Discussion: Endogenous Cannabinoid Receptors

Discussion: Endogenous Cannabinoid Receptors ORDER NOW FOR CUSTOMIZED AND ORIGINAL ESSAY PAPERS ON Discussion: Endogenous Cannabinoid Receptors I just need to answer this question and cite the article. Endogenous cannabinoid receptors are found throughout the brain, and are clustered in specific regions (see Iversen, 2003). Knowing the behavioral and cognitive effects of THC, what can we conclude about how the physiology of the brain (i.e. where receptors are found, and what NTs/drugs bind to them) informs us about effects psychoactive compounds have on the body? Discussion: Endogenous Cannabinoid Receptors attachment_1 DOI: 10.1093/brain/awg143 Advanced Access publication April 8, 2003 Brain (2003), 126, 1252±1270 INVITED REVIEW Cannabis and the brain Leslie Iversen Department of Pharmacology, University of Oxford, Oxford, UK Summary include disruption of psychomotor behaviour, shortterm memory impairment, intoxication, stimulation of appetite, antinociceptive actions (particularly against pain of neuropathic origin) and anti-emetic effects. Although there are signs of mild cognitive impairment in chronic cannabis users there is little evidence that such impairments are irreversible, or that they are accompanied by drug-induced neuropathology. A proportion of regular users of cannabis develop tolerance and dependence on the drug. Some studies have linked chronic use of cannabis with an increased risk of psychiatric illness, but there is little evidence for any causal link. The potential medical applications of cannabis in the treatment of painful muscle spasms and other symptoms of multiple sclerosis are currently being tested in clinical trials. Medicines based on drugs that enhance the function of endocannabinoids may offer novel therapeutic approaches in the future. Keywords: cannabinoid CB1 receptor; D9-tetrahydrocannabinol; rimonabant (SR141716A); anandamide; 2-arachidonylglycerol Abbreviations: 2-AG = 2-arachidonylglycerol; DSI = depolarization-induced suppression of inhibition; FAAH = fatty acid amide hydrolase; Gi/o = G-proteins negatively linked to adenylate cyclase or to inositol phosphates; LTD = long-term depression; LTP = long-term potentiation; mGlu = metabotropic glutamate; NMDA = N-methyl-D-aspartate; THC = D9tetrahydrocannabinol Introduction A large literature exists on the effects of cannabis, with many of the earlier studies conducted in human subjects (Mendelson et al., 1976; Jones, 1978; Hollister, 1986). Unfortunately, much of this research would now be regarded as inadequately controlled and poorly designed. However, research on cannabis has been stimulated in recent years by the recognition that speci®c receptors exist in the brain that Brain 126 ã Guarantors of Brain 2003; all rights reserved recognize cannabinoids, and by the discovery of a series of endogenous cannabinoids that act as ligands for these receptors. As was the case with opiate research in the 1970s, research on a psychoactive drug of plant origin has revealed a hitherto unknown physiological control mechanism. This review will focus mainly on the more recent literature in this ®eld. Downloaded from by guest on June 18, 2015 The active compound in herbal cannabis, D9-tetrahydrocannabinol, exerts all of its known central effects through the CB1 cannabinoid receptor. Research on cannabinoid mechanisms has been facilitated by the availability of selective antagonists acting at CB1 receptors and the generation of CB1 receptor knockout mice. Particularly important classes of neurons that express high levels of CB1 receptors are GABAergic interneurons in hippocampus, amygdala and cerebral cortex, which also contain the neuropeptides cholecystokinin. Activation of CB1 receptors leads to inhibition of the release of amino acid and monoamine neurotransmitters. The lipid derivatives anandamide and 2-arachidonylglycerol act as endogenous ligands for CB1 receptors (endocannabinoids). They may act as retrograde synaptic mediators of the phenomena of depolarizationinduced suppression of inhibition or excitation in hippocampus and cerebellum. Central effects of cannabinoids