Cannabinoid withdrawal is dependent upon PKA activation in the cerebellum

Mechanisms of cannabinoid tolerance

Cannabis has been used recreationally and medically for centuries, yet research into understanding the mechanisms of its therapeutic effects has only recently garnered more attention. There is evidence to support the use of cannabinoids for the treatment of chronic pain, muscle spasticity, nausea and vomiting due to chemotherapy, improving weight gain in HIV-related cachexia, emesis, sleep disorders, managing symptoms in Tourette syndrome, and patient-reported muscle spasticity from multiple sclerosis. However, tolerance and the risk for cannabis use disorder are two significant disadvantages for cannabinoid-based therapies in humans. Recent work has revealed prominent sex differences in the acute response and tolerance to cannabinoids in both humans and animal models. This review will discuss evidence demonstrating cannabinoid tolerance in rodents, non-human primates, and humans and our current understanding of the neuroadaptations occurring at the cannabinoid type 1 receptor (CB1R) that are responsible tolerance. CB1R expression is downregulated in tolerant animals and humans while there is strong evidence of CB1R desensitization in cannabinoid tolerant rodent models. Throughout the review, critical knowledge gaps are indicated and discussed, such as the lack of a neuroimaging probe to assess CB1R desensitization in humans. The review discusses the intracellular signaling pathways that are responsible for mediating CB1R desensitization and downregulation including the action of G protein-coupled receptor kinases, β-arrestin2 recruitment, c-Jun N-terminal kinases, protein kinase A, and the intracellular trafficking of CB1R. Finally, the review discusses approaches to reduce cannabinoid tolerance in humans based on our current understanding of the neuroadaptations and mechanisms responsible for this process.

ASRA Pain Medicine consensus guidelines on the management of the perioperative patient on cannabis and cannabinoids

BACKGROUND

The past two decades have seen an increase in cannabis use due to both regulatory changes and an interest in potential therapeutic effects of the substance, yet many aspects of the substance and their health implications remain controversial or unclear.

METHODS

In November 2020, the American Society of Regional Anesthesia and Pain Medicine charged the Cannabis Working Group to develop guidelines for the perioperative use of cannabis. The Perioperative Use of Cannabis and Cannabinoids Guidelines Committee was charged with drafting responses to the nine key questions using a modified Delphi method with the overall goal of producing a document focused on the safe management of surgical patients using cannabinoids. A consensus recommendation required ≥75% agreement.

RESULTS

Nine questions were selected, with 100% consensus achieved on third-round voting. Topics addressed included perioperative screening, postponement of elective surgery, concomitant use of opioid and cannabis perioperatively, implications for parturients, adjustment in anesthetic and analgesics intraoperatively, postoperative monitoring, cannabis use disorder, and postoperative concerns. Surgical patients using cannabinoids are at potential increased risk for negative perioperative outcomes.

CONCLUSIONS

Specific clinical recommendations for perioperative management of cannabis and cannabinoids were successfully created.

Protein kinases in natural versus drug reward

Natural and drug rewards act on the same neural pathway, the mesolimbic dopaminergic system. In brain regions such as the nucleus accumbens and ventral tegmental area, drugs of abuse-induced stimulation of signaling pathways can lead to synaptic reshaping within this system. This is believed to be underlying the maladaptive alterations in behaviors associated with addiction. In this review, we discuss animal studies disclosing the implication of several protein kinases, namely protein kinase A (PKA), extracellular signal regulated kinase (ERK) mitogen-activated protein kinases (MAPK), p38 MAPK, and calcium/calmodulin-dependent kinase II (CaMKII), in reward-related brain regions in drug and natural reward. Furthermore, we refer to studies that helped pave the way toward a better understanding of the neurobiology underlying non-drug and drug reward through genetic deletion or brain region-specific pharmacological inhibition of these kinases. Whereas the role of kinases in drug reward has been extensively studied, their implication in natural reward, such as positive social interaction, is less investigated. Discovering molecular candidates, recruited specifically by drug versus natural rewards, can promote the identification of novel targets for the pharmacological treatment of addiction with less off-target effects and being effective when used combined with behavioral-based therapies.

Evidence of familial confounding of the association between cannabis use and cerebellar-cortical functional connectivity using a twin study

Cerebellar-cortical resting-state functional connectivity (rsFC) has been reported to be altered in cannabis users. However, this association may be due to genetic and environmental confounding rather than a causal relationship between cannabis use and changes in rsFC. In this co-twin control study, linear mixed models were used to assess relationships between the number of lifetime cannabis uses (NLCU) and age of cannabis onset (ACO) with cerebellar-cortical rsFC. The rsFC with seven functional networks was evaluated in 147 monozygotic and 82 dizygotic twin pairs. Importantly, the use of genetically informed models in this twin sample facilitated examining whether shared genetic or environmental effects underlie crude associations between cannabis measures and connectivity. Individual-level phenotypic analyses (i.e., accounting for twin-pair non-independence) showed that individuals in the full sample with earlier ACO and higher NLCU had lower cerebellar rsFC within the VA, DA, and FP networks. Yet, there were no significant differences in cerebellar-cortical rsFC between monozygotic twins who were discordant for cannabis measures. These findings suggest shared genetic or environmental confounds contribute to associations between cannabis use and altered cerebellar-cortical rsFC, rather than unique causal impacts of cannabis use on cerebellar-cortical rsFC.

Cannabis use, abuse, and withdrawal: Cannabinergic mechanisms, clinical, and preclinical findings

Cannabis sativa is the most widely used illicit drug in the world. Its main psychoactive component is delta-9-tetrahydrocannabinol (THC), one of over 100 phytocannabinoid compounds produced by the cannabis plant. THC is the primary compound that drives cannabis abuse potential and is also used and prescribed medically for therapeutic qualities. Despite its therapeutic potential, a significant subpopulation of frequent cannabis or THC users will develop a drug use syndrome termed cannabis use disorder. Individuals suffering from cannabis use disorder exhibit many of the hallmarks of classical addictions including cravings, tolerance, and withdrawal symptoms. Currently, there are no efficacious treatments for cannabis use disorder or withdrawal symptoms. This makes both clinical and preclinical research on the neurobiological mechanisms of these syndromes ever more pertinent. Indeed, basic research using animal models has provided valuable evidence of the neural molecular and cellular actions of cannabis that mediate its behavioral effects. One of the main components being central action on the cannabinoid type-one receptor and downstream intracellular signaling related to the endogenous cannabinoid system. Back-translational studies have provided insight linking preclinical basic and behavioral biology research to better understand symptoms observed at the clinical level. This narrative review aims to summarize major research elucidating the molecular, cellular, and behavioral manifestations of cannabis/THC use that play a role in cannabis use disorder and withdrawal.

Cannabinoids, Endocannabinoids and Sleep

Sleep is a vital function of the nervous system that contributes to brain and bodily homeostasis, energy levels, cognitive ability, and other key functions of a variety of organisms. Dysfunctional sleep induces neural problems and is a key part of almost all human psychiatric disorders including substance abuse disorders. The hypnotic effects of cannabis have long been known and there is increasing use of phytocannabinoids and other formulations as sleep aids. Thus, it is crucial to gain a better understanding of the neurobiological basis of cannabis drug effects on sleep, as well as the role of the endogenous cannabinoid system in sleep physiology. In this review article, we summarize the current state of knowledge concerning sleep-related endogenous cannabinoid function derived from research on humans and rodent models. We also review information on acute and chronic cannabinoid drug effects on sleep in these organisms, and molecular mechanisms that may contribute to these effects. We point out the potential benefits of acute cannabinoids for sleep improvement, but also the potential sleep-disruptive effects of withdrawal following chronic cannabinoid drug use. Prescriptions for future research in this burgeoning field are also provided.

