Gregorio Cossu

Cannabinoid CB1 receptor knockout mice fail to self-administer morphine but not other drugs of abuse.

The rewarding effects of morphine, cocaine, amphetamine and nicotine were evaluated in CB1 receptor knockout mice by means of an intravenous self-administration model. Experiments were carried out on drug-naive animals using a nose-poking response (NPR)-like as operandum. The results of the present study indicate that morphine did not induce intravenous self-administration in mutant CB1 receptor knockout mice, whereas it was significantly self-administered by the corresponding wild type mice. On the contrary, cocaine, amphetamine and nicotine were self-administered to the same extent by both wild type and CB1 receptor knockout mice. These data clearly indicate that the CB1 cannabinoid receptor is essential not only for the expression of cannabinoid reinforcing effects but also for the modulation of morphine rewarding effects. The specificity of such interaction is supported by the finding that contrary to morphine, cocaine, d-amphetamine and nicotine were self-administered by mice at the same extent either in presence or in absence of the CB1 receptor.

Self-administration of the cannabinoid receptor agonist WIN 55,212-2 in drug-naive mice

Marijuana is one of the most widely used illicit recreational drugs. However, contrary to the majority of drugs abused by humans, there is a general opinion that rewarding effects are not manifested by animals. We studied a synthetic cannabinoid agonist WIN 55,212-2 using an intravenous self-administration model in drug-naive mice. The results of this study show that WIN 55,212-2 was intravenously self-administered by mice in a concentration-dependent manner according to a bell-shaped curve. Thus, self-administration of WIN 55,212-2 significantly increased, with respect to the vehicle self-administration control group, at concentrations of 0.5 and 0.1 mg/kg per injection. However, at WIN 55,212-2 concentration of 0.5 mg/kg per injection, self-administration significantly decreased. The results obtained show how WIN 55,212-2 is able to elicit both rewarding and aversive effects depending on the concentration used. Pretreatment of mice with the cannabinoid CB1 receptor antagonist SR 141716A (0.25 mg/kg, i.p.) completely prevented WIN 55,212-2 (0.1 mg/kg per injection) self-administration, indicating that WIN 55,212-2 rewarding effects are specifically mediated by cannabinoid CB1 receptors.

Cannabinoid mechanism in reinstatement of heroin‐seeking after a long period of abstinence in rats

Because opioid and cannabinoid systems have been reported to interact in the modulation of addictive behaviour, this study was aimed at investigating the ability of cannabinoid agents to reinstate or prevent heroin‐seeking behaviour after a prolonged period of extinction. In rats previously trained to self‐administer heroin intravenously, non‐contingent non‐reinforced priming administrations of heroin and cannabinoids were presented after long‐term extinction, and lever pressing following injections was observed. Results showed that: (i) intravenous priming infusions of heroin (0.1 and 0.2 mg/kg) lead to reinstatement of drug‐seeking behaviour; (ii) intraperitoneal priming injections of the central cannabinoid receptor agonists R‐(+)‐(2,3‐dihydro‐5‐methyl‐3‐[(4‐morpholinyl)methyl]pyrol[1,2,3‐de]‐1,4‐benzoxazinyl) (1‐naphthalenyl)methanonemesylate (WIN 55,212‐2, 0.15 and 0.3 mg/kg) and (–)‐cis‐3‐[2‐hydroxy‐4(1,1‐dimethyl‐heptyl)phenyl]‐trans‐4‐(3‐hydroxypropyl) cyclohexanol (CP 55,940, 0.05 and 0.1 mg/kg), but not Δ9‐tetrahydrocannabinol (Δ9‐THC, 0.1–1.0 mg/kg), effectively restored heroin‐seeking behaviour; (iii) intraperitoneal priming injection of the central cannabinoid receptor antagonist N‐(piperidin‐1‐yl)‐5‐(4‐chlorophenyl)‐1‐(2,4‐dichloro‐phenyl)4‐methyl‐1H‐pyrazole‐3‐carboxamide (SR 141716A, 0.3 mg/kg) did not reinstate responding, but (iv) completely prevented heroin‐induced reinstatement of drug‐seeking behaviour. Moreover, heroin‐seeking behaviour was still present for a few days following cannabinoid primings, indicating a long‐lasting effect of cannabinoids on responding for heroin. These findings indicate that relapse to heroin after an extended drug‐free period is triggered by cannabinoid agonists and that SR 141716A prevents drug‐seeking behaviour, suggesting that the use of the cannabinoid antagonist could have some therapeutic benefits in heroin‐induced relapse.

