Rat brain cannabinoid receptors are N-linked glycosylated proteins

All-Atom Molecular Dynamics Simulations Indicated the Involvement of a Conserved Polar Signaling Channel in the Activation Mechanism of the Type I Cannabinoid Receptor

The type I cannabinoid G protein-coupled receptor (CB1, GPCR) is an intensely investigated pharmacological target, owing to its involvement in numerous physiological functions as well as pathological processes such as cancers, neurodegenerative diseases, metabolic disorders and neuropathic pain. In order to develop modern medications that exert their effects through binding to the CB1 receptor, it is essential to understand the structural mechanism of activation of this protein. The pool of atomic resolution experimental structures of GPCRs has been expanding rapidly in the past decade, providing invaluable information about the function of these receptors. According to the current state of the art, the activity of GPCRs involves structurally distinct, dynamically interconverting functional states and the activation is controlled by a cascade of interconnecting conformational switches in the transmembrane domain. A current challenge is to uncover how different functional states are activated and what specific ligand properties are responsible for the selectivity towards those different functional states. Our recent studies of the μ-opioid and β₂-adrenergic receptors (MOP and β₂AR, respectively) revealed that the orthosteric binding pockets and the intracellular surfaces of these receptors are connected through a channel of highly conserved polar amino acids whose dynamic motions are in high correlation in the agonist- and G protein-bound active states. This and independent literature data led us to hypothesize that, in addition to consecutive conformational transitions, a shift of macroscopic polarization takes place in the transmembrane domain, which is furnished by the rearrangement of polar species through their concerted movements. Here, we examined the CB1 receptor signaling complexes utilizing microsecond scale, all-atom molecular dynamics (MD) simulations in order to see if our previous assumptions could be applied to the CB1 receptor too. Apart from the identification of the previously proposed general features of the activation mechanism, several specific properties of the CB1 have been indicated that could possibly be associated with the signaling profile of this receptor.

Subsynaptic Distribution, Lipid Raft Targeting and G Protein-Dependent Signalling of the Type 1 Cannabinoid Receptor in Synaptosomes from the Mouse Hippocampus and Frontal Cortex

Numerous studies have investigated the roles of the type 1 cannabinoid receptor (CB1) in glutamatergic and GABAergic neurons. Here, we used the cell-type-specific CB1 rescue model in mice to gain insight into the organizational principles of plasma membrane targeting and Gαi/o protein signalling of the CB1 receptor at excitatory and inhibitory terminals of the frontal cortex and hippocampus. By applying biochemical fractionation techniques and Western blot analyses to synaptosomal membranes, we explored the subsynaptic distribution (pre-, post-, and extra-synaptic) and CB1 receptor compartmentalization into lipid and non-lipid raft plasma membrane microdomains and the signalling properties. These data infer that the plasma membrane partitioning of the CB1 receptor and its functional coupling to Gαi/o proteins are not biased towards the cell type of CB1 receptor rescue. The extent of the canonical Gαi/o protein-dependent CB1 receptor signalling correlated with the abundance of CB1 receptor in the respective cell type (glutamatergic versus GABAergic neurons) both in frontal cortical and hippocampal synaptosomes. In summary, our results provide an updated view of the functional coupling of the CB1 receptor to Gαi/o proteins at excitatory and inhibitory terminals and substantiate the utility of the CB1 rescue model in studying endocannabinoid physiology at the subcellular level.

Fit-for-purpose based testing and validation of antibodies to amino- and carboxy-terminal domains of cannabinoid receptor 1

Specific and selective anti-CB₁ antibodies are among the most powerful research tools to unravel the complex biological processes mediated by the CB₁ receptor in both physiological and pathological conditions. However, low performance of antibodies remains a major source of inconsistency between results from different laboratories. Using a variety of techniques, including some of the most commonly accepted ones for antibody specificity testing, we identified three of five commercial antibodies against different regions of CB₁ receptor as the best choice for specific end-use purposes. Specifically, an antibody against a long fragment of the extracellular amino tail of CB₁ receptor (but not one against a short sequence of the extreme amino-terminus) detected strong surface staining when applied to live cells, whereas two different antibodies against an identical fragment of the extreme carboxy-terminus of CB₁ receptor (but not one against an upstream peptide) showed acceptable performance on all platforms, although they behaved differently in immunohistochemical assays depending on the tissue fixation procedure used and showed different specificity in Western blot assays, which made each of them particularly suitable for one of those techniques. Our results provide a framework to interpret past and future results derived from the use of different anti-CB₁ antibodies in the context of current knowledge about the CB₁ receptor at the molecular level, and highlight the need for an adequate validation for specific purposes, not only before antibodies are placed on the market, but also before the decision to discontinue them is made.

Cannabinoid receptor CB1 and CB2 interacting proteins: Techniques, progress and perspectives

Cannabinoid receptors 1 and 2 (CB₁ and CB₂) are implicated in a range of physiological processes and have gained attention as promising therapeutic targets for a number of diseases. Protein-protein interactions play an integral role in modulating G protein-coupled receptor (GPCR) expression, subcellular distribution and signaling, and the identification and characterization of these will not only improve our understanding of GPCR function and biology, but may provide a novel avenue for therapeutic intervention. A variety of techniques are currently being used to investigate GPCR protein-protein interactions, including Förster/fluorescence and bioluminescence resonance energy transfer (FRET and BRET), proximity ligation assay (PLA), and bimolecular fluorescence complementation (BiFC). However, the reliable application of these methodologies is dependent on the use of appropriate controls and the consideration of the physiological context. Though not as extensively characterized as some other GPCRs, the investigation of CB₁ and CB₂ interacting proteins is a growing area of interest, and a range of interacting partners have been identified to date. This review summarizes the current state of the literature regarding the cannabinoid receptor interactome, provides commentary on the methodologies and techniques utilized, and discusses future perspectives.

Neuromolecular Mechanisms of Cannabis Action

Most of our current understanding of the neuromolecular mechanisms of Cannabis action focusses on two plant cannabinoids, THC and CBD. THC acts primarily through presynaptic CB cannabinoid receptors to regulate neurotransmitter release in the brain, spinal cord and peripheral nerves. CBD action, on the other hand, is probably mediated through multiple molecular targets.

Regulation of cannabinoid CB1 and CB2 receptors, neuroprotective mTOR and pro-apoptotic JNK1/2 kinases in postmortem prefrontal cortex of subjects with major depressive disorder

BACKGROUND

Dysregulations of endocannabinoids and/or cannabinoid (CB) receptors have been implicated in the pathophysiology and treatment of major depressive disorder (MDD).

METHODS

CB₁ and CB₂ receptors, neuroprotective mTOR (mechanistic target of rapamycin) and pro-apoptotic JNK1/2 (c-Jun-N-terminal kinases) were quantified by immunoblotting in postmortem prefrontal cortex of MDD and controls, and further compared in antidepressant (AD)-free and AD-treated subjects. Neuroplastic proteins (PSD-95, Arc, spinophilin) were quantified in MDD brains.

RESULTS

Total cortical CB₁ glycosylated (≈54/64 kDa) receptor was increased in MDD (+20%, n=23, p=0.02) when compared with controls (100%, n=19). This CB₁ receptor upregulation was quantified in AD-treated (+23%, n=14, p=0.02) but not in AD-free (+14%, n=9, p=0.34) MDD subjects. In the same MDD cortical samples, CB₂ glycosylated (≈45 kDa) receptor was unaltered (all MDD: +11%, n=23, p=0.10; AD-free: +12%, n=9, p=0.31; AD-treated: +10%, n=14, p=0.23). In MDD, mTOR activity (p-Ser2448 TOR/t-TOR) was increased (all MDD: +29%, n=18, p=0.002; AD-free: +33%, n=8, p=0.03; AD-treated: +25%, n=10, p=0.04). In contrast, JNK1/2 activity (p-Thr183/Tyr185/t-JNK) was unaltered in MDD subjects. Cortical PSD-95, Arc, and spinophilin contents were unchanged in MDD.

LIMITATIONS

A relative limited sample size. Some MDD subjects had been treated with a variety of ADs. The results must be understood in the context of suicide victims with MDD.

CONCLUSIONS

The upregulation of CB₁ receptor density, but not that of CB₂ receptor, as well as the increased mTOR activity in PFC/BA9 of subjects with MDD (AD-free/treated) support their contributions in the complex pathophysiology of MDD and in the molecular mechanisms of antidepressant drugs.

