Memory reactivation mediates emotional valence updating of contextual memory in mice with protracted morphine...

Mice subjected to morphine locomotor sensitization develop increased anxiety-behavior expression during protracted morphine withdrawal. This behavioral change is dependent on reexposure to the context of locomotor sensitization and reflects a state of conditioned anxiety. In this study, the effect of memory reconsolidation on the expression of conditioned anxiety in mice with protracted morphine withdrawal was examined. Five experimental protocols involving male C57BL/6 mice were used in which the animals were subjected to locomotor sensitization induced by morphine and reexposed to the context associated with the drug effect 28 days after locomotor sensitization and immediately after subjected to elevated plus maze. In experiment 1, mice were subjected or not to memory reactivation session and was observed that memory reactivation 27 days after sensitization reduced conditioned anxiety. In experiment 2, mice were subjected to memory reactivation, 24 h, 6 h or 1 h before contextual reexposure, and the effect of memory reactivation coincided with the temporal requirement for reconsolidation. In experiment 3, which involved exposure to a situation of acute stress immediately before memory reactivation, the mice demonstrated a return to increased conditioned anxiety. To confirm the influence of reconsolidation, in experiments 4 and 5, mice subjected to memory reactivation were treated with Nimodipine, diazepam or cyclohexamine, substances commonly used as pharmacological controls in reconsolidation experiments. Treatment with each substance separately inhibited the effect of reactivation in experiment 5 (presence of acute stressor) but not in experiment 4 (absence of acute stressor). These results suggest that, in our experimental model, reconsolidation is mediated through updating of the emotional valence of contextual memory associated with the administration of morphine.