UCM707, a potent and selective inhibitor of endocannabinoid uptake, potentiates hypokinetic and antinociceptive effects of anandamide

CB2 receptor in the CNS: from immune and neuronal modulation to behavior

Despite low levels of cannabinoid receptor type 2 (CB2R) expression in the central nervous system in human and rodents, a growing body of evidence shows CB2R involvement in many processes at the behavioral level, through both immune and neuronal modulations. Recent in vitro and in vivo evidence have highlighted the complex role of CB2R under physiological and inflammatory conditions. Under neuroinflammatory states, its activation seems to protect the brain and its functions, making it a promising target in a wide range of neurological disorders. Here, we provide a complete and updated overview of CB2R function in the central nervous system of rodents, spanning from modulation of immune function in microglia but also in other cell types, to behavior and neuronal activity, in both physiological and neuroinflammatory contexts.

On the Biomedical Properties of Endocannabinoid Degradation and Reuptake Inhibitors: Pre-clinical and Clinical Evidence

The endocannabinoid system (ECS) is composed of endogenous cannabinoids; components involved in their synthesis, transport, and degradation; and an expansive variety of cannabinoid receptors. Hypofunction or deregulation of the ECS is related to pathological conditions. Consequently, endogenous enhancement of endocannabinoid levels and/or regulation of their metabolism represent promising therapeutic approaches. Several major strategies have been suggested for the modulation of the ECS: (1) blocking endocannabinoids degradation, (2) inhibition of endocannabinoid cellular uptake, and (3) pharmacological modulation of cannabinoid receptors as potential therapeutic targets. Here, we focused in this review on degradation/reuptake inhibitors over cannabinoid receptor modulators in order to provide an updated synopsis of contemporary evidence advancing mechanisms of endocannabinoids as pharmacological tools with therapeutic properties for the treatment of several disorders. For this purpose, we revisited the available literature and reported the latest advances regarding the biomedical properties of fatty acid amide hydrolase and monoacylglycerol lipase inhibitors in pre-clinical and clinical studies. We also highlighted anandamide and 2-arachidonoylglycerol reuptake inhibitors with promising results in pre-clinical studies using in vitro and animal models as an outlook for future research in clinical trials.

Review of the Endocannabinoid System

The endocannabinoid system (ECS) is a widespread neuromodulatory network involved in the developing central nervous system as well as playing a major role in tuning many cognitive and physiological processes. The ECS is composed of endogenous cannabinoids, cannabinoid receptors, and the enzymes responsible for the synthesis and degradation of endocannabinoids. In addition to its endogenous roles, cannabinoid receptors are the primary target of Δ⁹-tetrahydrocannabinol, the intoxicating component of cannabis. In this review, we summarize our current understanding of the ECS. We start with a description of ECS components and their role in synaptic plasticity and neurodevelopment, and then discuss how phytocannabinoids and other exogenous compounds may perturb the ECS, emphasizing examples relevant to psychosis.

Neuroprotection or Neurotoxicity of Illicit Drugs on Parkinson's Disease

Parkinson's Disease (PD) is currently the most rapid growing neurodegenerative disease and over the past generation, its global burden has more than doubled. The onset of PD can arise due to environmental, sporadic or genetic factors. Nevertheless, most PD cases have an unknown etiology. Chemicals, such as the anthropogenic pollutant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and amphetamine-type stimulants, have been associated with the onset of PD. Conversely, cannabinoids have been associated with the treatment of the symptoms'. PD and medical cannabis is currently under the spotlight, and research to find its benefits on PD is on-going worldwide. However, the described clinical applications and safety of pharmacotherapy with cannabis products are yet to be fully supported by scientific evidence. Furthermore, the novel psychoactive substances are currently a popular alternative to classical drugs of abuse, representing an unknown health hazard for young adults who may develop PD later in their lifetime. This review addresses the neurotoxic and neuroprotective impact of illicit substance consumption in PD, presenting clinical evidence and molecular and cellular mechanisms of this association. This research area is utterly important for contemporary society since illicit drugs' legalization is under discussion which may have consequences both for the onset of PD and for the treatment of its symptoms.

An Endocannabinoid Uptake Inhibitor from Black Pepper Exerts Pronounced Anti-Inflammatory Effects in Mice

Guineensine is a dietary N-isobutylamide widely present in black and long pepper (Piper nigrum and Piper longum) previously shown to inhibit cellular endocannabinoid uptake. Given the role of endocannabinoids in inflammation and pain reduction, here we evaluated guineensine in mouse models of acute and inflammatory pain and endotoxemia. Significant dose-dependent anti-inflammatory effects (95.6 ± 3.1% inhibition of inflammatory pain at 2.5 mg/kg ip and 50.0 ± 15.9% inhibition of edema formation at 5 mg/kg ip) and acute analgesia (66.1 ± 28.1% inhibition at 5.0 mg/kg ip) were observed. Moreover, guineensine inhibited proinflammatory cytokine production in endotoxemia. Intriguingly, guineensine and LPS independently induced catalepsy, but in combination this effect was abolished. Both hypothermia and analgesia were blocked by the CB1 receptor inverse agonist rimonabant, but the pronounced hypolocomotion was CB1 receptor-independent. A subsequent screen of 45 CNS-related receptors, ion channels, and transporters revealed apparent interactions of guineensine with the dopamine transporter DAT, 5HT2A, and sigma receptors, uncovering its prospective polypharmacology. The described potent pharmacological effects of guineensine might relate to the reported anti-inflammatory effects of pepper.

Marijuana Compounds: A Nonconventional Approach to Parkinson's Disease Therapy

Parkinson's disease (PD), a neurodegenerative disorder, is the second most common neurological illness in United States. Neurologically, it is characterized by the selective degeneration of a unique population of cells, the nigrostriatal dopamine neurons. The current treatment is symptomatic and mainly involves replacement of dopamine deficiency. This therapy improves only motor symptoms of Parkinson's disease and is associated with a number of adverse effects including dyskinesia. Therefore, there is unmet need for more comprehensive approach in the management of PD. Cannabis and related compounds have created significant research interest as a promising therapy in neurodegenerative and movement disorders. In this review we examine the potential benefits of medical marijuana and related compounds in the treatment of both motor and nonmotor symptoms as well as in slowing the progression of the disease. The potential for cannabis to enhance the quality of life of Parkinson's patients is explored.

Functionally Optimized Neuritogenic Farinosone C Analogs: SAR-Study and Investigations on Their Mode of Action

Several natural products derived from entomopathogenic fungi have been shown to initiate neuronal differentiation in the rat pheochromocytoma PC12 cell line. After the successful completion of the total synthesis program, the reduction of structural complexity while retaining biological activity was targeted. In this study, farinosone C served as a lead structure and inspired the preparation of small molecules with reduced complexity, of which several were able to induce neurite outgrowth. This allowed for the elaboration of a detailed structure-activity relationship. Investigations on the mode of action utilizing a computational similarity ensemble approach suggested the involvement of the endocannabinoid system as potential target for our analogs and also led to the discovery of four potent new endocannabinoid transport inhibitors.

