Chronic blockade of cannabinoid CB2 receptors induces anxiolytic-like actions associated with alterations in GABA(A) receptors

BACKGROUND AND PURPOSE

The aim of this study was to explore the effects of CB₂ receptor agonist and antagonist in the regulation of anxiety-like behaviours.

EXPERIMENTAL APPROACHES

Effects of acute and chronic treatment with the CB₂ receptor agonist JWH133 and CB₂ receptor antagonist AM630 were evaluated in the light-dark box (LDB) and elevated plus maze (EPM) tests in Swiss ICR mice. CB₂ receptor, GABA_Aα₂ and GABA_Aγ₂ gene and protein expression in the cortex and amygdala of mice chronically treated with JWH133 or AM630 were examined by RT-PCR and Western blot. Effects of chronic AM630 treatment were evaluated in spontaneously anxious DBA/2 mice in LDB.

KEY RESULTS

Acute JWH133 treatment failed to produce any effect. Acute AM630 treatment increased anxiety and was blocked by pre-treatment with JWH133. Chronic JWH133 treatment increased anxiety-like behaviour whereas chronic AM630 treatment was anxiolytic in LDB and EPM tests. Chronic AM630 treatment increased gene and reduced protein expression of CB₂ receptors, GABA_Aα₂ and GABA_Aγ₂ in cortex and amygdala. Chronic JWH133 treatment resulted in opposite gene and protein alterations. In addition, chronic AM630 administration decreased the anxiety of DBA/2 mice in the LDB test.

CONCLUSIONS AND IMPLICATIONS

The opposing behavioural and molecular changes observed after chronic treatment with AM630 or JWH133 support the key role of CB₂ receptors in the regulation of anxiety. Indeed, the efficacy of AM630 in reducing the anxiety of the spontaneously anxious DBA/2 strain of mice strengthens the potential of the CB₂ receptor as a new target in the treatment of anxiety-related disorders.     

Anandamide mediates hyperdynamic circulation in cirrhotic rats via CB(1) and VR(1) receptors

BACKGROUND AND PURPOSE: Hyperdynamic circulation and mesenteric hyperaemia are found in cirrhosis. To delineate the role of endocannabinoids in these changes, we examined the cardiovascular effects of anandamide, AM251 (CB(1) antagonist), AM630 (CB(2) antagonist) and capsazepine (VR1 antagonist), in a rat model of cirrhosis. EXPERIMENTAL APPROACH: Cirrhosis was induced by bile duct ligation. Controls underwent sham operation. Four weeks later, diameters of mesenteric arteriole and venule (intravital microscopy), arterial pressure, cardiac output, systemic vascular resistance and superior mesenteric artery (SMA) flow were measured after anandamide, AM251 (with or without anandamide), AM630 and capsazepine administration. CB(1), CB(2) and VR1 receptor expression in SMA was assessed by western blot and RT-PCR. KEY RESULTS: Anandamide increased mesenteric vessel diameter and flow, and cardiac output in cirrhotic rats, but did not affect controls. Anandamide induced a triphasic arterial pressure response in controls, but this pattern differed markedly in cirrhotic rats. Pre-administration of AM251 blocked the effects of anandamide. AM251 (without anandamide) increased arterial pressure and systemic vascular resistance, constricted mesenteric arterioles, decreased SMA flow and changed cardiac output in a time-dependent fashion in cirrhotic rats. Capsazepine decreased cardiac output and mesenteric arteriolar diameter and flow, and increased systemic vascular resistance in cirrhotic rats, but lacked effect in controls. Expression of CB(1) and VR1 receptor proteins were increased in cirrhotic rats. AM630 did not affect any cardiovascular parameter in either group. CONCLUSIONS AND IMPLICATIONS: These data suggest that endocannabinoids contribute to hyperdynamic circulation and mesenteric hyperaemia in cirrhosis, via CB(1)- and VR1-mediated mechanisms.     

