Identification of medium/high-threshold extrinsic mechanosensitive afferent nerves to the gastrointestinal tract.

BACKGROUND & AIMS: Large distentions reliably evoke sensation from the noninflamed, nonischemic bowel, but the specialized afferent axonal structures responsible have not been morphologically identified. We investigated whether their transduction sites are located on major blood vessels close to and within the gut wall. METHODS: In vitro extracellular recordings were made from mesenteric nerve trunks in guinea pig ileum, combined with rapid axonal dye filling and immunohistochemical analysis of nerve trunks. RESULTS: Recordings revealed sensory fibers with focal mechanosensitive sites in the mesenteries that could be activated by von Frey hairs and by stretch. Dye filling revealed varicose branching sensory axons on mesenteric blood vessels but no other anatomically specialized structures in mesenteric membranes or the serosa. Large-amplitude stretch and von Frey hairs also activated sensory endings within the gut wall itself but only if the submucosa was present; mechanotransduction sites in the serosa or outer muscle layers were sparse. Mechanosensitive sites in submucosa were exclusively associated with submucosal blood vessels. Submucosal endings had significantly higher thresholds to stretch than specialized low-threshold mechanoreceptors characterized previously in the rectum (P < .05) and were therefore classified as medium/high-threshold mechanoreceptors. Capsaicin (0.3-1 micromol/L) activated most mechanosensitive mesenteric (68%) and submucosal (85%) afferent endings. Similar intramural mechanosensitive afferent endings on blood vessels also exist in the colon and bladder. CONCLUSIONS: Varicose branching axons of sensory neurons on intramural blood vessels, previously shown to mediate sensory vasodilation, are transduction sites for medium/high-threshold, stretch-sensitive mechanoreceptors, encoding large distentions in hollow viscera.

Rectal intraganglionic laminar endings are transduction sites of extrinsic mechanoreceptors in the guinea pig rectum.

BACKGROUND & AIMS: Vagal afferent mechanoreceptors in the upper gut have recently been identified morphologically as intraganglionic laminar endings (IGLEs), but little is known about the structure of mechanoreceptive endings elsewhere in the gastrointestinal tract. We have morphologically characterized the nerve endings of specialized mechanoreceptors in the rectum. METHODS: Extracellular recordings from guinea pig rectal and colonic nerves were made, in vitro, in combination with rapid anterograde transport of biotinamide, to reveal the morphology of recorded fibers. Controlled distentions were used to activate mechanoreceptive afferent units, and von Frey hairs were used to identify their transduction sites. RESULTS: Rectal mechanoreceptors were present in high density, had low thresholds, and adapted slowly to maintained distention. Each afferent unit had multiple small (<200 microm diameter) transduction sites ("hot spots") at which they could be activated locally by application of a light von Frey hair (0.08-7 mN). Anterograde dye filling revealed characteristic rectal intraganglionic laminar endings (rIGLEs) in myenteric ganglia, significantly associated with hot spots, comparable to the IGLEs of vagal tension receptors, but smaller and less complex. Afferent fibers with these morphologic and physiologic features could not be recorded from colonic nerves innervating the large bowel proximal to the rectum. CONCLUSIONS: The rectum receives a dense afferent innervation by a distinct population of low-threshold, slowly adapting mechanoreceptors with specialized intraganglionic laminar endings (rIGLEs), which are not found more proximally in the colon.

Excitation of rat colonic afferent fibres by 5-HT(3) receptors.

The gastrointestinal tract contains most of the body's 5-hydroxytryptamine (5-HT) and releases large amounts after meals or exposure to toxins. Increased 5-HT release occurs in patients with irritable bowel syndrome (IBS) and their peak plasma 5-HT levels correlate with pain episodes. 5-HT(3) receptor antagonists reduce symptoms of IBS clinically, but their site of action is unclear and the potential for other therapeutic targets is unexplored. Here we investigated effects of 5-HT on sensory afferents from the colon and the expression of 5-HT(3) receptors on their cell bodies in the dorsal root ganglia (DRG). Distal colon, inferior mesenteric ganglion and the lumbar splanchnic nerve bundle (LSN) were placed in a specialized organ bath. Eighty-six single fibres were recorded from the LSN. Three classes of primary afferents were found: 70 high-threshold serosal afferents, four low-threshold muscular afferents and 12 mucosal afferents. Afferent cell bodies were retrogradely labelled from the distal colon to the lumbar DRG, where they were processed for 5-HT(3) receptor-like immunoreactivity. Fifty-six percent of colonic afferents responded to 5-HT (between 10(-6) and 10(-3) M) and 30 % responded to the selective 5-HT(3) agonist, 2-methyl-5-HT (between 10(-6) and 10(-2) M). Responses to 2-methyl-5-HT were blocked by the 5-HT(3) receptor antagonist alosetron (2 x 10(-7) M), whereas responses to 5-HT were only partly inhibited. Twenty-six percent of L1 DRG cell bodies retrogradely labelled from the colon displayed 5-HT(3) receptor-like immunoreactivity. We conclude that colonic sensory neurones expressing 5-HT(3) receptors also functionally express the receptors at their peripheral endings. Our data reveal actions of 5-HT on colonic afferent endings via both 5-HT(3) and non-5-HT(3) receptors.