Involvement of catecholaminergic neurons in motor innervation of striated muscle in the mouse esophagus

Immunohistochemical characterization of interstitial cells and their spatial relationship to motor neurons within the mouse esophagus

Interstitial cells of Cajal (ICC) and PDGFRα+ cells regulate smooth muscle motility in the gastrointestinal (GI) tract, yet their function in the esophagus remains unknown. The mouse esophagus has been described as primarily skeletal muscle; however, ICC  have been identified in this region. This study characterizes the distribution of skeletal and smooth muscle cells (SMCs) and their spatial relationship to ICC, PDGFRα+ cells, and intramuscular motor neurons in the mouse esophagus. SMCs occupied approximately 30% of the distal esophagus, but their density declined in more proximal regions. Similarly, ANO1+ intramuscular ICC (ICC-IM) were distributed along the esophagus, with density decreasing proximally. While ICC-IM were closely associated with SMCs, they were also present in regions of skeletal muscle. Intramuscular, submucosal, and myenteric PDGFRα+ cells were densely distributed throughout the esophagus, yet only intramuscular PDGFRα+ cells in the lower esophageal sphincter (LES) and distal esophagus expressed SK3. ICC-IM and PDGFRα+ cells were closely associated with intramuscular nNOS+, VIP+, VAChT+, and TH+ neurons and GFAP+ cells resembling intramuscular enteric glia. These findings suggest that ICC-IM and PDGFRα+ cells may have roles in regulating esophageal motility due to their close proximity to each other and to skeletal muscle and SMCs, although further functional studies are needed to explore their role in this region. The mixed muscular composition and presence of interstitial cells in the mouse distal esophagus is anatomically similar to the transitional zone found in the human esophagus, and therefore, motility studies in the mouse may be translatable to humans.

Neurochemical classification of serotonin-immunoreactive neurons co-innervating motor endplates in the mouse esophagus

Serotonin immunoreactivity was previously found in myenteric neurons co-innervating motor endplates in the mouse esophagus striated muscle and an involvement in motility control was suggested. However, it is not known if other neuroactive substances are present in these neurons and to what extent they co-localize. First, vasoactive intestinal peptide (VIP) was established as a bona fide marker for putative inhibitory myenteric neurons by evaluating co-localization with neuronal nitric oxide synthase (nNOS) and neuropeptide Y (NPY). Then, co-localization of serotonin and VIP was tested in co-innervating axons on motor endplates, which were visualized with α-bungarotoxin (α-BT) by multilabel immunofluorescence. Myenteric ganglia were also surveyed for co-localization in neuronal perikarya and varicosities. nNOS, NPY, and VIP were completely co-localized in enteric co-innervating nerve terminals on motor endplates. After co-staining with VIP, we found (a) serotonin (5-HT)-positive nerve endings without VIP (44% of 5-HT-positively innervated endplates), (b) 5-HT- and VIP-positive endings without co-localization (35%), and (c) 5-HT- and VIP-positive endings with co-localization (21%). About one-fifth of nerve terminals on motor endplates containing 5-HT originate from putative inhibitory peptidegic nitrergic neurons. However, the majority represents a different population presumably subserving different functions.

Enteric co-innervation of striated muscle in the esophagus: still enigmatic?

The existence of a distinct ganglionated myenteric plexus between the two layers of the striated tunica muscularis of the mammalian esophagus has represented an enigma for quite a while. Although an enteric co-innervation of vagally innervated motor endplates in the esophagus has been suggested repeatedly, it was not possible until recently to demonstrate this dual innervation. Twenty-two years ago, we were able to demonstrate that motor endplates in the rat esophagus receive dual innervation from both vagal nerve fibers originating in the brain stem and from varicose enteric nerve fibers originating in the myenteric plexus. Meanwhile, a considerable amount of data has been gathered on enteric co-innervation and its occurrence in the esophagus of a variety of species including humans, its neurochemistry, spatial relationships on motor endplates, ontogeny and possible functional roles. These data underline the significance of this newly discovered innervation component, although its function in vivo is still largely unknown. The aim of this review, which is an update of our previous paper (Wörl and Neuhuber in Histochem Cell Biol 123(2):117-130. doi: 10.1007/s00418-005-0764-7 , 2005a), is to summarize the current knowledge about enteric co-innervation of esophageal striated muscle and to provide some hints as to its functional significance.