Study of cannabinoid dependence in animals

Endocannabinoids and addiction memory: Relevance to methamphetamine/morphine abuse

AIM

This review aims to summarise the role of endocannabinoid system (ECS), incluing cannabinoid receptors and their endogenous lipid ligands in the modulation of methamphetamine (METH)/morphine-induced memory impairments.

METHODS

Here, we utilized the results from researches which have investigated regulatory role of ECS (including cannabinoid receptor agonists and antagonists) on METH/morphine-induced memory impairments.

RESULTS

Among the neurotransmitters, glutamate and dopamine seem to play a critical role in association with the ECS to heal the drug-induced memory damages. Also, the amygdala, hippocampus, and prefrontal cortex are three important brain regions that participate in both drug addiction and memory task processes, and endocannabinoid neurotransmission have been investigated.

CONCLUSION

ECS can be regarded as a treatment for the side effects of METH and morphine, and their memory-impairing effects.

Daily THC and withdrawal increase dopamine D1-D2 receptor heteromer to mediate anhedonia and anxiogenic-like behavior through a dynorphin and kappa opioid receptor mechanism

Background

Frequent cannabis use is associated with a higher risk of developing cannabis use disorder and other adverse consequences. However, rodent models studying the underlying mechanisms of the reinforcing and withdrawal effects of the primary constituent of cannabis, Δ⁹-tetrahydrocannabinol (THC), have been limited.

Methods

This study investigated the effects of daily THC (1 mg/kg, intraperitoneal, 9 days) and spontaneous withdrawal (7 days) on hedonic and aversion-like behaviors in male rats. In parallel, underlying neuroadaptive changes in dopaminergic, opioidergic, and cannabinoid signaling in the nucleus accumbens were evaluated, along with a candidate peptide designed to reverse altered signaling.

Results

Chronic THC administration induced anhedonic- and anxiogenic-like behaviors not attributable to altered locomotor activity. These effects persisted after drug cessation. In the nucleus accumbens, THC treatment and withdrawal catalyzed increased cannabinoid CB₁ receptor activity without modifying receptor expression. Dopamine D₁-D₂ receptor heteromer expression rose steeply with THC, accompanied by increased calcium-linked signaling, activation of BDNF/TrkB (brain-derived neurotrophic factor/tropomyosin receptor kinase B) pathway, dynorphin expression, and kappa opioid receptor signaling. Disruption of the D₁-D₂ heteromer by an interfering peptide during withdrawal reversed the anxiogenic-like and anhedonic-like behaviors as well as the neurochemical changes.

Conclusions

Chronic THC increases nucleus accumbens dopamine D₁-D₂ receptor heteromer expression and function, which results in increased dynorphin expression and kappa opioid receptor activation. These changes plausibly reduce dopamine release to trigger anxiogenic- and anhedonic-like behaviors after daily THC administration that persist for at least 7 days after drug cessation. These findings conceivably provide a therapeutic strategy to alleviate negative symptoms associated with cannabis use and withdrawal.

Evidence for spontaneous cannabinoid withdrawal in mice

Although the behavioral effects of acute and chronic exposure to cannabinoids have been extensively studied in mice, spontaneous withdrawal following exposure to cannabinoids has not been well characterized in this species. To address this issue, different groups of mice were treated for 5 days with saline, 20-36 mg/kg/day of the CB partial agonist Δ9-tetrahydrocannabinol (Δ9-THC), or 0.06-0.1 mg/kg/day of the CB high-efficacy agonist AM2389. Initial studies assessed changes in observable behavior (paw tremors) that were scored from the recordings taken at 4 or 24 h after the last injection. Subsequently, radiotelemetry was used to continuously measure body temperature and locomotor activity before (baseline), during, and after the 5-day dosing regimens. Results show that increases in paw tremors occurred following 5-day exposure to AM2389 or Δ9-THC. In telemetry studies, acute AM2389 or THC decreased both temperature and activity. Rapid tolerance occurred to the hypothermic effects of the cannabinoids, whereas locomotor activity continued to be suppressed following each drug injection. In contrast, increases in locomotor activity were evident 12-72 h after discontinuing daily injections of either 0.06 or 0.1 mg/kg/day AM2389. Increases in locomotor activity were also noted in mice treated daily with 30 or 36, but not 20 mg/kg/day Δ9-THC; these effects were smaller and appeared later than effects seen in AM2389-treated mice. These results indicate that the discontinuation of daily treatment with a CB high-efficacy agonist will yield evidence of spontaneous withdrawal that may reflect prior dependence, and that the degree of cannabinoid dependence may vary in relation to the dose or efficacy of the agonist injected daily.

Chronic migraine caused a higher rate of tendency to cannabinoid agonist compared to morphine

Opioid and cannabinoid systems have considerable roles in the modulation of chronic pain as well as regulation reward circuit and addiction responses. This study investigated the effect of nitroglycerin (NTG)-induced migraine attack on the acquisition of morphine and cannabinoid-induced conditioned place preference (CPP) in male rats. Adult male rats (230-250 gr) were used. Experimental groups were included (n=10): control, opioid receptor agonist morphine (10 mg/kg), WIN55,212-2 (1 mg/kg) as a cannabinoid receptor agonist, NTG + morphine (10 mg/kg) and NTG + WIN55,212-2 (1 mg/kg). Nitroglycerin (10 mg/kg) was used to induce migraine attacks every other day for 9 days. After migraine induction, conditioning performance was assessed by CPP test. During conditioning days, morphine and WIN55,212-2 were injected subcutaneously and intraperitoneally, respectively. Anxiety and locomotor activity were evaluated using open field test (OFT). According to data, conditioning score for morphine-treated rats was significantly decreased following NTG-induced migraine (p<0.01). However, NTG-induced migraine was able to increase the conditioning score in WIN55,212-2 as compared to the control group (p<0.05). In OFT, there were no significant differences in locomotor activity and grooming behaviors between experimental groups. However, time spent in the center of the OFT box was significantly decreased in NTG plus morphine-treated rats as compared to control (p<0.05). Moreover, rearing response in NTG - treated groups which received either morphine or WIN55,212-2 decreased as compared to the control group (p<0.01). NTG induced migraine prompts a decrease in morphine and an increase in cannabinoid performances. So, these compounds’ effects on drug dependency during migraine attacks may occur at different mechanisms. (www.actabiomedica.it)

Assessing physiological dependence and withdrawal potential of mitragynine using schedule-controlled behaviour in rats

RATIONALE

Kratom is proposed to exhibit therapeutic potential as an opium substitute, but little is known about its dependence-producing profile, particularly of its main psychoactive compound, mitragynine (MG).

OBJECTIVES

This study examined the dependence-producing effects of MG using operant-scheduled behaviour in rats and investigated the potential therapeutic effect of MG by comparing effects to buprenorphine in morphine-dependent rats using the same schedule-controlled behavioural task.

METHODS

The effects of acutely administered MG and morphine were determined in rats trained to respond under fixed-ratio (FR) 10 schedule of food reinforcement. Next, the rats were administered MG and morphine twice daily for 14 consecutive days to determine if physiological dependence would develop by examining cessation of drug treatment and following antagonist-precipitated withdrawal. The study then examined the effects of MG substitution to suppress naloxone-precipitated morphine withdrawal effects on scheduled responding.

RESULTS

Acute doses of MG did not produce dose-related decreases on FR schedules of responding compared to morphine. Unlike morphine, MG-treated rats showed no suppression of response rates following cessation of MG treatment. However, withdrawal effects were evident for MG after precipitation by either naloxone or SR141716A (rimonabant), similar to morphine-treated rats. MG in higher doses (10 and 30 mg/kg) attenuated the naloxone-precipitated morphine withdrawal effects while smaller doses of buprenorphine (0.3 and 1.0 mg/kg) were necessary to alleviate these effects.

CONCLUSION

The findings suggest that MG does not induce physiological dependence but can alleviate the physical symptoms associated with morphine withdrawal which represent the desired characteristics of novel pharmacotherapeutic interventions for managing opioid use disorder (OUD).

