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

Cannabinoids: Role in Neurological Diseases and Psychiatric Disorders

An impact of legalization and decriminalization of marijuana is the gradual increase in the use of cannabis for recreational purposes, which poses a potential threat to society and healthcare systems worldwide. However, the discovery of receptor subtypes, endogenous endocannabinoids, and enzymes involved in synthesis and degradation, as well as pharmacological characterization of receptors, has led to exploration of the use of cannabis in multiple peripheral and central pathological conditions. The role of cannabis in the modulation of crucial events involving perturbed physiological functions and disease progression, including apoptosis, inflammation, oxidative stress, perturbed mitochondrial function, and the impaired immune system, indicates medicinal values. These events are involved in most neurological diseases and prompt the gradual progression of the disease. At present, several synthetic agonists and antagonists, in addition to more than 70 phytocannabinoids, are available with distinct efficacy as a therapeutic alternative in different pathological conditions. The present review aims to describe the use of cannabis in neurological diseases and psychiatric disorders.

The Impacts of Adolescent Cannabinoid Exposure on Striatal Anxiety- and Depressive-Like Pathophysiology Are Prevented by the Antioxidant N-Acetylcysteine

Background

Exposure to Δ9-tetrahydrocannabinol (THC) is an established risk factor for later-life neuropsychiatric vulnerability, including mood- and anxiety-related symptoms. The psychotropic effects of THC on affect and anxiogenic behavioral phenomena are known to target the striatal network, particularly the nucleus accumbens, a neural region linked to mood and anxiety disorder pathophysiology. THC may increase neuroinflammatory responses via the redox system and dysregulate inhibitory and excitatory neural balance in various brain circuits, including the striatum. Thus, interventions that can induce antioxidant effects may counteract the neurodevelopmental impacts of THC exposure.

Methods

In the current study, we used an established preclinical adolescent rat model to examine the impacts of adolescent THC exposure on various behavioral, molecular, and neuronal biomarkers associated with increased mood and anxiety disorder vulnerability. Moreover, we investigated the protective properties of the antioxidant N-acetylcysteine against THC-related pathology.

Results

We demonstrated that adolescent THC exposure induced long-lasting anxiety- and depressive-like phenotypes concomitant with differential neuronal and molecular abnormalities in the two subregions of the nucleus accumbens, the shell and the core. In addition, we report for the first time that N-acetylcysteine can prevent THC-induced accumbal pathophysiology and associated behavioral abnormalities.

Conclusions

The preventive effects of this antioxidant intervention highlight the critical role of redox mechanisms underlying cannabinoid-induced neurodevelopmental pathology and identify a potential intervention strategy for the prevention and/or reversal of these pathophysiological sequelae.

Effects of Cannabis on Glutamatergic Neurotransmission: The Interplay between Cannabinoids and Glutamate

There has been a significant increase in the consumption of cannabis for both recreational and medicinal purposes in recent years, and its use can have long-term consequences on cognitive functions, including memory. Here, we review the immediate and long-term effects of cannabis and its derivatives on glutamatergic neurotransmission, with a focus on both the presynaptic and postsynaptic alterations. Several factors can influence cannabinoid-mediated changes in glutamatergic neurotransmission, including dosage, sex, age, and frequency of use. Acute exposure to cannabis typically inhibits glutamate release, whereas chronic use tends to increase glutamate release. Conversely, the postsynaptic alterations are more complicated than the presynaptic effects, as cannabis can affect the glutamate receptor expression and the downstream signaling of glutamate. All these effects ultimately influence cognitive functions, particularly memory. This review will cover the current research on glutamate-cannabis interactions, as well as the future directions of research needed to understand cannabis-related health effects and neurological and psychological aspects of cannabis use.

Addictive drugs modify neurogenesis, synaptogenesis and synaptic plasticity to impair memory formation through neurotransmitter imbalances and signaling dysfunction

Drug abuse changes neurophysiological functions at multiple cellular and molecular levels in the addicted brain. Well-supported scientific evidence suggests that drugs negatively affect memory formation, decision-making and inhibition, and emotional and cognitive behaviors. The mesocorticolimbic brain regions are involved in reward-related learning and habitual drug-seeking/taking behaviors to develop physiological and psychological dependence on the drugs. This review highlights the importance of specific drug-induced chemical imbalances resulting in memory impairment through various neurotransmitter receptor-mediated signaling pathways. The mesocorticolimbic modifications in the expression levels of brain-derived neurotrophic factor (BDNF) and the cAMP-response element binding protein (CREB) impair reward-related memory formation following drug abuse. The contributions of protein kinases and microRNAs (miRNAs), along with the transcriptional and epigenetic regulation have also been considered in memory impairment underlying drug addiction. Overall, we integrate the research on various types of drug-induced memory impairment in distinguished brain regions and provide a comprehensive review with clinical implications addressing the upcoming studies.

Endocannabinoid signaling in oligodendroglia

In the central nervous system, oligodendrocytes synthesize the myelin, a specialized membrane to wrap axons in a discontinuous way allowing a rapid saltatory nerve impulse conduction. Oligodendrocytes express a number of growth factors and neurotransmitters receptors that allow them to sense the environment and interact with neurons and other glial cells. Depending on the cell cycle stage, oligodendrocytes may respond to these signals by regulating their survival, proliferation, migration, and differentiation. Among these signals are the endocannabinoids, lipidic molecules synthesized from phospholipids in the plasma membrane in response to cell activation. Here, we discuss the evidence showing that oligodendrocytes express a full endocannabinoid signaling machinery involved in physiological oligodendrocyte functions that can be therapeutically exploited to promote remyelination in central nervous system pathologies.

THC and CBD produce divergent effects on perception and panic behaviours via distinct cortical molecular pathways

Clinical and pre-clinical evidence demonstrates divergent psychotropic effects of THC vs. CBD. While THC can induce perceptual distortions and anxiogenic effects, CBD displays antipsychotic and anxiolytic properties. A key brain region responsible for regulation of cognition and affect, the medial prefrontal cortex (PFC), is strongly modulated by cannabinoids, suggesting that these dissociable THC/CBD-dependent effects may involve functional and molecular interplay within the PFC. The primary aim of this study was to investigate potential interactions and molecular substrates involved in PFC-mediated effects of THC and CBD on differential cognitive and affective behavioural processing. Male Sprague Dawley rats received intra-PFC microinfusions of THC, CBD or their combination, and tested in the latent inhibition paradigm, spontaneous oddity discrimination test, elevated T-maze and open field. To identify local, drug-induced molecular modulation in the PFC, PFC samples were collected and processed with Western Blotting. Intra-PFC THC induced strong panic-like responses that were counteracted with CBD. In contrast, CBD did not affect panic-like behaviours but blocked formation of associative fear memories and impaired latent inhibition and oddity discrimination performance. Interestingly, these CBD effects were dependent upon 5-HT_1A receptor transmission but not influenced by THC co-administration. Moreover, THC induced robust phosphorylation of ERK1/2 that was prevented by CBD, while CBD decreased phosphorylation of p70S6K, independently of THC. These results suggest that intra-PFC infusion of THC promotes panic-like behaviour associated with increased ERK1/2 phosphorylation. In contrast, CBD impairs perceptive functions and latent inhibition via activation of 5-HT_1A receptors and reduced phosphorylation of p70S6K.