Cannabinoid Receptors: An Update on Cell Signaling, Pathophysiological Roles and Therapeutic Opportunities in Neurological, Cardiovascular, and Inflammatory Diseases

The identification of the human cannabinoid receptors and their roles in health and disease, has been one of the most significant biochemical and pharmacological advancements to have occurred in the past few decades. In spite of the major strides made in furthering endocannabinoid research, therapeutic exploitation of the endocannabinoid system has often been a challenging task. An impaired endocannabinoid tone often manifests as changes in expression and/or functions of type 1 and/or type 2 cannabinoid receptors. It becomes important to understand how alterations in cannabinoid receptor cellular signaling can lead to disruptions in major physiological and biological functions, as they are often associated with the pathogenesis of several neurological, cardiovascular, metabolic, and inflammatory diseases. This review focusses mostly on the pathophysiological roles of type 1 and type 2 cannabinoid receptors, and it attempts to integrate both cellular and physiological functions of the cannabinoid receptors. Apart from an updated review of pre-clinical and clinical studies, the adequacy/inadequacy of cannabinoid-based therapeutics in various pathological conditions is also highlighted. Finally, alternative strategies to modulate endocannabinoid tone, and future directions are also emphasized.

Development and Validation of CRISPR Activator Systems for Overexpression of CB1 Receptors in Neurons

Gene therapy approaches using viral vectors for the overexpression of target genes have been for several years the focus of gene therapy research against neurological disorders. These approaches deliver robust expression of therapeutic genes, but are typically limited to the delivery of single genes and often do not manipulate the expression of the endogenous locus. In the last years, the advent of CRISPR-Cas9 technologies have revolutionized many areas of scientific research by providing novel tools that allow simple and efficient manipulation of endogenous genes. One of the applications of CRISPR-Cas9, termed CRISPRa, based on the use of a nuclease-null Cas9 protein (dCas9) fused to transcriptional activators, enables quick and efficient increase in target endogenous gene expression. CRISPRa approaches are varied, and different alternatives exist with regards to the type of Cas9 protein and transcriptional activator used. Several of these approaches have been successfully used in neurons in vitro and in vivo, but have not been so far extensively applied for the overexpression of genes involved in synaptic transmission. Here we describe the development and application of two different CRISPRa systems, based on single or dual Lentiviral and Adeno-Associated viral vectors and VP64 or VPR transcriptional activators, and demonstrate their efficiency in increasing mRNA and protein expression of the Cnr1 gene, coding for neuronal CB1 receptors. Both approaches were similarly efficient in primary neuronal cultures, and achieved a 2–5-fold increase in Cnr1 expression, but the AAV-based approach was more efficient in vivo. Our dual AAV-based VPR system in particular, based on Staphylococcus aureus dCas9, when injected in the hippocampus, displayed almost complete simultaneous expression of both vectors, high levels of dCas9 expression, and good efficiency in increasing Cnr1 mRNA as measured by in situ hybridization. In addition, we also show significant upregulation of CB1 receptor protein in vivo, which is reflected by an increased ability in reducing neurotransmitter release, as measured by electrophysiology. Our results show that CRISPRa techniques could be successfully used in neurons to target overexpression of genes involved in synaptic transmission, and can potentially represent a next-generation gene therapy approach against neurological disorders.

Protein Interactors and Trafficking Pathways That Regulate the Cannabinoid Type 1 Receptor (CB1R)

The endocannabinoid system (ECS) acts as a negative feedback mechanism to suppress synaptic transmission and plays a major role in a diverse range of brain functions including, for example, the regulation of mood, energy balance, and learning and memory. The function and dysfunction of the ECS are strongly implicated in multiple psychiatric, neurological, and neurodegenerative diseases. Cannabinoid type 1 receptor (CB1R) is the most abundant G protein-coupled receptor (GPCR) expressed in the brain and, as for any synaptic receptor, CB1R needs to be in the right place at the right time to respond appropriately to changing synaptic circumstances. While CB1R is found intracellularly throughout neurons, its surface expression is highly polarized to the axonal membrane, consistent with its functional expression at presynaptic sites. Surprisingly, despite the importance of CB1R, the interacting proteins and molecular mechanisms that regulate the highly polarized distribution and function of CB1R remain relatively poorly understood. Here we set out what is currently known about the trafficking pathways and protein interactions that underpin the surface expression and axonal polarity of CB1R, and highlight key questions that still need to be addressed.

Long-term application of cannabinoids leads to dissociation between changes in cAMP and modulation of GABAA receptors of mouse trigeminal sensory neurons

Antinociception caused by cannabinoids may have a partial peripheral origin in addition to its central site of action. In fact, we have observed that anandamide selectively and reversibly inhibits GABA_A receptors of putative nociceptive neurons of mouse trigeminal sensory ganglia via CB1 receptor activation to inhibit adenylyl cyclase and decrease cAMP with downstream posttranslational alterations. Since cannabinoids are often used chronically, we studied changes in cAMP levels and GABA-mediated currents of trigeminal neurons following 24 h application of anandamide (0.5 μM) or the synthetic cannabinoid WIN 55,212-2 (5 μM). With this protocol GABA responses were similar to control despite persistent fall in cAMP levels. Inhibition by WIN 55,212-2 of GABA effects recovered after 30 min washout and was not associated with changes in CB1 receptor expression, indicating lack of CB1 receptor inactivation and transient loss of negative coupling between CB1 receptors and GABA_A receptors. The phosphodiesterase inhibitor rolipram (100 μM; 24 h) enhanced cAMP levels and GABA-mediated currents, suggesting GABA_A receptors were sensitive to persistent upregulation via cAMP. While the adenylyl cyclase activator forskolin (1-20 μM) facilitated cAMP levels and GABA currents following 30 min application, this action was lost after 24 h in line with the drug limited lifespan. The PKA inhibitor PKI 14-22 (10 μM) increased cAMP without changing GABA currents. These data indicate that modulation of GABA_A receptors by intracellular cAMP could be lost following persistent application of cannabinoids. Thus, these observations provide an insight into the waning antinociceptive effects of these compounds.

Daytime-Dependent Changes of Cannabinoid Receptor Type 1 and Type 2 Expression in Rat Liver

The present study was performed to investigate the diurnal expression pattern of cannabinoid receptor type 1 (CB₁) and type 2 (CB₂) in liver tissue of 12- and 51-week-old normoglycemic Wistar rats. By using real-time RT-PCR, daytime dependent changes in both age groups and, for both, hepatic Cnr1 and Cnr2 receptor mRNA levels were measured. Highest amount of mRNA was detected in the light period (ZT3, ZT6, and ZT9) while the lowest amount was measured in the dark period (ZT18 and ZT21). Diurnal transcript expression pattern was accompanied by comparable changes of protein level for CB₁, as shown by Western blotting. The current results support the conclusion that expression pattern of cannabinoid receptors are influenced by light/dark cycle and therefore seems to be under the control of a diurnal rhythm. These findings might explain the differences in the efficacy of cannabinoid receptor agonists or antagonists. In addition, investigation of liver of streptozotocin (STZ)-treated 12- and 51-week-old rats show alterations in the diurnal profile of both receptors Cnr1 and Cnr2 compared to that of normoglycemic Wistar rats. This suggests an influence of diabetic state on diurnal expression levels of cannabinoid receptors.

Interaction between Cannabinoid Type 1 and Type 2 Receptors in the Modulation of Subventricular Zone and Dentate Gyrus Neurogenesis

Neurogenesis in the adult mammalian brain occurs mainly in two neurogenic niches, the subventricular zone (SVZ) and the subgranular zone (SGZ) of the dentate gyrus (DG). Cannabinoid type 1 and 2 receptors (CB₁R and CB₂R) have been shown to differently modulate neurogenesis. However, low attention has been given to the interaction between CB₁R and CB₂R in modulating postnatal neurogenesis (proliferation, neuronal differentiation and maturation). We focused on a putative crosstalk between CB₁R and CB₂R to modulate neurogenesis and cultured SVZ and DG stem/progenitor cells from early postnatal (P1-3) Sprague-Dawley rats. Data showed that the non-selective cannabinoid receptor agonist WIN55,212-2 promotes DG cell proliferation (measured by BrdU staining), an effect blocked by either CB₁R or CB₂R selective antagonists. Experiments with selective agonists showed that facilitation of DG cell proliferation requires co-activation of both CB₁R and CB₂R. Cell proliferation in the SVZ was not affected by the non-selective receptor agonist, but it was enhanced by CB₁R selective activation. However, either CB₁R or CB₂R selective antagonists abolished the effect of the CB₁R agonist in SVZ cell proliferation. Neuronal differentiation (measured by immunocytochemistry against neuronal markers of different stages and calcium imaging) was facilitated by WIN55,212-2 at both SVZ and DG. This effect was mimicked by either CB₁R or CB₂R selective agonists and blocked by either CB₁R or CB₂R selective antagonists, cross-antagonism being evident. In summary, our findings indicate a tight interaction between CB₁R and CB₂R to modulate neurogenesis in the two major neurogenic niches, thus contributing to further unraveling the mechanisms behind the action of endocannabinoids in the brain.

