Effects of 2-arachidonylglycerol, an endogenous cannabinoid, on neuronal activity in rat hippocampal slices

Dual effects of anandamide in the antiepileptic activity of diazepam in pentylenetetrazole-induced seizures in mice

The prototype endocannabinoid, anandamide activates both CB 1 and transient receptor potential vanilloid type 1 channels (TRPV1) receptor at different concentrations. At high concentrations, anandamide-mediated TRPV1 effects are opposite to its effects at low concentrations via CB 1 receptor. Thus, synaptic concentrations of anandamide govern the neuronal activity and consequently might affect the response of a drug. This study was undertaken to investigate the influence of high and low doses of anandamide on the anticonvulsant action of diazepam on the subcutaneous dose of pentylenetetrazole (PTZ) in Swiss mice weighing 20-25 g. Results revealed that intracerebroventricular administration of capsazepine (a TRPV1 antagonist: 1, 10, or 100 µg/mouse) and the low doses (10 µg/mouse) of anandamide, AM404 (anandamide transport inhibitor), or URB597 (fatty acid amide hydrolase inhibitor) augmented the anticonvulsant effect of diazepam. Conversely, higher dose of anandamide, AM404, URB597 (100 µg/mouse) as well as capsaicin (a TRPV1 agonist: 1, 10, or 100 µg/mouse) attenuated the protective effect of diazepam against PTZ-induced seizures. Thus, this study demonstrates that the effects of diazepam may be augmented by activating CB 1 receptors or dampened via TRPV1 receptors. The findings of the present study can be extrapolated to understand the use of TRPV1 blockers alone or in combination of benzodiazepines in the treatment of benzodiazepines-refractory status epilepticus, a condition associated with maladaptive trafficking of synaptic gamma-aminobutyric acid and glutamate receptors. However, potential clinical applications are needed to further support such preclinical studies.

Depolarization-initiated endogenous cannabinoid release and underlying retrograde neurotransmission in interneurons of amygdala

The depolarization is also important for the short-term synaptic plasticity, known as depolarization-induced suppression of excitation (DSE). The two major types of neurons and their synapses in the lateral nucleus of amygdala (LA) are prone to plasticity. However, DSE in interneurons has not been reported in amygdala in general and in LA in particular. Therefore, we conducted the patch-clamp experiments with LA interneurons. These neurons were identified by lack of adaptation in firing rate of action potentials. In this study, we show for the first time a transient suppression of neurotransmission at synapses both within the local network and between cortical inputs and interneurons of the LA. The retrograde neurotransmission from GABAergic interneurons were comparable with that of glutamatergic pyramidal cells. That is the axonal terminals of cortical inputs do not posses selectivity toward two neuronal subtypes. However, the DSE of both types of neurons involve an increase in intracellular Ca²⁺ and the release of endogenous cannabinoids (eCB) and activation of presynaptic CB1 receptors. The magnitude of DSE was significantly higher in interneurons compared with pyramidal cells, though developed with some latency.

Cannabinoids as hippocampal network administrators

Extensive pioneering studies performed in the hippocampus have greatly contributed to our knowledge of an endogenous cannabinoid system comprised of the molecular machinery necessary to process endocannabinoid lipid messengers and their associated cannabinoid receptors. Moreover, a foundation of knowledge regarding the function of hippocampal circuits, and its role in supporting synaptic plasticity has facilitated our understanding of the roles cannabinoids play in the diverse behaviors in which the hippocampus participates, in both normal and pathological states. In this review, we present an historical overview of research pertaining to the hippocampal cannabinoid system to provide context in which to understand the participation of the hippocampus in cognition, behavior, and epilepsy. We also examine potential roles for the hippocampal formation in mediating dysfunctional behavior, and assert that these phenomena reflect disordered physiological activity within the hippocampus and its interactions with other brain regions after exposure to synthetic cannabinoids, and the phytocannabinoids found in marijuana, such as Δ⁹-THC and cannabidiol. In this regard, we examine contemporary hypotheses concerning the hippocampal endocannabinoid system's participation in psychotic disorders, schizophrenia, and epilepsy, and examine cannabinoid-sensitive cellular mechanisms contributing to coherent network oscillations as potential contributors to these disorders. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

Inhibition of monoacylglycerol lipase mediates a cannabinoid 1-receptor dependent delay of kindling progression in mice

Endocannabinoids, including 2-arachidonoylglycerol (2-AG), activate presynaptic cannabinoid type 1 receptors (CB1R) on inhibitory and excitatory neurons, resulting in a decreased release of neurotransmitters. The event-specific activation of the endocannabinoid system by inhibition of the endocannabinoid degrading enzymes may offer a promising strategy to selectively activate CB1Rs at the site of excessive neuronal activation with the overall goal to prevent the development epilepsy. The aim of this study was to investigate the impact of monoacylglycerol lipase (MAGL) inhibition on the development and progression of epileptic seizures in the kindling model of temporal lobe epilepsy. Therefore, we selectively blocked MAGL by JZL184 (8mg/kg, i.p.) in mice to analyze the effects of increased 2-AG levels on kindling acquisition and to exclude an anticonvulsive potential. Our results showed that JZL184 treatment significantly delayed the development of generalized seizures (p=0.0066) and decreased seizure (p<0.0001) and afterdischarge duration (p<0.001) in the kindling model of temporal lobe epilepsy, but caused only modest effects in fully kindled mice. Moreover, we proved that JZL184 treatment had no effects in conditional CB1R knockout mice lacking expression of the receptor in principle neurons of the forebrain. In conclusion, the data demonstrate that indirect CB1R agonism delays the development of generalized epileptic seizures but has no relevant acute anticonvulsive effects. Furthermore, we confirmed that the effects of JZL184 on kindling progression are CB1R mediated. Thus, the data indicate that the endocannabinoid 2-AG might be a promising target for an anti-epileptogenic approach.

