A new model of visceral pain and referred hyperalgesia in the mouse

The generation of transgenic mice that lack or overexpress genes relevant to pain is becoming increasing common. However, only one visceral pain model, the writhing test, is widely used in mice. Here we describe a novel model, chemical stimulation of the colon, which we have developed in mice. Mice of either sex were injected i.v. with 30 mg/kg Evan's Blue for subsequent determination of plasma extravasation. For behavioural testing, they were placed on a raised grid and 50 microl of saline, mustard oil (0.25-2.5%) or capsaicin (0.03-0.3%) was administered by inserting a fine cannula into the colon via the anus. Visceral pain-related behaviours (licking abdomen, stretching, contractions of abdomen etc) were counted for 20 min. Before intracolonic administration, and 20 min after, the frequency of withdrawal responses to the application of von Frey probes to the abdomen was tested. The colon was removed post-mortem and the Evan's Blue content measured. Mustard oil and capsaicin administration evoked dose-dependent visceral pain behaviours, referred hyperalgesia (significant increase in responses to von Frey hairs) and colon plasma extravasation. The peak behavioural responses were evoked by 0.1% capsaicin and by 1% mustard oil respectively. The nociceptive behavioural responses were dose-dependently reversed by morphine (ED50 = 1.9 +/- 1 mg/kg s.c.). We conclude that this model represents a useful tool both for phenotyping mutant mice and for classical pharmacology since information on visceral pain, referred hyperalgesia and colon inflammation can all obtained from the same animal.