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The gastrointestinal pharmacology of cannabinoids
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The gastrointestinal pharmacology of cannabinoids Angelo A Izzo*, Nicola Mascolo and Francesco Capasso The digestive tract contains endogenous cannabinoids (anandamide and 2-arachidonylglycerol) and cannabinoid CB1 receptors can be found on myenteric and submucosal nerves. Activation of CB1 receptors inhibits gastrointestinal motility, intestinal secretion and gastric acid secretion. The enteric location of CB1 receptors could provide new strategies for the management of gut disorders Addresses Department of Experimental Pharmacology, University of Naples Federico II, Via D Montesano 49, 80131 Naples, Italy *e-mail: [email protected] Current Opinion in Pharmacology 2001, 1:597–603 1471-4892/01/$ — see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations 2-AG 2-arachidonyl glycerol ChAT choline acetyltransferase ∆9-THC ∆9-tetrahydrocannabinol i.p. intraperitoneal NANC non-adrenergic non-cholinergic PMSF phenylmethylsulfonyl fluoride VIP vasoactive intestinal peptide
Introduction Marijuana is a well known recreational drug that has for centuries been prescribed therapeutically in herbal medicine for the treatment of a wide array of health problems, including a range of gastrointestinal disorders [1]. A century ago in the United States extracts of Cannabis Sativa were indicated for gastrointestinal pain, gastroenteritis and diarrhoea, and there are anecdotal reports suggesting that marijuana may be effective in alleviating symptoms of Crohn’s disease and diabetic gastroparesis. Cannabis contains more than 60 cannabinoids that might, in a synergistic (or antagonistic) way, contribute to its pharmacological activity. In recent years, several investigations have shown that many of the pharmacological effects of Cannabis, or its main active component ∆9-tetrahydrocannabinol (∆9-THC), are mediated by at least two types of receptors, both of which are coupled to Gi/o proteins ([2,3]; Figure 1). CB1 cannabinoid receptors are expressed by central and peripheral neurones and CB2 receptors are expressed mostly by immune cells. With the discovery of receptors for plant-derived cannabinoids, the search for the endogenous ligands for these receptors began. Arachidonyl ethanolamine (anandamide, selectivity: CB1>>CB2), 2-arachidonylglycerol (2-AG, non-selective) [2–4] and, more recently, 2-arachidonyl glyceryl ether (noladin ether, CB1-selective) [5•] have been detected in mammalian tissues (Table 1, Figure 2). Anandamide undergoes depolarisationinduced synthesis and release from neurones and can be
removed from its sites of action by carrier-mediated transport (anandamide transport), which can be inhibited by AM404 [2]. Once within the cell, anandamide can by hydrolysed by a membrane-bound anandamide amidohydrolase [2], which can be inhibited by the anandamide hydrolase inhibitor phenylmethylsulfonyl fluoride (PMSF). Cannabinoid receptors and endogenous cannabinoids (endocannabinoids) are referred to as the ‘endogenous cannabinoid system’. As detailed elsewhere [6], synthetic cannabinoid receptor agonists (mostly non-selective) and antagonists (selective) are now available. Cannabinoid receptor agonists are usually classified, according to chemical structure, into one of four main groups: classical (e.g. ∆9-THC, cannabinol, HU 210), non-classical (e.g. CP 55,940), aminoalkylindoles (e.g. WIN 55,212-2) and eicosanoids (e.g. anandamide, 2-AG, noladin ether). The selectivity and effects of these ligands are summarised in Table 1. Although cannabinoids have a wide variety of biological effects — with potential therapeutic applications including the treatment of pain, neurodegenerative and musculoskeletal disorders, inflammation and cancer [7–10] — this review focuses on recent cannabinoid research findings with possible therapeutic applications for the treatment of gastrointestinal disorders. Some cannabinoid agonists are already used clinically. For example, nabilone is used to suppress nausea and vomiting provoked by anticancer durgs and ∆9-THC is used to boost the appetite of AIDS patients [6].
Cannabinoid receptors in the gut The densities of CB1 receptors in peripheral tissues are lower than those found in areas of the brain such as the cerebellum or cerebral cortex. This is because some peripheral tissues may contain high concentrations of the CB1 receptor localised in discrete regions such as nerve terminals but which form only a small part of the total tissue mass. Griffin et al. [11] detected mRNA encoding the CB1 receptor but not CB2 receptor mRNA in the myenteric plexus of the guinea-pig small intestine. However, both types of mRNA were detected in guinea-pig whole gut, possibly because of the presence in the whole tissue of resident macrophages and other immune cells that are known to contain CB2 receptors. CB1 mRNA has also been detected in human stomach and colon [12] and in the myenteric and submucosal plexus of rat embryo digestive tract [13]. Immunohistochemical studies have revealed CB1 receptor immunoreactivity in the myenteric and submucosal ganglionated plexuses of porcine ileum and colon [14•]. In the ileum, all CB1 receptor immunoreactive neurones were also immunoreactive to the cholinergic marker choline acetyltransferase (ChAT). CB1 receptor and ChAT immunoreactive neurones appeared to be in close apposition to ileal Peyer’s patches,
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Figure 1 Cannabinoid agonists
Ca2+
K+ Extracellular –
γ β Calcium channels blocked [Ca2+]
CB1 α Gi/o
γ –
AC
β
CB2 α
–
Gi/o
Potassium channels activated [K+]
[cAMP] ERK cascade activated
Intracellular
Signal transduction pathways activated by cannabinoid receptor agonists. Cannabinoid receptor agonists activate cannabinoid CB1 and/or CB2 receptors, both coupled to Gi/o proteins. This leads to inhibition of adenylyl cyclase (AC) and activation of the extracellular signal-regulated kinase (ERK) cascade. Furthermore, the CB1 receptor can induce inhibition of N-type and P/Q-type voltage sensitive Ca2+ channels and activate G-protein-activated inwardly rectifying K+ channels. This leads to membrane hyperpolarisation and inhibition of activity.
AC
[cAMP] ERK cascade activated Current Opinion in Pharmacology
submucosal blood vessels, and intestinal crypts. In the distal colon, CB1 receptor immunoreactive neurones were also immunoreactive to ChAT, although coexpression was less frequent than in the ileum. Immunoreactivity to vasoactive intestinal peptide (VIP) or nitric oxide (NO) synthase was not found in ileal or colonic CB1 receptor immunoreactive neurones. A preliminary report [15] has shown immunohistochemical localisation of CB1 receptors on acetylcholine-containing neurones that innervate smooth muscle, mucosa and submucosal blood vessels of the rat stomach.
