immediately after, retrieval disrupted the reconsolidation of social play-induced CPP. Mifepristone did not affect social play-induced CPP in the absence of memory retrieval. Treatment with the NMDA receptor antagonist MK-801 modestly affected the reconsolidation of social play-induced CPP. However, the reconsolidation of social play-induced CPP was not affected by treatment with the mineralocorticoid and CB1 cannabinoid receptor antagonists spironolactone and rimonabant, respectively. We conclude that glucocorticoid neurotransmission mediates the reconsolidation of social reward-related memories in rats. These data indicate that the neural mechanisms of the reconsolidation of social reward-related memories only partially overlap with those underlying the reconsolidation of other reward-related memories.

CB1 antagonism: interference with affective properties of acute naloxone-precipitated morphine withdrawal in rats

RATIONALE

Modulation of the endocannabinoid system has been found to interfere with opiate withdrawal. The potential of activation and blockade of the endocannabinoid system to prevent the aversive-affective state of naloxone-precipitated morphine withdrawal (MWD) was investigated in a one-trial conditioned place aversion (CPA) paradigm.

OBJECTIVE

CPA provides a sensitive measure of the motivational effects of acute MWD. The potential of the fatty acid amide hydrolase (FAAH) inhibitors, URB597 and PF-3845, the CB1 antagonist/inverse agonist, AM251, and the neutral CB1 antagonists, AM4113 and AM6527 (oral), to interfere with establishment of a MWD-induced CPA was investigated. As well, the potential of AM251 and AM4113 to interfere with reinstatement of a previously established MWD-induced CPA was investigated.

MATERIALS AND METHODS

Using a one-trial place conditioning paradigm, rats were administered naloxone (1 mg/kg, subcutaneous (s.c.)) 24 h after receiving a high dose of morphine (20 mg/kg, s.c.) and were placed on the conditioning floor. To determine the effect of each pretreatment drug on the establishment of the MWD-induced CPA, URB597 (0.3 mg/kg, intraperitoneally (i.p.)), PF-3845 (10 mg/kg, i.p.), AM251 (1 or 2.5 mg/kg, i.p.), AM4113 (1 or 2.5 mg/kg, i.p.), and AM6527 (5 mg/kg, oral) were administered prior to conditioning.

RESULTS

AM251 (2.5, but not 1 mg/k), AM4113, and AM6527, but not URB597 or PF-3845, interfered with the establishment of the MWD-induced CPA. AM251 and AM4113 did not prevent reinstatement of the CPA.

CONCLUSIONS

Neutral antagonism of the CB1 receptor reduces the aversive affective properties of morphine withdrawal.

Endocannabinoids underlie reconsolidation of hedonic memories in Wistar rats

RATIONALE

Drug addicts constantly relapse to drug seeking after recall of memories linked to the drug experience. It is believed that a successful application of therapies that block memory reconsolidation may end the continuous cycle of drug relapse.

OBJECTIVES

The purpose of this study is to investigate whether modulation of the endocannabinoid system would impact the reconsolidation of opioid-related hedonic memories in rats previously paired to morphine context.

METHODS

Male Wistar rats were trained to acquire a morphine-conditioned place preference (CPP). One week later, morphine-CPP memory was reactivated by a brief exposure to a drug-paired context. Immediately after the memory reactivation session, independent groups of morphine-trained rats received a single subcutaneous injection of different doses of cannabinoid CB1 receptor antagonist rimonabant, CB2-selective antagonist AM630, potent CB1/CB2 agonist WIN 55,212-2, inhibitor of enzyme fatty acid amide hydrolase URB597, or vehicle. Morphine-CPP was retested 1 and 2 weeks after reactivation.

RESULTS

Blockade of CB1 (but not CB2) cannabinoid receptors impaired CPP reconsolidation of morphine-CPP at both tests 1 and 2 weeks post-reactivation, whereas direct activation of cannabinoid receptors did not produce significant effects on morphine-induced CPP. However, boosting endocannabinoid signaling by inhibition of anandamide metabolism promoted a transient CB1-dependent enhancement of the CPP.

