Attenuation of cue-induced heroin-seeking behavior by cannabinoid CB1 antagonist infusions into the nucleus accumbens core and prefrontal cortex, but not basolateral amygdala

SNAP25 differentially contributes to Gi/o-coupled receptor function at glutamatergic synapses in the nucleus accumbens

The nucleus accumbens (NAc) guides reward-related motivated behavior implicated in pathological behavioral states, including addiction and depression. These behaviors depend on the precise neuromodulatory actions of G_i/o-coupled G-protein-coupled receptors (GPCRs) at glutamatergic synapses onto medium spiny projection neurons (MSNs). Previous work has shown that discrete classes of G_i/o-coupled GPCR mobilize Gβγ to inhibit vesicular neurotransmitter release via t-SNARE protein, SNAP25. However, it remains unknown which Gαi/o systems in the NAc utilize Gβγ-SNARE signaling to dampen glutamatergic transmission. Utilizing patch-clamp electrophysiology and pharmacology in a transgenic mouse line with a C-terminal three-residue deletion of SNAP25 (SNAP25Δ3) weaking the Gβγ-SNARE interaction, we surveyed a broad cohort of G_i/o-coupled GPCRs with robust inhibitory actions at glutamatergic synapses in the NAc. We find that basal presynaptic glutamate release probability is reduced in SNAP25Δ3 mice. While κ opioid, CB1, adenosine A1, group II metabotropic glutamate receptors, and histamine H3 receptors inhibit glutamatergic transmission onto MSNs independent of SNAP25, we report that SNAP25 contributes significantly to the actions of GABA_B, 5-HT1_B/D, and μ opioid receptors. These findings demonstrate that presynaptic G_i/o-coupled GPCRs recruit heterogenous effector mechanisms at glutamatergic synapses in the NAc, with a subset requiring SNA25-dependent Gβγ signaling.

Negative allosteric modulation of CB1 cannabinoid receptor signaling suppresses opioid-mediated reward

Blockade of cannabinoid type 1 (CB₁)-receptor signaling decreases the rewarding properties of many drugs of abuse and has been proposed as an anti-addiction strategy. However, psychiatric side-effects limit the clinical potential of orthosteric CB₁ antagonists. Negative allosteric modulators (NAMs) represent a novel and indirect approach to attenuate CB₁ signaling by decreasing affinity and/or efficacy of CB₁ ligands. We hypothesized that a CB₁-NAM would block opioid reward while avoiding the unwanted effects of orthosteric CB₁ antagonists. GAT358, a CB₁-NAM, failed to elicit cardinal signs of direct CB₁ activation or inactivation when administered by itself. GAT358 decreased catalepsy and hypothermia but not antinociception produced by the orthosteric CB₁ agonist CP55,940, suggesting that a CB₁-NAM blocked cardinal signs of CB₁ activation. Next, GAT358 was evaluated using in vivo assays of opioid-induced dopamine release and reward in male rodents. In the nucleus accumbens shell, a key component of the mesocorticolimbic reward pathway, morphine increased electrically-evoked dopamine efflux and this effect was blocked by a dose of GAT358 that lacked intrinsic effects on evoked dopamine efflux. Moreover, GAT358 blocked morphine-induced reward in a conditioned place preference (CPP) assay without producing reward or aversion alone. GAT358-induced blockade of morphine CPP was also occluded by GAT229, a CB₁ positive allosteric modulator (CB₁-PAM), and absent in CB₁-knockout mice. Finally, GAT358 also reduced oral oxycodone (but not water) consumption in a two-bottle choice paradigm. Our results support the therapeutic potential of CB₁-NAMs as novel drug candidates aimed at preventing opioid reward and treating opioid abuse while avoiding unwanted side-effects.

Altered brain intrinsic functional hubs and connectivity associated with relapse risk in heroin dependents undergoing methadone maintenance treatment: A resting-state fMRI study

BACKGROUND

The neural substrates underlying the relapse behavior of heroin dependents (HD) who received long-term methadone maintenance treatment (MMT) have yet to be thoroughly expounded. This study investigated the relapse-related intrinsic functional hubs of HD and their functional integration feature at whole brain network level.

METHODS

57 male HD receiving MMT and 49 matched healthy controls (HC) were enrolled. All of the subjects received resting-state functional magnetic resonance imaging scan. And the 57 patients were assigned a 26-month follow-up for collecting illegal drug use information. Of them, 11 were non-relapsers and 46 relapsers. We analyzed the voxel-based degree centrality (DC) to reveal the differences in nodule centrality between HD and HC, conducted Pearson partial-correlation analysis to confirm the relationship between relapse frequency and DC value of the nodes with significant intergroup differences, and finally compared the functional connectivity (FC) of the relapse-related hubs between non-relapsers and relapsers.

RESULTS

We found the DC values of right insula and left nucleus accumbens (NAc) were negatively correlated with relapse frequency. Compared with the non-relapsers, the relapsers had a significant decreased FC between left NAc and inhibitory control circuitry, including left dorsolateral prefrontal cortex, left inferior frontal gyrus and motor regions.

CONCLUSIONS

These findings suggest that the neural substrates of relapse vulnerability in HD undergoing MMT are the intrinsic functional hubs of introceptive and reward systems and the latter modulates relapse behavior via interaction with inhibitory control circuit.

Non-Opioid Treatments for Opioid Use Disorder: Rationales and Data to Date

Opioid use disorder (OUD) represents a major public health problem that affects millions of people in the USA and worldwide. The relapsing and recurring aspect of OUD, driven by lasting neurobiological adaptations at different reward centres in the brain, represents a major obstacle towards successful long-term remission from opioid use. Currently, three drugs that modulate the function of the opioidergic receptors, methadone, buprenorphine and naltrexone have been approved by the US Food and Drug Administration (FDA) to treat OUD. In this review, we discuss the limitations and challenges associated with the current maintenance and medication-assisted withdrawal strategies commonly used to treat OUD. We further explore the involvement of glutamatergic, endocannabinoid and orexin signaling systems in the development, maintenance and expression of addiction-like behaviours in animal models of opioid addiction, and as potential and novel targets to expand therapeutic options to treat OUD. Despite a growing preclinical literature highlighting the role of these potential targets in animal models of opioid addiction, clinical and translational studies for novel treatments of OUD remain limited and inconclusive. Further preclinical and clinical investigations are needed to expand the arsenal of primary treatment options and adjuncts to maximise efficacy and prevent relapse.

