Cannabinoid Receptor 1 Receptor Research Articles

End of the line for cannabinoid receptor 1 as an anti-obesity target? An opinion

The recent News and Analysis article, “End of the line for cannabinoid receptor 1 as an antiobesity target?” (Nature Rev. Drug Discov. 7, 961–963; 2008)1 highlighted the downfall of cannabinoid receptor 1 (CB1) blockers for the treatment of obesity, largely owing to psychiatric side effects. In this correspondence, we consider whether it seems possible to save the CB1 blocking strategy, and the steps that might be needed to achieve this. First, CB1 blockers that do not cross the blood–brain barrier need to be developed. Their application to pathologies in which blockade of CB1 receptors in the central nervous system is required would therefore be excluded, limiting the anti-obesity efficacy to an improvement of metabolic factors that can lead to diabetes and cardiovascular diseases, independently of central control. In this respect, it is noteworthy that peripheral blockade of CB1 receptors has been reported to have a therapeutic effect in several pathologies, such as hepatic fibrosis, chronic inflammatory conditions, diabetes and cancer2,3. So, in these diseases, highly selective peripheral CB1 antagonists, the use of lower doses or local administration of CB1 blockers could be strategies to minimize psychotropic side effects. The potential of targeting the CB1 receptor in cancer therapy is particularly intriguing. CB1 blockers might be used in combination with chemotherapy, in order to reduce doses, avoid resistance and exert a more potent clinical effect. Interestingly, obesity is a well-recognized factor associated with both increased cancer incidence and unfavourable prognosis in several cancers4. Obesity, the weight gain following chemotherapy and the frequency of cancer recurrence linked to this weight gain make strategies to mitigate the adverse metabolic effects of obesity a priority for cancer prevention and treatment. So, CB1 blockers, which possess anti-obesity, anti-inflammatory and anticancer properties, might be a promising treatment option. Moreover, it would be interesting to perform observational studies on patients treated with CB1 blockers for a substantial period of time, to determine whether there is an association between the therapy and the incidence of all and site-specific cancers, which might indicate a protective effect of CB1 blockade. Further studies are needed to assess the potential of CB1 blockers as anticancer drugs, as well as their possible application in other peripheral pathologies. For example, recent studies have provided evidence of a role for CB1 receptor signalling in angiogenesis5. Because angiogenesis is a fundamental process in cancer progression and in the pathogenesis of ocular, joint and skin disorders, for which a locally delivered therapy is End of the line for cannabinoid receptor 1 as an anti-obesity target? An opinion

A Cytochrome P450‐derived Epoxygenated Metabolite of Anandamide is a Potent, Cannabinoid Receptor 2 Selective Agonist

Oxidation of the endocannabinoid anandamide by cytochrome P450 (P450) enzymes has the potential to affect signaling pathways within the endocannabinoid system and pharmacological responses to novel drug candidates targeting this system. We previously reported that the human cytochrome P450s 4F2, 3A4 and 2D6 are high‐affinity, high‐turnover anandamide oxygenases in vitro, forming the novel metabolites hydroxyeicosatetraenoic acid ethanolamides (HETE‐EAs) and epoxyeicosatrienoic acid ethanolamides (EET‐EAs). The objective of this study was to investigate the possible biological significance of this metabolic pathway. We report that the anandamide epoxide 5,6‐EET‐EA is a potent and selective cannabinoid receptor 2 (CB2) agonist. The Ki values for binding of 5,6‐EET‐EA to membranes from Chinese hamster ovary (CHO) cells recombinantly expressing either the human CB1 of the CB2 receptor were 11.4 μM and 8.9 nM, respectively. In addition, 5,6‐EET‐EA inhibited the forskolin‐stimulated cyclic AMP accumulation in CHO cells stably expressing the CB2 receptor (IC50 = 9.8 ± 1.3 nM). Within the central nervous system the CB2 receptor is expressed on activated microglia and it is considered a possible target for the treatment of neuroinflammation. BV‐2 microglial cells stimulated with low doses of interferon‐gamma exhibited increased capacity for converting anandamide to 5,6‐EET‐EA which correlated with increased protein expression of microglial P450s 4F13 and 3A1. Finally, we demonstrate that 5,6‐EET‐EA is more stable than anandamide in brain homogenate and it is primarily metabolized by epoxide hydrolase. Combined, our results suggest that epoxidation of anandamide by P450s to form 5,6‐EET‐EA represents an endocannabinoid bioactivation pathway in context of immune cell function.