Changes in CB1 and CB2 receptors in the post-mortem cerebellum of humans affected by spinocerebellar ataxias

Background and PurposeSpinocerebellar ataxias (SCAs) are a family of chronic progressive neurodegenerative diseases, clinically and genetically heterogeneous, characterized by loss of balance and motor coordination due to degeneration of the cerebellum and its afferent and efferent connections. Unlike other motor disorders, the possible role of changes in the endocannabinoid system in the pathogenesis of SCAs has not been investigated.Experimental ApproachThe status of cannabinoid receptor type 1 (CB₁) and cannabinoid receptor type 2 (CB₂) receptors in the post-mortem cerebellum of SCA patients and controls was investigated using immunohistochemical procedures.Key ResultsImmunoreactivity for the CB₁ receptor, and also for the CB₂ receptor, was found in the granular layer, Purkinje cells, neurons of the dentate nucleus and areas of white matter in the cerebellum of SCA patients at levels notably higher than controls. Double-labelling procedures demonstrated co-localization of CB₁ and, in particular, CB₂ receptors with calbindin, supporting the presence of these receptors in Purkinje neurons. Both receptors also co-localized with Iba-1 and glial fibrillary acidic protein in the granular layer and white matter areas, indicating that they are present in microglia and astrocytes respectively.Conclusions and ImplicationsOur results demonstrate that CB₁ and CB₂ receptor levels are significantly altered in the cerebellum of SCA patients. Their identification in Purkinje neurons, which are the main cells affected in SCAs, as well as the changes they experienced, suggest that alterations in endocannabinoid receptors may be related to the pathogenesis of SCAs. Therefore, the endocannabinoid system could provide potential therapeutic targets for the treatment of SCAs and its progression.Linked ArticlesThis article is part of a themed section on Cannabinoids 2013. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-6     

Cannabinoid CB1 receptors mediate the effects of corticotropin-releasing factor on the reinstatement of cocaine seeking and expression of cocaine-induced behavioural sensitization

BACKGROUND AND PURPOSE

The endocannabinoid and corticotropin-releasing factor (CRF) systems have been implicated in several long-lasting behavioural effects of prior cocaine experience. The present experiments were designed to probe functional interactions between endocannabinoids and CRF by testing the role of cannabinoid CB₁ receptors in cocaine-related behaviours induced or mediated by CRF.

EXPERIMENTAL APPROACH

In Experiment 1, rats trained to self-administer cocaine were pretreated with the CB₁ receptor antagonist, AM251 (0, 10, 100 or 200 µg, i.c.v.), before tests for reinstatement in response to CRF (0, 0.5 µg, i.c.v.), intermittent footshock stress (0, 0.9 mA) or cocaine (0, 10 mg·kg⁻¹, i.p.). In Experiment 2, rats pre-exposed to cocaine (15–30 mg·kg⁻¹, i.p.) or saline for 7 days were pretreated with AM251 (0, 10 or 100 µg, i.c.v.) before tests for locomotion in response to CRF (0.5 µg, i.c.v.), cocaine (15 mg·kg⁻¹, i.p.) or saline (i.c.v.).

KEY RESULTS

Pretreatment with AM251 selectively interfered with CRF-, but not footshock- or cocaine-induced reinstatement. AM251 blocked the expression of behavioural sensitization induced by challenge injections of both CRF and cocaine.

CONCLUSIONS AND IMPLICATIONS

These findings reveal a mediating role for CB₁ receptor transmission in the effects of CRF on cocaine-related behaviours.     

CB1 receptor blockade reduces the anxiogenic-like response and ameliorates the neurochemical imbalances associated with alcohol withdrawal in rats

There is strong evidence that blocking CB1 receptors may reduce alcohol intake in alcohol-dependent individuals. However, there is still limited evidence that CB1 receptor antagonists may also be beneficial in the attenuation of alcohol withdrawal syndrome, even though alcohol withdrawal appears to be milder in CB1 receptor knockout mice. Here we have examined whether the CB1 receptor antagonist rimonabant (SR141716) can alleviate the behavioral symptoms and revert the neurochemical imbalance elicited by a 3-h interruption of chronic alcohol exposure (7.2% in the drinking water for 10 days) in male Wistar rats. Administration of rimonabant attenuated the strong anxiogenic traits of the animals that developed when regular alcohol intake was interrupted. This may reflect the correction of the GABA/glutamate imbalances developed by the animals that received rimonabant in various brain regions involved in emotional (e.g. prefrontal cortex) and motor (e.g. caudate-putamen and globus pallidus) responses. In addition, rimonabant also affected the dopamine deficits generated by alcohol abstinence in the amygdala and ventral-tegmental area, albeit to a lesser extent. However, this antagonist was unable to correct the impairment caused by alcohol abstinence in serotonin and neuropeptide Y. The endocannabinoid activity in the brain of alcohol-abstinent rats indicated that the behavioral and neurochemical improvements caused by rimonabant were not related to the attenuation of a possible increase in this activity generated by alcohol withdrawal. Conversely, the density of CB1 receptors was reduced in alcohol-abstinent animals (e.g. globus pallidus, substantia nigra), as were the levels of endocannabinoids and related N-acylethanolamines (e.g. amygdala, caudate-putamen). Thus, rimonabant possibly enhances an endogenous response generated by interrupting the regular use of alcohol. In summary, rimonabant might attenuate withdrawal symptoms associated with alcohol abstinence, an effect that was presumably due to the normalization of GABA and glutamate, and to a lesser extent, dopamine transmission in emotion- and motor-related areas.     