The role of the mTOR pathway in models of drug-induced reward and the behavioural constituents of addiction

BACKGROUND

Exposure to drugs of abuse induces neuroadaptations in critical nodes of the so-called reward systems that are thought to mediate the transition from controlled drug use to the compulsive drug-seeking that characterizes addictive disorders. These neural adaptations are likely to require protein synthesis, which is regulated, among others, by the mechanistic target of the rapamycin kinase (mTOR) signalling cascade.

METHODS

We have performed a narrative review of the literature available in PubMed about the involvement of the mTOR pathway in drug-reward and addiction-related phenomena.

AIMS

The aim of this study was to review the underlying architecture of this complex intracellular network and to discuss the alterations of its components that are evident after exposure to drugs of abuse. The aim was also to delineate the effects that manipulations of the mTOR network have on models of drug reward and on paradigms that recapitulate some of the psychological components of addiction.

RESULTS

There is evidence for the involvement of the mTOR pathway in the acute and rewarding effects of drugs of abuse, especially psychostimulants. However, the data regarding opiates are scarce. There is a need to use sophisticated animal models of addiction to ascertain the real role of the mTOR pathway in this pathology and not just in drug-mediated reward. The involvement of this pathway in behavioural addictions and impulsivity should also be studied in detail in the future.

CONCLUSIONS

Although there is a plethora of data about the modulation of mTOR by drugs of abuse, the involvement of this signalling pathway in addictive disorders requires further research.

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.

Epigenetic mechanisms associated with addiction-related behavioural effects of nicotine and/or cocaine: implication of the endocannabinoid system

The addictive use of nicotine (NC) and cocaine (COC) continues to be a major public health problem, and their combined use has been reported, particularly during adolescence. In neural plasticity, commonly induced by NC and COC, as well as behavioural plasticity related to the use of these two drugs, the involvement of epigenetic mechanisms, in which the reversible regulation of gene expression occurs independently of the DNA sequence, has recently been reported. Furthermore, on the basis of intense interactions with the target neurotransmitter systems, the endocannabinoid (ECB) system has been considered pivotal for eliciting the effects of NC or COC. The combined use of marijuana with NC and/or COC has also been reported. This article presents the addiction-related behavioural effects of NC and/or COC, based on the common behavioural/neural plasticity and combined use of NC/COC, and reviews the interacting role of the ECB system. The epigenetic processes inseparable from the effects of NC and/or COC (i.e. DNA methylation, histone modifications and alterations in microRNAs) and the putative therapeutic involvement of the ECB system at the epigenetic level are also discussed.

Neurobiology of Propofol Addiction and Supportive Evidence: What Is the New Development?

Propofol is a short-acting intravenous anesthetic agent suitable for induction and maintenance of general anesthesia as well as for procedural and intensive care unit sedation. As such it has become an unparalleled anesthetic agent of choice in many institutional and office practices. However, in addition to its idealistic properties as an anesthetic agent, there is accumulating evidence suggesting its potential for abuse. Clinical and experimental evidence has revealed that not only does propofol have the potential to be abused, but also that addiction to propofol shows a high mortality rate. Based on this evidence, different researchers have shown interest in determining the probability of propofol to be an addictive agent by comparing it with other drugs of abuse and depicting a functional similitude that involves the mesocorticolimbic pathway of addiction. In light of this, the Drug Enforcement Agency and the American Society of Anesthesiologists have put forth certain safety recommendations for the use of propofol. Despite this, the abuse potential of propofol has been challenged at different levels and therefore the preeminent focus will be to further validate the linkage from medicinal and occasional use of propofol to its addiction, as well as to explore the cellular and molecular targets involved in establishing this linkage, so as to curb the harm arising out of it. This review incorporates the clinical and biomolecular evidence supporting the abuse potential of propofol and brings forth the promising targets and the foreseeable mechanism causing the propofol addiction phenotypes, which can be called upon for future developments in this field.

Δ9-Tetrahydrocannabinol Experience Influences ΔFosB and Downstream Gene Expression in Prefrontal Cortex

Introduction: Repeated administration of abused drugs, including Δ⁹-tetrahydrocannabinol (THC), induces the stable transcription factor ΔFosB in dopaminergic terminal field regions of the mesolimbic system. These studies investigated the effect of prior repeated THC treatment on THC-induced ΔFosB expression and regulation of downstream targets in the forebrain.

Methods: Mice received THC (10 mg/kg) or vehicle twice daily for 13 days, and then half of each group received a single injection of THC or vehicle 45 min before brain collection. ΔFosB messenger RNA (mRNA) and protein were measured by polymerase chain reaction and immunoblotting, respectively. Potential downstream targets of ΔFosB induction were measured by immunoblot.

Results: THC injection in mice with a history of repeated THC treatment enhanced ΔFosB expression as compared with vehicle in the prefrontal cortex (PFC), nucleus accumbens (NAc), and amygdala. This change occurred concomitantly with an increase in ΔFosB mRNA in the PFC and NAc. THC injection in mice with a history of repeated THC treatment increased expression of cyclin-dependent kinase 5 (Cdk5) and its regulatory protein p35 only in the PFC. This increase in Cdk5 and p35 expression in PFC was also found in mice that had only received repeated THC administration, suggesting that this effect might be due to induction of ΔFosB. Extracellular signal-regulated kinase (ERK) phosphorylation was increased in PFC after THC injection in repeated THC-treated mice. Phosphorylation of glycogen synthase kinase-3β (GSK3β), a Cdk5 target, was reduced in PFC after repeated THC treatment regardless of THC history, and phosphorylation of dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) at the Cdk5-regulated threonine 75 site was unchanged.

Conclusion: These results suggest that a history of repeated THC administration primes THC-mediated induction of ΔFosB in the NAc and PFC, and that expression of both downstream targets of ΔFosB (e.g., Cdk5 and p35) and upstream activators (e.g., pERK) in the PFC is dependent on THC history, which might have functional implications in addiction and neuropsychiatric disease.

