The Small Intestine

Distension [and resection (26)] of the ileum, jejunum [including traction of the mesentery (27)], and duodenum has been described in rodents; for the purposes of this review, we shall

Figure 2 Acetic acid produces gastric ulcers in the rat. Kissing ulcers (A), induced in a rat three days after intraluminal application of 60% acetic acid. Arrows indicate round ulcers on the posterior and anterior walls. Using a different method (injection of 20% acetic acid into the stomach wall), the visceromotor response to gastric distension is significantly enhanced from three days (B) to 60 days (C) after acetic acid treatment (p< 0.05). Source: (A) From Ref. 22; (B) Redrawn and adapted from Ref. 24.

Figure 2 Acetic acid produces gastric ulcers in the rat. Kissing ulcers (A), induced in a rat three days after intraluminal application of 60% acetic acid. Arrows indicate round ulcers on the posterior and anterior walls. Using a different method (injection of 20% acetic acid into the stomach wall), the visceromotor response to gastric distension is significantly enhanced from three days (B) to 60 days (C) after acetic acid treatment (p< 0.05). Source: (A) From Ref. 22; (B) Redrawn and adapted from Ref. 24.

concentrate on just one—the duodenum. As described by Colburn et al. (28), a balloon catheter is surgically inserted into the duodenum through the stomach and exteriorized at the base of the skull. The animal is allowed to recover following closure of the incisions, and testing is initiated 5 to 14 days later. Inflation of the balloon results in writhing behaviors, the score of which (higher scores relating to pain-like behavior) increases with increasing distension volume (28,29) and are dose-dependently inhibited by morphine (28). Passive avoidance testing also provides behavioral evidence that duodenal distension in the rat is aversive (30,31).

The response to distension of the viscera is often assessed in restrained or lightly anesthetized animals using chronically implanted EMG electrodes in the abdominal wall. This method has been modified to allow the study of duodenal distension-evoked responses in the freely moving rat (30,31). Abdominal EMG electrodes and an arterial catheter (tunneled through the femoral artery to the aorta) are connected to a telemetry transmitter, and EMG, blood pressure, and heart rate are recorded simultaneously before, during, and after duodenal distension. Nijsen et al. report a volume-dependent increase in the EMG activity, blood pressure, and heart rate; morphine inhibited the EMG increases during distension (30). A similar method has been used to examine the cardiovascular response to distensions lasting both 5 and 20 seconds (31).

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