Increased sensitivity to AM251 in aged rats in the prevention of high‐fat diet‐induced hyperphagia

Energy balance is regulated, in part, by the leptin and endocannabinoid (EC) systems. These systems interact, in that leptin inhibits endocannabinoid levels in the hypothalamus. Little is known about the EC system in aged, leptin resistant rats. Three‐ and 30‐month‐old male F344xBN rats were given ad libitum access to chow or 60% high‐fat (HF) diet during 6‐day daily i.p. administration of cannabinoid receptor‐1 (CB1) antagonist, AM251, and HF diet‐induced hyperphagia was examined. In young, the lower (0.83mg/kg/day) and higher (2.78mg/kg/day) doses of AM251 reduced the peak high‐fat diet‐induced hyperphagia by 24% and 100%, respectively. In aged, the lower and higher doses reduced the peak by 12% and 98%, respectively. However, when the total period of hyperphagia was considered, the low dose did not decrease cumulative caloric intake in young, but reduced it by 45% in aged rats. The lower dose of AM251 did not affect the length of hyperphagia period in young, but the period was reduced from 30 to only 6 days in aged rats. These data indicate that aged rats are more sensitive to AM251. One possible mechanism is that aged rats, which are leptin resistant, lack the leptin negative regulation of endocannabinoid levels leading to hyperactive CB1 receptors, thus increasing the susceptibility to CB1 blockade in aged rats. Supported by NIH AG‐26159, P30 AG028740, and AG00196.

CNR1 Gene is Associated with High Neuroticism and Low Agreeableness and Interacts with Recent Negative Life Events to Predict Current Depressive Symptoms

Cannabinoid receptor 1 (CB1) gene (CNR1) knockout mice are prone to develop anhedonic and helpless behavior after chronic mild stress. In humans, the CB1 antagonist rimonabant increases the risk of depressed mood disorders and anxiety. These studies suggest the hypothesis that genetic variation in CB1 receptor function influences the risk of depression in humans in response to stressful life events. In a population sample (n=1269), we obtained questionnaire measures of personality (Big Five Inventory), depression and anxiety (Brief Symptom Inventory), and life events. The CNR1 gene was covered by 10 SNPs located throughout the gene to determine haplotypic association. Variations in the CNR1 gene were significantly associated with a high neuroticism and low agreeableness phenotype (explained variance 1.5 and 2.5%, respectively). Epistasis analysis of the SNPs showed that the previously reported functional 5' end of the CNR1 gene significantly interacts with the 3' end in these phenotypes. Furthermore, current depression scores significantly associated with CNR1 haplotypes but this effect diminished after covariation for recent life events, suggesting a gene x environment interaction. Indeed, rs7766029 showed highly significant interaction between recent negative life events and depression scores. The results represent the first evidence in humans that the CNR1 gene is a risk factor for depression--and probably also for co-morbid psychiatric conditions such as substance use disorders--through a high neuroticism and low agreeableness phenotype. This study also suggests that the CNR1 gene influences vulnerability to recent psychosocial adversity to produce current symptoms of depression.