Characterization of the effects of reuptake and hydrolysis inhibition on interstitial endocannabinoid levels in the brain: an in vivo microdialysis study

The present experiments employed in vivo microdialysis to characterize the effects of commonly used endocannabinoid clearance inhibitors on basal and depolarization-induced alterations in interstitial endocannabinoid levels in the nucleus accumbens of rat brain. Compounds targeting the putative endocannabinoid transporter and hydrolytic enzymes (FAAH and MAGL) were compared. The transporter inhibitor AM404 modestly enhanced depolarization-induced increases in 2-arachidonoyl glycerol (2-AG) levels but did not alter levels of N-arachidonoyl-ethanolamide (anandamide, AEA). The transport inhibitor UCM707 did not alter dialysate levels of either endocannabinoid. The FAAH inhibitors URB597 and PF-3845 robustly increased AEA levels during depolarization without altering 2-AG levels. The MAGL inhibitor URB602 significantly enhanced depolarization-induced increases in 2-AG, but did not alter AEA levels. In contrast, the MAGL inhibitor JZL184 did not alter 2-AG or AEA levels under any condition tested. Finally, the dual FAAH/MAGL inhibitor JZL195 significantly enhanced depolarization-induced increases in both AEA and 2-AG levels. In contrast to the present observations in rats, prior work in mice has demonstrated a robust JZL184-induced enhancement of depolarization-induced increases in dialysate 2-AG. Thus, to further investigate species differences, additional tests with JZL184, PF-3845, and JZL195 were performed in mice. Consistent with prior reports, JZL184 significantly enhanced depolarization-induced increases in 2-AG without altering AEA levels. PF-3845 and JZL195 produced profiles in mouse dialysates comparable to those observed in rats. These findings confirm that interstitial endocannabinoid levels in the brain can be selectively manipulated by endocannabinoid clearance inhibitors. While PF-3845 and JZL195 produce similar effects in both rats and mice, substantial species differences in JZL184 efficacy are evident, which is consistent with previous studies.

Endocannabinoid regulation of spinal nociceptive processing in a model of neuropathic pain

Models of neuropathic pain are associated with elevated spinal levels of endocannabinoids (ECs) and altered expression of cannabinoid receptors on primary sensory afferents and post-synaptic cells in the spinal cord. We investigated the impact of these changes on the spinal processing of sensory inputs in a model of neuropathic pain. Extracellular single-unit recordings of spinal neurones were made in anaesthetized neuropathic and sham-operated rats. The effects of spinal administration of the cannabinoid CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) and the cannabinoid receptor type 2 (CB(2)) receptor antagonist N-[(1S)-endo-1,3,3-trimethylbicycloheptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528) on mechanically-evoked responses of spinal neurones were determined. The effects of spinal administration of (5Z,8Z11Z,14Z)-N-(3-furanylmethyl)-5,8,11,14-eicosatetraenamide (UCM707), which binds to CB(2) receptors and alters transport of ECs, on evoked responses of spinal neurones and spinal levels of ECs were also determined. The cannabinoid CB(1) receptor antagonist AM251, but not the CB(2) receptor antagonist, significantly facilitated 10-g-evoked responses of spinal neurones in neuropathic, but not sham-operated, rats. Spinal administration of UCM707 did not alter spinal levels of ECs but did significantly inhibit mechanically-evoked responses of neurones in neuropathic, but not sham-operated, rats. Pharmacological studies indicated that the selective inhibitory effects of spinal UCM707 in neuropathic rats were mediated by activation of spinal CB(2) receptors, as well as a contribution from transient receptor potential vanilloid 1 (TRPV1) channels. This work demonstrates that changes in the EC receptor system in the spinal cord of neuropathic rats influence the processing of sensory inputs, in particular low-weight inputs that drive allodynia, and indicates novel effects of drugs acting via multiple elements of this receptor system.

The endocannabinoid system and pain

The therapeutic potential of cannabinoids has been the topic of extensive investigation following the discovery of cannabinoid receptors and their endogenous ligands. Cannabinoid receptors and their endogenous ligands are present at supraspinal, spinal and peripheral levels. Cannabinoids suppress behavioral responses to noxious stimulation and suppress nociceptive processing through activation of cannabinoid CB(1) and CB(2) receptor subtypes. Endocannabinoids, the brain's own cannabis-like substances, share the same molecular target as Delta(9)-tetrahydrocannabinol, the main psychoactive component in cannabis. Endocannabinoids serve as synaptic circuit breakers and regulate multiple physiological and pathological conditions, e.g. regulation of food intake, immunomodulation, inflammation, analgesia, cancer, addictive behavior, epilepsy and others. This review will focus on uncovering the roles of anandamide and 2-arachidonoylglycerol, the two best characterized endocannabinoids identified to date, in controlling nociceptive responding. The roles of anandamide and 2-arachidonoylglycerol, released under physiological conditions, in modulating nociceptive responding at different levels of the neuraxis will be emphasized in this review. Effects of modulation of endocannabinoid levels through inhibition of endocannabinoid hydrolysis and uptake is also compared with effects of exogenous administration of synthetic endocannabinoids in acute, inflammatory and neuropathic pain models. Finally, the therapeutic potential of the endocannabinoid signaling system is discussed in the context of identifying novel pharmacotherapies for the treatment of pain.

A potent and selective inhibitor of endocannabinoid uptake, UCM707, potentiates antinociception induced by cholestasis

Cholestasis is associated with changes including analgesia. The endocannabinoid system can reduce pain sensitivity. Considering the interaction between the endogenous opioid and endocannabinoid systems in nociception processing, we studied the effect of UCM707 as a potent and selective inhibitor of endocannabinoid uptake on modulation of nociception in a model of elevated endogenous opioid tone, cholestasis. Cholestasis was induced in male Sprague-Dawley rats by ligation of the main bile duct using two ligatures and transecting the duct at the midpoint between them. Seven days later, tail-flick latencies were measured 10 min after injection of UCM707 (0.1, 1 and 10 mg/kg, i.p.) alone or with co-administration of a CB(1) receptor antagonist, AM251 (1 mg/kg, i.p.), with UCM707 (10 mg/kg, i.p.) in experimental groups. A significant increase (P < 0.01) in tail-flick latency was observed in cholestatic rats compared with rats belonging to unoperated and sham groups. Administration of UCM707 (1 and 10 mg/kg) to cholestatic animals significantly increased tail-flick latency compared with the vehicle-treated cholestatic group (P < 0.05 and P < 0.001, respectively). UCM707 injection in unoperated and sham groups did not alter baseline tail-flick latency compared with vehicle-treated groups. The effect of UCM707 in the cholestatic group was blocked by co-administration of AM251 (1 mg/kg, i.p.) with UCM707. These data showed that the endocannabinoid system is involved in nociception processing during cholestasis and that the effects of UCM707 on the pain threshold in cholestatic rats may be a result of CB(1) receptor activation by the increased extracellular levels of endocannabinoids.

Effects of the endocannabinoid transport inhibitors AM404 and UCM707 on diabetic neuropathy in rats

1. Diabetic rats display increased pain responses following injection of formalin into the paw, suggesting the presence of hyperalgesia. In the present study, we investigated the efficacy of the systemic administration of the endocannabinoid transport inhibitors UCM707 and AM404 (1, 10 and 50 mg/kg, i.p.) on hyperalgesia during the formalin test in streptozocin (STZ)-induced diabetic rats. 2. Nociceptive testing was performed in male adult Wistar rats 4 weeks after the onset of hyperglycaemia. At the end of the experiment, all rats were weighed and then underwent plasma glucose measurements. 3. Diabetes caused significant hyperalgesia during both phases of the formalin test. At 10 and 50 mg/kg, both UCM707 and AM404 reversed chemical hyperalgesia in diabetic rats. UCM707 (10 and 50 mg/kg) caused less intensive nociceptive behaviour during both phases of the test, whereas AM404 (10 and 50 mg/kg) only affected pain scores during Phase 1 of the formalin test. At 1 mg, neither drug had any effect on pain behaviour in control and diabetic groups compared with their respective controls. Neither UCM707 nor AM404 had any effect on bodyweight or plasma glucose levels of treated compared with non-treated rats at any of the doses tested. 4. The results of the present study indicate that systemic administration of UCM707 and AM404 is effective in ameliorating chemical hyperalgesia in STZ-diabetic rats. Thus, endocannabinoid transport inhibitors may have potential in the treatment of painful diabetic neuropathy.

