Evidence that adaptation to cold water swim-induced analgesia is a learned response

Controlling for novelty of the test context, the present experiment determined if adaptation to forced cold water swim stress-induced analgesia is learned through Pavlovian conditioning. Following baseline measurement of pain sensitivity, Group A swam in Context A for 3 min and, 8 h later, sat in Context B for 3 min. The conditions were reversed for Group B. All rats were given a tail-withdrawal test immediately after swimming or sitting in each context. On the first test day, conducted 24 h after the completion of the adaptation phase, all rats swam in Context A for 3 min, and tail-withdrawal latencies were immediately obtained after the swim. On the second test day, 24 h later, all rats swam in Context B, with tail-withdrawal latencies measured immediately thereafter. Both groups showed significantly less analgesia when tested in the adaptation context in which they previously swam than in the other context. These data provide strong evidence that adaptation to stress-induced analgesia is a learned response.