Signaling molecules targeting cannabinoid receptors: Hemopressin and related peptides

Cannabinoid receptors (CBRs) are part of the endocannabinoid system, which is involved in various physiological processes such as nociception, inflammation, appetite, stress, and emotion regulation. Many studies have linked the endocannabinoid system to neuroinflammatory and neurodegenerative disorders such as Parkinson's disease, Huntington's chorea, Alzheimer's disease, and multiple sclerosis. Hemopressin [Hp; a fragment of the hemoglobin α1 chain (95-103 amino acids)] and related peptides [VD-Hpα and RVD-Hpα] are peptides that bind to CBRs. Hp acts as an inverse agonist to CB₁ receptor (CB₁R), VD-Hpα acts as an agonist to CB₁R, and RVD-Hpα acts as a negative allosteric modulator of CB₁R and a positive allosteric modulator of CB₂R. Because of the critical roles of CBRs in numerous physiological processes, it is appealing to use Hp and related peptides for therapeutic purposes. This review discusses their discovery, structure, metabolism, brain exposure, self-assembly characteristics, pharmacological characterization, and pharmacological activities.

Unidirectional opioid-cannabinoid cross-tolerance in the modulation of social play behavior in rats

RATIONALE

The endocannabinoid and the endogenous opioid systems interact in the modulation of social play behavior, a highly rewarding social activity abundantly expressed in young mammals. Prolonged exposure to opioid or cannabinoid receptor agonists induces cross-tolerance or cross-sensitization to their acute behavioral effects.

OBJECTIVES AND METHODS

Behavioral and biochemical experiments were performed to investigate whether cross-tolerance or cross-sensitization occurs to the play-enhancing effects of cannabinoid and opioid drugs on social play behavior, and the possible brain substrate involved.

RESULTS

The play-enhancing effects induced by systemic administration of JZL184, which inhibits the hydrolysis of the endocannabinoid 2-AG, were suppressed in animals repeatedly pretreated with the opioid receptor agonist morphine. Conversely, acute morphine administration increased social play in rats pretreated with vehicle or with either JZL184 or the cannabinoid agonist WIN55,212-2. Acute administration of JZL184 increased the activation of both CB1 receptors and their effector Akt in the nucleus accumbens and prefrontal cortex, brain regions important for the expression of social play. These effects were absent in animals pretreated with morphine. Furthermore, only animals repeatedly treated with morphine and acutely administered with JZL184 showed reduced activation of CB1 receptors and Akt in the amygdala.

CONCLUSIONS

The present study demonstrates a dynamic opioid-cannabinoid interaction in the modulation of social play behavior, occurring in limbic brain areas strongly implicated in social play behavior. A better understanding of opioid-cannabinoid interactions in social play contributes to clarify neurobiological aspects of social behavior at young age, which may provide new therapeutic targets for social dysfunctions.

Positive Allosteric Modulation of Cannabinoid Receptor Type 1 Suppresses Pathological Pain Without Producing Tolerance or Dependence

BACKGROUND

Activation of cannabinoid CB₁ receptors suppresses pathological pain but also produces unwanted central side effects. We hypothesized that a positive allosteric modulator of CB₁ signaling would suppress inflammatory and neuropathic pain without producing cannabimimetic effects or physical dependence. We also asked whether a CB₁ positive allosteric modulator would synergize with inhibitors of endocannabinoid deactivation and/or an orthosteric cannabinoid agonist.

METHODS

GAT211, a novel CB₁ positive allosteric modulator, was evaluated for antinociceptive efficacy and tolerance in models of neuropathic and/or inflammatory pain. Cardinal signs of direct CB₁-receptor activation were evaluated together with the propensity to induce reward or aversion and physical dependence. Comparisons were made with inhibitors of endocannabinoid deactivation (JZL184, URB597) or an orthosteric cannabinoid agonist (WIN55,212-2). All studies used 4 to 11 subjects per group.

RESULTS

GAT211 suppressed allodynia induced by complete Freund's adjuvant and the chemotherapeutic agent paclitaxel in wild-type but not CB₁ knockout mice. GAT211 did not impede paclitaxel-induced tumor cell line toxicity. GAT211 did not produce cardinal signs of direct CB₁-receptor activation in the presence or absence of pathological pain. GAT211 produced synergistic antiallodynic effects with fatty acid amide hydrolase and monoacylglycerol lipase inhibitors in paclitaxel-treated mice. Therapeutic efficacy was preserved over 19 days of chronic dosing with GAT211, but it was not preserved with the monoacylglycerol lipase inhibitor JZL184. The CB₁ antagonist rimonabant precipitated withdrawal in mice treated chronically with WIN55,212-2 but not in mice treated with GAT211. GAT211 did not induce conditioned place preference or aversion.

CONCLUSIONS

Positive allosteric modulation of CB₁-receptor signaling shows promise as a safe and effective analgesic strategy that lacks tolerance, dependence, and abuse liability.

Chronic, intermittent treatment with a cannabinoid receptor agonist impairs recognition memory and brain network functional connectivity

Elucidating how cannabinoids affect brain function is instrumental for the development of therapeutic tools aiming to mitigate 'on target' side effects of cannabinoid-based therapies. A single treatment with the cannabinoid receptor agonist, WIN 55,212-2, disrupts recognition memory in mice. Here, we evaluate how prolonged, intermittent (30 days) exposure to WIN 55,212-2 (1 mg/kg) alters recognition memory and impacts on brain metabolism and functional connectivity. We show that chronic, intermittent treatment with WIN 55,212-2 disrupts recognition memory (Novel Object Recognition Test) without affecting locomotion and anxiety-like behaviour (Open Field and Elevated Plus Maze). Through 14 C-2-deoxyglucose functional brain imaging we show that chronic, intermittent WIN 55,212-2 exposure induces hypometabolism in the hippocampal dorsal subiculum and in the mediodorsal nucleus of the thalamus, two brain regions directly involved in recognition memory. In addition, WIN 55,212-2 exposure induces hypometabolism in the habenula with a contrasting hypermetabolism in the globus pallidus. Through the application of the Partial Least Squares Regression (PLSR) algorithm to the brain imaging data, we observed that prolonged WIN 55,212-2 administration alters functional connectivity in brain networks that underlie recognition memory, including that between the hippocampus and prefrontal cortex, the thalamus and prefrontal cortex, and between the hippocampus and the perirhinal cortex. In addition, our results support disturbed lateral habenula and serotonin system functional connectivity following WIN 55,212-2 exposure. Overall, this study provides new insight into the functional mechanisms underlying the impact of chronic cannabinoid exposure on memory and highlights the serotonin system as a particularly vulnerable target.

Novel behavioral assays of spontaneous and precipitated THC withdrawal in mice

BACKGROUND

A subset of cannabis users develop some degree of Cannabis Use Disorder (CUD). Although behavioral therapy has some success in treating CUD, many users relapse, often citing altered sleep, mood, and irritability. Preclinical animal tests of cannabinoid withdrawal focus primarily on somatic-related behaviors precipitated by a cannabinoid receptor antagonist. The goal of the present study was to develop novel cannabinoid withdrawal assays that are either antagonist-precipitated or spontaneously induced by abstinence.

METHODS

C57BL/6 J mice were repeatedly administered the phytocannabinoid Δ⁹-tetrahydrocannabinol (THC; 1, 10 or 50 mg/kg, s.c.), the synthetic cannabinoid receptor agonist JWH-018 (1 mg/kg, s.c.), or vehicle (1:1:18 parts ethanol:Kolliphor EL:saline, s.c.) for 6 days. Withdrawal was precipitated with the cannabinoid receptor inverse agonist rimonabant (3 mg/kg, i.p.) or elicited via abstinence (i.e., spontaneous withdrawal), and putative stress-related behavior was scored. Classic somatic signs of cannabinoid withdrawal were also quantified.