Attenuation of kainic acid-induced status epilepticus by inhibition of endocannabinoid transport and degradation in guinea pigs

Status epilepticus (SE) is a medical emergency associated with a high rate of mortality if not treated promptly. Exogenous and endogenous cannabinoids have been shown to possess anticonvulsant properties both in vivo and in vitro. Here we study the influence of endocannabinoid metabolism on the development of kainic acid-induced SE in guinea pigs. For this purpose, the inhibitors of endocannabinoid transport, AM404, and enzymatic (fatty acid amide hydrolase) degradation, URB597, were applied. Cannabinoid CB1 receptor antagonist, AM251, was also tested. Animal behavior as well as local electric field potentials in four structures: medial septum, hippocampus, entorhinal cortex and amygdala were analyzed when AM404 (120nmol), URB597 (4.8nmol) or AM251 (20nmol) were administrated alone or together with 0.4μg of kainic acid. All substances were injected i.c.v. AM404, URB597 or AM251 administered alone did not alter markedly local field potentials of all four studied structures in the long-term compared with their basal activity. AM404 and URB597 significantly alleviated kainic acid-induced SE, decreasing behavioral manifestations, duration of seizure events and SE in general without changing the amplitude of local field potentials. AM251 did not produce distinct effects on SE in terms of our experimental paradigm. There was no apparent change of the seizure initiation pattern when kainic acid was coadministrated with AM404, URB597 or AM251. The present study provides electrophysiologic and behavioral evidences that inhibition of endocannabinoid metabolism plays a protective role against kainic acid-induced SE and may be employed for therapeutic purposes. Further investigations of the influences of cannabinoid-related compounds on SE genesis and especially epileptogenesis are required.

Low-frequency stimulation evokes serotonin release in the nucleus accumbens and induces long-term depression via production of endocannabinoid

The nucleus accumbens (NAc), a major component of the mesolimbic system, is involved in the mediation of reinforcing and addictive properties of many dependence-producing drugs. Glutamatergic synapses within the NAc can express plasticity, including a form of endocannabinoid (eCB)-long-term depression (LTD). Recent evidences demonstrate cross talk between eCB signaling pathways and those of other receptor systems, including serotonin (5-HT); the extensive colocalization of CB1 and 5-HT receptors within the NAc suggests the potential for interplay between them. In the present study, we found that 20-min low-frequency (4 Hz) stimulation (LFS-4Hz) of glutamatergic afferences in rat brain slices induces a novel form of eCB-LTD in the NAc core, which requires 5-HT2 and CB1 receptor activation and L-type voltage-gated Ca2+ channel opening. Moreover, we found that exogenous 5-HT application (5 μM, 20 min) induces an analogous LTD (5-HT-LTD) at the same synapses, requiring the activation of the same receptors and the opening of the same Ca2+ channels; LFS-4Hz-LTD and 5-HT-LTD were mutually occlusive. Present results suggest that LFS-4Hz induces the release of 5-HT, which acts at 5-HT2 postsynaptic receptors, increasing Ca2+ influx through L-type voltage-gated channels and 2-arachidonoylglycerol production and release; the eCB travels retrogradely and binds to presynaptic CB1 receptors, causing a long-lasting decrease of glutamate release, resulting in LTD. These observations might be helpful to understand the neurophysiological mechanisms underlying drug addiction, major depression, and other psychiatric disorders characterized by dysfunction of 5-HT neurotransmission in the NAc.

Neuron to astrocyte communication via cannabinoid receptors is necessary for sustained epileptiform activity in rat hippocampus

Astrocytes are integral functional components of synapses, regulating transmission and plasticity. They have also been implicated in the pathogenesis of epilepsy, although their precise roles have not been comprehensively characterized. Astrocytes integrate activity from neighboring synapses by responding to neuronally released neurotransmitters such as glutamate and ATP. Strong activation of astrocytes mediated by these neurotransmitters can promote seizure-like activity by initiating a positive feedback loop that induces excessive neuronal discharge. Recent work has demonstrated that astrocytes express cannabinoid 1 (CB1) receptors, which are sensitive to endocannabinoids released by nearby pyramidal cells. In this study, we tested whether this mechanism also contributes to epileptiform activity. In a model of 4-aminopyridine induced epileptic-like activity in hippocampal slice cultures, we show that pharmacological blockade of astrocyte CB1 receptors did not modify the initiation, but significantly reduced the maintenance of epileptiform discharge. When communication in astrocytic networks was disrupted by chelating astrocytic calcium, this CB1 receptor-mediated modulation of epileptiform activity was no longer observed. Thus, endocannabinoid signaling from neurons to astrocytes represents an additional significant factor in the maintenance of epileptiform activity in the hippocampus.

Involvement of transient receptor potential vanilloid type 1 channels in the pro-convulsant effect of anandamide in pentylenetetrazole-induced seizures

Anandamide, an endogenous agonist of CB(1) receptors, also activates TRPV1 but at a higher concentration. Studies demonstrate the anticonvulsant activity of anandamide via CB(1) receptors, while its action through TRPV1 is still ambiguous. Thus, the present study investigated the influence of anandamide on pentylenetetrazole-induced seizures in mice pretreated with TRPV1 or CB(1) receptor antagonists. Acute intracerebroventricular administration of low doses of anandamide (10, 20, or 40μg/mouse) produced anticonvulsant effect, while the pro-convulsant effect was evident at high doses (80 or 100μg/mouse). Interestingly, AM251 (2μg/mouse), a CB(1) antagonist pretreatment blocked the anticonvulsant effect, but augmented the pro-convulsant effect. Conversely, in the presence of inactive dose of capsazepine (1μg/mouse), a TRPV1 antagonist, anandamide exhibited significant anticonvulsant effect even at high doses with no change in its anticonvulsant effect. Moreover, mice treated with capsaicin, a TRPV1 agonist (10, or 100μg/mouse) exhibited pro-convulsant activity that was blocked by capsazepine pretreatment. However, capsazepine, per se at doses 10 or 100μg/mouse exhibited anticonvulsant effect. Like anandamide, the agents (AM404 and URB597), which increase its synaptic concentrations produced similar biphasic effects. Thus, these results indicate that anandamide exhibits both pro- and anticonvulsant activities by activating TRPV1 and CB(1) receptor respectively.

