Tachykininergic and serotonergic control of the migrating motor complex : studies in rat and man

Small bowel motility has been studied in rats and in humans by means of electromyography and manometry. The aim was to study the effects of mammalian tachykinins and 5 hydroxytryptamine(5-HT, serotonin) on small bowel motility, with special regard to the migrating motility complex (MMC), contractile force, and transit of small bowel contents. In rats, the natural tachykinins substance P, neurokinin B, neuropeptide K, neuropeptide y and neurokinin A, interrupted the migrating myoelectric complex and induced phaseII-like irregular myoelectric activity. The effect is mediated by all three subsets of NK receptors but the insensitivity to cholinergic blockade indicates a major role for neurokinin-2 receptors directly on smooth muscle cells. In rats, neurokinin A and neurokinin B also accelerated transit of small bowel contents. In humans, neurokinin A interrupted the migrating motor complex, induced phase II-like activity, and increased the contraction frequency and amplitude of phase II-like contractions. 5-hydroxytryptamine dose-dependently stimulated the initiation of phase m of the migrating motor complex in rats as well as in humans. In rats, this effect was mediated mainly by 5-HT3- receptors which are dependent on cholinergic pathways. Thus, in the regulation of the migrating motor complex, the tachykinins stimulate phase II contractions, while 5-HT acts as an initiator of phase III. It can be speculated that the major importance of tachykinins in the regulation of small bowel motility is to enhance contractile activity, while 5-hydroxytryptamine exerts its major influence on the rhythm of the migrating motor complex.