A Cytochrome P450-Derived Epoxygenated Metabolite of Anandamide Is a Potent Cannabinoid Receptor 2-Selective Agonist

Oxidation of the endocannabinoid anandamide by cytochrome P450 (P450) enzymes has the potential to affect signaling pathways within the endocannabinoid system and pharmacological responses to novel drug candidates targeting this system. We previously reported that the human cytochromes P450 2D6, 3A4, and 4F2 are high-affinity, high-turnover anandamide oxygenases in vitro, forming the novel metabolites hydroxyeicosatetraenoic acid ethanolamides and epoxyeicosatrienoic acid ethanolamides. The objective of this study was to investigate the possible biological significance of these metabolic pathways. We report that the 5,6-epoxide of anandamide, 5,6-epoxyeicosatrienoic acid ethanolamide (5,6-EET-EA), is a potent and selective cannabinoid receptor 2 (CB2) agonist. The K(i) values for the binding of 5,6-EET-EA to membranes from Chinese hamster ovary (CHO) cells expressing either recombinant human CB1 or CB2 receptor were 11.4 microM and 8.9 nM, respectively. In addition, 5,6-EET-EA inhibited the forskolin-stimulated accumulation of cAMP in CHO cells stably expressing the CB2 receptor (IC(50) = 9.8 +/- 1.3 nM). Within the central nervous system, the CB2 receptor is expressed on activated microglia and is a potential therapeutic target for neuroinflammation. BV-2 microglial cells stimulated with low doses of interferon-gamma exhibited an increased capacity for converting anandamide to 5,6-EET-EA, which correlated with increased protein expression of microglial P450 4F and 3A isoforms. Finally, we demonstrate that 5,6-EET-EA is more stable than anandamide in mouse brain homogenates and is primarily metabolized by epoxide hydrolase. Combined, our results suggest that epoxidation of anandamide by P450s to form 5,6-EET-EA represents an endocannabinoid bioactivation pathway in the context of immune cell function.

The Role of Protein Kinase A in the Ethanol-Induced Increase in Spontaneous GABA Release Onto Cerebellar Purkinje Neurons

Ethanol increases miniature inhibitory postsynaptic current frequency and decreases the paired-pulse ratio, which suggests that ethanol increases both spontaneous and evoked GABA release, respectively. We have shown previously that ethanol increases GABA release at the rat interneuron-Purkinje cell synapse and that this ethanol effect involves calcium release from internal stores; however, further exploration of the mechanism responsible for ethanol-enhanced GABA release was needed. We found that a cannabinoid receptor 1 (CB1) agonist, WIN-55212, and a GABA(B) receptor agonist, baclofen, decreased baseline spontaneous GABA release and prevented ethanol from increasing spontaneous GABA release. The CB1 receptor and GABA(B) receptor are Galpha i-linked G protein-coupled receptors with common downstream messengers that include adenylate cyclase and protein kinase A (PKA). Adenylate cyclase and PKA antagonists blocked ethanol from increasing spontaneous GABA release, whereas a PKA antagonist limited to the postsynaptic neuron did not block ethanol from increasing spontaneous GABA release. These results suggest that presynaptic PKA plays an essential role in ethanol-enhanced spontaneous GABA release. Similar to ethanol, we found that the mechanism of the cannabinoid-mediated decrease in spontaneous GABA release involves internal calcium stores and PKA. A PKA antagonist decreased baseline spontaneous GABA release. This effect was reduced after incubating the slice with a calcium chelator, BAPTA-AM, but was unaffected when BAPTA was limited to the postsynaptic neuron. This suggests that the PKA antagonist is acting through a presynaptic, calcium-dependent mechanism to decrease spontaneous GABA release. Overall, these results suggest that PKA activation is necessary for ethanol to increase spontaneous GABA release.

Reciprocal changes in vanilloid (TRPV1) and endocannabinoid (CB1) receptors contribute to visceral hyperalgesia in the water avoidance stressed rat

Background:Increasing evidence suggests that chronic stress plays an important role in the pathophysiology of several functional gastrointestinal disorders. We investigated whether cannabinoid receptor 1 (CB1) and vanilloid receptor 1 (TRPV1;...