Epileptiform activity in the CA1 region of the hippocampus becomes refractory to attenuation by cannabinoids in part because of endogenous γ-aminobutyric acid type B receptor activity

The anticonvulsant properties of marijuana have been known for centuries. The recently characterized endogenous cannabinoid system thus represents a promising target for novel anticonvulsant agents; however, administration of exogenous cannabinoids has shown mixed results in both human epilepsy and animal models. The ability of cannabinoids to attenuate release of both excitatory and inhibitory neurotransmitters may explain the variable effects of cannabinoids in different models of epilepsy, but this has not been well explored. Using acute mouse brain slices, we monitored field potentials in the CA1 region of the hippocampus to characterize systematically the effects of the cannabinoid agonist WIN55212-2 (WIN) on evoked basal and epileptiform activity. WIN, acting presynaptically, significantly reduced the amplitude and slope of basal field excitatory postsynaptic potentials as well as stimulus-evoked epileptiform responses induced by omission of magnesium from the extracellular solution. In contrast, the combination of omission of magnesium plus elevation of potassium induced an epileptiform response that was refractory to attenuation by WIN. The effect of WIN in this model was partially restored by blocking γ-aminobutyric acid type B (GABA(B) ), but not GABA(A) , receptors. Subtle differences in models of epileptiform activity can profoundly alter the efficacy of cannabinoids. Endogenous GABA(B) receptor activation played a role in the decreased cannabinoid sensitivity observed for epileptiform activity induced by omission of magnesium plus elevation of potassium. These results suggest that interplay between presynaptic G protein-coupled receptors with overlapping downstream targets may underlie the variable efficacy of cannabinoids in different models of epilepsy.

Inverse relationship of cannabimimetic (R+)WIN 55, 212 on behavior and seizure threshold during the juvenile period

Cannabinoids have anti-convulsant effects in both in vivo and in vitro models of status epilepticus. Since the development of spontaneous seizures and neuronal vulnerability are age-dependent, we hypothesized that the anti-convulsant effects of cannabimimetics are also age-dependent. We administered a single injection of varied doses of (R+)WIN 55,212 (0.5, 1, 5 mg/kg) to postnatal (P) day 20 rats 90 min prior to induction of kainate (KA)-induced status epilepticus. The highest dose of (R+)WIN 55,212 (5 mg/kg) resulted in rapid onset of behavioral stupor, loss of balance, stiffening and immobility while standing on hind legs or laying flat in prone position; lower doses had minimal or no behavioral effect. After KA administration, seizure scores and electroencephalography (EEG) recordings were inversely related to (R+)WIN 55,212 dosage whereby higher doses were associated with high seizures scores and synchronous epileptiform activity and low doses with low seizure scores and diminished spiking in the EEG. Immunohistochemistry revealed a dose-dependent reduction in CB1 receptor expression with increasing concentrations of (R+)WIN 55,212 in presence or absence of KA seizures. Nissl and NeuN staining showed hippocampal injury was attenuated only when seizures were mild following low doses of WIN 55,212 (0.5, 1 mg/kg), consistent with the level of CB1 expression. Since low doses abolished seizures without psychotropic side-effects further study may facilitate a groundbreaking cannabamimetic therapeutic strategy to treat early-life seizures. Higher doses had adverse effects on behavior and failed to prevent seizures and protect CA1 neurons possibly due to inactivation or loss of CB1 receptors.

Chronic stress differentially regulates cannabinoid CB1 receptor binding in distinct hippocampal subfields

Exposure to chronic stress has been found to decrease cannabinoid CB(1) receptor expression in the hippocampus; however, the specificity of this phenomenon to specific subfields of the hippocampus has not been characterized. To this extent, male Sprague-Dawley rats were exposed to 21 days of restraint stress (6 h/day), after which autoradiographical analysis of cannabinoid CB(1) receptor binding site densities were examined in the CA1, CA3 and dentate gyrus subfields of the hippocampus. Chronic stress was found to produce a significant reduction in cannabinoid CB(1) receptor binding in the dentate gyrus, while increasing cannabinoid CB(1) receptor binding in the CA3. There was no effect of chronic stress on cannabinoid CB(1) receptor binding in the CA1, or two other proximal regions, the retrosplenial cortical gyrus and the laterodorsal thalamus. Given the role of hippocampal cannabinoid CB(1) receptor activity in the maintenance of synaptic integrity and neuroplasticity in the hippocampus, these data suggest that changes in cannabinoid CB(1) receptor activity following stress may contribute to stress-induced modulation of these processes.

