Current Neuropharmacology - Volume 2, Issue 1, 2004

Cannabinoid Special Issue The progress of cannabinoid research is of growing interest to both government and the general public. This is evidenced by the plethora of articles on the topic in popular science press. Here are some recent quotes: “The painkilling effect of cannabis can be reproduced by boosting the effect of the body's own cannabislike chemicals. The finding raises the prospect of painkillers that do the same trick as smoking a joint but without any side-effects” New Scientist, July, 2001. “Do decades of dope-smoking wreck cannabis users' memory and concentration? Or is this just another anti-marijuana myth?” New Scientist, August 2002. Anecdotal evidence for the medical applications of cannabis that dates back hundreds of years, can increasingly be viewed in the context of a growing body of scientific and clinical research literature. A brief search of web of science reveals that the number of publications on cannabinoids has doubled in four years to 407 in 2002. This should convince even the most sceptical of the huge potential of the endocannabinoid system as a novel pharmacological target. Contrary to popular opinion cannabinoid researchers are not a company of ageing hippies clad in psychedelic lab coats! We have, however, transformed from being the beleaguered last speaker at biomedical conferences, addressing a snoozing, sparsely-populated lecture theatre, to being at the centre of an exciting ‘hot topic’ in pharmacology. In this special issue of Current Neuropharmacology we seek to give a flavour of the most exciting new developments in the pharmacology of cannabinoids. The contents of this special issue signal the huge scope and implications of the endocannabinoid system from the receptor and enzyme level to the enticing novel therapeutic applications.

Cannabis is not only a widely abused drug, but also has the potential for the development of useful agents for the treatment of emesis, anorexia and several neurological disorders. In this article we will review the biology of endogenous cannabinoid system and the effects of modulation of transmitter release by cannabinoids in the nervous system. During the past decade, two cannabinoid receptors, named CB1 and CB2, were identified. Putative endogenous ligands at these receptors were discovered as anandamide and 2-arachidonoylglycerol, and several research tools were identified, including receptor agonists and antagonists, antibodies, antisense oligodeoxynucleotides, and CB1 and CB2 receptor knockout mice. CB1 receptor is negatively coupled to adenylate cyclase and is either negatively or positively associated to ion channels. The localization of CB1 receptors justifies the effects of cannabinoids in the central nervous system, including the control of movement, memory impairment, analgesia and addiction. The main function of the endocannabinoid system is to regulate synaptic transmission in excitatory and inhibitory pathways in the brain. In this respect, it is relevant that the majority of cannabinoid receptors is located presynaptically on neurons where their activation causes the inhibition of the release of the respective neurotransmitter, an action that is shared in common with opioid receptors. CB1 receptor-mediated inhibition of transmitter release might explain the reinforcing properties and memory impairment caused by cannabinoids. Moreover, it may be relevant to the therapeutic potentials of cannabinoids.

There is convincing evidence that mammalian tissues express at least two types of cannabinoid receptor, CB1 and CB2, and that the endogenous cannabinoid, anandamide, and certain other eicosanoid agonists for known cannabinoid receptors can also activate vanilloid (VR1) receptors. Evidence is now also emerging that in addition to these established receptors for cannabinoids, other pharmacological targets for eicosanoid and / or non-eicosanoid cannabinoids are present in neuronal or non-neuronal tissues that include brain, spinal cord, microglial cells, heart, certain arteries, small intestine, vas deferens and peritoneum. Among new receptors to have been proposed for cannabinoids are CB2-like receptors in mouse paw and peritoneum, receptors for abnormal-cannabidiol in microglial cells and in arterial endothelial and non-endothelial cells, Gprotein coupled receptors for R-(+)-WIN55212 and anandamide in brain and spinal cord, receptors for Δ9- tetrahydrocannabinol and cannabinol on perivascular sensory nerves, α2-adrenoceptor-like (imidazoline) receptors at sympathetic nerve terminals and VR1-like receptors on glutamatergic neurons in hippocampus and dentate gyrus. The presence of novel allosteric sites for cannabinoids on delayed rectifier potassium channels and on 5-HT3, muscarinic M1 and M4, and glutamate GLUA1 and GLUA3 receptors has also been proposed. Current evidence for the existence of these new molecular targets for cannabinoids is summarized in this review. This evidence is largely pharmacological in nature, much of it coming from functional or binding assays with established or novel ligands, sometimes performed using tissues or cell lines that do not express CB1 or CB2 receptors. None of the proposed new cannabinoid receptors have yet been cloned.

G protein-coupled CB1 cannabinoid receptors are found in high density in the nervous system. CB1 cannabinoid receptors have the ability to change conformation between inactive and active receptor states in the absence of agonists. The ability to adopt an active conformation in the absence of agonists results in constitutive receptor signaling. Inverse agonists reverse the constitutive activity of the receptor in the absence of agonists also antagonize receptor activity due to the presence of agonists. This dual effect of the CB1 cannabinoid receptor inverse agonist SR141716A is a result of its ability to bind both inactive and active receptors, but with a relatively higher affinity for the inactive receptor. The higher affinity of SR141716A for the inactive receptor is due to its ability to hydrogen bond to lysine 3.28 in transmembrane helix 3, a residue available to SR141716A only in the inactive state.

