Aims: 

Transient receptor potential A1 (TRPA1) channels are expressed by many afferent neurons supplying the colon and may play a role in pain signalling. This study examined whether stimulation of TRPA1 by intracolonic mustard oil (MO) causes afferent input to the spinal cord and whether this effect is changed by mild colitis.

Methods: 

Female Him: OF1 mice were treated with intracolonic MO (2% in peanut oil). C-Fos expression in the superficial layers of the spinal dorsal horn (laminae I-IIo) at the level of S1 was measured immunocytochemically 1h post-treatment. Morphine (10mg/kg in saline) was administered subcutaneously 1h before the MO stimulus. Mild colitis was induced by adding dextrane sulphate sodium (DSS, 36-50kDa, 2%) to the drinking water for 1 week. The effect of DSS on stool consistency, colon length, colonic myeloperoxidase (MPO), homecage-activity, weight, feeding and drinking was monitored.

Results: 

Relative to vehicle, MO increased the number of c-Fos-positive cells in laminae I-IIo, the main termination site of visceral afferent neurons, by 77.1% (t-test: t8.784 = 4.851, p < 0.05). DSS-induced mild colitis was characterized by increased myeloperoxidase (MPO) levels, shortened colon length and loose bloody stool, whereas homecage-activity, body weight, feeding and drinking remained unchanged. Compared to control, the effect of MO to induce spinal c-Fos was enhanced after DSS pretreatment by 41.1% (t-test: t12 = 3.387, p < 0.05). Morphine reduced the spinal c-Fos response to MO by 43.5% (t-test: t11 = -4.688, p < 0.05). Conclusion:

TRPA1 stimulation by intracolonic MO induces afferent input to the spinal cord. This response is exaggerated in mice with mild colitis. This observation and the inhibition of the spinal c-Fos response by morphine reflect the nociceptive nature of MO-induced afferent input.