RESULTS

Precipitated THC withdrawal significantly increased plasma corticosterone. Precipitated withdrawal from either THC or JWH-018 suppressed marble burying, increased struggling in the tail suspension test, and elicited somatic withdrawal behaviors. The monoacylglycerol lipase inhibitor JZL184 attenuated somatic precipitated withdrawal but had no effect on marble burying or struggling. Spontaneous THC or JWH-018 withdrawal-induced paw tremors, head twitches, and struggled in the tail suspension test after 24-48 h abstinence. JZL184 or THC attenuated these spontaneous withdrawal-induced behaviors.

CONCLUSION

Outcomes from tail suspension and marble burying tests reveal that THC withdrawal is multifaceted, eliciting and suppressing behaviors in these tests, in addition to inducing well-documented somatic signs of withdrawal.

Role of the endocannabinoid system in drug addiction

Drug addiction is a chronic relapsing disorder that produces a dramaticglobal health burden worldwide. Not effective treatment of drug addiction is currently available probably due to the difficulties to find an appropriate target to manage this complex disease raising the needs for further identification of novel therapeutic approaches. The endocannabinoid system has been found to play a crucial role in the neurobiological substrate underlying drug addiction. Endocannabinoids and cannabinoid receptors are widely expressed in the main areas of the mesocorticolimbic system that participate in the initiation and maintenance of drug consumption and in the development of compulsion and loss of behavioral control occurring during drug addiction. The identification of the important role played by CB1 cannabinoid receptors in drug addiction encouraged the possible used of an early commercialized CB1 receptor antagonist for treating drug addiction. However, the incidence of serious psychiatric adverse events leaded to the sudden withdrawal from the market of this CB1 antagonist and all the research programs developed by pharmaceutical companies to obtain new CB1 antagonists were stopped. Currently, new research strategies are under development to target the endocannabinoid system for drug addiction avoiding these side effects, which include allosteric negative modulators of CB1 receptors and compounds targeting CB2 receptors. Recent studies showing the potential role of CB2 receptors in the addictive properties of different drugs of abuse have open a promising research opportunity to develop novel possible therapeutic approaches.

Preclinical Studies of Cannabinoid Reward, Treatments for Cannabis Use Disorder, and Addiction-Related Effects of Cannabinoid Exposure

Cannabis use has become increasingly accepted socially and legally, for both recreational and medicinal purposes. Without reliable information about the effects of cannabis, people cannot make informed decisions regarding its use. Like alcohol and tobacco, cannabis can have serious adverse effects on health, and some people have difficulty discontinuing their use of the drug. Many cannabis users progress to using and becoming addicted to other drugs, but the reasons for this progression are unclear. The natural cannabinoid system of the brain is complex and involved in many functions, including brain development, reward, emotion, and cognition. Animal research provides an objective and controlled means of obtaining information about: (1) how cannabis affects the brain and behavior, (2) whether medications can be developed to treat cannabis use disorder, and (3) whether cannabis might produce lasting changes in the brain that increase the likelihood of becoming addicted to other drugs. This review explains the tactics used to address these issues, evaluates the progress that has been made, and offers some directions for future research.

Adolescent Exposure to the Synthetic Cannabinoid WIN 55212-2 Modifies Cocaine Withdrawal Symptoms in Adult Mice

Chronic cannabinoid consumption is an increasingly common behavior among teenagers and has been shown to cause long-lasting neurobehavioral alterations. Besides, it has been demonstrated that cocaine addiction in adulthood is highly correlated with cannabis abuse during adolescence. Cocaine consumption and subsequent abstinence from it can cause psychiatric symptoms, such as psychosis, cognitive impairment, anxiety, and depression. The aim of the present research was to study the consequences of adolescent exposure to cannabis on the psychiatric-like effects promoted by cocaine withdrawal in adult mice. We pre-treated juvenile mice with the cannabinoid CB1 receptor agonist WIN 55212-2 (WIN) and then subjected them to a chronic cocaine treatment during adulthood. Following these treatments, animals were tested under cocaine withdrawal in the following paradigms: pre-pulse inhibition, object recognition, elevated plus maze, and tail suspension. The long-term psychotic-like actions induced by WIN were not modified after cocaine cessation. Moreover, the memory impairments induced by cocaine withdrawal were not altered by previous adolescent WIN intake. However, WIN pre-treatment prevented the anxiogenic effects observed after cocaine abstinence, and led to greater depressive-like symptoms following cocaine removal in adulthood. This study is the first to show the long-lasting behavioral consequences of juvenile exposure to WIN on cocaine withdrawal in adult mice.

New vistas on cannabis use disorder

Cannabis sativa preparations are the most consumed illicit drugs for recreational purposes worldwide, and the number of people seeking treatment for cannabis use disorder has dramatically increased in the last decades. Due to the recent decriminalization or legalization of cannabis use in the Western Countries, we may predict that the number of people suffering from cannabis use disorder will increase. Despite the increasing number of cannabis studies over the past two decades, we have gaps of scientific knowledge pertaining to the neurobiological consequences of long-term cannabis use. Moreover, no specific treatments for cannabis use disorders are currently available. In this review, we explore new research that may help fill these gaps. We discuss and provide a solution to the experimental limitation of a lack of rodent models of THC self-administration, and the importance this model can play in understanding the neurobiology of relapse and in providing a biological rationale for potential therapeutic targets. We also focus our attention on glial cells, commenting on recent preclinical evidence suggesting that alterations in microglia and astrocytes might contribute to the detrimental effects associated with cannabis abuse. Finally, due to the worrisome prevalence rates of cannabis use during pregnancy, we highlight the associations between cannabis use disorders during pregnancy and congenital disorders, describing the possible neuronal basis of vulnerability at molecular and circuit level. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

Synthetic Pot: Not Your Grandfather's Marijuana

In the early 2000s in Europe and shortly thereafter in the USA, it was reported that 'legal' forms of marijuana were being sold under the name K2 and/or Spice. Active ingredients in K2/Spice products were determined to be synthetic cannabinoids (SCBs), producing psychotropic actions via CB1 cannabinoid receptors, similar to those of Δ9-tetrahydrocannabinol (Δ9-THC), the primary active constituent in marijuana. Often abused by adolescents and military personnel to elude detection in drug tests due to their lack of structural similarity to Δ9-THC, SCBs are falsely marketed as safe marijuana substitutes. Instead, SCBs are a highly structural diverse group of compounds, easily synthesized, which produce very dangerous adverse effects occurring by, as of yet, unknown mechanisms. Therefore, available evidence indicates that K2/Spice products are clearly not safe marijuana alternatives.

Effects of daily delta-9-tetrahydrocannabinol treatment on heroin self-administration in rhesus monkeys

Opioid abuse remains a significant public health problem; together with the greater availability of marijuana in some regions there is an increasing likelihood that opioids and marijuana will be used together. Polydrug abuse is associated with increased toxicity and poorer treatment outcome; thus, a better understanding of the consequences of repeated coadministration of these drugs will facilitate the development of better prevention and treatment strategies. This study examined the effects of daily treatment with the cannabinoid receptor agonist delta-9-tetrahydrocannabinol (Δ-THC) and its discontinuation on self-administration of heroin in rhesus monkeys (n=4) lever-pressing under a fixed-ratio 30 schedule. Heroin self-administration (0.32-32 μg/kg/infusion, intravenously) generated an inverted U-shaped dose-effect curve. Administered acutely, Δ-THC (0.01-0.32 mg/kg, subcutaneously) dose dependently decreased responding for heroin and flattened the self-administration dose-effect curve. Daily treatment with Δ-THC (0.01-0.1 mg/kg/12 h, subcutaneously) either had no effect on or decreased responding for heroin. In addition, daily treatment did not significantly impact extinction of heroin self-administration or resumption of responding for heroin after extinction. Discontinuation of daily Δ-THC treatment did not systematically impact rates of heroin self-administration. These data suggest that repeated administration of a cannabinoid receptor agonist likely does not increase, and possibly decreases, the positive reinforcing effects of a mu opioid receptor agonist.