Discovery and characterization of two novel CB1 receptor splice variants with modified N-termini in mouse

Numerous studies have been carried out in the mouse model, investigating the role of the cannabinoid receptor type 1 (CB1). However, mouse CB1 (mCB1) receptor differs from human CB1 (hCB1) receptor in 13 amino acid residues. Two splice variants, hCB1a and hCB1b, diverging in their amino-termini, have been reported to be unique for hCB1 and, via different signaling properties, contribute to CB1 receptor physiology and pathophysiology. We hypothesized that splice variants also exist for the mCB1 receptor and have different signaling properties. On murine hippocampal cDNA, we identified two novel mCB1 receptor splice variants generated by splicing of introns with 117 bp and 186 bp in the N-terminal domain, corresponding to deletions of 39 or 62 amino acids, respectively. The mRNAs for the splice variants mCB1a and mCB1b are expressed at low levels in different brain regions. Western blot analysis of protein extracts from stably transfected HEK293 cells indicates a strongly reduced glycosylation because of the absence of two glycosylation sites in mCB1b. On-cell western analysis in these stable lines revealed increased internalization of mCB1a and mCB1b upon stimulation with the agonist WIN55,212-2 as compared to mCB1. Results also point toward an increased affinity to SR141716 for mCB1a, as well as slightly enhanced inhibition of neurotransmission compared to mCB1. In mCB1b, agonist-induced MAPK phosphorylation was decreased compared to mCB1 and mCB1a. Identification of mouse CB1 receptor splice variants may help to explain differences found between human and mouse endocannabinoid systems and improve the understanding of CB1 receptor signaling and trafficking in different species.

Mouse Neuroblastoma CB1 Cannabinoid Receptor-Stimulated [35S]GTPɣS Binding: Total and Antibody-Targeted Gα Protein-Specific Scintillation Proximity Assays

G protein-coupled receptors (GPCRs) are important regulators of cellular signaling functions and therefore are a major target for drug discovery. The CB1 cannabinoid receptor is among the most highly expressed GPCRs in neurons, where it regulates many differentiated neuronal functions. One model system for studying the biochemistry of neuronal responses is the use of neuroblastoma cells originating from the C1300 tumor in the A/J mouse, including cloned cell lines NS20, N2A, N18TG2, N4TG1, and N1E-115, and various immortalized hybrids of neurons with N18TG2 cells. GPCR signal transduction is mediated through interaction with multiple types and subtypes of G proteins that transduce the receptor stimulus to effectors. The [35S]GTPɣS assay provides a valuable pharmacological method to evaluate efficacy and potency in the first step in GPCR signaling. Here, we present detailed protocols for the [35S]GTPɣS-binding assay to measure the total G protein binding and the antibody-targeted scintillation proximity assay to measure specific Gα proteins in neuroblastoma cell membrane preparations. This chapter presents step-by-step methods from cell culture, membrane preparation, assay procedures, and data analysis.

Heterologous regulation of the cannabinoid type 1 receptor by angiotensin II in astrocytes of spontaneously hypertensive rats

Brainstem and cerebellar astrocytes have critical roles to play in hypertension and attention-deficit hyperactivity disorder, respectively. Angiotensin (Ang) II, via the astroglial Ang type 1 receptor (AT1R), has been demonstrated to elevate pro-inflammatory mediators in the brainstem and the cerebellum. The activation of astroglial cannabinoid type 1 receptor (CB1R), a master regulator of homeostasis, has been shown to neutralize inflammatory states. Factors that drive disease progression are known to alter the expression of CB1Rs. In this study, we investigated the role of Ang II in regulating CB1R protein and mRNA expression in astrocytes isolated from the brainstem and the cerebellum of spontaneously hypertensive rats (SHRs). The results were then compared with their normotensive counterpart, Wistar rats. Not only was the basal expression of CB1R protein and mRNA significantly lower in SHR brainstem astrocytes, but treatment with Ang II resulted in lowering it further in the initial 12 h. In the case of cerebellum, Ang II up-regulated the CB1R protein and mRNA in SHR astrocytes. While the effect of Ang II on CB1R protein was predominantly mediated via the AT1R in SHR brainstem; both AT1R- and AT2R-mediated Ang II's effect in the SHR cerebellum. These data are strongly indicative of a potential new mode of cross-talk between components of the renin angiotensin system and the endocannabinoid system in astrocytes. The consequence of such a cross-talk could be a potential reduced endocannabinoid tone in brainstem in hypertensive states, but not in the cerebellum under the same conditions.

Cannabinoid control of brain bioenergetics: Exploring the subcellular localization of the CB1 receptor

Brain mitochondrial activity is centrally involved in the central control of energy balance. When studying mitochondrial functions in the brain, however, discrepant results might be obtained, depending on the experimental approaches. For instance, immunostaining experiments and biochemical isolation of organelles expose investigators to risks of false positive and/or false negative results. As an example, the functional presence of cannabinoid type 1 (CB₁) receptors on brain mitochondrial membranes (mtCB₁) was recently reported and rapidly challenged, claiming that the original observation was likely due to artifact results. Here, we addressed this issue by directly comparing the procedures used in the two studies. Our results show that the use of appropriate controls and quantifications allows detecting mtCB₁ receptor with CB₁ receptor antibodies, and that, if mitochondrial fractions are enriched and purified, CB₁ receptor agonists reliably decrease respiration in brain mitochondria. These data further underline the importance of adapted experimental procedures to study brain mitochondrial functions.

Antibodies to cannabinoid type 1 receptor co-react with stomatin-like protein 2 in mouse brain mitochondria

Anti-cannabinoid type 1 receptor (CB1 ) polyclonal antibodies are widely used to detect the presence of CB1 in a variety of brain cells and their organelles, including neuronal mitochondria. Surprisingly, we found that anti-CB1 sera, in parallel with CB1 , also recognize the mitochondrial protein stomatin-like protein 2. In addition, we show that the previously reported effect of synthetic cannabinoid WIN 55,212-2 on mitochondrial complex III respiration is not detectable in purified mitochondrial preparations. Thus, our study indicates that a direct relationship between endocannabinoid signaling and mitochondrial functions in the cerebral cortex seems unlikely, and that caution should be taken interpreting findings obtained using anti-CB1 antibodies.

Cannabinoid receptor 1 signaling in cardiovascular regulating nuclei in the brainstem: A review

Cannabinoids elicit complex hemodynamic responses in experimental animals that involve both peripheral and central sites. Centrally administered cannabinoids have been shown to predominantly cause pressor response. However, very little is known about the mechanism of the cannabinoid receptor 1 (CB₁R)-centrally evoked pressor response. In this review, we provided an overview of the contemporary knowledge regarding the cannabinoids centrally elicited cardiovascular responses and the possible underlying signaling mechanisms. The current review focuses on the rostral ventrolateral medulla (RVLM) as the primary brainstem nucleus implicated in CB₁R-evoked pressor response.

Cannabinoids and bone: endocannabinoids modulate human osteoclast function in vitro

BACKGROUND AND PURPOSE

Both CB(1) and CB(2) cannabinoid receptors have been shown to play a role in bone metabolism. Crucially, previous studies have focussed on the effects of cannabinoid ligands in murine bone cells. This study aimed to investigate the effects of cannabinoids on human bone cells in vitro.

