Conditioned changes in pain reactivity: II. In search of the elusive phenomenon of conditioned hyperalgesia.

Previous research suggests that a stimulus that has been paired with an aversive event can elicit either an increase (hyperalgesia) or decrease (hypoalgesia) in pain reactivity in rats. Attempts have recently been made to isolate the variables that determine the direction of the conditioned response. Very little evidence has been found for conditioned hyperalgesia when a spinally mediated measure of pain reactivity (the tail-flick test) was used. In the present study, the impact of a conditioned stimulus was assessed using a procedure modeled after one in which conditioned hyperalgesia was obtained with the tail-flick test (Davis & Hendersen, 1985, Experiment 4). With this training paradigm, a discrete conditioned stimulus was found to elicit hypoalgesia but not hyperalgesia.