Some neurohistochemical properties of nerve elements in myenteric plexus of rabbit ileum: similarities and dissimilarities to the rodent pattern.

Enteric neurons have distinct neurochemical codings in each species. The basal tone of the gastrointestinal tract of the rabbit is low and produces neurally evoked pendular movements. Therefore, it might have an innervation pattern different from that of other laboratory animals. We have characterised myenteric neuron populations in rabbit ileum with neurochemical markers that are known to be associated with distinct cell types and/or fibre systems in the myenteric plexus. The density of nerve cells estimated with the NADH-diaphorase technique was about 2500 cells/cm2 and most, if not all, neurons contained microtubule-associated protein 2. NADPH-diaphorase-positive cells were numerous. One cell type was large and emitted long straight processes, whereas small cells bore thin filamentous dendrites. Neurons immunoreactive for 28-kDa calcium-binding protein were rare. Over 70% of them had very strongly labelled lamellar dendrites. Their axons were beaded and formed pericellular baskets around unstained somata. We found very few small tyrosine-hydroxylase-positive cells. The fibre network in the plexus was very strong; the axons formed many pericellular baskets. In double labelling studies, no co-localisation was revealed between the 28-kDa calcium-binding protein and NADPH-diaphorase. Some fibres containing 28-kDa calcium-binding protein formed only a few contacts on somata of NADPH-diaphorase-positive cells. None of the NADPH-diaphorase-labelled cells were found to be stained for tyrosine hydroxylase. Tyrosine-hydroxylase-positive fibres rarely made pericellular baskets on the surface of NADPH-diaphorase-positive somata. Strongly immunolabelled pericellular baskets were never observed around NADPH-diaphorase-positive cell somata. The results suggest that myenteric neurons in rabbit comprise distinct and characteristic neurochemical properties that are different from the rodent pattern. Therefore, the explanation of the motility pattern of rabbit intestine can be approached on a chemical neuroanatomical basis.