Neurohormonal peptides, serotonin, and nitric oxide synthase in the enteric nervous system and endocrine cells of the gastrointestinal tract of neotenic and thyroid hormone-treated axolotls (Ambystoma mexicanum)

Immunoreactivity against vasoactive intestinal polypeptide (VIP), neurotens in (NT), substance P (SP), calcitonin gene-related peptide (CGRP), gastrin/ cholecystokinin (GAS/CCK), somatostatin (SOM), serotonin (SER), and nitric oxide synthase (NOS) was investigated in the gastrointestinal tract of the urodele Ambystoma mexicanum, the axolotl, by the use of immunohistochemical techniques. The study also compares the distribution patterns and frequenc ies of the neurohormones, and NOS in neotenic and thyroxine-treated (metamo rphosed) individuals. GAS/CCK, SP, NT SOM, and SER immunoreactivities occur red in endocrine mucosal cells and VIP, SP, CGRP, NTSER, SER, and NOS immun oreactivities in the enteric nervous system. The GAS/CCK-immunoreactive (-I R) cells were restricted to the upper small intestine. NT-IR and SP-IR endo crine cells were found in the entire gastrointestinal tract and were most p rominent in the distal large intestine. The density of the SOM-IR cells dec reased from the stomach toward the large intestine. SER-IR endocrine cells were found throughout the gastrointestinal tract, with particularly high de nsities in the stomach and distal large intestine. The VIP-IR enteric nerve fibers were the most prominent ones, present in all layers of the entire g astrointestinal tract, and supplied the smooth muscle and the vasculature. The SER-IR fibers exhibited similar distribution patterns but were less num erous. Very few NT-IR but many SP-IR fibers were found in the muscle and su bmucosal layers. The NT-IR fibers mainly supplied blood vessels, while the SP-IR fibers were also in contact with the smooth muscle. In the muscle and submucosal layers, CGRP-IR fibers were associated to the vasculature; CGRP immunoreactivity occurred also in a minority of SP-IR fibers. NOS-IR nerve fibers were in contact with submucosal arteries but were the least frequen t. After metamorphosis provoked by exogenous thyroxine, the number of SOM-I R endocrine cells in the stomach mucosa was increased as well as the densit y of VIP-IR, SER-IR, and SP-IR nerve fibers in the gastrointestinal tract. It is proposed that the observed increases may reflect refinements of the n eurohormonal system after metamorphosis.