BACKGROUND Inactivity of the gut leads to atrophic changes of which little is known.

AIMS To investigate structural, neuronal, and functional changes occurring in bypassed rat ileum.

RESULTS Adaptive changes in bypassed ileum include atrophy and remodelling of the gut wall. The total numbers of submucous and myenteric neurones per unit length increased one and four weeks after bypass but were identical to sham operated intestine 10 weeks after bypass. Neurones expressing vasoactive intestinal peptide, neuropeptide Y, or pituitary adenylate cyclase activating peptide decreased gradually in number in bypassed ileum. Nitric oxide synthase expressing neurones were increased, particularly in the myenteric ganglia. No change in the frequency and distribution of interstitial cells of Cajal was noted. The contractile response elicited by electrical stimulation of sham operated ileum consisted of a fast cholinergic twitch followed by a slower non-adrenergic, non-cholinergic contraction. In the bypassed ileum an identical biphasic contraction was elicited; however, the entire response was non-adrenergic, non-cholinergic. The relaxatory response to electrical stimulation in sham operated ileum was nitric oxide mediated; after bypass it was non-nitrergic.

CONCLUSIONS Notable atrophic changes were seen in the rat ileum after bypass. The enteric nervous system reacted with neuronal cell death and plasticity in terms of release and expression of neurotransmitters.