Exacerbated sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia and hyperalgesia) occurs during inflammation and underlies painful diseases. Proteases that are generated during inflammation and disease cleave protease-activated receptor 2 (PAR 2 ) on afferent nerves to cause mechanical hyperalgesia in the skin and intestine by unknown mechanisms. We hypothesized that PAR 2 -mediated mechanical hyperalgesia requires sensitization of the ion channel transient receptor potential vanilloid 4 (TRPV4). The ability to detect mechanical stimuli allows organisms to respond to their environment. High-intensity mechanical stimuli can damage tissue and provoke pain, leading to avoidance behaviours. Inflammatory mediators enhance sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia or hyperalgesia, respectively), resulting in pain associated with disorders such as arthritis, inflammatory bowel disease and irritable bowel syndrome. However, the ion channels that transduce mechanical stimuli are

To investigate the possible contribution of peripheral sensory mechanisms to abdominal pain following infectious colitis, we examined whether the Citrobacter rodentium mouse model of human E. coli infection caused hyperexcitability of nociceptive colonic dorsal root ganglion (DRG) neurons and whether these changes persisted following recovery from infection. Mice were gavaged with C. rodentium or distilled water. Perforated patch clamp recordings were obtained from acutely dissociated Fast Blue labelled colonic DRG neurons and afferent nerve recordings were obtained from colonic afferents during ramp colonic distensions. Recordings were obtained on day 10 (acute infection) and day 30 (infection resolved). Following gavage, colonic weights, myeloperoxidase (MPO) activity, stool cultures, and histological scoring established that infection caused colitis at day 10 which resolved by day 30 in most tissues. Electrophysiological recordings at day 10 demonstrated hyperexcitability of colonic DRG neurons (40% mean decrease in rheobase, P = 0.02; 50% mean increase in action potential discharge at twice rheobase, P = 0.02). At day 30, the increase in action potential discharge persisted (∼150% increase versus control; P = 0.04). In voltage clamp studies, transient outward (I A ) and delayed rectifier (I K ) currents were suppressed at day 10 and I A currents remained suppressed at day 30. Colonic afferent nerve recordings during colonic distension demonstrated enhanced firing at day 30 in infected animals. These studies demonstrate that acute infectious colitis evokes hyperexcitability of colonic DRG neurons which persists following resolution of the infection and that suppression of I A currents may play a role. Together, these findings suggest that peripheral pain mechanisms could contribute to post-infectious symptoms in conditions such as post-infectious irritable bowel syndrome.

The antinociceptive mechanism underlying protease-activated receptor-4 (PAR(4)) activation was studied in Fast Blue-labelled dorsal root ganglia (DRG) neurons from mouse colon which expressed transcript for PAR(4). Whole cell perforated patch clamp recordings were obtained from these neurons and the effects on neuronal excitability of PAR(4) activating peptides (AP) and reverse peptides (RP) were examined. A 3-min application of PAR(4)-AP (100 micromol L(-1)) markedly suppressed the number of action potential discharged at twice rheobase for up to 60 min. PAR(4)-RP had no effect. PAR(4) application suppresses the excitatory effects of PAR(2). These findings demonstrated that activation of PAR(4) on colonic DRG neurons suppresses their excitability, suggesting these receptors could provide important targets for modifying pain in colonic GI disorders such as IBS and IBD.