Time-dependent differences of repeated administration with Delta9-tetrahydrocannabinol in proenkephalin and cannabinoid receptor gene expression and G-protein activation by mu-opioid and CB1-cannabinoid receptors in the caudate-putamen

Mechanisms of cannabinoid tolerance

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

Cannabidiol regulates behavioral and brain alterations induced by spontaneous alcohol withdrawal

The main goal of this study was to evaluate if the administration of cannabidiol (CBD) regulates behavioral and gene expression alterations induced by spontaneous alcohol withdrawal (SAW) in mice. Increasing doses of ethanol were administered to C57BL/6J male mice for 15 days (2.5, 3 and 3.5 g/kg/12 h, p. o.), and SAW was studied at 6, 12, 24, and 72 h after the last ethanol administration. The efficacy of acute CBD (10, 20, and 40 mg/kg, i. p.) to regulate behavioral changes induced by SAW was explored at 6 h. Gene expression analyses of cannabinoid receptors 1 (Cnr1) and 2 (Cnr2), mu-opioid receptor (Opmr1), and proopiomelanocortin (Pomc) in the nucleus accumbens (NAcc), and Pomc and tyrosine hydroxylase (Th) in the ventral tegmental area (VTA), were carried out by real time-PCR. Pearson correlation was used to identify potential associations between the gene expression data and the anxiety-like behaviors. Biostatistical studies suggest associations between gene expression data and the anxiogenic behaviors in mice exposed to the SAW model and treated with VEH and 40 mg/kg of CBD. Mice exposed to the SAW model showed significant somatic withdrawal signs, anxiety-like behaviors, and remarkable changes in the gene expression of all brain targets at 6 h. CBD dose-dependently normalized the behavioral, somatic withdrawal signs and anxiety-like behaviors and modulated gene expression changes in the NAcc, but not in the VTA. The results of this study suggest that CBD may regulate specific alcohol withdrawal-associated alterations. However, further studies are required to explore the possible mechanisms involved.

Cannabidiol Interferes with Establishment of Δ9-Tetrahydrocannabinol-Induced Nausea Through a 5-HT1A Mechanism

Introduction: Cannabinoid hyperemesis syndrome (CHS) is characterized by intense nausea and vomiting brought on by the use of high-dose Δ⁹-tetrahydrocannabinol (THC), the main psychotropic compound in cannabis. Cannabidiol (CBD), a nonpsychotropic compound found in cannabis, has been shown to interfere with some acute aversive effects of THC. In this study, we evaluated if CBD would interfere with THC-induced nausea through a 5-HT_1A receptor mechanism as it has been shown to interfere with nausea produced by lithium chloride (LiCl). Since CHS has been attributed to a dysregulated stress response, we also evaluated if CBD would interfere with THC-induced increase in corticosterone (CORT). Materials and Methods: The potential of CBD (5 mg/kg, ip) to suppress THC-induced conditioned gaping (a measure of nausea) was evaluated in rats, as well as the potential of the 5-HT_1A receptor antagonist, WAY-100635 (WAY; 0.1 mg/kg, ip), to reverse the suppression of THC-induced conditioned gaping by CBD. Last, the effect of CBD (5 mg/kg, ip) on THC-induced increase in serum CORT concentration was evaluated. Results: Pretreatment with CBD (5 mg/kg, ip) interfered with the establishment of THC-induced conditioned gaping (p=0.007, relative to vehicle [VEH] pretreatment), and this was reversed by pretreatment with 0.1 mg/kg WAY. This dose of WAY had no effect on gaping on its own. THC (10 mg/kg, ip) significantly increased serum CORT compared with VEH-treated rats (p=0.04). CBD (5 mg/kg, ip) pretreatment reversed the THC-induced increase in CORT. Conclusions: CBD attenuated THC-induced nausea as well as THC-induced elevation in CORT. The attenuation of THC-induced conditioned gaping by CBD was mediated by its action on 5-HT_1A receptors, similar to that of LiCl-induced nausea.

Role of the stress response and the endocannabinoid system in Δ9-tetrahydrocannabinol (THC)-induced nausea

RATIONALE

Dysregulation of the endocannabinoid (eCB) system by high doses of Δ⁹-tetrahydrocannabinol (THC) is hypothesized to generate a dysfunctional hypothalamic-pituitary-adrenal (HPA) axis contributing to cannabinoid hyperemesis syndrome (CHS).

OBJECTIVES AND METHODS

Using the conditioned gaping model of nausea, we aimed to determine if pre-treatments that interfere with stress, or an anti-emetic drug, interfere with THC-induced nausea in male rats. The corticotropin-releasing hormone (CRH) antagonist, antalarmin, was given to inhibit the HPA axis during conditioning. Since eCBs inhibit stress, MJN110 (which elevates 2-arachidonylglycerol (2-AG)) and URB597 (which elevates anandamide (AEA)) were also tested. Propranolol (β-adrenergic antagonist) and WAY-100635 (5-HT_1A antagonist) attenuate HPA activation by cannabinoids and, therefore, were assessed. In humans, CHS symptoms are not alleviated by anti-emetic drugs, such as ondansetron (5-HT₃ antagonist); however, benzodiazepines are effective. Therefore, ondansetron and chlordiazepoxide were tested. To determine if HPA activation by THC is dose-dependent, corticosterone (CORT) was analyzed from serum of rats treated with 0.0, 0.5, or 10 mg/kg THC.

RESULTS

Antalarmin (10 and 20 mg/kg), MJN110 (10 mg/kg), URB597 (0.3 mg/kg), propranolol (2.5 and 5 mg/kg), WAY-100635 (0.5 mg/kg), and chlordiazepoxide (5 mg/kg) interfered with THC-induced conditioned gaping, but the anti-emetic ondansetron (0.1 and 0.01 mg/kg) did not. THC produced significantly higher CORT levels at 10 mg/kg than at 0.0 and 0.5 mg/kg THC.

CONCLUSIONS

Treatments that interfere with the stress response also inhibit THC-induced conditioned gaping, but a typical anti-emetic drug does not, supporting the hypothesis that THC-induced nausea, and CHS, is a result of a dysregulated stress response.

Cannabinoid Hyperemesis Syndrome: A Review of Potential Mechanisms

Introduction: Cannabinoids have long been known for their ability to treat nausea and vomiting. Recent reports, however, have highlighted the paradoxical proemetic effects of cannabinoids. Cannabinoid hyperemesis syndrome (CHS) is characterized by cyclical episodes of nausea and vomiting, accompanied by abdominal pain following prolonged, high-dose cannabis use, which is alleviated by hot baths and showers. Little is known about the cause of this syndrome. Discussion: Cannabinoids produce a biphasic effect on nausea and vomiting, with low doses having an antiemetic effect and high doses producing emesis. Presentation and treatment of CHS are similar to cyclical vomiting syndrome as well as chemotherapy-related anticipatory nausea and vomiting, suggesting that these phenomena may share mechanisms. The prevalence of CHS is not known because of the symptomatic overlap with other disorders and the lack of knowledge of the syndrome by the public and physicians. Treatment with typical antiemetic drugs is ineffective for CHS, but anxiolytic and sedative drugs, along with hot showers, seem to be consistently effective at reducing symptoms. The only known way to permanently end CHS, however, is abstinence from cannabinoids. Case studies and limited pre-clinical data on CHS indicate that prolonged high doses of the main psychotropic compound in cannabis, Δ9-tetrahydrocannabinol (THC), result in changes to the endocannabinoid system by acting on the cannabinoid 1 (CB1) receptor. These endocannabinoid system changes can dysregulate stress and anxiety responses, thermoregulation, the transient receptor potential vanilloid system, and several neurotransmitters systems, and are thus potential candidates for mediating the pathophysiology of CHS. Conclusions: Excessive cannabinoid administration disrupts the normal functioning of the endocannabinoid system, which may cause CHS. More clinical and pre-clinical research is needed to fully understand the underlying pathophysiology of this disorder and the negative consequences of prolonged high-dose cannabis use.

Modulation of Pain Sensitivity by Chronic Consumption of Highly Palatable Food Followed by Abstinence: Emerging Role of Fatty Acid Amide Hydrolase

There is a strong relationship between palatable diet and pain sensitivity, and the cannabinoid and opioid systems might play an important role in this correlation. The palatable diet used in many animal models of obesity is the cafeteria (CAF) diet, based on human food with high sugar, salt, and fat content. In this study, we investigated whether long-term exposure to a CAF diet could modify pain sensitivity and explored the role of the cannabinergic system in this modification. Male Sprague–Dawley rats were divided into two groups: one fed with standard chow only (CO) and the other with extended access (EA) to a CAF diet. Hot plate and tail flick tests were used to evaluate pain sensitivity. At the end of a 40-day CAF exposure, EA rats showed a significant increase in the pain threshold compared to CO rats, finding probably due to up-regulation of CB1 and mu-opioid receptors. Instead, during abstinence from palatable foods, EA animals showed a significant increase in pain sensibility, which was ameliorated by repeated treatment with a fatty acid amide hydrolase inhibitor, PF-3845 (10 mg/kg, intraperitoneally), every other day for 28 days. Ex vivo analysis of the brains of these rats clearly showed that this effect was mediated by mu-opioid receptors, which were up-regulated following repeated treatment of PF-3845. Our data add to the knowledge about changes in pain perception in obese subjects, revealing a key role of CB1 and mu-opioid receptors and their possible pharmacological crosstalk and reinforcing the need to consider this modulation in planning effective pain management for obese patients.

