Extrinsic denervation increases myenteric nitric oxide synthase-containing neurons and inhibitory neuromuscular transmission in guinea pig

Enteric nerves can function normally without connections with the central nervous system. A contributing component of the functional autonomy exhibited by enteric nerves is their plasticity. In the present study, the number of nitric oxide synthase-immunoreactive (NOS-ir) myenteric neurons and inhibitory neuromuscular transmission were studied in extrinsically denervated ileal segments. Segments of ileum were extrinsically denervated by crushing the mesenteric blood vessels supplying a loop of ileum in anesthetized guinea pigs. Some unoperated animals were treated with capsaicin or 6-hydroxydopamine (6-OHDA) to disrupt primary afferent and sympathetic nerves, respectively. NOS-ir was localized using indirect immunofluorescence. Nerve-mediated relaxations of longitudinal muscle were studied in vitro using standard methods. At 7 weeks after extrinsic denervation there was a 93% increase in the number of NOS-ir myenteric neurons. The number of neurons containing detectable vasoactive intestinal peptide-ir neurons was not changed after extrinsic denervation. Neurogenic relaxations caused by 10, 20 and 50 Hz transmural stimulation were larger in extrinsically-denervated tissues compared to control tissues. The NOS antagonist, nitro-L-arginine (300 microM) inhibited neurogenic relaxations in control and extrinsically-denervated tissues. Capsaicin- but not 6-OHDA-treatment mimicked the effects of extrinsic denervation on NOS-ir and neurogenic relaxations of the longitudinal muscle. Active or passive properties of the longitudinal muscle were unaffected by extrinsic denervation. These data indicate that extrinsic denervation is associated with an increase in the number of myenteric neurons expressing detectable NOS-ir and potentiation of inhibitory transmission to longitudinal muscle. This effect is due to loss of extrinsic sensory nerves.