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Background: A patient underwent repair of a thoracoabdominal aortic aneurysm. Epidural morphine, 4 mg, was given for pain relief. After anesthesia, the patient displayed lower extremity paraparesis. This effect was reversed by naloxone. The authors sought to confirm these observations using a rat spinal ischemia model to define the effects of intrathecal morphine administered at various times after reflow on behavior and spinal histopathology., Methods: Spinal cord ischemia was induced for 6 min using an intraaortic balloon. Morphine or saline, 30 microg, was injected intrathecally at 0.5, 2, or 24 h after reflow. In a separate group, spinal cord temperature was decreased to 27 degrees C before ischemia. After ischemia, recovery of motor function was assessed periodically using the motor deficit index (0 = complete recovery; 6 = complete paraplegia)., Results: After ischemia, all rats showed near-complete recovery of function by 4-6 h. Intrathecal injection of morphine at 0.5 or 2 h of reflow (but not at 24 h) but not saline caused a development of hind limb dysfunction and lasted for 4.5 h (motor deficit index score = 4-6). This effect was reversed by intrathecal naloxone (30 microg). Intrathecal morphine administered after hypothermic ischemia was without effect. Histopathological analysis in animals that received intrathecal morphine at 0.5 or 2 h after ischemia (but not at 24 h) revealed dark-staining alpha motoneurons and interneurons. Intrathecal saline or spinal hypothermia plus morphine was without effect., Conclusions: These data indicate that during the immediate reflow following a noninjurious interval of spinal ischemia, intrathecal morphine potentiates motor dysfunction. Reversal by naloxone suggests that this effect results from an opioid receptor-mediated potentiation of a transient block of inhibitory neurons initiated by spinal ischemia.