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

Prenatal exposure to Cannabis smoke induces early and lasting damage to the brain

Cannabis is the most widely used illegal drug during pregnancy, however, the effects of gestational exposure to Cannabis smoke (CS) on the central nervous system development remain uncharacterised. This study investigates the effects of maternal CS inhalation on brain function in the offspring. Pregnant mice were exposed daily to 5 min of CS during gestational days (GD) 5.5-17.5. On GD 18.5 half of the dams were euthanized for foetus removal. The offspring from the remaining dams were euthanized on postnatal days (PND) 20 and 60 for evaluation. Brain volume, cortex cell number, SOX2, histone-H3, parvalbumin, NeuN, and BDNF immunoreactivity were assessed in all groups. In addition, levels of NeuN, CB1 receptor, and BDNF expression were assessed and cortical primary neurons from rats were treated with Cannabis smoke extract (CSE) for assessment of cell viability. We found that male foetuses from the CS exposed group had decreased brain volume, whereas mice at PND 60 from the exposed group presented with increased brain volume. Olfactory bulb and diencephalon volume were found lower in foetuses exposed to CS. Mice at PND 60 from the exposed group had a smaller volume in the thalamus and hypothalamus while the cerebellum presented with a greater volume. Also, there was an increase in cortical BDNF immunoreactivity in CS exposed mice at PND 60. Protein expression analysis showed an increase in pro-BDNF in foetus brains exposed to CS. Mice at PND 60 presented an increase in mature BDNF in the prefrontal cortex (PFC) in the exposed group and a higher CB1 receptor expression in the PFC. Moreover, hippocampal NeuN expression was higher in adult animals from the exposed group. Lastly, treatment of cortical primary neurons with doses of CSE resulted in decreased cell viability. These findings highlight the potential negative neurodevelopmental outcomes induced by gestational CS exposure.

Chronic use of cannabis might impair sensory error processing in the cerebellum through endocannabinoid dysregulation

Chronic use of cannabis leads to both motor deficits and the downregulation of CB₁ receptors (CB₁R) in the cerebellum. In turn, cerebellar damage is often related to impairments in motor learning and control. Further, a recent motor learning task that measures cerebellar-dependent adaptation has been shown to distinguish well between healthy subjects and chronic cannabis users. Thus, the deteriorating effects of chronic cannabis use in motor performance point to cerebellar adaptation as a key process to explain such deficits. We review the literature relating chronic cannabis use, the endocannabinoid system in the cerebellum, and different forms of cerebellar-dependent motor learning, to suggest that CB₁R downregulation leads to a generalized underestimation and misprocessing of the sensory errors driving synaptic updates in the cerebellar cortex. Further, we test our hypothesis with a computational model performing a motor adaptation task and reproduce the behavioral effect of decreased implicit adaptation that appears to be a sign of chronic cannabis use. Finally, we discuss the potential of our hypothesis to explain similar phenomena related to motor impairments following chronic alcohol dependency.

An Approach to Biomarker Discovery of Cannabis Use Utilizing Proteomic, Metabolomic, and Lipidomic Analyses

Introduction: Relatively little is known about the molecular pathways influenced by cannabis use in humans. We used a multi-omics approach to examine protein, metabolomic, and lipid markers in plasma differentiating between cannabis users and nonusers to understand markers associated with cannabis use. Methods: Eight discordant twin pairs and four concordant twin pairs for cannabis use completed a blood draw, urine and plasma toxicology testing, and provided information about their past 30-day cannabis use and other substance use patterns. The 24 twins were all non-Hispanic whites. Sixty-six percent were female. Median age was 30 years. Fifteen participants reported that they had used cannabis in the last 30 days, including eight participants that used every day or almost every day (29-30 of 30 days). Of these 15 participants, plasma 11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol (THC-COOH) and total tetrahydrocannabinol (THC) concentrations were detectable in 12 participants. Among the eight "heavy users" the amount of total THC (sum of THC and its metabolites) and plasma THC-COOH concentrations varied widely, with ranges of 13.1-1713 ng/mL and 2.7-284 ng/mL, respectively. A validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay measured plasma THC-COOH, THC, and other cannabinoids and metabolites. Plasma THC-COOH was used as the primary measure. Expression levels of 1305 proteins were measured using SOMAScan assay, and 34 lipid mediators and 314 metabolites were measured with LC-MS/MS. Analyses examined associations between markers and THC-COOH levels with and without taking genetic relatedness into account. Results: Thirteen proteins, three metabolites, and two lipids were identified as associated with THC-COOH levels. Myc proto-oncogene was identified as associated with THC-COOH levels in both molecular insight and potential marker analyses. Five pathways (interleukin-6 production, T lymphocyte regulation, apoptosis, kinase signaling pathways, and nuclear factor kappa-light-chain-enhancer of activated B cells) were linked with molecules identified in these analyses. Conclusions: THC-COOH levels are associated with immune system-related pathways. This study presents a feasible approach to identify additional molecular markers associated with THC-COOH levels.