Inhaled delivery of Δ(9)-tetrahydrocannabinol (THC) to rats by e-cigarette vapor technology

Most human Δ(9)-tetrahydrocannabinol (THC) use is via inhalation, and yet few animal studies of inhalation exposure are available. Popularization of non-combusted methods for the inhalation of psychoactive drugs (Volcano(®), e-cigarettes) further stimulates a need for rodent models of this route of administration. This study was designed to develop and validate a rodent chamber suitable for controlled exposure to vaporized THC in a propylene glycol vehicle, using an e-cigarette delivery system adapted to standard size, sealed rat housing chambers. The in vivo efficacy of inhaled THC was validated using radiotelemetry to assess body temperature and locomotor responses, a tail-flick assay for nociception and plasma analysis to verify exposure levels. Hypothermic responses to inhaled THC in male rats depended on the duration of exposure and the concentration of THC in the vehicle. The temperature nadir was reached after ∼40 min of exposure, was of comparable magnitude (∼3 °Celsius) to that produced by 20 mg/kg THC, i.p. and resolved within 3 h (compared with a 6 h time course following i.p. THC). Female rats were more sensitive to hypothermic effects of 30 min of lower-dose THC inhalation. Male rat tail-flick latency was increased by THC vapor inhalation; this effect was blocked by SR141716 pretreatment. The plasma THC concentration after 30 min of inhalation was similar to that produced by 10 mg/kg THC i.p. This approach is flexible, robust and effective for use in laboratory rats and will be of increasing utility as users continue to adopt "vaping" for the administration of cannabis.

Preclinical studies on the reinforcing effects of cannabinoids. A tribute to the scientific research of Dr. Steve Goldberg

RATIONALE

The reinforcing effects of most abused drugs have been consistently demonstrated and studied in animal models, although those of marijuana were not, until the demonstration 15 years ago that delta-9-tetrahydrocannabinol (THC) could serve as a reinforcer in self-administration (SA) procedures in squirrel monkeys. Until then, those effects were inferred using indirect assessments.

OBJECTIVES

The aim of this manuscript is to review the primary preclinical procedures used to indirectly and directly infer reinforcing effects of cannabinoid drugs.

METHODS

Results will be reviewed from studies of cannabinoid discrimination, intracranial self-stimulation (ICSS), conditioned place preference (CPP), as well as change in levels of dopamine assessed in brain areas related to reinforcement, and finally from self-administration procedures. For each procedure, an evaluation will be made of the predictive validity in detecting the potential abuse liability of cannabinoids based on seminal papers, with the addition of selected reports from more recent years especially those from Dr. Goldberg's research group.

RESULTS AND CONCLUSIONS

ICSS and CPP do not provide consistent results for the assessment of potential for abuse of cannabinoids. However, drug discrimination and neurochemistry procedures appear to detect potential for abuse of cannabinoids, as well as several novel "designer cannabinoid drugs." Though after 15 years transfer of the self-administration model of marijuana abuse from squirrel monkeys to other species remains somewhat problematic, studies with the former species have substantially advanced the field, and several reports have been published with consistent self-administration of cannabinoid agonists in rodents.

Adolescent delta-9-tetrahydrocannabinol (THC) exposure fails to affect THC-induced place and taste conditioning in adult male rats

BACKGROUND

Adolescent initiation of drug use has been linked to problematic drug taking later in life and may represent an important variable that changes the balance of the rewarding and/or aversive effects of abused drugs which may contribute to abuse vulnerability. The current study examined the effects of adolescent THC exposure on THC-induced place preference (rewarding effects) and taste avoidance (aversive effects) conditioning in adulthood.

METHODS

Forty-six male Sprague-Dawley adolescent rats received eight injections of an intermediate dose of THC (3.2mg/kg) or vehicle. After these injections, animals were allowed to mature and then trained in a combined CTA/CPP procedure in adulthood (PND ~90). Animals were given four trials of conditioning with intervening water-recovery days, a final CPP test and then a one-bottle taste avoidance test.

RESULTS

THC induced dose-dependent taste avoidance but did not produce place conditioning. None of these effects was impacted by adolescent THC exposure.

CONCLUSIONS

Adolescent exposure to THC had no effect on THC taste and place conditioning in adulthood. The failure to see an effect of adolescent exposure was addressed in the context of other research that has assessed exposure of drugs of abuse during adolescence on drug reactivity in adulthood.

Assessment of the Abuse Liability of Synthetic Cannabinoid Agonists JWH-030, JWH-175, and JWH-176

The emergence and use of synthetic cannabinoids have greatly increased in recent years. These substances are easily dispensed over the internet and on the streets. Some synthetic cannabinoids were shown to have abuse liability and were subsequently regulated by authorities. However, there are compounds that are still not regulated probably due to the lack of abuse liability studies. In the present study, we assessed the abuse liability of three synthetic cannabinoids, namely JWH-030, JWH-175, and JWH-176. The abuse liability of these drugs was evaluated in two of the most widely used animal models for assessing the abuse potential of drugs, the conditioned place preference (CPP) and self-administration (SA) test. In addition, the open-field test was utilized to assess the effects of repeated (7 days) treatment and abrupt cessation of these drugs on the psychomotor activity of animals. Results showed that JWH-175 (0.5 mg/kg), but not JWH-030 or JWH-176 at any dose, significantly decreased the locomotor activity of mice. This alteration in locomotor activity was only evident during acute exposure to the drug and was not observed during repeated treatment and abstinence. Similarly, only JWH-175 (0.1 mg/kg) produced significant CPP in rats. On the other hand, none of the drugs tested was self-administered by rats. Taken together, the present results indicate that JWH-175, but not JWH-030 and JWH-176, may have abuse potential. More importantly, our findings indicate the complex psychopharmacological effects of synthetic cannabinoids and the need to closely monitor the production, dispensation, and use of these substances.

Analgesic tolerance to morphine is regulated by PPARγ

BACKGROUND AND PURPOSE

Opioid drugs are potent analgesics. However, their chronic use leads to the rapid development of tolerance to their analgesic effects and subsequent increase of significant side effects, including drug dependence and addiction. Here, we investigated the role of PPARγ in the development of analgesic tolerance to morphine in mice.

EXPERIMENTAL APPROACH

We monitored analgesia on alternate days using the tail immersion test.

KEY RESULTS

Daily administration of morphine (30 mg·kg(-1) , bid) resulted in the rapid development of tolerance to thermal analgesia. Co-administration of pioglitazone (10 and 30 mg·kg(-1) , bid) significantly attenuated the development and expression of tolerance. However, pretreatment with GW-9662 (5 mg·kg(-1) , bid), a selective PPARγ antagonist, completely abolished this effect. Injection of GW-9662 and a lower dose of morphine (15 mg·kg(-1) , bid) accelerated the development of tolerance to its antinociceptive effect. Subsequently, we found that conditional neuronal PPARγ knockout (KO) mice develop a more rapid and pronounced tolerance to morphine antinociception compared with wild-type (WT) controls. Moreover, in PPARγ KO mice, pioglitazone was no longer able to prevent the development of morphine tolerance.

CONCLUSIONS AND IMPLICATIONS

Overall, our results demonstrate that PPARγ plays a tonic role in the modulation of morphine tolerance, and its pharmacological activation may help to reduce its development. These findings provide new information about the role of neuronal PPARγ and suggest that combining PPARγ agonists with opioid analgesics may reduce the development of tolerance and possibly attenuate the potential for opioid abuse.

Cannabinoid abuse and addiction: Clinical and preclinical findings

Cannabinoid abuse disorders represent a widespread public health issue, but there are no approved medications for their treatment. This review describes efforts to understand the mechanisms of cannabinoid abuse and its adverse effects, to identify molecular targets for pharmacotherapy, and to evaluate potential treatments in human volunteers and animal models of cannabinoid reward, withdrawal, and relapse.