Have we been ignoring the elephant in the room? Seven arguments for considering the cerebellum as part of addiction circuitry

Addiction involves alterations in multiple brain regions that are associated with functions such as memory, motivation and executive control. Indeed, it is now well accepted that addictive drugs produce long-lasting molecular and structural plasticity changes in corticostriatal-limbic loops. However, there are brain regions that might be relevant to addiction other than the prefrontal cortex, amygdala, hippocampus and basal ganglia. In addition to these circuits, a growing amount of data suggests the involvement of the cerebellum in many of the brain functions affected in addicts, though this region has been overlooked, traditionally, in the addiction field. Therefore, in the present review we provide seven arguments as to why we should consider the cerebellum in drug addiction. We present and discuss compelling evidence about the effects of drugs of abuse on cerebellar plasticity, the involvement of the cerebellum in drug-induced cue-related memories, and several findings showing that the instrumental memory and executive functions also recruit the cerebellar circuitry. In addition, a hypothetical model of the cerebellum's role relative to other areas within corticostriatal-limbic networks is also provided. Our goal is not to review animal and human studies exhaustively but to support the inclusion of cerebellar alterations as a part of the physiopathology of addiction disorder.

Molecular Mechanism: ERK Signaling, Drug Addiction, and Behavioral Effects

Addiction to psychostimulants has been considered as a chronic psychiatric disorder characterized by craving and compulsive drug seeking and use. Over the past two decades, accumulating evidence has demonstrated that repeated drug exposure causes long-lasting neurochemical and cellular changes that result in enduring neuroadaptation in brain circuitry and underlie compulsive drug consumption and relapse. Through intercellular signaling cascades, drugs of abuse induce remodeling in the rewarding circuitry that contributes to the neuroplasticity of learning and memory associated with addiction. Here, we review the role of the extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase, and its related intracellular signaling pathways in drug-induced neuroadaptive changes that are associated with drug-mediated psychomotor activity, rewarding properties and relapse of drug seeking behaviors. We also discuss the neurobiological and behavioral effects of pharmacological and genetic interferences with ERK-associated molecular cascades in response to abused substances. Understanding the dynamic modulation of ERK signaling in response to drugs may provide novel molecular targets for therapeutic strategies to drug addiction.

Mechanisms of Action and Persistent Neuroplasticity by Drugs of Abuse

Adaptation of the nervous system to different chemical and physiologic conditions is important for the homeostasis of brain processes and for learning and remembering appropriate responses to challenges. Although processes such as tolerance and dependence to various drugs of abuse have been known for a long time, it was recently discovered that even a single pharmacologically relevant dose of various drugs of abuse induces neuroplasticity in selected neuronal populations, such as the dopamine neurons of the ventral tegmental area, which persist long after the drug has been excreted. Prolonged (self-) administration of drugs induces gene expression, neurochemical, neurophysiological, and structural changes in many brain cell populations. These region-specific changes correlate with addiction, drug intake, and conditioned drugs effects, such as cue- or stress-induced reinstatement of drug seeking. In rodents, adolescent drug exposure often causes significantly more behavioral changes later in adulthood than a corresponding exposure in adults. Clinically the most impairing and devastating effects on the brain are produced by alcohol during fetal development. In adult recreational drug users or in medicated patients, it has been difficult to find persistent functional or behavioral changes, suggesting that heavy exposure to drugs of abuse is needed for neurotoxicity and for persistent emotional and cognitive alterations. This review describes recent advances in this important area of research, which harbors the aim of translating this knowledge to better treatments for addictions and related neuropsychiatric illnesses.

Sex-dependent psychoneuroendocrine effects of THC and MDMA in an animal model of adolescent drug consumption

Ecstasy is a drug that is usually consumed by young people at the weekends and frequently, in combination with cannabis. In the present study we have investigated the long-term effects of administering increasing doses of delta-9-tetrahydrocannabinol [THC; 2.5, 5, 10 mg/kg; i.p.] from postnatal day (pnd) 28 to 45, alone and/or in conjunction with 3,4-methylenedioxymethamphetamine [MDMA; two daily doses of 10 mg/kg every 5 days; s.c.] from pnd 30 to 45, in both male and female Wistar rats. When tested one day after the end of the pharmacological treatment (pnd 46), MDMA administration induced a reduction in directed exploration in the holeboard test and an increase in open-arm exploration in an elevated plus maze. In the long-term, cognitive functions in the novel object test were seen to be disrupted by THC administration to female but not male rats. In the prepulse inhibition test, MDMA-treated animals showed a decrease in prepulse inhibition at the most intense prepulse studied (80 dB), whereas in combination with THC it induced a similar decrease at 75 dB. THC decreased hippocampal Arc expression in both sexes, while in the frontal cortex this reduction was only evident in females. MDMA induced a reduction in ERK1/2 immunoreactivity in the frontal cortex of male but not female animals, and THC decreased prepro-orexin mRNA levels in the hypothalamus of males, although this effect was prevented when the animals also received MDMA. The results presented indicate that adolescent exposure to THC and/or MDMA induces long-term, sex-dependent psychophysiological alterations and they reveal functional interactions between the two drugs.

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.

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

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

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.

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.

Brain regional differences in CB1 receptor adaptation and regulation of transcription

Cannabinoid CB1 receptors (CB1Rs) are expressed throughout the brain and mediate the central effects of cannabinoids, including Δ(9)-tetrahydrocannabinol (THC), the main psychoactive constituent of marijuana. Repeated THC administration produces tolerance to cannabinoid-mediated effects, although the magnitude of tolerance varies by effect. Consistent with this observation, CB1R desensitization and downregulation, as well as induction of immediate early genes (IEGs), vary by brain region. Zif268 and c-Fos are induced in the forebrain after acute THC administration. Phosphorylation of the cAMP response-element binding protein (CREB) is increased in a region-specific manner after THC administration. Results differ between acute versus repeated THC injection, and suggest that tolerance to IEG activation might develop in some regions. Repeated THC treatment produces CB1R desensitization and downregulation in the brain, although less adaption occurs in the striatum as compared to regions such as the hippocampus. Repeated THC treatment also induces expression of ΔFosB, a very stable isoform of FosB, in the striatum. Transgenic expression of ∆FosB in the striatum enhances the rewarding effects of several drugs, but its role in THC-mediated effects is not known. The inverse regional relationship between CB1R desensitization and ∆FosB induction suggests that these adaptations might inhibit each other, although this possibility has not been investigated. The differential regional expression of individual IEGs by acute or repeated THC administration suggests that regulation of target genes and effects on CB1R signaling will contribute to the behavioral effects of THC.