Physiological roles of 2-arachidonoylglycerol, an endogenous cannabinoid receptor ligand

2-Arachidonoylglycerol is an arachidonic acid-containing monoacylglycerol isolated from the rat brain and canine gut as an endogenous ligand for the cannabinoid receptors (CB1 and CB2). 2-Arachidonoylglycerol binds to both the CB1 receptor, abundantly expressed in the nervous system, and the CB2 receptor, mainly expressed in the immune system, with high affinity, and exhibits a variety of cannabimimetic activities. Notably, anandamide, another endogenous ligand for the cannabinoid receptors, acts as a partial agonist at these cannabinoid receptors, whereas 2-arachidonoylglycerol acts as a full agonist. The results of structure-activity relationship experiments strongly suggested that 2-arachidonoylglycerol rather than anandamide is the true natural ligand for both the CB1 and the CB2 receptors. Evidence is gradually accumulating which shows that 2-arachidonoylglycerol plays physiologically and pathophysiologically essential roles in various mammalian tissues and cells.

Status epilepticus causes a long-lasting redistribution of hippocampal cannabinoid type 1 receptor expression and function in the rat pilocarpine model of acquired epilepsy

Activation of the cannabinoid type 1 (CB1) receptor, a major G-protein-coupled receptor in brain, acts to regulate neuronal excitability and has been shown to mediate the anticonvulsant effects of cannabinoids in several animal models of seizure, including the rat pilocarpine model of acquired epilepsy. However, the long-term effects of status epilepticus on the expression and function of the CB1 receptor have not been described. Therefore, this study was initiated to evaluate the effect of status epilepticus on CB1 receptor expression, binding, and G-protein activation in the rat pilocarpine model of acquired epilepsy. Using immunohistochemistry, we demonstrated that status epilepticus causes a unique "redistribution" of hippocampal CB1 receptors, consisting of specific decreases in CB1 immunoreactivity in the dense pyramidal cell layer neuropil and dentate gyrus inner molecular layer, and increases in staining in the CA1-3 strata oriens and radiatum. In addition, this study demonstrates that the redistribution of CB1 receptor expression results in corresponding functional changes in CB1 receptor binding and G-protein activation using [3H] R+-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl](1-napthalen-yl)methanone mesylate (WIN55,212-2) and agonist-stimulated [35S]GTPgammaS autoradiography, respectively. The redistribution of CB1 receptor-mediated [35S]GTPgammaS binding was 1) attributed to an altered maximal effect (Emax) of WIN55,212-2 to stimulate [35S]GTPgammaS binding, 2) reversed by the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A), 3) confirmed by the use of other CB1 receptor agonists, and 4) not reproduced in other G-protein-coupled receptor systems examined. These results demonstrate that status epilepticus causes a unique and selective reorganization of the CB1 receptor system that persists as a permanent hippocampal neuronal plasticity change associated with the development of acquired epilepsy.

Depolarization-induced Rapid Generation of 2-Arachidonoylglycerol, an Endogenous Cannabinoid Receptor Ligand, in Rat Brain Synaptosomes

2-arachidonoylglycerol (2-AG) is an endogenous ligand for the cannabinoid receptors with a variety of potent biological activities. In this study, we first examined the effects of potassium-induced depolarization on the level of 2-AG in rat brain synaptosomes. We found that a significant amount of 2-AG was generated in the synaptosomes following depolarization. Notably, depolarization did not affect the levels of other molecular species of monoacylglycerols. Furthermore, the level of anandamide was very low and did not change markedly following depolarization. It thus appeared that the depolarization-induced accelerated generation is a unique feature of 2-AG. We obtained evidence that phospholipase C is involved in the generation of 2-AG in depolarized synaptosomes: U73122, a phospholipase C inhibitor, markedly reduced the depolarization-induced generation of 2-AG, and the level of diacylglycerol was rapidly elevated following depolarization. A significant amount of 2-AG was released from synaptosomes upon depolarization. Interestingly, treatment of the synaptosomes with SR141716A, a CB1 receptor antagonist, augmented the release of glutamate from depolarized synaptosomes. These results strongly suggest that the endogenous ligand for the cannabinoid receptors, i.e. 2-AG, generated through increased phospholipid metabolism upon depolarization, plays an important role in attenuating glutamate release from the synaptic terminals by acting on the CB1 receptor.

Role of the endocannabinoid system in regulating cardiovascular and metabolic risk factors

Increased endocannabinoid (EC) system activity promotes excessive food intake and obesity in animals and humans. The EC system regulates food intake and hedonic reward through central mechanisms located within the hypothalamus and limbic forebrain. In rodent models, cannabinoid1 (CB1) receptor blockade reduces appetite and weight and prevents obesity and insulin resistance. The EC system also regulates food intake and metabolic factors through peripheral CB1 receptors located at multiple sites throughout the body, including adipose tissue, skeletal muscle, liver, and the gastrointestinal (GI) tract. In rodent models, CB1 receptor antagonists act in the liver to decrease lipogenesis, act in the GI tract to increase satiety, and function in adipose tissue to normalize adiponectin levels and reduce fat storage. The CB1 receptor antagonist rimonabant has been shown to reduce food intake and improve metabolic parameters, such as insulin resistance and fatty liver, in animal models of obesity. In preliminary human studies, upregulation of the EC system has been linked to obesity through mechanisms that include high-fat diet, insulin resistance, and genetic malfunction of an EC inactivation enzyme. Evidence suggests that CB1 receptor blockade is a novel therapeutic strategy that addresses the underlying mechanisms of both obesity and cardiometabolic risk.