Correspondence to: Leslie Iversen, Department of Pharmacology, University of Oxford, Mans®eld Road, Oxford OX1 3QT, UK E-mail: [email protected] Cannabis and the brain 1253 Fig. 2 Chemical structure of the CB1 selective antagonist drug rimonabant (SR141716A). Discussion: Endogenous Cannabinoid Receptors Fig. 1 Chemical structures of THC, the synthetic CB1 receptor agonist WIN 55,2122 and the endocannabinoids. Endogenous cannabinoids The cannabinoid system in brain Exogenous cannabinoids and their receptors The principal active component in the complex mixture of cannabinoids present in extracts of the plant Cannabis sativa is D9-tetrahydrocannabinol (THC) (Mechoulam, 1970) (Fig. 1). THC is a sticky resin that is not soluble in water. Smoking remains the most ef®cient means of delivering the drug and experienced users can titrate the dose by adjusting the frequency and depth of inhalation (Iversen, 2000). THC or cannabis extracts can also be taken orally in fat-containing foods or dissolved in a suitable pharmaceutical oil, but absorption is delayed and variable (Iversen, 2000). A series of man-made synthetic cannabinoids, some of which are more potent and more water soluble than THC, is also available (Pertwee, 1999) (Fig. 1). All of these compounds act as agonists at the CB1 cannabinoid receptor (Matsuda et al., 1990), which is the only one known to be expressed in the brain. A second cannabinoid receptor, CB2, is expressed only in peripheral tissues, principally in the immune system (Munro et al., 1993; Felder and Glass, 1998; Pertwee, 1999). THC and Following the discovery of speci®c cannabinoid receptors, a search was made for naturally occurring ligands of these receptors in mammalian tissues. This led to the discovery of a series of arachidonic acid derivatives with potent actions at cannabinoid receptors. These are: anandamide (N-arachidonyl-ethanolamine; Devane et al., 1992), 2-arachidonylglycerol (2-AG; Mechoulam et al., 1995; Sugiura et al., 1995; Stella et al., 1997) and 2-arachidonylglyceryl ether (HanusÏ et al., 2001) (Fig. 1). Of these, anandamide is the ligand that has been most extensively studied so far. The endogenous cannabinoids known as `endocannabinoids’ are present only in small amounts in the brain or other tissues. Like other lipid mediators (e.g. prostaglandins) they appear to be synthesized and released locally on demand (see below). Anandamide and the other endogenous cannabinoids are rapidly inactivated by a combination of a transporter mechanism and by the enzyme fatty acid amide hydrolase (FAAH) (Di Marzo et al., 1994; Piomelli et al., 1998; Giuffrida et al., 2001). Genetically engineered mice lacking FAAH displayed elevated levels of anandamide in brain and were supersensitive to the biological actions of anandamide (Cravatt et al., 2001). The discovery of agents that could interfere with the inactivation of endogenous cannabinoids may provide a novel means of pharmaco- Downloaded from by guest on June 18, 2015 the synthetic cannabinoids also act to some extent as agonists at the CB2 receptor. Both cannabinoid receptors are members of the G-protein coupled class, and their activation is linked to inhibition of adenylate cyclase activity (Howlett et al., 1988). A series of synthetic drugs is also now available that act as speci®c antagonists at CB1 or CB2 receptors (D’Souza and Kosten, 2001). One of these compounds, rimonabant(SR141716A), which acts selectively to block CB1 receptors (Rinaldi-Carmona et al., 1994; Compton et al., 1996), has been widely used in studies of the actions of cannabinoids in the CNS (Fig. 2). 