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.

MicroRNA let-7d is a target of cannabinoid CB1 receptor and controls cannabinoid signaling

Cannabinoid CB1 receptor, the molecular target of endocannabinoids and cannabis active components, is one of the most abundant metabotropic receptors in the brain. Cannabis is widely used for both recreational and medicinal purposes. Despite the ever-growing fundamental roles of microRNAs in the brain, the possible molecular connections between the CB1 receptor and microRNAs are surprisingly unknown. Here, by using reporter gene constructs that express interaction sequences for microRNAs in human SH-SY5Y neuroblastoma cells, we show that CB1 receptor activation enhances the expression of several microRNAs, including let-7d. This was confirmed by measuring hsa-let-7d expression levels. Accordingly, knocking-down CB1 receptor in zebrafish reduced dre-let-7d levels, and knocking-out CB1 receptor in mice decreased mmu-let-7d levels in the cortex, striatum and hippocampus. Conversely, knocking-down let-7d increased CB1 receptor mRNA expression in zebrafish, SH-SY5Y cells and primary striatal neurons. Likewise, in primary striatal neurons chronically exposed to a cannabinoid or opioid agonist, a let-7d-inhibiting sequence facilitated not only cannabinoid or opioid signaling but also cannabinoid/opioid cross-signaling. Taken together, these findings provide the first evidence for a bidirectional link between the CB1 receptor and a microRNA, namely let-7d, and thus unveil a new player in the complex process of cannabinoid action.

The Role of Cannabinoids in Neuroanatomic Alterations in Cannabis Users

The past few decades have seen a marked change in the composition of commonly smoked cannabis. These changes primarily involve an increase of the psychoactive compound ∆(9)-tetrahydrocannabinol (THC) and a decrease of the potentially therapeutic compound cannabidiol (CBD). This altered composition of cannabis may be linked to persistent neuroanatomic alterations typically seen in regular cannabis users. In this review, we summarize recent findings from human structural neuroimaging investigations. We examine whether neuroanatomic alterations are 1) consistently observed in samples of regular cannabis users, particularly in cannabinoid receptor-high areas, which are vulnerable to the effects of high circulating levels of THC, and 2) associated either with greater levels of cannabis use (e.g., higher dosage, longer duration, and earlier age of onset) or with distinct cannabinoid compounds (i.e., THC and CBD). Across the 31 studies selected for inclusion in this review, neuroanatomic alterations emerged across regions that are high in cannabinoid receptors (i.e., hippocampus, prefrontal cortex, amygdala, cerebellum). Greater dose and earlier age of onset were associated with these alterations. Preliminary evidence shows that THC exacerbates, whereas CBD protects from, such harmful effects. Methodologic differences in the quantification of levels of cannabis use prevent accurate assessment of cannabis exposure and direct comparison of findings across studies. Consequently, the field lacks large "consortium-style" data sets that can be used to develop reliable neurobiological models of cannabis-related harm, recovery, and protection. To move the field forward, we encourage a coordinated approach and suggest the urgent development of consensus-based guidelines to accurately and comprehensively quantify cannabis use and exposure in human studies.