Inhibition of FAAH and activation of PPAR: new approaches to the treatment of cognitive dysfunction and drug addiction

Enhancing the effects of endogenously-released cannabinoid ligands in the brain might provide therapeutic effects more safely and effectively than administering drugs that act directly at the cannabinoid receptor. Inhibitors of fatty acid amide hydrolase (FAAH) prevent the breakdown of endogenous ligands for cannabinoid receptors and peroxisome proliferator-activated receptors (PPAR), prolonging and enhancing the effects of these ligands when they are naturally released. This review considers recent research on the effects of FAAH inhibitors and PPAR activators in animal models of addiction and cognition (specifically learning and memory). These studies show that FAAH inhibitors can produce potentially therapeutic effects, some through cannabinoid receptors and some through PPAR. These effects include enhancing certain forms of learning, counteracting the rewarding effects of nicotine and alcohol, relieving symptoms of withdrawal from cannabis and other drugs, and protecting against relapse-like reinstatement of drug self-administration. Since FAAH inhibition might have a wide range of therapeutic actions but might also share some of the adverse effects of cannabis, it is noteworthy that at least one FAAH-inhibiting drug (URB597) has been found to have potentially beneficial effects but no indication of liability for abuse or dependence. Although these areas of research are new, the preliminary evidence indicates that they might lead to improved therapeutic interventions and a better understanding of the brain mechanisms underlying addiction and memory.

Medications development for opioid abuse

Here we describe methods for preclinical evaluation of candidate medications to treat opioid abuse and dependence. Our perspective is founded on the propositions that (1) drug self-administration procedures provide the most direct method for assessment of medication effects, (2) procedures that assess choice between opioid and nondrug reinforcers are especially useful, and (3) the states of opioid dependence and withdrawal profoundly influence both opioid reinforcement and the effects of candidate medications. Effects of opioid medications on opioid choice in nondependent and opioid-dependent subjects are reviewed. Various nonopioid medications have also been examined, but none yet have been identified that safely and reliably reduce opioid choice. Future research will focus on (1) strategies for increasing safety and/or effectiveness of opioid medications, and (2) continued development of nonopioids such as inhibitors of endocannabinoid catabolic enzymes or inhibitors of opioid-induced glial activation.

Loss of CB1 receptors leads to differential age-related changes in reward-driven learning and memory

Previous studies have shown that cannabinoid 1 (CB1) receptor signaling dissociates between reward-associated and aversive memories. The influence of CB1 receptors on the aversion-driven spatial learning in the Morris water maze test is strongly age-dependent: mice with genetic deletion of CB1 receptors (Cnr1^−/−) show superior learning when young but inferior learning when old compared to age-matched wild-type mice. Whether the reward-driven spatial learning is influenced in the same way by CB1 receptor signaling as the aversion-driven learning remains unclear. Thus, we examined the performance of Cn1^−/− and their wild-type littermates at ages of 2-, 5-, and 12-months-old in the eight-arm radial maze test—a reward-motivated model of spatial learning. Interestingly, 2-months-old Cnr1^−/− mice had a superior learning ability to wild-type mice. At the age of 5-months, Cnr1^−/− mice showed the same performance as the wild-type littermates. However, 12-months-old Cnr1^−/− mice showed significantly impaired performances in each parameter of the test. Accordingly, this study provides compelling support for our previous result that genetic deletion of CB1 receptor leads to early onset of age-related memory decline, similarly affecting both reward and aversion-driven learning.

Monoacylglycerol lipase - a target for drug development?

The endocannabinoid (eCB) system is involved in processes as diverse as control of appetite, perception of pain and the limitation of cancer cell growth and invasion. The enzymes responsible for eCB breakdown are attractive pharmacological targets, and fatty acid amide hydrolase inhibitors, which potentiate the levels of the eCB anandamide, are now undergoing pharmaceutical development. 'Drugable' selective inhibitors of monoacylglycerol lipase, a key enzyme regulating the levels of the other main eCB, 2-arachidonoylglycerol, were however not identified until very recently. Their availability has resulted in a large expansion of our knowledge concerning the pharmacological consequences of monoacylglycerol lipase inhibition and hence the role(s) played by the enzyme in the body. In this review, the pharmacology of monoacylglycerol lipase will be discussed, together with an analysis of the therapeutic potential of monoacylglycerol lipase inhibitors as analgesics and anticancer agents.

The endocannabinoid system and the brain

The psychoactive constituent in cannabis, Δ(9)-tetrahydrocannabinol (THC), was isolated in the mid-1960s, but the cannabinoid receptors, CB1 and CB2, and the major endogenous cannabinoids (anandamide and 2-arachidonoyl glycerol) were identified only 20 to 25 years later. The cannabinoid system affects both central nervous system (CNS) and peripheral processes. In this review, we have tried to summarize research--with an emphasis on recent publications--on the actions of the endocannabinoid system on anxiety, depression, neurogenesis, reward, cognition, learning, and memory. The effects are at times biphasic--lower doses causing effects opposite to those seen at high doses. Recently, numerous endocannabinoid-like compounds have been identified in the brain. Only a few have been investigated for their CNS activity, and future investigations on their action may throw light on a wide spectrum of brain functions.

