The amygdala is essential for the expression of conditional hypoalgesia

Two experiments were conducted to determine whether the amygdala is involved in the performance of hypoalgesia as a Pavlovian conditional response. Rats were trained by pairing a distinctive observation chamber with a series of 3 footshocks. Rats were returned to the chamber 24 hr later, and the time spent engaged in freezing behavior and stereotyped behavioral reactions to a subcutaneous injection of dilute formalin was recorded. Sham-operated subjects spent large amounts of time freezing and were hypoalgesic on the formalin test in relation to nonshocked controls. Small electrolytic lesions of the amygdala eliminated both defensive freezing behavior and hypoalgesia without altering baseline reactions to formalin. Larger lesions made with ibotenic acid produced a similar pattern of results implicating neurons intrinsic to the amygdala. These results indicate that the amygdala may represent a forebrain site critical for the activation of descending antinociceptive systems in response to certain classes of environmental stressors.