Endocannabinoid system and opioid addiction: Behavioural aspects

Cannabinoids produce a variety of pharmacological effects very similar to those elicited by opioids. Direct and indirect interactions with opioid system have been proposed to explain some cannabinoid effects such as analgesia and attenuation of opioid-withdrawal syndrome, and evidence has been provided in support to the notion that rewarding properties of cannabinoids and opioids might be functionally linked. In particular, a growing body of studies points to an important role of the endogenous cannabinoid system in the modulation of opioid rewarding and addictive effects. The current review examines progresses in the past few years in the elucidation of cannabinoid-opioid interactions in drug abuse and dependence, focusing on recent findings from behavioural studies using different animal models of addiction. Specifically, here we review data on the behavioural aspects (i.e., drug abuse, dependence, tolerance, sensitization, relapse and drug vulnerability) of the specific, often reciprocal, cross-talk between cannabinoids and opioids with particular reference to the role of the endocannabinoid system in opioid addiction. The potential biochemical mechanisms involved in these pharmacological interactions are discussed together with possible therapeutic implications in the pharmacotherapy of opioid dependence. However, individuation of the precise anatomical substrates and molecular mechanisms of such interaction still remains a complex and challenging field for future research.

CB1 receptor agonist and heroin, but not cocaine, reinstate cannabinoid-seeking behaviour in the rat.

We recently provided evidence for a functional link between cannabinoid and opioid endogenous systems in relapse to heroin‐seeking behaviour in rats. In the present study, we aimed at investigating whether the previously observed cross‐talk between cannabinoids and opioids could be extended to mechanisms underlying relapse to cannabinoid‐seeking behaviour after a prolonged period of abstinence. In rats previously trained to intravenously self‐administer the synthetic cannabinoid receptor (CB1) agonist WIN 55,212‐2 (12.5 μg kg−1 inf−1) under a fixed ratio (FR1) schedule of reinforcement, noncontingent nonreinforced intraperitoneal (i.p.) priming injections of the previously self‐administered CB1 agonist (0.25 and 0.5 mg kg−1) as well as heroin (0.5 mg kg−1), but not cocaine (10 mg kg−1), effectively reinstate cannabinoid‐seeking behaviour following 3 weeks of extinction. The selective CB1 receptor antagonist SR 141716A (0.3 mg kg−1 i.p.) does not reinstate responding when given alone, but completely prevents the cannabinoid‐seeking behaviour triggered by WIN 55,212‐2 or heroin primings. The nonselective opioid antagonist naloxone (1 mg kg−1 i.p.) has no effect on operant behaviour per sè, but significantly blocks cannabinoid‐ and heroin‐induced reinstatement of cannabinoid‐seeking behaviour. These results provide the first evidence of drug‐induced reinstatement of cannabinoid‐seeking behaviour, and further strengthen previous findings on a cross‐talk between the endogenous cannabinoid and opioid systems in relapse mechanisms to drug‐seeking.

Cannabinoid CB1 antagonist SR 141716A attenuates reinstatement of heroin self-administration in heroin-abstinent rats

Rats with a previous history of heroin self-administration were studied to assess interactions occurring between cannabinoids and opioids in an animal model of reinstatement of heroin-seeking behaviour. Rats were trained to self-administer heroin and after a long-term extinction were primed with one of the following non-contingent non-reinforced drug administrations: saline (or vehicle), heroin, synthetic cannabinoid CB(1) receptor agonists (WIN 55,212-2 or CP 55,940), opioid antagonist (naloxone) or CB(1) antagonist (SR 141716A), alone or in combination. After primings, lever-pressing activity was recorded and compared to those observed during previous phases of training and extinction. Results of this study showed that (i) priming injections of heroin (0.1 mg/kg) as well as CB(1) agonists WIN 55,212-2 (0.15 or 0.30 mg/kg) and CP 55,940 (0.05 or 0.1 mg/kg) completely restore heroin-seeking behaviour; (ii) primings of naloxone (1 mg/kg) and SR 141716A (0.3 mg/kg) had no effect when administered alone; (iii) heroin-induced reinstatement was fully prevented by pre-treatment with either naloxone or SR 141716A; (iv) pre-treatment with SR 141716A significantly reduced WIN 55,212-2 and CP 55,940 priming effects. These results suggest that cannabinoid CB(1) receptors play an important role in the mechanisms underlying relapse to heroin-seeking and depict CB(1) antagonists as possible therapeutic agents for use in the prevention of relapse to heroin abuse.