2-Arachidonoylglycerol Enhances the Phagocytosis of Opsonized Zymosan by HL-60 Cells Differentiated into Macrophage-Like Cells

2-Arachidonoylglycerol is an endogenous ligand for the cannabinoid receptors (CB1 and CB2). While evidence is accumulating that the CB1 receptor plays important regulatory roles in various nervous tissues and cells, the physiological roles of the CB2 receptor, which is abundantly expressed in the immune system, are yet to be determined. In this study, we examined in detail the effect of 2-arachidonoylglycerol on the phagocytosis of opsonized zymosan by HL-60 cells that had differentiated into macrophage-like cells. We found that the addition of 2-arachidonoylglycerol augmented the phagocytosis of opsonized zymosan by the differentiated HL-60 cells. The effect was observed from 1 nM and increased with increasing concentrations of 2-arachidonoylglycerol. Treatment of the cells with SR144528 or pertussis toxin abolished the effect of 2-arachidonoylglycerol, indicating that the CB2 receptor and Gi/o are involved in the augmented phagocytosis. Phosphatidylinositol 3-kinase and extracellular signal-regulated kinase were also suggested to be involved; treatment of the cells with wortmannin or PD98059 abrogated the 2-arachidonoylglycerol-augmented phagocytosis. These results strongly suggest that 2-arachidonoylglycerol, derived from stimulated inflammatory cells, has an important role in augmenting the phagocytosis of invading microorganisms by macrophages/monocytes thereby stimulating inflammatory reactions and immune responses.

Synthesis and biological evaluation of several structural analogs of 2-arachidonoylglycerol, an endogenous cannabinoid receptor ligand

2-Arachidonoylglycerol (2-AG (1)) is an endogenous ligand for the cannabinoid receptors (CB1 and CB2). There is growing evidence that 2-arachidonoylglycerol plays important physiological and pathophysiological roles in various mammalian tissues and cells, though the details remain to be clarified. In this study, we synthesized several remarkable analogs of 2-arachidonoylglycerol, closely related in chemical structure to 2-arachidonoylglycerol: an analog containing an isomer of arachidonic acid with migrated olefins (2-AGA118 (3)), an analog containing a one-carbon shortened fatty acyl moiety (2-AGA113 (4)), an analog containing an one-carbon elongated fatty acyl moiety (2-AGA114 (5)), a hydroxy group-containing analog (2-AGA105 (6)), a ketone group-containing analog (2-AGA109 (7)), and a methylene-linked analog (2-AGA104 (8)). We evaluated their biological activities as cannabinoid receptor agonists using NG108-15 cells which express the CB1 receptor and HL-60 cells which express the CB2 receptor. Notably, these structural analogs of 2-arachidonoylglycerol exhibited only weak agonistic activities toward either the CB1 receptor or the CB2 receptor, which is in good contrast to 2-arachidonoylglycerol which acted as a full agonist at these cannabinoid receptors. These results clearly indicate that the structure of 2-arachidonoylglycerol is strictly recognized by the cannabinoid receptors (CB1 and CB2) and provide further evidence that the cannabinoid receptors are primarily the intrinsic receptors for 2-arachidonoylglycerol.

Endocannabinoids block status epilepticus in cultured hippocampal neurons

Status epilepticus is a serious neurological disorder associated with a significant morbidity and mortality. Antiepileptic drugs such as diazepam, phenobarbital and phenytoin are the mainstay of status epilepticus treatment. However, over 20% of status epilepticus cases are refractory to the initial treatment with two or more antiepileptic drugs. Endocannabinoids have been implicated as playing an important role in regulating seizure activity and seizure termination. This study evaluated the effects of the major endocannabinoids methanandamide and 2-arachidonylglycerol (2-AG) on status epilepticus in the low-Mg(2+) hippocampal neuronal culture model. Status epilepticus in this model was resistant to treatment with phenobarbital and phenytoin. Methanandamide and 2-AG inhibited status epilepticus in a dose-dependent manner with an EC(50) of 145+/-4.15 nM and 1.68+/-0.19 microM, respectively. In addition, the anti-status epilepticus effects of methanandamide and 2-AG were mediated by activation of the cannabinoid CB(1) receptor since they were blocked by the cannabinoid CB(1) receptor antagonist AM251. These results provide the first evidence that the endocannabinoids, methanandamide and 2-AG, are effective inhibitors of refractory status epilepticus in the hippocampal neuronal culture model and indicate that regulating the endocannabinoid system may provide a novel therapeutic approach for treating refractory status epilepticus.

Arachidonyl-2'-chloroethylamide, a highly selective cannabinoid CB1 receptor agonist, enhances the anticonvulsant action of valproate in the mouse maximal electroshock-induced seizure model

Endogenous cannabinoid ligands and cannabinoid CB(1) receptor agonists have been shown to exert potent anticonvulsant effects in various experimental models of epilepsy. The purpose of this study was to determine the effects of arachidonyl-2'-chloroethylamide (ACEA; N-(2-chloroethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide, a highly selective cannabinoid CB(1) receptor agonist) on the threshold for electroconvulsions and the anticonvulsant activity of valproate in the maximal electroshock-induced seizures in mice. To inhibit the rapid metabolic degradation of ACEA by the fatty-acid amide hydrolase, phenylmethylsulfonyl fluoride (PMSF) was used at a constant ineffective dose of 30 mg/kg (i.p.). Moreover, the effects of ACEA and PMSF on the acute adverse-effect profile of valproate were determined in the chimney test. Additionally, the adverse-effect potentials of combination of ACEA, PMSF with valproate were examined in the step-through passive avoidance task (long-term memory) and grip-strength test (neuromuscular strength). To ascertain any pharmacokinetic contribution of ACEA and PMSF to the observed interaction between tested drugs, both free (non-protein bound) plasma and total brain concentrations of valproate were estimated. Results indicated that ACEA (5 and 7.5 mg/kg; i.p.) combined with PMSF increased significantly (P<0.001) the electroconvulsive threshold in mice. ACEA at low doses of 1.25 and 2.5 mg/kg, i.p., with PMSF had no impact on threshold for electroconvulsions. Similarly, neither PMSF (30 mg/kg) nor ACEA (15 mg/kg) administered alone affected the electroconvulsive threshold in mice. Moreover, ACEA (at a subthreshold dose of 2.5 mg/kg; i.p.) co-administered with PMSF potentiated significantly the antielectroshock activity of valproate by reducing its ED(50) from 258.3 to 195.1 mg/kg (P<0.01). Isobolographic transformation of data revealed that the interactions between valproate and ACEA (at 1.25 and 2.5 mg/kg) combined with PMSF were additive. In the chimney test, the combination of ACEA (2.5 mg/kg) and PMSF (30 mg/kg) had no effect on acute adverse effect of valproate and its TD(50) (356.4 mg/kg) did not differ significantly from that for valproate administered alone (TD(50)=404.4 mg/kg). Moreover, none of the examined drugs administered either alone or in combinations produced long-term memory deficits in the step-through passive avoidance task and impaired neuromuscular strength in the grip-strength test in mice. In contrast, ACEA (2.5 mg/kg; i.p.) combined with PMSF (30 mg/kg; i.p.) considerably increased both, the free plasma (by 42%; P<0.01) and total brain (by 49%; P<0.001) concentrations of valproate (administered at 195 mg/kg; i.p.) in mice. Hence, the observed interaction between valproate and ACEA with PMSF in the maximal electroshock test was pharmacokinetic in nature. Finally, based on this preclinical study, one can conclude that ACEA--a cannabinoid CB(1) receptor agonist co-administered with PMSF pharmacokinetically interacted with valproate and thus, providing the enhancement of the antielectroshock activity of valproate in mice, although, the isobolographically determined interaction between drugs was additive. To elucidate the protective role of cannabinoids in the brain during seizures, more advanced neurochemical studies are required.