[18F]MK-9470 PET measurement of cannabinoid CB1 receptor availability in chronic cannabis users

Δ(9) -Tetrahydrocannabinol, the main psychoactive component of cannabis, exerts its central effects through activation of the cerebral type 1 cannabinoid (CB1 ) receptor. Pre-clinical studies have provided evidence that chronic cannabis exposure is linked to decreased CB1 receptor expression and this is thought to be a component underlying drug tolerance and dependence. In this study, we make first use of the selective high-affinity positron emission tomography (PET) ligand [(18) F]MK-9470 to obtain in vivo measurements of cerebral CB1 receptor availability in 10 chronic cannabis users (age = 26.0 ± 4.1 years). Each patient underwent [(18) F]MK-9470 PET within the first week following the last cannabis consumption. A population of 10 age-matched healthy subjects (age = 23.0 ± 2.9 years) was used as control group. Parametric modified standardized uptake value images, reflecting CB1 receptor availability, were calculated. Statistical parametric mapping and volume-of-interest (VOI) analyses of CB1 receptor availability were performed. Compared with controls, cannabis users showed a global decrease in CB1 receptor availability (-11.7 percent). VOI-based analysis demonstrated that the CB1 receptor decrease was significant in the temporal lobe (-12.7 percent), anterior (-12.6 percent) and posterior cingulate cortex (-13.5 percent) and nucleus accumbens (-11.2 percent). Voxel-based analysis confirmed this decrease and regional pattern in CB1 receptor availability in cannabis users. These findings revealed that chronic cannabis use may alter specific regional CB1 receptor expression through neuroadaptive changes in CB1 receptor availability, opening the way for the examination of specific CB1 -cannabis addiction interactions which may predict future cannabis-related treatment outcome.

Interactions of the opioid and cannabinoid systems in reward: Insights from knockout studies

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides (enkephalins, endorphins, and dynorphins). The endogenous cannabinoid system comprises lipid neuromodulators (endocannabinoids), enzymes for their synthesis and their degradation and two well-characterized receptors, cannabinoid receptors CB1 and CB2. These systems play a major role in the control of pain as well as in mood regulation, reward processing and the development of addiction. Both opioid and cannabinoid receptors are coupled to G proteins and are expressed throughout the brain reinforcement circuitry. Extending classical pharmacology, research using genetically modified mice has provided important progress in the identification of the specific contribution of each component of these endogenous systems in vivo on reward process. This review will summarize available genetic tools and our present knowledge on the consequences of gene knockout on reinforced behaviors in both systems, with a focus on their potential interactions. A better understanding of opioid-cannabinoid interactions may provide novel strategies for therapies in addicted individuals.

Antinociceptive effects of central administration of the endogenous cannabinoid receptor type 1 agonist VDPVNFKLLSH-OH [(m)VD-hemopressin(α)], an N-terminally extended hemopressin peptide

The cannabinoid system has been demonstrated to modulate the acute and chronic pain of multiple origins. Mouse VD-hemopressin(α) [(m)VD-Hpα], an 11-residue α-hemoglobin-derived peptide, was recently reported to function as a selective agonist of the cannabinoid receptor type 1 (CB₁) in vitro. To characterize its behavioral and physiological properties, we investigated the in vivo effects of (m)VD-Hpα in mice. In the mouse tail-flick test, (m)VD-Hpα dose-dependently induced antinociception after supraspinal (EC₅₀ = 6.69 nmol) and spinal (EC₅₀ = 2.88 nmol) administration. The antinociceptive effects of (m)VD-Hpα (intracerebroventricularly and intrathecally) were completely blocked by N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3- carboxamide (AM251; CB₁ antagonist), but not by 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl(4-methoxyphenyl)-methanone (AM630; CB₂ antagonist) or naloxone (opioid antagonist), showing its selectivity to the CB₁ receptor. Furthermore, the central nervous system (CNS) effects of (m)VD-Hpα were evaluated in body temperature, locomotor activity, tolerance development, reward, and food intake assays. At the highly antinociceptive dose (3 × EC₅₀), (m)VD-Hpα markedly exerted hypothermia and hypoactivity after supraspinal administration. Repeated intracerebroventricular injection of (m)VD-Hpα resulted in both development of tolerance to antinociception and conditioned place aversion. In addition, central injection of (m)VD-Hpα dose-dependently stimulated food consumption. These findings demonstrate that this novel cannabinoid peptide agonist induces CB₁-mediated central antinociception with some CNS effects, which further supports a CB₁ agonist character of (m)VD-Hpα. Moreover, the current study will be helpful to understand the in vivo properties of the endogenous peptide agonist of the cannabinoid CB₁ receptor.

Biphasic effects of Δ9-tetrahydrocannabinol on brain stimulation reward and motor activity

Δ(9)-tetrahydrocannabinol (Δ(9)-THC), the main psychoactive ingredient of marijuana, has led to equivocal results when tested with the intracranial self-stimulation (ICSS) procedure or the open-field test for motor activity, two behavioural models for evaluating the reward-facilitating and locomotor stimulating effects of drugs of abuse, respectively. Therefore, in the present study, the effects of high and low doses of Δ(9)-THC were compared in the ICSS procedure and the open-field test. Moreover, the involvement of CB(1) receptors in tentative Δ(9)-THC-induced effects was investigated by pre-treating the animals with the CB(1) receptor antagonist SR141716A (rimonabant). The results obtained show that low doses of Δ(9)-THC induce opposite effects from high doses of Δ(9)-THC. Specifically, 0.1 mg/kg Δ(9)-THC decreased ICSS thresholds and produced hyperactivity, whereas 1 mg/kg increased ICSS thresholds and produced hypoactivity. Both effects were reversed by pre-treatment with SR141716A, indicating the involvement of CB(1) receptors on these actions. Altogether, our results indicate that Δ(9)-THC can produce acute activating effects in locomotion that coincide with its reward-facilitating effects in the ICSS paradigm. The present findings provide further support that Δ(9)-THC induces behaviours typical of abuse and substantiate the notion that marijuana resembles other drugs of abuse.

Distinct pharmacology and metabolism of K2 synthetic cannabinoids compared to Δ(9)-THC: mechanism underlying greater toxicity?

K2 or Spice products are emerging drugs of abuse that contain synthetic cannabinoids (SCBs). Although assumed by many teens and first time drug users to be a "safe" and "legal" alternative to marijuana, many recent reports indicate that SCBs present in K2 produce toxicity not associated with the primary psychoactive component of marijuana, ∆(9)-tetrahydrocannabinol (Δ(9)-THC). This mini-review will summarize recent evidence that use of K2 products poses greater health risks relative to marijuana, and suggest that distinct pharmacological properties and metabolism of SCBs relative to Δ(9)-THC may contribute to the observed toxicity. Studies reviewed will indicate that in contrast to partial agonist properties of Δ(9)-THC typically observed in vitro, SCBs in K2 products act as full cannabinoid receptor type 1 (CB1R) and type 2 (CB2R) agonists in both cellular assays and animal studies. Furthermore, unlike Δ(9)-THC metabolism, several SCB metabolites retain high affinity for, and exhibit a range of intrinsic activities at, CB1 and CB2Rs. Finally, several reports indicate that although quasi-legal SCBs initially evaded detection and legal consequences, these presumed "advantages" have been limited by new legislation and development of product and human testing capabilities. Collectively, evidence reported in this mini-review suggests that K2 products are neither safe nor legal alternatives to marijuana. Instead, enhanced toxicity of K2 products relative to marijuana, perhaps resulting from the combined actions of a complex mixture of different SCBs present and their active metabolites that retain high affinity for CB1 and CB2Rs, highlights the inherent danger that may accompany use of these substances.

Prediction and Prevention of Prescription Drug Abuse: Role of Preclinical Assessment of Substance Abuse Liability

In 2011, the prevalence of prescription drug abuse exceeded that of any other illicit drug except marijuana. Consequently, efforts to curtail abuse of new medications should begin during the drug development process, where abuse liability can be identified and addressed before a candidate medication has widespread use. The first step in this process is scheduling with the Drug Enforcement Agency so that legal access is appropriately restricted, dependent upon levels of abuse risk and medical benefit. To facilitate scheduling, the Food and Drug Administration (FDA) has published guidance for industry that describes assessment of abuse liability. The purpose of this paper is to review methods that may be used to satisfy the FDA's regulatory requirements for animal behavioral and dependence pharmacology. Methods include psychomotor activity, self-administration (an animal model of the rewarding effects of a drug), drug discrimination (an animal model of the subjective effects of a drug), and evaluation of tolerance and dependence. Data from tests conducted at RTI with known drugs of abuse illustrate typical results, and demonstrate that RTI is capable of performing these tests. While using preclinical data to predict abuse liability is an imperfect process, it has substantial predictive validity. The ultimate goal is to increase consumer safety through appropriate scheduling of new medications.