Sex difference in brain CB1 receptor availability in man

The endocannabinoid system (ECS) has a widespread neuromodulatory function in the central nervous system and is involved in important aspects of brain function including brain development, cortical rhythms, plasticity, reward, and stress sensitivity. Many of these effects are mediated via the cannabinoid CB1 receptor (CB1R) subtype. Animal studies convincingly show an interaction between the ECS and sex hormones, as well as a sex difference of higher brain CB1R in males. Human in vivo studies of sex difference have yielded discrepant findings. Gender differences in CB1R availability were investigated in vivo in 11 male and 11 female healthy volunteers using a specific CB1R tracer [¹⁸F]FMPEP-d2 and positron emission tomography (PET). Regional [¹⁸F]FMPEP-d2 distribution volume was used as a proxy for CB1R availability. In addition, we explored whether CB1R availability is linked to neuropsychological functioning. Relative to females, CB1R availability was on average 41% higher in males (p = 0.002) with a regionally specific effect larger in the posterior cingulate and retrosplenial cortices (p = 0.001). Inter-subject variability in CB1R availability was similar in both groups. Voxel-based analyses revealed an inverse association between CB1R availability and visuospatial working memory task performance in both groups (p < 0.001). A CB1R sex difference with a large effect size was observed and should be considered in the design of CB1R-related studies on neuropsychiatric disorders. The behavioural correlates and clinical significance of this difference remain to be further elucidated, but our studies suggest an association between CB1R availability and working memory.

Cannabidiol regulates behavioural alterations and gene expression changes induced by spontaneous cannabinoid withdrawal

BACKGROUND AND PURPOSE

Cannabidiol (CBD) represents a promising therapeutic tool for treating cannabis use disorder (CUD). This study aimed to evaluate the effects of CBD on the behavioural and gene expression alterations induced by spontaneous cannabinoid withdrawal.

EXPERIMENTAL APPROACH

Spontaneous cannabinoid withdrawal was evaluated 12 h after cessation of CP-55,940 treatment (0.5 mg·kg^-1 every 12 h, i.p.; 7 days) in C57BL/6J mice. The effects of CBD (5, 10 and 20 mg·kg^-1 , i.p.) on withdrawal-related behavioural signs were evaluated by measuring motor activity, somatic signs and anxiety-like behaviour. Furthermore, gene expression changes in TH in the ventral tegmental area, and in the opioid μ receptor (Oprm1), cannabinoid CB₁ receptor (Cnr1) and CB₂ receptor (Cnr2) in the nucleus accumbens, were also evaluated using the real-time PCR technique.

KEY RESULTS

The administration of CBD significantly blocked the increase in motor activity and the increased number of rearings, rubbings and jumpings associated with cannabinoid withdrawal, and it normalized the decrease in the number of groomings. However, CBD did not change somatic signs in vehicle-treated animals. In addition, the anxiogenic-like effect observed in abstinent mice disappeared with CBD administration, whereas CBD induced an anxiolytic-like effect in non-abstinent animals. Moreover, CBD normalized gene expression changes induced by CP-55,940-mediated spontaneous withdrawal.

CONCLUSIONS AND IMPLICATIONS

The results suggest that CBD alleviates spontaneous cannabinoid withdrawal and normalizes associated gene expression changes. Future studies are needed to determine the relevance of CBD as a potential therapeutic tool for treating CUD.

Oral haloperidol or olanzapine intake produces distinct and region-specific increase in cannabinoid receptor levels that is prevented by high fat diet

Clinical studies show higher levels of cannabinoid CB1 receptors (CB1R) in the brain of schizophrenic patients while preclinical studies report a significant functional interaction between dopamine D2 receptors and CB1Rs as well as an upregulation of CB1Rs after antipsychotic treatment. These findings prompted us to study the effects of chronic oral intake of a first and a second generation antipsychotic, haloperidol and olanzapine, on the levels and distribution of CB1Rs in the rat brain. Rats consumed either regular chow or high-fat food and drank water, haloperidol drinking solution (1.5mg/kg), or olanzapine drinking solution (10mg/kg) for four weeks. Motor and cognitive functions were tested at the end of treatment week 3 and upon drug discontinuation. Two days after drug discontinuation, rats were euthanized and brains were processed for in vitro receptor autoradiography. In chow-fed animals, haloperidol and olanzapine increased CB1R levels in the basal ganglia and the hippocampus, in a similar, but not identical pattern. In addition, olanzapine had unique effects in CB1R upregulation in higher order cognitive areas, in the secondary somatosensory cortex, in the visual and auditory cortices and the geniculate nuclei, as well as in the hypothalamus. High fat food consumption prevented antipsychotic-induced increase in CB1R levels in all regions examined, with one exception, the globus pallidus, in which they were higher in haloperidol-treated rats. The results point towards the hypothesis that increased CB1R levels could be a confounding effect of antipsychotic medication in schizophrenia that is circumveneted by high fat feeding.

Turning Down the Thermostat: Modulating the Endocannabinoid System in Ocular Inflammation and Pain

The endocannabinoid system (ECS) has emerged as an important regulator of both physiological and pathological processes. Notably, this endogenous system plays a key role in the modulation of pain and inflammation in a number of tissues. The components of the ECS, including endocannabinoids, their cognate enzymes and cannabinoid receptors, are localized in the eye, and evidence indicates that ECS modulation plays a role in ocular disease states. Of these diseases, ocular inflammation presents a significant medical problem, given that current clinical treatments can be ineffective or are associated with intolerable side-effects. Furthermore, a prominent comorbidity of ocular inflammation is pain, including neuropathic pain, for which therapeutic options remain limited. Recent evidence supports the use of drugs targeting the ECS for the treatment of ocular inflammation and pain in animal models; however, the potential for therapeutic use of cannabinoid drugs in the eye has not been thoroughly investigated at this time. This review will highlight evidence from experimental studies identifying components of the ocular ECS and discuss the functional role of the ECS during different ocular inflammatory disease states, including uveitis and corneal keratitis. Candidate ECS targeted therapies will be discussed, drawing on experimental results obtained from both ocular and non-ocular tissue(s), together with their potential application for the treatment of ocular inflammation and pain.

Effects of neuropeptide FF and related peptides on the antinociceptive activities of VD-hemopressin(α) in naive and cannabinoid-tolerant mice

Neuropeptide FF (NPFF) system has recently been reported to modulate cannabinoid-induced antinociception. The aim of the present study was to further investigate the roles of NPFF system in the antinociceptive effects induced by intracerebroventricular (i.c.v.) administration of mouse VD-hemopressin(α), a novel endogenous agonist of cannabinoid CB1 receptor, in naive and VD-hemopressin(α)-tolerant mice. The effects of NPFF system on the antinociception induced by VD-hemopressin(α) were investigated in the radiant heat tail-flick test in naive mice and VD-hemopressin(α)-tolerant mice. The cannabinoid-tolerant mice were produced by given daily injections of VD-hemopressin(α) (20 nmol, i.c.v.) for 5 days and the antinociception was measured on day 6. In naive mice, intracerebroventricular injection of NPFF dose-dependently attenuated central analgesia of VD-hemopressin(α). In contrast, neuropeptide VF (NPVF) and D.NP(N-Me)AFLFQPQRF-NH2 (dNPA), two highly selective agonists for Neuropeptide FF1 and Neuropeptide FF2 receptors, enhanced VD-hemopressin(α)-induced antinociception in a dose-dependent manner. In addition, the VD-hemopressin(α)-modulating activities of NPFF and related peptides were antagonized by the Neuropeptide FF receptors selective antagonist 1-adamantanecarbonyl-RF-NH2 (RF9). In VD-hemopressin(α)-tolerant mice, NPFF failed to modify VD-hemopressin(α)-induced antinociception. However, both neuropeptide VF and dNPA dose-dependently potentiated the antinociception of VD-hemopressin(α) and these cannabinoid-potentiating effects were reduced by RF9. The present works support the cannabinoid-modulating character of NPFF system in naive and cannabinoid-tolerant mice. In addition, the data suggest that a chronic cannabinoid treatment modifies the pharmacological profiles of NPFF, but not the cannabinoid-potentiating effects of neuropeptide VF and dNPA.

