Vagal and spinal afferent innervation of the rat esophagus: A combined retrograde tracing and immunocytochemical study with special emphasis on calcium‐binding proteins

Abstract

Vagal afferent neurons contain a variety of neurochemical markers and neuroactive substances, most of which are present also in dorsal root ganglion cells. To test for the suitability of the calcium-binding protein calretinin as a specific marker for vagal afferent fibers in the periphery, immunocytochemistry for this protein was combined with retrograde tracing. Nerve fibers in the rat esophagus, as well as vagal and spinal sensory neurons innervating the esophagus, were investigated for co-localization of calretinin with calbindin, calcitonin gene-related peptide, and NADPH diaphorase. The results indicated that calretinin immunocytochemistry demonstrates neuronal structures known as vagal afferent from other studies, in particular intraganglionic laminar endings. A few enteric neurons whose distribution was unrelated to intraganglionic laminar endings also stained for calretinin. Strikingly, calretinin immunoreactivity was absent from spinal afferent neurons innervating the rat esophagus. In intraganglionic laminar endings and nodose ganglion cells calretinin was highly co-localized with calbindin but not with calcitonin gene-related peptide. On the other hand, calbindin was also found in spinal afferents to the esophagus where it was co-localized with calcitonin gene-related peptide. Vagal afferent neurons innervating the esophagus were never positive for NADPH diaphorase. Thus, calretinin appears to be a more specific marker for vagal afferent structures in the esophagus than calbindin, which is expressed by both vagal and spinal sensory neurons. Calretinin immunocytochemistry may be utilized as a valuable tool for investigations of subpopulations of vagal afferents in certain viscera.

Nitric Oxide as an Endogenous Peripheral Modulator of Visceral Sensory Neuronal Function

“…Our data indicate that nodose nNOS is not transported to the stomach, because peripheral endings of vagal afferents were nNOS negative. This corroborates tracing data from the rat, in which peripheral vagal afferent terminals in the esophagus were NOS negative (Dütsch et al, 1998), as are those in rat lung (Brouns et al, 2002). Therefore, although nNOS is three times more likely to be expressed in gastric neurons compared with the general vagal afferent population, this is not transported to the peripheral endings and is therefore unlikely to be responsible for the generation of NO involved in modulating afferent mechanosensitivity.…”

Key Role of Mucosal Primary Afferents in Mediating the Inhibitory Influence of Capsaicin on Vagally Mediated Contractions in the Mouse Esophagus

“…Vagal innervation of the mucosa shows the greatest density in the upper cervical esophagus which steeply decreases in the lower cervical and thoracic esophagus [10,20,32]. On the other hand, spinal afferents are distributed rather evenly from oral to aboral [10]. Since we have used thoracic esophageal segments in the current study, we can suggest that mucosal capsaicinsensitive primary afferents, mainly of spinal origin, play an important role in mediating the inhibitory influence of capsaicin on vagally mediated contractions.…”

“…The mucosa of the alimentary tract, in addition to its absorptive and secretive functions, serves as a sensory organ that perceives different sensory modalities and transducts them to the enteric as well as the central nervous systems. Vagal innervation of the mucosa shows the greatest density in the upper cervical esophagus which steeply decreases in the lower cervical and thoracic esophagus [10,20,32]. On the other hand, spinal afferents are distributed rather evenly from oral to aboral [10].…”

Extrinsic primary afferent signalling in the gut

“…77,78 In the mouse colon, most of the thoracolumbar spinal afferent endings can be activated by strong local compression of the mesenteries or wall of the gut. 46 First described in the 1960s, these endings are associated with branch points of mesenteric arteries and encode both contraction and distension of the gut wall and traction on the mesenteries, 79 with relatively low sensitivity.…”