Effects of a nonpeptide motilin receptor antagonist on proximal gastric motor function

Physiological functions and potential clinical applications of motilin

Motilin is a gastrointestinal hormone secreted by the duodenum. This peptide regulates a characteristic gastrointestinal contraction pattern, called the migrating motor complex, during the fasting state. Motilin also affects the pressure of the lower esophageal sphincter, gastric motility and gastric accommodation in the gastrointestinal tract. Furthermore, motilin induces bile discharge into the duodenum by promoting gallbladder contraction, pepsin secretion in the stomach, pancreatic juice and insulin secretion from the pancreas. In recent years, it has been shown that motilin is associated with appetite, and clinical applications are expected for diseases affected by food intake, e.g. obesity, by regulating motilin levels. Gastric acid and bile are the two major physiological regulators for motilin release. Caloric foods have varying effects on motilin levels, depending on their composition. Among non-caloric foods, bitter substances reduce motilin levels and are therefore expected to have an appetite-suppressing effect. Various motilin receptor agonists and antagonists have been developed but have yet to reach clinical use.

Motilin: from gastric motility stimulation to hunger signalling

After the discovery of motilin in 1972, motilin and the motilin receptor were studied intensely for their role in the control of gastrointestinal motility and as targets for treating hypomotility disorders. The genetic revolution - with the use of knockout models - sparked novel insights into the role of multiple peptides but contributed to a decline in interest in motilin, as this peptide and its receptor exist only as pseudogenes in rodents. The past 5 years have seen a major surge in interest in motilin, as a series of studies have shown its relevance in the control of hunger and regulation of food intake in humans in both health and disease. Luminal stimuli, such as bitter tastants, have been identified as modulators of motilin release, with effects on hunger and food intake. The current state of knowledge and potential implications for therapy are summarized in this Review.

Motilin: towards a new understanding of the gastrointestinal neuropharmacology and therapeutic use of motilin receptor agonists

The gastrointestinal hormone motilin has been known about for >40 years, but after identification of its receptor and subsequent development of new tools and methods, a reappraisal of its actions is required. Firstly, it is important to note that motilin and ghrelin receptors are members of the same family (similar genomic organization, gastrointestinal distribution and abilities to stimulate gastrointestinal motility), yet each fails to recognize the ligand of the other; and whereas ghrelin and ghrelin receptors are widespread outside the gastrointestinal tract, motilin and its receptors are largely restricted to the gastrointestinal tract. Secondly, although some studies suggest motilin has activity in rodents, most do not, and receptor pseudogenes exist in rodents. Thirdly, motilin preferentially operates by facilitating enteric cholinergic activity rather than directly contracting the muscle, despite the relatively high expression of receptor immunoreactivity in muscle. This activity is ligand-dependent, with short-lasting actions of 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. Replacement motilin receptor agonists derived from erythromycin (motilides) have been unsuccessful. New, non-motilide, small molecule receptor agonists, designed to minimize self-desensitization, are now entering 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 part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.170.issue-7.