Localization of substance P binding sites in submucous plexus of guinea pig ileum, using whole‐mount autoradiography

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Abstract
Whole mounts of guinea pig ileum submucosa were incubated with radiolabeled tachykinins, and binding sites were visualized using autoradiography. Very dense specific binding for [125I]-Bolton-Hunter substance P (BHSP) was observed over ganglia of the submucous plexus, with weaker binding over internodal strands. Dense specific binding was also seen over occasional strands of circular muscle, with weak binding over clumps of mucosa. Although very weak binding was seen over some large blood vessels, no binding was associated with smaller blood vessels. Localization of binding was absent in whole-mounts coincubated with 1 nM substance P, used to define nonspecific binding. Localization of BHSP-specific binding was also abolished in whole-mounts coincubated with 1 nM substance P, but not with 1 nM neurokinin B, suggesting that binding was probably to an NK-1 tachykinin receptor. In whole-mounts incubated in [125I]-iodohistidyl neurokinin A (INKA) or [125I]-Bolton-Hunter neurokinin B (BHNKB), no specific binding over ganglia was observed. These binding sites for BHSP are probably identical with the neuronal substance P receptors mediating mucosal ion transport.