EXPERIMENTAL APPROACH

Quantitative RT-PCR was used to determine expression of cannabinoid receptors and liquid chromatography-electrospray ionization tandem mass spectrometry was used to determine the presence of endocannabinoids in human bone cells. The effect of cannabinoids on human osteoclast formation, polarization and resorption was determined by assessing the number of cells expressing α(v) β(3) or with F-actin rings, or measurement of resorption area.

KEY RESULTS

Human osteoclasts express both CB(1) and CB(2) receptors. CB(2) expression was significantly higher in human monocytes compared to differentiated osteoclasts. Furthermore, the differentiation of human osteoclasts from monocytes was associated with a reduction in 2-AG levels and an increase in anandamide (AEA) levels. Treatment of osteoclasts with LPS significantly increased levels of AEA. Nanomolar concentrations of AEA and the synthetic agonists CP 55 940 and JWH015 stimulated human osteoclast polarization and resorption; these effects were attenuated in the presence of CB(1) and/or CB(2) antagonists.

CONCLUSIONS

AND IMPLICATIONS Low concentrations of cannabinoids activate human osteoclasts in vitro. There is a dynamic regulation of the expression of the CB(2) receptor and the production of the endocannabinoids during the differentiation of human bone cells. These data suggest that small molecules modulating the endocannabinoid system could be important therapeutics in human bone disease.

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.

Characterisation of cannabinoid 1 receptor expression in the perikarya, and peripheral and spinal processes of primary sensory neurons

The cannabinoid 1 (CB1) receptor is expressed by a sub-population of primary sensory neurons. However, data on the neurochemical identity of the CB1 receptor-expressing cells, and CB1 receptor expression by the peripheral and central terminals of these neurons are inconsistent and limited. We characterised CB1 receptor expression in dorsal root ganglia (DRG) and spinal cord at the lumbar 4-5 level, as well as in the urinary bladder and glabrous skin of the hindpaw. About 1/3 of DRG neurons exhibited immunopositivity for the CB1 receptor, the majority of which showed positivity for the nociceptive markers calcitonin gene-related peptide (CGRP) or/and Griffonia (bandeiraea) simplicifolia IB4 isolectin-binding. Virtually all CB1 receptor-immunostained fibres showed immunopositivity for CGRP in the skin, while very few did in the urinary bladder. No CB1 receptor-immunopositive nerve fibres were IB4 positive in either peripheral tissue. Spinal laminae I and II-outer showed the highest density of CB1 receptor-immunopositive punctae, the majority of which showed positivity for CGRP or/and IB4 binding. These data indicate that a major sub-population of nociceptive primary sensory neurons expresses CB1 receptors that are transported to both peripheral and central terminals of these cells. Therefore, the present data suggest that manipulation of endogenous CB1 receptor agonist levels in these areas may significantly reduce nociceptive input into the spinal cord.

Cannabinoid receptor expression in HIV encephalitis and HIV-associated neuropathologic comorbidities

AIMS

Cannabinoids have been proposed for treating various neurodegenerative disorders and as adjunct therapy for HIV+ patients with neurologic sequelae. The expression of cannabinoid receptors (CB1 and CB2) has been reported in neurodegenerative diseases and in simian immunodeficiency virus encephalitis, yet the receptor expression in the central nervous system of HIV+ individuals is not known.

METHODS

An anti-CB1 antibody and two anti-CB2 antibodies were employed for immunohistochemistry in the cerebral cortex and white matter of HIV encephalitis (HIVE) and HIV-associated comorbidities, as well as control brains (HIV- and HIV+).

RESULTS

By quantitative image analysis, we observed that CB1 was increased in HIVE brains and those with comorbidities, while CB2 was significantly increased in the white matter of HIVE. Morphologically, CB1 was present in neurones, and both CB1 and CB2 were present in meningeal macrophages and subpial glia in all brains. In HIVE, CB1 was found in white matter microglia and perivascular cells, while CB2 was increased in microglia, astrocytes and perivascular macrophages. Double immunofluorescence with cell-specific markers and immunoblots on primary cultured microglia and astrocytes substantiated the glial localization of the cannabinoid receptors and specificity of the antibodies.

CONCLUSIONS

Our study indicates that cannabinoid receptor expression occurs in glia in HIVE brains, and this may have ramifications for the potential use of cannabinoid ligands in HIV-infected patients.

Human sperm anatomy: ultrastructural localization of the cannabinoid1 receptor and a potential role of anandamide in sperm survival and acrosome reaction

Recently, the endocannabinoid (EC) system and the presence of CB1 receptor (CB1-R), have been identified in human sperm. However, the effects of EC receptor ligands such as anandamide (N-arachidonoylethanolamine) and the role of EC system in male fertility is still largely unexplored. In the present study, we investigated the ultrastructural compartmentalization of CB1-R and analyzed the effects of its stimulation by using a stable analog of anandamide, 2-methylarachidonyl-2'-fluoro-ethylamide (MET-F-AEA). We focused particularly on sperm survival and acrosin activity. The study of human sperm anatomy by transmission electron microscopy with immunogold analysis revealed the location of the CB1-R prevalently in the sperm membranes of the head and interestingly on the mitochondria. The effect of different concentrations of MET-F-AEA from 100 nM to 1 microM evidenced a significant decrease of sperm survival. Interestingly, we analyzed this negative effect at molecular level, testing the EC action on different known sperm survival targets. MET-F-AEA-treatment decreased both pBCL2 and pAkt, two prosurvival proteins, and increased pPTEN expression which is the main regulator of the PI3K/Akt pathway. Moreover, a biphasic effect was observed with increasing MET-F-AEA concentrations on the acrosin activity. The blockage of the CB1-R by using its selective antagonist SR141716 (rimonabant) induced an opposite action on sperm survival supporting a role for this receptor in the biology of the male gamete.

Neuronal and glial localization of the cannabinoid-1 receptor in the superficial spinal dorsal horn of the rodent spinal cord

A long line of experimental evidence indicates that endogenous cannabinoid mechanisms play important roles in nociceptive information processing in various areas of the nervous system including the spinal cord. Although it is extensively documented that the cannabinoid-1 receptor (CB(1)-R) is strongly expressed in the superficial spinal dorsal horn, its cellular distribution is poorly defined, hampering our interpretation of the effect of cannabinoids on pain processing spinal neural circuits. Thus, we investigated the cellular distribution of CB(1)-Rs in laminae I and II of the rodent spinal dorsal horn with immunocytochemical methods. Axonal varicosities revealed a strong immunoreactivity for CB(1)-R, but no CB(1)-R expression was observed on dendrites and perikarya of neurons. Investigating the co-localization of CB(1)-R with markers of peptidergic and non-peptidergic primary afferents, and axon terminals of putative glutamatergic and GABAergic spinal neurons we found that nearly half of the peptidergic (immunoreactive for calcitonin gene-related peptide) and more than 20% of the non-peptidergic (binding isolectin B4) nociceptive primary afferents, more than one-third and approximately 20% of the axon terminals of putative glutamatergic (immunoreactive for vesicular glutamate transporter 2) and GABAergic (immunoreactive for glutamic acid decarboxylase; GAD65 and/or GAD67) spinal interneurons, respectively, were positively stained for CB(1)-R. In addition to axon terminals, almost half of the astrocytic (immunoreactive for glial fibrillary acidic protein) and nearly 80% of microglial (immunoreactive for CD11b) profiles were also immunolabeled for CB(1)-R. The findings suggest that the activity-dependent release of endogenous cannabinoids activates a complex signaling mechanism in pain processing spinal neural circuits into which both neurons and glial cells may contribute.

Gz mediates the long-lasting desensitization of brain CB1 receptors and is essential for cross-tolerance with morphine

Background

Although the systemic administration of cannabinoids produces antinociception, their chronic use leads to analgesic tolerance as well as cross-tolerance to morphine. These effects are mediated by cannabinoids binding to peripheral, spinal and supraspinal CB1 and CB2 receptors, making it difficult to determine the relevance of each receptor type to these phenomena. However, in the brain, the CB1 receptors (CB1Rs) are expressed at high levels in neurons, whereas the expression of CB2Rs is marginal. Thus, CB1Rs mediate the effects of smoked cannabis and are also implicated in emotional behaviors. We have analyzed the production of supraspinal analgesia and the development of tolerance at CB1Rs by the direct injection of a series of cannabinoids into the brain. The influence of the activation of CB1Rs on supraspinal analgesia evoked by morphine was also evaluated.