Cannabinoid receptor CB2 localisation and agonist-mediated inhibition of capsaicin responses in human sensory neurons

Cannabinoid receptor 2 (CB2) agonists provide the potential for treating chronic pain states without CNS effects associated with CB1 receptor activation. Animal models suggest that they act mainly via non-neuronal cells, possibly inhibition of inflammatory cells in the periphery or CNS, or via release of β-endorphin; however, the clinical relevance and mechanism of analgesic action is uncertain. Here, we demonstrate colocalisation of CB2 with CB1 and the capsaicin receptor TRPV1 in human dorsal root ganglion (DRG) sensory neurons and increased levels of CB2 receptors in human peripheral nerves after injury, particularly painful neuromas. In primary cultures of human DRG neurons, selective CB2 agonists blocked activation of inward cation currents and elevation of cytoplasmic Ca 2+ in response to capsaicin. These inhibitory effects were reversed by GW818646X a CB2 antagonist, and 8-bromo cAMP, but not by SR141716 a CB1 antagonist, or naloxone. Thus CB2 receptor agonists functionally inhibited nociceptive signalling in human primary sensory neurons via a mechanism shared with opioids, of adenylyl cyclase inhibition, but not via μ-opioid receptors. We conclude that CB2 agonists deserve imminent clinical trials for nociceptive, inflammatory and neuropathic chronic pain, in which capsaicin or heat-activated responses via TRPV1 may provide a clinical marker.

The CB1 Endocannabinoid System Modulates Adipocyte Insulin Sensitivity

Mounting evidence suggests that the endocannabinoid system regulates energy metabolism through direct effects on peripheral tissues as well as central effects that regulate appetite. Here we examined the effect of cannabinoid receptor 1 (CB1) signaling on insulin action in fat cells. We examined effects of the natural CB1 agonist, 2-Arachidonoylglycerol (2-AG), and the synthetic CB1 antagonist, SR141716, on insulin action in cultured adipocytes. We used translocation of glucose transporter GLUT4 to plasma membrane (PM) as a measure of insulin action. 2-AG activation of the CB1 receptor promoted insulin sensitivity whereas antagonism by SR141716 reduced insulin sensitivity. Neither drug affected GLUT4 translocation in the absence of insulin or with high doses of insulin. Consistent with these results we found that insulin-stimulated phosphorylation of the protein kinase Akt was increased by 2-AG, attenuated by SR141716, and unaffected in the absence of insulin or by addition of high-dose insulin. These data provide a functional and molecular link between the CB1 receptor and insulin sensitivity, because insulin-stimulated phosphorylation of Akt is required for GLUT4 translocation to the PM. The sensitizing effects of 2-AG were abrogated by SR141716 and Pertussis toxin, indicating that the effects are mediated by CB1 receptor. Importantly, neither 2-AG nor SR141716 alone or in combination with maximal dose of insulin had effects on GLUT4 translocation and Akt phosphorylation. These data are consistent with a model in which the endocannabinoid system sets the sensitivity of the insulin response in adipocytes rather than directly regulating the redistribution of GLUT4 or Akt phosphorylation.

Design and Synthesis of a Novel Series of N-Alkyl Isatin Acylhydrazone Derivatives that Act as Selective Cannabinoid Receptor 2 Agonists for the Treatment of Neuropathic Pain

There is growing interest in using cannabinoid receptor 2 (CB2) agonists for the treatment of neuropathic pain. We have synthesized a novel series of N-alkyl isatin acylhydrazone derivatives and have identified and characterized several of them as novel analogues with high functional activity and selectivity at human CB2 receptors using [(35)S]GTP-gamma-S assays. Binding affinities at human CB2 and CB1 were determined for compounds 28, 33, 40, 48, and 58. Structure-activity relationship studies of this novel series led to optimization of our lead compound, compound 33 (MDA19). Compound 33 possessed potent antiallodynic effects in a rat model of neuropathic pain but did not affect rat locomotor activity. More potent and more CB2-receptor-selective compounds, including compounds 37, 40, and 48, were also discovered.

