The defecation reflex in rats: fundamental properties and the reflex center

While pharmacological and physiological studies in rats are now increasing, physiological properties of their defecation have been scarcely investigated. This study was performed to define the properties of defecation in decerebrate rats, with special reference to the pontine defecation reflex center, which has been postulated in dogs. Intraluminal pressure was recorded from the colon and rectum with balloon-pressure transducer method using balloons of 15–20 mm in length and 0.1–0.3 ml in volume. Distention of a balloon in the descending colon and rectum with an additional injection of 0.03–0.1 ml air induced propulsive contractions on the descending colon and rectum. The mean of threshold pressures to induce propulsive contraction was 17.0±5.8 mm Hg (mean±S.E.) in the proximal part and 18.3±3.3 mm Hg in the distal part of the descending colon, and 11.8±1.3 mm Hg in the rectum. The maximum amplitude of propulsive contractions was 55 mm Hg in the rectum, 47 mm Hg in the distal part of the descending colon and 38 mm Hg in the proximal part. Similar colorectal propulsive contractions were produced by gastric distention (5–10 ml, 20–30 mm Hg) and electrical stimulation of the anal canal. Contrarily, spontaneous contractions of the proximal colon were suppressed by rectal distention and anal-canal stimulation. These results suggest that the descending colon and rectum, but not the proximal colon, were innervated by the pelvic afferent and efferent fibers mediating the defecation reflex. Pontine transection at the cerebellar peduncle level abolished colorectal propulsive contractions induced by distention of the stomach, descending colon and rectum, and stimulation of the anal canal, although much smaller contractions were still induced after the pontine transection. These results suggest that the pontine defecation reflex center exists and works in rats, as in dogs.