Regional quantitation of preprodynorphin mRNA in guinea pig gastrointestinal tract

Biological redundancy of endogenous GPCR ligands in the gut and the potential for endogenous functional selectivity

This review focuses on the existence and function of multiple endogenous agonists of the somatostatin and opioid receptors with an emphasis on their expression in the gastrointestinal tract. These agonists generally arise from the proteolytic cleavage of prepropeptides during peptide maturation or from degradation of peptides by extracellular or intracellular endopeptidases. In other examples, endogenous peptide agonists for the same G protein-coupled receptors can be products of distinct genes but contain high sequence homology. This apparent biological redundancy has recently been challenged by the realization that different ligands may engender distinct receptor conformations linked to different intracellular signaling profiles and, as such the existence of distinct ligands may underlie mechanisms to finely tune physiological responses. We propose that further characterization of signaling pathways activated by these endogenous ligands will provide invaluable insight into the mechanisms governing biased agonism. Moreover, these ligands may prove useful in the design of novel therapeutic tools to target distinct signaling pathways, thereby favoring desirable effects and limiting detrimental on-target effects. Finally we will discuss the limitations of this area of research and we will highlight the difficulties that need to be addressed when examining endogenous bias in tissues and in animals.

Orally administered soymorphins, soy-derived opioid peptides, suppress feeding and intestinal transit via gut mu(1)-receptor coupled to 5-HT(1A), D(2), and GABA(B) systems

We previously reported that soymorphins, mu-opioid agonist peptides derived from soy beta-conglycinin beta-subunit, have anxiolytic-like activity. The aim of this study was to investigate the effects of soymorphins on food intake and gut motility, along with their mechanism. We found that soymorphins decreases food intake after oral administration in fasted mice. Orally administered soymorphins suppressed small intestinal transit at lower dose than that of anorexigenic activity. Suppression of food intake and small intestinal transit after oral administration of soymorphins was inhibited by naloxone or naloxonazine, antagonists of mu- or mu(1)-opioid receptor, respectively, after oral but not intraperitoneal administration. The inhibitory activities of small intestinal transit by soymorphins were also inhibited by WAY100135, raclopride, or saclofen, antagonists for serotonin 5-HT(1A), dopamine D(2), or GABA(B) receptor, respectively. We then examined the order of activation of 5-HT(1A), D(2), and GABA(B) receptors, using their agonists and antagonists. The inhibitory effect of 8-hydroxy-2-dipropylaminotetralin hydrobromide, a 5-HT(1A) agonist, after oral administration on small intestinal transit was blocked by raclopride or saclofen. Bromocriptine, a D(2) agonist-induced small intestinal transit suppression, was inhibited by saclofen, but not by WAY100135. Baclofen, a GABA(B) agonist-induced small intestinal transit suppression, was not blocked by WAY100135 or raclopride. These results suggest that 5-HT(1A) activation elicits D(2) followed by GABA(B) activations in small intestinal motility. We conclude that orally administered soymorphins suppress food intake and small intestinal transit via mu(1)-opioid receptor coupled to 5-HT(1A), D(2), and GABA(B) systems.

Involvement of mu-receptor in endogenous opioid peptide-mediated inhibition of acetylcholine release from the rat stomach

We examined the effect of endogenous opioid peptides on vagally evoked release of acetylcholine (ACh) from the isolated, vascularly perfused rat stomach. The vagus nerves were electrically stimulated twice at 2.5 Hz for 2 min, and test substances were administered during the second stimulation. beta-Endorphin (10(-7) and 3 x 10(-7) M), an endogenous nonselective agonist of mu-receptors, inhibited the release of ACh. However, [Leu5]-enkephalin, an endogenous nonselective agonist of delta-receptors, and U-50488, a kappa-receptor agonist, had no effect at a higher dose of 10(-6) M. Beta-endorphin-induced inhibition was abolished by naloxone. Endomorphins 1 and 2 (3 x 10(-7) and 10(-6) M), endogenous selective agonists of mu-receptors, also inhibited the release of ACh. These results suggest that the mu-receptor is involved in the endogenous opioid peptide-induced inhibition of the release of ACh from the rat stomach.