Examining the effects of pain on morphine self-administration following a spinal contusion injury

Morphine is one of the most effective treatments for pain after spinal cord injury (SCI), but clinicians are concerned about addiction when prescribing this drug. Experimental evidence suggests that the presence of pain lowers the potential for addiction. Indeed, we have shown that morphine self-administration is lowered after SCI, but we do not know whether pain is driving this effect. We conducted two experiments to test this. First, we examined morphine self-administration in the acute phase of injury. Rats were given a sham, mild, moderate, or severe contusion injury. They were placed into self-administration chambers for 7-days (beginning 24-hours after injury). Irrespective of injury severity, all subjects displayed the same pattern of morphine administration: animals given access to a higher concentration of morphine (3.0 mg/lever press) administered more than those given access to a moderate dose (1.5 mg). This dose-dependent effect suggests that the subjects were not simply administering the drug for analgesia. Self-administration was then assessed in the chronic phase of injury, when SCI animals begin to develop neuropathic pain. Rats were placed into self-administration chambers for 7 days beginning 14- or 28-days following injury. In contrast to the acute phase of injury, we found contused animals administered the full amount of morphine available to them each session (even at the moderate, 1.5mg, dose), at a rate commensurate with sham controls and an amount that far exceeded that needed for analgesia. Together, these results suggest the addictive potential of morphine is altered in the acute, but not the chronic phase of injury. This is in contrast to what has been reported using other models of neuropathic pain. Neuropathic pain does not appear to reduce the addictive potential of morphine in an SCI model.