Assessment of Acute Motor Effects and Tolerance Following Self-Administration of Alcohol and Edible ∆9 -Tetrahydrocannabinol in Adolescent Male Mice

BACKGROUND

Cannabinoids and their principle psychoactive target, the cannabinoid type 1 receptor (CB1R), impact a number of alcohol-related properties, and although alcohol and cannabis are often co-used, particularly in adolescence, few animal models of this phenomenon exist. We modeled the co-use of alcohol and ∆⁹ -tetrahydrocannabinol (THC) in adolescent mice using ingestive methods popular during this developmental period in humans, namely binge-drinking and edible THC. With this model, we assessed levels of use, acute effects, and tolerance to each substance.

METHODS

Adolescent male C57BL/6J mice had daily, limited access to 1 of 2 edible doughs (THC or control), to 1 of 2 fluids (ethanol (EtOH) or water), and in 1 of 2 orders (dough-fluid or fluid-dough). Home cage locomotor activity was recorded both during access and after access. On the day following the final access session, a subset of mice were assessed for functional and metabolic tolerance to alcohol using accelerating rotarod and blood EtOH concentrations, respectively. The remaining mice were assessed for tolerance to THC-induced hypothermia, and whole-brain CB1R expression was assessed in all mice.

RESULTS

EtOH intake was on par with levels previously reported in adolescent mice. Edible THC was well-consumed, but consumption decreased at the highest dose provided. Locomotor activity increased following EtOH intake and decreased following edible THC consumption, and edible THC increased fluid intake in general. The use of alcohol produced neither functional nor metabolic tolerance to an alcohol challenge. However, the use of edible THC impaired subsequent drug-free rotarod performance and was associated with a reduction in THC's hypothermic effect.

CONCLUSIONS

Adolescent mice self-administered both alcohol and edible THC to a degree sufficient to acutely impact locomotor activity. However, only edible THC consumption had lasting effects during short-term abstinence. Thus, this adolescent co-use model could be used to explore sex differences in self-administration and the impact substance co-use might have on other domains such as mood and cognition.

Expression of protein kinase A and the kappa opioid receptor in selected brain regions and conditioned place aversion in morphine-dependent rats

This study examined adaptive changes in protein kinase A (PKA) and kappa opioid receptor (KOR) in selected addiction-related brain regions before and after conditioned place aversion (CPA). Seventy-two male SD rats were randomly assigned to an experimental group (morphine + naloxone, “MN”) and 2 control groups: MS (morphine + saline) and SN (saline + naloxone). MN rats were intraperitoneally injected with morphine twice per day for 6.5 days and naloxone once and trained to establish CPA model. MS and SN rats were injected with equivalent volumes of morphine plus saline and saline plus naloxone, respectively. PKA and KOR in AcbSH, CeA and VTA were measured by immunohistochemistry. Before CPA, there were no significant differences in PKA and KOR expression levels in the AcbSH, CeA and VTA between MN and 2 control groups. After CPA, significant differences in PKA expression were detected in the AcbSH (P<0.001) and VTA (P=0.018) between MN and 2 control groups. The average gray intensity of MN group (109.50±4.66) in AcbSH was significantly higher than that of MS (126.50±3.70, P<0.001) and MN (133.50±6.364, P<0.001) groups. Significant differences in KOR expression were also detected between MN and 2 control groups in CeA (P<0.001). In MN group, PKA and KOR expression levels showed adaptive changes at different points of CPA. These findings demonstrated that neuroadaptation mediated by PKA and KOR may be an important molecular neurobiology basis for CPA. The upregulation of AC-cAMP-PKA-CREB signaling pathway in AcbSH and VTA has some role in the neurobiological mechanism of CPA.

Marijuana dependence: not just smoke and mirrors

Marijuana (Cannabis sativa) is the most commonly used illicit drug worldwide as well as in the Unites States. Prolonged use of marijuana or repeated administration of its primary psychoactive constituent, Δ9-tetrahydrocannabinol (THC), can lead to physical dependence in humans and laboratory animals. The changes that occur with repeated cannabis use include alterations in behavioral, physiological, and biochemical responses. A variety of withdrawal responses occur in cannabis-dependent individuals: anger, aggression, irritability, anxiety and nervousness, decreased appetite or weight loss, restlessness, and sleep difficulties with strange dreams. But the long half-life and other pharmacokinetic properties of THC result in delayed expression of withdrawal symptoms, and because of the lack of contiguity between drug cessation and withdrawal responses the latter are not readily recognized as a clinically relevant syndrome. Over the past 30 years, a substantial body of clinical and laboratory animal research has emerged supporting the assertion that chronic exposure to cannabinoids produces physical dependence and may contribute to drug maintenance in cannabis-dependent individuals. However, no medications are approved to treat cannabis dependence and withdrawal. In this review, we describe preclinical and clinical research that supports the existence of a cannabinoid withdrawal syndrome. In addition, we review research evaluating potential pharmacotherapies (e.g., THC, a variety of antidepressant drugs, and lithium) to reduce cannabis withdrawal responses and examine how expanded knowledge about the regulatory mechanisms in the endocannabinoid system may lead to promising new therapeutic targets.

Cannabinoid receptor-interacting protein 1a modulates CB1 receptor signaling and regulation

Cannabinoid CB1 receptors (CB1Rs) mediate the presynaptic effects of endocannabinoids in the central nervous system (CNS) and most behavioral effects of exogenous cannabinoids. Cannabinoid receptor-interacting protein 1a (CRIP1a) binds to the CB1R C-terminus and can attenuate constitutive CB1R-mediated inhibition of Ca(2+) channel activity. We now demonstrate cellular colocalization of CRIP1a at neuronal elements in the CNS and show that CRIP1a inhibits both constitutive and agonist-stimulated CB1R-mediated guanine nucleotide-binding regulatory protein (G-protein) activity. Stable overexpression of CRIP1a in human embryonic kidney (HEK)-293 cells stably expressing CB1Rs (CB1-HEK), or in N18TG2 cells endogenously expressing CB1Rs, decreased CB1R-mediated G-protein activation (measured by agonist-stimulated [(35)S]GTPγS (guanylyl-5'-[O-thio]-triphosphate) binding) in both cell lines and attenuated inverse agonism by rimonabant in CB1-HEK cells. Conversely, small-interfering RNA-mediated knockdown of CRIP1a in N18TG2 cells enhanced CB1R-mediated G-protein activation. These effects were not attributable to differences in CB1R expression or endocannabinoid tone because CB1R levels did not differ between cell lines varying in CRIP1a expression, and endocannabinoid levels were undetectable (CB1-HEK) or unchanged (N18TG2) by CRIP1a overexpression. In CB1-HEK cells, 4-hour pretreatment with cannabinoid agonists downregulated CB1Rs and desensitized agonist-stimulated [(35)S]GTPγS binding. CRIP1a overexpression attenuated CB1R downregulation without altering CB1R desensitization. Finally, in cultured autaptic hippocampal neurons, CRIP1a overexpression attenuated both depolarization-induced suppression of excitation and inhibition of excitatory synaptic activity induced by exogenous application of cannabinoid but not by adenosine A1 agonists. These results confirm that CRIP1a inhibits constitutive CB1R activity and demonstrate that CRIP1a can also inhibit agonist-stimulated CB1R signaling and downregulation of CB1Rs. Thus, CRIP1a appears to act as a broad negative regulator of CB1R function.