Cannabinoid CB2 receptors in human brain inflammation

The presence of functional cannabinoid CB2 receptors in the CNS has provoked considerable controversy over the past few years. Formerly considered as an exclusively peripheral receptor, it is now accepted that it is also present in limited amounts and distinct locations in the brain of several animal species, including humans. Furthermore, the inducible nature of these receptors under neuroinflammatory conditions, in contrast to CB1, makes them attractive targets for the development of novel therapeutic approaches. In fact, the undesired psychoactive effects caused by CB1 activation have largely limited the clinical use of cannabinoid-related compounds that act on these receptors. In this review some recent findings on the antiinflammatory properties of CB2 receptors are presented, as well as new perspectives that have been obtained based on studies of human postmortem brain samples. In addition, various working hypotheses are also proposed and discussed.     

Inverse agonism of cannabinoid CB1 receptors potentiates LiCl-induced nausea in the conditioned gaping model in rats

BACKGROUND AND PURPOSE

Cannabinoid CB₁ receptor antagonists/inverse agonists, potentiate toxin-induced nausea and vomiting in animal models. Here, we sought to determine if this potentiated nausea was mediated by inverse agonism or neutral antagonism of the CB₁ receptor, and if the potentiated nausea would be produced by intracerebroventricular (icv) administration of an inverse agonist.

EXPERIMENTAL APPROACH

The conditioned gaping model of nausea in rats was used to compare the CB₁ receptor antagonist/inverse agonist, AM251, and the CB₁ receptor neutral antagonists, AM6527 (centrally and peripherally active) and AM6545 (peripherally active), in potentiating conditioned gaping produced by lithium chloride (LiCl) solution. The effect of icv (lateral ventricle and 4th ventricle) administration of AM251 on LiCl-induced gaping in this model was also evaluated.

KEY RESULTS

At a dose that did not produce conditioned gaping on its own, systemically administered AM251 (1.25 mg·kg⁻¹) potentiated LiCl-induced conditioned gaping and reduced sucrose palatability; however, even doses as high as 8 mg·kg⁻¹ of AM6545 and AM6527 neither potentiated LiCl-induced conditioned gaping nor reduced sucrose palatability. Infusions of AM251 into the lateral ventricles (1.25, 12.5 and 125 µg) or the 4th ventricle (2.5, 12.5 and 125 µg) did not potentiate LiCl-induced conditioned gaping reactions, but all doses attenuated saccharin palatability during the subsequent test.

CONCLUSIONS AND IMPLICATIONS

Inverse agonism, but not neutral antagonism, of CB₁ receptors potentiated toxin-induced nausea. This effect may be peripherally mediated or may be mediated centrally by action on CB₁ receptors, located distal to the cerebral ventricles.     

High hopes for CB2 receptors in neurogenesis

During life, new neurons are continually added to hippocampal circuitry, with evidence suggesting that these adult-born neurons are functionally linked to cognition and emotion. The mammalian brain contains actively dividing neural stem cells in discrete regions, including the subventricular zone of the lateral ventricles and the subgranular zone of the dentate gyrus. Once mature, these neurons integrate into neuronal networks, forming synaptic connections with interneurons, mossy cells and CA3 pyramidal cellsLinked ArticleThis article is a commentary on Avraham et al., pp. 468–479 of volume 171 issue 2. To view this paper visit http://dx.doi.org/10.1111/bph.12478     

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 CB₁ receptor-mediated extracellular signal-regulated kinase (ERK) activation in neurons are poorly understood. We investigated the cellular mechanisms involved in CB₁ 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 CB₁ 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 CB₁ 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 CB₁ receptor-stimulated ligand-independent transactivation of multiple receptor tyrosine kinases (RTKs). Phase I requires G_i/oβγ 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 CB₁ receptor-stimulated, ligand-independent, transactivation of multiple RTKs.

