Insulin-like peptide (INSL5), a gut hormone produced by colonic L-cells acts through a G protein-coupled receptor, RXFP4 which is present in the enteric neurons. Although a recent report indicated INSL5 to be orexigenic, our recent unpublished data suggest that INSL5 is an important regulator of colonic motility. Therefore, further studies are required to fully validate this system as a drug target. In a major advance, we recently chemically synthesized a minimized INSL5 analog which is significantly easier to assemble in large quantity compared with native INSL5. In this project, we have further optimized this peptide to produce the smallest possible agonist with high RXFP4 affinity. We also have developed RXFP4-specific antagonists. These analogs will be very useful as tools to understand physiological roles of INSL5/RXFP4 and as drug leads for the treatment of colon motility disorders.