A method is described to measure visceral nociception following implantation of remote balloon devices into the descending colons of rats. Male Sprague Dawley rats were fed ‘Transgel’ for 20-24 h. Rats were subsequently immobilized by inhalation of O₂:CO₂ for 40 s and a small deflated balloon device was introduced by the intrarectal route approximately 2 cm into the descending colon. The balloon was made by placing a 27x3 mm section of polyethylene transfer pipette inside a finger cot. This was pushed into a small cylindrical PTFE collar 7.5x7.5 mm and an 8 mm aluminum cap with rubber septum was then crimped over the end. The balloon was inflated by an injection of saline. Rats were given 15 min to fully recover. The effects of balloon inflation on spontaneous exploratory rearing was measured for 5 min using a photocell activity monitor. Normal rats without rectal balloons exhibited frequent rearing activity within the 5 min observation time (244±13 infra-red beam breaks, n=8). The presence of an inflated balloon in the descending colon of rats suppressed spontaneous rearing activity compared to control animals without balloons. Increased balloon inflation volumes significantly (p<0.005) reduced rearing activity (178±14, 1 ml; 151±11, 1.5 ml; 145±13, 2.0 ml; 90±10, 2.5 ml) demonstrating a stimulus response relationship.

This method has advantages over those previously described. It does not involve indwelling cannulae which may be pulled or chewed [2]. Invasive surgery is not required to implant electrodes to measure visceral or muscular activity [1]. The rats are conscious and unrestrained. Observation time is short and large numbers of animals can be monitored quickly. The methodology parallels clinical diagnostic measurements of visceral pain in humans [3] and the readout is objective. This method should allow mechanisms of visceral nociception to be assessed more easily in vivo.

References

1. Ness, T.J.; Gebhart, G.F. (1988). Colorectal distension as a noxious visceral stimulus. Brain Research, 450, 153-169.
2. Ness, T.J.; Randitch, A.; Gebhart, G.F. (1991). Further behavioral evidence that colorectal distension is a ‘noxious’ visceral stimulus in rats. Neuroscience Letters, 131, 113-116.
3. Ritchie, J. (1973). Pain from distension of the pelvic colon by inflating a balloon in the irritable colon syndrome. Gut, 14, 125-132.