Endocannabinoid LTD in Accumbal D1 Neurons Mediates Reward-Seeking Behavior

The nucleus accumbens (NAc) plays a key role in drug-related behavior and natural reward learning. Synaptic plasticity in dopamine D1 and D2 receptor medium spiny neurons (MSNs) of the NAc and the endogenous cannabinoid (eCB) system have been implicated in reward seeking. However, the precise molecular and physiological basis of reward-seeking behavior remains unknown. We found that the specific deletion of metabotropic glutamate receptor 5 (mGluR5) in D1-expressing MSNs (D1^miRmGluR5 mice) abolishes eCB-mediated long-term depression (LTD) and prevents the expression of drug (cocaine and ethanol), natural reward (saccharin), and brain-stimulation-seeking behavior. In vivo enhancement of 2-arachidonoylglycerol (2-AG) eCB signaling within the NAc core restores both eCB-LTD and reward-seeking behavior in D1^miRmGluR5 mice. The data suggest a model where the eCB and glutamatergic systems of the NAc act in concert to mediate reward-seeking responses.

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mGluR5-D1-CB1-induced eCB-LTD mediates drugs of abuse and natural reward seeking

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eCB-LTD in D2-MSNs plays no important role in processing of reward-seeking responses

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Loss of eCB-LTD is a consequence of higher MAGL activity and lower CB1R expression

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Acute drug administration stops craving for alternative rewards on following days

Role of endocannabinoid system in dopamine signalling within the reward circuits affected by chronic pain

The association between chronic pain, depression and anxiety has gained particular attention due to high rates of comorbidity. Recent data demonstrated that the mesolimbic reward circuitry is involved in the pathology of chronic pain. Interestingly, the mesolimbic reward circuit participates both in pain perception and in pain relief. The endocannabinoid system (ECS) has emerged as a highly relevant player involved in both pain perception and reward processing. Targeting ECS could become a novel treatment strategy for chronic pain patients. However, little is known about the underlying mechanisms of action of cannabinoids at the intersection of neurochemical changes in reward circuits and chronic pain. Because understanding the benefits and risks of cannabinoids is paramount, the aim of this review is to evaluate the state-of-art knowledge about the involvement of the ECS in dopamine signalling within the reward circuits affected by chronic pain.

Cannabinoid CB1 receptor neutral antagonist AM4113 inhibits heroin self-administration without depressive side effects in rats

Cannabinoid CB₁ receptors (CB₁Rs) have been shown to be a promising target in medication development for the treatment of addiction. However, clinical trials with SR141716A (rimonabant, a selective CB₁R antagonist/inverse agonist) for the treatment of obesity and smoking cessation failed due to unwanted side effects, such as depression, anxiety, and suicidal tendencies. Recent preclinical studies suggest that the neutral CB₁R antagonist AM4113 may retain the therapeutic anti-addictive effects of SR141716A in nicotine self-administration models and possibly has fewer unwanted side effects. However, little is known about whether AM4113 is also effective for other drugs of abuse, such as opioids and psychostimulants, and whether it produces depressive side effects similar to SR141716A in experimental animals. In this study, we demonstrated that systemic administration of AM4113 (3 and 10 mg/kg) dose-dependently inhibited the self-administration of intravenous heroin but not cocaine or methamphetamine, whereas SR141716A (3 and 10 mg/kg) dose-dependently inhibited the self-administration of heroin and methamphetamine but not cocaine. In the electrical brain-stimulation reward (BSR) paradigm, SR141716A (3 and 10 mg/kg) dose-dependently increased the BSR stimulation threshold (i.e., decreased the stimulation reward), but AM4113 had no effect on BSR at the same doses, suggesting that SR141716A may produce aversive effects while AM4113 may not. Together, these findings show that neutral CB₁R antagonists such as AM4113 deserve further research as a new class of CB₁R-based medications for the treatment of opioid addiction without SR141716A-like aversive effects.

Relapse to opioid seeking in rat models: behavior, pharmacology and circuits

Lifetime relapse rates remain a major obstacle in addressing the current opioid crisis. Relapse to opioid use can be modeled in rodent studies where drug self-administration is followed by a period of abstinence and a subsequent test for drug seeking. Abstinence can be achieved through extinction training, forced abstinence, or voluntary abstinence. Voluntary abstinence can be accomplished by introducing adverse consequences of continued drug self-administration (e.g., punishment or electric barrier) or by introducing an alternative nondrug reward in a discrete choice procedure (drug versus palatable food or social interaction). In this review, we first discuss pharmacological and circuit mechanisms of opioid seeking, as assessed in the classical extinction-reinstatement model, where reinstatement is induced by reexposure to the self-administered drug (drug priming), discrete cues, discriminative cues, drug-associated contexts, different forms of stress, or withdrawal states. Next, we discuss pharmacological and circuit mechanisms of relapse after forced or voluntary abstinence, including the phenomenon of "incubation of heroin craving" (the time-dependent increases in heroin seeking during abstinence). We conclude by discussing future directions of preclinical relapse-related studies using opioid drugs.

Role of prefrontal cortex in the extinction of drug memories

It has been recognized that drug addiction engages aberrant process of learning and memory, and substantial studies have focused on developing effective treatment to erase the enduring drug memories to reduce the propensity to relapse. Extinction, a behavioral intervention exposing the individuals to the drug-associated cues repeatedly, can weaken the craving and relapse induced by drug-associated cues, but its clinic efficacy is limited. A clear understanding of the neuronal circuitry and molecular mechanism underlying extinction of drug memory will facilitate the successful use of extinction therapy in clinic. As a key component of mesolimbic system, medial prefrontal cortex (mPFC) has received particular attention largely in that PFC stands at the core of neural circuits for memory extinction and manipulating mPFC influences extinction of drug memories and subsequent relapse. Here, we review the recent advances in both animal models of drug abuse and human addicted patients toward the understanding of the mechanistic link between mPFC and drug memory, with particular emphasis on how mPFC contributes to the extinction of drug memory at levels ranging from neuronal architecture, synaptic plasticity to molecular signaling and epigenetic regulation, and discuss the clinic relevance of manipulating the extinction process of drug memory to prevent craving and relapse through enhancing mPFC function.

Functional Relevance of Endocannabinoid-Dependent Synaptic Plasticity in the Central Nervous System

The endocannabinoid (eCB) signaling system plays a key role in short-term and long-term synaptic plasticity in brain regions involved in various neural functions ranging from action selection to appetite control. This review will explore the role of eCBs in shaping neural circuit function to regulate behaviors. In particular, we will discuss the behavioral consequences of eCB mediated long-term synaptic plasticity in different brain regions. This review brings together evidence from in vitro and ex vivo studies and points out the need for more in vivo studies.