Therapeutic potential of cannabinoids in gastrointestinal disorders Antiulcer activity
The cannabinoid receptor agonist WIN 55,212-2 has been shown to reduce stress-induced gastric ulcers in rats [16]. This protective effect of WIN 55,212-2 was counteracted by SR141716A but not by SR144528 (CB1 and CB2 receptor antagonists respectively), thus indicating the involvement of CB1 receptors. The antiulcerative effect of WIN 55,212-2 could be related to its antisecretory effects because cannabinoid agonists reduced in vivo acid secretion stimulated by pentagastrin (but not by histamine) in the rat via activation of CB1 receptors [17,18]. The lack of effect on histamineinduced acid secretion tends to exclude the presence of CB1 receptors on parietal cells, which is consistent with immunohistochemical studies (discussed earlier). This finding supports the possibility of CB1-modulated control of vagally-stimulated acid secretion. Gastric motility
Cannabinoid receptors modulate gastric emptying in the rat. Indeed, the cannabinoid agonists WIN 55,212-2
(i.p., intraperitoneal), cannabinol (i.p.) and ∆9-THC (intravenously) reduced gastric motility via CB1 but not CB2 cannabinoid receptors [19,20]. ∆9-THC evoked longlasting decreases in intragastric pressure and pyloric contractility and these changes may contribute to the antiemetic properties of the drug [20]. The sites of action of ∆9-THC are probably at the level of the dorsal vagal complex, the vagus nerves and within the gastric myenteric plexus. Intestinal motility Excitatory transmission in the small intestine
Cannabinoid receptor agonists affect isolated intestinal segments in a manner that resembles the action of µ opioid receptor (MOR) agonists and α2-adrenoceptor agonists. Thus, a number of cannabinoid receptor agonists show high potency as inhibitors of electrically-induced contractions in the guinea-pig longitudinal-muscle/myenteric-plexus preparation [21–23]. As these agonists do not inhibit the contractions produced by exogenously applied acetylcholine in this preparation, it is likely that cannabinoids inhibit the twitch response by acting prejunctionally rather than via a direct action on intestinal smooth muscle. The inhibitory action of cannabinoid agonists was competitively antagonised by the CB1 receptor antagonist SR141716A but was unaffected by the opioid receptor antagonist naloxone or the α2-adrenoceptor antagonist yohimbine. Furthermore, the inhibitory effect of WIN 55,212-2 was also attenuated by compounds that increase cAMP (IBMX, forskolin) or by increasing the intracellular Ca2+ concentration [24]. This is consistent with the transduction mechanism associated with CB1 receptors — inhibition of adenylate cyclase and block of calcium channels (see Figure 1). The cannabinoid agonists WIN 55,212-2 and CP 55,940, at concentrations that inhibit the electrically-induced twitch response in the guinea-pig
The gastrointestinal pharmacology of cannabinoids Izzo et al.
ileum, produce a corresponding reduction in acetylcholine release from guinea-pig enteric nerves and this effect is mediated by CB1 receptors [25]. Electrophysiological studies have shown that WIN 55,212-2 and CP 55,940 inhibit fast (cholinergic) and slow (nonadrenergic non-cholinergic [NANC]) excitatory synaptic transmission in myenteric S-neurones of the guinea-pig small intestine [26]. This inhibitory effect occurred, at least for fast cholinergic synaptic transmission, by reversible activation of both presynaptic and postsynaptic CB1 receptors. However, it is possible that some myenteric neurones are devoid of cannabinoid receptors, as detectable effects were observed in only one-third of S-neurones. WIN 55,212-2 and anandamide inhibit cholinergic and NANC excitatory responses in the circular muscle of the guinea-pig ileum via activation of CB1 receptors [27]; the effect is not antagonised by naloxone, yohimbine or the NO synthase inhibitor L-NAME (L-N-nitro arginine methyl ester). Interestingly, the inhibitory effect of WIN 55,212-2 on cholinergic (but not on NANC) transmission was reduced by apamin (an inhibitor of small conductance Ca2+-dependent K+ channels), which blocks the inhibitory actions of ATP or related purines on the neuromuscular junction. Croci et al. [28] provided the first functional evidence of the presence of prejunctional cannabinoid CB1 receptors in human ileum longitudinal smooth muscle, through which the cannabinoid agonist WIN 55,212.2 inhibited electrically evoked contractile responses. This effect was reversed by the CB1 receptor antagonist SR141716A, but not by the CB2 receptor antagonist SR144528 or by naloxone. Peristalsis in the isolated guinea-pig ileum and mouse colon
Peristalsis can be reproduced in vitro by slowly infusing Krebs solution into the intestinal lumen to radially stretch the intestinal wall. Two phases of peristalsis have been described in response to this stretch: a preparatory phase, in which the intestine gradually distends until a threshold distension is reached, and an emptying phase, in which the circular muscle at the oral end of the intestine contracts. The cannabinoid receptor agonists WIN 55,212-2 and CP 55,940 have been shown to inhibit peristalsis in the guinea-pig ileum [29]. This was deduced from the decreased longitudinal smooth-muscle reflex contraction and the increased threshold pressure and volume required to elicit peristalsis during the preparatory phase of peristalsis and from the observed reduction of the maximal ejection pressure during the emptying phase of peristalsis. At the highest concentration tested (1 µM) both WIN 55,212-2 and CP 55,940 completely shut down peristaltic activity. These effects are counteracted by SR141716A but not SR144528 and are therefore mediated by CB1 but not CB2 receptors. Heinemann et al. [30] showed that the antiperistaltic activity of methanandamide (a stable analogue of anandamide)
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Figure 2
O N H
OH
Anandamide
OH
O O
OH
2-Arachidonylglycerol
OH C O H2
OH
Noladin ether Current Opinion in Pharmacology
Chemical structures of endogenous cannabinoids.