Activation of the cannabinoid type-1 receptor mediates the anticonvulsant properties of cannabinoids in the hippocampal neuronal culture models of acquired epilepsy and status epilepticus

Cannabinoids have been shown to have anticonvulsant properties, but no studies have evaluated the effects of cannabinoids in the hippocampal neuronal culture models of acquired epilepsy (AE) and status epilepticus (SE). This study investigated the anticonvulsant properties of the cannabinoid receptor agonist R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolol[1,2,3 de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone (WIN 55,212-2) in primary hippocampal neuronal culture models of both AE and SE. WIN 55,212-2 produced dose-dependent anticonvulsant effects against both spontaneous recurrent epileptiform discharges (SRED) (EC50 = 0.85 microM) and SE (EC50 = 1.51 microM), with total suppression of seizure activity at 3 microM and of SE activity at 5 microM. The anticonvulsant properties of WIN 55,212-2 in these preparations were both stereospecific and blocked by the cannabinoid type-1 (CB1) receptor antagonist N-(piperidin-1-yl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride (SR141716A; 1 microM), showing a CB1 receptor-dependent pathway. The inhibitory effect of WIN 55,212-2 against low Mg2+-induced SE is the first observation in this model of total suppression of SE by a selective pharmacological agent. The clinically used anticonvulsants phenytoin and phenobarbital were not able to abolish low Mg2+-induced SE at concentrations up to 150 microM. The results from this study show CB1 receptor-mediated anticonvulsant effects of the cannabimimetic WIN 55,212-2 against both SRED and low Mg2+-induced SE in primary hippocampal neuronal cultures and show that these in vitro models of AE and SE may represent powerful tools to investigate the molecular mechanisms mediating the effects of cannabinoids on neuronal excitability.

Effects of cannabinoids on neurotransmission

The CB1 cannabinoid receptor is widely distributed in the central and peripheral nervous system. Within the neuron, the CB1 receptor is often localised in axon terminals, and its activation leads to inhibition of transmitter release. The consequence is inhibition of neurotransmission via a presynaptic mechanism. Inhibition of glutamatergic, GABAergic, glycinergic, cholinergic, noradrenergic and serotonergic neurotransmission has been observed in many regions of the central nervous system. In the peripheral nervous system, CB1 receptor-mediated inhibition of adrenergic, cholinergic and sensory neuroeffector transmission has been frequently observed. It is characteristic for the ubiquitous operation of CB1 receptor-mediated presynaptic inhibition that antagonistic components of functional systems (for example, the excitatory and inhibitory inputs of the same neuron) are simultaneously inhibited by cannabinoids. Inhibition of voltage-dependent calcium channels, activation of potassium channels and direct interference with the synaptic vesicle release mechanism are all implicated in the cannabinoid-evoked inhibition of transmitter release. Many presynaptic CB1 receptors are subject to an endogenous tone, i.e. they are constitutively active and/or are continuously activated by endocannabinoids. Compared with the abundant data on presynaptic inhibition by cannabinoids, there are only a few examples for cannabinoid action on the somadendritic parts of neurons in situ.

Depolarization-induced suppression of excitation in murine autaptic hippocampal neurones

Depolarization-induced suppression of excitation and inhibition (DSE and DSI) appear to be important forms of short-term retrograde neuronal plasticity involving endocannabinoids (eCB) and the activation of presynaptic cannabinoid CB1 receptors. We report here that CB1-dependent DSE can be elicited from autaptic cultures of excitatory mouse hippocampal neurones. DSE in autaptic cultures is both more robust and elicited with a more physiologically relevant stimulus than has been thus far reported for conventional hippocampal cultures. An additional requirement for autaptic DSE is filled internal calcium stores. Pharmacological experiments favour a role for 2-arachidonyl glycerol (2-AG) rather than arachidonyl ethanolamide (AEA) or noladin ether as the relevant endocannabinoid to elicit DSE. In particular, the latter two compounds fail to reversibly inhibit EPSCs, a quality inconsistent with the role of bona fide eCB mediating DSE. Delta9-Tetrahydrocannabinol (delta9-THC) fails to inhibit EPSCs, yet readily occludes both DSE and EPSC inhibition by a synthetic CB1 agonist, WIN 55212-2. With long-term exposure (approximately 18 h), delta9-THC also desensitizes CB1 receptors. Lastly, a functional endocannabinoid transporter is necessary for the expression of DSE.

