Such activity contrasts with that in humans where the effects of motilin agonists about lower bowel functions have been found to vary (Jamesonet al.,1992; Sharmaet al.,1995; Bassottiet al.,1998; Emmanuelet al.,2004; Venkatasubramaniet al.,2008). Motilin and its receptor are found mostly within the GI tract (see Introduction). motilin contrasting with longer-lasting actions of the non-selective and selective motilin receptor agonists erythromycin and GSK962040. Finally, the use of erythromycin (also an antibiotic drug) to treat patients requiring acceleration of gastric emptying has led to concerns over safety and potential exacerbation of antibiotic resistance. Alternative motilin receptor agonists derived from erythromycin (motilides) have been unsuccessful. New, non-motilide, small molecule receptor agonists, designed to PF-02575799 minimize self-desensitization, are now entering PF-02575799 clinical trials for treating patients undergoing enteral feeding or with diabetic gastroparesis. Thus, for the translational pharmacologist, the study of motilin illustrates the need to avoid overreliance on artificial systems, on structural information and on animal studies. == LINKED ARTICLES == This article is usually a part of a themed section on Neuropeptides. To view the other articles in this section visithttp://dx.doi.org/10.1111/bph.2013.170.issue-7 Keywords:translational sciences, gastrointestinal, neuropharmacology, motilin, ghrelin == Introduction == The gastrointestinal (GI) hormone motilin was identified over 40 years ago (Brownet al.,1973) following suggestions that a substance Jun was released from the duodenum to increase gastric emptying (Shay and Gershon-Cohen,1935) and gastric motor activity in denervated gastric pouches PF-02575799 (Brownet al.,1966). Motilin is usually a 22-amino-acid peptide, synthesized and secreted by specific endocrine cells in the epithelia of human upper small intestine, most notably the jejunum and duodenum, with smaller amounts elsewhere, such as the gastric antrum (Christofides,1978). In humans, motilin is usually released during fasting and after eating. The hormone is also released in response to air-filled balloons (Boivinet al.,1992a) or by drinking water (Christofides,1978), suggesting that this stimulus for its release after eating is usually mechanical, although its release may also be influenced by particular nutrients such as fat (Christofides,1978). The amount of motilin released is not thought to be high enough to affect gastric motility in healthy individuals. However, in patients with delayed gastric emptying, it is still possible that endogenous motilin may have an effect because of the greater potential to observe stimulation (Boivinet al.,1992b; see later). The release of motilin during fasting occurs in association with phase III of the migrating motor complex (MMC). In humans, MMC activity begins in the upper gut. It is characterized by four distinct phases. The first and longest is usually a period of near quiescence, followed by a period of small-amplitude contractions of irregular frequency known as phase II, and then a burst of high-amplitude propulsive contractions (phase III), which move down the intestine and terminate in the distal small intestine; phase IV is sometimes used to describe the decline of activity back to baseline (Husebye,1999). Phase III activity is usually thought to help clear the stomach and intestine from any undigested material, prevent bacterial overgrowth in the upper gut and perhaps help to develop the sensation of hunger (Sanger and Lee,2008). Studies in dogs (Nakajimaet al.,2010) suggest that phase II of the MMC is usually caused by a gradual build-up of 5-HT, which acts at 5-HT4receptors within the enteric nervous system (ENS) to increase contractile activity. This PF-02575799 leads to further release of 5-HT from enterochromaffin cells, by a similar process to the release of motilin. The former acts at 5-HT3receptors to help initiate phase III activity (5-HT3receptor antagonists reduce phase III periodicity; Wilmeret al.,1993), whereas the latter helps sustain the contractile activity in the stomach (rabbit anti-motilin serum blocks phase III activity in doggie stomach; Leeet al.,1983) but not the small intestine. The reason why two different mediators are involved is usually unclear. Nevertheless, it is worth noting that there is a correlation between gastric MMCs and feelings of hunger (Anget al.,2008), suggesting that this released motilin could have an additional role to enhance appetite, perhaps by releasing ghrelin (Zeitlowet al.,2010) and/or by directly activating the vagus nerve (Mochikiet al.,1997; Suzukiet al.,1998) to signal information to the brain..