Cannabinoids and Epilepsy

Cannabis has been used for centuries to treat seizures. Recent anecdotal reports, accumulating animal model data, and mechanistic insights have raised interest in cannabis-based antiepileptic therapies. In this study, we review current understanding of the endocannabinoid system, characterize the pro- and anticonvulsive effects of cannabinoids [e.g., Δ9-tetrahydrocannabinol and cannabidiol (CBD)], and highlight scientific evidence from pre-clinical and clinical trials of cannabinoids in epilepsy. These studies suggest that CBD avoids the psychoactive effects of the endocannabinoid system to provide a well-tolerated, promising therapeutic for the treatment of seizures, while whole-plant cannabis can both contribute to and reduce seizures. Finally, we discuss results from a new multicenter, open-label study using CBD in a population with treatment-resistant epilepsy. In all, we seek to evaluate our current understanding of cannabinoids in epilepsy and guide future basic science and clinical studies.

Impact of stress on prefrontal glutamatergic, monoaminergic and cannabinoid systems

Stress has been shown to have marked and divergent effects on learning and memory which involves specific brain regions, such as spatial and declarative memory involving the hippocampus, memory of emotional arousing experiences and fear involving the amygdala, and executive functions and fear extinction involving the prefrontal cortex or the PFC. Response to stress involves a coordinated activation of a constellation of physiological systems including the activation of the hypothalamic-pituitary-adrenal (HPA) axis and other modulatory neurotransmitters and signaling systems. This paper presents a concise review of the effects of stress and glucocorticoids on the glutamatergic and monoaminergic (including noradrenergic, dopaminergic, and serotonergic systems) neurotransmitter systems as well as endocannabinoid signaling. Because of the breadth of the scope of this topic, the review is limited to the effects of stress on these brain systems on the prefrontal cortex, and where relevant, the hippocampus and the amygdala.

Trajectory of adolescent cannabis use on addiction vulnerability

The adolescent brain is a period of dynamic development making it vulnerable to environmental factors such as drug exposure. Of the illicit drugs, cannabis is most used by teenagers since it is perceived by many to be of little harm. This perception has led to a growing number of states approving its legalization and increased accessibility. Most of the debates and ensuing policies regarding cannabis were done without consideration of its impact on one of the most vulnerable population, namely teens, or without consideration of scientific data. We provide an overview of the endocannabinoid system in relation to adolescent cannabis exposure and provide insights regarding factors such as genetics and behavioral traits that confer risk for subsequent addiction. While it is clear that more systematic scientific studies are needed to understand the long-term impact of adolescent cannabis exposure on brain and behavior, the current evidence suggests that it has a far-reaching influence on adult addictive behaviors particularly for certain subsets of vulnerable individuals. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Cannabinoid and opioid interactions: implications for opiate dependence and withdrawal

Withdrawal from opiates, such as heroin or oral narcotics, is characterized by a host of aversive physical and emotional symptoms. High rates of relapse and limited treatment success rates for opiate addiction have prompted a search for new approaches. For many opiate addicts, achieving abstinence may be further complicated by poly-drug use and co-morbid mental disorders. Research over the past decade has shed light on the influence of endocannabinoids (ECs) on the opioid system. Evidence from both animal and clinical studies point toward an interaction between these two systems, and suggest that targeting the EC system may provide novel interventions for managing opiate dependence and withdrawal. This review will summarize the literature surrounding the molecular effects of cannabinoids and opioids on the locus coeruleus-norepinephrine system, a key circuit implicated in the negative sequelae of opiate addiction. A consideration of the trends and effects of marijuana use in those seeking treatment to abstain from opiates in the clinical setting will also be presented. In summary, the present review details how cannabinoid-opioid interactions may inform novel interventions in the management of opiate dependence and withdrawal.

Cannabinoids increase type 1 cannabinoid receptor expression in a cell culture model of striatal neurons: implications for Huntington's disease

The type 1 cannabinoid receptor (CB1) is a G protein-coupled receptor that is expressed at high levels in the striatum. Activation of CB1 increases expression of neuronal trophic factors and inhibits neurotransmitter release from GABA-ergic striatal neurons. CB1 mRNA levels can be elevated by treatment with cannabinoids in non-neuronal cells. We wanted to determine whether cannabinoid treatment could induce CB1 expression in a cell culture model of striatal neurons and, if possible, determine the molecular mechanism by which this occurred. We found that treatment of STHdh(7/7) cells with the cannabinoids ACEA, mAEA, and AEA produced a CB1receptor-dependent increase in CB1 promoter activity, mRNA, and protein expression. This response was Akt- and NF-κB-dependent. Because decreased CB1 expression is thought to contribute to the pathogenesis of Huntington's disease (HD), we wanted to determine whether cannabinoids could increase CB1 expression in STHdh(7/111) and (111/111) cells expressing the mutant huntingtin protein. We observed that cannabinoid treatment increased CB1 mRNA levels approximately 10-fold in STHdh(7/111) and (111/111) cells, compared to vehicle treatment. Importantly, cannabinoid treatment improved ATP production, increased the expression of the trophic factor BDNF-2, and the mitochondrial regulator PGC1α, and reduced spontaneous GABA release, in HD cells. Therefore, cannabinoid-mediated increases in CB1 levels could reduce the severity of some molecular pathologies observed in HD.

The dynamic nature of type 1 cannabinoid receptor (CB(1) ) gene transcription

The type 1 cannabinoid receptor (CB(1) ) is an integral component of the endocannabinoid system that modulates several functions in the CNS and periphery. The majority of our knowledge of the endocannabinoid system involves ligand-receptor binding, mechanisms of signal transduction, and protein-protein interactions. In contrast, comparatively little is known about regulation of CB(1) gene expression. The levels and anatomical distribution of CB(1) mRNA and protein are developmental stage-specific and are dysregulated in several pathological conditions. Moreover, exposure to a variety of drugs, including cannabinoids themselves, alters CB(1) gene expression and mRNA levels. As such, alterations in CB(1) gene expression are likely to affect the optimal response to cannabinoid-based therapies, which are being developed to treat a growing number of conditions. Here, we will examine the regulation of CB(1) mRNA levels and the therapeutic potential inherent in manipulating expression of this gene.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8.

Naltrexone does not attenuate the effects of intravenous Δ9-tetrahydrocannabinol in healthy humans

Although a wealth of preclinical evidence indicates an interplay between the μ-opioid (MOR) and cannabinoid 1 receptor (CB1R) systems, the precise nature of the cross modulation in humans is unclear. The objective of this study was to evaluate the effects of pretreatment with the MOR antagonist, naltrexone, on the subjective, behavioural and cognitive effects of the CB1R agonist, Δ9-tetrahydrocannabinol (THC), in healthy human subjects. Healthy human subjects, screened carefully for any medical or psychiatric illness, were administered either placebo or active naltrexone (25 mg) orally on each test day, followed 45 min later by placebo and 165 min later by active i.v. THC (0.025 mg/kg) in a randomized, fixed-order, double-blind manner. Subjective, behavioural and cognitive effects were assessed before and at several points after each drug administration. THC produced expected effects, including euphoria, anxiety, transient perceptual alterations, transient psychotomimetic effects and cognitive impairments. However, naltrexone did not produce any effects alone, nor did it attenuate any of THC's effects. Thus, in healthy human subjects who use cannabis intermittently, MOR antagonism does not modulate the common acute subjective, behavioural and cognitive effects of THC.

Increased ethanol intake in prodynorphin knockout mice is associated to changes in opioid receptor function and dopamine transmission

The purpose of this study was to examine the role of the prodynorphin gene in alcohol sensitivity, preference and vulnerability to alcohol consumption. Handling-induced convulsion (HIC) associated to alcohol, alcohol-induced loss of righting reflex (LORR), hypothermic effects in response to acute ethanol challenge, blood ethanol levels (BELs), conditioned place preference, voluntary ethanol consumption and preference, tyrosine hydroxylase (TH), dopamine transporter (DAT) and proenkephalin (PENK) gene expression, and µ-, δ- and κ-opioid agonist-stimulated [S(35) ]- guanosine 5'-triphosphate-binding autoradiography were studied in prodynorphin knockout (PDYN KO) and wild-type (WT) mice. There were no differences in HIC, LORR or the decrease in body temperature in response to acute ethanol challenge between PDYN KO and WT mice. PDYN KO mice presented higher BEL, higher ethanol-conditioned place preference and more ethanol consumption and preference in a two-bottle choice paradigm than WT mice. These findings were associated with lower TH and higher DAT gene expression in the ventral tegmental area and substantia nigra, and with lower PENK gene expression in the caudate-putamen (CPu), accumbens core (AcbC) and accumbens shell (AcbSh) in PDYN KO. The functional activity of the µ-opioid receptor was lower in the CPu, AcbC, AcbSh and cingulate cortex (Cg) of PDYN KO mice. In contrast, δ- and κ-opioid receptor-binding autoradiographies were increased in the CPu and Cg (δ), and in the CPu, AcbC and Cg (κ) of PDYN KO. These results suggest that deletion of the PDYN gene increased vulnerability for ethanol consumption by altering, at least in part, PENK, TH and DAT gene expression, and µ-, δ- and κ-opioid receptor functional activity in brain areas closely related to ethanol reinforcement.