Prefrontal cortex stimulation induces 2-arachidonoyl-glycerol-mediated suppression of excitation in dopamine neurons

Endocannabinoids form a novel class of retrograde messengers that modulate short- and long-term synaptic plasticity. Depolarization-induced suppression of excitation (DSE) and inhibition (DSI) are the best characterized transient forms of endocannabinoid-mediated synaptic modulation. Stimulation protocols consisting of long-lasting voltage steps to the postsynaptic cell are routinely used to evoke DSE-DSI. Little is known, however, about more physiological conditions under which these molecules are released in vitro. Moreover, the occurrence in vivo of such forms of endocannabinoid-mediated modulation is still controversial. Here we show that physiologically relevant patterns of synaptic activity induce a transient suppression of excitatory transmission onto dopamine neurons in vitro. Accordingly, in vivo endocannabinoids depress the increase in firing and bursting activity evoked in dopamine neurons by prefrontal cortex stimulation. This phenomenon is selectively mediated by the endocannabinoid 2-arachidonoyl-glycerol (2-AG), which activates presynaptic cannabinoid type 1 receptors. 2-AG synthesis involves activation of metabotropic glutamate receptors and Ca2+ mobilization from intracellular stores. These findings indicate that dopamine neurons release 2-AG to shape afferent activity and ultimately their own firing pattern. This novel endocannabinoid-mediated self-regulatory role of dopamine neurons may bear relevance in the pathogenesis of neuropsychiatric disorders such as schizophrenia and addiction.

Cannabinoid function in learning, memory and plasticity

Marijuana and its psychoactive constituents induce a multitude of effects on brain function. These include deficits in memory formation, but care needs to be exercised since many human studies are flawed by multiple drug abuse, small sample sizes, sample selection and sensitivity of psychological tests for subtle differences. The most robust finding with respect to memory is a deficit in working and short-term memory. This requires intact hippocampus and prefrontal cortex, two brain regions richly expressing CB1 receptors. Animal studies, which enable a more controlled drug regime and more constant behavioural testing, have confirmed human results and suggest, with respect to hippocampus, that exogenous cannabinoid treatment selectively affects encoding processes. This may be different in other brain areas, for instance the amygdala, where a predominant involvement in memory consolidation and forgetting has been firmly established. While cannabinoid receptor agonists impair memory formation, antagonists reverse these deficits or act as memory enhancers. These results are in good agreement with data obtained from electrophysiological recordings, which reveal reduction in neural plasticity following cannabinoid treatment, and increased plasticity following antagonist exposure. The mixed receptor properties of the pharmacological tool, however, make it difficult to define the exact role of any CB1 receptor population in memory processes with any certainty. This makes it all the more important that behavioural studies use selective administration of drugs to specific brain areas, rather than global administration to whole animals. The emerging role of the endogenous cannabinoid system in the hippocampus may be to facilitate the induction of long-term potentiation/the encoding of information. Administration of exogenous selective CB1 agonists may therefore disrupt hippocampus-dependent learning and memory by 'increasing the noise', rather than 'decreasing the signal' at potentiated inputs.

Prolonged cannabinoid treatment results in spatial working memory deficits and impaired long-term potentiation in the CA1 region of the hippocampus in vivo

Adult male Long-Evans rats were administered the potent cannabinoid 1 receptor agonist HU-210 (100 microg/kg, i.p.) for 15 days continuously and their performance on a matching-to-place version of the Morris water maze was subsequently evaluated. Overall, experimental animals performed significantly worse initially on the reference memory component of this task, but their performance improved over 5 days until it was indistinguishable from that of control animals. Animals given HU-210 did not exhibit working memory impairments at short intertrial delays (30 s); however, significant impairments were observed in learning performance with longer intertrial delays (300 s). In vivo electrophysiological analyses revealed that long-term potentiation in the CA1 region of the hippocampus was significantly impaired following the administration of HU-210 for 15 days. These results indicate that long-term cannabinoid exposure can produce marked deficits in reference and working memory performance, and also impair hippocampal synaptic plasticity in vivo.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.

The endocannabinoid system: function in survival of the embryo, the newborn and the neuron

Since the identification and cloning of the first cannabinoid (CB1) receptor and the subsequent discovery of the endogenous cannabinoid ligands (endocannabinoids), anandamide, 2-arachidonoyl glycerol (2-AG) and noladin ether, a intensive search for their function in health and disease has been launched. The endocannabinoids in the central nervous system bind Gi/o coupled CB1 receptors that modulate adenylyl cyclase, ion channels and extracellular signal-regulated kinases. The present review discusses the nature of endocannabinoid (anandamide and 2-AG) neurotransmission, the activity of cannabinoids and the possibility that some of these activities are mediated via a receptor, yet to be discovered, which is distinct from the brain specific CB1 receptor. Three physiological functions in which the endocannabinoids play a critical role are also discussed: embryonal implantation, feeding and appetite, and neuroprotection.