Effect of cannabis on glutamate signalling in the brain: A systematic review of human and animal evidence

Use of cannabis or delta-9-tetrahydrocannabinol (Δ9-THC), its main psychoactive ingredient, is associated with psychotic symptoms or disorder. However, the neurochemical mechanism that may underlie this psychotomimetic effect is poorly understood. Although dopaminergic dysfunction is generally recognized as the final common pathway in psychosis, evidence of the effects of Δ9-THC or cannabis use on dopaminergic measures in the brain is equivocal. In fact, it is thought that cannabis or Δ9-THC may not act on dopamine firing directly but indirectly by altering glutamate neurotransmission. Here we systematically review all studies examining acute and chronic effects of cannabis or Δ9-THC on glutamate signalling in both animals and man. Limited research carried out in humans tends to support the evidence that chronic cannabis use reduces levels of glutamate-derived metabolites in both cortical and subcortical brain areas. Research in animals tends to consistently suggest that Δ9-THC depresses glutamate synaptic transmission via CB1 receptor activation, affecting glutamate release, inhibiting receptors and transporters function, reducing enzyme activity, and disrupting glutamate synaptic plasticity after prolonged exposure.

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.

Interplay between synaptic endocannabinoid signaling and metaplasticity in neuronal circuit function and dysfunction

Synaptic neuromodulation acts across different functional domains to regulate cognitive processing and behavior. Recent challenges are related to elucidating the molecular and cellular mechanisms through which neuromodulatory pathways act on multiple time scales to signal state-dependent contingencies at the synaptic level or to stabilise synaptic connections during behavior. Here, we present a framework with the synaptic neuromodulators endocannabinoids (eCBs) as key players in dynamic synaptic changes. Modulation of various molecular components of the eCB pathway yields interconnected functional activation states of eCB signaling (prior, tonic, and persistent), which may contribute to metaplastic control of synaptic and behavioral functions in health and disease. The emerging picture supports aberrant metaplasticity as a contributor to cognitive dysfunction associated with several pathological states in which eCB signaling, or other neuromodulatory pathways, are deregulated.

Δ9-THC-caused synaptic and memory impairments are mediated through COX-2 signaling

Marijuana has been used for thousands of years as a treatment for medical conditions. However, untoward side effects limit its medical value. Here, we show that synaptic and cognitive impairments following repeated exposure to Δ(9)-tetrahydrocannabinol (Δ(9)-THC) are associated with the induction of cyclooxygenase-2 (COX-2), an inducible enzyme that converts arachidonic acid to prostanoids in the brain. COX-2 induction by Δ(9)-THC is mediated via CB1 receptor-coupled G protein βγ subunits. Pharmacological or genetic inhibition of COX-2 blocks downregulation and internalization of glutamate receptor subunits and alterations of the dendritic spine density of hippocampal neurons induced by repeated Δ(9)-THC exposures. Ablation of COX-2 also eliminates Δ(9)-THC-impaired hippocampal long-term synaptic plasticity, working, and fear memories. Importantly, the beneficial effects of decreasing β-amyloid plaques and neurodegeneration by Δ(9)-THC in Alzheimer's disease animals are retained in the presence of COX-2 inhibition. These results suggest that the applicability of medical marijuana would be broadened by concurrent inhibition of COX-2.

Role of ERK signaling in activity-dependent modifications of histone proteins

It is well-established that neuronal intracellular signaling governed by the extracellular signal-regulated kinase (ERK/MAPK) plays a crucial role in long-term adaptive changes that occur during cognitive processes. ERK is a downstream component of a conserved signaling module that is activated by the serine/threonine kinase, Raf, which activates the MAPK/ERK kinase (MEK)1/2 protein kinases, which, in turn, activate ERK1/2. This signaling pathway has been reported to be activated in numerous physiological conditions due to a variety of stimuli, ranging from the activation of ionotropic glutamatergic receptors to metabotropic dopaminergic receptors and neurotrophin receptors. Interestingly, activated ERK can have early and late downstream effects at both the nuclear and synaptic levels. Locally, ERK signaling results in transient changes in the efficacy of synaptic transmission by modifying both pre- and post-synaptic targets. Once translocated into the nucleus, ERK signaling may control transcription by targeting several different regulators of gene expression such as transcription factors and histone proteins. ERK function is considered fundamental in processes such as long-term memory storage and drug addiction, by means of its role in activity-dependent epigenetic modifications that occur in the brain. In this review, we summarize the current understanding of ERK action in the neuroepigenetic processes underlying physiological responses, cognitive processes and drug addiction.