1254 L. Iversen logically modifying cannabinoid function in the brain (Piomelli et al., 2000). Neuroanatomical distribution of CB1 receptors in brain The distribution of cannabinoid receptors was ®rst mapped in rat brain in autoradiographic studies, using the radioligand [H3]CP-55,940, which binds with high af®nity to CB1 sites (Herkenham et al., 1991) (Fig. 3). Discussion: Endogenous Cannabinoid Receptors The validity of using this radioligand was con®rmed by autoradiographic studies in CB1 receptor knockout mice, in which no detectable [H3]CP55,940 binding sites were observed (Zimmer et al., 1999). More recently, antibodies that target the C- or N-terminal regions of the CB1 receptor protein have been used for immunohistochemical mapping studies (Egertova et al., 1998; Pettit et al., 1998; Egertova and Elphick, 2000). Immunohistochemistry provides a superior degree of spatial resolution to autoradiography, but the overall pattern of distribution of CB1 receptors revealed by the two approaches is very similar (Elphick and EgertovaÂ, 2001). The mapping studies in rat brain showed that CB1 receptors are mainly localized to axons and nerve terminals and are largely absent from the neuronal soma or dendrites. The ®nding that cannabinoid receptors are predominantly presynaptic rather than postsynaptic is consistent with the postulated role of cannabinoids in modulating neurotransmitter release (see below). In both animals and man the cerebral cortex, particularly frontal regions, contains high densities of CB1 receptors. There are also very high densities in the basal ganglia and in the cerebellum (Fig. 3). In the limbic forebrain CB1 receptors are found particularly in the hypothalamus and in the anterior cingulate cortex. The hippocampus also contains a high density of CB1 receptors. The relative absence of the cannabinoid receptors from brainstem nuclei may account for the low toxicity of cannabinoids when given in overdose. The regional distribution of the CB1 receptor in brain correlates only poorly with the levels of anandamide and other endocannabinoids in different brain regions (Felder et al., 1996; Bisogno et al., 1999). However, measurements of endocannabinoids have yielded variable results, and a strict correlation would not be expected for ligands that are only produced on demand. There is a better correlation between the regional distribution of CB1 receptors and the enzyme FAAH. FAAH is widely distributed in CNS and other tissues, suggesting that its role is not con®ned to inactivating endogenous cannabinoids. Nevertheless, particularly high levels of FAAH were found in brain regions that are enriched in CB1 receptors, and immunohistochemical staining suggested a complementary relationship between FAAH and CB1 receptors at the synaptic level (Egertova et al., 1998; Elphick and EgertovaÂ, 2001). In cerebellum, hippocampus and neocortex FAAH was expressed at high levels in the somato-dendritic regions of neurons that were postsynaptic to CB1-positive axon terminals. The close and complementary relationship between CB1 receptors and FAAH led to the hypothesis that FAAH may participate in the inactivation of endogenous cannabinoids released locally at synapses Downloaded from by guest on June 18, 2015 Fig. 3 Distribution of cannabinoid CB1 receptors in rat brain revealed by an autoradiograph of the binding of radioactively labeled CP-55940 (a high af®nity agonist ligand) to a sagittal brain section. The brain regions labelled are: Cb = cerebellum; CbN = deep cerebellar nucleus; cc = corpus callosum; EP = entopeduncular nucleus; ® = ®mbria hippocampus; Fr = frontal cortex; FrPaM = frontoparietal cortex motor area; GP = globus pallidus; Hi = hippocampus; IC = inferior colliculus; LP = lateral posterior thalamus; Me = medial amygdaloid nucleus; PO = primary olfactory cortex; PCRt = parvocellular reticular nucleus; SNR = substantia nigra reticulate; Tu = olfactory tubercle; VP = ventroposterior thalamus. Discussion: Endogenous Cannabinoid Receptors Photograph kindly supplied by Dr Miles Herkenham, National Institute of Mental Health, USA. Cannabis and the brain (Elphick and EgertovaÂ, 2001). These authors postulated a retrograde cannabinoid signalling mechanism, whereby endogenous cannabinoids are released in response to synaptic activation, feedback to presynaptic receptors on these axon terminals, and are subsequently inactivated by FAAH after their uptake into the postsynaptic compartment. This hypothesis has been supported independently by neurophysiological ®ndings, as described below. Effects of cannabinoids on synaptic function Inhibition of neurotransmitter release of the duration of presynaptic action potentials as they invade axon terminals. Biosynthesis of endocannabinoids Despite their similar chemical structures, the endocannabinoids are produced through distinct biochemical pathways. The formation of anandamide is thought to result from the hydrolysis of the precursor N-arachidonoyl phophatidylethanolamine, catalysed by the phosphodiesterase enzyme phospholipase D (Di Marzo et al., 1994; Cadas et al., 1997). 