CONCLUSIONS AND IMPLICATIONS

The complex expression of CB₁ 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 CB₁-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     

Modulation of gastric emptying and gastrointestinal transit in rats through intestinal cannabinoid CB(1) receptors

We studied the delay in gastric emptying and gastrointestinal transit induced by the cannabinoid receptor agonists (+)-WIN 55,212-2 (R(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate) and CP 55,940 ((-)-cis-3[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl) cyclohexanol), as prevented by the selective cannabinoid CB(1)-receptor antagonist SR141716 ((N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide)) in rats after systemic or central drug administration. Oral SR141716 showed comparable potency (ID(50) range 1.0-3.9 mg/kg) in antagonizing gastric emptying and gastrointestinal transit delay by (+)-WIN 55,212-2 or CP 55,940. Gastric emptying and gastrointestinal transit delay after intracerebroventricular (i.c.v.) (+)-WIN 55,212-2 was prevented by oral or i.c.v. SR141716, but i.c.v. SR141716 did not significantly reduce the effect of i.p. (+)-WIN 55,212-2. Pertussis toxin prevented the delaying action of i.c.v. (+)-WIN 55,212-2 on both gastric emptying and gastrointestinal transit, but had no effect on (+)-WIN 55,212-2 i.p. These findings are consistent with a primary role of peripheral cannabinoid CB(1) receptor mechanisms in gastrointestinal transit delay by specific agonists.     

Inhibition of fatty acid amide hydrolase unmasks CB1 receptor and TRPV1 channel-mediated modulation of glutamatergic synaptic transmission in midbrain periaqueductal grey

BACKGROUND AND PURPOSE

While arachidonyl ethanolamine (anandamide) produces pharmacological effects mediated by cannabinoid CB1 receptors, it is also an agonist at the transient receptor potential vanilloid type 1 (TRPV1) ion channel. This study examined the cellular actions of anandamide in the midbrain periaqueductal grey (PAG), a region implicated in the analgesic actions of cannabinoids, and which expresses both CB1 receptors and TRPV1.

EXPERIMENTAL APPROACH

In vitro whole cell patch clamp recordings of glutamatergic excitatory postsynaptic currents (EPSCs) were made from rat and mouse PAG slices.

KEY RESULTS

Capsaicin (1 µM) increased the rate, but not the amplitude of miniature EPSCs in subpopulations of neurons throughout the rat and mouse PAG. Capsaicin had no effect on miniature EPSCs in PAG neurons from TRPV1 knock-out mice. In mouse PAG neurons, anandamide (30 µM) had no effect on the rate of miniature EPSCs alone, or in the presence of either the CB1 antagonist AM251 (3 µM) or the TRPV1 antagonist iodoresiniferatoxin (300 nM). Anandamide produced a decrease in miniature EPSC rate in the presence of the fatty acid amide hydrolase (FAAH) inhibitor URB597 (1 µM). By contrast, anandamide produced an increase in miniature EPSC rate in the presence of both URB597 and AM251, which was absent in TRPV1 knock-out mice.

CONCLUSIONS AND IMPLICATIONS

These results suggest that the actions of anandamide within PAG are limited by enzymatic degradation by FAAH. FAAH blockade unmasks both presynaptic inhibition and excitation of glutamatergic synaptic transmission which are mediated via CB1 receptors and TRPV1 respectively.     

Novel selective cannabinoid CB(1) receptor antagonist MJ08 with potent in vivo bioactivity and inverse agonistic effects

Aim:

To characterize the biological profiles of MJ08, a novel selective CB₁ receptor antagonist.

Methods:

Radioligand binding assays were performed using rat brain and spleen membrane preparations. CB₁ and CB₂ receptor redistribution and intracellular Ca²⁺ ([Ca²⁺]_i) assays were performed with IN CELL Analyzer. Inverse agonism was studied using intracellular cAMP assays, and in guinea-pig ileum and mouse vas deferens smooth muscle preparations. In vivo pharmacologic profile was assessed in diet-induced obesity (DIO) mice.