Identification of intracellular carriers for the endocannabinoid anandamide

The endocannabinoid anandamide (arachidonoyl ethanolamide, AEA) is an uncharged neuromodulatory lipid that, similar to many neurotransmitters, is inactivated through its cellular uptake and subsequent catabolism. AEA is hydrolyzed by fatty acid amide hydrolase (FAAH), an enzyme localized on the endoplasmic reticulum. In contrast to most neuromodulators, the hydrophilic cytosol poses a diffusional barrier for the efficient delivery of AEA to its site of catabolism. Therefore, AEA likely traverses the cytosol with the assistance of an intracellular carrier that increases its solubility and rate of diffusion. To study this process, AEA uptake and hydrolysis were examined in COS-7 cells expressing FAAH restricted to the endoplasmic reticulum, mitochondria, or the Golgi apparatus. AEA hydrolysis was detectable at the earliest measurable time point (3 seconds), suggesting that COS-7 cells, normally devoid of an endocannabinoid system, possess an efficient cytosolic trafficking mechanism for AEA. Three fatty acid binding proteins (FABPs) known to be expressed in brain were examined as possible intracellular AEA carriers. AEA uptake and hydrolysis were significantly potentiated in N18TG2 neuroblastoma cells after overexpression of FABP5 or FABP7, but not FABP3. Similar results were observed in COS-7 cells stably expressing FAAH. Consistent with the roles of FABP as AEA carriers, administration of the competitive FABP ligand oleic acid or the selective non-lipid FABP inhibitor BMS309403 attenuated AEA uptake and hydrolysis by approximately 50% in N18TG2 and COS-7 cells. Taken together, FABPs represent the first proteins known to transport AEA from the plasma membrane to FAAH for inactivation and may therefore be novel pharmacological targets.

Pharmacological and therapeutic secrets of plant and brain (endo)cannabinoids

Research on the chemistry and pharmacology of cannabinoids and endocannabinoids has reached enormous proportions, with approximately 15,000 articles on Cannabis sativa L. and cannabinoids and over 2,000 articles on endocannabinoids. The present review deals with the history of the Cannabis sativa L. plant, its uses, constituent compounds and their biogeneses, and similarity to compounds from Radula spp. In addition, details of the pharmacology of natural cannabinoids, as well as synthetic agonists and antagonists are presented. Finally, details regarding the pioneering isolation of the endocannabinoid anandamide, as well as the pharmacology and potential therapeutic uses of endocannabinoid congeners are presented.

The endocannabinoid system as a target for the treatment of motor dysfunction

There is evidence that cannabinoid-based medicines that are selective for different targets in the cannabinoid signalling system (e.g. receptors, inactivation mechanism, enzymes) might be beneficial in basal ganglia disorders, namely Parkinson's disease (PD) and Huntington's disease (HD). These benefits not only include the alleviation of specific motor symptoms [e.g. choreic movements with cannabinoid receptor type 1 (CB(1))/transient receptor potential vanilloid type 1 agonists in HD; bradykinesia with CB(1) antagonists and tremor with CB(1) agonists in PD], but also the delay of disease progression due to the neuroprotective properties demonstrated for cannabinoids (e.g. CB(1) agonists reduce excitotoxicity; CB(2) agonists limit the toxicity of reactive microglia; and antioxidant cannabinoids attenuate oxidative damage). In addition, extensive biochemical, anatomical, physiological and pharmacological studies have demonstrated that: (i) the different elements of the cannabinoid system are abundant in basal ganglia structures and they are affected by these disorders; (ii) the cannabinoid system plays a prominent role in basal ganglia function by modulating the neurotransmitters that operate in the basal ganglia circuits, both in healthy and pathological conditions; and (iii) the activation and/or inhibition of the cannabinoid system is associated with important motor responses that are maintained and even enhanced in conditions of malfunctioning and/or degeneration. In this article we will review the available data regarding the relationship between the cannabinoid system and basal ganglia activity, both in healthy and pathological conditions and will also try to identify future lines of research expected to increase current knowledge about the potential therapeutic benefits of targeting this system in PD, HD and other basal ganglia disorders.

The role of TRPV1 receptors in the antinociceptive effect of anandamide at spinal level

While it is well known that the endogenous cannabinoid receptor ligand anandamide also activates the transient receptor potential vanilloid1 (TRPV1) receptors, there has been no in vivo study indicating the role of the TRPV1 receptors in the antinociceptive effect of anandamide at spinal level. The goal of this study was to determine the effect of inhibition of TRPV1 receptors by capsazepine on the antinociceptive potency of anandamide after intrathecal administration. Anandamide alone (1, 30 or 100 microg) dose-dependently decreased carrageenan-induced thermal hyperalgesia, however, the highest dose caused temporary excitation and vocalization, suggesting the pain-inducing potential of anandamide. Capsazepine (10 or 20 microg) by itself did not change the pain sensitivity markedly, but the lower dose increased it, and the higher dose decreased the antinociceptive effect of 30 microg anandamide. Furthermore, both doses of capsazepine decreased the efficacy of the largest dose of anandamide. These results show that TRPV1 receptor activation plays a substantial role in the antinociceptive effects of anandamide at spinal level. The effect of the inhibition on TRPV1 receptors depended on the dose applied. We presume that coactivation of the cannabinoid and TRPV1 receptors by anandamide provides elevated antinociception through the release of antinociceptive endogenous ligands at spinal level.

Endocannabinoid metabolism and uptake: novel targets for neuropathic and inflammatory pain

Cannabinoid CB1 and CB2 receptors are located at key sites involved in the relaying and processing of noxious inputs. Both CB1 and CB2 receptor agonists have analgesic effects in a range of models of inflammatory and neuropathic pain. Importantly, clinical trials of cannabis-based medicines indicate that the pre-clinical effects of cannabinoid agonists may translate into therapeutic potential in humans. One of the areas of concern with this pharmacological approach is that CB1 receptors have a widespread distribution in the brain and that global activation of CB1 receptors is associated with adverse side effects. Studies of the endogenous cannabinoids (endocannabinoids) have demonstrated that they are present in most tissues and that in some pain states, such as neuropathic pain, levels of endocannabinoids are elevated at key sites involved in pain processing. An alternative approach that can be used to harness the potential therapeutic effects of cannabinoids is to maximise the effects of the endocannabinoids, the actions of which are terminated by re-uptake and metabolism by various enzymes, including fatty acid amide hydrolase (FAAH), monoacylglycerol lipase (MAGL) and cyclooxygenase type 2 (COX2). Preventing the metabolism, or uptake, of endocannabinoids elevates levels of these lipid compounds in tissue and produces behavioural analgesia in models of acute pain. Herein we review recent studies of the effects of inhibition of metabolism of endocannabinoids versus uptake of endocannabinoids on nociceptive processing in models of inflammatory and neuropathic pain.

Effects of anandamide and noxious heat on intracellular calcium concentration in nociceptive drg neurons of rats

As an endogenous agonist at the cannabinoid receptor CB1 and the capsaicin-receptor TRPV1, anandamide may exert both anti- and pronociceptive actions. Therefore we studied the effects of anandamide and other activators of both receptors on changes in free cytosolic calcium ([Ca(2+)](i)) in acutely dissociated small dorsal root ganglion neurons (diameter: < or =30 microm). Anandamide (10 microM) increased [Ca(2+)](i) in 76% of the neurons. The EC(50) was 7.41 microM, the Hill slope was 2.15 +/- 0.43 (mean +/- SE). This increase was blocked by the competitive TRPV1-antagonist capsazepine (10 microM) and in Ca(2+)-free extracellular solution. Neither exclusion of voltage-gated sodium channels nor additional blockade of voltage-gated calcium channels of the L-, N-, and/or T-type, significantly reduced the anandamide-induced [Ca(2+)](i) increase or capsaicin-induced [Ca(2+)](i) transients (0.2 microM). The CB1-agonist HU210 (10 microM) inhibited the anandamide-induced rise in [Ca(2+)](i). Conversely, the CB1-antagonist AM251 (3 microM) induced a leftward shift of the concentration-response relationship by approximately 4 microM (P < 0.001; Hill slope, 2.17 +/- 0.75). Intracellular calcium transients in response to noxious heat (47 degrees C for 10 s) were highly correlated with the anandamide-induced [Ca(2+)](i) increases (r = 0.84, P < 0.001). Heat-induced [Ca(2+)](i) transients were facilitated by preincubation with subthreshold concentrations of anandamide (3 microM), an effect that was further enhanced by 3 microM AM251. Although anandamide acts on both TRPV1 and CB1 receptors in the same nociceptive DRG neurons, its pronociceptive effects dominate. Anandamide triggers an influx of calcium through TRPV1 but no intracellular store depletion. It facilitates the heat responsiveness of TRPV1 in a calcium-independent manner. These effects of anandamide differ from those of the classical exogenous TRPV1-agonist capsaicin and suggest a primarily modulatory mode of action of anandamide.