Synaptic targets of Δ9-tetrahydrocannabinol in the central nervous system

The availability of potent synthetic agonists for cannabinoid receptors has facilitated our understanding of cannabinoid actions on synaptic transmission in the central nervous system. Moreover, the ability of these compounds to inhibit neurotransmitter release at many central synapses is thought to underlie most of the behavioral effects of cannabinoid agonists. However, despite the widespread use and misuse of marijuana, and recognition of its potential adverse psychological effects in humans, comparatively few studies have examined the actions of its primary psychoactive constituent, Δ(9)-tetrahydrocannabinol (THC), at well-defined synaptic pathways. Here we examine the recent literature describing the effects of acute and repeated THC exposure on synaptic function in several brain regions and explore the importance of these neurobiological actions of THC in drug addiction.

Molecular mechanisms of cannabinoid addiction

Cannabis is the world's most widely used illicit substance, with an estimated number of 119-224 million users worldwide. In recent years we assisted to an increased effort aimed to individuate the brain circuits underlying cannabis addiction and dependence. Similarly to other drugs of abuse, repeated exposure to cannabinoids causes brain neuroadaptations that persist long after drug effects, contribute to the negative affective states during withdrawal, and ultimately facilitate relapse. Recently, considerable progress has been made in understanding the cellular and molecular consequences of prolonged cannabis use, among which is the identification of specific set of transcriptional regulations that develop differently after chronic cannabinoids and in the abstinent brain.

Interactions between the cannabinoid and dopaminergic systems: evidence from animal studies

There is a prominent role of the cannabinoid system to control basal ganglia function, in respect to reward, psychomotor function and motor control. Cannabinoid dysregulations might have a pathogenetic role in dopamine- and basal ganglia related neuropsychiatric disorders, such as drug addiction, psychosis, Parkinson's disease and Huntington's disease. This review highlights interactions between cannabinoids, and dopamine, to modulate neurotransmitter release and synaptic plasticity in the context of drug addiction, psychosis and cognition. Modulating endocannabinoid function, as a plasticity based therapeutic strategy, in the above pathologies with particular focus on cannabinoid receptor type 1 (CB1 receptor) antagonists/inverse agonists, is discussed. On the basis of the existing literature and of new experimental evidence presented here, CB1 receptor antagonists might be beneficial in disease states associated with hedonic dysregulation, and with cognitive dysfunction in particular in the context of psychosis. It is suggested that this effects might be mediated via a hyperglutamatergic state through metabotropic glutamate activation. Indications for endocannabinoid catabolism inhibitors in psychiatric disorders, that might be CB1 receptor independent and might involve TRPV1 receptors, are also discussed.

delta(9)-Tetrahydrocannabinol-dependent mice undergoing withdrawal display impaired spatial memory

RATIONALE

Cannabis users display a constellation of withdrawal symptoms upon drug discontinuation, including sleep disturbances, irritability, and possibly memory deficits. In cannabinoid-dependent rodents, the CB(1) antagonist rimonabant precipitates somatic withdrawal and enhances forskolin-stimulated adenylyl cyclase activity in cerebellum, an effect opposite that of acutely administered ∆(9)-tetrahydrocannabinol (THC), the primary constituent in cannabis.

OBJECTIVES

Here, we tested whether THC-dependent mice undergoing rimonabant-precipitated withdrawal display short-term spatial memory deficits, as assessed in the Morris water maze. We also evaluated whether rimonabant would precipitate adenylyl cyclase superactivation in hippocampal and cerebellar tissue from THC-dependent mice.

RESULTS

Rimonabant significantly impaired spatial memory of THC-dependent mice at lower doses than those necessary to precipitate somatic withdrawal behavior. In contrast, maze performance was near perfect in the cued task, suggesting sensorimotor function and motivational factors were unperturbed by the withdrawal state. Finally, rimonabant increased adenylyl cyclase activity in cerebellar, but not in hippocampal, membranes.

CONCLUSIONS

The memory disruptive effects of THC undergo tolerance following repeated dosing, while the withdrawal state leads to a rebound deficit in memory. These results establish spatial memory impairment as a particularly sensitive component of cannabinoid withdrawal, an effect that may be mediated through compensatory changes in the cerebellum.

FAAH-/- mice display differential tolerance, dependence, and cannabinoid receptor adaptation after delta 9-tetrahydrocannabinol and anandamide administration

Repeated administration of Delta(9)-tetrahydrocannabinol (THC), the primary psychoactive constituent of Cannabis sativa, induces profound tolerance that correlates with desensitization and downregulation of CB(1) cannabinoid receptors in the CNS. However, the consequences of repeated administration of the endocannabinoid N-arachidonoyl ethanolamine (anandamide, AEA) on cannabinoid receptor regulation are unclear because of its rapid metabolism by fatty acid amide hydrolase (FAAH). FAAH(-/-) mice dosed subchronically with equi-active maximally effective doses of AEA or THC displayed greater rightward shifts in THC dose-effect curves for antinociception, catalepsy, and hypothermia than in AEA dose-effect curves. Subchronic THC significantly attenuated agonist-stimulated [(35)S]GTP gamma S binding in brain and spinal cord, and reduced [(3)H]WIN55,212-2 binding in brain. Interestingly, AEA-treated FAAH(-/-) mice showed less CB(1) receptor downregulation and desensitization than THC-treated mice. Experiments examining tolerance and cross-tolerance indicated that the behavioral effects of THC, a low efficacy CB(1) receptor agonist, were more sensitive to receptor loss than those of AEA, a higher efficacy agonist, suggesting that the expression of tolerance was more affected by the intrinsic activity of the ligand at testing than during subchronic treatment. In addition, the CB(1) receptor antagonist, rimonabant, precipitated a markedly reduced magnitude of withdrawal in FAAH(-/-) mice treated subchronically with AEA compared with mice treated repeatedly with THC. The findings that repeated AEA administration produces lesser adaptive changes at the CB(1) receptor and has reduced dependence liability compared with THC suggest that pharmacotherapies targeting endocannabinoid catabolic enzymes are less likely to promote tolerance and dependence than direct acting CB(1) receptor agonists.