Prodynorphin gene deletion increased anxiety-like behaviours, impaired the anxiolytic effect of bromazepam and altered GABAA receptor subunits gene expression in the amygdala

This study evaluated the role of prodynorphin gene in the regulation of anxiety and associated molecular mechanisms. Emotional responses were assessed using the light-dark test, elevated plus maze and social interaction tests in prodynorphin knockout and wild-type mice. Corticotrophin releasing factor and proopiomelanocortin gene expressions in the hypothalamus were evaluated after restraint stress using in situ hybridization. The anxiolytic efficacy of bromazepam and GABA(A) receptor subunits gene expression in the amygdala were also assessed in both genotypes. The deletion of prodynorphin increased anxiety-like behaviours and proopiomelanocortin gene expression in the arcuate nucleus (two-fold). Moreover, the anxiolytic action of bromazepam was significantly attenuated in the mutant mice. Decreased GABA(A)γ(2) and increased GABA(A)β(2) gene expression receptor subunits were found in the amygdala of prodynorphin knockout mice. These results indicate that deletion of prodynorphin gene is associated with increased anxiety-like behaviours, enhanced sensibility response to stress stimuli, reduced anxiolytic efficacy of bromazepam and altered expression of the GABA(A) receptor subunits.

Evaluation of the role of striatal cannabinoid CB1 receptors on movement activity of parkinsonian rats induced by reserpine

It has been observed cannabinoid CB1 receptor signalling and the levels of endocannabinoid ligands significantly increased in the basal ganglia and cerebrospinal fluids of Parkinson's disease (PD) patients. These evidences suggest that the blocking of cannabinoid CB1 receptors might be beneficial to improve movement disorders as a sign of PD. In this study, a dose-response study of the effects of intrastriatal injection of a cannabinoid CB1 receptor antagonist, AM251 and agonist, ACPA, on movement activity was performed by measuring the catalepsy of reserpinized and non-PD (normal) rats with bar test. Also the effect of co-administration the most effective dose of AM251 and several doses of ACPA were assessed. AM251 decreases the reserpine induced catalepsy in dose dependent manner and ACPA causes catalepsy in normal rats in dose dependant manner as well. AM251 significantly reverse the cataleptic effect in all three groups (1, 10, 100 ng/rat) that received ACPA. These results support this theory that cannabinoid CB1 receptor antagonists might be useful to alleviate movement disorder in PD. Also continuance of ACPA induced catalepsy in rats after AM251 injection can indicate that other neurotransmitters or receptors interfere in ACPA induced catalepsy. Based on the present finding there is an incomplete overlapping between cannabinoid CB1 receptor agonist and antagonist effects.

The cannabinoid CB1 receptor is involved in the anxiolytic, sedative and amnesic actions of benzodiazepines

Previous studies in our laboratory showed that cannabinoid CB1 receptor knockout mice (CB1-/-) presented increased anxiety-like behaviours that did not respond to the anxiolytic actions of benzodiazepines. These results suggest that the pharmacological effects of benzodiazepines may involve the participation of cannabinoid CB1 receptors. Therefore, the purpose of this study was to examine the effects of alprazolam and the cannabinoid CB1 receptor antagonist AM251 on behavioural assays (light-dark box test, neurological severity score and step-down inhibitory avoidance test) and on the functional activity of the CB1 receptor (WIN-55,212-stimulated [(35)S] guanosine triphosphate (GTP) gamma binding autoradiography).The administration of alprazolam (40 microg/kg, intraperitoneal (i.p.)) decreased anxiety-like behaviours in the light-dark box test and significantly reduced WIN-55,212-stimulated [(35)S]GTPgamma binding autoradiography in the amygdala and in the CA1 field of the hippocampus, but was without effects on CA2, CA3 and the dentate gyrus (DG) of the hippocampus. The administration of AM251 (3 mg/kg, i.p.) blocked the anxiolytic action of alprazolam (40 microg/kg, i.p.), significantly reduced the sedative (ataxia, neurological severity score in the 0.5 cm bar) and the amnesic actions (short time term memory (1 h after electric shock)) of alprazolam (0.5 mg/kg, i.p.).Taken together, these findings revealed that cannabinoid CB1 receptor plays a pivotal role in the pharmacological actions of benzodiazepines. Furthermore, these results suggest that blockade of cannabinoid CB1 receptors may be useful in the treatment of patients with problems related to the consumption of benzodiazepines. Further clinical trials are needed to test this hypothesis.

Examining the roles of cannabinoids in pain and other therapeutic indications: a review

IMPORTANCE OF THE FIELD

In recent times, our knowledge of cannabinoids and the endocannabinoid system has greatly advanced. With expanding knowledge, synthetic cannabinoids - including nabilone, dronabinol and a combination of synthetic Delta9-THC and cannabidiol - have been developed and tested for benefit in a variety of therapeutic indications.

AREAS COVERED IN THIS REVIEW

The aim of this article is to provide a summative review of the vast amount of clinical trial data now available on these agents.

WHAT THE READER WILL GAIN

To locate clinical trials for review, a literature search was performed using PubMed between the dates of 25 May and 30 June 2009. Search parameters were set to isolate only human randomized controlled trials (RCTs) published between 1990 and 2009. Keywords consistently used for each search include: cannabinoids, marijuana, THC, nabilone and dronabinol. Preferential selection was given to the best-designed trials, focusing on placebo-controlled, double-blind RCTs with the largest patient populations, if available.

TAKE HOME MESSAGE

As efficacy and tolerability of these agents remain questionable, it is important that cannabinoids not be considered 'first-line' therapies for conditions for which there are more supported and better-tolerated agents. Instead, these agents could be considered in a situation of treatment failure with standard therapies or as adjunctive agents where appropriate.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

CB1 receptor blockade decreases ethanol intake and associated neurochemical changes in fawn-hooded rats

BACKGROUND

This study was undertaken to identify the neurochemical changes underlying the attenuation of voluntary ethanol intake induced by the cannabinoid CB1 receptor antagonist AM251 in fawn-hooded rats.

METHODS

Rats were exposed to the 2-bottle-choice paradigm (ethanol 10% v/v or water) for 15 days. After this period, rats received AM251 (3 to 6 mg/kg, i.p.) or vehicle.

RESULTS

Voluntary ethanol intake decreased (30%) with the administration of incremental dosages of AM251 (3 mg/kg, 5 days and 6 mg/kg, 5 days) in rats with acquired high preferring ethanol consumption (>3.5 g of ethanol/kg/d). Ethanol intake significantly decreased proopiomelanocortin expression in the arcuate nucleus (38.31%) and micro-opioid-DAMGO-stimulated [(35)S]-GTPgamma binding in the caudate-putamen (40%), nucleus accumbens core (AccC) (32.87%), and shell (AccS) (34.21%). Moreover, ethanol intake increased tyrosine hydroxylase (TH) gene expression in the substantia nigra (24%) and ventral tegmental area (23%) and corticotrophin-releasing gene expression in the paraventricular hypothalamic nucleus (41.6%). The reduction of ethanol intake induced by AM251 was associated with blockade or significant reduction of the changes produced by ethanol in the expression of these genes in key regions related to drug dependence. Interestingly, treatment with AM251 reduced (20%) TH gene expression in rats drinking only water. In this respect, the action of AM251 in reducing TH gene expression may not be specific.

CONCLUSION

Taken together, these results revealed that blockade of cannabinoid CB1 receptors (CB1r) decreased voluntary ethanol intake in ethanol-habituated rats by normalizing the neurochemical alterations induced by ethanol.

