Current Drug Targets-CNS & Neurological Disorders - Current Issue

This theme issue is dedicated to my children, Giuseppe and Claudia and to my wife, Gianna. Marijuana is one of the most widely used drugs in the world, and its extracts have been used for centuries in folklore medicine. Yet the identity of its psychoactive constituent, delta-9-tetrahydrocannabinol (Δ9-THC), remained elusive until 1964, when Raphael Mechoulam's group revealed it. Then, the field had to wait until 1988 for the first evidence, presented by Allyn Howlett and colleagues, that a specific cannabinoid receptor exists in the brain. This discovery raised the question of the reason why our brain, as well as most peripheral tissues, contain cannabinoid receptors, that are unlikely to exist in mammalian tissues for the sake of a plant constituent like Δ9-THC. On this background, several groups have started intense research aimed at finding the endogenous ligands of cannabinoid receptors, the so-called "endocannabinoids". Two of such molecules were discovered in the early '90s: N-arachidonoylethanolamine (anandamide) was found in 1992 by Mechoulam's group, and 2- archidonoylglycerol was reported in 1995, independently by the group of Takayuki Sugiura and by that of Mechoulam. Since then the field has attracted growing interest, and a bunch of molecules and proteins have been discovered. These include endocannabinoids and endocannabinoid-like compounds (i.e., amides, esters and ethers of (poly-un)saturated fatty acids), cannabinoid and non-cannabinoid receptors, synthetic and degradative enzymes, as well as membrane transporters, that are at the moment the most debated topic in the field. Overall, these components form the "endocannabinoid system". The growing interest towards this endogenous system of lipids and proteins is demonstrated by the number of papers that appeared in PubMed over the last decade. The word "endocannabinoid" scores 37 entries in 1992-1995, 240 in 1997-2000, and 659 in 2002-2005 ("cannabinoid" scores 428, 911 and 1600 in the same periods of time). If "endocannabinoid" is matched with "central nervous system (CNS)" the entries are 15, 83 and 246, suggesting that more than one third of the field is strictly related to the CNS. The same indication comes from the analysis of "cannabinoid", that counts 106, 298 and 466 entries, when matched with "CNS". Of interest is also the fact that the therapeutic implications of (endo)cannabinoid-oriented drugs have expanded over the last few years, thus the match "endocannabinoid" and "drug development" leads to 0 entries in 1992-1995, 15 in 1997-2000, and 65 in 2002-2005 (for the match with "cannabinoid" the figures are 23, 57 and 146, respectively). As a matter of fact, at least two synthetic cannabinoids are in advanced phase III clinical trials: SR141716 (rimonabant), and HU-211 (dexanabinol). The first compound has been developed by Sanofi-Aventis, and is an appetite modulator; the second compound, developed by Pharmos, is a neuroprotectant in head trauma. Both drugs may represent pharmaceutical breakthroughs in important therapeutic areas of human disease. This brings to the present book. This theme-issue is an outstanding collection of hot topics in the endocannabinoid research, and the review articles should form the basis to better understand the molecular background of the ongoing clinical applications of endocannabinoid-targeted drugs, which have generated great expectations as conceptually new therapeutics. I hope that the aspects of endocannabinoid research summarized in this book may foster novel ideas, boosting investigations into new directions of basic science and clinic, thus ultimately improving our understanding and therapeutic exploitation of the endocannabinoid system.

Endocannabinoids, defined in 1995 as endogenous agonists of cannabinoid receptors, their anabolic and catabolic pathways, and the enzymes involved in these pathways (the "endocannabinoid enzymes"), are the subject of this review. A general strategy seems to apply to the regulation of the levels of the two major endocannabinoids, anandamide and 2-arachidonoylglycerol (2-AG). Five endocannabinoid enzymes have been cloned to date: two are responsible for the biosynthesis and degradation of anandamide, the NAPE-selective phospholipase D and the fatty acid amide hydrolase, respectively; the other three catalyse the biosynthesis and degradation of 2-AG, the sn-1-selective diacylglycerol lipases α and β and the monoacylglycerol lipase, respectively. The major features of these five proteins, their relative weight in determining endocannabinoid levels, and the possible targeting of some of them for therapeutic purpose, as well as the possibility of the existence of alternative anabolic and catabolic pathways are discussed.

