Cannabinoid inhibition of guinea-pig intestinal peristalsis via inhibition of excitatory and activation of inhibitory neural pathways

Abstract

Since activation of cannabinoid CB₁ receptors inhibits gastrointestinal transit in the mouse, this study analyzed the action of the cannabinoid receptor agonist methanandamide on distension-induced propulsive motility. Peristalsis in luminally perfused segments of the guinea-pig isolated ileum was elicited by a rise of the intraluminal pressure. The pressure threshold at which peristaltic contractions were triggered was used to quantify drug effects. Methanandamide (0.1–3 μM) inhibited peristalsis as deduced from a concentration-related increase in the peristaltic pressure threshold, an action that was prevented by the CB₁ receptor antagonist SR141716A (1 μM) per se, which had no effect on peristalsis. The distension-induced ascending reflex contraction of the circular muscle was likewise depressed by methanandamide in a SR141716A-sensitive manner, whereas indomethacin-induced phasic contractions of the circular muscle were left unchanged by methanandamide. The anti-peristaltic action of methanandamide was inhibited by apamin (0.5 μM), attenuated by N-nitro-l-arginine methyl ester (300 μM) and left unaltered by suramin (300 μM), pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid (150 μM) and naloxone (0.5 μM). It is concluded that methanandamide depresses intestinal peristalsis via activation of CB₁ receptors on enteric neurons, which results in blockade of excitatory motor pathways and facilitation of inhibitory pathways operating via apamin-sensitive K⁺ channels and nitric oxide.

Introduction

Cannabinoids are known to inhibit intestinal motor activity (Chesher et al., 1973), which is of physiological and pharmacological interest because two cannabinoid receptors, denoted as CB₁ and CB₂ and coupled to G proteins, and endogenous cannabinoid receptor ligands such as anandamide have been identified (Devane et al., 1992, Mechoulam et al., 1995, Pertwee, 1997). The action of cannabinoids on intestinal motility is brought about by CB₁ receptors, since the inhibitory effect of cannabinoid receptor agonists on gastrointestinal transit in the mouse is prevented by the CB₁ receptor antagonist SR141716A (Calignano et al., 1997, Colombo et al., 1998). This finding is in keeping with the presence of CB₁-like, but not CB₂-like, cannabinoid receptor messenger RNA in the myenteric plexus of the guinea-pig intestine (Griffin et al., 1997). The observation that SR141716A is able to facilitate gastrointestinal transit (Calignano et al., 1997, Colombo et al., 1998) has been taken to implicate endogenous cannabinoids in the control of gastrointestinal motility, but this conjecture is inconclusive in view of the inverse agonist properties of SR141716A (MacLennan et al., 1998).

The mechanisms by which cannabinoids inhibit intestinal motility have not been fully elucidated. One relevant mode of action is the prejunctional inhibition of acetylcholine release from myenteric neurons in the guinea-pig small intestine, which is mediated by CB₁ receptors (Pertwee et al., 1996, Coutts and Pertwee, 1997). This action is consistent with the ability of cannabinoids to block excitatory synaptic transmission within the myenteric plexus (López-Redondo et al., 1997) and likely to be an explanation for why cannabinoids depress neurogenic contractions of the gut (Pertwee et al., 1996, Griffin et al., 1997, Izzo et al., 1998), in which acetylcholine is the major neuro–neuronal and neuro–muscular transmitter (Tonini and Costa, 1990).

In an attempt to analyze the action of cannabinoids on propulsive motility in the gut we set out to examine the effect of a cannabinoid receptor agonist on distension-induced peristalsis in the guinea-pig small intestine. This study was carried out with methanandamide, a derivative of anandamide that is metabolically more stable than the parent compound (Abadji et al., 1994). Having seen that methanandamide inhibits peristalsis via CB₁ receptor activation we went on to investigate the site and mechanism of this cannabinoid action. Two possibilities were envisaged, one being that methanandamide interrupts excitatory motor pathways of peristalsis, the other being that methanandamide stimulates inhibitory motor pathways. The former possibility was tested by studying the effect of methanandamide on the distension-induced ascending reflex contraction of the circular muscle, which represents a neurogenic response important for initiation of peristalsis (Tonini et al., 1996) and involves sensory neurons, interneurons and motor neurons (Tonini and Costa, 1990, Johnson et al., 1998). The latter possibility was addressed by examining whether apamin, an inhibitor of fast inhibitory junction potentials mediated by transmitters of inhibitory motor neurons in the guinea-pig small intestine (Niel et al., 1983, Crist et al., 1992, Zagorodnyuk and Maggi, 1998), or blockade of the formation of nitric oxide (NO), another transmitter of inhibitory motor neurons (Lyster et al., 1992, Waterman and Costa, 1994), would prevent the anti-peristaltic action of methanandamide.