Prior exposure to THC increases the addictive effects of nicotine in rats

Although it is more common for drug abuse to progress from tobacco to cannabis, in many cases cannabis use develops before tobacco use. Epidemiological evidence indicates that prior cannabis use increases the likelihood of becoming dependent on tobacco. To determine whether this effect might be due to cannabis exposure per se, in addition to any genetic, social, or environmental factors that might contribute, we extended our series of studies on 'gateway drug' effects in animal models of drug abuse. Rats were exposed to THC, the main psychoactive constituent of cannabis, for 3 days (two intraperitoneal injections/day). Then, starting 1 week later, they were allowed to self-administer nicotine intravenously. THC exposure increased the likelihood of acquiring the nicotine self-administration response from 65% in vehicle-exposed rats to 94% in THC-exposed rats. When the price of nicotine was manipulated by increasing the response requirement, THC-exposed rats maintained higher levels of intake than vehicle-exposed rats, indicating that THC exposure increased the value of nicotine reward. These results contrast sharply with our earlier findings that prior THC exposure did not increase the likelihood of rats acquiring either heroin or cocaine self-administration, nor did it increase the reward value of these drugs. The findings obtained here suggest that a history of cannabis exposure might have lasting effects that increase the risk of becoming addicted to nicotine.

Long-term CB₁ receptor blockade enhances vulnerability to anxiogenic-like effects of cannabinoids

Compelling evidence has documented the anxiolytic and mood-enhancing properties of cannabis. In susceptible users, however, consumption of this drug is conducive to panic, paranoia and dysphoria. We hypothesized that the up-regulation of CB₁ receptors (CB₁Rs) in select brain regions may enhance the vulnerability to cannabinoid-induced anxiety. To test this possibility, we assessed the behavioral impact of a potent cannabinoid agonist (CP55,940; 0.05-0.1 mg/kg, IP) on C57BL/6 male mice, respectively subjected to a prolonged pre-treatment of either the selective CB₁R antagonist/inverse agonist AM251 (1 mg/kg/day IP, for 21 days, followed by a 3-day clearance period before testing) or its vehicle (VEH1). Anxiety-like responses were studied in the novel open field, elevated plus maze (EPM) and social interaction assays. While CP55,940 induced anxiolytic-like effects in the EPM in VEH1-exposed animals, it elicited opposite actions in AM251-exposed mice. In this last group, CP55,940 also reduced rearing and social interaction in comparison to its vehicle (VEH2). The divergent effects of CP55,940 in AM251- and VEH1-pretreated animals were confirmed in 129SvEv mice. Immunoblotting analyses on brain samples of C57BL/6 mice revealed that AM251 pre-treatment caused a significant up-regulation of CB₁R expression in the prefrontal cortex and striatum, but also a down-regulation of these receptors in the hippocampus and midbrain. Notably, CB₁R levels in the prefrontal cortex were negatively correlated with anxiolysis-related indices in the EPM; furthermore, midbrain CB₁R expression was positively correlated with the total duration of social interaction. These results suggest that regional variations in brain CB₁R expression may differentially condition the behavioral effects of cannabinoids with respect to anxiety-related responses.

Analysis of tolerance and behavioral/physical dependence during chronic CB1 agonist treatment: effects of CB1 agonists, antagonists, and noncannabinoid drugs

Behavioral studies of chronic CB(1) receptor activation may provide a pharmacological approach to understanding efficacy-related differences among CB(1) ligands as well as mechanistic commonalities between cannabinoid and noncannabinoid drugs. In the present studies, the effects of CB(1) agonists [(6aR,10aR)-3-(1-adamantyl)-6,6,9-trimethyl-6a,7,10,10a-tetrahydrobenzo[c]chromen-1-ol (AM411), 9β-(hydroxymethyl)-3-(1-adamantyl)-hexahydrocannabinol (AM4054), R-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate (WIN55,212.2), Δ(9)-tetrahydrocannabinol (Δ(9)-THC), (R)-(+)-arachidonyl-1'-hydroxy-2'-propylamide (methanandamide)], CB(1) antagonists [5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide (SR141716A), 5-(4-alkylphenyl)-1-(2,4-dichlorophenyl)-4-methyl-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide (AM4113)], and dopamine (DA)-related [methamphetamine, (±)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF82958), (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390), (6aR)-5,6,6a,7-tetrahydro-6-propyl-4H-dibenzo[de,g]quinoline-10,11-diol (R-(-)-NPA), haloperidol] and opioid (morphine, naltrexone) drugs on scheduled-controlled responding under a 30-response fixed ratio schedule of stimulus-shock termination in squirrel monkeys were compared before and during chronic treatment with the long-acting CB(1) agonist AM411 (1.0 mg/kg per day, i.m.). Prechronic treatment with all drugs except naltrexone (1-10 mg/kg) produced dose-related decreases in responses rates. Dose-response re-determinations during chronic treatment revealed the following: 1) >250-fold (AM411, methanandamide) and >45-fold (AM4054, WIN55,212.2, Δ(9)-THC) rightward shifts in the ED(50) values for CB(1) agonists; 2) >100-fold and >20-fold leftward shifts in the ED(50) values for SR141716A and AM4113, respectively; and 3) approximately 4.8-fold and 10-fold rightward shifts in the ED(50) values for methamphetamine and the DA D(2) agonist R-(-)-NPA, respectively. Dose-response relationships for other DA-related and opioid drugs were unchanged by chronic CB(1) agonist treatment. Differences in the magnitude of tolerance among CB(1) agonists during chronic treatment may be indicative of differences in their pharmacological efficacy, whereas the enhanced sensitivity to behaviorally disruptive effects of CB(1) antagonists may provide evidence for CB(1)-related behavioral and/or physical dependence. Finally, the development of cross-tolerance to methamphetamine and R-(-)-NPA bolsters previous evidence of interplay between CB(1) and DA D(2) signaling mechanisms.

Biphasic effects of cannabinoids in anxiety responses: CB1 and GABA(B) receptors in the balance of GABAergic and glutamatergic neurotransmission

Biphasic effects of cannabinoids have been shown in processes such as feeding behavior, motor activity, motivational processes and anxiety responses. Using two different tests for the characterization of anxiety-related behavior (elevated plus-maze and holeboard), we first identified in wild-type C57BL/6N mice, two doses of the synthetic CB1 cannabinoid receptor agonist CP-55,940 with anxiolytic (1 μg/kg) and anxiogenic properties (50 μg/kg), respectively. To clarify the role of CB1 receptors in this biphasic effect, both doses were applied to two different conditional CB1 receptor knockout (KO) mouse lines, GABA-CB1-KO (CB1 receptor inactivation in forebrain GABAergic neurons) and Glu-CB1-KO (CB1 receptor inactivation in cortical glutamatergic neurons). We found that the anxiolytic-like effects of the low dose of cannabinoids are mediated via the CB1 receptor on cortical glutamatergic terminals, because this anxiolytic-like response was abrogated only in Glu-CB1-KO mice. On the contrary, the CB1 receptor on the GABAergic terminals is required to induce an anxiogenic-like effect under a high-dose treatment because of the fact that this effect was abolished specifically in GABA-CB1-KO mice. These experiments were carried out in both sexes, and no differences occurred with the doses tested in the mutant mice. Interestingly, the positive allosteric modulation of GABA(B) receptor with GS-39783 was found to largely abrogate the anxiogenic-like effect of the high dose of CP-55,940. Our results shed new light in further understanding the biphasic effects of cannabinoids at the molecular level and, importantly, pave the way for the development of novel anxiolytic cannabinoid drugs, which may have favorable effect profiles targeting the CB1 receptor on glutamatergic terminals.