Functions of cannabinoid receptors in the hippocampus

Marijuana smoking is recognised to impair human cognition and learning, but the mechanisms by which this occurs are not well characterised. This article focuses exclusively on the hippocampus to review the effects of cannabinoids on hippocampal function and evaluate the evidence that hippocampal cannabinoid receptors play a role in learning and formation of memory. Activation of cannabinoid receptors inhibits release of a variety of neurotransmitters, and modulates a number of intrinsic membrane conductances. Suppression of inhibitory GABAergic synaptic transmission has been repeatedly described, but whether there is also control of excitatory glutamatergic transmission is more controversial. The recognition that the commonly used WIN55,212-2 also acts via non-cannabinoid receptors may help resolve this issue. The involvement of endocannabinoids in depolarisation induced suppression of inhibition (DSI) and the demonstration that activation of metabotropic glutamate receptors can stimulate endocannabinoid release have provided the first insights into the physiological roles of the cannabinoids. Cannabinoids have consistently been reported to inhibit high frequency stimulation induced synaptic long-term potentiation but the experimental design of most behavioural experiments have meant it is not possible to categorically demonstrate a role for hippocampal cannabinoid receptors in learning and memory.

Biosynthesis and degradation of anandamide and 2-arachidonoylglycerol and their possible physiological significance

N -arachidonoylethanolamine (anandamide) was the first endogenous cannabinoid receptor ligand to be discovered. Dual synthetic pathways for anandamide have been proposed. One is the formation from free arachidonic acid and ethanolamine, and the other is the formation from N -arachidonoyl phosphatidylethanolamine (PE) through the action of a phosphodiesterase. These pathways, however, do not appear to be able to generate a large amount of anandamide, at least under physiological conditions. The generation of anandamide from free arachidonic acid and ethanolamine is catalyzed by a degrading enzyme anandamide amidohydrolase/fatty acid amide hydrolase operating in reverse and requires large amounts of substrates. As for the second pathway, arachidonic acids esterified at the 1-position of glycerophospholipids, which are mostly esterified at the 2-position, are utilized for the formation of N -arachidonoyl PE, a stored precursor form of anandamide. In fact, the actual levels of anandamide in various tissues are generally low except in a few cases. 2-Arachidonoylglycerol (2-AG) was the second endogenous cannabinoid receptor ligand to be discovered. 2-AG is a degradation product of arachidonic acid-containing glycerophospholipids such as inositol phospholipids. Several investigators have demonstrated that 2-AG is produced in a variety of tissues and cells upon stimulation. 2-AG acts as a full agonist at the cannabinoid receptors (CB1 and CB2). Evidence is gradually accumulating and indicates that 2-AG is the most efficacious endogenous natural ligand for the cannabinoid receptors. In this review, we summarize the tissue levels, biosynthesis, degradation and possible physiological significance of two endogenous cannabimimetic molecules, anandamide and 2-AG.

Endocannabinoids in the central nervous system--an overview

Many aspects of the physiology and pharmacology of anandamide (arachidonoyl ethanol amide), the first endogenous cannabinoid ligand ("endocannabinoid") isolated from pig brain, have been studied since its discovery in 1992. Ethanol amides from other fatty acids have also been identified as endocannabinoids with similar in vivo and in vitro pharmacological properties. 2-Arachidonoyl glycerol and noladin ether (2-arachidonyl glyceryl ether), isolated in 1995 and 2001, respectively, so far, display pharmacological properties in the central nervous system, similar to those of anandamide. The endocannabinoids are widely distributed in brain, they are synthesized and released upon neuronal stimulation, undergo reuptake and are hydrolyzed intracellularly by fatty acid amide hydrolase (FAAH). For therapeutic purposes, inhibitors of FAAH may provide more specific cannabinoid activities than direct agonists, and several such molecules have already been developed. Pharmacological effects of the endocannabinoids are very similar, yet not identical, to those of the plant-derived and synthetic cannabinoid receptor ligands. In addition to pharmacokinetic explanations, direct or indirect interactions with other receptors have been considered to explain some of these differences, including activities at serotonin and GABA receptors. Binding affinities for other receptors such as the vanilloid receptor, have to be taken into account in order to fully understand endocannabinoid physiology. Moreover, possible interactions with receptors for the lysophosphatidic acids deserve attention in future studies. Endocannabinoids have been implicated in a variety of physiological functions. The areas of central activities include pain reduction, motor regulation, learning/memory, and reward. Finally, the role of the endocannabinoid system in appetite stimulation in the adult organism, and perhaps more importantly, its critical involvement in milk ingestion and survival of the newborn, may not only further our understanding of the physiology of food intake and growth, but may also find therapeutic applications in wasting disease and infant's "failure to thrive".

Assessment of the role of CB1 receptors in cannabinoid anticonvulsant effects

The cannabinoid CB1 receptor has been shown to be the primary site of action for cannabinoid-induced effects on the central nervous system. Activation of this receptor has proven to dampen neurotransmission and produce an overall reduction in neuronal excitability. Cannabinoid compounds like delta9-tetrahydrocannabinol and cannabidiol have been shown to be anticonvulsant in maximal electroshock, a model of partial seizure with secondary generalization. However, until now, it was unknown if these anticonvulsant effects are mediated by the cannabinoid CB1 receptor. Likewise, (R)-(+)-[2,3-Dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN 55,212-2), a cannabimimetic compound that has been shown to decrease hyperexcitability in cell culture models via the cannabinoid CB1 receptor, has never been evaluated for anticonvulsant activity in an animal seizure model. We first show that the cannabinoid compounds delta9-tetrahydrocannabinol (ED50 = 42 mg/kg), cannabidiol (ED50 = 80 mg/kg), and WIN 55,212-2 (ED50 = 47 mg/kg) are anticonvulsant in maximal electroshock. We further establish, using the cannabinoid CB1 receptor specific antagonist N-(piperidin-1-yl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride (SR141716A) (AD50 = 2.5 mg/kg), that the anticonvulsant effects of delta9-tetrahydrocannabinol and WIN 55,212-2 are cannabinoid CB1 receptor-mediated while the anticonvulsant activity of cannabidiol is not. This study establishes a role for the cannabinoid CB1 receptor in modulating seizure activity in a whole animal model.