Acute morphine dependence: Effects observed in shock and light discrimination tasks

Alterations in shock discrimination accuracy in the rat, indicative of hyperalgesia, have been noted 1–3 days following a single injection of morphine. To establish the extent to which these “withdrawal-like” effects were specific to the shock discrimination paradigm, rats were trained in two separate discrimination tasks. The discriminative stimuli (SD's) for correct lever presses were mild electric shocks of different intensities in one task and were short duration lights over the levers in the other. After achieving comparable accuracy levels in the two tasks, the animals were injected SC with 30 mg/kg morphine sulfate and performance levels assessed 1, 2, 3, and 7 days later. Shock discrimination accuracy was significantly enhanced on post-morphine day 2, while accuracy in the light position task was not significantly affected on any of the post-morphine test days. The results indicated that increased pain sensitivity, as well as other signs of dependence, can occur following acute exposure to morphine.