The following new observations were made in our immunohistochemical and immunocytochemical investigations of the neural elements of the gastrointestinal tract:

• The distribution and density of the nitrergic nerve elements are different in the gastrointestinal tract but greater numbers of NO containing nerve elements are found in the myenteric plexuses and in the inner, circular smooth muscle layer of the sphincter-regions. This distribution pattern suggests that nitrergic nerves cause smooth muscle relaxation in the sphincter regions.

• In the pylorus we have demonstrated NOS immunoreactive nerve terminals in both intramural nerve plexuses forming intreneuronal synapses. These results suggest the postsynaptic effect of NO on these plexus neurones and thus participation in the local neural reflexes.

• The similar distribution and localization of VIP and NOS immunoreactive nerve elements suggests the possible colocalization of these transmitters in the inhibitory neurons of the pylorus.

• We have observed nitrergic nerve terminals in close apposition to the smooth muscle cells of the blood vessels, suggesting the regulating effect of NO on the blood flow and thus on the gastroprotection. NO containing nerve terminals were also found around the pyloric glands and beneath the surface epithelium suggesting the nitrergic modulation of gastric secretion.

• In our experimental colitis model we have demonstrated the marked decrease of the number of nitrergic nerve terminals and their synapses during moderate inflammation. We found that some of the interstitial cells of Cajal in the inner, circular muscle layer of the inflamed colon became NOS immunoreactive. This change can modulate the motility during inflammation and can be a sign of innervation plasticity of the enteral system.