in the guinea-pig ileum was inhibited by apamin and attenuated by the NO synthase inhibitor L-NAME. Furthermore, methanandamide, acting on CB1 receptors, inhibited atropine-resistant peristalsis (which is mediated by the release of endogenous tachykinins), a result that is consistent with the ability of cannabinoid receptor agonists to suppress NANC excitatory transmission in guinea-pig circular smooth muscle [27]. The authors proposed that activation of CB1 receptors inhibits peristalsis by two distinct mechanisms of action: interruption of excitatory motor pathways and activation of inhibitory motor pathways that cause peristaltic inhibition through release of apamin-sensitive transmitters and NO [30]. A functional role for CB1 receptors in the regulation of motility in the colon was recently reported [31]. The cannabinoid receptor agonist WIN 55,212-2 was found to attenuate peristaltic activity in the mouse colon and this effect was associated with a decrease in circular and longitudinal smooth muscle contractile strength. Upper gastrointestinal transit in vivo
Application of the endogenous cannabinoid ligand anandamide (subcutaneous or i.p.) [32,33], the natural agonist cannabinol (i.p.) [34,35••] or the synthetic agonists WIN 55,212-2 (i.p.) [34,36–38] or CP 55,940 (i.p.) [35••] inhibited intestinal motility in mice. This effect was counteracted by SR141716A but not SR144528, thereby indicating an involvement of CB1 but not CB2 receptors. WIN 55,212-2 and cannabinol were significantly more effective when administered intracerebroventricularly than when administered intraperitoneally [34]. The higher potency of cannabinoid agonists administered centrally than peripherally suggests a central site of action. However, central CB1 receptors probably contribute little to the effects of peripherally administered cannabinoids as the
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Table 1 Cannabinoid drugs and their effects on the digestive tract. Drug
Category
In vitro effect
In vivo effect
WIN 55,212-2
Non-selective Inhibits excitatory (cholinergic and NANC) Reduces gastrointestinal transit and [17,19,22, cannabinoid receptor transmission, acetylcholine release from defaecation in mice; reduces 25,26,28, agonist (synthetic) guinea-pig enteric nerves, peristalsis in the gastrointestinal transit and fluid 31,34,35••, guinea-pig ileum and mouse colon; inhibits accumulation in the rat small intestine; 36–38,41•] fast and slow excitatory synaptic transmission reduces pentagastrin-stimulated in myenteric S-neurones in the guinea-pig gastric acid secretion and stress-induced ileum; inhibits cholinergic transmission in the ulceration in the rat human ileum; reduces electrically stimulated ion transport in the rat ileum
CP 55,940
Non-selective Inhibits cholinergic trasmission, peristalsis and cannabinoid receptor acetylcholine release in the guinea-pig ileum; agonist (synthetic) inhibits fast and slow excitatory synaptic transmission in myenteric S-neurones in the guinea-pig ileum
Reduces gastrointestinal transit in rats and mice
[22,25,26, 29,38,46]
Cannabinol
Non-selective Inhibits cholinergic transmission in the guineacannabinoid receptor pig ileum agonist (plant-derived)
Reduces gastric emptying in the rat and reduces gastrointestinal transit in mice
[19,22,34, 35••]
∆9-THC
Non-selective Inhibits cholinergic transmission in the guineacannabinoid receptor pig ileum agonist (plant-derived)
Reduces gastric and intestinal motility in the rat
[20,22,39]
HU-210
Non-selective cannabinoid receptor agonist (synthetic)
Reduces pentagastrin-stimulated gastric [18] acid secretion in the rat
Anandamide
Endogenous Inhibits cholinergic and NANC excitatory cannabinoid receptor transmission in the guinea-pig ileum agonist (selectivity: CB1>> CB2)
Reduces gastrointestinal transit in mice
[27,32,33]
Noladin ether
Endogenous selective CB1 receptor agonist
Reduces defaecation in mice
[5•]
Methanandamide Synthetic cannabinoid Inhibits peristalsis in the guinea-pig small receptor agonist intestine; inhibits ascending excitatory reflex (selectivity: produced by balloon distension CB1>> CB2)
References
[30]
JWH-015
Selective CB2 receptor agonist (synthetic)
SR141716A
Selective CB1 receptor antagonist (synthetic)
AM281
Selective CB1 receptor Facilitation of peristalsis in the guinea-pig small antagonist (synthetic) intestine
LY320135
Selective CB1 receptor antagonist (synthetic)
No effect on rat gastric acid secretion
SR144528
Selective CB2 receptor No effect on peristalsis in the guinea-pig small antagonist (synthetic) intestine and no effect on human cholinergic transmission
No effect on gastric and intestinal motility [19,28,29, in mice and rats 34,46]
AM404
Anandamide uptake inhibitor (synthetic)
No effect on anandamide-induced delay in motility
PMSF
Anandamide hydrolase Potentiates anandamide-induced inhibition of inhibitor (synthetic) cholinergic transmission in the guinea-pig ileum
No effect on pentagastrin-induced gastric acid secretion in rats Increases excitatory (cholinergic and NANC) Increases gastrointestinal transit and transmission and acetylcholine release in the defaecation in mice; increases guinea-pig ileum; facilitation of peristalsis in gastrointestinal transit and fluid the guinea-pig small intestine and mouse colon; accumulation in the rat small intestine; no effect on human cholinergic transmission no effect on gastric emptying
Potentiates anandamide-induced inhibition of acetylcholine release in the guinea-pig ileum
effect of i.p.-injected cannabinoid agonists was not modified by the ganglionic blocker hexamethonium [34,38,39]. Recent evidence indicates that anandamide is also an endogenous ligand of vanilloid (capsaicin) receptors [40], which are expressed predominantly on primary afferent
[17]
[19,22,25, 27,29–31, 34,35••,36, 38,46,47] [29] [17]
[32,45] [21]
neurones. However, it is unlikely that vanilloid receptors could mediate anandamide-induced changes in motility, as chronic treatment with capsaicin (to ablate capsaicinsensitive afferent neurones) or the vanilloid receptor antagonist capsazepine failed to modify the anandamide-induced delay in gastrointestinal transit in vivo ([33]; see also Update).
The gastrointestinal pharmacology of cannabinoids Izzo et al.
Motility in pathophysiological states
Croton oil is a well known intestinal irritant used experimentally to induce inflammation in the mouse small intestine. This inflammation is characterised by disruption of the mucosa and an infiltration of lymphocytes into the submucosa. The cannabinoid agonists CP 55,940 (i.p.) and cannabinol (i.p.) blocked the increase in intestinal motility induced by croton oil, an effect mediated by CB1, but not CB2 receptors [35••]. In addition, chronic inflammation enhanced the potency of cannabinoid agonists, an effect that was associated with an increased expression of CB1 receptors in the inflamed small intestine [35••]. The low doses of cannabinoid agonists that are needed to reduce motility during chronic inflammation are of interest in the light of possible therapeutic applications of such compounds in inflammatory bowel diseases.
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Figure 3
Endogenous cannabinoids
Synthetic cannabinoids
Plant-derived cannabinoids
Intestinal secretion
Using an enteropooling method, it was demonstrated that the cannabinoid agonist WIN 55,212-2 decreased basal fluid accumulation in the rat small intestine and this effect was counteracted by the CB1 receptor antagonist SR141716A [38]. Furthermore, WIN 55,212-2 also attenuated electrically-induced, but not acetylcholine-induced secretion in the rat ileum in vitro, again in a SR141716A-sensitive manner [41•]. Thus, activation of CB1 receptors may produce an antisecretory effect through a neuronal mechanism, which in all likelihood involves the inhibition of acetylcholine release from submucosal plexus neurones.