Results:

In radioligand binding assay, MJ08 selectively antagonized CB₁ receptor (IC₅₀=99.9 nmol/L). In EGFP-CB₁_U2OS cells, its IC₅₀ value against CB₁ receptor activation was 30.23 nmol/L (SR141716A: 32.16 nmol/L). WIN 55,212-2 (1 μmol/L) increased [Ca²⁺]_i in the primary cultured hippocampal neuronal cells and decreased cAMP accumulation in CHO-hCB₁ cells. MJ08 (10 nmol/L–10 μmol/L) blocked both the WIN 55,212-2-induced effects. Furthermore, MJ08 reversed the inhibition of electrically evoked twitches of mouse vas deferens by WIN 55,212-2 (pA₂=10.29±1.05). MJ08 and SR141716A both showed an inverse agonism activity by markedly promoting the contraction force and frequency of guinea pig ileum muscle. MJ08 significantly increased the cAMP level in CHO-hCB₁ cells with an EC₅₀ value of 78.6 nmol/L, which was lower than the EC₅₀ value for SR141716A (159.2 nmol/L). Besides the more potent pharmacological effects of cannabinoid CB₁ receptor antagonism in DIO mice, such as reducing food intake, decreasing body weight, and ameliorating dyslipidemia, MJ08 (10 mg/kg) unexpectedly raised the fasted blood glucose in vivo.

Conclusion:

MJ08 is a novel, potent and selective CB₁ receptor antagonist/inverse agonist with potent bioactive responses in vitro and in vivo that may be useful for disclosure the versatile nature of CB₁ receptors.     

Cannabinoid CB1 receptor antagonist rimonabant disrupts nicotine reward-associated memory in rats

Exposure to cues previously associated with drug intake leads to relapse by activating previously acquired memories. Based on previous findings, in which cannabinoid CB₁ receptors were found to be critically involved in specific aspects of learning and memory, we investigated the role of CB₁ receptors in nicotine reward memory using a rat conditioned place preference (CPP) model. In Experiment 1, rats were trained for CPP with alternating injections of nicotine (0.5 mg/kg, s.c.) and saline to acquire the nicotine-conditioned memory. To examine the effects of rimonabant on the reconsolidation of nicotine reward memory, rats were administered rimonabant (0, 0.3, and 3.0 mg/kg, i.p.) immediately after reexposure to the drug-paired context. In Experiment 2, rats were trained for CPP similarly to Experiment 1. To examine the effects of rimonabant on the reinstatement of nicotine reward memory, rimonabant (0, 0.3, and 3.0 mg/kg, i.p.) was administered before the test of nicotine-induced CPP reinstatement. In Experiment 3, to evaluate whether rimonabant itself produces a reward memory, rats were trained for CPP with alternating injections of different doses of rimonabant (0, 0.3, and 3.0 mg/kg) and saline. Rimonabant at a dose of 3.0 mg/kg significantly disrupted the reconsolidation of nicotine memory and significantly blocked the reinstatement of nicotine-induced CPP. However, rimonabant itself did not produce CPP. These findings provide clear evidence that CB₁ receptors play a role in nicotine reward memory, suggesting that CB₁ receptor antagonists may be a potential target for managing nicotine addiction.     

Antipsychotic-induced alterations in CB1 receptor-mediated G-protein signaling and in vivo pharmacology in rats

Dysregulation of the endocannabinoid and dopamine systems has been implicated in schizophrenia. The purpose of this study was to examine the effects of sub-chronic treatment with two antipsychotics on CB₁ receptor-mediated in vitro and in vivo effects. Adult and adolescent male and female rats were injected twice daily with haloperidol (0.3 mg/kg), clozapine (10 mg/kg), or saline for 10 days. Subsequently, CB₁ receptor number and function were assessed by [³H]SR141716 and WIN55,212-2-stimulated [³⁵S]GTPγS binding, respectively. The effects of sub-chronic antipsychotic treatment on the in vivo actions of Δ⁹-tetrahydrocannabinol (Δ⁹-THC) were also evaluated. In adult female rats, antipsychotic treatment attenuated maximal stimulation of CB₁ receptor-mediated G-protein activity in the striatum (clozapine) and prefrontal cortex (both antipsychotics), but not in the ventral midbrain. Associated changes in CB₁ receptor number were not observed, suggesting that this attenuation was not due to downregulation. In vivo, sub-chronic treatment with clozapine, but not haloperidol, attenuated Δ⁹-THC-induced suppression of activity in adult females, whereas neither drug altered hypothermia or catalepsy. In contrast, antipsychotic treatment did not change CB₁ receptor-mediated G-protein activation in any brain region in adult male rats and in adolescents of either sex. In vivo, haloperidol, but not clozapine, enhanced Δ⁹-THC-mediated suppression of activity and hypothermia in adult male rats whereas neither antipsychotic affected Δ⁹-THC-induced in vivo effects in adolescent rats. These findings suggest that modulation of the endocannabinoid system might contribute in a sex- and age-selective manner to differences in motor side effects of clozapine versus haloperidol.     

