MORPHOLOGICAL AND IMMUNOHISTOCHEMICAL IDENTIFICATION OF NEURONS AND THEIR TARGETS IN THE GUINEA-PIG DUODENUM

Nerve circuits within the proximal duodenum were investigated using a combination of immunohistochemistry for individual neuron markers and lesion of intrinsic nerve pathways to determine axon projections. Cell shapes and axonal projections were also studied in cells that had bee n injected with a marker substance. Several major neuron populations w ere identified. Calbindin immunoreactivity occurred in a population of myenteric nerve cells with Dogiel type 11 morphology. These had axons that projected to other myenteric ganglia, to the circular muscle end to the mucosa. All were immunoreactive for the synthesizing enzyme fo r acetylcholine, choline acetyltransferase, and some were also immunor eactive for calretinin. Myenteric neurons with nitric oxide synthase i mmunoreactivity projected anally to the circular muscle. These were al so immunoreactive for vasoactive intestinal peptide, and proportions o f them had enkephalin and/or neuropeptide Y immunoreactivity. It is su ggested that they are inhibitory motor neurons to the circular muscle. A very few (about 2%) of nitric oxide synthase-immunoreactive neurons had choline acetyltransferase immunoreactivity. Tachykinin (substance P)-immunoreactive nerve cells were numerous in the myenteric plexus. Some of these projected orally to the circular muscle and are conclude d to be excitatory motor neurons. Others projected to the tertiary ple xus which innervates the longitudinal muscle and others provided termi nals in the myenteric plexus. Two groups of descending interneurons we re identified, one with somatostatin immunoreactivity and one with vas oactive intestinal peptide immunoreactivity. The two most common nerve cells in submucous ganglia were neuropeptide Y- and vasoactive intest inal peptide-immunoreactive nerve cells. Both provided innervation of the mucosa. There was also a population of calretinin-immunoreactive s ubmucous neurons that innervated the mucosal glands, but not the villi . Comparison with the ileum reveals similarities in the chemistries an d projections of neurons. Differences include the almost complete abse nce of nitric oxide synthase immunoreactivity from vasoactive intestin al peptide-immunoreactive interneurons in the duodenum, the projection of calbindin-immunoreactive Dogiel type II neurons to the circular mu scle and the absence of tachykinin immunoreactivity from these neurons . (C) 1998 IBRO. Published by Elsevier Science Ltd.