CB1
Physiological role for endocannabinoids There is evidence that intestinal motility could be physiologically inhibited by endogenous cannabinoids. Indeed, the presence of the endogenous cannabinoid receptor agonist 2-AG has been demonstrated in the canine gut [42], whereas mouse [35••] and rat [43] small intestine contains significant concentrations of anandamide hydrolase, the enzyme responsible for the inactivation of anandamide [44]. We have revealed the presence, in the small intestine of mice, of levels of anandamide (0.036 nmol/g tissue) and particularly, 2-AG (44 nmol/g tissue) that are higher than those measured in the rat brain and sufficient to activate cannabinoid receptors [35••]. This is compatible with a tonic activation of CB1 receptors in the small intestine. There is also evidence for the presence of anandamide uptake and metabolising mechanisms in guinea-pig ileum. Indeed, the inhibitory effect of anandamide on cholinergic transmission [21] or acetylcholine release [45] can be potentiated by the anandamide hydrolase inhibitor phenylmethylsulfonyl fluoride (PMSF) or by the anandamide uptake inhibitor AM404. Functional studies indicate that the CB1 receptor antagonist SR141716A, administered alone, can produce motility changes that are opposite in direction to those of cannabinoid agonists. In fact, SR141716 has been shown to increase cholinergic transmission in the guinea-pig longitudinal smooth muscle [22], cholinergic and NANC excitatory
Antiulcer effect
Reduction of gastric and intestinal motility
Reduction of intestinal secretion
Current Opinion in Pharmacology
Summary of the effects of cannabinoid drugs on the digestive system. Endogenous, synthetic and plant-derived cannabinoid agonists act on CB1 receptors located on enteric nerves of the digestive tract. Pharmacological effects include gastroprotection, reduction of gastric and intestinal motility and reduction of intestinal secretion.
transmission in the guinea-pig circular smooth muscle [27], electrically-evoked acetylcholine release from guinea-pig myenteric nerves [25], ascending excitatory reflex produced by balloon distension of the guinea-pig ileum [30], peristaltic efficiency in the isolated guinea-pig ileum [29] and mouse colon [31], upper gastrointestinal transit in mice [34,35••,36,38] and defaecation in rats and mice [38,46,47]. It should be kept in mind, however, that these effects cannot be attributed unequivocally to the displacement of endogenous cannabinoids because SR141716A behaves as an inverse agonist at CB1 receptors [48].
Conclusions There is a growing body of evidence to support a CB1 receptor mediated modulation of gastrointestinal motility,
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intestinal secretion and gastroprotection (Table 1 and Figure 3). More detailed studies have indicated that the gut endocannabinoid system is a potential target to decrease intestinal motility, especially during inflammatory states. Two possible strategies might be pursued to obtain drugs able to reduce intestinal motility without provoking unacceptable systemic (i.e. psychotropic, cardiovascular) side effects [49,50]. First, to develop selective CB1 receptor agonists that have minimal capacity to cross the blood–brain barrier, in a manner similar to the anti-diarrhoeal opiate loperamide and second, to develop inhibitors of endocannabinoid transport and/or enzymatic degradation which, by increasing local levels of endocannabinoids (where ongoing production is occurring), can have greater pharmacological selectivity than direct-acting cannabinoid receptor drugs. Finally, it is worth noting that cannabinoids possess analgesic, anti-inflammatory and antiulcerative activities, which could be of particular therapeutic importance given the ulcerogenic effects of many of the anti-inflammatory drugs such as aspirin used in modern medicine.
Update A recent study performed in the isolated guinea-pig ileum demonstrated that anandamide inhibited electrically stimulated acetylcholine release via activation of non-CB1 and non-CB2 receptors and increased basal acetylcholine release via activation of vanilloid receptors [51]
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest •• of outstanding interest
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29. Izzo AA, Mascolo N, Tonini M, Capasso F: Modulation of peristalsis by cannabinoid CB1 ligands in the isolated guinea-pig ileum. Br J Pharmacol 2000, 129:984-990. 30. Heinemann A, Shahbazian A, Holzer P: Cannabinoid inhibition of guinea-pig intestinal peristalsis via inhibition of excitatory and activation of inhibitory neural pathways. Neuropharmacology 1999, 38:1289-1297. 31. Mancinelli R, Fabrizi A, Del Monaco S, Azzena GB, Vargiu R, Colombo GC, Gessa GL: Inhibition of peristaltic activity by cannabinoids in the isolated distal colon of mouse. Life Sci 2001, 69:101-111. 32. Calignano A, La Rana G, Makriayannis A, Lin SY, Beltramo M, Piomelli D: Inhibition of intestinal motility by anandamide, an endogenous cannabinoid. Eur J Pharmacol 1997, 340:R7-R8. 33. Izzo AA, Capasso R, Pinto L, Di Carlo G, Mascolo N, Capasso F: Effect of vanilloid drugs on gastrointestinal transit in mice. Br J Pharmacol 2001, 132:1411-1416. 34. Izzo AA, Pinto L, Borrelli F, Capasso R, Mascolo N, Capasso F: Central and peripheral cannabinoid modulation of gastrointestinal transit in physiological states or during the diarrhoea induced by croton oil. Br J Pharmacol 2000, 129:1627-1632. 35. Izzo AA, Fezza F, Capasso F, Bisogno T, Pinto L, Iuvone T, Esposito G, •• Mascolo N, Di Marzo V: Cannabinoid CB1-receptor mediated regulation of gastrointestinal motility in mice in a chronic model of intestinal inflammation. Br J Pharmacol 2001, in press. This paper reports for the first time the presence of anandamide in the gut and presents evidence for the existence of an endocannabinoid tone, which regulates intestinal motility. Furthermore, using Western blot analysis, an upregulation of CB1 receptors during intestinal inflammation is demonstrated. 36. Colombo G, Agabio R, Lobina C, Reali R, Gessa GL: Cannabinoid modulation of intestinal propulsion in mice. Eur J Pharmacol 1998, 344:67-69. 37.
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38. Izzo AA, Mascolo N, Borrelli F, Capasso F: Defaecation, intestinal fluid accumulation and motility in rodents: implications of cannabinoid CB1 ligands in the isolated guinea pig ileum. Naunyn Schmiedebergs Arch Pharmacol 1999, 359:65-70. 39. Izzo AA, Addeo D, Amato M, Borrelli F, Capasso R, Di Carlo G, Nocerino E, Mascolo N, Pinto L, Capasso F: In vivo modulation of gastrointestinal motility by cannabinoid drugs. Pharm Pharmacol Commun 2000, 6:255-258.
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40. Zygmunt PM, Petersson J, Andersson DA, Chuang H, Sorgard M, Di Marzo V, Julius D, Hogestatt ED: Vanilloid receptors on sensory nerves mediate the vasodilatator action of anandamide. Nature 1999, 400:452-457. 41. Tyler K, Hillard CJ, Greenwood-Van Meerveld B: Inhibition of small • intestinal secretion by cannabinoids is CB1 receptor-mediated in rats. Eur J Pharmacol 2000, 409:207-211. Using an Ussing chamber, the authors show that cannabinoid receptor agonists affect intestinal mucosal transport through a neuronal mechanism involving submucosal CB1 receptors. 42. Mechoulam R, Ben-Shabat S, Hanus L, Ligumsky M, Kaminski NE, Schatz AR, Gopher A, Almog S, Marting BR, Compton DR et al.: Identification of an endogenous 2-monoglyceride, present in canine gut, that binds to a cannabinoid receptor. Biochem Pharmacol 1995, 50:83-90. 43. Katayama K, Ueda N, Kurahashi Y, Suzuki H, Yamamoto S, Kato I: Distribution of anandamide amidohydrolase in rat tissues with a special reference to small intestine. Biochim Biophys Acta 1997, 1347:212-218. 44. Ueda N, Yamamoto S: Anandamide amidohydrolase (fatty acid amide hydrolase). Prostaglandins Other Lipid Mediat 2000, 61:19-28. 45. Yal WM, Reese LA: Cannabinoid receptor: inhibition of ACh release from myenteric plexus of small intestine. Gastroenterology 1999, 116:G4791. 46. Izzo AA, Mascolo N, Pinto L, Capasso R, Capasso F: The role of cannabinoid receptors in intestinal motility, defaecation and diarrhoea in rats. Eur J Pharmacol 1999, 384:37-42. 47.
