Behavioral evidence for an opiate pituitary mechanism subserving conditioned analgesia

The effect of a naloxone or a dexamethasone pretreatment on the conditioned analgesia resulting from the exposure of rats to an experimental chamber repeatedly paired with a grid shock was investigated. Shock induced disruption of bar pressing activity was taken as a behavioral measure of pain responsiveness. It was found that a 6, but not a 3, mg/kg dose of naloxone i.p. is hyperalgesic in unconditioned rats and effectively antagonizes conditioned analgesia. Dexamethasone (up to a dose of 2 + 1 mg/kg i.p. 23 and 1 h prior) did not alter the pain responsiveness of unconditioned rats, but caused a dose dependent suppression of conditioned analgesia. These results are discussed in terms of an opiate pituitary mechanism subserving conditioned analgesia.