Results

Intracerebroventricular (icv) administration of cannabinoid receptor agonists, WIN55,212-2, ACEA or methanandamide, generated a dose-dependent analgesia. Notably, a single administration of these compounds brought about profound analgesic tolerance that lasted for more than 14 days. This decrease in the effect of cannabinoid receptor agonists was not mediated by depletion of CB1Rs or the loss of regulated G proteins, but, nevertheless, it was accompanied by reduced morphine analgesia. On the other hand, acute morphine administration produced tolerance that lasted only 3 days and did not affect the CB1R. We found that both neural mu-opioid receptors (MORs) and CB1Rs interact with the HINT1-RGSZ module, thereby regulating pertussis toxin-insensitive Gz proteins. In mice with reduced levels of these Gz proteins, the CB1R agonists produced no such desensitization or morphine cross-tolerance. On the other hand, experimental enhancement of Gz signaling enabled an acute icv administration of morphine to produce a long-lasting tolerance at MORs that persisted for more than 2 weeks, and it also impaired the analgesic effects of cannabinoids.

Conclusion

In the brain, cannabinoids can produce analgesic tolerance that is not associated with the loss of surface CB1Rs or their uncoupling from regulated transduction. Neural specific Gz proteins are essential mediators of the analgesic effects of supraspinal CB1R agonists and morphine. These Gz proteins are also responsible for the long-term analgesic tolerance produced by single doses of these agonists, as well as for the cross-tolerance between CB1Rs and MORs.

Characterisation of the cannabinoid receptor system in synovial tissue and fluid in patients with osteoarthritis and rheumatoid arthritis

Introduction

Cannabis-based medicines have a number of therapeutic indications, including anti-inflammatory and analgesic effects. The endocannabinoid receptor system, including the cannabinoid receptor 1 (CB₁) and receptor 2 (CB₂) and the endocannabinoids, are implicated in a wide range of physiological and pathophysiological processes. Pre-clinical and clinical studies have demonstrated that cannabis-based drugs have therapeutic potential in inflammatory diseases, including rheumatoid arthritis (RA) and multiple sclerosis. The aim of this study was to determine whether the key elements of the endocannabinoid signalling system, which produces immunosuppression and analgesia, are expressed in the synovia of patients with osteoarthritis (OA) or RA.

Methods

Thirty-two OA and 13 RA patients undergoing total knee arthroplasty were included in this study. Clinical staging was conducted from x-rays scored according to Kellgren-Lawrence and Larsen scales, and synovitis of synovial biopsies was graded. Endocannabinoid levels were quantified in synovial fluid by liquid chromatography-mass spectrometry. The expression of CB₁ and CB₂ protein and RNA in synovial biopsies was investigated. Functional activity of these receptors was determined with mitogen-activated protein kinase assays. To assess the impact of OA and RA on this receptor system, levels of endocannabinoids in the synovial fluid of patients and non-inflamed healthy volunteers were compared. The activity of fatty acid amide hydrolase (FAAH), the predominant catabolic endocannabinoid enzyme, was measured in synovium.

Results

CB₁ and CB₂ protein and RNA were present in the synovia of OA and RA patients. Cannabinoid receptor stimulation of fibroblast-like cells from OA and RA patients produced a time-dependent phosphorylation of extracellular signal-regulated kinase (ERK)-1 and ERK-2 which was significantly blocked by the CB₁ antagonist SR141716A. The endocannabinoids anandamide (AEA) and 2-arachidonyl glycerol (2-AG) were identified in the synovial fluid of OA and RA patients. However, neither AEA nor 2-AG was detected in synovial fluid from normal volunteers. FAAH was active in the synovia of OA and RA patients and was sensitive to inhibition by URB597 (3'-(aminocarbonyl) [1,1'-biphenyl]-3-yl)-cyclohexylcarbamate).

Conclusion

Our data predict that the cannabinoid receptor system present in the synovium may be an important therapeutic target for the treatment of pain and inflammation associated with OA and RA.

Cannabinoid regulation in identified synapse of terrestrial snail

In the terrestrial snail a direct monosynaptic glutamatergic connection between the primary sensory neuron and a premotor interneuron involved in withdrawal behaviour can be functionally identified using electrophysiological techniques. We investigated the involvement of cannabinoids in regulation of this synaptic contact. The results demonstrate that the specific binding sites for agonists to mammalian type 1 cannabinoid receptors (CB1Rs) exist in the snail's nervous system. Application of a synthetic cannabinoid agonist anandamide selectively changed the efficacy of synaptic contacts between the identified neurons. A decrease in the long-term synaptic facilitation of the synaptic contact elicited by high-frequency nerve tetanization in the presence of cannabinoid agonist anandamide was observed, suggesting a possible role of endocannabinoids in regulation of plasticity at this synaptic site. The selective antagonist of CB1Rs [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide] AM251 bath application was changing the efficacy of the synaptic contact only when the postsynaptic neuron had been intracellularly activated before its application. This observation implies an involvement of endocannabinoids in plasticity phenomena induced by activity in the postsynaptic target. Additional support of endocannabinoid involvement in synaptic function at this site was given by experiments in which AM251 blocked the short-term suppression of synaptic excitation evoked by low-frequency nerve tetanization, a phenomenon qualitatively similar to cannabinoid-dependent synaptically evoked suppression of excitation demonstrated in the mammalian nervous system. The results of the present study suggest an involvement of cannabinoids in the regulation of synaptic efficacy. Further, anandamide could be a candidate for an endogenous neuromessenger involved in plasticity processes.

BfCBR: a cannabinoid receptor ortholog in the cephalochordate Branchiostoma floridae (Amphioxus)

A gene encoding an ortholog of vertebrate CB(1)/CB(2) cannabinoid receptors was recently identified in the urochordate Ciona intestinalis (CiCBR; [Elphick, M.R., Satou, Y., Satoh, N., 2003. The invertebrate ancestry of endocannabinoid signalling: an orthologue of vertebrate cannabinoid receptors in the urochordate Ciona intestinalis. Gene 302, 95-101.]). Here a cannabinoid receptor ortholog (BfCBR) has been identified in the cephalochordate Branchiostoma floridae. BfCBR is encoded by a single exon and is 410 amino acid residue protein that shares 28% sequence identity with CiCBR and 23% sequence identity with human CB(1) and human CB(2). The discovery of BfCBR and CiCBR and the absence of cannabinoid receptor orthologs in non-chordate invertebrates indicate that CB(1)/CB(2)-like cannabinoid receptors originated in an invertebrate chordate ancestor of urochordates, cephalochordates and vertebrates. Furthermore, analysis of the relationship of BfCBR and CiCBR with vertebrate CB(1) and CB(2) receptors indicates that the gene/genome duplication that gave rise to CB(1) and CB(2) receptors occurred in the vertebrate lineage. Identification of BfCBR, in addition to CiCBR, paves the way for comparative analysis of the expression and functions of these proteins in Branchiostoma and Ciona, respectively, providing an insight into the ancestral functions of cannabinoid receptors in invertebrate chordates prior to the emergence of CB(1) and CB(2) receptors in vertebrates.

Characterization of CB1 cannabinoid receptor immunoreactivity in postmortem human brain homogenates

The CB1 cannabinoid receptor (CB1) is the predominant type of cannabinoid receptor in the CNS, in which it displays a unique anatomical distribution and is present at higher densities than most other known seven transmembrane domain receptors. Nevertheless, as with almost all seven transmembrane domain receptors, the tertiary and quaternary structure of this receptor is still unknown. Studies of CB1 in rat cerebral tissue are scarce, and even less is known regarding the expression of CB1 in the human brain. Thus, the aim of the present work was to characterize CB1 expression in membranes from postmortem human brain using specific antisera raised against this protein. Western blot analysis of P1 and P2 fractions, and crude plasma membrane preparations from the prefrontal cortex showed that CB1 migrated as a 60 kDa monomer under reducing conditions. These data were confirmed by blotting experiments carried out with human U373MG astrocytoma cells as a positive control for CB1 expression and wild-type CHO cells as negative control. In addition, when proteins were solubilized in the absence of dithiothreitol, the anti-human CB1 antiserum detected a new band migrating at around 120 kDa corresponding in size to a putative CB1 dimer. This band was sensitive to reducing agents (50 mM dithiothreitol) and showed sodium dodecylsulphate stability, suggesting the existence of disulfide-linked CB1 dimers in the membrane preparations. Important differences in the anatomical distribution of CB1 were observed with regard to that described previously in monkey and rat; in the human brain, CB1 levels were higher in cortex and caudate than in the cerebellum.

