Previous studies have shown that anandamide as well as other natural and synthetic cannabinoid agonists, by activating enteric CB1 receptors, can reduce intestinal motility either in vitro (Pertwee et al., 1995; Izzo et al., 1998) or in vivo (Fride, 1995; Calignano et al., 1997; Izzo et al., 2001a). In the present study we have shown that the selective cannabinoid CB1 receptor antagonist SR141716A, at doses able to counteract the inhibitory effect of anandamide (Izzo et al., 2001a), was not able to counteract the inhibitory effect of PEA on intestinal motility. There is also some evidence in literature that some effect of PEA can be mediated by as-yet uncharacterized ‘CB2-like’ receptors, because some pharmacological effects of PEA can be counteracted by the selective CB2 receptor antagonist SR144528 (Facci et al., 1995; Calignano et al., 1998). In the present study, however, the effect of PEA on intestinal motility was not modified by SR144528. The dose of SR144528 used in the present study was 10 fold higher than the dose of SR144528 able to counteract the analgesic effect of PEA (Calignano et al., 1998). Collectively, these results indicate that the effect of PEA on intestinal motility is not mediated by activation of cannabinoid receptors. Presynaptic/prejunctional systems, such as α2-adrenoceptors or opioid receptors, which, if activated, are known to inhibit intestinal motility, are not involved in the inhibitory effect of PEA. In fact, naloxone or yohimbine, antagonists of opioid or α2-adrenoceptors, respectively, failed to modify PEA-induced changes in motility. In addition, the effect of PEA was not modified by the ganglion blocker hexamethonium, thus suggesting a peripheral site of action. Moreover, it is unlikely that the inhibitory effect of PEA could derive from modulation of NO production, as pre-treatment of mice with the NO synthase inhibitor L-NAME did not modify PEA-induced changes in motility. Others have shown that PEA inhibits NO production in murine macrophages and that this effect does not appear to be mediated by cannabinoid receptors (Ross et al., 2000).
PMSF is a non-specific irreversible amidase inhibitor that inhibits the action of fatty acid amide hydrolase. Previous investigators have shown that PMSF enhanced the pharmacological activity of anandamide (Wiley et al., 2000; Lambert & Di Marzo, 1999), including its ability to reduce intestinal motility (Pertwee et al., 1995). In the present study, PMSF, at doses previously shown to be effective (Wiley et al., 2000), did not modify the inhibitory effect of PEA on intestinal motility. The lack of effect of PMSF is not surprising in the light of the observation the PEA is not hydrolyzed by fatty acid amide hydrolase as efficiently as anandamide (Lambert & Di Marzo, 1999), and that another amidase insensitive to PMSF has been identified for PEA (Ueda et al., 1999).