Endocannabinoid circuits have been shown to regulate a number of important pathways including pain, feeding, memory and motor coordination. Direct manipulation of endocannabinoid tone, therefore, may relieve disease symptoms related to analgesia, obesity, Alzheimer's and Parkinson's in humans. The endocannabinoid circuit involves two cloned receptors: CB1 in the CNS and CB2 in the periphery; endogenously produced ligands including anandamide, 2-arachidonylglycerol and palmitoylethanolamide; and enzymes that degrade endocannabinoid ligands to terminate signaling. Currently, three enzymes have been characterized with the ability to hydrolyze endocannabinoids: fatty acid amide hydrolase (FAAH), monoglyceride lipase (MGL) and N-acylethanolamine-hydrolyzing acid amidase (NAAA). The purpose of this review is to examine the molecular biology for the enzymes that hydrolyze endocannabinoids covering the protein activity and expression, mRNA characterization, genomic locus organization, promoter analysis and knockout phenotypes.

Retrograde synaptic signaling influences both short-term and long-term plasticity of the brain, in both excitatory and inhibitory synapses. During the last few years it has become apparent that the endogenous ligands for the cannabinoid CB1 receptor, the "endocannabinoids", fulfill an essential role in the brain as retrograde synaptic messengers, in a number of structures including the hippocampus, cerebellum and the limbic and mesocortical systems. This seminal discovery provides a cellular basis for the well known ubiquitous role of the endocannabinoids and their receptors (together, the "ECBR" system) in virtually all brain functions studied. This review will relate the anatomical distribution of the endocannabinoids and their CB1 receptors to functions of the ECBR system, as much as possible in light of the endocannabinoids as retrograde synaptic messengers. Functional implications of the high rates of co-localization with cholecystokinin (CCK), will also be considered. The most obvious function to be profoundly affected by the retrograde synaptic role of the endocannabinoids is memory. However, additional functions and dysfunctions such as reward and addiction, motor coordination, pain perception, feeding and appetite, coping with stress, schizophrenia and epilepsy will also be reviewed. Finally, the widespread presence of the ECBR system in the brain also lends a scientific basis for the development of cannabinoid-based medicines. The same ubiquity of the ECBR system however, should also be taken into consideration with respect to possible adverse side effects and addictive potential of such pharmaceutical developments.

Drug dependence is a chronically relapsing disorder, manifested as an intense desire for the drug, with impaired ability to control the urges to take the drug, even at the expense of serious adverse consequences. These behavioral abnormalities develop gradually during repeated exposure to a drug of abuse, and can persist for months or years after discontinuation of use, suggesting that this addiction can be considered a form of drug-induced neural plasticity. Many neurotransmitters, including gamma-aminobutyrric acid (GABA), glutamate, acetylcholine, dopamine, serotonin and endogenous opioid peptides, have been implicated in the effects of the various drugs of abuse. Dopamine has been consistently associated with the reinforcing effects of most of them. There is, in addition, a growing body of evidence that the endogenous cannabinoid system might participate in the motivational and dopamine-releasing effects of several drugs of abuse. This review will discuss the latest advances on the mechanisms of cannabinoid dependence and the possible role of the endocannabinoid system in the treatment of addiction, not only to marijuana but also to the other common illicit drugs.

Two topics are presented in this review. In the first section, we review data regarding the effects of the endocannabinoids (eCBs) and cannabinoid receptors on neuroimmune function. The function of eCBs in the interaction between the immune system and the central nervous system (CNS) is of particular interest, since the CNS itself is a rich source of eCBs while being exquisitely sensitive to inflammation. There are several sites at which cannabinoids can influence neuroinflammation. Microglial cells express both CB receptors and make eCBs. Activation of CB receptors on these cells seems to promote migration and proliferation but to reduce activation to macrophages. In several neurodegenerative diseases, up-regulation of microglial CB2 receptors have been observed. It is our hypothesis that microglial CB receptor activity is anti-inflammatory and could be exploited to manipulate neuroinflammatory processes with a minimum of unwanted effects. The second topic discussed suggests that the eCB/CB1 receptor pair is involved in the responses of animals to acute, repeated and variable stress. The roles of this pair are complex and dependent upon previous stress, among other things. Dysfunctional responding to stress is a component of several human neuropsychiatric disorders, including anxiety and panic disorders, post-traumatic stress disorders, premenstrual dysphoria and quite possibly, drug abuse. While it is too early to say with certainty, it is very possible that either inhibition or potentiation of endocannabinoid signaling will be an efficacious novel therapeutic approach to more than one human psychiatric disease.

An Increasing body of evidence suggests that cannabinoids have beneficial effects on the symptoms of multiple sclerosis, including spasticity and pain. Endogenous molecules with cannabinoid-like activity, such as the "endocannabinoids", have been shown to mimic the anti-inflammatory properties of cannabinoids through the cannabinoid receptors. Several studies suggest that cannabinoids and endocannabinoids may have a key role in the pathogenesis and therapy of multiple sclerosis. Indeed, they can down regulate the production of pathogenic T helper 1- associated cytokines enhancing the production of T helper 2-associated protective cytokines. A shift towards T helper 2 has been associated with therapeutic benefit in multiple sclerosis. In addition, cannabinoids exert a neuromodulatory effect on neurotransmitters and hormones involved in the neurodegenerative phase of the disease. In vivo studies using mice with experimental allergic encephalomyelitis, an animal model of multiple sclerosis, suggest that the increase of the circulating levels of endocannabinoids might have a therapeutic effect, and that agonists of endocannabinoids with low psychoactive effects could open new strategies for the treatment of multiple sclerosis.