Self-medication of a cannabinoid CB2 agonist in an animal model of neuropathic pain

Drug self-administration methods were used to test the hypothesis that rats would self-medicate with a cannabinoid CB(2) agonist to attenuate a neuropathic pain state. Self-medication of the CB(2) agonist (R,S)-AM1241, but not vehicle, attenuated mechanical hypersensitivity produced by spared nerve injury. Switching rats from (R,S)-AM1241 to vehicle self-administration also decreased lever responding in an extinction paradigm. (R,S)-AM1241 self-administration did not alter paw withdrawal thresholds in sham-operated or naive animals. The percentage of active lever responding was similar in naive groups self-administering vehicle or (R,S)-AM1241. The CB(2) antagonist SR144528 blocked both antiallodynic effects of (R,S)-AM1241 self-medication and the percentage of active lever responding in neuropathic (but not naive) rats. Neuropathic and sham groups exhibited similar percentages of active lever responding for (R,S)-AM1241 on a fixed ratio 1 (FR1) schedule. However, neuropathic animals worked harder than shams to obtain (R,S)-AM1241 when the schedule of reinforcement was increased (to FR6). (R,S)-AM1241 self-medication on FR1, FR3, or FR6 schedules attenuated nerve injury-induced mechanical allodynia. (R,S)-AM1241 (900μg intravenously) failed to produce motor ataxia observed after administration of the mixed CB(1)/CB(2) agonist WIN55,212-2 (0.5mg/kg intravenously). Our results suggest that cannabinoid CB(2) agonists may be exploited to treat neuropathic pain with limited drug abuse liability and central nervous system side effects. These studies validate the use of drug self-administration methods for identifying nonpsychotropic analgesics possessing limited abuse potential. These methods offer potential to elucidate novel analgesics that suppress spontaneous neuropathic pain that is not measured by traditional assessments of evoked pain.

Animal models of cannabinoid reward

The endogenous cannabinoid system is involved in numerous physiological and neuropsychological functions. Medications that target this system hold promise for the treatment of a wide variety of disorders. However, as reward is one of the most prominent of these functions, medications that activate this system must be evaluated for abuse potential. Meanwhile, cannabis is already being used chronically by millions of people, many of whom eventually seek treatment for cannabis dependence. Therefore, there is a need for procedures that can be used to: (i) better understand the mechanisms of cannabinoid reward; (ii) evaluate the abuse potential of new medications; and (iii) evaluate the effectiveness of medications developed for treating cannabis dependence. Animal models of cannabinoid reward provide a means of accomplishing these goals. In this review, we briefly describe and evaluate these models, their advantages and their shortcomings. Special emphasis is placed on intravenous cannabinoid self-administration in squirrel monkeys, a valid, reliable and flexible model that we have developed over the past decade. Although the conditions under which cannabinoid drugs have rewarding effects may be more restricted than with other drugs of abuse such as cocaine and heroin, work with these models indicates that cannabinoid reward involves similar brain mechanisms and produces the same kinds of reward-related behaviour. By continuing to use these animal models as tools in the development of new medications, it should be possible to take advantage of the potential benefits provided by the endocannabinoid system while minimizing its potential for harm.

How is substance use linked to psychosis? A study of the course and patterns of substance dependence in psychosis

Substance use in mentally ill patients is now a major problem that influences the course and outcome of psychosis. With prevalence ranging up to 60%, several theories were postulated to explain the link. It would be interesting to know if substances have different effects in persons with psychosis than in those without. This study aimed to explore patterns of symptomatology of dependence and comorbid psychiatric illness by comparing and contrasting it with a group suffering from pure substance dependence. Consecutively admitted patients who were matched for age, sex, and tobacco use were divided into 3 groups. These were substance dependence without any comorbid psychiatric disorder (SD; n = 32), schizophrenia with substance dependence (SC; n = 31), and bipolar disorder with substance dependence (BD; n = 31). Patients were administered the Schedules for Clinical Assessment in Neuropsychiatry (SCAN) and Mini International Neuropsychiatric Inventory (MINI) to evaluate the chronology of criterion of International Classification of Diseases (ICD)-10 dependence. Results showed that cannabis was the most common substance used by both the SC (100%) and BD (80%) groups. This was followed by alcohol as the most common substance used, with prevalence of 87% in SC and 77% in BD groups. There was a significant difference in the pattern of use of cannabis in patients with psychosis, who developed tolerance much faster (P = .018) and had longer durations of cannabis use (P = .001) than the SD group. The presence of "loss of control" over drug use criterion seems to be a specific marker predicting development of dependence and psychosis. Cannabis use is more strongly associated with development of psychosis than any other substance.

Differential regulation of behavioral tolerance to WIN55,212-2 by GASP1

Cannabinoid agonists have shown some promise clinically as analgesics, in particular for cancer pain, in which they have the additional benefit of decreasing nausea. However, as for most other drugs, the long-term use of cannabinoids is limited by the development of tolerance. Several molecular mechanisms have been proposed to explain drug tolerance, including receptor downregulation. The cannabinoid 1 (CB1) receptors can be downregulated in vitro through an interaction with the G-protein-coupled receptor-associated sorting protein1, GASP1, that targets CB1 receptors for degradation after their agonist-mediated endocytosis. To investigate whether GASP1-mediated postendocytic sorting of the CB1 receptor contributes to tolerance to cannabinoid drugs in vivo, we generated a mouse with a disruption of GASP1. In wild-type mice, repeated administration of the cannabinoid agonist WIN55,212-2 promoted downregulation of CB1 receptor levels and concomitant tolerance to the effects of drug on antinociception, motor incoordination, and locomotor hypoactivity. In contrast, GASP1 knockout mice did not develop tolerance to any of these effects and showed no significant receptor downregulation. Taken together, this study provides evidence that GASP1 regulates CB1 receptor downregulation in vivo, and that postendocytic receptor trafficking has a key role in the development of tolerance to WIN55,212-2.

Anxiety-like effects of SR141716-precipitated delta9-tetrahydrocannabinol withdrawal in mice in the elevated plus-maze

Marijuana discontinuation has been recently reported to be anxiogenic in humans, which may predict relapse. Limited animal research has been carried out to model this withdrawal-associated negative affect. The current study sought to investigate the potential anxiety-like effects of cannabinoid withdrawal in mice. Male ICR mice were injected s.c. with delta9-tetrahydrocannabinol (THC) at 10mg/kg or vehicle once daily for 10 days. To precipitate withdrawal, the cannabinoid CB1 antagonist SR141716 (0.3, 1.0, or 3.0mg/kg) or vehicle was administrated i.p. 4h following the last THC or vehicle treatment. Thirty minutes later, mice were tested on the elevated plus-maze (EPM) for 5min. SR141716 did not significantly change EPM behaviors in vehicle-treated mice. In contrast, SR141716 precipitated a reduction in exploration of the open arms of EPM in mice repeatedly treated with THC vs vehicle. At 3.0mg/kg, SR141716 significantly reduced % open arm entries of the total arm entries, % open arm time of total time in arms, and the absolute time spent in open arms. No significant differences in the number of closed or total arm entries were observed, indicating that the behavioral changes were not due to altered motor activity. Collectively, the present results constitute the first evidence that cannabinoid withdrawal produces anxiety-like effects in mice. This animal model may help to identify the mechanisms that contribute to adaptations in the neuronal circuitry of the brain that are expressed as emotional symptoms of cannabinoid withdrawal.

