MODULATION OF HYPOALGESIA BY MORPHINE AND NUMBER OF SHOCK TRIALS - COVARIATION OF A MEASURE OF CONTEXT FEAR AND HYPOALGESIA

In a recent series of studies, we observed that exposure to prolonged foot shock increased hypoalgesia induced by morphine. This increase wa s observed only when testing was conducted in the presence of shock-as sociated cues, suggesting that it resulted from context-conditioned fe ar. However, we do not know whether the extended stressor parameters e mployed in that study are necessary for an observance of the effect. T herefore, in the present study, we assessed the effect of the number o f shock trials (either 0, 20, 100, or 200) on the hypoalgesia observed following morphine administration. In addition, we measured activity as an independent index of context-conditioned fear, because in prior studies there had been no independent behavioral assessment of the con ditioning of fear to the context. Although others have shown a covaria tion of conditioned fear and context-induced hypoalgesia using shock p arameters and test paradigms different from our own, we sought to asse ss whether the same covariation would herd for conditioned fear and th e hypoalgesia observed following the administration of morphine. The r esults showed increased hypoalgesia in all groups exposed to foot shoc k, demonstrating that prolonged exposure to foot shock is not necessar y for an observance of this effect. In addition, the results revealed a linear relationship between number of trials of shock and hypoalgesi a, but a U-shaped relationship between trials and activity. The patter n of results is considered in light of Fanselow's Perceptual-Defensive -Recuperative model.