Inhibitors of endocannabinoid-metabolizing enzymes reduce precipitated withdrawal responses in THC-dependent mice

Abstinence symptoms in cannabis-dependent individuals are believed to contribute to the maintenance of regular marijuana use. However, there are currently no medications approved by the FDA to treat cannabis-related disorders. The only treatment currently shown consistently to alleviate cannabinoid withdrawal in both animals and humans is substitution therapy using the psychoactive constituent of marijuana, Delta(9)-tetrahydrocannabinol (THC). However, new genetic and pharmacological tools are available to increase endocannabinoid levels by targeting fatty acid amide hydrolase (FAAH) or monoacylglycerol lipase (MAGL), the enzymes responsible for the degradation of the endogenous cannabinoid ligands anandamide and 2-arachidonoylglycerol, respectively. In the present study, we investigated whether increasing endogenous cannabinoids levels, through the use of FAAH (-/-) mice as well as the FAAH inhibitor URB597 or the MAGL inhibitor JZL184, would reduce the intensity of withdrawal signs precipitated by the CB(1) receptor antagonist rimonabant in THC-dependent mice. Strikingly, acute administration of either URB597 or JZL184 significantly attenuated rimonabant-precipitated withdrawal signs in THC-dependent mice. In contrast, FAAH (-/-) mice showed identical withdrawal responses as wild-type mice under a variety of conditions, suggesting that the absence of this enzyme across the development of dependence and during rimonabant challenge does not affect withdrawal responses. Of importance, subchronic administration of URB597 did not lead to cannabinoid dependence and neither URB597 nor JZL184 impaired rotarod motor coordination. These results support the concept of targeting endocannabinoid metabolizing enzymes as a promising treatment for cannabis withdrawal.

Protein kinases and addiction

Although drugs of abuse have different chemical structures and interact with different protein targets, all appear to usurp common neuronal systems that regulate reward and motivation. Addiction is a complex disease that is thought to involve drug-induced changes in synaptic plasticity due to alterations in cell signaling, gene transcription, and protein synthesis. Recent evidence suggests that drugs of abuse interact with and change a common network of signaling pathways that include a subset of specific protein kinases. The best studied of these kinases are reviewed here and include extracellular signal-regulated kinase, cAMP-dependent protein kinase, cyclin-dependent protein kinase 5, protein kinase C, calcium/calmodulin-dependent protein kinase II, and Fyn tyrosine kinase. These kinases have been implicated in various aspects of drug addiction including acute drug effects, drug self-administration, withdrawal, reinforcement, sensitization, and tolerance. Identifying protein kinase substrates and signaling pathways that contribute to the addicted state may provide novel approaches for new pharmacotherapies to treat drug addiction.

Advances in the field of cannabinoid--opioid cross-talk

A remarkable amount of literature has been generated demonstrating the functional similarities between the endogenous opioid and cannabinoid systems. Anatomical, biochemical and molecular data support the existence of reciprocal interactions between these two systems related to several pharmacological responses including reward, cognitive effects, and the development of tolerance and dependence. However, the assessment of the bidirectionality of these effects has been difficult due to their variety and complexity. Reciprocal interactions have been well established for the development of physical dependence. Cross-tolerance and cross-sensitization, although not always bidirectional, are also supported by a number of evidence, while less data have been gathered regarding the relationship of these systems in cognition and emotion. Nevertheless, the most recent advances in cannabinoid-opioid cross-modulation have been made in the area of drug craving and relapse processes. The present review is focused on the latest developments in the cannabinoid-opioid cross-modulation of their behavioural effects and the possible neurobiological substrates involved.

An NMDA antagonist (LY 235959) attenuates abstinence-induced withdrawal of planarians following acute exposure to a cannabinoid agonist (WIN 55212-2)

The mechanisms that facilitate the development and expression of cannabinoid physical dependence in humans and other mammals are poorly understood. The present experiments used a planarian model to provide evidence that pharmacological antagonism of NMDA receptors significantly attenuates the development of cannabinoid physical dependence. Abstinence-induced withdrawal from the cannabinoid agonist WIN 55212-2 (10 microM) was manifested as a significant (P<0.05) decrease in the rate of planarian spontaneous locomotor velocity (pLMV) when WIN 55212-2 (10 microM)-exposed planarians were placed into drug-free water. No change in pLMV occurred when WIN 55212-2 (10 microM)-exposed planarians were placed into water containing WIN 55212-2 (10 microM). WIN 55212-2 (10 microM)-exposed planarians placed into water containing LY 235959 (1 or 10 microM) did not display withdrawal (no significant difference, P>0.05, in pLMV). In addition, withdrawal was not observed (no significant difference, P>0.05, in pLMV) in planarians that were co-exposed to a solution containing WIN 55212-2 (10 microM) and LY 235959 (10 microM). The present results reveal that NMDA receptor activation mediates the development of cannabinoid physical dependence and the expression of cannabinoid withdrawal in planarians.

ERK-dependent modulation of cerebellar synaptic plasticity after chronic Delta9-tetrahydrocannabinol exposure

Chronic exposure to Delta9-tetrahydrocannabinol (THC) induces tolerance to cannabinoid-induced locomotor effects, which are mediated by cannabinoid receptors (CB1Rs) located in motor control regions, including the cerebellum. There is substantial evidence of cerebellar CB1R molecular adaptation and modifications in receptor signaling after prolonged cannabinoid exposure. However, very little is known about the effects of chronic cannabinoid administration on cerebellar synaptic plasticity, which may contribute to the development of cannabinoid behavioral tolerance. In the cerebellar cortex, activation of CB1R inhibits excitatory synaptic transmission at parallel fiber (PF)-Purkinje cell (PC) synapses by decreasing neurotransmitter release. Our study aimed to investigate the neurophysiological adaptive responses occurring at cerebellar PF-PC cell synapses after repeated THC exposure. In THC-tolerant mice, an increase of the basal release probability was found at PF-PC synapses, in parallel with a facilitation of slow mGluR1 (metabotropic glutamate receptor type 1)-mediated excitatory postsynaptic currents and a reduced sensitivity to the inhibitory effects of the CB1R agonist CP55,940 [(-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol]. Additionally, after repeated THC exposures, presynaptic PF-PC long-term potentiation was blocked by A1R (adenosine receptor-1) activation. Inhibition of the extracellular signal regulated kinase (ERK) pathway prevented these alterations of cerebellar synaptic transmission and plasticity. In summary, we provide evidence for ERK-dependent modulatory mechanisms at PF-PC synapses after chronic THC administration. This contributes to generation of forms of pathological synaptic plasticity that might play a role in cannabinoid dependence.

Adenosine A2a blockade prevents synergy between mu-opiate and cannabinoid CB1 receptors and eliminates heroin-seeking behavior in addicted rats

Relapse is the most serious limitation of effective medical treatment of opiate addiction. Opiate-related behaviors appear to be modulated by cannabinoid CB1 receptors (CB1) through poorly understood cross-talk mechanisms. Opiate and CB1 receptors are coexpressed in the nucleus accumbens (NAc) and dorsal striatum. These regions also have the highest density of adenosine A2a receptors (A2a) in the brain. We have been investigating the postsynaptic signaling mechanisms of mu-opiate receptors (MORs) and CB1 receptors in primary NAc/striatal neurons. In this article, we present evidence that MOR and CB1 act synergistically on cAMP/PKA signaling in NAc/striatal neurons. In addition, we find that synergy requires adenosine and A2a. Importantly, an A2a antagonist administered either directly into the NAc or indirectly by i.p. injection eliminates heroin-induced reinstatement in rats trained to self-administer heroin, a model of human craving and relapse. These findings suggest that A2a antagonists might be effective therapeutic agents in the management of abstinent heroin addicts.