The cannabinoid system and its pharmacological manipulation--a review, with emphasis upon the uptake and hydrolysis of anandamide

Although cannabis has been used both recreationally and for medicinal purposes since ancient times, it was not until the 1990s that the receptors responsible for many of the actions of Delta(9)-tetrahydrocannabinol, the main psychoactive ingredient of cannabis, were cloned. Since then, our knowledge of the endogenous cannabinoid system, its physiology, pharmacology and therapeutic potential have expanded enormously. In the present review, the cannabinoid system is described, with particular emphasis on the mechanisms of removal and metabolism of the endocannabinoid signalling molecule anandamide. The current literature shows that cells can accumulate anandamide, and that this process can be disrupted pharmacologically, but that the nature of the mechanism(s) involved remains a matter of some debate. The main enzyme for the hydrolysis of anandamide, fatty acid amide hydrolase, is well characterized, and molecules selectively inhibiting this enzyme have potential therapeutic utility in a number of areas, in particular for the treatment of pain conditions.

Evaluation of the neuroprotective effect of cannabinoids in a rat model of Parkinson's disease: importance of antioxidant and cannabinoid receptor-independent properties

We have recently demonstrated that two plant-derived cannabinoids, Delta9-tetrahydrocannabinol and cannabidiol (CBD), are neuroprotective in an animal model of Parkinson's disease (PD), presumably because of their antioxidant properties. To further explore this issue, we examined the neuroprotective effects of a series of cannabinoid-based compounds, with more selectivity for different elements of the cannabinoid signalling system, in rats with unilateral lesions of nigrostriatal dopaminergic neurons caused by local application of 6-hydroxydopamine. We used the CB1 receptor agonist arachidonyl-2-chloroethylamide (ACEA), the CB2 receptor agonist HU-308, the non-selective agonist WIN55,212-2, and the inhibitors of the endocannabinoid inactivation AM404 and UCM707, all of them administered i.p. Daily administration of ACEA or WIN55,212-2 did not reverse 6-hydroxydopamine-induced dopamine (DA) depletion in the lesioned side, whereas HU-308 produced a small recovery that supports a possible involvement of CB2 but not CB1 receptors. AM404 produced a marked recovery of 6-hydroxydopamine-induced DA depletion and tyrosine hydroxylase deficit in the lesioned side. Possibly, this is caused by the antioxidant properties of AM404, which are derived from the presence of a phenolic group in its structure, rather than by the capability of AM404 to block the endocannabinoid transporter, because UCM707, another transporter inhibitor devoid of antioxidant properties, did not produce the same effect. None of these effects were observed in non-lesioned contralateral structures. We also examined the timing for the effect of CBD to provide neuroprotection in this rat model of PD. We found that CBD, as expected, was able to recover 6-hydroxydopamine-induced DA depletion when it was administered immediately after the lesion, but it failed to do that when the treatment started 1 week later. In addition, the effect of CBD implied an upregulation of mRNA levels for Cu,Zn-superoxide dismutase, a key enzyme in endogenous defenses against oxidative stress. In summary, our results indicate that those cannabinoids having antioxidant cannabinoid receptor-independent properties provide neuroprotection against the progressive degeneration of nigrostriatal dopaminergic neurons occurring in PD. In addition, the activation of CB2 (but not CB1) receptors, or other additional mechanisms, might also contribute to some extent to the potential of cannabinoids in this disease.

Anxiolytic-like properties of the anandamide transport inhibitor AM404

The endocannabinoids anandamide and 2-arachidonoyglycerol (2-AG) may contribute to the regulation of mood and emotion. In this study, we investigated the impact of the endocannabinoid transport inhibitor AM404 on three rat models of anxiety: elevated plus maze, defensive withdrawal and separation-induced ultrasonic vocalizations. AM404 (1-5 mg kg(-1), intraperitoneal (i.p.)) exerted dose-dependent anxiolytic-like effects in the three models. These behavioral effects were associated with increased levels of anandamide, but not 2-AG, in the prefrontal cortex and were prevented by the CB(1) cannabinoid antagonist rimonabant (SR141716A), suggesting that they were dependent on anandamide-mediated activation of CB(1) cannabinoid receptors. We also evaluated whether AM404 might influence motivation (in the conditioned place preference (CPP) test), sensory reactivity (acoustic startle reflex) and sensorimotor gating (prepulse inhibition (PPI) of the startle reflex). In the CPP test, AM404 (1.25-10 mg kg(-1), i.p.) elicited rewarding effects in rats housed under enriched conditions, but not in rats kept in standard cages. Moreover, AM404 did not alter reactivity to sensory stimuli or cause overt perceptual distortion, as suggested by its lack of effect on startle or PPI of startle. These results support a role of anandamide in the regulation of emotion and point to the anandamide transport system as a potential target for anxiolytic drugs.

Effects of inhibition of fatty acid amide hydrolase vs. the anandamide membrane transporter on TRPV1-mediated calcium responses in adult DRG neurons; the role of CB1receptors

The aim of the present study was to investigate the relationship between TRPV1 stimulation and endocannabinoid-driven CB(1) receptor-mediated inhibition of activity in adult rat dorsal root ganglion (DRG) neurons, a model of primary afferent nociceptors. Calcium-imaging studies were performed to compare the effects of the fatty acid amide hydrolase (FAAH) inhibitor URB597 (1 microm) vs. the anandamide (AEA) uptake inhibitor UCM707 (1 microm) on capsaicin (100 nm) and N-arachidonoyl dopamine (NADA; 1 microm)-evoked changes in intracellular calcium [Ca(2+)](i) in DRG neurons. The ability of the CB(1) receptor antagonist AM251 (1 microm) to modulate the effects of URB597 and UCM707 was also determined. Suprafusion of NADA and capsaicin evoked robust increases in [Ca(2+)](i) in DRG neurons (89 +/- 4% and 132 +/- 6% of the depolarizing KCl response, respectively). Co-incubation with URB597 significantly attenuated both NADA and capsaicin-evoked increases in [Ca(2+)](i) (39 +/- 3% and 79 +/- 4% of KCl response, respectively). Similarly, co-incubation with UCM707 significantly attenuated both NADA and capsaicin-evoked increases in [Ca(2+)](i) (59 +/- 7% and 72 +/- 4% of KCl response, respectively). The CB(1) receptor antagonist AM251 significantly attenuated the effects of URB597 on NADA-evoked increases in [Ca(2+)](i) but not the effects of URB597 on capsaicin-evoked increases in [Ca(2+)](i). By contrast, AM251 significantly attenuated the inhibitory effects of UCM707 on both NADA and capsaicin-evoked increases in [Ca(2+)](i.) These data suggest that transport of both NADA and capsaicin into DRG neurons and the subsequent activation of TRPV1 is partly governed by FAAH-dependent mechanisms as well as via the putative AEA membrane transporter.

