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 was observed only when testing was conducted in the presence of shock-associated cues, suggesting that it resulted from context-conditioned fear. However, we do not know whether the extended stressor parameters employed in that study are necessary for an observance of the effect. Therefore, in the present study, we assessed the effect of the number of 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 conditioning of fear to the context. Although others have shown a covariation of conditioned fear and context-induced hypoalgesia using shock parameters and test paradigms different from our own, we sought to assess whether the same covariation would hold for conditioned fear and the hypoalgesia observed following the administration of morphine. The results showed increased hypoalgesia in all groups exposed to foot shock, demonstrating that prolonged exposure to foot shock is not necessary for an observance of this effect. In addition, the results revealed a linear relationship between number of trials of shock and hypoalgesia, but a U-shaped relationship between trials and activity. The pattern of results is considered in light of Fanselow's Perceptual-Defensive-Recuperative model.