Cannabinoid tolerance and dependence

The use of marijuana for recreational and medicinal purposes has resulted in a large prevalence of chronic marijuana users. Consequences of chronic cannabinoid administration include profound behavioral tolerance and withdrawal symptoms upon drug cessation. A marijuana withdrawal syndrome is only recently gaining acceptance as being clinically significant. Similarly, laboratory animals exhibit both tolerance and dependence following chronic administration of cannabinoids. These animal models are being used to evaluate the high degree of plasticity that occurs at the molecular level in various brain regions following chronic cannabinoid exposure. In this review, we describe recent advances that have increased our understanding of the impact of chronic cannabinoid administration on cannabinoid receptors and their signal transduction pathways. Additionally, we discuss several potential pharmacotherapies that have been examined to treat marijuana dependence.

The use of cannabinoids in multiple sclerosis

Naturally occurring cannabinoids including Delta9-tetrahydrocannabinol and cannabidiol as well as endocannabinoids and synthetic cannabinoids may have a role in modulating experimental models of multiple sclerosis. Recent clinical studies to treat symptoms of multiple sclerosis have shown varying results, which may reflect issues relating to the way in which such studies were conducted. There is now increasing interest in the potential role of cannabinoids not only in symptom relief, but also for their possible neuroprotective actions.

Proteomic analysis of nicotine-associated protein expression in the striatum of repeated nicotine-treated rats

Through the proteomic analysis using 2-dimensional electrophoresis, the nicotine addiction-associated proteins were extensively screened in the striatum of rat brains. The nicotine addiction was developed by repeated nicotine injection (0.4mg/kg s.c.), twice daily for 7 days, followed by one challenge injection after a 3 day withdrawal period, and then confirmed by observing a 2.3-fold increase in locomoter activity. The 3 up- and 4 down-regulated proteins were selected and identified to be zinc-finger binding protein-89 (ZBP-89), 2'3'-cyclic nucleotide 3'-phosphodiesterase 1, deoxyribonuclease 1-like 3 (DNase1l3), tandem pore domain halothane inhibited K(+) channel (THIK-2), brain-specific hyaluronan-binding protein (BRAL-1), death effector domain-containing DNA binding protein (DEDD), and brain-derived neurotrophic factor (BDNF) by mass spectrophotometric fingerprinting. Among them, the expression patterns of ZEB-89, DNase1l3, THIK-2, DEDD, and BDNF mRNAs were found to be coincident with those of cognate proteins, by using RT-PCR analysis. These proteins could be suggested as drug targets to develop a new therapy for nicotine-associated diseases, as well as the clues to understand the mechanism of nicotine.

Adenosine A2A receptors are involved in physical dependence and place conditioning induced by THC

A2A adenosine and CB1 cannabinoid receptors are highly expressed in the central nervous system, where they modulate numerous physiological processes including adaptive responses to drugs of abuse. Both purinergic and cannabinoid systems interact with dopamine neurotransmission (through A2A and CB1 receptors, respectively). Changes in dopamine neurotransmission play an important role in addictive-related behaviours. In this study, we investigated the contribution of A2A adenosine receptors in several behavioural responses of Delta9-tetrahydrocannabinol (THC) related to its addictive properties, including tolerance, physical dependence and motivational effects. For this purpose, we first investigated acute THC responses in mice lacking A2A adenosine receptors. Antinociception, hypolocomotion and hypothermia induced by acute THC administration remained unaffected in mutant mice. Chronic THC treatment developed similar tolerance to these acute effects in wild-type and A2A-knockout mice. However, differences in the body weight pattern were found between genotypes during such chronic treatment. Interestingly, the somatic manifestations of SR141716A-precipitated THC withdrawal were significantly attenuated in mutant mice. The motivational responses of THC were also evaluated by using the place-conditioning paradigm. A significant reduction of THC-induced rewarding and aversive effects was found in mice lacking A2A adenosine receptors in comparison with wild-type littermates. Binding studies revealed that these behavioural changes were not associated with any modification in the distribution and/or functional activity of CB1 receptors in knockout mice. Therefore, this study shows, for the first time, a specific involvement of A2A receptors in the addictive-related properties of cannabinoids.

Long-term administration of Delta9-tetrahydrocannabinol desensitizes CB1-, adenosine A1-, and GABAB-mediated inhibition of adenylyl cyclase in mouse cerebellum

Cannabinoid CB(1) receptors in the cerebellum mediate the inhibitory effects of Delta(9)-tetrahydrocannabinol (THC) on motor coordination. Intracellular effects of CB(1) receptors include inhibition of adenylyl cyclase via activation of G(i/o) proteins. There is evidence for the convergence of other neuronal receptors, such as adenosine A(1) and GABA(B), with the cannabinoid system on this signaling pathway to influence motor function. Previous studies have shown that brain CB(1) receptors are desensitized and down-regulated by long-term THC treatment, but few studies have examined the effects of long-term THC treatment on downstream effector activity in brain. Therefore, these studies examined the relationship between CB(1), adenosine A(1), and GABA(B) receptors in cerebella of mice undergoing prolonged treatment with vehicle or THC at the level of G protein activation and adenylyl cyclase inhibition. In control cerebella, CB(1) receptors produced less than additive inhibition of adenylyl cyclase with GABA(B) and A(1) receptors, indicating that these receptors are localized on overlapping populations of cells. Long-term THC treatment produced CB(1) receptor down-regulation and desensitization of both cannabinoid agonist-stimulated G protein activation and inhibition of forskolin-stimulated adenylyl cyclase. However, G protein activation by GABA(B) or A(1) receptors was unaffected. It is noteworthy that heterologous attenuation of GABA(B) and A(1) receptor-mediated inhibition of adenylyl cyclase was observed, even though absolute levels of basal and forskolin- or G(s)-stimulated activity were unchanged. These results indicate that long-term THC administration produces a disruption of inhibitory receptor control of cerebellar adenylyl cyclase and suggest a potential mechanism of cross-tolerance to the motor incoordinating effects of cannabinoid, GABA(B), and A(1) agonists.

Role of different brain structures in the behavioural expression of WIN 55,212-2 withdrawal in mice

We have evaluated several responses induced by the cannabinoid agonist WIN 55,212-2 related to its addictive properties, including rewarding effects and the development of physical dependence in mice. Moreover, we have studied the specific involvement of several brain regions with high density of CB1 cannabinoid receptors, such as striatum, hippocampus, amygdala and cerebellum, in the behavioural expression of SR 141716A-precipitated WIN 55,212-2 withdrawal. The systemic administration of the CB1 receptor antagonist SR 141716A (10 mg kg(-1), s.c.) precipitated behavioural signs of withdrawal in mice chronically treated with WIN 55,212-2 (1 and 2 mg kg(-1), intraperitoneal (i.p.)), revealing the development of physical dependence. The microinjection of SR 141716A (1.5 and 3 micrograms) into the cerebellum induced severe manifestations of abstinence in mice dependent on WIN 55,212-2 (1 mg kg(-1), i.p.). Out of 10 signs evaluated, seven were statistically significant: wet dog shakes, body tremor, paw tremor, piloerection, mastication, genital licks and sniffing. When the cannabinoid antagonist was administered into the hippocampus and the amygdala, a moderate but significant withdrawal syndrome was also observed. However, no signs of abstinence were induced when SR 141716A was microinjected into the striatum. WIN 55,212-2 produced rewarding effects in the place-conditioning paradigm in mice pre-exposed to a priming injection of the drug. These results show a reliable behavioural model to reveal rewarding effects and physical dependence induced by the repeated administration of WIN 55,212-2 in mice. The cerebellum and to a lesser extent the hippocampus and the amygdala participate in the behavioural expression of cannabinoid withdrawal.