Cannabinoid CB1 receptors transactivate multiple receptor tyrosine kinases and regulate serine/threonine kinases to activate ERK in neuronal cells

BACKGROUND AND PURPOSE

Signalling networks that regulate the progression of cannabinoid CB(1) receptor-mediated extracellular signal-regulated kinase (ERK) activation in neurons are poorly understood. We investigated the cellular mechanisms involved in CB(1) receptor-stimulated ERK phosphorylation in a neuronal cell model.

EXPERIMENTAL APPROACH

Murine N18TG2 neuronal cells were used to analyse the effect of specific protein kinase and phosphatase inhibitors on CB(1) receptor-stimulated ERK phosphorylation. The LI-COR In Cell Western assay and immunoblotting were used to measure ERK phosphorylation.

KEY RESULTS

The time-course of CB(1) receptor-stimulated ERK activation occurs in three phases that are regulated by distinct cellular mechanisms in N18TG2 cells. Phase I (0-5 min) maximal ERK phosphorylation is mediated by CB(1) receptor-stimulated ligand-independent transactivation of multiple receptor tyrosine kinases (RTKs). Phase I requires G(i/o) βγ subunit-stimulated phosphatidylinositol 3-kinase activation and Src kinase activation and is modulated by inhibition of cAMP-activated protein kinase A (PKA) levels. Src kinase activation is regulated by the protein tyrosine phosphatases 1B and Shp1. The Phase II (5-10 min) rapid decline in ERK phosphorylation involves PKA inhibition and serine/threonine phosphatase PP1/PP2A activation. The Phase III (>10 min) plateau in ERK phosphorylation is mediated by CB(1) receptor-stimulated, ligand-independent, transactivation of multiple RTKs.

CONCLUSIONS AND IMPLICATIONS

The complex expression of CB(1) receptor-stimulated ERK activation provides cellular selectivity, modulation of sensitivity to agonists, and coincidence detection with RTK signalling. RTK and PKA pathways may provide routes to novel CB(1) -based therapeutic interventions in the treatment of addictive disorders or neurodegenerative diseases. LINKED ARTICLES This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Social isolation stress down-regulates cortical early growth response 1 (Egr-1) expression in mice

Social isolation stress induces behavioral disturbances such as aggression, cognitive impairments, and deficits in prepulse inhibition in mice. Social isolation mice have, therefore, been studied as an animal model of neuropsychiatric disorders such as schizophrenia. Recently, the decrease in early growth response (Egr) gene expression levels were reported in the post-mortem brains of schizophrenia patients. In this study, we investigate the effects of social isolation stress on the expression levels of Egr mRNA and protein in the frontal cortex. Social isolation stress exposure significantly down-regulated the expression of Egr-1 protein and Egr-1 gene transcript in nucleus of cortical neurons in a manner dependent on a social isolation period. This stress had no effect on the expression level of Egr-1 in the striatum or the expression levels of other Egr family members (Egr-2, -3, and -4) in the frontal cortex. These results suggest that the decrease in Egr-1 expression in the frontal cortex may be involved in social isolation stress-induced behavioral abnormalities.

β-arrestin2 regulates cannabinoid CB1 receptor signaling and adaptation in a central nervous system region-dependent manner

BACKGROUND

Cannabinoid CB(1) receptors (CB(1)Rs) mediate the effects of ▵(9)-tetrahydrocannabinol (THC), the psychoactive component in marijuana. Repeated THC administration produces tolerance and dependence, which limit therapeutic development. Moreover, THC produces motor and psychoactive side effects. β-arrestin2 mediates receptor desensitization, internalization, and signaling, but its role in these CB(1)R effects and receptor regulation is unclear.

METHODS

CB(1)R signaling and behaviors (antinociception, hypothermia, catalepsy) were assessed in β-arrestin2-knockout (βarr2-KO) and wild-type mice after THC administration. Cannabinoid-stimulated [(35)S]GTPγS and [(3)H]ligand autoradiography were assessed by statistical parametric mapping and region-of-interest analysis.

RESULTS

β-arrestin2 deletion increased CB(1)R-mediated G-protein activity in subregions of the cortex but did not affect CB(1)R binding, in vehicle-treated mice. βarr2-KO mice exhibited enhanced acute THC-mediated antinociception and hypothermia, with no difference in catalepsy. After repeated THC administration, βarr2-KO mice showed reduced CB(1)R desensitization and/or downregulation in cerebellum, caudal periaqueductal gray, and spinal cord and attenuated tolerance to THC-mediated antinociception. In contrast, greater desensitization was found in hypothalamus, cortex, globus pallidus, and substantia nigra of βarr2-KO compared with wild-type mice. Enhanced tolerance to THC-induced catalepsy was observed in βarr2-KO mice.

CONCLUSIONS

β-arrestin2 regulation of CB(1)R signaling following acute and repeated THC administration was region-specific, and results suggest that multiple, overlapping mechanisms regulate CB(1)Rs. The observations that βarr2-KO mice display enhanced antinociceptive responses to acute THC and decreased tolerance to the antinociceptive effects of the drug, yet enhanced tolerance to catalepsy, suggest that development of cannabinoid drugs that minimize CB(1)R interactions with β-arrestin2 might produce improved cannabinoid analgesics with reduced motor suppression.

The dual neuroprotective-neurotoxic profile of cannabinoid drugs

Extensive in vitro and in vivo studies have shown that cannabinoid drugs have neuroprotective properties and suggested that the endocannabinoid system may be involved in endogenous neuroprotective mechanisms. On the other hand, neurotoxic effects of cannabinoids in vitro and in vivo were also described. Several possible explanations for these dual, opposite effects of cannabinoids on cellular fate were suggested, and it is conceivable that various factors may determine the final outcome of the cannabinoid effect in vivo. In the current review, we focus on one of the possible reasons for the dual neuroprotective/neurotoxic effects of cannabinoids in vivo, namely, the opposite effects of low versus high doses of cannabinoids. While many studies reported neuroprotective effects of the conventional doses of cannabinoids in various experimental models for acute brain injuries, we have shown that a single administration of an extremely low dose of Δ(9) -tetrahydrocannabinol (THC) (3-4 orders of magnitude lower than the conventional doses) to mice induced long-lasting mild cognitive deficits that affected various aspects of memory and learning. These findings led to the idea that this low dose of THC, which induces minor damage to the brain, may activate preconditioning and/or postconditioning mechanisms and thus will protect the brain from more severe insults. Indeed, our recent findings support this assumption and show that a pre- or a postconditioning treatment with extremely low doses of THC, several days before or after brain injury, provides effective long-term cognitive neuroprotection. The future therapeutical potential of these findings is discussed.

