Immunohistochemical localization of nicotinic acetylcholine receptors in the guinea pig bowel and pancreas

Abstract

Although nicotinic acetylcholine receptors (nAChRs) are known to be present on neural elements in both the bowel and the pancreas, the precise location of these receptors has not previously been determined. Immunocytochemistry, by using a rat monoclonal antibody (mAb35), which recognizes alpha-bungarotoxin (alpha-Bgt)-insensitive nAChRs, and a polyclonal antibody raised against the alpha-Bgt-sensitive receptor subunit, alpha7, was used to locate receptor protein in guinea pig gut and pancreas. mAb35-receptor (mAb35-R) immunoreactivity was abundant in both enteric plexuses, enterochromaffin cells, and pancreatic ganglia. Immunostaining was associated with the cell membrane, and clusters of mAb35-R were observed on cell somas and dendrites. Receptor immunoreactivity was also observed on terminals and axons, suggesting that a subset of nAChRs is presynaptic. Internal sites of mAb35-R were observed in permeabilized ganglia. Cells expressing the receptors were closely associated with ChAT-immunoreactive nerve fibers. In addition, the majority of ChAT-positive neurons expressed both cell surface and internal stores of mAb35-R. In the bowel, clusters of mAb35-R were present on the soma and dendrites of Dogiel type I motorneurons and secretomotor neurons. Receptors were detected at the plasma membrane of calbindin-immunoreactive myenteric neurons. In contrast, calbindin-immunoreactive submucosal neurons did not express cell surface mAb35-R, supporting the idea that they are sensory neurons. A subset of enteric neurons expressed both mAb35-R and glutamate receptor (GluR1) immunoreactivity. In the pancreas, mAb35-R immunoreactivity was only observed in ganglia. Alpha7-immunoreactivity was found on both enteric cell bodies and nerve fibers. Based on these results, it appears that visceral nAChRs are composed of at least four subunits and that both pre- and postsynaptic nAChRs are present in the gut and pancreas.