The role of cannabinoid receptors in intestinal motility, defaecation and diarrhoea in rats
Introduction
Cannabinoid receptors are the molecular targets of hashish and marijuana, the drugs of abuse obtained from Cannabis sativa L. (Cannabinaceae). Two classes of cannabinoid receptors, named CB1 and CB2, have been identified (Pertwee, 1997). The cannabinoid CB1 receptor occurs in the brain (Matsuda et al., 1990), where it is responsible for psychoactive effects of cannabis and in certain peripheral tissues Gérard et al., 1991, Kaminski et al., 1992, Schlicker et al., 1996, whereas the cannabinoid CB2 receptor is present outside the nervous system (Munro et al., 1993), mostly in cells of the immune system, presumably mediating cannabinoid-induced immunosuppression and possibly also antinflammatory effects (Pertwee, 1997). The guinea-pig myenteric plexus-longitudinal muscle preparation contains cannabinoid binding sites that closely resemble cannabinoid CB1 receptors in the guinea-pig brain (Ross et al., 1998).
Results from functional experiments have led to the conclusion that activation of cannabinoid CB1 receptors can mediate the inhibition of electrically evoked contractions in the guinea-pig Pertwee et al., 1996, Izzo et al., 1998b and human ileum (Croci et al., 1998), while the selective cannabinoid CB1 receptor antagonist, (N-piperidin-1-yl)-5-(4-chlorophenyl)-1-2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A), potentiates excitatory transmission in the guinea-pig ileum Pertwee et al., 1996, Izzo et al., 1998b but not in the human ileum (Croci et al., 1998).
Cannabinoids also modulate intestinal motility in vivo. Activation of cannabinoid CB1 receptors inhibits while blockade of cannabinoid CB1 receptors increases defaecation (Izzo et al., 1999) and upper gastrointestinal transit in mice Calignano et al., 1997, Colombo et al., 1998, Izzo et al., 1999. From the above, it appears that the role of cannabinoid receptors in intestinal motility in vivo has been studied exclusively in the mouse. The present study was performed to investigate the role of cannabinoid receptors in intestinal motility and defaecation in the rat. In addition, since marijuana is a traditional remedy to treat diarrhoea (Grinspoon and Bakalar, 1993), the potential antidiarrhoeal activity of cannabinoids was also evaluated. For these purposes, the cannabinoid receptor agonists, (R)-(+)[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN 55,212-2) and (−)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol) (CP 55,940) Compton et al., 1992, Pertwee, 1997, the cannabinoid CB1 receptor antagonist, SR141716A (Rinaldi-Carmona et al., 1995), and the cannabinoid CB2 receptor antagonist, N-[-(1S)-endo-1,3,3-trimethyl bicyclo [2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528) (Rinaldi-Carmona et al., 1998), were used. Antidiarrhoeal activity was studied using the castor-oil test. This test has been used extensively in several laboratories as a basic pharmacological test to screen and evaluate antidiarrhoeal drugs.
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Animals
Male Sprague–Dawley rats (Fondazione “Mario Negri” Sud, Imbaro, Chieti), weighing 170–200 g were used after 1 week of adaptation to the housing conditions (23±2°C; 60% humidity). The animals were deprived of food 3 h before the experiments on faecal excretion and 12 h before the experiments on intestinal motility and diarrhoea. All animal experiments complied with the Italian D.L. no. 116 of 27 January 1992 and associated guidelines in the European Communities Council Directive of 24 November
Upper gastrointestinal transit
Administration of the cannabinoid CB1 receptor antagonist, SR141716A (0.3–5 mg/kg), dose-dependently enhanced upper gastrointestinal transit (Fig. 1). This effect was significant (P<0.05) for the 1–5 mg/kg doses. In contrast, WIN 55,212-2 and CP 55,940 significantly inhibited intestinal motility from 1 mg/kg and 0.1 mg/kg onward respectively and this effect was dose-related (Fig. 1). WIN 55,212-3, the less active isomer of WIN 55,212-2, did not significantly affect intestinal motility (%
Discussion
Recent findings indicate that cannabinoid CB1 receptors can modulate intestinal motility, based on the findings that cannabinoid receptor agonists are able to depress intestinal motility both in vitro and in vivo and that these effects are counteracted by SR141716A, a specific cannabinoid CB1 receptor antagonist Pertwee et al., 1996, Colombo et al., 1998, Izzo et al., 1998b, Izzo et al., 1999. In the mouse, activation of cannabinoid CB1 receptors decreases, while blockade of cannabinoid CB1
Acknowledgements
This work was supported by CNR, MURST and the Enrico and Enrica Sovena Foundation. The authors thank Mr. Domenico Addeo for his help.
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