Increase of brain endocannabinoid anandamide levels by FAAH inhibition and alcohol abuse behaviours in the rat

A major clinical concern with the use of cannabinoid receptor 1 (CB1) direct agonists is that these compounds increase alcohol drinking and drug abuse-related behaviours. As an alternative approach, CB1-receptor-mediated activity can be facilitated by increasing anandamide levels with the use of hydrolase fatty acid amide hydrolase (FAAH) inhibitors. Using the selective FAAH inhibitor URB597, we investigated whether activation of the endogenous cannabinoid tone increases alcohol abuse liability, as what happens with the CB1 receptor direct agonists. URB597 was tested on alcohol self-administration in Wistar rats and on homecage alcohol drinking in genetically selected Marchigian Sardinian alcohol-preferring (msP) rats. In Wistar rats, URB597 effects on alcohol-induced anxiety and on stress-, yohimbine- and cue-induced reinstatement of alcohol seeking were also evaluated. For comparison, the effect of the CB1 receptor antagonist rimonabant on ethanol self-administration was also tested. Under our experimental condition, intraperitoneal (IP) administration of URB597 (0.0, 0.3 and 1.0 mg/kg) neither increased voluntary homecage alcohol drinking in msP rats nor facilitated fixed ratio 1 and progressive ratio alcohol self-administration in nonselected Wistars. In the reinstatement tests, the compound did not have effects on cue-, footshock stress- and yohimbine-induced relapse. Conversely, URB597 completely abolished the anxiogenic response measured during withdrawal after an acute IP administration of alcohol (3.0 g/kg). Rimonabant (0.0, 0.3, 1.0 and 3.0 mg/kg) significantly reduced ethanol self-administration. Results demonstrate that activation of the endocannabinoid anandamide system by selective inhibition of FAAH does not increase alcohol abuse risks but does reduce anxiety associated to alcohol withdrawal. We thus can speculate that medication based on the use of endocannabinoid system modulators such as URB597 may offer important advantages compared to treatment with direct CB1 receptor activators.

Biosynthesis, purification, and characterization of a cannabinoid receptor 2 fragment (CB2 271–326)

Obtaining sufficient amount of purified G-protein coupled receptors (GPCRs) is almost always one of the major challenges for their structural studies. CB2 271–326, a human cannabinoid receptor 2 (CB2) fragment comprising part of the third extracellular loop (EL3), the seventh transmembrane domain (TM7) and C-terminal juxtamembrane region of the receptor, was over-expressed as a fusion protein into inclusion body (IB) of Escherichia coli. The fusion protein was purified by histidine-selected nickel affinity chromatography under denaturing conditions. Then, the fusion protein IBs were solubilized in detergent (Brij58) and the expression fusion leader sequence (TrpLE) was specifically cleaved with tobacco etch virus (TEV) protease. The target fragment, CB2 271–326, was subsequently purified by reverse-phase HPLC and confirmed by SDS–PAGE and mass spectrometry. This hydrophobic fragment can refold in mild detergents digitonin and Brij58. Circular dichroism (CD) spectroscopy of CB2 271–326 in digitonin and Brij58 micelles showed that the fragment adopts a more than 75% α-helical structure, with the remainder having β-strand structure. Fluorescence spectroscopy and quenching studies suggested that the C-terminal region lies near the surface of the digitonin micelles and the TM7 region is folded relatively close to the center of the micelles. This study may provide an alternative strategy for the production and structure/functional studies of GPCRs such as CB2 receptor protein produced in the form of IBs.