Costa B, Colleoni M: SR141716A induces in rats a bahavioral pattern opposite to that of CB1 receptor agonists. Zhongguo Yao Li Xue Bao 1999, 20:1103-1108.
48. MacLennan SJ, Reyen PH, Bonhaus DW: Evidence for inverse agonism of SR141716A at human recombinant cannabinoid CB1 and CB2 receptors. Br J Pharmacol 1998, 124:619-622. 49. Izzo AA, Mascolo N, Capasso F: Forgotten target for marijuana: the endocannabinoid system in the gut. Trends Pharmacol Sci 2000, 21:372-373. 50. Piomelli D, Giuffrida A: Reply: Cannabinoid paths to anti-diarrheal drugs. Trends Pharmacol Sci 2000, 21:373. 51. Mang CF, Erbelding D, Kilbinger H: Differential effects of anandamide on acetylcholine release in the guinea-pig ileum mediated via vanilloid and non-CB1 receptors. Br J Pharmacol 2001, 134:161-167.
The gastrointestinal pharmacology of cannabinoids Angelo A Izzo*, Nicola Mascolo and Francesco Capasso The digestive tract contains endogenous cannabinoids (anandamide and 2-arachidonylglycerol) and cannabinoid CB1 receptors can be found on myenteric and submucosal nerves. Activation of CB1 receptors inhibits gastrointestinal motility, intestinal secretion and gastric acid secretion. The enteric location of CB1 receptors could provide new strategies for the management of gut disorders Addresses Department of Experimental Pharmacology, University of Naples Federico II, Via D Montesano 49, 80131 Naples, Italy *e-mail: [email protected] Current Opinion in Pharmacology 2001, 1:597–603 1471-4892/01/$ — see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations 2-AG 2-arachidonyl glycerol ChAT choline acetyltransferase ∆9-THC ∆9-tetrahydrocannabinol i.p. intraperitoneal NANC non-adrenergic non-cholinergic PMSF phenylmethylsulfonyl fluoride VIP vasoactive intestinal peptide
Introduction Marijuana is a well known recreational drug that has for centuries been prescribed therapeutically in herbal medicine for the treatment of a wide array of health problems, including a range of gastrointestinal disorders [1]. A century ago in the United States extracts of Cannabis Sativa were indicated for gastrointestinal pain, gastroenteritis and diarrhoea, and there are anecdotal reports suggesting that marijuana may be effective in alleviating symptoms of Crohn’s disease and diabetic gastroparesis. Cannabis contains more than 60 cannabinoids that might, in a synergistic (or antagonistic) way, contribute to its pharmacological activity. In recent years, several investigations have shown that many of the pharmacological effects of Cannabis, or its main active component ∆9-tetrahydrocannabinol (∆9-THC), are mediated by at least two types of receptors, both of which are coupled to Gi/o proteins ([2,3]; Figure 1). CB1 cannabinoid receptors are expressed by central and peripheral neurones and CB2 receptors are expressed mostly by immune cells. With the discovery of receptors for plant-derived cannabinoids, the search for the endogenous ligands for these receptors began. Arachidonyl ethanolamine (anandamide, selectivity: CB1>>CB2), 2-arachidonylglycerol (2-AG, non-selective) [2–4] and, more recently, 2-arachidonyl glyceryl ether (noladin ether, CB1-selective) [5•] have been detected in mammalian tissues (Table 1, Figure 2). Anandamide undergoes depolarisationinduced synthesis and release from neurones and can be
removed from its sites of action by carrier-mediated transport (anandamide transport), which can be inhibited by AM404 [2]. Once within the cell, anandamide can by hydrolysed by a membrane-bound anandamide amidohydrolase [2], which can be inhibited by the anandamide hydrolase inhibitor phenylmethylsulfonyl fluoride (PMSF). Cannabinoid receptors and endogenous cannabinoids (endocannabinoids) are referred to as the ‘endogenous cannabinoid system’. As detailed elsewhere [6], synthetic cannabinoid receptor agonists (mostly non-selective) and antagonists (selective) are now available. Cannabinoid receptor agonists are usually classified, according to chemical structure, into one of four main groups: classical (e.g. ∆9-THC, cannabinol, HU 210), non-classical (e.g. CP 55,940), aminoalkylindoles (e.g. WIN 55,212-2) and eicosanoids (e.g. anandamide, 2-AG, noladin ether). The selectivity and effects of these ligands are summarised in Table 1. Although cannabinoids have a wide variety of biological effects — with potential therapeutic applications including the treatment of pain, neurodegenerative and musculoskeletal disorders, inflammation and cancer [7–10] — this review focuses on recent cannabinoid research findings with possible therapeutic applications for the treatment of gastrointestinal disorders. Some cannabinoid agonists are already used clinically. For example, nabilone is used to suppress nausea and vomiting provoked by anticancer durgs and ∆9-THC is used to boost the appetite of AIDS patients [6].
Cannabinoid receptors in the gut The densities of CB1 receptors in peripheral tissues are lower than those found in areas of the brain such as the cerebellum or cerebral cortex. This is because some peripheral tissues may contain high concentrations of the CB1 receptor localised in discrete regions such as nerve terminals but which form only a small part of the total tissue mass. Griffin et al. [11] detected mRNA encoding the CB1 receptor but not CB2 receptor mRNA in the myenteric plexus of the guinea-pig small intestine. However, both types of mRNA were detected in guinea-pig whole gut, possibly because of the presence in the whole tissue of resident macrophages and other immune cells that are known to contain CB2 receptors. CB1 mRNA has also been detected in human stomach and colon [12] and in the myenteric and submucosal plexus of rat embryo digestive tract [13]. Immunohistochemical studies have revealed CB1 receptor immunoreactivity in the myenteric and submucosal ganglionated plexuses of porcine ileum and colon [14•]. In the ileum, all CB1 receptor immunoreactive neurones were also immunoreactive to the cholinergic marker choline acetyltransferase (ChAT). CB1 receptor and ChAT immunoreactive neurones appeared to be in close apposition to ileal Peyer’s patches,
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Figure 1 Cannabinoid agonists
Ca2+
K+ Extracellular –
γ β Calcium channels blocked [Ca2+]
CB1 α Gi/o
γ –
AC
β
CB2 α
–
Gi/o
Potassium channels activated [K+]
[cAMP] ERK cascade activated
Intracellular
Signal transduction pathways activated by cannabinoid receptor agonists. Cannabinoid receptor agonists activate cannabinoid CB1 and/or CB2 receptors, both coupled to Gi/o proteins. This leads to inhibition of adenylyl cyclase (AC) and activation of the extracellular signal-regulated kinase (ERK) cascade. Furthermore, the CB1 receptor can induce inhibition of N-type and P/Q-type voltage sensitive Ca2+ channels and activate G-protein-activated inwardly rectifying K+ channels. This leads to membrane hyperpolarisation and inhibition of activity.