Endocannabinoids are thought to act as retrograde messengers in the central nervous system. By activating presynaptic cannabinoid CB1 receptors they can reduce neurotransmitter release and modulate synaptic plasticity. To date, anandamide and 2-arachidonoylglycerol (2- AG) are the best studied endocannabinoids. The life span of these lipid molecules in the extracellular space is an important factor in the regulation of their cellular responses. In this review we will discuss the metabolic fate of endocannabinoids, i.e. the mechanisms leading to the termination and / or modification of their actions. It is thought that endocannabinoids can be inactivated via a two-step mechanism. First, endocannabinoids are proposed to be translocated into the cell via selective transporter(s). However, the elusive nature of the putative protein responsible for endocannabinoid uptake has initiated a debate on its existence. Evidence in favor and against will be discussed. Once inside the cell, two major metabolic pathways act upon endocannabinoids: hydrolysis and oxygenation. Hydrolysis of the amide or ester function in anandamide and 2-AG, respectively, terminates their activity on cannabinoid receptors. The proteins responsible for their hydrolysis, fatty acid amide hydrolase and monoacylglycerol lipase, have been cloned and studied in detail. Much less is known about the oxygenation pathways. Lipoxygenase- and cycloxygenase-catalyzed oxygenation of endocannabinoids has been shown to generate a new array of possible biologically active compounds, such as the prostamides and the prostaglandin-glycerols, acting upon novel molecular targets. We will discuss the formation and the possible actions of these novel endocannabinoid derivatives.

Recent electrophysiological studies have clarified that endogenous cannabinoids (endocannabinoids) mediate retrograde signals from postsynaptic neurons to presynaptic terminals in the CNS. Endocannabinoids can be released from postsynaptic neurons following depolarization-induced elevation of intracellular Ca2+ concentration. The released endocannabinoids act retrogradely onto presynaptic cannabinoid CB1 receptors and suppress inhibitory or excitatory neurotransmitter release. This type of modulation has been termed depolarizationinduced suppression of inhibition (DSI) or excitation (DSE). Endocannabinoid release and resultant retrograde suppression of transmitter release are also triggered by activation of group I metabotrophic glutamate receptors (mGluRs) or muscarinic acetylcholine receptors (mAChRs) in the postsynaptic neurons. This pathway can work independently of the depolarization-induced mechanism, because group I mGluR activation can produce endocannabinoids when the postsynaptic neuron is loaded with the fast Ca2+ buffer BAPTA. Nevertheless, evidence suggests that these two pathways for endocannabinoid production can work cooperatively in neurons. It is shown that DSI is enhanced significantly when group I mGluRs, or mAChRs is activated simultaneously by its specific agonists. This enhancement is much more prominent than what is expected from the simple summation of depolarization-induced and group I mGluR / mAChR-induced endocannabinoid release. Activation of group I mGluRs or mAChRs causes no significant change in depolarization-induced Ca2+ transients, indicating that the enhanced DSI does not result from the augmentation of Ca2+ influx. The CB1, group I mGluRs, mAChRs and voltage-gated Ca2+ channels are all expressed widely in the CNS. Thus, the endocannabinoid-mediated retrograde modulation is an important and widespread mechanism for the regulation of synaptic transmission in the CNS.

DRG neurones are a heterogeneous population of neurones that vary in sensory transduction mechanisms, myelination, axon diameter, cell soma size, neurotransmitter content, receptor and ion channel expression. The generation, conduction, integration and transmission of pain signals begin in DRG neurones and they remain an important target for therapeutic intervention in pain disorders. In this review we discuss the expression of cannabinoid receptors in DRG neurones. We will also discuss the actions and molecular mechanisms of cannabinoid receptor ligands in the context of modulating sensory neurone excitability. Evidence suggests that vanilloid and cytokine receptor functions, voltageactivated Ca2+ and K + channels and Ca2+-activated channels are all either direct or indirect sites that might be modulated by synthetic and endogenous cannabinoid ligands.

The endocannabinoid system, consisting of cannabinoid1 (CB1) and cannabinoid2 (CB2) receptors, endogenous cannabinoid ligands and metabolising enzymes, is present throughout the pathways mediating responses to painful stimuli. Electrophysiological and behavioural studies have demonstrated that endocannabinoids, as well as plant-derived and synthetic cannabinoid agonists have anti-nociceptive effects in animal models of acute, inflammatory and neuropathic pain. CB1 receptors located at peripheral, spinal and supra-spinal sites are an important target mediating the antinociceptive effects of cannabinoid agonists. Recent evidence points to an additional role of CB2 receptors, particularly during inflammatory and neuropathic pain conditions. The mechanisms underlying the analgesic effects of cannabinoids are likely to involve inhibition of pre-synaptic neurotransmitter and neuropeptide release in addition to modulation of post-synaptic neuronal excitability. Modulation of pro-nociceptive and pro-inflammatory mediators from immune cells may be another mechanism by which cannabinoids exert their effects in the periphery. Points of convergence between cannabinoids and other classes of analgesic agents including the opioids and cyclooxygenase inhibitors offer further insight into the potential sites and mechanisms of interactions between these systems. The large body of pre-clinical evidence in support of cannabinoids as potential analgesic agents provides the functional framework for large-scale controlled clinical trials of the effects of cannabinoids and modulation of the endocannabinoid system during painful conditions.