Neurochemical effects of the endocannabinoid uptake inhibitor UCM707 in various rat brain regions

To date, UCM707, (5Z,8Z,11Z,14Z)-N-(3-furylmethyl)eicosa-5,8,11,14-tetraenamide, has the highest potency and selectivity in vitro and in vivo as inhibitor of the endocannabinoid uptake. Its biochemical, pharmacological and therapeutic properties have been intensely studied recently, but the information on its capability to modify neurotransmitter activity, which obviously underlies the above properties, is still limited. In the present study, we conducted a time-course experiment in rats aimed at examining the neurochemical effects of UCM707 in several brain regions following a subchronic administration (5 injections during 2.5 days) of this inhibitor in a dose of 5 mg/kg weight. In the hypothalamus, the administration of UCM707 did not modify GABA contents but reduced norepinephrine levels at 5 h after administration, followed by an increase at 12 h. Similar trends were observed for dopamine, whereas serotonin content remained elevated at 1 and, in particular, 5 and 12 h after administration. In the case of the basal ganglia, UCM707 reduced GABA content in the substantia nigra but only at longer (5 or 12 h) times after administration. There were no changes in serotonin content, but a marked reduction in its metabolite 5HIAA was recorded in the substantia nigra. The same pattern was found for dopamine, contents of which were not altered by UCM707 in the caudate-putamen, but its major metabolite DOPAC exhibited a marked decrease at 5 h. In the cerebellum, UCM707 reduced GABA, serotonin and norepinephrine content, but only the reduction found for norepinephrine at 5 h reached statistical significance. The administration of UCM707 did not modify the contents of these neurotransmitters in the hippocampus and the frontal cortex. Lastly, in the case of limbic structures, the administration of UCM707 markedly reduced dopamine content in the nucleus accumbens at 5 h, whereas GABA content remained unchanged in this structure and also in the ventral-tegmental area and the amygdala. By contrast, norepinephrine and serotonin content increased at 5 h in the nucleus accumbens, but not in the other two limbic structures. In summary, UCM707 administered subchronically modified the contents of serotonin, GABA, dopamine and/or norepinephrine with a pattern strongly different in each brain region. So, changes in GABA transmission (decrease) were restricted to the substantia nigra, but did not appear in other regions, whereas dopamine transmission was also altered in the caudate-putamen and the nucleus accumbens. By contrast, norepinephrine and serotonin were altered by UCM707 in the hypothalamus, cerebellum (only norepinephrine), and nucleus accumbens, exhibiting biphasic effects in some cases.

The endocannabinoid system as an emerging target of pharmacotherapy

The recent identification of cannabinoid receptors and their endogenous lipid ligands has triggered an exponential growth of studies exploring the endocannabinoid system and its regulatory functions in health and disease. Such studies have been greatly facilitated by the introduction of selective cannabinoid receptor antagonists and inhibitors of endocannabinoid metabolism and transport, as well as mice deficient in cannabinoid receptors or the endocannabinoid-degrading enzyme fatty acid amidohydrolase. In the past decade, the endocannabinoid system has been implicated in a growing number of physiological functions, both in the central and peripheral nervous systems and in peripheral organs. More importantly, modulating the activity of the endocannabinoid system turned out to hold therapeutic promise in a wide range of disparate diseases and pathological conditions, ranging from mood and anxiety disorders, movement disorders such as Parkinson's and Huntington's disease, neuropathic pain, multiple sclerosis and spinal cord injury, to cancer, atherosclerosis, myocardial infarction, stroke, hypertension, glaucoma, obesity/metabolic syndrome, and osteoporosis, to name just a few. An impediment to the development of cannabinoid medications has been the socially unacceptable psychoactive properties of plant-derived or synthetic agonists, mediated by CB(1) receptors. However, this problem does not arise when the therapeutic aim is achieved by treatment with a CB(1) receptor antagonist, such as in obesity, and may also be absent when the action of endocannabinoids is enhanced indirectly through blocking their metabolism or transport. The use of selective CB(2) receptor agonists, which lack psychoactive properties, could represent another promising avenue for certain conditions. The abuse potential of plant-derived cannabinoids may also be limited through the use of preparations with controlled composition and the careful selection of dose and route of administration. The growing number of preclinical studies and clinical trials with compounds that modulate the endocannabinoid system will probably result in novel therapeutic approaches in a number of diseases for which current treatments do not fully address the patients' need. Here, we provide a comprehensive overview on the current state of knowledge of the endocannabinoid system as a target of pharmacotherapy.

The emerging role of the endocannabinoid system in endocrine regulation and energy balance

During the last few years, the endocannabinoid system has emerged as a highly relevant topic in the scientific community. Many different regulatory actions have been attributed to endocannabinoids, and their involvement in several pathophysiological conditions is under intense scrutiny. Cannabinoid receptors, named CB1 receptor and CB2 receptor, first discovered as the molecular targets of the psychotropic component of the plant Cannabis sativa, participate in the physiological modulation of many central and peripheral functions. CB2 receptor is mainly expressed in immune cells, whereas CB1 receptor is the most abundant G protein-coupled receptor expressed in the brain. CB1 receptor is expressed in the hypothalamus and the pituitary gland, and its activation is known to modulate all the endocrine hypothalamic-peripheral endocrine axes. An increasing amount of data highlights the role of the system in the stress response by influencing the hypothalamic-pituitary-adrenal axis and in the control of reproduction by modifying gonadotropin release, fertility, and sexual behavior. The ability of the endocannabinoid system to control appetite, food intake, and energy balance has recently received great attention, particularly in the light of the different modes of action underlying these functions. The endocannabinoid system modulates rewarding properties of food by acting at specific mesolimbic areas in the brain. In the hypothalamus, CB1 receptor and endocannabinoids are integrated components of the networks controlling appetite and food intake. Interestingly, the endocannabinoid system was recently shown to control metabolic functions by acting on peripheral tissues, such as adipocytes, hepatocytes, the gastrointestinal tract, and, possibly, skeletal muscle. The relevance of the system is further strenghtened by the notion that drugs interfering with the activity of the endocannabinoid system are considered as promising candidates for the treatment of various diseases, including obesity.

CANNABINOID RECEPTORS AS THERAPEUTIC TARGETS

CB1 and CB2 cannabinoid receptors are the primary targets of endogenous cannabinoids (endocannabinoids). These G protein-coupled receptors play an important role in many processes, including metabolic regulation, craving, pain, anxiety, bone growth, and immune function. Cannabinoid receptors can be engaged directly by agonists or antagonists, or indirectly by manipulating endocannabinoid metabolism. In the past several years, it has become apparent from preclinical studies that therapies either directly or indirectly influencing cannabinoid receptors might be clinically useful. This review considers the components of the endocannabinoid system and discusses some of the most promising endocannabinoid-based therapies.

Inhibition of fatty acid amide hydrolase and monoacylglycerol lipase by the anandamide uptake inhibitor VDM11: evidence that VDM11 acts as an FAAH substrate

There is some dispute concerning the extent to which the uptake inhibitor VDM11 (N-(4-hydroxy-2-methylphenyl) arachidonoyl amide) is capable of inhibiting the metabolism of the endocannabinoid anandamide (AEA) by fatty acid amide hydrolase (FAAH). In view of a recent study demonstrating that the closely related compound AM404 (N-(4-hydroxyphenyl)arachidonylamide) is a substrate for FAAH, we re-examined the interaction of VDM11 with FAAH. In the presence of fatty acid-free bovine serum albumin (BSA, 0.125% w v(-1)), both AM404 and VDM11 inhibited the metabolism of AEA by rat brain FAAH with similar potencies (IC(50) values of 2.1 and 2.6 microM, respectively). The compounds were about 10-fold less potent as inhibitors of the metabolism of 2-oleoylglycerol (2-OG) by cytosolic monoacylglycerol lipase (MAGL). The potency of VDM11 towards FAAH was dependent upon the assay concentration of fatty acid-free bovine serum albumin (BSA). Thus, in the absence of fatty acid-free BSA, the IC(50) value for inhibition of FAAH was reduced by a factor of about two (from 2.9 to 1.6 microM). A similar reduction in the IC(50) value for the inhibition of membrane bound MAGL by both this compound (from 14 to 6 microM) and by arachidonoyl serinol (from 24 to 13 microM) was seen. An HPLC assay was set up to measure 4-amino-m-cresol, the hypothesised product of FAAH-catalysed VDM11 hydrolysis. 4-Amino-m-cresol was eluted with a retention time of approximately 2.4 min, but showed a time-dependent degradation to compounds eluting at peaks of approximately 5.6 and approximately 8 min. Peaks with the same retention times were also found following incubation of the membranes with VDM11, but were not seen when the membranes were preincubated with the FAAH inhibitors URB597 (3'-carbamoyl-biphenyl-3-yl-cyclohexylcarbamate) and CAY10401 (1-oxazolo[4,5-b]pyridin-2-yl-9-octadecyn-1-one) prior to addition of VDM11. The rate of metabolism of VDM11 was estimated to be roughly 15-20% of that for anandamide. It is concluded that VDM11 is an inhibitor of FAAH under the assay conditions used here, and that the inhibition may at least in part be a consequence of the compound acting as an alternative substrate.