Sex-dependent effects of periadolescent exposure to the cannabinoid agonist CP-55,940 on morphine self-administration behaviour and the endogenous opioid system

Early cannabinoid consumption may predispose individuals to the misuse of addictive drugs later in life. However, there is a lack of experimental evidence as to whether cannabinoid exposure during adolescence might differently affect opiate reinforcing efficacy and the opioid system in adults of both sexes. Our aim was to examine whether periadolescent chronic exposure to the cannabinoid agonist CP-55,940 could exert sex-dependent effects on morphine reinforcing and the opioid system in adulthood. Morphine reinforcing was studied under a progressive ratio (PR) reinforcement schedule in adult male and female rats that previously acquired morphine self-administration under a fixed ratio 1 (FR1) schedule. Binding levels and functionality of mu-opioid receptors were also evaluated. Periadolescent cannabinoid exposure altered morphine self-administration and the opioid system in adult rats in a sex-dependent manner. CP-55,940-exposed males exhibited higher self-administration rates under a FR1, but not under a PR schedule. In females, CP-55,940 did not modify morphine self-administration under either schedule. Moreover, CP-55,940 also increased mu-opioid receptor levels in the subcallosal streak of pre-treated animals and decreased mu-opioid receptor functionality in the nucleus accumbens shell but again, only in males. Our data indicate that adult male rats exposed to the cannabinoid in adolescence self-administer more morphine than females, but only when the demands required by the schedule of reinforcement are low, which might be related to the decrease in mu-opioid receptor functionality in the NAcc-shell observed in these animals.

Manipulation of fatty acid amide hydrolase functional activity alters sensitivity and dependence to ethanol

The aim of this study was to examine the role of fatty acid amide hydrolase (FAAH) on ethanol sensitivity, preference, and dependence. The deletion of FAAH gene or the inhibition of FAAH by carbamoyl-biphenyl-3-yl-cyclohexylcarbamate (URB597) (0.1 mg/kg) markedly increased the preference for ethanol. The study further reveals that URB597 specifically acts through FAAH and that cannabinoid-1 (CB(1)) receptor is critical for N-arachidonoyl ethanolamide (AEA) mediated ethanol-reinforced behavior as revealed by lack of URB597 effect in both FAAH and CB(1)-/- mice compared with vehicle-treated -/- mice. The FAAH -/- mice displayed a lower sensitivity to hypothermic and sedative effects to acute ethanol challenge. The FAAH -/- mice also exhibited a reduction in the severity of handling-induced convulsions following withdrawal from chronic ethanol exposure. The CB(1) receptor and proenkephalin gene expressions, and CB(1) receptor and mu-opioid (MO) receptor-mediated G-protein activation were found to be significantly lower in the caudate-putamen, nucleus accumbens core and shell of FAAH -/- than +/+ mice. Interestingly, the MO receptor-stimulated G-protein signaling was greater in the striatum of FAAH -/- than +/+ mice following voluntary ethanol consumption. These findings suggest that an elevation in the AEA content and its action on the limbic CB(1) receptor and MO receptor might contribute to ethanol-reinforced behavior. Treatment with drugs that decrease AEA tone might prove useful in reducing excessive ethanol consumption.

Endocannabinoid regulation of relapse mechanisms

Addiction involves a complex neuropharmacologic behavioural cycle, in which positive reinforcement exerted by the drug and the negative state of withdrawal drive the user to extremes to obtain the drug. Comprehensive studies have established that relapse is the most common outcome of recovery programs treating addictive behaviours. Several types of anticraving medication are available nowadays, such as naltrexone for the treatment of alcoholism, bupropion for nicotine, methadone or buprenorphine for heroin. This review focuses on recent behavioural data providing a rationale for an endocannabinoid mechanism underlying reinstatement of compulsive drug seeking. Studies supporting the contention that reinstatement of extinguished drug self-administration behaviour may be generated by cannabinoid CB1 receptor agonists and attenuated, if not blocked, by CB1 receptor antagonists, are here reviewed. In support to these findings, conditioned place preference studies substantiate the involvement of the endocannabinoid system in recidivism mechanisms by demonstrating that motivation to relapse can be triggered by CB1 receptor activation while blockade of such receptors may prevent reinstatement of place conditioning induced by either drug primings or drug-associated cues. Finally, biochemical studies evaluating changes in endocannabinoid levels, CB1 receptor density and CB1 mRNA expression during re-exposure to drug following extinction are also examined. Taken together, the evidence available has important implications in the understanding and treatment of relapsing episodes in patients undergoing detoxification.

Opioid antagonism of cannabinoid effects: differences between marijuana smokers and nonmarijuana smokers

In non-human animals, opioid antagonists block the reinforcing and discriminative-stimulus effects of Delta(9)-tetrahydrocannabinol (THC), while in human marijuana smokers, naltrexone (50 mg) enhances the reinforcing and subjective effects of THC. The objective of this study was to test a lower, more opioid-selective dose of naltrexone (12 mg) in combination with THC. The influence of marijuana-use history and sex was also investigated. Naltrexone (0, 12 mg) was administered 30 min before oral THC (0-40 mg) or methadone (0-10 mg) capsules, and subjective effects, task performance, pupillary diameter, and cardiovascular parameters were assessed in marijuana smoking (Study 1; n=22) and in nonmarijuana smoking (Study 2; n=21) men and women. The results show that in marijuana smokers, low-dose naltrexone blunted the intoxicating effects of a low THC dose (20 mg), while increasing ratings of anxiety at a higher THC dose (40 mg). In nonmarijuana smokers, low-dose naltrexone shifted THC's effects in the opposite direction, enhancing the intoxicating effects of a low THC dose (2.5 mg) and decreasing anxiety ratings following a high dose of THC (10 mg). There were no sex differences in these interactions, although among nonmarijuana smokers, men were more sensitive to the effects of THC alone than women. To conclude, a low, opioid-selective dose of naltrexone blunted THC intoxication in marijuana smokers, while in nonmarijuana smokers, naltrexone enhanced THC intoxication. These data demonstrate that the interaction between opioid antagonists and cannabinoid agonists varies as a function of marijuana use history.

Gene transcription alterations associated with decrease of ethanol intake induced by naltrexone in the brain of Wistar rats

Preclinical and clinical studies suggest that the administration of the opioid antagonist naltrexone decreases the intake of ethanol. However, the neuroplastic adaptations in the brain associated to reduction of ethanol consumption remains to be elucidated. The aim of the study was to identify gene transcription alterations underlying the attenuation of voluntary ethanol intake by administration of naltrexone in rats. Increasing doses of naltrexone (0.7 mg/kg, 4 days and 1.4 mg/kg/day, 4 days) to rats with acquired high preferring ethanol consumption (>3.5 g of ethanol/kg/day) decreased voluntary ethanol intake (50%). Voluntary ethanol consumption altered mu-opioid receptor function in the cingulate cortex, caudate-putamen (CPu), nucleus accumbens core (Acb C) and shell (Acb S), the expression of tyrosine hydroxylase (TH) in the ventral tegmental area and substantia nigra, proenkephalin (PENK) in the piriform cortex, olfactory tubercle, CPu, Acb C and Acb S, ventromedial nucleus (VMN) and paraventricular nucleus (PVN) of the hypothalamus, corticotropin releasing factor (CRF) in PVN, cannabinoid CB(1) receptor (CB1-R) in the CPu, hippocampus and VMN, and serotonin transporter (5-HTT) in the dorsal and median raphe nuclei. The reduction of ethanol intake induced by naltrexone was associated with a blockade or significant reduction of the changes produced by ethanol in the expression of these genes in key regions related to drug dependence. These results point to a role for the mu-opioid receptor, TH, PENK, CRF, CB1-R, and 5-HTT genes in specific brain regions in the modulation of neuroadaptative mechanisms associated to the decrease of ethanol intake induced by naltrexone.

Activation of G-proteins in brain by endogenous and exogenous cannabinoids

The biological response to cannabinoid agonist begins when the agonist-bound receptor activates G-protein G(alpha) subunits, thus initiating a cascade of signal transduction pathways. For this reason, information about cannabinoid receptors/G-protein coupling is critical to understand both the acute and chronic actions of cannabinoids. This review focuses on these mechanisms, predominantly examining the ability of cannabinoid agonists to activate G-proteins in brain with agonist-stimulated [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) binding. Acute efficacies of cannabinoid agonists at the level of G-protein activation depend not only on the ability of the agonist to induce a high affinity state in G(alpha) for GTP, but also to induce a low affinity for GDP. When several agonists are compared, it is clear that cannabinoid agonists differ considerably in their efficacy. Both WIN 55212-2 and levonantradol are full agonists, while Delta(9)-tetrahydrocannabinol is a weak partial agonist. Of interest, anandamide and its stable analog methanandamide are partial agonists. Chronic treatment in vivo with cannabinoids produces significant tolerance to the physiological and behavioral effects of these drugs, and several studies have shown that this is accompanied by a significant loss in the ability of cannabinoid receptors to couple to G-proteins in brain. These effects vary across different brain regions and are usually (but not always) accompanied by loss of cannabinoid receptor binding. Although the relationship between cannabinoid receptor desensitization and tolerance has not yet been established, these mechanisms may represent events that lead to a loss of cannabinoid agonist response and development of tolerance.