Evidence for inverse agonism of SR141716A at human recombinant cannabinoid CB1 and CB2 receptors

The cannabinoid receptor antagonist SR141716A has been suggested to be an inverse agonist at CB₁ receptors in some isolated intact tissues. We found that the basal incorporation of [³⁵S]-GTPγS in Chinese hamster ovary cells expressing human recombinant CB₁ and CB₂ receptors was inhibited by SR141716A (mean pEC₅₀s 8.26 and 6.00, respectively), whereas cannabinol (10 μM) had no significant effect at hCB₁ receptors but inhibited the binding at hCB₂ receptors. As cannabinol had no effect on basal [³⁵S]-GTPγS binding at hCB₁ at a concentration 100 fold higher than its binding affinity (K_i=0.1 μM), we conclude that endogenous cannabinoid receptor agonists are not a confounding factor and suggest the actions of SR141716A at the hCB₁ receptor, and the actions of SR141716A and cannabinol at the hCB₂ receptor, are due to inverse agonism.     

The diverse CB1 and CB2 receptor pharmacology of three plant cannabinoids: delta9-tetrahydrocannabinol, cannabidiol and delta9-tetrahydrocannabivarin

Cannabis sativa is the source of a unique set of compounds known collectively as plant cannabinoids or phytocannabinoids. This review focuses on the manner with which three of these compounds, (-)-trans-delta9-tetrahydrocannabinol (delta9-THC), (-)-cannabidiol (CBD) and (-)-trans-delta9-tetrahydrocannabivarin (delta9-THCV), interact with cannabinoid CB1 and CB2 receptors. Delta9-THC, the main psychotropic constituent of cannabis, is a CB1 and CB2 receptor partial agonist and in line with classical pharmacology, the responses it elicits appear to be strongly influenced both by the expression level and signalling efficiency of cannabinoid receptors and by ongoing endogenous cannabinoid release. CBD displays unexpectedly high potency as an antagonist of CB1/CB2 receptor agonists in CB1- and CB2-expressing cells or tissues, the manner with which it interacts with CB2 receptors providing a possible explanation for its ability to inhibit evoked immune cell migration. Delta9-THCV behaves as a potent CB2 receptor partial agonist in vitro. In contrast, it antagonizes cannabinoid receptor agonists in CB1-expressing tissues. This it does with relatively high potency and in a manner that is both tissue and ligand dependent. Delta9-THCV also interacts with CB1 receptors when administered in vivo, behaving either as a CB1 antagonist or, at higher doses, as a CB1 receptor agonist. Brief mention is also made in this review, first of the production by delta9-THC of pharmacodynamic tolerance, second of current knowledge about the extent to which delta9-THC, CBD and delta9-THCV interact with pharmacological targets other than CB1 or CB2 receptors, and third of actual and potential therapeutic applications for each of these cannabinoids.     