Signaling pathways involved in the development of cannabinoid tolerance

Considerable plasticity exists in the endogenous cannabinoid system, as evidenced by the high degree of tolerance that develops following repetitive exposure to exogenously administered cannabinoid receptor agonists. This tolerance development is accompanied by cannabinoid CB(1) receptor downregulation and attenuation of G-protein activation. The biological processes responsible for CB(1) receptor downregulation remain to be fully understood. However, recent evidence suggests that several protein kinases participate in the development of cannabinoid tolerance. These observations implicate a role for protein kinases in cannabinoid signaling pathways. It remains to be established whether these protein kinases are directly involved in CB(1) receptor regulation or whether they contribute to tolerance by modulating additional signaling pathways.

Behavioral and molecular changes elicited by acute administration of SR141716 to Delta9-tetrahydrocannabinol-tolerant rats: an experimental model of cannabinoid abstinence

Whether chronic cannabinoid consumption produces a dependent state comparable to that occurring with other drugs (e.g. the appearance of withdrawal signs when consumption is interrupted), and whether chronic cannabinoid consumption increases the risk of consuming other drugs of greater addictive power, are probably the two questions relating to cannabinoid addiction that provoke the most controversy. The present study was designed to further explore these two questions in laboratory animals. Firstly, we examined the effects of an acute challenge with SR141716 (an antagonist for the cannabinoid CB(1) receptor) in Delta(9)-tetrahydrocannabinol (Delta(9)-THC)-tolerant rats. This antagonist has been reported to precipitate a cannabinoid withdrawal syndrome. Thus, the administration of SR141716 to Delta(9)-THC-tolerant rats reduced inactivity in the open-field test and enhanced responses as tremor, turning and retropulsion-these responses that were only slightly enhanced in control rats. The administration of SR141716 increased the plasma prolactin and the corticosterone concentration in controls, but these increases were much lesser in Delta(9)-THC-tolerant rats. In addition, CRF-mRNA levels in the paraventricular hypothalamic nucleus, while reduced in SR141716-treated controls, were significantly increased in Delta(9)-THC-tolerant rats. The analysis of endocannabinoids also revealed that the administration of SR141716, which was mostly inactive in control rats, was able to reverse the changes in anandamide or 2-arachidonoylglycerol concentrations found in Delta(9)-THC-tolerant rats, in the striatum, limbic forebrain, diencephalon, cerebellum and brainstem, but not in the midbrain and hippocampus. As a second objective, we evaluated whether Delta(9)-THC-tolerant rats were more vulnerable to morphine in a self-administration paradigm. The Delta(9)-THC-tolerant and control rats self-administered morphine to a similar extent, in concordance with the similar values of dopaminergic activity in limbic and motor regions. In summary, our data indicate that Delta(9)-THC-tolerant rats were not more vulnerable to the reinforcing properties of morphine. However, they responded to the blockade of CB(1) receptors by exhibiting slightly but possibly relevant differences in behavioral, endocrine and molecular parameters compared to the response in non-tolerant rats. This is indicative of the existence of a withdrawal syndrome in cannabinoid-tolerant rats that is mild compared with abstinence in opioid-dependent rats.

Modulation of extracellular signal-regulated kinases cascade by chronic Δ9-tetrahydrocannabinol treatment

Acute Delta(9)-tetrahydrocannabinol (THC) injection increased ERK pathway (ERK, pCREB, and c-fos) mostly in the caudate putamen and cerebellum. This effect underwent to homeostatic adaptation after chronic treatment. Moreover, chronic THC exposure induced increases in the ERK cascade (ERK, pCREB, and Fos B) in the prefrontal cortex and hippocampus, suggesting that different neuronal circuits seem to be involved in the early phase and late phase of exposure. The involvement of ERK pathway in cannabinoid chronic exposure was also confirmed in Ras-GRF1 knock out mice, a useful model where cannabinoid-induced ERK activation is lost. In fact, Ras-GRF1 ko mice did not develop tolerance to THC analgesic and hypolocomotor effect. Our data suggest that ERK cascade could play a pivotal role in the induction of synaptic plasticity due to cannabinoid chronic exposure.

The neurobiology of addiction

Drug addiction includes complex neurobiological and behavioural processes. Acute reinforcing effects of drugs of abuse are responsible for the initiation of drug addiction, whereas the negative consequences of drug abstinence have a crucial motivational significance for relapse and maintenance of the addictive process. The mesocorticolimbic system represents a common neuronal substrate for the reinforcing properties of drugs of abuse. Both dopamine and opioid transmission play a crucial role in this reward pathway. Common neuronal changes have also been reported during the abstinence to different drugs of abuse that could underlie the negative motivational effects of withdrawal. These changes include decreased dopaminergic activity in the mesolimbic system and a recruitment of the brain stress pathways. All drugs of abuse interact with these brain circuits by acting on different molecular and neurochemical mechanisms. The existence of bidirectional interactions between different drugs of abuse, such as opioids and cannabinoids, provides further findings to support this common neurobiological substrate for drug addictive processes.

Cellular mechanisms of Delta 9-tetrahydrocannabinol behavioural sensitization

We investigated the cellular events linked to the induction of cannabinoid behavioural sensitization. In sensitized rats, autoradiographic binding studies with [3H]CP-55,940 showed a significant increase in cannabinoid receptor binding, specifically in the cerebellum, with no changes in the other brain areas where basal CB1-receptor expression is observed. In vitro autoradiography of CP-55,940-stimulated [35S]GTP gamma S binding provided a picture of cannabinoid receptor-mediated G protein activation. Basal [35S]GTP gamma S binding was not affected, whereas sensitized rats showed a significant increase of net [35S]GTP gamma S binding in the caudate putamen and cerebellum. Autoradiographic studies suggested that only these two areas had altered receptor functionality. We therefore focused our intracellular investigations only there, first surveying the responsiveness of the cAMP system to cannabinoids. CP-55,940 was unable to inhibit forskolin-induced cAMP accumulation in the cerebellum of sensitized animals, but no difference was observed between groups in the caudate putamen. Finally, we surveyed the levels of CREB and AP-1 binding activity, in the same two areas and found no difference in sensitized rats. The intracellular picture in sensitized rats suggests that besides the cAMP cascade, other signalling pathways may participate in the development of cannabinoid sensitization.

Addictive potential of cannabinoids: the underlying neurobiology

Drugs that are addictive in humans have a number of commonalities in animal model systems-(1). they enhance electrical brain-stimulation reward in the core meso-accumbens reward circuitry of the brain, a circuit encompassing that portion of the medial forebrain bundle (MFB) which links the ventral tegmental area (VTA) of the mesencephalic midbrain with the nucleus accumbens (Acb) of the ventral limbic forebrain; (2). they enhance neural firing of a core dopamine (DA) component of this meso-accumbens reward circuit; (3). they enhance DA tone in this reward-relevant meso-accumbens DA circuit, with resultant enhancement of extracellular Acb DA; (4). they produce conditioned place preference (CPP), a behavioral model of incentive motivation; (5). they are self-administered; and (6). they trigger reinstatement of drug-seeking behavior in animals behaviorally extinguished from intravenous drug self-administration behavior and, perforce, pharmacologically detoxified from their self-administered drug. Cannabinoids were long considered 'anomalous', in that they were believed to not interact with these brain reward processes or support drug-seeking and drug-taking behavior in these animal model systems. However, it is now clear-from the published data of several research groups over the last 15 years-that this view of cannabinoid action on brain reward processes and reward-related behaviors is untenable. This paper reviews those data, and concludes that cannabinoids act on brain reward processes and reward-related behaviors in strikingly similar fashion to other addictive drugs.