Regulation of Neuronal Function by Ras-GRF Exchange Factors

Ras-GRF1 (GRF1) and Ras-GRF2 (GRF2) constitute a family of guanine nucleotide exchange factors (GEFs). The main isoforms, p140-GRF1 and p135-GRF2, have 2 GEF domains that give them the capacity to activate both Ras and Rac GTPases in response to signals from a variety of neurotransmitter receptors. GRF1 and GRF2 proteins are found predominantly in adult neurons of the central nervous system, although they can also be detected in a limited number of other tissues. p140-GRF1 and p135-GRF2 contain calcium/calmodulin-binding IQ domains that allow them to act as calcium sensors to mediate the actions of NMDA-type and calcium-permeable AMPA-type glutamate receptors. p140-GRF1 also mediates the action of dopamine receptors that signal through cAMP. Although p140-GRF1 and p135-GRF2 have similar functional domains, studies of GRF knockout mice show that they can play strikingly different roles in regulating MAP kinase family members, neuronal synaptic plasticity, specific forms of learning and memory, and behavioral responses to psychoactive drugs. In addition, the function of GRF proteins may vary in different regions of the brain. Alternative splice variants yielding smaller GRF1 gene isoforms with fewer functional domains also exist; however, their distinct roles in neurons have not been revealed. Continuing studies of these proteins should yield important insights into the biochemical basis of brain function as well as novel concepts to explain how complex signal transduction proteins, like Ras-GRFs, integrate multiple upstream signals into specific downstream outputs to control brain function.

Pre- and post-conditioning treatment with an ultra-low dose of Δ9-tetrahydrocannabinol (THC) protects against pentylenetetrazole (PTZ)-induced cognitive damage

Preconditioning, a phenomenon where a minor noxious stimulus protects from a subsequent more severe insult, and post-conditioning, where the protective intervention is applied following the insult, offer new insight into the neuronal mechanism(s) of neuroprotection and may provide new strategies for the prevention and treatment of brain damage. We have previously reported that a single administration of an extremely low dose of Δ(9)-tetrahydrocannabinol (THC; the psychoactive ingredient of marijuana) to mice induced minor long-lasting cognitive deficits. In the present study we examined the possibility that such a low dose of THC will protect the mice from more severe cognitive deficits induced by the epileptogenic drug pentylenetetrazole (PTZ). THC (0.002 mg/kg, a dose that is 3-4 orders of magnitude lower than the doses that induce the conventional effects of THC) was administered 1-7 days before, or 1-3 days after the injection of PTZ (60 mg/kg). The consequences of this treatment were studied 3-7 weeks later by various behavioral tests that evaluated different aspects of memory and learning. We found that a single administration of THC either before or after PTZ abolished the PTZ-induced long-lasting cognitive deficits. Biochemical studies indicated a concomitant reduction in phosphorylated-ERK (extracellular signal-regulated kinase) in the cerebella of mice 7 weeks following the injection of THC. Our results suggest that a pre- or post-conditioning treatment with extremely low doses of THC, several days before or after brain injury, may provide safe and effective long-term neuroprotection.

The RasGrf family of mammalian guanine nucleotide exchange factors

RasGrf1 and RasGrf2 are highly homologous mammalian guanine nucleotide exchange factors which are able to activate specific Ras or Rho GTPases. The RasGrf genes are preferentially expressed in the central nervous system, although specific expression of either locus may also occur elsewhere. RasGrf1 is a paternally-expressed, imprinted gene that is expressed only after birth. In contrast, RasGrf2 is not imprinted and shows a wider expression pattern. A variety of isoforms for both genes are also detectable in different cellular contexts. The RasGrf proteins exhibit modular structures composed by multiple domains including CDC25H and DHPH motifs responsible for promoting GDP/GTP exchange, respectively, on Ras or Rho GTPase targets. The various domains are essential to define their intrinsic exchanger activity and to modulate the specificity of their functional activity so as to connect different upstream signals to various downstream targets and cellular responses. Despite their homology, RasGrf1 and RasGrf2 display differing target specificities and non overlapping functional roles in a variety of signaling contexts related to cell growth and differentiation as well as neuronal excitability and response or synaptic plasticity. Whereas both RasGrfs are activatable by glutamate receptors, G-protein-coupled receptors or changes in intracellular calcium concentration, only RasGrf1 is reported to be activated by LPA, cAMP, or agonist-activated Trk and cannabinoid receptors. Analysis of various knockout mice strains has uncovered a specific functional contribution of RasGrf1 in processes of memory and learning, photoreception, control of post-natal growth and body size and pancreatic β-cell function and glucose homeostasis. For RasGrf2, specific roles in lymphocyte proliferation, T-cell signaling responses and lymphomagenesis have been described.

Long-term fluoxetine treatment modulates cannabinoid type 1 receptor-mediated inhibition of adenylyl cyclase in the rat prefrontal cortex through 5-hydroxytryptamine 1A receptor-dependent mechanisms

Increasing data indicate that brain endocannabinoid system plays a role in the effects of antidepressant medications. Here we examined the effect of in vivo exposure to the selective serotonin uptake inhibitor fluoxetine on cannabinoid type 1 (CB(1)) receptor density and functionality in the rat prefrontal cortex (PFC) and cerebellum. Long-term treatment with fluoxetine (10 mg/kg/day) enhanced CB(1) receptor inhibition of adenylyl cyclase (AC) in the PFC and reduced it in the cerebellum without altering receptor density and agonist stimulation of guanosine 5'-O-(3-[(35)S]thio) triphosphate ([(35)S]GTP gamma S) in either area. Analysis of [(35)S]GTP gamma S-labeled G alpha subunits allowed for the detection of up-regulated CB(1) receptor coupling to G alpha(i2), G alpha(i3) in the PFC, and reduced coupling to G alpha(i3) in the cerebellum of fluoxetine-treated rats. Concomitant administration of the 5-HT(1A) receptor antagonist N-[2-[4- (2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate (WAY100635; 0.1 mg/kg/day) reduced fluoxetine-induced modulation of CB(1) receptor coupling to G alpha subunits and AC in the PFC but not in the cerebellum. These results indicate that increased CB(1) receptor signaling at the G alpha(i)-AC transduction level is a long-term adaptation induced by fluoxetine in the PFC and point to a role for 5-HT(1A) receptors in this effect. Basal AC activity, protein kinase A (PKA) catalytic subunit expression, and phospho-cAMP response element-binding protein (pCREB)/CREB ratio were also up-regulated in the PFC of fluoxetine-treated animals, whereas no differences were detected in the cerebellum. It is interesting that long-term Delta(9)-tetrahydrocannabinol treatment did not elicit antidepressant-like effects or modulated behavioral responses of fluoxetine in an animal model of depression (olfactory bulbectomy). These data suggest that altered signal transduction through CB(1) receptors in the PFC may participate in the regulation of the AC-PKA-CREB cascade induced by fluoxetine in this brain area.

