Abstract We have evaluated the effect of the vanilloid receptor agonists resiniferatoxin (RTX), capsaicin and piperine and of the vanilloid receptor antagonist cap- sazepine on the resting tone in the isolated rat ileum. Cap- sazepine (10 –8 –3×10 –5 M) produced a concentration-re- lated relaxation (8±3%–49±3%) of the rat ileum. By con- trast RTX (up to 10 –8 M), capsaicin (up to 10 –6 M) and piperine (up to 10 –5 M) were without effect. Pre-treatment with capsaicin [either in vivo (50 mg/kg s.c.) or in vitro (10 –6 M)] did not modify the inhibitory effect of cap- sazepine. The L-type Ca 2+ channel antagonist nifedipine (10 –6 M), but not the N-type Ca 2+ channel antagonist ω-conotoxin GVIA (3×10 –8 M) nor the Na + channel blocker tetrodotoxin (3×10 –7 M), counteracted the inhibitory ef- fect of capsazepine. The NK 1 receptor antagonist SR 140333 (10 –7 M), the NK 2 receptor antagonist SR 48968 (10 –6 M), the NK 3 receptor antagonist SR 142801 (10 –7 M), atropine (10 –6 M), hexamethonium (10 –4 M), phentol- amine (10 –6 M) plus propranolol (10 –6 M), N G -nitro-L- arginine methyl ester (L-NAME 3×10 –4 M), apamin (10 –7 M), methysergide (10 –6 M), the calcitonin gene-re- lated peptide (CGRP) antagonist hCGRP 8–37 (1.5× 10 –6 M), the VIP antagonist hGRF 1–29 (10 –5 M) did not modify the inhibitory effect of capsazepine. Capsazepine (2.5–40 mg/kg) also decreased upper gastrointestinal tran- sit in vivo. It is concluded that the vanilloid antagonist capsazepine has a direct relaxing effect on rat intestinal smooth muscle which could involve L-type calcium chan- nels. We found no evidence to suggest that capsazepine is antagonizing an endogenous vanilloid. Keywords Vanilloid (capsaicin) receptors · Enteric nervous system · Piperine · Resiniferatoxin · Intestine · Capsazepine · Intestinal motility · Primary afferent neurones Introduction Capsaicin, from hot peppers of the Capsicum family, piperine, present in black pepper (Piper nigrum), and resiniferatoxin (RTX), an extremely irritant diterpene pre- sent in the latex of several members of the genus Euphor- bia, are naturally occurring agonist ligands of the vanil- loid receptor (Sterner and Szallasi 1999). Capsaicin and piperine are structurally similar. RTX combines structural features of two classes of irritant compounds, capsaici- noids and phorbol ester, but it failed to induce typical phorbol ester effects (e.g. promoting activity or activation of EB virus, release of bronectin, competition for phorbol ester binding to protein kinase C; Szallasi et al. 1989). Several other plant-derived compounds (e.g. zingerone from ginger or scutigeral from the edible mushroom Alba- trellus ovinus), synthetic compounds (e.g. olvanil and ar- vanil) as well as the endocannabinoid anandamide (Zyg- munt et al. 1999; Smart et al. 2000), have been shown to possess agonist activity at the vanilloid receptor (Sterner and Szallasi 1999); in addition a synthetic antagonist, cap- sazepine, has also been developed (Bevan et al. 1992). A functional vanilloid receptor termed VR1 (vanilloid receptor subtype 1), which is activated not only by vanil- loids but also by noxious heat and low pH, has been cloned (Caterina et al. 1997; Helliwell et al. 1998; Kress and Zeilhofer 1999; Szallasi and Di Marzo 2000; Caterina and Julius 2001). VR1 is a nonselective cation channel with a preference for Ca 2+ and it accounts for both the binding and functional activity of vanilloid ligands that show RTX- and capsaicin-type structrure-activity rela- tions (Szallasi et al. 1999). VR1 is expressed predominantly, although not exclu- sively, by primary afferent neurones (Sterner and Szallasi 1999). These neurones can convey signals coming from Emilia Nocerino · Angelo A. Izzo · Francesca Borrelli · Francesco Capasso · Raffaele Capasso · Aldo Pinto · Lidia Sautebin · Nicola Mascolo Relaxant effect of capsazepine in the isolated rat ileum Naunyn-Schmiedeberg’s Arch Pharmacol (2002) 365 : 187–192 DOI 10.1007/s00210-001-0522-x Received: 27 July 2001 / Accepted: 29 November 2001 / Published online: 1 February 2002 ORIGINAL ARTICLE E. Nocerino · A.A. Izzo (✉) · F. Borrelli · F. Capasso · L. Sautebin · N. Mascolo Department of Experimental Pharmacology, University of Naples “Federico II”, via D. Montesano 49, 80131 Naples, Italy e-mail: aaizzo@unina.it R. Capasso · A. Pinto Department of Pharmaceutical Sciences, University of Salerno, via Ponte Don Melillo, 84084 Fisciano (SA), Italy © Springer-Verlag 2002