AC
[cAMP] ERK cascade activated Current Opinion in Pharmacology
submucosal blood vessels, and intestinal crypts. In the distal colon, CB1 receptor immunoreactive neurones were also immunoreactive to ChAT, although coexpression was less frequent than in the ileum. Immunoreactivity to vasoactive intestinal peptide (VIP) or nitric oxide (NO) synthase was not found in ileal or colonic CB1 receptor immunoreactive neurones. A preliminary report [15] has shown immunohistochemical localisation of CB1 receptors on acetylcholine-containing neurones that innervate smooth muscle, mucosa and submucosal blood vessels of the rat stomach.
Therapeutic potential of cannabinoids in gastrointestinal disorders Antiulcer activity
The cannabinoid receptor agonist WIN 55,212-2 has been shown to reduce stress-induced gastric ulcers in rats [16]. This protective effect of WIN 55,212-2 was counteracted by SR141716A but not by SR144528 (CB1 and CB2 receptor antagonists respectively), thus indicating the involvement of CB1 receptors. The antiulcerative effect of WIN 55,212-2 could be related to its antisecretory effects because cannabinoid agonists reduced in vivo acid secretion stimulated by pentagastrin (but not by histamine) in the rat via activation of CB1 receptors [17,18]. The lack of effect on histamineinduced acid secretion tends to exclude the presence of CB1 receptors on parietal cells, which is consistent with immunohistochemical studies (discussed earlier). This finding supports the possibility of CB1-modulated control of vagally-stimulated acid secretion. Gastric motility
Cannabinoid receptors modulate gastric emptying in the rat. Indeed, the cannabinoid agonists WIN 55,212-2
(i.p., intraperitoneal), cannabinol (i.p.) and ∆9-THC (intravenously) reduced gastric motility via CB1 but not CB2 cannabinoid receptors [19,20]. ∆9-THC evoked longlasting decreases in intragastric pressure and pyloric contractility and these changes may contribute to the antiemetic properties of the drug [20]. The sites of action of ∆9-THC are probably at the level of the dorsal vagal complex, the vagus nerves and within the gastric myenteric plexus. Intestinal motility Excitatory transmission in the small intestine
Cannabinoid receptor agonists affect isolated intestinal segments in a manner that resembles the action of µ opioid receptor (MOR) agonists and α2-adrenoceptor agonists. Thus, a number of cannabinoid receptor agonists show high potency as inhibitors of electrically-induced contractions in the guinea-pig longitudinal-muscle/myenteric-plexus preparation [21–23]. As these agonists do not inhibit the contractions produced by exogenously applied acetylcholine in this preparation, it is likely that cannabinoids inhibit the twitch response by acting prejunctionally rather than via a direct action on intestinal smooth muscle. The inhibitory action of cannabinoid agonists was competitively antagonised by the CB1 receptor antagonist SR141716A but was unaffected by the opioid receptor antagonist naloxone or the α2-adrenoceptor antagonist yohimbine. Furthermore, the inhibitory effect of WIN 55,212-2 was also attenuated by compounds that increase cAMP (IBMX, forskolin) or by increasing the intracellular Ca2+ concentration [24]. This is consistent with the transduction mechanism associated with CB1 receptors — inhibition of adenylate cyclase and block of calcium channels (see Figure 1). The cannabinoid agonists WIN 55,212-2 and CP 55,940, at concentrations that inhibit the electrically-induced twitch response in the guinea-pig
The gastrointestinal pharmacology of cannabinoids Izzo et al.
ileum, produce a corresponding reduction in acetylcholine release from guinea-pig enteric nerves and this effect is mediated by CB1 receptors [25]. Electrophysiological studies have shown that WIN 55,212-2 and CP 55,940 inhibit fast (cholinergic) and slow (nonadrenergic non-cholinergic [NANC]) excitatory synaptic transmission in myenteric S-neurones of the guinea-pig small intestine [26]. This inhibitory effect occurred, at least for fast cholinergic synaptic transmission, by reversible activation of both presynaptic and postsynaptic CB1 receptors. However, it is possible that some myenteric neurones are devoid of cannabinoid receptors, as detectable effects were observed in only one-third of S-neurones. WIN 55,212-2 and anandamide inhibit cholinergic and NANC excitatory responses in the circular muscle of the guinea-pig ileum via activation of CB1 receptors [27]; the effect is not antagonised by naloxone, yohimbine or the NO synthase inhibitor L-NAME (L-N-nitro arginine methyl ester). Interestingly, the inhibitory effect of WIN 55,212-2 on cholinergic (but not on NANC) transmission was reduced by apamin (an inhibitor of small conductance Ca2+-dependent K+ channels), which blocks the inhibitory actions of ATP or related purines on the neuromuscular junction. Croci et al. [28] provided the first functional evidence of the presence of prejunctional cannabinoid CB1 receptors in human ileum longitudinal smooth muscle, through which the cannabinoid agonist WIN 55,212.2 inhibited electrically evoked contractile responses. This effect was reversed by the CB1 receptor antagonist SR141716A, but not by the CB2 receptor antagonist SR144528 or by naloxone. Peristalsis in the isolated guinea-pig ileum and mouse colon
Peristalsis can be reproduced in vitro by slowly infusing Krebs solution into the intestinal lumen to radially stretch the intestinal wall. Two phases of peristalsis have been described in response to this stretch: a preparatory phase, in which the intestine gradually distends until a threshold distension is reached, and an emptying phase, in which the circular muscle at the oral end of the intestine contracts. The cannabinoid receptor agonists WIN 55,212-2 and CP 55,940 have been shown to inhibit peristalsis in the guinea-pig ileum [29]. This was deduced from the decreased longitudinal smooth-muscle reflex contraction and the increased threshold pressure and volume required to elicit peristalsis during the preparatory phase of peristalsis and from the observed reduction of the maximal ejection pressure during the emptying phase of peristalsis. At the highest concentration tested (1 µM) both WIN 55,212-2 and CP 55,940 completely shut down peristaltic activity. These effects are counteracted by SR141716A but not SR144528 and are therefore mediated by CB1 but not CB2 receptors. Heinemann et al. [30] showed that the antiperistaltic activity of methanandamide (a stable analogue of anandamide)
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Figure 2
O N H
OH
Anandamide
OH
O O
OH
2-Arachidonylglycerol
OH C O H2
OH
Noladin ether Current Opinion in Pharmacology
Chemical structures of endogenous cannabinoids.