The CB(1) receptor antagonist, AM281, improves recognition loss induced by naloxone in morphine withdrawal mice

Morphine withdrawal leads to the activation of endocannabinoid system and cognitive deficits. The aim of this study was to evaluate the effects of AM281, a cannabinoid antagonist/inverse agonist, on memory deficit following naloxone-precipitated morphine withdrawal in mice. Male mice were made dependent by increasing doses of morphine (30-90 mg/kg) twice daily for 3 days. The object recognition task was used to evaluate memory dysfunction. The test comprised three sections: habituation for 15 min., first trial for 12 min. and test trial for 5 min. In this learning paradigm, the difference in exploration between a previously seen object and a new object is taken as an index of memory performance (recognition index). The recognition index was assessed on the third day of morphine treatment by the injection of 0.1 mg/kg naloxone 3 hr after the last dose of morphine. Chronic administration of AM281 at 2.5 mg/kg significantly improved the memory impairment, producing a recognition index of 36.0 ± 3.9 as compared with vehicle-treated data (recognition index = -3.1 ± 8.2%). A single dose of AM281 at 5 mg/kg improved the recognition index from -1.5 ± 3.9% in morphine withdrawal animals to 18.5 ± 11.6%. Concurrent administration of AM281 with morphine proved to be more effective in protecting the animals from losing their memory compared to acute action of AM281. These results indicate that the contribution of the cannabinoid system to memory deficit is attributable to morphine withdrawal. By blocking cannabinoid receptors, AM281 may become useful in preventing memory deficit after morphine withdrawal.

The endocannabinoid transport inhibitor AM404 differentially modulates recognition memory in rats depending on environmental aversiveness

Cannabinoid compounds may influence both emotional and cognitive processes depending on the level of environmental aversiveness at the time of drug administration. However, the mechanisms responsible for these responses remain to be elucidated. The present experiments investigated the effects induced by the endocannabinoid transport inhibitor AM404 (0.5-5 mg/kg, i.p.) on both emotional and cognitive performances of rats tested in a Spatial Open Field task and subjected to different experimental settings, named High Arousal (HA) and Low Arousal (LA) conditions. The two different experimental conditions influenced emotional reactivity independently of drug administration. Indeed, vehicle-treated rats exposed to the LA condition spent more time in the center of the arena than vehicle-treated rats exposed to the HA context. Conversely, the different arousal conditions did not affect the cognitive performances of vehicle-treated animals such as the capability to discriminate a spatial displacement of the objects or an object substitution. AM404 administration did not alter locomotor activity or emotional behavior of animals exposed to both environmental conditions. Interestingly, AM404 administration influenced the cognitive parameters depending on the level of emotional arousal: it impaired the capability of rats exposed to the HA condition to recognize a novel object while it did not induce any impairing effect in rats exposed to the LA condition. These findings suggest that drugs enhancing endocannabinoid signaling induce different effects on recognition memory performance depending on the level of emotional arousal induced by the environmental conditions.

The endocannabinoid system in anxiety, fear memory and habituation

Evidence for the involvement of the endocannabinoid system (ECS) in anxiety and fear has been accumulated, providing leads for novel therapeutic approaches. In anxiety, a bidirectional influence of the ECS has been reported, whereby anxiolytic and anxiogenic responses have been obtained after both increases and decreases of the endocannabinoid tone. The recently developed genetic tools have revealed different but complementary roles for the cannabinoid type 1 (CB1) receptor on GABAergic and glutamatergic neuronal populations. This dual functionality, together with the plasticity of CB1 receptor expression, particularly on GABAergic neurons, as induced by stressful and rewarding experiences, gives the ECS a unique regulatory capacity for maintaining emotional homeostasis. However, the promiscuity of the endogenous ligands of the CB1 receptor complicates the interpretation of experimental data concerning ECS and anxiety. In fear memory paradigms, the ECS is mostly involved in the two opposing processes of reconsolidation and extinction of the fear memory. Whereas ECS activation deteriorates reconsolidation, proper extinction depends on intact CB1 receptor signalling. Thus, both for anxiety and fear memory processing, endocannabinoid signalling may ensure an appropriate reaction to stressful events. Therefore, the ECS can be considered as a regulatory buffer system for emotional responses.