New Drugs for Type 2 Diabetes Mellitus

Oral therapy for type 2 diabetes mellitus, when used appropriately, can safely assist patients to achieve glycaemic targets in the short to medium term. However, the progressive nature of type 2 diabetes usually requires a combination of two or more oral agents in the longer term, often as a prelude to insulin therapy. Issues of safety and tolerability, notably weight gain, often limit the optimal application of anti-diabetic drugs such as sulfonylureas and thiazolidinediones. Moreover, the impact of different drugs, even within a single class, on the risk of long-term vascular complications has come under scrutiny. For example, recent publication of evidence suggesting potential detrimental effects of rosiglitazone on myocardial events generated a heated debate and led to a reduction in use of this drug. In contrast, current evidence supports the view that pioglitazone has vasculoprotective properties. Both drugs are contraindicated in patients who are at risk of heart failure. An additional recently identified safety concern is an increased risk of fractures, especially in postmenopausal women.Several new drugs with glucose-lowering efficacy that may offer certain advantages have recently become available. These include (i) injectable glucagon-like peptide-1 (GLP-1) receptor agonists and oral dipeptidyl peptidase-4 (DPP-4) inhibitors; (ii) the amylin analogue pramlintide; and (iii) selective cannabinoid receptor-1 (CB1) antagonists. GLP-1 receptor agonists, such as exenatide, stimulate nutrient-induced insulin secretion and reduce inappropriate glucagon secretion while delaying gastric emptying and reducing appetite. These agents offer a low risk of hypoglycaemia combined with sustained weight loss. The DPP-4 inhibitors sitagliptin and vildagliptin are generally weight neutral, with less marked gastrointestinal adverse effects than the GLP-1 receptor agonists. Potential benefits of GLP-1 receptor stimulation on beta cell neogenesis are under investigation. Pancreatitis has been reported in exenatide-treated patients. Pramlintide, an injected peptide used in combination with insulin, can reduce insulin dose and bodyweight. The CB1 receptor antagonist rimonabant promotes weight loss and has favourable effects on aspects of the metabolic syndrome, including the hyperglycaemia of type 2 diabetes. However, in 2007 the US FDA declined approval of rimonabant, requiring more data on adverse effects, notably depression. The future of dual peroxisome proliferator-activated receptor-alpha/gamma agonists, or glitazars, is presently uncertain following concerns about their safety.In conclusion, several new classes of drugs have recently become available in some countries that offer new options for treating type 2 diabetes. Beneficial or neutral effects on bodyweight are an attractive feature of the new drugs. However, the higher cost of these agents, coupled with an absence of long-term safety and clinical outcome data, need to be taken into consideration by clinicians and healthcare organizations.

Role of cortical cannabinoid CB1 receptor in conditioned taste aversion memory

The brain endocannabinoid system has been shown to play a role in memory, though the extent to which this role generalizes over different types and processes of memory is not yet determined. Here we show that the cannabinoid receptor 1 (CB1) plays differential roles in acquisition, extinction and reconsolidation of conditioned taste aversion (CTA) memory in the rat insular cortex, which contains the taste cortex. Activation of the CB1 receptor in the insular cortex inhibits acquisition and reconsolidation but not extinction, whereas blockade of the CB1 receptor promotes memory and blocks extinction of CTA, while having no apparent effect on reconsolidation. The CB1 ligands used in this study were incapable of substituting the unconditioned stimulus in CTA training. All in all, the data raise the possibility that the state of activity of the CB1 receptor in the insular cortex contributes to the encoding of hedonic valence that enters into association with taste items.

The CB2 cannabinoid agonist AM‐1241 prolongs survival in a transgenic mouse model of amyotrophic lateral sclerosis when initiated at symptom onset

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive motor neuron loss, paralysis and death within 2-5 years of diagnosis. Currently, no effective pharmacological agents exist for the treatment of this devastating disease. Neuroinflammation may accelerate the progression of ALS. Cannabinoids produce anti-inflammatory actions via cannabinoid receptor 1 (CB1) and cannabinoid receptor 2 (CB2), and delay the progression of neuroinflammatory diseases. Additionally, CB2 receptors, which normally exist primarily in the periphery, are dramatically up-regulated in inflamed neural tissues associated with CNS disorders. In G93A-SOD1 mutant mice, the most well-characterized animal model of ALS, endogenous cannabinoids are elevated in spinal cords of symptomatic mice. Furthermore, treatment with non-selective cannabinoid partial agonists prior to, or upon, symptom appearance minimally delays disease onset and prolongs survival through undefined mechanisms. We demonstrate that mRNA, receptor binding and function of CB2, but not CB1, receptors are dramatically and selectively up-regulated in spinal cords of G93A-SOD1 mice in a temporal pattern paralleling disease progression. More importantly, daily injections of the selective CB2 agonist AM-1241, initiated at symptom onset, increase the survival interval after disease onset by 56%. Therefore, CB2 agonists may slow motor neuron degeneration and preserve motor function, and represent a novel therapeutic modality for treatment of ALS.