Spontaneous network activity of cerebellar granule neurons: impairment by in vivo chronic cannabinoid administration

Synchronized activity of neuronal networks has been proposed to be essential for cerebellar function. To examine the occurrence and organization of spontaneous neuronal activity in the cerebellum in vivo, we imaged mouse cerebellar slices loaded with the intracellular Ca2+ concentration indicator, fura-2. Recordings were then analysed statistically to identify correlated network activity. Ca2+ imaging revealed consistent spontaneous correlated network activity of granule cells (GC), which often occurred in clusters of coactivated GC. The number of spontaneously active GC, their activation frequency and correlation, were controlled by glutamate and GABA ionotropic receptors. These findings indicate that distinctive patterns of correlated activity between GC networks may be relevant for cerebellar circuit function. Cannabinoid antagonist-precipitated delta9-tetrahydrocannabinol (THC) withdrawal impaired motor coordination. Given that the cerebellum has been suggested recently to be a main substrate for cannabinoid withdrawal, we used imaging of spontaneous network activity to examine whether GC, which contain CB1 cannabinoid receptors, respond to chronic THC treatment and withdrawal. Acute administration of THC had no effect on patterns of spontaneous GC network activity. In contrast, chronic THC administration severely impaired GC activity and network coordination. Incubation of cerebellar slices, from chronically THC-treated mice, with the cannabinoid antagonist, SR141716A increased the number and network correlation of active GC. These data provide physiological evidence of the involvement of cerebellar circuits in the adaptive changes occurring during chronic THC exposure and withdrawal.

Marijuana withdrawal syndrome in the animal model

Although the proposition that repeated marijuana use can lead to marijuana dependence has long been accepted, only recently has evidence emerged suggesting that abstinence leads to clinically significant withdrawal symptoms. Converging evidence from human and animal studies has increased our understanding of cannabinoid dependence. One of the most powerful tools to advance this area of research is the CB1 cannabinoid receptor antagonist SR 141716A, which reliably precipitates withdrawal syndromes in mice, rats, and dogs that have been treated repeatedly with cannabinoids. In addition, the use of CB1 receptor knockout mice has revealed that not only cannabinoid dependence is mediated through a CB1 receptor mechanism of action, but CB1 receptors also modulate opioid dependence. Moreover, the results of other genetically altered mouse models suggest the existence of a reciprocal relationship between cannabinoid and opioid systems in drug dependence. Undoubtedly, these animal models will play pivotal roles in further characterizing cannabinoid dependence and elucidating the mechanisms of action, as well as developing potential pharmacotherapies for cannabinoid dependence.

Characterization of the mouse adenylyl cyclase type VIII gene promoter: regulation by cAMP and CREB

Adenylyl cyclase (AC) type VIII has been implicated in several forms of neural plasticity, including drug addiction and learning and memory. In the present study, we directly examined the role for the transcription factor CREB (cAMP response element binding protein) in regulating ACVIII expression by cloning a 5.2 kilobase region upstream of the translation start site of the mouse ACVIII gene. Analysis of this fragment revealed consensus elements for several transcription factors, including a canonical cAMP response element (CRE) in close proximity to the transcription initiation region. Next, ACVIII promoter activity was studied in two neural-derived cell lines and in primary cultures of rat striatal neurons. Activation of the cAMP pathway by forskolin treatment increased promoter activity, and a series of deletion and point mutants demonstrated that this activation is mediated specifically via the canonical CRE site. Gel shift assays confirmed that this site can bind CREB and several CREB family proteins. Further, activation of the ACVIII promoter by forskolin was potentiated by expression of a constitutively active form of CREB, CREB-VP16, whereas it was inhibited by expression of a dominant-negative form of CREB, A-CREB. Finally, over-expression of CREB in vivo, by viral-mediated gene transfer, induced ACVIII promoter activity in the brains of ACVIII-LacZ transgenic mice. These results suggest that the ACVIII gene is regulated by CREB in vitro and in vivo and that this regulation may contribute to CREB-dependent neural plasticity.

Heterologous sensitization of adenylate cyclase is protein kinase A-dependent in Cath.a differentiated (CAD)-D2L cells

Persistent activation of Galphai/o-coupled receptors results in a paradoxical enhancement of subsequent drug-stimulated adenylate cyclase activity. The exact mechanism of this up-regulation in the cyclic AMP signaling pathway, known as heterologous sensitization, remains undefined. The present study was designed to investigate the involvement of cyclic AMP-dependent protein kinase in D2L receptor-mediated sensitization in a neuronal cellular environment. The current studies were conducted in the Cath.a differentiated (CAD) cell line transfected stably with the D2L dopamine receptor (CAD-D2L). Long-term 18 h treatment with the D2 receptor agonist, quinpirole, resulted in a two-fold enhancement of forskolin-stimulated cyclic AMP accumulation. Similarly, long-term treatment with the PKA inhibitors, H89 or Rp-8Br-cAMP, also enhanced adenylate cyclase activity. In contrast, long-term activation of protein kinase A (PKA) by forskolin, isobutylmethylxanthine (IBMX), or dibutyryl cyclic AMP caused a significant reduction in subsequent forskolin-stimulated cyclic AMP accumulation and reduced both quinpirole- and H89-induced heterologous sensitization. The effects of PKA inhibitors and activators did not involve changes in PKA subunit expression. RT-PCR analysis of adenylate cyclase isoform expression patterns revealed the expression of mRNA for ACVI and ACIX in CAD-D2L cells. The ability of ACVI to be negatively regulated by PKA is consistent with the observation that inhibition of PKA results in heterologous sensitization of adenylate cyclase activity in CAD-D2L cells.

Study of cannabinoid dependence in animals

Different animal models have been used to clarify the consequences of chronic exposure to cannabinoid agonists and their abuse liability. Following the chronic administration of cannabinoids, tolerance develops to most of their pharmacological effects. The development of cannabinoid tolerance is particularly rapid, and seems to be due to pharmacodynamic events. A cross-tolerance among different exogenous cannabinoid agonists has been reported. Somatic signs of spontaneous withdrawal have not been reported after chronic Delta(9)-tetrahydrocannabinol (THC) treatment, but were observed after chronic treatment with the cannabinoid agonist WIN-55,212-2. The administration of the CB(1) cannabinoid antagonist SR141716A in animals chronically treated with THC and other cannabinoid agonists precipitated somatic manifestations of withdrawal. The potential ability of anandamide to induce physical dependence has not been clarified. Subjective drug effects of cannabinoids have been reported by drug discrimination studies, which show cross discrimination among different natural and synthetic agonists. The rewarding effects of cannabinoids have been revealed by using several paradigms: place conditioning, intracranial self-stimulation, and self-administration. Cannabinoids have been reported to lower intracranial self-stimulation thresholds in rats. However, particular experimental conditions are required to induce conditioned place preference with cannabinoids. Numerous studies have shown that THC is unable to induce a self-administration behaviour in animals. However, WIN-55,212-2 was intravenously self-administered in mice, and monkeys that had a previous history of cocaine self-administration also self-administered THC. The mesolimbic dopaminergic system seems to be the substrate for the rewarding properties of cannabinoids.