Signal transduction via cannabinoid receptors

The endocannabinoids anandamide and 2-arachidonoylglycerol are lipid mediators that signal via CB(1) and CB(2) cannabinoid receptors and Gi/o-proteins to inhibit adenylyl cyclase and stimulate mitogen-activated protein kinase. In the brain, CB(1) receptors interact with opioid receptors in close proximity, and these receptors may share G-proteins and effector systems. In the striatum, CB(1) receptors function in coordination with D(1) and D(2) dopamine receptors, and combined stimulation of CB(1)-D(2) receptor heteromeric complexes promotes a unique interaction to stimulate cAMP production. CB(1) receptors also trigger growth factor receptor signaling cascades in cells by engaging in cross-talk or interreceptor signal transmission with the receptor tyrosine kinase (RTK) family. Mechanisms for CB(1) receptor-RTK transactivation can include stimulation of signal transduction pathways regulated by second messengers such as phospholipase C, metalloprotease cleavage of membrane-bound precursor proteins such as epidermal growth factor which activate RTKs, RTK autophosphorylation, and recruitment of non-receptor tyrosine kinases. CB(1) and CB(2) receptors are expressed in peripheral tissues including liver and adipose tissue, and are induced in pathological conditions. Novel signal transduction resulting from endocannabinoid regulation of AMP-regulated kinase and peroxisome proliferator-activated receptors have been discovered from studies of hepatocytes and adipocytes. It can be predicted that drug discovery of the future will be based upon these novel signal transduction mechanisms for endocannabinoid mediators.

Long-term consequences of a single treatment of mice with an ultra-low dose of Delta9-tetrahydrocannabinol (THC)

A single administration of an extremely low dose (0.002 mg/kg) of Delta9-tetrahydrocannabinol (THC; the psychoactive ingredient of marijuana) to ICR mice induced long-term cognitive deficits that lasted for at least 5 months. The behavioral deficits were detected by several tests that evaluated different aspects of memory and learning, including spatial navigation and spatial and non-spatial recognition. Our findings point to possible deficits in attention or motivation that represent a common upstream cognitive process that may affect the performance of the mice in the different behavioral assays. Similar ultra-low doses of THC (3-4 orders of magnitude lower than doses that are known to evoke the acute effects of THC) also induced sustained activation of extracellular-regulated kinase (ERK1/2) in the cerebellum, indicating that a single injection of such low doses of the cannabinoid drug can stimulate neuronal regulatory mechanisms. The relevance of these findings to the behavioral consequences of chronic exposure to marijuana is discussed.

Cannabinoid modulation of hippocampal long-term memory is mediated by mTOR signaling

Cognitive impairment is one of the most important negative consequences associated with cannabis consumption. We found that CB1 cannabinoid receptor (CB1R) activation transiently modulated the mammalian target of rapamycin (mTOR)/p70S6K pathway and the protein synthesis machinery in the mouse hippocampus, which correlated with the amnesic properties of delta9-tetrahydrocannabinol (THC). In addition, non-amnesic doses of either the mTOR blocker rapamycin or the protein synthesis inhibitor anisomycin abrogated the amnesic-like effects of THC, pointing to a mechanism involving new protein synthesis. Moreover, using pharmacological and genetic tools, we found that THC long-term memory deficits were mediated by CB1Rs expressed on GABAergic interneurons through a glutamatergic mechanism, as both the amnesic-like effects and p70S6K phosphorylation were reduced in GABA-CB1R knockout mice and by NMDA blockade.

Characterization of anandamide-stimulated cannabinoid receptor signaling in human ULTR myometrial smooth muscle cells

Accumulating evidence highlights the importance of the endocannabinoid anandamide (AEA) as a key mediator in reproductive physiology. Current data suggest potential roles for AEA in gametogenesis, fertilization, and parturition. AEA exerts its actions through two G protein-coupled receptors, termed cannabinoid receptor 1 (CB1), and 2 (CB2), and the ligand-gated transient receptor potential vanilloid receptor type 1 (TRPV1) ion channel. At present, the cellular mechanism(s) and consequences of AEA signaling in reproductive tissues, especially the myometrium, are poorly understood. Here, we examine the expression of CB1, CB2, and TRPV1 in the human myometrial smooth muscle cell-line (ULTR) and characterize intracellular signaling after stimulation with AEA. Radioligand binding analysis revealed a total CB receptor expression of 76 +/- 24 fmol/mg protein, with both quantitative PCR and competition binding studies indicating a negligible CB2 component. AEA caused Galpha(i/o)-dependent inhibition of adenylate cyclase to reduce intracellular cAMP levels. In addition, AEA caused a 2.5- to 3.5-fold increase in ERK activation, which was ablated by inhibition of Galpha(i/o), phosphoinositide-3-kinase and Src-kinase activities, but not by inhibition of Ca(2+)/calmodulin-dependent protein kinase or protein kinase C activities. TRPV1 channel activation with capsaicin failed to activate ERK. Consistent with these findings, the selective agonists, arachidonyl-2-chloroethylamide (CB1) and L759656 (CB2), and selective antagonists AM251 (CB1) and JTE907 (CB2), provided pharmacological evidence that the ERK signaling pathway is activated through endogenously expressed CB1. These findings provide an insight into myometrial AEA signaling, highlighting a potential role for endocannabinoids in the regulation of gene expression in myometrial smooth muscle cells.

Preliminary evidence of cannabinoid effects on brain-derived neurotrophic factor (BDNF) levels in humans

BACKGROUND

Acute and chronic exposure to cannabinoids has been associated with cognitive deficits, a higher risk for schizophrenia and other drug abuse. However, the precise mechanism underlying such effects is not known. Preclinical studies suggest that cannabinoids modulate brain-derived neurotrophic factor (BDNF). Accordingly, we hypothesized that Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the principal active component of cannabis, would alter BDNF levels in humans.

MATERIALS AND METHODS

Healthy control subjects (n = 14) and light users of cannabis (n = 9) received intravenous administration of (0.0286 mg/kg) Delta(9)-THC in a double-blind, fixed order, placebo-controlled, laboratory study. Serum sampled at baseline, after placebo administration, and after Delta(9)-THC administration was assayed for BDNF using ELISA.

RESULTS

Delta(9)-THC increased serum BDNF levels in healthy controls but not light users of cannabis. Further, light users of cannabis had lower basal BDNF levels. Delta(9)-THC produced psychotomimetic effects, perceptual alterations, and "high" and spatial memory impairments.