Intravenous self-administration of gamma-hydroxybutyric acid in drug-naive mice

The reinforcing effects of gamma-hydroxybutyric acid (GHB) were studied by means of intravenous self-administration in drug-naive mice. GHB self-administration was concentration-dependent (0.01-0.5 mg/kg/inj.) according to a bell-shaped curve. Pretreatment with the specific GHB receptor antagonist NCS-382 at a dose of 12.5 mg/kg i.p. completely antagonized the reinforcing effects of GHB. These data suggest that GHB is able to induce reinforcing effects in mice and support the hypothesis of an abuse liability of this drug.

Strain and schedule-dependent differences in the acquisition, maintenance and extinction of intravenous cannabinoid self-administration in rats

Cannabinoids have been reported to sustain self-administration in laboratory animals; however, genetic differences and environmental factors critical in the initiation and retention of such behaviour are yet to be defined. This study investigated the acquisition, maintenance and extinction of self-administration of the cannabinoid CB1 receptor agonist WIN 55,212-2 (6.25-25 microg/kg/inf) in Long Evans, Lister Hooded and Sprague-Dawley rats under a continuous schedule of reinforcement and two different response-like operanda, nose-poking and lever-pressing. Results showed that Long Evans and Lister Hooded, but not Sprague Dawley, rats acquired and retained stable cannabinoid self-administration behaviour under both modus operandi, as defined by significant differences between responding in the active versus the inactive hole/lever. In rats developing firm self-administration, substitution of saline for WIN 55,212-2 extinguished the responding, supporting the notion that cannabinoids may serve as a positive reinforcer in laboratory animals. Nevertheless, significant differences among strains and responding modalities were observed in the percentage of acquisition, amount of drug intake during maintenance and timing of extinction. In addition, no significant strain differences were found in motor response to WIN 55,212-2 (0.3 and 3.0 mg/kg), thus excluding that strain differences observed during cannabinoid self-administration could be related to different cannabinoid-induced locomotor effects.

Baclofen prevents drug-induced reinstatement of extinguished nicotine-seeking behaviour and nicotine place preference in rodents

The gamma-aminobutyric acid(GABA)-B receptor agonist baclofen is known to reduce drug intake in both animals and humans and to prevent reinstatement of cocaine-, opioid-, and alcohol-seeking in rats after a period of extinction, but its effect on nicotine reinstatement is unknown. This study investigated the effect of baclofen on nicotine-seeking reinstatement both using the extinction/reinstatement model of nicotine self-administration and conditioned place preference (CPP). Results showed that in rats previously trained to intravenously self-administer nicotine (30 microg/kg/inf) under a FR-1 schedule of reinforcement, acute nicotine (0.15 mg/kg) priming effectively reinstates nicotine-seeking behaviour following extinction. At doses used in this study (up to 2.5 mg/kg) baclofen alone did not affect locomotor activity and did not reinstate responding. However, baclofen dose-dependently attenuated drug-induced reinstatement of nicotine-seeking in rats. Moreover, baclofen (1.25 mg/kg) completely blocked nicotine-induced reinstatement of extinguished nicotine (0.3 mg/kg) CPP in mice. Altogether, our results showed that baclofen is able to antagonise reinstatement of nicotine-seeking and CPP triggered by nicotine primings, suggesting its potential clinical utility as an anti-relapse agent.

Rewarding properties of gamma-hydroxybutyric acid: an evaluation through place preference paradigm.

Abstract Gamma-hydroxybutyric acid (GHB), a putative neurotransmitter or neuromodulator found in the mammalian brain, has been successfully used in clinical practice to alleviate both alcohol and opiate withdrawal symptoms. In the present study we used a conditioned place preference (CPP) paradigm to investigate whether GHB possesses rewarding properties in rats. In order to exclude possible artifacts due to experimental design, we evaluated the possibility of a shift in preference when rats are conditioned either on their non-preferred side or on a randomly assigned side of conditioning. In both experiments GHB was seen to induce CPP. Although to date the physiological role of this compound still remains unclear, there is no doubt that GHB, further to its proven effect on alcohol and opiates, possesses rewarding properties of its own. The abuse liability afforded by this drug suggests the use of particular caution in handling GHB as a clinically useful drug.