Cannabis sativa has many common names: hemp, marijuana, bhang, ganja, hashish, etc. It is an annual herb in the hemp family (Cannabaceae) which grows 3-10 feet tall and has hairy leaves divided into 5 to 7 serrated leaflets; the leaves are often sticky with resin.
The tetrahydrocannabinols (THCs) in hemp have documented medicinal value. Its leaves and sap have been universally used as a painkiller and sedative, and ancient herbalists to make salves for burns and other wounds used its roots. In 1965, scientists isolated THC and later discovered that the compound can lower pressure in the eyes of glaucoma patients and has thus far been an excellent drug for glaucoma treatment. They also discovered THC dilates bronchial tubes and can therefore benefit asthma sufferers. Later, they discovered that THC has antispasmodic properties that could possibly benefit people with epilepsy. And finally, THC has proven to give relief to patients undergoing chemotherapy by helping to alleviate the nausea they often suffer, and many AIDS and cancer patients have found it stimulates their appetite, thus combating the wasting associated with their illnesses.
Several cells in the brain and other organs contain specific protein receptors that recognize THC and some other cannabinoids and trigger cell responses. Other cannabinoids do not bind to these cannabinoid receptors and exert their effects by other ways. The discovery of specific cannabinoid receptors prompted the search for putative naturally occurring chemicals that interact with the receptors, the endocannabinoids. There are at least two cannabinoid receptor types, CB1 receptors and CB2 receptors. CB1 receptors are found in high concentrations within the brain and spinal cord. They are also present in certain peripheral cells and tissues (some neurons, some endocrine glands, leukocytes, spleen, heart and parts of the reproductive, urinary and gastrointestinal tracts). CB2 receptors are expressed primarily by immune cells und tissues (leukocytes, spleen and tonsils).
Cannabinoid receptors are part of the endocannabinoid system, which consists of cannabinoid receptors, endogenous cannabinoids (endocannabinoids), and the enzymes that synthesize and degrade endocannabinoids. Emerging evidence implicates endocannabinoids in a wide variety of physiological and pathophysiological processes. To date, most drugs used therapeutically that interact with the endocannabinoid system are derived from cannabis and produce their effects by activation of cannabinoid receptors
Components of endocannabinoid signalling system
The past 15 years have seen a tremendous advance in our understanding of the elements of the endocannabinoid system. During this period two cannabinoid receptors, CB1 and CB2, have been cloned; several endogenous cannabinoids have been identified; and the synthetic and degradative pathways for the endocannabinoids have been partially elucidated.
Cannabinoid receptors
The CB1 receptor was first cloned as an orphan receptor from a rat cDNA library based on its homology to the bovine substance K receptor. By combining anatomical, molecular, and pharmacological approaches, the distribution and primary signalling mechanisms of the CB1 receptor were determined. Second cannabinoid receptor was found in a human promyelocytic cDNA library and designated the CB2 receptor on the basis of its homology to the CB1 receptor and similar ligand binding profile. In the CNS, CB1 receptors are most highly expressed on axons and nerve terminals, but ample functional evidence also supports their expression on somata. CB2 receptors are primarily found on immune cells. The highest levels of CB2mRNA in peripheral blood cells are found in B-lymphocytes > natural killer cells > neutrophils > T8 lymphocytes >T4 lymphocytes. Both CB1 and CB2 receptors belong to the super family of G protein-coupled receptors, coupling to inhibitory G proteins (Gi/o). As such, CB1 and CB2 receptors inhibit adenylyl cyclase and activate MAP kinase. In addition, CB1 receptors inhibit presynaptic N- and P/Q-type calcium channels and activate inwardly rectifying potassium channels. Beyond CB1 and CB2, several intriguing studies support the existence of additional cannabinoid receptors.