in the guinea-pig ileum was inhibited by apamin and attenuated by the NO synthase inhibitor L-NAME. Furthermore, methanandamide, acting on CB1 receptors, inhibited atropine-resistant peristalsis (which is mediated by the release of endogenous tachykinins), a result that is consistent with the ability of cannabinoid receptor agonists to suppress NANC excitatory transmission in guinea-pig circular smooth muscle [27]. The authors proposed that activation of CB1 receptors inhibits peristalsis by two distinct mechanisms of action: interruption of excitatory motor pathways and activation of inhibitory motor pathways that cause peristaltic inhibition through release of apamin-sensitive transmitters and NO [30]. A functional role for CB1 receptors in the regulation of motility in the colon was recently reported [31]. The cannabinoid receptor agonist WIN 55,212-2 was found to attenuate peristaltic activity in the mouse colon and this effect was associated with a decrease in circular and longitudinal smooth muscle contractile strength. Upper gastrointestinal transit in vivo
Application of the endogenous cannabinoid ligand anandamide (subcutaneous or i.p.) [32,33], the natural agonist cannabinol (i.p.) [34,35••] or the synthetic agonists WIN 55,212-2 (i.p.) [34,36–38] or CP 55,940 (i.p.) [35••] inhibited intestinal motility in mice. This effect was counteracted by SR141716A but not SR144528, thereby indicating an involvement of CB1 but not CB2 receptors. WIN 55,212-2 and cannabinol were significantly more effective when administered intracerebroventricularly than when administered intraperitoneally [34]. The higher potency of cannabinoid agonists administered centrally than peripherally suggests a central site of action. However, central CB1 receptors probably contribute little to the effects of peripherally administered cannabinoids as the
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Table 1 Cannabinoid drugs and their effects on the digestive tract. Drug
Category
In vitro effect
In vivo effect
WIN 55,212-2
Non-selective Inhibits excitatory (cholinergic and NANC) Reduces gastrointestinal transit and [17,19,22, cannabinoid receptor transmission, acetylcholine release from defaecation in mice; reduces 25,26,28, agonist (synthetic) guinea-pig enteric nerves, peristalsis in the gastrointestinal transit and fluid 31,34,35••, guinea-pig ileum and mouse colon; inhibits accumulation in the rat small intestine; 36–38,41•] fast and slow excitatory synaptic transmission reduces pentagastrin-stimulated in myenteric S-neurones in the guinea-pig gastric acid secretion and stress-induced ileum; inhibits cholinergic transmission in the ulceration in the rat human ileum; reduces electrically stimulated ion transport in the rat ileum
CP 55,940
Non-selective Inhibits cholinergic trasmission, peristalsis and cannabinoid receptor acetylcholine release in the guinea-pig ileum; agonist (synthetic) inhibits fast and slow excitatory synaptic transmission in myenteric S-neurones in the guinea-pig ileum
Reduces gastrointestinal transit in rats and mice
[22,25,26, 29,38,46]
Cannabinol
Non-selective Inhibits cholinergic transmission in the guineacannabinoid receptor pig ileum agonist (plant-derived)
Reduces gastric emptying in the rat and reduces gastrointestinal transit in mice
[19,22,34, 35••]
∆9-THC
Non-selective Inhibits cholinergic transmission in the guineacannabinoid receptor pig ileum agonist (plant-derived)
Reduces gastric and intestinal motility in the rat
[20,22,39]
HU-210
Non-selective cannabinoid receptor agonist (synthetic)
Reduces pentagastrin-stimulated gastric [18] acid secretion in the rat
Anandamide
Endogenous Inhibits cholinergic and NANC excitatory cannabinoid receptor transmission in the guinea-pig ileum agonist (selectivity: CB1>> CB2)
Reduces gastrointestinal transit in mice
[27,32,33]
Noladin ether
Endogenous selective CB1 receptor agonist
Reduces defaecation in mice
[5•]
Methanandamide Synthetic cannabinoid Inhibits peristalsis in the guinea-pig small receptor agonist intestine; inhibits ascending excitatory reflex (selectivity: produced by balloon distension CB1>> CB2)
References
[30]
JWH-015
Selective CB2 receptor agonist (synthetic)
SR141716A
Selective CB1 receptor antagonist (synthetic)
AM281
Selective CB1 receptor Facilitation of peristalsis in the guinea-pig small antagonist (synthetic) intestine
LY320135
Selective CB1 receptor antagonist (synthetic)
No effect on rat gastric acid secretion
SR144528
Selective CB2 receptor No effect on peristalsis in the guinea-pig small antagonist (synthetic) intestine and no effect on human cholinergic transmission
No effect on gastric and intestinal motility [19,28,29, in mice and rats 34,46]
AM404
Anandamide uptake inhibitor (synthetic)
No effect on anandamide-induced delay in motility
PMSF
Anandamide hydrolase Potentiates anandamide-induced inhibition of inhibitor (synthetic) cholinergic transmission in the guinea-pig ileum
No effect on pentagastrin-induced gastric acid secretion in rats Increases excitatory (cholinergic and NANC) Increases gastrointestinal transit and transmission and acetylcholine release in the defaecation in mice; increases guinea-pig ileum; facilitation of peristalsis in gastrointestinal transit and fluid the guinea-pig small intestine and mouse colon; accumulation in the rat small intestine; no effect on human cholinergic transmission no effect on gastric emptying
Potentiates anandamide-induced inhibition of acetylcholine release in the guinea-pig ileum
effect of i.p.-injected cannabinoid agonists was not modified by the ganglionic blocker hexamethonium [34,38,39]. Recent evidence indicates that anandamide is also an endogenous ligand of vanilloid (capsaicin) receptors [40], which are expressed predominantly on primary afferent
[17]
[19,22,25, 27,29–31, 34,35••,36, 38,46,47] [29] [17]
[32,45] [21]
neurones. However, it is unlikely that vanilloid receptors could mediate anandamide-induced changes in motility, as chronic treatment with capsaicin (to ablate capsaicinsensitive afferent neurones) or the vanilloid receptor antagonist capsazepine failed to modify the anandamide-induced delay in gastrointestinal transit in vivo ([33]; see also Update).
The gastrointestinal pharmacology of cannabinoids Izzo et al.
Motility in pathophysiological states
Croton oil is a well known intestinal irritant used experimentally to induce inflammation in the mouse small intestine. This inflammation is characterised by disruption of the mucosa and an infiltration of lymphocytes into the submucosa. The cannabinoid agonists CP 55,940 (i.p.) and cannabinol (i.p.) blocked the increase in intestinal motility induced by croton oil, an effect mediated by CB1, but not CB2 receptors [35••]. In addition, chronic inflammation enhanced the potency of cannabinoid agonists, an effect that was associated with an increased expression of CB1 receptors in the inflamed small intestine [35••]. The low doses of cannabinoid agonists that are needed to reduce motility during chronic inflammation are of interest in the light of possible therapeutic applications of such compounds in inflammatory bowel diseases.
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Figure 3
Endogenous cannabinoids
Synthetic cannabinoids
Plant-derived cannabinoids
Intestinal secretion
Using an enteropooling method, it was demonstrated that the cannabinoid agonist WIN 55,212-2 decreased basal fluid accumulation in the rat small intestine and this effect was counteracted by the CB1 receptor antagonist SR141716A [38]. Furthermore, WIN 55,212-2 also attenuated electrically-induced, but not acetylcholine-induced secretion in the rat ileum in vitro, again in a SR141716A-sensitive manner [41•]. Thus, activation of CB1 receptors may produce an antisecretory effect through a neuronal mechanism, which in all likelihood involves the inhibition of acetylcholine release from submucosal plexus neurones.