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.

Effects of menthol and its interaction with nicotine-conditioned cue on nicotine-seeking behavior in rats

RATIONALE

Increasing clinical evidence suggests that menthol, a significant flavoring additive in tobacco products, may contribute to smoking and nicotine dependence. Relapse to smoking behavior presents a formidable challenge for the treatment of tobacco addiction. An unresolved issue is whether the mentholation of tobacco products precipitates relapse to tobacco use in abstinent smokers.

OBJECTIVES

The present study examined the effects of menthol on the perseverance and relapse of nicotine-seeking behavior in rats.

METHODS

Male Sprague-Dawley rats were trained to press a lever for intravenous nicotine self-administration (0.03 mg/kg/infusion) under a fixed-ratio five schedule of reinforcement. Each nicotine infusion was signaled by the presentation of a sensory stimulus that was established as a discrete nicotine-conditioned cue. Five minutes prior to the sessions, the rats received an intraperitoneal injection of menthol (0.1 mg/kg) or vehicle. In the subsequent extinction test sessions, nicotine was unavailable with or without menthol and/or the nicotine-conditioned cue. The reinstatement tests were performed the following day after the extinction criterion was met. Menthol was also tested on food-seeking responses. In a subset of nicotine-trained rats, a transient receptor potential melastatin 8 (TRPM8) antagonist RQ-00203078 was given prior to menthol administration.

RESULTS

Continued administration of menthol sustained responses on the previously active and nicotine-reinforced lever in the extinction tests. The readministration of menthol after extinction reinstated active lever responses. In both the extinction and the reinstatement tests, a combination of pre-session menthol administration and cue representation during the session produced a more robust behavioral effect than either menthol or the cue alone. No such effects of menthol was observed in food trained rats. RQ-00203078 did not change menthol effect on nicotine seeking.

CONCLUSION

These data demonstrated that menthol specifically sustained and reinstated nicotine-seeking behavior, and this effect was independent of TRPM8 activity. These findings suggest that menthol in most tobacco products, even not menthol labeled, may contribute to the perseverance of and relapse to tobacco-seeking behavior.

Differential expression of endocannabinoid system-related genes in the dorsal hippocampus following expression and reinstatement of morphine conditioned place preference in mice

The endocannabinoid signaling plays a critical role in mediating rewarding effects to morphine. The relative stability for the expression and reinstatement of morphine conditioned place preference (CPP) suggests the involvement of differential neuroadaptations in learned associations between environmental cues and morphine. Changes in gene expression in hippocampus through the endogenous cannabinoid system (eCB) may accompany and mediate the development of such neuroadaptations to repeated morphine stimulation. To test this possibility, we systematically compared the expression of eCB-related genes in the dorsal hippocampus following the expression, extinction, and reinstatement of morphine CPP using quantitative RT-PCR analyses. We found that expression of morphine CPP was associated with significant increases in mRNA expression for the primary clearance routes for anandamide (AEA) and 2-AG (fatty acid amide hydrolase [FAAH] and monoacylglycerol lipase [MAGL], respectively), but with reductions in cannabinoid 1 receptors (CB1R) and CB2R in dorsal hippocampus following the expression of CPP. However, our results indicated that decreased in MAGL and increased CB1R mRNA levels were accompanied with morphine CPP reinstatement. No significant changes in mRNA expression for enzymes involved in AEA and 2-AG biosynthesis (N-acylphosphatidylethanolamine phospholipase D [NAPEPLD] and diacylglycerol lipase-α/β [DAGLα/β], respectively) were found in all conditions. These results suggest that differential regulation of the synthesis and/or degradation of the eCB system contribute to the expression and reinstatement of morphine CPP.

The endocannabinoid hydrolysis inhibitor SA-57: Intrinsic antinociceptive effects, augmented morphine-induced antinociception, and attenuated heroin seeking behavior in mice

Although opioids are highly efficacious analgesics, their abuse potential and other untoward side effects diminish their therapeutic utility. The addition of non-opioid analgesics offers a promising strategy to reduce required antinociceptive opioid doses that concomitantly reduce opioid-related side effects. Inhibitors of the primary endocannabinoid catabolic enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) show opioid-sparing effects in preclinical models of pain. As simultaneous inhibition of these enzymes elicits enhanced antinociceptive effects compared with single enzyme inhibition, the present study tested whether the dual FAAH-MAGL inhibitor SA-57 [4-[2-(4-chlorophenyl)ethyl]-1-piperidinecarboxylic acid 2-(methylamino)-2-oxoethyl ester] produces morphine-sparing antinociceptive effects, without major side effects associated with either drug class. SA-57 dose-dependently reversed mechanical allodynia in the constriction injury (CCI) of the sciatic nerve model of neuropathic pain and carrageenan inflammatory pain model. As previously reported, SA-57 was considerably more potent in elevating anandamide (AEA) than 2-arachidonyl glycerol (2-AG) in brain. Its anti-allodynic effects required cannabinoid (CB)₁ and CB₂ receptors; however, only CB₂ receptors were necessary for the anti-edematous effects in the carrageenan assay. Although high doses of SA-57 alone were required to produce antinociception, low doses of this compound, which elevated AEA and did not affect 2-AG brain levels, augmented the antinociceptive effects of morphine, but lacked cannabimimetic side effects. Because of the high abuse liability of opioids and implication of the endocannabinoid system in the reinforcing effects of opioids, the final experiment tested whether SA-57 would alter heroin seeking behavior. Strikingly, SA-57 reduced heroin-reinforced nose poke behavior and the progressive ratio break point for heroin. In conclusion, the results of the present study suggest that inhibition of endocannabinoid degradative enzymes represents a promising therapeutic approach to decrease effective doses of opioids needed for clinical pain control, and may also possess therapeutic potential to reduce opioid abuse.