This work summarises recent findings relating to the activation of CB1 cannabinoid receptors in the gastrointestinal (GI) tract with a particular emphasis on the endocannabinoid and endovanilloid systems. Of the endogenous cannabinoid ligands discovered thus far (anandamide, 2-arachidonoyl glycerol (2-AG), noladin ether, virodhamine and N-arachidonoyl dopamine), only anandamide and 2-AG have been investigated in the GI tract. Recent experimental evidences regarding the inhibitory influence of CB1 receptor stimulation on fluid secretion and the reduction on GI motility and transit are discussed in the light of their possible interactions with the endocannabinoid system. Recent reports of the actions of anandamide both at cannabinoid and vanilloid receptors in the GI tract are described with particular reference to the distribution of CB1 and VR1 receptors in the intestine, vagal afferent fibres and their central projections. A consistent finding is the presence of CB1 receptors on excitatory cholinergic neurones particularly in the myenteric ganglia of a variety of species. Colocalisation studies of cannabinoid receptor-expressing cells with specific cell markers have been used in an attempt to correlate the immunohistochemical data with functional studies in order to determine the physiological significance of subsets of cholinergic neurones. Evidence regarding cannabinoids as antiemetics and the role of cannabinoid CB1 receptor activation in emesis in vomiting species and nausea in non-vomiting animal models is presented. Observed changes in vanilloid and cannabinoid expression in certain disease states, for example in inflammation, are discussed with a view to their possible therapeutic potential in the treatment of inflammatory bowel conditions.

The use of cannabinoids for medicinal purposes has attracted wide interest in recent years, especially after the discovery of the endocannabinoid system. However, mechanisms relevant for neuroprotection and recovery of both exogenously applied and endogenous cannabinoids are only partly established. Supported by the observation that cannabinoids are released after neuronal injury and disorder, various CNS applications are currently investigated and both clinical and experimental data are now emerging. The present review describes possible mechanisms of cannabinoid actions relevant for neuroprotection, with a focus on excitotoxic cascades, oxidative stress and inflammation. The complexity of the system is outlined with regards to different cannabinoids, the receptors and cascades they activate, as well as the cell types and experimental approaches utilised to study their action. Furthermore, the unresolved problem of targeting protective pathways, while avoiding psychoactive and detrimental cascades is discussed.

Cannabinoids are well known for their effects on neurotransmission in the central nervous system. Whereas the components of cannabinoid signaling have been intensively studied in neurons, recent data suggest that a cannabinoid signaling system, i.e. the endogenous cannabinoid ligands, the receptors that they activate and the components that degrade them, also exist in the non-neuronal cells of the brain, the glia. Because of their abundance and because of their importance in maintaining brain homeostasis, glial cells are involved in and affected by virtually all neurological diseases. Interestingly, a lot of neurological conditions, chronic and acute, are associated with a disturbance of the endocannabinoid system. This raises the possibility that effects of cannabinoids on glial cells have an important impact in these conditions and could potentially be exploited therapeutically. Cannabinoids regulate various physiological functions of glial cells, such as glucose metabolism and gap junction permeability and also have a major influence on pathophysiological functions of glial cells, like migration, cytokine production and nitric oxide release. In this review we summarize the current knowledge about cannabinoids and glial cells with a special focus on neurological conditions, especially neurodegenerative, demyelinating and neoplastic diseases.

It has long been known that cannabinoids can alter perception and cognitive function. In this review article, we first discuss the current knowledge of both plant-derived and synthetic cannabinoid effects on learning and memory formation in animals with a particular emphasis on spatial mapping. It appears that stimulation of the cannabinoid receptors in the brain is detrimental to spatial learning, and recent advances suggest that working memory is particularly vunerable and in some cases the involvement of the cannabinoid system in memory consolidation. Most of these effects are mediated by CB1 receptors, and blockade of these receptors can, under certain conditions enhance learning. In the second part, we investigate whether cannabinoid effects can be explained in terms of modulation of other transmitter systems. Since CB1 is G-protein-coupled and predominantly presynaptically localised, the primary effect may lie in a down / up-regulation of other transmitters, resulting in memory deficits. Such an analysis proved useful as it revealed that modulation of the noradrenergic and cholinergic system is readily able to explain deficits reported in memory consolidation, while cholinergic, dopaminergic, and GABAergic regulation may explain working memory deficits.