Vagal intraganglionic laminar endings and intramuscular arrays mature at different rates in pre-weanling rat stomach

To assess whether vagal afferents in the gastrointestinal (GI) tract mature postnatally and differentiate at different rates, potentially reflecting the changing functional requirements of weaning and independence, the vagal afferent innervation of the stomach was inventoried in pre-weanling and adult rats. Wheatgerm agglutinin-horseradish peroxidase was injected into the nodose ganglia of 9-day-old and adult rats, and after tracer transport, the animals were sacrificed. Their stomachs were prepared as wholemounts and processed with tetramethylbenzidine. Inventories were obtained with a counting grid that was systematically positioned throughout the wholemounts by the use of a sampling template that was adjusted for stomach size and shape. Densities in the gastric antrum, corpus, and forestomach were determined for (1) afferent bundles, (2) individual fibers separated from the bundles and presumably located near their targets, (3) differentiated intraganglionic laminar endings (IGLEs) associated with myenteric ganglia, and (4) differentiated intramuscular arrays (IMAs) situated within the smooth muscle layers. In pre-weanling rats, which were 10 days old at perfusion, the distributions of vagal bundles and fibers were similar to those of adults, suggesting that the basic vagal architecture develops early. Differentiated IGLEs were also distributed in a mature pattern in 10 day olds, whereas few IMAs had yet been distributed and differentiated in the forestomach of the pre-weanlings. The authors hypothesize that these different developmental patterns for the two types of vagal afferents are consistent with their putative functional roles as, respectively, mechanoreceptors (IGLEs) that coordinate rhythmic motor function needed early for the digestion of milk and stretch receptors (IMAs) needed later as the GI tract natures for the transition to solid food at weaning.