Memory encoding in hippocampal ensembles is negatively influenced by cannabinoid CB1 receptors

It has previously been demonstrated that the detrimental effect on the performance of a delayed nonmatch to sample (DNMS) memory task by exogenously administered cannabinoid (CB1) receptor agonist, WIN 55212-2 (WIN), is reversed by the receptor antagonist rimonabant. In addition, rimonabant administered alone elevates DNMS performance, presumably through the suppression of negative modulation by released endocannabinoids during normal task performance. Other investigations have shown that rimonabant enhances encoding of DNMS task-relevant information on a trial-by-trial, delay-dependent basis. In this study, these reciprocal pharmacological actions were completely characterized by long-term, chronic intrahippocampal infusion of both agents (WIN and rimonabant) in successive 2-week intervals. Such long-term exposure allowed extraction and confirmation of task-related firing patterns, in which rimonabant reversed the effects of CB1 agonists. This information was then utilized to artificially impose the facilitatory effects of rimonabant and to reverse the effects of WIN on DNMS performance, by delivering multichannel electrical stimulation in the same firing patterns to the same hippocampal regions. Direct comparison of normal and WIN-injected subjects, in which rimonabant injections and ensemble firing facilitated performance, verified reversal of the modulation of hippocampal memory processes by CB1 receptor agonists, including released endocannabinoids.

Behavioral effects of fatty acid amide hydrolase inhibition on morphine withdrawal symptoms

Chronic morphine exposure causes tolerance and dependence. The cessation of morphine consumption induces a withdrawal syndrome that may involve cannabinoids and is characterized by undesirable psychological and physical signs. The present study examined whether augmentation of the endocannabinoid system by inhibition of fatty acid amide hydrolase could suppress the morphine withdrawal syndrome in morphine-addicted rats. Morphine dependency was induced by 7 consecutive days of morphine injection. The morphine-addicted rats received URB597 (1, 0.5, 0.3, 0.1, 0.03 mg/kg), a fatty acid amide hydrolase inhibitor, before the precipitation of morphine withdrawal syndromes by naloxone. Withdrawal symptoms including jumping, teeth chattering, paw tremor, wet dog shakes, face grooming, penis licking, standing, rearing, sniffing and percent of weight loss were recorded during 30 min after naloxone injection. The results showed that the morphine withdrawal precipitated rats had significantly more withdrawal symptoms than naive control rats and the administration of URB597 (all doses except 0.03 mg/kg) reduced most of the morphine withdrawal symptoms. We conclude that the administration of URB597 modulated morphine withdrawal symptoms. This finding shows that endocannabinoids interact with the opioid system during the morphine withdrawal period and that potentiation of the endogenous cannabinoid system by URB597 may be a new target strategy for the management of morphine addiction.

[11C]CURB: Evaluation of a novel radiotracer for imaging fatty acid amide hydrolase by positron emission tomography

INTRODUCTION

Fatty acid amide hydrolase (FAAH) is the enzyme responsible for metabolising the endogenous cannabinoid, anandamide, and thus represents an important target for molecular imaging. To date, no radiotracer has been shown to be useful for imaging of FAAH using either positron emission tomography (PET) or single photon emission computed tomography (SPECT). We here determine the suitability of a novel carbon-11-labeled inhibitor of FAAH via ex vivo biodistribution studies in rat brain in conjunction with pharmacological challenges.

METHODS

A potent irreversible inhibitor of FAAH, URB694, radiolabeled with carbon-11 in the carbonyl position ([(11)C]CURB), was administered to male rats via tail-vein injection. Rats were sacrificed at various time points postinjection, and tissue samples were dissected, counted and weighed. Specific binding to FAAH was investigated by pretreatment of animals with URB694 or URB597. For metabolism and mechanism of binding studies, whole brains were excised post-radiotracer injection, homogenised and extracted exhaustively with 80% aq. acetonitrile to determine the time course and fraction of radioactivity that was irreversibly bound to brain parenchyma.

