TRPA1 activation induces visceral hypersensitivity in mice

Moh Panesar, Elliot Lilley, Claire Mathers, Mark Nash, Alyson Fox

Novartis Institutes for Biomedical Research, Horsham, UK

There are currently few treatments available for Irritable Bowel Syndrome (IBS) as the underlying mechanisms remain unclear. Visceral hypersensitivity and hyperalgesia have been well documented in IBS (Mertz et al.,1995).The Transient Receptor Potential ion channel, TRPA1, is expressed in nociceptive sensory neurones where it acts as a sensor for pungent chemicals and is implicated in pain processing (Bautista et al., 2006). Activation of TRPA1 by irritants including mustard oil (allyl isothiocyanate) induces somatic pain and hyperalgesia (Kwan et al., 2006) that is inhibited by the TRPA1 antagonist (AP18) in animals (Petrus et al., 2007). Here, we have examined the role of TRPA1 in a commonly used model of visceral hypersensitivity induced by intracolonic mustard oil (Laird et al., 2001) in TRPA1 knockout (KO) and wild-type (WT) mice.

Male TRPA1 KO/WT litter-mate mice (129p / C57Bl/6J, 25-35g) were used for the study. Under inhalation anaesthesia, mustard oil (0.25%MO; 50μl, intracolonic) was administered by inserting a fine cannula via the anus. Following administration of MO the animals were observed for spontaneous pain-related behaviors (abdominal retractions) for 10 to 30 minutes post dose. The referred allodynia was inferred from mechanical withdrawal thresholds tested prior to and 48h after MO administration by application of von Frey filaments to the abdomen of the mouse in a raised perspex ventilated box with a wire mesh floor. A sharp abdominal retraction upon application of the filament was taken as a withdrawal response. The TRPA1 antagonist, AP18 (10 mg/kg, iv) was dosed 48h post MO administration and it’s effect on referred allodynia in WT mice was assessed. Blood and colon were taken for MPO analysis and histology to confirm the absence of inflammation. AP18 was dissolved in 25% cremophor and saline. MO was dissolved in 70% ethanol/ saline. The treatment groups (n=7-19) were compared to the relevant control using ANOVA followed by Dunnett’s test (p<0.05 was considered significant).

Intracolonic administration of MO produced a significant increase in the number of spontaneous pain behaviors in TRPA1 WT (7.12 ± 1.3) but not the KO mice (0.37 ± 0.48). MO produced a decrease in mechanical withdrawal threshold in WT (vehicle; 10.6 ± 1.4g, MO; 4.75 ± 0.69 g) but not KO (vehicle; 12.2 ± 0.67g, MO; 9.7 ± 0.66g) mice. AP18 reversed the MO-induced referred mechanical allodynia in the WT (vehicle; 4 ± 0.5g, AP18; 9 ± 0.9g) mice.

The results show that MO induces visceral pain and referred allodynia in mice. TRPA1 deficient mice did not show pain to MO and a TRPA1 inhibitor reversed the referred abdominal pain. These data demonstrate that MO induces visceral pain via TRPA1 and that this channel may represent a potential target for treatment of visceral hypersensitivity.

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Petrus et al., 2007. Mol Pain; 3 (40)