Cannabidiol inhibits human glioma cell migration through a cannabinoid receptor-independent mechanism

We evaluated the ability of cannabidiol (CBD) to impair the migration of tumor cells stimulated by conditioned medium. CBD caused concentration-dependent inhibition of the migration of U87 glioma cells, quantified in a Boyden chamber. Since these cells express both cannabinoid CB1 and CB2 receptors in the membrane, we also evaluated their engagement in the antimigratory effect of CBD. The inhibition of cell was not antagonized either by the selective cannabinoid receptor antagonists SR141716 (CB1) and SR144528 (CB2) or by pretreatment with pertussis toxin, indicating no involvement of classical cannabinoid receptors and/or receptors coupled to Gi/o proteins. These results reinforce the evidence of antitumoral properties of CBD, demonstrating its ability to limit tumor invasion, although the mechanism of its pharmacological effects remains to be clarified.

Cannabinoid receptor expression in peripheral arterial chemoreceptors during postnatal development

Prenatal exposure to tobacco smoke increases risk of sudden infant death syndrome (SIDS). Marijuana is frequently smoked in conjunction with tobacco, and perinatal exposure to marijuana is associated with increased incidence of SIDS. Abnormalities in peripheral arterial chemoreceptor responses during sleep may be operative in infants at risk for SIDS, and nicotine exposure adversely affects peripheral arterial chemoreceptor responses. To determine whether marijuana could potentially affect the activity of peripheral arterial chemoreceptors during early postnatal development, we used in situ hybridization histochemistry to characterize the pattern and level of mRNA expression for cannabinoid type 1 receptor (CB1R) in the carotid body, superior cervical ganglia (SCG), and nodose-petrosal-jugular ganglia (NG-PG-JG) complex in newborn rats. We used immunohistochemistry and light, confocal, and electron microscopy to characterize the pattern of CB1R and tyrosine hydroxylase protein expression. CB1R mRNA expression was intense in the NG-PG-JG complex, low to moderate in the SCG, and sparse in the carotid body. With maturation, CB1R gene expression significantly increased (P < 0.01) in the NG-PG-JG complex. CB1R immunoreactivity was localized to nuclei of ganglion cells in the SCG and NG-PG-JG complex, whereas tyrosine hydroxylase immunoreactivity was localized to the cytoplasm. Exposure to marijuana during early development could potentially modify cardiorespiratory responses via peripheral arterial chemoreceptors. The novel finding of nuclear localization of CB1Rs in peripheral ganglion cells suggests that these receptors may have an, as yet, undetermined role in nuclear signaling in sensory and autonomic neurons.

Immunohistochemical characterisation and localisation of cannabinoid CB1 receptor protein in the rat vestibular nucleus complex and the effects of unilateral vestibular deafferentation

CB1 receptor expression has been reported to be low in the brainstem compared with the forebrain, and low in the vestibular nucleus complex (VNC) compared with other regions in the brainstem. However, a frequent effect of cannabis is dizziness and loss of balance. This may be due to the activation of cannabinoid receptors in the central vestibular pathways. We used immunohistochemistry to study the distribution of CB1 receptor protein in the VNC, and Western blotting to measure CB1 receptor expression in the VNC following unilateral vestibular deafferentation (UVD); the hippocampal CA1, CA2/3 and dentate gyrus (DG) regions were also analysed for comparison. This study confirms a previous electrophysiological demonstration that CB1 receptors exist in significant densities in the VNC and are likely to contribute to the neurochemical control of the vestibular reflexes. Nonetheless, CB1 receptor expression did not change significantly in the VNC during vestibular compensation. In addition, despite some small but significant changes in CB1 receptor expression in the CA2/3 and the DG following UVD, in no case were these differences statistically significant in comparison to both control groups.

Cannabinoids suppress synaptic input to neurones of the rat dorsal motor nucleus of the vagus nerve

Cannabinoids bind central type 1 receptors (CB1R) and modify autonomic functions, including feeding and anti-emetic behaviours, when administered peripherally or into the dorsal vagal complex. Western blots and immunohistochemistry indicated the expression of CB1R in the rat dorsal vagal complex, and tissue polymerase chain reaction confirmed that CB1R message was made within the region. To identify a cellular substrate for the central autonomic effects of cannabinoids, whole-cell patch-clamp recordings were made in brainstem slices to determine the effects of CB1R activation on synaptic transmission to neurones of the dorsal motor nucleus of the vagus (DMV). A subset of these neurones was identified as gastric related after being labelled retrogradely from the stomach. The CB1R agonists WIN55,212-2 and anandamide decreased the frequency of spontaneous excitatory or inhibitory postsynaptic currents in a concentration-related fashion, an effect that persisted in the presence of tetrodotoxin. Paired pulse ratios of electrically evoked postsynaptic currents were also increased by WIN55,212-2. The effects of WIN55,212-2 were sensitive to the selective CB1R antagonist AM251. Cannabinoid agonist effects on synaptic input originating from neurones in the nucleus tractus solitarius (NTS) were determined by evoking activity in the NTS with local glutamate application. Excitatory and inhibitory synaptic inputs arising from the NTS were attenuated by WIN55,212-2. Our results indicate that cannabinoids inhibit transfer of synaptic information to the DMV, including that arising from the NTS, in part by acting at receptors located on presynaptic terminals contacting DMV neurones. Inhibition of synaptic input to DMV neurones is likely to contribute to the suppression of visceral motor responses by cannabinoids.

Differential distribution of functional cannabinoid CB1 receptors in the mouse gastroenteric tract

Recently, the gastrointestinal pharmacology of cannabinoid CB(1) receptors has been extensively explored. We employed western blotting and immunohistochemistry techniques to study the distribution of the cannabinoid CB(1) receptor protein in the mouse gastroenteric tract. The cannabinoid CB(1) receptor peptide was detected by western blotting only in its glycosylated form (63 kDa) with a significant differential distribution. The highest levels of expression were detected in the stomach and in the colon, while the pyloric valve was devoid of any cannabinoid CB(1) receptor protein. The immunohistochemical study showed intense cannabinoid CB(1) receptor immunoreactivity in ganglia subadjacent to the gastric epithelium and in the smooth muscle layers of both the small and large intestine. Only the small intestine showed (-)-3-[2-hydroxyl-4-(1,1-dimethylheptyl)-phenyl]-4-(3-hydroxylpropyl) cyclohexan-1-ol) ([3H]CP 55,940) specific binding (27%). These receptors mediated pharmacologically significant effects since the cannabinoid CB(1) receptor agonist R(-)-7-hydroxy-delta-6-tetra-hydrocannabinol-dimethylheptyl (HU 210) dose dependently inhibited gastrointestinal transit up to 70%, while the cannabinoid CB(1) receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide (SR 141716A) increased gastrointestinal transit. Moreover, the dose of 0.3 microg/kg of HU 210, devoid per se of any activity on mouse intestinal propulsion, blocked the increased gastroenteric transit induced by the cannabinoid CB(1) antagonist SR 141716A.

Dimerization of G protein-coupled receptors: CB1 cannabinoid receptors as an example

A polyclonal antibody directed towards the last 73 amino acid residues of the rat type 1 cannabinoid (CB1) receptor strongly and exclusively labels a high molecular weight (between 160 and 200 kDa) form of the receptor in Western analysis. In contrast, a human CB1 polyclonal antibody identifies both monomeric CB1 as well as the high molecular weight form. The carboxy terminus (CT) antibody was also used in immunocytochemistry of rat hippocampal sections. Sections probed with CT antibody show intense staining of a meshwork of fibers and occasional interneurons of the stratum oriens, stratum pyramidal, and stratum radiatum of the CA1 and CA3 regions while mossy fibers and granule cells of the internal stratum appear unstained. These data provide evidence that CB1 likely exists as a dimer in vivo and that the carboxy end of the receptor may play a role in the assembly of the oligomer.