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

Drug abuse and addiction cause widespread social and public health problems, and the neurobiology underlying drug actions and drug use and abuse is an area of intensive research. Drugs of abuse alter synaptic transmission, and these actions contribute to acute intoxication as well as the chronic effects of abused substances. Transmission at most mammalian synapses involves neurotransmitter activation of two receptor subtypes, ligand-gated ion channels that mediate fast synaptic responses and G protein-coupled receptors (GPCRs) that have slower neuromodulatory actions. The GPCRs represent a large proportion of neurotransmitter receptors involved in almost all facets of nervous system function. In addition, these receptors are targets for many pharmacotherapeutic agents. Drugs of abuse directly or indirectly affect neuromodulation mediated by GPCRs, with important consequences for intoxication, drug taking and responses to prolonged drug exposure, withdrawal and addiction. Among the GPCRs are several subtypes involved in presynaptic inhibition, most of which are coupled to the Gi/o class of G protein. There is increasing evidence that these presynaptic Gi/o-coupled GPCRs have important roles in the actions of drugs of abuse, as well as behaviors related to these drugs. This topic will be reviewed, with particular emphasis on receptors for three neurotransmitters, Dopamine (DA; D1- and D2-like receptors), Endocannabinoids (eCBs; CB1 receptors) and glutamate (group II metabotropic glutamate (mGlu) receptors). The focus is on recent evidence from laboratory animal models (and some evidence in humans) implicating these receptors in the acute and chronic effects of numerous abused drugs, as well as in the control of drug seeking and taking. The ability of drugs targeting these receptors to modify drug seeking behavior has raised the possibility of using compounds targeting these receptors for addiction pharmacotherapy. This topic is also discussed, with emphasis on development of mGlu2 positive allosteric modulators (PAMs).

Going and stopping: Dichotomies in behavioral control by the prefrontal cortex

The rodent dorsal medial prefrontal cortex (PFC), specifically the prelimbic cortex (PL), regulates the expression of conditioned fear and behaviors interpreted as reward-seeking. Meanwhile, the ventral medial PFC, namely the infralimbic cortex (IL), is essential to extinction conditioning in both appetitive and aversive domains. Here we review evidence that supports, or refutes, this "PL-go/IL-stop" dichotomy. We focus on the extinction of conditioned fear and the extinction and reinstatement of cocaine- or heroin-reinforced responding. We then synthesize evidence that the PL is essential for developing goal-directed response strategies, while the IL supports habit behavior. Finally, we propose that some functions of the orbital PFC parallel those of the medial PFC in the regulation of response selection. Integration of these discoveries may provide points of intervention for inhibiting untethered drug seeking in drug use disorders, failures in extinction in Post-traumatic Stress Disorder, or co-morbidities between the two.

Differential roles of medial prefrontal subregions in the regulation of drug seeking

The prefrontal cortex plays an important role in shaping cognition and behavior. Many studies have shown that medial prefrontal cortex (mPFC) plays a key role in seeking, extinction, and reinstatement of cocaine seeking in rodent models of relapse. Subregions of mPFC appear to play distinct roles in these behaviors, such that the prelimbic cortex (PL) is proposed to drive cocaine seeking and the infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction. This dichotomy of mPFC function may be a general attribute, as similar dorsal-ventral distinctions exist for expression vs. extinction of fear conditioning. However, other results indicate that the role of mPFC neurons in reward processing is more complex than a simple PL-seek vs. IL-extinguish dichotomy. Both PL and IL have been shown to drive and inhibit drug seeking (and other types of behaviors) depending on a range of factors including the behavioral context, the drug-history of the animal, and the type of drug investigated. This heterogeneity of findings may reflect multiple subcircuits within each of these PFC areas supporting unique functions. It may also reflect the fact that the mPFC plays a multifaceted role in shaping cognition and behavior, including those overlapping with cocaine seeking and extinction. Here we discuss research leading to the hypothesis that dorsal and ventral mPFC differentially control drug seeking and extinction. We also present recent results calling the absolute nature of a PL vs. IL dichotomy into question. Finally, we consider alternate functions for mPFC that correspond less to response execution and inhibition and instead incorporate the complex cognitive behavior for which the mPFC is broadly appreciated.

Animal models of drug relapse and craving: From drug priming-induced reinstatement to incubation of craving after voluntary abstinence

High rates of relapse to drug use during abstinence is a defining feature of drug addiction. In abstinent drug users, drug relapse is often precipitated by acute exposure to the self-administered drug, drug-associated cues, stress, as well as by short-term and protracted withdrawal symptoms. In this review, we discuss different animal models that have been used to study behavioral and neuropharmacological mechanisms of these relapse-related phenomena. In the first part, we discuss relapse models in which abstinence is achieved through extinction training, including the established reinstatement model, as well as the reacquisition and resurgence models. In the second part, we discuss recent animal models in which drug relapse is assessed after either forced abstinence (e.g., the incubation of drug craving model) or voluntary (self-imposed) abstinence achieved either by introducing adverse consequences to ongoing drug self-administration (e.g., punishment) or by an alternative nondrug reward using a discrete choice (drug vs. palatable food) procedure. We conclude by briefly discussing the potential implications of the recent developments of animal models of drug relapse after voluntary abstinence to the development of medications for relapse prevention.

Endocannabinoid signalling in reward and addiction

Brain endocannabinoid (eCB) signalling influences the motivation for natural rewards (such as palatable food, sexual activity and social interaction) and modulates the rewarding effects of addictive drugs. Pathological forms of natural and drug-induced reward are associated with dysregulated eCB signalling that may derive from pre-existing genetic factors or from prolonged drug exposure. Impaired eCB signalling contributes to dysregulated synaptic plasticity, increased stress responsivity, negative emotional states and cravings that propel addiction. Understanding the contributions of eCB disruptions to behavioural and physiological traits provides insight into the eCB influence on addiction vulnerability.

The endocannabinoid system regulates synaptic transmission in nucleus accumbens by increasing DAGL-α expression following short-term morphine withdrawal

BACKGROUND AND PURPOSE

The endocannabinoid (eCB) system is involved in pathways that regulate drug addiction and eCB-mediated synaptic plasticity has been linked with addictive behaviours. Here, we investigated the molecular mechanisms underlying the changes in eCB-dependent synaptic plasticity in the nucleus accumbens core (NAcc) following short-term withdrawal from repeated morphine treatment.

EXPERIMENTAL APPROACH

Conditioned place preference (CPP) was used to evaluate the rewarding effects of morphine in rats. Evoked inhibitory postsynaptic currents of medium spiny neurons in NAcc were measured using whole-cell patch-clamp recordings. Changes in depolarization-induced suppression of inhibition (DSI) in the NAcc were assessed to determine the effect of short-term morphine withdrawal on the eCB system. To identify the potential modulation mechanism of short-term morphine withdrawal on the eCB system, the expression of diacylglycerol lipase α (DGL-α) and monoacylglycerol lipase was detected by Western blot analysis.