Cannabinoid CB1 receptor antagonists cause status epilepticus-like activity in the hippocampal neuronal culture model of acquired epilepsy

Status epilepticus (SE) is a major medical emergency associated with a significant morbidity and mortality. Little is known about the mechanisms that terminate seizure activity and prevent the development of status epilepticus. Cannabinoids possess anticonvulsant properties and the endocannabinoid system has been implicated in regulating seizure duration and frequency. Endocannabinoids regulate synaptic transmission and dampen seizure activity via activation of the presynaptic cannabinoid receptor 1 (CB1). This study was initiated to evaluate the role of CB1 receptor-dependent endocannabinoid synaptic transmission towards preventing the development of status epilepticus-like activity in the well-characterized hippocampal neuronal culture model of acquired epilepsy using patch clamp electrophysiology. Application of the CB1 receptor antagonists SR141716A (1 μM) or AM251 (1 μM) to “epileptic” neurons caused the development of continuous epileptiform activity, resembling electrographic status epilepticus. The induction of status epilepticus-like activity by CB1 receptor antagonists was reversible and could be overcome by maximal concentrations of CB1 agonists. Similar treatment of control neurons with CB1 receptor antagonists did not produce status epilepticus or hyperexcitability. These findings suggest that CB1 receptor-dependent endocannabinoid endogenous tone plays an important role in modulating seizure frequency and duration and preventing the development of status epilepticus-like activity in populations of epileptic neurons. The regulation of seizure activity and prevention of status epilepticus by the endocannabinoid system offers an important insight into understanding the basic mechanisms that control the development of continuous epileptiform discharges.

Differential mechanisms mediating depressor and diuretic effects of anandamide

Anandamide (AEA), one of the endocannabinoid compounds, has an important regulatory function by serving as an autocrine/paracrine or endocrine factor throughout the body via activation of the cannabinoid receptor 1 (CB1) and/or the transient receptor potential vanilloid type 1 (TRPV1) channels. However, the role of AEA in the regulation of renal excretory function is largely unknown. The present study was designed to test the hypothesis that intrarenal administration of AEA enhances renal excretory function leading to a decrease in blood pressure. A metabolically stable analog of AEA, methanandamide (MethA, 300 nmol/kg per min), was infused into the left renal medulla of anesthetized Wistar rats with or without a selective TRPV1 antagonist, capsazepine (Capz, 150 nmol/kg per min) or a selective cannabinoid receptor 1 (CB1) antagonist, AM251 (Am, 150 nmol/kg per min). Ureters were cannulated for collection of urine. A laser-Doppler flowmeter was used to determine the changes of blood flow in the cortex and medulla of the infused kidney. In the absence of the changes in cortical and medullary blood flow, unilateral intramedullary infusion of MethA significantly increased urine flow rate by 64% ipsilaterally and 62% contralaterally without changing sodium excretion when compared to vehicle controls (P < 0.01). Neither Capz nor Am blocked the MethA-induced increases in urine flow rate bilaterally. Intramedullary infusion of MethA significantly decreased mean arterial pressure (MAP) (P < 0.01), which was blocked by Am but not Capz. Denervation of the infused kidney blocked the MethA-induced increases in urine flow rate bilaterally without altering MethA-induced decreases in MAP. Therefore, our data show that intramedullary infusion of AEA increases urine volume excretion and decreases blood pressure via distinct operational mechanisms. While activation of the CB1 receptor may underlie AEA-induced depressor effects, the diuretic effect of AEA appears to be mediated by neuronal reflex of the kidney, which is not sensitive to blockade of the TRPV1 or CB1 receptor.

