Excitation of rat colonic afferent fibres by 5‐HT₃ receptors

Abstract

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‐HT3 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‐HT3 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‐HT3 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‐HT3 agonist, 2‐methyl‐5‐HT (between 10−6 and 10−2 M). Responses to 2‐methyl‐5‐HT were blocked by the 5‐HT3 receptor antagonist alosetron (2 × 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‐HT3 receptor‐like immunoreactivity. We conclude that colonic sensory neurones expressing 5‐HT3 receptors also functionally express the receptors at their peripheral endings. Our data reveal actions of 5‐HT on colonic afferent endings via both 5‐HT3 and non‐5‐HT3 receptors.

TRPV1 Function in Mouse Colon Sensory Neurons Is Enhanced by Metabotropic 5-Hydroxytryptamine Receptor Activation

“…Previous studies demonstrated the presence of metabotropic 5-HT receptors in intestinal afferent neurons and suggest that only a subgroup express 5-HT 3 receptors (Hillsley and Grundy, 1998; Hicks et al, 2002). In addition to differential expression of 5-HT receptors, the onset and duration of 5-HT effects differed with stimulation of metabotropic receptors, causing a slower and longer-lasting stimulation.…”

“…Although these mediators play an important role in changes attributable to injury and inflammation, the effects of 5-HT on colon sensory neurons provides a mechanism by which physiological stimuli can trigger enhanced responses in normal tissue. Mucosal stimuli trigger 5-HT release from enteroendocrine cells, which in turn activates intrinsic and extrinsic primary afferent neurons within the gut wall (Pan and Gershon, 2000;Hicks et al, 2002). Given the apparent expression of TRPV1 in nearly all C57BL/6 colon DRG neurons, and the generation of spontaneous action potentials at normal body temperature after exposure to 5-HT, the activation of peripheral afferents may contribute to sensation and even conscious perception of visceral stimuli in the uninflamed gut.…”

“…Visceral sensory neurons express several 5-HT receptors Hicks et al, 2002;Mazzia et al, 2003), and we therefore examined the role of different 5-HT receptors in the modulation of TRPV1 function using pharmacological tools. As described above, concentrations of 5-HT Ն1.0 M did not affect resting membrane potential or baseline currents.…”

“…Serotonin excites mechanosensitive afferent fibers innervating the colon through effects on the ionotropic 5-HT 3 as well as metabotropic 5-HT receptors (Hicks et al, 2002). Although sev-eral studies have demonstrated effects of metabotropic 5-HT receptors on voltage-dependent ion channels in neurons (Braha et al, 1993;Torres et al, 1995;Cardenas et al, 2001), the mechanisms by which G-protein-coupled 5-HT receptors trigger action potentials are not known (Christian et al, 1989;Cardenas et al, 1997a,b).…”

Extrinsic primary afferent signalling in the gut

“…They are excited by stroking the mucosa with very light von Frey hairs (10 mg) and are strongly activated by 5-hydroxytryptamine or 5-HT3 agonists. 68 Spinal mucosal afferents are more abundant in lumbosacral than thoracolumbar spinal pathways, 46 but the detailed structure of their endings…”