Endogenous cannabinoids (endocannabinoids)
The widespread distribution of cannabinoid receptors suggests the presence of an endogenous ligand, or endogenous cannabinoid (endocannabinoid). The prototypical endocannabinoid, anandamide, the amide of ethanolamine and arachidonic acid, was first isolated and identified as an endogenous cannabinoid. A series of similar compounds, varying in the nature of the (unsaturated) fatty acid are also found in tissues. Some of these, for example homo-? - linolenoyl ethanol amide and docosatetraenoyl ethanol amide, engage CB1 receptors. Others, such as palmitoyl ethanol amide (PEA) and oleoyl ethanol amide (OEA) do not, but have profound analgesic or anorexic effects, respectively. The second endogenous cannabinoid, 2-arachidonyl glycerol (2-AG), was identified and its importance as an endogenous cannabinoid was established. In part because of its non signalling role as an intermediate in several lipid metabolic pathways, 2-AG is far more abundant than anandamide. Recently, amides of arachidonic acid such as N-arachidonyl dopamine, serine, and Glycine have been described. Although some of these compounds have activity at cannabinoid receptors, they also affect many other targets. Despite their superficial structural similarity, substantial differences between the anandamide and 2-AG families of endogenous cannabinoids must be noted and are an important source of therapeutic specificity. The first difference is the route of synthesis. Anandamide and related acylamides are made following the hydrolysis of N-arachidonyl (or another unsaturated fatty acid) phosphatidyl ethanolamine (NAPE) by a specific phosopholipase D. The major synthetic pathway of 2-AG formation is the hydrolysis of phosphatidyl inositol by phosopholipase C and diacylglycerol lipase (DGL) with phosphatidylinositol hydrolysis by phosopholipase A1 and lyso-phospholipase C being important in some tissues. Another important difference between the 2-AG and anandamide families is efficacy. Multiple studies with CB1 and CB2 receptors have found that anandamide is a low efficacy agonist, whereas 2-AG is highly efficacious. The efficacy of anandamide at CB1 receptors is similar to that of 9tetrahydrocannabinol ( 9THC), the major psychoactive component of cannabis. This leads to the intriguing possibility that the psychoactive effects of cannabis and 9THC are due to a combination of mimicking anandamide action at CB1 receptors while antagonizing 2-AG actions at these same receptors.
Endocannabinoids and neuronal plasticity
The prominent presynaptic localization of CB1 receptors and their inhibition of calcium channels and activation of potassium channels suggest that they may modulate neurotransmission and affect neuronal excitability. Indeed, a large number of studies show that activation of CB1 receptors by both exogenous and endogenous cannabinoids suppresses neurotransmission. Furthermore, the enhancement of endocannabinoid synthesis during neural activity suggests that these ligands may inhibit neurotransmission. Indeed, endocannabinoid involvement in neuronal plasticity has now been shown to occur at many synapses. Endocannabinoid-mediated inhibition of neurotransmission comes in two forms, transient and long lasting. Transient, also termed DSI (depolarization-induced suppression of inhibition) or DSE (depolarization-induced suppression of excitation), relies on generation of endocannabinoids following increases in intracellular calcium - often from entry through calcium- permeant cell surface channels or release from intracellular stores, sometimes by activation of metabotropic receptors. DSI and DSE are of short duration, lasting tens of seconds, and localized; thus they may serve to rapidly modulate small ensembles of synapses. Long lasting endocannabinoid-mediated inhibition of neurotransmission, one form of long-term depression (LTD), is also ubiquitous. Here, endocannabinoids, often produced by group I metabotropic receptors during prolonged low-frequency stimulation, activate presynaptic CB1 receptors. Endocannabinoid LTD (eLTD) only requires CB1 receptor activation for its initiation; once established eLTD is independent of CB1 receptor activation. A substantial literature describes a role for endocannabinoids in vascular regulation. Examples include vasodilatation during sepsis or cirrhosis and a role in the regulation of cerebral blood flow. In some cases CB1 receptors have been implicated, in others endocannabinoids appear to be interacting with novel receptors. Possible cardiovascular actions of endocannabinoids and the implication of their antagonism need to be considered when using CB1 antagonists. For example, inhibition of FAAH and raising acyl ethanol amine levels can decrease blood pressure in hypertensive rats. However, to date, there is no suggestion of an increased incidence of hypertension in patients treated for up to a year with the CB1 antagonist, rimonabant.
CB1 receptor antagonists
CB1 receptor antagonists have received the most attention and are the furthest along in clinical studies. The primary indication is for obesity with secondary indications for disorders that have a prominent craving component. Rimonabant, also known as SR141716 or Acomplia®, was the first CB1 antagonist reported. Another chemical series that has given rise to relatively selective CB1 receptor antagonists are the substituted benzofurans (LY320135).
" CB1 antagonists as anti obesity drugs
Low doses of CB1 receptor agonists enhance food consumption. Conversely, CB1 receptor antagonists decrease food consumption and body weight.
" CB1 antagonists and craving
CB1 antagonists are also used in the treatment of drug abuse.