Design, expression, and characterization of a synthetic human cannabinoid receptor and cannabinoid receptor/ G-protein fusion protein

We report here the synthesis and characterization of two gene constructs designed to facilitate structure/function studies of the human neuronal cannabinoid receptor, CB1. The first gene, which we call shCB1, is a synthetic gene containing unique restriction sites spaced roughly 50-100 bases apart to facilitate rapid mutagenesis and cloning. A nine amino acid epitope tag (from the rhodopsin C-terminus) is also present in the shCB1 C-terminal tail to enable detection and purification using the monoclonal antibody 1D4. We find that that the shCB1 gene can be transiently expressed in COS cells with yield of approximately 10-15 micro g receptor per 15 cm plate and is wild type like in its ability to bind cannabinoid ligands. Our confocal microscopy studies indicate shCB1 targets to the membrane of HEK293 cells and is internalized in response to agonist. To facilitate functional studies, we also made a chimera in which the C-terminus of shCB1 was fused with the N-terminus of a G-protein alpha subunit, Galphai. The shCB1/Galphai chimera shows agonist stimulated GTPgammaS binding, and thus provides a simplified way to measure agonist induced CB1 activation. Taken together, the shCB1 and shCB1/Galphai gene constructs provide useful tools for biochemical and biophysical examinations of CB1 structure, activation and attenuation.

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.

Agonist-induced internalization and trafficking of cannabinoid CB1 receptors in hippocampal neurons

Agonist-induced internalization of G-protein-coupled receptors is an important mechanism for regulating receptor abundance and availability at the plasma membrane. In this study we have used immunolabeling techniques and confocal microscopy to investigate agonist-induced internalization and trafficking of CB(1) receptors in rat cultured hippocampal neurons. The levels of cell surface CB(1) receptor immunoreactivity associated with presynaptic GABAergic terminals decreased markedly (by up to 84%) after exposure to the cannabinoid agonist (+)-WIN55212, in a concentration-dependent (0.1-1 microm) and stereoselective manner. Inhibition was maximal at 16 hr and abolished in the presence of SR141716A, a selective CB(1) receptor antagonist. Methanandamide (an analog of an endogenous cannabinoid, anandamide) also reduced cell surface labeling (by 43% at 1 microm). Differential labeling of cell surface and intracellular pools of receptor demonstrated that the reduction in cell surface immunoreactivity reflects agonist-induced internalization and suggests that the internalized CB(1) receptors are translocated toward the soma. The internalization process did not require activated G-protein alpha(i) or alpha(o) subunits. A different pattern of cell surface CB(1) receptor expression was observed using an undifferentiated F-11 cell line, which had pronounced somatic labeling. In these cells substantial CB(1) receptor internalization was also observed after exposure to (+)-WIN55212 (1 microm) for relatively short periods (30 min) of agonist exposure. In summary, this dynamic modulation of CB(1) receptor expression may play an important role in the development of cannabinoid tolerance in the CNS. Agonist-induced internalization at presynaptic terminals has important implications for the modulatory effects of G-protein-coupled receptors on neurotransmitter release.

Localisation of cannabinoid receptors in the rat brain using antibodies to the intracellular C-terminal tail of CB

The CB(1)-type cannabinoid receptor mediates physiologic effects of Delta(9)-tetrahydrocannabinol, the psychoactive ingredient of the drug marijuana. In this report, the authors analyse the expression of CB(1) in the rat brain by using antibodies to the C-terminal 13 amino acids of the receptor. Western blot analysis of rat brain membranes revealed a prominent immunoreactive band with a molecular mass ( approximately 53 kDa) consistent with that predicted for CB(1) from the rat cDNA sequence. In addition, however, less intense immunoreactive bands corresponding to glycosylated ( approximately 62 kDa) and putative N-terminally shorter ( approximately 45 kDa) isoforms of CB(1) were detected. The distribution of CB(1)-immunoreactivity in rat brain was similar to the distribution of binding sites for radiolabelled cannabinoids, with high levels of expression in the olfactory system, the hippocampal formation, the basal ganglia, the cerebellum, and the neocortex. This provides important evidence that CB(1) is likely to be largely responsible for mediating effects of cannabinoids in the brain. CB(1) immunoreactivity was associated with nerve fibre systems and axon terminals but was not detected in neuronal somata. This is consistent with the presynaptic inhibitory effects of cannabinoids on neurotransmitter release in the brain. Detailed immunocytochemical analysis of anatomically or functionally related regions of the brain revealed the location of CB(1) receptors within identified neural circuits. Determination of the cellular and subcellular location of CB(1) within known neuronal circuits of the brain provides an anatomic framework for interpretation of the neurophysiologic and behavioural effects of cannabinoids.

Cannabinoid CB(1) receptor expression in rat spinal cord

While evidence implicates the endogenous cannabinoid system as a novel analgesic target at a spinal level, detailed analysis of the distribution of the cannabinoid receptor CB(1) in spinal cord has not been reported. Here, immunocytochemical studies were used to characterize the CB(1) receptor expression in rat spinal cord. Staining was found in the dorsolateral funiculus, the superficial dorsal horn (a double band of CB(1) immunoreactivity (ir) in laminae I and II inner/III transition), and lamina X. Although CB(1)-ir was present in the same laminae as primary afferent nociceptor markers, there was limited colocalization at an axonal level. Interruption of both primary afferent input by dorsal root rhizotomy and descending input by rostral spinal cord hemisection produced minor changes in CB(1)-ir. This and colocalization of CB(1)-ir with interneurons expressing protein kinase C subunit gamma-ir suggest that the majority of CB(1) expression is on spinal interneurons. These data provide a framework and implicate novel analgesic mechanisms for spinal actions of cannabinoids at the CB(1) receptor.

The human eye expresses high levels of CB1 cannabinoid receptor mRNA and protein

We used reverse transcriptase polymerase chain reaction to detect the expression of the central and peripheral cannabinoid receptors (CB1 and CB2, respectively) mRNA, and Western blotting to show the presence of the CB1 protein in subregions of the human eye. CB2 mRNA transcripts were undetectable, while levels of CB1 mRNA were significantly expressed in the human retina (25.8 +/- 2.46%), ciliary body (210 +/- 11.55%) and iris (62.7 +/- 5.94%) when compared with those of the normalizing reference gene beta2 microglobulin. The CB1 gene encodes a functional protein which is detected in its glycosylated (63 kDa) and unglycosylated (54 kDa) form in the same areas by a specific purified antibody raised against the amino terminus (residues 1-77) of the CB1 receptor. These results further support the proposed role of the CB1 receptor in controlling intraocular pressure, helping to explain the antiglaucoma properties of marijuana.

Immunocytochemical localization of cannabinoid CB1 receptor and fatty acid amide hydrolase in rat retina

Cannabinoids have major effects on central nervous system function. Recent studies indicate that cannabinoid effects on the visual system have a retinal component. Immunocytochemical methods were used to localize cannabinoid CB1 receptor immunoreactivity (CB1R-IR) and an endocannabinoid (anandamide and 2-arachidonylglycerol) degradative enzyme, fatty acid amide hydrolase (FAAH)-IR, in the rat retina. Double labeling with neuron-specific markers permitted identification of cells that were labeled with CB1R-IR and FAAH-IR. CB1R-IR was observed in all cells that were protein kinase C-immunoreactive (rod bipolar cells and a subtype of GABA-amacrine cell) as well as horizontal cells (identified by calbindin-IR). There was also punctate CB1R-IR in the distal one-third of the inner plexiform layer (IPL) that could not be assigned to a cell type. FAAH-IR was most prominent in large ganglion cells, whose dendrites projected to a narrow band in the proximal IPL. Weaker FAAH-IR was observed in the soma of horizontal cells (identified by calbindin-IR); the soma of large, but not small, dopamine amacrine cells (identified by tyrosine hydroxylase-IR); and dendrites of orthotopic- and displaced-starburst amacrine cells (identified by choline acetyltransferase-IR) but in less than 50% of the starburst amacrine cell somata. The extensive distribution of CB1R-IR on horizontal cells and rod bipolar cells indicates a role of endocannabinoids in scotopic vision, whereas the more widespread distribution of FAAH-IR indicates a complex control of endocannabinoid release and degradation in the retina.

