Differential effects of anandamide on acetylcholine release in the guinea‐pig ileum mediated via vanilloid and non‐CB 1 cannabinoid receptors

The effects of anandamide on [ 3 H]‐acetylcholine release and muscle contraction were studied on the myenteric plexus‐longitudinal muscle preparation of the guinea‐pig ileum preincubated with [ 3 H]‐choline. Anandamide increased both basal [ 3 H]‐acetylcholine release (pEC 50 6.3) and muscle tone (pEC 50 6.3). The concentration‐response curves for anandamide were shifted to the right by 1 μ M capsazepine (p K B 7.5 and 7.6), and by the combined blockade of NK 1 and NK 3 tachykinin receptors with the antagonists CP99994 plus SR142801 (each 0.1 μ M ). The CB 1 and CB 2 receptor antagonists, SR141716A (1 μ M ) and SR144528 (30 n M ), did not modify the facilitatory effects of anandamide. Anandamide inhibited the electrically‐evoked release of [ 3 H]‐acetylcholine (pEC 50 5.8) and contractions (pEC 50 5.2). The contractile response to the muscarinic agonist methacholine was not significantly affected by 10 μ M anandamide. The inhibitory effects of anandamide were not changed by either capsazepine (1 μ M ), SR144528 (30 n M ) or CP99994 plus SR142801 (each 0.1 μ M ). SR141716A (1 μ M ) produced rightward shifts in the inhibitory concentration‐response curves for anandamide yielding p K B values of 6.6 and 6.2. CP55940 inhibited the evoked [ 3 H]‐acetylcholine release and contractions, and SR141716A (0.1 μ M ) shifted the concentration‐response curves of CP55940 to the right with p K B values of 8.4 and 8.9. The experiments confirm the existence of release‐inhibitory CB 1 receptors on cholinergic myenteric neurones. We conclude that anandamide inhibits the evoked acetylcholine release via stimulation of a receptor that is different from the CB 1 and CB 2 receptor. Furthermore, anandamide increases basal acetylcholine release via stimulation of vanilloid receptors located at primary afferent fibres.British Journal of Pharmacology (2001) 134 , 161–167; doi: 10.1038/sj.bjp.0704220