CB1
Physiological role for endocannabinoids There is evidence that intestinal motility could be physiologically inhibited by endogenous cannabinoids. Indeed, the presence of the endogenous cannabinoid receptor agonist 2-AG has been demonstrated in the canine gut [42], whereas mouse [35••] and rat [43] small intestine contains significant concentrations of anandamide hydrolase, the enzyme responsible for the inactivation of anandamide [44]. We have revealed the presence, in the small intestine of mice, of levels of anandamide (0.036 nmol/g tissue) and particularly, 2-AG (44 nmol/g tissue) that are higher than those measured in the rat brain and sufficient to activate cannabinoid receptors [35••]. This is compatible with a tonic activation of CB1 receptors in the small intestine. There is also evidence for the presence of anandamide uptake and metabolising mechanisms in guinea-pig ileum. Indeed, the inhibitory effect of anandamide on cholinergic transmission [21] or acetylcholine release [45] can be potentiated by the anandamide hydrolase inhibitor phenylmethylsulfonyl fluoride (PMSF) or by the anandamide uptake inhibitor AM404. Functional studies indicate that the CB1 receptor antagonist SR141716A, administered alone, can produce motility changes that are opposite in direction to those of cannabinoid agonists. In fact, SR141716 has been shown to increase cholinergic transmission in the guinea-pig longitudinal smooth muscle [22], cholinergic and NANC excitatory
Antiulcer effect
Reduction of gastric and intestinal motility
Reduction of intestinal secretion
Current Opinion in Pharmacology
Summary of the effects of cannabinoid drugs on the digestive system. Endogenous, synthetic and plant-derived cannabinoid agonists act on CB1 receptors located on enteric nerves of the digestive tract. Pharmacological effects include gastroprotection, reduction of gastric and intestinal motility and reduction of intestinal secretion.
transmission in the guinea-pig circular smooth muscle [27], electrically-evoked acetylcholine release from guinea-pig myenteric nerves [25], ascending excitatory reflex produced by balloon distension of the guinea-pig ileum [30], peristaltic efficiency in the isolated guinea-pig ileum [29] and mouse colon [31], upper gastrointestinal transit in mice [34,35••,36,38] and defaecation in rats and mice [38,46,47]. It should be kept in mind, however, that these effects cannot be attributed unequivocally to the displacement of endogenous cannabinoids because SR141716A behaves as an inverse agonist at CB1 receptors [48].
Conclusions There is a growing body of evidence to support a CB1 receptor mediated modulation of gastrointestinal motility,
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intestinal secretion and gastroprotection (Table 1 and Figure 3). More detailed studies have indicated that the gut endocannabinoid system is a potential target to decrease intestinal motility, especially during inflammatory states. Two possible strategies might be pursued to obtain drugs able to reduce intestinal motility without provoking unacceptable systemic (i.e. psychotropic, cardiovascular) side effects [49,50]. First, to develop selective CB1 receptor agonists that have minimal capacity to cross the blood–brain barrier, in a manner similar to the anti-diarrhoeal opiate loperamide and second, to develop inhibitors of endocannabinoid transport and/or enzymatic degradation which, by increasing local levels of endocannabinoids (where ongoing production is occurring), can have greater pharmacological selectivity than direct-acting cannabinoid receptor drugs. Finally, it is worth noting that cannabinoids possess analgesic, anti-inflammatory and antiulcerative activities, which could be of particular therapeutic importance given the ulcerogenic effects of many of the anti-inflammatory drugs such as aspirin used in modern medicine.
Update A recent study performed in the isolated guinea-pig ileum demonstrated that anandamide inhibited electrically stimulated acetylcholine release via activation of non-CB1 and non-CB2 receptors and increased basal acetylcholine release via activation of vanilloid receptors [51]
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest •• of outstanding interest
10. De Petrocellis L, Melck D, Bisogno T, Di Marzo V: Endocannabinoids and fatty acid amides in cancer, inflammation and related disorders. Chem Phys Lipids 2000, 108:191-209. 11. Griffin G, Fernando SR, Ross RA, McKay NG, Ashford MLJ, Shire D, Huffman JW, Yu S, Lainton JAH, Pertwee RG: Evidence for the presence of CB2-like cannabinoid receptors on peripheral nerve terminals. Eur J Pharmacol 1997, 339:53-61. 12. Shire D, Carillon C, Kaghad M, Calandra B, Rinaldi-Carmona M, Le Fur G, Caput D, Ferrara P: An amino-terminal variant of the central cannabinoid receptor resulting from alternative splicing. J Biol Chem 1995, 270:3726-3731. 13. Buckley NE, Hansson S, Harta G, Mezey E: Expression of the CB1 and CB2 receptor messenger RNAs during embryonic development in the rat. Neuroscience 1998, 82:1131-1149. 14. Kulkarni-Narla A, Brown DR: Localization of CB1-cannabinoid • receptor immunoreactivity in the porcine enteric nervous system. Cell Tissue Res 2000, 302:73-80. This paper reports immunohistochemical localisation of CB1 cannabinoid receptors in the porcine enteric nervous system. The potential roles of these receptors in intestinal motility and epithelial transport, host defense and visceral pain transmission are described. 15. Izzo AA, Mascolo N, Capasso F: Marijuana in the new millennium: perspectives for cannabinoid research. Trends Pharmacol Sci 2000, 21:281-282. 16. Germanò MP, D’Angelo V, Mondello MR, Pergolizzi S, Capasso F, Capasso R, Izzo AA, Mascolo N, De Pasquale R: Cannabinoid CB1-mediated inhibition of stress-induced gastric ulcers in rats. Naunyn Schmiedebergs Arch Pharmacol 2001, 363:241-244. 17.
Coruzzi G, Adami M, Coppelli G, Frati P, Soldani G: Inhibitory effect of the cannabinoid receptor agonist WIN 55,212-2 on pentagastrin-induced gastric acid secretion in the anaesthetized rat. Naunyn Schmiedebergs Arch Pharmacol 1999, 360:715-718.
18. Adami M, Bertini S, Frati P, Soldani G, Coruzzi G: Cannabinoid CB1 receptors are involved in the regulation of rat gastric acid secretion. Pharm Pharmacol Commun 2000, 6:273-275. 19. Izzo AA, Mascolo N, Capasso R, Germanò MP, De Pasquale R, Capasso F: Inhibitory effect of cannabinoid agonists on gastric emptying in the rat. Naunyn Schmiedebergs Arch Pharmacol 1999, 360:221-223. 20. Krowicki ZK, Moerscbaecher JM, Winsauer PJ, Divagalli SV, Hornby PJ: ∆9-Tetrahydrocannabinol inhibits gastric motility in the rat through cannabinoid CB1 receptors. Eur J Pharmacol 1999, 37:187-196.
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