KEY RESULTS

Repeated morphine administration for 7 days induced stable CPP. Compared with the saline group, the level of DSI in the NAcc was significantly increased in rats after short-term morphine withdrawal. Furthermore, this increase in DSI coincided with a significant increase in the expression of DGL-α.

CONCLUSIONS AND IMPLICATIONS

Short-term morphine withdrawal potentiates eCB modulation of inhibitory synaptic transmission in the NAcc. We also found that DGL-α expression was elevated after short-term morphine withdrawal, suggesting that the eCB 2-arachidonyl-glycerol but not anandamide mediates the increase in DSI. These findings provide useful insights into the mechanisms underlying eCB-mediated plasticity in the NAcc during drug addiction.

LINKED ARTICLES

This article is part of a themed section on Endocannabinoids. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.7/issuetoc.

Effects of the cannabinoid CB₁ receptor allosteric modulator ORG 27569 on reinstatement of cocaine- and methamphetamine-seeking behavior in rats

BACKGROUND

Cannabinoid CB1 receptors play an essential role in drug addiction. Given the side effect profiles of orthosteric CB1 antagonism, new strategies have been attempted to modulate this target, such as CB1 receptor allosteric modulation. However, the effect of CB1 allosteric modulation in drug addiction is unknown. The present study examined the effects of the CB1 receptor allosteric modulator ORG27569 on the reinstatement of cocaine- and methamphetamine-seeking behavior in rats.

METHODS

Rats were trained to self-administer 0.75 mg/kg cocaine or 0.05 mg/kg methamphetamine in 2-h daily sessions for 14 days which was followed by 7 days of extinction sessions in which rats responded on the levers with no programmed consequences. On reinstatement test sessions, rats were administered ORG27569 (1.0, 3.2, 5.6 mg/kg, i.p.) or SR141716A (3.2 mg/kg, i.p.) 10 min prior to re-exposure to cocaine- or methamphetamine-paired cues or a priming injection of cocaine (10mg/kg, i.p.) or methamphetamine (1mg/kg, i.p.).

RESULTS

Both cues and a priming injection of cocaine or methamphetamine significantly reinstated the extinguished active lever responding. Pretreatment with ORG27569 resulted in a dose-related attenuation of both cue- and drug-induced reinstatement of cocaine- and methamphetamine-seeking behavior. SR141716A also exhibited similar inhibitory action on reinstatement of drug-seeking behavior.

CONCLUSION

Negative allosteric modulation of CB1 receptors can produce similar functional antagonism as orthosteric CB1 receptor antagonists on reinstatement of drug-seeking behavior. Thus, ORG27569 or other negative allosteric modulators deserve further study as potentially effective pharmacotherapy for drug addiction.

Effects of blockade of α4β2 and α7 nicotinic acetylcholine receptors on cue-induced reinstatement of nicotine-seeking behaviour in rats

Exposure to environmental stimuli conditioned to nicotine consumption critically contributes to the high relapse rates of tobacco smoking. Our previous work demonstrated that non-selective blockade of nicotinic acetylcholine receptors (nAChRs) reversed the cue-induced reinstatement of nicotine seeking, indicating a role for cholinergic neurotransmission in the mediation of the conditioned incentive properties of nicotine cues. The present study further examined the relative roles of the two major nAChR subtypes, α4β2 and α7, in the cue-induced reinstatement of nicotine seeking. Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion, free base) on a fixed-ratio 5 schedule of reinforcement. A nicotine-conditioned cue was established by associating a sensory stimulus with each nicotine infusion. After nicotine-maintained responding was extinguished by withholding the nicotine infusion and its paired cue, reinstatement test sessions were conducted with re-presentation of the cue but without the availability of nicotine. Thirty minutes before the tests, the rats were administered the α4β2-selective antagonist dihydro-β-erythroidine (DHβE) and α7-selective antagonist methyllycaconitine (MLA). Pretreatment with MLA, but not DHβE, significantly reduced the magnitude of the cue-induced reinstatement of responses on the active, previously nicotine-reinforced lever. In different sets of rats, MLA altered neither nicotine self-administration nor cue-induced reinstatement of food seeking. These results demonstrate that activation of α7 nAChRs participates in the mediation of the conditioned incentive properties of nicotine cues and suggest that α7 nAChRs may be a promising target for the development of medications for the prevention of cue-induced smoking relapse.

The reinstatement model of drug relapse: recent neurobiological findings, emerging research topics, and translational research

BACKGROUND AND RATIONALE

Results from many clinical studies suggest that drug relapse and craving are often provoked by acute exposure to the self-administered drug or related drugs, drug-associated cues or contexts, or certain stressors. During the last two decades, this clinical scenario has been studied in laboratory animals by using the reinstatement model. In this model, reinstatement of drug seeking by drug priming, drug cues or contexts, or certain stressors is assessed following drug self-administration training and subsequent extinction of the drug-reinforced responding.

OBJECTIVE

In this review, we first summarize recent (2009-present) neurobiological findings from studies using the reinstatement model. We then discuss emerging research topics, including the impact of interfering with putative reconsolidation processes on cue- and context-induced reinstatement of drug seeking, and similarities and differences in mechanisms of reinstatement across drug classes. We conclude by discussing results from recent human studies that were inspired by results from rat studies using the reinstatement model.

CONCLUSIONS

Main conclusions from the studies reviewed highlight: (1) the ventral subiculum and lateral hypothalamus as emerging brain areas important for reinstatement of drug seeking, (2) the existence of differences in brain mechanisms controlling reinstatement of drug seeking across drug classes, (3) the utility of the reinstatement model for assessing the effect of reconsolidation-related manipulations on cue-induced drug seeking, and (4) the encouraging pharmacological concordance between results from rat studies using the reinstatement model and human laboratory studies on cue- and stress-induced drug craving.

Role of amygdala in drug memory

Drug addiction is a chronic brain disorder with the hallmark of a high rate of relapse to compulsive drug seeking and drug taking even after long-term abstinence. Addiction has been considered as an aberrant memory that has been termed "addiction memory." Drug-related memory plays a critical role in the maintenance of learned addictive behaviors and emergence of relapse. Disrupting these long-lasting memories by administering amnestic agents or other manipulations during specific phases of drug memory is a promising strategy for relapse prevention. Recent studies on the processes of drug addiction and relapse have demonstrated that the amygdala is involved in associative drug addiction learning processes. In this review, we focus on preclinical studies that used conditioned place preference and self-administration models to investigate the differential roles of the amygdala in each phase of drug-related memory, including acquisition, consolidation, retrieval, reconsolidation, and extinction. These studies indicate that the amygdala plays a critical role in both cue-associative learning and the expression of cue-induced relapse to drug-seeking behavior.