Endocannabinoid Biosynthesis Proceeding through Glycerophospho-N-acyl Ethanolamine and a Role for α/β-Hydrolase 4 in This Pathway

N-Acyl ethanolamines (NAEs) are a large class of signaling lipids implicated in diverse physiological processes, including nociception, cognition, anxiety, appetite, and inflammation. It has been proposed that NAEs are biosynthesized from their corresponding N-acyl phosphatidylethanolamines (NAPEs) in a single enzymatic step catalyzed by a phospholipase D (NAPE-PLD). The recent generation of NAPE-PLD(-/-) mice has revealed that these animals possess lower brain levels of saturated NAEs but essentially unchanged concentrations of polyunsaturated NAEs, including the endogenous cannabinoid anandamide. These findings suggest the existence of additional enzymatic routes for the production of NAEs in vivo. Here, we report evidence for an alternative pathway for NAE biosynthesis that proceeds through the serine hydrolase-catalyzed double-deacylation of NAPE to generate glycerophospho-NAE, followed by the phosphodiesterase-mediated cleavage of this intermediate to liberate NAE. Furthermore, we describe the functional proteomic isolation and identification of a heretofore uncharacterized enzyme alpha/beta-hydrolase 4 (Abh4) as a lysophospholipase/phospholipase B that selectively hydrolyzes NAPEs and lysoNAPEs. Abh4 accepts lysoNAPEs bearing both saturated and polyunsaturated N-acyl chains as substrates and displays a distribution that closely mirrors lysoNAPE-lipase activity in mouse tissues. These results support the existence of an NAPE-PLD-independent route for NAE biosynthesis and suggest that Abh4 plays a role in this metabolic pathway by acting as a (lyso)NAPE-selective lipase.

Mutations of CB1 T210 Produce Active and Inactive Receptor Forms: Correlations with Ligand Affinity, Receptor Stability, and Cellular Localization

Human cannabinoid receptor 1 (CB(1)) has attracted substantial interest as a potential therapeutic target for treating obesity and other obsessive disorders. An understanding of the mechanism governing the transition of the CB(1) receptor between its inactive and active states is critical for understanding how therapeutics can selectively regulate receptor activity. We have examined the importance of the Thr at position 210 in CB(1) in this transition, a residue predicted to be on the same face of the helix as the Arg of the DRY motif highly conserved in the G protein-coupled receptor superfamily. This Thr was substituted with Ile and Ala via mutagenesis, and the receptors, T210I and T210A, were expressed in HEK 293 cells. The T210I receptor exhibited enhanced agonist and diminished inverse agonist affinity relative to the wild type, consistent with a shift toward the active form. However, treatment with GTPgammaS to inhibit G protein coupling diminished the affinity change for the inverse agonist SR141716A. The decreased thermal stability of the T210I receptor and increased level of internalization of a T210I receptor-GFP chimera were also observed, consistent with constitutive activity. In contrast, the T210A receptor exhibited the opposite profile: diminished agonist and enhanced inverse agonist affinity. The T210A receptor was found to be more thermally stable than the wild type, and high levels of a T210A receptor-GFP chimera were localized to the cell surface as predicted for an inactive receptor form. These results suggest that T210 plays a key role in governing the transition between inactive and active CB(1) receptor states.

The unfolding cannabinoid story on energy homeostasis: central or peripheral site of action?

Presentations in this symposium addressed effects and modes of action of endocannabinoids in various tissues in relation to metabolic disorders. Endocannabinoids are produced and exert their effect in various brain sites, including the mesolimbic reward circuitry and the hypothalamus. Both of these regions have direct ties to energy metabolism regulation, particularly food intake and energy expenditure. These data clearly suggest that the observed beneficial effects of CB1 (cannabinoid receptor 1) receptor antagonists on obesity may be related to the central endocannabinoid system. On the other hand, data presented on cannabinoid action in the liver and white adipose tissues clearly indicate that CB1-mediated events in affecting metabolic phenotype may occur in peripheral tissues as well. This together with the reported results from human trials on CB1 antagonists showing that the initial anorectic effect of rimonabant is diminished after the first weeks while longer lasting weight loss is achieved do indicate that peripheral action of cannabinoids are very important in body weight regulation. Should this hold true in the long run, antagonizing CB1 receptors with compound not crossing the blood-brain barrier could revolutionize pharmaceutical approaches to obesity by offering a tool that short cuts the central nervous system.