Tolerance to cannabinoid response on the myenteric plexus of guinea‐pig ileum and human small intestinal strips

Abstract

1 We studied tolerance to cannabinoid agonist action by comparing the in vitro inhibition of electrically evoked contractions of longitudinal muscle from small intestine of human and guinea-pig (myenteric plexus preparations) after 48-h incubation with the synthetic agonist ( þ ) WIN 55,212-2. We also investigated the intrinsic response to the selective cannabinoid CB 1 receptor antagonist rimonabant in control and tolerant strips. 2 ( þ ) WIN 55,212-2 inhibited guinea-pig (IC 50 4.8 nM) and human small intestine (56 nM) contractions with similar potency before or after 48-h incubation in drug-free conditions; this effect was competitively antagonized by rimonabant (pA 2 , 8.4, 8.2). A 48-h preincubation with ( þ ) WIN 55,212-2, but not with (À) WIN 55,212-3, completely abolished the acute agonist response in both tissue preparations. The opiate K-receptor agonist U69593 inhibited human small intestine contractions with a similar potency in control and strips tolerant to ( þ ) WIN 55,212-2, IC 50 39 and 43 nM. 3 Unlike human tissue, in guinea-pig small intestine, which has a high level of endocannabinoids, rimonabant alone increased the twitches induced by the electrical field stimulation (EC 50 100 nM) with a maximal effect of 123%. 4 In strips tolerant to ( þ ) WIN 55,212-2, rimonabant markedly increased (155%) the electrical twitches in human ileum and in guinea-pig myenteric plexus smooth muscle (133%). 5 This study shows tolerance can be induced to the cannabinoids' action in intestinal strips of human and guinea-pig by long in vitro incubation with the agonist ( þ ) WIN 55,212-2.

Pharmacological characterization of cannabinoid receptor activity in the rat‐isolated ileum myenteric plexus‐longitudinal muscle preparation

“…Moreover, rimonabant alone increased the amplitude of these contractions, and its interaction with WIN 55,212-2 yielded a Schild plot with a unit slope and pKB value of 8.4. Although the EFS parameters we used for eliciting the rebound contractions were almost identical to those used by Guagnini et al (2006), the interaction of WIN 55,212-2 with rimonabant yielded data that were similar to that observed under the twitch contractions conditions. The low pEC50 value reported by Guagnini et al (2006) may be attributed to the use of an isotonic recording technique and submaximal voltage for EFS, both of which yield steeper curves that are shifted towards the left (Barlow et al, 2001a,b;Cowie et al 1978).…”

“…Second, the pA2 value of rimonabant against WIN 55,212-2 was over a logarithmic unit less than its pKB value at the CB1 receptor. Recently, Guagnini et al (2006) showed that the ACh-mediated contractions of the guinea pig MPLM elicited by EFS with 10 s trains of pulses at 20 Hz were inhibited by WIN 55,212-2 with a pEC50 value of 8.32. Moreover, rimonabant alone increased the amplitude of these contractions, and its interaction with WIN 55,212-2 yielded a Schild plot with a unit slope and pKB value of 8.4.…”

“…Levels of both AEA and 2-AG high enough to cause the activation of CB1 receptors have been measured in both rat and guinea pig MPLM (Katayama et al, 1997;Valenti et al, 2005;Guagnini et al, 2006). Alternatively, the augmentation of the twitch contractions may be through an unmasking of a constitutive activity of the CB1 receptor through inverse agonism (Pertwee, 2005).…”

“…The former action has been invariably studied by evaluating either the distance travelled or the delay in the time taken for the transit of an orally or intra-duodenally administered non-absorbable marker from the pylorus to the caecum of the animal. While in vivo bioassays have played an important role in characterizing the pharmacology of cannabinoids on intestinal motility under both physiological and pathophysiological states, the mechanism of the depressant action has been inferred from in vitro studies performed on segments or strips of the isolated ileum of the guinea pig Coutts and Pertwee, 1997;Izzo et al, 1998;Mang et al, 2001), and, to a lesser extent, of a human Manara et al, 1998;Guagnini et al, 2006). Therefore, to date, the guinea pig ileum has been the standard in vitro bioassay for screening cannabinoids on the small intestine.…”

Effect of CB1 Ligands on Neurogenic and Myogenic Contractile Responses in the Guinea-Pig Ileum

“…The synthetic cannabinoid receptor agonist (+)-WIN 55212-2 and the cannabinoid CB1 receptor antagonist SR-141716A (rimonabant) were used to investigate CB1 functions [15]. In this context, an interaction of cannabinoids with other myorelaxant constituents of food and food supplements in the gastrointestinal tract is of general interest.…”