RESULTS

Upon intravenous injection into rats, [(11)C]CURB showed high brain uptake [standard uptake value (SUV) of 1.6-2.4 at 5 min] with little washout over time, which is characteristic of irreversible binding. Highest uptake of radioactivity was seen in the cortex, intermediate in the cerebellum and lowest in the hypothalamus, reflecting the reported distribution of FAAH. Brain uptake of radioactivity was decreased in a dose-dependent manner by pretreatment with increasing amounts of URB694, demonstrating that binding was saturable. Pretreatment with the well-characterised FAAH inhibitor, URB597, reduced binding in all brain regions by 70-80%. Homogenised brain extraction experiments demonstrated unequivocally that [(11)C]CURB was irreversibly bound to FAAH.

CONCLUSIONS

The title radiotracer demonstrates favourable properties such as good brain uptake, regional heterogeneity and specificity of binding based on ex vivo biodistribution studies in conscious rat brain. [(11)C]CURB represents a highly promising radiotracer for the imaging of FAAH using PET.

Cannabinoid receptor activation modifies NMDA receptor mediated release of intracellular calcium: implications for endocannabinoid control of hippocampal neural plasticity

Chronic activation or inhibition of cannabinoid receptors (CB1) leads to continuous suppression of neuronal plasticity in hippocampus and other brain regions, suggesting that endocannabinoids may have a functional role in synaptic processes that produce state-dependent transient modulation of hippocampal cell activity. In support of this, it has previously been shown in vitro that cannabinoid CB1 receptors modulate second messenger systems in hippocampal neurons that can regulate operation of intracellular processes including receptors which release calcium from intracellular stores. Here we demonstrate in hippocampal slices a similar endocannabinoid action on excitatory glutamatergic synapses via modulation of NMDA-receptor mediated intracellular calcium levels in confocal imaged neurons. Calcium entry through glutamatergic NMDA-mediated ion channels increases intracellular calcium concentrations by modifying release from ryanodine-sensitive channels in endoplasmic reticulum. The studies reported here show that NMDA-elicited increases in Calcium Green fluorescence are enhanced by CB1 receptor antagonists (i.e., Rimonabant), and inhibited by CB1 agonists (i.e., WIN 55,212-2). Suppression of endocannabinoid breakdown by either reuptake inhibition (AM404) or fatty-acid amide hydrolase inhibition (URB597) produced suppression of NMDA-elicited calcium increases comparable to WIN 55,212-2, while enhancement of calcium release provoked by endocannabinoid receptor antagonists (Rimonabant) was shown to depend on the blockade of CB1receptor mediated de-phosphorylation of Ryanodine receptors. Such CB1 receptor modulation of NMDA elicited increases in intracellular calcium may account for the respective disruption and enhancement by CB1 agents of trial-specific hippocampal neuron ensemble firing patterns during performance of a short-term memory task, reported previously from this laboratory.

Effects of the novel endocannabinoid uptake inhibitor, LY2183240, on fear-potentiated startle and alcohol-seeking behaviors in mice selectively bred for high alcohol preference

RATIONALE

Alcohol-use disorders often occur together with anxiety disorders in humans which may be partly due to common inherited genetic factors. Evidence suggests that the endocannabinoid system (ECS) is a promising therapeutic target for the treatment of individuals with anxiety and/or alcohol-use disorders.

OBJECTIVES

The present study assessed the effects of a novel endocannabinoid uptake inhibitor, LY2183240, on anxiety- and alcohol-seeking behaviors in a unique animal model that may represent increased genetic risk to develop co-morbid anxiety and alcohol-use disorders in humans. Mice selectively bred for high alcohol preference (HAP) show greater fear-potentiated startle (FPS) than mice selectively bred for low alcohol preference (LAP). We examined the effects of LY2183240 on the expression of FPS in HAP and LAP mice and on alcohol-induced conditioned place preference (CPP) and limited-access alcohol drinking behavior in HAP mice.

RESULTS

Repeated administration of LY2183240 (30 mg/kg) reduced the expression of FPS in HAP but not LAP mice when given prior to a second FPS test 48 h after fear conditioning. Both the 10 and 30 mg/kg doses of LY2183240 enhanced the expression of alcohol-induced CPP and this effect persisted in the absence of the drug. LY2183240 did not alter limited-access alcohol drinking behavior, unconditioned startle responding, or locomotor activity.

CONCLUSIONS

These findings suggest that ECS modulation influences both conditioned fear and conditioned alcohol reward behavior. LY2183240 may be an effective pharmacotherapy for individuals with anxiety disorders, such as post-traumatic stress disorder, but may not be appropriate for individuals with co-morbid anxiety and alcohol-use disorders.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).