IMPLICATIONS

The effects of socially relevant doses of cannabinoids on BDNF suggest a possible mechanism underlying the consequences of exposure to cannabis. This may be of particular importance for the developing brain and also in disorders believed to involve altered neurodevelopment such as schizophrenia. Larger studies to investigate the effects of cannabinoids on BDNF and other neurotrophins are warranted.

Drug addiction

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

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.

Regulation of CB1 cannabinoid receptor internalization by a promiscuous phosphorylation-dependent mechanism

Agonists stimulate cannabinoid 1 receptor (CB(1)R) internalization. Previous work suggests that the extreme carboxy-terminus of the receptor regulates this internalization - likely through the phosphorylation of serines and threonines clustered within this region. While truncation of the carboxy-terminus (V460Z CB(1)) and consequent removal of these putative phosphorylation sites prevents endocytosis in AtT20 cells, the residues necessary for CB(1)R internalization remain elusive. To determine the structural requirements for internalization, we evaluated endocytosis of carboxy-terminal mutant CB(1)Rs stably expressed in HEK293 cells. In contrast to AtT20 cells, V460Z CB(1)R expressed in HEK293 cells internalized to the same extent and with similar kinetics as the wild-type receptor. However, mutation of serine and/or threonine residues within the extreme carboxy-terminal attenuated internalization when these receptors were expressed in HEK293 cells. These results establish that the extreme carboxy-terminal phosphorylation sites are not required for internalization of truncated receptors, but are required for internalization of full-length receptors in HEK293 cells. Analysis of beta-arrestin-2 recruitment to mutant CB(1)R suggests that putative carboxy-terminal phosphorylation sites mediate beta-arrestin-2 translocation. This study indicates that the local cellular environment affects the structural determinants of CB(1)R internalization. Additionally, phosphorylation likely regulates the internalization of (full-length) CB(1)Rs.

Transcriptomic and proteomic analyses of mouse cerebellum reveals alterations in RasGRF1 expression following in vivo chronic treatment with delta 9-tetrahydrocannabinol

We have applied transcriptomic and proteomic techniques to identify changes in the RNA and the protein levels in the mouse cerebellum after chronic treatment with Delta(9)-tetrahydrocannabinol (THC). Among approximately 14,000 transcripts in a mouse cDNA microarray library, we found 11 genes with altered expression. RasGRF1, a neuron-specific Ras guanine nucleotide exchange factor, showed a reduction both at the RNA and protein levels with a specific decrease of the protein pool associated to cell membranes. In addition, proteomic analysis on cerebellum obtained from chronically THC-treated mice detected quantitative changes of additional 27 spots, mostly in the membranous fraction. We found enrichment of alpha (Galphao, Galphaq) and beta subunits (beta4/beta2 and beta5) of guanine nucleotide-binding proteins and of two calcium-binding proteins, calretinin and hippocalcin-like protein-1. In addition, we also detected a significant increase in the membrane fraction of proteins involved in exo-endocytosis such as septins, dynamin-1, and vesicle protein sorting 29. By western blotting, we confirmed increased membrane localization of calretinin and of dynamin-1 isoforms with higher isoelectric point, indicative for an underphosphorylated state of the molecule. In conclusion, our results indicate that chronic THC modulates the expression and subcellular localization of proteins implicated in Ras signaling, calcium-buffering potential, and trafficking.

Anandamide inhibits cholangiocyte hyperplastic proliferation via activation of thioredoxin 1/redox factor 1 and AP-1 activation

The endocannabinoid system regulates various aspects of hepatic fibrosis; however, nothing is known about its role in regulating cholangiocyte proliferation and function. We evaluated the effects of anandamide (AEA) on cholangiocyte proliferation and explored the effects of AEA on the thioredoxin 1 (TRX1)/redox factor 1 (Ref1)/activator protein-1 (AP-1) pathway. Mice underwent bile duct ligation (BDL) and were infused with AEA for 3 days postsurgery. Proliferation and apoptosis were evaluated in liver sections. Effects of in vitro AEA treatment on cholangiocyte proliferation and apoptosis were studied in purified cholangiocytes. The relative expression of cannabinoid receptors was also assessed in liver sections and cholangiocytes. mRNA expression of the cannabinoid receptors Cb1 and VR1 was decreased after BDL, whereas there was an upregulation of Cb2 mRNA. AEA decreased cholangiocyte growth and induced accumulation of reactive oxygen species, upregulation of TRX1, Ref1, c-Fos, and c-Jun expression, increased nuclear localization of TRX1, and increased AP-1 transcriptional activity. Specific knockdown of TRX1 or Ref1 expression ablated the AP-1 transcriptional activity and AEA-induced cell death but not expression of c-Fos and c-Jun. Knockdown of c-Fos and c-Jun expression also ablated AEA-induced apoptosis. We conclude that AEA suppresses cholangiocyte proliferation during cholestasis via a Cb2-dependent mechanism. Modulation of the endocannabinoid system may be important in the treatment of cholangiopathies.

Drug-induced alterations in the extracellular signal-regulated kinase (ERK) signalling pathway: implications for reinforcement and reinstatement

Drug addiction, characterized by high rates of relapse, is recognized as a kind of neuroadaptive disorder. Since the extracellular signal-regulated kinase (ERK) pathway is critical to neuroplasticity in the adult brain, understanding the role this pathway plays is important for understanding the molecular mechanism underlying drug addiction and relapse. Here, we review previous literatures that focus on the effects of exposure to cocaine, amphetamine, Delta(9)-tetrahydrocannabinol (THC), nicotine, morphine, and alcohol on ERK signaling in the mesocorticolimbic dopamine system; these alterations of ERK signaling have been thought to contribute to the drug's rewarding effects and to the long-term maladaptation induced by drug abuse. We then discuss the possible upstreams of the ERK signaling pathway activated by exposure of drugs of abuse and the environmental cues previously paired with drugs. Finally, we argue that since ERK activation is a key molecular process in reinstatement of conditioned place preference and drug self-administration, the pharmacological manipulation of the ERK pathway is a potential treatment strategy for drug addiction.