Discrete opioid gene expression impairment in the human fetal brain associated with maternal marijuana use

Fetal development is a period sensitive to environmental influences such as maternal drug use. The most commonly used illicit drug by pregnant women is marijuana. The present study investigated the effects of in utero marijuana exposure on expression levels of opioid-related genes in the human fetal forebrain in light of the strong interaction between the cannabinoid and opioid systems. The study group consisted of 42 midgestation fetuses from saline-induced voluntary abortions. The opioid peptide precursors (preprodynorphin and preproenkephalin (PENK)) and receptor (mu, kappa and delta) mRNA expression were assessed in distinct brain regions. The effect of prenatal cannabis exposure was analyzed by multiple regression controlling for confounding variables (maternal alcohol and cigarette use, fetal age, sex, growth measure and post-mortem interval). Prenatal cannabis exposure was significantly associated with increased mu receptor expression in the amygdala, reduced kappa receptor mRNA in mediodorsal thalamic nucleus and reduced preproenkephalin expression in the caudal putamen. Prenatal alcohol exposure primarily influenced the kappa receptor mRNA with reduced levels in the amygdala, claustrum, putamen and insula cortex. No significant effect of prenatal nicotine exposure could be discerned in the present study group. These results indicate that maternal cannabis and alcohol exposure during pregnancy differentially impair opioid-related genes in distinct brain circuits that may have long-term effects on cognitive and emotional behaviors.

Kappa- and delta-opioid receptor functional activities are increased in the caudate putamen of cannabinoid CB1 receptor knockout mice

The purpose of this study was to examine the functional interaction between endogenous opioid and cannabinoid receptor systems in the caudate putamen and nucleus accumbens. We therefore examined by autoradiography the functional activity and density of micro-, kappa- and delta-opioid receptors in both brain regions of cannabinoid CB1 receptor knockout mice. Functional activity was estimated by measuring agonist-stimulated [35S]GTPgammaS binding. Results showed that deletion of the CB1 cannabinoid receptor markedly increased kappa-opioid (50%) and delta-opioid (42%) receptor activities whereas no differences were found in micro-opioid receptor in the caudate putamen. In contrast, binding autoradiography showed a similar density of micro-, kappa- and delta-opioid receptors between mutant and wild-type mice. No differences were found in densities or activities of micro-, kappa- and delta-opioid receptors between mutant and wild-type mice in the nucleus accumbens. Taken together, our results revealed that deletion of CB1 cannabinoid receptors produced a pronounced increase in the activity of kappa- and delta-opioid receptors in the caudate putamen. This endogenous interaction between opioid and cannabinoid receptors may be relevant to further understand a variety of neuroadaptative processes involving the participation of opioid receptors, such as motor behaviour, emotional responses and drug dependence.

Molecular mechanisms involved in the asymmetric interaction between cannabinoid and opioid systems

The aim of this work was to study the mechanism of cross-modulation between cannabinoid and opioid systems for analgesia during acute and chronic exposure. Acute coadministration of ineffectual subanalgesic doses of the synthetic cannabinoid CP-55,940 (0.2 mg/kg i.p.) and morphine (2.5 mg/kg i.p.) resulted in significant antinociception. In chronic studies, a low dose of CP-55,940 (0.2 mg/kg, i.p.) that per se did not induce analgesia in naive animals produced a significant degree of antinociception in rats made tolerant to morphine, whereas in rats made tolerant to CP-55,940, morphine challenge did not produce any analgesic response. To identify the mechanism of these asymmetric interactions during chronic treatment, we investigated the functional activity of cannabinoid and mu opioid receptors and their effects on the cyclic AMP (cAMP) cascade. Autoradiographic-binding studies indicated a slight but significant reduction in cannabinoid receptor levels in the hippocampus and cerebellum of morphine-tolerant rats, whereas CP-55,940-stimulated [35S]GTPgammaS binding showed a significant decrease in receptor/G protein coupling in the limbic area. In CP-55,940 exposed rats, mu opioid receptor binding was significantly raised in the lateral thalamus and periaqueductal gray (PAG), with an increase in DAMGO-stimulated [35S]GTPgammaS binding in the nucleus accumbens. Finally, we tested the cAMP system's responsiveness to the cannabinoid and opioid in the striatum and dorsal mesencephalon. In vivo chronic morphine did not affect CP-55,940's ability to inhibit forskolin-stimulated cAMP production in vitro and actually induced sensitization in striatal membranes. In contrast, in vivo chronic CP-55,940 desensitized DAMGO's efficacy in inhibiting forskolin-stimulated cAMP production in vitro. The alterations to the cAMP system seem to mirror the behavioral responses, indicating that the two systems may interact at the postreceptor level. This might open up new therapeutic opportunities for relief of chronic pain through cannabinoid-opioid coadministration.

Time course of opioid and cannabinoid gene transcription alterations induced by repeated administration with fluoxetine in the rat brain

This study examined the time course effects (8, 16 and 31 days) of fluoxetine administration (1 mg/kg, p.o./day) on serotonin transporter (5-HTT), opioid, tyrosine hydroxylase (TH) and cannabinoid CB1 receptor gene expressions in selected regions of the rat brain. Treatment with fluoxetine progressively decreased (35-55%) 5-HTT gene expression in dorsal raphe nucleus at 8, 16 and 31 days. The results revealed that fluoxetine administration decreased (30%) proenkephalin gene expression in nucleus accumbens shell (AcbS) and caudate-putamen (CPu) (31 days) but was without effect in nucleus accumbens core AcbC. A pronounced and time related decrease (25-65%) in prodynorphin gene expression was detected in AcbC, AcbS, CPu, hypothalamic supraoptic and paraventricular nuclei at all time points as well as in proopiomelanocortin gene expression (20-30%) in the arcuate nucleus (ARC) of the hypothalamus. On days 16 and 31, tyrosine hydroxylase gene expression in ventral tegmental area and substantia nigra and cannabinoid CB1 receptor gene expression in the CPu decreased (approximately 45-50% from vehicle). In conclusion, fluoxetine by inhibiting the reuptake of serotonin produced pronounced and time related alterations in genes involved in the regulation of emotional behaviour, suggesting that these neuroplastic changes may be involved, at least in part, in the clinical efficacy of this drug in neuropsychiatric disorders.

Repeated administration with Delta9-tetrahydrocannabinol regulates mu-opioid receptor density in the rat brain

Several studies have demonstrated reciprocal, as well as synergistic interactions between cannabinoid and opioid systems. The aim of this study was to explore the time-related effects of repeated administration of Delta9-tetrahydrocannabinol on mu-opioid receptor autoradiography in various brain regions of the rat. To this aim, the effects of Delta9-tetrahydrocannabinol (Delta9-THC, 5 mg/kg/day; i.p.) were examined after 1, 3, 7 and 14 days of repeated administration on regions containing mu-opioid receptors: (i) forebrain [caudate-putamen, nucleus accumbens (core and shell) and piriform cortex]; (ii) amygdala (medial pars and cortical posteromedial pars), hypothalamus (ventromedial and dorsomedial nuclei, zona incerta), hippocampal regions (CA1, CA2, CA3, dentate girus), hindbrain (substantia nigra and ventral tegmental area); and (iii) thalamus, including 12 thalamic nuclei. In most of these regions, repeated cannabinoid administration increases mu-opioid receptor density; however, the onset, degree of magnitude reached and time-related effects produced by administration with Delta9-tetrahydrocannabinol are dependent upon the brain region examined. It appears that the major increase in mu-opioid receptor density occurs 1 and 3 days after Delta9-THC administration. In some regions, this increase is maintained and, for most of the brain areas examined, this effect is no longer significant by 14 days of administration, suggesting tolerance to cannabinoid treatment. Taken together, the results of this study suggest that cannabinoids produce a time-related differential responsiveness in mu-opioid receptor density in several brain areas that may be relevant to an understanding of the alterations associated with cannabinoid exposure.

Modulation of oral morphine antinociceptive tolerance and naloxone-precipitated withdrawal signs by oral Delta 9-tetrahydrocannabinol

Previous studies have demonstrated a functional interaction between cannabinoid and opioid systems in the development and expression of morphine tolerance and dependence. In these experiments, we examined the effect of a low oral dose of Delta 9-tetrahydrocannabinol (Delta 9-THC) on the development of oral morphine tolerance and the expression of naloxone-precipitated morphine withdrawal signs of jumping and diarrhea in ICR mice. Chronic treatment with high-dose oral morphine produced a 3.12-fold antinociceptive tolerance. Tolerance to morphine was prevented in groups receiving a daily cotreatment with a nonanalgetic dose (20 mg/kg p.o.) of Delta 9-THC, except when challenged with a very high dose of morphine. The chronic coadministration of low-dose Delta 9-THC also reduced naloxone-precipitated (1 mg/kg s.c.) platform jumping by 50% but did not reduce diarrhea. In separate experiments, mice treated chronically with high-dose morphine p.o. were not cross-tolerant to Delta 9-THC; in fact, these morphine-tolerant mice were more sensitive to the acute antinociceptive effects of Delta 9-THC. Delta 9-THC (20 mg/kg p.o.) also reduced naloxone-precipitated jumping but not diarrhea when administered acutely to morphine-tolerant mice. These results represent the first evidence that oral morphine tolerance and dependence can be circumvented by coadministration of a nonanalgetic dose of Delta 9-THC p.o. In summary, cotreatment with a combination of morphine and Delta 9-THC may prove clinically beneficial in that long-term morphine efficacy is maintained.

