Abdominal surgery decreases food-reinforced operant responding in rats: relevance of incisional pain.

BACKGROUND: Establishment of early oral nutrition after surgery is associated with a decrease in morbidity and mortality. The following studies were undertaken to determine how surgery influences food-reinforced behavior in rats and to determine the relevance of afferent input from the incision site on this behavior.
METHODS: Rats were trained to press a lever for food pellets to assess the effects of various abdominal surgical manipulations. Operant requirements and food availability were also manipulated. The effects of wound infiltration with bupivacaine and denervation of the abdominal musculature in the area of the incision were similarly examined.
RESULTS: Incision of the skin and abdominal musculature produced significant behavioral effects. Food pellets earned were significantly decreased, with gut manipulation producing effects of greater magnitude and duration than incision alone. Operant requirements or different schedules of food availability did not influence the effect of surgery on behavior. Infiltration of the wound with bupivacaine produced a reversal of the effects of surgery on behavior after skin and muscle incision but had minimal effects when the viscera were manipulated. Similarly, denervation of the abdominal musculature reversed the effects of abdominal incision on behavior.
CONCLUSIONS: Food maintained behavior is disrupted after laparotomy in rats. The time course and magnitude of this disruption, as well as its reversal by bupivacaine or denervation, are consistent with postoperative incisional pain. Manipulation of the viscera produces a greater effect than laparotomy alone, and additional mechanisms unrelated to incisional pain affect food reinforcement and feeding after surgery.