Control of gastrointestinal motility by peptides: old peptides, new tricks--new peptides, old tricks.

The discovery of new peptides that may or may not be members of existing peptide families is stimulating research in the field of gastrointestinal motility. Before their function in control of human motility can be predicted, both anatomic and functional pathways must be determined in a number of animal models. In many instances this has just begun. In other instances old concepts must be revised. This review examines the recent findings that motor actions attributable to VIP and by extension to its colocalized family member PHI may occur by turning off a tonic release that has held the muscle in a relaxed state. For the opioid family, some of the very complex actions are probably attributable to its action to inhibit the tonic release of VIP. For the tachykinin/neurokinin family, the focus is on the potential role as a sensory transmitter released antidromically from afferent capsaicin-sensitive nerve endings. In summarizing the actions of galanin, the reader is cautioned against any extrapolation to other species, because the actions and structure of the peptides have been found to be different in each species examined. CGRP, again a sensory transmitter found colocalized with substance P, tends to exert an opposite action on the smooth muscle from substance P (that of relaxation), and the interactions between these peptides may well prove to be important in gastrointestinal reflexes. The PP, PYY, and NPY family require much more study in gastrointestinal motor systems but appear to act as presynaptic inhibitory transmitters in a variety of local motor reflexes. One caveat from one who studies these systems is never to predict the action of a new or old peptide in your system of study, because the complexity of the system appears to determine the action expressed.