Enteric co-innervation of striated muscle fibers in the esophagus: Just a "hangover"?

Striated muscle of the esophagus was until recently considered to consist o f "classical" skeletal muscle fibers innervated by cholinergic vagal motone urons. The recently described co-innervation originating from enteric neuro ns expressing nNOS, VIP, NPY, and galanin added a new dimension of complexi ty. The aim of this study was to summarize current knowledge about, and to get further hints as to the possible function of enteric co-innervation of striated esophageal muscle fibers. Aldehyde fixed rat esophagi were processed for immunocytochemistry for CGRP or VAChT (to demonstrate vagal motor terminals), nNOS/NADPH-d, VIP, NPY, a nd galanin (to demonstrate enteric terminals), met-enkephalin, mu opiate re ceptor, muscarinic receptors m1-3, soluble guanylyl cyclase, and cGMP depen dent kinase type I and II. Motor endplates were visualized using fluorochro me tagged alpha -bungarotoxin to label nicotinic receptors, or with AChE hi stochemistry. Besides light and confocal laser scanning microscopy, immune electron microscopy was also employed. Up to 80% of motor endplates were co-innervated. In addition to nNOS, VIP, NPY, and galanin, many enteric terminals in esophageal motor endplates expr essed met-enkephalin. Some appeared to stain for the muscarinic m(2) recept or. There was prominent immunostaining for the mu opioid receptor in the sa rcolemma at both junctional and extrajunctional sites. Immunostaining for s oluble guanylyl cyclase was prominent immediately beneath the clusters of n icotinic receptors. Enteric varicosities and vagal terminals intermingled i n motor endplates often without intervening teloglial processes. During ont ogeny, initially high co-innervation rates were reduced to adult levels in a cranio-caudally progressing manner. We conclude that;, in addition to a possible nitrergic, VIP-, NPY-, and gal aninergic modulation of neuromuscular transmission by enteric neurons, opio idergic mechanisms could play a role. On the other hand, cholinergic influe nce on enteric neurons may be exerted also by the nucleus ambiguus via moto r endplates, in addition to the input from the dorsal motor nucleus. The ob servations that enteric nerve fibers contact striated muscle fibers at spec ialized sites, i.e., motor endplates, and that these contacts appear in an ordered cranio-caudal sequence after cholinergic motor endplates have been established point to a specific function in neuronal control of esophageal muscle rather than to be an unspecific "hangover" from the smooth muscle pa st of this organ. Anat Rec 262:41-46, 2001. (C) 2001 Wiley-Liss, Inc.