UCM707, an inhibitor of the anandamide uptake, behaves as a symptom control agent in models of Huntington's disease and multiple sclerosis, but fails to delay/arrest the progression of different motor-related disorders

To date, UCM707, (5Z,8Z,11Z,14Z)-N-(3-furylmethyl)eicosa-5,8,11,14-tetraenamide, has the highest potency and selectivity in vitro and in vivo as inhibitor of the endocannabinoid uptake. This may enable this compound to potentiate endocannabinoid transmission, with minimal side effects, in the treatment of several neurological disorders. In the present study, we examined whether the treatment with UCM707 produced beneficial effects, as other cannabinoid-related compounds have already shown, to alleviate motor deterioration or to delay/arrest neurodegeneration, in several models of neurological diseases such as Huntington's disease (HD), Parkinson's disease (PD) and multiple sclerosis (MS). UCM707 exhibited a notable anti-hyperkinetic activity in a rat model of HD generated by bilateral intrastriatal application of 3-nitropropionic acid. This effect was possibly associated with an amelioration of GABA and glutamate deficits induced by the toxin in the globus pallidus and the substantia nigra, respectively. However, UCM707 did not protect against the death of GABAergic neurons that occurs in rats with striatal atrophy generated by unilateral application of malonate, another animal model of HD, which is more useful to test neuroprotective strategies. In addition, UCM707 did not provide neuroprotection in rats with unilateral lesions of the nigrostriatal dopaminergic neurons caused by 6-hydroxydopamine, a rat model of PD. This was possibly due to the fact that UCM707 is devoid of anti-oxidant properties since another uptake inhibitor, AM404, that has these properties acted as a protective agent. Lastly, UCM707 was also unable to inhibit the development of the neurological impairment of rats with experimental autoimmune encephalomyelitis (EAE), an acute model of MS. However, UCM707, like other endocannabinoid uptake inhibitors reported previously, significantly reduced spasticity of the hindlimbs in a chronic relapsing EAE mice, a chronic model of MS. In summary, UCM707 might be a promising compound in HD to alleviate motor symptoms, which represents an important goal considering the current lack of efficient pharmacological treatments in this basal ganglia disorder. However, the compound was unable to delay neurodegeneration in this disorder and also in PD. In addition, UCM707 did not produce any neurological recovery from inflammatory attack in an EAE rat model of MS, although it retained the classic anti-spastic action shown by other uptake inhibitors in the EAE mouse model of this disease.

Anandamide transport: A critical review

Anandamide (AEA) uptake has been described over the last decade to occur by facilitated diffusion, but a protein has yet to be isolated. In some cell types, it has recently been suggested that AEA, an uncharged hydrophobic molecule, passively diffuses through the plasma membrane in a process that is not protein-mediated. Since that observation, recent kinetics studies (using varying assay conditions) have both supported and denied the presence of an AEA transporter. In this review, we analyze the current literature exploring the mechanism of AEA uptake and endeavor to explain the reasons for the divergent views. One of the main variables among laboratories is the incubation time of the cells with AEA. Initial kinetics (at time points <1 min depending upon the cell type) isolate events that occur at the plasma membrane and are most useful to study saturability of uptake and effects of purported transport inhibitors upon uptake. Results with longer incubation times reflect events not only at the plasma membrane but also interactions at intracellular sites that may include enzyme(s), other proteins, or specialized lipid-binding domains. Furthermore, at long incubation times, antagonists to AEA receptors reduce AEA uptake. Another complicating factor in AEA transport studies is the nonspecific binding to plastic culture dishes. The magnitude of this effect may exceed AEA uptake into cells. Likewise, AEA may be released from plastic culture dishes (without cells) in such a manner as to mimic efflux from cells. AEA transport protocols using BSA, similar to the method used for fatty acid uptake studies, are gaining acceptance. This may improve AEA solution stability and minimize binding to plastic, although some groups report that BSA interferes with uptake. In response to criticisms that many transport inhibitors also inhibit the fatty acid amide hydrolase (FAAH), new compounds have recently been synthesized. Following their characterization in FAAH+/+ and FAAH-/- cells and transgenic mice, several inhibitors have been shown to have physiological activity in FAAH-/- mice. Their targets are now being characterized with the possibility that a protein transporter for AEA may be characterized.

Effect of repeated systemic administration of selective inhibitors of endocannabinoid inactivation on rat brain endocannabinoid levels

Several selective inhibitors of endocannabinoid inactivation via either the fatty acid amide hydrolase (FAAH) or the putative endocannabinoid transporter have been developed so far. Here, we have studied the effect in rats of a subchronic intraperitoneal treatment with three recently developed selective inhibitors of endocannabinoid uptake (VDM-11, UCM-707 and OMDM-2) or with a selective FAAH inhibitor (N-arachidonoyl-serotonin, AA-5-HT), on the brain levels of anandamide and 2-arachidonoylglycerol (2-AG) measured by means of isotope dilution LC-MS 1, 5 and 12 h after the last treatment. OMDM-2 was the most efficacious compound at enhancing the levels of anandamide at all time points, with a maximal effect (1.9-fold enhancement) after 5h. This compound also enhanced 2-AG levels by approximately 1.3-fold, but only 5 and 12h from administration. VDM-11 slightly, albeit significantly, enhanced anandamide levels (1.3-fold) only at 1h from administration and 2-AG levels (1.3-fold) only after 5h. Finally, UCM-707 only affected 2-AG levels (by two-fold) at only 1h from administration. FAAH inhibition by AA-5-HT significantly enhanced the levels of both anandamide (between 1.3- and 1.5-fold, maximal effect after 1 h) and 2-AG (between 1.3- and 1.6-fold, maximal effect after 12 h) at all time points. Brains from rats treated with AA-5-HT did never exhibit enhanced levels of serotonin, thus pointing to the metabolic stability of this FAAH inhibitor. These data indicate that: (1) the pharmacological effects reported so far for the four compounds under study in animal models of diseases may be due to enhancement of both anandamide and 2-AG levels; (2) 2-AG seems to need a longer time after the last administration in order to be augmented; (3) OMDM-2 and AA-5-HT should be regarded as enhancers of endocannabinoid levels suitable for use in vivo.

Activation of the endocannabinoid system as a therapeutic approach in a murine model of multiple sclerosis

Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) is a well-characterized murine model of human multiple sclerosis (MS) that closely resembles the chronic and progressive clinical form of the disease. Recent studies have described the involvement of the cannabinoid system in the progression of the disease and the benefits associated with the administration of cannabinoid agonists. With the objective to study whether "indirect" agonists, that is, compounds able to reinforce the physiological endocannabinoid transmission and, therefore, devoid of the psychotropic effects of "direct" agonists, could be suitable agents for the amelioration of MS neurological deficits, we administered the potent and selective anandamide uptake inhibitor UCM707 to TMEV-infected mice. Our results indicate that treatment during established disease significantly improves the motor function of the diseased mice. At the histological level, UCM707 is able to reduce microglial activation, diminish major histocompatibility complex class II antigen expression, and decrease cellular infiltrates in the spinal cord. Additionally, in microglial cells, UCM707 decreases the production of the proinflammatory cytokines tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-6; reduces nitric oxide levels and inducible nitric oxide synthase expression; and is able to potentiate the action of a subeffective dose of the endocannabinoid anandamide. Overall, these results suggest that agents able to activate the endocannabinoid system could constitute a new series of drugs for the treatment of MS.