Effects of dopamine D1 and D2 receptor blockade on MK-801-induced hyperlocomotion in rats

Blocking glutamatergic transmission at the N-methyl-d-aspartate (NMDA) receptor complex with MK-801 (0.15–0.5 mg/kg, IP) was found to induce a robust, dose-dependent increase in locomotor activity. This behavioural activation was similar in intensity to that observed afterd-amphetamine (1 mg/kg, SC). The locomotor stimulation induced by MK-801 at 0.3 mg/kg was significantly inhibited by the D₂ dopamine receptor antagonist raclopride (0.1–0.3 mg/kg, SC) and by the D₁ receptor antagonist SCH 23390 (0.04 mg/kg, SC). The locomotor activity induced by a higher dose of MK-801 (0.5 mg/kg) was reduced by higher doses of raclopride or SCH 23390 administered alone (0.3 and 0.08 mg/kg, respectively), and was inhibited by simultaneous administration of ineffective doses. Raclopride significantly reducedd-amphetamine-induced locomotor activity at a dose (0.2 mg/kg) that also blocked the effects of a low dose of MK-801. In contrast, SCH 23390 blocked the effects ofd-amphetamine at a dose (i.e. 0.01 mg/kg) lower than that needed to block MK-801. These results suggest that the dopaminergic system may in part mediate the locomotor effects induced by the NMDA antagonist, MK-801, in rats. However, the locomotor activity induced by MK-801 appears to be less sensitive to dopaminergic receptor blockade than that induced byd-amphetamine, suggesting that the underlying mechanisms, although similar, are not identical.     

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.     

Cannabinoid CB(1) receptor antagonist rimonabant attenuates reinstatement of ketamine conditioned place preference in rats

Recent evidence suggests that cannabinoid CB(1) receptors may represent effective targets for therapeutic agents used to treat cocaine and heroin relapse. However, the role of cannabinoid CB(1) receptors in the potential treatment for other drugs of abuse is still largely unknown. The present study was conducted to determine whether cannabinoid CB(1) receptors play a similar role in relapse to ketamine abuse. To establish a ketamine reinstatement model in the conditioned place preference paradigm, rats were trained to develop place preference conditioned by ketamine, which was subsequently extinguished through daily exposure to the test chambers in the absence of ketamine. On the day following the last extinction session, four groups of rats were injected with ketamine (1, 5, 10 and 15 mg/kg, i.p.) to reinstate previously extinguished conditioned place preference. To investigate the effects of rimonabant, a cannabinoid CB(1) receptor antagonist, on reinstatement of ketamine-induced place preference, different doses of rimonabant (0.1, 0.5 and 3 mg/kg, i.p) were injected 30 min prior to the ketamine (5 and 15 mg/kg, i.p.) priming injection in a separate group of rats. To determine whether rimonabant itself produces conditioned place preference or conditioned place aversion, rats were trained for conditioned place preference or place aversion with rimonabant (0, 0.1, 0.5, 3.0 mg/kg, i.p.). While ketamine priming injections reinstated extinguished place preference, rimonabant administration significantly attenuated the reinstatement of ketamine-induced place preference in a dose-dependent manner. Importantly, rimonabant itself did not produce conditioned place preference or place aversion. Since the reinstatement effects of ketamine administration were inhibited by rimonabant, these findings suggest that a cannabinoid CB(1) receptor antagonist may be useful in preventing relapse to ketamine abuse.     

Subchronic nicotine exposure in adolescence induces long-term effects on hippocampal and striatal cannabinoid-CB1 and mu-opioid receptors in rats

There is evidence for the existence of functional interactions between nicotine and cannabinoids and opioid compounds in adult experimental animals. However, there is scarce information about these relationships in young animals. In the present study we evaluated short and long-term effects of a subchronic nicotine treatment [0.4 mg/kg daily i.p. injections from postnatal day (PND) 34 to PND 43], upon hippocampal and striatal cannabinoid-CB(1) and mu-opioid receptors in Wistar rats of both genders. Rats were sacrificed 2 h after the last nicotine injection (short-term effects, PND 43) or one month later (long-term effects, PND 75). Hippocampal and striatal cannabinoid CB(1) and mu-opioid receptors were quantified by Western blotting. The subchronic nicotine treatment induced a region-dependent long-lasting effect in cannabinoid CB(1) receptor: a significant increase in hippocampal cannabinoid CB(1) receptors and a significant decrease in striatal cannabinoid CB(1) receptors, with these effects being similar in males and females. With respect to mu-opioid receptors, subchronic nicotine induced a significant down-regulation in hippocampal and striatal mu-opioid receptors in the long-term, and within the striatum the effects were more marked in adult males than in females. The present results indicate that juvenile nicotine taking may have implications for the endocannabinoid and endogenous opioid function and for the behaviors served by those systems, this includes possible modification of the response of adults to different psychotropic drugs, i.e. cannabis and morphine/heroin when taken later in life.