Nicotine withdrawal syndrome: behavioural distress and selective up-regulation of the cyclic AMP pathway in the amygdala

Nicotine addiction is a major public health issue. The use of laboratory animal models is a crucial tool in research aiming at understanding the pathophysiological mechanisms of nicotine dependence and at proposing new therapies. In rodents, cessation of nicotine exposure or administration of the nicotinic antagonist mecamylamine induces a nicotine withdrawal syndrome. Antagonist-precipitated withdrawal from other abused drugs such as opiates or cannabinoids has been associated with region-specific modifications of the activity of the cyclic AMP pathway. Here we show that mecamylamine-precipitated nicotine withdrawal in the rat is characterized by an increase in thigmotaxis (time spent in the periphery of an open field) that may be indicative of behavioural distress and can be associated with a selective up-regulation of adenylyl cyclase activity in the amygdala, a region implicated in the regulation of negative affect in response to aversive stimuli, including withdrawal. Adenylyl cyclase activity that is increased during precipitated nicotine withdrawal is stimulated by calcium/calmodulin, as is also the case for opioid and cannabinoid abstinence. This suggests that directly or indirectly mediated increases in the activity of the cyclic AMP pathway could constitute a possible common molecular mechanism underlying neuroadaptive changes following abstinence from different abused drugs.

Cannabinoid addiction: behavioral models and neural correlates

The use of cannabis sativa preparations as recreational drugs can be traced back to the earliest civilizations. However, animal models of cannabinoid addiction allowing the exploration of neural correlates of cannabinoid abuse have been developed only recently. We review these models and the role of the CB1 cannabinoid receptor, the main target of natural cannabinoids, and its interaction with opioid and dopamine transmission in reward circuits. Extensive reviews on the molecular basis of cannabinoid action are available elsewhere (Piomelli et al., 2000; Schlicker and Kathmann, 2001).

Novel, not adenylyl cyclase-coupled cannabinoid binding site in cerebellum of mice

In this study we report data suggesting the presence of a non-CB1, non-CB2 cannabinoid site in the cerebellum of CB1-/- mice. We have carried out [(35)S]GTPgammaS binding experiments in striata, hippocampi, and cerebella of CB1-/- and CB1(+/+) mice with Delta(9)-THC, WIN55,212-2, HU-210, SR141716A, and SR144528. In CB1-/- mice Delta(9)-THC and HU-210 did not stimulate [(35)S]GTPgammaS binding. However, WIN55,212-2 was able to stimulate [(35)S]GTPgammaS binding in cerebella of CB1-/- mice. The maximal effect of this stimulation was 31% that of wild type animals. This effect was reversible neither by CB1 nor CB2 receptor antagonists. Similar results were obtained with the endogenous cannabinoid, anandamide. However, adenylyl cyclase was not inhibited by WIN55,212-2 or anandamide in the CB1(minus sign/minus sign) animals. In striata and hippocampi of CB1-/- mice no [(35)S]GTPgammaS stimulation curve could be obtained with WIN55,212. Our findings suggest that there is a non-CB1 non-CB2 receptor present in the cerebellum of CB1-/- mice.

Endocannabinoids in cognition and dependence

Cannabis use is associated with a wide range of pharmacological effects, some of which have potential therapeutic benefit while others result in negative outcomes. Acute cannabinoid intoxication has been well documented to produce deficits in cognitive functioning with concomitant changes in glutamatergic, GABAergic, and cholinergic neurochemical systems in the hippocampus, each of which has been implicated in memory. Additionally, cannabis-dependent individuals abstaining from this drug can undergo a constellation of mild withdrawal effects. The use of the CB(1) cannabinoid receptor antagonist SR141716A and transgenic mice lacking the CB(1) receptor are critical tools for investigating the role of the endocannabinoid system in cognition, drug dependence, and other physiological processes. Converging evidence in which performance in a variety of memory tasks is enhanced following either SR141716A treatment or in CB(1) receptor knockout mice indicates that this system may play an important role in modulating cognition. There are also indications that this system may function to modulate opioid dependence. The purpose of this review is to describe recent advances that have furthered our understanding of the roles that the endocannabinoid system play on both cognition and drug dependence.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

The psychoactive ingredient of marijuana induces behavioural sensitization

Here we describe, for the first time, the occurrence of behavioural sensitization after chronic exposure to Delta9-tetrahydrocannabinol. Rats were treated twice a day, for five days, with increasing doses (5, 10, 20, 40, 40 mg/kg i.p.) of Delta9-tetrahydrocannabinol or its vehicle and after 20 days of withdrawal, animals were challenged with 5 mg/kg (i.p.) of the drug and their behaviour was assessed. Contrary to the motor inhibition induced in control rats, challenge with Delta9-tetrahydrocannabinol in pre-exposed animals elicited a complex behavioural syndrome mainly characterized by oral stereotyped items. Due to the relevance of behavioural sensitization in drug-seeking behaviour that persists long after discontinuation of drug use, our findings suggest that cannabinoids could trigger neurobiological alteration not dissimilar from those observed with more harmful abused drugs.

The cannabinoid CB(1) receptor antagonist SR141716A increases norepinephrine outflow in the rat anterior hypothalamus

The effects of the selective cannabinoid CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide-hydrochloride (SR141716A) on extracellular concentrations of norepinephrine and 5-hydroxytryptamine (5-HT) were assessed by in vivo microdialysis in the anterior hypothalamus of freely moving rats. SR14716A (0.3, 1, 3 mg/kg, i.p.) dose-dependently increased norepinephrine efflux to about 300% of baseline, without affecting 5-HT levels. This increase in norepinephrine outflow could play an important role in the pharmacological and potentially therapeutic actions of SR141716A.

Regulation and role of adenylyl cyclase isoforms

At least nine closely related isoforms of adenylyl cyclases (ACs), the enzymes responsible for the synthesis of cyclic AMP (cAMP) from ATP, have been cloned and characterized in mammals. Depending on the properties and the relative levels of the isoforms expressed in a tissue or a cell type at a specific time, extracellular signals received through the G-protein-coupled receptors can be differentially integrated. The present review deals with various aspects of such regulations, emphasizing the role of calcium/calmodulin in activating AC1 and AC8 in the central nervous system, the potential inhibitory effect of calcium on AC5 and AC6, and the changes in the expression pattern of the isoforms during development. A particular emphasis is given to the role of cAMP during drug and ethanol dependency and to some experimental limitations (pitfalls in the interpretation of cellular transfection, scarcity of the invalidation models, existence of complex macromolecular structures, etc).

Effects of cannabinoids in Krox-24 targeted mice

Krox-24 is an immediate early gene encoding a zinc-finger transcription factor implicated in several adaptive responses, and its induction by cannabinoids has been reported. We used mice targeted in the Krox-24 gene to specifically dissect the role of this protein in the acute and chronic central actions of cannabinoids. We report here on the ability of cannabinoids to activate G-proteins and to inhibit adenylyl cyclase, and to elicit behavioral responses in wild-type and mutant mice. The behavioral parameters and the biochemical correlates of abstinence after delta9-THC withdrawal were evaluated. We show that Krox-24 is not involved in the acute analgesic effects of delta9-THC and in the SR precipitated delta9-THC withdrawal syndrome.