Chronic THC intake modifies fundamental cerebellar functions

Delta9-tetrahydrocannabinol (THC), the principal bioactive component in the Cannabis plant, is truly a captivating drug. Acute and chronic THC intake produces a spectrum of biological effects ranging from transient psychotropic effects to prolonged medicinal benefits, many of which have been fostered for centuries by our society. In the July 2013 issue of the JCI, Cutando et al. combined mouse genetics with classic mouse behavioral analysis to deepen our understanding of the physiological consequence of subchronic THC intake on eyeblink reflexes, a fundamental neuronal adaptive response, revealing that this regimen leads to downregulation of the cannabinoid CB1 receptor (referred to as CB1 in the Cutando et al. article) in cerebellar stress fibers and the activation of microglia, raising provocative new questions about the safety profile of regimented THC intake.

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.

Microglial activation underlies cerebellar deficits produced by repeated cannabis exposure

Chronic cannabis exposure can lead to cerebellar dysfunction in humans, but the neurobiological mechanisms involved remain incompletely understood. Here, we found that in mice, subchronic administration of the psychoactive component of cannabis, delta9-tetrahydrocannabinol (THC), activated cerebellar microglia and increased the expression of neuroinflammatory markers, including IL-1β. This neuroinflammatory phenotype correlated with deficits in cerebellar conditioned learning and fine motor coordination. The neuroinflammatory phenotype was readily detectable in the cerebellum of mice with global loss of the CB1 cannabinoid receptor (CB1R, Cb1(-/-) mice) and in mice lacking CB1R in the cerebellar parallel fibers, suggesting that CB1R downregulation in the cerebellar molecular layer plays a key role in THC-induced cerebellar deficits. Expression of CB2 cannabinoid receptor (CB2R) and Il1b mRNA was increased under neuroinflammatory conditions in activated CD11b-positive microglial cells. Furthermore, administration of the immunosuppressant minocycline or an inhibitor of IL-1β receptor signaling prevented the deficits in cerebellar function in Cb1(-/-) and THC-withdrawn mice. Our results suggest that cerebellar microglial activation plays a crucial role in the cerebellar deficits induced by repeated cannabis exposure.

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.

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.

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.

SK channel modulation rescues striatal plasticity and control over habit in cannabinoid tolerance

Endocannabinoids (eCBs) regulate neuronal activity in the dorso-lateral striatum (DLS), a brain region that is involved in habitual behaviors. How synaptic eCB signaling contributes to habitual behaviors under physiological and pathological conditions remains unclear. Using a mouse model of cannabinoid tolerance, we found that persistent activation of the eCB pathway impaired eCB-mediated long-term depression (LTD) and synaptic depotentiation in the DLS. The loss of eCB LTD, occurring preferentially at cortical connections to striatopallidal neurons, was associated with a shift in behavioral control from goal-directed action to habitual responding. eCB LTD and behavioral alterations were rescued by in vivo modulation of small-conductance calcium activated potassium channel (SK channel) activity in the DLS, which potentiates eCB signaling. Our results reveal a direct relationship between drug tolerance and changes in control of instrumental performance by establishing a central role for eCB LTD in habit expression. In addition, SK channels emerge as molecular targets to fine tune the eCB pathway under pathological conditions.

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.

Reduced expression of glutamate receptors and phosphorylation of CREB are responsible for in vivo Delta9-THC exposure-impaired hippocampal synaptic plasticity

Chronic use of marijuana impairs synaptic plasticity and cognitive function. However, the molecular mechanisms by which marijuana alters long-term synaptic plasticity are largely unknown. Here, we show that repeated in vivo exposures to Delta9-THC for 7 consecutive days significantly impaired hippocampal long-term potentiation (LTP) of excitatory glutamatergic synaptic transmission. The Delta9-THC exposure-induced decrease in LTP was prevented by pharmacological inhibition or deletion of the cannabinoid 1 receptor (CB1R). To determine the molecular mechanisms underlying Delta9-THC-altered LTP, we targeted expression and function of the glutamate receptors (GluR) and phosphorylation status of cAMP-response element-binding protein (CREB). Chronic in vivo exposure to Delta9-THC produced CB1R-dependent decreases in expression of hippocampal GluR1, NR2A, and NR2B, the ratio of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/NMDA receptor-gated currents, and phosphorylation of CREB. Our results suggest that reduced expression and function of the GluR subunits and phosphorylation of CREB may underlie the impaired long-term synaptic plasticity induced by repeated in vivo exposure to Delta9-THC.

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.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

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.