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.

Endocannabinoids shape accumbal encoding of cue-motivated behavior via CB1 receptor activation in the ventral tegmentum

Transient increases in nucleus accumbens (NAc) dopamine concentration are observed when animals are presented with motivationally salient stimuli and are theorized to energize reward seeking. They arise from high-frequency firing of dopamine neurons in the ventral tegmental area (VTA), which also results in the release of endocannabinoids from dopamine cell bodies. In this context, endocannabinoids are thought to regulate reward seeking by modulating dopamine signaling, although a direct link has never been demonstrated. To test this, we pharmacologically manipulated endocannabinoid neurotransmission in the VTA while measuring transient changes in dopamine concentration in the NAc during reward seeking. Disrupting endocannabinoid signaling dramatically reduced, whereas augmenting levels of the endocannabinoid 2-arachidonoylglycerol (2AG) increased, cue-evoked dopamine concentrations and reward seeking. These data suggest that 2AG in the VTA regulates reward seeking by sculpting ethologically relevant patterns of dopamine release during reward-directed behavior.

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.

Role of projections from ventral medial prefrontal cortex to nucleus accumbens shell in context-induced reinstatement of heroin seeking

In humans, exposure to contexts previously associated with heroin use can provoke relapse. In rats, exposure to heroin-paired contexts after extinction of drug-reinforced responding in different contexts reinstates heroin seeking. This effect is attenuated by inhibition of glutamate or dopamine transmission in nucleus accumbens shell, or inactivation of ventral medial prefrontal cortex (mPFC). Here, we used an anatomical asymmetrical disconnection procedure to demonstrate that an interaction between glutamatergic projections from ventral mPFC to accumbens shell and local dopamine D(1) postsynaptic receptors contributes to context-induced reinstatement of heroin seeking. We also combined the marker of neuronal activity, Fos, with the retrograde tracer Fluoro-Gold to assess activation in this pathway during context-induced reinstatement. Rats were trained to self-administer heroin for 12 d; drug infusions were paired with a discrete tone-light cue. Lever pressing was subsequently extinguished in a nondrug-associated context in the presence of the discrete cue. Rats were then tested in the heroin- or extinction-associated contexts under extinction conditions. Injections of muscimol + baclofen into ventral mPFC in one hemisphere and D(1)-family receptor antagonist SCH 23390 into the contralateral or ipsilateral accumbens shell decreased context-induced reinstatement. Unilateral injections of muscimol + baclofen into ventral mPFC or SCH 23390 into the accumbens shell had no effect. Context-induced reinstatement was associated with increased Fos expression in ventral mPFC neurons, including those projecting to accumbens shell, with higher double-labeling in the ipsilateral projection than in the contralateral projection. Our results demonstrate that activation of glutamatergic projections from ventral mPFC to accumbens shell, previously implicated in inhibition of cocaine relapse, promotes heroin relapse.

Effects of caffeine on persistence and reinstatement of nicotine-seeking behavior in rats: interaction with nicotine-associated cues

RATIONALE

Caffeine and nicotine are the most commonly co-used psychostimulants. However, it is still unclear whether caffeine exposure enhances nicotine-seeking behavior.

OBJECTIVE

The present study examined the effects of caffeine on nicotine-seeking in rats trained to self-administer nicotine with and without presession administration of caffeine.

METHODS

Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion, freebase) on a fixed ratio 5 schedule of reinforcement and associate a stimulus cue with each nicotine administration. Five minutes before the sessions, the rats received an intraperitoneal administration of caffeine (5 mg/kg). Extinction tests were conducted under four conditions: presession caffeine administration, response-contingent presentation of nicotine cues, neither condition, or both conditions. Reinstatement tests were conducted after responding was extinguished by withholding presession caffeine, nicotine, and its cues. A separate group of rats trained without presession caffeine exposure was also subjected to the reinstatement tests.

RESULTS

In the rats trained with presession caffeine exposure, continued caffeine administration sustained nicotine-seeking responses and interacted with nicotine cues to significantly delay the extinction of nicotine-seeking behavior. Readministration of caffeine after extinction effectively reinstated nicotine-seeking behavior. In caffeine-naive rats, caffeine administration did not reinstate extinguished nicotine-seeking behavior but significantly potentiated the cue-induced reinstatement of nicotine-seeking.

CONCLUSION

These data demonstrate that caffeine administration sustained and reinstated nicotine-seeking behavior, possibly via its acquired discriminative-stimulus properties predictive of nicotine availability. These findings suggest that smokers who attempt to quit may benefit from stopping caffeine consumption.

Regulation of cocaine-induced reinstatement by group II metabotropic glutamate receptors in the ventral tegmental area

RATIONALE

A high rate of relapse is a daunting challenge facing clinical treatment of cocaine addiction. Recent studies have shown that drugs of abuse enhance glutamate neurotransmission in dopamine neurons in the ventral tegmental area (VTA) and such enhancement may contribute to the risk of relapse.

OBJECTIVES

Given the important role of group II metabotropic glutamate receptors (mGluR2/3s) in regulating glutamate release from the glutamatergic terminals, this study aimed to test whether activation of mGluR2/3s in the VTA can inhibit cocaine-induced reinstatement of cocaine-seeking behavior, a model of relapse to drug-seeking behavior.

METHODS

Rats were trained to self-administer intravenous cocaine (0.25 mg/infusion) under a modified fixed-ratio 5 schedule. After rats reached the training criteria, they went through extinction training to extinguish cocaine-seeking behavior. Then the dose-response effects of a selective mGluR2/3 agonist LY 379268 microinjected into the VTA on cocaine-induced reinstatement of cocaine-seeking behavior were assessed.

RESULTS

LY 379268 (0.032-0.1 μg/side) dose-dependently decreased cocaine-induced reinstatement. The effect could not be fully attributed to diffusion of the drug to the neighboring substantia nigra or to motor impairment. Interestingly, LY 379268 has a less potent effect on cocaine-induced reinstatement than on sucrose-induced reinstatement of sucrose-seeking behavior.

CONCLUSIONS

Our data support the idea that glutamate release in the VTA is critically involved in cocaine-induced reinstatement and indicate that loss of mGluR2/3-mediated regulation of glutamate release in the VTA may critically contribute to the risk of relapse.