Chronic WIN55,212-2 elicits sustained and conditioned increases in intracranial self-stimulation thresholds in the rat

The present study sought to examine whether repeated administration of the CB(1) receptor agonist WIN55,212-2 affected intracranial self-stimulation (ICSS) behavior and induced phenomena of tolerance or sensitization, similar to typical addictive drugs. Rats received intraperitoneal injections of vehicle for 5 days, vehicle or WIN55,212-2 (0.1, 0.3 or 1mg/kg) for 20 subsequent days, and vehicle for 5 additional days. Thresholds for ICSS were measured before and after each injection. The initial five injections of vehicle did not affect ICSS thresholds. WIN55,212-2 (1mg/kg) significantly increased ICSS thresholds from the first day of administration, an effect that remained stable across the subsequent days of administration. During the 5 additional days, where WIN55,212-2 was substituted with vehicle, rats demonstrated a conditioned increase in postinjection thresholds that was significant the first 3 days of this period. These findings indicate that repeated WIN55,212-2 administration elicited a sustained increase in ICSS, i.e., phenomena of tolerance or sensitization were not observed. The present data demonstrate cannabinoid-predictive stimuli that may gain affective salience and play an important role in maintaining cannabinoid administration.

Hyperlocomotion and paw tremors are two highly quantifiable signs of SR141716-precipitated withdrawal from delta9-tetrahydrocannabinol in C57BL/6 mice

Increasing evidence suggests that marijuana abstinence leads to clinically significant withdrawal symptoms in humans. In mouse models, following chronic treatment with delta9-tetrahydrocannabinol (THC), administration of the selective cannabinoid CB1 receptor antagonist SR141716 (rimonabant) elicited varying behavioral responses, depending on mouse strain and dosing regimen. In the present study, C57BL/6 mice were injected s.c. with THC (25 mg/kg) or vehicle twice daily for 4.5 days. SR141716 (15 mg/kg) was administrated i.p. 4 h following the last THC treatment. During a 2-h observation period immediately following the SR141716 challenge, the total locomotor, ambulatory and stereotypic activities of THC-treated mice were 4.1, 3.3, and 3.8 times those of vehicle-treated mice, respectively. The number of paw tremors elicited in THC-treated mice was 111+/-11 during the 45 min immediately following SR141716, whereas only 1.1+/-0.4 was associated with vehicle-treated animals. In contrast, the number of scratching bouts was higher in vehicle-treated (182+/-20) vs THC-treated (17+/-4) mice. The present study is the first to demonstrate hyperlocomotion as an explicit sign of THC abstinence in mice. Together with paw tremors, the two unambiguous withdrawal signs may permit highly quantitative investigation of THC abstinence in C57BL/6 mice and may facilitate investigation of the mechanisms involved via both pharmacological and genetic manipulations, and ultimately potential treatments for cannabis dependence.

Association between ABCB1 C3435T polymorphism and increased risk of cannabis dependence

Prolonged cannabis use has a significant impact on health and well-being. Genetic factors are known to influence cannabis dependence, but few specific genetic markers have been identified. ABCB1 polymorphisms are known to modify drug pharmacokinetics but have yet to be studied for their role in generating and maintaining cannabis dependence. The objective of this study is to determine if ABCB1 C3435T polymorphism may represent an independent genetic marker for cannabis dependence risk. An open bi-centric association study was conducted in two French Addiction Centres. Caucasian patients diagnosed with isolated cannabis dependence were compared with healthy age-matched controls for socio-demographic, clinical and genetic data using chi-square test, Fisher's exact test, or Mann-Whitney U test. Independent association between ABCB1 C3435T SNP marker and cannabis dependence was evaluated using multiple logistic regression analysis. Versus controls (n=40), patients with cannabis dependence (n=40) had a significantly higher 3435C allele frequency (62.5% versus 43.8% respectively, P=0.017) and CC genotype (50% versus 20%, P=0.005, OR=4.00 [1.50-10.60]). Multiple logistic regression analysis of the C3435T SNP and variables identified in univariate analyses indicated that the CC genotype was independently associated with cannabis dependence (P=0.045, OR=6.61 [1.05-46.58]). This is the first time a significant specific genetic marker has been shown in cannabis dependence. ABCB1 polymorphisms may alter Delta9THC distribution, its psychoactive effects and individual vulnerability to dependence. These results pave the way to a new pharmacogenetic hypothesis in cannabis dependence.

PKCepsilon regulates behavioral sensitivity, binding and tolerance to the CB1 receptor agonist WIN55,212-2

The cannabinoid CB1 receptor (CB1) is one of the most abundant G protein-coupled receptors in the brain, but little is known about the mechanisms that modulate CB1 receptor signaling. Here, we show that inhibition or null mutation of the epsilon isozyme of protein kinase C (PKCepsilon) selectively enhances behavioral responses to the CB1 agonist WIN55,212-2 in mice, but not to the structurally unrelated CB1 agonist CP55,940. Binding affinity for [(3)H] WIN55,212-2 was increased in brain membranes from PKCepsilon(-/-) mice compared with PKCepsilon(+/+) mice. There was no difference in binding of the inverse agonist [(3)H] SR141716A. In addition, repeated administration of WIN55,212-2 produced greater analgesic and thermal tolerance in PKCvarepsilon(-/-) mice compared with PKCepsilon(+/+)mice. These results indicate that PKCvarepsilon selectively regulates behavioral sensitivity, CB1 receptor binding and tolerance to WIN55,212-2.

The adverse health effects of cannabis use: what are they, and what are their implications for policy?

BACKGROUND

The adverse health effects of cannabis are a source of contention in debates about policies towards the drug.

METHODS

This paper provides a review of epidemiological evidence on the major adverse health effects of cannabis use and considers its implications for policy.

RESULTS

The evidence strongly suggests that cannabis can adversely affect some users, especially adolescents who initiate use early and young adults who become regular users. These adverse effects probably include increased risks of: motor vehicle crashes, the development of cannabis dependence, impaired respiratory function, cardiovascular disease, psychotic symptoms, and adverse outcomes of adolescent development, namely, poorer educational outcomes and an increased likelihood of using other illicit drugs.

CONCLUSIONS

Politically, evidence of adverse health effects favours the status quo in developed countries like Australia where cannabis policy has been framed by the media as a choice between two views: (1) either cannabis use is largely harmless to most users and so we should legalize, or at the very least decriminalize its use; or (2) it harms some of its users so we should continue to prohibit its use.

Effects of rimonabant on the reinstatement of nicotine-conditioned place preference by drug priming in rats

Drug addiction is a chronic disorder characterized by a relatively high rate of relapse even after long period of abstinence. In the present study, we used the conditioned place preference (CPP) paradigm to investigate the establishment, extinction, reinstatement and cross-reinstatement of nicotine-induced place conditioning in rats. Nicotine produced a place preference to the initially less-preferred compartment paired with its injections during conditioning (0.5mg/kg, i.p., three drug sessions). Once established, nicotine CPP was extinguished by repeated testing. Following this extinction phase, the reinstatement of CPP was investigated. Nicotine-experienced rats were challenged with nicotine (0.5mg/kg, i.p.) or morphine (10mg/kg, i.p.). These priming injections of both drugs induced a marked preference for the compartment previously paired with nicotine. Furthermore, the objective of the present study was to evaluate the efficacy of CB1 cannabinoid receptor antagonist rimonabant (0.5, 1 and 2mg/kg, i.p.) in blocking the reinstatement of nicotine-induced CPP provoked by nicotine and morphine. It was shown that rimonabant attenuated the reinstatement of nicotine-conditioned response induced by both drugs. The outcome of our studies may suggest that CB1 receptor antagonists may become a promising target for effective pharmacotherapy of tobacco addiction and polydrug abuse.

Drug addiction

Many drugs of abuse, including cannabinoids, opioids, alcohol and nicotine, can alter the levels of endocannabinoids in the brain. Recent studies show that release of endocannabinoids in the ventral tegmental area can modulate the reward-related effects of dopamine and might therefore be an important neurobiological mechanism underlying drug addiction. There is strong evidence that the endocannabinoid system is involved in drug-seeking behavior (especially behavior that is reinforced by drug-related cues), as well as in the mechanisms that underlie relapse to drug use. The cannabinoid CB(1) antagonist/inverse agonist rimonabant has been shown to reduce the behavioral effects of stimuli associated with drugs of abuse, including nicotine, alcohol, cocaine, and marijuana. Thus, the endocannabinoid system represents a promising target for development of new treatments for drug addiction.