Opposing actions of chronic Delta9-tetrahydrocannabinol and cannabinoid antagonists on hippocampal long-term potentiation

Memory deficits produced by marijuana arise partly via interaction of the psychoactive component, Delta(9)-tetrahydrocannabinol (Delta(9)-THC), with cannabinoid receptors in the hippocampus. Although cannabinoids acutely reduce glutamate release and block hippocampal long-term potentiation (LTP), a potential substrate for learning and memory, the consequences of prolonged exposure to Delta(9)-THC for hippocampal function are poorly understood. Rats were injected with Delta(9)-THC (10 mg/kg, i.p., q.d.) for 1, 3, or 7 d, and electrophysiological recordings were performed in hippocampal slices 1d after the final injection. At this time, Delta(9)-THC was undetectable in hippocampus using liquid chromatography-mass spectrometry (LC-MS). Hippocampal LTP generated using high-frequency (HFS) or theta burst stimulation was not observed in brain slices from the 7-d Delta(9)-THC-treated animals. Delta(9)-THC also blocked HFS-LTP after 3 d, but not 1 d of treatment. The complete blockade of LTP persisted for 3 d after the last Delta(9)-THC injection, and full reversal of the LTP deficit was not observed up to 14 d following Delta(9)-THC withdrawal. The cannabinoid antagonist AM251 (2 mg/kg), administered before each Delta(9)-THC injection prevented the blockade of LTP, and 7-d treatment with AM251 alone significantly increased the level of LTP. Chronic Delta(9)-THC also produced tolerance to the inhibition of synaptic GABA, but not glutamate release by the agonist WIN55,212-2. These data define consequences of repeated Delta(9)-THC exposure for synaptic plasticity in the hippocampus that may help explain memory impairments in humans following chronic marijuana use.

SCLIP, a Microtubule-destabilizing Factor, Interacts with RasGRF1 and Inhibits Its Ability to Promote Rac Activation and Neurite Outgrowth

RasGRF1 is a neuron-specific guanine nucleotide exchange factor for the small GTPases Ras and Rac. It is implicated in the regulation of memory formation and in the development of tolerance to drug abuse, although the mechanisms have been elucidated only in part. Here we report the isolation, by the yeast two-hybrid screen, of the microtubule-destabilizing factor SCLIP (SCG10-like protein) as a novel RasGRF1-interacting protein. This interaction requires the region spanning the Dbl-homology domain of RasGRF1, endowed with catalytic activity on Rac. In search for a possible function we found by biochemical means that SCLIP influences the signaling properties of RasGRF1, greatly reducing its ability to activate the Rac/p38 MAPK pathway, while the Ras/Erk one remains unaffected. Moreover, a potential role is suggested by transfection studies in neuronal PC12 cells in which RasGRF1 induces neurite outgrowth, and coexpression of SCLIP counteracts this effect, causing a dramatic decrease in the percentage of cells bearing neurites, which also appear significantly shortened. This study unveils a physical and functional interaction between RasGRF1 and SCLIP. We suggest that this novel interplay may have possible implications in mechanisms that regulate neuronal morphology and structural plasticity.

Regulation of mitogen-activated protein kinases by glutamate receptors

Glutamate receptors regulate gene expression in neurons by activating intracellular signaling cascades that phosphorylate transcription factors within the nucleus. The mitogen-activated protein kinase (MAPK) cascade is one of the best characterized cascades in this regulatory process. The Ca(2+)-permeable ionotropic glutamate receptor, mainly the NMDA receptor subtype, activates MAPKs through a biochemical route involving the Ca(2+)-sensitive Ras-guanine nucleotide releasing factor, Ca(2+)/calmodulin-dependent protein kinase II, and phosphoinositide 3-kinase. The metabotropic glutamate receptor (mGluR), however, activates MAPKs primarily through a Ca(2+)-insensitve pathway involving the transactivation of receptor tyrosine kinases. The adaptor protein Homer also plays a role in this process. As an information superhighway between surface glutamate receptors and transcription factors in the nucleus, active MAPKs phosphorylate specific transcription factors (Elk-1 and CREB), and thereby regulate distinct programs of gene expression. The regulated gene expression contributes to the development of multiple forms of synaptic plasticity related to long-lasting changes in memory function and addictive properties of drugs of abuse. This review, by focusing on new data from recent years, discusses the signaling mechanisms by which different types of glutamate receptors activate MAPKs, features of each MAPK cascade in regulating gene expression, and the importance of glutamate/MAPK-dependent synaptic plasticity in memory and addiction.