Vagal afferents innervating the gastrointestinal tract and CCKA-receptor immunoreactivity.
A large body of evidence derived from electrophysiological recording and pharmacological/behavioral experiments suggests the presence of CCKA-receptors on vagal primary afferent fibers innervating the gastrointestinal tract. With the availability of antibodies specific for the CCKA-receptor, we wanted to demonstrate its presence and distribution on identified vagal afferent fibers and different types of terminals in the mucosa, myenteric plexus, and external muscle layers of the stomach and duodenum. In the duodenal mucosa, neither a C-terminal (Ab-1) nor an N-terminal (Ab-2) specific antibody produced any specific staining; in the myenteric plexus, non-vagal enteric neurons and their processes, but not vagal intraganglionic laminar endings (IGLEs), exhibited CCKAR-immunoreactivity. Similarly, in the gastric myenteric plexus, a population of enteric neurons and their processes, but not identified vagal IGLEs, were labeled by both antibodies. In both external muscle layers of the stomach, CCKAR-immunoreactive axons were in close register with labeled vagal afferent intramuscular arrays, but the two labels were not contained in the same varicosities. Ab-1 immunoreactivity was found in the cell membrane of vagal afferent perikarya in the nodose ganglia and in pancreatic acinar cells. The failure to detect CCKAR-immunoreactivity in peripheral vagal afferent terminals cannot be due to methodological problems because it was present in enteric neurons in the same sections, and because it did not stain structures resembling IGLEs in material without the potentially masking vagal afferent label. We conclude that CCKA-receptors on vagal afferent terminals: 1) are below the immunohistochemical detection threshold, 2) exhibit a conformation or affinity state inaccessible to the two antibodies, or 3) are not transported to the peripheral terminals.