The effects of cannabinoids on the brain

Cannabinoids have a long history of consumption for recreational and medical reasons. The primary active constituent of the hemp plant Cannabis sativa is delta9-tetrahydrocannabinol (delta9-THC). In humans, psychoactive cannabinoids produce euphoria, enhancement of sensory perception, tachycardia, antinociception, difficulties in concentration and impairment of memory. The cognitive deficiencies seem to persist after withdrawal. The toxicity of marijuana has been underestimated for a long time, since recent findings revealed delta9-THC-induced cell death with shrinkage of neurons and DNA fragmentation in the hippocampus. The acute effects of cannabinoids as well as the development of tolerance are mediated by G protein-coupled cannabinoid receptors. The CB1 receptor and its splice variant CB1A, are found predominantly in the brain with highest densities in the hippocampus, cerebellum and striatum. The CB2 receptor is found predominantly in the spleen and in haemopoietic cells and has only 44% overall nucleotide sequence identity with the CB1 receptor. The existence of this receptor provided the molecular basis for the immunosuppressive actions of marijuana. The CB1 receptor mediates inhibition of adenylate cyclase, inhibition of N- and P/Q-type calcium channels, stimulation of potassium channels, and activation of mitogen-activated protein kinase. The CB2 receptor mediates inhibition of adenylate cyclase and activation of mitogen-activated protein kinase. The discovery of endogenous cannabinoid receptor ligands, anandamide (N-arachidonylethanolamine) and 2-arachidonylglycerol made the notion of a central cannabinoid neuromodulatory system plausible. Anandamide is released from neurons upon depolarization through a mechanism that requires calcium-dependent cleavage from a phospholipid precursor in neuronal membranes. The release of anandamide is followed by rapid uptake into the plasma and hydrolysis by fatty-acid amidohydrolase. The psychoactive cannabinoids increase the activity of dopaminergic neurons in the ventral tegmental area-mesolimbic pathway. Since these dopaminergic circuits are known to play a pivotal role in mediating the reinforcing (rewarding) effects of the most drugs of abuse, the enhanced dopaminergic drive elicited by the cannabinoids is thought to underlie the reinforcing and abuse properties of marijuana. Thus, cannabinoids share a final common neuronal action with other major drugs of abuse such as morphine, ethanol and nicotine in producing facilitation of the mesolimbic dopamine system.

The CB1 cannabinoid receptor in the brain

The CB1 cannabinoid receptor in brain is a G-protein-coupled receptor that exists as a protein possessing seven transmembrane helices that span the membrane. The intracellular surface is able to interact with f1p4oteins of the Gi/o family to regulate effector proteins, including adenylate cyclase, Ca2+ channels, and K+ channels, and to stimulate the mitogen-activated protein kinase pathway. The CB1 cannabinoid receptor recognizes three classes of agonist ligands: cannabinoid, eicosanoid, and aminoalkylindole. These agonist subtypes may interact with the CB1 cannabinoid receptor by some common points of association, yet may have subtle differences in the way that they interact with the receptor protein. This may be evident in the allosteric regulation by monovalent cations and individual agonists. The juxtamembrane region of the C-terminal is able to activate G-proteins. It is proposed that conformational changes in the receptor induced by agonist ligands may alter the conformation or exposure of the juxtamembrane C-terminal region extending from helix VII.

Delta9-tetrahydrocannabinol induces apoptosis in C6 glioma cells

delta9-Tetrahydrocannabinol (THC), the major active component of marijuana, induced apoptosis in C6.9 glioma cells, as determined by DNA fragmentation and loss of plasma membrane asymmetry. THC stimulated sphingomyelin hydrolysis in C6.9 glioma cells. THC and N-acetylsphingosine, a cell-permeable ceramide analog, induced apoptosis in several transformed neural cells but not in primary astrocytes or neurons. Although glioma C6.9 cells expressed the CBI cannabinoid receptor, neither THC-induced apoptosis nor THC-induced sphingomyelin breakdown were prevented by SR141716, a specific antagonist of that receptor. Results thus show that THC-induced apoptosis in glioma C6.9 cells may rely on a CBI receptor-independent stimulation of sphingomyelin breakdown.

Pharmacological evidence for anandamide amidase in human cardiac and vascular tissues

The present report demonstrates the presence of antianandamide and anticannabinoid receptor 1 immunopositive material on the saphenous vascular endothelium. The endogenous cannabinoid, anandamide, in a dose-dependent manner stimulated the release of nitric oxide (NO) from saphenous vein, internal thoracic artery and right atrium tissue segments in vitro. This process can be antagonized by the nitric oxide synthase (NOS) inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME) (10(-4) M; 3.4+/-0.9 nM NO; P<0.01 compared to anandamide alone), as well as by the cannabinoid receptor I antagonist SR 141716A (2.9+/-1.0 nM NO; P<0.01). Furthermore, in the presence of varying concentrations of methylarachidonylfluorophosphonate, an anandamide amidase inhibitor, 10(-8) M anandamide stimulates a higher peak level of NO that remains elevated for a longer period of time (P<0.05) compared to anandamide alone, demonstrating the presence of anandamide amidase in human vascular tissues. Morphine, as anandamide, can stimulate the release of NO from right atria. This process can be inhibited by the opiate receptor antagonist naloxone and the NOS inhibitor L-NAME. As expected SR 141716A (10(-6) M; 26+3.8 NO nM in the presence of 10(-7) M morphine) did not antagonize morphine's ability to release NO. Taken together, the data demonstrate that cannabinoid signalling is involved with the regulation of the microvascular environment.

Immunohistochemical distribution of cannabinoid CB1 receptors in the rat central nervous system

Immunohistochemical distribution of cannabinoid receptors in the adult rat brain was studied using specific purified antibodies against the amino-terminus of the CB1 receptor. Our results generally agree well with the previous studies using CB1 receptor autoradiography and messenger RNA in situ hybridization. However, because of its greater resolution, immunohistochemistry allowed identification of particular neuronal cells and fibers that possess cannabinoid receptors. CB1-like immunoreactivity was found in axons, cell bodies and dendrites, where it appeared as puncta in somata and processes. Both intensely and moderately or lightly stained neurons were observed. The intensely stained neurons were dispersed and only occur in cortical structures including hippocampal formation and olfactory bulb. Moderately or lightly stained neurons were found in caudate-putamen and amygdala. In the hippocampal formation only intensely stained neurons were observed. The cell bodies of pyramidal neurons in CA1 and CA3 fields appeared to be unstained but surrounded by a dense plexus of immunoreactive fibers. The granule cells in the dentate area were also immunonegative. Many intensely stained neurons were located at the base of the granule cell layer. CB1-like immunoreactive neurons and fibers were also found in the somatosensory, cingulate, perirhinal, entorhinal and piriform cortices, in claustrum, amygdaloid nuclei, nucleus accumbens and septum. Beaded immunoreactive fibers were detected in periaqueductal gray, nucleus tractus solitarius, spinal trigeminal tract and nucleus, dorsal horn and lamina X of the spinal cord. A triangular cap-like mass of immunoreactivity was found to surround the basal part of the Purkinje cell body in the cerebellum. Only small, lightly stained cells were found in the molecular layer in the cerebellum close to the Purkinje cell layer. The CB1 receptor is widely distributed in the forebrain and has a more restricted distribution in the hindbrain and the spinal cord. It appears to be expressed on cell bodies, dendrites and axons. According to the location and morphology, many, but not all, CB1-like immunoreactive neurons appear to be GABAergic. Therefore, cannabinoids and cannabinoid receptors may play a role in modulating GABAergic neurons.

Characterization of CB1 cannabinoid receptors using receptor peptide fragments and site-directed antibodies

The mechanism by which CB1 cannabinoid receptors are coupled to the Gi/Go class of G proteins was studied. A peptide representing the juxtamembrane carboxyl terminus robustly stimulated guanosine-5'-O-(3-thio)triphosphate binding. Peptides simulating subdomains of the third intracellular loop (IL3) activated minimally when present alone but produced additive effects when present in combination. Peptides representing the amino-side IL3 and the juxtamembrane carboxyl terminus autonomously inhibited adenylate cyclase, and this response was not significantly augmented or inhibited by peptides representing other intracellular domains. Site-directed antipeptide antibodies developed against the domains of the amino terminus, first extracellular loop, amino-side IL3, and juxtamembrane carboxyl terminus of CB1 receptors failed to influence binding of [3H]CP-55940. However, IgG raised against the amino-side IL3 diminished the agonist-dependent inhibition of adenylate cyclase. These experiments suggest that the juxtamembrane carboxyl terminus is critical for G protein activation by CB1 cannabinoid receptors and that the amino-side IL3 also may interact with Gi proteins leading to inhibition of adenylate cyclase.