Vagal afferent innervation of the proximal gastrointestinal tract mucosa: chemoreceptor and mechanoreceptor architecture.

The vagus nerve supplies most of the extrinsic low-threshold chemosensitive (e.g., Mei, 1985; Grundy and Scratcherd, 1989) and mechanosensitive (e.g., Clarke and Davison, 1978; Grundy, 1988) afferents innervating the mucosa of the proximal gastrointestinal (GI) tract. Both modalities, which have been described electrophysiologically, have been implicated as important in controlling the digestion and absorption of nutrients and in generating feedback that influences ingestion (Mei, 1985; Janig, 1996; Bates et al., 1998; Raybould, 1999; Beyak et al., 2006). The vagal chemosensitive afferents appear to be broadly tuned, responding to a variety of nutritive (e.g., Randich et al., 2000; Cuche et al., 2001), osmotic (e.g., Cottrell and Iggo, 1984c; Mei and Garnier, 1986), and other chemical (e.g., Mei, 1985; Page et al., 2002) properties of chyme. The mechanosensitive afferents innervating the mucosa are typically characterized as low-threshold, often fast-adapting, fibers (e.g., Clarke and Davison, 1978; Page et al., 2002) that presumably monitor grinding, movement, and emptying of material from the stomach into the lumen of the intestine. At least some vagal afferents innervating the mucosa appear to be polymodal, responding to both chemical and mechanical events (e.g., Cottrell and Iggo, 1984a,c; Grundy, 1988) and, though less studied, apparently thermal stimuli as well (cf. Mei and Garnier, 1986; Janig, 1996). Little is known about the morphologies of these visceral afferents. Conventional histological analyses have documented a variety of neural elements in the mucosa and submucosa (e.g., Berkley, 1893; Hill, 1927; Palay and Karlin, 1959; Stach, 1973; Keast, 1987), but these early observations were typically unable to distinguish fibers based on their source (e.g., vagal or dorsal root, intrinsic or extrinsic, efferent or afferent, etc.). More recently, limited observations on vagal afferents in the mucosa and submucosa of the duodenum have been made with tracers, including autoradiographic markers (Sato and Koyano, 1987) and the carbocyanine label “Dil” (Powley et al., 1994; Berthoud et al., 1995; Berthoud and Patterson, 1996). These initial analyses are instructive, but incomplete. They focused more on the population of afferents than on the morphology of the individual neuron, and they were based on tracer protocols that did not routinely yield the highest definition of terminal fields. Thus, the present experiment was designed to extend previous descriptions of primary vagal afferents in the submucosa and mucosa of the GI tract. The experiment employed the marker dextran conjugated with tetramethylrhodamine and biotin to circumvent the low resolution and other limitations of early autoradiographic analyses and in vivo DiI protocols (cf. Powley et al., 1994; Berthoud et al., 1995; Powley and Phillips, 2005). By employing injection parameters yielding strong labeling of only a few fibers in any one specimen, combined with permanentlabeling histochemistry, the experimental protocol made it practical to examine and characterize the terminal fields of individual fibers. Serial series of thick sections of the gut wall were collected to facilitate following and reconstructing the labeled afferents.