Cannabinoid control of motor function at the basal ganglia

Classic and novel data strengthen the idea of a prominent role for the endocannabinoid signaling system in the control of movement. This finding is supported by three-fold evidence: (1) the abundance of the cannabinoid CB1 receptor subtype, but also of CB2 and vanilloid VR1 receptors, as well as of endocannabinoids in the basal ganglia and the cerebellum, the areas that control movement; (2) the demonstration of a powerful action, mostly of an inhibitory nature, of plant-derived, synthetic, and endogenous cannabinoids on motor activity, exerted by modulating the activity of various classic neurotransmitters; and (3) the occurrence of marked changes in endocannabinoid transmission in the basal ganglia of humans affected by several motor disorders, an event corroborated in animal models of these neurological diseases. This three-fold evidence has provided support to the idea that cannabinoid-based compounds, which act at key steps of the endocannabinoid transmission [receptors, transporter, fatty acid amide hydrolase (FAAH)], might be of interest because of their potential ability to alleviate motor symptoms and/or provide neuroprotection in a variety of neurological pathologies directly affecting basal ganglia structures, such as Parkinson's disease and Huntington's chorea, or indirectly, such as multiple sclerosis and Alzheimer's disease. The present chapter will review the knowledge on this issue, trying to establish future lines for research into the therapeutic potential of the endocannabinoid system in motor disorders.

The biosynthesis, fate and pharmacological properties of endocannabinoids

The finding of endogenous ligands for cannabinoid receptors, the endocannabinoids, opened a new era in cannabinoid research. It meant that the biological role of cannabinoid signalling could be finally studied by investigating not only the pharmacological actions subsequent to stimulation of cannabinoid receptors by their agonists, but also how the activity of these receptors was regulated under physiological and pathological conditions by varying levels of the endocannabinoids. This in turn meant that the enzymes catalysing endocannabinoid biosynthesis and inactivation had to be identified and characterized, and that selective inhibitors of these enzymes had to be developed to be used as (1) probes to confirm endocannabinoid involvement in health and disease, and (2) templates for the design of new therapeutic drugs. This chapter summarizes the progress achieved in this direction during the 12 years following the discovery of the first endocannabinoid.

THE ENDOCANNABINOID SYSTEM: PHYSIOLOGY AND PHARMACOLOGY

The endogenous cannabinoid system is an ubiquitous lipid signalling system that appeared early in evolution and which has important regulatory functions throughout the body in all vertebrates. The main endocannabinoids (endogenous cannabis-like substances) are small molecules derived from arachidonic acid, anandamide (arachidonoylethanolamide) and 2-arachidonoylglycerol. They bind to a family of G-protein-coupled receptors, of which the cannabinoid CB(1) receptor is densely distributed in areas of the brain related to motor control, cognition, emotional responses, motivated behaviour and homeostasis. Outside the brain, the endocannabinoid system is one of the crucial modulators of the autonomic nervous system, the immune system and microcirculation. Endocannabinoids are released upon demand from lipid precursors in a receptor-dependent manner and serve as retrograde signalling messengers in GABAergic and glutamatergic synapses, as well as modulators of postsynaptic transmission, interacting with other neurotransmitters, including dopamine. Endocannabinoids are transported into cells by a specific uptake system and degraded by two well-characterized enzymes, the fatty acid amide hydrolase and the monoacylglycerol lipase. Recent pharmacological advances have led to the synthesis of cannabinoid receptor agonists and antagonists, anandamide uptake blockers and potent, selective inhibitors of endocannabinoid degradation. These new tools have enabled the study of the physiological roles played by the endocannabinoids and have opened up new strategies in the treatment of pain, obesity, neurological diseases including multiple sclerosis, emotional disturbances such as anxiety and other psychiatric disorders including drug addiction. Recent advances have specifically linked the endogenous cannabinoid system to alcoholism, and cannabinoid receptor antagonism now emerges as a promising therapeutic alternative for alcohol dependence and relapse.

Role of the endocannabinoid system in Alzheimer's disease: New perspectives

The role of the endocannabinoid system in several diseases is currently under intense study. Among these, Alzheimer's disease may be a new promising area of research. We have recently reported the existence of profound changes in the location and density of several elements of this system in Alzheimer's disease tissue samples, indicating that a non-neuronal endocannabinoid system is up-regulated in activated glia. Additional data from other groups suggest that glial cells may be important elements in the regulation of endocannabinoid system activity, both in health as in disease. Some of these aspects are briefly discussed in the present review.

Involvement of vanilloid-like receptors in the effects of anandamide on motor behavior and nigrostriatal dopaminergic activity: in vivo and in vitro evidence

The administration of the endocannabinoid anandamide to rats produces hypokinesia in parallel to a decrease in the activity of nigrostriatal dopaminergic neurons. It was earlier hypothesized that this effect was mediated through the activation of CB(1) receptors, although these receptors have not been found in dopaminergic neurons, but in striatal projection neurons connected with them. However, two recent discoveries: (i) that anandamide is also able to activate vanilloid VR(1) receptors, and (ii) that VR(1) receptors are located on nigrostriatal dopaminergic neurons, allow to re-evaluate this hypothesis and suggest that the activation of vanilloid-like receptors rather than CB(1) receptors might be responsible of anandamide-induced hypokinesia and decreased nigrostriatal dopaminergic activity. To validate this new hypothesis, we carried out two different experiments. First, we explored whether the inhibitory effects of anandamide on motor activity and dopaminergic transmission were reversed by capsazepine, an antagonist for vanilloid-like receptors. Our data demonstrated that anandamide reduced ambulation, stereotypies and exploration, measured in the open-field test, whereas it increased the time spent in inactivity. All these effects were completely reversed by capsazepine, which had no effect by itself. Anandamide also caused a significant decrease in nigrostriatal dopaminergic activity, reflected by a reduction in DOPAC contents in the caudate-putamen, which was also reversed by capsazepine. As a second objective, we explored whether anandamide is able to directly influence nigrostriatal dopaminergic function by examining its effects on in vitro dopamine (DA) release using perifused striatal fragments. Our data confirmed that anandamide significantly decreased K(+)-stimulated dopamine release from nigrostriatal terminals and that this effect was vanilloid-like receptor-mediated since it was prevented by capsazepine. This in vitro inhibitory effect was not seen with a classic cannabinoid agonist that does not bind vanilloid-like receptors. In summary, anandamide behaves as a hypokinetic substance, thus producing motor depression in the open-field test, presumably related to a decrease in nigrostriatal dopaminergic activity. These effects were completely reversed by the vanilloid-like receptor antagonist capsazepine, thus indicating a role of these receptors, which are located on dopaminergic neurons, in mediating hypokinetic effects of anandamide. In vitro studies, using perifused striatal fragments, support this vanilloid-like receptor-mediated direct action, which would not be available for classic cannabinoid agonists.

In vivo pharmacological actions of two novel inhibitors of anandamide cellular uptake

Two inhibitors of the cellular uptake of the endocannabinoid anandamide, (R)-N-oleoyl-(1'-hydroxybenzyl)-2'-ethanolamine and (S)-N-oleoyl-(1'-hydroxybenzyl)-2'-ethanolamine (OMDM-1 and OMDM-2, respectively), were recently synthesized, and their in vitro pharmacological activity described. Here we have assessed their activity in two typical pharmacological responses of cannabimimetic compounds. We first examined whether these compounds exert any effect per se on locomotion and pain perception in rats, and/or enhance the effects of anandamide on these two processes. We compared the effects of the novel compounds with those produced by a previously developed selective inhibitor, N-arachidonoyl-(2-methyl-4-hydroxyphenyl)amine (VDM-11). When assayed alone, OMDM-1 and OMDM-2 (1-10 mg/kg, i.p.) did not affect any of the five motor parameters under investigation, although the former compound exhibited a trend for the inhibition of ambulation, fast movements, and speed in rats. OMDM-2 and, to a lesser extent, VDM-11 (5 mg/kg, i.p.) enhanced the motor-inhibitory effects of a noneffective dose (2 mg/kg, i.p.) of anandamide, while OMDM-1 did not. In a typical test of acute analgesia, OMDM-2 and VDM-11 (1-10 mg/kg, i.p.), but not OMDM-1, significantly enhanced the time spent by rats on a "hot plate." However, the same compounds (5 mg/kg, i.p.) did not enhance the analgesic effect of a subeffective dose (2 mg/kg, i.p.) of anandamide, whereas OMDM-1 exerted a strong trend towards potentiation (P=0.06). We next explored the possible use of the two novel compounds in a pathological condition. Thus, we determined if, like other previously developed anandamide reuptake inhibitors, OMDM-1 and OMDM-2 inhibit spasticity in an animal model of multiple sclerosis-the chronic relapsing experimental allergic encephalomyelitis in mice. As previously shown with a higher dose of VDM-11, both novel compounds (5 mg/kg, i.v.) significantly reduced spasticity of the hindlimb in mice with chronic relapsing experimental allergic encephalomyelitis. We suggest that OMDM-1 and, particularly, OMDM-2 are useful pharmacological tools for the study of the (patho)physiological role of the anandamide cellular uptake process, and represent unique templates for the development of new antispastic drugs.