Rapid CB1 cannabinoid receptor desensitization defines the time course of ERK1/2 MAP kinase signaling

Molecular mechanisms regulating the development of physiological and behavioral tolerance to cannabinoids are not well understood. Two cellular correlates implicated in the development and maintenance of tolerance are CB(1) cannabinoid receptor internalization and uncoupling of receptor signal transduction. Both processes have been proposed as mediators of tolerance because of observations that chronic Delta(9)-tetrahydrocannabinol (THC) treatment causes both region-specific decreases in CB(1) receptors and G-protein coupling in the brain. To determine the balance of these two processes in regulating CB(1) receptor signaling during sustained receptor stimulation, we evaluated the parameters affecting ERK1/2 MAP kinase activity in HEK293 cells stably expressing CB(1) receptors. CB(1) receptor stimulation by the potent CB(1) receptor agonist, CP 55,940 transiently activated ERK1/2. To determine if CB(1) receptor desensitization or internalization was responsible for the transient nature of ERK1/2 activation, we evaluated ERK1/2 phosphorylation in HEK293 cells expressing a desensitization-deficient CB(1) receptor (S426A/S430A CB(1)). Here, the duration of S426A/S430A CB(1) receptor-mediated activation of ERK1/2 was markedly prolonged relative to wild-type receptors, and was dynamically reversed by SR141716A. Interestingly, the S426A/S430A CB(1) receptor was still able to recruit betaarrestin-2, a key mediator of receptor desensitization, to the cell surface following agonist activation. In contrast to a central role for desensitization, pharmacological and genetic approaches suggested CB(1) receptor internalization is dispensable in the transient activation of ERK1/2. This study indicates that the duration of ERK1/2 activation by CB(1) receptors is regulated by receptor desensitization and underscores the importance of G-protein uncoupling in the regulation of CB(1) receptor signaling.

Modulation of extracellular signal-regulated kinase (ERK) by opioid and cannabinoid receptors that are expressed in the same cell

In the present study we investigated the signal transduction pathways leading to the activation of extracellular signal-regulated kinase (ERK) by opioid or cannabinoid drugs, when their receptors are coexpressed in the same cell-type. In N18TG2 neuroblastoma cells, the opioid agonist etorphine and the cannabinoid agonist CP-55940 induced the phosphorylation of ERK by a similar mechanism that involved activation of delta-opioid receptors or CB1 cannabinoid receptors coupled to Gi/Go proteins, matrix metalloproteases, vascular endothelial growth factor (VEGF) receptors and MAPK/ERK kinase (MEK). In HEK-293 cells, these two drugs induced the phosphorylation of ERK by separate mechanisms. While CP-55940 activated ERK by transactivation of VEGFRs, similar to its effect in N18TG2 cells, the opioid agonist etorphine activated ERK by a mechanism that did not involve transactivation of a receptor tyrosine kinase. Interestingly, the activation of ERK by etorphine was resistant to the inhibition of MEK, suggesting the possible existence of a novel, undescribed yet mechanism for the activation of ERK by opioids. This mechanism was found to be specific to etorphine, as activation of ERK by the micro-opioid receptor (MOR) agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol] enkephalin) was mediated by MEK in these cells, suggesting that etorphine and DAMGO activate distinct, ligand-specific, conformations of MOR. The characterization of cannabinoid- and opioid-induced ERK activation in these two cell-lines enables future studies into possible interactions between these two groups of drugs at the level of MAPK signaling.

Cellular mechanisms underlying the anxiolytic effect of low doses of peripheral Delta9-tetrahydrocannabinol in rats

We investigated the effect of low doses of intraperitoneal Delta(9)-tetrahydrocannabinol (THC) on anxiety behavior in rats using the elevated plus maze (EPM). An anxiolytic effect was obtained in a range of doses between 0.075 and 1.5 mg/kg, the 0.75 dose being the most effective. Pretreatment with the CB1 receptor antagonist AM251 fully reversed THC's effect, suggesting CB1 receptors were involved. In order to elucidate the neuroanatomical substrates underlying the effect of the maximal effective dose of THC, we investigated cFos expression in anxiety-related brain regions (prefrontal cortex, nucleus accumbens, amygdala, and hippocampus) of rats exposed to the EPM. THC significantly lowered the amount of cFos in prefrontal cortex and amygdala without affecting the other cerebral areas. As there is increasing evidence that CREB function regulates anxiety-like behavior in rats, the second biochemical parameter we measured was phosphorylated CREB in the same brain areas. Rats treated with THC showed a significant increase in CREB activation in the prefrontal cortex and hippocampus. In the prefrontal cortex this increased activation was linked to an increase in ERK activation, whereas in the hippocampus there was a drop in the activity of CAMKII, a kinase with inhibitory effect on CREB activation. All these effects were reversed by AM251 pretreatment, suggesting that stimulation of CB1 receptors is fundamental for triggering the biochemical events. Our results suggest that the stimulation of these receptors in the prefrontal cortex, amygdala, and hippocampus with the subsequent activation of different signaling pathways is the first event underlying the effects of cannabinoids on anxious states.

Acute, chronic and withdrawal effects of the cannabinoid receptor agonist WIN55212-2 on the sequential activation of MAPK/Raf-MEK-ERK signaling in the rat cerebral frontal cortex: short-term regulation by intrinsic and extrinsic pathways

The cannabinoids (CB) modulate the extracellular signal-regulated kinase (ERK), leading to various forms of plasticity in the brain. Little is known, however, on the in vivo short- and long-term activation and regulation of the components of mitogen-activated protein kinase (MAPK)/ERK signaling by CB. The CB agonist WIN55212-2 (8 mg/kg) increased the immunodensities of phosphorylated c-Raf-1 (42%), MEK1/2 (63%), ERK1 (24%), and ERK2 (28%) in the rat cerebral frontal cortex. These effects were antagonized by SR141716A (rimonabant, 10 mg/kg), a selective CB(1) receptor antagonist. Repeated WIN55212-2 treatment (2-8 mg/kg for 5 days) resulted in tachyphylaxis to the acute activation of Raf-MEK-ERK signaling. Acute WIN55212-2 also induced a hypothermic effect in rats, which was reduced after repeated administration (tolerance). Treatment with SR141716A after chronic WIN55212-2 resulted in the expected cannabinoid withdrawal syndrome, without concomitant alterations in the phosphorylation state of c-Raf-1, MEK1/2, or ERK1/2. Pretreatment with SL327 (20 mg/kg, a MEK1/2 inhibitor) increased the basal phosphorylation of c-Raf-1 (40%) and MEK1/2 (74%; feedback regulation) and fully prevented the up-regulation of ERK1/2 (23-31%) induced by WIN55212-2. Pretreatment with MK801 (1 mg/kg, a NMDA receptor antagonist) effectively blocked the up-regulation c-Raf-1 (41%), MEK1/2 (57%) and ERK1/2 (25-30%) induced by the CB agonist. The main findings demonstrate that the acute stimulation of CB(1) receptors in the frontal cortex results in the sequential phosphorylation of Raf-MEK-ERK cascade, in which c-Raf-1 activation (rate-limiting process) plays a crucial role. Moreover, the in vivo stimulating effect of WIN55212-2 on Raf-MEK-ERK signaling is under the extrinsic regulation of an excitatory glutamatergic mechanism.