Integrative Control of Gastrointestinal Motility by Nitric Oxide

In the gastrointestinal (GI) tract, nitric oxide (NO) has been shown over the last 25 years to exert a prominent function as inhibitory neurotransmitter. Apart from the regulation of secretion and resorption, NO from nitrergic neurons has been demonstrated to be crucial for GI smooth muscle relaxation and motility. In fact, several human diseases such as achalasia, gastroparesis, slow transit constipation or Hirschsprung's disease may involve dysfunctional nitrergic signaling. Most of NO's effects as neurotransmitter are mediated by NO-sensitive guanylyl cyclase (NO-GC) and further transduced by cGMP-dependent mechanisms. In contrast to the vascular system where NO from the endothelium induces relaxation by acting on NO-GC solely in smooth muscle cells, GI tissues contain several different NO-GC-expressing cell types that include smooth muscle cells, interstitial cells of Cajal and fibroblast-like cells. Based on this diverse localization of the NO receptor, the exact pathway(s) leading to NO-induced relaxation are still unknown. Global and cell-specific knockout mouse strains have been generated that lack enzymes participating in nitrergic signaling. These animals have been helpful in examining the role of NO in smooth muscle of the GI tract. Here, we discuss the current knowledge on NO-mediated mechanisms in the relaxation of GI smooth muscle in stomach, small and large intestine including sphincters. Special focus is placed on the integration of nitrergic signals by specialized cell types within the gut smooth muscle layers.