CB1 cannabinoid receptors and on-demand defense against excitotoxicity

Abnormally high spiking activity can damage neurons. Signaling systems to protect neurons from the consequences of abnormal discharge activity have been postulated. We generated conditional mutant mice that lack expression of the cannabinoid receptor type 1 in principal forebrain neurons but not in adjacent inhibitory interneurons. In mutant mice,the excitotoxin kainic acid (KA) induced excessive seizures in vivo. The threshold to KA-induced neuronal excitation in vitro was severely reduced in hippocampal pyramidal neurons of mutants. KA administration rapidly raised hippocampal levels of anandamide and induced protective mechanisms in wild-type principal hippocampal neurons. These protective mechanisms could not be triggered in mutant mice. The endogenous cannabinoid system thus provides on-demand protection against acute excitotoxicity in central nervous system neurons.

Role of endogenous cannabinoids in synaptic signaling

Research of cannabinoid actions was boosted in the 1990s by remarkable discoveries including identification of endogenous compounds with cannabimimetic activity (endocannabinoids) and the cloning of their molecular targets, the CB1 and CB2 receptors. Although the existence of an endogenous cannabinoid signaling system has been established for a decade, its physiological roles have just begun to unfold. In addition, the behavioral effects of exogenous cannabinoids such as delta-9-tetrahydrocannabinol, the major active compound of hashish and marijuana, await explanation at the cellular and network levels. Recent physiological, pharmacological, and high-resolution anatomical studies provided evidence that the major physiological effect of cannabinoids is the regulation of neurotransmitter release via activation of presynaptic CB1 receptors located on distinct types of axon terminals throughout the brain. Subsequent discoveries shed light on the functional consequences of this localization by demonstrating the involvement of endocannabinoids in retrograde signaling at GABAergic and glutamatergic synapses. In this review, we aim to synthesize recent progress in our understanding of the physiological roles of endocannabinoids in the brain. First, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. The fine-grain anatomical distribution of the neuronal cannabinoid receptor CB1 is described in most brain areas, emphasizing its general presynaptic localization and role in controlling neurotransmitter release. Finally, the possible functions of endocannabinoids as retrograde synaptic signal molecules are discussed in relation to synaptic plasticity and network activity patterns.

Design, synthesis, and biological evaluation of new inhibitors of the endocannabinoid uptake: comparison with effects on fatty acid amidohydrolase

A new series of arachidonic acid derivatives were synthesized and evaluated as inhibitors of the endocannabinoid uptake. Most of them are able to inhibit anandamide uptake with IC(50) values in the low micromolar range (IC(50) = 0.8-24 microM). In general, the compounds had only weak effects upon CB(1), CB(2), and VR(1) receptors (K(i) > 1000-10000 nM). In addition, there was no obvious relationship between the abilities of the compounds to affect anandamide uptake and to inhibit anandamide metabolism by fatty acid amidohydrolase (FAAH; IC(50) = 30-113 microM). This indicates that the compounds do not exert their effects secondarily to FAAH inhibition. It is hoped that these compounds, particularly the most potent in this series (compound 5, UCM707, with IC(50) values for anandamide uptake and FAAH of 0.8 and 30 microM, respectively), will provide useful tools for the elucidation of the role of the anandamide transporter system in vivo.

Design, synthesis and biological evaluation of new endocannabinoid transporter inhibitors

In the present work we describe the synthesis and the in vitro evaluation of a series of arachidonic acid derivatives of general structure I as endocannabinoid transporter inhibitors. In addition, we report the first in vivo studies of the most potent derivative (4, UCM707) within this series. The majority of compounds studied are highly potent (IC(50)=24-0.8 micro M) and selective endocannabinoid uptake inhibitors with very low affinities for either the enzyme fatty acid amide hydrolase (IC(50)=30-113 micro M) or for cannabinoid receptor subtype 1 (CB(1)), cannabinoid receptor subtype 2 (CB(2)) and vanilloid receptor subtype 1 (VR(1)) (K(i)=1000-10000 nM). Among them, (5Z,8Z,11Z,14Z)-N-(fur-3-ylmethyl)icosa-5,8,11,14-tetraenamide (UCM707) behaves as the most potent endocannabinoid transporter inhibitor described to date (IC(50)=0.8 micro M) and exhibits improved potency for the anandamide transporter, high selectivity for CB(1) and VR(1) receptors, and modest selectivity for CB(2). In vivo it enhances the analgesia and hypokinetic effects induced by a subeffective dose of anandamide.

Endogenous cannabinoid system as a modulator of food intake

The ability of Cannabis sativa (marijuana) to increase hunger has been noticed for centuries, although intensive research on its molecular mode of action started only after the characterization of its main psychoactive component Delta(9)-tetrahydrocannabinol in the late 1960s. Despite the public concern related to the abuse of marijuana and its derivatives, scientific studies have pointed to the therapeutic potentials of cannabinoid compounds and have highlighted their ability to stimulate appetite, especially for sweet and palatable food. Later, the discovery of specific receptors and their endogenous ligands (endocannabinoids) suggested the existence of an endogenous cannabinoid system, providing a physiological basis for biological effects induced by marijuana and other cannabinoids. Epidemiological reports describing the appetite-stimulating properties of cannabinoids and the recent insights into the molecular mechanisms underlying cannabinoid action have proposed a central role of the cannabinoid system in obesity. The aim of this review is to provide an extensive overview on the role of this neuromodulatory system in feeding behavior by summarizing the most relevant data obtained from human and animal studies and by elucidating the interactions of the cannabinoid system with the most important neuronal networks and metabolic pathways involved in the control of food intake. Finally, a critical evaluation of future potential therapeutical applications of cannabinoid antagonists in the therapy of obesity and eating disorders will be discussed.

Cannabinergic ligands

The understanding of the pharmacology surrounding the cannabinergic system has seen many advances since the discovery of the CB1 receptor in the mammalian brain and the CB2 receptor in the periphery. Among these advances is the discovery of the endogenous ligands arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol amide (2-AG), which are selective agonists for the CB1 and CB2 receptors, respectively. These endogenous neuromodulators involved in the cannabinergic system are thought to be produced on demand and are metabolized by the enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAG lipase). Recently, we characterized a reuptake system that facilitates the transport of anandamide across the cell membrane and subsequently developed selective inhibitors of this transport, which have been found to have therapeutic potential as analgesic and peripheral vasodilators. The cannabinergic proteins currently being explored, which include the CB1 and CB2 receptors, FAAH and the anandamide transporter, are excellent targets for the development of therapeutically useful drugs for a range of conditions including pain, loss of appetite, immunosuppression, peripheral vascular disease and motor disorders. As cannabinoid research has progressed, various potent and selective cannabimimetic ligands, targeting these four cannabinoid proteins, have been designed and synthesized. Many of these ligands serve as important molecular probes, providing structural information regarding the binding sites of the cannabinergic proteins, as well as pharmacological tools, which have been playing pivotal roles in research aimed at understanding the biochemical and physiological aspects of the endocannabinoid system. This review will focus on some of the current cannabinergic ligands and probes and their pharmacological and therapeutic potential.