Chronic pain involves a spontaneous component of pain as well as an affective component of pain. In an effort to understand the underlying mechanisms involved, Gary Strichartz, PhD, professor of anesthesia at Brigham and Women’s Hospital in Boston, Massachusetts, and colleagues developed an animal model for chronic post-thoracotomy pain that evaluated both evoked mechanical sensitivity and spontaneous pain.1
Using this animal model, Dr. Strichartz and colleagues not only measured evoked pain responses but also resting (non-evoked) pain, which is rarely tested in animal models.
“The mechanism used to assess this is conditioned place preference and relies on the conditioned association between having less pain from analgesic drug delivery and being in a particular place – one chamber of a two-chambered container,” Strichartz said. “The principle of this method is, if an animal gets relief from resting pain when it is placed in chamber A – identified by the rat by its wall pattern, floor composition, and odor – and no relief from a drug-free placebo injection when it has been placed in chamber B, then the rat will choose to spend more time in A than in B. But if there is no resting pain, the analgesic drug will not have any sensory effect and the place preference will be equal between A and B.”
Both Evoked and Non-Evoked Pain Present
This particular animal model showed spontaneous pain one to two weeks after surgery, which is a symptom reported by patients after a thoracotomy. Specifically, the investigators found that there does appear to be resting pain by conditioned place preference when using the well-established analgesic drug gabapentin, given systemically. “It reduces the evoked response that is heightened by surgery and also produces a preference in the animal’s choice of where to be, which demonstrates that it relieves resting pain,” he said.