Endocannabinoid signaling in the amygdala: anatomy, synaptic signaling, behavior, and adaptations to stress

The molecular constituents of endocannabinoid (eCB) signaling are abundantly expressed within the mammalian amygdaloid complex, consistent with the robust role of eCB signaling in the modulation of emotional behavior, learning, and stress-response physiology. Here, we detail the anatomical distribution of eCB signaling machinery in the amygdala and the role of this system in the modulation of excitatory and inhibitory neuroplasticity in this region. We also summarize recent findings demonstrating dynamic alternations in eCB signaling that occur in response to stress exposure, as well as known behavioral consequences of eCB-mediated modulation of amygdala function. Finally, we discuss how integrating anatomical and physiological data regarding eCB signaling in the amygdala could help elucidate common functional motifs of this system in relation to broader forebrain function.

The endocannabinoid 2-arachidonoylglycerol mediates D1 and D2 receptor cooperative enhancement of rat nucleus accumbens core neuron firing

Many motivated and addiction-related behaviors are sustained by activity of both dopamine D1- and D2-type receptors (D1Rs and D2Rs) as well as CB1 receptors (CB1Rs) in the nucleus accumbens (NAc). Here, we use in vitro whole-cell patch-clamp electrophysiology to describe an endocannabinoid (eCB)-dopamine receptor interaction in adult rat NAc core neurons. D1R and D2R agonists in combination enhanced firing, with no effect of a D1R or D2R agonist alone. This D1R+D2R-mediated firing increase required CB1Rs, since it was prevented by the CB1R antagonists AM251 and Rimonabant. The D1R+D2R firing increase also required phospholipase C (PLC), the major synthesis pathway for the eCB 2-arachidonoylglycerol (2-AG) and one of several pathways for anandamide. Further, inhibition of 2-AG hydrolysis with the monoglyceride lipase (MGL) inhibitor JZL184 allowed subthreshold levels of D1R+D2R receptor agonists to enhance firing, while inhibition of anandamide hydrolysis with the fatty acid amide hydrolase (FAAH) inhibitors URB597 or AM3506 did not. Filling the postsynaptic neuron with 2-AG enabled subthreshold D1R+D2R agonists to increase firing, and the 2AG+D1R+D2R increase in firing was prevented by a CB1R antagonist. Also, the metabotropic glutamate receptor 5 (mGluR5) blocker MPEP prevented the ability of JZL184 to promote subthreshold D1R+D2R enhancement of firing, while the 2-AG+D1R+D2R increase in firing was not prevented by the mGluR5 blocker, suggesting that mGluR5s acted upstream of 2-AG production. Thus, our results taken together are consistent with the hypothesis that NAc core eCBs mediate dopamine receptor (DAR) enhancement of firing, perhaps providing a cellular mechanism underlying the central role of NAc core D1Rs, D2Rs, CB1Rs, and mGluR5s during many drug-seeking behaviors.

Endocannabinoid influence in drug reinforcement, dependence and addiction-related behaviors

The endogenous cannabinoid system is an important regulatory system involved in physiological homeostasis. Endocannabinoid signaling is known to modulate neural development, immune function, metabolism, synaptic plasticity and emotional state. Accumulating evidence also implicates brain endocannabinoid signaling in the etiology of drug addiction which is characterized by compulsive drug seeking, loss of control in limiting drug intake, emergence of a negative emotional state in the absence of drug use and a persistent vulnerability toward relapse to drug use during protracted abstinence. In this review we discuss the effects of drug intake on brain endocannabinoid signaling, evidence implicating the endocannabinoid system in the motivation for drug consumption, and drug-induced alterations in endocannabinoid function that may contribute to various aspects of addiction including dysregulated synaptic plasticity, increased stress responsivity, negative affective states, drug craving and relapse to drug taking. Current knowledge of genetic variants in endocannabinoid signaling associated with addiction is also discussed.

Endocannabinoid-mediated synaptic plasticity and addiction-related behavior

Endogenous cannabinoids (eCBs) are retrograde messengers that provide feedback inhibition of both excitatory and inhibitory transmission in brain through the activation of presynaptic CB₁ receptors. Substantial evidence indicates that eCBs mediate various forms of short- and long-term plasticity in brain regions involved in the etiology of addiction. The present review provides an overview of the mechanisms through which eCBs mediate various forms of synaptic plasticity and discusses evidence that eCB-mediated plasticity is disrupted following exposure to a variety of abused substances that differ substantially in pharmacodynamic mechanism including alcohol, psychostimulants and cannabinoids. The possible involvement of dysregulated eCB signaling in maladaptive behaviors that evolve over long-term drug exposure is also discussed, with a particular focus on altered behavioral responses to drug exposure, deficient extinction of drug-related memories, increased drug craving and relapse, heightened stress sensitivity and persistent affective disruption (anxiety and depression).

Endogenous GDNF in ventral tegmental area and nucleus accumbens does not play a role in the incubation of heroin craving

Glial cell line-derived neurotrophic factor (GDNF) activity in ventral tegmental area (VTA) mediates the time-dependent increases in cue-induced cocaine-seeking after withdrawal (incubation of cocaine craving). Here, we studied the generality of these findings to incubation of heroin craving. Rats were trained to self-administer heroin for 10 days (6 hours/day; 0.075 mg/kg/infusion; infusions were paired with a tone-light cue) and tested for cue-induced heroin-seeking in extinction tests after 1, 11 or 30 withdrawal days. Cue-induced heroin seeking was higher after 11 or 30 days than after 1 day (incubation of heroin craving), and the time-dependent increases in extinction responding were associated with time-dependent changes in GDNF mRNA expression in VTA and nucleus accumbens. Additionally, acute accumbens (but not VTA) GDNF injections (12.5 µg/side) administered 1-3 hours after the last heroin self-administration training session enhanced the time-dependent increases in extinction responding after withdrawal. However, the time-dependent increases in extinction responding after withdrawal were not associated with changes in GDNF protein expression in VTA and accumbens. Additionally, interfering with endogenous GDNF function by chronic delivery of anti-GDNF monoclonal neutralizing antibodies (600 ng/side/day) into VTA or accumbens had no effect on the time-dependent increases in extinction responding. In summary, heroin self-administration and withdrawal regulate VTA and accumbens GDNF mRNA expression in a time-dependent manner, and exogenous GDNF administration into accumbens but not VTA potentiates cue-induced heroin seeking. However, based on the GDNF protein expression and the anti-GDNF monoclonal neutralizing antibodies manipulation data, we conclude that neither accumbens nor VTA endogenous GDNF mediates the incubation of heroin craving.