2-AG, on the other hand, is produced by cleavage of an inositol-1,2-diacylglycerol, catalysed by phospholipase C. Although both anandamide and 2-AG can activate CB1 receptors, it is not clear whether both function as endocannabinoids, and whether their synthesis and release are independently controlled. The levels of 2-AG found in brain (2±10 nmol/g) are 50±1000 times higher than those of anandamide (10±50 pmol/g). There is some evidence for separate control of their biosynthesis. Stimulation of glutamate release from Schaffer collaterals in rat hippocampal slices increased levels of 2-AG, but not anandamide (Stella et al., 1997). On the other hand, another study using in vivo microdialysis probes showed that local administration of the dopamine D2 receptor agonist quinpirole caused an increased release of anandamide from rat striatum without affecting levels of 2-AG (Giuffrida et al., 1999). Indeed, despite the much higher tissue levels of 2-AG relative to anandamide and the availability of a very sensitive assay, no 2-AG could be detected at all in the striatal dialysate samples. In cultured rat cortical neurons activation of Ca2+ in¯ux by stimulation of glutamate N-methyl-D-aspartate (NMDA) receptors caused an increase in 2-AG formation but not anandamide (Stella and Piomelli, 2001). However, if NMDA activation was combined with a cholinergic agonist (carbachol) the formation of both endocannabinoids was increased. In both cases Ca2+ in¯ux was required for endocannabinoid synthesis. It is clear that much remains to be learned about the relative roles played by the different endocannabinoids. The biosynthesis of the most recently discovered third endocannabinoid, 2-arachidonylglyceryl ether, remains to be characterized. Endogenous cannabinoids act as retrograde signal molecules at synapses Important new insights into the physiological role of cannabinoids has emerged from neurophysiological studies published independently by three different research groups in 2001. A phenomenon known as depolarization-induced suppression of inhibition (DSI) has been known to neurophysiologists for some years (Alger and Pitler, 1995). It is a form of fast retrograde signalling from postsynaptic neurons back to inhibitory cells that innervate them, and is particularly prominent in the hippocampus and cerebellum. Three prop- Downloaded from by guest on June 18, 2015 The presynaptic localization of CB1 receptors suggests a role for cannabinoids in modulating the release of neurotransmitters from axon terminals, and this has been con®rmed by a substantial body of experimental data. Early reports (Gill et al., 1970; Roth, 1978) showed that THC inhibited acetylcholine release from electrically stimulated guinea pig ileum. Similar inhibitory effects of THC and other cannabinoids on the release of a variety of neurotransmitters from CNS neurons have been observed in many subsequent studies (Schlicker and Kathmann, 2001).Discussion: Endogenous Cannabinoid Receptors The neurotransmitters involved include L-glutamate, GABA, noradrenaline, dopamine, 5-HT and acetylcholine. The brain regions most often studied in vitro, usually in tissue slice preparations, have been cerebellum, hippocampus or neocortex. Neurotransmitter release has been studied directly in superfused preparations, and indirectly by measuring postsynaptic currents. Although most of these studies involved rat or mouse brain, a few studies have shown similar results using human brain tissue (Katona et al., 2000; Schlicker and Kathmann, 2001). Because THC is only poorly water soluble, the more soluble synthetic CB1 receptor agonists WIN552123, HU210 or CP55-2940 were used in these in vitro studies. The speci®city of the cannabinoid effects were con®rmed by demonstrating that the inhibitory