Behavioural and gene transcription alterations induced by spontaneous cannabinoid withdrawal in mice

This study examined behavioural signs that occur during tolerance development to cannabinoid treatment and hormonal and gene expression alterations induced by spontaneous cannabinoid withdrawal in mice. Tolerance to CP-55,940 treatment developed for hypothermia, ambulatory and exploratory locomotor activity. Cessation of cannabinoid treatment resulted in a behavioural withdrawal syndrome characterized by a pronounced increase in ambulatory activity and rearings. Corticosterone plasma concentrations dramatically increased 24 and 72 h after cessation of cannabinoid treatment. Similarly, an increase (40%) in cannabinoid [35S]GTPgammaS binding autoradiography was detected on days 1 and 3 of abstinence. Spontaneous cannabinoid withdrawal produced time-related significant alterations in gene transcription: (i) decreased (20%) tyrosine hydroxylase (TH) mRNA levels in the ventral tegmental area and increased (50%) in substantia nigra; (ii) increased proenkephalin (PENK) gene expression more than 100% in caudate-putamen, nucleus accumbens, olfactory tubercle and piriform cortex; (iii) increased (20-40%) pro-opiomelanocortin (POMC) gene expression in the arcuate nucleus of the hypothalamus. These results suggest that spontaneous cannabinoid withdrawal occur after cessation of CP-55,940 treatment. This 'syndrome' includes behavioural, hormonal and gene transcription alterations that seems to be part of the regulation of neuronal plasticity induced by spontaneous cannabinoid withdrawal.

The effects of delta9-tetrahydrocannabinol physical dependence on brain cannabinoid receptors

The effects of chronic Delta(9)-tetrahydrocannabinol on cannabinoid receptor levels and receptor-G-protein coupling were investigated. Male Sprague-Dawley rats were infused continuously with low or high dose regimens of Delta(9)-tetrahydrocannabinol or vehicle for 4 days. Following treatment, rats were sacrificed for cannabinoid CB(1) receptor binding analysis or challenged with the cannabinoid CB(1) receptor antagonist, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide HCl (SR141716A). The rats receiving Delta(9)-tetrahydrocannabinol exhibited antagonist-precipitated withdrawal signs. Each brain region (cerebellum, cortex, hippocampus and basal ganglia) from high-dose rats showed 30-70% decreases in [3H] (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxyphenyl)cyclohexanol (WIN55212-2) B(max) values, indicating receptor down-regulation. Most regions showed decreased WIN55212-2-stimulated [35S]guanosine-5'-O-3-thiotriphosphate (GTPgammaS) binding, indicating desensitization of cannabinoid CB(1) receptors. Additional receptor binding assays in cerebellar membranes showed a significantly greater decrease in agonist than in antagonist B(max) values, indicating a lower fraction of coupled receptors after treatment. Concentration-effect analysis of five agonists revealed that the treatment resulted in greater decreases in the efficacy of low-efficacy agonists.

Gender differences in proenkephalin gene expression response to delta9-tetrahydrocannabinol in the hypothalamus of the rat

Chronic exposure to delta9-tetrahydrocannabinol (delta9-THC) produces an activation of preproenkephalin (PENK) gene expression in the rat hypothalamus. The levels of circulating gonadal steroids concurrently modulate this neuropeptide in male and female rats. However, whether gonadal steroids regulate delta9-THC effects on PENK gene expression in the hypothalamus of male and female rats remains unknown. To test this hypothesis, experiments were carried out on intact, 2-week-gonadectomized, 1-week-gonadectomized, 1-week-dihydrotestosterone (DHT) replaced male rats, and 2-week-gonadectomized, 1-week-gonadectomized, 1-week-oestradiol replaced female rats. One week after hormonal replacement, animals were treated with vehicle or delta9-THC (5 mg/kg/day, i.p. 7 days). In males, delta9-THC administration to intact animals induced PENK mRNA in the paraventricular nucleus (PVN) and ventromedial nucleus (VMN) of the hypothalamus. Orchidectomy did not affect basal PENK mRNA levels in the PVN, but reduced PENK mRNA levels in the VMN. However, delta9-THC treatment induced PENK gene expression to the same extent in both hypothalamic nuclei of intact, castrated and DHT-replaced males. In females, ovariectomy decreased PENK gene expression in PVN and VMN. delta9-THC administration increased PENK gene expression in castrated females, but had no effect in the oestradiol-replaced group. Taken together, these results suggest gender differences in the response of chronic exposure to cannabinoids on PENK gene expression in the hypothalamus. Furthermore, it appears that alterations in opioid gene expression induced by cannabinoids in female rats depend upon the presence or absence of circulating oestradiol.

Acute or chronic effects of cannabinoids on spontaneous or pharmacologically induced yawning in rats

Yawning is a reflex or event that is not fully understood. It is controlled by many neurotransmitters and neuropeptides and can be induced pharmacologically by cholinergic or dopaminergic agonists. Amongst their many actions, cannabinoids acting on cannabinoid (CB(1) or CB(2)) receptors can alter cholinergic and/or dopaminergic activity. This study examined the effects of Delta(8)-tetrahydrocannabinol (Delta(8)-THC) administered acutely (2.5 mg/kg intraperitoneally [ip], 15 min before test) or chronically (5 mg/kg for 30 days followed by 24 h or 7 days of discontinuation) on yawning induced by pilocarpine, a cholinergic agonist (0, 1, 2, 4 or 8 mg/kg ip), or apomorphine, a dopaminergic agonist (0, 20, 40 or 80 microg/kg subcutaneously [sc]). Acute effects of different doses of Delta(9)-tetrahydrocannabinol (Delta(9)-THC: 0, 0.5, 1.25 or 2.5 mg/kg ip) on yawning induced by pilocarpine (2 mg/kg ip) or apomorphine (40 microg/kg sc) were also investigated. Both pilocarpine and apomorphine produced yawning in a dose-related manner. Acute administration of Delta(8)-THC and Delta(9)-THC significantly reduced yawning induced by both pilocarpine and apomorphine. Chronic administration of Delta(8)-THC did not change yawning induced by either agonist 24 h or 7 days after discontinuation of Delta(8)-THC. However, a high frequency of spontaneous yawning was observed 7 days after Delta(8)-THC discontinuation. These results suggest that cannabinoid agonists inhibited yawning induced by cholinergic or dopaminergic agonists. In addition, the increased frequency of spontaneous yawning following cessation of chronic administration of a cannabinoid agonist may be of importance as a withdrawal sign for these drugs.

Molecular biology of cannabinoid receptors

During the last decade, research on the molecular biology and genetics of cannabinoid receptors has led to a remarkable progress in understanding of the endogenous cannabinoid system, which functions in a plethora of physiological processes in the animal. At present, two types of cannabinoid receptors have been cloned from many vertebrates, and three endogenous ligands (the endocannabinoids arachidonoyl ethanolamide, 2-arachidonoyl glycerol and 2-arachidonoyl-glycerol ether) have been characterized. Cannabinoid receptor type 1 (CB(1)) is expressed predominantly in the central and peripheral nervous system, while cannabinoid receptor type 2 (CB(2)) is present almost exclusively in immune cells. Cannabinoid receptors have not yet been cloned from invertebrates, but binding proteins for endocannabinoids, endocannabinoids and metabolic enzyme activity have been described in a variety of invertebrates except for molting invertebrates such as Caenorhabditis elegans and Drosophila. In the central nervous system of mammals, there is strong evidence emerging that the CB(1) and its ligands comprise a neuromodulatory system functionally interacting with other neurotransmitter systems. Furthermore, the presynaptic localization of CB(1) together with the results obtained from electrophysiological experiments strengthen the notion that in cerebellum and hippocampus and possibly in other regions of the central nervous system, endocannabinoids may act as retrograde messengers to suppress neurotransmitter release at the presynaptic site. Many recent studies using genetically modified mouse lines which lack CB(1) and/or CB(2) finally could show the importance of cannabinoid receptors in animal physiology and will contribute to unravel the full complexity of the cannabinoid system.