CB1 receptors regulate alcohol-seeking behavior and alcohol self-administration of alcohol-preferring (P) rats

RATIONALE

The endogenous cannabinoid (CB) system mediates a number of behaviors associated with drug-seeking and drug self-administration. In this study the effects of CB1 receptor manipulations on operant ethanol (EtOH) responding during EtOH-seeking, EtOH-relapse as well as on-going EtOH self-administration were determined.

METHODS

Alcohol-preferring (P) rats were trained in 2-lever operant chambers to self-administer 15% EtOH (v/v) and water on a concurrent fixed-ratio 5-fixed-ratio 1 (FR5-FR1) schedule of reinforcement in daily 1-h sessions. After 10 weeks, rats underwent 7 extinction sessions, followed by 2 weeks in their home cages without access to EtOH or operant chambers. Rats were then returned to the operant chambers for testing of EtOH-seeking behavior (no EtOH present) for 4 sessions. After a week in their home cages following the EtOH-seeking test, rats were returned to the operant chambers with access to EtOH and water (relapse). Rats were then maintained in the operant chambers for daily 1-h sessions with access to 15% EtOH and water for several weeks.

RESULTS

The CB1 receptor antagonist (SR141716A), at doses of 1 and 2 mg/kg, i.p. reduced EtOH-seeking and transiently reduced EtOH self-administration during relapse and maintenance. Conversely, treatment with the CB1 receptor agonist CP 55, 940, at doses of 1 and 10 μg/kg i.p., increased EtOH-seeking and EtOH self-administration during relapse.

CONCLUSIONS

The results of this study demonstrate that activation of CB1 receptors are involved in regulating EtOH-seeking as well as the reinforcing effects of EtOH under relapse and on-going self-administration conditions.

Preclinical evidence of new opioid modulators for the treatment of addiction

IMPORTANCE OF THE FIELD

Addiction to opiates is one of the most severe forms of substance dependence, and despite a variety of pharmacological approaches to treat it, relapse is observed in a high percentage of subjects. New pharmacological compounds are necessary to improve the outcome of treatments and reduce adverse side effects. Moreover, drugs that act on the opioid system can also be of benefit in the treatment of alcohol or cocaine addiction. AREA COVERED BY THIS REVIEW: Recent preclinical studies of pharmacological agents for the treatment of opiate addiction (2008 to the present date).

WHAT THE READER WILL GAIN

The reader will be informed of the latest drugs shown in animal models to modify dependence on opiates and the reinforcing effects of these drugs. In addition, reports of the latest studies to test these compounds in models of other drug addictions are reviewed.

TAKE HOME MESSAGE

The classic clinical pharmacotherapy for opiate dependence, involving mu-opioid receptor agonists or antagonists, has not yielded a high success rate in humans. In pharmacotherapy for opioid dependence, new options are emerging and different pharmacological strategies are now being tested.

Prefrontal cortical regulation of drug seeking in animal models of drug relapse

Prefrontal cortical dysfunction is thought to underlie maladaptive behaviors displayed by chronic drug users, most notably the high propensity for relapse that severely impedes successful treatment of drug addiction. In animal models of drug relapse, exposure to drug-associated stimuli, small amounts of drug, and acute stressors powerfully reinstate drug seeking by critically engaging the prefrontal cortex, with the anterior cingulate, prelimbic, infralimbic, and orbitofrontal subregions making distinct contributions to drug seeking. Hence, from an addiction treatment perspective, it is necessary to fully explicate the involvement of the prefrontal cortex in drug relapse.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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).

Neurochemistry underlying relapse to opiate seeking behaviour

Relapse is a major clinical problem and remains a major challenge in the treatment of addictions. A goal of current research is to gain a greater understanding of the neurochemistry underlying relapse to opiate use. Factors which trigger relapse in humans such as stress, exposure to opiates and/or drug-associated cues, can also trigger opiate-seeking in animals. This review will overview preclinical studies relating to the neurochemistry of opiate-seeking with a focus on studies published from 2005 to present.

Extinction circuits for fear and addiction overlap in prefrontal cortex

Extinction is a form of inhibitory learning that suppresses a previously conditioned response. Both fear and drug seeking are conditioned responses that can lead to maladaptive behavior when expressed inappropriately, manifesting as anxiety disorders and addiction, respectively. Recent evidence indicates that the medial prefrontal cortex (mPFC) is critical for the extinction of both fear and drug-seeking behaviors. Moreover, a dorsal-ventral distinction is apparent within the mPFC, such that the prelimbic (PL-mPFC) cortex drives the expression of fear and drug seeking, whereas the infralimbic (IL-mPFC) cortex suppresses these behaviors after extinction. For conditioned fear, the dorsal-ventral dichotomy is accomplished via divergent projections to different subregions of the amygdala, whereas for drug seeking, it is accomplished via divergent projections to the subregions of the nucleus accumbens. Given that the mPFC represents a common node in the extinction circuit for these behaviors, treatments that target this region may help alleviate symptoms of both anxiety and addictive disorders by enhancing extinction memory.

Differential response to a selective cannabinoid receptor antagonist (SR141716: rimonabant) in female mice from lines selectively bred for high voluntary wheel-running behaviour

Exercise is a naturally rewarding behaviour in human beings and can be associated with feelings of euphoria and analgesia. The endocannabinoid system may play a role in the perception of neurobiological rewards during and after prolonged exercise. Mice from lines that have been selectively bred for high voluntary wheel running (high runner or HR lines) may have evolved neurobiological mechanisms that increase the incentive salience of endurance-type exercise. Here, we test the hypothesis that endocannabinoid signalling has been altered in the four replicate HR lines as compared with four nonselected control lines. After 18 days of acclimation to cages with attached wheels, we injected mice with rimonabant (SR141716), a selective cannabinoid CB1 receptor antagonist. During the time of normal peak running, each mouse received, in a randomized order, intraperitoneal injection of rimonabant (0.1 or 3.0 mg/kg) or vehicle, over 9 days. Drug response was quantified as wheel revolutions, time and speed 10-70 min postinjection. Rimonabant decreased running in all mice; however, female HR mice differentially decreased running speed and distance (but not time) as compared with control females. We conclude that altered endocannabinoid signalling plays a role in the high wheel running of female HR mice.