effects of the agonists were completely blocked by the CB1-selective antagonist rimonabant. The cellular mechanisms involved in the inhibition of neurotransmitter release by cannabinoids remain unclear. Some have suggested that there is a direct inhibitory effect of CB1 receptor activation on N-type Ca2+ currents (Caul®eld and Brown, 1992; MacKie and Hill, 1992). However, the effect appears more likely to involve sites downstream of voltage-dependent Ca2+ channels, since a number of studies have shown that cannabinoids reduce the frequencies of miniature excitatory or inhibitory synaptic currents, which are Ca2+ independent, rather than altering their amplitude, which is Ca2+ sensitive (Schlicker and Kathmann, 2001). Deadwyler et al. (1995) suggested that the inhibitory effect of CB1 receptor activation on adenylate cyclase activity causes a decreased phosphorylation of A-type K+ channels by the cAMP-dependent enzyme protein kinase A. This, in turn, would activate the A-type K+ channels and cause a shortening 1255 1256 L. Iversen occluded by the CB1 receptor agonist WIN55,2122. Kreitzer and Regehr (2001b) went on to show that inhibitory inputs to rat cerebellar Purkinje cells from basket cells and stellate cells were subject to DSI, and that this was also blocked by AM-251 and occluded by WIN55,2122. The DSE phenomenon in the cerebellum is also linked to mGlu receptors. Maejima et al. (2001) reported that mGlu agonists acting on mouse Purkinje cells mimicked DSE, and the effects could be blocked by CB1 antagonists. These ®ndings suggest that endocannabinoids are involved in the rapid modulation of synaptic transmission in CNS by a retrograde signalling system that can in¯uence synapses in a local region of some 40 mm diameter, causing inhibitory effects on both excitatory and inhibitory neurotransmitter release that persist for tens of seconds. This may play an important role in the control of neural circuits, particularly in cerebellum and hippocampus (see below).Discussion: Endogenous Cannabinoid Receptors Exogenously administered THC or other cannabinoids cannot mimic the physiological effects of locally released endocannabinoids. Since they cause long-lasting activation of CB1 receptors in all brain regions, their overall effect is to cause a persistent inhibition of neurotransmitter release from those nerve terminals that express CB1 receptors, and as a consequence they temporarily occlude and prevent the phenomena of DSI and DSE. Effects of cannabinoids on CNS function Psychomotor control CB1 receptors are expressed at particularly high densities in the basal ganglia and cerebellum, so it is not surprising that cannabinoids have complex effects on psychomotor function (reviewed by RodrõÂguez de Fonseca et al., 1998). One of the earliest reports of the effects of cannabis extracts in experimental animals described the awkward swaying and rolling gait caused by the drug in dogs, with periods of intense activity provoked by tactile or auditory stimuli, and followed eventually by catalepsy and sleep (Dixon, 1899). In rodents cannabinoids tend to have a triphasic effect. Thus in rats low doses of THC (0.2 mg/kg) decreased locomotor activity, while higher doses (1±2 mg/kg) stimulated movements, and catalepsy emerged at doses of 2.5 mg/kg (SanÄudo-PenÄa et al., 2000). Similarly in mice, Adams and Martin (1996) described a `popcorn effect’ in animals treated with THC. Groups of mice are sedated by the drug, but will jump in response to auditory or tactile stimuli, as they fall into other animals these in turn jump, resembling corn popping in a popcorn machine. Interestingly, the CB1 receptor antagonist rimonabant stimulated locomotor activity in mice, suggesting that there is tonic activity in the endocannabinoid system that contributes to the control of spontaneous levels of activity (Compton et al., 1996). These effects of cannabinoids may be due, in part, to actions at cerebellar or striatal receptors. Patel and Hillard (2001) used tests of speci®c cerebellar functions to show that Downloaded from by guest on June … Get a 10 % discount on an order above $ 100 Use the following coupon code : NURSING10