Cannabinoids, such as the Δ9- tetrahydrocannabinol (THC), present in the cannabis plant, as well as anandamide and 2-arachidonoyl glycerol, produced by the mammalian body, have been shown to protect the brain from various insults and to improve several neurodegenerative diseases. The current review summarizes the evidence for cannabinoid neuroprotection in vivo, and refers to recent in vitro studies, which help elucidate possible molecular mechanisms underlying this protective effect. Some of these mechanisms involve the activation of CB1 and CB2 cannabinoid receptors, while others are not dependent on them. In some cases, protection is due to a direct effect of the cannabinoids on neuronal cells, while in others, it results from their effects on non-neuronal elements within the brain. In many experimental set-ups, cannabinoid neurotoxicity, particularly by THC, resides side by side with neuroprotection. The current review attempts to shed light on this dual activity, and to dissociate between the two contradictory effects.

Clinical trial data are beginning to emerge with respect to the therapeutic efficacy of cannabis extracts for the treatment of chronic pain. Although there is some evidence of efficacy, a major issue concerns the narrow margin between doses producing therapeutic effects and those producing the "highs" associated with cannabis misuse. In addition, longterm use is associated with an increased risk of psychiatric illness. These negative aspects constrain the doses of cannabis extracts and psychoactive cannabinoids that can be given to patients, and raise the risk that properly conducted clinical trials with too low dosages will impact negatively on subsequent drug development in this field. However, recent research has opened up a number of avenues whereby compounds acting directly upon cannabinoid (CB) receptors may have therapeutic potential. In this review, two such areas are discussed, namely a) the possible use of peripherally acting CB agonists and CB2 receptor-selective agonists for the treatment of pain, and b) the possible utility of CB2 receptor agonists for the prevention of stress-induced exacerbations of skin disorders such as psoriasis. A second area of drug development at present is that of CB1 receptor antagonists / inverse agonists, spearheaded by rimonabant, for the treatment of obesity and as an aid for smoking cessation. An important aspect of these compounds is their efficacy and selectivity, and this is discussed in detail in the present review.

The study of the cannabinoids can be established in the middle sixties with the elucidation of the structure of the active principle of Cannabis sativa plant, the Δ9-tetrahydrocannabinol. However, the existence of an endogenous cannabinoid system (ECS) has not been unequivocally accepted until recently. The last two decades have witnessed an impressive advance in the knowledge about cannabinoids, their chemistry, the enzymes involved in their metabolism, and their physiological and pathological roles. In particular, we have made progress in modifying the activity of the ECS with selective compounds, validating the ECS as a new therapeutic target. Endocannabinoids play a role in physiological and pathological processes, and their levels are affected in several disorders. Therefore, it should be possible to ameliorate these pathologies by correcting their altered levels. This review focuses on the current therapeutic opportunities of endocannabinoid-directed drugs, and pays special attention to the therapeutic possibilities underlying the inhibition of the endocannabinoid inactivation. The strategy of manipulating the ECS might open new avenues in the development of therapeutic approaches for a number of disorders, both central and peripheral, that lack as yet effective treatments.

The endogenous cannabinoids (endocannabinoids) are bioactive signaling molecules, that show diverse cellular and physiological effects and play various roles in the central nervous system, as well as in the periphery. The discovery of N-arachidonoylethanolamine (anandamide, AEA) and of the enzyme that terminates its signaling, i. e. fatty acid amide hydrolase (FAAH), has inspired pharmacological strategies to augment endocannabinoid tone and biological activity through inhibition of FAAH. Here we discuss the role of natural endocannabinoid derivatives, like the hydroxyanandamides (OH-AEAs) generated from AEA via lipoxygenase activity, as powerful inhibitors of FAAH. We propose that these compounds, by reversibly inhibiting FAAH, may control in vivo the endocannabinoid tone. We consider the theoretical structural properties of OH-AEAs and other natural inhibitors of FAAH, based on the calculation of theoretical molecular descriptors commonly used in Quantitative Structure Activity Relationship (QSAR) studies. The QSAR properties of OH-AEAs and congeners suggest that they could act at different specific sites of FAAH, thus confirming their potential value as templates for the development of next-generation therapeutics.