CB2 receptor agonists
The physiological role of CB2 receptors remains to be fully defined. However, several intriguing preclinical studies suggest that agonists at this receptor may be clinically useful. Multiple animal studies suggest that chronic pain may be one such indication. Still more preliminary studies also suggest a role of CB2 receptors in the maintenance of bone density and the progression of atherosclerotic lesions. A particularly attractive feature of selective CB2 agonists, such as AM1241, HU308, and JWH133, as therapeutics is that they are devoid of known psycho activity. Cannabis and its extracts have long been used to treat painful conditions. Although CB1 activation may be analgesic, several studies clearly show that CB2 agonists are also effective in chronic pain models. Specifically, CB2 agonists are analgesic in neuropathic pain models, peripheral inflammatory models, and some sensitization models. In thermal hypersensitivity models, CB2 agonists enhance beta-endorphin release from keratinocytes. In addition, CB2 agonists are analgesic in neuropathic pain models. At the present time, a synthesis of these studies suggests that in neuropathic pain models following nerve injury, CB2 agonists may be analgesic by their effects on spinal microglia, whereas in the peripheral models CB2 agonists might be acting by decreasing beta-endorphin release in the dermis. PEA is another lipid mediator that should be considered in the context of the peripheral actions of CB2 receptors. Acting locally, PEA is an effective analgesic in inflammatory pain models, and CB2 antagonists block this analgesia. However, PEA does not bind to cannabinoid receptors. CB2 agonist (HU308) decreases bone loss following ovariectomy in mice. CB2 ligands may have therapeutic utility in other chronic inflammatory diseases. CB2 signalling may also be involved in the neurodegeneration associated with plaque development in Alzheimer's disease and the inflammatory response accompanying retroviral encephalitis, such as that occurring with HIV. CB2 receptors rapidly desensitize, at least when expressed in heterologous expression systems. CB2 receptors in different species vary considerably in their distal carboxyl termini. As this region is important in many aspects of GPCR signalling, it will be necessary to determine that the regulation and function of CB2 signalling important for its therapeutic actions are maintained across species.
Fatty acid amino hydrolase (FAAH)
FAAH is the major degradative enzyme for anandamide and related amides in vivo. In contrast to the findings of in vitro studies, FAAH does not appear to metabolize 2-AG to a significant extent in vivo. Thus, drugs that selectively inhibit FAAH would increase N-acyl ethanolamine levels without affecting those of 2-AG. Indeed, in addition to the less specific Trifluoro Methyl Ketones inhibitors, at least two families of FAAH inhibitors have been developed, the alpha-keto hetero cycles and the carbamates. Endocannabinoid release is enhanced following nociceptive stimulation and that inhibiting the degradation of these endocannabinoids (here, presumably an N-acyl ethanol amine) might be therapeutically beneficial. The analgesia produced by selective carbamates-based FAAH inhibitors (URB532, URB597), as well as the reversible alpha-keto Hetrocyclic FAAH inhibitors (OL92, OL135), is blocked by CB1 antagonists. The carbamates FAAH inhibitors are also efficacious in two anxiolytic models. Importantly for their therapeutic application, selective FAAH inhibitors do not produce the catalepsy, hypothermia, or hyperphagia that are seen with direct CB1 receptor agonists. Taken together, these observations emphasize that with FAAH inhibitors, it is feasible to produce local increases in endocannabinoids associated with behaviorally meaningful effects and that these inhibitors activate cannabinoid signaling in a much more selective way than is possible with CB1 agonists.
Endocannabinoid transport inhibitors
It is commonly assumed although not rigorously proven that endocannabinoid action is terminated, in part, by their uptake into cells. This putative mechanism appears similar for anandamide and 2-AG, with some mild differences in the effect of the degraded endocannabinoid on the process. The initial pharmacological probe used to study the putative endocannabinoid membrane transporter (EMT), AM404, inhibited uptake, but it also interacted with CB1 receptors and activated VR1 channels at higher concentrations. In addition, it is a substrate for FAAH. These multiple actions often made it difficult to interpret studies done with AM404, particularly in vivo studies. This led to a hypothesis that endocannabinoid passage across membranes was passive and carrier independent, solely as a consequence of their metabolism by FAAH (or MAG lipase). However, the development of more potent and specific EMT inhibitors that have less activity toward FAAH, VR1, and CB1 clearly shows that it is possible to dissociate inhibition of FAAH (which would decrease endocannabinoid transport because of intracellular accumulation of endocannabinoid) from authentic transmembrane endocannabinoid transport. Although pointing toward a transport process, it needs to be recognized that these inhibitor studies do not differentiate between the possibilities that the EMT is a transmembrane transporter, a carrier protein, or another entity.
Perspectives for the future
These are exciting times for drugs targeting cannabinoid receptors and the endocannabinoid system. A rich variety of drugs are being developed and novel indications elucidated. Based on preclinical studies and their lack of psycho activity, CB2 agonists have a strong potential for treatment of pain and some promise in osteoporosis and cardiovascular disease. With their high specificity, FAAH inhibitors remain an exciting potential therapeutic, possibly for pain or anxiety.
The authors are with Pharmacy Group, Birla Institute of Technology and Science, Pilani 333 031, Rajasthan. Email: anantha@bits-pilani.ac.in