Immunohistochemical identification of cholecystokinin A receptors on interstitial cells of Cajal, smooth muscle, and enteric neurons in rat pylorus

One of the physiological functions of circulating cholecystokinin (CCK) is in the control of the pyloric sphincter and the subsequent delivery of nutrients to the small intestine. In order to identify the site(s) of action of CCK in the gastropyloric region, we performed immunohistochemistry using an antibody directed to the C-terminal region of the cholecystokinin A receptor (CCKAR). In the rat, cells that display strong CCKAR immunoreactivity and fit the morphological description of interstitial cells of Cajal (ICC) were found in the distal sphincter muscle and in the circular muscle of the proximal duodenum. Double labeling showed that these cells coexpressed vimentin, but that not all vimentin-positive cells expressed CCKAR. Confirmation that the CCKAR-expressing cells were ICC also came from kit double-labeling experiments in mice. In addition to ICC, circular smooth muscle cells at the tip of the comma-shaped sphincter muscle, but not elsewhere, also exhibited strong, membrane-bound CCKAR immunoreactivity. With higher antibody concentrations, the entire circular muscle displayed moderate CCKAR immunoreactivity, suggesting that circular smooth muscle cells express low levels of CCKAR. Select neurons in the myenteric ganglia near the sphincter muscle proper, the distal antrum, and proximal duodenum, as well as a few single neurons in the submucosa, also expressed strong CCKAR immunoreactivity. Finally, CCKAR-immunoreactive ICC and neurons were not specifically related to vagal afferent intramuscular and intraganglionic endings, and vagal afferents themselves did not exhibit any CCKAR immunoreactivity. These results suggest a role for ICC and enteric neurons in the mediation of CCK effects on pyloric sphincter pressure in addition to direct effects of the hormone on circular smooth muscle.