Ultrastructural localization of the CB1 cannabinoid receptor in mu-opioid receptor patches of the rat Caudate putamen nucleus

Cannabinoids and opioids are widely consumed drugs of abuse that produce motor depression, in part via respective activation of the cannabinoid subtype 1 receptor (CB1R) and the mu-opioid receptor (muOR), in the striatal circuitry originating in the caudate putamen nucleus (CPN). Thus, the CB1R and muOR may show similar targeting in the CPN. To test this hypothesis, we examined the electron microscopic immunocytochemical labeling of CB1R and muOR in CPN patches of rat brain. Of the CB1R-labeled profiles, 34% (588) were dendrites, presumably arising from spiny as well as aspiny-type somata, which also contained CB1R immunoreactivity. In dendrites, CB1R often was localized to nonsynaptic and synaptic plasma membranes, particularly near asymmetric excitatory-type junctions. Almost one-half of the CB1R-labeled dendrites contained muOR immunoreactivity, whereas only 20% of all muOR-labeled dendrites expressed CB1R. Axons and axon terminals as well as abundant glial processes also showed plasmalemmal CB1R and were mainly without muOR immunoreactivity. Many CB1R-labeled axon terminals were small and without recognizable synaptic junctions, but a few also formed asymmetric, or more rarely symmetric, synapses. The CB1R-labeled glial processes were often perivascular or perisynaptic, surrounding asymmetric excitatory-type axospinous synapses. Our results show that in CPN patches CB1R and muOR are targeted strategically to some of the same postsynaptic neurons, which may account for certain similarities in motor function. Furthermore, they also provide evidence that CB1R may play a major role in the modulation of presynaptic transmitter release and glial functions that are unaffected in large part by opioids active at muOR in CPN.

Differential role of the nitric oxide pathway on delta(9)-THC-induced central nervous system effects in the mouse

This study investigated whether the nitric oxide pathway was involved in the central effects of Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the major psychoactive constituent of cannabis sativa. Body temperature, nociception and locomotion were measured in neuronal nitric oxide synthase (nNOS) knock-out (KO) mice and wild-type (WT) controls after intraperitoneal application of Delta(9)-THC. These Delta(9)-THC-induced effects are known to be mediated through the brain-type cannabinoid receptor 1 (CB1). Therefore, in situ hybridization (ISH) experiments were performed in the adult murine brain to determine possible changes in CB1 mRNA levels in nNOS-KO, compared with WT mice, and to reveal brain areas where CB1 and nNOS were coexpressed in the same neurons. We found that an intraperitoneal injection of 10 mg/kg Delta(9)-THC led to the same increase in the hot plate latencies in both genotypes, suggesting that Delta(9)-THC-mediated antinociception does not involve nNOS. In contrast, a significant Delta(9)-THC-induced decrease of body temperature and locomotor activity was only observed in WT, but not in nNOS-KO mice. ISH revealed significantly lower levels of CB1 mRNA in the ventromedial hypothalamus (VMH) and the caudate putamen (Cpu) of the nNOS-KO animals, compared with WT mice. Both areas are known to be among the regions involved in cannabinoid-induced thermoregulation and decrease of locomotion. A numerical evaluation of nNOS/CB1 coexpression showed that approximately half of the nNOS-positive cells in the dorsolateral Cpu also express low levels of CB1. ISH of adjacent serial sections with CB1 and nNOS, revealed expression of both transcripts in VMH, suggesting that numerous nNOS-positive cells of VMH coexpress CB1. Our findings indicate that the nitric oxide pathway is involved in some, but not all of the central effects of Delta(9)-THC.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Enhancement of anandamide formation in the limbic forebrain and reduction of endocannabinoid contents in the striatum of delta9-tetrahydrocannabinol-tolerant rats

Recent studies have shown that the pharmacological tolerance observed after prolonged exposure to synthetic or plant-derived cannabinoids in adult rats is accompanied by down-regulation/desensitization of brain cannabinoid receptors. However, no evidence exists on possible changes in the contents of the endogenous ligands of cannabinoid receptors in the brain of cannabinoid-tolerant rats. The present study was designed to elucidate this possibility by measuring, by means of isotope dilution gas chromatography/mass spectrometry, the contents of both anandamide (arachidonoylethanolamide; AEA) and its biosynthetic precursor, N-arachidonoylphosphatidylethanolamine (NArPE), and 2-arachidonoylglycerol (2-AG) in several brain regions of adult male rats treated daily with delta9-tetrahydrocannabinol (delta9-THC) for a period of 8 days. The areas analyzed included cerebellum, striatum, limbic forebrain, hippocampus, cerebral cortex, and brainstem. The same regions were also analyzed for cannabinoid receptor binding and WIN-55,212-2-stimulated guanylyl-5'-O-(gamma-[35S]thio)-triphosphate ([35S]GTPgammaS) binding to test the development of the well known down-regulation/desensitization phenomenon. Results were as follows: As expected, cannabinoid receptor binding and WIN-55,212-2-stimulated [35S]GTPgammaS binding decreased in most of the brain areas of delta9-THC-tolerant rats. The only region exhibiting no changes in both parameters was the limbic forebrain. This same region exhibited a marked (almost fourfold) increase in the content of AEA after 8 days of delta9-THC treatment. By contrast, the striatum exhibited a decrease in AEA contents, whereas no changes were found in the brainstem, hippocampus, cerebellum, or cerebral cortex. The increase in AEA contents observed in the limbic forebrain was accompanied by a tendency of NArPE levels to decrease, whereas in the striatum, no significant change in NArPE contents was found. The contents of 2-AG were unchanged in brain regions from delta9-THC-tolerant rats, except for the striatum where they dropped significantly. In summary, the present results show that prolonged activation of cannabinoid receptors leads to decreased endocannabinoid contents and signaling in the striatum and to increased AEA formation in the limbic forebrain. The pathophysiological implications of these findings are discussed in view of the proposed roles of endocannabinoids in the control of motor behavior and emotional states.

Activational role of cannabinoids on movement

Cannabinoid's major effect on movement is hypoactivity. Nevertheless, a biphasic excitatory/inhibitory effect of cannabinoids on movement has been repeatedly acknowledged. However, the literature is lacking a detailed description of such an effect. In this study, we performed a dose-response study of the effects of Delta(9)-tetrahydrocannabinol on movement. Immediately after the administration of vehicle or a dose of Delta(9)-tetrahydrocannabinol (0.2, 0.5, 1, 1.5, 2, 2.5, 3, 4, or 5 mg/kg), the animal was placed in an activity monitor and observed for 1 h. Several parameters were recorded. The horizontal and vertical activities were measured as the number of photobeams broken between the photocells on the walls of an activity monitor. The number of wet dog shakes, scratches with hindpaw, mouth movements, forepaw flutters were also recorded, as was the amount of time in minutes that each subject spent grooming. The number of fecal boluses was recorded as an index of autonomic activity. Each animal was subsequently tested for catalepsy in the bar test. A triphasic effect was observed: low doses of the cannabinoid receptor agonist Delta(9)-tetrahydrocannabinol (0.2 mg/kg) decreased locomotor activity while higher doses (1-2 mg/kg) dose-dependently stimulated movement until catalepsy emerged (2.5 mg/kg) accompanied by decreases in activity.

Chronic delta9-tetrahydrocannabinol treatment produces a time-dependent loss of cannabinoid receptors and cannabinoid receptor-activated G proteins in rat brain

Chronic treatment of rats with delta9-tetrahydrocannabinol (delta9-THC) results in tolerance to its acute behavioral effects. In a previous study, 21-day delta9-THC treatment in rats decreased cannabinoid activation of G proteins in brain, as measured by in vitro autoradiography of guanosine-5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS) binding. The present study investigated the time course of changes in cannabinoid-stimulated [35S]GTPgammaS binding and cannabinoid receptor binding in both brain sections and membranes, following daily delta9-THC treatments for 3, 7, 14, and 21 days. Autoradiographic results showed time-dependent decreases in WIN 55212-2-stimulated [35S]GTPgammaS and [3H]WIN 55212-2 binding in cerebellum, hippocampus, caudate-putamen, and globus pallidus, with regional differences in the rate and magnitude of down-regulation and desensitization. Membrane binding assays in these regions showed qualitatively similar decreases in WIN 55212-2-stimulated [35S]GTPgammaS binding and cannabinoid receptor binding (using [3H]SR141716A), and demonstrated that decreases in ligand binding were due to decreases in maximal binding values, and not ligand affinities. These results demonstrated that chronic exposure to delta9-THC produced time-dependent and region-specific down-regulation and desensitization of brain cannabinoid receptors, which may represent underlying biochemical mechanisms of tolerance to cannabinoids.